节点文献
低频水下声源的激光探测研究
【作者】 许佳婷;
【导师】 苗润才;
【作者基本信息】 陕西师范大学 , 光学, 2012, 硕士
【摘要】 利用激光探测水下声波信号不仅可以实现激光遥感技术,同时也可以进一步探测水下目标。具有非常广阔的应用前景,因而引起了广泛的重视。本文提出了一种低频水下声信号的激光探测技术,并建立了实验装置。水下声源发出的声波通过水介质传播到水表面,形成表面声波。根据表面波声光效应的原理,当激光束照射到表面声波时,表面波调制入射光束,产生光的衍射。实验上观察到了清晰的衍射图样。并进一步得到了衍射图样随声源频率的变化关系。根据波长与光斑间距的解析关系式,计算出了低频水下声源所引起的表面波波长,利用色散关系提出了探测水下声源频率的方法。并分析了液体表面声波振幅随波传播距离的衰减关系。全文由以下四个部分组成:本文第一部分重点介绍了水下探测的意义及背景,提出了利用光学方法探测水下目标的优势。详细介绍了激光声纳探测技术与遥感测距的进展。进一步总结了目前光学测距方法,大体分为相干测量和非相干测量两种,重点介绍了激光多普勒测量技术、激光三角法和激光直接强度调制方法。本文第二部分详细介绍了表面声波的产生及机理,并从流体力学的角度介绍了液体表面声波的波动及边界条件,由此得到了一维表面声波可以看作是理想的正弦反射式位相型光栅,其对入射光束有调制效应。并详细介绍了声光效应原理及其研究应用的意义,并从公式推导中得到了声表面波与光波相互作用,其反射光波是由一组以n为衍射级次的衍射光波所叠加而成的,并以一定的衍射角传播。本文第三部分提出了一种利用激光衍射法探测低频水下声信号的实验装置,对于频率为几十赫兹的低频声表面波,实验中观察到了清晰稳定的衍射图样,当改变表面波频率时,衍射图样中的条纹间距也在变化,并随着频率的增大而变宽。依据波动光学的原理,分析了表面波波长随条纹间距变化的解析关系式,并据此对表面波波长和波矢进行了测定。进一步从波动角度出发,得到了液体表面张力波的色散关系式,并根据计算机程序拟合出了角频率相对表面波波矢的双对数曲线,得到了实验值的回归方程,发现与理论色散关系非常吻合。从而验证了本实验装置的可行性,通过本实验系统可以探测到水下声信号的频率信息。本文第四部分根据本实验所建立的探测装置,测量了当表面波频率一定时,改变表面波传播距离,采集到了不同的衍射图样。从衍射强度理论出发,得到了光衍射的强度公式,根据此公式通过计算机程序可以得到不同传播距离时的表面波振幅值。对该振幅分析发现,当传播距离增大时,振幅值在减小,即液体表面波振幅具有衰减效应。根据最小二乘法拟合得到了振幅的对数相对传播距离的回归曲线,并计算出了该频率下的衰减系数。同时我们发现了随着频率的增大,表面波衰减系数也在增大,即表面波衰减随着频率的增大而增强。本文以实验为主,利用所设计的激光探测装置进行了水下声信号的探测,得到了由水下声场所产生的表面波的信息。数据结果表明,利用该装置可以探测水下低频声源的频率信息。与其他探测水下的方法相比,具有无损、非接触、实时、灵活性强的特点。
【Abstract】 By using laser detection underwater acoustic signal can not only realize the laser remote sensing technology, but also it can further underwater target detection. Because it has the wide prospects of application, so the extensive attention was caused. A technology has been developed to detect the low-frequency underwater acoustic signal by laser and a detection apparatus is developed in this paper. The sound wave produced by sound source under water propagates to the water surface through water medium and then forms the surface acoustic wave (SAW). When the laser beam illuminates obliquely on the surface acoustic waves at a certain angle, these waves result in the modulation effect on a laser beam reflected by the water surface. Then the steady and visible diffraction pattern is formed on the observing screen. The relationship between the diffraction fringe spacing and the wave frequency was obtained in experiment. A method that can calculate the wavelength produced by the low frequency underwater acoustic is proposed. According to the dispersion relations, a method for detecting underwater sound frequency is presented. The attenuation relationship between the SAW amplitude and the propagation distance is analyzed.This paper is composed with the following four parts:The first part in this paper, the significance and the background was mainly introduced for the underwater detection. And a method using laser is putted forward to detect the underwater target. The progress of the laser sonar detection technology and the remote sensing was detailed. The optical ranging method was summarized to divided into two kinds:coherent measurement and incoherent measurement. Laser doppler measurement technology, laser triangulation and laser direct intensity modulation was especially focused on.In this second part, the generation and mechanism of the surface acoustic wave was introduced. Based on the fluid mechanics, the liquid surface acoustic wave and boundary condition was provided. Therefore one-dimensional surface sound waves that can be seen as the ideal sine type reflex type the phase grating have a modulation effect for the incident beam. The principle of the acoustic-optic effect and its application significance was given. The formula from the surface acoustic wave and light interaction was got. The reflected light was composed of a set of grades added by the waves in n diffraction light, and spread with a certain diffraction angle.The third part in this paper, a technology has been developed to detect the low-frequency underwater acoustic signal by laser. And a detection apparatus is developed. For dozens of Hertz low frequency surface acoustic wave, the clear stability diffraction pattern was observed. When the surface wave frequency was changing, the stripe spacing of the diffraction pattern was also varied, and became wide with the increase of frequency. Based on the principle of wave optics, the analytical formula of the surface with fringe spacing was analyzed. Then the wavelength and the wave vector were determined. From the angle of the fluctuation, the dispersion relationship with SAW was given. According to the computer program, the log-log plot of the angular frequency versus the wave number k on the liquid surface was fitting. And the regression equation of the value was got. the experimental result coincided with theory dispersion relationship. Then the feasibility of the experiment equipment was validated. Through the experiment system, we can detect the frequency information with the SAW.The fourth part in this paper, based on the detection devices, the diffraction patterns collected when the propagation distance was changed and the surface wave frequency was keeping constant. From diffraction intensity theory, the strength of the light diffraction formula was got. According to the formula, the surface wave amplitude values were provided under the different propagation distance through the computer program. When the propagation distance was increased, the amplitude values were decreased. Therefore the attenuation effect existed in the liquid surface wave amplitude. According to the least-square method, the relative transmission distance curve about the amplitude logarithm with the propagation distance was obtained. The corresponding attenuation coefficient was calculated. And we find that when the frequency was increased, the attenuation coefficient of the surface wave was increased. The surface wave attenuation was enhanced as frequency increased.Based on the experiment in this paper, the low-frequency acoustic signal was produced through stimulate the underwater source. The surface wave signal was formed. The liquid surface acoustic wave developed the diffraction effects, and the diffraction pattern was formed. The fringe spacing and the wavelength values were obtained. And the regression equation of the dispersion relations was fitting out. And the surface wave amplitude attenuation characteristics was analysed. The liquid surface wave amplitude was decreased when the propagation distance was increased, and the attenuation coefficient was increased with the frequency.
【Key words】 underwater sound source; acoustic-optic effect; surfaceacoustic wave; dispersion relationship; attenuation coefficient;