Significance Yb-doped fiber lasers operating around 980nm have attracted much attention because of their extensive application as high-brightness pumping sources. In addition, Yb-doped fiber lasers operating at this wavelength are potential sources for blue-green light. They can generate blue light around 488nm for underwater communications and sensing by frequency doubling. However, it is challenging to achieve a high brightness with Yb-doped fiber lasers operating near 980nm. The main reasons for this are that Yb-doped fiber lasers when operating near 980nm not only have a high pumping threshold but also have serious gain competition with the amplified spontaneous emission (ASE) at around 1030nm. A number of studies have been conducted on achieving high brightness with these lasers because of their wide application potential. The output power of continuous-wave (CW) Yb-doped fiber lasers operating near 980nm was previously low, which limited their applicability. Researchers have enhanced their peak power by pulsing, realizing blue-green light output by frequency doubling. Although development of pulsed Yb-doped fiber lasers is advancing, people are still aiming to achieve a higher output power with CW fiber lasers. Early research progresses of CW Yb-doped fiber lasers operating near 980nm have been reviewed in Ref.[16] and Ref.[17]. However, these studies only review the research progress before 2011. In recent years, significant progress has been made with CW Yb-doped fiber lasers operating near 980nm. In this study, the development of CW Yb-doped fiber lasers operating near 980nm after 2011 is reviewed. The technical schemes and important experimental results are summarized and discussed. The future development of CW Yb-doped fiber lasers operating near 980nm is also discussed.

Progress In recent years, research outside China on CW Yb-doped fiber oscillators operating near 980nm has involved optimization of the fiber design and significantly progressed fiberization of the fiber laser system. Thus far, an output power of 151W at 978nm with slope efficiency of 63% has been achieved with a CW single-mode all-fiber oscillator operating near 980nm. In this scheme, the high output power was obtained by using a Yb-doped, all-solid, double-cladding photonic bandgap fiber. However, because of the complex structure of the Yb-doped all-solid double-cladding photonic bandgap fiber, optimizing the design of this fiber will also be an important research direction. Most recently, a scheme based on a Yb-doped fiber with core and inner cladding diameters varying along the fiber length in the shape of a saddle has been used for a fiber oscillator. This oscillator achieves a continuous output power of 10.6W at 976nm with laser slope efficiency of 18.4%. Compared with the Yb-doped all-solid double-cladding photonic bandgap fiber, the saddle-shaped double-cladding Yb-doped fiber is easier to fabricate. However, the limited pump power and low efficiency are common problems in this kind of scheme. Improving the output power and slope efficiency by further optimizing the double-cladding Yb-doped fiber should be the next research focus in this direction. In addition to fiber oscillators, fiber amplifiers operating near 980nm are also important. With the main oscillator power amplifier (MOPA) structure, the amplifier can achieve higher output power. Furthermore, the amplifier with the MOPA structure allows for flexibility with the chosen seed laser. At present, double-cladding phosphate fiber is mainly used in CW Yb-doped fiber amplifiers, and the output wavelength is around 976nm. There are two research directions for fiber amplifiers operating near 980nm. First, with regards to single-mode amplifiers, based on the optimization design of Yb-doped fibers, improving the output power and slope efficiency are research focuses. The latest single-mode amplifier reported has achieved an output power of 39W at 976nm with laser slope efficiency of 19%. In this scheme, the slope efficiency of the amplifier is low. Therefore, increasing the slope efficiency will be the emphasis of future research. Second, for multimode fiber amplifiers, the research direction has been to achieve high efficiency output power by increasing the core-cladding diameter ratio of the fiber. More recently, the achieved output power of this kind of fiber amplifier has been over 30W with a slope efficiency of 66%. The emphasis of future research will be to improve the beam quality together with the efficiency.

Nationally,there has been significant research progress on CW Yb-doped fiber lasers in recent years. Most recently, a CW output power of 113W at 976nm with a slope efficiency of 45% has been achieved. It is the first time that a 100-W fiber amplifier operating near 980nm has been achieved, and it provides an important basis for the next step of power upscaling. With regards to fiber oscillators operating near 980nm, progress is relatively poor because of limitations with the fabrication technology of special Yb-doped fibers (e.g., Yb-doped photonic bandgap fibers). In addition, another important research direction is optimization of the design of the double-cladding fiber based on current fiber fabrication technology to improve the output characteristics.

Conclusion and Prospect In summary, the development of CW Yb-doped fiber oscillators and amplifiers operating near 980nm has progressed rapidly in the past 10 years, where those with an output power around 100 W have been demonstrated. However, scaling up of the output power is still a challenge. The design and fabrication of Yb-doped fibers will also play an important role in the future development of fiber lasers operating near 980nm.