We experimentally observe polarization-locked vector solitons in a passively mode-locked fiber laser. The vector soliton pulse is composed of components along both principal polarization axes of the linearly birefringent laser cavity. For certain values of birefringence and pulse energy these components propagate with a constant relative optical phase of ±π/2, and hence the pulse has a fixed elliptical polarization state. The linear birefringence of the cavity is canceled by the nonlinear birefringence created by the unequal amplitudes of the two polarization components. This dynamic equalization of the phase velocities of the components results in the stable propagation of an elliptically polarized vector soliton pulse. Under different conditions we also observe the nonlinear instability of the fast principal axis as an intracavity pulse linearly polarized along the slow axis of the cavity. We present the experimental characterization of both the polarization-locked vector soliton and the fast axis instability and discuss the nonlinear mechanism creating both phenomena.
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