Moving photoacoustic sources: Acoustic waveforms in one, two, and three dimensions and application to trace gas detection

Abstract

The photoacoustic effect is governed by a wave equation with a source term proportional to the time derivative of the optical heat deposition per unit volume and time. Although the typical configuration for generation of the photoacoustic effect makes use of pulsed or amplitude modulated optical beams, the form of the source term in the wave equation indicates that a continuous optical source moving in an absorbing medium is capable of sound generation as well. Here, the properties of simple sources moving in one, two, and three space dimensions are reviewed. The salient feature of sources moving in one-dimension at sound speed is that the amplitude of the acoustic wave increases with time without bound according to linear acoustics. Two schemes, one in the time-domain and the other in the frequency-domain, that take advantage of this principle for increasing the sensitivity of trace gas detection are reviewed.