Yb-doped fiber amplifiers and their applications in the radiation environment become more and more attractive in recent years. However, the radiation effect will do harm to the Yb-doped fibers, which can give negative effect on the output properties of Yb-doped fiber amplifiers.In this paper, the radiation effect on the transverse mode instability (TMI) of Yb-doped fiber amplifiers is studied. TMI can make the single light coupled from the fundamental mode to high-order mode, and thus degenerate the beam quality of fiber amplifier. TMI is considered as one key limitation of power up-scaling of fiber amplifiers.
In this paper, the radiation effect on the TMI is studied theoretically, and a formula of TMI threshold is presented with the radiation-indued attenuation (RIA, the most important radiation effect for the TMI) into account. The formula is deduced by introducing the loss of signal light induced by RIA into the formerly reported TMI-threshold formula which can be obtained by the linear stability analysis of the numerical model studying the TMI. Then, the relationship between the TMI and radiation dose is also given with the help of Power-Law describing the relationship between the RIA and radiation dose.
With the formula, the variations of TMI threshold with the radiation dose and RIA are studied. It is found, as expect, that the TMI threshold decreases monotonously with the increment of RIA or radiation dose. In spite of that, it is also found, unexpectedly to some extent, that the gain coefficient of fiber amplifiers will also affect the radiation effect on TMI threshold. The results reveal that the increment of gain coefficient will lower the sensitivity of TMI threshold to the radiation dose. In spite of that, it is also implied that the gain coefficient cannot be too large because it can also make the TMI threshold lowered. Therefore, the well-enough radiation resistance of Yb-doped fiber should be indispensable, in order to keep high TMI threshold in the radiation environment.
Because the RIA cannot only affect the TMI threshold but can also affect the output power or efficiency of Yb-doped fiber amplifier, the comparison of two effects of RIA is also discussed. It is found that the TMI threshold is more sensitive to the radiation than the output power or efficiency (see the abstract figure). It means that the TMI can be present in the irradiated Yb-doped fiber amplifier, although the output power is lowered because of RIA. This result can be verified by the experimental observation formerly reported. As a result, TMI can become the key limitation to the output power of Yb-doped fiber amplifiers in the radiation environment. The pertinent results can provide significant guidance for the applications of Yb-doped fiber amplifiers in the radiation environment.