Significance In recent years, with the continually improved power and beam quality of the high-power Ytterbium-doped fiber laser oscillator, laser oscillators are being extensively used in industry, scientific research, and other areas. Compared with the master oscillator power amplification (MOPA) fiber laser configuration, fiber laser oscillators hold the advantages of compact volume, easy control logic, low cost, anti-reflection, and high stability. With the development of fiber components and processing technology, the output power and beam quality of fiber laser oscillators will improve, and may be used instead of the MOPA fiber laser in the future.

Progress In scientific research, early in 2012, the Alfalight company reported all fiber laser oscillators with output power of 1 kW. Since then, the output power of the laser oscillator continued to increase every year and increased a lot in the last two years. In 2014, the Coherent company reported single-mode fiber laser with power of 3 kW in spatial structure. Soon, Laserline Gmbh reported 17.5 kW multi-mode fiber laser in spatial structure in 2019 ( Fig. 2). Compared with the oscillators in spatial structure, all fiber laser oscillators got more attention by the researchers. After 2016, many institutions studied single-mode laser oscillators in detail, and the output power increased from 2 kW in 2015 to 8 kW in 2020. In 2020, Fujikura reported the highest single-mode, all-fiber laser oscillator with output power of 8 kW ( Fig. 9). In our group, we are studying laser oscillator from 2010. In 2012—2020, we also demonstrated all-fiber laser oscillators with output power from 1 kW to 6 kW ( Fig. 6).

In 2010, the CoreLase company launched the fiber laser oscillator product with output power of 1 kW. After 5 years, the CoreLase company launched a 2 kW laser oscillator product. In 2015, Maxphotonics company in China also launched a 1.5 kW laser oscillator product in cooperation with our group. Since 2017, a lot of laser companies such as Lumentum, GW laser, Reci laser, Feibo Laser, and DK laser launched fiber laser oscillator products with output power from 2 kW to 4 kW (Table 2).

As we know, IPG photonics demonstrated a 10 kW single mode fiber laser with MOPA configuration. After that time, fiber laser with MOPA configuration was being developed. Many institutions demonstrated output power from 5 kW to 10 kW in recent years (Table 3). In China, some institutions including CEAP, Tsinghua University, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, and our group demonstrated MOPA laser with power more than 10 kW (Table 3). However, in the industry, only after 2019, Raycus laser and Maxphotonics launched MOPA laser products with power 3--6 kW (Table 3), and most of these were not single mode products. On comparing the production of fiber laser in scientific research and in industry, we can see that the time from scientific reported laser to the industry product in fiber laser with MOPA configuration needs almost three years, which is longer than that of the fiber laser oscillator. The possible reason could be that the laser oscillator holds the advantages of anti-reflection and high stability than the fiber laser with MOPA configuration, which gives the fiber laser oscillator a more practical option in industry. So, we can see that laser oscillators are widely used than the MOPA fiber lasers in industry. And also, in some experiment, researchers found that the fiber laser oscillator held a higher mode instability threshold than that in fiber laser with the MOPA configuration (Fig. 11).

For the future, the develop tendency of Ytterbium-doped fiber laser includes scaling both power and efficiency with good beam quality, generating special beam pattern in practice application, and extending the laser wavelength to short and long wavelengths.

To scale the fiber laser performance, close attention needs to be paid to these key technologies: high efficiency and loss pump & signal combiner are the essential preconditions for high power and good beam quality fiber laser; high efficiency and relative low absorption laser diode as the pump source for the gain fiber is the key component for increasing laser power and efficiency; new types of gain fiber such as spindly fiber are an effective method for balancing the nonlinear effect and mode instability; specific fiber grating is an effective way for lasers with controllable beam quality. Considering these technologies, a technical proposal for 10 kW high-power Ytterbium-doped laser oscillator is provided. In this proposal, we have used most of the above-mentioned technologies.

Conclusions and Prospect With further expansion in the fiber laser, the requirements for the power and beam quality of the fiber laser oscillator will increase. If using the conventional technology method for the near single-mode fiber laser oscillator, technical bottleneck will be encountered during the power increasing. Our new technical proposal combined the special high efficiency and loss pump & signal combiner, high efficiency and relative low absorption laser diode, gain fiber with vibrational core diameter and end cap with tapered fiber, which can provide a breakthrough regarding the power limitation of the conventional fiber laser, and help scale the power of single mode fiber laser oscillator to more than 10 kW.