Heterodyne detection of laser radiation

The principles of heterodyne detection are summarized. Theoretical and experimental aspects concerning infrared heterodyne detection near 10 um are discussed with emphasis on (1) the dependence of signal-to-noise ratio (SNR) phase front assignment of the beams and on spot diameters; (2) the antenna profile of a heterodyne receiver; and (3) phase front distortion of spherical wave fronts due to transmission through an oblique beam splitter. CO2-laser radiation was shifted in frequency by means of an acousto-optical modulator, in order to be used as a local oscillator in a heterodyne test set-up. An infrared CW-Doppler radar test set-up is described which allows detection of targets at distances up to several kilometers and with velocities between 1 and 200 km/h.