Investigation of Pumping Ratio Effects on the TMI induced by Quantum Defect and Photodarkening in a 4 kW Bidirectionally Hybrid-Wavelength Pumped Fiber Laser

Abstract Transverse mode instability (TMI) is a dominant bottleneck that restricts the stable operation and power-scaling potential of high-power fiber lasers. In this work, we experimentally investigated the impacts of pumping ratio on both quantum defect-induced and photodarkening-induced TMI in a 4 kW bidirectionally pumped fiber laser, which adopted a hybrid-wavelength pumping scheme consisting of 915 nm co-pumps and 976 nm counter-pumps with a total pump power of 6 kW. At a pumping ratio of 0.58 corresponding to 2.2 kW co-pump power and 3.8 kW counter-pump power, the initial TMI threshold was measured at 4.16 kW, whereas it increased slightly to 4.2 kW at a pumping ratio of 0.667. Power stability tests were conducted at an output power of 4 kW to investigate the photodarkening induced TMI. At a pumping ratio of 0.58, severe power fluctuations with characteristic frequency peaks at 1.4 kHz and 2.65 kHz emerged after only 22 minutes of laser operation, indicating a notable TMI threshold reduction triggered by photodarkening-related thermal accumulation. In contrast, increasing the pumping ratio to 0.667 delayed the onset of photodarkening-induced TMI to 76 minutes. These results demonstrate that although optimizing the pumping ratio yields only a marginal improvement in the initial TMI threshold, it plays a critical role in suppressing photodarkening-induced TMI and ensuring long-term operational stability. This study provides a practical and cost-effective strategy for the stability and reliability of high-power fiber lasers.