Abstract:
A new noncontacting optical method of vibration detection has been developed that utilizes the photorefractive effect in select materials to form an optical ``lock-in'' amplifier. The method synchronously detects the optical phase shift of an object beam scattered from a vibrating specimen surface. Four-wave mixing and conventional sychronous detection allow measurement of both the vibration amplitude and phase directly as a function of the excitation frequency. Narrow bandwidth detection can be achieved at frequencies from the photorefractive response limit to the reciprocal of the photoinduced carrier recombination time. The method has been implemented using Bismuth silicon oxide providing a resolution bandwidth of about 130 Hz and flat frequency response up to the MHz region. Both synchronous and random vibration excitation methods can be used providing a minimum detectable displacement amplitude of 0.002 nm, at present, with the possibility of further improvement. Application ofthe method to resonant ultrasound spectroscopy is described using materials of various microstructures and shapes. [Work supported by the U.S. Department of Energy.]