Measurement of refractive index profiles in optical waveguides by moiré deflectometry technique

Measurement of refractive index profiles in optical waveguides is an important issue for controlling manufacturing procedures and determination of some important characteristics of waveguides. In this paper, an optical waveguide is considered as a phase object. When collimated laser beams are illuminated on waveguides, the refractive index gradient distorts the plane waves, then the distortion is determined by use of the moiré deflectometry method. Dimensions of the waveguides under study are on the order of 10 μm, so the moiré deflectometry setup was changed and optimized for measuring in the micrometer dimensions. This modified and generalized moiré deflectometry technique helps to intensify plane wave distortion thousands of times and expands it to be measurable. Finally, by analysis of moiré fringes, we succeeded in determining the refractive index profile of an optical waveguide by approximation of semi-cylindrical symmetry.