Birefringence engineering-enabled spectral detuning in a thulium-doped ultrafast fiber laser

Wavelength-tunable soliton generation from the thulium-doped fiber laser (TDFL) is the key optical carrier for the high-speed photonic applications in new-era communication, bio-imaging, and range-sensing systems covering the 2.0-μm band. To meet these wavelength-switching/scanning demands, this work demonstrates the intracavity birefringent polarization detuning mechanism for self-starting and wavelength-tuning of the sub-picosecond soliton from the TDFL. With the unique intracavity-birefringence-manipulated polarization detuning feature, such a TDFL soliton further exhibits a broadband wavelength tunability of up to 22 nm (from 1.936 μm to 1.958 μm) without any physical filters, and the obtained time-bandwidth product of 0.319 remains almost the transform-limited condition. The 5%-coupled output power is 3.2 mW with a slope efficiency of 0.94% under pumping at 0.5 W. The pumping threshold is only 0.25 W for self-starting the TDFL soliton repeated at 28.55 MHz with a self-amplitude-modulation coefficient γ SAM of −1.89 × 10 −1 sin2θ p , and a Kelly sideband frequency spacing of 0.5 THz. The birefringent polarization detuning angle is up to 18 degrees to allow the TDFL soliton wavelength tolerance to be stabilized during environmental perturbations.