Thermal noise spectrum of a fiber-optic magnetostrictive transducer

We report the observation of the thermal equilibrium strain fluctuation spectrum in a transducer using a fiber interferometer. The thermal noise power spectrum in the range of 20-30 kHz of the metallic glass cylinder in this experiment is determined by the dynamic elastic susceptibility and has peak value corresponding to approximately 0.8 microrad/ radicalHZ at 25.5 kHz. The data are analyzed by using the fluctuation-dissipation theorem and a coupled-mode model for dynamic response in metallic glass.