ECEG 287 Optical Detection Course Notes Part 11
- Slides: 19
ECEG 287 Optical Detection Course Notes Part 11: Coherent Detection Profs. Charles A. Di. Marzio and Stephen W. Mc. Knight Northeastern University, Spring 2004 February 2004 Charles A. Di. Marzio, Northeastern University 1
Overview • • Coherent Detection Fundamentals Signals and Noise Balanced Detection Resolving Ambiguities – Heterodyne – Offset Homodyne – Optical Quadrature February 2004 Charles A. Di. Marzio, Northeastern University 2
Coherent Laser Radar Optional Pulsed Amplifier /4 Laser /2 February 2004 Charles A. Di. Marzio, Northeastern University 3
Mixing Equations February 2004 Charles A. Di. Marzio, Northeastern University 4
Coherent Detection Advantages f. W m. W n. W • SNR Enhancement wrt Detector Noise • Dynamic Range Reduction • Phase Information (at least partial) February 2004 Charles A. Di. Marzio, Northeastern University 5
Coherent Detection Model Signal Local Oscillator Noise: hn. B Detector Noise February 2004 Charles A. Di. Marzio, Northeastern University 6
Coherent Detection Noise Issues 10000 Signal 8000 4 k. TB 6000 4000 2000 0 0 6 x 10 20 40 4 Psig 60 Quantum Noise 80 100 Dotted Lines: Higher LO 5 4 3 2 1 0 0 20 40 60 P February 2004 Charles A. Di. Marzio, Northeastern University sig 80 100 7
Balanced Mixing 4 Re(sig*ref*) |sig-ref|2 sig-ref February 2004 Subtract =|sig|2+|ref|22 Re(sig*ref*) sig+ref |sig+ref|2 =|sig|2+|ref|2+ 2 Re(sig*ref*) sig Charles A. Di. Marzio, Northeastern University 8 Stopped Mon 23 Feb 04
Balanced Mixing Implementation sig-ref February 2004 sig+ref sig Charles A. Di. Marzio, Northeastern University 9
Another Implementation sig-ref February 2004 sig+ref sig Charles A. Di. Marzio, Northeastern University 10
Direction (Phase) Ambiguity Transmitter Signal (Postive Doppler) Transmitter LO LO Signal (Negative Doppler) (Postive Doppler) Charles A. Di. Marzio, Northeastern University 11 (Negative Doppler) 0 February 2004 f
Offset Local Oscillator Transmitter Signal Transmitter LO LO (Postive Doppler) Signal (Postive Doppler) 0 February 2004 Charles A. Di. Marzio, Northeastern University 12 f
Offset LO Implementation (1) Optional Pulsed Amplifier /4 Laser /2 Laser February 2004 Charles A. Di. Marzio, Northeastern University 13
Offset LO Implementation (2) Optional Pulsed Amplifier /4 Laser /2 February 2004 Bragg Cell Charles A. Di. Marzio, Northeastern University 14
A Very Simple Laser Radar Laser Current Monitor February 2004 i Charles A. Di. Marzio, Northeastern University t 15
Quadrature Demodulation I Signal LO (Ref) 90 Deg z=I+i. Q Q February 2004 Charles A. Di. Marzio, Northeastern University 16
Optical Quadrature Demodulation (End View of Polarization) Reference (Circular Polarization) February 2004 Signal (Linear Polarization) Charles A. Di. Marzio, Northeastern University 17
Optical Quadrature and Balanced Mixing • Based on Mach-Zehdner interferometer. • Signal Arm: 45 -Degree Linear Polarization (Ex=Ey) • Local Oscillator: Circular Polarization (Ex=i. Ey) • Both Interferometer Outputs used for balanced detection (common mode rejection) • Light source is low power He. Ne or diode laser. February 2004 To Cameras Objective From Laser SIG LO Charles A. Di. Marzio, Northeastern University Condenser /4 18
Optical Quadrature Microscope Unwraped Phase from Experiment, Radians 60 50 40 30 20 10 0 60 40 20 0 -20 -40 -60 Charles A. Di. Marzio, Northeastern University n=1. 6 d=53 mm February 2004 -10 . 19
