Nonlinear Fiber Optics - 4 ed. Agrawal

206 Chapter 6. Polarization Effects
when modulation instability occurs in high-birefringence fibers [62]. The formation of
dark solitons is not surprising if we recall from Chapter 5 that optical fibers support
only dark solitons in their normal-GVD regime.
In all of these experiments, fiber birefringence plays an important role. As discussed
before, vector modulation instability can occur in isotropic fibers (n x = n y ) such
that the gain spectrum depends on the polarization state of the input CW beam. Unfortunately,
it is difficult to make birefringence-free fibers. As an alternative, modulation
instability was observed in a bimodal fiber in which the input beam excited two
fiber modes (the LP 01 and LP 11 modes) with nearly equal power levels, and the two
modes had the same group velocity [60]. In a 1999 experiment [63], a nearly isotropic
fiber was realized by winding 50 m of “spun” fiber with a large radius of curvature
of 25 cm. The beat length for this fiber was ∼1 km, indicating a birefringence level