Instabilities for all-optical switching in two-mode nonlinear waveguides

The coupling of two codirectional modes in waveguides exhibiting, intensity-dependent refractive index may be spatially unstable: small changes in the input light field yeld large variations at the output of the guiding structure. Based on this effect, a number of devices can be designed whose common feature is that an intense beam can be switched by means of a much weaker controlling light field. Fast responding nonlinear glass fibers require high switching power and large dimensions. Moreover, for certain incidence conditions fiber imperfections may cause chaotic switching at the output of a few beat lengths long fiber. Sub-watt switching powers are possible with resonant nonlinearities. In this case, however, large absorptive losses and saturation of the nonlinear index changes may inhibit the instability (and nonlinear switching). We show that using a directional coupler with a two-level saturable nonlinearity imposes rather strict allowances on the material characteristics, as well as considerable wavelength selectivity.