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Photorefractive Phase Reversal Speckle Photography for Evaluation of In-Plane Displacement
Nandigana Krishna Mohan and Rekha Mathew

A real-time, photorefractive phase shifting read-out system was developed for quantitative evaluation of fringe patterns obtained in speckle photography. The optical system consists of a phase reversal speckle photography (PRSP) arrangement with a narrow laser beam as the light source and the photorefractive crystal, BaTiO3 as recording medium. The phase (contrast) reversal is accomplished by varying the pressure within an air-filled quartz cell inserted in the pump beam (reference beam) of a two-beam coupling configuration. The pump beam interferes with the imaged speckle pattern (object beam), and creates a dynamic grating inside the BaTiO3 crystal. This work shows that phase reversal is achieved when a p-phase shifted speckle pattern overlaps an unshifted speckle pattern in the observation plane. A detailed analysis of the speckle pattern analysis is carried out using the PRSP technique. The experimental results are shown on a diffuse surface which is subjected to rotation in its own plane using the four-frame phase shifting technique. With the slow response time of the BaTiO3 crystal, and by skillful management of the exposure times, the proposed technique is a simple, attractive and alternative method for fringe analysis.

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