Refractive index measurement of optical waveguides using modified end-fire coupling method.

We present a modified end-fire coupling method to reconstruct two-dimensional index profiles of optical waveguides. The reconstruction is based on the measured differential optical fields and an inverse algorithm for the Helmholtz equation. The differential fields are obtained from spatially perturbed images recorded by a CCD camera. A closed-loop piezoelectric stage given a sinusoidal wave and an oil-immersion objective lens for high-resolution images are used in the measurement. A single-mode fiber and a Ti:LiNbO(3) waveguide with known index profile at 632.8 nm are measured for demonstration. The measured index profiles agree quite well with core regions of known index profiles.