Indirect optical control of microwave circuits and antennas

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Indirect optical control of microwave circuits
and antennas

• Amit S. Nagra
• ECE Dept.
• University of California Santa Barbara

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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
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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

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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

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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

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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

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Direct Optical Control Schemes
Direct control of bulk semiconductor devices
Direct control of junction devices
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Indirect Optical Control Schemes
Indirect control using biased detectors
Indirect control using photovoltaic detectors
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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

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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

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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

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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 21
  capacitance tuning range
• Miniature PV array capable of generating greater
  than 7 V
• RF blocking resistor gt 1 K? to act as broadband
  open circuit

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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

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Key Challenges for the Miniature PV arrays
Incompatibility of conventional GaAs PV cell and
Schottky varactor
Failure of mesa isolation under illumination
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Solutions
Developed planar PV cell that shares epitaxial
layers with Schottky varactor
Lateral oxidation of buried AlGaAs layer for
isolation
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Combined Epitaxial Structure
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

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Fabrication of the Monolithic OVC
(a) Mesa etch and lateral oxidation
(b) Expose top of Schottky mesa
(c) Self aligned N-ohmic contacts
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Fabrication of the Monolithic OVC
(d) Schottky contact
(e) P-ohmic contacts
(f) AR coating and NiCr resistors
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Fabrication of the Monolithic OVC
(g) CPW metal and resistor pads
(h) Air bridge interconnections
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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

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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

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Measured PV array Performance
Control
Oxidized
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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

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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)

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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
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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

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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

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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

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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

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

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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

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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

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Characterization of the Transient Response of the
Monolithic OVC
Measured data
Simplified models
Rise time
Fall time
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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

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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