All-phase FFT based distance measurement in laser self-mixing interferometry

摘要

As is widely known in the field of laser self-mixing interferometry (SMI), a commercial laser diode (LD) under linear injection current modulation can serve as a low-cost laser rangefinder, whose self-mixed signal exhibits a beat frequency proportional to the distance to be measured. So exact frequency extraction becomes a significant issue in this measurement scheme. In order to obtain better frequency and then better distance estimation performance, a new frequency extraction technique based on all-phase FFT (apFFT) is proposed to deal with SMI signals to suppress the influence of spectrum leakage and signal noise. Related mathematical derivation shows the advantages of the apFFT, then the simulation analysis indicates that the relative frequency errors can be evaluated to be lower than 10(-3) even under the signal-to-noise ratio (SNR) of 5 dB. While in the experiments, the corresponding distance resolution is about 30 mu m in the range examined. Additionally, the discussions of several influencing factors are included. The method proposed is effective in detecting target distance and can provide an interesting and attractive technical route for SMI distance measurement.