Optical Laboratory 1
Fibre numerical aperture measurement using a HeNe laser
OBJECTIVE:
To use a HeNe laser to measure the acceptance angle and thus the
numerical aperture of three different types of fibre; 50/125 µm glass
fibre, 200 µm core polymer clad silica fibre and 1 mm core plastic
fibre.
EQUIPMENT:
•
•
•
•
•
•
HeNe. Laser with SMA connector, emitting at a wavelength of 632.8 nm
50/125 graded index silica fibre (orange cable).
200/400 step index polymer clad silica fibre (white cable).
1 mm core step index plastic fibre (black cable)
Circle target, 5 mm spacing
Vernier Calipers (Ruler)
PROCEDURE:
This exercise is best carried out in semi-darkness, for example in the screened room. The
numerical aperture is a parameter normally associated with light entering a fibre, however
to measure numerical aperture it is easier to investigate the characteristics of the light
leaving the fibre, which will provide a reasonable approximation of the numerical
aperture. The background to fibre propagation and numerical aperture is explained in the
Appendix to this exercise.
Important: Do not look into the light beam emitted from the laser or the fibre at
any time, as this could cause eye damage.
Connect one of the fibres to the HeNe laser, noting that it is easier to start with one of the
fibres with a large core diameter, such as a 1mm core plastic fibre. Take the other end of
the fibre and project the light output on to the 5mm circle target provided. Determine the
circle diameter R of the light and the distance L from the fibre end to the circle.
Hence calculate the acceptance angle and by taking the sine of the acceptance
angle find the numerical aperture of the fibre. Now for the same fibre repeat this
procedure for at least four other values of distance L, calculating the acceptance angle
and numerical aperture in each case. Finally take the average of the four numerical
aperture values to yield a final result. Now repeat the above procedure for the remaining
fibres.
RESULTS & CONCLUSIONS:
1. Tabulate your results and plot the NA as a function of the distance L for each
fibre.
2. How does the fibre core diameter influence the NA?
3. Comment on the different factors influencing any inaccurcies you may find.
4. Compare and comment on your results by comparison with those from typical
equivalent cables to be found in supplier catalogues.
Page 1 of 20
Optical Laboratory 1
Background information on Fibre Numerical Aperture
An optical fibre consists of a core that is surrounded by a cladding. The core and cladding
are normally made of silica glass, although polymer materials are also in use. The
function of the core is to transmit an optical signal while the purpose of the
cladding is to guide the light within the core, in effect to confine the light within
the core. A fibre is sometimes called an optical waveguide because light is guided
through the fibre. The basic construction of a fibre is shown in figure 1(a). In order to
confine the optical signal to the core of the fibre the core and cladding materials are
deliberately given different refractive indices, so that the refractive index of the core (N1)
is higher than that of the cladding (N2). The refractive index of a material
decides whether the material transmits or reflects a light ray that intersects the surface
of the material. The simplest type of fibre is called a step index fibre, since in such a fibre
there is a step in the value of the refractive index at the boundary between the core and
the cladding. This is shown in figure 1(b) which displays the so called refractive index
profile of a step index fibre. The refractive index profile of a fibre is a graph which shows
how the refractive index varies with distance from the centre of the fibre. In a step index
fibre the refractive index is constant at N1 until the core cladding boundary is reached,
where the refractive index falls to N2 The core diameter of step index multimode fibre is
typically 200nm, with a cladding diameter of 300nm. A light ray that enters the fibre does
not merely travel straight down through the centre of the core. Instead light rays within
the core are continually reflected at the core/cladding boundary so that the rays
remain within the core. This process is called total internal reflection and is the
means by which an optical signal is confined to the core of a fibre.
Figure 2 illustrates the process for a step index fibre.
Cladding
Core
Figure 1 Core with refractive index N1 and
0
N2 N1
Cladding
Diameter
Cladding
Core
Diameter
Core
Fibre side view
Fibre end view
Figure 2 Propagation of light in an optical fibre profile for a step index optical fibre
Page 2 of 20
Optical Laboratory 1
Cladding
Fibre Axis
B
A
C
Core
(i)
(ii)
Figure 3 Propagation of light in an optical fibre
In order to understand the process in more detail consider in figure 2 a light ray (i)
entering the core at point A and then travelling through the core until it reaches the core
cladding boundary at point B. As long as the light ray intersects the core/cladding
boundary at a small enough angle the ray will be reflected back into the core to travel on
to point C where the process of reflection is repeated. If a ray enters the fibre at a steep
angle, for example light ray (ii), then when this ray intersects the core/cladding boundary
the angle of intersection is too large and reflection back in to the core does not
take place and the light ray is lost in the cladding. This means that to be guided through a
fibre a light ray must enter the core with an angle that is less than the so called acceptance
angle for the fibre. A ray which enters the fibre with an angle greater than the acceptance
angle will be lost in the cladding. By convention the acceptance angle for a fibre can also
be described by the term "numerical aperture". The fibre acceptance angle can be
calculated from the refractive indices of the core and cladding using the formula:
θ1 = Sin −1  n1 2 − n2 2 


The numerical aperture of a fibre is simply equal to the mathematical sine of the
fibre acceptance angle hence the numerical aperture (NA) is given by:
2
2
NA =  n1 − n2 


Page 3 of 20
Optical Laboratory 2
Fibre coupler characterisation
OBJECTIVE:
To measure the insertion loss, excess loss and directivity of an
unknown commercial singlemode coupler.
EQUIPMENT:
•
•
•
•
Optical splitter, manufactured by Sumicem, terminated with FC/PC connectors
Tunable laser
OSA
2 x FC/PC patch leads
PROCEDURE:
Before measuring the coupler characteristics it should be observed that each of the
couplers outputs are terminated with FC/PC connectors. These connectors have a fixed
loss which adds to that of the coupler itself. During manufacture the essential
parameters of the coupler are measured before they are terminated with connectors. As
the coupler is now connectorised for the purpose of the experiment we will assume 0dB
loss at the connectors (FC/PC loss is very low).
To carry out this exercise a stable reference source of light at 1550 nm is needed.
A tunable laser source is provided for this purpose. Connect the laser to the OSA using
the fibre patch cord provided and adjust the laser output power level to 0dBm. Record the
exact value of fibre output power for reference. This optical power level is now the
reference source power for the rest of the exercise and should not be altered again.
The peak power of the laser can be measured using the OSA by pressing the [Markers]
button followed by [Peak Search].
Note: Most couplers/splitters when manufactured have 4 ports but one is removed giving
one input and two outputs.
1. Insertion Loss
Using a pair of patch cords connect the laser source to the input fibre and measure the
output levels from the two output fibres in dBm. Now using the reference power
measured above calculate the insertion loss of the coupler from the input port to output
port 3 and from the input port to output port 4.
2. Excess loss
The excess loss is the loss in the coupler in addition to the combined insertion losses. By
adding the output powers at ports 3 & 4 we would expect to get a total power equal to the
reference power. The combined powers will be less than the reference power and this
difference is the excess loss.
3. Directivity (also called Crosstalk)
Develop a strategy for yourself to determine the directivity, based on the definitions
found in the Appendix, of the coupler for optical powers into output ports #1 and #2
Page 4 of 20
Optical Laboratory 2
RESULTS & CONCLUSIONS:
1. Record all of the results listed in the paragraphs above and carefully document
your method of measuring the directivity.
2. Calculate the split ratio of the coupler.
3. Compare your insertion loss, excess loss and directivity values with the typical
data supplied from the manufacturer.
Background information on Fibre Couplers
Four port coupler used as a splitter for illustration, with power applied to port 1 Power in and out of ports is given by
P1, P2 etc..
Split Ratio (or Coupling Ratio)
Defined as the percentage division of optical power between the output ports. Most
common split ratio available is 50%,
i.e. P3 = P4. Split ratio frequently given as X/Y, where X and Y are the output ports.
Insertion Loss (dB)
Defined generally as the loss between two particular ports. It depends on the splitting
ratio and any imperfections in the
coupler (excess loss). For example for a 50/50 split ratio a reasonable insertion loss is <
3.4 dB
Excess Loss (dB)
In an ideal coupler the insertion loss would be set by the splitting ratio only, so
for a 50/50 splitter, the insertion loss
would be 3 dB. In practice scattering, absorption and imperfections raise the loss above
the theoretical value. This is the
so-called excess loss. The excess loss for a particular port is the difference
between the ideal insertion loss calculated
from the split ratio and the measured insertion loss. For example for a 50/50 split ratio a
reasonable excess loss is < 0.4 dB
Crosstalk or Directivity (dB)
Crosstalk is a measure of the isolation between two input or two output ports. Power
from port 1 may be backscattered to
port 2 and vice-versa. Crosstalk is the dB log ratio of the power into port 1, by
comparison with the power leaving pot 2.
For a quality 50/50 splitter a typical crosstalk is normally less than -60 dB.
Directivity is crosstalk written as a positive
value.
Page 5 of 20
Optical Laboratory 3
Bend loss/attenuation in fibre
OBJECTIVE:
To measure the bend loss in two samples of graded index fibre cable
as a function of bend radius and hence determine the 50% or 3 dB
attenuation bend radius.
EQUIPMENT:
•
Bend loss mandrel (see note 1. below)
•
62.5/125 µm core ST-to-ST 900 µm buffered fibre patch lead.
•
62.5/125 µm core ST-to-ST 2.5 mm fibre patch lead.
•
LED 850 nm reference light source, using ST output.
•
Optical power meter.
Note 1: The bend loss mandrel offers bend diameters of 2.5 mm to 25 mm in 2.5mm
steps thus: 2.5 mm, 5 mm, 7.5 mm, 10 mm, 12.5 mm, 15mm, 17.5 mm, 20 mm, 25 mm.
PROCEDURE:
The white nylon mandrel allow a precise bend radius to be set. Start with the 62.5 µm
core fibre 900µm buffered fibre. Connect the fibre to the ST source and to the power
meter, making sure the power meter is set to dBm (dB/dBm button) and a wavelength of
850 nm (λ button). Try to ensure that there are as few bends as possible in the fibre at
this point. Record the power measured on the power meter as the reference (no bend)
power level. Now starting with the 25 mm diameter mandrel wrap the fibre once around
the mandrel. Try to keep the fibre in place on the mandrel without to much stress and also
within reason try to ensure that there is exactly one turn of fibre on the mandrel. Record
the power level in dBm and note the difference in dB with respect to the no bend
reference power level. Now move on to the next smallest mandrel diameter and repeat the
measurement. As you approach the smallest mandrel diameter be careful not to damage
the fibre (particularly for the smallest 2.5 mm mandrel diameter).
Draw up a table thus:
Mandrel diameter
Mandrel Radius
Level in dBm
Attenuation in dB
relative to the
reference
25mm
12.5mm
xxx
xxx
22.5mm
11.25mm
xxx
xxx
Etc.
Etc.
Etc.
Etc.
Now repeat the above for the second fibre sample, drawing up a separate table. Because
of variations in connectors and launch power the reference (no bend power level) is likely
to be different and must be measured again.
Page 6 of 20
Optical Laboratory 3
RESULTS & CONCLUSIONS:
1. Carefully plot on a single graph (a sample is included below) using the data from
the tables above the bend loss in dB as a function of bend radius for all the fibre
samples used. Hence:
2. Compare and comment on the difference between the fibre samples and why the
difference exists.
3. Estimate and record the 3 dB loss/attenuation radius and comment again on the
differences between fibre samples.
4. Verify the rule of thumb that an adequate (low loss) minimum bend radius is 10 to
20 times the diameter of the cable.
Page 7 of 20
Optical Laboratory 4
Connector Inspection
OBJECTIVE:
To inspect industry manufactured single-mode and multimode fiber
connectors
EQUIPMENT:
•
Fiber connector samples
•
TV monitor
•
Microscopic inspection kit with fiber connector adaptor
•
IPA and lint free wipes
•
Camera
PROCEDURE:
Connect the monitor and the microscopic inspection kit using BNC leads. Recognize and
sort numbered fiber connector samples in the work place. Now insert the sample
connector into the adaptor of the microscopic inspection kit. Adjust the knob to align the
focus of the object lens with the connector endface. This will be shown in the monitor
with a clear image of the connector endface. Now adjust the screws in the front of the
microscopic inspection kit to centre the connector endface in the monitor screen. Now
inspecting the image on the monitor screen distinguish between a perfect connector and
an imperfect connector and recognize the cracks, chips and dirt on an imperfect connector
endface.
RESULTS & CONCLUSIONS:
1. Research, on the web and in text books, what type of imperfections exist on
connector end faces and how to identify them. Also comment on any standards
manufactures may use in inspecting their connectors.
2. Inspect every connector sample and identify any imperfections in the connector
end face.
3. Using a camera take a picture of the endface.
4. In your report identify the imperfections on the pictures and any steps that may be
taken to remove them. State whether the connector can be used and comment on
why or why not.
Page 8 of 20
Optical Laboratory 5
Connector Misalignment Loss
OBJECTIVE:
To measure the insertion loss of a connector joint as a function of
axial misalignment and lateral misalignment using 1mm core plastic
fibres.
EQUIPMENT:
•
Variable X-Y-Z positioning assembly
•
Mains powered cased LED optical power source (820 nm)
•
Fibre optic power meter
•
2 plastic optical fibre patch cords, 1 mm core
PROCEDURE:
Connect the SMA-to-SMA adapter with the variable X-Y-Z positioning assembly to the
optical source and optical power meter. Ensure the optical power meter is set to measure
in the 820 nm window. This assembly has one fixed adapter and one adapter which can
be moved in space in the X, Y and Z dimensions. Thus one connector ferrule/fibre is
fixed in space, while the other can be moved in space in 3 dimensions. The micrometers
on the X-Y-Z stages are calibrated in two ways. The linear scale is in mm, while the
rotating scale has 50 divisions and two rotations equals 1 mm, so each increment on the
rotating scale represents 10 µm.
Start by aligning very carefully the two ferrule ends containing the fibres to get the
highest power reading on the optical power meter. This will take care and patience. Note
the optical power level on the meter as the reference value.
Measure the optical power level as the movable ferrule/fibre vary the position of the
movable ferrules using the X-Y-Z stages thus:
1. Measure and tabulate the received power level as a function of Z motion (endseparation or longitudinal misalignment) using a table similar to that shown
below. Take great care not to alter the X or Y positions as this could invalidate the
results and avoid moving the leads and assembly while taking measurements.
Axial Misalignment (µm)
Power level measured (dBm)
0
-
Reference value
-
Attenuation relative to
reference (dB)
0
-
2. Now plot the data for longitudinal misalignment with the misalignment values
plotted along the horizontal axis of the graph and the attenuation values along the
vertical axis.
Page 9 of 20
Optical Laboratory 5
3. Restore the Z dimension to the reference value. Now measure the received power
level as a function of X firstly and then Y motion (both are lateral misalignment)
using a table similar to that shown below. Again take great care not to alter the Z
position in this case and avoid moving the leads and assembly while taking
measurements.
Lateral Misalignment
(µm)
0
-
Power level measured (dBm)
Reference value
-
Attenuation relative to
reference (dB)
0
-
4. Now plot the data for X and Y lateral misalignment on the same graph, with the
misalignment values plotted along the horizontal axis of the graph and the
attenuation values along the vertical axis.
RESULTS & CONCLUSIONS:
1. Predict the expected misalignment losses using the equations from the notes or
from a text book for a lateral misalignment of 100 µm and for an axial
misalignment of 50 µm.
2. Comment on your results and the graphs plotted, comparing your experimental
results to the predicted results.
3. Comment on how a significant change in the fibre NA and core size might alter
your results.
Page 10 of 20
Optical Laboratory 5
Background information on Joint Attenuation
Since optical fibre has a low loss, by comparison with other transmission media, a major consideration with
all types of fibre joint is the minimisation of the loss of optical power or attenuation that occurs at the joint.
There are a number of reasons why optical power loss occurs at a joint. The most fundamental type of loss
is so called Fresnel reflection which occurs if a small air gap exists between the ends of the two fibres at the
joint. The attenuation is typically a fraction of a decibel. Fibre connectors are prone to loss due to Fresnel
reflection, whereas in a fibre splice since there is no air gap Fresnel reflection is eliminated.
Optical power loss also occurs at a joint when there is a physical difference or incompatibility between the
two fibres which are jointed, for example:
- Different core and/or cladding diameters.
- Different fibre numerical apertures or refractive indices.
- Fibre faults, for example where the core and cladding are not concentric
Together with loss caused by Fresnel reflection the above factors are usually referred to as intrinsic joint
losses. Even when the intrinsic joint loss is eliminated there is still the problem of the quality of the fibre
alignment provided by the jointing mechanism. Examples of possible misalignment between coupled
compatible fibres are shown in figure 1 below. Misalignment can occur in three ways:
- The separation between the fibres (longitudinal misalignment).
- The offset perpendicular to the fibre axis (lateral or axial misalignment)
- The angle between the core axes (angular misalignment)
The optical loss that results from these three types of misalignment is called extrinsic loss and it depends
upon factors which include the fibre type and core diameter.
Z
Y
(a) Longitudinal
misalignment
(b) Lateral
misalignment
α
(c) Angular
misalignment
Fibre Axis
Figure 1 The three possible types of fibre misalignment
Lateral misalignment is more serious than longitudinal, for example in figure 1 for multimode graded index
optical fibres with a core diameter of 50 µm if the lateral misalignment "Y" is 10 µm the attenuation at the
joint is about 1 dB, whereas if the longitudinal misalignment "Z" is 10 µm the attenuation is only 0.1 dB.
For angular misalignment an attenuation of 1 dB would result from an angle of misalignment of 4 degrees.
In the case of singlemode fibres with much smaller core sizes, in the order of 8 µm, an attenuation of 1 dB
would result from a lateral misalignment of only 2 µm. It is for this reason that the jointing of singlemode
optical fibres needs to be much more precise than the jointing of multimode optical fibres.
Page 11 of 20
Optical Laboratory 6
Coupler wavelength dependence
OBJECTIVE:
To measure the wavelength dependence of an unknown commercial
optical coupler.
EQUIPMENT:
•
unknown optical coupler.
•
Tunable laser
•
Optical spectrum analyser
•
FC/PC patch leads
PROCEDURE:
Connect the optical coupler to the optical spectrum analyser (OSA) and the tunable laser
using the supplied patch leads. User instructions to the tunable laser and OSA are
supplied. The coupler has 3 ports, an input port and 2 output ports. The laser should be
connected to the input port and the OSA to one of the output ports. The laser output
power should be set to 0dBm (1 mW). The laser should then be swept in steps of 4 nm
from 1500nm to 1600nm and the peak power measured on the OSA. This should be
carried out again with the second output port connected to the OSA.
The peak power of the laser can be measured using the OSA by pressing the [Markers]
button followed by [Peak Search].
RESULTS & CONCLUSIONS:
1. Carefully plot on a single graph the wavelength versus the insertion loss for
both ports
2. Calculate how flat is the wavelength dependence over the C-Band.
3. Comment on the insertion loss over the spectrum and specifically at 1550nm.
Page 12 of 20
Optical Laboratory 7
Optical Return loss measurement
OBJECTIVE:
To measure the return loss at a fibre air interface.
EQUIPMENT:
•
99/1 optical coupler
•
Laser set
•
Optical power meter
•
2x FC/PC patch leads
•
One FC/PC patch lead with endface under test
PROCEDURE:
Set up the equipment as follows.
Coupler
Power
meter
2
Laser
1
99%
3
1%
Connector
endface
1. Connect the patch lead with the connector endface under test to port 3 of the
coupler.
2. Connect the laser and power as in the diagram above.The laser will launch light
into the port 1 (1% port) of the coupler. This light passes through the coupler and
down to the connector endface. Due to the glass air interface a Fresnel reflection
will occur. This reflected light is called return loss and will travel back towards
the coupler splitting with 99% of it arriving at the power meter at port 2. The
optical power Pr at port 2 which results from reflections caused by the
components and the coupler, is thus measured.
3. Next the cable (containing the connector endface under test is removed and
replaced by a non-reflecting termination. (To do this we leave the cable in place
and wind it on a mandral. This causes excessive bend loss and stops any
Page 13 of 20
Optical Laboratory 7
reflections from the end face.) This allows Pc due only to the coupler to be
measured at port 2.
4. The detector is then transferred to port 3 and the power incident upon the endface
Pref is measured. (note the bending is undone).
5. Apart from the loss in transmission between port 3 and port 2, the fraction of the
reflected power from the components under test is (Pr-Pc)/Pref. To obtain the ports
3 to 2 loss, the source and detector are connected to ports 3 and 2 respectively,
providing a measurement Pout.
6. Finally a Pin is measured by connecting the source directly to the detector such
that Pout/Pin is the fraction of the optical power transmitted between port 3 and
port 2. Hence the optical return loss is given by:
 Pout Pref 

ORL = 10 Log10 
 Pin (Pr − Pc ) 
Note: the values in this equation are in Watts and not dBm.
RESULTS & CONCLUSIONS:
1. Carefully take the measurements as instructed above and calculate the optical
return loss.
2. By analysing the experimental careful can you identify any other factors that may
be influencing the measured result.
3. What are typical ORL measurements for the following components:
a. FC/PC connector
b. FC/APC connector
c. Splice
Page 14 of 20
Optical Laboratory 8
Fibre core mismatch measurement
OBJECTIVE:
To measure the insertion loss due to the core size mismatch of a
SMF28 and HP980 single mode fibres.
EQUIPMENT:
• SMF28 single mode fibre
• HP980 single mode fibre
• Source
• Power meter
• Fusion splicer
PROCEDURE:
Connect the SMF28 pigtail to the optical source and optical power meter in a darkened
room (or under black out material). The unterminated end of the pigtail should be aligned
with the photodiode in the power meter. This will give us the value of the optical power
in the fibre just prior to it leaving the fibre (this is due to the fact that as the photodiode
surface is large so all the light leaving the fibre is collected). Note the output optical
power from the fibre pigtail. Splice the piece of HP980 fibre to the SMF28 pigtail(see
background material). Again connect the SMF28 pigtail to the optical source and the
output HP980 fibre to the optical power meter measure the output power. This will give
us the value of the optical power in the HP980 fibre just prior to it leaving the fibre. Due
to the mismatch of the core diameters of SMF28 to HP980 fibre we ill get a core size
mismatch loss. If we subtract your two results and you have your insertion loss due to
core size mismatch..
Photodiode
Optical
Source
Optical
Power
Meter
x
SMF 28
Fibre pigtail end
Photodiode
Optical
Source
x
SMF 28
y
HP980
Fibre splice
Optical
Power
Meter
Fibre pigtail end
RESULTS & CONCLUSIONS:
1. Experimentally measure the insertion loss of the splice of two fibres spliced with
mismatched cores.
2. Analytically verify your answer using the method below. Fibre specifications can
be found below also.
3. Discuss what other factors may be influencing the insertion loss of the splice.
Page 15 of 20
Optical Laboratory 8
Analytical verification
Using an equation developed by a W. C. Young the insertion loss can be calculated using
a Gaussian field approximation principle:
 w2 w1 
Toff ( d ) = 4 ⋅ 
+

 w1 w2 
−2
where w1 and w2 are defined as the mode field diameters (MFDs) of the fibres that are
mismatched. The mode fiels diameters of the SMF28 and the HP980 can be calculated
using the equations and fibre specifications below.
w =

 1.619  2.879 
0.65 +  3 2  +

V6 
 V 

2
1
2
⋅a
The V number (normalized frequency) is defined as:
 2 ⋅π ⋅ a 
V =
 ⋅ NA
 λ 
Recall that the core-cladding index difference or simply index difference ∆ is given by
n2 = n1 ⋅ (1 − ∆)
And the numerical aperture NA is given by:
NA = n12 − n 22
Fibre Specifications
SMF28 singlemode fiber:
Fiber core radius: 4.15 µm
Refractive index of fiber cladding: 1.4447 (@1550nm)
Refractive index of fiber core: 1.4504 (@1550nm)
HP980 singlemode fiber:
Fiber core radius: 2.875 µm
Refractive index of fiber cladding: 1.4440 (@1550nm)
Refractive index of fiber core: 1.4578 (@1550nm)
Page 16 of 20
Optical Laboratory 8
Splicing background
1.
i.
ii.
iii.
2.
Preparation of the fiber:
Remove the outer buffer using mechanical stripper.
Clean the removed buffer area with alcohol or acetone solution.
Cleave the fiber end in order to obtain a flat end-face. This will reduce the
scattering loss while coupling light into the fiber.
Cleaving operation:
Cleaver works on the basis of scribe and break technique, illustrated in Fig.1. The carbide
blade is used to start a small crack in the fiber. Stress is evenly applied by pulling the
fiber, causing the crack to propagate through the fiber and resulting in a cleaver across a
flat cross section of the fiber perpendicular to the fiber axis.
Carbide blade
Pull
Pull
Fiber
Tip to crack propagates
As fiber is pulled
Fig.1. Scribe and break technique of fiber cleaving
Page 17 of 20
Optical Laboratory 8
Misalignment types
There are several types of misalignment possibilities between a fiber optic source and a
fiber optic receiver. In Fig. 4 we show the typical 3 cases of mechanical misalignments.
Fig. 4. Fiber Misalignments
Lateral or axial Misalignment
The lateral or axial offset condition shown in Fig 4 (a) is further demonstrated in Fig. 5,
where we show the motivation for the mathematical model to predict the insertion loss
due to this type of misalignment.
Fig. 5. Axial/ Lateral Offset Condition
Page 18 of 20
Optical Laboratory 9
Mechanical splice
OBJECTIVE:
To carry out a mechanical splice
EQUIPMENT:
•
Mechanical splice
•
Stripper
•
Cleaver etc.
•
Fibre
PROCEDURE:
Follow the instructions as per the AMP mechanical splice.
RESULTS & CONCLUSIONS:
1. Discuss any advantages a mechanical splice may have over a fusion splice or
connector pair.
2. What type of insertion loss mechanisms are at play and how do they compare to a
fusion splice or connector pair.
Page 19 of 20
Optical Laboratory 10
Fibre numerical aperture measurement using standardised technique
OBJECTIVE:
To measure the NA of a number of fibres using the technique
described in the attached Corning document which is based on the
following standards.
EIA/TIA-455-47B (FOTP-47), Output Far-Field Radiation Pattern Measurement,
Method C.
EIA/TIA-455-177A (FOTP-177), Numerical Aperture Measurement of Graded-Index
Optical Fibers, Method A.
EQUIPMENT:
•
•
•
•
•
HeNe. Laser with SMA connector, emitting at a wavelength of 632.8 nm
50/125 graded index silica fibre (orange cable).
200/400 step index polymer clad silica fibre (white cable).
1 mm core step index plastic fibre (black cable)
Photodiode target on a translation stage.
PROCEDURE:
As in the attached corning document but using a simplified setup.(Lenses and mode
strippers not available)This exercise is best carried out in semi-darkness, for example in
the screened room. The numerical aperture is a parameter normally associated with light
entering a fibre, however to measure numerical aperture it is easier to investigate the
characteristics of the light leaving the fibre, which will provide a reasonable
approximation of the numerical aperture.
Important: Do not look into the light beam emitted from the laser or the fibre at
any time, as this could cause eye damage.
RESULTS & CONCLUSIONS:
1. Compare and comment on your results by comparison to those found in Optical
Laboratory 1 (Fibre numerical aperture measurement using a HeNe laser).
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