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ECEG287 Optical Detection Course Notes Part 11: Coherent Detection

Learn about signals, noise, and balanced detection in coherent laser radar systems. Explore advantages, mixing equations, noise issues, and implementations of coherent detection techniques. Enhance SNR and dynamic range while resolving phase ambiguities.

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ECEG287 Optical Detection Course Notes Part 11: Coherent Detection

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  1. ECEG287 Optical Detection Course NotesPart 11: Coherent Detection Profs. Charles A. DiMarzio and Stephen W. McKnight Northeastern University, Spring 2004 Charles A. DiMarzio, Northeastern University

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

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

  4. Mixing Equations Charles A. DiMarzio, Northeastern University

  5. Coherent Detection Advantages mW nW fW • SNR Enhancement wrt Detector Noise • Dynamic Range Reduction • Phase Information (at least partial) Charles A. DiMarzio, Northeastern University

  6. Coherent Detection Model Signal Local Oscillator Noise: hnB Detector Noise Charles A. DiMarzio, Northeastern University

  7. Coherent Detection Noise Issues 10000 Signal 8000 4kTB 6000 4000 Quantum Noise 2000 0 0 20 40 60 80 100 P sig 4 x 10 6 Dotted Lines: Higher LO 5 4 3 2 1 0 0 20 40 60 80 100 P sig Charles A. DiMarzio, Northeastern University

  8. Balanced Mixing 4Re(sig*ref*) |sig-ref|2 Subtract =|sig|2+|ref|2- 2Re(sig*ref*) sig-ref sig+ref ref |sig+ref|2 =|sig|2+|ref|2+ 2Re(sig*ref*) sig Charles A. DiMarzio, Northeastern University Stopped Mon 23 Feb 04

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

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

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

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

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

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

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

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

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

  18. Optical Quadrature and Balanced Mixing To Cameras • 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. Objective Condenser LO SIG From Laser /4 Charles A. DiMarzio, Northeastern University

  19. 60 50 40 30 20 10 0 60 40 n=1.6 d=53 mm -10 20 0 -20 -40 -60 Optical Quadrature Microscope Unwraped Phase from Experiment, Radians . Charles A. DiMarzio, Northeastern University

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