Widely wavelength-tunable femtosecond soliton in the C + L band using an intracavity birefringence-induced filtering effect

We propose and demonstrate a widely wavelength-tunable nonlinear polarization evolution (NPE) mode-locked fiber laser. The NPE structure serves as both a mode-locking mechanism and intracavity birefringence-induced filtering (IBIF) effect. Without requiring additional filtering devices, the center wavelength of conventional soliton can be tuned from 1552.28 nm to 1604.32 nm by simply adjusting the polarization states and pump power, corresponding to a tuning range of 52.04 nm that covers the C + L band. The proposed laser could output sub-380 fs pulses with 3-dB spectral bandwidths exceeding 10 nm over the whole wavelength tuning range. To the best of our knowledge, this is the shortest pulse duration achieved in a wavelength-tunable Er-doped fiber laser based on NPE mode-locking and the IBIF effect with a wavelength tuning range exceeding 50 nm. This compact, all-fiber, and widely wavelength-tunable laser holds significant potential for applications in LiDAR and bio-imaging.