Effect of linewidth enhancement factor on Doppler beat waveform obtained from a self-mixing laser diode

The asymmetry and fringe inclination of the sawtooth-like waveform observed in the self-mixing signal obtained from a semiconductor laser with optical feedback enable the direction discrimination of line-of-sight motion of ex ternal target surfaces. We investigated the effects of linewidth enhancemen t factor and feedback strength on the asymmetry and the direction of fringe inclination of the signal by numerical simulations. For the first time, we show that direction of the fringe inclination is determined not only by th e direction of the target motion but also by the value of the linewidth enh ancement factor.