Modeling the mode competition of Yb-doped fiber amplifiers in consideration of photodarkening

In this paper, the numerical steady-state model of mode competition in high power ytterbium-doped fiber amplifiers (YDFA) which considering the distributed photodarkening (PD) loss has been studied and developed for the first time. According to the relationship between PD loss and the upper-state population fraction, the formula of saturated PD loss which can distinguish different modes is established. Based on the rate-equation model considering transverse spatial-hole burning, we calculate the longitudinal distribution and quantitative results of each mode’s PD loss for the first time to the best of our knowledge. The dependence of parameters such as pumping direction, fiber length, and fiber doping composition on YDFA is also investigated and discussed. According to our simulation results, the output power of YDFA is strongly affected by PD loss. Meanwhile, PD increases the loss of higher-order mode (HOM), as well as enhances the ratio between fundamental mode (FM) and HOM. Thus, HOM suppression and improvement of beam quality can be expected within PD effect. In our work, the mode competition of high power YDFA under varying degrees of PD loss has been simulated. Obviously the output power will decrease with increasing the PD loss. However the ratio between FM and HOM has no evident changes with varying the PD loss, which providing guideline on appropriately using PD loss to regulate the mode competition between FM and HOM aiming at effective single-mode output of YDFA.

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