Phase Demodulation Method for Low SNR Laser Doppler Signals.

All Fiber Displacement Interferometer System for Any Reflector (AFDISAR) provides outstanding measurement accuracy and resolution for recording the dynamic damage evolution and fracture of materials in the shock wave and detonation physics research. Based on sine curve fitting, a new numerical phase demodulation method is presented for the low SNR AFDISAR Doppler signal while measuring low speed moving target. The phase difference between reference and measurement beam is determined by the displacement of the measurement target and changes the output current of the photodetector. Sine curve fitting method is introduced to modulate the phase from the differential frequency of Doppler signal. Traditional Hilbert transform and new sine curve fitting method are simulated separately for modulating the displacement from the low SNR laser Doppler signals with DC-bias offset. The simulation results show that the decoding accuracy of sine curve fitting method is better than the accuracy of Hilbert transform. When SNR downing to -5dB, the error of sine curve fitting is only 1°, while the value for Hilbert transform is as large as58.19°. The measurement results shows that sine curve fitting method has better robustness and tolerance to noise.