水下低频液体表面波的声光衍射及衍射条纹的非对称性

【作者】 谢甜；

【导师】 苗润才；

【作者基本信息】 陕西师范大学 ， 光学工程， 2012， 硕士

【摘要】 用光学技术测量水下声信号的方法有很多,包括激光干涉法、激光多普勒法,和通量变化法。这些方法的测量精度非常高,但是这些方法对于目标的反射面有十分严格的要求,在实际应用中存在诸多不便。针对于此,我们提出了利用激光衍射的方法测量表面波,该方法可测量频率范围比较大,并且具有实时、无接触和无损等诸多优点,应用性比较广泛。本文根据表面波声光效应的原理,对水下声信号进行探测与处理,主要包括以下四个部分：1、介绍了液体表面波,对液体表面波的光学检测方法做了归纳,重点介绍了六种表面波的光学检测方法,并对这些方法的检测原理,适用范围,优缺点进行了总结。最后对液体表面波的激光探测技术的未来做了展望。2、重点介绍了声光效应,理论上分析了声光效应的原理和表面波光栅,为后面的分析做了铺垫。3、对低频液体表面波的衍射级和入射角的关系进行了讨论。对入射光场进行傅里叶变换,得到了衍射光场的解析表达式。进一步分析得到,衍射条纹的间隔不仅与光栅常数有关,而且与入射角有关。当光栅常数远大于入射光波长时,要观察到清晰的衍射条纹,入射角存在一个临界值,当入射角大于该临界值时,可观察到条纹,反之,则观察不到条纹。本文理论上给出了临界值的大小。4、根据液体表面波声光效应原理,建立了一种探测水下低频声信号的实验装置。水下声信号经过液体传播,在液体表面形成表面声波,当激光束照射液面时,从液面反射的光经表面波调制后,形成清晰的衍射图样。用光电探测器采集该衍射图样,并传输给计算机。实验发现,根据声光效应,介质中的声波类似于衍射光栅。对于几十赫兹的低频液体表面波,它所形成的光栅常数在毫米量级,远大于入射光波波长。当光栅常数远大于入射光波长时,观察到了清晰的衍射图样。并在激光束斜入射的条件下,实验上观察到衍射条纹具有明显的不对称性。根据光栅方程一般表达式,分析得到了衍射条纹的解析表达式,由该表达式可以解释衍射条纹位置和光斑强度的不对称性,及其产生的机理。更多还原

【Abstract】 The methods for using optical to measure underwater acoustic signal has a lot of kinds, such as laser interference method, laser doppler method, and flux variation method. The measurement accuracy is very high. But it is very strict for the target reflective surface. There have a lot of inconvenience in the actual application. Therefore the method for measuring surface wave is put forward by using of laser diffraction. It’s measure frequency range is big. And it has many advantages of real-time, no contact, nondestructive and more extensive application. Based on the acousto-optic effect, the underwater acoustic signals has detected and treated. In this paper it mainly includes the following four parts:1、The liquid surface wave was introduced. The optical detection method for liquid surface wave was inducted. Six kinds of optical methods has been introduced to detect the surface wave. And the of detection principle, suitable scope, advantages and disadvantages are summarized. The future of the detection technology for the liquid surface wave is discussed by laser.2、The acousto-optic effect was emphasis on. the principle of effect and surface wave grating were analysised. And it was laying the ground work for the behind of the analysis.3、The relationship between the order of liquid surface wave diffraction and the incidence angle was discussed. To Fourier transformation for the incident light field, the analytical expression of the light field was obtained. We analysis further that the interval of the diffraction stripe is not only related to the grating constant, but also to the incident angle. When was much more than the incident light wavelength, there is a critical value to observe the clear diffraction stripe; when the incident Angle was more than the critical value, we can observed the stripe, conversely can’t. The critical value was given theoretically in this paper.4、According to the principle of the acousto-optic effect for the liquid surface wave, a experimental set up to detect the low frequency underwater sound signals. Underwater acoustic signal spread to the surface, and the sound wave is formed in the liquid surface. When the laser beam irradiated the liquid surface, the diffraction pattern is formed after the reflected light modulate by the surface wave. It is collected with photoelectric detector to transfer to the computer. Based on the acousto-optic effect the sound wave in the medium is similar to a diffraction grating. For dozens of Hertz of low frequency liquid surface wave, it’s grating constant is in mm magnitude. And this is much more than the incident light wavelength. Under the condition of incidence obliquely for the laser beam, the asymmetry of the diffraction stripe was observed obviously. According to the general formula of the grating equation, the analytical expression of the diffraction stripe is received. The position of the diffraction stripe and the flare strength asymmetry and its mechanism can be explained by this expression.更多还原