Numerical maps for fiber lasers mode locked with nonlinear polarization evolution: Comparison with semi-analytical models

We have used a fully vectorial model based on two coupled nonlinear Schrodinger equations to study mode locking and pulse generation initiated and stabilized by nonlinear polarization evolution in a stretched pulse, double-clad, Yb-doped, fiber laser. The model takes explicitly into account gain saturation, finite amplification bandwidth, Kerr-induced self- and cross-phase modulations, group velocity dispersion, polarization control, and linear birefringence. Complete maps versus the orientation of intra-cavity wave-plates have been established. They comprise a large variety of pulse regimes that can be simply obtained by turning the intracavity wave-plate: stable single pulse per round trip, multiple pulsing, unstable pulsing on a continuous wave (CW) background, as well as limit cycles. In addition, we have demonstrated that linear birefringence plays a key role in the pulse-shaping mechanism induced by nonlinear polarization evolution.