Effective mitigation of the transverse mode instability by optimizing the fiber coiling configuration in a counter pumped fiber oscillator

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Transverse mode instability (TMI) limits power scaling of high power fiber lasers by degrading beam quality and compromising operational stability. In this study, we demonstrated that optimizing the coiling configuration of ytterbium doped fiber (YDF) effectively suppressed the TMI induced by the photodarkening and the quantum defect in a 915 nm counter-pumped fiber oscillator. With the YDF output end positioned on the outer coil, the oscillator underwent a three-period degradation in the power stability test consisting of an initial stable period, a quasi-static power decline, and a final dynamic TMI with kHz-scale fluctuations. A position-resolved numerical model is developed, and the results support a positive feedback loop mediated by the thermally induced mode coupling, in which the forward higher-order mode (HOM) growing in the low-loss output-end section is partially reflected by the output coupler and the amplified backward HOM reinforces the thermally induced grating. When the YDF output end was repositioned onto the inner coil, the enhanced HOM bending loss near the output end stripped the forward HOM before it reached the output coupler, so the feedback loop was broken at its starting point. Consequently, the inner-coil configuration accumulated about 6 h of stable operation at 3.74 kW without measurable fluctuations, whereas recoiling the same fiber to the outer-coil configuration reproduced the degradation, with the beam quality factor M 2 degrading from 1.25 to 2.2 and backward spectral broadening from 3.1 to 9.2 nm. These comparative results provide a simple and practical coiling strategy to mitigate the TMI and enhance the reliability of high-power fiber lasers.