Engineering short-wavelength emission in Tm³⁺-doped silica fibers via ring-shaped core doping

Thulium-doped silica fibers are attractive candidates for light sources with emission in the 1.7-1.9 µm spectral range. Still, short-wavelength efficient generation is hindered by strong reabsorption associated with the quasi-three-level nature of Tm³⁺ ions. In this work, numerical modeling is used to investigate how a ring-shaped radial distribution of Tm³⁺ ions in the core can favor short-wavelength emission. The results show that, under cladding pumping, moving the doped region from the center toward the edge broadens the ASE spectrum, shifts its spectral centroid toward shorter wavelengths, and delays the onset of spectral narrowing associated with weak cavity feedback, while requiring a longer optimal fiber length and maintaining a high gain level. With increasing pump power, the ring-doped geometry preserves a broader short-wavelength ASE contribution than the centrally doped configuration, whereas the latter becomes spectrally narrowed much more rapidly. A double-clad Tm/Al/Ge-doped silica fiber was fabricated by modified chemical vapor deposition with chelate doping technology (MCVD-CDT), with a core/cladding diameter of 17/105 µm, and its performance was verified experimentally through luminescence measurements, broadband emission, and lasing near 1.8 µm.