The area of nonlinear pulse interactions in Bragg grating structures, although largely unexplored, contains many interesting and novel effects. Perhaps the simplest of these is the CW switching of a weak probe by a strong pump. Recall that a Bragg grating reflects strongly around the Bragg resonance frequency ω0 which is inversely proportion to the average refractive index. In a nonlinear medium the presence of a strong pump alters the refractive index, and thus frequencies which were reflected (or transmitted) by the grating can be transmitted (or reflected). This effect was first seen by LaRochelle et al. in 19901 and to date this is the only experimental work done in this area. However since then considerable theoretical work has been done onpulse interactions2 in fibre Bragg gratings (FBGs). Also experimental reports of nonlinear propagation in FBGs have started appearing in the literature3,4. A major factor in this upsurge of interest has been the development of techniques for writing long gratings at arbitrary wavelengths using the side illumination of fibres with UV light. This fact coupled with the latest generation of high power fibre sources allows the exploration of pulse interactions in FBGs in great detail.