Abstract: 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.