Thermal nonlinear absorption and power limiting in waveguides

Power limiting in semiconductor-doped glass waveguides in the cw regime is explained on the basis of thermal effects. From the measurement of the temperature dependence of the refractive index and transmission as a function of wavelength, we show that the change in the absorption coefficient that is due to heating of the bulk-glass substrates at high optical powers accounts for the nonlinear behavior of the waveguides. The power limiting observed in planar waveguides was calculated, and excellent agreement with experiment was obtained. We also report the first measurement to our knowledge of the temperature and spectral dependence of the refractive index of a bulk-glass substrate. These data are useful in interpreting thermal nonlinear effects in the waveguides reported in the literature.