Determination of Laser Beam Divergence Using the Acousto-Optic Interaction with Surface Acoustic Waves

Abstract

Laser probing of surface acoustic waves (SAW) using proximity reference gratings was found to be sensitive to the divergence of the probing laser beam. The probing sensitivity was observed to fall off with increased separation between the reference grating and the SAW substrate due to beam divergence 11). In this paper a brief theoretical analysis using generalized plane waves has been extended to divergent Gaussian beams. The sensitivity to the probing laser beam divergence was found to increase for higher frequency SAW. Experimental measurements were made using y-cut LINb03 for z-propagation at frequencies of 17.4 MHz and 87.2 MHz corresponding to SAW wavelengths of 200 pm and 40 pm respectively. The essential features of the analysis were confirmed for the TEMoo mode of a HeNe laser for beam divergence from 300 p rad to 20 m rad.