Some possible novel configurations for optico‐acoustic transducer arrays created by controlled motion of laser beams across water surfaces

Abstract

Laser beams impinging on water surfaces will generate underwater sound if the beam intensity is modulated or if the beam is moving, although the optical‐to‐acoustical energy conversion efficiency is typically extremely low. Previous analyses have been confined to modulated beams impinging at a fixed point or moving rectilinearly at constant speed, but the present paper adopts a futuristic view that intricate moving and rotating mirror systems can be designed that will allow the entry point to move in any desired pattern (including skips and zig‐zags) and with any velocity versus time profile; the only constraints are that the beam entry angle be fixed and that the exponential decay rate of beam intensity with depth be fixed. This latitude in beam configuration allows a systematic exploitation of the pumping principle, that local energy conversion efficiency is proportional to product of acoustic pressure and beam intensity, by placing the beam wherever and whenever the acoustic pressure is currently the highest. Directed acoustic waves pointed obliquely downwards can be created by moving an obliquely incident beam through a sequence of short parallel path segments, the first sequence pattern simulating a sinusoidal heat deposition wave of fixed wavelength and beam width, the next sequence causing this pattern to move at the sound speed in the desired propagation direction. [Work supported by ONR.]