Automatic phase stepping in fiber-optic ESPI by closed-loop gain switching

In this paper we describe a new technique for automatic phase stepping In f iber-optic Electronic Speckle Pattern Interferometry, (ESPI), useful in bot h single-point and imaging configurations. The proposed technique is based on closed-loop switching of the gain sign in the control loop used to stabi lize the reference wavefront phase, with the aid of a Proportional-Integral (P.I.) controller. Simulation and experimental results are compared, showi ng that by a synchronization signal or by a proper choice of the feedback l oop 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 s tep amounts to +/-2.5 mr over a bandwidth of 200 Hz, This level of phase st ability is well suited for ESPI aplications.