Indirect optical control of microwave circuits and antennas Amit S. Nagra ECE Dept. University of...
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Transcript of Indirect optical control of microwave circuits and antennas Amit S. Nagra ECE Dept. University of...
Indirect optical control of microwave circuits and antennas
Amit S. NagraECE 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 ResistivitySubstrate
PhotoconductiveAntenna
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
Photovoltaicdetectors
Biaseddetectors
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 GroundSignal
Source DrainGate
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
ElectricalControl
Input
OpticalControl
Input
PhotovoltaicArray
MicrowaveDevice
BiasSignal
+
_
OpticalControl
Input
Indirect control using biased detectors
Indirect control using photovoltaic detectors
Comparison of Optical Control Schemes
Control Technique Mechanism Optical PowerRequirements
ExternalBias
Response Time
Direct illumination ofbulk semiconductors
Photoconductive High 0.1-100 W
Optional Limited by carrierlifetimes in substrate(s–ps)
Direct illumination ofjunction devices
Photovoltaic &Photoconductive
Moderate 1-10 mW
Required Photovoltaic(>100 ns)Photoconductive(50-100 ps)
Indirect control usingphotovoltaic detectors
Photovoltaic Low 0.1-1 mW
Notrequired
Limited by PV arrayjunction capacitance(> 100 ns)
Indirect control usingbiased detectors
Photoconductive Low 0.1-1 mW
Required Limited by opticalmodulation anddetection 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
RF BlockResistor
MicrowaveCircuit
Ph
oto
volt
aic
Ar r
ay
DC
Lo
ad
Var
act o
rInc
ide
nt
Lig
ht
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
Semi-insulating GaAs
N-GaAs
P-GaAs
Airbridge
P-GaAs
N-GaAs
Ohmic Contacts
Substrate Leakage
Nextdevice
Nextdevice
Failure of mesa isolation under illumination
Incompatibility of conventional GaAs PV cell and Schottky varactor
PassivationLayer
N- GaAs
P GaAs
N+ GaAs Substrate
P-Contact Fingers
Large Area N-Ohmic Contact Ac
tiv
e R
eg
ion
(3
-5µ
m)
N+ GaAs
Semi-insulating GaAs Substrate
SchottkyContact
N- GaAs
OhmicContact
OhmicContact
Solutions
N- GaAs
P GaAs
Semi-insulating GaAs Substrate
P-Contact FingersAnti ReflectionCoating
PassivationLayer
N+ GaAs
N-OhmicContact
Va
rac
tor
la
ye
rs
Semi-insulating GaAs
N-GaAs
P-GaAs Nextdevice
Airbridge
P-GaAs
N-GaAs
Ohmic Contacts
Oxidized AlGaAs Oxidized AlGaAs
Nextdevice
Lateral oxidation of buried AlGaAs layer for isolation
Developed planar PV cell that shares epitaxial layers with Schottky varactor
Combined Epitaxial Structure
N+ GaAs (Nd = 3 1018) 7000Å
Semi-insulating GaAs Substrate
N- GaAs (Nd = 2 1017) 7000Å
P- GaAs (Na = 5 1017) 6000Å
P+ GaAs (Na = 5 1018) 500Å
N+ GaAs (Nd = 3 1018) 7000Å
Semi-insulating GaAs Substrate
N- GaAs (Nd = 2 1017) 7000Å
P- GaAs (Na = 5 1017) 6000Å
P+ GaAs (Na = 5 1018) 500Å
Al.98Ga.02As 500Å
Al.85Ga.15As 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-ohmicN-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-ohmicN-ohmic
Schottkycontact
(d) Schottky contact
N+ GaAs
N- GaAs
P- GaAs
N+ GaAs
N- GaAsN-ohmicN-ohmic
Schottkycontact
P-ohmic
N+ GaAs
N- GaAs
P- GaAs
N+ GaAs
N- GaAsN-ohmicN-ohmic
Schottkycontact
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- GaAsN-ohmicN-ohmic
Schottkycontact
P-ohmicAR coatingResistor
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 blockresistor
Airbridge RF BlockResistor
10-C
ell G
aAs
PV
Arr
ay
Ext
erna
lL
oad
Sch
ottk
yV
a ra c
t or
MonolithicOVC
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
Stage OVC wafer
CPW probe Fiber
• 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
-160
-120
-80
-40
0
0 2 4 6 8 10
Cu
rren
t (
A)
Voltage (V)
Popt
= 5.1 mW
Popt
= 2.7 mW
Popt
=1.3 mW
Popt
=310 W
-160
-120
-80
-40
0
0 2 4 6 8 10
Cu
rren
t (
A)
Voltage (V)
Popt
= 5.1 mW
Popt
= 2.7 mW
Popt
=1.3 mW
Popt
=310 W
Sample Open circuitvoltage
Fill Factor Conversionefficiency
Oxidized 10.5 V 0.84 26.8%
Control 9.95 V 0.44 13.3%
Control Oxidized
Measured PV Array Performance
0
2
4
6
8
10
12
0 1 2 3 4 5
Mea
esu
red
Ou
tpu
t V
olt
age
(V)
Optical Power (mW)
______ Oxidized Sample- - - - - - Control Sample
Load=100 k
Load=500 k
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
7.5
8
8.5
9
9.5
10
10.5
-10 -5 0 5
Op
en C
ircu
it v
olt
age
(V)
Optical Power (dBm)
OxidizedSample
ControlSample
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)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 0.05 0.1 0.15 0.2 0.25
Modeled
Extracted from s-parameters
Cap
acit
ance
(p
F)
Optical Power (mW)
Optically Tunable Band Reject Filter
Resonator
RFinput
OVC
RFoutput
Picture of monolithically fabricated circuit
RFinput
MonolithicOVC
Zo=80 40° @ 5GHz
RFoutput
C0=0.85 pF
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
-20
-15
-10
-5
0
0 2 4 6 8 10
Inse
rtio
n L
oss
(d
B)
Frequency (GHz)
Popt= 0 W Popt= 450 W
Popt= 70 W
-20
-15
-10
-5
0
0 2 4 6 8 10
Inse
rtio
n L
oss
(d
B)
Frequency (GHz)
Popt= 0 W Popt= 450 W
Popt= 70 W
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
RFinput
Zo=76 37.3° @ 12 GHz
RFoutput
SchottkyVaractor
PhotovoltaicArray
RF blockresistor
C0=0.28 pF
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
PV array
Varactors
RF input RF output
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
-50
0
50
100
150
200
250
0 2 4 6 8 10 12 14
Popt
=0 W
Popt
=70 W
Popt
=450 W
Dif
fere
nti
al P
has
e S
hif
t (D
egre
es)
Frequency (GHz)
-50
0
50
100
150
200
250
0 2 4 6 8 10 12 14
Popt
=0 W
Popt
=70 W
Popt
=450 W
Dif
fere
nti
al P
has
e S
hif
t (D
egre
es)
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
Measured Simulated
Insertion Loss and Return Loss as a Function of Optical Power
-4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0 2 4 6 8 10 12 14
Popt
= 0 W
Popt
= 70 W
Popt
= 450 W
Ins
ert
ion
Lo
ss
(d
B)
Frequency (GHz)
-4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0 2 4 6 8 10 12 14
Popt
=0 W
Popt
=70 W
Popt
=450 W
Ins
ert
ion
Lo
ss
(d
B)
Frequency (GHz)
0 2 4 6 8 10 12 14
Popt
= 0 W
Popt
= 70 W
Popt
= 450 W
-50
-40
-30
-20
-10
0
Re
turn
Lo
ss
(d
B)
Frequency (GHz)
-50
-40
-30
-20
-10
0
0 2 4 6 8 10 12 14
Popt
= 0 W
Popt
=70 W
Popt
=450 W
Re
turn
Lo
ss
(d
B)
Frequency (GHz)
Measured Simulated
Ret
urn
Los
sIn
sert
ion
Los
s
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
OVC
Folded Slot Antenna
-25
-20
-15
-10
-5
0
10 12 14 16 18 20
Ret
urn
Lo
ss (
dB
)
Frequency (GHz)
Popt
= 0 W Popt
= 450 W
Popt
= 70 W
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
PulseGenerator
LaserDriver
SemiconductorLaser Diode
DUT
DigitizingOscilloscope
ActiveProbes
ModulatedLight
OutputVoltage
• 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
0
2
4
6
8
10
12
0 4 8 12 16
C0= 1.3 pF
C0= 0.6 pF
Ou
tpu
t V
olt
age
(V)
Time (s)
0
2
4
6
8
10
12
0 2 4 6 8 10 12 14 16
Popt
=900 W, load=330 k
Popt
=600 W, load=1 M
Ou
tpu
t V
olt
age
(V)
Time (s)
Measured data
Simplified models
I pho
to
C arr
ay(v
)
C var
acto
r(v)
R loa
d
C arr
ay(v
)
C var
acto
r(v)
tC V
Ireff
sc
t R Cf load eff
Rise time Fall time
Characterization of the Transient Response of the Monolithic OVC
Zero biascapacitance
DC loadresistance
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