Adaptive photodetectors for vibration monitoring

We report on simple high-sensitivity interferometric technique of detecting vibrations and present characteristics of laser vibrometer using GaAs, CdTe and CdZnTe adaptive photodetectors based on the effect of the non-steady-state photoelectromotive force. It enables efficient direct conversion of high-frequency phase modulation of speckle-like optical wave reflected from the vibrating object into an output electrical signal with concomitant setting of optimal operation point of the interferometer and suppression of amplitude laser noise. The results of measurements of semiconductor material's parameters of CdTe and CdZnTe are presented. The experiments are carried out for diffusion regime of signal excitation at light wavelength &lgr;=1.15 &mgr;m. The sign, conductivity and diffusion length are estimated from the dependencies of the signal on the temporal and spatial frequencies. Preliminary studies at 1.06 &mgr;m showed that it is possible to detect ultrasonic vibrations with the amplitude of 0.2 Å with a signal power of 20 mW and a bandwidth of 15.5 MHz. This optical phase-to-electrical signal converter is not sensible to ambient vibrations, thermal drift, amplitude laser noise and is therefore appropriated for industrial applications.