Out-Of-Plane Displacement Measurement Using Multiple-Exposure Speckle Recording In Batio3 Crystal: Use Of Converging Beam Illumination In Free Space Geometry

We propose a multiple speckle recording scheme for quasi-real-lime out-of-plane displacement measurement. The technique is based on two-wave mixing in a photorefractive crystal. A coverging beam is employed for illuminating the diffuse surface. Objective speckles thus generated are recorded as signal information inside a BaTiO3 crystal. Amplification mode of two-wave mixing geometry enables the system to operate even on very weak signals.Finite storage time of the crystal is exploited to record multiple signal information using sequential recording procedure. Circular fringes formed as a result of out-of-plane displacement, are used for displacement measurements. Multiple recording mechanism is employed to obtain sharpened fringes enabling accurate measurements. Theory has been worked out to explain the scheme. Experimental results have been obtained for multiple recording (as many as 10 recordings) case to show accurate measurement of out-of-plane displacement. Improvement in accuracy, readability as well as stability make the system very attractive for future applications.