Precision Measurement of Deformation Using a Self-calibrated Digital Speckle Pattern Interferometry (DSPI)

A self-calibrating phase-shifting technique using a Michelson Interferometer is presented to measure phase distribution more accurately in Digital Speckle Pattern Interferometry (DSPI). DSPI is a well-established technique for the determination of whole field deformation via quantitatively measuring the phase distribution of speckle interferograms that use the phase shifting technique. In the phase shifting technique, the phase distribution in a speckle interferogram is quantitatively determined by recording multiple intensity images (usually four images) in which a constant phase shift, e.g. 90 degrees, is introduced between each consecutive image. A precise phase determination is greatly dependent on the accuracy of the phase shift introduced. The popular methods to minimize the error resulting from inaccurate phase shift use various algorithms and need to record five or eight images (rather than four images). This requires a higher demand on system stabilization which increases measurement difficulty. Moreover, these methods are passive. In this paper, we present an active feedback control method to minimize the error induced by inaccurate phase shift. It utilizes a Michelson Interferometer for feedback and controlling. The Michelson Interferometer is integrated into the DSPI system and shares a common optical path. When the optical path length of the reference arm in the DSPI is changed by a piezoelectric transducer or environmental disturbance, the output of Michelson Interferometer will be changed accordingly. The output of Michelson Interferometer acts as a feedback and then controls the piezoelectric transducer. Thus the phase shift is stabilized by the embedded Michelson Interferometer. An experiment is conducted and the results are presented in this paper.