White-light geometric phase interferometer for surface profiling

Abstract

Even though interferometric profilers offer excellent axial resolution, their application is limited by ambiguities which arise if the measurement range involves a change in the optical path difference greater than a wavelength. This limitation has been overcome by using white light and scanning the object in height to locate the fringe-visibility peak. In this case, the measurement range can be increased to many wavelengths while maintaining high axial resolution. While various digital filtering techniques have been used to recover the fringe visibility curve from the sampled intensity data, they tend to be numerically intensive. The use of phase-shifting techniques to simplify the processing is complicated by the fact that the phase change introduced by changing the optical path is wavelength dependent, leading to systematic errors in the values of the fringe visibility. We show how an achromatic phase-shifter which operates on the geometric phase and generates the required phase shifts without any change in the optical path difference, can be used to evaluate the fringe visibility directly and locate the position of the fringe-visibility peak along the scanning axis.