DETERMINATION OF ORDINARY REFRACTIVE-INDEX PROFILE FOR A PLANAR WAVE-GUIDE BY TRANSMISSION SPECTRUM ANALYSIS

We introduce a new method to determine the ordinary refractive index p rofile 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 an d numerically analyzed to fit the measured transmission spectrum. We d emonstrated this method on a proton exchanged planar waveguide in z-cu t LiTaO3. We found that the ordinary refractive index profile of this waveguide can very well be described by a Fermi-Dirac function. The in dex profile evolution with proton exchange time and anneal time were o btained together with the diffusion coefficients for the proton exchan ge and anneal processes. We discovered that, for the proton exchange p rocess in LiTaO3, there exists a surface diffusion phenomenon which ha s a much smaller diffusion coefficient than that of the bulk diffusion . (C) 1998 American Institute of Physics. [S0021-8979(98)04911-1].