Acoustic-based measurements of material absorption coefficients: Relationship between laser pulse duration and stress confinement time

Ideally, to use photoacoustics to determine the absorption coefficient mu(a) of a medium, the laser pulse duration tau(p) is much shorter than the stress confinement time tau(sr) required for a laser-induced stress wave to propagate a distance equal to the light penetration depth delta. However, without prior knowledge of delta (equal to 1/mu(a)), it is not clear whether a given photoacoustic measurement is indeed performed under stress-confined conditions. The purpose of this study was to explore the effects of tau(p) and tau(sr) upon efforts to obtain estimates of mu(a) using photoacoustics. A numerical model was developed to simulate stress signals and investigate how measurements of mu(a) are related to the ratio tau=tau(p)/tau(sr). Experimental photoacoustic measurements at several values of tau were performed to estimate mu(a) of water, and a deconvolution model was applied to correct the measured mu(a) without prior knowledge of tau. Under the conditions simulated in this study, tau(p) must be less than similar to0.1tau(sr) for optimal photoacoustic measurements of mu(a). Since it is difficult to achieve such conditions at midinfrared wavelengths for accurate soft tissue characterization due to strong water absorption bands, a numerical deconvolution technique was implemented to overcome this limitation of conventional photoacoustics, resulting in up to a 30% improvement in photoacoustic-based estimates of the sample mu(a). (C) 2004 American Institute of Physics.