Nonlinear Absorption and Power Limiting in Waveguides

Nonlinear waveguides are considered to be potential candidates for all-optical signal processing applications if suitable substrate/film materials can be identified. The material parameters of primary interest are the nonlinear coefficient n2, the absorption coefficient α and the relaxation time τ [1]. For fast switching with low power requirements, n2/ατ should be as large as possible. Although several different materials have been explored, the quantum-confined nonlinearity such as observed in multiple quantum wells and in microcrystallites of II-VI semiconductor-doped glasses appears to be very attractive. However, in such materials near-band gap enhancement of the nonlinearity is accompanied by absorption and consequent heating of the substrate. The thermal effect is much slower than, for example, the band-filling fast (ps) nonlinearity in doped glasses and consequently it is likely to interfere in high-speed operation of nonlinear devices unless it can be isolated. Thus it is important that its magnitude and role be understood. In this paper, we present measurement of temperature dependence of refractive index and spectral transmission and explain the power limiting observed in planar and channel waveguides in II-VI doped glasses. The results are analyzed on the basis of thermal effects associated with the absorption and the consequent red-shift of the band-edge in the bulk glass. Our quantitative analysis explains not only the nonlinear prism coupler data of other authors [2,3], but also is in excellent agreement with our results of power limiting in planar and channel waveguides [4].