Automatic Phase Stepping in Fiber Optic Closed-loop Gain Switching

In this paper we describe a new technique for automatic phase stepping in fiber-optic ESPI (Electronic Speckle Pattern Interferometry), useful in both single-point and imaging configurations. The proposed technique is based on closed-loop switching of the gain sign in the control loop used to stabilize the reference wave front phase, with the aid of a P.I. (Proportional-Integral) controller. Simulation and experimental results are compared, showing that by a synchronization signal or by a proper choice of the feedback loop parameters, a sequence of pi/2 steps of the reference wavefront phase, all of the same sign, is obtained. The residual fluctuation of each phase step amounts to +-2.5 mr over a bandwidth of 200Hz. This level of phase stability is well suited for ESPI aplications. This work was partially supported by the Measurement and Testing section of the Fourth framework program of the European Union.