Theoretical Comparison of NPR and Hybrid Mode-Locked Soliton Thulium-Doped Fiber Lasers

The evolutions and properties of solitons in a thulium-doped fiber laser hybrid mode-locked by the coaction of nonlinear polarization rotation (NPR) and a semiconductor saturable absorber mirror (SESAM) have been numerically investigated. The mode-locking threshold can be greatly reduced by inserting a SESAM into the NPR-based mode-locked laser cavity. The features of solitons in this hybrid mode-locked laser with different cavity lengths and linear cavity phase delay bias have also been analyzed. The simulations show that this hybrid mode-locking technique is beneficial for shortening the cavity and has excellent phase delay tolerance. We further show the relationship among the linear cavity phase delay bias, the cavity length and the single soliton energy in hybrid mode-locked lasers, which can help control the maximum achievable soliton energy for different applications.