Abstract

Nonlinear directional couplers (NLDCs) have been shown to have great potential as the basis for all-optical switching devices (reviewed, for example by Agrawal and Boyd [1]). The potential for nonlinear switching has been demonstrated in InGaAsP NLDCs, by utilizing resonant non-linearities near the band gap, i.e., λ = 1.55μm, thereby requiring moderate switching powers [2]. However, the required swiching power is higher than that ideally required for semiconductor laser optical sources and so it is important that this is minimised. Significant reductions in switching power have been predicted in coupled fibers with asymmetric configurations when compared to corresponding symmetric designs [3]. Chen et al also forecast further reductions in switching power by matching gain in one guide to loss in the other. We have studied the design of asymmetric semiconductor couplers, where the coupling is finely balanced, and predict the switching power to be substantially reduced when compared with symmetric designs. These investigations are now being extended to vertical configurations, which are expected to have significant advantages over the more conventional horizontal designs. By adopting a vertical configuration, the waveguiding regions can be realistically closer together, thereby reducing the coupling length, which will allow for a minimum of loss over the distance required for switching. Furthermore, the geometrical asymmetry of the coupler can be maximized, by coupling a planar-guide to a rib-guide. Likewise material asymmetry can be introduced easily into a vertical configuration. Finally, by having the guides in different planes, the doping requirements needed for biasing in order to induce gain (at transparency), can be more easily realised.

© 1996 Optical Society of America