February 2004 Charles A. DiMarzio, Northeastern University 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 Overview Coherent Detection Fundamentals Signals and Noise Balanced Detection Resolving Ambiguities –Heterodyne –Offset Homodyne –Optical Quadrature

February 2004 Charles A. DiMarzio, Northeastern University Coherent Laser Radar Laser /4 /2 Optional Pulsed Amplifier

February 2004 Charles A. DiMarzio, Northeastern University Mixing Equations

February 2004 Charles A. DiMarzio, Northeastern University Coherent Detection Advantages SNR Enhancement wrt Detector Noise Dynamic Range Reduction Phase Information (at least partial) fW mWnW

February 2004 Charles A. DiMarzio, Northeastern University Coherent Detection Model Detector Noise Signal Local Oscillator Noise: h B

February 2004 Charles A. DiMarzio, Northeastern University Coherent Detection Noise Issues P sig Signal 4kTB Quantum Noise x 10 4 P sig Dotted Lines: Higher LO

February 2004 Charles A. DiMarzio, Northeastern University Balanced Mixing sig ref sig+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 Balanced Mixing Implementation sig ref sig+ref sig-ref

February 2004 Charles A. DiMarzio, Northeastern University Another Implementation sig ref sig+ref sig-ref

February 2004 Charles A. DiMarzio, Northeastern University Direction (Phase) Ambiguity 0 f Transmitter LO Signal (Postive Doppler) Transmitter LO Signal (Postive Doppler) (Negative Doppler) (Negative Doppler)

February 2004 Charles A. DiMarzio, Northeastern University Offset Local Oscillator 0 f Transmitter LO Signal (Postive Doppler) Transmitter LO Signal (Postive Doppler)

February 2004 Charles A. DiMarzio, Northeastern University Offset LO Implementation (1) Laser /4 /2 Optional Pulsed Amplifier Laser

February 2004 Charles A. DiMarzio, Northeastern University Offset LO Implementation (2) Laser /4 /2 Optional Pulsed Amplifier Bragg Cell

February 2004 Charles A. DiMarzio, Northeastern University A Very Simple Laser Radar Laser Current Monitor i t

February 2004 Charles A. DiMarzio, Northeastern University Quadrature Demodulation Signal LO (Ref) I 90 Deg Q z=I+iQ

February 2004 Charles A. DiMarzio, Northeastern University Optical Quadrature Demodulation (End View of Polarization) Reference (Circular Polarization) Signal (Linear Polarization)

February 2004 Charles A. DiMarzio, Northeastern University Optical Quadrature and Balanced Mixing Based on Mach-Zehdner interferometer. Signal Arm: 45-Degree Linear Polarization (E x =E y ) Local Oscillator: Circular Polarization (E x =iE y ) Both Interferometer Outputs used for balanced detection (common mode rejection) Light source is low power HeNe or diode laser. LO SIG From Laser To Cameras /4 Objective Condenser

February 2004 Charles A. DiMarzio, Northeastern University Optical Quadrature Microscope Unwraped Phase from Experiment, Radians n=1.6 d=53  m