A cw CO2 laser rangefinder using heterodyne detection and on-off amplitude modulation

A CO₂ laser rangefinder has been built that uses an acousto-optic modulator to obtain an amplitude modulated optical beam from a cw laser. An on-off deflected beam was obtained by driving the modulator with a carrier modulated by a periodic binary code. The received infra-red radiation was heterodyne detected. After envelope detection of the heterodyne signal, the signal-to-noise ratio was improved by applying a digital integrator. Ranging was carried out by calculating the delay of the received code by means of a cross-correlation technique. For this purpose a minicomputer was implemented in the system. Using 1 W average beam power and measuring times less than 1 s, ranges of several kilometers have been determined from stationary targets.