Out-of-plane vibration measurement by speckle photography using photorefractive BaTiO3 crystal

The present work describes the measurement of small out-of-plane vibrations in quasi real time using a photorefractive crystal as a medium for recording speckle patterns. Vibrating circular fringes are generated by interference between the reconstructed speckles recorded for a stationary object beam, and vibrating speckles transmitted through a BaTiO3 crystal after introducing a small out-of-plane shift in the diffuser position. Vibration amplitudes, of the order of the wavelength of light, have been measured by counting of fringes evolving from, and sinking into the center of the circular fringe pattern at the observation plane. The vibration frequency is measured by the number of times the change takes place in the fringes from evolution to sinking mode per unit lime. Object vibration in an arbitrary direction is also analyzed and found to produce similar results. (C) 2001 Society of Photo-Optical Instrumentation Engineers.