LASER-INDUCED ACOUSTIC SHOCK WAVES.

Results are presented on the generation of high-pressure nonlinear acoustic waves by the use of high-intensity laser pulses. The laser used was a Nd:YAG Q-switched laser with a wavelength of 1. 06 mu m, peak energy of 700 mJ, peak power of 10**8 W/cm**2, Gaussian intensity profile, and pulsewidth of 10 ns. In the first phase of the experiments, a water volume was chosen as a model propagation medium to simulate a homogeneous, isotropic medium supporting only longitudinal waves. The beam was focused onto the water to achieve the intensity necessary to cause dielectric breakdown, evidenced by optical emissions. The resulting acoustic shock waves were detected with a commercial hydrophone PK14 (2. 54 mm diameter) with a frequency response flat to 300 kHz. This was used to measure the frequency-dependent ultrasonic radiation pattern at a distance of 1 m as a function of elevation angle, laser intensity, and depth of focal point below the surface of the water. Similar but somewhat sharper results were obtained when the focus was changed from near surface to 30 mm below the surface.