Indirect optical control of microwave circuits and antennas

1. Indirect optical control of microwave circuits and antennas Amit S. Nagra ECE Dept. University of California Santa Barbara

2. Acknowledgements Ph.D. Committee Professor Robert York Professor Nadir Dagli Professor Umesh Mishra ECE Dept. UCSB Dr. Michael VanBlaricum Toyon Research Corporation Goleta, CA MBE material Prashant Chavarkar ECE Dept. UCSB AlGaAs OxidationJeff Yen Primit Parikh Varactor loaded linesProfessor Rodwell ECE Dept. UCSB

3. Motivation for Optical Control • Advantages • Low loss distribution of control signals over optical fibers • Optical fibers and optical sources have high bandwidths  optical control attractive where high speed is required • Optical fibers are light and compact  weight and volume savings crucial for airborne and space applications • Optical fibers are immune to EMI  attractive for secure control (military applications) • Extremely high isolation between microwave circuit and control circuit • Optical fibers are non-invasive (do not significantly perturb fields in the vicinity of radiating structures)  ideal for control of antennas • Optical fiber links have been deployed in several antennas for distribution of the microwave signal (information to be radiated)  control signal can be distributed over same link

4. High ResistivitySubstrate Opaque Mask Illumination PhotoconductiveAntenna Opening in Mask Applications of Optical Control • Functions / Applications • Optical control of amplifiers, switches,phase shifters, filters  remote control of microwave antennas and circuits • Optical reference signal distribution, optical injection locking of microwave oscillators  beam scanning arrays, power combining arrays • Optical control of antennas  reconfigurable and frequency agile antennas • Photoconductive antennas • Illumination of bulk substrates • Photogenerated plasma acts as radiating surface • Very versatile • High optical power requirement

5. Optically Controlled Synaptic Elements Optical Fiber Conducting Branches RF input Applications of Optical Control • Optically reconfigurable synaptic antenna • Conductive grid with optically controlled synaptic elements (switches/reactive loads) • Current path / current amplitude phase on sections of grid can be varied optically • Efficient use of optical power • Elements must not require DC bias

6. Optical control schemes Direct control Indirect control Bulk semiconductors Junction devices Photovoltaicdetectors Biaseddetectors Introduction to Optical Control Schemes • Desirable properties in an optical control scheme for microwave circuits and antennas • Low optical power consumption • Bias free operation for antenna applications • Sensitive to light in the 600 nm to 700 nm range where cheap sources are available • Ease of coupling light into device being controlled • No RF performance penalties for using optical control

7. Focussing Optics Illumination Illumination Source Drain Gate Channel Ground Signal Ground Insulating Buffer/Substrate 2-5 m High Resistivity Semiconductor Direct Optical Control Schemes Direct control of bulk semiconductor devices Direct control of junction devices

8. Bias Supply Bias Supply MicrowaveDevice OpticalControlInput Gain / Level Shifting Microwave Circuit PhotovoltaicArray ElectricalControlInput Reverse Biased Photodetector + BiasSignal _ OpticalControlInput Indirect Optical Control Schemes Indirect control using biased detectors Indirect control using photovoltaic detectors

9. Comparison of Optical Control Schemes • Photovoltaic control is a bias free technique that requires low optical power • Most suitable for optical control of microwave circuits and antennas

10. Photovoltaic Control using the OVC • Key features of the Optically Variable Capacitor (OVC) • PV array controls reverse bias voltage across a varactor diode • Varactor junction capacitance can be controlled optically • No external bias required • RF block resistor keeps PV array out of microwave signal path • DC load resistor improves transient response and enables better voltage control

11. Photovoltaic Control using the OVC • Advantages of the OVC • Reverse biased varactor dissipates very little power  optical power required for control is small • Optical and microwave functions performed in separate devices that can be independently optimized • Varactor diode designed to produce desired capacitance swing with lowest possible RF insertion loss • PV array designed to generate desired output voltage range using the smallest optical power • Hybrid OVC • Commercially available PV arrays used to control discrete varactor diode • Hybrid version of OVC demonstrated in tunable loop antenna at 800 MHz • Large PV array requires beam shape/ expanding optics • Transient speed limited by PV array junction capacitance

12. Monolithic OVC • Motivation for the monolithic OVC • Small size OVC required for high frequency circuits/antennas • Miniature PV array matched to fiber spot size for ease of optical coupling • Small connection parasitics extends the range of usable frequencies and capacitance values • Monolithic OVC has faster transient response due to smaller PV array capacitance • Components for the monolithic OVC • High Q-factor varactor diode with a minimum 2:1 capacitance tuning range • Miniature PV array capable of generating greater than 7 V • RF blocking resistor > 1 K to act as broadband open circuit

13. Key Design issues for the Monolithic OVC • Choice of material system • GaAs has several desirable properties for the monolithic OVC • semi-insulating substrate, high-Q varactors, compatible with MMICs, well developed photovoltaic technology • Choice of device technology and integration techniques • Schottky diodes on n-type GaAs as varactors • high cut-off frequency, planar design, easily integrated with circuits • GaAs PN homojunction diodes for PV array • high open circuit voltages, efficient optical absorption in band of interest, good conversion efficiency • Airbridge interconnection scheme • low connection parasitics, can be used with small features

14. SchottkyContact OhmicContact OhmicContact N- GaAs PassivationLayer P-Contact Fingers N+ GaAs Semi-insulating GaAs Substrate P GaAs N- GaAs Active Region (3-5µm) N+ GaAs Substrate Large Area N-Ohmic Contact Key Challenges for the Miniature PV arrays Incompatibility of conventional GaAs PV cell and Schottky varactor Failure of mesa isolation under illumination

15. P-Contact Fingers Anti ReflectionCoating PassivationLayer P GaAs N-OhmicContact Varactor layers N- GaAs N+ GaAs Semi-insulating GaAs Substrate Solutions Developed planar PV cell that shares epitaxial layers with Schottky varactor Lateral oxidation of buried AlGaAs layer for isolation

16. Al.85Ga.15As 500Å P+ GaAs (Na = 5 1018) 500Å P- GaAs (Na = 5 1017) 6000Å N- GaAs (Nd = 2 1017) 7000Å N+ GaAs (Nd = 3 1018) 7000Å Al.98Ga.02As 500Å P+ GaAs (Na = 5 1018) 500Å Semi-insulating GaAs Substrate P- GaAs (Na = 5 1017) 6000Å N- GaAs (Nd = 2 1017) 7000Å N+ GaAs (Nd = 3 1018) 7000Å Semi-insulating GaAs Substrate Combined Epitaxial Structure Control sample Oxidized sample • Layout of the miniature PV array • Circular array with pie shaped cells for effective optical absorption • Contacts on periphery to minimize blockage • Fabricated using oxidized and control epitaxial layers shown above

17. N- GaAs N+ GaAs N+ GaAs N+ GaAs N- GaAs N+ GaAs P- GaAs N- GaAs N+ GaAs N+ GaAs P- GaAs N- GaAs P- GaAs PV cell mesa Schottky diode mesa Oxidized AlGaAs PV cell mesa Schottky diode mesa Oxidized AlGaAs P- GaAs N- GaAs N- GaAs N-ohmic N-ohmic Oxidized AlGaAs Fabrication of the Monolithic OVC (a) Mesa etch and lateral oxidation (b) Expose top of Schottky mesa (c) Self aligned N-ohmic contacts

18. N+ GaAs N+ GaAs N+ GaAs N+ GaAs N+ GaAs N+ GaAs Schottkycontact N-ohmic N-ohmic Oxidized AlGaAs P-ohmic Schottkycontact N-ohmic N-ohmic P- GaAs P- GaAs P- GaAs N- GaAs N- GaAs N- GaAs N- GaAs N- GaAs N- GaAs AR coating P-ohmic Schottkycontact NiCr Resistor N-ohmic N-ohmic Fabrication of the Monolithic OVC (d) Schottky contact (e) P-ohmic contacts (f) AR coating and NiCr resistors

19. N+ GaAs N+ GaAs N+ GaAs N+ GaAs AR coating P-ohmic Schottkycontact Resistor pads N-ohmic N-ohmic CPW P- GaAs P- GaAs N- GaAs N- GaAs N- GaAs N- GaAs Air Bridges AR coating CPW Fabrication of the Monolithic OVC (g) CPW metal and resistor pads (h) Air bridge interconnections

20. PV array Airbridge RF blockresistor Varactor Monolithic OVC Fabricated at UCSB • Salient features • 10 cell GaAs PV-array, Schottky varactor diode, RF blocking resistor, CPW pads integrated on same wafer • DC load provided by measurement setup or wire bonded using chip resistor

21. Measurement Setup • Light from 670 nm semiconductor laser diode coupled into 200 m core diameter multi-mode fiber • Fiber positioned over OVC with fiber probe mounted on XYZ stage • DC I-V measurements on a semiconductor parameter analyzer • RF measurements using CPW on wafer probes attached to a vector network analyzer

22. Measured PV array Performance Control Oxidized

23. Measured PV Array Performance • Summary • Substrate leakage reduces output voltage, fill factor and efficiency of array • Buried oxide effective in eliminating substrate leakage • Array with oxide has higher open circuit voltage, fill factor, efficiency and can drive load impedances more effectively • DC load helps linearize the array response

24. Microwave Characterization of the Monolithic OVC • S-parameter data recorded for different illumination intensities • Converted to equivalent capacitance by fitting to series R-C model • Capacitance tuning from 0.85 pF to 0.38 pF • Only 200 W of optical power required for full tuning range (under 1 M  external DC load)

25. Optically Tunable Band Reject Filter • Circuit schematic • Single shunt resonator loaded with the monolithic OVC for tuning • At resonance, circuit presents short circuit circuit causing signal to be reflected • By varying the capacitive loading, resonant frequency can be adjusted Picture of monolithically fabricated circuit

26. Optically Tunable Band Reject Filter Simulated Measured • Rejection frequency tunable from 3.8 GHz to 5.2 GHz (31% tuning range) • No external bias required • Maximum optical power of 450 W for full tuning range (lowest reported) • Greater than 15 dB of rejection- better rejection possible by using multiple resonator sections

27. Optically Controlled X-band Analog Phase Shifter Circuit Schematic • Basic Principle • Varactor loaded line behaves like synthetic transmission line with modified capacitance per unit length • Phase velocity on the synthetic line is a function of varactor capacitance • By varying the bias, phase delay for a given length of line can be varied

28. Optically Controlled X-band Analog Phase Shifter Optically controlled phase shifter fabricated at UCSB • CPW line periodically loaded with shunt varactor diodes connected in parallel to preserve circuit symmetry • All the varactors require identical bias • Single PV array controls several varactor diodes simultaneously

29. Phase Shift as a Function of Optical Power Measured Simulated • Differential phase shift increases linearly with frequency (attractive for wide band radar) • Maximum differential phase shift of 175 degrees at 12 GHz using just 450 W of optical power

30. Insertion Loss and Return Loss as a Function of Optical Power Measured Simulated Insertion Loss Return Loss

31. Optically Controlled X-band Analog Phase Shifter • Summary of phase shifter performance • Bias free control • Only 450 W of optical power needed (lowest reported) • Maximum differential phase shift of 175 degrees at 12 GHz with insertion loss less than 2.5 dB • Return loss lower than -12 dB over all phase states • Best loss performance for an optically controlled phase shifter • Loss performance comparable to the state of the art electronic phase shifters • Demonstrates potential of varactor loaded transmission lines for linear applications • Further work needs to be done to study ways to improve the design of varactor loaded lines for even better performance

32. Optical Impedance Tuning of a Folded Slot Antenna Optically tunable antenna fabricated at UCSB • Resonant folded slot antenna on GaAs (half wavelength long at 18 GHz) • Resonant frequency shifted down to 14.5 GHz due to capacitive loading (OVC) • Tuning of match frequency from 14.5 to 16 GHz using just 450 W of optical power • Lowest reported power requirement for bias free optical control of antennas

33. PulseGenerator DigitizingOscilloscope LaserDriver Semiconductor Laser Diode Active Probes OutputVoltage ModulatedLight DUT Characterization of the Transient Response of the Monolithic OVC • Intensity modulated light (square wave) used as input to the OVC • Rise and fall times of optical signal ~ 200 ns (limited by driver circuit) • OVC output voltage used as measure of response speed • OVC voltage measured using active probes (1 MegaOhm, 0.1 pF) to prevent loading

34. (v) (v) (v) (v) load photo array array varactor varactor R I C C C C Characterization of the Transient Response of the Monolithic OVC Measured data Simplified models Rise time Fall time

35. Characterization of the Transient Response of the Monolithic OVC • Summary of transient response characterization • Rise time limited primarily by measurement setup - unable to verify scaling laws - circuit response faster than 300 ns • Fall time scales with DC load and total OVC capacitance • Miniature PV array with small junction capacitance responsible for improved switching response compared to hybrid OVC • Possible to obtain switching times faster than 1 microsecond

36. Conclusions • Monolithic OVC effort • Identified suitable technology for the bias free control of microwave circuits and antennas • Developed components for the monolithic OVC and successfully integrated them on wafer • Incorporated the monolithic OVC in microwave circuits and antennas • Demonstrated bias free optical control using lowest reported optical power