Evaluation of high frequency vibrations using electronic speckle pattern interferometry

Abstract

The available techniques for the study of high frequency vibration using electronic speckle pattern interferometry (ESPI) are briefly surveyed. We concentrate on two methods in particular. The first is a straightforward approach in which a CCD camera is used having a frame rate of at least twice the highest vibration frequency so that the sampling criterion is satisfied. The images are processed and analysed off-line. Digital phase shifting can be also implemented for detailed fringe analysis. The second approach is time-averaged ESPI in which the Bessel fringe function can be analysed in real time by modulating the optical path difference in the interferometer. This can be done either by using a vibrating component or, as in the present work, by direct modulation of the laser wavelength at the frequency of the vibrating mode.