The development of mid-infrared lasers operating in the infrared region around 2.7 μm is an active research field due to its applications including military and remote sensing, atmosphere pollution monitoring, eye-safe radars, and medical surgery. In the search of materials for the development of efficient optical devices based on rare-earth ions, two key factors are considered: glass host and the active ions. The glass host should have a high transparency in the mid-infrared region and thus should have a minimal absorption in the typical OH⁻ absorption band at 3 μm, low-nonradiative decay rates, high-radiative emission rates, and compatibility with fiber and waveguide fabrication processes. As the choice of ion for emission in the mid-infrared, Er³⁺ is well known for emission around 1.55 μm and 2.7 μm. However, the 2.7 μm emission is usually not efficient unless co-doped with other rare-earth ions such as Pr³⁺, Tm³⁺, Yb³⁺, Nd³⁺, or Ho³⁺.

In the January 15th issue of Optics Letters (DOI: 10.1364/OL.36.000109; p. 109), Y. Tian, R. Xu, L. Zang, L. Hu, and J. Zhang in a joint collaboration of the Chinese Academy of Sciences in Shangai and Beijing report the observation of 2.7 μm emission from diode pumped Er³⁺/Pr³⁺ co-doped fluorophaspate glass. They said that the good thermal properties and spectroscopic characteristics make this material promising for the development of a 2.7 μm laser.

The researchers have produced a fluorophosphate glass with the molar composition 20Al(PO₃)_3- 80RF₂– 3ErF₃–0.6PrF₃ (R = Mg, Ca, Sr, Ba). The thermal analysis of the prepared glass reveals that ΔT defined as T _x−T _g, wherein T _x is the onset crystallization temperature and T _g is the glass transition temperature, is 238.7°C. This value is significantly higher than that reported for other fluorophosphates and fluoride glasses and indicates that the investigated fluorophosphates glass composition possess a better thermal stability which can achieve a larger working range during optical fiber drawing. Investigation of the doped glass IR transmission spectrum shows that the transmittance of the glass in the infrared is as high as 90% and that the absorption around 3 μm, related to the OH⁻ absorption band, is negligible. Finally the infrared photoluminescence studies show a strong 2.7 μm emission band for the codoped Er³⁺/Pr³⁺ glass, which is not present for the only Er-doped glass. Analysis of the upconversion spectra in the green visible range shows that this behavior is due to an energy transfer process in which the presence of Pr ions participate, and as a result the 1.55 μm Er emission is weakened while the 2.7 μm emission is enhanced. Furthermore, peak of calculated emission cross-section in Er³⁺/Pr³⁺ doped fluorophosphates glass at 2708 nm achieves (6.57 ± 0.11) × 10⁻²¹ cm² which is higher than the result of Er³⁺ doped oxyfluoride transparent glass ceramics (4.3 × 10⁻²¹ cm²) and ZBLAN glass (5.7 × 10⁻²¹ cm²).