Observation of Polarization-Locked Vector Solitons in Optical Fiber

We experimentally observe polarization-locked vector solitons in a passively modelocked fiber laser. The intracavity vector soliton consists of two components orthogonally polarized along the principal axes of the linearly birefringent laser cavity and propagates with a fixed relative optical phase of +- pi/2 and hence, a constant elliptical polarization state. The linear birefringence of the cavity is cancelled by a difference between the nonlinear phase shifts of the two polarization components due to their unequal amplitudes. This dynamic equalization of their phase velocities results in the stable propagation of an elliptically polarized vector soliton. In addition, we observe the nonlinear instability of the fast birefringent axis manifested as a intracavity pulse, linearly polarized along the slow axis of the cavity. We present the experimental characterization of both the polarization-locked vector soliton and fast axis instability and discuss the nonlinear mechanism creating both phenomena.