Order-of-magnitude power enhancement of an Er3+ 2.7-µm ZBLAN laser utilizing lifetime quenching by energy transfer to Pr3+

In recent years, there have been enormous research efforts to improve the performance of lasers emitting around 3 µm mainly because of their potential applications in medicine. The erbium-doped ZBLAN fiber is a promising candidate for the construction of a compact and efficient all-solid-state laser emitting on the transition at 2.7 µm. However, a high excitation density with the consequences of pump excited-state absorption (ESA) can lead to output-power saturation in the fiber laser. This saturation was overcome in a cascade lasing regime and 150 mW of output power was achieved under Ti:sapphire pumping at 791 nm. Here, we investigate experimentally a theoretical proposal to scale the output power of the Er3+ 2.7-µm fiber laser to the 1-W region: Ground-state bleaching, large excitation of the Er3+ 4I11/2 and 4I13/2 laser levels, and consequent ESA losses are avoided by an active reduction of the excitation density due to a Förster-Dexter-type energy transfer from the Er3+ 4I13/2 lower laser level to the 3F3 level of a Pr3+ codopant and subsequent fast multiphonon relaxation of the Pr3+ ion. With concentrations of 35000 ppm mol. Er3+ and 3000 ppm mol. Pr3+, the Er3+ 4I13/2 lifetime in ZBLAN is quenched from 8.7 ms to less than 300 µs. Utilizing this approach, we demonstrate 1.7 W of output power in a near transverse-fundamental mode and 17% slope efficiency at 2.71 µm from an erbium ZBLAN fiber pumped at 790 nm by 22 W from a diode source. This result represents more than an order-of-magnitude improvement in output power over previous work. The double-clad fiber consisted of a circular core of 15 µm diameter and a rectangular inner cladding of 100×200 µm2. Since also ESA from the 4I11/2 upper laser level is avoided due to the threshold condition for the laser levels, further power scaling seems possible by pumping at 980 nm directly into 4I11/2