Transfer Characteristics and Frequency Response of Wide-Band Amorphous-Solid Debye-Sears Light Modulators

Abstract

The operation of fused-silica Debye-Sears light modulators is investigated for normal light incidence over a range of conditions such that the effect of amplitude modulation of the light beam by the acoustic wave, as well as phase modulation, must be taken into account. Using the approximations of Raman and Nath, for which the ultrasonic wave can be treated as a phase grating, the relationships between input electrical excitation and the resulting spatial modulation are determined for shear and compression waves. The effect of the length of the light path on the operation of the light modulator is then considered for a range of values for which Raman and Nath's simplified approach is no longer valid. From the results of Bhatia and Noble it is shown theoretically and verified experimentally that, for single frequency excitation, a value of this dimension can be specified such that the effective spatial modulation of the light beam can be maximized. The case of electrical excitation over a finite frequency band is also considered, and a simple expression describing the frequency response of the light modulator, in terms of the total spatial modulation of the light beam, is derived. Measurements of light-modulator frequency response under conditions such that the amplitude-modulation effects are not signficant are presented. These measurements are found to agree with theoretical predictions.