Experiments on thermoacoustic arrays with laser excitation

Abstract

A recent theoretical formulation by Westervelt and Larson predicts the development of highly directive sound beams in water through the thermalization of modulated laser light. The present paper reports the experimental validation of this theory with measurements carried out in a fresh water lake. Additional theory pertinent to practical implementation is also presented and verified. The experimental apparatus includes an optically pumped laser system operating in the conventional mode at wavelengths in either the red or infrared regions of the optical spectrum. Modulation of the stochastic light burst is provided by a half-wave Pockels cell. The acoustic measurements began with an existence test which confirmed that an acoustic signal is produced by thermo-optic demodulation of laser light in the water. The sound pulse was generated in a beam with a width and source level that are in reasonable agreement with theory. Acoustic diffraction effects associated with laser beams of finite size were also examined, as were the nearfield effects of this type of array. The relation of laser technology to further work in the blue–green spectrum is analyzed. Subject Classification: [43]20.55; [43]85.40; [43]35.65.