Optical Determination Of A Surface Acoustic Wave Using Light Interference And Light Diffraction

Optical method is an important technique to detect the substance properties. Of course, this method is a good choice for the surface research as well. When the optical effects, including interference and diffraction, occuring on the liquid surface, many fantastic phenomena will emerge,which may be used to measure some parameters of liquid, such as surface tension, SAW wavelength, amplitude, and so on. Therefore, optical methods have been adopted extensively in many fields.In this thesis, we studied liquid surface using optical methods: When a capillary wave propagating along the air-liquid interface, a grating will form on the surface under certain condition. When a collimated-beam illuminated upon the liquid surface, interference or diffraction patterns may be observed. Based on the relationship between the distrib- ution of intensity and the surface wave, we can study the liquid properties.In our research, the major phenomena and conclusions were obtained as follows:1 For the SAWs at the region of a few tens Hertz, neither diffraction technique nor the laser scanning slope method is suitable since the scan- ning laser beam neither illuminates a part of the acoustical wavelength (geometrical optics case), nor illuminates many wavelengths (pure diffraction case). Thus, few techniques are available for SAW at these frequencies. A practicable method for liquid-surface acoustic-wave at the frequencies of a few tens Hertz, based on the combination of optical interference and scanning wave slope, has been developed.2 High visibility stationary interference fringes and two extremely bright spots at the edges in the reflection field were experimentally observed. The SAW characteristics of wavelength and amplitude are obtained directly from the measured divergence angle and angular separation of the interference fringes. And we have derived the expression of the light intensity.3 Light interference method is used to determine the attenuation coefficient of the tens hertz Liquid Surface Acoustic Wave in real time.4 The high visibility diffraction patterns were observed experimentally in the case of the hundreds hertz Liquid Surface Acoustic Wave. Furthermore, the disappearance of zero-order diffraction was obtained, which was corresponding to the 100% diffraction efficiency. We can use Bessel function to calculate attenuation coefficient.