532†nm pump induced photo-darkening inhibition and photo-bleaching in high power Yb-doped fiber amplifiers.

A novel method for mitigating photo-darkening and the effective photo-bleaching phenomenon by 532 nm cladding pump in Yb-doped fiber were herein reported. Compared with the pristine fiber, beyond 30% of photo-darkening induced excess loss was suppressed by 532 nm pretreatment. Moreover, the excess loss in the photo-darkened fiber was completely bleached with 532 nm pump. Additionally, the bleached fiber exhibited better photo-darkening resistance. Therefore, for high power application, a 20/400 gamma irradiated fiber was bleached in situ by 532 nm pump and the laser properties were explored. The output power restored to 421W accounting for 82% of the pristine fiber, with the mode instability threshold rising to over 2.6 times and the efficiency increasing from 37% to 63%. The results indicate 532 nm pump has bright prospects for the stable operation of high power fiber lasers.

Mitigation of mode instability in laser oscillators based on deuterium loading.

Ytterbium-doped fiber (YDF) loaded with deuterium is used herein to mitigate mode instability. Experimental results reveal that this method can increase the mode instability threshold in a laser oscillator. Specifically, when the YDF was loaded with deuterium over two- and four-week periods, the mode instability threshold power increased from ∼459 W to ∼533 W (16%) and to ∼622 W (35%), respectively, but the respective laser efficiencies were almost unaffected (71.5% vs. 72.9% and 75.4%). In conclusion, deuterium loading is effective in the mitigation of mode instability. It is envisaged to be applied in the power scaling of high-power fiber lasers.

Radical passive bleaching of Tm-doped silica fiber with deuterium.

We demonstrate the almost complete 2 µm laser power recovery of the gamma-ray-irradiated thulium (Tm)-doped silica fiber under deuterium loading. The optical-optical slope efficiency and the cladding absorption spectra of the Tm-doped fiber with gamma-ray irradiation and deuterium treatment have been measured for comparison. It was found that the slope efficiency of the irradiated Tm-doped fiber could be recovered to 96.1% of the pristine after deuterium bleaching, which significantly degraded from 60.7% to 25.3% after irradiation. Meanwhile, the additional absorption attenuation of the irradiated Tm-doped with D2 treatment completely vanished. Based on the comprehensive comparison of cladding absorption spectra, the probable mechanism of the deuterium bleaching effect on irradiated Tm-doped fiber has also been discussed.