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

The influence of the beam width of a Gaussian laser beam on its acousto-opt ical diffraction by a travelling sound wave under Raman-Nath conditions (Kl ein-Cook parameter Q << 1) has been studied. We develop a new general analy tical formula that has the merit of being valid for arbitrary bundle widths . If the beam width is smaller than the ultrasonic wavelength, ray bending, f ocusing/defocusing effects and propagational diffraction inside the sound c olumn are shown to appear and are fully quantitatively described. As a cons equence the intensity at the exit plane and in the far field become time de pendent. When the bundle width becomes higher than the sonic wavelength, first the b roadened far field diffraction pattern is recovered and finally, for nearly infinite beam widths, the sharply resolved Bessel lines are found.