The Propagation Of Intense Ultrashort Laser Pulse And Applications Of Laser Pulse In Underwater Sound Source

In recent years, with the progress of laser technology, the study of ultrashort intense laser pulse in transparent media, filamentation control and laser induced underwater acoustic source has become a hot research topic. Based on this, this paper will study the propagation of ultrashort intense laser pulse, the control of multifilaments and the application of laser induced underwater acoustic sources.First, this paper studied the effects including scattering, the space-time induced defocusing, group velocity dispersion, high order dispersion, Kerr effect induced self-focusing, self-phase modulation, Raman effect, multiphoton ionization, etc. Subsequently, this paper also analyzed the interaction between plasma and laser pulse. A specific mathematical model had been given by combining all these effects.The model which described the propagation of ultrashort intense laser pulse in transparent media had been solved numerically. In the two-dimensional simulations, the affections of pulse peak power, pule width, pulse shape, wavelength to the propagation of laser pulse, filamentation and plasma channel had been studied. Simulation results showed that, the higher of pulse peak power, the more favors of the formation of a continuous plasma channel; The Bessel-shaped laser pulse might lead a narrower, longer and more uniform distributed plasma channel. If the laser pulse is a ultraviolet laser pulse (λ=248nm), the plasma density kept at the same order of about1023m-3, while the corresponding peak power intensity would be two orders lower. Keeping the other parameters unchanged, increasing the pulse width would make the plasma more evenly distributed. In the three-dimensional simulations, methods of changing the initial laser pulse to ellipse and ring-like pulse, using black slices, amplitude mask and phase mask had been studied for the control of multifilamentation. The results showed that, when the initial laser was ellipse, the filaments arranged along the major axis, while in the direction of minor axis, the distribution area of filaments was compressed. Filaments located in the shape of a ring during the propagation of a ring-like laser pulse. By using black slices, filaments arranged the same shape with that in the slices. For example, by graving line, square and ellipse shape on the slices would lead to line, square and ellipse distributed filaments. The number of filaments could be controlled accurately by using amplitude mask, and the filaments locations formed a regular geometric pattern. Filaments would arrange along the direction of phase changing axis when phase mask was applied.This paper also studied the applications of laser pulse in the field of underwater acoustic sources. The wave shapes, spectrums and directivities of laser induced underwater acoustic waves had been analyzed theoretically and numerically. The collapse of double bubbles and corresponding induced sound pressures had been studied for the first time. The results showed that, under thermal expansion mechanism, by controlling the time varying pulse intensities, beam shapes, shapes and repletion rates of laser strings, one could effectively control the waveforms, spectrums and directivities of laser induced underwater sources. Under vaporization mechanism, the waveforms, spectrums and directivities of laser induced underwater sources were varied with observation position, detection frequency and beam shape. The directivity characteristics could be studied by using the established plasma column model and plasma ellipsoid model under breakdown mechanism. During double bubbles movement, the nearer of the two bubble, the smaller of the minimum radius they would reach during collapse and the higher sound pressure they radiated.Finally, experiments had been designed to research the wave shapes and spectrums of laser induced underwater acoustic sources. Besides, relationships between sound pressure levels and pulse energies, salinities, particle concentration densities and focusing positions were studied. The experiment results showed that, the higher of the pulse energy, the higher sound pressure level it would be generated. With the increasing of salinity, the laser pulse tended to generated higher sound pressure level. At a certain observation point, there would be an exact focusing point that one could get the highest sound pressure. The sound pressure level showed no significant upward or downward trend with the increasing of particle concentration density. The prediction results showed that in turbid seawater, high frequency oscillation of the waveforms disappeared and the waveforms were broadened with the increasing of propagation distances. Also, the high frequency part of the spectrum decayed rapidly with the increasing of propagation distances. Considering the affection of the sandy seabed, the waveforms and spectrums of the acoustic waves did not change much, but their amplitude became smaller.