# Progress and Summary of Photodarkening in Rare Earth Doped Fiber
## 2 Mechanism of Photodarkening  
The PD effect is regarded as a relaxation process of a statistical ensemble of micro-optical centers (PD complexes). These micro-optical centers (PD complexes) are reversibly converted into color centers by the energy of pump photons, resulting in measurable PD-induced excess loss [33]. The color centers lead to the increase of broadband absorption centered on the visible range. The color centers absorb the pump and the signal light,which reduces the power conversion efficiency of the fiber laser and can generate excess heat in the fiber laser [38; 39]. In some optical materials, the color center is produced by irradiation, which leads to excessive loss of broadband. In rare-doped laser materials, this effect is referred to as PD, which are inevitably exposed to intense pump radiation. The irradiation of the materials by the pump will directly or indirectly produce short band intrinsic absorption and produce electronic transitions of atoms or ions, i.e., the electrons in the valence band are excited to the conduction band, while holes are left in the valence band. After the electrons in the conduction band and holes in the valence band are trapped by the intrinsic defects and impurity defects of glass, the electron capture type color center (F Center) and the hole capture type color center (V Center) are formed respectively. It is well known that high-energy radiation will produce an absorption band in a matrix such as silica glass [40; 41], and ultraviolet light is also used to make Bragg mirrors into fiber [42]. Therefore PD will appear in optical fiber, with ultraviolet laser [43], and even with an ultraviolet lamp [20; 44]. In the case of \(\gamma\)-ray, first, \(\gamma\)-ray interacts directly with optical fiber through the photoelectric effect, Compton scattering, and electron pair effect, which leads to the ionization of electron-hole pair in fiber; secondly, when the electron-hole pair is captured by the intrinsic defect, doping defect, impurity defect and radiation-induced defect in fiber, a special point defect with effective charge will be formed. This kind of defect can bind electrons and holes, cause the electrons or holes in the fiber to excite, and produce light absorption. Because its absorption band often falls in the visible light range, it is called the color center [45]. The oxygen-deficient center (ODC) in the germanium silicate glass has its characteristic absorption peak at 240 nm, which releases free electrons when irradiated at resonance wavelength. The released electrons are trapped near the GeO\({}_{4}\) tetrahedron, forming the Ge(I) color center and an absorption peak at 280 nm. Its absorption range extends to the near-infrared band, showing PD [44]. There is a lot of evidence showing that the charge-transfer (CT) band appears in Yb-doped aluminosilicate glass near 230 nm. The transition of Yb\({}^{3+}\) ion from trivalent state to low valence state leads to the formation of Yb\({}^{2+}\)[3; 20; 46; 47; 48; 49; 50]. When stable Yb\({}^{2+}\) is formed, free holes are generated and excited to a higher energy level CT state, which leads to the formation of color center and the induced loss at ultraviolet and infrared wavelengths [46; 51; 52]. It is found that in the Yb/Al co-doped fiber working in the amplifier, the PD absorption spectrum becomes uniform with time, and the absorption loss at 477 nm and 558 nm increases with the extension of working time. These phenomena are considered to be caused by the transfer of inter-center excitation from type II oxygen deficiency centers (ODC (II)) to Tm\({}^{3+}\) and Er\({}^{3+}\) impurities, respectively. ODC (II) can cooperate with the valence electrons of Yb\({}^{3+}\) ions to form color centers on the matrix, resulting in visible to near-infrared light [15]. In this section, we will focus on the conditions leading to defects (so-called PD) in the literature rather than the process of forming defects themselves.