  We study the modulational instability induced by periodic variations of
group-velocity dispersion and nonlinear coefficients in a highly birefringent
fiber. We observe, for each resonance order, the presence of two pairs of
genuine vector type sidebands, which are spectrally unbalanced between the
polarization components for nonzero group-index mismatch, and one pair of
balanced sidebands emerging and dominating at increasing group-index mismatch.
As the conventional modulational instability manifests itself, it is partially
suppressed by the proximity of these new unstable regions.

Optics;Pattern Formation and Solitons