Study of Optical Sound Generation and Amplification.

This project involves three separate tasks with the following titles: Task I - Generation of low frequency sound from optical pulses; Task II - Propagation of sound through a gas with an overpopulation of vibrationally excited states; and Task III - Optoacoustic studies in liquids. The three tasks, funded by the physics division of ONR, represent a three-pronged study of the generation of sound by the absorption of light and amplification of a propagating sound wave. Generation of low frequency sound from a series of optical pulses involves optimizing the density of pulses to obtain maximum energy at low frequencies. The present effort is devoted to numerical simulation. With present modulation schemes, as much as 50% of the energy appears in the desired fundamental, but predicted harmonic distortion is unacceptable. Amplification of sound propagating through gas with an overpopulation of vibrationally excited states has now been observed in N2/H2,N2/He, and N2/CH4 mixtures. The gas is excited with an electrical discharge. Present efforts are devoted to achieving better agreement between theory and experiment. Optoacoustic pulse have been observed in several alcohols and water. The optoacoustic amplitude has been observed as a function of laser energy, distance from excitation zone to detection region, and optical absorption coefficient. (Author)