Photodarkening(激光)

英国南安普顿S. Yoo

1. [Photodarkening in Yb-doped aluminosilicate fibers induced by 488 nm irradiation-OL-2007]

It is believed that the photodarkening (induced optical losses) includes color centers, but no study has been reported to identify the color centers or explain the route of the photodarkening. 

增强光暗化：Yb clusters or Yb pairs were reported to suffer from increased photodarkening. 
抑制光暗化：
(1) a high Al concentration. 因为可以增加more oxygen surrounding spaces, thus reducing the number of ODCs。
(2) annealing in an O₂ atmosphere(高压)，因为reduces the number of ODCs。

辐照条件：446 mW/mm² of 488 nm radiation from an Ar⁺ ion laser for 5 h.
对Yb-free的fiber，辐照前后的吸收谱基本没有变化，但是对于Yb-doped fiber，~220 nm的宽峰明显增强，说明这个峰和Yb有关。对于Yb³⁺掺杂的玻璃来说，一般Yb周围都是氧原子，但是如果Yb的掺杂浓度足够大，就会出现氧deficiency的情况，此时可能形成诸如Yb-Yb或者Yb-Al一样的ill-valenced bonds。人们在这锗硅酸盐玻璃中发现存在240 nm的吸收，用此波长的光辐照，那么产生的电子就会被trapped at nearby GeO₄ tetrahedra, forming a Ge(1) color center that induces absorption peaking at 280 nm. 基于此，本文作者也hypothesize同样的光暗化机理：

• 488 nm激光，双光子过程去break the hypothesized ODCs(oxygen deficiency center)，释放电子。作者变功率测试表明slope约为2，证实双光子过程。
• Electrons may be trapped at Al or Yb sites and form a color center resulting in photodarkening.

作者认为不应该考虑Yb²⁺的生成，因为because the observed absorption curve does not follow the typical divalent Yb absorption。对比Yb-free和Yb-doped的样品，作者认为Yb and possibly ODCs, play a direct role。

In summary, photodarkening in Yb-doped aluminosilicate fibers induced by 488 nm irradiation was investigated and compared with that in Yb-free aluminosilicate fibers. The Yb-doped fibers showed significant photodarkening. Yb-associated oxygendeficiency centers, with an absorption peak at 220 nm, were proposed as a precursor to the photodarkening in Yb-doped aluminosilicate fibers. O₂ loading was found to reduce the photodarkening, as expected for an ODC-dependent process. Thus, O₂ loading may provide a solution to the photodarkening problem in Yb-doped aluminosilicate fibers. This also explained how a higher Al concentration can mitigate photodarkening, as has been reported.

Comments [Comment on “Photodarkening in Yb-doped aluminosilicate fibers induced by 488 nm irradiation”-2008]

上文中，230 nm的吸收被认为是Yb-related oxygen deficiency centers (ODCs)，这种说法是错误的。The 230 nm absorption band actually consists of several overlapping charge-transfer (CT) absorption bands. 

• Excitation to the first (lowest in energy) CT state results in the temporal formation of an Yb²⁺ ion and a bound hole. Characteristic Yb³⁺ luminescence near 1 μm is observed upon recombination.
• A stable Yb²⁺ ion is formed, and a free hole is generated upon excitation to a second (higher in energy) CT state, which result in color center formation with induced absorption observed at visible and nearIR wavelengths.

如果230 nm吸收的确对应于ODCs，那么掺杂其他稀土也应该有类似的现象，但是这里掺杂Nd的并没有这个吸收峰。另外O₂处理的样品230 nm吸收峰实际上是增强了，which provides further evidence against the assignment of an ODC。

Reply [Reply to comment on “Photodarkening in Yb-doped aluminosilicate fibers induced by 488 nm irradiation”-OL-2008]

 

瑞典中部大学Engholm

1. [Strong UV absorption and visible luminescence in ytterbium-doped aluminosilicate glass under UV excitation-OL-2007]

2. [Preventing photodarkening in ytterbium-doped high power fiber lasers; correlation to the UV-transparency of the core glass-OE-2008]

其他

1. [The Yb-doped aluminosilicate fibers photodarkening mechanism based on the charge-transfer state excitation.-Fiber lasers XI: technology, systems, and applications. SPIE-2014]

2. [掺镱光纤光暗化研究进展-材料科学-胡丽丽-2017]

3. [掺Yb³⁺光纤中光暗化的研究概述-2015]    [掺镱光纤激光器光子暗化现象的研究进展—2016]

光暗化简介及原因

Yb³⁺双能级结构，没有激发态吸收和交叉弛豫，所以量子效率高，Yb3+掺杂的光纤也广泛应用于高功率的光纤激光器。

• 光暗化现象：Yb3+光纤随着运行时间的增加，输出功率降低，Yb³⁺掺杂浓度越高，这种现象越明显，限制了光纤的输出功率，也影响寿命和稳定性。

主要影响因素

抑制措施

4. [Revisiting literature observations on photodarkening in Yb3+ doped fiber considering the possible presence of Tm impurities]

5. [Analysis of photodarkening effects in ytterbium-doped laser fibers-PHD-2013]

6. [Radiation induced losses in Ytterbium doped laser materials-PHD-2013]

7. [Progress and Summary of Photodarkening in Rare Earth Doped Fiber—2021]