Properties of normal-dispersion femtosecond fiber lasers

We report a systematic study of all-normal-dispersion mode-locked fiber lasers. Spectral filtering of a chirped pulse in the cavity is a major component of the pulse shaping in these lasers. We identify the nonlinear phase shift accumulated by the pulse, spectral filter bandwidth, and group-velocity dispersion as the key parameters that determine the behavior and properties of these lasers. Trends in the performance as these parameters are varied are summarized. A wide range of pulse shapes and evolutions can occur. Experimental results from Yb-doped all-normal-dispersion fiber lasers agree reasonably well with the results of numerical simulations.