Abstract

Soliton propagation in transparent media was believed to be a feature of materials with cubic nonlinearity. However, solitary wave propagation in the form of mutually trapped spatial envelopes (i.e., spatial simultons) have been recently demonstrated also in quadratic media [1]. Although such trapping was originally predicted nearly two decades ago [2], it attracted a lot of theoretical and experimental efforts only recently, due to the growing activity on cascading, and the impact that solitons might have for potential applications. As far as the latter aspect is concerned, one of the key issues is the possibility of all-optical switching and routing of optical signals. Before the advent of solitons, several schemes employing quadratic materials have already been proposed, and some of them have also been succesfully demonstrated [3-4]. However, the major drawback, common to all the proposed devices is the fact that their operation was proposed and/or investigated within the cw regime. As a consequence, in the pulsed regime, the performances of these devices are essentially limited by the fact that the pulses could not be switched as units or particles, as it would be desirable in a pure digital processing scheme. Viceversa, we show in this communication, that solitary waves of quadratic media have the capability to perform nearly ideal digital operation. In particular we focus our attention on type II SHG in the 1+1 dimensional case, which applies to either temporal solitons, or spatial solitons in waveguides [1]. We investigated the most appealing case for which the second-harmonic (SH) is not launched at the input. Switching is implemented at fundamental frequency (FF) by acting over the two FF input components only. Two different regimes are considered (see Fig. 1): a) for nearly symmetric excitation of the FF soliton components, the output polarization state is uniform and very sensitive to small input imbalances, (i.e., to the input polarization angle), b) the polarization rotation of an intense soliton component may be controlled in a phase-insensitive way by a weaker control pulse. In both cases, the nonlinear crystal in conjunction with a polarization analyzer behaves as a switch. Note that similar results are valid also for the nondegenerate three-wave which is governed by formally equivalent However in this case, the soliton input is constituted by two envelopes at different frequency. Therefore, since switching relies on the nonlinear phase-shifts experienced by the components, soliton switching requires the nonlinear crystal to be placed in an interferometric device.

© 1996 Optical Society of America