Laser-Induced Acoustic Shock Waves

The use of laser pulses to induce elastic waves in materials has many applications, ranging from weapons damage to noncontact NDE. This paper presents preliminary results on the generation of high-pressure nonlinear acoustic waves by the use of high-intensity laser pulses. The laser used was a Nd:YAC Q-switched laser with a wavelength of 1.06 microns, peak energy of 700 mj, peak power of - 108 W/cm2, 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 focussed 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 PKI4 (2.54 mm diameter) with a frequency response flat to 300 kHz. This was used to measure the frequencydependent 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. The data substantiate that the effective source of the shock waves is in the shape of a small cylindrical column approximately 20 mm long in length, oriented perpendicular to the surface, and some distance below it, depending upon the focal point of the laser beam.