Development and Refinement of Long Period Fiber Grating (LPFG)
Manufacture and Characterization Techniques
Kevin Edmonds, Patrick Chan, Dr. Ivan Tomov, Dr. H.P. Lee
Abstract
Long period fiber gratings (LPFGs) operate as band rejection filters in optical fibers, lending
themselves to sensory and signal-shaping applications. We explore a novel approach to LPFG
manufacture via CO2 laser etching; by angling the optic fiber during inscription, we modulate
laser intensity through distance attenuation. Ultimately, we conclude that this approach is not
efficiently implementable due to the increased software complexity required to maintain laser
focus throughout the writing process. Adhering to our discussion of LPFGs we shift our attention
to acousto-optic tunable filters (AOTFs). AOTFs are a subset of LPFGs that are not fixed to any
specific wavelength when constructed and function by vibrating the optic fiber, thereby creating a
standing wave in the fiber that acts as the filtering mechanism. AOTFs are therefore tunable by
altering their vibrations, but manufacturing inconsistencies mean that, given a constant amount of
power, two AOTFs may tune to two different wavelengths. We investigate an interferometric
approach to quantifying AOTF efficiency through measurement of AOTF vibration amplitudes.
We successfully set up and implement an interferometer in measuring vibrations on proxy
samples, and are re-running the experiment on actual AOTFs. Successful completion of this study
will simplify the AOTF tuning process and consequently simplify future AOTF research.
Key Words: Long period fiber grating (LPFG), acousto-optic tunable filter, CO2
laser etching, LPFG Sensing, Vibrometer, Lock-in Amplifier
Introduction
The use of optic fibers as tunable filters offers a low-cost, easy-to-manufacture solution
for signal shaping and sensing applications. We aim to develop and refine manufacturing
and characterization techniques for two types of long period fiber grating (LPFG): CO2etched gratings and Acousto-optic Tunable Filters (AOTFs). Both types of LPFG serve as
band rejection filters in optic devices, but support this functionality through different
mechanisms. We will briefly explain important characteristics of etched gratings and
AOTFs, and then shift our discussion to current research tasks.
Long Period Fiber Gratings at a Glance
In general, LPFGs are optic fibers with periodically varying indexes of refraction through
a section of the fiber. These varying indexes of refraction will be referred to as interfaces.
When light travels through the fiber, each successive interface the light passes through
results in increased attenuation of that light at some specified wavelength. If the
interfaces are all the same, this attenuation causes the fiber to act as a band-rejection filter
centered about the specified wavelength. When light is shined through the grating into an
optical spectrum analyzer, the intensity of the light at different wavelengths can be
measured, and as expected, light at the wavelength rejected by the grating will have much
lower intensity than light at other wavelengths. The point of low intensity will be referred
to as a notch.
Etched Gratings
Etched gratings are passive LPFGs that operate by periodically physically deforming an
optic fiber to create the interfaces of the grating. Etched gratings are compact and easy to
manufacture. However, because the affected wavelength is specified when the grating is
written, etched gratings are not tunable after creation.
Manufacture of Etched Gratings
Etched gratings can be written by placing an optic fiber under constant tension and
periodically heating the fiber with a laser along the length of the fiber. The result of the
heating is malleability of the fiber, and stretching of the fiber at the heated points due to
the applied tension. These points have different indexes of refraction than the rest of the
fiber and thereby form the interfaces of the grating.
We employ two methods in controlling the heating of the fiber. The first method is
“point-by-point” heating of the fiber, in which a CO2 laser scans to a point on the fiber,
turns on, turns off, and moves to a new point. This process is repeated some set number
of times. The technique is simple to implement but results in sharp differences in
refractive indexes between the interfaces and the rest of the fiber. These differences lead
to high insertion loss, or general signal degradation, which is undesirable.
The second method is a modulation scan of the fiber, in which the CO2 laser is constantly
on but varies in intensity as it scans across the fiber. The result is more gradual changes
in the refractive index, which causes lower insertion loss in the fiber. This benefit comes
at the cost of increased software complexity.
Etched Gratings as Sensors
An important property of etched gratings is the linearity of the amount by which its notch
shifts when placed under tension or heat. This linearity property means that the notch will
shift by a predictable amount when under tension or temperature stress, governed by
some measurable constant. Thus, if we know that the notch shifts by an amount d when
under some tension T, if we see that the notch has shifted by some amount d2, we can
calculate the amount of tension T2 that the notch must be under due to this linearity
property.
Acousto-Optic Tunable Filters
AOTFs are active LPFGs, which operate by sending an acoustic wave through an optical
fiber. This acoustic wave becomes a standing wave in the fiber, which then serves to
form the interfaces of the grating. AOTFs can be tuned by varying the amplitude and/or
frequency of the acoustic wave sent through the fiber. A sample AOTF consists of a fiber
attached to a piezoelectric transducer (PZT) which causes the fiber to vibrate. The
necessity of the fiber’s being attached to the PZT means the AOTF is not a compact
device. However, because it is tunable, one AOTF can do the job of many etched
gratings. AOTFs are not well suited for sensing applications, but are better suited for
signal-shaping applications.
Research Objectives
Our research aims to produce high-quality and well-characterized LPFGs to be used in
communications and sensing industries. Research projects towards this end involve:
i.
Simultaneous remote computer control of multiple instruments
ii.
Development of an apparatus for measurement of the shifting constant of
an etched-fiber grating
iii.
Investigation of a novel way of writing broadband rejection filters based
on varying the distance of the fiber from the writing laser
iv.
Interferometric measurement of the vibration amplitude of an AOTF to
determine mounting efficiency
Simultaneous Remote Computer Control of Multiple Instruments
It is desirable to control all of the instruments used in our experiments without physically
adjusting parameters at each instrument. Further, certain experiments require sweeping
across a range of parameters, which is awkward or impossible to do directly on the
instruments. A remote computer control suite was thus developed to speed up and enable
automation of experiments.
Method
Separate programs were written to control each instrument individually. Some newer
instruments had pre-written LabVIEW drivers, which made interfacing simpler. Older
instruments required consulting their respective programmers’ manuals to find
appropriate command strings. Each subprogram was tested and verified to operate
correctly. After each subprogram was complete, they were combined into one unified
program.
Materials
The program was written in LabVIEW 8.0 and interfaces with the instruments via their
GPIB (General Purpose Interface Bus) ports. The instruments plug into the USB port of a
standard PC using a GPIB-to-USB cable.
The instruments interfaced with were:
Agilent Technologies DSO6034a Optical Spectrum Analyzer
Agilent Technologies 33120a 15MHz Arbitrary Waveform Generator
Agilent 8164a Lightwave Measurement System
HP3325A Synthesizer/Function Generator
Results
The program was tested and verified to work correctly. The program is easily expandable
to include automation of experiments, but because at present no experiments require
automation, that functionality is left out of the program.
Figure 1: Sample screenshot of instrument control program
Development of an Apparatus for Measurement of the Shifting Constant of
an Etched-Fiber Grating
Pursuit of a low-cost means of characterizing the response of etched gratings to tension
was motivated by the high cost of existing devices for achieving the same end. The
resulting apparatus was constructed based on commercially available devices, but
excludes the expensive electronics required for precise tension measurements.
Method
The apparatus operates through tension equations from classic Newtonian mechanics. An
optic fiber with etched grating is clamped at both ends, with both ends fed into an optical
spectrum analyzer (OSA). A weight is suspended from the fiber as seen in Figure 2,
resulting in a calculable amount of tension in the fiber.
Figure 2: Apparatus schematic
Materials
The apparatus was built out of stock hardware store aluminum and screen door sliding
wheels and is fastened together with stock hardware store ¼” bolts. It is mounted to a
ThorLabs optics platform. The fibers are held by ThorLabs optic clamps and are fed into
an ACQU AQ6317B Optical Spectrum Analyzer.
Testing Procedure
The waveform of the grating under no tension is recorded from the OSA. Mass
increments of 2.5g are then added to the fiber and the waveform is recorded at each
increment.
Results
1546.3
1546.2
1546.1
Wavelength (nm)
1546
1545.9
1545.8
1545.7
1545.6
1545.5
1545.4
0
20
40
60
80
100
120
Strain (g)
We see that the LPFG tested is insensitive to strain less than 50 grams, and has two
pictured linear regions. Despite the change in linear constant at around 80 grams, the
linearity property of the tested LPFG is nevertheless preserved throughout the test. Thus,
the waveforms recovered successfully demonstrate the linear response of the gratings to
tension.
Discussion
This method trades precise measurement electronics for well-known equations that
describe behavior of the apparatus. This necessarily sacrifices some measurement
accuracy. The pulleys in the device, for example, are neither mass-less nor frictionless,
and formulas that treat them as such will yield only approximate results. We have
concluded, however, that these approximate results are sufficiently accurate for our
purposes since we are successfully able to predict how much tension a fiber is under
based on how much its notch shifts.
Investigation of a Novel Way of Writing Broadband Rejection Filters Based
on Variable Distances of the Fiber from the Writing Laser
Rather than attenuate a light signal at one wavelength very strongly, it is possible to vary
the interfaces of an LPFG to attenuate a signal at multiple wavelengths more weakly. The
result is a broadband filter, also known as a chirped grating. We present a novel approach
of varying the interfaces of the LPFG by angling the fiber relative to the CO2 laser while
writing. The distance of the fiber from the laser varies, thereby increasing the intensity of
the laser at points closer to it and varying the interfaces during the writing process. We
hypothesize that the resulting grating should have the characteristics of a chirped grating.
Method
We position the fiber at approximately a 12° (11.92°) angle relative to the horizontal. The
CO2 laser is positioned perpendicularly above the fiber, which causes the distance from
the laser to the fiber to change throughout the writing process.
Figure 3: Experimental Setup
Alignment
Because the laser is invisible, we use a red HeNe laser aligned with the CO2 laser and
align our fiber with the HeNe laser. We determine that the fiber is aligned when the
diffraction grating formed by the HeNe laser hitting the fiber is symmetric and most welldefined. We use a customized linear translation stage to make fine adjustments to the
alignment of the fiber.
Tension
The fiber is clamped at one end with a vacuum clamp. In the original experiment, both
ends are clamped with vacuum clamps and tension is applied by pulling one clamp from
the other via a sliding translation stage. To mimic this technique, the strength of the
vacuum clamps is measured, and a weight is chosen to emulate the pulling force of the
clamps. In this setup, the other end is slung over the translation micrometer and tension is
applied via our chosen weight at the end.
Writing
Writing was first done using a point-by-point heating method, and was done again using
a modulation scan. We wrote 20 interfaces into the gratings. We ran the experiment
multiple times, varying the intensity of the laser from 6-9%, and varying the exposure
time per interface from 0.1 to .5 seconds.
Materials
SynRad J48-1 CO2 Laser
PI-M415.DG Translation Stages
JDSU 1126p HeNe Laser
Modified Newport M-461 Linear Translation Stage
Results
Systematic adjustment of writing parameters yielded consistently inadequate gratings.
We have concluded that varying the distance of the fiber from the writing laser causes the
laser to fall out of focus during the writing process, yielding bad interfaces when written
by the out-of-focus laser. Rather than angling the fiber, we instead produced chirped
gratings through rewriting the controlling software for the CO2 laser.
Discussion
The increased software complexity required to maintain laser focus for the entirety of the
writing process negatively offsets any simplicity gains resulting from writing the fiber at
an angle. It is simpler to vary laser intensity in software than it is to both vary laser
intensity through distance attenuation and simultaneously maintain laser focus throughout
the writing process. Further, even if the angling process worked without rewriting
software, the added complexity of the experimental setup was much more prone to failure
and more expensive than the comparatively simple and inexpensive software revision.
Interferometric Measurement of the Vibration Amplitude of an AOTF to
Determine Mounting Efficiency
AOTFs consist of an optic fiber attached to a PZT and operate by injecting a standing
wave into the fiber via an external adjustable vibration from the PZT. Note however that
for a PZT vibrating at a given frequency and amplitude, the actual vibration transferred to
the optic fiber is dependent on the connection between the optic fiber and the PZT. We
will call the quality of this connection the mounting efficiency. We seek to quantify the
mounting efficiency of our AOTFs via an interferometric approach to vibration
measurement.
Experimental Setup
Figure 4 shows the setup of our interferometer. A 633 nm laser is sent through a beam
splitter, creating two beams referred to here as the “sample beam” and “reference beam.”
A sample is connected to a PZT, which is in turn connected to an HP3325a
Synthesizer/Function Generator, which controls the vibration of the sample. The beam
hits the vibrating sample and is reflected back to the beam splitter where it recombines
with the reference beam. The reference beam is modulated to 80 MHz via a Brimrose
Acousto-optic Driver. The recombined signal is directed to and captured by a ThorLabs
PDA10a photodetector. Using the interference pattern generated by the recombination of
the sample beam with the reference beam, we are able to calculate the amplitude of the
vibration of the sample.
Figure 4: Vibrometer Schematic. Source: Andy Chen, Sam Pan, David Tseng, H.P. Lee. Non-Contact
Vibrometer.
The Photodetector Signal
We reference Martinussen et al. in interpreting the signal on the photodetector. We use an
HP8752a Network Analyzer to recover the signal from the photodetector. The
recombined signal shows three peaks on the network analyzer in the frequency domain.
The first peak occurs at the frequency of the reference laser –80 MHz in this case. The
second and third peaks occur at the reference laser frequency plus and minus the
frequency of the vibrating sample. The second and third peaks are equal in magnitude, so
we ignore the third peak and look instead at the first and second peaks. The ratio between
the first and second peaks is given by  /4a, where a is the amplitude of the vibrating
sample. Knowing this ratio, we need only measure the amplitude of the first and second
peaks and, knowing the wavelength of our laser, we can calculate the amplitude of the
vibrating sample.
Lock In Amplifier Development for Signal Processing
In anticipation of signal retrieval from a high noise environment, we set forth to develop
a software-based lock in amplifier due to the unavailability of necessary mixers required
to use a real one.
Lock in amplifiers offer a means of extracting a potentially weak signal from an
otherwise overpoweringly noisy environment. A lock in amplifier is fed a noisy signal
and a reference signal tuned to the desired frequency to detect. The output of the lock in
amplifier is given by the equation
Source [7]
The output Uout(t) is given by the average value of the input signal multiplied against the
reference signal. Using this output, we are able to calculate the amplitude of the input
signal at the frequency of the reference signal.
Figure 5: Software lock in amplifier output for a test signal composed of sinusoids with frequency and
amplitudes as shown
Our lock in amplifier was developed in LabVIEW and verified to work with test input
signals. However, during the course of our interferometer tests we were clearly able to
see the signal on the network analyzer, thus temporarily eliminating the need for the lock
in amplifier. For future samples, we expect that our signal will be weaker and thus our
lock in amplifier is still necessary.
Results
We have calculated the vibration of our sample given a range of PZT amplitudes. We see
from the results the expected linear relationship between sample vibration and PZT
amplitude. Presently, we are re-running tests to ensure consistency among measured
Vibration Amplitude
results.
20
18
16
14
12
10
8
6
4
2
0
Series1
1 2 3 4 5 6 7 8 9 10
PZT Amplitude (*10 V)
Figure 6: Expected linear relationship between PZT Amplitude and Vibration Amplitude. Slight
fluctuations may be attributed to measurement noise.
Future Research
We aim to perfect our detection technique, and then proceed to measurements of real
AOTF samples. Currently we are devising a reliable way of stabilizing the signal from
our photodetector to ensure consistent measurements. In addition, we aim to automate the
data acquisition process from our network analyzer to increase the rate at which we can
take measurements and thereby analyze measurement consistency.
Acknowledgements
We would like to thank the NSF for their generous funding. Further, we would like to
acknowledge and graciously thank the IM-SURE program and especially Said M.
Shokair for his earnest involvement in the quality of the IM-SURE experience for faculty
and fellows alike.
References
1. Carlson, A. Bruce. Communication Systems – Introduction to Signals and Noise in Electrical
Communication. Singapore: McGraw Hill Book Co., 1986
2. Chen, Andy, Sam Pan, David Tseng, and H.P. Lee: Non-Contact Vibrometer.
3. Kartalopoulos, Stamatios V. Introduction to DWDM Technology – Data in a Rainbow. Piscataway, NJ:
IEEE Press, 1999.
4. Lopez-Higuera, Jose Miguel. Handbook of Optical Fibre Sensing Technology. West Sussex, England:
John Wiley and Sons Ltd., 2002.
5. Martinussen, Hanne, Astrid Aksnes and Helge E. Engan: High Sensitivity Vibration Measurements with
Absolute Calibration. 2006 Optical Society of America, OCIS codes (120.3940) Metrology; (120.3180)
Interferometry.
6. Othonos, Andreas, and Kyriacos Kalli. Fiber Bragg Gratings – Fundamentals and Applications in
Telecommunications and Sensing. Norwood, MA: Artech House Inc., 1999.
7. "Lock In Amplifier." Wikipedia.org. <http://en.wikipedia.org/wiki/Lock-in_amplifier>.
8. "What is a Lock-In Amplifier?." Center for Precision Metrology.
<http://www.cpm.uncc.edu/programs/tn1000.pdf>.