  Concept of spectral soliton coupling is proposed to explain the
group-velocity-matching dispersive wave generation and the soliton spectral
tunneling effect. Soliton eigen state is found corresponding to a spectral
phase profile under which the soliton phase shifts induced by the dispersion
and the nonlinearity are instantaneously counterbalanced. A local but non-eigen
soliton pulse will shed off energy and form radiations at other wavelengths,
during the coupling to the soliton eigen state. Group-velocity-mathing between
the local and generated wave is necessary not only as it produces a
coupler-like phase profile which supports soliton coupling but also because it
physically supports a soliton phase matching condition, which means the
generated wave has a soliton state phase matched to the local exciting pulse.
Examples in realistic photonic crystal fiber structures are also presented.
