Polarization Switching in a Mode-Locked Fiber Laser Based on Reduced Graphene Oxide

We present an experimental study of the polarization switching process in a passively mode-locked fiber laser that incorporates a highly nonlinear element formed by depositing reduced graphene oxide onto a fiber taper, which provides both saturable absorption effect and high nonlinearity. Increasing pump power, the laser moves from the self-pulsing mode to the rectangular-pulse mode and shows different polarization switching characteristics. At low pump powers, the polarization switching effects are weak. At a high enough pump power, the polarization switching becomes significant and the switching frequency can be twice of the fundamental cavity round-trip frequency. Beyond a certain high pump power, the polarization switching becomes intense and the switching frequency is equal to the fundamental cavity round-trip frequency. To the best of our knowledge, it is the first time to report such a process, which provides new information useful for the development of a more complete theory for the polarization switching dynamics in a mode-locked laser.