THERMAL NON-LINEAR EFFECTS ON UNDERWATER SOUND GENERATION BY LASER RADIATION

Abstract

In the present paper, the authors study the thermal non-linear effects on the excitation of sound generation which is applicable to the determination of the pressure in the direction of the laser beam and at an angle with the laser beam. An additional pressure term appears due to this non-linear effect. Considering the small amplitude waves, the impulse responses of the system corresponding to both the pressure terms are determined. The pressure corresponding to the main term is expressed as convolution between the impulse response and the time derivative of the laser intensity and that corresponding to the additional term is given as a convolution between the system's impulse response and double time derivative of the square of the energy released in timet. The effects of the thermal non-linearity on the sound pressure are studied, considering the different time profiles of the laser pulse. It is found that besides the thermal non-linearity, the pressure profile depends on laser parameters and the shape. It is found that the pressure profiles are drastically different for the small and long thermoacoustic arrays. The thermal non-linearity affects the pressure waveform and the effect is pronounced for a short source. The results obtained are compared with those of previous studies. A reasonable explanation of the experimental observations of anomalous behaviour of underwater sound generation is also provided, on the basis of thermal non-linearity.