Incoherent light interferometer for single-shot, dual wavelength, areal topography measurements of a reflective surface

This paper introduces an interferometer for single-shot areal quantitative phase imaging at two wavelengths simultaneously, suitable for use with low coherence sources. It operates in reflection geometry with on-axis illumination, so that it can be conveniently applied to surface texture measurements. The system consists of two identical 4f systems forming the reference and sample arm. The reference arm is terminated by an optical assembly consisting of a dichroic mirror and two diffraction gratings, providing two separate dispersed off-axis reference beams. Two sets of distinct, separable interference fringes allow the use of the synthetic wavelength technique to obtain an extended unambiguous range of measurement recorded in a single frame. The implementation presented here was operating with superluminescent diode (SLED) sources with central wavelengths of 757 nm and 841 nm, giving a theoretical unambiguous range of 3.79 µm. The system was experimentally tested by measuring a sample with a 1 µm deep groove in electroformed nickel. The result shows good agreement with a contact stylus profiler measurement, although further work is needed to limit the noise in the interference signals.