<title>In-line phase-shifter calibration and drift measurement and compensation for digital speckle pattern interferometry (DSPI)</title>

Abstract

Few conditions are absolutely necessary to improve the accuracy of quantitative measurements of displacements in an interferometer: the accurate calibration of the phase- shifter, the control and the suppression of the interferometers drift. Calibration consists in measuring phase shifts generated by piezoelectric transducers (PZT). We have developed an original in-line PZT calibration based on speckles intensity modulation. A new method to measure the phase drift versus time in the whole interferometer (reference and object arm) is presented. Two innovative methods to compensate phase drift in real time allow accurate displacements measurements. Both compensation methods do not use a feedback loop active control during DSPI measurements. So it allows us to avoid overshoot or oscillation problems linked to the feedback loop. All presented methods allow inexpensive data processing and were successfully applied to an out-of-plane sensitive interferometer. PZT calibration, drift measurements and real time compensation are automated.