Thermoacoustic radiation of underwater sound by a moving high‐power laser pulse

Directive sound beams can be generated by modulating the intensity of a laser beam illuminating an optically absorbing medium, usually water. The thermal mechanism inducing the acoustic wave can be optimized by moving the laser beam through the water at velocities close to transonic. However, most theories are not valid at transonic source velocity or for nearfield radiation. These limitations can be removed by using a time domain analysis based on the impulse response of the thermoacoustic array. Although this approach requires the use of a computer, it is also suitable for the complicated (but promising) case of a source moving at nonconstant velocity along a curved path. Experimental results have been obtained with a 1‐ms laser pulse at two optical wavelengths (ruby and Nd: Glass) inducing a long and a short array. A rotating mirror achieved source velocities up to Mach 2. Experimental results include pressure waveforms, directivity patterns in the vertical, and horizontal planes of the source, and the sound level dependence on range and Mach number. [Work supported by ONR.]