Understanding Soliton Spectral Tunneling as a Spectral Coupling Effect

Soliton eigenstate is found corresponding to a dispersive phase profile under which the soliton phase changes induced by the dispersion and nonlinearity are instantaneously counterbalanced. Much like a waveguide coupler relying on a spatial refractive index profile that supports mode coupling between channels, here we suggest that the soliton spectral tunneling effect can be understood supported by a spectral phase coupler. The dispersive wave number in the spectral domain must have a coupler-like symmetric profile for soliton spectral tunneling to occur. We show that such a spectral coupler exactly implies phase as well as group-velocity matching between the input soliton and tunneled soliton, namely a soliton phase matching condition. Examples in realistic photonic crystal fibers are also presented.