Wavelength-tunable mode-locked thulium-doped fiber laser in the 1.7 μm spectral region

We report a continuously tunable, fiberized mode-locked thulium-doped fiber laser operating in the 1.7 µm region, capable of delivering both conventional and dissipative solitons across a > 50 nm tuning span. To overcome the strong gain competition at longer wavelengths, a decoupled composite filtering strategy is employed. Specifically, a statically coiled standard single-mode fiber (SMF) paired with a short-pass filter (SPF) is used to flatten the gain profile efficiently. Independent of this fixed filtering, smooth, continuous wavelength selection is achieved via an intracavity Fabry-Pérot (FP) etalon formed by simply butt-coupling a gradient-index (GRIN) lens to a semiconductor saturable absorber mirror (SESAM). Furthermore, through systematic intracavity dispersion management, broadband stretched pulses are generated and externally compressed to 277 fs. This robust and versatile architecture provides a highly practical source for demanding ultrafast optical applications, such as multiphoton microscopy.