1.7 μm all-fiber Tm-doped mode-locked laser with narrow bandwidth filtering based on the nonlinear multimode interference effect

The mode-locked (ML) all-fiber lasers with a linear cavity, using a single-mode-graded-index multimode-single-mode (SMS) device or a single-mode-few-mode-single-mode (SFS) device, are proposed and experimentally demonstrated. The homemade SMS or SFS devices work on the principle of nonlinear multimode interference. The linear cavity was constructed with two fiber Bragg gratings. By inserting an optimized length SMS into a Thulium-doped cavity, we achieved a 1724.9 nm Q-switched ML pulsed laser at the pump threshold of 29.9 dBm. A ML laser was observed when the multimode fiber in the SMS was stretched with a pump power of 31 dBm. The pulse width was about 326.3 ps, and the repetition rate was 16.5 MHz. The SMS was replaced with the SFS for comparison, and it was observed that the ML laser could be obtained with a better tolerance of the few-mode fiber length. Additionally, the SFS device offered a lower pump threshold and higher slope efficiency.