Theory of thick and thin reference gratings for enhanced detection of acoustic waves

Abstract

Acoustic waves propagating in elastic media are used for many practical devices such as acoustooptic modulators, beam deflectors, and switches. The generation and detection of acoustic waves is also important in the study and characterization of materials. Typically, acoustic waves produce small effective refractive-index variations and can, thus, be difficult to detect by simple optical probing. It has been shown that use of stationary reference gratings can greatly facilitate the detection of these waves. In this paper, the application of thick (Bragg diffraction regime, i.e., two diffracted waves) and thin (Raman-Nath diffraction regime, i.e., multiple diffracted waves) sinusoidal phase reference gratings is considered for enhanced detection of acoustic waves. The coupled-wave theory is used to analyze the thick-grating case, while the transmittance approach is used for the thin-grating case. Simple, analytical expressions are obtained that describe the diffraction characteristics in each case. It is shown that thick reference gratings produce a purely sinusoidal temporal light-intensity modulation while thin gratings, in general, produce nonsinusoidal modulation.