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Light valves based on the ultrasonic-diffraction effect are capable of producing high-index intensity modulation of a light beam, but they are subject to a fundamental bandwith limitation associated with the finite time of transit of an ultrasonic wave across the light beam. It is shown theoretically that modulation bandwidth can be maximized by converging the incident light beam to a focus to minimize beam diameter and by confining the ultrasonic beam to the vicinity of the focal plane. Under properly chosen conditions, modulation bandwidth can be as large as 1/7.7 times the ultrasonic carrier frequency. An experimental investigation was carried out using water as the ultrasonic medium to verify the theoretical dependence of modulation bandwidth on ultrasonic- and optical-beam parameters and to evaluate constants of the theory. The experiments also showed that the wavefronts near the focus of a convergent light beam are sufficiently plane to produce Bragg-type diffraction when conditions are otherwise favorable. Bragg diffraction was used in the modulation bandwidth measurements. The dependence of diffracted-beam intensity on ultrasonic amplitude was investigated experimentally and, in accordance with theory, it was found that nearly all of the incident (zero-order) light can be transferred to a selected first-order diffracted beam. This fact permits either the zero-order or a first-order beam to be chosen as the useful output of the modulator.
Hance et al. (1965) studied this question.