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Characterization of Optical Devices Using Magnitude and Group Delay Measurement
Haiqiao Lin, Undergraduate Student
Dr. Christi Madsen, Professor
INTRODUCTION
POLARIZATION CONTROL
Mid-Infrared optical devices are currently being developed in the fabrication
facilities at Texas A&M University. As the fabrication of these devices nears
completion, it is necessary to develop the ability to accurately measure their
performance.
In his masters thesis, former graduate student Mike Thompson originally
proposed a method for characterizing optical devices using magnitude and
phase delay measurements. My project seeks to improve upon his process
in two major ways:
-Improve accuracy and repeatability of wavelength-dependent
measurements by adding gas cell and interferometer reference signals.
-Enable the characterization of polarization-dependent optical devices by
adding mechanisms to control the input polarization to a device under test.
In addition to wavelength dependence, some optical devices are also have
behaviour that varies according to the polarization of the incoming light
signal. This is very problematic, because the polarization of light in an optical
fiber can change dramatically depending on the length of the fiber and how it
is arranged physically. Even a small bump on the fiber can cause a
significant change in polarization.
An electronic polarization controller was the key to overcoming this problem.
By sweeping through various settings of the waveplates on the polarization
controller, we can determine the maximum and minimum transmission ratios
for the a polarization-dependent device, as well as their associated states of
polarization. From this, we can calculate the values for the waveplate
settings in order to obtain the maximum and minimum transmission. Thus,
we have two known, stable polarization states for which we can test a
device.
Wiltron 6637A-40 RF Sweep Generator. This RF Generator was used
to generate the modulating signal for the laser.
HP 83410C Lightwave Reciever. This is a high-speed optical detector that
converts an optical signal into an electrical signal.
Additionally, many steps were taken to automate the measurement process
by controlling devices through the computer using National Instruments
LabVIEW software.
JDS Uniphase PR2000 Polarization Controller. This is the electronic
polarization controller used as described in previous sections. It is remotely
controlled on the computer through a GPIB interface.
Gas Cell, Interferometer, and Photodetector Board. This box houses the
gas cell and interferometer, which are used as described in previous
sections. It also contains a photodetector board which converts the outputs
of these optical devices into an electrical voltage which is then read into the
computer.
Fig. 1 Block diagram of group delay measurement technique
WAVELENGTH REFERENCES
Many interesting optical devices have a great deal of wavelength-dependent
behaviour. Because our current laser sources are not perfect, there are small
variations in wavelength that occur from one measurement to the next. To
solve this problem, I added two references: a gas cell and an interferometer.
Fig. 3 Example of Calculated vs Measured Max and Min Transmissions
The gas cell is a simple optical device that attenuates the incoming signal at
certain known “absorption wavelengths.” As we sweep our laser source
across a wavelength range, we can compare the spots where we observe an
absorption peak from the gas cell to the known absorption wavelengths
available from a data sheet, in order to obtain a precise reference point for our
wavelength measurement.
National Instruments PCI-6110 DAQ Board. This is a data acquisition
board that is used as the interface to input all of our measurements into the
computer for post-processing.
EQUIPMENT
Agilent 8164A Laser. This is the primary laser source used throughout this
project. It features a GPIB interface through which it can be remotely
controlled on the computer.
ACKNOWLEDGEMENTS
Optical Modulator and AD8302 Detector. This box, built by Mike
Thompson, houses the optical modulator and AD8302 Detector. The laser
output is modulated by a RF signal before going into the device under test
(DUT). After going through the DUT, the optical signal is then converted back
into an electrical signal. The AD8302 detector compares this signal to the
original RF modulating signal to determine the magnitude and phase
changes produced by the DUT.
Fig. 2 Sample Gas Cell Response
The interferometer, on the other hand, has a wavelength response that is
periodic, with a very high frequency. Once we have a set reference point from
the gas cell, we can accurately measure the location of any point on our
wavelength spectrum by calculating the number of interferometer periods in
between that point and the reference point set by the gas cell.
Department of Electrical and Computer Engineering
Texas A&M University
College Station, TX 77843-3128
Dr. Christi Madsen, Ph.D, my faculty advisor: for her insight,
troubleshooting ability, and fast responses to emails…
J. Paul Chambers, soon-to-be MS: for his helpful suggestions, willingness
to answer my stupid questions, and availability to bail me out when I lock
myself out of the lab…
Mike Thompson, MS: even though I have never met him, he laid the ground
work for everything that I did, so this project would not exist if it weren’t for
him!
Mid-Infrared Technologies for Health and the Environment, REU Program