Endcapping of high-power 3 µm fiber lasers

Fiber tip photodegradation through OH diffusion currently limits the long term operation of high-power fiber lasers and amplifiers operating near 3 µm. To address this issue, we investigate the resistance to OH diffusion of fluoride and oxide endcaps manufactured out of ZrF$_4$₄, AlF$_3$₃, GeO$_2$₂, SiO$_2$₂ and Al$_2$₂O$_3$₃ fibers. To this extent, the endcaps are spliced at the output of a 20 W continuous-wave fiber laser operating at 2.8 µm and their degradation over a 100 h time period is monitored. While the fluoride-based endcaps underwent failure during the first 10 h, their oxide counterparts survived the experiment, although showcasing degradation which was reflected as an increase of the endface temperature over time. To overcome this issue, we propose a novel method to completely suppress OH diffusion which consists in sputtering a nanoscopic diffusion barrier film made of silicon nitride (Si$_3$₃N$_4$₄) on the output face of the endcap. The effectiveness of the approach is validated on Al$_2$₂O$_3$₃, ZrF$_4$₄ and AlF$_3$₃ endcaps which show no sign of degradation after being used for more than a 100 h at the output of a 3 µm high-power fiber laser.