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Digital shearing speckle interferometry applied to optical diagnostics in flow

For optical diagnostics of fluid mechanics and thermal flow it is difficult to use holographic interferometry or digital speckle pattern interferometry under the harsh engineering environments because there are various disturbances in two dividing coherent beams caused by ambient refractive-index change, optical component disturbance, etc. The digital shearing speckle pattern interferometry (DSSPI), proposed in this paper, uses the interference phenomena between two objective means which pass through a slightly different area in space. In this paper, the experimental arrangement and basic principle of the digital shearing speckle pattern interferometry using optical polarization phase shift are presented in detail. The mathematical models of experimental DSSPI data processing for 2D and axis-symmetrical flow fields are proposed respectively. The technique of the optical polarization phase shift is described to extract quantitatively the phase information form shearing speckle interferograms. Compared with PZT phase-shifter, the optical polarization phase-shifter is more convenient because it does not need a complex calibration. Last, the shearing speckle interferograms and phase images for a candle flame and a natural convection of a heated vertical cylindrical tube are presented. The research show that the DSSPI is an ideal observation and measurement technique for optical diagnostics of fluid mechanics and thermal flow.