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Indirect optical control of microwave circuits and antennas Amit S. Nagra ECE Dept. University of California Santa Barbara Acknowledgements Ph.D. Committee MBE material Professor Robert York Prashant Chavarkar Professor Nadir Dagli ECE Dept. UCSB Professor Umesh Mishra ECE Dept. UCSB Dr. Michael VanBlaricum Toyon Research Corporation Goleta, CA 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 Photoconductive antennas • Illumination of bulk substrates Opaque Mask High Resistivity Substrate • Photogenerated plasma acts as radiating surface Illumination • Very versatile • High optical power requirement Photoconductive Antenna Opening in Mask 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 Optically Controlled Synaptic Elements Optical Fiber Conducting Branches RF input 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 Optical control schemes Direct control Bulk semiconductors Junction devices Indirect control Photovoltaic detectors Biased detectors Direct Optical Control Schemes Illumination Direct control of bulk semiconductor devices Ground Signal Ground High Resistivity Semiconductor Focussing Optics Source Direct control of junction devices Illumination Gate Channel Insulating Buffer/Substrate 2-5 m Drain Indirect Optical Control Schemes Bias Supply Indirect control using biased detectors Optical Control Input Bias Supply Gain / Level Shifting Microwave Circuit Electrical Control Input Reverse Biased Photodetector + Indirect control using photovoltaic detectors Optical Control Input Bias Signal _ Photovoltaic Array Microwave Device Comparison of Optical Control Schemes Control Technique Mechanism Optical Power External Requirements Bias Response Time Direct illumination of bulk semiconductors Photoconductive High 0.1-100 W Optional Limited by carrier lifetimes in substrate (s–ps) Direct illumination of junction devices Photovoltaic & Photoconductive Moderate 1-10 mW Required Photovoltaic (>100 ns) Photoconductive (50-100 ps) Indirect control using photovoltaic detectors Photovoltaic Low 0.1-1 mW Not required Limited by PV array junction capacitance (> 100 ns) Indirect control using biased detectors Photoconductive Low 0.1-1 mW Required Limited by optical modulation and detection speeds (> 10 ps) • 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 Varactor DC Load Photovoltaic Array Incident Light RF Block Resistor Microwave Circuit 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 Incompatibility of conventional GaAs PV cell and Schottky varactor P GaAs N- GaAs N+ GaAs Substrate Large Area N-Ohmic Contact Schottky Contact Active Region (3-5µm) Passivation Layer P-Contact Fingers Ohmic Contact N- GaAs N+ GaAs Semi-insulating GaAs Substrate Failure of mesa isolation under illumination Ohmic Contacts Next device Ohmic Contact Airbridge P-GaAs P-GaAs Next device N-GaAs N-GaAs Substrate Leakage Semi-insulating GaAs Solutions Developed planar PV cell that shares epitaxial layers with Schottky varactor Anti Reflection Coating P-Contact Fingers Passivation Layer Varactor layers P GaAs N- GaAs N-Ohmic Contact N+ GaAs Semi-insulating GaAs Substrate Lateral oxidation of buried AlGaAs layer for isolation Ohmic Contacts Next device Airbridge P-GaAs P-GaAs N-GaAs N-GaAs Oxidized AlGaAs Oxidized AlGaAs Semi-insulating GaAs Next device Combined Epitaxial Structure Al.85Ga.15As 500Å P+ GaAs (Na = 5 1018) 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 P- GaAs (Na = 5 1017) 6000Å N- GaAs (Nd = 2 1017) 7000Å N+ GaAs (Nd = 3 1018) 7000Å Al.98Ga.02As 500Å Semi-insulating GaAs Substrate Oxidized sample Semi-insulating GaAs Substrate 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 PV cell mesa Schottky diode mesa P- GaAs P- GaAs N- GaAs N- GaAs N+ GaAs N+ GaAs Oxidized AlGaAs (a) Mesa etch and lateral oxidation PV cell mesa P- GaAs Schottky diode mesa N- GaAs N- GaAs N+ GaAs N+ GaAs Oxidized AlGaAs (b) Expose top of Schottky mesa P- GaAs N-ohmic N-ohmic N- GaAs N+ GaAs N- GaAs N+ GaAs Oxidized AlGaAs (c) Self aligned N-ohmic contacts Fabrication of the Monolithic OVC Schottky contact P- GaAs N-ohmic N-ohmic N- GaAs N+ GaAs N- GaAs N+ GaAs Oxidized AlGaAs (d) Schottky contact P-ohmic Schottky contact P- GaAs N-ohmic N-ohmic N- GaAs N+ GaAs N- GaAs N+ GaAs (e) P-ohmic contacts AR coating P-ohmic P- GaAs N-ohmic N- GaAs N+ GaAs Schottky contact NiCr Resistor N-ohmic N- GaAs N+ GaAs (f) AR coating and NiCr resistors Fabrication of the Monolithic OVC AR coating P-ohmic P- GaAs N-ohmic Resistor pads N- GaAs N+ GaAs Schottky contact N-ohmic N- GaAs N+ GaAs CPW (g) CPW metal and resistor pads AR coating Air Bridges P- GaAs N- GaAs N+ GaAs N- GaAs N+ GaAs (h) Air bridge interconnections CPW Monolithic OVC Fabricated at UCSB PV array Schottky Varactor 10-Cell GaAs PV Array RF block resistor Varactor Monolithic OVC 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 External Load RF Block Resistor Airbridge Measurement Setup Fiber CPW probe Stage OVC wafer • 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 0 0 P =310 W P =310 W opt opt -40 -40 P =1.3 mW -80 Current (A) Current (A) opt P = 2.7 mW opt -120 P =1.3 mW opt -80 P = 2.7 mW opt -120 P = 5.1 mW P = 5.1 mW -160 opt opt 0 2 -160 4 6 8 10 0 Voltage (V) 2 4 6 Voltage (V) Sample Open circuit voltage Fill Factor Conversion efficiency Oxidized 10.5 V 0.84 26.8% Control 9.95 V 0.44 13.3% 8 10 Measured PV Array Performance 12 Meaesured Output Voltage (V) Open Circuit voltage (V) 10.5 10 Oxidized Sample 9.5 9 Control Sample 8.5 8 7.5 -10 -5 0 5 Optical Power (dBm) Load=500 k 10 8 Load=100 k 6 4 ______ Oxidized Sample - - - - - - Control Sample 2 0 0 1 2 3 4 5 Optical Power (mW) 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 0.9 Modeled 0.8 Capacitance (pF) Extracted from s-parameters 0.7 0.6 0.5 0.4 0.3 0.2 0 0.05 0.1 0.15 0.2 0.25 Optical Power (mW) • 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 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 RF input Zo=80 40° @ 5GHz Monolithic OVC Picture of monolithically fabricated circuit RF output OVC RF output Resonator RF input C0=0.85 pF Optically Tunable Band Reject Filter Simulated Measured 0 Insertion Loss (dB) Insertion Loss (dB) 0 -5 -10 Popt= 0 W Popt= 450 W -15 -5 -10 Popt= 450 W Popt= 0 W -15 Popt= 70 W Popt= 70 W -20 -20 0 2 4 6 8 Frequency (GHz) 10 0 2 4 6 8 10 Frequency (GHz) • 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 Photovoltaic Array RF block resistor RF input C0=0.28 pF Zo=76 37.3° @ 12 GHz RF output Schottky Varactor 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 PV array RF input RF output Varactors • 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 Simulated 250 Differential Phase Shift (Degrees) Differential Phase Shift (Degrees) Measured P =0 W opt 200 P =70 W opt 150 P =450 W opt 100 50 0 -50 0 2 4 6 8 10 Frequency (GHz) 12 14 250 P =0 W opt 200 P =70 W opt P =450 W 150 opt 100 50 0 -50 0 2 4 6 8 10 12 Frequency (GHz) • 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 14 Insertion Loss and Return Loss as a Function of Optical Power Simulated 0 0 -0.5 -0.5 -1 -1 Insertion Loss (dB) Insertion Loss (dB) Insertion Loss Measured -1.5 -2 -2.5 P -3 P P -3.5 =0 W opt -1.5 -2 -2.5 P -3 =70 W P opt =450 W P -3.5 opt = 0 W opt = 70 W opt = 450 W opt -4 -4 0 2 4 6 8 10 12 0 14 2 4 10 12 14 0 0 -10 Return Loss (dB) -10 Return Loss (dB) 8 Frequency (GHz) Frequency (GHz) Return Loss 6 -20 -30 -20 -30 P P = 0 W opt -40 P = 70 W -40 P opt P P = 450 W opt 2 4 6 8 10 12 14 0 2 4 6 8 Frequency (GHz) Frequency (GHz) =70 W opt =450 W opt -50 -50 0 = 0 W opt 10 12 14 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 0 OVC Folded Slot Antenna Return Loss (dB) -5 -10 -15 P opt = 0 W P opt = 450 W -20 P -25 10 Optically tunable antenna fabricated at UCSB 12 14 opt = 70 W 16 18 Frequency (GHz) • 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 20 Characterization of the Transient Response of the Monolithic OVC Pulse Generator Digitizing Oscilloscope Laser Driver Semiconductor Laser Diode Active Probes Output Voltage Modulated Light DUT • 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 10 10 Output Voltage (V) 12 Output Voltage (V) 12 8 6 4 C = 1.3 pF 0 2 8 6 4 P =900 W, load=330 k opt 2 C = 0.6 pF P =600 W, load=1 M opt 0 0 0 4 0 8 12 0 16 2 4 Time (s) 6 8 10 12 14 16 Time (s) Rise time Rload Cvaractor (v) tr Ceff V Carray (v) Cvaractor (v) Carray (v) Iphoto Simplified models t f Rload Ceff Isc Fall time Characterization of the Transient Response of the Monolithic OVC Zero bias capacitance DC load resistance Rise time Fall time 1.4 pF 1 M 290 ns 4.1 s 0.7 pF 1 M 270 ns 2.3 s 0.7 pF 330 k 290 ns 780 ns 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