LD泵浦高亮度光纤激光器:设计、仿真与实现(特邀)

Design,simulation and implementation of direct LD pumped highbrightness fiber laser(invited)

LD泵浦掺镱光纤激光器具有低成本、高效率、高光束质量等优点,在工业、科研、国防等领域有着广泛的应用。在大部分实际应用中,由功率和光束质量决定的亮度是影响光纤激光器实际作用性能的核心指标。受到非线性效应(尤其是受激拉曼散射)和模式不稳定效应的限制,当前高亮度掺镱光纤激光器输出功率提升遭遇了明显的技术瓶颈。为了抑制非线性效应和模式不稳定效应,在传统方法的基础上,提出了变纤芯直径光纤和优化泵浦波长等成体系的方法以提升光纤激光器的输出功率;为了有效提高对光纤激光器的设计研发能力,提出并开发了具有自主知识产权的光纤激光仿真软件SeeFiberLaser。首先,介绍了影响宽谱高功率掺镱光纤激光器亮度提升的主要限制因素,给出了各个限制因素的抑制方法;其次,利用自研光纤激光仿真软件SeeFiberLaser对提升光纤激光器功率的方法进行优化设计,并对工业常用的振荡器和高亮度光纤激光放大器进行仿真优化;然后,介绍课题组采用后向泵浦、变纤芯直径光纤和优化泵浦波长等方法提升激光功率,实现的6~10 kW高亮度功率光纤激光器;最后,对更高亮度光纤激光器的技术方案进行讨论和展望,提出了无源器件集成化、增益传能光纤一体化等思路,提出了基于变纤芯直径增益传能一体化光纤和集成化无源器件的新型高功率近单模光纤激光器技术方案。

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 spindleshaped 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.