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A switched-voltage technique is described which makes it possible to set the pass-band of an electrically tunable electro-optic birefringent filter over a considerable range. A model incorporating the above technique and using dihydrogen phosphate-type crystals is investigated. The filter scans the optical spectrum from 250 nm to 750 nm with a bandwidth ranging in value from 8 nm at the violet end to 48 nm at the red end. The transmission losses for light in the principal plane of polarization for the five-unit filter considered are less than 25%. The switched-voltage program is obtained from an analogue computer controlled by a linear wavelength-voltage ramp. It is therefore possible to select a centre wavelength by manual control or alternatively to time-control the filter.

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In this paper we describe a novel method of probing a sound field in three dimensions with a focused light beam. In our method, the spatial-temporal modulation impressed upon the light by the sound is extracted by the use of either polarization sensitive elements or spatial filters placed in front of a photodiode. The technique is inherently capable of high-resolution sampling because, as is shown, phase and amplitude of the resulting rf current are indicative of the complex sound pressure in a narrow region around the focus of the light beam. This is in contrast to earlier methods using a narrow collimated light beam which only give an indication of sound-pressure amplitude and phase integrated over the entire width of the sound field traversed by the light beam. The first part of the paper analyzes the mode of operation and discusses sensitivity and resolution in a heuristic manner. The second part provides experimental results for various frequencies and sound media. The third part treats the rigorous theory.