Significance Ytterbium-doped fiber lasers have gained rapid development in the past two decades. Since the breakthrough of hundred-watt-level power at the end of the last century, nowadays the output power of the fiber lasers can exceed ten-kilowatt level. Thanks to the waveguide nature of optical fibers and the laser diode pumping techniques, high-power ytterbium-doped fiber laser owns several advantageous features, including high conversion efficiency, high beam brightness, compact and flexible architecture, easy thermal management, and stable operation. High-power ytterbium-doped fiber lasers have become preferred laser sources for many applications in various fields, such as industrial manufacturing, biomedical treatment, scientific researches, and defense.

Technologies for improving the output characteristics of high-power ytterbium-doped fiber lasers, especially for scaling output power, realizing more control of the output spectrum and expanding the range of output wavelengths, have received considerable interests from the fields. The characteristics are influenced by pumping schemes of the lasers, of which there are two major kinds as direct pumping and tandem-pumped. Direct pumping generally means that fiber lasers are pumped by laser diodes, which usually emit at around 915 nm and 976 nm for ytterbium-doped fibers (YDFs). In contrast, tandem-pumped means that fiber lasers are pumped by other fiber lasers, of which the wavelength generally ranges from 1000 nm to 1030 nm. Both pumping schemes have demonstrated high capability for realizing high-performance ytterbium-doped fiber lasers, and have realized outstanding advances in increasing power and brightness, controlling output spectrum, and expanding the available range of output wavelength. Particularly, tandem-pumped scheme has achieved increasing performance from recent studies. A few 20 kW fiber lasers have been proposed in the last several years.

Owing to the rapid development of light sources such as laser diodes, direct pumping has been the majority of the adopted pumping schemes. However, for higher power levels, namely 10 kW level, direct pumping may be a bottleneck to ensuring safe operation. Direct pumping offers usually low pump brightness [typically <0.2 W/(μm²·sr)]. For gaining a higher absorption, the available pump wavelength will be very limited, as it needs to be at around the absorption peak of the gain media (for YDFs, ~976 nm). This causes a large quantum defect in the pump-to-laser conversion process and thus severe heat management problem; for higher-power fiber lasers, the problem can be too hard to controll. In contrast, in tandem-pumped scheme where pump brightness can be enhanced by 3 orders to even >1000 W/(μm²·sr), it is possible to use double cladding YDFs of much smaller cladding diameters while having a good pump-gain overlap. In this way, it is possible to use longer pump wavelengths while having good pump absorption, and the heat problem can be mitigated. In recent years, an increasing number of high-power fiber lasers at from 1000 nm to 1030 nm has been proposed and demonstrated. There is an important landmark, as in 2009 when the IPG company for the first time announced their 10 kW fiber laser that was pumped by combined 47 fiber lasers at 1018 nm. The news has pushed the studies of fiber lasers for tandem-pumped (including 1018 nm fiber lasers) to the fast lane.

Progress This paper reviews the latest research progress about tandem-pumped high-power ytterbium-doped fiber lasers. We discuss the key technologies in realizing the tandem-pumped high-power fiber lasers with leading performances, and look forward to possible directions and challenges in future studies. In the second chapter, we review recent development of high-power 1018 nm fiber lasers. The features and performance of the lasers are introduced (Table 1, Fig. 1—3); the main problem, amplified spontaneous emission (ASE), that hinders further power scaling and several promising methods to mitigate it are discussed. In the third chapter, the recent development of tandem-pumped high-power ytterbium-doped fiber lasers working at traditional wavelengths, which have scaled the output powers, are introduced (Fig. 4—7). The fourth chapter discusses recent results of tandem-pumped high-power random fiber lasers, which can offer more stable spectral characteristics (Table 2, Fig. 8—9). The fifth chapter discusses tandem-pumped high-power Raman fiber lasers, which expands the range of output wavelength of ytterbium-doped fiber lasers to around 1.2 μm (Table 3, Fig. 10—11).

Conclusions and Prospects The advances in high-power fiber lasers continues to be driven by novel concepts and innovative techniques. Studies on high-power 1018 nm fiber lasers and the various tandem-pumped high-power fiber lasers have contributed a lot of useful solutions for the future development of many fields. In scaling output power, the application of the high-power 1018 nm fiber lasers in tandem-pumped high-power fiber lasers has been making excellent results. However, development of the high-power 1018 nm fiber lasers, as well as the others for tandem-pumped, is the key for further increasing the final performance. In controlling output spectral characteristics, tandem-pumped high-power random fiber lasers have shown promising effects in stable time-domain characteristics and can be used as seed in main oscillator power amplifier (MOPA) configuration for controlling the broadening of laser bandwidth. In expanding the range of output wavelength, tandem-pumped high-power Raman fiber lasers have exhibited outstanding performance in realizing high-power output at up to 1.2 μm. Meanwhile, it is further possible to combine the merits of erbium-ytterbium-co-doped fibers to reach the output of >1.5 μm. The future of tandem-pumped high-power lasers and the fiber lasers for tandem-pumped is bright and awaiting further studies.