Polarization instabilities in birefringent nonlinear media: application to fiber-optic devices.

A medium with linear birefringence will preserve the polarization state of a light beam whose electric-field vector is oriented along either principal axis of the medium. In that regard, the two principal axes are entirely equivalent. It is usually assumed that this equivalence also holds at high intensities and that an intense beam oriented along either axis will suffer no changes in its polarization, barring imperfections that cause scattering between the axes. In this Letter, we show that the intensity-dependent refractive index leads to an instability in the polarization state of a light wave oriented along the fast axis of a birefringent medium. The slow axis remains a stable guiding center. Depending on the input intensity and the polarization state, the polarization ellipse can execute either oscillatory or rotatory motions about the slow axis in a manner analogous to the motion of a nonlinear pendulum. The results presented here are important both for fiber-optic devices that rely on an intensitydependent polarization state in a birefringent fiber1 -3 and for those nonlinear devices in which the preservation of linear polarization is essential. The spatial evolution of the polarization state in a birefringent nonlinear medium is conveniently described by coupled wave equations for the orthogonal circularly polarized modes c+ and c(Ref. 4):