Spherical Model of an Acoustical Wave Generated by Rapid Laser Heating in a Liquid

Abstract

A spherical model for the acoustic pressure-wave generated by the rapid thermal expansion owing to laser heating is analytically investigated. The physics of the wave generation is extensively discussed, and from this point of view the mathematical boundary value problem is formulated. A Laplace-transform-Green-function method is applied to solve this problem, and the resultant pressure wave is expressed as a function of time, space, and input parameters. The result shows that any point in the medium outside the laser focus experiences, first, a strong positive pressure pulse, and then, a strong tension (negative pressure) pulse. The shape of these two pulses is skew symmetric with respect to the central time between the two peaks of the pulses. These two peaks occur respectively at the retarded starting and extinguishing edges of the laser pulse. The magnitudes of these two peaks are equal. They are proportional to the laser intensity, and inversely proportional to the distance of observation. Numerical examples are given.