All-normal dispersion fiber lasers with magneto-optical polarization controllers.

An all-normal dispersion fiber laser mode-locked by nonlinear polarization evolution is presented in this paper. A magneto-optical polarization controller is employed in a fiber laser to optimize the polarization state and mode-locking operation. In order to provide an adjustable and large enough magnetic field for magneto-optical crystals, a magnetic yoke is designed with silicon steel and copper coil. The Q-switched and continuous-wave mode-locking regime of the fiber laser are investigated experimentally. At a pumping power of 2.08 W, the laser can generate stable mode-locking pulses with an average power of 213 mW. The repetition rate and the pulse duration are 63.7 MHz and 6.2 ps, respectively, corresponding to a pulse energy of 3.34 nJ and a peak power of 539 W. The laser can operate continuously in mode-locking states over five hours with almost no variation in pulse profile, optical spectrum, and output power.