声表面波在线性调频声栅上作Bragg衍射后的聚焦声场

本文应用准周期概念给出声表面波在线性调频声栅上Bragg衍射的透射函数。将一级衍射波的分布作为有限孔径内的已知分布。将求解透射区的一级声束的场分布问题作为求上述有限孔的衍射问题来处理。分析结果表明,对偏转声束来说,在Fresnel区存在一个Fraunhofer平面。线性调频声栅同时起两个作用:它既相当于一个等周期的声束偏转器,同时又是一个Fresnel透镜,在焦平面上的声场分布,是声栅出射平面上偏转声束幅度分布的Fourier变换。分析结果还表明:聚焦声轴随入射声波频率而平行移动。这一现象属首次发现,实验结果证实了上述理论分析预计。关键词：

FOCUSING BEHAVIOURS OF THE DIFFRACTED BEAM OF SURFACE ACOUSTIC WAVE ON CHIRPED GRATING IN BRAGG REGIME

The acoustic field distribution of the deflected wave beam in transmission region from a chirped acoustic grating is described by using Fresnel-Kirchhoff diffraction integral expression. The transmission function of the SAW beam which is diffracted by a linearly chirped grating in Bragg regime is obtained by using quasi-periodicity approximation and this function is considered as the field distribution function in the aperture of the exit plane. It is shown from the analysis that in Fresnel region there is a Fraunhofer plane where the field distribution is the Fourier transformation of the field along the aperture. It is demonstrated that the role played by the grating is two fold i.e. a deflector and a lens. The most important point we found is that the focusing axis of the focused wave beam will translate parallelly when the frequency of the input signal is changed. This scanning configuration is quite different from the traditional deflection scanning. The theoretical predictions are verified by experimental observations and measurements.