# Progress and Summary of Photodarkening in Rare Earth Doped Fiber
## 1 Introduction
### Research Progress of Ytterbium-Doped Fiber (YDF) Photodarkening  
Although the discovery and research of PD in TDF is earlier than that in YDF, at present, the research on the PD effect and laser mode instability of rare earth doped fiber is mainly aimed at YDF laser [15]. Compared with Tm\({}^{3+}\) ion, Yb\({}^{3+}\) ion has a great difference in energy level structure. Yb\({}^{3+}\) ion belongs to a simple energy level structure, with no excited-state absorption, long fluorescence life, and small quantum defects [16]. In 1997, [PERSON] et al. [17] found pump-induced absorption in Yb-doped silica fiber. In 2005, [PERSON] et al. [18] first called it PD in YDF research. In 2007, [PERSON] et al. [1] reported experiments showing new characteristics of PD and PB of high concentration Yb-doped silicon fibers exposed to 977 nm and 543 nm in core irradiation. The results showed that pumping the fiber at 977 nm would increase the absorption in a wide spectral range from 400 to 1000 nm (PD), and simultaneously decrease the resonant (Yb\({}^{3+}\)) absorption co-efficient near 1 \(\upmu\)m (PB). In 2007, [PERSON] et al. [19] first observed the partial bleaching of PD loss by pump power itself. The effects of temperature and pump wavelength on the equilibrium state of additional loss were studied. From the dependence of the measured rate constant on the density of excited Yb ions, they concluded that an average of 3 to 4 excited Yb ions produced or bleached a color center, which was the cause of additional loss. [PERSON] et al. [2] studied the temporal evolution of the PD effect in an Yb-doped silica large-mode-area (LMA) fiber. When the fiber was exposed to a 980 nm pumped laser, the absorption spectrum showed an increase in the visible and near-infrared spectrum. In 2007, [PERSON] et al. [3] studied the evolution of the PD effect with time in Yb-doped aluminosilicate fiber irradiated at 488 nm. In Yb-doped aluminosilicate fiber, irradiation caused significant excess loss in the ultraviolet-visible spectrum, while the PD-induced loss of undoped aluminosilicate fiber can be ignored. In 2008, [PERSON] et al. [20] carried out PD experiments with Yb-doped silicate fiber samples. Very low-level PD was observed in Yb-doped phosphosilicate fiber pumped by a 915 nm high-power diode. In 2008, [PERSON] et al. [21] used Yb-doped fiber pumped by a 915 nm laser to study the change of PD with the concentration of co-dopant aluminum and phosphorus. Although high concentration co-doping was required, the core composition with equal aluminum and phosphorus content was the most promising to achieve YDF with low PD, high laser efficiency, and low laser core numerical aperture. In 2009, [PERSON] et al. [22] reported the experiment of thermal treatment of photodarkened YDF samples. The non-isothermal bleaching method at different temperature ramp rates could be used to determine the thermal energy distribution of color centers. The average value of the distributed activation energy was about 1.3 eV and the FWHM is 0.5 eV. In 2009, [PERSON] et al. [23] studied thermal-bleaching (TB) of PD-induced loss in LMA Yb-doped silica fiber with 20 \(\upmu\)m core diameter. In 2009, [PERSON] et al. [24] showed that the PD resistivity of YDF lasers could be greatly improved by cerium co-doping. In 2009, [PERSON] et al. [25] proposed a method of TB for LMA Yb-doped fiber. The photodarkened YDF sample was recovered to original state by thermal annealing. Next year, [PERSON] et al. [26] examined the effect of PD-induced pump light absorption on YDF thermal loading. In these experiments, the fiber was cladding-pumped at 915 nm, air-cooled by natural convection, and monitored with a mid-infrared thermal camera. It was found that fiber temperature was related to progressive PD. In 2010, [PERSON] et al. [27] reported the PD behavior of Yb-doped aluminosilicate fiber at high temperature. In 2010, [PERSON] et al. [28] measured and analyzed the change of refractive index caused by PD and TB in YDF with high accuracy, based on a modal interference method.