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NASA Technical Memorandum 101328 

High Frequency GaAlAs Modulator 

and Photodetector for Phased Array 

Antenna Applications 

(bASA-TM- 10 1328) hXGk FRECUkhCY GdAlAS N8 8-3 CCJd 

kLDilLBflCH ALC YkiC5CLEIECTCE FCE ERASED AfifiAY 

6lcIEhNA BIPZICAlICNE (NASA) 1; F CSCL C9C Unclas 

G3/33 0165556 

P.C. Claspy, C.M. Chorey, and S.M. Hill 

Case Western Reserve University 

Cleveland, Ohio 

and 

K.B. Bhasin 

Lewis Research Center 

Cleveland, Ohio 

Prepared for the 

International Symposium and Exhibition on Fiber Optics, 

Optoelectronics, and Laser Applications 

sponsored by the Society of Photo-Optical Instrumentation Engineers 

Boston, Massachusetts, September 6-9, 1988 
HIGH FReQUENCY GaAlAs MODULATOR AND PHOTODETECTOR FOR PHASED ARRAY ANTENNA APPLICATIONS 

P.C. Claspy, C.M. Chorey, and S.M. Hill 

Cleveland, Ohio 44106 

Case Western Reserve University, Department of Electrical Engineering and Applied Physics 

and 

K.B. Bhasin 

National Aeronautics and Space Administration, Lewis Research Center, 

Cleveland, Ohio 44135 

ABSTRACT 

A waveguide Mach-Zehnder electro-optic modulator and an interdigitated photoconductive 

detector designed to operate at 820 nm, fabricated on different GaAlAs/GaAs heterostructure 

materials, are being investigated for use in optical interconnects in phased array antenna 

systems. Measured optical attenuation effects in the modulator are discussed and the 

observed modulation performance up to 1 GHz is presented. Measurements of detector frequency response are described and results of these measurements are presented. 

1. INTRODUCTION AND BACKGROUND 

Considerable attention has been devoted over the past few years to the use of monolithic 

microwave integrated circuits (MMIC's) in phased array antenna systems. Although phased 

arrays consisting of hundreds of elements are becoming a reality, questions regarding practical interconnection methods for information and control signal distribution to these elements remain. 1#2 Waveguide, coaxial cable, and microstrip transmission lines suffer from 

various combinations of high weight, mechanical inflexibility, and high loss, whereas fiber 

optic interconnects have the potential to overcome these limitations if various technological issues can be successfully addressed. Among these issues are the efficient high speed 

modulation of the output of semiconductor lasers, and the efficient detection and demodulation of that light at the array element. To enhance the usefulness of modulators and 

detectors in the proposed MMIC application they should be chip-level integrable with the 

GaAlAs/GaAs heterostructure devices, such as high electron mobility transistors (HEMT's), 

that have been demonstrated to have a significant speed advantage over MESFET-type devices 

in MMIC's. 

High speed modulation of the output of a diode laser can be achieved either by direct modulation of the driving current or by use of an external device such as an electro-optic modulator. 

modulated lasers with modulation bandwidths in the 10 to 20 GHz range are commercially available at both 820 nm and 1.33 pm.4 Various authors have also reported external electrooptic modulators based on Li~bO3~ and on GaA1As/GaAs.6 

level integrable with semiconductor lasers using current technology, and most of the 

research on GaAlAs/GaAs waveguide modulators has been at 1.33 pm. 

Schottky hotodiode that is useful to 100 GHz,~ and GaAlAs/GaAs-based detectors have been 

re~0rted.g.~ 

material and device geometry on the performance of the latter detector type had been 

reported . 

In this paper we discuss the results obtained to date on the performance of a triple heterostructure GaAlAs traveling wave electro-optic modulator and of a series of interdigitated 

photoconductive detectors fabricated on a HEMT-type heterostructure material. An objective 

of our current research is to investigate detectors and external modulators that operate at 

820 nm and which are potentially easily integrated with HEMT-type MMIC's. The modulator is 

a waveguide Mach-Zehnder device that is driven by a 50 R coplanar electrode structure. 

Detailed discussions of the design and fabrications of the modulator have been presented 

previously,10-12 so in this paper we address primarily the observed performance and an 

analysis of those results. The detectors, on the other hand, have not been described previously, so the discussion will encompass their design and fabrication as well as their performance. In Section 2, the configuration and performance of the modulator is discussed, 

Section 3 describes the design and fabrication of the detectors, and Section 4 presents an 

Significant successes using the former approach have been rep~rted,~ and directly 

The LiNbO3 devices are not chip 

Numerous authors have also described high speed GaAs-based optical detectors, including a 

At the onset of our research, however, no systematic study of the effects of 

1 
overview of the observed detector performance. Finally, in Section 5 the results will be 

summarized and concluding remarks will be made. 

2. MODULATOR CHARACTERISTICS AND PERFORMANCE 

The electro-optic modulator, shown schematically in Fig. 1, consists of two features, 

namely, the strip-loaded, single-mode optical waveguide Mach-Zehnder interferometer, and 

the coplanar waveguide electrodes. As shown in Fig. 2, the three layers of the MBE-grown 

material are GaAlAs, with the concentrations of A1 chosen to shift the absorption edge to 

shorter wavelengths while still providing adequate guiding. The upper layer was etched to 

form the loading ridge, and the optical signal propagates in the middle layer. This configuration concentrates the light energy away from the etched edges of the loading ridge to 

reduce the light scattering. To prevent additional losses the first layer above the semiinsulating GaAs was chosen to be thick enough that the evanescent optical field does not 

extend to the substrate. A coplanar traveling wave electrode structure was chosen to provide the microwave field in the required direction. It was designed for 50 R impedance on 

semi-insulating GaAs and it was assumed, since there is no doping in any layer and the A1 

fraction is small, that the design remains valid when implemented on GaAlAs. The calculated 

half-wave voltage is 18 V and the 3 dB bandwidth-length product, which is determined by the 

phase shift between the light and the microwave drive signal, was calculated to be 12 GHz 

for 100 percent intensity modulation. 

mental arrangement shown in Fig. 3. Electrical bias was supplied through a biasing tee and 

the drive signal was the amplified output of a HP 8350B Sweep Oscillator. Light from the 

"pigtailed" single-mode output fiber of an Ortel Model SL-620s diode laser was butt coupled 

to the modulator input and the output from the modulator was focused into a single-mode 

fiber. The modulated light was detected by a Newport Model 877 avalanche photodiode or by 

an Ortel Model RSL-25 photodiode. The input fiber was aligned to the waveguide by imaging 

the modulator output surface onto an IR-sensitive camera connected to a video monitor. When 

alignment was correct a single intensity peak, with little scattered light, was observed. 

DC measurements yielded a half-wave voltage of approximately 30 V, and the modulation 

bandwidth is of the order of 500 MHz, as shown in Fig. 4. Because both of these results 

differ significantly from the design values, further study has been undertaken. Since no 

increase in transmitted intensity is observed for bias voltage greater than 30 V and no 

polarization effects are observed at the output, it appears that the dominant effect in the 

modulation is electro-absorption (Franz-Keldish effect) rather than the electro-optic 

effect.13 Further evidence of this is that the absorption losses in the modulator, with no 

applied voltage, are much larger than expected. While this may explain the large extinction 

voltage, the observed frequency response characteristics remain unexplained at present. 

The DC and high frequency characteristics of the modulator were measured using the experi3. DETECTOR GEOMETRY ANTI FABRICATION 

An interdigitated electrode geometry with 1 pm dimensions (Figure 5) was chosen for the 

detectors to ensure short transit distances for photon-generated carriers while also maximizing the active area. Although this geometry results in a higher intrinsic capacitance 

than that for a straight-gap device, it is not enough to significantly affect device 

performance. 

cally in Fig. 6. Standard photolithography and wet etching techniques were used for the 

process, and the evaporated AuGe/Ni/Au ohmic contacts were flash annealed in a 700 OC furnace. 

The detectors were fabricated on the HEMT-type, MBE-grown material that is shown schematiAs a final processing step the 400-8, cap layer of ntGaAIAs was removed. 

4. DETECTOR PERFORMANCE 

The detectors were mounted on an alumina substrate on which a 50 R gold microstrip transmission line had been deposited, as shown schematically in Fig. 7. Gold wire-bonded connections were made between the detectors and the transmission line, and the output was through 

a SMA end launcher that was in pressure contact with the line. 

DC I-V curves were obtained using a curve tracer while the detectors were illuminated by 

white light through a microscope. A typical example of an I-V curve at two different illumination levels, shown in Fig. 8, demonstrates that the detectors are symmetric devices and 

that the contacts are ohmic. 

Frequency response measurements were made over the range 0.01 to 10 GHz using the experimental arrangement shown schematically in Fig. 9. The detectors were dc biased through a 

biasing tee and a 50 R load was used to impedance match the detector output to the measurement system. Modulated light was from a directly modulated, fiber-pigtailed Ortel Model 

2 
TSL-1000 diode laser, and the rf output signal from the detector was amplified and measured 

using an HP 8566A spectrum analyzer. 

A typical example of the measured frequency response of a detector is given in Fig. 10. 

As shown in the figure, the 3 dB cutoff frequency is at approximately 185 MHz, with the 

response falling at about 12 dB/decade from 185 MHz to near 1 GHz. Beyond 1 GHz the 

response falls at the 20 dB/decade rate expected of a single-pole device. Preliminary measurements on devices with larger geometry (up to 4 pm electrode spacing) that were fabricated 

on the same material show a similar response, with only the overall amplitude being higher. 

These results suggest that device speed is limited by minority carrier trapping rather than 

by geometry, by majority carrier mobility, or by saturation ve10city.l~ 

Measurements have also been made of the responsivity at 500 MHz using an Ortel Model 

SL-620 diode laser as a source. A typical result of the measurement of detector current as 

a function of average incident optical power is shown in Fig. 11. The responsivity at 

500 MHz, measured as the slope of this curve in its most linear region, is 0.21 A/W. Since 

responsivity has the same frequency dependence as gain,15 the low frequency responsivity of 

this device is 2.5 A/W, giving an external quantum efficiency of 3.81, which is comparable 

to that found in GaAs detectors. 

5. SUMMARY AND CONCLUSIONS 

The performance of a waveguide Mach-Zehnder electro-optic modulator and of a set of interdigitated photoconductive detectors is being studied at frequencies to 10 GHz. Although 

modulation of light has been observed up to about 500 MHz, neither the half-wave voltage nor 

the modulation bandwidth are as predicted by a theoretical model. The device exhibits a 

higher than predicted absorption at the 820 nm design wavelength and the observed modulation 

appears to be from electro-absorption rather than from the expected electro-optic effect. A 

spectroscopic study is presently underway in an effort to understand the observed behavior. 

The observed performance of the detectors is comparable to that of GaAs-based photoconductive devices of similar geometry. While they did not exhibit an extremely high bandwidth, 

they did possess usable gain well into the GHz range. The bandwidth appears to be limited 

by minority carrier trapping, and a study is presently underway to evaluate this effect. 

6. ACKNOWLEDGEMENT 

This work was conducted under a cooperative research agreement between NASA Lewis Research 

Center and Case Western Reserve University (NCC3-99). During part of the period of the 

research, Prof. P.C. Claspy was a participant in the NASA/ASEE Summer Faculty Fellowship 

Program. Portions of the work on the detectors are from a thesis submitted to Case Western 

Reserve University by S.M. Hill in partial fulfillment of the requirements for the MS degree 

in Electrical Engineering. 

7. REFERENCES 

1. K.B. Bhasin, G. Anzic, R.R. Kunath, and D.J. Connolly, "Optical Techniques to Feed and 

Control GaAs MMIC Modules for Phased Array Antenna Applications," 11th AIAA Communications 

Satellite Systems Conference, AIM, New York, 506-514 (1986). 

2. K.B. Bhasin and D.J. Connolly, "Advances in Gallium Arsenide Monolithic Microwave 

Integrated-Circuit Technology for Space Communications Systems," IEEE Trans. Microwave 

Theory Tech., 34(10), 994-1001 (1986). 

3. K.Y. Lau, and A. Yariv, "Ultra-High Speed Semiconductor Lasers," IEEE J. Quantum Electron., 21(2), 121-138 (1985). 

4. For example, Ortel Corp., Alhambra, CA. 

5. C.M. Gee and G.D. Thurmond, "Wideband Traveling-Wave Electro-optic Modulator," in 

6. P. Buchmann, H. Kaufman, H. Melchior, and G. Guekos, "Broadband Y-Branch Electro-optic 

7. S.Y. Wang and D.M. Bloom, "100 GHz Bandwidth Planar GaAs Schottky Photodiode," Elec8. C.Y. Chen, A.Y. Cho, C.G. Bethea, P.A. Garbinski, Y.M. Pang, B.F. Levine, and 

9. T. Umeda, Y. Cho, and A. Shibatomi, "Picosecond HEMT Photodetector," Japan. J. Appl. 

10. D.M. Materna, "Investigation of a GaAlAs Mach-Zehnder Electro-optic Modulator," M.S. 

Thesis, Case-Western Reserve University, Cleveland, OH, 1986. (NASA CR-179573). 

11. D.M. Materna, A.M. Ferendeci, and K.B. Bhasin, "Design of a GaAlAs Traveling Wave 

Mach-Zehnder Electro-optic Modulator," in Optoelectronic Materials, Devices, Packaqinq, and 

Interconnects, T.E. Batchman, R.F. Carson, R.L. Gallawa, and H.J. Wojtunik, eds., Proc. 

Optical Technoloqy for Microwave Applications, S.K. Yao, ed., Proc. SPIE Vol. 477, 17-22 

(1984). 

GaAs Waveguide Interferometer for 1.3 pn," Appl. Phys. Lett., 46(5), 462-464 (1985). 

tron. Lett., 19(14), 554-555 (1983). 

K. Ogawa, "Ultrahigh Speed Modulation-Doped Heterostructure Field-Effect Photodetectors," 

Appl. Phys. Lett., 42(12), 1040-1042 (1983). 

Phys. Lett., 25(10), L801-L803 (1986). 

SPIE Vol. 836, 195-198 (1987). 

3 
12. C.M. Chorey, A. Ferendeci, and K. Bhasin, "A High Frequency GaAlAs Traveling Wave 

Electro-Optic Modulator at 0.82 pm," in IEEE MTT-S International Microwave Symposium Digest, 

Vol. 2, IEEE, Piscataway, NJ, 735-738 (1988). 

Appl. Phys. Lett., 22(8), 372-374 (1973). 

Coaxial Semiconductor Detectors," IEEE Trans. Electron Devices, 29(9), 1431-1441 (1982). 

and Deep Traps Effect," IEEE Trans. Electron Devices, 33(3), 373-378 (1986). 

13. F.K. Reinhart, "Electroabsorption in AlyGa~-yAs-A1xGa~-xAs Double Heterostructures," 

14. H. Beneking, "On the Response Behavior of Fast Photoconductive Optical Planar and 

15. G.J. Papaioannou and J.R. Forrest, "On the Photoresponse of GaAs MESFET's: Backgating 

k 

INTENSITY 

MODULATED 

LIGHT OUT 

TRANSMISSION 

LINE ELECTRODES 7 

-0.82 

FIG. 1. SCHEMATIC OF MACH-ZEHNDER ELECTROOPTIC MODULATOR. 

0.12 pM7 

f Ga(0.87)AI (0.13)As 

Ga(0.9>Al(O. 1)As 

3.5 pM Ga(0.87)AI (0.13)As 

FIG. 2. ELECTROOPTIC MODULATOR MATERIAL SCHEMATIC. 

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FIG. 5. 1 PM x 1 PM DETECTOR. 

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6 
SCHEMATIC OF HEMT DETECTOR 

FIG. 6. DETECTOR HATER I AL STRUCTURE. 

CENTER 

'm 

FIG . 7. DETECTOR MOUNT CONFIGURATION. 

7 
FIG. 8. I-V CHARACTERISTIC OF 

1 VM x 1 p~ DETECTOR AT THREE 

DIFFERENT I LLUM I NATION LEVELS. 

8 
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FREQUENCY, GHz 

FIG, 10. FREQUENCY RESPONSE OF 1 pM X 1 pM 

DETECTOR. 

10 
NASA National Aeronautics and Report Documentation Page 

Space Administration 

1. Report No. I 2. Government Accession No. 

17. Key Words (Suggested by Author@)) 

Galium arsenide 

GaAlAs modulator 

HEMT photodetector 

Phased array antenna 

NASA TM-101328 

4. Title and Subtitle 

High Frequency GaAlAs Modulator and Photodetector for 

Phased Array Antenna Applications 

18. Distribution Statement 

Unclassified - Unlimited 

Subject Category 33 

I 

19. Security Classif. (of this report) 20. Security Classif. (of this page) 

Unclassified Unclassified 

7. Author@) 

P.C. Claspy, C.M. Chorey, S.M. Hill, and K.B. Bhasin 

21. No of pages 22. Price’ 

12 A02 

9. Performing Organization Name and Address 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135-3191 

12. Sponsoring Agency Name and Address 

National Aeronautics and Space Administration 

Washington, D.C. 20546-0001 

3. Recipient’s Catalog No. 

5. Report Date 

6. Performing Organization Code 

8. Performing Organization Report No. 

E-4339 

10. Work Unit No. 

506-44-2 1 

11. Contract or Grant No. 

13. Type of Report and Period Covered 

Technical Memorandum 

14. Sponsoring Agency Code 

15. Supplementary Notes 

Prepared for the International Symposium and Exhibition on Fiber Optics, Optoelectronics, and Laser Applications 

sponsored by the Society of Photo-Optical Instrumentation Engineers, Boston, Massachusetts, September 6-9, 1988. 

P.C. Claspy, C.M. Chorey, and S.M. Hill, Case Western Reserve University, Dept. of Electrical Engineering 

and Applied Physics, Cleveland, Ohio 44106; K.B. Bhasin, NASA Lewis Research Center. 

16. Abstract 

A waveguide Mach-Zehnder electro-optic modulator and an interdigitated photoconductive detector designed to 

operate at 820 nm, fabricated on different GaAlAs/GaAs heterostructure materials, are being investigated for use 

in optical interconnects in phased array antenna systems. Measured optical attenuation effects in the modulator are 

discussed and the observed modulation performance up to 1 GHz is presented. Measurements of detector 

frequency response are described and results of these measurements are presented. 

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