Enhanced long-wavelength emission and 2109 nm lasing in Tm ³⁺ /Ho ³⁺ co-doped germanate-core/silicate-cladding glass fiber

Fiber lasers emitting beyond 2.1 μm remain attractive owing to their superior performance in molecular "fingerprint" gas sensing, supercontinuum generation, and advanced bioimaging applications. The Ho³⁺: ⁵I₇→⁵I₈ transition offers a larger emission cross-section and a longer metastable lifetime than Tm³⁺ at 2.1 μm. However, the scarcity of commercial pump sources has hindered the widespread adoption of Ho³⁺-doped fiber lasers. Tm³⁺/Ho³⁺ co-doping has emerged as an effective strategy to address this limitation. In this work, a Tm³⁺/Ho³⁺ co-doped BaO-Sb₂O₃-GeO₂ (BSG) germanate glass exhibiting low phonon energy (∼810 cm^-1) and high crystallization resistance (Δ<i>T</i> = 250°C) was designed. To optimize long-wavelength emission from Ho³⁺, both forward and reverse energy transfer processes were systematically investigated using spectroscopic measurements and molecular dynamics simulations. Employing a melt-in-tube technique, we fabricated a germanate-core/silicate-cladding glass fiber that combines the extended fluorescence lifetime of Ho³⁺: ⁵I₇→⁵I₈ (4.60 ms) with the mechanical robustness of a silicate cladding. Compared with previously reported Tm³⁺/Ho³⁺ co-doped fibers, the custom-designed fiber exhibits an amplified spontaneous emission peak dominated by Ho³⁺ near 2.1 μm. Laser output at 2109 nm was achieved in a 2 cm fiber, approximately 1/50 the length of a typical silica fiber, underscoring its high potential as a gain medium for integrated fiber devices operating beyond 2.1 μm.