Operation of Ultrasonic Light Modulators under Conditions of Random Electrical Excitation

Abstract

Excitation of an ultrasonic light modulator by random electrical signals will cause a random modulation to be imposed on the emerging light wavefront. A general solution for the resulting light diffraction patterns is obtained, from which the conditions are determined for which the diffracted light-intensity distribution will be a valid representation of the power spectral density of the input electrical noise signal. The necessary experimental conditions for which the variance of the spectral measurements will be small are determined. The percentage of the light-modulator bandwidth over which measurements will be valid is found to be dependent on the product of the spatial-frequency bandwidth of the light modulator and the length of the optical aperture; this effect is evaluated and numerical results are presented. The case in which the input to the ultrasonic light modulator consists of a deterministic signal and noise is also considered. Using these results, the diffracted light levels due to signal and noise are obtained as a function of the input signal and noise power levels.