Effect of mode competition on photodarkening distribution of Yb-doped fiber laser

Effect of mode competition on photodarkening distribution of Yb-doped fiber lasers is studied. The mode competition model is introduced into photodarkening research to compare gain and inversion rate of different modes. Based on the power-law dependence of photodarkening rate on inversion, the distribution of photodarkening could be derived from that of inversion, which is calculated from the mode competition model. Transverse and longitudinal variations of photodarkening are obtained for fibers with different parameters. The photodarkening rate is not identical across and along the active fiber, and it is higher in the core rim and the pump injection end of the fiber. Reasons for these distributions are discussed and analyzed, including differences in gain and inversion rate for each mode, differences in field intensity distribution of each mode, and way of pumping. A three-dimensional distribution of photodarkening could be acquired by combining the transversal and longitudinal photodarkening distributions. Besides, the relationship between light intensity and photodarkening is illustrated. And the impact of core pumping on photodarkening is discussed.