Spectroscopy of Ho3+-doped fluoride glasses for green double-clad fiber lasers

Holmium ions (Ho³⁺) are attractive for generation of green emission according to the ⁵F₄+ ⁵S₂ → ⁵ I₈ transition. We report on the assessment of the potential of Ho³⁺ -doped fluoride glasses for green fiber lasers exploiting the double-clad fiber geometry. ZBLAN glasses doped with HoF3 (0.1 – 0.9 mol%) were studied. The absorption cross-section for the ⁵ I₈ → ⁵F₁+ ⁵G₆ transition is 1.75×10^-20 cm² at 448.5 nm. Under excitation in the blue, the glasses exhibit intense green luminescence. The stimulated-emission cross-section for the ⁵F₄+ ⁵S₂ → ⁵ I₈ transition is 0.67×10^-20 cm² at 549 nm. With increasing the HoF₃ doping level, the luminescence lifetime of the ⁵F₄+ ⁵S₂ states slowly decreases in the range of 291 – 180 μs. The output performance of a diode-pumped green Ho fiber laser was simulated for a fiber geometry with a double D-shaped inner cladding. The variable parameters were the HoF3 doping level, the fiber length, and the output coupling. It is shown that the generation of watt-level green output from such a laser is possible when using relatively short (<1 m) fibers with low doping levels (about 0.5 mol% HoF₃), as well as high transmissions of the output coupler (<60%, depending on the passive losses in the fiber).