Infrared-to-Visible Upconversion and 1.53-μm Emission of Er³⁺-Doped Al(PO₃)₃-Based Fluorophosphate Glass

We investigate the spectroscopic properties of the 1.53-μm emission from the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions in Al(PO₃)₃-based fluorophosphate glass for applications in broadband fibre amplifiers. The emission peak locates at ～1530 nm with a full width at half-maximum of ～65 nm. The measured lifetime and the calculated emission cross-section of this transition are ～8.1 ms and ～7.6 x 10^-21cm², respectively. Frequency upconversion to the ⁴F_7/2 state occurs simultaneously upon excitation of the 1.53-μm emission with a 977-nm laser diode. Three infrared-to-visible upconversion emissions centred at around 524, 546 and 668 nm have been clearly observed. The quadratic dependence of fluorescence on excitation laser power confirms the fact that a two-photon process will contribute to the infrared-to-visible upconversion emissions.