Elevating the TMI threshold by incorporating QCW pumping in a high-power CW fiber oscillator

Transverse mode instability (TMI) significantly limits the power scaling of ytterbium-doped fiber lasers. In this Letter, what we believe to be a novel TMI mitigation strategy is proposed and demonstrated in a bidirectional output fiber laser. On the basis of the continuous wave (CW) pump, integrating a quasi-continuous wave (QCW) pump can effectively improve the TMI threshold of the system. In the experiment, a QCW pump with a repetition rate of 1 kHz and a duty cycle of 10% was injected into a CW-pumped fiber laser oscillator, which successfully broke the power limit and alleviated the beam quality degradation. Moreover, when applying a pump modulation depth of approximately 68%, the TMI threshold was notably increased from 624 W in the free-running condition to 1093 W at end B, which is a 469 W (75%) improvement. At this time, the beam quality factor M² at end B is approximately 1.48, and the stimulated Raman scattering suppression ratio is about 28 dB. Compared to conventional TMI mitigation methods, this approach provides active control in the time domain of fiber lasers, without compromising the optical structures, thereby effectively mitigating TMI and offering broad application prospects.