Experiments on a laser-excited thermoacoustic array

Abstract

The optical and acoustic fields associated with the thermalization of modulated laser light in a freshwater lake were measured in accordance with the procedures set forth in the previous paper. A single, modulated laser beam entering the water in one direction was found to generate a propagating CW pulse at the modulating frequency. An optical filter test showed this pulse to emanate from the thermal volume. The amplitude of the acoustic pulse wag directly proportional to the laser power. The acoustical pulse was generated in a “doughnut”-shaped pattern, having its maximum in a plane perpendicular to the optical axis. Beam-pattern measurements show a steep monotonic decay with increasing angle from the acoustic axis. The acoustic beamwidth was found to be proportional to the optical attenuation of the medium and inversely proportional to the acoustic frequency. The dependence of sound-pressure level on the size of the optical beam was also measured. These data support the general validity of the Westervelt-Larson theory for thermoacoustic arrays that employ laser excitation. Extrapolation of the theory indicates that highly directional sound beams could be generated in clear sea water with lasers operating in the blue-green spectrum. [This work supported by the U. S. Navy Office of Naval Research.]