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