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