Multi-kilowatt (kW) (≥3kW) level fiber lasers with high stability are significant in many applications, and Yb-doped fiber is the key device in such fiber lasers. The incredible advances of the past few decades in fiber fabrication technology have led to an exponential increase in the output power of continuous-wave (CW) fiber lasers. However, with further scaling the output power, photodarkening (PD) was found to be one of the critical limit factors for long-term laser reliability under multi kW level output power.