Experimental GaAlAs modulators operating at 0.82 micrometers using a Mach-Zehnder interferometer configuration were designed and fabricated. Coplanar 50 ohm travelling wave microwave electrodes were used to obtain a bandwidth length product of 11.95 GHz-cm. The design, fabrication and dc performance of the GaAlAs travelling wave modulator is presented

Bhasin, Kul B.

Chorey, Christopher M.

Ferendeci, Altan

English

NASA Technical Reports Server

NASA Technical Memorandum 100970
A High Frequency GaAlAs Travelling
Wave Electro-Optic Modulator
A r\ c\r\at 0.82
Christopher M. Chorey and Altan Ferendeci
Case Western Reserve University
Cleveland, Ohio
and
Kul B. Bhasin
Lewis Research Center
Cleveland, Ohio
Presented at the
1988 IEEE MTT-S International Microwave Symposium
New York, New York, May 25-27, 1988
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https://ntrs.nasa.gov/search.jsp?R=19880018856 2020-03-20T05:38:07+00:00Z
A HIGH FREQUENCY GaAlAs TRAVELLING WAVE ELECTRO-OPTIC MODULATOR AT 0.82 jam
. : f i . . . . . . '
:
 ; Christopher M, Chorey and'Altan Ferendeci
Electrical Engineering and Applied Physics
Case Western Reserve University
.. .Cleveland, Ohio 44106
and
. . Kul Bhasin
National Aeronautics and Space Administration
Lewis Research Center •
Cleveland, Ohio 44135
SUMMARY
Experimental GaAlAs modulators operating at 0.82 |im using a Mach-Zehnder
interferometer configuration were designed and fabricated. Coplanar 50 Q trav-
e l l i n g wave microwave electrodes were used to obtain a bandwidth-length product
of 11.95 GHz-cm. The design, fabrication and DC performance of the GaAlAs
travelling wave modulator, is presented.
INTRODUCTION
The development of high frequency integrated microwave-optical components
and circuits on GaAs substrates is required for highly agile and light weight
optically controlled GaAs MMIC based phased array antennas for future space
communication and tracking systems (ref. 1). External modulation of a GaAlAs
solid state laser output by a modulator is a means to convert a microwave sig-
nal, to an optical.signal, thus allowing the benefits of low weight and low
crosstalk to. be applied to these systems.
Waveguide electro-optic indirect modulators fabricated on LiNb03 and GaAs
substrates and operating at 1.3 vim have previously been reported in the litera-
ture (refs. 2 to 4). It is unlikely that with the present technology, LiNb03
modulators can easily be integrated with the present day GaAs MMIC sub-
strates. On the other hand, GaAs modulators cannot be integrated with GaAlAs
laser sources due to high absorption of the GaAs at 0.82 \im wavelength.
In this paper, we present the fabrication of a triple heterostructure
GaAlAs travelling wave electro-optic modulator which operates at 0.82 jam wave-
length and avoids the absorption -problems associated with pure GaAs. The
detailed analysis and design of the electro-optic modulator has been previously
reported (refs. 5 and 6). The modulator uses a 50 Q travelling wave coplanar
waveguide electrode and needs no matching network. It has a high bandwidth
and is capable of integration with GaAs/GaAlAs laser diodes. Although GaAs
has a smaller electro-optic coefficient and requires higher microwave drive
power levels than LiNb03, the modulator presented here allows true integration
with GaAs MMIC technology.. • ,
DESIGN OF THE MODULATOR
The electro-optic modulator, shown schematically In figure 1, is comprised
of two essential components: (a) the optical waveguide Mach-Zehnder interfer-
ometer and <b> the microwave coplanar waveguide.
A material problem associated with 0.82 pm operation of GaAs modulator is
that GaAs is highly absorptive at this wavelength. This is avoided by the sub-
stitution of a small percentage of aluminum to form GaAlAs which shifts the
absorption edge of the compound semiconductor to shorter wavelengths. In the
design of the present modulator, aluminum concentrations greater than 10 per-
cent were chosen so that all three GaAlAs layers are transparent at 0.82 pm.
In determining the response of the modulator, the :electro-optic coefficient
for GaAs is used in the assumption that the small percentage of the aluminum
does not significantly alter the electro-optic coefficient.
Optical propagation losses in semiconductor waveguides are also a disad-
vantage of GaAs modulators. In our design, a strip loaded ridge structure was
chosen for the optical waveguide. Three layers of GaAlAs with differing alumi-
num concentrations were grown over a semi-insulating substrate (fig. 2). The
upper layer was etched to form the loading ridge, but the optical signal propa-
gates in the middle layer. This configuration concentrates the light energy
away from the etched edges of the loading ridge, reducing the intensity of
light scattered by the edges and therefore reducing the propagation losses.
To prevent additional losses, the first layer above the semi-insulating GaAs
is chosen to be thick enough so that the signal w i l l not extend into the sub-
strate and be absorbed by the GaAs.
To provide the microwave field in the required orientation, a coplanar
travelling .wave electrode structure was chosen. The electrodes were designed
for 50 Q impedance on semi-insulating GaAs. Since there is no doping in any
layer, the low intrinsic carrier concentration of GaAs allows the substrate to
be treated as a dielectric. The absence of ohmic and rectifying behavior makes
the transmission line electrodes possible. The conductor and gap widths are
chosen to provide the 50 Q characteristic impedance. The greatest advantage in
choosing the coplanar transmission line structure is the elimination of the
matching networks throughout the operating bandwidth of the modulator.
The bandwidth of the proposed device is determined by the walkoff between
the light and microwave signal which propagates through the device at different
velocities. The calculated 3 dB bandwidth-length product of the 11.95 GHz-cm
is for full scale intensity modulation without any optimization of the wave-
guiding structure. This may further be accomplished by finding the proper
coplanar waveguide dimensions so that the propagation constant of the microwave
signal can be brought closer to that of the optical signal. If the modulation
depth is reduced, the corresponding bandwidth-length product is increased.
FABRICATION
The modulator was fabricated in a three layer Ga}_xAlxAs heterostructure
grown on a semi-insulating GaAs substrate. The GaAlAs layers were grown by
MBE at Perkin Elmer and consisted of a 3.5-vim thick lower cladding layer of
a 0.8-nm thick middle guiding layer of Ga.grjAl; iQAs, and an
0.8-vim thick top cladding layer of Ga.87A.1 .i3As. The entire GaAlAs heter-
ostructure was capped by 100 A of GaAs for protection prior to fabrication.
. i'
Fabrication took place by standard photolithography using an E-beam gener-
ated mask. The waveguiding ridges were defined'by wet chemical etching in a
solution of H3P04:H202:H20::3:1:25 to a ridge height of 1400 A. The "Y" branch
of the interferometer was defined to a feature size of <1 urn (fig. 2) and the
edges of the waveguiding ridges were generally smooth and clean (fig. 3). The
travelling waveguide electrode pattern was produced by liftoff techniques using
E-beam evaporated metals. Two types of electrodes were applied; rectifying,
Schottky (Ti/Au,AU) and ohmic (Au/Ge, Ni, Au>.
MODULATOR CHARACTERISTICS
Testing of the modulator is accomplished by butt coupling of the laser
radiation to the cleaved end facet of the device from a single mode optical
fiber pig-tailed to a GaAlAs laser diode (Ortel-SL620s). The output facet of
the modulator is imaged by an IR sensitive camera and displayed on a video mon-
itor. Successful coupling to the modulator produces a single intensity peak
at the output facet with little lateral leakage observable. A modulator
cleaved midway along the parallel branches of the Mach-Zehnder .interferometer
shows two intensity peaks, one under each optical waveguide (fig. 4).
DC measurements on modulators have shown that the turn-off voltage, Vp,
is approximately 30 V.
CONCLUSION
A GaAlAs electro-optic Mach-Zehnder modulator operating at 0.82 ^m has
been designed and fabricated. Because the waveguiding layer is transparent at
0.82 urn the design is potentially integratable with a GaAlAs laser diode.
Theoretical calculations indicate a 3 dB bandwidth-length product of 11.95 GHz-
cm for 100 percent modulation depth.
REFERENCES :
1. Bhasin, K.B., et al.: Optical Techniques to Feed and Control GaAs MMIC
modules for Phased Array Antenna Applications. Eleventh AIAA Communication
Satellite Systems Conference, AIAA, 1986, pp. 506-513. (NASA TM-87218).
2. Gee, C.M.; Thurmond, G.D.; and Yen, H.W.: 17-GHz Bandwidth Electro-Optic
Modulator. Appl. Phys. Lett., vol. 43, no. 11, Dec. 1, 1983, pp. 998-1000.
3. Buchmann, P., et al.: Broadband Y-Branch Electro-Optic GaAs Waveguide
Interferometer for 1.3 jam. Appl. Phys. Lett., vol. 46, no. 5, Mar. 1,
1985, pp. 462-464.
4. Donnelly, J.P., et al.: A High Frequency GaAs Optical Guided-Wave
Electro-Optic Interferometric Modulator. IEEE J. Quantum Electron,
vol. QE-21, no. 1, Jan. 1985, pp. 18-21.
5. Materna, D.M.: Investigation of a GaAlAs Mach-Zehnder Electro-Optic Modu-
lator. M.S Thesis, Case Western Reserve University, 1986. (NASA
CR-179573).
6. Materna, D:M.; Ferendeci, A.M.; and Bhasin, K.B.: Design of a GaAlAs Trav-
e l l i n g Wave Mach-Zehnder Electro-Optic Modulator. Optoelectronic Materi-
als, Devices, Packaging, and Interconnects, Proc. SPIE Vol. 836, T.E.
Batchman, et al., eds., SPIE, 1987, pp. 195-198.
COPLANAR TRANSMISSION
LINE ELECTRODES
->
Ga1-xAlxAs
GaAs—
INTENSITY
MODULATED
LIGHT OUT
FIGURE 1. - SCHEMATIC VIEW OF GAAtAs MACH-ZAHNDER ELECTRO-OPTIC MODULATOR. DIMENSIONS OF
COPLANAR WAVEGUIDE ARE CHOSEN FOR 500 CHARACTERISTIC IMPEDANCE.
ORIGINAL PAGE IS
OF POOR QUALITY
-»+« w
Ga1-xAlxAs
METAL | j | METAL |_
€
u -•
, | METAL
= 0
= 0.5
= 1.3
= a «
(DIMENSIONS IN MICRONS)
GaAs
FIGURE 2.- CROSS-SECTIONAL SCHEMATIC OF MODULATOR STRUCTURE.
FIGURE 3. - "Y" BRANCH OF HACH-ZEHNDER INTERFEROMETER. GAP
BETWEEN BRANCHES IS APPROXIMATELY 1 MM AT IT'S NARROWEST
POINT.
ORIGINAL PAGE IS
DB POOR QUALITY
. Au
Electrodes
-
28KU 2 . 5 1 K X 5U 8048 CEH
FIGURE 4. - PARALLEL WAVEGUIDES OF INTERFEROMETER WITH COPLANAR
ELECTRODES. EDGES OF WAVEGUIDES ARE GENERALLY SMOOTH AND FREE
OF DEFECTS.
FIGURE S. - OUTPUT OF MODULATOR CLEAVED MIDWAY ALONG INTERFERO-
METER. EACH OF THE INTENSITY PEAKS LIES UNDER ONE OF THE
PARALLEL WAVEGUIDES.
fWNSA
National .Aeronautics and
Space Administration
Report Documentation Page
1. Report No.
NASA'TM-100970
2. Government Accession No. 3. Recipient's Catalog No.
4. Title and Subtitle
A High Frequency GaAlAs Travelling Wave Electro-Optic
Modulator at 0.82 \M
5. Report Date
1988
6. Performing Organization Code
7. Author(s)
Christopher M. Chorey, Altan Ferendeci, and
Kul Bhasin
8. Performing Organization Report No.
E-4261
9. Performing Organization Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135-3191
10. Work Unit No.
506-44-2C
11. Contract or Grant No.
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D.C. 20546-0001
13. Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
15. Supplementary Notes
Presented at the 1988 IEEE MTT-S International Symposium, New York, New York,
May 25-27, 1988. Christopher M. Chorey and Altan Ferendeci, Electrical
Engineering and Applied Physics Dept., Case Western Reserve University,
Cleveland, Ohio 44106; Kul B. Bhasin, NASA Lewis Research Center.
16. Abstract
Experimental GaAlAs modulators operating at 0.82 vim using a Mach-Zehnder
interferometer configuration were designed and fabricated. Coplanar 50 fl
travelling wave microwave electrodes were used to obtain a bandwidth-length
product of 11.95 GHz-cm. The design, fabrication and DC performance of the
GaAlAs travelling wave modulator is presented.
\o 6
t PC
IV t. --'0
I -.? C-V r
17. Key Words (Suggested by Author(s))
Gallium arsenide
Optical wave guide
GaAlAs modulator
18. Distribution Statement
Unclassified - Unlimited
Subject Category 33
19. Security Classif. (of this report)
Unclassi f ied
20. Security Classif. (of this page)
Unclassified
21. No of pages
8
22. Price'
A02
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A high frequency GaAlAs travelling wave electro-optic modulator at 0.82 micrometers

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