Experimental Exploration of the Limits of Achievable Q of Grooved Surface-Wave Resonators.
MASSACHUSETTS INST OF TECH LEXINGTON LINCOLN LAB
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An important basic question that remains to be answered for the surface-wave resonator is that of maximum achievable Q. Intrinsic material loss imposes a limit of Q approximately equal to 100,000 at 150 MHz on LiNbO3 and quartz. However, devices produced to date have been limited by grating reflection loss to Q approximately equal to 20,000. This paper compares two alternative approaches to the problem of minimizing the grating reflection loss and hence maximizing the Q of the grooved Fabry-Perot resonator. In one approach, the number of elements grooves in the reflecting arrays is modest, and increasing array reflectivity and hence Q is obtained by increasing the element reflectivity deepening the grooves. Experimental measurements show that Q can be increased in this way until a groove depth of roughly 0.025 Lambda or a step reflectivity of about 1 is attained on a 300-groove YZ LiNbO3 resonator, at which point excessive losses due to bulk-wave scattering preclude the existence of a resonance. In the second approach, the element reflectivity groove depth is kept at a minimum and increasing reflectivity and Q are obtained by increasing the number of elements in the array. In either approach, element reflectivity is increased by a closer control of groove width-to-period ratio to values of 0.5 or less. Measurements of Q for various devices on LiNbO3 and ST quartz are compared with theory. In addition, the maximum Q values measured will be compared with the theoretical maximum Qs set by intrinsic material losses. Author
- Line, Surface and Bulk Acoustic Wave Devices