Convenient method for calibrating nonlinear phase modulators for use in phase-shifting interferometry

A method to calibrate a nonlinear phase modulator is de- scribed that is based on the minimization of a function obtained from a sequence of interferometric images. The method is equally applicable to smooth wavefront and speckled wavefront interferometry, allowing con- venient in situ calibration for a wide range of interferometer configura- tions. The main advantage over previous calibration methods is that both the linear and quadratic phase variation terms are measured. The per- formance of the proposed method is verified by experiments in which the phase shift is determined independently by a Fourier transform method.