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

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 Oxidation Jeff Yen Primit Parikh Varactor loaded lines Professor Rodwell ECE Dept. UCSB

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

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 Illumination High Resistivity Substrate Photoconductive Antenna Opaque Mask Opening in Mask Photoconductive antennas Illumination of bulk substrates Photogenerated plasma acts as radiating surface Very versatile High optical power requirement

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 Conducting Branches Optically Controlled Synaptic Elements Optical Fiber RF input

Introduction to Optical Control Schemes Indirect control Photovoltaic detectors Biased detectors Direct control Bulk semiconductors Junction devices 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

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

Indirect Optical Control Schemes Bias Supply Gain / Level Shifting Microwave Circuit Reverse Biased Photodetector Bias Supply Electrical Control Input Optical Control Input Photovoltaic Array Microwave Device Bias Signal + _ Optical Control Input Indirect control using biased detectors Indirect control using photovoltaic detectors

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

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

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

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

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

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

Solutions N - GaAs P GaAs Semi-insulating GaAs Substrate P-Contact Fingers Anti Reflection Coating Passivation Layer N + GaAs N-Ohmic Contact Varactor layers Lateral oxidation of buried AlGaAs layer for isolation Developed planar PV cell that shares epitaxial layers with Schottky varactor

Combined Epitaxial Structure N + GaAs (N d = ) 7000Å Semi-insulating GaAs Substrate N - GaAs (N d = ) 7000Å P - GaAs (N a = ) 6000Å P + GaAs (N a = ) 500Å N + GaAs (N d = ) 7000Å Semi-insulating GaAs Substrate N - GaAs (N d = ) 7000Å P - GaAs (N a = ) 6000Å P + GaAs (N a = ) 500Å Al.98 Ga.02 As 500Å Al.85 Ga.15 As 500Å Oxidized sample Control 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

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

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

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

Monolithic OVC Fabricated at UCSB Varactor PV array RF block resistor Airbridge 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

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

Measured PV array Performance Control Oxidized

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

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)

Optically Tunable Band Reject Filter Picture of monolithically fabricated circuit 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

Optically Tunable Band Reject Filter Measured Simulated 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

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

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

Phase Shift as a Function of Optical Power 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 MeasuredSimulated

Insertion Loss and Return Loss as a Function of Optical Power MeasuredSimulated Return Loss Insertion Loss

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

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

Characterization of the Transient Response of the Monolithic OVC Pulse Generator Laser Driver Semiconductor Laser Diode DUT Digitizing Oscilloscope Active Probes Modulated Light Output Voltage 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

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

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

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