Slowdown and splitting of gap solitons in apodized Bragg gratings

We study the motion of gap solitons in two models of apodized nonlinear fibre Bragg gratings (BGs), with the local reflectivity kappa varying along the fibre. A single step of kappa, and a periodic array of alternating steps with opposite signs (a 'Bragg superstructure') are considered. These structures may be used in the design of various optical elements employing the gap solitons. A challenging possibility is to slow down and eventually halt the soliton by passing it through a step of increasing reflectivity, thus capturing a pulse of standing light. First, we develop an analytical approach, assuming adiabatic evolution of the soliton, and making use of the energy conservation and balance equation for the momentum. Comparison with simulations shows that the analytical approximation is quite accurate, unless the inhomogeneity is too narrow, or the step is too high: the soliton is either transmitted across the step or bounces back from it. If the step is narrow, systematic simulations demonstrate that the soliton splits into transmitted and reflected pulses (splitting of a BG soliton which hits a chirped grating was observed in experiments). Moving through the periodic 'superstructure', the soliton accumulates distortion and suffers radiation loss if the structure is composed of narrow steps. The soliton moves without any loss or irreversible deformation through the array of sufficiently broad steps.