Near transform-limited nanosecond pulse generation in mode-locked fiber lasers.

By introducing significant anomalous dispersion in a fiber laser, we numerically obtained a mode-locked pulse with a pulse duration of 1.9 ns and 3-dB bandwidth of 1.3 pm, which corresponds to a time-bandwidth product of about 0.317. The output pulse energy is 1.9 nJ. The variation of output pulses versus introduced dispersion is presented. It is found that, determined by the soliton area theorem, transform-limited nanosecond mode-locked pulses could be expected. In addition, we numerically observed the period doubling phenomenon by increasing the pump power, which indicates that fiber lasers operating in the regime of significant anomalous dispersion can still exhibit rich nonlinear dynamics based on nanosecond pulses. For a fixed cavity dispersion, the pulse energy achievable is limited by the appearance of pulse nonlinear dynamics.