Switched Directional Coupler with Stepped Δβ-Reversal

Abstract

It has been proposed1,2 that optical directional couplers with dielectric waveguides fabricated in electro-optic material and controlled by an applied electric field can serve as amplitude modulators or as switches of information-carrying optical channels. In principle, the switching function can be obtained by control of either the coupling (κ) between the two waveguides or of the difference Δβ in the two propagation constants (phase mismatch), but it was soon recognized3-6 that the control of phase mismatch is preferable as it requires drive voltages that are about 2 orders of magnitude smaller. Recently, Papuchon et al7 and Campbell et al8 have reported the first implementations of such devices using diffused strip guides in LiNbO3 and metal-gap strip guides in GaAs, respectively. These first devices may perform well as amplitude modulators, but it still appears difficult to produce optical switches that can meet the stringent cross-talk requirements demanded by the various communication systems, namely some 25 to 70 decibels depending on the application. One of the reasons for this difficulty is the fact that the Δβ-switched directional coupler allows the electrical adjustment of only one of the two switching states. As a step toward easing this difficulty, we propose here a modified switched coupler configuration, which allows the electrical adjustment of both switching states. This property is obtained by constructing a coupler consisting of two sections with phase mismatch Δβ of opposite sign.