叉指换能器激发表面声波的生长特性

本文利用暗场法和光探针技术对叉指换能器,特别是线性调频叉指换能器,激发表面声波的生长特性进行了观察和测量。并且利用有关的理论进行了计算。实验和理论结果一致表明在共振激发指对的中心位置,声强极小;离开中心位置,声强迅速增长。对于周期性叉指换能器,向两侧增长是对称的;而对于线性调频叉指换能器,向两侧增长是不对称的,向高频端的增长比向低频端的更强,因而形成线性调频叉指换能器向两端辐射的不对称性。但是在实验结果与理论结果的比较中,也出现一些问题,我们对此进行了初步的讨论。

GENERATION CHARACTERISTICS OF SAW IN THE EXCITED INTERDIGITAL TRANSDUCERS

The characteristics of the spatial growth of surface acoustic wave (SAW) in the excited interdigital transducers (IDT) are observed and measured quantitatively. Both the method of dark field imaging and that of simple optical probing have been used. In the experimental setup, the detected optical intensity is the result of superposition of both diffracted optical beams produced by the acoustic waves propagating in two opposite directions. The periodic IDT and linear FM IDT deposited on the substrates of Y-cut Z-pro-pagation LiNbO₃ have been investigated. For the periodic IDT, the experimental investigations show that the superposed intensity of SAW in the central position of IDT is minimum, then it grows with the distance from the centre section. The growth is symmetric in both directions of the excited IDT. In the case of Linear FM IDT, the experimental results indicate that in the central position of the resonantly excited finger pairs the superposed intensity of SAW is minimum too, but then withthe distance from the central position, the spatial growth of SAW in two directions are asymmetric.
Theoretical calculations are also made by two ways. One of them is performed by the multiplication of the admittance matrixes as employed by Smith, considering imped-.ance discontinuity at each interdigital electrode edge. The other can be obtained from the analytical expression of the generation characteristic of SAW for periodic IDT, which is derived by Shui. The results obtained by both methods roughly agreed each other. The experimental data may be explaned qualitatively by these theories. But some discre-pencies are presented and discussed breifly.