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In recent years, the transverse mode instability (TMI) has been a main limiting factor of average power scalability of high-power fiber lasers with near-diffraction-limited beam quality. A number of strategies in experimental or theoretical scopes have been proposed to mitigate the TMI, especially in the bulk-optic systems. In this paper, the enhancement of the TMI threshold is to be systematically investigated in a monolithic high-power large-mode-area (LMA) Yb-doped fiber laser system, by all-optical strategies including optimizing the pump distribution as well as adapting the seed power during operation. It is demonstrated that the TMI threshold can be dramatically increased under counter-pump and bi-directional pump regime compared with the co-pump regime. Further enhancement of the TMI threshold is achieved via optimizing the ratio of the forward pump power to the total pump power under bi-directional pump regime. As a result, a ~3 kW TMI-free high-power output is realized. Moreover, the systematical study on the effect of the seed laser on the TMI threshold reveals a rising trend of the TMI threshold with the seed power increased. At the same time, a semi-analytical model considering both the gain saturation and the photo-darkening effect is set to theoretically analyze the TMI threshold, manifesting good agreements with the experimental data. This paper provides a valuable reference for the construction of monolithic high-power LMA fiber-based laser system in terms of effectively mitigating the TMI threshold.

Towards the Enhancement of the TMI Threshold in Monolithic High-Power Fiber System by Controlling the Pump Distribution and the Seed Power