THERMAL NONLINEAR EFFECTS ON UNDERWATER SOUND GENERATION BY LASER-RADIATION

In the present paper, the authors study the thermal non-linear effects on the excitation of sound generation which is applicable to the dete rmination 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 im pulse responses of the system corresponding to both the pressure terms are determined. The pressure corresponding to the main term is expres sed as convolution between the impulse response and the time derivativ e of the laser intensity and that corresponding to the additional term is given as a convolution between the system's impulse response and d ouble time derivative of the square of the energy released in time t. The effects of the thermal non-linearity on the sound pressure are stu died, considering the different time profiles of the laser pulse. It i s found that besides the thermal non-linearity, the pressure profile d epends on laser parameters and the shape. It is found that the pressur e profiles are drastically different for the small and long thermoacou stic arrays. The thermal non-linearity affects the pressure waveform a nd the effect is pronounced for a short source. The results obtained a re compared with those of previous studies. A reasonable explanation o f the experimental observations of anomalous behaviour of underwater s ound generation is also provided, on the basis of thermal non-linearit y. (C) 1998 Academic Press Limited.