声表面波透过理想化线性调频声栅后的Raman-Nath衍射特性

本文将声表面波声栅上的Raman-Nath衍射现象研究发展到其周期的空间频率为线性变化的声栅情形。在理论上,首先将声栅的调制函数理想化为正弦函数,然后将声栅出射平面上相应区域作为无穷障板上有限孔径内的场分布,采用光学中熟知的Kirchhoff衍射积分公式来求透射区的衍射场。给出了物理概念清晰的分析结果。表明声栅同时起两个作用:一是相当于一个等周期声栅作Raman-Nath衍射;二是对各衍射级声波又同时是具各自不同周期变化率的Fresnel透镜,实验上采用特殊结构叉指换能器来模拟上述理论要求的出射波阵面。实验结果良好地证实了上述理论分析结果。

RAMAN-NATH DIFFRACTION FIELDS OF SURFACE ACOUSTIC WAVES BY CHIRPED ACOUSTIC GRATING

The diffraction of surface acoustic waves in Raman-Nath regime on acoustic grating with linearly chirped spatial frequency is investigated.Theoretically,the transfer function of the Raman-Nath grating is considered as the distribution function in the finite aperture and by using the Kirchhoff diffraction integral expression,the acoustic field distribution in transmission region can be considered as the diffraction patterns of the distribution function in the finite aperture.It is shown from the analysis that the Raman-Nath diffraction grating with chirped spatial frequency can perform two functions simultaneously,i.e.,firstly,it looks like a Raman-Nath diffraction grating with constant period and secondly,for every diffracted beam,it is a Fresnel lens.Experimentally,an unique IDT with chirped spatial frequency in transversal direction and with the shape of the sine-function for its fingers is used to simulate the diffracted fields of the grating analyzed theoretically.The diffraction characteristics analyzed theoryetically are confirmed by experiments.