Vibration frequency measurement with speckle interferometry

We propose a new method for measuring vibration frequency with electronic speckle pattern interferometry. In this method, laser beam wavelength is modulated with an independent frequency. In accordance with a frequency difference between the modulation and the vibration, a maximum intensity in a fringe pattern image changes. When the modulation magnitude is reduced, the frequency difference between the vibration and the modulation, in which the fringe pattern diminishes, widens and the maximum intensity in the fringe pattern image alters gently while the modulation frequency is scanned. Then we can search the vibration frequency, in which the maximum intensity in the image has a peak when the modulation frequency equals to the vibration frequency, with a hill-climbing method. It is confirmed in an experiment that the vibration frequency can be measured in a short time sufficient for practical use.