Optical Heterodyning with Noncritical Angular Alignment Using a Pair of Ultrasonic Light Modulators

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Abstract
Dual diffraction of a coherent light beam passing through a pair of ultrasonic light modulators (ULM's) are studied analytically for optical heterodyning on the basis of geometrical optics. Since every collinearly diffracted light beam experiences double frequency shifts through the ULM's, a pair of such beams can produce a beating signal having a frequency equal to the difference between their frequencies. Some factors which influence the phase stability of the beating signal as well as the spatial requirement for coincidence between the two wave-fronts of photomixed light are clarified. A numerical calculation using a pair of the first order diffracted light beams of 6328 Å predicts that the inclination tolerance of the second ULM is about ±3° for the fixed first ULM, when the first and second sound frequencies are 8 and 7 MHz respectively, and the receiving area of a detector is 1 cm in width.

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