112°-LiTaO3 periodic waveguides

The waveguide properties of periodic waveguide structures on 112-LiTaO₃ are studied experimentally and theoretically at frequencies close to the Bragg stopband. On admittance curves of long transducer type test structures it is seen that additional attenuation appears at both the high and low frequency sides of the stopband in that the resonances at these frequencies are suppressed. In addition, strong spurious resonances are seen in the lower half of the stopband region. Direct laser probe measurements have shown that the amplitude distribution in the transverse direction across the waveguide changes significantly versus frequency. Propagation ranges from (a) weak single mode guiding at low frequency to (b) strong multi-mode guiding near the lower stopband frequency then (c) a region without any guided modes and strong side radiation near the upper stopband frequency and (d) a return to weak single mode guiding at frequencies well above the stopband. The theoretical explanation of the observed effect is proposed based on Haus and Wang (1978) and Schwelb (178) models. This effect “side-radiation” from waveguide structures in devices at the frequencies corresponding to the upper edge of the stopband can give rise to spurious responses due to acoustic coupling between different parts of the device