Mode-locked thulium-doped fiber laser using a stretched few-mode fiber as a saturable absorber

In this work, an all-fiber saturable absorber (SA) based on nonlinear multimode interference in a short few-mode fiber (FMF) was proposed. Saturable absorption in this all-fiber device was demonstrated with an absorption modulation depth and a saturation fluence of 14.3% and 55.6 mu J cm-2, respectively. A stable mode-locking operation was observed in a thulium-doped passively mode-locked fiber laser using the FMF-based FMF SA. Ultrafast pulses were generated at 1911 nm with a pulse width of 1.96 ps and a 15.69 MHz repetition rate, respectively. The output average power was 2.8 mW, corresponding to a pulse energy of 178 pJ, which was significantly improved compared to graded-index multimode fiber-based SA. The experimental results demonstrate that FMF SA has great potential in eye-safe ultrafast photonics.