Significance Compared with the laser diode (LD)-pumped scheme, the tandem-pumped scheme has the advantages of high pump brightness, small quantum defect, and low thermal load, making it the main technical solution for ultra-high power ytterbium-doped fiber lasers (YDFLs). Nevertheless, due to the weak absorption of Yb-ions at 1018 nm, it is necessary to enlarge the core diameter or increase the length of YDF to improve the total pump absorption, resulting in more severe nonlinear effects [mainly stimulated Raman scattering (SRS)] and transverse mode instability (TMI). Despite the 20 kW YDFLs have been obtained based on YDF with a core diameter of about 50 mu m and tandem-pumped scheme, the beam quality is poor for the lacking of efficient mode control. It is very challenging to maintain excellent beam quality with increasing power actually, due to the contradictions between the SRS and TMI suppression strategies. Although IPG photonics has announced a 10 kW single-mode YDFL tandem pumped by 1018 nm YDFLs as early as 2009, no other institutions have ever reported >5 kW YDFL with high beam quality (M-2<2) as of 2021. We presented the research progress on tandem-pumped YDFLs achieved by National University of Defense Technology in the past three years. Possible approaches to enhance power and beam quality were also discussed. Progress To balance high power and high beam quality, we proposed a backward/bidirectional tandem-pumped solution. Although backward/bidirectional pumping schemes are widely applied in LD-pumped laser systems, investigations on their technicalities in tandem-pumped YDFL are very rare in open publications. The primary reason is that in the tandem- pumped YDFL, the 1018 nm fiber laser which functions as the pump source, is highly susceptible to the signal light. Even a small proportion of signal light coupled into the 1018 nm fiber laser might cause significant power decline, even leading to the destruction of the 1018 nm laser. By optimizing the 1018 nm laser oscillator and the backward combiner, we successfully reduced the adverse effects of signal laser on the pump source and ensured the stable operation of the backward/bidirectional pumping system. Then by employing the optimized 1018 nm fiber lasers as a pump source, the benefits of a backward tandem pump were first demonstrated with conventional 25/250 mu m YDF. The signal power was boosted to 5 kW (M-2=1. 54) free of TMI or SRS. On the contrary, the SRS threshold of the YDFL was only 3. 94 kW when the YDF was forward pumped. Afterward, the bidirectional pump scheme was also tested with 30/250 mu m YDF. A 6.22 kW laser output with M-2=1. 53 was obtained by using a 30/250 mu m YDF, but further power scaling was limited by SRS. For higher power, the backward pump scheme was applied and the maximum laser power reached 10. 03 kW (M-2 = 1. 92) without SRS or TMI. In addition to optimizing the pump scheme, special fiber designs were also considered. The geometric or optical structure in the transverse or longitudinal direction of the YDF was modified for SRS suppression and mode control. By using confine-doped YDF (CYDF) of which only part of the core was selectively doped, it was possible to tailor the gain of high-order modes for better beam quality. The influence of key parameters, including the doping ratio, core diameter of CYDF, and mode content of seed laser, on the beam cleanup effect of CYDF was numerically analyzed. According to the simulation, the CYDF with core/inner cladding diameter of 40/250 <mu>m and a relative doping ratio of 0. 75 was designed, fabricated, and applied in a backward tandem-pumped YDFL. 10. 1 kW laser power at 1080 nm was realized with M-2 = 2. 16. The beam quality was superior to that of conventional double-clad YDF with an equivalent core diameter. Besides, tapered YDF (TYDF) that has varied core and cladding diameter along the longitudinal direction was also fabricated and used in tandem pump for the first time. The core/inner cladding diameter of the input and output end of the TYDF was 30/ 250 mu m and 48/400 mu m respectively. The beam quality of the signal laser was well maintained during the high-power scaling process. The M-2 factor was measured to be about 2. 2 at 10. 13 kW, which was much better than that of the 48/ 400 mu m YDF drawn from the same fiber preform. Our special structured fibers have demonstrated superior SRS suppression and mode control compared with conventional double-clad fibers. Conclusions and Prospects Our team has taken a lead in conducting research on high-power backward/bidirectional tandem pumps in China, which has resulted in a significant improvement in the SRS threshold while keeping good beam quality of the YDFL. We have achieved fiber laser output of tens of kilowatts based on conventional YDF, CYDF, and STYDF, with significantly better beam quality than fiber lasers pumped by LD of the same power. However, it should be noted that further improvements to the beam quality and power of the tandem-pumped YDFL still pose significant challenges. Existing solutions have not been able to achieve >10 kW single-mode laser. Next, the team will continue to deepen the research on the evolution of SRS and TMI at extremely high-power levels. The focus of our future work is to improve the pump absorption of the gain fiber, reduce the NA of the core, and increase the loss of higher-order modes. With the help of high-performance gain fibers and advanced fiber optic devices, we hope to steadily improve the output power and beam quality of tandem-pumped YDFLs.
