Photodarkening (PD) is a critical issue for the lifetime and modal stability of high-power Yb-doped fiber (YDF) lasers. Despite extensive investigation in last decade, the physical origin of the pump-induced losses remains a subject of discussion. Currently, though PD-resistance can be improved via doping with P and/or Al, or co-doped Ce or other means, each mean has its own disadvantage. Therefore, higher stability of YDF laser requires further upgrading the PD-resistance of YDF. Recently, researchers led by Professor HU Lili at Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, have provided a more effective method, i.e., the sol-gel method combined with high-temperature sintering, for preparing PD-resistant YDF. The results showed a better PD-resistance of Yb3+/Al 3+co-doped silica fibers. In their experiment, the PD-induced loss of YDFs prepared by the sol-gel is lower than that of YDFs prepared by MCVD method under the same experimental conditions. The mechanism of the PD-resistance enhancement caused by the sol-gel preparation method was studied using cooperative luminescence spectra of Yb3+ions, electron paramagnetic resonance (EPR) spectroscopy, and electron probe microanalysis (EPMA). A model of the color-center generation in the PD process was proposed to explain the mechanism of PD resistance improvement for the YDF fabricated via the sol-gel method. Obviously, this work suggests that the sol–gel method may be effective for preparing PD-resistant YDFs. This research, entitled "Photodarkening-resistance improvement of Yb3+/Al 3+ co-doped silica fibers fabricated via sol-gel method" has been published in Optics Express. This work was supported by Advanced Electronic Materials of the Ministry of Science and Technology, China (Grant No. 2016YFB0402201); National Natural Science Foundation of China (Grant No.61775224); National Natural Science Foundation of China (Grant No. 61505232).
Fig. (a) Schematic of the PD measurement. (b) Experimental setup for core-pumped fiber lasers.