Nonlinear cross-polarization switching and domain wall solitons with counterpropagating laser beams in optical fibers

Summary form only given. We report detailed experimental studies of polarization instabilities and domain wall generation with quasi-CW beams that counterpropagate in a polarization isotropic optical fiber. In such a fiber four mutual arrangements of polarizations exist that permit a beam to travel spatially unchanged in the presence of an intense counterpropagating wave. These eigenpolarizations correspond to relative polarizations of the beams which are either linear and parallel or perpendicular, or circular and corotating or counterrotating. A full spatiotemporal stability analysis reveals that only corotating circularly polarized waves maintain their state of polarization unchanged upon counterpropagation. On the contrary, the other eigenpolarizations are unstable with respect to the growth of polarization perturbations. Whenever such unstable polarization configurations were injected into the fiber, we observed in each case that the spatiotemporal instability leads to a spontaneous switching of both polarizations throughout the fiber. In the particular case of two counterrotating circular input waves, the polarization instability of the beams evolves into stable polarization domain wall solitons. We present what we believe is a first complete experimental observation of the stable and unstable polarization configurations for counterpropagating beams in optical fibers. We experimentally confirmed that only the corotating circular polarizations are temporally stable. On the other hand, we also observed that all the other eigenpolarizations are unstable and lead to significant nonlinear cross-polarization switching.