Determination of ordinary refractive index profile for a planar waveguide by transmission spectrum analysis

We introduce a new method to determine the ordinary refractive index profile of a planar waveguide on a crystal plate. The index profile of the planar waveguide can be numerically divided into multilayers; the transmission spectrum of the multilayer waveguide can be calculated and numerically analyzed to fit the measured transmission spectrum. We demonstrated this method on a proton exchanged planar waveguide in z-cut LiTaO3. We found that the ordinary refractive index profile of this waveguide can very well be described by a Fermi-Dirac function. The index profile evolution with proton exchange time and anneal time were obtained together with the diffusion coefficients for the proton exchange and anneal processes. We discovered that, for the proton exchange process in LiTaO3, there exists a surface diffusion phenomenon which has a much smaller diffusion coefficient than that of the bulk diffusion. (C) 1998 American Institute of Physics. [S0021-8979(98)04911-1].