Microwave Frequency Acoustic Surface-Wave Loss Mechanisms on LiNbO3

The temperature and frequency dependence of the attenuation of 0.5–5-GHz acoustic surface waves on LiNbO3 has been measured. For propagation in vacuum a frequency-squared dependence of the total attenuation is obtained with a value at 1 GHz of 0.9 dB/μsec. Temperature-dependence measurements using a novel three-transducer technique show the dominant loss (0.7 dB/μsec at 1 GHz) mechanism to be the interaction with thermally excited elastic waves. Propagation in air results in an additional loss linearly proportional to frequency with a value of 0.2 dB/μsec at 1 GHz. The effects of beam steering and diffraction losses are also investigated both theoretically and experimentally. Both misalignment of transducers with respect to pure mode propagation axes and misalignment of the propagation-plane perpendicular can add significantly to delay line insertion loss. This beam steering loss on Y-cut Z-propagating LiNbO3 is considerably higher than on the 41.5° rotated-cut X-propagating orientation. The loss mechanisms measured in this paper are sufficient to completely account for the insertion loss of surface-wave delay lines.