Speckle shear photography using photorefractive crystals for in-plane strain measurement

Abstract

This paper present new optical configurations in speckle shear photography to measure in-plan displacement and the strain in real time using photorefractive (PR) crystals as recording medium. Speckle photography studies are made using a simple two-beam coupling configuration PR beam coupling in barium titanate crystals is characterized experimentally by measuring the signal beam gain and the exponential gain coefficient at 440 nm, 632.8 nm and 780 nm. In speckle shear photography, a diffused object illuminated with two parallel narrow laser beams is imaged inside the crystal, and a pump beam is added at this plane. The speckle patterns due to each beam and the pump beam produce index gratings. When the object is deformed, the speckle patterns shift consequently. We now have four speckle fields two generated from the interaction of pump beam with the index gratings and two pertaining to deformed states directly transmitted through the crystal. Thus, the fields from respective points on the object interfere after passage through the crystal and produce the Young's fringe patterns. Due to strain, the fringes in each pattern are of different width and orientation, resulting in the generation of a moire pattern. The strain is obtained from the width and orientation of the fringes in the moire pattern. The experiments are conducted on a specimen with a notch, which is subjected to tensile loading. The above studies are carried out at three different laser wavelengths, namely, 441.6 nm, 632.8 nm and 780 nm and the results are compared.