Polarization rotation in nonlinear bimodal optical fibers

The effect of nonlinear interactions on second-order modes in circular cross-section optical fibers is considered. In the linear case, analysis using the proper modes reveals that the fiber is a natural polarization rotator. An initial linearly polarized (LP) mode is actually a mixture of proper modes and recurs only periodically as the field propagates; at half-periods the input polarization and field pattern are rotated by 90°. Complete analytical expressions and numerical examples describing the modification of this phenomenon by nonlinearity are presented. For lower powers, the phenomenon persists with period increasing toward infinity at a transition point, beyond which complete polarization rotation is suppressed. At very high powers, the Kerr effect dominates: polarization is fixed in one direction and an LP mode does become a real mode of the system