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Millimeter-wave GaAs Schottky barrier diodes were used as fast light detectors. It was possible to observe beat signals between a HeNe laser oscillating at 633 nm and a cw dye laser with approximately 80-GHz frequency offset. Thus, great frequency differences between lasers emitting in the visible can be determined with the high accuracy inherent in heterodyning techniques.
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Burghardt et al. (1979) studied this question.
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