# Progress and Summary of Photodarkening in Rare Earth Doped Fiber
## 1 Introduction
### Research Progress of Ytterbium-Doped Fiber (YDF) Photodarkening
The influence of the PD-induced refractive index change (RIC) on fiber lasers was discussed. In 2011, [PERSON] et al. [29] reported PD kinetics characterization of YDF samples in a wide range of temperatures from 77 to 773 K and with respect to different concentrations of Yb\({}_{2}\)O\({}_{3}\). When the time went to infinity, the saturation loss was obviously depended on the fiber temperature. In 2011, [PERSON] et al. [30] examined the temperature dependence of PD in Yb-doped silica fibers. A series of continuous PD experiments were carried out on the same fiber samples. The results showed that it had good repeatability without an obvious change of glass structure. They found that in the process of infrared irradiation, the level of saturation of the PD losses could be determined by the core temperature. They observed that the change of core temperature, which was caused by pump absorption due to PD, would affect the inversion level and PD processes. In 2011, [PERSON] [15] observed a characteristic line at 2.6 eV (477 nm) for Yb/Al and co-doped silica fibers. This linewas proposed to be due to inter-center excitation transfer from type II oxygen deficiency centers ODC(II) to Tm\({}^{3+}\) trace impurities. In 2012, [PERSON] et al. [31] examined the reduction of PD loss in Yb-doped aluminosilicate fibers using light irradiation with a wavelength of 633 nm. It was demonstrated that the final PB value depended on the irradiation intensity of bleaching light, but for the determined bleaching light intensity, the percentage of PD reduction was independent of the dopant concentration. In 2014, [PERSON] et al. [32] discussed the effect of visible light irradiation on PD in 1070 nm YDF laser. Simultaneous PD and PB effects caused by 976 nm and 405 nm or 550 nm radiations respectively were investigated. They observed a significant PB effect due to 405 nm radiation but it did not completely recover. A strong absorption of the 405 nm radiation by the excited ions (Excited-State Absorption) was also observed, which was found to be the main limiting factor of bleaching performance together, and PD losses caused by ground-state absorption were observed. In 2015, [PERSON] et al. [33] reported significant bleaching of Yb-doped fiber pumped at 793 nm. Up to 68% PD loss at 810 nm was bleached. The bleaching experiment under different powers showed that the pump power was positively correlated with the bleaching. In 2018, [PERSON] et al. [34] presented a microscopic model, which fully described the experimental experiences of PD research and was based on appropriate micro-optical mechanisms. The PD effect was regarded as a relaxation process of a statistical ensemble of micro-optical centers (PD complexes). These micro-optical centers (PD complexes) were reversibly converted into color centers by the energy of pump photons, resulting in the measurable PD loss. In 2019, [PERSON] et al. [35] reported the fundamental elimination of the PD effect in YDF by deuterium loading. No PD phenomenon was observed in TDF by deuterium pre-loading under 915 nm pumping conditions. In 2019, [PERSON] et al. [36] proposed a phase-separation method of nanoporous glass to prepare low PD YDF. Compared with the improved conventional vapor deposition method, equilibrium PD-induced excess loss of the nanoporous glass fiber was reduced to 35.07, 12.49, and 1.69 dB/m at 702, 810, and 1041 nm, respectively, despite higher Yb\({}^{3+}\) concentration. In 2020, [PERSON] et al. [37] found that in high-power YDF amplifiers, H\({}_{2}\) loading could fundamentally inhibit the deterioration of the optical aging effect and laser performance.