Experiment and modeling of passively Q-switched ytterbium doped double-clad fiber lasers

High power passively Q-switched fiber lasers have been demonstrated with different ytterbium doped double-clad fibers by using Cr4+:YAG saturable absorber. With small (5.4 mum) and medium (10 mum) core fibers, single mode outputs have been obtained with pulse energy of tens of muJ, pulse duration of the order of a hundred nanoseconds and pulse-repetition-rate of up to 300 kHz. A theoretical model is developed to predict the laser spectrum and numerically simulate the output characteristics versus pump power. The focused beam in the Cr4+:YAG crystal is assumed to have a Gaussian profile and is analyzed using traveling wave rate equations. The effect of amplified spontaneous emission is also investigated in the simulation, which shows reasonable agreement with experimental observations. With large-mode-area (25 mum) polarization maintaining ytterbium doped fiber, the Q-switched output shows stimulated Brillouin scattering. Linearly polarized output with >300 kW peak power and a pulse duration as short as 490 ps have been obtained. A theoretical model is developed to simulate passive Q-switching with the stimulated Brillouin scattering, which shows good agreement with the experiment.;A compact, adjustment free self Q-switched ytterbium doped fiber laser has been demonstrated with fiber ring mirror, in which cooperative Rayleigh-Brillouin scattering was employed as a gain switching mechanism. As short as 600 ps pulses have been obtained at both stimulated Brillouin and Raman frequencies, with peak powers estimated to be up to 26 kW and 5 kW, respectively.;A high power two-wavelength passively Q-switched Yb doped fiber laser has been achieved with a very simple cavity configuration. Single transverse mode output at 1040 nm and 1070 nm was obtained with tens of muJ pulse energies and hundred nanosecond pulse durations. Modeling of the transient build up of the free-running laser shows the two-wavelength behavior can be attributed to the existence of two gain peaks of Yb-doped fiber under intermediate pump conditions. A theoretical simulation model qualitatively explains the dynamics of the two-wavelength Q-switching behavior observed in the experiment indicating that longer fibers and higher pump powers favor 1070 nm laser output and reproduces the pulse pattern where two wavelength pulses alternate at an intermediate pump power.