Significance Power scalability of fiber lasers have attracted a great deal of attention for its remarkable features, such as excellent beam quality, high conversion efficiency, flexible operation, and wide applications in biomedicine, intelligent manufacturing, energy exploration, defense and security. However, there have been no reports of significant operation power breakthroughs of near-single-mode fiber laser since the first demonstration of 10 kW-level system in 2009. The wasted heat accumulation, which can induce thermal lens and transverse mode instability effects, is one of the most important limitation factors. Quantum defect, defined as (1-λp/λs), where λp is the pump wavelength and λs is the lasing wavelength, has always been a key parameter in high-power fiber lasers. High quantum defect not only limits the conversion efficiency but also increases the thermal load in fiber lasers. In hence, much research on low quantum defect fiber laser has been reported in the past decades.
Progress This paper first introduces the performance exploration of high-power fiber laser at 1 μm band, including the power scaling and corresponding quantum defect decrease. It can be said that the power scaling progress of fiber lasers is also a continuous struggle against waste heat and other factors. As to low quantum defect fiber laser, the reported works mainly focus on two different technical schemes based on rare earth doped fiber and passive fiber. For the convenience of description, this article stipulates that the quantum defect of low quantum defect fiber lasers is ≤ 4.50%, and the quantum defect of ultra-low quantum defect fiber lasers is ≤ 1%. Then, ytterbium-doped fiber lasers with low quantum defect are summarized. In 2011, Wirth et al. demonstrated a 2.9 kW fiber laser operating at 1071 nm that is tandem-pumped by a 1030 nm thin-disk laser, and the corresponding quantum defect is about 3.83% (Fig.1). In 2014, Chang et al. presented a fiber laser with a maximal output power of 5.7 W and a quantum defect of 1.9% (Fig.2). To further reduce the quantum defect of fiber lasers, some specially designed active fibers and high pumping intensity methods are adopted. For example, in 2018, Yu et al. demonstrated a 400 mW-level fiber lasers with less than 1% quantum defect via ytterbium-doped multicomponent fluorosilicate fibers (Fig.5). Additionally, Raman fiber lasers with low quantum defect are reviewed. Based on common silicon fiber, a maximal power of 3 kW-level with a quantum defect of 4.42% (Fig.7) and a maximal power of 6.2 W with a quantum defect of 0.56% were achieved. To further improve the operation power of ultra-low quantum defect fiber laser, the scheme enabled by boson peak in phosphosilicate fiber was presented and realized by Zhang et al. in 2020 (Fig.14). In 2021, Ma et al. demonstrated a 100 W-level ultra-low quantum defect fiber laser with a quantum defect of 0.97%. What's more, cladding pump scheme was also been validated (Fig.15).
Conclusions and Prospects The important progress of low quantum defect fiber laser operating at 1 μm band is reviewed. And the manuscript mainly focuses on two different technical schemes based on rare earth doped fiber and passive fiber. In rare earth doped fiber based lasers, the utilization of tandem-pumping, multi-component doping and strong pumping schemes can reduce the quantum defect, and the related ytterbium-doped fiber lasers with quantum defect ≤1% have achieved 400 mW-level output power. In Raman fiber lasers, the maximal output power of 100 W-level with a quantum defect of ≤1% has been demonstrated with the aid of techniques such as special doping, pump spectrum regulation, and gain competition suppression. The feasibility of cladding pumping scheme has also been verified successfully, indicating its significant potential in achieving high-power and low quantum loss output.
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