Bulk-Acoustic-Wave Resonators Using Holographic Reflection Gratings in Photorefractive Materials.

摘要

Bulk-acoustic-wave resonators that make use of reflection gratings written holographically in the bulk of photorefractive materials such as LiNbO3 and LiTaO3 have been shown to have high performance and much promise as the stabilizing element in low-phase-noise oscillators. The hologram interacts with the acoustic wave through the electroacoustic effect to form a reflection grating for acoustic waves of the frequency at which the Bragg condition is satisfied. Such devices have high Q and operate at fundamental frequencies greater than 1 GHz. The material-limited acoustic Q in LiNbO3 is greater than 10,0000 at 1 GHz and the value for LiTaO3 is even higher. Fabrication is relatively simple since all high-resolution features are incorporated in the hologram. Deposited thin-film ZnO compressional-wave transducers are used. The resonators have application in oscillators for doppler-radar and communication systems, in which the important measure of performance is the single-sideband (SSB) phase noise. Measurements of the phase noise of oscillators built with LiNbO3 resonators and operating at 1 GHz will be presented. Presently, the SSB phase noise at 1-GHz fundamental frequency is - 165 dBc.Hz at 100-kHz offset from the carrier and - 150 dBc/Hz and 10-kHz offset. This is comparable to other resonator technologies and is limited by the currently achievable Q of 1000. Effects limiting the Q are discussed and projections of improved phase noise are presented. Also, as possible extensions of the technology, the use of the grating as an acoustic transducer and the use of other photorefractive materials such as strontium barium niobate (SBN) are also discussed. Reprints. (JHD)