OPTICAL FIBER EXPERIMENTS
FOR UNDERGRADUATE
ENGINEERS
by
DAVID LEO NELSON, B.S. IN E.E.
A THESIS
IN
ELECTRICAL ENGINEERING
Submitted to t h e Graduate F a c u l t y
of Texas Tech U n i v e r s i t y i n
P a r t i a l F u l f i l l m e n t of
the Requirements f o r
t h e Degree of
MASTER OF SCIENCE
IN
ELECTRICAL ENGINEERING
Approved
August 1982
C^ri^ * ^
ACKNOWLEDGEMENTS
I dedicate this work to my wife and companion Juliet Martin Nelson
for her belief in and love for me, and to my daughter Melodee Elisabeth
for providing the impetus to finish.
I would like to thank Dr. Thomas F. Krile for his guidance and interest in my education.
I also thank my colleagues in the Optical Systems
Lab — particularly, Dr. Gary K. Froehlich, Larry M. Baker and Bailey H.
Jones for many helpful conversations, and Fred Finlay and Fernando Bermudez
for important physical assistance.
Thanks are also due Telesforo Delacruz
and Lloyd Gordon who provided the technical assistance necessary for constructing the lab-built equipment.
Special thanks to Don Johnson for do-
ing the lion's share of the work for Chapter 10.
Much of the fiber used in the experiments was provided by Corning
Glass Works through the University Grant Program.
I will state here
that, in the course of these experiments, the gifts were not always
used for the purposes for which they were designed.
Any deficiencies
are, therefore, mine.
To Drs. R. H. Seacat, William M. Portnoy, and T, R, Burkes, thank
you for providing me with a strong sense of purpose and excellent examples.
With appreciation, I acknowledge Drs. John Craig, Truman Lewis
and John Walkup for serving on my committee.
Finally, thanks are due the National Science Foundation which supported the development of the projects under Grant y/SER-8001394.
11
CONTENTS
ACKNOWLEDGEMENTS
ii
ABSTRACT
iy
LIST OF TABLES
v
LIST OF FIGURES
vi
I.
II.
INTRODUCTION
1
MEASUREMENT OF NUMERICAL APERTURE
6
III. MEASUREMENT OF SPECTRAL ATTENUATION
IV.
MEASUREMENT OF FIBER LOSS. . '
19
32
V.
CHARACTERIZATION OF DETECTORS FOR OPTICAL FIBER SYSTEMS. . 45
VI.
CHARACTERIZATION OF SOURCES FOR OPTICAL FIBER SYSTEMS. . . 60
VII.
VIII.
IX.
WAVELENGTH MULTIPLEXING IN AN OPTICAL FIBER
78
OPTICAL LINK DESIGN
96
FIBER PARAMETERS BY SCATTERING MEASUREMENTS
118
X.
AN OPTICAL FIBER ACOUSTIC SENSOR
138
XI.
A HOLOGRAPHIC COUPLER FOR FIBERS
150
CONCLUSION
164
XII.
APPENDICES
I.
169
BEAM LAUNCHER
169
A LAB-BUILT GONIOMETER
172
A LAB-BUILT FIBER CLEAVER
175
A LAB-BUILT PHOTOMULTIPLIER TUBE HOUSING
178
A LAB-BUILT LASER DIODE PULSER
181
A LAB-BUILT FIBER STRETCHER
188
VII.
TRANS IMPEDANCE AMPLIFIERS
190
VIII.
A LAB-BUILT LIGHT CHOPPER
196
II.
III.
IV.
V.
VI.
BIBLIOGRAPHY
199
iii
ABSTRACT
This report describes a set of ten experiments designed to introduce undergraduate engineering students to the area of fiber optics.
The projects include measurement of pertinent parameters of optical
fibers, sources and detectors (the major components of fiber optic
systems), the construction of a simple fiber optic communication link,
the use of an optical fiber as a sensor of acoustic waves, and the making of a holographic optical element to serve as a fiber coupler.
Each
experiment is self-contained with subsections relating to theory and experimental practice. Although the experiments are expressly designed for
a project-type laboratory, it is hoped that they will prove useful in
other situations such as classroom demonstrations. A special effort has
been made to employ equipment which might be expected to be in undergraduate engineering departments.
Appendices cover the construction of
certain useful auxiliary equipment.
IV
LIST OF TABLES
Table
Page
1-1.
Comparison of Communications Cable Types,
2
5-1.
Photodetectors Compared
50
5-2.
Rise-Time Comparison
51
5-3.
Relative Responsivity
53
5-4.
Dark-Current Comparison
7-1.
Some Optical Filters in the IR
8-1.
Power and Loss Relationships
.....
54
88
,
v
9"
LIST OF FIGURES
Figure
Page
2-1. Meridional rays and skew rays in a step-index fiber. "I"
2-2
denotes an intersection with the fiber axis
15
Geometry for calculating the acceptance angle
15
2-3.
Different angles of entrance lead to different path lengths
in the fiber
2-4. Index of refraction versus wavelength. . ,
2-5.
16
16
Equipment setup: a) schematic, b) actual L, laser; C, collimator; BS, beamsplitter; R, reference detector; X, microscope objective for overfilled launch condition; M, fiber
micropositioners; MS, mode stripper; D, detector
17
2-6.
Output pattern for overfilled launch
18
2-7.
Output pattern for underfilled launch
18
3-1. Measured source spectral sensitivity
28
3-2.
Absolute spectral sensitivity of S-1 phosphor
28
3-3.
Calibrated source spectral sensitivity.
29
3-4.
Equipment setup for the measurement of spectral attenuation
CO, collimating optics; PH, pinhole; X, microscope objective;
IF," interference filter and holder; MS, mode stripper; OF
optical fiber ; PMT, photomultiplier
30
3-5.
Relative fiber spectral loss
31
4-1.
Equipment setup
43
4-2.
Loss vs. lateral misalignment
43
4-3.
Loss vs. angular misalignment
44
4-4.
Loss vs. longitudinal misalignment
44
5-1.
Equipment setup for a) rise-time measurement, b) spectral
sensitivity measurement
57
5-2.
S-1 response curve
58
5-3.
"Calibrated" tungsten-halogen source curve
58
vi
5-4.
6-1.
6-2.
Spectral sensitivity curves: a) FPT-100 phototransistor,
b) C-30808 PIN photodiode, c) C-30817 avalanche photodiode
59
Three types of input-coupling loss: a) unintercepted illumination, b) NA mismatch, and c) reflection.
72
Intensity profiles for various values of m. Circles are
drawn for values of m from 1 (Lambertian) to 30
73
6-3.
Equipment setup for measuring intensity vs. drive current. , 73
6-4.
Output current in RCA C-30808 PIN diode vs. drive current in
source. (45V back bias on PIN, 82kJ2 in series.) a) RCA
C-30123 IR-LED, b) RCA SG2001 ILD
74
6-5.
Equipment setup for measuring intensity profile
75
6-6.
Experimentally determined intensity profiles, a) RCA
C-30123 IR-LED, b) RCA SG2001 laser diode
76
Equipment setup for measuring spectral distribution of
sources
77
7-1.
Frequency- or wavelength-division multiplexing
91
7-2.
a) Spectrum of band-limited signal (modulated cosine); b)
spectrum of band-limited signal multiplied by square-pulse
of frequency oj . .'
91
7-3.
An envelope detector
92
7-4.
Equipment setup
92
7-5.
LED driver circuit for both analog and digital inputs. . . .
93
7-6.
Crude wavelength-division multiplexer
93
7-7.
Alternative WDM schemes: a) beamsplitter, b) wavelengthselective mirror
94
6-7.
7-8. Multiplexed output. [Red: 14mV _ , -10 Hz sinusoid; IR:
lOmV
, 120 Hz, square wave]. .^.^
94
P-P
7-9. Demultiplexed IR signal. [10 mV _ , 120 Hz, square wave], , 95
7-10.
8-1.
Transmission characteristics of Kodak Wratten filter No.
47A
Basic transmission system,
L = Loss in dB
Vll
95
HI
8-2.
Alternative formats for digital data, a) non-return-tozero (NRZ); b) return-to-zero (RZ)
2^2
8-3.
Required optical power, a) vs. bit rate for digital
system, b) vs. bandwidth for analog system with APD, and
c) vs. bandwidth for analog system with PIN detector. . . . 113
8-4.
Optical power throughput worksheet. From 12]
114
8-5.
Rise-time analysis worksheet. From I2J
115
8-6.
Electronic schematic for the system
116
8-7.
Transmitted IF signal and detected audio signal. Upper
trace — IF signal; lower trace - audio output
117
9-1.
Setup to observe backscattered light
I33
9-2.
Incident, reflected, refracted, and emergent ray paths. . . 133
9-3.
Rays incident upon fiber, traced for a single internal
reflection
Plot of $ and 6 versus 6 for a fiber of n = 1.5
o
Ray considerations to determine fiber diameter
9-4.
9-5.
9-6.
Cross section of fiber, showing paths of refracted and
reflected rays that leave the fiber at the same scattering
angle 6
-|^^,
134
135
I35
9-7.
Cross section of fiber, showing refracted ray at the angle of incidence that just grazes the core. Bounds for angles 9 and 6
are shown. Dashed ray is cladding ray which leaves at *the same
scattering angle as the core ray
136
9-8.
Composite graph of experimental and theoretical scattering
patterns, a) experimental results, b) fringe position calculated from geometric ray-tracing, and c) calculated fringe
modulation. From [4J
136
9-9.
Backscatter pattern of unclad step-index fiber
I37
9-10.
Backscatter pattern of clad graded-index fiber
137
10-1.
Homodyne acoustic sensor configuration
147
10-2.
a) fringe field at detector showing pinhole position, b)
light intensity vs. distance from pinhole, showing operating
point
'
147
viii
10-3.
Heterodyne acoustic sensor configuration, , , ,
10-4.
Fringe pattern at detector, , . ,
10-5.
Recovered acoustic signal at 1 kHz. , . , ,
I49
10-6.
Recovered signal at "transition" showing instability
149
11-1.
Setup for HOE^ recording. Plane-wave reference. L, laser;
C, collimator; Ml, M2, M3, mirrors; BS, beamsplitter; 0,
object; H, holographic plate, a) schematic, b) actual , . , , 160
11-2.
Schematic setup for HOE playback. Spherical playback, L,
laser; C, collimator; Ml, M3 mirrors; LI, converging lens
of NA equal to fiber which is to be coupled; H, hologram. , . 161
11-3.
Coordinate systems for spherical wave holography,
ing, b) playback, . . , ,
,
11-4.
I43
,,,,,,,
148
a) record161
Geometry used for calculating position of object and the
reference source given the desired positions of the playback
source and the real images, . ,
162
Virtual images of four point sources as played back from the
HOE
-
162
Real images of two of the four point sources obtained upon
playback of the HOE
163
I-l.
A typical beam launcher
171
1-2.
Bent fiber serving as a mode scrambler
171
A lab-built goniometer
174
A lab-built fiber cleaver
177
Components of the PMT housing. From left; base plate with
BNC connectors, pedestal for PMT socket, PMT and magnetic
shield, outer case and top plate with cover
180
The assembled PMT housing
180
V-1.
The discharge circuit
186
V-2.
The charging circuit
186
V-3.
The trigger circuit
187
V-4.
Waveform of the current pulse through the laser diode
(50 nsec/div)
187
11-5.
11-6.
II-l.
III-l.
IV-1.
IV-2.
IX
VI-1.
A lab-built fiber stretcher
189
VII-1.
The simplest photodiode bias circuit
193
VII-2.
Solid-state photodiode equivalent circuit when operating
into low impedance
. 193
VII-3.
The transimpedance amplifiei:
193
VII-4.
VII-5.
Poor front-end amplifier design . . . . . .
...
Better choices for front-end amplifiers using transimpedance techniques. Where two photodiodes are shown,
one of the two is shielded from all light for dark current
compensation
194
195
A lab-built light chopper
198
VIII-1.
X
CHAPTER I
INTRODUCTION
The field of fiber optics is presently experiencing a concentration
of research and application which perhaps has not been seen since the
introduction of the transistor.
The advances in silicon technology in
the electronics industry accelerated the development of low-loss optical
fibers, and communication system applications soon followed.
The elec-
tronics industry also added to the growth of the fiber optics industry
with the production of solid-state sources and detectors especially
tailored to the needs of fiber systems, and progress in fiber optic connectors promises to maintain this growth.
The advantages which optical fiber systems hold over conventional
metallic communication systems, summarized in Table 1-1, assure that
fibers will continue to see new applications [1], This, in turn, suggests that the newly-graduated electrical engineer, whatever his specialization, will inevitably encounter a design problem which is suitably
handled by a fiber system.
Any prior knowledge of such topics as the
vocabulary of fiber optics, requirements for fiber optic system design,
and relative values of parameters for the various components will be
invaluable.
With this in mind, a set of experiments is presented here to permit
undergraduate electrical engineering students to gain experience handling,
evaluating, and using optical fiber and associated sources and detectors.
The report is the fourth in a series sponsored by the National Science
Foundation for the purpose of speeding the transfer of information about
TABLE 1-1.
Characteristic
Comparison of Communications Cable Types
Twisted
Pair
Length-Bandwidth
Product (MHz-km):
1
Repeater Spacing(km) :
1-2
System Cost:
Low, slow
increase in
future
•
System Lifetime
(Years):
20-40
Crosstalk:
Noise immunity:
Electrical
input-output
insulation:
Vibration Tolerance:
Weight, size:
Cable connections:
High
Low
No
Good
High
Soldering,
standard
connectors
Coaxial
Cable
Fiber
Optics
Cable
20
1-2
Medium, slow
increase in
future
400
2-10
High, now.
steep decrease
in future
20-40
1-2, 10-40 in
future
Negligible
High
Low
Medium
No
Good
High
Soldering,
standard
connectors
Complete
Good
Low
Splicing, wellaligned
connectors
new technologies from industry to the universities [2], [3], [4]. An underlying goal in the development of these projects was to provide experiments which can be performed with relatively inexpensive equipment which
might reasonably be expected to be found in university undergraduate
laboratories.
In some cases, specialized equipment was "home-made,"
and construction details are found in the appendices.
The chapters are grouped basically by measurement and application.
Chapters II through IV deal with the measurements of fiber properties
while Chapters V and VI are concerned with the sources and detectors
which are candidates for fiber optic systems.
Chapter VII illustrates
an important property of optical fiber, namely, that many signals can
propagate through a fiber without interference.
Chapter VIII combines
knowledge obtained in the preceding chapters to construct a simple fiber
link.
Chapter IX introduces an elegant measurement technique capable of
determining an optical fiber*s dimensions and index of refraction by
observing the way in which the fiber scatters light incident on its
side.
Chapter X demonstrates the use of an optical fiber as a sensor
for acoustic waves, and Chapter XI relies on Froehlich's work [4] for
the construction of a holographic optical element which can be used as
a multi-fiber coupler.
Even with lab-built equipment, not all types of experiments are
within the reach of undergraduate students.
The most notable experi-
ments absent from this report are those dealing with the measurement of
fiber dispersion, fiber index profiling, single-mode fibers, and optical
time-domain reflectometry — all of which are important current areas of
study.
However, it is hoped that the experiments presented here will
serve to instill an appreciation for the capabilities of fiber optic
systems and to whet the students' appetite for further work.
The experiments are designed to be performed as three-week project
labs in the Electrical Engineering Department at Texas Tech University,
Typically, two students, working as partners, are given the project
assignment plus a few selected references.
They are left very much to
themselves as far as researching the material and deciding how to attack
the problem.
At the end of the three-week period, the students meet with
an appointed faculty advisor who grades them on how well their solution
meets the specifications of the problem statement.
Following this, the
students meet once more with the advisor for an oral examination in which
they are asked about the underlying principles associated with the lab
project and design and measurement details.
They receive a second grade
for this oral examination.
The experiments are adaptable to other formats, and each experiment is self-contained.
The philosophy of having the students research
the required information should, however, be left intact; i.e. this should
not be used as a "cookbook."
Students should be strongly encouraged to
read the references presented at the end of each experiment.
Sample ques-
tions after each experiment represent what might be asked at an oral examination, and may indicate ways in which the experiment might be expanded.
A bibliography is also included for general information.
Reference
[1] Wolf, H. F., editor. Handbook of Fiber Optics, Garland STPM Press,
New York, 1979, p. 8.
[2]
Peckham, L. N., M. 0. Hagler, and M. Kristiansen, "Laser Experiments for Undergraduate Electrical Engineering Students," Technical Report #1, NSF Grant GY-4761, June 1969, Texas Tech University.
[3] Molen, G. M., C. R. Parten, M. 0. Hagler, and M, Kristiansen, "Laser
Experiments for Undergraduate Electrical Engineering Students,"
Technical Report y/2, NSF Grant GY-4761, May 1971, Texas Tech University.
[4]
Froehlich, G. K., J. F. Walkup, and M. 0, Hagler, "Optical Information Processing Experiments for Undergraduate Engineers," Final
Technical Report, NSF Grant SER75-17673, January 1977, Texas Tech
University.
Copies of this report are available from the National
Technical Information Service, Springfield, VA, 22151 Caccession
No. PB 264356).
Chapter II
MEASUREMENT OF NUMERICAL APERTURE
Project Assignment
You are to measure the numerical aperture of two representative
types of optical filler using two techniques. You should take care
that the measurements you make can be repeated with good accuracy.
You may use a laser as the source.
Demonstrate how the measurement
varies with the input launch conditions.
Objectives of the Experiment
1.
To acquaint the student with the characterization of an
optical fiber by measurement of its numerical aperture.
2.
To familiarize the student with the problems of fiber
handling, inputting and extracting light, and different
types of optical fibers.
Equipment Needed
1.
Short (1 meter) length of optical fiber.
2.
Fiber cleaving tool and polishing equipment.
3.
Laser.
4.
Collimating optics.
5.
Assorted focal length lenses.
6.
Two photodetectors and power supply.
7.
Electrometer or digital voltmeter.
8.
Goniometer (see Appendix II).
9.
Simple mode-strippers.
10.
Fiber-optic micropositioners.
Theory
Introduction
The measurement of numerical aperture is probably the most
fundamental and productive measurement which can be performed on
a fiber in terms of the amount of information obtained for the
effort.
It is a first-order measure of the light-acceptance capa-
bility of the fiber, and immediately characterizes the fiber as
single-mode or multi-mode.
The measurement of numerical aperture
(or NA) is analogous to the measurement of the light-gathering
power of an optical system in lens optics [l].
The numerical aperture must be known if the system source
and detector are to be optimally matched to the fiber with respect
to coupling parameters.
It is important to choose a numerical
aperture whose value is an acceptable compromise among the normally
conflicting requirements of large bandwidth, large optical acceptance
angle, and minimum bending loss.
Basic Concepts
In optical waveguides, meridional rays
planes which also contain the waveguide axis.
enter the fiber in
If the fiber is per-
fectly straight and the rays entering are exactly parallel to the
fiber axis, the rays will propagate through the fiber with no deflection (assuming there is also no scattering).
If the fiber is
not straight, or the rays enter the fiber at an angle with respect
to the fiber axis, the rays will follow a wavelike course through
the fiber, due to reflections at (or bending near) the core-cladding
interface.
8
Skew rays describe all rays which do not pass through the fiber
axis.
Rays of this type follow continuous helical paths inside the
fiber core. Meridional rays represent the more important propagation mechanism and are easier to describe [2]. Both types are
shown in Figure 2-1.
Information carried by light in an optical fiber is constrained
to remain in the fiber by the property of total internal reflection.
At the interface of two materials of differing indices of refraction,
say n, and n„, light passing from material 1 at an angle of 6. with
respect to the normal at the interface will travel in material 2
at an angle 9^ with respect to the normal as given by Snell's law:
sin 62 = (n./n2) sin-e^
(2-1)
The maximum value of the angle of incidence 0^ for which a
ray will be totally internally reflected can be derived from Snell's
law and simple trigonometry.
With reference to Figure 2-2,
n^sin BQ = n^sin 6^ = n^cos 0^
= n^ [1 - (sin 0^)^f'^
(2-2)
At the critical acceptance angle 0^^, sin 0^^ = (n2/n^)sin 90 .
Therefore,
NA = n sin6
o
oc
r
,
/
N2n0.5
= n^ [1 - (n2/n^) J
2
2n0.5
= [n^ - nj ]
(2-3)
Notice that light entering the fiber at angles greater than the
critical angle 6^^ are "leaked" into the cladding and later lost
to the environment due to bending of the fiber and imperfections
(scratches, dust, microbends, etc.) on the cladding surface. Although the figure has been drawn for the case of a step-index fiber,
it should be obvious that there is a critical angle associated with
graded-index fibers also.
Strictly speaking, the above holds only for meridional rays
in an ideal fiber.
For this reason, the NA calculated in Equation
(2-3) is often called the nominal !^.
However, skew rays at inci-
dent angles greater than the critical acceptance angle can also be
conducted by fibers of circular cross section.
In practice, one finds that the limiting angle 6
is not as
sharply defined in real fibers as indicated by Equation (2-3).
Diffraction, striae, and irregularities at the core-cladding interface all tend to decollimate the transmitted light and, in so doing,
increase the effective NA.
There is an important connection between numerical aperture
and dispersion, both modal and material [3]. The number of internal
reflections and therefore the total length of the path which the
light travels in a given length of fiber is smaller for smaller
angles of incidence 0 , as in Figure 2-3.
Because the velocity of
light in a material is given by:
V = c/n
,
(2-4)
where n is the refractive index of the material and c is the speed
10
of light in a vacuum, monochromatic rays which are very nearly
parallel to the fiber axis will reach the output end of the fiber
faster than rays which enter the fiber at high angles of incidence.
If the range of angles at which light is allowed to enter the fiber
is very broad (i.e., if the NA is very large), and if the fiber is
very long, the puls^ of light detected at the output will be much
longer than the pulse of light injected at the input due to wavepackets arriving at different times. This is the phenomenon known
as modal dispersion.
It imposes a maximum bandwidth restriction
on the system [4].
For most materials, the refractive index is not a constant
but is a function of the wavelength of light. Hecht and Zajac [5]
have examples for several optical materials; the curves are of the
form given in Figure 2-4.
It is seen from the curves that longer-
wavelength radiation "sees" a smaller refractive index while traveling through the material than shorter-wavelength
tion.
radia-
From Equation (2-4), one infers that longer wavelengths tra-
el faster through this medium than shorter wavelengths.
This gives
another component to pulse-spreading, owing to the fact that no
sources are perfectly spectrally pure but have a spread of wavelengths
present.
This phenomenon is called material dispersion, and of
course the longer the path length, the worse will be the effect.
Because of this, high NA fibers, with the associated longer path
lengths can have larger material dispersion effects than lower M
fibers of the same material.
11
Experimental Procedure and Results
Two techniques are typically used to measure a fiber's numerical
aperture.
The first method specifies overfilling the fiber's input
and uses a white card to allow the spot size to be viewed at the output.
A fiber is said to be "overfilled" when the launching numerical
aperture is larger Chan that of the fiber. This spot size represents
approximately the width of the 90% intensity points. Thus the human
eye is the measuring instrument.
The radius of the spot divided by
the distance of the spot from the end of the fiber will be the tangent of the acceptance-cone half angle.
The second method utilizes the lab-built goniometer (Appendix II).
A PIN detector mounted in an L-shaped bracket is swept around the
fiber's endface in increments of two degrees.
In both methods, the angular extent of the cone of radiation
exiting the fiber is assumed to be the same as the acceptance cone.
The equipment configuration of Figure 2-5 is used. A laser source is
used because of the narrow spectral line width which places less
stringent requirements on the optics. If a broadband source, such as
a tungsten source, were used, special (and expensive) achromatic
optics would be required to ensure that all wavelengths were brought
to a focus at the same place on the fiber endface.
With the fiber overfilled, the curve of Figure 2-6 was obtained.
Narrowing the angle of incident radiation at the fiber output results
in a narrowing of the cone of the detected radiation at the output
and a subsequent lower measurement for numerical aperture, as indicated
in Figure 2-7.
12
Special Problems
It is imperative that some form of mode-stripping be used
to normalize the input launch conditions.
The fiber may be passed
through a pair of large black rubber stoppers with several drops
of glycerin which provides an index-match with the cladding and
strips away light which is present in the cladding.
Alternatively,
the fiber may be pressed between layers of glycerin-soaked black
velvet.
An important phenomenon to observe while performing this
experiment is the dependence of the measurement on the input launch
conditions.
It was specified that the fiber be overfilled, but
what if it is not?
Assuming appropriate mode-stripping, a narrowing
of the input cone of light entering the fiber should result in a
narrowing of the output cone and hence a smaller measured value for
fiber NA.
If mode-stripping is not done, the
high-angle
light modes propagating in the cladding may lead to an erroneous
measurement.
If a beamsplitter setup is not used to simultaneously monitor
the output power of the laser source, power fluctuations in the
source may also obscure the effect.
If a single detector is used,
the intensity readings taken from the photodiode at the output must
be averaged at each angular position to minimize the effects of
power fluctuations.
If the statistics of the laser power fluctua-
tion are known, the readings may be subjected to a weighted averaging
method.
If, for example, the source power fluctuation is known to
be Gaussian, simple averaging is sufficient.
Another problem may appear when the MA of a very long fiber is
13
measured.
If there is a strong tendency for mode coupling in the
fiber, i.e., a tendency for "low-angle" rays to be coupled into
"high-angle" rays by scattering mechanisms or microbending, the
measured NA may be larger than the true value.
In addition, the
output cone may not be noticeably narrower for a corresponding
narrowing of the input excitation angle.
Sample Questions
1.
A phenomenon called "NA-dependent loss" is often referred
to in the literature.
2.
To what does this refer?
How would you automate this measurement for a company
which manufactures optical fibers?
3.
A thin pencil of light of diameter much smaller than the
diameter of the fiber is incident on a fiber endface.
What do you predict will be the output pattern of radiation?
4.
It has been mentioned that some skew rays with angles of
incidence greater than that of the critical acceptance
angle can be transmitted.
This suggests a second critical
acceptance angle for skew rays.
What is the equation for
this angle in terms of the relevant fiber parameters?
References
[l] Smith, Warren J.,"Image Formation:
Geometrical and Physical
Optics", Handbook of Optics, Walter G. Driscoll, Ed., McGrawHill, New York, 1978, p. 2.5.
[2] Wolf, Helmut F.,"Optical Waveguides", Handbook of Fiber Optics.
H. F. Wolf, Ed., Garland STPM Press, New York, 1979, pp. 5^-57.
[3]
Ibid., pp. 59 and 61.
14
[4] Wolf, Helmut F.,"System Aspects", Handbook of Fiber Optics,
H. F. Wolf, Ed., Garland STPM Press, New York, New York, 1979,
pp. 385-386.
r5] Hecht, Eugene and Alfred Zajac, Optics, Addison-Wesley,
Reading, Massachusetts, p. 42.
15
Core
Cladding
Figure 2-1. Meridional (M) and skew (S) rays in a stepindex fiber, "i" denotes an intersection
with the fiber axis.
Figure 2-2. Geometry for calculating the acceptance angle.
16
Figure 2-3. Different angles of entrance lead to
different path lengths in the fiber.
Dense Flint
Glass
1.7
c
o
a
CO
Light Flint
Glass
1.6
u
0)
u
Crystal Quartz
Crown Glass
1.5
Vitreous
Quartz
l."*
20 0
'+00
600
3 00
1000
Wavelength X (nm)
Fi<yure 2-4. Index of refraction versus wavelength.
17
s a
\
CO
/
ia
/
en
1/ S
D
s
CO
A
X
en
pa
ij.
18
100
^s
^1
80
<»
l»
•
CO
c
>
60
1
t
0)
•H
4J
3
40
'
15.4J
3
O
>
u
20
rt
0
1
-10
-8
-6
-4
-2
0
+2
+4
Degrees off-axis
+6
+8
+10
Figure 2-6. Output pattern for overfilled launch.
100
^s
•H
CO
C
QJ
<
80
•= .
60
4-1
<1
a
40
8 4'
4J
<»
o
>
•H
(•
('
-
<
20
yi
4J
rt
0
-10
-8
-6
-4
-2
0
+2
+4
Degrees off-axis
+6
+8
+10
Figure 2-7. Output pattern for underfilled launch,
Chapter III
MEASUREMENT OF SPECTRAL ATTENUATION
Project Assignment
You are to measure the spectral attenuation of an optical fiber
utilizing a photomultiplier tube as your detector. Plot the attenuation in dB/km as a function of optical wavelength.
Be prepared to
discuss the various components of spectral loss.
Objectives of the Experiment
1.
To acquaint the student with loss in optical fibers,
particularly, spectral loss,
2.
To familiarize the student with the experimental procedure for measuring fiber attenuation.
3.
To provide the student with experience in using a
photomultiplier tube.
Equipment Needed
1.
Tungsten-halogen or other "white-light" source.
2.
A set of narrowband interference filters or a monochroma tor .
3.
A long length (approx. 100 meter) of optical fiber.
4.
Photomultiplier tube with an S-1.phosphor and high
negative voltage power supply.
(RCA 7102)
5.
Assorted optical lenses.
6.
Pinhole.
7.
Second photodetector of any type (for reference).
8.
Fiber cutter (see Appendix III).
19
20
9.
Beam splitter.
10.
Two micropositioners.
Theory
Introduction
If optical fibers attenuated light uniformly at all wavelengths,
the need to characterize spectral attenuation would not exist.
How-
ever, fibers do exhibit different losses for light of different wavelengths.
The spectral loss curve will typically show regions of
relatively lower attenuation (measured in dB/km) than others.
For
maximum efficiency, it is desirable to transmit information in the
region of lowest attenuation.
This has not been possible until
relatively recently when diode sources (both laser and LED) were
designed to emit radiation in the far infrared where many fibers
have attenuation minima.
Important also is the consideration that sources do not emit
radiation of one pure wavelength, but in fact, emit radiation in
a region about a central wavelength.
If fiber attenuation is not
uniform for the region of emitted radiation, unnecessary power
losses occur.
Basic Concepts
Spectral loss measurements are useful for showing wavelengthdependent attenuation in optical fibers due to absorption and
scattering.
phenomena:
Spectral loss is known [l] to be composed of three
(1) fundamental material scattering (Rayleigh scatter-
ing), (2) fundamental material absorption by the glass due to elec-
21
tronic transitions (in the UV region) and vibrational energy transitions (in the IR), and (3) impurity absorption due to overtones of
impurity atom vibrations.
Rayleigh scattering (named for Lord Rayleigh, who observed the
scattered flux density to be inversely proportional to the fourth
power of the wavelength) occurs when light scatters from particles
which are small in comparison to the wavelength of the light being
scattered.
In glass, Rayleigh scattering can arise from two separate
effects, density and composition fluctuations, and it tends to dominate the shape of the loss curve.
All transparent matter scatters
light due to fluctuations in the density (and hence, the refractive
index) which result
from fluctuations in temperature.
Glass differs
in that these fluctuations are "frozen-in" when the glass is cooled
in the annealing process.
The attenuation coefficient (base e)
which characterizes the scattering loss is called the turbidity [2].
A second major source of loss is material absorption.
The in-
dividual atoms of the optical material are held together by chemical
bonds and thermal energy maintains them in a state of random motion
or stretching vibration.
I^en an electromagnetic wave impinges on
an atom or a molecule, it interacts with the bound electron cloud,
imparting energy to the material; i.e., some of the light is absorbed
The oscillatory frequency of the electron cloud is equal to the
driving frequency, that is, the frequency of the electric field of
the light.
The amplitude of the oscillation will be large only
when the frequency is near the resonant frequency of the atom.
At
frequencies above or below resonance, the electrons vibrating with
respect to the nucleus can be regarded as oscillating electric di-
22
poles, and as such, they will reradiate energy at a frequency which
coincides with that of the incident light.
In addition to these
electron-oscillators which generally have resonances in the ultraviolet, there are atomic oscillators which correspond to the vibration of the constituent atoms within a molecule.
Because of the
large atomic masses, these oscillators have resonances in the infrared.
A primary contributor to this type
of loss is contamination
due to the OH- ion.
Losses due to impurity absorption arise predominantly from
transition-metal ion contamination such as iron, cobalt, and chromium.
The reason for these absorptions is that the impurities have incompletely filled inner electron shells.
Transitions between levels
of unfilled shells give rise to the characteristic absorptions.
The most widely used method for transmission loss measurement,
of which spectral loss is one type, is the "cut-back method" [3]
although other methods have been developed [4],[5].
A calorimetric
method is presented by Midwinter [6]. A fiber of length L is excited
by a suitable broadband
source (see Appendix I) such as a xenon-
arc or tungsten-halogen lamp.
The detector, located at a position
X = L measures the intensity output of the fiber I(L).
Without dis-
turbing the input launch conditions on the fiber, the fiber is cut
to a length m, where m is much less than L, and the detector at
position X = m again measures the intensity output I(m). The loss
for the fiber length, L - m, is given approximately by
A = I(m)/I(L) , or,
a = 10 Log [l(m)/I(L)] measured in dB.
(3-1)
(3-2)
23
Experimental Procedure and Results
In this experiment the fiber will be characterized by a loss
curve:
attenuation of transmitted light versus wavelength.
A
broadband ("white-light") source will be needed for excitation and
a sensitive, broadband detector, such as the RCA type 7102 photomultiplier tube, is needed at the output.
If a single broadband
detector is not available, two solid-state detectors may be used
if the respective responses cover the range of interest.
A long (>100 meter) fiber is needed:
the fiber must be long
enough that losses will be measurable on the equipment available,
yet short enough so that the signal will still be detectable.
Strictly speaking, absolute intensity measurements are not necessary
because a spectral curve will be generated for each of two fiber
lengths:
the resulting ratio of intensities at each wavelength
gives the spectral loss.
However, the method of calibrating the
spectral output of the source is included as it would have to be
for absolute loss measurements.
The subject of absolute loss mea-
surement is covered in Chapter IV.
For true losses to be measured, the source would properly need
to be calibrated spectrophotometrically, but for the purposes of
this experiment, the source can be calibrated to the spectral curve
of the phosphor of the photomultiplier tube as supplied by the manufacturer.
If the curve is not supplied, it may easily be obtained
from The Handbook of Optics [7]. There is an implicit assumption
that the curve is representative of all phosphors of this type, and
this is not a bad one for this application.
24
A curve of output intensity versus wavelength is generated for
the source using the narrow-band interference filters and PMT, as
in Figure 3-1.
This curve is divided point-by-point by the "known"
curve of the PMT phosphor. Figure 3-2, to get the calibrated source
curve shown in Figure 3-3.
The calibrated spectral output of the
white-light source"is then used to correct the loss curve of the
fiber.
As mentioned above, this is not strictly necessary.
For
example, consider the measured intensity exiting from a long length
of fiber:
^n,l ^h^ = hn,l ^h^h,l
where I.
(H>'^d(^l>
"-3)
/> is the actual optical power input into the long fiber,
L- p is the fiber loss factor for the long length, and G, is the
wavelength-sensitive gain of the detector for this particular wavelength X^ .
This expression will be divided by the measured inten-
sity exiting from a short length of fiber:
I
(X.) = I.
(X,)L. ^ (X.)G (X )
m , s l
in,s
1 f,s
1 d 1
(3-4)
where I
is the optical power launched into the short fiber, G,
in,s
"
is the same value defined above, and L-^
is the fiber loss factor
I, s
for the short length.
If the launching conditions are not disturbed,
I
« = I.
, and therefore:
in,-c
in,s
It is seen that the spectral output of the source at each wavelength
is not relevant to the measurement.
However, once again, if abso-
lute power measurements are to be made, it will be necessary to
25
know the absolute power supplied by the source at each wavelength
of interest.
Now referring to Figure 3-4, a beam launcher of the type described in Appendix I is used to excite the fiber with a tungstenhalogen lamp as the source.
Narrow-band interference filters select
the wavelength of the excitation for each data point.
Mode stripping
(see Chapter II) is used at both the input and output ends of the
fiber for standardization of the measurement.
A photomultiplier tube with S-1 panchromatic phosphor is utilized as the detector, and the fiber output is butted to the tube
glass.
Standard precautions for use of photomultiplier tubes are
followed, e.g. when measurements are not being taken, all light to
the PMT input is blocked to reduce dark current effects.
If necessary,
neutral density filters are inserted in the optical path to guard
against overexciting the PMT.
Since the cut-back method is a destructive one, planning is required if a number of measurements are to be made at once.
All
power measurements are recorded for the long length of fiber while
the source power is simultaneously monitored with the reference detector.
After the data are taken for the discrete wavelengths re-
presented by the interference filters, the fiber is cleaved at a
point near the input end.
Data are again taken at this nearer point
at all wavelengths of interest together with the power in the reference detector.
For the system described, the loss curve shown in Figure 3-5
was obtained.
Notice the characteristic Rayleigh-scattering shape
and the OH- peak.
26
Special Problems
On a practical note, care must be taken to limit the amount
of light reaching the PMT. Because of the large avalanche gain,
the last djmode in the chain is in danger of burning out for too
great an excitation.
This is not a serious limitation because neu-
tral density filters may be added in the input optics section for the
short fiber measurement. Appendix IV contains a detailed description of a lab-built housing for the PMT.
If narrow-band interference filters are used, it must be ascertained that they are placed only in collimated light beams. Their
performance is uncertain in other situations.
When the detector is moved from one position to another in the
fiber optics system, it is desirable to have the same area of the
detector illuminated for the measurement because variations may
exist over the sensitive surface of the detector.
With any measurement of transmission loss, much importance is
placed on controlling the launch conditions.
For this reason, both
a mode stripper and a mode scrambler (Appendix I) are recommended [8]
A reference detector is necessary to correct anomalies which are due
to intensity fluctuations of the white-light source.
Sample Questions
1.
In what ways do you expect the launch conditions to
affect the measurement of spectral attenuation?
2.
What mechanisms are responsible for fiber loss?
3.
What is responsible for the attenuation peak at
approximately 950 nm?
27
4.
What relative weights do you attach to the various
components of spectral loss; i.e., which factors."
are relatively strong and which are relatively weak?
References
[l]
Sandbank, C. P> , Editor, Optical Fibre Communication Systems,
John Wiley and Sons, Chichester, 1980, pp. 44-46.
[2] Maurer, Robert D., "Glass Fibers for Optical Communications,"
Proceedings of the IEEE, vol 61, April 1973, p. 454.
[3] Marcuse, D., Principles of Optical Fiber Measurements, Academic
Press, New York, 1981, pp. 226-230.
[4] Technical Staff of CSELT [Centro Studio e Laboratorio Telecomunicazioni]. Optical Fibre Communication, McGraw-Hill,
New York, 1981, pp. 160-ff.
[5] Miller, Stewart E., and Alan G. Chynoweth, editors. Optical
Fiber Telecommunications, Academic Press, New York, 1979,
p. 355.
[6] Midwinter, John E., Optical Fibers for Transmission, John
Wiley and Sons, New York, 1979, pp. 197-204.
[7] Driscoll, Walter G., Handbook of Optics, McGraw-Hill, New York,
1978, Section 4, p. 23.
[8] Marcuse, op. cit., p. 197 and p. 201..
28
100
/ ^
rJ
80
6
3
E
\
N
k-
/
\
/
\
»6 0
6
/
\
O
CO
c §40
CO
/
o
u
V
\
/
(U
0L,
/
20
\
•
\
/
i+OO
<,
600
500
700
800
900
1000
1100
Wavelength (nm)
Figure 3-1.
Measured source spectral sensitivity.
£. » >3
•
?. 2.0
1
/
\
"s
•
/
\
>>
/
\
\
r
•H
U
/
\
CO
c
/
\
CO 1 . 0
•u
\
/
3
.
/
0
\
CO 0 . 5
V
<
\ ^
'^.
t+00
500
600
700
800
Wavelength (nm)
900
1000
1100
Figure 3-2. Absolute spectral sensitivity of S-L phosphor.
29
100
/
/
80
/
e
X u
rt -H
60
\
\
\
/
f
e >
•H
/
O -H
CO
•u C
C (U
(U CO
a
;^
(U
04
/
1*0
\
/
^
20
t+00
500
600
700
800
900
1000
Wavelength (nm)
Figure 3-3. Calibrated source spectral sensitivity,
30
r
MS
FKQ
X
TungstenHalogen
Source
CO
IF
PH
Stabilized
Power
Supply
OF
V
MS
Electrometer
Figure 3-4.
Equipment setup for the measurement of spectral
attenuation. CO, collimating optics; PH, pinhole; X, microscope objective; IF, interference
filter and holder; MS, mode stripper; OF, optical
fiber; PMT, photomultiplier.
31
en
n
o
it)
-i
m
CO
u
u
o
o.
CO
u ^.
12
10
••
\
•\
^
.a -a -8
0) P3
•H »-'
t4-t
-6
>
_?
0
i+O0
500
60 0
7 00
800
900
Wavelength (nm)
Figure 3-5.
Relative fiber spectral loss.
1000
CHAPTER IV
MEASUREMENT OF FIBER LOSS
Project Assignment
You are to measure the fiber loss (in dB/km) for a length of multimode optical fiber.
Repeatability of the measurement should be emphasized
and demonstrated.
Construct a short 2-meter fiber link.
Cleave the fiber carefully in
the middle and measure the loss for the butt joint. Plot the effects of
lateral and angular misalignment as well as the effects of longitudinal
displacement of the fibers at the joint.
Objectives of the Experiment
1.
To acquaint the student with the various factors contributing to
power loss of signal in optical fibers.
2.
To develop an intuitive feeling for the relative sizes of these
losses.
3.
To give students an appreciation for how fiber coupling affects
loss.
4.
To give students experience in handling optical fibers.
Equipment Needed
1.
Long (glass: >100 meters; plastic: >10 meters) of step- or gradedindex optical fiber.
2.
Fiber cleaver (Ap'pendix III).
3.
Avalanche photodiode (APD) and bias supply.
4. Oscilloscope.
5.
Index-matching fluid (glycerin).
6.
Mode-strippers (Appendix I).
32
33
7. Micropositioners (3 needed),
8.
Optical source and launching optics (Appendix I).
9.
Narrow-band interference filter (800 to 900 nm).
10.
Light chopper (Appendix VIII).
11.
Goniometer (Appendix II),
Theory
Introduction
There are many ways to measure fiber loss, but not all of them are
suitable for a laboratory program for undergraduates.
Possible methods
are the cutback method, which is still probably the most widely used in
industry, and the integrating-sphere method.
For either of these methods,
it is necessary for the input end of the fiber to remain fixed, overfilled, and undisturbed.
Mode stripping, as discussed in Appendix I, at
both the input and output is necessary to remove extraneous light in the
cladding which would lead to a measurement error.
The cutback method begins with a relatively long fiber. With the
input overfilled, the intensity of the output light is measured by a
detector butted to the output of the fiber. Overfilling the input refers
to the mismatch between source numerical aperture, or NA, and fiber numerical aperture; the source NA is larger than the fiber NA, and hence some
light is launched into the fiber at angles larger than the acceptance angle
of the fiber.
This light will ultimately leak into the cladding and be
lost, and it may seem wasteful to overfill the input. However, this method
guarantees that as many modes as possible will be launched into the fiber.
34
Without disturbing the launch end of the fiber, the fiber is carefully cleaved two meters away from the input, and an intensity measurement is again made with the same detector butted to the new shorter length.
If the input has remained undisturbed during the measurements, the ratio
of the two powers (intensities) may be converted to a power dB loss per
kilometer II].
The integrating-sphere method takes a slightly different approach.
A silicon photodetector is mounted in the wall of the sphere while the
optical fiber being measured passes through the sphere along a diameter,
A narrow interior baffle is used to block direct light from the fiber
from reaching the photodetector.
The detector measures the average
amount of light escaping from the fiber over a length equal to the diameter of the sphere.
per kilometer.
This loss can be converted into a power dB loss
However, this measurement of loss ignores the loss due
to absorption centers in the material because it is only concerned with
light that is escaping from the cladding, the scattering loss.
A prob-
lem arises here due to the large amount of light lost by absorption at
the inner surface of the sphere.
The integrating sphere is primarily
passive, relying on the wall of the sphere to perfectly reflect light
to the one photosensitive element.
Tynes [2] describes an integrating-
cube method where all of the inner area of the cube is photosensitive.
A third, calorimetric, method exists which measures the amount of
light power absorbed in the fiber per unit length I3J,
The sum of the
losses measured by the integrating-sphere method and the calorimetric
method should be the total loss number measured by the cutback method.
Unfortunately, the calorimetric method requires the construction or
35
purchase of a rather sophisticated device to measure the heat gain of a
fiber directly attributable to the light passing through it, a subtle
effect.
Basic Concepts
It should be noted that there is a tendency in the literature for
the words "mode" and "ray" to be used interchangeably. Marcuse provides a good explanation of the difference 14],
However, for our pur-
poses, we can think of low-order modes as families of rays traveling
through the fiber which strike the core-cladding boundary at small angles
of incidence.
High-order modes would then be families of rays which
travel at relatively higher angles. The high-order modes are most susceptible to loss by being leaked into the cladding and ultimately out of
the fiber (by Snell's law; see Chapter II),
Because each mode is subject
to a different loss, the distribution of modes will affect the loss measured.
Except in rare instances, communications engineers are not particularly interested in isolating the various losses contributing to total
loss:
the single total loss number is good enough. For purposes of
developing better fibers, it would of course be necessary to be able
to evaluate each type of loss independently.
Thus stated, only the-measurement of total loss will be discussed.
Conceptually, the measurement of total fiber "insertion loss" is one of
the easiest to grasp.
One starts with a long length of fiber and meas-
ures the optical power exiting the fiber.
The fiber is then cut to a
shorter length, and a power measurement is again made.
The change in
36
optical power detected (assuming the input has remained unchanged, and
the detector is illuminated over the same area) must be the amount of power
"lost" in the removed section (for whatever reasons), and is defined to
be
^=^°^°^>(^long/^hort> ^^-
^^-1>
The loss per kilometer will be L divided by the length of the section
removed.
The technique works well for high-loss fibers: say, fibers
with losses around 100 dB/km,
The results are easily repeatable, too.
However, when extremely low-loss fibers (1 dB/km) are to be measured,
the problem becomes much more difficult. To compound the problem,
fibers are often supplied in lengths no greater than 1 km, meaning that
the measurement technique must be capable of greater precision. At these
levels, the condition of the endfaces of the fiber becomes important.
One other annoyance is that there is still no agreement as to what
one number constitutes the loss figure for a multi-mode fiber.
This is
due primarily to the fact that it is not clear how the fiber should be
excited.
Should all modes be excited or just the low-order modes?
should some happy medium be reached?
Or
The consensus now appears to be
to measure the fiber loss for the steady-state modal distribution in
the fiber; i.e., the distribution of light among the various modes which
is most likely to occur after the light has' propagated through a very
long section of the fiber.
If it is assumed that this is the most
desirable distribution (overfilling the fiber now would be wasteful),
steps must be taken to launch a distribution in the fiber which approximates the steady-state distribution. Various techniques have been sug-
37
gested:
sending the light first through a long section of dummy fiber,
using a mode scrambler (see Appendix I), or sending the light first
through a section which consists of step-graded-step-index fibers
spliced together.
Some fibers show a tendency to couple energy from one group of modes
to other groups:
this is the phenomenon referred to as mode coupling.
By no means do all fibers exhibit this.
Fibers which show a large degree
of mode coupling will tend to rapidly establish a steady-state modal
distribution.
Fibers which show no significant coupling of energy
between modes even after several kilometers may not be characterizable
by a single number for the loss figure.
With all of this uncertainty about what fiber loss really means,
manufacturers have taken to specifying an "at worst" attenuation which
provides an upper bound for the loss.
experiment.
This is the approach taken in the
Exciting only the low-order modes would be overly optimistic;
launching a steady-state distribution would require that the distribution
be known a priori.
Therefore, the loss will be measured for the case of
an overfilled input, which is to say that more modes will be launched
than can be guided by the fiber.
Simply put, the source numerical aper-
ture will be larger than the fiber numerical aperture.
Experimental Procedure and Results
The experiment proceeds much the same as the experiment of Chapter
III, with careful attention paid to the order in which different measurements are performed,
A long length of type 1504 silica graded-
index 63 ym core fiber was unwound from a spool donated by Corning.
38
The longest length which could be obtained before a break was encountered
was 142.5 m.
Two persons were needed to perform the unrolling;
one un^
wound the fiber from the stock spool while the other took it up on a
spare spool. The new spool was marked with the length and dated and
signed.
The long length of fiber was placed in the experimental setup
of Figure 4-1 with 2 meters unwound and marked with tape at the input
end to permit easy cleaving of the 2-meter length later. An avalanche
photodiode was utilized as the detector due to the anticipated small intensity at the output end of the long fiber. For illustrative purposes,
the loss was measured not at a single wavelength but, instead, for the
total light input into the fiber from a broad-band tungsten-halogen
source.
The addition of the narrow-band interference filter in the in-
put optics resulted in an intensity too low to be detected,
The light
chopper of Appendix YIII was also used to permit the signal to be seen
above the relatively high dark voltage of the APD load resistor.
Careful movement of the micromanipulators at the input and output
ends of the fiber resulted in signal maximization at the detector. After
this, the input optical alignment was not disturbed. Mode stripping
was accomplished by placing the fiber and a drop of index-matching
fluid (glycerin) between two large black rubber stoppers. With the
long-length measurement having been made, the fiber was cleaved 2 meters
from the input end. Another measurement was taken. The total length
of the section which was removed was 142.5 - 2 = 140,5 m.
obtained was
The loss
39
L = 10 log (V /V ) = 10 log (17m'V/5QmV)
= -4.7 dB/140.5 km
= -33.4 dB/km.
The fiber is rated at -16 dB/km at 900 nm, and for the crude setup
used, and the broad-band launch, the value obtained is certainly within
expectation.
At this point, the 2-meter length was cleaved as closely as possible in half.
The ends were butted together after mounting each in a
micropositioner and these ends and the end at the detector were adjusted
until once again a maximum signal was received at the detector. The
reading of 25 mV across the detector load was used with the previously
obtained value of 50 mV for no joint in the 2-meter section to obtain
the loss of the dry joint which was
L. = 10 log (25mV/50mV)
= -3 dB,
a reasonable value for a dry joint.
With the break now made in the 2-meter section, it is a straightforward problem to obtain curves of loss versus the three types of misalignment:
lateral, angular and longitudinal. To obtain the sensitivity
necessary to move one fiber a fraction of a micron laterally with respect to the other, a modification was made to one of the micromanipulators as discussed in the section Special Problems.
The goniometer of
Appendix II was used to position one fiber at an obtuse angle
with respect to the other.
The goniometer yielded a resolution of
40
1/2 degree. For the longitudinal misalignment, the fibers were
butted, the signal was maximized, and the fibers were then gradually separated with the help of a micrometer on the goniometer.
The micrometer had a resolution of 0.001".
Figures 4-2, 4-3, and
4-4 show both the geometries used and the loss vs. misalignment
curves for the three cases. The input optics were still unmoved
from the measurement of the long length of fiber. The mode
strippers of Figure 4-1 were used at the input and output only not at the joint.
Special Problems
The use of the avalanche photodiode is recommended due to
the likelihood of very small signals.
There was a problem in getting enough resolution in the
lateral misalignment experiment.
This was solved by noting that
one turn of the screw on the micromanipulator resulted in a
displacement of 0.013".
By mounting a 360° protractor (incre-
mented by degrees) onto this screw and adding a pointer, a
precision of 0.013"/360 was obtained.
Although the narrow-band interference filter resulted in
a signal which was too small to be detected over the long length
of fiber, it should be noted that the long length represented
a loss of over 33 dB. This suggests that the long length could
have been made much shorter with no danger of being unable to
measure a loss.
This would allow the fiber loss to be measured
41
at 850 nm which is a popular wavelength for specification.
It is believed by us that an integrating-sphere or an integrating-cube experiment would be instructive for those desiring to expand
upon this project. A suggested starting point for the sphere would be
a ping-pong ball coated on the inside with Eastman 6080 White Reflectance Coating.
Or a Christmas tree ornament with no coating?
Sample Questions
1.
Loss in dB is calculated from Equation (4-1) as L = 10 log
P
/P. , Why then was the loss calculated as L = 10 log
out in
"^
V
out
/v. , instead of the standard L = 20 log V ^/V. which
m'
out in
you learned in introductory electrical engineering?
2.
What is optical time-domain reflectometry?
Describe the funda-
mentals of the theory.
3.
For conventional copper telephone systems in your area, how
closely are repeaters spaced?
How closely are microwave re-
peaters spaced for cross-country runs?
4.
With regard to the spectral sensitivity curve measured in
Chapter III, what does the loss measured in Chapter IV represent?
References
[1] Marcuse, D., Principles of_ Optical Fiber Measurements, Academic
Press, New York, 1981, pp. 226-230.
12]
Tynes, A, R,, "Integrating Cube Detector," Applied Optics, vol. 9,
1970, p. 2706.
42
[3] Stone, F, T., W, B, Gardner, and C, R, Lovelace, "Calorimetric
measurement of absorption losses in optical fibers," Optics Letters,
vol. 2, 1978, p. 48.
14] Marcuse, D., op. cit,, p." 245-249.
43
tape
marking
2-m length
APD
TungstenHalogen
Source
Micropositioner
Micropositioner
and mode stripper and mode stripper
Light
Chopper
Figure 4-1. Equipment setup.
t
1
-0 C3
-2 •u
C
-i+
v
^
no
-6 -
fl
CO
—4
E
-8 -
1—i
cC
1-1
-10 -
•u
c^
—"
.
-12 -
CO
>
X
-11+
.
0
1—1
-16 -
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Figure 4-2. Loss vs. lateral misalignment.
0.7
0.8
^
/D
44
T
-5
1
r
r
+2
-2 -1
0 +1
Degrees Misaligned
T
T
T
+3
+4 +5
Figure 4-3. Loss vs. angular misalignment.
1 1 \ T
14
16
4
6
8
10
12
End separation (in mils)
T — I — I — I — I — I — I — I
0
2
r
Figure 4-4. Loss vs. longitudinal misalignment
CHAPTER V
CHARACTERIZATION OF DETECTORS FOR OPTICAL FIBER SYSTEMS
Project Assignment
In comparative measurements, you are to investigate four types of
optical detectors with respect to rise time, relative responsivity,
spectral sensitivity, and dark current.
The detectors are to include
a phototransistor, a PIN photodiode, an avalanche photodiode and a
photomultiplier tube.
Objectives of the Experiment
1.
To familiarize the student with devices presently in use for
detecting optical signals,
2.
To acquaint the student with the operation, advantages, and
disadvantages of each type.
Equipment Needed
1.
Injection laser diode (pulsed),
2.
Phototransistor (PT).
3.
PIN photodiode.
4.
Avalanche photodiode (APD).
5.
Photomultiplier tube (PMT) with S-1 phosphor (RCA 7102).
6.
High voltage negative power supply.
7.
0 - 50V power supply.
8. Monochromator or set of narrow-band interference filters.
9.
Mode stripper (see Appendix I).
10.
Tungsten-halogen or other white-light source.
11.
Oscilloscope, or digital voltmeter,
12.
Light chopper (see Appendiit VIII).
45
46
Theory
Introduction
At present, information transmitted via light through an optical
fiber must be converted to electrical energy at a receiver to be meaningful.
another:
This conversion is effected in photodetectors of one type or
which type is used depends a great deal on the characteristics
of the communication link.
With regard to suitability for fiber optics
applications, photodetectors must have:
high sensitivity (to be able
to detect weak optical signals), peak efficiency near the wavelength
of the system source, high speed, large signal-to-noise ratio, roomtemperature operation capability, and high reliability.
Present systems utilize solid-state p-n junction devices for
detection because they satisfy many of the above requirements and
typically are smaller and consume less power than nonsemiconductor detectors.
Optical detectors can be characterized by four parameters:
rise-
time (or bandwidth), spectral sensitivity, responsivity (at a given
wavelength), and dark current [1]. The first represents a limitation
on the overall system frequency response.
The second is necessary
to optimally match detector to both source and fiber.
The third and
fourth dictate the minimum detectable signal for the link for which the
detector is to be used.
These quantities aid in the proper choice of
a detector for use in a particular fiber optic communication channel.
Other detector parameters are important also.
The noise equiva-
lent power (NEP) is one typically specified by the manufacturer.
It also
47
is useful for determining the "minimum detectable signal" which defines
the amount of incident optical power necessary to generate a photocurrent equal to the photodiode noise current.
To ensure integrity of
the link, the detector is operated with a signal level above the minimum detectable signal. Unfortunately, the detector NEP measurement is
too difficult to be performed without sophisticated measurement equipment.
Basic Concepts
The response time of the photodetector is the transit time of the
generated charge carriers to the output terminals of the device.
It is
dependent on the device construction as well as the external circuitry.
It is a measure of how fast the detector can respond to changes in the
incident light intensity, and hence a measure of the bandwidth.
In lieu of measuring the detector 3-dB bandwidth directly, the
bandwidth information may be inferred from the measurement of the rise
time of the device [2]. For rise-time measurements, an optical signal
with a rise time faster than that expected for the detector is required.
This implies an optical step function or a fast-rising optical pulse.
The pulses supplied by a pulsed injection laser diode are usually adequate for this purpose.
In any cascade-connected system, the overall system rise time, t ,
is approximately
t = 1.1 (t? + t^ + ... + t5°'^
r
where t
1
2
C5-1)
m
is the total system rise time, and t. is the rise time associ-
ated with the element i of the system.
For fiber optic systems, these
f
t
48
typically include the rise time of the source, the dispersion effects
of the fiber, and the rise time of the receiver.
If the rise time of
the source is known (say, from measurement in Chapter VI), and if no
fiber is used, the rise time of the receiver can be obtained from a
measurement of the system rise time. The detector 3-dB bandwidth in
Hz is then approximately 0.35 divided by the rise time in seconds 13].
The measurement of absolute spectral sensitivity for the various
detectors requires the use of a standard source emitting many wavelengths
of light where the output intensity at each wavelength is known. Responsivity, which has units of amps/watt, is an absolute spectral sensitivity
defined for the wavelength where the response is a maximum [4].
A de-
tector with a relatively higher responsivity will be able to resolve
lower-power signals than one with relatively lower responsivity.
Standard sources are, in general, expensive and sensitive to temperature and aging effects.
In this experiment, only relative sensi-
tivity curves will be obtained and relative responsivity data will be
inferred by comparing the spectral response curves for the different
detectors.
With a tungsten-halogen broad-band source in place, a device
for individually selecting single wavelengths follows, allowing a sensitivity of the detector at each wavelength to be determined. Figure
5-lb.
The device for selecting individual wavelengths may be a mono-
chromator or a set of narrow-band interference filters.
For the purposes of comparison, it is only required that all of
the detectors be referenced to the same spectral curve. If the curve
for the tungsten-halogen source is not available, the response of each
49
detector may be compared to that of the photomultiplier tube. To do
this, the PMT is used to "calibrate" the output of the tungsten source,
and this "standard" source is measured by the other detectors.
(If
the chopper of Appendix VIII is also used for all four detectors, it
should be included as part of the "standard" source.) All spectral
responses may be compared in this fashion.
Of course, no information
is obtained for the PMT because its spectral response (obtained from
a standard phosphor curve) was tacitly assumed to be exact.
Dark current is a very simple quantity to obtain with the electrometer. With only the detector circuit operational and the detector
input blocked with opaque paper, the output current is measured.
Experimental Procedure and Results
The equipment setups of Figure 5-1 were used for all measurements.
For measurements which did not require the laser diode pulser to be the
source, an improvement in signal-to-noise ratio was achieved with the
use of a lab-built light chopper (Appendix VIII).
This allowed the
relatively larger dc bias voltage to be subtracted from the signal
by ac coupling to the oscilloscope.
The four phiotodetectors to be compared
are listed in Table 5-1 with a few of the important parameters. The
photodetectors were positioned a distance from the source as necessary
to avoid saturation.
The rise-time measurement required the use of a
source of fast-rising optical pulses. The pulsed laser diode of Chapter
VI is used for this purpose.
The 10%-90% rise time for the laser diode
as used in the pulser of Appendix V was about 8 ns. A tungsten-halogen
lamp served as the source for the spectral sensitivity measurements.
50
Table 5 - 1 .
P h o t o d e t e c t o r s Compared by Manufacturer
Area (mm )
Specifications
Spectral
Sensitivity ((? 900nm)
Fairchild FPT-100
phototransistor (PT)
not available
RCA C-30808 PIN
photodiode:
0.6 A/W
RCA C-30817
avalanche
photodiode (APD):
0,5
75 A/W
780
450 A/W
RCA 7102
photomultiplier
tube (PMT):
A set of narrow-band interference filters was required to select
individual wavelengths to excite the detectors.
The detectors were operated in the simplest circuits possible; a
bias voltage was applied to the series combination of the detector and
a resistor.
The response times for the system of source and detector
were measured from oscillograms and the response times of the detectors were calculated from Equation (5-1).
Table 5-2.
The results are collected in
Both the APD and the PMT are capable of much faster response
time, but require more sophisticated circuitry to achieve it. For this
reason, the comparison is somewhat unfair.
The bandwidth is taken to
be 0.35 divided by the 10%-90% rise time, as appropriate for a simple
first-order RC circuit model.
51
TABLE 5-2.
Detector:
Rise-Time Comparison
PT
PIN
APD
PMT
10%-90% rise time:
1.64 ys
17 ns
15 ns
30 ns
Bandwidth:
0.213
20,6
23.3
11.7 MHz
It should be cautioned that the response time of the detector in
the receiver is but one component of the receiver rise time.
tributing factors include:
Other con-
the input impedance of the receiver amplifier,
the bias circuitry of the detector, the input impedance of the measuring
instrument, and stray capacitances and inductances.
The detector rise
time for a p-n junction device is largely a function of the junction
capacitance and as such cannot be improved greatly without replacing
the device.
Although it is most desirable to have on hand a calibrated standard
source of light of various wavelengths, for the purposes of this comparison, it will suffice to relate the measurement of spectral sensitivity
to some other basis.
The spectral response curve of an S-1 phosphor.
Figure 5-2, (which is used in photomultiplier tubes, such as the RCA 7102
used here, to give broad spectral response) is readily obtainable from
manufacturer's data sheets [5],
If this curve is taken to be the ref-
erence, all succeeding measurements may be referred to it.
assumed that the PMT chosen has this spectral response,
It will be
Notice, then,
that instead of relating everything to the known output of a standard
source, all measurements are referred to the assumed response of a
particular material, namely, the S-1 phosphor.
52
With this in mind, the output intensities of the wavelengths present in an uncalibrated tungsten-halogen source are measured with the
PMT.
Knowing the response curve of the PI-IT, we work backwards to pre-
dict what the relative intensity of the source at each wavelength
must be.
This rough^ standard source is then used to-measure the spec-
tral sensitivities of the remaining three detectors.
The curve of Fig-
ure 5-3 results for the source, and the curves of Figure 5-4 represent
the spectral responses of the phototransistor, PIN diode, and APD, with
care taken that the full area of the photosensitive surface in the detector was covered.
The curves are normalized to the peak sensitivity
for each detector.
Common sense is necessary for making this comparison due to some of
the requirements of the PMT bias.
Before biasing the PMT, it is nec"111
essary to have some idea what order of magnitude of light intensity
^™
fa]]]
is to be detected.
The design of the bias circuitry is dependent
on this because of the rather limited dynamic range of the PMT,
It is
best to use the PMT to measure light intensities in the range for which
the bias circuit was designed.
The solid-state device character-
istics should be measured after the PMT characteristics have been
measured because these devices are typically less sensitive to light
than is the PMT.
In this order, it will become obvious that the PMT
is the most light-sensitive of the devices being compared.
In fact, for this experiment, the procedure was reversed with some
unhappy consequences.
fore the PMT.
The solid-state photodetectors were measured be-
They were positioned a distance, d, from the tungsten-halo-
gen source as indicated by Figure 5-lb which shows the equipment setup
53
for the spectral-sensitivity comparison.
The distance d was dictated
by the minimum intensity which could be detected for the least sensitive of the devices:
in this case, the PIN diode.
Finally, the PMT
was placed at the same location and unusual behavior was noted.
The
spectral output of the source appeared to have shifted from what had
been previously measured in Chapter III.
The peak output power still oc-
curred at a wavelength of 700 nm as before, but, as an example, the ratio
R = I(A = 400 nm)/I(X = 700 nm)
measured 0.374 rather than the 0,10 measured previously (cf. Figure
3-1).
The PMT was originally biased to detect a light intensity which
would result in a maximum output anode current of 5 yA.
At the posi-
tion d from the source, the output anode current at 700.:.nm was 15.5 uA
suggesting that the PMT was saturated and operating nonlinearly.
Indeed,
when the intensities were reduced (by positioning the PMT further from
the source), the expected behavior returned, indicating that the responsivity of the PMT is probably an order of magnitude better than the
other detectors.
Ideally, all four detectors should be exposed to the
same power density to determine the relative responsivities, but with
this caveat in mind, the four detectors are ranked in order of responsivity to radiation at 900 nm in Table 5-3.
TABLE 5-3.
Responsivity:
Detector:
Relative Responsivity
Best
PMT
Worst
APD
PT
PIN
54
The dark current measurements were made during the spectral sensitivity measurements with all light blocked from entering the detector.
Note that a screen which blocks visible light may not block infrared or ultraviolet.
The results are tabulated in Table 5-4,
In
practical measurement systems, however, it is more appropriate to measure
the current due to ambient light, which includes dark current as well as
current resulting from stray light which does not originate from the
source.
When this ambient current value is subtracted from the meas-
ured value with the source in place, the numbers obtained are said to
be "corrected for ambient light,"
The value appearing for the APD is
excessively large and may be the result of prior exposure of the detector to light which was too intense,
TABLE 5-4.
Detector:
Dark Current:
PT
933 nA
Dark-Current Comparison
PIN
APD
PMT
400 nA
1.6 mA
95 na
Special Problems
Before any measurements are attempted, all necessary equipment —
power supplies, biasing circuits, test equipment, etc. - should be
assembled and made ready.
The measurements of each device should fol-
low one another quickly in order to minimize system drifts.
Also,
major anomalies can be more easily recognized in this manner.
The use
of oscilloscope photographs is not strictly necessary, but may aid the
student in side-by-side comparisons of rise times.
55
To form the narrow optical pulses necessary for rise-time measurements, a pulsed laser diode is used.
Appendix V discusses the construc-
tion of a pulse-forming circuit for an RCA SG2001 laser diode.
Another
design is provided by Andrews 16],
Once again, a certain natural order to the experiments presents
itself.
The rise times measured for each detector depend in part on
the rise time of the source.
Therefore it is desirable to have a light-
intensity-out vs. drive current curve available for the source so that
the lasing threshold for the source may be noted on the output current
waveform from the laser diode pulser as discussed in Chapter VI.
This
will allow the rise time to be measured.
The PMT presents a special problem because the fall time is likely
to be an order of magnitude below that of the solid-state devices.
If
the pulse repetition rate (PRR) of the ILD is not adjusted to allow the
accumulated charge in the PMT to dissipate, the rise time can not be
accurately depicted.
This is not necessarily a problem because the PRR
of the ILD may have to be chosen very small axiyvray for power dissipation
considerations in the ILD itself.
Sample Questions
1.
Following the results of your experimentation, what recommendations do you make as to the applicability of the various
types of detectors with regard to cost, system length, choice
of analog or digital transmission schemes, and other relevant
parameters?
56
2.
It was observed that the ILD device exhibited an extremely fast
rise time, but by its nature, it is a pulsed device. Although
it is not normally used to transmit information, can you suggest a modulation scheme to allow it to transmit information?
3. What modifications to the PMT biasing circuitry can you suggest
to improve the rise time?
4.
The spectral sensitivity measurements were tied to a typical
curve for an S-1 phosphor. What would be the most precise way
to do it?
References
[1] Saxena, A. N., and H. F. Wolf; "Optical Detectors," Handbook of
Fiber Optics, H. F. Wolf, ed.. Garland STPM Press, New York, 1979,
p. 225.
12] ITT, "Optical Fiber Communications Link Design," Technical Note
R-1, 1978, pp. 6-7.
[3] Peebles, Peyton Z., Jr., Communication System Principles, AddisonWesley, Reading, Massachusetts, 1976, p. 65.
[4] Applications Engineering Staff of the Hewlett-Packard Optoelectronics Division, Optoelectronics/Fiber-Optics Applications
Manual, 2nd Edition, McGraw-Hill, New York, 1981, p. 4.3.
[5] RCA, "Photomultiplier Tubes, Image-Converter Tubes, Photodiodes,"
Publication No. PIT-700A, RCA Electronic Components, Harrison, New
Jersey, September 1969, p. 5,
[6] Andrews, J., "An Inexpensive Laser Diode Pulser," Review of
Scientific Instruments, Vol. 45, No. 1, January 1974, pp. 22-25.
57
Photodetector
Current
Pulser
Injection Laser
Diode
V
B
A
to
scope
a)
It
ft
(IS
Interference
Filter
Broadband
Light
Source
Photodetector
Light Chopper
d
b)
Figure 5-1.
Equipment setup for a) rise-time measurement, b)
spectral sensitivity measurement.
58
100 -r
80 •^5
•H
>.-6 0 +
CO
c
w
kO
••
>
•H
CC
•0)
P^ 20
H
i+00
H—I ^
500
Figure 5-2.
1—I
600
H-H
700
1
1—I
800
900
1
1 1
1000
X(nm)
S-1 response curve.
100 -r
^s
£^80
•H
CO
C
<u
c 60 -3
a
u
3
O
01 i+O ->
•H
oi 20
i
1 1 1
1 1
1 1
1 1 1
1 1 1
700
800
900
1000
X(nm)
»+00
500
600
Figure 5-3.
"Calibrated" tungsten-halogen source curve,
59
100_
e^s
80>
•H
4J
•H
a)
60-
CO
c
a;
CO
1+0-
>
•H
4J
'2 0 -
CO
iH
CU
0
T
•
I
1
i+OO
'
11
500
I—I—I—I—r—r
6O0
700-
800
900
"1—r
T—\—r
1000
X
(nm)
' 100 J
4-1
80-
>
b)
•H
iJ
•H
W
60-
CO
i+0-
c
a;
>
•H
U
Ct
20-
rH
1+00
T—I—I—I—I—I—r
500
600
700
800
900
1—I—r
T—J—r
1000
X
(nm)
100 &«s
£:•
60-
•H
>
60c)
CO
c
i+020-
>
•H
1—I—1—I—I—I—r
1—\—r " 1 — r
1
I—r
X
(nm)
Figure 5 - 4 . Spectral s e n s i t i v i t y curves: a) FPT-100 p h o t o t r a n s i s t o r ,
b) C-30808 PIN photodiode, c) C-30817 avalanche photodiode.
i+00
500
600
700
800
900
1000
Chapter VI
CHARACTERIZATION OF SOURCES FOR OPTICAL FIBER
SYSTEMS
Project Assignment
In a comparative study, you are to investigate the operating
parameters of a light-emitting diode and a laser diode. These parameters are to include radiation pattern, peak-intensity wavelength,
and spectral bandwidth.
Generate an intensity vs. drive current curve
for each device.
Objectives of the Experiment
1.
To acquaint the student with the characteristics of
various optical sources as well as the advantages and
disadvantages of each.
2.
To give the student practical experience in making
source measurements.
3.
To acquaint the student with losses associated with
input source coupling.
Equipment Needed
1.
IR-LED of the type designed for fiber optics applications.
2.
Laser diode (pulsed or cw).
3.
IR viewing screen.
4.
Two PIN photodetectors or avalanche photodiodes.
5.
ILD pulse driver circuit (see Appendix V) or dc current
source.
60
61
6.
Monochromator.
7.
Goniometer (see Appendix II).
8.
Light chopper (see Appendix VIII). or square-wave generator.
Theory
Introduction
Optical sources are rather easily characterized by parameters
such as total output power, radiation pattern, spectral bandwidth,
rise time, and intensity-out vs. drive current.
As in the previous
experiment on detector measurements, the thrust of this experiment
is to do a comparative study of different sources:
namely, the light-
emitting diode (LED) and the injection laser diode (ILD), both of
which are suitable for specific fiber optic applications.
With this in mind, total power measurements are not absolutely
necessary because there is usually some flexibility remaining in the
choice of the system detector.
Therefore, within the system, the draw-
back of a less powerful source may to some extent be corrected with
a more sensitive detector.
However, a relative power comparison can
be inferred from the intensity-out versus drive current plots for
both devices.
Basic Concepts
Perhaps second only to attenuation loss in the optical fiber,
the coupling of source to fiber contributes most to system loss.
Coupling losses are of three types [l]:
1.
Unintercepted illumination (UI) loss resulting from a
geometrical area mismatch between the source's illumination
62
spot (in the plane of the fiber face) and the area of the
fiber core.
2.
Numerical aperture (NA) loss arising from light rays with
angles of incidence outside the acceptance cone of the
fiber transmission line.
3.
Reflection (R) loss from the endface of the fiber.
The three types are illustrated in Figure 6-1.
Note that parameters
such as source area, radiation pattern, and source-fiber separation
can be selected to reduce input coupling loss.
UI loss can be estimated from the expression:
UI loss = 10 log (A /A ) dB,
c p
where A
c
is the area of the fiber core and A
p
(6-1)
is the area of the
projected spot of the source.
The radiation pattern of a uniform surface emitter (a reasonable assumption for the LED) is approximately that of a Lambertian source and is
given by:
1(0) = I^ cos (c}i) ,
(6-2)
where c|> is the angle measured between a line perpendicular to the
emitter and a line drawn from the source to the detector, and I is
o
the intensity at (
|
) = 0°.
Narrower beam patterns of other devices can be reasonably approximated by the expression:
I((j>) = I
[cos((J))]°' .
(6-3)
The exponent m can be determined from experiment for a particular
63
device by interpolating on the curve of Figure 6-2 which gives intensity profiles for various values of m.
This value of m is
necessary for calculating the NA loss.
The optical power coupled into the fiber can be obtained from
[1]:
I^ = I^. [1 - (cose)°^^]
where I
,
(6-4)
i s the t o t a l source power and 0 is the f i b e r ' s acceptance
cone h a l f - a n g l e .
Then:
NA loss = 10 log (I /IJ
c
dB
.
(6-5)
t
Reflection loss is almost negligible in comparison to the losses
associated with unintercepted illumination and numerical aperture,
but it is important in fiber splices. The reflection coefficient,
p, gives the fraction of incident light reflected from the fiber endface.
It is found approximately from:
p = [(n^-l)/(n^+ 1)]2
,
(6-6)
where n, is the index of refraction of the fiber core, for step-index
fibers.
The R loss is then:
R loss = 10 log (1 - p)
.
(6-7)
From this discussion of source-fiber coupling loss, it is seen
that the proper choice of a source can significantly improve system
performance.
be rewarding.
Therefore, an investigation of source parameters will
64
At present, there are only two candidates being seriously considered for fiber optic sources:
the light-emitting diode (LED) and
the injection laser diode (ILD).
They are being designed to have
characteristics much different than those of solid-state sources
for other applications.
In the noncommunications fields, the goal
has been to get the maximum visibility with minimum power consumption,
and this has led to the development of large-area, low-radiance
LEDs and pulsed lasers operating at high peak currents [2]. Sources
for fiber optic communication systems, however, do not necessarily
require high power; instead, it is important that the source power
be distributed spatially for maximum acceptance by the optical fiber.
They must also be stable and capable of continuous (CW) operation
at room temperature.
If analog transmission is desired, of course the source must
be linear, and in LEDs, the light output power is very nearly a
linear function of the drive current.
Digital information trans-
mission requires only that the source be capable of being driven
between multiple (usually two) stable states.
For best source-fiber
match, the intensity of the source should be maximum at a wavelength
where the fiber has a minimum in attenuation.
The LED is typically the more simply constructed of the two
types of sources although recent, more sophisticated heterojunction
structures provide desirable high radiance and high speed.
The
spectral bandwidth is of the order of 25 to 40 nanometers at the
3 dB points at room temperature.
Material dispersion effects (discussed
in Chapter VIII) therefore make them undesirable for very long distance
systems.
65
LEDs are further categorized as surface or edge emitters.
Marcuse [3] compares the amount of power each type is capable of
launching into the fiber core. Edge emitters, with the attendant
narrower beam patterns, have the advantage of higher efficiency in
coupling light into the fiber, and hence, they also provide significantly reduced NA losses.
By adding a cavity to provide feedback, the LED becomes a
laser at high current densities, substituting stimulated emission
for spontaneous emission.
Because the radiation is emitted from a
very small area, it is not as collimated as laser radiation from
other laser sources but diverges somewhat. However, there is still
a significant improvement over a Lambertian emitter.
Laser diodes offer the benefits of narrow spectral bandwidth
(typically 2 to 10 nm at the 3 dB points), more directionality to
the radiation, and fast rise time. They are typically both more
complex and expensive.
The latest laser diodes being developed
are designed to operate continuously on a few milliamps of current.
The older, pulsed devices have limitations of duty cycle, slow pulse
repetition rate (PRR), and high (greater than 4 amp) threshold current:
indicating they will probably not be used in the future in
communications applications.
Because of the cost of these new devices,
however, the characteristics of laser diodes may be observed on the
less expensive pulsed devices.
Experimental Procedure and Results
The two devices investigated were an RCA C-30123 IR-LED and an
RCA SG2001 injection laser diode. When power levels were adequate.
66
an RCA C-30808 PIN photodiode was employed as the detector; an RCA
C-30817 avalanche photodiode was used in the spectral bandwidth
measurements.
Although phototransistors are attractive detectors
because of price, the associated nonlinear response renders them
inappropriate for these measurements.
The measurement of output intensity versus drive current was
attempted first using the equipment setup of Figure 6-3.
In the
case of the pulsed injection laser diode, the building of a laser
diode pulser, as described in Appendix V, was required.
Current
amplitude is varied to a maximum of 8 A by controlling the voltage
to which the storage element charges.
(The maximum allowable
peak current is 10 A at 0.1% duty cycle and PRR maximum of 50 kHz
for the RCA SG2001 device.)
The LED is driven by a dc current source with variable output
up to 200 ma, the maximum allowable continuous current for this
device.
After warmup, the current through the diode is monitored
along with the output current in a photodetector.
Plots of output
detector current (proportional to illuminating intensity) versus
source drive current for both LED and ILD are depicted in Figure 6-4,
Observe that the laser requires a minimum threshold current before
lasing occurs.
Monitoring the drive current through the laser is
equivalent to monitoring the laser output power.
When the current
is above the threshold current, the diode must be lasing.
The radiation patterns were observed using the lab-built goniometer (Appendix II) to sweep a detector in an arc around the source
as the equipment setup in Figure 6-5 indicates.
The polar plots
67
of Figure 6-6 show the results.
Absolute intensity
measurements are not recorded; instead, all intensities
are recorded as a percent of the maximum observed intensity.
However,
the output power of the laser is
much greater than that of the LED.
The system shown in Figure 6-7 was used to measure the spectral
bandwidth of the sources. The monochromator selects only certain
lines in the spectral output of the source (the spectral bandwidth
of the monochromator must be narrower than that of the source) ,
The avalanche photodetector of course has a range of sensitivity
which includes the range of interest.
There is usually sufficient power for the laser light to be
observed even after passing through the monochromator, but if the
LED is driven with a dc current, its radiation may not be detectable
above the relatively high bias voltage on the output resistor. Techniques as discussed in the section Special Problems may remove this
trouble.
The parameter of peak-intensity wavelength is also measured with
this setup. For the C-30123 LED, peak intensity was found to occur
at 855 nm while for the SG2001 ILD peak intensity was found at 908 nm.
These compare well with the manufacturer's specifications of 830 nm
and 904 nm respectively.
Spectral bandwidths, measured between the
half-power points, were determined to be 40 nm for the LED and 4.8 nm
for the ILD, comparing well to the previous experiments [2]. Values
are typically in the range of nanometers for laser diodes and tens
68
of nanometers for LEDs.
Special Problems
When obtaining the intensity-out vs. drive-current curve for the
laser diode, it is necessary to remember that some ILDs are intended
for pulsed operatiort. For our purposes, we may assume that the shape
of the curve is known to have the characteristic shape for a laser
diode [4]. With this simplification, the problem becomes one of
"finding the knee" of the curve. Knowing where the lasing threshold
of the device is will permit us to later determine at what point on
the output current waveform the device actually begins lasing. A
useful graph for this purpose is a plot of the output current in a
(linear) photodetector versus the drive current in the laser diode.
Laser drive circuits are available from several laser ma.nufacturers including RCA [5] and Laser Diode Laboratories. An example is
given in Appendix V.
It should be emphasized that the objective of
the experiment is to investigate characteristics of optical sources,
• ,<:
Lj
•«
not characteristics of electronic pulse-forming networks. Because
of the extremely narrow pulses which are needed, an attempt should
be made to adhere closely to proven designs.
A driver circuit suggested by J. Andrews [6] utilizes a delay
line as a storage element ^nd dumps through the laser by driving a
transistor into avalanche breakdown. This circuit was constructed and worked well, but it is slightly troublesome in that the
length of the delay line must be destructively changed to adjust the
current pulse width. Most commercially available circuits charge a
69
storage capacitor for the energy dump.
SCRs are often chosen as
the switching elements.
Other practical concerns include allowing a reasonable warmup
time for the laser:
30 minutes is suggested.
Also, the current
pulse should be observed on the oscilloscope as the magnitude is
varied to ensure that the basic pulse shape does not change. Because of the high-frequency nature of the measurements to be performed, a thorough understanding of the performance of the oscilloscope chosen is encouraged; in particular, watch the triggering!
The high voltage supply chosen should contribute as little as possible
to the noise, and shielding to prevent stray inductive pickup should
be used as needed.
Subtractive techniques may aid measurement at
low signal levels, e.g. the signal obtained with the signal absent
is subtracted from that obtained with the signal present.
The most
difficult part of the measurement seems to be choosing a consistent
measurement scheme, e.g. deciding what point on the noisy waveform is
representative of the quantity being determined.
It is desirable that a permanent laser diode pulser be built
by the first students assigned to the experiment.
The pulser should
be tested with a small resistance (5 to 10 ohms) in place of the ILD
until pulse widths and magnitudes can be determined.
The photodetector used to measure the output radiation from both
sources must be positioned far enough away that it will not saturate
at maximum drive current in the source.
In the measurement of spectral bandwidth, the biggest problem
will be in overcoming the light loss obtained by passing the light
70
through the monochromator.
Alignment should be done carefully,
tracing the beam through the monochromator with an IR viewing card.
For the weaker LED signal, chopping the beam with a light chopper
(Appendix VIII) as previously suggested will allow the use of ac
coupling of the oscilloscope.
Another approach would be to drive
the LED with a square-wave generator.
Both of these approaches are
aimed at allowing the use of the more powerful avalanche photodiode
for detection.
The disadvantage of an APD is the relatively high
bias voltage present; without the chopping techniques, it may be
impossible to see the information.
Sample Questions
1.
From your experience with this project, list the comparative
advantages and disadvantages of the LED and ILD.
2.
3.
What values of m, as explained in the text, best describe
'^^
the devices you measured?
Cl«5
What bearing do the incoherence of the LED emission and the
coherence of the laser diode have on the discussion?
4.
Perform sample calculations of input coupling loss assuming
"typical" LED and ILD sources and a multimode step-index
125 micron core fiber.
5.
How do you expect the expression for R loss to differ for
graded-index fibers?
References
[l]
Kleekamp, Charles and Bruce Metcalf, "Designer's Guide to Fiber Optics —
Part 2," Electronic Design News. January 20, 1978, p. 46.
l,tj
71
[2]
Burrus, C. A,, H.C. Casey, Jr., and Tingye Li, "Optical Sources,"
Optical Fiber Telecommunications, Stewart E. Miller and A. Q.
Chynoweth, Editors, Academic Press, New York, 1979, p. 499.
[3J
Marcuse, D.,"LED Fundamentals:
Comparison of Front- and Edge-
Emitting Diodes," IEEE Journal of Quantum Electronics, Vol. QE-13,
No. 10, October 1977, pp. 819-827.
[4] Selway, P. R., A. R. Goodwin, and P. A. Kirkby, "Semiconductor
Laser Light Sources for Optical Fibre Communications," Optical
Fibre Communication Systems, C. P. Sandbank, editor, John Wiley
and Sons, Chichester, 1980, Figure 114, p. 158.
[5] RCA, "Solid State Emitters," Publication SSE-100, pp. 19-20.
[6] Andrews, J., "An Inexpensive Laser Diode PiiLser," Review of
Scientific Instruments, Vol. 45, No. 1, January 1974, pp. 22-25.
;-33
[j HI
I. i
72
Optical Spot
Emitting
Source
Fiber
a)
b)
c)
Figure 6-1. Three types of input-coupling loss: a) uninterceoted illumination, b) NA mismatch, and c)
reflection.
73
(^, Angle of Radiation, degrees
Figure 6-2,
Intensity profiles for various values of m. Circles
are drawn for values of m from 1 (Lambertian) to
30.
RCA
C-30808
Detector
1
Laser diode
Pulser
,.45V
'
V-t
. -J —
IRCA
SG2001
ILD
"^i
>
scope
82kr^
Variable
Voltage
Supply
Figure 6-3. Equipment setup for measuring intensity vs. drive current.
74
120
160
200
Drive Current (mA)
Pulsed, 44% duty cycle PRR=lkHz
a)
30
28
26
24
< 22
3.
20
C 18
u 16
u
14
It /IK
•
:'¥>
o
Z 12
a. 10
5
MB
86.4-
1
0
1
2
3
H
5
6
Drive Current (A)
Pulsed
b)
Figure 6-4. Output current in RCA C-30808 PIN diode vs. drive
current in source. (45V back bias on PIN, 82kP. in
series.) a) RCA C-30123 IR-LED, b) RCA SG2001 ILD.
75
45V
Driver
Source
82kJ2
Figure 6-5.
Equipment setup for measuring intensity profile.
11
i
H
76
100%
15"
t)Ml
a)
.••••'
• '»
. *•]
il
M
100% ^^^
'5
r
••i
.1
Experimentally determined intensity profiles.
a) RCA C-30123 IR-LED, b) RCA SG2001 laser
diode.
77
•
n
• il
—Detector
Driver
LED or
ILD
Wavelength
Selector
•ii3
82kQ
Figure 6-7. Equipment setup for measuring spectral distribution
of sources.
Chapter VII
WAVELENGTH J4ULTIPLEXING IN Al^ OPTICAL FIBER
Project Assignment
You are to generate two optical signals, one visible and one
infrared, and wavelength-division multiplex them in an optical fiber.
In addition, one of the signals is to be analog vrfiile the other signal must be digital.
After transmitting the signals through the
fiber, separate the signals and show that there is no interference
between them.
Objectives of the Experiment
1.
To exhibit the wavelength-division multiplexing capability of an optical fiber.
2.
To demonstrate two ways information may be transmitted
with an optical signal through an
optical
fiber.
3.
To provide an introduction to fiber optics as related
to communications considerations.
»
Equipment Needed
1.
Pigtailed visible-LED.
2.
Pigtailed infrared-LED.
3.
Two LED drivers (one analog and one digital).
4.
Fiber coupler for launching signals.
5.
Beamsplitter.
6.
IR viewing screen.
7.
Short length of optical fiber.
78
79
8.
Micromanipulator.
9.
Optical detector.
10.
Oscilloscope.
11.
Red-blocking filter (e.g. Wratten 47A).
Theory
Introduction
Tne present interest in optical communication systems owes
its existence to the development of the laser, the first coherent
source of light.
Midwinter [l] suggests the primary impact of
lasers on present systems was as an encouragement for researchers
to consider the optical frequency spectrum as an extension of the
radio and microwave spectra.
The result of this new attitude was
the increase in study of all optical components:
sources, detectors,
modulators, lenses, mirrors, waveguides, and systems.
A promising recent development is the announcement by researchers
at the Nagoya Institute of Technology that a diode laser has been
developed which emits simultaneously at wavelengths of 1.17 and 1.3
microns [2] .
The device consist of two lasers lying alongside
each other, and eliminates the need for a multiplexer to put the
signals in a fiber.
Basic Concepts
The study of multiplexing signals in an optical fiber constitutes a review of the principles of communication engineering which
concerns itself with the transmission of various signals between
points.
The signals to be transmitted will be sent through a channel
of some sort, either in the form of a guiding transmission line or
80
merely an open space throughout which the signals are radiated. Each
of the signals generally has a small finite bandwidth compared to the
bandwidth of the channel itself.
It is wasteful to use the channel for one signal only because
the channel is being operated very much below its capacity to transmit information.
However, if two signals are sent simultaneously,
they will mutually interfere, destroying the information. This
desire to more efficiently exploit the "bandwidth-space" of the
transmission line led to the introduction of multiplexing techniques.
There are a number of ways to perform this multiplexing operation
physically [3], but all may be treated mathematically in the same
;,ii
•;2
manner.
Modulation Theory
Signals may share a transmission channel provided that they
can be separated at the receiver.
There are two important ways in
>
which this is accomplished.
If the signals occupy different ranges
in the frequency domain, they may be separated with bandpass filters
which pass signals only in carefully defined ranges.
If the signals
occupy different time intervals, they may be separated with a synchronous detector.
The first approach, where the frequency spectra
of the signals are interleaved, is known as frequency-division or
wavelength-division multiplexing.
The latter approach, in which
samples of each signal are interleaved in the time domain, is called
time-division multiplexing.
The technique of multiplexing signals in time is more a digital
electronic project than fiber-optic and, for this reason, will not
-X
,',1
81
be discussed here.
However, frequency- or wavelength-division multi-
plexing, or WDM, offers an opportunity to exhibit one of the most
attractive features of optical fibers;
namely, that two signals
can exist in the same fiber at the same time (without mutually
interfering) without any need for sampling techniques.
It is possible to shift the frequency spectrum of a signal,
usually centered about frequency zero, to a position centered about
a new nonzero frequency by modulating it (multiplying the signal
by a sinusoid of the desired new frequency).
The convolution
theorem [4] states that if f, (t) has Fourier transform F^(ai) and
•1*4
f^(t) has Fourier transform F2 (o)), then
J *
-111
f^(t)*f2(t) ' ^ F^(a))F2(w)
(7-1)
and
f^(t)f2(t) <^
(l/2Tr)F^(a))*F2(a»),
(7-2)
•iC
where
F[f(t)] = F(ca) =/'*f(t)exp(-ja)t)dt
is defined to be the Fourier transfom of f(t).
may then be defined in terms of ?M,
(7-3)
The function f(t)
using the inverse transfom
relationship
F((o)exp(ja>t)da)
(7-4)
—00
A direct result of the convolution theorem indicates that the convolution of a function f(x) with a unit impulse function reproduces
82
the function itself.
That is,
f(x)*6(x) = f(x)
(7-5)
and
f(x)*5(x-T) = f(x-T) .
(7-6)
A sinusoidal signal cos(a3 t) is said to be amplitude-modulated
c
^
by a signal f(t) when it is multiplied by f(t).
The Fourier trans-
form of cos (oj t) indicates the frequency spectrum is composed of
two impulse functions of amplitude ir: one located at o) = uci , the
other at 0) = -oj^. By the convolution theorem:
c
f(t)cos(a)^t) <-* (l/2Tr)[F(a))**(a))] =
(l/2)F(a))*[6((o+u) )+6(a)-a) )],
c
c
(7-7)
where $(u)) is the frequency spectrxim (Fourier transform) of cos(a) t)
In communication applications, the cosine function is known as the
Ml
carrier and f(t) as the modulation fimction.
From Equation (7-6) above it follows that
f(t)cos(aj t) <-^ (l/2)[F(o)+a)^)+F(a)-aj^)] .
c
c
c
(7-8)
f(t)sin(a)^t) ^ > (j/2)[F(a)-Hia^)-F(a3-u)^)].
(7-9)
Similarly,
Therefore multiplication of a signal by a sinusoid results in a
translation of the spectrum of that signal by ±a)^.
Because the
length of the antenna needed to transmit a signal is inversely
83
proportional to the frequency, this shifting also has th.e practical
advantage of allowing the transmission of the signal with a shorter
antenna.
When many signals are to simultaneously share a transmission
line, as in WDM, the spectrum of each signal is shifted to a unique
position by multiplying by a carrier wave of the appropriate frequency.
If the shifted spectra of the various signals do not over-
lap, all may be transmitted simultaneously.
It is easiest to regard the transmission medium as a band-pass
filter of finite bandwidth.
Obviously, signals whose frequency
spectra lie outside this bandwidth will not be transmitted.
The introduction of the laser made available coherent carriers
of frequencies on the order of 10
Hz.
J
il
>'
••'
n
If the bandwidths of the
signals to be transmitted were restricted to just 0.1% of the carrier
"j
f
I
frequency, these band-limited signals would be permitted to have
bandwidths of 1000 GHz.
Because an optical fiber transmits optical
'^
• <
signals well into the infrared, the attendant information-carrying
capacity of the fiber is astounding.
Suppose it is desired to transmit n band-limited signals.
simplicity, assume identical bandwidths of o)^ rad/sec.
For
To guarantee
the frequency spectra are separated upon modulation, each signal is
modulated with a carrier of a unique frequency o)^, ^2^ - • • * ^^» where
each carrier is separated from adjacent carriers by at least 2aj^
rad/sec.
Each of the modulated signals has bandwidth 2a)^ centered
at 0),. as shown in Figure 7-1.
1
At the receiver, the signals are separated by using the proper
f
84
set of bandpass filters, each of bandwidth 2a) centered at co .
m
i
Finally, each filtered signal is demodulated by an AM detector.
The detection scheme for amplitude-modulated signals is rather
simple.
One of two methods
i s used.
In a rectifier detector, the
modulated carrier is rectified; i.e., the negative portion of the
carrier is set to zero.
This is equivalent to multiplying the modu-
lated carrier by a square wave of the same frequency as the carrier.
Again, by the convolution theorem, the spectrum of the rectified
signal is shown in Figure 7-2 for the band-limited signal f ( t ) . A
bandpass filter centered at co = iu) will recover the transmitted
information f ( t ) .
In an envelope detector, the output of the detector reproduces
,A
Jj
: "1
the envelope of the modulated carrier.
7-3.
The circuit is as in Figure
On the positive cycle of the input, the capacitor charges to
the peak voltage of the input.
As the input signal falls below
this peak value, the diode is reverse-biased.
The capacitor dis-
I •)
'J
„.•»•
<
charges through the resistor at a rate adjusted for the characteristics of the signal to be detected.
During each positive cycle,
the capacitor charges to the peak input voltage, and when the diode
becomes reverse-biased, the capacitor voltage changes very little.
The result is the capacitor voltage follows the envelope of the input.
For commercially broadcast signals, the frequency spectrum of
the transmitted signal is first shifted back down in frequency by
"mixing" it with the sinusoidal output of a local oscillator at
the receiver (heterodyning).
This translates the frequency spectrum
downward in frequency to a fixed intermediate frequency for which
85
amplification is more easily provided.
The information is then de-
tected from this newer intermediate-frequency carrier by one of the
techniques described above.
For fiber optic systems applications, information is transmitted digitally in the form of a coded set of pulses, except for
very short links.
This is a direct result of the inherently more
reliable nature of digital communications.
A detector of optical radiation outputs a current of electrons
for an input of photons.
an intensity variation.
the detector.
The incoming light field is seen only as
As such, any phase information is lost in
When sources of light are modulated for communication
purposes, therefore, they are intensity-modulated.
For a given
source, the modulation may be either digital where the device is
driven exclusively between two states: ON and OFF, or analog where
the device is driven continuously in the region between OFF and
fully ON.
When digital modulation is used, the information is en-
coded in the pattern of pulses which the detector sees.
For analog
modulation, the information is sent directly as intensity variation.
Experimental Procedure and Results
In the system to be considered here, two signals of different
types (one analog and one digital), are used to amplitude-modulate
two LEDs of differing wavelengths.
To guarantee that the shifted
frequency spectra do not overlap, the signals should be band-limited,
For the sake of simplicity, the analog signal was chosen to be a
sinusoid of frequency f = 10 Hz which
of approximate wavelength 640 nm.
modulated a red LED
The IR-LED of approximate wave-
86
length 950 nm was modulated by a square-wave of
frequency
120 Hz.
The experimental setup used is shown in Figure 7-4.
nals were driven by the circuit of Figure 7-5.
Both sig-
The two light
sources correspond to carriers of different frequencies.
Each of
the sources was a pigtailed LED, prepared as described in Special
Problems.
The two signals are introduced into an optical fiber in
a simple fashion. Figure 7-6.
Two grooves were made in a piece of
balsa wood by pressing the leads of a resistor into it.
The fibers
were placed in these grooves and covered with a piece of plexiglas
cut from a drafting triangle.
after alignment.
One screw clamps the fibers in place
The angle between the grooves matches the accep-
tance cone angle of the fiber.
Because the fiber is short, the
fact that very little optical power is coupled into the fiber is
immaterial; there is enough for the receiver to detect.
Of course,
for longer runs of fiber, more care would have to be exercised in
the input coupling.
One approach would be to collimate the outputs of each source
and combine the two spatially in a beamsplitter, as in Figure 7-7a;
a lens following this focuses the light onto the core.
A third
method might use a reflective filter which passes one of the beams
yet reflects the other as in Figure 7-7b.
At the receiving end, a Bell and Howell 509-50 detector with
integrated op-amp provides an output electrical signal.
Because
of the inefficient input coupling, some amplification will be needed.
Figure 7-8 shows the signal detected with both sources present.
87
men
a Kodak 47A filter is used to block the red components of
the multiplexed signal, the output of Figure 7-9 results.
To re-
cover the red signal, without the use of an IR filter (none of which
were available), a beamsplitter and two detectors are used.
beamsplitter transmittance must equal the reflectance.
The
On one leg,
the red-blocking filter allows only the IR signal to reach the detector.
This output is fed into the inverting input of a differential
amplifier.
The second leg would employ a neutral density filter
(to match the attenuation of the red-blocking filter in the first
leg) before the detector; both red and IR signals reach the detector.
The output of this detector is fed into the non-inverting input of
the differential amp.
The information contained in the red signal
therefore appears at the output of the amplifier.
Special Problems
The most serious problem associated with this experiment is
obtaining filters vdiich properly match the spectral characteristics
of the sources used:
blocking one and passing the other.
The
Kodak 47A is readily obtainable from Kodak; the transmission characteristics are as shown in Figure 7-10 [5]. However, no filter was
found
which
would pass red and block the near-infrared at 950 nm.
If a longer IR wavelength source is used, filtering may be possible
with the materials of Table 7-1 [6].
88
TABLE 7-1.
Some Optical Filters in the IR
Low-Pass Filter w/37% cutoff at 1.2 ym-20 mm of water.
1.0 ym-0.5 mm of water.
2.2 ym-1 mm of Dupont
Lucite.
ii.DD ym-lO mm of common
glass.
High-Pass Filter
tl
M
"
"
II
II
" 1 . 2 ym-2 mm of silicon.
II
1.8 ym-2 mm of germanium,
Positioning the fibers at the launching end is very critical
Tne alignment was chosen to equalize the signals detected from
each source with the other disconnected.
To pigtail the devices,
the ends of inexpensive LEDs were first clipped off with diagonal
pliers.
With the driver connected, a miniature drill (such as
that available with a Dremel tool) was used to drill a well over
the emitting area of the LED.
A short (6 cm) piece of multimode
glass fiber was temporarily affixed into the well with a drop of
clear instant glue.
Silicone sealant was added around the joint
for strength.
It is further necessary to protect the input coupler from
gross vibrations and air currents which introduce noise in the
system due to relative motion of the fibers in the coupler.
At
the same time, the detector should be shielded from stray-light
89
sources, both visible and infrared.
If narrow-band interference filters are used, the signal may
be too weak to detect.
PIN photodiodes are extremely amenable to
use in op-amp circuits (see Appendix VII).
If no more amplification
can be provided, and no avalanche photodiode is available, the input coupling will have to be improved.
Sample Questions
1.
What other ways can be used to introduce the two signals
into the fiber [3]?
2.
What is the primary, fundamental advantage fiber optic
systems have over all other communications systems?
3.
Why would you expect laser diodes to be more appropriate
sources for a system where many signals are to be multiplexed?
4.
For multiplexed signals in a fiber, which would be the
more reliable way to transmit information, time-division
multiplexing or wavelength-division multiplexing?
5.
You have seen that signals transmitted via optical fibers
represent AM modulation of an optical-frequency carrier.
How does the detector remove the signal from the carrier?
Has any mixing taken place in the detector?
What type
of detection scheme is provided by an optical radiation
detector?
6.
In what ways does the real fiber optic communication
system depart from the ideal assumptions of the section
on Theory?
90
References
[l] Midwinter,
J. E.. Optical Fibers for Transmission, John Wiley
and Sons, New York, 1979, p. 1.
[2] Sakai,S., T. Aoki, and M. Umeno, as reported in "Dual-Wavelength
Laser Needs No Multiplexer," Fiberoptic Technology, March 1982,
p. 117.
[3] Tomlinson, W.J., "Wavelength multiplexing in multimode optical
fibers," Applied Optics, Vol 16, No. 8, August 1977, pp. 21802194.
[4] Lathi, B.P., Signals, Systems, and Communication, John Wiley
and Sons, New York, 1965, Chapter 11.
[5] Eastman Kodak Co., "Kodak Filters for Scientific and Technical
Uses," Kodak Publication Bo. B-3, Eastman Kodak Co., New York,
June 1973.
[6]
DePalma,
James,
editor, "Filters," SPSE Handbook of Photo-
graphic Science and Engineering. John Wiley and Sons, New York,
1973, Section 4.
91
,Fi(a))
0)
0)
m
m
0)
0
m
(D
m
F
((D)
mux
^
(JL)
n
Figure 7-1. Frequencyplexing.
or wavelength-division multi-
F{A+f(t)}cos(x)^t = F{g(t)}
0)
03
a)
F{g(t)-p(t)}
Figure 7-2.
a) Spectrum of band-limited signal (modulated cosine);
b) Spectrum of modulated signal multiplied by
square-pulse train of frequency 03 .
92
Figure 7-3. An envelope detector.
Analog
Input
LED
Driver
Circuit
Fiber
Digital
Input
LED
Driver
Circuit
Figure 7-4. Equipment setup
i
Fiber
Blocking
Pigtails
Filter
Bell and
Howell
509-50 Photo
diode and
Op Amp
93
Voltage
Signal >
In
Figure 7-5. LED driver circuit for both analog and
digital inputs.
Figure 7-6. Crude wavelength-division multiplexer
94
t
i
..
ir ' M i l
I
I N M 'M' 1 ., ',
ViV< I,
a)
b)
Figure 7-7. Alternative WDM schemes: a) beamsplitter,
b) wavelength-selective mirror.
Figure 7-8. Multiplexed output. [Red: l^mV
lOmV
, 120 Hz square wave]
P-P
, 10 Hz sinusoid; IR:
95
Figure 7-9. Demultiplexed IR signal. [lO mV ,,
120 Hz, square wave]
^"^^
0.1 —I
c
1
4J
s
c 10 —
OJ
U
H
100 —»
20 0
30 0
'•00
500
600
700
800
900
Wavelength (nm)
Figure 7-10. Transmission characteristics of Kodak
Wratten filter No. 47A.
CHAPTER VIII
OPTICAL LINK DESIGN
Project Assignment
Design, construct, and test a communications link for transmission of an IF analog signal through an optical fiber at least 2
meters in length.
The IF signal may be taken directly from the out-
put of the IF amplifier in an AM radio. Justify your choice of source
and detector in your design.
As an exercise, design a 2-km 20 Mb/s NRZ digital fiber optic
link with one permanent splice. The fiber specified is graded-index,
_q
with a loss of 4 dB/km. The bit error rate should be less than 10 .
Objectives of the Experiment
1.
To acquaint students with the practical nature of the
use of fiber optics in communications.
2.
To acquaint students with the use of both an opticalpower budget and a rise-time budget.
Equipment Needed
1.
Source of information-carrying IF signal.
2.
Optical source.
3.
Optical detector.
4.
2-meters of optical fiber.
5.
Appropriate transmit and receive electronics.
6.
Loudspeaker.
7.
Oscilloscope.
96
97
Theory
Introduction
The preceding chapters have dealt with the various properties
of optical fibers, sources, and detectors, but presently, the most
important, application of fiber optics lies in the transmission of
information.
To construct a transmission link, it will be necessary
to draw from the results of the prior experiments.
Optical links can be configured to accept information in either
digital or analog form.
While there is still some debate as to the
proper format to be used for optimal transmission, it is generally
agreed that simpler analog modulation is appropriate for shorter
links while digital schemes should be used for longer distances [l].
In an analog scheme, the transmitter modulates the intensity of
a light source, usually by controlling the source drive current.
In
the digital scheme, the source intensity is pulsed to transmit
binary bits of information.
Both formats have common design
approaches up to a point although signal-to-noise ratio (SNR) is
more appropriately associated with analog systems and bit-error rate
(BER) with digital systems.
Typically, a design engineer will be asked to design a communication channel for which he is given:
a) the characteristics of the
signal to be transmitted, b) the channel length, and c) the allowable
signal degradation in passing through the channel.
The design pro-
cedure then becomes one of choosing the optimum source-link-detector
combination.
The choice must include cost, and fortunately the
prices of fiber-optic components continue to drop as the production
98
volume increases.
Basic Concepts
Much of the following Information is taken from Technical Note
R-1 from ITT [2].
Other component manufacturers also provide such
literature. [3], [4], and are eager to provide design aids to prospective users of their products, and to familiarize students with
fiber-optic systems.
A basic transmission system using a current-modulated light
source is shown in Figure 8-1, and Table 8-1 shows the power and
loss relationships for each point in the line. Each functional
block in the diagram is specified by a loss transfer function, L,
where
L = 10 log (P^^^/P^^) dB
( 8-1)
(as in Chapters III and IV). The combined transfer function for
several stages in sequence will be the sum of the losses of each of
the stages. All powers are expressed in dBm (dB with respect to one
milliwatt).
With the understanding that each system component will have some
non-negligible optical insertion loss and a finite bandwidth, analyses
must be done to account for both the optical power loss in the system
and the bandwidth restriction.
The designer, working with a finite
optical power at the input, must "throw some away" for each stage of
the design.
Similarly, the "slowest" elements in the system will ulti-
mately control the system's final bandwidth.
Optical-power budgeting
and system rise-time budgeting are simple bookkeeping techniques to
99
account for the power losses and bandwidth restrictions.
TABLE 8-1. Power and Loss Relationships
(Two sections of fiber only)
Parameter
Equation
Power from light source
P
s
P =P - L
0
s
0
P, = P - (L« + L )
Power coupled into fiber
Power after first fiber section
I s
0
1
P« = P - (L« + L, + L^)
Power after first joint
Power after second fiber section
Power c o u p l e d i n t o d e t e c t o r
^ 3
0
1 2
P, = P - (L« + L, + L^ + L )
J
s
0
1 2
3
P, = P - (L^ + L, + L^ + L^ + L , )
if
s
U
1
Z
3
4
To determine the performance of an optical fiber system, four
main areas must be considered:
source power characteristics, optical
power coupling losses, intrinsic fiber losses, and losses due to receiver sensitivity.
The source power characteristics (Chapter VI)
include total output power, wavelength of peak power, physical size
of the emitting surface and output port, and output radiation profile.
The intrinsic fiber loss (Chapter IV) is the loss in dB/km times the
length of the link at the particular wavelength of transmission. The
receiver also contributes to the loss due to the "quantum efficiency"
of the detector.
In addition to these losses, there are the losses
due to source-fiber coupling and fiber-detector coupling as well as
splice losses.
For purposes of bandwidth evaluation, the overall bandwidth
limitation is conveniently analyzed in terms of rise and fall times
100
of the i n d i v i d u a l components.
For an analog system, the o v e r a l l
r i s e time of t h e r e c e i v e r must be l e s s than the s p e c i f i e d r i s e time
of the t r a n s m i s s i o n system.
i n t o two formats:
D i g i t a l communication systems a r e divided
r e t u m - t o - z e r o (RZ) and n o n - r e t u m - t o - z e r o (NRZ).
In the former, t h e r e i s a r e t u r n to the zero s t a t e (no l i g h t p r e s e n t )
between two s u c c e s s i v e high s t a t e s ( l i g h t p r e s e n t ) .
In the l a t t e r
format, t h i s t r a n s i t i o n back to zero i s not r e q u i r e d .
i s shown i n Figure
8-2.
A comparison
The r i s e time for d i g i t a l systems should
be l e s s than a b i t i n t e r v a l for NRZ data where a b i t i n t e r v a l i s defined to be the r e c i p r o c a l of the b i t r a t e .
A r u l e of thumb allowing
for the f i n i t e r i s e time of the a m p l i f i e r s i n the r e c e i v e r permits
the r i s e time of the system up to and including the d e t e c t o r to be
no more than 70% of a b i t i n t e r v a l for NRZ data or 35% of a b i t i n t e r v a l for RZ d a t a .
Spreading
places
of
the o p t i c a l
a severe r e s t r i c t i o n
pulses
in
the fiber
on t h e maximum
also
bandwidth.
For example, c o n s i d e r two p u l s e s of width At in time sent AT a p a r t .
If the spreading caused by d i s p e r s i o n causes the p u l s e s to spread to
a width AT/2 in time, they w i l l be i n d i s t i n g u i s h a b l e a t the r e c e i v e r .
Sources
The first considerations are the characteristics of the source
which include - total output power, wavelength of peak power, physical
size of the emitting area and output port, and output intensity profile.
A comparison between LEDs and laser diodes has been done in
Chapter VI.
LEDs and lasers are available with peak-emission wavelengths in
101
the near-infrared in the regions of low intrinsic fiber loss.
Lasers
can produce about 10 dB more optical power than LEDs, and due to the
narrower angular radiation pattern, are capable of injecting about
10 dB more power into an optical fiber than do LEDs.
However,
lasers are also more expensive, more temperature-sensitive, and less
reliable (due to the generally higher lasing currents), LEDs are
still more attractive for shorter links where sophistication is not
required.
Both devices require drivers which are capable of supplying the
required drive current to the low dynamic impedance of the device at
the rate of modulation.
In addition, the laser diode driver may be"
required to provide a dc bias current just below the lasing threshold
of the laser diode to reduce the turn-on time.
Fibers
As an optical signal propagates along the fiber, its amplitude
and bandwidth are reduced.
Attenuation is expressed in dB/km while
the bandwidth reduction, or dispersion, increases with length and is
expressed in MHz-km.
For pulsed signals, this results in time spread-
ing or "smearing" of the pulses given in ns/km.
Several types of fiber are manufactured for differing applications
The least expensive is generally made of plastic, either totally or
in the form of a plastic-clad silica core (PCS).
Plastics usually
have a higher attenuation and dispersion than other types. Of the
glass types of fiber, there is a choice between a step-index (SI)
core or graded-index (GI) core (see Chapter II). Both exhibit lowloss, but the refractive-index distribution of the GI core is tailored
102
to reduce modal dispersion.
Coupling/Connecting/Splicing
Whenever the optical signal propagates from one system component
to another in the diagram of Figure 8-1, there are associated
coupling losses.
These include area mismatch loss, NA loss, fiber
misalignment and separation loss, and Fresnel loss.
Therefore these
losses are shown as functional blocks as well.
Although the double-crucible method for drawing fibers theoretically allows fibers of any length to be drawn, fibers are typically sold in lengths of 1 km.
splices must be made.
For runs greater than this length,
Connectors are defined to be non-permanent
junctions which can be taken apart repeatedly.
Splices are defined
to be permanent junctions of fibers which are held together by
epoxy or a fusion weld.
Losses result from the discontinuity at
the junction; splice losses are typically lower than connector losses
because discontinuity reflections are reduced by the bonding agent.
Detectors
Optical-power Budget
Design of a fiber link typically starts at the detector.
Of
course, if the designer has any of the components already on-hand,
the link may be designed around them.
The curves of Figure 8-3 indicate the required optical power
at the detector for a given SNR or BER for both PIN and APD detectors.
The worksheet published by ITT in [2] is extremely useful
for computing power throughput and is therefore reproduced here
in Figure
8-4.
103
Using the worksheet, all known quantities are first entered.
A fiber is selected based on considerations of attenuation and bandwidth.
Once a fiber has been selected, a source/detector combination
is chosen.
The designer optimizes performance versus cost in the
selection, using the LEDs and PIN diodes if practical, and laser
diodes and APDs if necessary.
The optical power margin; i.e., the
difference between transmitted optical power and required optical
power at the receiver, may then be computed.
The power margin is
the power the designer has available to "spend" on each functional
block in the link.
It is calculated by taking the optical power
coupled into the fiber by the source and subtracting from that
number all the succeeding losses in the system.
After the analysis
is completed, individual components may be changed as practicable
to improve the margin.
For the case of a simple butt joint of LED source to a fiber,
a nomograph is available for estimating the amount of optical power
coupled into the fiber [6]. However, it is known that the insertion
of a small lens between source and fiber significantly reduces the
coupling loss to about 12 dB for LED-to-fiber and about 2 dB for
ILD-to-fiber [7]. Pigtailed sources also reduce the loss, and
manufacturers publish peak output power for these devices.
For
digital data with a 50% density of ON states, the average source
output power is 3 dB less for NRZ format and 6 dB less for RZ format.
For an analog system, the average output power is 3 dB less than
the peak output power, and in addition the source should not be
operated at peak current for lifetime considerations.
This implies
104
a reduction in the output power as well.
Connector loss is typically 0.1 to 1.0 dB per connector, and
splice loss is typically 0.01 to 0.5 dB. Worst-case design indicates the largest value for loss should be used.
Both losses vary
with different fibers and specifications for joints are usually provided by the manufacturer.
If they are not, a loss of 1 dB for
connector and 0.3 dB per splice may be assumed.
Fiber loss is given by multiplying the loss in dB/km at the
operating wavelength by the length of the fiber.
It is suggested
that an extra 3 dB of loss be allowed for long-term degradation of
source power output.
3 dB may also be necessary to allow for de-
gradation due to temperature. Finally, fiber-to-detector loss is
taken to be about 1 dB (typ.).
Rise-time Budget
A system design is not complete until a rise-time analysis
has been performed to ensure that the system will have the necessary
bandwidth.
The overall bandwidth limitation is conveniently analyzed
in terms of rise/fall times. In any cascade-connected system, the
overall rise-time is approximately
t
7
2
2 05
= 1.1 (t^ + t2 + . . .+ t^) '
(8-2)
where t. is the rise time associated with the system element i. The
rise-time analysis worksheet of Figure 8-5 is used to compute the
bandwidth.
Rise times for sources, detectors, and receivers are
specified by the manufacturers.
The bandwidth restriction placed
on the system by pulse-spreading is accounted for by dividing the
fiber bandwidth factor in MHz-km by the length in km.
To obtain
105
the accompanying rise time, multiply the rise-time factor in
ns/km by the length of the fiber in km.
width
The total system band-
(-3 dB) for an analog system is approximately equal to 0.35
divided by the system rise time.
Once again, if the result of the analysis proves to be unsatisfactory, individual components of the link may be replaced to upgrade the performance.
Experimental Procedure and Results
The experiment is performed in steps. The transmitter was the
first section built and tested. The source for the IF signal was taken
directly out of the IF amplifier in an inexpensive ($6.95) pocket
AM radio. A review of AM radio circuits and a look at the schematic
indicated the proper place to break the circuit for installing a
fiber optic link between the output of the IF amplifier and the
speaker.
Referring to the transmitter portion of the system cir-
cuitry of Figure 8-6, it is seen that an op-amp amplifies the voltage from the radio while providing a large input impedance to the
radio to avoid loading.
A voltage-follower boosts the
current level of the signal in order to drive the
LED.
The second-stage also- provides a non-zero quiescent current
to prevent the LED from rectifying the signal. The quiescent current is 350 mA, and the signal current is 50 mA p-p. For the LED
used (XC-880A, peak IR wavelength of 880 nm), this current corresponds to an output flux of 1.88 mW peak and 1.75 mW average power.
106
As is often done, these are related to a 1 mW level as 2.73 and
2.43 dBm respectively.
The average source coupled power will be
smaller; for this application, the fiber was pigtailed to the LED
after the end of the LED was sawed off and a locating hole drilled
into the plastic capsule just above the emitting area.
A drop of
instant glue held the fiber temporarily while a coating of silicone sealant was applied around the butt joint to provide strain
relief.
fashion.)
(The fiber was pigtailed to the detector in a similar
For this type of interface, two losses are important:
1) area mismatch loss, and 2) NA loss.
previously in Chapter VI.
These have been discussed
Here, total launching losses were esti-
mated to be 6 dB which would result in -3.6 dBm being launched.
The optical-power budget analysis is not necessary for such
a short link, as will be seen.
example.
However, it is included as an
From the curve of Figure
8-3b, it is seen that for a
transmission bandwidth of 455 kHz (the IF carrier frequency), the
average received optical power must be greater than -85 dBm for
an SNR greater than unity.
For a desired SNR of 30 dB, approxi-
mately -55 dBm are required at the receiver.
This represents an
optical power margin of
-3.6 - (-55) = 51.4 dBm.
The fiber available for the experiment (Corning type 1504)
was multi-mode graded-index with a loss factor of 10.3 dB/km and
a bandwidth factor of 740 MHz-km.
For the 2-meter demonstration
link, the attenuation is seen to be negligible.
107
The detector (Centronics OSDl-1) was optimized for operation
in the blue-UV region of the spectrum, and hence, no data were
published on the responsivity at 880 nm.
However, it has a quantum
efficiency of 65% at 880 nm.
The fiber-to-detector coupling loss was taken to be 1 dB;
3 dB are allowed for time degradation of the source and another
3 dB for temperature degradation of the total system.
The excess
optical power is then seen to be
51.4 - 1 - 3 - 3 = 44.4 dB
for the system.
5 to 10 dB.
In real systems, this would be on the order of
Obviously, the link could be made much longer.
The rise time analysis is not necessary in such a simple link.
The rise time of the LED was unknown, but after installation in
the transmitter circuit it was checked to see if the current waveform was a faithful reproduction of the information signal.
The
740 MHz-km bandwidth factor for the fiber indicates the bandwidth
will be even greater for a 2-meter length.
The rise time of the
detector is listed at 15 ns, more than fast enough for the 769 ns
rise time required to detect a 455 kHz signal.
To continue with the circuitry of Figure
8-6, LF353 op-amps
were chosen for the relatively high (4 MHz) gain-bandwidth product
they exhibit and for the fact that they have FET-inputs.
It was
known that the receiver would require quite a lot of gain, and it
was desirable to achieve this without affecting the bandwidth
seriously.
In the receiver portion of the circuit, the PIN photo-
108
diode operates into a transimpedance amplifier front-end which is
followed by another op-amp stage.
To increase the power level of
the detected signal, a common-emitter amplifier is used as the third
stage.
The fourth stage common-emitter amplifier provides voltage
gain and operated as a rectifier detector as well.
Because the
operating point is located at cutoff, only the positive-going parts
of the information are amplified at the output.
A low-pass filter
picks the audio information out of the output and this is fed into
the LM386 audio amplifier chip to drive the speaker.
The photograph
of Figure 8-7 shows the amplitude-modulated IF carrier transmitted as
well as the audio signal detected and driving the speaker.
The voice
represented is that of news commentator Paul Harvey.
Special Problems
Before a fiber is ever pigtailed into the system, the system
should be checked with just the source and detector forming the optical
link.
Care must be exercised that the receiver circuitry is not satur-
ated by the relatively larger optical signal received from the source
with the fiber absent.
One way to do this is to position the source a
distance away which avoids saturation; in this case, a ball-point pen
tube was cut off to the proper length and the source and detector fitted
into the ends.
This allowed adjusting the circuit variables without the
need for constantly checking alignment.
The solution of biasing the fourth stage of the receiver circuit
at cutoff was happened upon quite by accident.
common method of audio detection in AM radios.
Research shows this is a
109
Sample Questions
1.
If, instead of an AM signal, you were asked to
design a link to transmit a television signal,
how would your design be affected?
2.
In your opinion, which components represent the
highest cost in the installation of a fiber-based
system?
Of the components you have to assemble to
build a system, which would you design around?
Why?
3.
What are amplitude-shift keying and phase-shift
keying and of what importance are they?
References
[l]
Wolf, H. F., "System Aspects," in Handbook of Fiber Optics,
H. F. Wolf, Editor, Garland.STPM Press, New York, 1979, pp.
380-381.
[2] ITT, "Optical Fiber Communications Link Design," Technical
Note R-1, 8/78.
[3] Hewlett-Packard,"Flux Budget Considerations for Fiber Optic
Link Design," Application Bulletin 57, in Optoelectronics
Designer's Catalog, Hewlett Packard, 1981.
[4]
Belden Corporation Fiber Optic Group, "Use of Decibel Units
in Fiber Optic Systems," Fiber Optical Technical Bulletin
A/GI, not dated.
[5]
Swindell, W., "Circuits for Detectors of Visible Radiation,"
in Applied Optics and Optical Engineering, vol. VIII, R. R.
110
Shannon and J. C. Wyant, editors. Academic Press, New York,
1980, p. 323.
[6] Storozum, S. L., "Estimating the power coupled into an
optical fiber," Electronics, May 22, 1980, pp. 154-155.
[7] Wolf, H. F., op. cit., p. 392.
Ill
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113
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- 6 0- 7 0-
<:
-80-90-
I
.01
— r
.1
1
1
1
10
1
100
Bandwidth (Mhz)
c)
Figure 8-3. Required optical power, a) vs. bit rate for digital
svstem, b) vs. bandwidth for analog system with APD, and c) vs
bandwidth for analog system with PIN detector.
114
Required bandwidth or bit rate:
Required distance:
Required SNR or BER:
Fiber type:
Total fiber bandwidth @
Source type:
Detector type:
MHz-km:
MHz
Average source coupled power,-P,;
dBm
Receiver sensitivity, PR :
dBm
Total Margin,
Total fiber loss @
No. connectors:
No. splices:
Figure
(PR-PS)
dB
dB/km:
dB
Total connector loss @
dB/conn:
dB
Total splice loss@
dB/spiice:
dB
Detector coupler loss:
dB
Allowance for temperature degradation:
dB
Allowance for time degradation:
dB
Total Attenuation:
dB
Excess power (total margin-total attenuation):
dB
8-4. Optical power throughput worksheet. From [2].
115
Required system rise time:
Required fiber length, type:
RISE TIME
Source type:
RISE TIME
SQUARED
ns
Total fiber rise time due to multimode
dispersron @
ns/km:
Total fiber rise time due to material
dispersion @
ns/km:
ns
ns
(Typically 5.5 ns/km for LEDs,
negligible for lasers.)
Detector type:
ns
Receiver (if analog):
ns
SUM OF SQUARES:
System rise time, (1.1) (Square root of sum):
ns
Analog system -3dB electrical bandwidth, (.35 -;- system rise time):
Figure
8-5. Rise-time analysis worksheet. From [2].
116
E
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117
Figure
8-7.
Transmitted IF signal and detected
audio signal.
Upper trace - IF signal; lower
trace - audio output.
CHAPTER IX
FIBER PARAMETERS BY SCATTERING MEASUREMENTS
Project Assignment
You are to measure two important optical fiber parameters for
an unclad optical fiber, namely, index of refraction and fiber diameter, utilizing the back-scatter measurement technique. Compare
the pattern you obtain with that obtained for a clad step-index
fiber or a clad graded-index fiber. Photograph the interference
patterns you see.
Be prepared to discuss the forward-scatter tech-
nique used on clad step-index fibers.
Objectives of the Experiment
1.
To acquaint the student with current nondestructive
methods for measuring certain fiber parameters.
2.
To give practical experience with the easier of the
two, the back-scatter method.
Equipment Needed
1.
Short pieces of unclad step-index optical fiber and
clad step-index or clad graded-index fiber.
2.
Helium-neon laser.
3.
Opaque viewing screen.
4.
Camera and film.
5.
Ruler.
6.
Polarizer with known direction of polarization.
118
119
Theory
Introduction
The refractive indices of the cladding and core and the
diameter of the core are three fundamental parameters which can specify
the transmission characteristics of an optical fiber.
Unless they
are rigidly controlled, excessive signal degradation and loss in
the fiber may result.
Therefore, manufacturers have been interested
in nondestructive ways to monitor fibers for variations in the
desired values.
Several mechanical methods have been proposed [l],
but most suffer from a lack of sensitivity.
However, two optical
methods have been proposed which may have inherently better sensitivities due to the interference phenomenon.
The back-scatter method [2] observes the way coherent laser
light is scattered from a side-illuminated fiber back toward the
source and provides information about fiber diameter and index.
In its simplest form, it is useful only for unclad fibers of uniform
refractive index.
A more complex scattering theory has been devel-
oped which is capable of predicting the back-scatter pattern from
clad fibers of arbitrary refractive-index profile [3].
The forward-scatter technique [4] is useful for clad step-index
fibers.
It can be used for measuring the core/cladding diameter
ratio, and the indices of'both the cladding and the core although
it suffers from the necessity that one of the three quantities must
be known.
This method has also been suggested for determining the
refractive index profile for a fiber of arbitrary profile [5].
The importance of both of these methods lies in the fact that
120
they are noncontact and nondestructive as well as reasonably precise.
The information provided by these methods may be utilized in a feedback control operation to maintain quality during the fiber-drawing
process.
It is also used to test the quality of preforms before
they are drawn into fibers.
Watkins [6] reported the development
of an instrument to determine the core diameter and outer diameter
of an optical fiber using the forward-scatter technique.
Basic Concepts
Back-Scatter Measurements
For the discussion to follow, it is assumed that light from a
cw laser of wavelength X is incident on an unclad fiber in a direction perpendicular to the fiber axis.
lel to the fiber axis.
reflected back.
The light is polarized paral-
Some of the light will be transmitted, some
Of these components of the light, some will have
passed through the fiber and some will simply have been reflected
from the surface of the fiber.
The derivation for determining the
fiber index and diameter from a back-scatter pattern is due to
Presby [2].
In back-scatter measurements, we are interested in the scattering pattern resulting from radiation scattered back toward the
source.
As in Figure 9-1, light from a collimated cw laser source
passes through an aperture in an opaque viewing screen and impinges
on a fiber a distance h from the screen.
The back-scattered light
falls on the screen and is observed visually or photographically.
The resulting scattering pattern formed has several interesting
features.
First, it is seen that most of the radiation is localized
in a region nearest the center of the pattern with obvious transitions
121
to a lower-level continuum further from the center.
The bright
central region is usually called region A and the outer region,
region B.
Second, there is a modulation of intensity along the
pattern.
Sharp fringes whose spacing AL decreases with distance
L from the center of the pattern into region A are observed.
Finally, it is noticed that the pattern may change noticeably with
rotation of the fiber about its axis.
The distance from the center
of region A to the boundary between regions A and B is L . This
defines the angular extent of region A:
$ = arctan (L /h) ,
m
m
(9-1)
typically, ±20** from the incident direction.
From the measurement of L , h, AL, and L it is desired to
m
determine the index of refraction and the diameter of the fiber.
This may be done with a geometrical-optics analysis.
To do this,
consider the geometry and nomenclature of Figure 9-2.
Collimated
light from a coherent source strikes the fiber from the left.
ray gives rise to both a reflected ray and a refracted ray.
Each
At
the boundary between two isotropic media, the angle of incidence
equals the angle of reflection for a reflected ray and the angle of
the refracted ray obeys Snell's law,
n sin e
o
o
= n^sin 9^ ,
I
(9-2)
I
where all angles are referred to the normal at the interface.
Use
of these two relations allows each ray to be traced through the
fiber.
The same approach is applicable to fibers of any cross-
122
section, whether clad or unclad, but the results are not as easy to
interpret.
Emerging rays contribute to the scattered light and the
interference of these waves results in the fringe pattern of region A.
Refractive Index Determination
Consider a set of rays incident on the fiber on the upper halfsection, as shown in Figure 9-3.
The set incident on the lower half
is identical and is omitted for clarity.
A few of the reflected rays
are shown as a reminder that they also are a part of the scattered
light.
Observe that as the point of incidence moves in a clockwise
direction around the top left quadrant of the section, the angle of
incidence is monotonically increasing.
The angle deviation 9 of
the back-scattered ray from its incident direction at first decreases,
reaches a minimum, and then increases.
Referring back to Figure 9-2, consider a general ray whose angle
of incidence is 9^.
The angle of refraction, by Snell's law, is 9^.
Therefore, the ray is advanced through an angle Q - Q. upon entering
the fiber.
Reflection off of the back surface of the fiber advances
the ray through an additional angle of TT - 29,;, and upon emergence
from the fiber, a final increment of 9 - 9- is added.
o
t
The total
angular deviation of the ray for the trip through the fiber is
therefore
9 = IT -I- 29^ - 49^
.
(9-3)
To find the incident point at which minimum angular deviation
occurs, set d9/d9
= 0, keeping in mind that 9^ is also a function
of 9 , as Equation (9-2) implies.
With n
= 1 (for air) and n^ equal
to the index of refraction of the fiber, the angle 9 . which satis°
min
123
fies the minimum deviation criterion is
®min ' ^ = ^^^^°s [(^f^ - l)/3]°*^
(9-4)
It is more convenient to continue the discussion in terms of $ where
$
= TT - 9.
Then $ ^ ^ = $^ = ^ _ Q^. A plot of $ (and 9) versus
the angle of incidence 9
^
Figure 9-4, shows a sharp cutoff for $ .
m
From Equation (9-4) it is obvious that 9 and $ are independent of
the fiber diameter.
Using Equations (9-1) and (9-4),
$^ = 4 arcsin[Q/n^] - 2 arcsin[Q]
(9-5)
where the dummy variable
Q = 2 [0.333 - (n^^)/12]°-^
.
For given values of $ , n^ can be easily found by iterative techniques
and a programmable calculator.
Diameter Determination
The spatial coherence of the illumination has not entered the
discussion so far, but will now be seen to be fundamental to the
determination of the fiber diameter.
Again, following [2], consider
two parallel rays AC and HI coming from the same source and incident
upon the fiber at angles 9
and 9 ' as in Figure 9-5.
Ray AC emerges
at point E after refraction at the front surface and reflection at
the rear surface.
The portions of the ray which are externally re-
flected at point C and internally reflected at point E are ignored.
Ray HI is reflected at I on the front surface at an angle 9^', and
again, the part of the ray entering the fiber at that point is ignored.
If
124
9 ' = $/2 =
o
(TT
- 9)/2 = 2 9 . - 9 ,
t o
(9-6)
the two rays will interfere. The interference pattern from many such
groups of rays gives rise to the backscatter pattern of region A.
The path difference between the two rays ACDEG and HIJ is BC -ICD -I- DE -H FE = 2(BC -H CD) . The optical paths are
BC = r[cos (29- - 9 ) - cos 9 ] ,
r
o
o
(9-7)
and
CD = 2(nj)r cos 9^
.
(9-8)
Therefore, the optical path difference S is
S = 4 n^ r cos(9^)-4 n^ sin(9^)sin(9^ - 9^)
.
(9-9)
Using Equation (9-1) to eliminate 9^ yields
S = 4r[(l-R/n^^)°*^ *(n^+R/np-Rcos9^/n^^]
(9-10)
where the dummy variable
2
R = [sin 9^]
Using Taylor series expansions for sin 9^ and cos 9^ in Equation (9-10),
2
and neglecting terms of order greater than 9^ gives
S = 4r [n^ - $^/(16 (l-n^/2) )]
in terms o
f $.
,
The corresponding phase shift is therefore
(9-11)
125
<J) = 27rS/X
= [8Trr/x][n^ - $^/(16 (l-n^/2) )]
.
(9-12)
The distance between any two successive maxima or minima in the
pattern of region A can be found by setting the phase difference
between those two points equal to In.
That is.
"^1 " *2 ^ 27r
= [8iTr/X][*^^ -*2^]/[l6 (l-n^/2) ] ,
(9-13)
and
^^ = [$2 +(4X/r)(l-n^/2)]°-^
(9-14)
The approximate expression obtained in terms of the measured
parameters h, AL, and L, and utilizing Equation (9-1) and the fact
that, for small angles, tan(x) approximately equals x is
r =« [2(X)h^ /L(AL)](l-n^/2)
,
(9-15)
where L is measured from the center of region A to the center of AL.
Therefore, the radius of the fiber (and thus the diameter) is obtained
from a measurement of h, L, AL, and the computation of n-.
An alternative interpretation of the features of the backscatter pattern is presented in [7J.
Forward-scatter Measurements
The following discussion and derivation is due to Watkins [4].
Consider a step-index fiber of radius b with a core of radius a and
refractive index n^, and with a cladding of index n^.
Again, a
126
geometrical-optics approach for obtaining fiber parameters
from the forward-scattered light will be attempted because it is
easier. However, it should be pointed out that a wave solution
does exist [8].
Figure 9-6 is a diagram of the fiber cross-section
showing two rays leaving the fiber at a scattering angle of 9 in the
forward direction.
One ray is refracted through the fiber; the
other is reflected from the fiber surface. Far forward of the
fiber, the rays will interfere.
The optical path length of the re-
fracted ray in the core is given by
P^ = 2(n^)a cos6
(9-16)
and that of the ray in the cladding by
P
= 2(n2)[ b cos3
- a xosy ] .
(9-17)
The total path length of the refracted ray is
P = P^ -H P2 - X/4
,
(9-18)
where a correction has been added because the geometrical-optics
approach fails to predict the correct outcome when rays converge
to a focus.
The optical path length of the reflected ray is given by
U = 2[b cosa - b sin 9/2 ] + X/2
,
(9-19)
where X/2 accountr. for a phase shift of TT upon reflection.
The optical path difference between the two rays will be
S = P - U
,
(9-20)
127
and this can be used to predict maxima and minima in the scattering
pattern.
There is a range of angles where two refracted rays can traverse
the fiber and leave at the same scattering angle.
One of the rays
will go through the core and cladding (core ray) and the other through
the cladding only (cladding ray). The range of angles, 9 to 9 .
c
u
for which this is possible lies between
9^ = 2[arcsin(a(n^)/b) - arcsin(a/b)]
(9-21)
and
9
=9
-H TT - 2[arcsin(n2/n )]
This range is shown in Figure 9-7.
.
(9-22)
These two rays also interfere in
the far field and cause the modulation of the fringe pattern between
9 and 9 .
c
u
Figure 9-8 shows an experimentally measured intensity pattern
versus scattering angle (Figure 9-8a) to be compared with the fringe
position predicted by path difference analysis between refracted
and reflected rays using geometric ray tracing (Figure 9-8b) and
the fringe modulation calculated from interference between core and
cladding rays (Figure 9-8c).
The important results are as follows.' For fibers of relatively
small core diameter, the total diameter may be found independently
of core parameters, by measuring the fringe pattern at a scattering
angle greater than 9 . The ratio of core diameter to fiber diameter must
be less than 0.6 for 9 to exist.
c
This makes possible the separation
128
of fiber diameter from the core diameter, based on measurement of
the fringe-minima positions at large scattering angles.
The core
diameter is determined by measuring the positions and periods of
modulation of the interference pattern between 9 and 9 .
c
u
However, from Equations (9-21) and (9-22), there are seen to
be three unknowns in the case of a general step-index fiber:
core-dia-
meter/fiber-diameter ratio, and the refractive indices of the cladding
and core.
If any one of the three is known from some other measure-
ment, the other two may be determined.
Experimental Procedure and Results
The configuration of Figure 9-1 is utilized for the unclad
fiber measurement using back-scattered light with the exception
that the fiber is horizontal.
laser.
The source'is a 5-mW helium-neon
The beam passes through an aperture in an opaque viewing
screen which is simply a blank computer card.
the fiber a distance approximately 79 mm away.
From there, it strikes
With the fiber in
the horizontal position, the scattering pattern will be in the
vertical direction.
This is advantageous from a photographic aspect
because the camera may be placed at any angular position in the plane
containing both the fiber and source and depth-of-field considerations
may be ignored.
The entire pattern will be the same distance from
the lens so there will be no focusing or fore-shortening problems.
Use of a camera requires a macro or close-focusing lens.
In
addition, forward-scattered light must be blocked from entering the
camera.
When the intensity pattern is recorded directly on instant-
type film placed at the position of the computer card, these two
129
cpncems disappear.
However, it is then necessary to tilt the fiber
slightly to move the scattering pattern to the side of the source
beam so that an aperture need not be cut in the film.
This has the
disadvantage that the pattern seen is slightly distorted.
No unclad fiber was readily available, but the glass step-index
core of a plastic clad step-index fiber was used after the plastic
cladding was carefully stripped away.
The stripping may be accom-
plished with a solvent or a pair of automatic wire strippers.
back-scatter pattern of Figure 9-9 resulted.
The
Because the pattern
was recorded on a 35-mm negative, it could be placed in an enlarger
to make the measurement of the relative spacings much easier.
L was measured at the time the photograph was recorded and was
m
later used to scale the enlarged measurements.
For the configuration depicted, L
and AL = 2.52mm.
= 45mm, h = 79mm, L = 11.25mm
Therefore, <l> =29.7°, and n^ is calculated from
m
I
Equation (9-5) to be 1.43; d is calculated from Equation (9-15) to
be 158 microns.
By way of comparison. Figure 9-10 is the back-scatter pattern
of a Corning type SDF CDC-clad graded-index fiber of known diameter
125 microns.
The distance h and the magnification are the same as
in Figure 9-9.
cutoff L .
m
Note the lack both of fringes and of an identifiable
Special Problems
Most fibers deviate noticeably from their nominally circular
cross-sections.
Fiber ellipticity may obscure or distort the pattern,
Oftentimes, simply moving the beam to another, more circular, section
130
of the fiber is enough to improve the pattern.
To check the ellipti-
city, rotate the fiber about its axis in the beam and watch for significant changes in the scattering pattern, particularly changes in
L . A thorough discussion of characterization of optical fibers by
m
scattering is given by Marcuse [9].
For reasons given in [2], the laser light must be polarized in
a direction parallel to the fiber axis.
This is most easily done
with a piece of polarizing material in which the direction of polarization is known.
When doing this, remember that it is easier for
the eye to see a minimum of intensity rather than a maximum of intensity.
Place the polarization of the polarizing sheet perpendicular
to the fiber axis and rotate the laser until a minimum is reached.
Stretching the fiber did most to improve the pattern.
It was
noticed that taping the fiber to a standard lens holder was not
sufficient to keep the fiber taut.
in this way was not stable.
The scattering pattern obtained
Tne construction of a jig for stretching
the fiber is detailed in Appendix IV.
Sample Questions
1.
In what ways do you expect ellipticity of the unclad
fiber to affect the scattering pattern?
2.
Is it possible to use back-scatter measurements to
obtain information about the diameter of an unclad
graded-index fiber where the radial refractive-index
distribution is known?
3.
Draw three back-scatter rays through the fiber, one at
minimum angular deviation, one in the region of in-
131
creasing deviation, and one in the region of decreasing
deviation.
4.
Write the expression for the angle of incidence of a ray
as a function of its vertical distance above the centerline of the fiber, with reference to Figure 9-2.
5.
Trace the path of a general back-scattered ray through
the fiber for the case where the fiber is tilted to make
the scattering pattern appear away from the source.
(In
this way, the light beam does not have to pass through
the viewing screen.)
How do you expect this to affect the
scattering pattern?
6.
What effect would expanding the laser beam to, say,
100 times the fiber diameter have on the appearance of
the scattering pattern?
7.
What effect will eccentricity of the fiber core have on
the appearance of the forward-scatter pattern?
References
[l]
Kapany, N.S., Fiber Optics, Academic Press, New York, 1967.
[2]
Presby, H.M., "Refractive index and diameter measurements of
unclad optical fibers," Journal of the Optical Society of
America (JOSA), vol. 64, 1974, pp. 280-284.
[3]
Marcuse, D.and H. M. Presby, "Light scattering from optical
fibers with arbitrary refractive-index distribution," JOSA,
vol. 65, 1975, pp. 367-375.
[4]
Watkins, L.S., "Scattering from side-illuminated clad glass
fibers for determination of fiber parameters," JOSA, vol. 64,
132
1974, p. 767.
[5]
Okoshi, T.and K. Hotate, "Refractive index profile of an
optical fiber:
its measurement by the scattering pattern
method," Applied Optics (AO), vol. 15, 1976, pp. 2756-2764.
[6] Watkins, L.S., "Instrument for Continuously Monitoring Fiber
Core and Outer^Diameters," Optical Fiber Transmission Technical Digest, January 1975, pp. TuA4-l & ff.
[7] Lit, John W.Y., "Radius of uncladded optical fiber from
back-scattered radiation pattern," JOSA, vol. 65, 1975,
pp. 1311-1315.
[8] Kerker, M. and E. Matijevic, "pattering of Electromagnetic
Waves from Concentric Infinite Cylinders," JOSA, vol 51, 1961,
p. 506.
[9] Marcuse, D., "Light Scattering from Elliptical Fibers," AO,
vol. 13, 1974, pp. 1903-1905.
133
Viewing
Screen
Jt~
Incident
Unclad
Optical
Fiber
Camera
Backscattered
Radiation
Figure 9-1. Setup to observe backscattered light,
Reflected Ray
Refracted Ray
Incident
Ray
n = 1
o
Internally
Reflected
Ray
Emergent
Ray
Figure 9-2. Incident, reflected, refracted, and emergent
ray paths.
134
Figure 9-3. Rays incident upon fiber, traced for a
single internal reflection.
— i l 56
zn
^
o
20
-160
16
- le**
1 2 -
-4168
8
—
-17 2
U
-
rsO
9'
- 176
1 80
Figure 9-4. Plot of $ and 9 versus 9^ for a fiber of
n = 1.5.
135
Figure 9-5. Ray considerations to determine fiber diamet er,
Refracted Ray
Reflected Ray
Figure 9-6. Cross section of fiber, showing paths of
refracted and reflected rays that leave the
fiber at the same scattering angle 9.
136
Figure 9-7.
U
u
<
Cross section of fiber, showing refracted ray at the
angle of incidence that just grazes the core. Bounds
for angles 9 and 9 are shown. Dashed ray is cladding ray which leaves at the same scattering angle as
the core ray.
U
'
\
-\/\/\/\/v\AM;r
5.0
10.0
15.0
20.0
in.n
Scattering Angle 9, degrees
(c)
25.0
Figure 9-8. Composite graph of experimental and theoretical scattering patterns, a) experimental results, b) fringe
position calculated from geometric ray-tracing and c)
calculated fringe modulation. From [4].
137
Figure 9-9. Backscatter pattern of unclad step-index
fiber.
Figure 9-10
Backscatter pattern of clad graded-index
fiber.
CHAPTER X
AN OPTICAL FIBER ACOUSTIC SENSOR
Project Assignment
You are to design and construct a homodyne interferometer system incorporating optical fibers to detect an acoustic pressure wave,
The source of the acoustic wave may be taken from either a piezoelectric element or a speaker. Determine the useful frequency
range which your fiber sensor can detect.
Objectives of the Experiment
1.
To acquaint the student with the use of an optical
fiber as a sensor.
2.
To illustrate the utility of interferometric techniques for performing measurements.
Equipment Needed
1.
Two 2-meter lengths of glass optical fiber.
2.
Piezo-electric transducer or a 0.5 W, 2 in. diameter
speaker.
(If the transducer is used, a dummy mandrel
of the same shape will also be needed.)
3.
Oscilloscope.
4.
Micropositioners (3 needed).
5.
5 mW laser.
6.
Microscope objective.
7.
Beamsplitter.
8.
Photomultiplier tube (PMT) and bias supply.
138
139
9.
Pinhole.
10.
Fiber cleaver (Appendix II).
11.
Prism tables (2 needed).
12.
Audio oscillator.
Theory
Introduction
An optical fiber which is exposed to pressure variations, such
as from an incident acoustic wave, undergoes geometrical deformations
and changes in refractive index.
These changes result in phase
modulation of the light being transmitted by the optical fiber.
Detection of the phase changes, and thus the pressure variations,
can be accomplished interferometrically as follows. A coherent
light beam from a laser is made to pass through two similar fibers
one of which is immersed in the acoustic field and the other of which
is a control or reference path.
The two output beams are combined
collinearly and an interference pattern (fringe pattern) results.
Now if the incident acoustic pressure field changes, the phase shift
of the light in the detection path will change, and the interference
pattern will change.
By observing the changing interference pattern,
one is able to obtain information about the incident acoustic wave.
Theoretically, acoustic sensors with extremely high sensitivities
are achievable with this interferometric technique [l].
Basic Concepts
Changes of phase in an optical fiber are due to changes in
140
length (strain), changes in refractive index, and changes in diameter. All three perturbations change the effective optical path
length in the detection arm of the fiber interferometer. The
total phase change can be expressed as
A(|,^ = 3AL + LA3
(10-1)
where 3, the phase change per unit length of the fiber, is given
by
3 = nk
= (index of r e f r a c t i o n ) ( w a v e number)
= n(2Tr/X) and L i s t h e a c o u s t o - o p t i c i n t e r a c t i o n l e n g t h [ l ] .
A(|)^ =
0AL + L[(d3/dn)
Then
A n -f- (d3/dD) A D ]
(10-2)
where D is the diameter of the fiber core. The third term in
Equation (10-2) is negligible while the first two have similar
magnitudes but opposite signs. The effect due to the change in
length (the first term) is the dominant effect, and thus a net phase
modulation does result.
Two types of configurations are possible for an interferometric
fiber-optic acoustic sensor:
homodyne or heterodyne. The homodyne sys-
tem is the"simpler of the.two, but as might be expected, the heterodyne
system is more sensitive and has greater iimnunity to noise [2].
The
homodyne system is sketched in Figure 10-1. The light field output
141
from the reference fiber may be taken to be
E^ exp(ja)t)
(10-3)
where o) is the frequency of the laser light. The light field output
from the sensor fiber, however, will be phase-modulated by the pressure
wave p(t) impinging from the interaction medium and therefore will be
E
where $
and E
exp(j(a)t + $ + p(t))
(10-4)
are constants. The output photocurrent from the photo-
detector is proportional to the squared magnitude of the total incident
light field, and thus will be
I « K [ 3 - 2 [sin({.Jp(t)] ,
assuming narrow-band phase modulation.
(10-5)
The fringe pattern at the de-
tector will be as shown in Figure 10-2a;^ pinhole placed in front of
the detector has dimensions much less than the width of one fringe to
ensure that the fringe edges will be detected. For the pinhole position shown, the operating point of Figure 10-2b results. Observe that
a change in distance is equivalent to a change in phase. The homodyne
system has disadvantages:
operating-point instability, nonlinearity
and limited dynamic range; it is, however, frequently discussed in
the literature [3], [4].
The heterodyne system of Figure 10-3 offers improved performance.
As the term heterodyne suggests, some frequency shifting is involved
in this configuration.
It is provided by the acousto-optic modulator
in the reference branch of the interferometer. Because this shifting
142
is- introduced, however, more sophisticated detection techniques will
be required.
Now the field exiting the reference fiber is
E Q exp(j(a)t + (D^t))
(10-6)
while the field from the detection fiber is
E^ exp[j(a)t -f <\>^ + p(t))]
as before.
(10-7)
The output photocurrent is then
I « K^ -I- K2 cos(aj t H- 4) + p(t))
ci K^ -f K2 cos[9(t)] .
(10-8)
This has the form of a wideband phase-modulated communications
signal, so a standard wideband phase detector is used to obtain p(t).
The photocurrent is first passed through an FM limiter/discriminator
whose output is proportional to the derivative of 9(t), which is
also proportional to the derivative of p(t). An integrator then
returns a signal proportional to p(t). The advantages gained justify
this procedure:
the relative phase shift <J) is no longer a factor,
the linearity and bandwidth restrictions now lie with the receiver
system, and the la
carrier allows the detector to be operated in a
low-noise region.
Experimental Procedure and Results
The experiment was arranged as in Figure 10-1.
Two types of
devices were used as sources of the acoustic pressure wave.
A cylin-
143
drical piezo-electric transducer was first used with 10 turns of
Coming Type 1504 63-ym core graded-index fiber wound around it and
spot glued.
In order that the losses in each arm would be nearly
equal, a dummy mandrel cut from 1/2" electrical conduit was also
wrapped with 10 turns of the reference fiber. A two-meter length of
fiber was used for each path.
It was anticipated that the piezo-
electric transducer used here, a ferroelectric ceramic shell used in
sonar hydrophones, might not be readily obtainable. Thus the experiment was repeated with the detection fiber taped at both ends of a
diameter of a 2" diameter, 0.5-watt audio speaker. Very similar
results were obtained with each transducer, but the amplitude of the
detected signal was lower for the speaker source than for the piezoelectric shell source, indicating a shorter interaction length. For
the latter, a fiber length equal to
(no. of turns)
(TT)
(diameter) = 10(TT) (0.75") = 23.55"
(10-9)
interacted with the acoustic signal, and for the former, a length
of only 2" interacted.
Also, the interaction modes were different.
In the first case, the fiber was stretched by an expanding cylinder;
in the second case, the fiber was stretched as a violin string might
be stretched.
In both instances, the acoustic source was mounted on a prism
table away from the reference fiber and insulated from the table with
rubber tape. Both sources were driven with a ^5V
nal.
sinusoidal sig-
No gross crosstalk was observed.
It was found that a beamsplitter was not necessary at the launch-
144
ing end because the diameter of the beam exiting the 5 mW He-Ne laser
amply covered both fiber ends when the fibers were placed side-by-side
However, the "beam-combiner" at the output ends of the fibers was
necessary to obtain "nice" interference fringes. Fringes do result
when the fibers are simply placed side-by-side, but for the pinhole
used, the PMT had to be placed so far away for the proper fringe
spacing that it was impossible to see the fringe movement due to
intensity fall-off. Placing a beam-splitter at the output allowed
the PMT to be placed a reasonable distance away from the fiber ends.
Figure 10-4 shows the fringe pattern (with no acoustic signal present).
Figure 10-5 shows the recovered 1 kHz acoustic signal. The noise is
inherent in the PMT used.
However, the temperature sensitivity of
the fiber as well as sensitivity to small air currents caused the
operating point to move about at a low frequency. The photograph of
Figure 10-6 shows the resultant signal during one of these transients.
Special Problems
The first order of business is to obtain two pieces of fiber
which are the same length. After cleaving one end of the fiber, it
can be taped to a 2-meter scale. A small peice of tape marks the
2-meter length.
The piece can now be carefully positioned in the
fiber cleaver of Appendix III and cleaved.
the second fiber.
This is repeated for
If any mistakes are made, the process is repeated
until two fibers of the same length are produced.
As Figure 10-4 shows, the fringe pattern is not perfect but
contains regions where laser speckle dominates (lower right-hand
145
corner of the figure).
are present.
However, regions where good fringes exist
Care should be taken to avoid the speckle regions.
When the fibers are moved, the whole pattern changes, so avoid air
currents and large vibrations. Position the photodetector in a
region that looks uniform and then fine-position it to maximize the
output signal.
The temperature sensitivity of the fiber interferometer is
large for the homodyne configuration [s].
Therefore the operating
point of Figure 10-2b shifts with temperature. Fortunately, the
occurrence is a slowly varying one, and is rather easily recognized,
High-pass filtering will remove it from the desired signal.
Sample Questions
1.
Comment on the stability problems- you encountered in your
experiment. What were the sources?
How can the stability
be improved?
2.
When the experiment is performed with a speaker as the
acoustic wave source, the fiber is taped to the speaker
at both ends of a diameter. Need it be taped, or could
it simply lie on the surface of the speaker to achieve
the same results.
3. What types of non-fiber acoustic sensors are available
today?
4.
What advantages does a fiber-based sensor offer?
What other quantities might be amenable to detection with
fiber sensors?
5. What signal processing would you recommend to improve the
recovery of the acoustic signal?
146
References
[l] Shajenko, P., J. P. Flatley, and M. B. Moffett, "On fiber-optic
hydrophone sensitivity," Journal of the Acoustic Society of
America (JASA), November 1978, pp. 1286-1288.
[2] Eberhardt, F. J. and F. A. Andrews, "Laser heterodyne system
for measurement and analysis of vibrations," JASA, September
1977, pp. 603-609.
[3] Cole, J. H., R. L. Johnson, and P. G. Bhuta, "Fiber-optic
detection of sound," JASA, November 1977, pp. 1136-1138.
[4] Bucaro, J. A. and H. D. Dardy, "Fiber-optic hydrophone," JASA,
November 1977, pp. 1302-1304.
147
Piezoelectric
Transducer
or
Acoustic Environment
Detection
^ Fiber
Reference
Fiber
Dummy
Mandrel
Pinhole
Beam
Splitter
Figure 10-1. Homodyne acoustic sensor configuration.
Operating
Point
Detector
Aperture
a)
Distance
Figure 10-2
a) fringe field at detector showing pinhole
position, b) light intensity vs. distance from
pinhole, showing operating point.
148
Beamsplitter
Laser
s
U)
ezoelectric transducer
or acoustic environment, p(t)
0)
CO
Acousto-optic
Modulator
Detection
Fiber
Reference
Fiber
Dummy
Mandrel
Pinhole
Beamsplitter
dp(t)
dt
p(t)
Integrator
FM Limiter/
Discriminator
rf
Figure 10-3. Heterodyne acoustic sensor configuration.
Figure 10-4. Fringe pattern at detector.
Amp
149
Figure 10-5.
Recovered acoustic signal at 1 kHz.
Figure 10-6.
Recovered signal at "transition" showing instability.
CHAPTER XI
A HOLOGRAPHIC COUPLER FOR FIBERS
Project Assignment
You are to synthesize a direct-recorded holographic optical
element (HOE) to couple the output of one optical fiber into at
least two others.
Demonstrate to your advisor both the resulting
real and the virtual images.
Objectives of the Experiment
1.
To familiarize the student with an application of holography to the area of fiber optics.
2.
To demonstrate the holographic optical element as an
alternative to conventional optical couplers.
Equipment Needed
1.
Helium-neon laser.
2.
Collimating optics.
3.
Beam-splitter and prism table.
4.
Three front-surface mirrors.
5.
Assorted plate holders and optical benches.
6.
Laser power meter.
7.
Kodak 649F holographic plate and processing chemicals.
8.
Fly's-eye lens array or two lenses of short focal length.
Theory
Introduction
It is well known [l] that a hologram is able to reproduce both
150
151
real and virtual images of an object by reconstructing the wavefronts which are observed when the object is present.
Because
the hologram records both intensity and phase information about
the light incident upon it, all the information that can be known
about the object is present.
For that part of the system down-
stream of the object, it matters not at all whether the object
itself or the hologram is present.
Both result in light wavefronts
of exactly the same form.
A special area of holography is concerned with recording information where the "object" is an optical component such as a lens,
a mirror, or the like.
When information about one of these compo-
nents is recorded on a hologram, the component can be replaced by
the hologram.
The hologram performs exactly the same types of
operations on the incident light field as were performed by the
original elements of the system.
Holograms of this type are known
as holographic optical elements or HOEs [2].
A recent phenomenon has been the introduction of these holographic techniques into the area of fiber optics.
It is theoretically
possible to construct a holographic optical element (HOE) which will
direct the output light from one fiber to a group of others.
One
type has been demonstrated in the literature [3]. In the particular
work cited, the output of a fiber carrying a wavelength-multiplexed
signal is demultiplexed into several other fibers or detectors. A
HOE serves as a diffraction grating, directing the optical rays of
different wavelength into different output angles.
Detectors, or
other fibers, placed at these angular positions sense the demultiplexed signals.
152
In this experiment, a single HOE operates as a signal coupler,
rather than a demultiplexer, by focusing the output of one fiber
into a predetermined number of other fibers. A series of opaque
masks on a wheel can select the particular combination of fibers
or detectors to receive the information.
In a real system, the
light expanding from the output of one fiber in the form of a diverging spherical wave will be redirected by the hologram into the
acceptance cones of receiver fibers or onto detectors.
Parameters
of both the recording and playback geometries must be appropriately
chosen.
Basic Concepts
Hologram formation relies on the mutual spatial coherence of
an object wave and a reference wave to record amplitude and phase
information about light coming from an object.
To guarantee this
coherence, both waves are usually taken from the same coherent
source; i.e., a laser, by passing the laser output through a beamsplitter as in Figure 11-1.
Because lasers are not perfect, the
light is not coherent everywhere along the beam.
Manufacturers,
therefore, specify a "coherence length" within which the light is
spatially coherent.
For both object and reference waves to be
coherent at the plane where the hologram is to be recorded, they
must have traveled paths which do not differ by more than a coherence
length.
Simply stated, the distances from beamsplitter to hologram
along both paths must be roughly equal.
Again referring to Figure 11-1, the reference wave strikes the
holographic recording medium (typically Kodak 649F plates) directly.
153
The object wave, on the other hand, takes an indirect route, arriving
at the plate only after first striking an object or passing through
a system or medium which modulates the phase and intensity.
An in-
terference pattern results on the film which is analogous to a combination of amplitude and phase modulation of a carrier by the object
signal.
To prevent overmodulation, the reference beam (carrier)
should be three to eight times stronger than the object beam (signal).
Upon playback. Figure 11-2, only the reference beam is utilized; the
interference pattern on the hologram after it is developed modulates
the light of the reference beam in such a way that both real and
virtual images of the original object are formed.
A virtual image
is one which appears to be present but cannot be brought to a focus
on a screen without the use of a lens.
A real image, however, will
appear in focus on a screen without the aid of a lens.
The reference wave is typically a plane wave, but this is not
strictly necessary:
spherical waves, or waves which emanate from
a point source, are also used.
At times, it is useful to use the
combination of plane-wave recording and spherical-wave playback.
In general, the positions of the real and virtual images may be
calculated from [4]:
^i = ^ ^^i/^o^^o - ^"i/^^^r •" ^"i^'p^^
^^^"^^^
y. = T (^i/^o^yo ^ (^i/^>yr "" ^"i^'p^^P
^^^"^^^
z. = (1/z ± 1/z^ ? 1/z )
1
p
r
o
(11-lc)
where the coordinate systems of Figure 11-3 are used.
The positive
value for z^ refers to the virtual image and the negative value
154
refers to the real image.
With this notation, the plane wave is
seen to be a special case of the spherical wave with z
equal to
infinity.
Experimental Procedure and Results
In the experiment to be performed, certain constraints are imposed by the desire to use an optical fiber to access the hologram.
The fiber must be placed far enough from the hologram that the expanding spot size covers a "reasonable" (say, 3 sq. cm.) area of
the hologram for good diffraction efficiency.
This is because an
approximate spherical wave expanding from the "point" end of the
fiber will be used for playback.
Because it is desired to access other fibers with real images
of point sources, it is necessary that these real images lie outside the cone of undiffracted radiation to aid in positioning the
fibers, as in Figure 11-4.
This places restrictions on what the
coordinates of the real images may be.
As seen in the figure, the
numerical apertures of the images must match the NAs of the fibers
to be accessed.
This in turn imposes requirements on the input
optics, e.g. finding lenses which can be used to obtain "objects"
which are converging cones of light with the proper numerical
apertures.
In keeping with Equation (11-1), the other coordinates are
chosen, and the hologram is recorded.
[The reader is referred to
Froehlich's work [5] for a discussion of practical holography.]
It was found that recording with a plane wave reference (z^ =
infinity) gives the best results.
Thus, y^, z^, x^, y^, z^, x^, y^.
155
and z are known, and it remains to select x , v
P
0-^0
z
and v
o'
r*
As an example, consider the case where it is desired to
couple the output of a fiber of NA = 0.21 (which corresponds to an
acceptance cone half-angle of 12°) to two other fibers of similar
type. For simplicity, all sources, object and images are chosen to
lie in the x-z plane:
e.g.
Also, the calculations are performed for one image only.
In order for the exit cone of the input fiber to access a
circular area of 2-cm diameter on the hologram, it must be placed
a distance of 4.7 cm away.
It is assumed that this playback wave
is normally incident on the hologram. Therefore,
(Xp, yp, z ) = ( 0, 0, -1-4.7 ) cm.
The point at which it is desired for the real image to appear is
chosen with two criteria in mind.
First, it must lie outside the
diverging cone of undiffracted light resulting from the playback
source, and second, it must be in a location where the fibers to
be coupled may easily be placed.
The point:
(x^, y^, z^) = ( -1-1.75, 0, -2.5 ) cm.
is chosen to satisfy the two conditions.
The reference wave will be a plane wave, incident upon the
holographic plate at some angle a. Although
(x^, y^, z^) = ( -", 0, -Hco)
156
the ratio x^/z^ is not -1, but the tangent of angle a.
With these
values for the positions of the image, the playback source, and the
reference source. Equation (11-1) indicates the object should be
placed at
(x^, y^, z^) = (2.55, 0, 4.7) cm.
A fly's-eye array was used for obtaining the object points.
This
consisted of lenses of diameter 0.7 mm and focal length approximately
1 cm spaced in a square array on approximately 0.7 mm centers.
Therefore, the "objects" to be recorded are cones of light with
conical half-angles equal to
arctan ( 0.35/ 0.7 ) = arctan (0.5)
= 26.5°
The images of these cones of light will significantly overfill the
fibers to be coupled, and thus these lenses are not the optimum
choice for fibers of numerical aperture 0.21.
However, the con-
cept is easily observable.
After the hologram was recorded and developed in Kodak D-19
developer, it was played back first with the plane-wave reference
to ensure that indeed a hologram had been produced.
Bleaching in
Kodak Chromium Intensifier improved the brightness of the images.
The virtual images of Figure 11-5 were observed although no real
images were seen.
This is in keeping with the mathematics, which
predict (for the coordinates chosen) that the real images would
be formed at infinity.
157
When a diverging spherical wave, as formed by a lens, is
utilized for playback, the real images are observed.
It is neces-
sary, however, to vary the positions of the playback source and the
viewing screen to locate the images.
Uncertainties in the place-
ment of both the object and the reference beam in the recording
phase translate into uncertainty in the position of the real images.
The real images of Figure 11-6 were photographed as they appeared
on the viewing screen (two of the four images do not appear because
of the masking).
An optical fiber bundle placed at the points of
focus was able to transmit the signal from each real image.
When the output of an optical fiber bundle is used as the
source for playback, the real images are not as bright although
they are still observable.
Special Problems
The greatest problem with this experiment is locating the
real images upon playback.
The primary cause of the problem is
that of relative brightness.
When the human eye scans a scene for
dim objects, their presence can be hidden by the presence of other
brighter objects.
For this reason, the ambient light reaching the
viewing screen should be minimized.
The hologram is masked with
black construction paper in such a way that the only light present
behind the hologram comes through an "interesting" or "informationcarrying" part of the hologram.
Of course, care was taken in the
recording process to ensure that the real image would be far-removed
from the undiffracted portion of the playback light.
It has been mentioned that the intensity of the reference wave
158
should be three to eight times that of the object wave. Kodak type
649F emulsion requires an energy density of approximately 50 yJ/sq. cm.
for proper exposure.
The power density is measured at the plane of the
hologram with a laser power meter with both object and reference beams
present.
When the required energy density is divided by the power densi-
ty as measured by the detector in W/sq. cm., the exposure time results.
If an amplitude mask hologram is made, there will of course be absorption losses associated with its use. However, there are a number of
ways to improve the diffraction efficiency of a hologram, and these are
more fully covered in other sources [6], [7]. One of the easiest ways is
to bleach the hologram.
As mentioned previously, some care should be taken to measure the
path lengths from beamsplitter to film along both the object and reference paths. They must be very nearly equal.
Sample Questions
1. What other ways are being used for coupling optical fibers?
2. What factors would you control to optimize the efficiency
of the coupler?
3. What effect, if any, will be the results of playing back the
hologram with laser light of a different color than that with
which it was made?
4. How does fiber coupling differ from fiber switching?
5.
Will the coupler work for incoherent light?
References
[l] Goodman, J. W., Introduction to Fourier 02tics, McGraw-Hill, Inc.
San Francisco, 1968, pp. 198-ff.
159
[2]
Close, D.H., "Holographic Optical Elements," Optical Engineering,
vol. 14, 1975, p. 408.
[3] Horner, Joseph, and Jacques E. Ludman, "Single holographic
element wavelength demultiplexer," Applied Optics, vol. 20,
no. 10, May 18, 1981, pp. 1845-1847.
[4]
Goodman, J. W:, op. cit., p. 214-218.
[5]
Froehlich, G. K., J. F. Walkup, and M. 0. Hagler, Optical
Information Processing Experiments for Undergraduate Engineers,
Final Technical Report NSF Grant SER75-17673, Texas Tech Press,
Lubbock, Texas, January 1977, pp. 154-156.
[6]
Smith,
Howard, Principles of Holography, John Wiley and Sons,
New York, 1975.
[7]
Lehmann, M., Holography, Focal Press, New York, 1970.
160
b)
Figure 11-1. Setup for HOE recording. Plane-wave reference
L, laser; C, collimator; M1,M2,M3, mirrors;
BS, beamsplitter; 0, object; H, holographic
plate. a)schematic, b)actual.
161
Real
Images
Figure 11-2. Schematic setup for HOE playback. Spherical
playback. L, laser; C, collimator; M1,M3,
mirrors; LI, converging lens of NA equal to
fiber which is to be coupled; H, hologram.
Hologram
Reference
Source
(x^,y^,z
Recons true t ion
source
Object
Sourc
Film
Figure 11-3. Coordinate systems for spherical wave holography, a) recording, b) playback.
162
la
Real Images
Figure 11-4. Geometry used for calculating postion of
object and the reference source given
the desired positions of the playback
source and the real images.
Figure 11-5. Virtual images of four point sources as
played back from the HOE.
163
Figure 11-6. Real images of two of the four point
sources obtained upon playback of
the HOE.
CHAPTER XII
CONCLUSION
Testing
Although only two of the ten experiments presented in this
report have been tested by undergraduate students to date, the
feedback which we have received indicates that the information content within each chapter is sufficient to perform the project.
The
experiments performed were those of Chapter IX and Chapter XI, the
scattering-pattern experiment and holographic-optical-element experiment respectively.
The remainder of the experiments will be tested
and evaluated in the fall of 1982 and the spring of 1983.
In order
of difficulty, we rate them by chapter in this fashion (from least
difficult to most difficult):
VIII.
II, VI, IX, VII, X, III, IV, V, XI,
Chapter VI involves the construction of a pulser for the
laser diode, which will make it one of the more difficult experiments, but the measurements involved in the experiment are easy.
The most pleasing results were the spirit with which the two
sets of lab partners (all seniors) approached the tasks and the
constructive criticisms offered.
The experiments assigned were
delivered to the students before this report was completed,
and both sets of students wanted to know if appendices referred to
so freely in the chapters really existed.
Instead of including a
glossary of terms, which was suggested by one student, a special
effort was made to define terms in the text as they were encountered.
More thorough definitions can be obtained by consulting any of the
164
165
books on fiber optics in the bibliography.
In fact, when these
projects are assigned, it would be well to ascertain that students
have free access to several of the general fiber optics treatises.
In addition, free subscriptions to trade magazines, such as Electronic Desiffli News, Electro-Optical System Design. Laser Focus
and Fiberoptic Technology, and Photonics, are often available to
faculty members and professional.
These magazines often feature
tutorial articles in fiber optics which are a boon to students.
Also, many fiber-optics companies have free sales material of a
tutorial nature which they are eager to distribute.
Equipment
Attempts were continually made to construct, rather than purchase, suitable special equipment for the projects. We had at our
disposal a machine shop and a quantity of surplus aluminum from
which simple mounts, holders, etc. were made.
The goals of the
experiments are to illustrate salient features of fibers, sources
and detectors, and not necessarily to make precise measurements of
the quantities involved.
made cheaply, do so.
the demonstration.
If precise, accurate equipment can be
If not, buy the minimum necessary to perform
We found manufacturers most willing to pass
along literature, technical information, samples and out-of-spec
products to the undergraduate program.
They are eager to familiar-
ize future engineers and potential customers or employees with the
capabilities of their products.
We were the recipients of samples
of fiber from Corning Glass Works, sources and detectors from
166
Honeywell / Spectronics, Teflon samples
from DuPont (for building
a stripline for a laser diode pulser), samples of polarizers from
Polaroid, connectors from Deutsch, and a digital communications
link from an alumnus. The gifts were not solicited per se, but we
did inform these companies of our needs and they responded with
enthusiasm.
The minimum equipment requirements necessary to perform most
of the experiments dealing with fiber and optoelectronic measurements should include:
1 km of fiber, spooled
Four micromanipulators
A monochromator or set of interference filters (400-1000 nm)
A fiber polishing kit (or cleaver)
An IR viewing card
A white-light source (tungsten-halogen or arc)
and power supply
A selection of optoelectronic sources: LEDs, IR-LEDs,
and laser diodes
A selection of optoelectronic detectors:
phototransistors, photodiodes (both PIN and APD) and
a PMT
An optical bench and carriers
Assorted microscope objectives
A viewing telescope
Sophistication is not the rule; an interesting demonstration can
be presented to people who are completely unfamiliar with the area
167
of fiber optics using a red LED, a phototransistor and a piece of
lossy plastic fiber from a decorative fiber optic lamp.
In fact,
the experiment of Chapter VIII was done with fiber-optic components
which were not much more sophisticated than these.
For the holography experiment, a laser power meter, holographic
plates and chemicals, a vibration-free surface, and a 5 mW He-Ne
laser will also be needed.
The small helium-neon laser is also
needed for the acoustic sensor measurement, and its overall utility
is impressive.
It is wise to provide permanent mounts for the optoelectronic
components; they tend to be lost or broken easily.
The leads to
all of the photodetectors were the first casualties.
Repeated
soldering and desoldering of the leads as different groups perform
the same experiments can only exacerbate the problem.
We suggest
mounting the source or detector in a snug mounting hole drilled in
the short leg of an L-shaped bracket.
The long leg can be secured
to an optical-bench post so that the L is lying on its side, the
source or detector aimed horizontally and facing out from the L.
This allows the biasing resistor to be mounted above the long leg
of the L, and the leads are protected from strain.
The construction of a laser diode pulser and a photomultiplier
tube housing should be projects in and of themselves.
They require
proper design and careful construction to adequately protect the
user from the large dc voltages which will be encountered.
Any
electronic hardware constructed must be properly labeled if it is
to be useful to and safe for the next group of students to use it.
168
We are grateful for having the opportunity to work in this
area; we come away convinced that fiber optics is the technology
of the future and its potential is not yet even being approached. We
hope we have communicated a little of that feeling to our students.,
A demonstration paper will be presented at the 1982 Annual
Meeting of the Optical Society of America in Tucson, Arizona in
October 1982.
APPENDIX I
BEAM LAUNCHER
Marcuse [l] describes a typical beam launcher for launching light
into fibers for experimental purposes, and it is reproduced in Figure
I-l.
The light source may be a laser, a light-emitting diode, a xenon
arc lamp or a tungsten-halogen lamp.
For a broad distribution of
wavelengths, one of the latter two is chosen.
The main criterion
influencing arrangement of the launcher is repeatability.
The light from the source may first be focused onto a pinhole
to select a part of the image that is of more or less uniform intensity
as a new source for spherical waves. After collimation, narrow-band
interference filters mounted in a wheel or a monochromator may select
from among the various wavelengths.
Use of the filter wheel allows
highly reproducible measurements but permits only discrete wavelengths.
Monochromators, on the other hand, allow a continuous wavelength scan
but are difficult to reset to achieve precisely the same modal excitation, and severely limit the amount of light ^ i c h can be transmitted.
After filtration, the light passes through a rotary chopper.
While the chopper may easily supply an identifiable signal to a lockin amplifier for signal-to-noise considerations, it was found that
a light signal chopped by something as simple as rotating fan blades
was more easily detected from ambient light.
The beam splitter serves three purposes:
permitting most of
the light to fall on the fiber endface, reflecting some of the light
to a reference detector, and directing the light reflected from the
169
170
fiber endface to a viewing telescope (microscope eyepiece) to allow
observation of the position and size of the launched beam upon the
fiber face.
The reference detector may be used simply to monitor
the stability of the source or as the first element in a control
loop to improve that stability.
The light then passes through the variable aperture wheel
v^ich adjusts the numerical aperture of the launched beam, and finally,
a microscope objective focuses the beam onto the fiber core.
The
input fiber end is held in an xyz-positioner and the setup includes
a cladding-mode stripper.
An S-shaped mode stripper which also
functions as a mode scrambler to assure a mode distribution similar
to that present at steady state may also be used. Figure 1-2 [2].
The bent fiber section may be placed between two pieces of black
velvet soaked in index-matching fluid such as glycerin (or between
two rubber stoppers with a coating of the same fluid) allowing
cladding modes to leak out. Mode stripping may be necessary at the
output end of the fiber to remove cladding modes which may have
built up during the run through the fiber.
References
[l]
Marcuse, D., Principles of Optical Fiber Measurements, Academic
Press, New York, p. 203-205, 1981.
[2] Marcuse, op. cit., p. 204.
171
Viewing Telescope
n
Microscope
Objective
Beam
Splitter
Pinhole
Source
,,
RKtrz
>
Mode
Stripper
^
7
Optical
Filter
Light
Chopper
Figure I-l.
11
II
Reference Detector
A typical beam launcher.
Figure 1-2. Bent fiber serving as a mode scrambler.
7
\
APPENDIX II
A LAB-BUILT GONIOMETER
A goniometer is simply an instrument for measuring angles.
A
photodetector mounted on an xyz-micropositioner is swung in a circle
via a geared mechanism; the component of interest is positioned at
the center of the arc and intensity measurements are taken for a
set of angles.
Figure II-l.
The specific instrument constructed is depicted in
It consists of an L-shaped bracket and a plastic
drafting protractor mounted to a geared device which swings the
bracket/protractor in a circle.
The photodetector is sealed in a
hole in the vertical part of the bracket which was cut from an
alximinum I-beam.
The geared mechanism was taken from a government
surplus radio.
The assembly is attached to the micropositioner which was then
mounted to a standard optical-bench carrier.
(Although an xyz-micro-
positioner is included as an integral part of the goniometer described,
it could just as easily be used to manipulate the position of the
component being measured, rather than the position of the detector.)
A pointer made from a bent piece of thin copper wire is mounted on
a separate carrier and positioned just above the dial of the protractor.
The component for which an intensity-vs.-angular-displacement
profile is desired is centered on the axis of rotation of the detector
at the same height as the detector.
In the measurement of numerical
aperture (Chapter II), the end of a fiber is centered on the axis
172
173
of rotation.
For measurements of source radiation patterns
(Chapter VI), the emitting area of the source is positioned on
the axis.
And for the measurement of coupling loss vs. angular
mismatch (Chapter XI), two fibers are positioned there.
174
Figure II-l.
A lab-built goniometer.
APPENDIX III
A LAB-BUILT FIBER CLEAVER
Scribing and breaking techniques have long been used to fracture
glass.
They may also be used to produce acceptably clean, flat
surfaces at the ends'of optical fibers.
Theoretical investigation into the fracture mechanics of
optical fibers have predicted optimum conditions for obtaining smooth
endfaces [l]. All preparations are concerned with establishing
the proper stress distribution in the fiber so that a flaw started
on the surface with a diamond- or sapphire-tipped knife will propagate uniformly across the face.
Fiber cleaving tools produce the proper stress characteristic
by bending the fiber around a fixed-radius mandrel and pulling to
a prescribed tension. This is accomplished in different ways, and
hand-held cleaving tools are currently on the market [2].
For the
common 125-micron glass-clad glass-core fiber, a bend radius of between 5 and 6 cm and an applied tension of between 100 to 150 gm is
sufficient to produce smooth ends.
A satisfactory cleaver was built in the lab and is shown in
Figure III-l.
The mandrel M is a 5/8" thick disk cut from a 4-1/2"-
diameter (5.72 cm-radius) aluminum cylinder. A radial groove is
machined in one side to allow the positioning of the knife holder H.
The knife holder is a slotted rectangular piece which allows the
knife K (General Fiber Optics ^^1018) to move across the mandrel. The
clamp C holds the standing part of the fiber on the center line of the
mandrel.
A spring clip W mounted on an optical-bench post
175
176
(similar to the Ealing j'/22-7876; total weight approx. 130 gm) is
clamped onto the end of the fiber and suspended over the end of the
workbench to provide the tension.
The jaws of the spring clip are
faced with balsa wood to prevent fiber breakage at that point.
With the fiber inserted in the cleaver and tension applied,
the knife is lightly pressed into the surface of the fiber.
The
weight causes the fiber to break quickly and cleanly.
References
[l]
Gloge, D. et. al., "Optical Fiber End Preparation for Low-Loss
Splices." Bell System Technical Journal, vol. 52, No. 9, pp.
1579-1587, Nov. 1973.
[2]
For example, model DW9000, Deutsch Inc., Banning, CA; and
cat. no. 92203, Thomas and Betts Corp., Raritan, New Jersey.
177
Figure III-l. A lab-built fiber cleaver.
APPENDIX IV
A LAB-BUILT PHOTOMULTIPLIER TUBE HOUSING
The use of a photomultiplier tube (PMT) as a detector of
optical radiation requires special care in the design of a housing [ij
The housing provides protection for the user from the high operating
voltage, shielding from extraneous electrostatic and magnetic fields
and stray light, and a convenient place for the biasing circuitry.
In measurement applications, the dc component of the signal
output is important; hence the photocathode is normally operated at
a high negative voltage with respect to ground.
When a shield is
used in contact with the tube envelope, it must be connected to the
cathode potential.
As a result, the shield is at a high negative
voltage, and safety precautions are necessary to prevent shock to
the users. A 200-megohm resistor should be placed between the
high-voltage supply and the shield to eliminate the danger.
The photograph of Figure IV-1 shows the components of the
assembly for mounting in a cylindrical aluminum tube (2.825" O.D.,
2.475" I.D., 7" long, for the RCA 7102 tube used).
The base for
the PMT is mounted on a pedestal about 3 inches tall inside the
tube.
The voltage-divider network for the dynode chain is contained
inside the tube beneath the pedestal.
Only the negative HV-supply
and output signal voltage connections are brought outside of the
housing via BNC jacks.
Shielding is provided by a rolled tube of copper foil at cathode
potential completely encircling the glass envelope.
178
A dielectric
179
collar of polyethylene centers the PMT and shield inside the housing
and prevents shorting to the case. A cap on the input faceplate
protects the tube phosphor from ambient light when not in use.
Suitable warnings and information are affixed to the outside of
the housing. The complete assembly is shown in Figure IV-2.
References
[l] Engstrom, R. W., Photomultiplier Handbook, RCA Corp., pp. 80-ff,
1980.
180
Figure IV-1. Components of the PMT housing. From left;
base plate with BNC connectors, pedestal for
PMT socket, PMT and magnetic shield,
outer case and top plate with cover.
Figure IV-2.
The assembled PMT housing,
APPENDIX V
A LAB-BUILT LASER DIODE PULSER
This section is confined to a discussion of the SCR-type
pulser built.
The circuit described [l] generates the desired
pulses by discharging a capacitor through the SCR and the laser
diode, and meets the stringent requirements on pulse amplitude,
pulse width, and duty cycle, all of which are
laser diode parameters.
critical
Other circuits vary only in the methods by
which the capacitor is charged, the current is varied, and the SCR
is triggered.
All circuits consist of three basic sections:
(1) the dis-
charge circuit, (2) the charging circuit, and (3) the trigger circuit.
Discharge Circuit
The discharge circuit generates the current pulse in the laser,
and consequently, is the most important section of the design.
general configuration is as shown in
The
Figure V-1. The current pulse
is formed by discharging storage capacitor C through the SCR and the
laser diode.
The rise time of the pulse is usually determined by
the SCR while the fall time is usually set by the capacitor and the
total resistance in the discharge circuit.
The peak current, pulse
width, and voltage of the capacitor discharge circuit are interrelated
for various load and capacitance values.
Short pulse widths provide
less time for the SCR to turn on than longer pulses, therefore the
181
182
SCR impedance is higher and more voltage is required to generate
the same current.
In conventional operation, the anode current in the SCR, initiated
by a gate pulse, rises to its maximum value in about 1 us. During
this time, the anode-to-cathode impedance drops from open circuit
to a fraction of an ohm.
In injection-laser pulsers, however, the
duration of the anode-current pulse is imich less than the time required for the SCR to turn on completely.
Therefore the anode-to-
cathode impedance is at the level of 1 to 10 ohms throughout the
conduction period.
The major disadvantage of the high SCR impedance
is that it causes low circuit efficiency.
Because the SCR is being used in an unorthodox manner, many of
the traditional specifications such as peak current, reverse voltage,
on-state forward voltage, and turn-off time are not applicable.
In
fact, it is difficult to select an SCR for a pulsing circuit on the
basis of normally specified characteristics.
The specifications
important to laser pulser applications are forward-blocking voltage
and current rise time.
The voltage rating of the storage capacitor must be at least as
high as the supply voltage.
With the exception of ceramic types which
have noticeably greater series resistance, most capacitors (metallized
paper, mylar, mica, etc) perform well in this circuit.
Because of the fast rise times desired, lead inductance should
be minimized.
A well-built discharge circuit might have a total
lead length of only one inch and therefore an inductance of approxi-
183
mately 20 nanohenries [2].
A clamping diode is added in parallel with the laser to reduce
the current undershoot effects which are potentially damaging to the
laser.
A diode with a voltage rating equal to that of the storage
capacitor will not be destroyed if the laser is removed from the
circuit.
The polarity of the diode is opposite that of the laser
when installed.
The simplest current monitor is a resistor in series with the
laser and SCR in the discharge circuit.
It should however be of the
non-inductive type with a resistance of 0.1 to 3 ohms.
The inductance
of the resistor leads may cause a higher-than-actual current reading.
Charging Circuit
This section of the pulser charges the storage capacitor to
the supply voltage during the time interval between firings of the
SCR, and in addition, isolates the supply voltage from the discharge
circuit during the current pulse.
Because the response times of
the charging circuit are relatively long, lead lengths are not
critically important, and the charging circuit may be remote from
the discharge circuit.
The capacitor is charged through transistors Q^ and Q2* Figure V-2,
at times when the SCR is off.
D, ^,
No current passes through the diodes
so that Q2 is forward-biased into the saturation region.
If
enough time is allowed between trigger pulses, C charges to V^^.
When a trigger pulse reaches the SCR, current flows through diodes
D
^, and the voltage drop across those elements reverse-biases Q^
184
into cutoff so no current can flow into the supply voltage.
current flows only through the laser diode and the SCR.
The
When the
current decays to a value less than that required to hold the SCR
on, the SCR cuts off and C begins charging once again.
Trigger Circuit
There are several ways that the SCR can be triggered although
only one is discussed here. All circuits must meet the same
requirements.
The trigger circuit must provide a fast-rising
current pulse with an amplitude equal to at least five times the
minimum triggering current required for the SCR and a pulse width
of 0.2 to 10 us. The voltage required is only 10 to 15 volts
because the gate impedance is normally less than 100 ohms.
In Figure V-3, a 555 timer IC is used for the basic trigger:
R. is a potentiometer controlling the pulse repetition rate and Cj^
controls pulse width.
Testing
For protection of the laser diode, the pulse circuit should
be tested with a dummy load in place:
a short circuit or a one-ohm
load for the RCA SG2001 diode used here.
The SCR anode is a good
test point for pulser testing and trouble shooting.
The voltage
waveform at this point, as well as the current waveform of the
current monitor, should be as shown in the Figure V-4.
If the voltage on the SCR anode remains at the value of the
power supply, the discharge circuit is inoperative.
The discharge
circuit may be open, or the SCR trigger signal may be inadequate or
185
of reversed polarity.
If the anode voltage is constant at 1 or 2 volts, the SCR
is holding.
The charging circuit may be malfunctioning or the
SCR may be overheating.
If the anode voltage is zero, the SCR or capacitor is shorted
or the charging circuit is malfunctioning.
References
[l] RCA, "Solid-State Pulse Power Supplies for RCA GaAs and
GaAlAs Injection Lasers," Application Note AN4469, 1972.
[2] Terman, F. E., Radio Engineer's Handbook, McGraw-Hill,
New York, p. 48, 1943.
186
Figure V-1.
The discharge circuit.
e800V MAX
47 kfi
5W
.150 kJi
IW
1.5 kn
25W
|2N3439
'Heat Sinked
47 k^
5W
1.5 kfi
25W
Drive
Control
250 kO.
2N3439
'Heat Sinked
W
1N3563
B
Figure V-2.
The c h a r g i n g
circuit.
187
-H5V B
Figure V-3. The trigger circuit.
Figure V-4. Waveform of the current pulse through the laser
diode. (50 nsecs/div.)
APPENDIX VI
A LAB-BUILT FIBER STRETCHER
When performing scattering measurements on an optical fiber,
the assumption is made that the fiber is perfectly cylindrical.
For
this reason, it is necessary for the fiber to be stretched taut so
that the experiment will be repeatable.
This also reduces aberrations
caused by microbending.
Because of the susceptibility of the fiber to breakage from shear
stress, it is necessary that the fiber be stretched around a relatively large diameter.
Two short pieces of 3/4 in.-diameter electrical
conduit are mounted between two pieces of 1x2 lumber as in Figure VI-1.
Tightening the bolts keeps the conduit from turning.
Epoxy may also
be added to the assembly.
The short pieces of fiber are wrapped and taped to one piece
of tubing and stretched tightly between the posts before taping to
the second piece.
The complete assembly is mounted to a standard
optical post (13.7 mm diameter) for use with an optical bench.
188
189
Figure VI-1. A lab-built fiber stretcher.
APPENDIX VII
TRANSIMPEDANCE AMPLIFIERS
If the desired form for the output information signal transmitted by a fiber optic system is a voltage, it is necessary to
convert the output detector current to a voltage. The simplest
current-to-voltage converter is a resistor, as in Figure VII-1.
This, however, is useful only for very large signal levels which
almost certainly will not be the case for a fiber-transmitted signal.
The average sensitivity for a solid-state PIN diode is 500 mA/W
which indicates a 500 uA signal for an incident power of ImW.
(APDs have sensitivities ranging between 5 and 100 A/W.) To be
able to see such a small current, a large resistor must be used.
There are two disadvantages to doing so: 1) the photodiode model
may no longer be assumed to be the simple current source in parallel
with the junction capacitance of Figure VII-2, and 2) there will be
a large noise voltage present owing to the thermal noise in the
resistor.
An alternative approach uses a transimpedance amplifier as
synthesized with a standard op-amp. The term transimpedance arises
because an output ("trans-") voltage is caused by an input current
(V
/I
has the units of impedance).
out in
The following discussion of
the transimpedance amplifier is given by Swindell [l]. An example
is shown in Figure VII-3.
The circuit converts current into voltage
approximately as
V = - IRp
190
191
This results from the characteristic behavior of the op-amp in
attempting to maintain the voltage between the two inputs at
approximately zero volts. This voltage appears at the low-impedance
output of the op-amp and thus following amplifier stages will not
load the output significantly.
The photodiode works into an
equivalent resistance of
%
= V^/I = [V/A]/[V/R^] = Rp/A
CVII-1)
where A is the open-loop op-amp gain.
Because the op-amp is usually thought of as a voltage amplifier,
the first design attempt may lead the student to a circuit of the
form of Figure VII-4 [2].
The current generated in the photodiode
is converted to a voltage by the load resistor. The resulting volage is applied to the inverting input of the op-amp. The main disadvantage is that the voltage across the diode varies as a result
of the current-dependent voltage across R,.
Furthermore, the off-
set voltage will be amplified and cause an additional error. The
circuits of Figure VII-5 are better choices.
With present detectors and front-end transistors, FETs are
usually found to be more sensitive below 10 MHz, and BJTs are
usually more sensitive above 10 MHz [3].
References
[l] Swindell, W., "Circuits for Detectors of Visible Radiation,"
in Applied Optics and Optical Engineering, vol. VIII, R. R.
Shannon and J. C. Wyant, editors. Academic Press, New York,
192
pp. 317-334, 1980.
[2] Meiksin, Z. H., and P. C. Thackray, Electronic Design With
Off-the-shelf Integrated Circuits, Parker Publishing Co.,
West Nyack, New York, pp. 195-199, 1980.
[3]
ITT, "Optical Fiber Communications Link Design," Technical
Note R-1, 8/78.
193
l^^B
I
ZA
JPhotodiode
'••
• t o scope
\
Figure VII-1. The simplest photodiode bias circuit.
'photo
Figure VII-2. Solid-state photodiode equivalent circuit when
operating into low impedance.
Figure VII-3.
The transimpedance amplifier.
194
Photodiode
out
Figure VII-4. Poor front-end amplifier design.
195
out
a)
Photodiode
out
V
Blocke
B "TPhotodiode
b)
Photodiode
c)
Figure VII-5. Better choices for front-end amplifiers using transimpedance techniques. Where two photodiodes are shown, one of the two
is shielded from all light for dark current compensation.
APPENDIX VIII
A LAB BUILT LIGHT CHOPPER
In applications where small non-varying intensities must be
detected with a detector which requires a relatively large dc bias,
the relatively small dc signal voltage may not be detectable above
the bias voltage.
In these applications, it may be better to "chop"
the signal, that is, turn it on and off, so that it may be passed
through a blocking capacitor. One way to do this is with a simple
ac synchronous motor.
A disk of plexiglas is mounted on the shaft of the motor
and divided into sections which are alternately clear or opaque.
When placed in the optical path, the width of each section must be
sufficient to completely cut off or pass "the signal beam at that
point.
The chopping frequency will be n/2 times 60 Hz where n is
the number of sections on the disk. Figure VIII-1 is a photograph
of the completed chopper mounted on a standard optical-bench
carriage.
In this case, the motor was removed from a small fan.
If the disk is not carefully balanced, the resulting vibration
will destroy the integrity of the system alignment.
The simplest
solution requires one person to hold the motor in the optical path
while measurements are taken by another person.
It should be noted
that commercially available choppers are constructed of extremely
lightweight metal with slots cut out to pass the light. The metal
disks are usually painted black.
This construction chops the light
without altering the spectral distribution of the optical power.
However, plexiglas acts as an optical filter, attenuating light at
196
197
certain wavelengths more than others.
Therefore, if the chopper is
to be used in an experiment which is concerned with spectral response
(as in Chapter III or IV), it should be considered to be a part of
the source.
As such, the response of the chopper will be Itimped in
with the spectral output of the white-light source.
This combination
can be taken as a white-light source with a modified spectral output.
198
Figure VIII-1. A lab-built light chopper.
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