  A modification of the usual Type-II second-harmonic-generation model is
proposed, which includes two additional features: linear conversion and walkoff
(group-velocity difference) between two components of the fundamental-frequency
(FF) wave. Physical interpretations of the model are possible in both temporal
and spatial domains. In the absence of the intrinsic walkoff, the linear mixing
makes real soliton solutions stable or unstable, depending on the relative sign
of the two FF components. Unstable solitons spontaneously re-arrange themselves
into stable ones. Fundamental solitons change their shape (in particular, they
develop chirp) but remain stable if the intrinsic walkoff is included. In
addition, quasi-stable double-humped solitary waves are found in the latter
case.
