Handling unfavourable polarization states in a polarization-based shearing speckle interferometer

A polarization-based shearing speckle interferometer requires the incoming scattered light to have proper polarization characteristics: the light should be equally distributed over two orthogonal polarization states in order to obtain two interfering beams with equal intensities and the highest possible modulation. Many surfaces scatter linearly polarized light randomly, providing equal intensities for the interfering beams on average. However, when a diffusely reflecting metallic surface is illuminated by a linearly polarized laser beam, the polarization direction of the illuminating beam is retained to a large extent in the scattered light. As a result light entering the interferometer will not be randomly polarized, which may lead to low modulation due to unbalanced object and reference beams. A solution that handles the problem effectively is proposed. It consists of a quarter-wave plate positioned in front of the interferometer, oriented at 45°. It is shown that unfavourable predominant polarization states encountered when testing unprepared metallic surfaces can be converted into favourable ones, thereby obtaining well balanced object and reference beams, irrespective of the polarization direction of the incoming light.