Polarization instability of solitons in photonic crystal fibers

We present experimental and numerical results demonstrating the existence of polarization instability of femtosecond solitons in small core photonic crystal fibers. The frequency of the solitons in our setup shifts rapidly with propagation along the dispersion curve due to the Raman effect. This process can cause initially stable solitons to become unstable, or vice versa depending on the parameters.