【作者】 邓圆；

【导师】 金韬；

【作者基本信息】 浙江大学 ， 电子科学与技术， 2013， 博士

【摘要】 近年来,随着激光器技术的不断进步,超短脉冲强激光在介质中的传输、成丝控制以及激光在水下声源中的应用成为领域内的一个研究热点。本文在此基础上,开展超短脉冲强激光传输、成丝控制及激光水下声源应用的研究。本文首先研究了超短脉冲强激光在传输介质中传输时所引起的散射、空间-时间散焦、群速度色散、高阶色散、克尔自聚焦、自相位调制、拉曼效应、多光子电离等效应,对超短脉冲强激光在传输介质中的传播过程进行了全面的描述。本文还分析了等离子体与激光的相互作用过程,并综合超短脉冲强激光在传输介质中传输时的各种效应,给出了具体的数学模型。随后,本文采用数值仿真计算的方法,对超短脉冲强激光在传输介质中的传输过程进行了二维和三维仿真。在二维仿真中,研究了激光峰值功率、脉冲形状、激光波长、脉宽等因素对激光传输、成丝及产生的电离通道的影响。仿真结果表明,激光峰值功率越高,越有利于形成连续的电离通道；采用贝塞尔形激光脉冲,其产生的电离通道宽度更窄、长度更长、电子密度分布更均匀。改变激光波长为248nm的紫外激光,其产生的电子密度数量级保持不变,而相应的峰值功率密度减小两个数量级。保持激光其它参数不变,增大脉宽将使电离通道中的电子密度分布更均匀。在三维仿真中,研究了椭圆形激光脉冲、圆环形激光脉冲、黑色薄片、幅度盘及相位盘等手段对多丝分布的影响。结果表明,初始激光脉冲形状为椭圆形时,丝沿椭圆长轴方向分布,而在椭圆短轴方向上,丝分布区域被压缩。当初始激光脉冲形状为圆环形时,丝沿圆环轴线排列,从而在空间上形成一管道形电离通道。采用黑色薄片,对激光进行裁切,当激光脉冲被裁切为一维直线形、正方形及椭圆形时,相应形成的多丝也分别排列成直线形、正方形及椭圆形形状。通过采用幅度盘,可对丝的数量进行精确控制,此时,丝排列成规则的几何形状。改用相位盘,丝沿相位改变极大值方向进行排列。针对激光在水下声源的应用,本文分别研究了热膨胀机制、汽化机制及击穿机制下激光声源的波形、频谱、指向性等特性。本文还研究了双空泡情况下,激光致空化气泡溃灭及其产生的声波辐射。研究结果表明,通过控制激光光强的时域波形、光斑形状、激光串波形形状、重复频率等参数,可有效控制激光水下声源的时域波形、频谱及指向性。汽化机制下,激光声源产生声波的时域波形、频谱及指向性等随观测点位置、观测频率、光斑位置等不同而不同。击穿机制下,通过建立等离子体柱模型与等离子体椭球模型,研究了两种情况下声波的指向特性。此外,在激光致单空泡运动及其辐射声波的基础上,研究了双空泡运动情况下,空泡运动及其辐射声波。研究发现,空泡间距越小,空泡溃灭时达到的最小半径越小,辐射产生的声压越高。最后,本文设计实验,研究了激光水下声源的时域、频谱特性,并分别研究了水下声源辐射声压级与激光能量、水体盐度、颗粒物浓度、聚焦点位置等因素的关系。实验结果表明,激光能量越高,激光产生的声压级越高,两者近似成2次方关系；随水体盐度增加,激光声源辐射声压级呈上升趋势。当观测点位置一定时,聚焦点位置离水面越近,激光致声源辐射声波的声压级越高。激光声源辐射声压随颗粒物浓度的升高并没有明显的上升或下降趋势。预测结果表明,在浑浊海水中,随传输距离增加,激光声源辐射声波的时域波形中的高频振荡部分消失,脉宽展宽,频谱中的高频部分迅速衰减,直至消失。考虑沙质海底影响后,垂直入射时,激光水下声源辐射声波的时域波形、频谱分布并没有发生太大变化,然而其幅度变得更小。更多还原

【Abstract】 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.更多还原