Energy partitioning and optical‐to‐acoustic conversion efficiency during laser generation of underwater sound

Abstract

Two mechanisms (thermoacoustic and ablative) are considered with regard to how much optical energy is eventually transformed into sound. The analysis of the former begins with a fluid dynamic analysis of the generation of entropy and acoustic modal fields taking into account viscosity and thermal conduction. The acoustic modal field satisfies the Westervelt‐Larson inhomogeneous wave equation while the entropy modal field satisfies a transient heat conduction equation with similar source term. The derivation demonstrates that the Westervelt‐Larson equation is valid under much broader circumstances than was previously asserted by Bozhkov and Bunkin. The bulk of the absorbed optical energy goes into the entropy mode, but the acoustic portion can be enhanced if the spatial and temporal dependence of the energy deposition resembles a solution of the wave equation, such that initially generated waves are continually pumped by the source. An upper limit to the conversion efficiency is of order (β/ρcp)pmax, where...