Vertical coupler device design for reduced-power optical switching using nonlinearity mismatch

The potential of nonlinear directional couplers (NLDCs) as the basis for all-optical switching devices has been studied extensively [1]. The mode of operation of an NLDC as an all-optical switch is based on the optical intensity dependence of the nonlinear refractive indices of the two waveguiding regions of the coupler, where the modal propagation contants (β) of the waveguides are matched at low powers and mismatched at high powers. When the waveguides are in close proximity, the light in the input waveguide will couple to the other waveguide in a coupling distance Lc at low powers, but at high powers the waveguides will decouple and the light will remain in the input waveguide. The power at which the light is equally split between the waveguides is defined as the critical power, Pc [2]. Proposals to reduce Pc in fibers NLDCs have included asymmetric designs, matched gain in the input waveguide to loss in the other [3], and non-identical nonlinear refractive indices in the two waveguides [4], However, for true optoelectronic integration it is desirable to design NLDCs using semiconductor materials. Our previous studies of semiconductor NLDCs for the first time have focussed on the enhancement of asymmetry and realistic geometries for matching gain to loss [5]. This has been demonstrated by the use of vertical design configurations with a quasi- planar-waveguide coupled to a rib-waveguide (see figure 1). Such designs have a high degree of geometric asymmetry, and because the waveguides are in separate planes the doping and biasing requirements needed for inducing gain in only one waveguide are experimentally realisable. Furthermore, the separation of the waveguides is only limited by the epitaxial fabrication process. This indicates that the waveguide separation in a vertical NLDC can be much smaller than that for a conventional two-rib horizontal design, thereby further enhancing the predicted characteristics of the design by minimizing Lc. In this paper, we extend these studies to the design of vertical NLDC geometries with nonlinear refractive index mismatch between the two waveguides. This is achieved by having the two waveguiding layers made of different semiconductor materials. The structures have been designed and optimized using two computational modelling techniques, the Spectral Index method and Coupled Mode Theory. In this paper we report a fesibility study of the predicted advantages of such a design and the possible reductions in Pc.