Investigation of the acoustic source characteristics of high energy laser pulses models and experiment

Abstract

This thesis was motivated by the possibility of using high energy laser pulses as an acoustic source for naval applications. Research conducted in the 1970's and 80's shows that sound production from laser pulses is most efficient when the energy density of the pulse exceeds the threshold required for plasma formation. The resulting acoustic wave falls into the highly non-linear shock regime. Later work by Vogel et al. sought a more complete understanding of the non-linear dynamics and energy distribution of this process in an attempt to limit collateral tissue damage during laser surgery. This work includes detailed experimental data including photographs and hydrophone measurements as well as numerical calculations of expected pressures, bubble dynamics, and pulse shapes. The goal of this thesis was to investigate the characteristics of the laser generated acoustic pulse further through experimentation and modeling. Experiments were carried out with Ted Jones at the Naval Research Laboratory to investigate the directionality of the acoustic pulse produced by a 100fs 2mJ laser pulse focused just under the surface of water. The range dependence of the pressure amplitude was also examined. The amplitude of the pulse was found to vary with direction; however, this effect is considered likely to be a result of interference between the direct path and the surface reflection. A linear least-squares fit of the peak pressure amplitude with range revealed a 1/r1.2 relationship. This is consistent with the expected approximately 1/r relationship for pressure amplitudes under 100MPa. The modeling effort employed AUTODYN, a finite element program designed to handle the non-linear processes in explosions. The laser generated acoustic source was modeled using an explosive of the same volume as the laser spot reported by Vogel for his 10mJ 6ns pulse. The internal energy of the explosive was adjusted until the pressure amplitudes agreed with Vogel's measured values. The efficiency, pulse length, pulse shape, and variation of pressure amplitude with range achieved with AUTODYN are comparable to those reported by Vogel.