Significance  Direct LD pumped high power fiber lasers have the advantages of low cost, high conversion efficiency, and good beam quality, finding applications as diverse as industrial processing, medical treatment and fundamental research, etc. The brightness, which is determined by the output power and beam quality, is a crucial parameter of fiber lasers that affects the application effectiveness. However, the power scaling of high-brightness fiber lasers is mainly constrained by the nonlinear effects and the transverse mode instability (TMI). It is noteworthy that IPG Photonics announced the 10 kW and 20 kW single-mode fiber laser in 2009 and 2013, respectively, but no domestic fiber laser products with > 6 kW output power and beam quality factor M²<2.0 are commercially available until now (Mar.2023). There are multiple reasons for the huge gap between the domestic high-brightness fiber laser industry and its foreign counterparts. In addition to the late start time and the less advanced material processing technology, the lack of theoretical guidance and original theoretical-based solutions make it difficult to overcome the technical bottleneck for power scaling of high-brightness fiber lasers, and the lack of fiber laser simulation software also slowed down the process from fundamental research and laboratory demonstration to industrial products. Therefore, it is of significant necessity to develop fiber laser software to aid the fiber laser design and accelerate the process from simulation results to industrial products.

  Progress   To overcome the aforementioned difficulties, researchers from National University of Defense Technology have conducted fundamental theoretical research and developed fiber laser simulation software SeeFiberLaser with independent intellectual property rights. The software can simulate the generation, amplification and transmission of fiber lasers with different time domain characteristics and effects such as amplified spontaneous radiation, stimulated Raman scattering (SRS), stimulated Brillouin scattering and transverse mode competition can be considered in the simulation. The simulation results can output data of the power, spectrum, spot pattern, and time domain as required, which greatly helps the study of fiber laser theory, engineering design and scientific research. With the aim of suppressing SRS and TMI, systematic solutions are proposed, including exploiting the backward pump scheme, optimizing the pump wavelength, employing spindle-shaped fiber design, etc., which are proven effective to improve the performance of fiber lasers through theoretical simulation. Then, industrial fiber oscillators are simulated and optimized based on the SeeFiberLaser software by studying the effects of the active fiber length, operating wavelength, the reflectivity of the output coupling fiber Bragg grating, and the pump wavelength. Furthermore, high-brightness fiber amplifiers that are capable of delivering 8-10 kW output power are simulated and optimized based on the SeeFiberLaser software by considering the active fiber's length and pump absorption coefficient, the backward pump power as well as the core diameter and length of the quartz block head. Furthermore, experimental studies are carried out to verify the effectiveness of the above-mentioned theoretical solutions, including optimizing the pump wavelength as well as employing backward pumping for improved TMI threshold, and exploiting spindle-shaped fiber for SRS and TMI mitigation. Moreover, a 6 kW oscillating-amplifying high-brightness fiber laser based on pump wavelength optimization, 7 kW backward pumped high-brightness fiber laser, and 10 kW fiber laser based on fiber with a small core-to-cladding ratio are experimentally demonstrated, fully proving the functionality and great potential of the proposed solutions. Last but not least, the technical schemes of higher brightness fiber laser are prospected, which include employing integrated multifunctional passive devices on a single piece of passive fiber, adopting ytterbium-doped and energy transfer integrated fiber, and exploiting gain-resonator integrated design scheme, facilitating better good beam quality and improved stability.

  Conclusions and Prospects  Power scaling of high-brightness fiber laser is a complex yet challenging work, which requires comprehensive investigation and optimization. This paper analyzes the impact of various factors on the laser in the fiber laser design process and proposes methods, such as variable core diameter fiber, optimized pump wavelength, etc., to improve the performance of fiber lasers. Noteworthily, the SeeFiberLaser software is developed to bridge the fundamental research outcomes of fiber laser technology and industrial products, which were proven quite effective in high-power fiber laser optimization for both industrial and research use. In addition, 6-10 kW high-brightness fiber lasers have been experimentally demonstrated based on the proposed optimization solutions. Looking forward, higher-brightness robust fiber laser could be expected by developing integrated multifunctional passive devices, ytterbium-doped and energy transfer integrated fiber design, and gain-resonator integrated design scheme.