Effective mitigation of photodarkening-induced transverse mode instability in a monolithic fiber laser oscillator by 450 nm laser irradiation

The photodarkening (PD) and transverse mode instability (TMI) effects are two main factors limiting the power increase and long-term stability of high-power fiber lasers. A prolonged burn-in test for an all-fiber laser oscillator below the TMI threshold was carried out. We observed the PD-induced TMI effects, which manifested as a sudden decrease in the output power due to higher-order mode leakage. After several minutes of exposure to a high-power density 450 nm laser diode (LD), the output power returned to its initial state, significantly enhancing the oscillator's stability. The 450 nm LD probably mitigates the accumulation of thermal effects by inhibiting the photodarkening effect, thus preventing the occurrence of the TMI effects and improving the stability of the oscillator's output power. Our work provides useful guidance for the development of high-stability fiber laser oscillators.