Distance measurement with harmonic modulation of self-mixing laser wavelength at external optical feedback

Background and Objectives: Self-mixing interferometry of absolute distances is currently well represented by the method of frequency modulation of the laser diode supply current (FMCW). In recent years a harmonic modulation of the power current of a self-mixing laser began to be used. The advantage of the harmonic modulation method is due to the absence of the need to adjust the deviation of the radiation wavelength when changing the distance to the reflector. In the laser systems, the level of external optical feedback is an important parameter during interference measurements. The feedback level also affects the type of the self-mixing signal generated by modulation of the wavelength of laser radiation. The aim of this work was to study the effect of the feedback level on the accuracy of absolute distance measurements at the harmonic frequency modulation of laser radiation. Materials and Methods: The article presents a method of measuring distances using a semiconductor laser with a harmonic modulation of the radiation wavelength. The technique allows taking into account the influence of the feedback level to improve the accuracy of the measurements. In particular, it is proposed to reduce the feedback level in order to eliminate mode jumps and reduce the frequency shift of the laser diode radiation caused by a change in the concentration of charge carriers in the active region. Results: The results of the influence of the feedback level on the shape of the self-mixing signal and on the accuracy of distance measurements have been described. Conclusion: Evaluation of the feedback level can be carried out on the basis of the method of decomposition of the self-mixing signal in the Fourier and Bessel series and analysis of sets of spectral harmonics. The feedback level is determined using the values of the RMS deviation calculated from sets of spectral harmonics of the self-mixing signal.