Numerical study on the beam quality evolution of the thermally-guided very-large-mode-area fiber

The beam quality variations of transverse modes with thermal load in the thermally-guiding (TG) very-large-mode-area (VLMA) fiber are numerically investigated in this paper. It is found that the beam qualities of transverse modes can be optimized with the production of thermal load. The beam quality variations are not monotonous due to the thermally-induced index increment, which can be concluded into three stages (the “initial”, “transition”, and “steady” stage). The effects of fiber parameters such as index difference and core size are also studied. Furthermore, two practical cases of mode mixture are considered and the effects of higher-order mode content and the phase shift on the beam quality are revealed. It shows that the higher beam quality doesn't suggest lower higher-order mode content, and the phase shift will increase the uncertainty of beam quality. It is also suggested that by controlling the thermal effect, the overlap between two modes can be reduced to suppress the beam quality degradation caused by phase shift. The pertinent results can provide significant guidance on designing the high power TG VLMA fiber lasers with high beam quality preservation.