Described is FM modulation distortion in the light output of a semiconductor laser frequency-modulated with a triangular wave current. The variation of instantaneous FM modulation efficiency is caused by both the thermal effect of FM modulation and the optical feedback from a focusing lens.

The self-mixing type laser Doppler velocimter gives a simple method of directional velocimetry. The beat signal comprises the fundamental Doppler frequency as well as its harmonics, so that the waveform is like a sawtooth in which the sign of inclination reverses as the target changes the direction of its movement.