# Progress and Summary of Photodarkening in Rare Earth Doped Fiber
## 2 Mechanism of Photodarkening
Generally speaking, glass can absorb X-ray and 200-220 nm ultraviolet light to produce a color center, which greatly reduces the transmission performance of glass [45]. However, recent research shows that the transmission performance of Yb-doped fiber decreases significantly after irradiation at 488 nm [3; 46; 53], 793 nm [33], 915 nm [20; 25; 38; 54], 920 nm [5], 977 nm [1; 27], 980 nm [2] and thulium-doped fiber at 475 nm [8], 790 nm [13], 1064 nm [10; 55], 1080 nm [14], 1120 nm [4], 1140 nm [11], 1175 nm [56]. We believe that under the condition of the laser working normally in the ordinary environment, the color center is produced by the interaction between high-energy (short-wavelength) photons and ions, atoms, or electrons in the crystal. This means that not all wavelengths (especially long wavelengths) leading to PD are directly involved in the process of generating color centers, and the conversion process from low-energy photons to high-energy photons needs to be carried out first. Furthermore, the ways of generating high-energy photons in the laser are different, mainly includingexcited-state absorption (ESA) [10], energy transfer upconversion (ETU) [13], avalanche upconversion [13], cooperative luminescence [57], and so on. It is known that Tm\({}^{3+}\) ions can easily up-convert infrared light to visible or ultraviolet light [58; 59], especially, when they are coupled with Yb\({}^{3+}\) ions [60]. It is known that Tm ions with \({}^{1}\)G\({}_{4}\) level show a strong absorption band at about 488 nm and Tm ions at \({}^{1}\)G\({}_{4}\) level can be excited to \({}^{1}\)I\({}_{6}\)-\({}^{3}\)P\({}_{0}\) level by ESA, so as to up-convert 488 nm radiation to its ultraviolet [61]. PD was observed in Tm-doped silica optical fibers pumped at 475 nm wavelength [8; 9]. With respect to the sample fiber test experiment mentioned above, ultraviolet and blue light are rarely used as the pump in the actual fiber laser, in fact, they are generated by the upconversion of the pump. Blue luminescence was observed in Tm-doped fiber pumped at 1.06-1.14 \(\upmu\)m [4; 10; 11; 14; 55; 56], as the three-photon-induced \({}^{1}\)G\({}_{4}\)-\({}^{3}\)H\({}_{6}\) transition of Tm\({}^{3+}\) (Figure 1). The same Tm ion continuously absorbs multiple photons to reach a higher excited-state energy level, thus emitting ultraviolet and blue light. In 2010, [PERSON] et al. [13] discussed the processes of generating blue light in 790 nm pumped Tm-doped fibers pumped and how fiber composition influenced these processes (Figure 2). Figure 2a shows the avalanche upconversion process. Tm ions absorb a photon with a wavelength of 790 nm, transiting from \({}^{3}\)H\({}_{6}\) to \({}^{3}\)H\({}_{4}\), and then reach \({}^{3}\)H\({}_{5}\) through a non-radiative transition. Next, the ions at the \({}^{3}\)H\({}_{5}\) absorb a photon with a wavelength of 790 nm again, so as to enter a higher energy excited-state (\({}^{1}\)G\({}_{4}\)). Figure 2b shows the energy transfer upconversion. Tm ions absorb a photon with a wavelength of 790 nm, transiting from \({}^{3}\)H\({}_{6}\) level to \({}^{3}\)H\({}_{4}\) level. During the transition to \({}^{3}\)H\({}_{5}\), the energy generated is transferred to another Tm ion excited to \({}^{3}\)H\({}_{6}\), thus entering a higher energy excited-state (\({}^{1}\)G\({}_{4}\)). These pieces of evidence clearly indicate that PD is related to the presence of Tm\({}^{3+}\). In addition, some defects in fiber can also absorb pump power and generate blue photons [7]. When we pump TDF at 793 nm, we also observe the peak spectrum at 470 nm. Compared with Tm\({}^{3+}\) ions, Yb\({}^{3+}\) ions have different energy level structures. Its energy level structure is simple, and there is no ESA and other phenomena [16]. Therefore, we cannot explain the phenomenon of PD in YDF pumped in the blue and infrared bands. In 2007, [PERSON] et al. [1] showed that the phenomenon of PD was related to a multi-excitation of ytterbium clusters emitting light in the ultraviolet region.