Download Indirect optical control of microwave circuits and antennas

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