February 2004 Charles A. DiMarzio, Northeastern University 10464-11-1
ECEG287 Optical Detection Course Notes
Part 11: Coherent Detection
Profs. Charles A. DiMarzio
and
Stephen W. McKnight
Northeastern University, Spring 2004
February 2004 Charles A. DiMarzio, Northeastern University 10464-11-2
Overview
• Coherent Detection Fundamentals
• Signals and Noise
• Balanced Detection
• Resolving Ambiguities– Heterodyne– Offset Homodyne– Optical Quadrature
February 2004 Charles A. DiMarzio, Northeastern University 10464-11-3
Coherent Laser Radar
.2V
f
Laser
/4
/2
Optional Pulsed
Amplifier
February 2004 Charles A. DiMarzio, Northeastern University 10464-11-4
Mixing Equations
losig EEE 22
losig EEE
*2
losiglosig EEEEE
losiglosiglosig EEEEEEE **222
*Re2222
losiglosig EEEEE
2
iφiφ
losiglosig
eePPPPP
February 2004 Charles A. DiMarzio, Northeastern University 10464-11-5
Coherent Detection Advantages
2
iφiφ
losiglosig
eePPPPP
• SNR Enhancement wrt Detector Noise• Dynamic Range Reduction• Phase Information (at least partial)
fW mW nW
February 2004 Charles A. DiMarzio, Northeastern University 10464-11-6
Coherent Detection Model
Detector Noise
Signal
Local OscillatorNoise: hB
February 2004 Charles A. DiMarzio, Northeastern University 10464-11-7
Coherent Detection Noise Issues
0 20 40 60 80 1000
2000
4000
6000
8000
10000
Psig
Signal4kTB
QuantumNoise
0 20 40 60 80 1000
1
2
3
4
5
6x 10
4
Psig
Dotted Lines:Higher LO
February 2004 Charles A. DiMarzio, Northeastern University 10464-11-8
Balanced Mixing
sig
refsig+ref
sig-ref
Subtract|sig-ref|2
|sig+ref|2
=|sig|2+|ref|2-2Re(sig*ref*)
=|sig|2+|ref|2+2Re(sig*ref*)
4Re(sig*ref*)
Stopped Mon 23 Feb 04
February 2004 Charles A. DiMarzio, Northeastern University 10464-11-9
Balanced Mixing Implementation
sig
refsig+ref
sig-ref
February 2004 Charles A. DiMarzio, Northeastern University 10464-11-10
Another Implementation
sig
refsig+ref
sig-ref
February 2004 Charles A. DiMarzio, Northeastern University 10464-11-11
Direction (Phase) Ambiguity
0f
Transmitter
LO Signal(PostiveDoppler)
Transmitter
LOSignal(PostiveDoppler)
(NegativeDoppler)
(NegativeDoppler)
February 2004 Charles A. DiMarzio, Northeastern University 10464-11-12
Offset Local Oscillator
0f
Transmitter
LO Signal(PostiveDoppler)
Transmitter
LOSignal(PostiveDoppler)
February 2004 Charles A. DiMarzio, Northeastern University 10464-11-13
Offset LO Implementation (1)
.2V
f
Laser
/4
/2
Optional Pulsed
Amplifier
Laser
February 2004 Charles A. DiMarzio, Northeastern University 10464-11-14
Offset LO Implementation (2)
.2V
f
Laser
/4
/2
Optional Pulsed
Amplifier
Bragg Cell
February 2004 Charles A. DiMarzio, Northeastern University 10464-11-15
A Very Simple Laser Radar
.2V
f
Laser
Current Monitor
i
t
February 2004 Charles A. DiMarzio, Northeastern University 10464-11-16
Quadrature Demodulation
Signal titi eez
LO (Ref)
I
titi eec 90 Deg
Q
z=I+iQ
iees titi /
February 2004 Charles A. DiMarzio, Northeastern University 10464-11-17
Optical Quadrature Demodulation
tcos
tsin
(End View ofPolarization)
tcos
tcos
Reference(CircularPolarization) Signal (Linear
Polarization)
February 2004 Charles A. DiMarzio, Northeastern University 10464-11-18
Optical Quadrature and Balanced Mixing
• Based on Mach-Zehdner interferometer.
• Signal Arm: 45-Degree Linear Polarization (Ex=Ey)
• Local Oscillator: Circular Polarization (Ex=iEy)
• Both Interferometer Outputs used for balanced detection (common mode rejection)
• Light source is low power HeNe or diode laser.
LO SIGFromLaser
ToCameras
/4
Objective
Condenser
E ESIG LO
E ESIG LO
February 2004 Charles A. DiMarzio, Northeastern University 10464-11-19
Optical Quadrature Microscope
.
-10
0
10
20
30
40
50
60
Unwraped Phasefrom Experiment, Radians
-60-40-200204060
n=1.6d=53 m
