Populations and optical gain properties of Ho³⁺ in sulfide glasses for U-band (1625-1675 nm) fibre optic amplifiers

Spectroscopic properties of Ho³⁺-doped sulfide and chalcohalide glasses were investigated for their potential for U-band fibre optic amplifiers. Emissions centered at ∼1.67 μm from the ⁵I ₅→⁵I₇ transition in Ho³⁺ were recorded. Population inversion was achieved between the ⁵I ₅ and ⁵I₇ levels in Ho³⁺ with approximately 0.05 mol% Tb³⁺ codoping. Calculations based on cross sections and population densities show that a positive gain can be realised in the U-band region upon codoping of Tb³⁺.