Experimental observation of polarization instability in a birefringent optical fiber. fiber.

We present the first experimental demonstration of a spatial instability in the nonlinear evolution of the state of polarization of an intense light beam in a birefringent Kerr-like medium. As the peak power crosses the threshold for the instability, we observed strong intensity-dependent power transfer between the two counterrotating circularly polarized waves propagating along a birefringent optical fiber. The experimental results agree well with the theory.