To achieve a 3.02 kW Yb-doped fiber laser oscillator, the behavior of transverse mode instability (TMI) is experimentally studied in different pumping configurations; co, hybrid, counter, and bidirectional. A comparative analysis showed that population inversion saturation has a substantial impact on TMI threshold enhancement in high power fiber oscillators. Monitoring the dynamic power exchange of fundamental mode and higher-order mode of laser output beam indicates that in a hybrid pumping scheme, simultaneous pumping with two different wavelengths enhances the TMI threshold to a great stand. Moreover, injecting a few watts of pumping light in the counter direction mitigates the TMI caused by pumping in the co-direction. Calculation of population inversion in different pumping configurations using simulation shows that higher population inversion saturation leads to increasing the TMI threshold.

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A 1030 nm ytterbium doped fiber laser (YDFL), made by using a 20/400 μm YDF, was studied both theoretically and experimentally in this article. From the theoretical point of view, it is found that the optimum length for the active fiber with higher than 30 dB suppression of amplified spontaneous emission (ASE) effect is about 7.3 m. However, an output beam with a diffraction limited quality (M 2 = 1.16) and 735 W power with a slope efficiency of 77% and over 30 dB of signal to noise ratio without any sign of ASE was extracted from this YDFL.