Spectroscopic analysis and laser simulations of Yb³⁺/Ho³⁺ co-doped lead-germanate glass

The potential of a Yb³⁺/Ho³⁺ co-doped lead-germanate glass as a laser gain medium around 2 µm is investigated by spectroscopic measurements and rate equation modelling. The glass, based on the molar composition of 56GeO₂-31PbO-4Ga₂O₃-9Na₂O and co-doped with 1.5 mol% Yb₂O₃ and 0.4 mol% Ho₂O₃, possesses a broad Ho³⁺ emission spectrum covering ∼1.8 µm to 2.2 µm for the Ho³⁺:⁵I₇→⁵I₈ transition, and a long ⁵I₇ fluorescence lifetime of (7.74 ± 0.03) ms. We estimate a competitive 2 µm quantum efficiency (76%) compared to other germanate glasses. The intensity parameters are calculated to be Ω₂ = 3.0×10⁻²⁰ cm², Ω₄ = 1.2×10⁻²⁰ cm² and Ω₆ = 2.0×10⁻²⁰ cm². The energy transfer analysis from Ho³⁺ to OH group represents a low Ho³⁺-OH clustering factor (γ = 0.15) compared to phosphate and other germanate glasses. Applying these parameters to the laser model predicts > 15% laser slope efficiency for cavity losses ≤ 0.5 dB using 976 nm pumping. The results show that this Yb³⁺/Ho³⁺ co-doped lead-germanate glass is a promising candidate for efficient lasing around 2 µm.