SYNTHETIC-ARRAY HETERODYNE-DETECTION - A SINGLE-ELEMENT DETECTOR ACTSAS AN ARRAY

A simple technique, synthetic-array heterodyne detection, permits an o rdinary single-element optical detector to behave as though it were a coherent array. A successful experimental implementation of a syntheti c two-pixel array, using a CO2 laser and a single-element HgCdTe photo diode is reported. A different heterodyne local oscillator frequency i s incident upon each resolvable region of the detector surface. Thus d ifferent regions are mapped to different heterodyne beat frequencies. One can determine where the photons struck the detector surface even t hough a single electrical connection to the detector is used. This als o prevents the destructive interference that occurs when multiple spec kles are imaged (akin to spatial diversity). In coherent lidar this pe rmits a larger field of view. An acousto-optic modulator produces the local oscillator frequencies and can achieve good spatial separation o f optical frequencies of the order of a megahertz apart.