Development of High-Efficiency High-Frequency Ultrasonic Transducers for Acousto-Optic Devices.

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In order to realize high-performance acousto-optic devices such as light modulators and deflectors, high-efficiency broadband ultrasonic transducers operating at ultra-high frequencies up to 10 GHz are required. The purpose of this research project is to propose and develop a new type ultrasonic transducer for acousto-optic devices consisting of interdigital electrodes on a grooved surface of LiNb <O_3> . The results obtained in this project can be summarized as follows:1. Optimum surface orientations and groove directions for the transducer were investigated. It was shown that the 36゜ rotated Y-cut with grooves parallel to the Z' axis is suitable for shear-wave transduction and the X-cut with grooves making an angle of 37.86゜ with the Z axis is suitable for longitudinal-wave transduction.2. Piezoelectrically-induced stress distributions, radiated acoustic wave fields, and conversion loss of the transducers on the optimum cuts were analysed using a finite element method. The dependence … More of the frequency characteristics on the depth, width, and period of grooves was studied. The center frequency was shown to be situated near the frequency where the groove depth is equal to a galf-wavelength.3. A fabrication process for transducers up to 300 MHz, using dicing saws and photolithographic techniques, was developed. It was demonstrated that a shear-wave transducer fabricated on a X-cut of LiNb <O_3> by this process had a conversion loss as low as 2.5 dB and a large specific bandwidth as large as 110%.4. As a fabrication process for transducers operating at high frequencies up to 10 GHz, we investigated an ion-etching technique using <C_2F_6> gas. Aluminum and chromium films were shown to be suitable as an etching mask. For an RF power of 100 W, the etch rate of <LiNbO_3> was about 3 <micro> m/h and the ratio of the etch rate to that of Al masks was about 3.5. The analysis of anisotropic Bragg diffraction of light beams showed that the Bragg angle of the incident light beam has a stationary value near several GHz for X-cut and 163゜ rotated Y-cut transducers, which leads to broadband acousto-optic devices. Less