Coherent photodetection with the aid of a gas laser

An analysis is made of the operation of a gas laser both as a source of radiation and as a detector of a reflected signal characterized by a Doppler frequency shift. It is shown that coherent photodetection occurs in the active medium of the laser. The Lamb theory of a laser is used to obtain expressions for the conversion efficiency and the width of the frequency band. Measurements are reported of the responsivity of a helium-neon laser used as a detector: its value was 1 MV/W and the width of the frequency band was about 100 kHz.