The acousto-optical interaction of narrow laser beams under Raman-Nath conditions

摘要

The influence of the beam width of a Gaussian laser beam on its acousto-optical diffraction by a travelling sound wave under Raman-Nath conditions (Klein-Cook parameter Q 1) has been studied. We develop a new general analytical formula that has the merit of being valid for arbitrary bundle widths. If the beam width is smaller than the ultrasonic wavelength, ray bending, focusing/defocusing effects and propagational diffraction inside the sound column are shown to appear and are fully quantitatively described. As a consequence the intensity at the exit plane and in the far field become time dependent. When the bundle width becomes higher than the sonic wavelength, first the broadened far field diffraction pattern is recovered and finally, for nearly infinite beam widths, the sharply resolved Bessel lines are found.