Refractive-index measurement of high-refractive-index integrated-optic components

The most commonly-used technique for measuring the refractive index profile of an optical fiber is the refracted near-field method. This standard method cannot be directly used for integrated optical waveguides such as silica-on-silicon or LiNbO3 because of the geometrical constraints imposed by the slab waveguide. A modified method was described in previous work and subsequently implemented with some improvements (e.g. use of a calibrated solid refractive-index reference element: a simplified waveguide identification) in a commercial apparatus. However, the non-availability of suitable index-matching liquids having an index of refraction greater than about 1.8 prevent this apparatus being used with high-index DUTs. In this paper, we propose and experimentally verify a modified instrument that permits the characterization of the index profile of high refractive-index waveguides such as LiNbO3. Provided that the waveguide is written in a homogeneous bulk substrate with a known index, this modified approach allows for spatial and refractive-index resolutions that are practically as good as those obtained with the standard technique applied to optical fibers.