【作者】 陈清明；

【导师】 程祖海； 朱海红；

【作者基本信息】 华中科技大学 ， 物理电子学， 2009， 博士

【摘要】 由于TEA脉冲CO₂激光器易获得高能量和高峰值功率的输出，而且输出激光波长10.6μm处于大气窗口，水对该波长吸收大，因而TEA脉冲CO₂激光致声在激光水声应用特别是海洋遥感探测中具有广阔的应用前景。对脉冲CO₂激光在水中激发的声波特性进行理论和实验研究，具有重要的军事价值和现实意义。本论文围绕脉冲CO₂激光水中光声信号的产生机理，转换效率，传播特性和测量方法等基础问题进行了系列研究，主要研究内容和结论有：从理论上研究了常见的三种尖峰-拖尾型波形的CO₂脉冲激光在水中产生的热弹光声波的波形、幅度、能量转换效率及频谱特征，以及激光脉冲波形参数变化对光声信号的影响规律。研究结果表明：热弹光声波形由激光波形函数的一阶导数决定；其声脉冲由CO₂激光的尖峰和拖尾两部分共同产生；波形和幅度主要由脉冲尖峰的宽度及形状决定；激光脉冲尖峰决定光声信号的高频段，而拖尾部分产生的光声信号对应于低频段。拖尾的宽度与高度和尖峰接近时，拖尾作用加强。分析了表面和整体汽化两种光声机制的特点，计算出了整体汽化的阈值，给出了液体温度不同时的蒸汽流密度及光声信号求解的理论模式和汽化光声信号的特征及随激光参数的变化规律。在对光击穿机制的阈值和波形特征进行分析的基础上，得到了汽化和击穿的光声能量转换效率，并通过快速傅里叶分析，得出了汽化和击穿光声信号的频谱特征。采用衰减系数与频率平方成正比的模型，得出了在纯水中传输情况下，热弹、汽化、击穿光声信号传输的波形、幅度与频谱的变化情况。结果表明：光声信号水中衰减对热弹光声信号影响较大，而对低频的汽化光声和击穿光声中的低频成分（频率低于50kHz）影响较小。通过改变激光器工作电压、混合气体成分、气压等，来改变激光脉冲特性。实验研究了不同的脉冲CO₂激光器光声信号特征，确定了光声信号特征与激光脉冲参数的对应关系。通过监测实验研究容器底部放置待检测物前后的反射光声信号的幅度及频谱变化和采用ICCD摄像机拍摄了液体表面蒸汽反冲运动的方向和速度的方法，确定了光声信号的激发机制、蒸汽的反冲压力和利用光声效应检测水底或水中目标方法的可行性。通过在改变盐水的浓度模拟海水的特征的方法，实验测得了光声信号的波形、幅度、频谱等特征与盐水浓度的关系。结果表明：CO₂激光脉冲在盐水中激发光声信号时，盐浓度越高，光声信号幅度与频谱越低。上述研究结果，为脉冲激光致声水下目标探测等应用提供了理论和实验依据。更多还原

【Abstract】 Since the output energy and peak power of the TEA-CO₂ pulse laser are high, its wavelength is 10.6μm which is the atmospheric window, absorption coefficience of air to CO₂ laser is low and absorption coefficience of water to CO₂ laser is high, photoacoustic detection using TEA-CO₂ pulse laser for remote sensing in ocean obtains more and more attentions. Invetigation of laser acoustics induced by TEA-CO₂ pulse laser in water is practical and important in military and remote detection. This paper investigates the characteristics of laser acoustic signal induced by pulsed CO₂ laser in water.Firstly, the waveform, amplitude, laser acoustic energy conversion efficiency, spectra of thermoelastic regime, induced by the three common waveforms of CO₂ laser, e.g., an initial gain-switched spike of approximately several tens ns FWHM （full width at half-maximum） in duration and followed by a several us tail, as well as their relations to the laser waveforms and laser waveform parameters, were investigated theoretically in this paper. Some valuable results are obtained. Thermoelastic laser acoustic signal is demermined by the first derivative of laser waveform; the acoustic signal is contributed to the combination effects of the spike peak and the tail of the laser pulse, the waveform amplitude and profile are mainly determined by the duration and the shape of the spike, high frequence singals are induced by the spike, while the low frequence singals are induced by the tail. The role of the tail becomes important when its relative intensity to the peak increased or its duration been near the duration of the peak.The flux density of vapor for different liquid temperature and theoretical model for solving optoacoustic induced by surface and body vaporization mechanisms are proposed. Characteristic of laser acoustics of vaporization and relation with the parameters of laser pulse and medium are discussed. The mechanism of breakdown and laser acoustic characteristics is researched. Waveforms and spectra of laser acoustics induced by vaporization and laser breakdown are discussed according to the theoretical results, and conversion efficiency is also acquired.Using attenuation model, e.g., absorption coefficient proportional to square of frequency, the transmission and propagation of the laser acoustics in water were theoretically analyzed. The effect of attenuation on the laser acoustics induced by different mechanisms is studied in detail. The results show that the thermoelastic acoustic signals with higher frequency attenuate significantly, while laser acoustic signals with frequency under 50 kHz attenuate slowly, which are induced by vaporization and breakdown of water.The parameters of CO₂ laser pulsed were changed by choosing different lasers and working voltage, composition of the hybrid gas or pressure of the working gas. The experimental study of laser acoustic induced by pulsed CO₂ laser has been performed in this thesis. The influence of the laser parameters on the characteristics of the acoustic signals were investigated in detail. By comparing the difference of the waveforms and the spectra if there was an object at the bottom of the container, the scheme of detecting object in water is proposed.By analyzing the photos taken by ICCD, the movement veolocity of the steam is got and the mechanism of the laser acoustics can be deduced.By solving salt in water as seawater, the influence of salinity on laser acoustics is studied experimentally.The results show that the amplitude and peak frequency of the acoustic signals decrease with the increase of the salt concentration.These results can provide theoretical and experimental proof for application of laser acoustics in under-water object detecting.更多还原