【作者】 李杰；

【导师】 许晓军；

【作者基本信息】 国防科学技术大学 ， 光学工程， 2013， 博士

【摘要】 光纤激光器具有转换效率高、光束质量好、结构紧凑、易散热、工作稳定性好等优点，已经广泛应用于工业和国防领域。然而，受光纤非线性效应及光学热损伤等机制的限制，单根光纤激光器的输出功率不可能无限提升。为了获得更大的光纤激光输出，对多个中等功率的光纤激光进行合束是一种有效手段，其中所需的关键元器件就是光纤合束器。目前，美国IPG公司基于光纤合束器的多模连续光纤激光输出功率已高达50kW，但是光束质量很差，M2约为32.7。如何利用光纤合束器在获得大功率激光输出的同时实现高光束质量，进一步扩大其应用范围，是本技术领域需要解决的关键问题。本文围绕光纤合束器模式转换与控制展开了相关理论和实验研究，主要内容包括：首先回顾了光纤合束器的发展及其在光纤激光系统上的应用，指出利用光纤合束器可以实现大功率光纤激光输出，但是输出光光束质量较差，严重限制了其应用范围。据此提出了本文研究内容——研究光纤合束器的模式转换，探索通过模式控制改善光纤合束器输出光束质量。进一步对光纤合束器在不同入射条件下的输出特性展开了数值模拟研究。应用模式分析法分析了多模光纤和71光纤合束器在理想入射、离轴偏移入射和倾斜入射条件下的输出光场和激发模式特性。引入功率谱密度分析方法分析了理想入射、离轴偏移入射和倾斜入射对多模光纤和71光纤合束器的输出光场强度分布和相位分布的影响，探索光纤合束器输出光光束质量退化的机理。根据光纤合束器的两个设计准则，实现了低损耗并可承受百瓦量级输出功率光纤合束器的研制。针对不同功能、不同类型的光纤合束器，提出了四种光纤组束方法。设计加工了一种新型的封装制冷装置，通过实验论证了在光纤合束器传输效率大于95%的前提下，这种热管理方式能够实现4kW的有效制冷。通过分析比较现有的几种光束质量评价方法，选择修正的BPF参数和M2因子一起作为评价合束器输出光光束质量的参考因子。定量分析了光纤熔锥束的占空比和相位误差对输出光光束质量的影响。修正了高增益耦合模型，分析了熔锥光纤束被动相干合成的机制，定量分析了随机相位大小对5路、10路和19路熔锥光纤束阵列锁相状态的影响，以及光纤阵列路数对随机相位的容忍度。最后提出了一种基于光纤合束器全光纤结构的主动相干自适应优化控制方案。实现了单根多模光纤的模式控制，验证了光束净化可以实现高亮度的输出。在光纤合束器相干锁相基础上，应用光束净化系统对光纤合束器的相干输出光场进行模式控制。对自制的21光纤合束器在非相干输入、相干输入锁相前和锁相后的光束净化进行了实验研究，验证了此模式控制方案可以实现高亮度激光输出的光纤合束器。更多还原

【Abstract】 Fiber laser has become increasingly popular for industrial and defense applicationsdue to the advantages of high effciency, good beam quality, compactness, convenientheat management and high reliability. However, the power scaling of the fber laser islimited by the thermal damage and the nonlinear effects. The all-fiber fiber combiner,which merges the outputs of a multitude of single-mode fiber lasers, is the keycomponent for high power output. At present, multimode fiber lasers have been scaledto output power of50kW by IPG. However, the beam quality is poor and the M2factoris32.7. How to scale power while simultaneously achieving high brightness by a fbercombiner for more applications is a challenge. The mode transformation and modecontrol of the fiber combiner are investigated and analyzed in theory, simulation andexperiment in this thesis as follows.The paper begins with reviewing the history of fiber combiners and theirapplications on the fiber lasers. It puts forward that the fiber combiner is an effectiveapproach for high power output. Owing to the poor beam quality of the combiningoutput, the range of the applications are narrowed severely. Therefore, the modetransformation should be studied and the way of the mode control should be explored toimprove the beam quality.The output properties of the fiber combiner under different incidence are simulatedand studied numerically. The output field and mode properties of the single multimodefiber and the71fiber combiner are analyzed under perfect incidence, off-axisincidence and tilted incidence by the mode analysis method. The power spetral density(PSD) method is introduced, and the effect with different incidence on the intensitydistribution and phase distribution are researched. The mechanism that the beam qualityof output beam of the fiber combiner degenerates is exploredAccording to two design rules, the low loss fiber combiners with hundreds wattpower available are fabricated. Four methods of bundling fibers are brought forwardaccording to the types of the fiber combiner. A new package for combiner with excellentcooling are devised and the package is tested for the capability of4kW output powerwhen the efficiency is larger than95%。Several methods for evaluating the beam qualityare compared and analyzed. The modified beam propagation factor (BPF) and M2factorare selected as the appropriate beam quality factors for the fiber combiner. Theinfluence of the TFB duty ratio and phase errors on the beam quality are studied indetail.The model for high-gain fiber laser arrays is revised to analyze the passivecoherent combining of the tapered fused fiber bundles. The influence of the magnitudeof the phase errors on the output phase properties of the5-,10-and19-fused fiber bundle arrays are discussed. The relationship between the tolerance of the phase errorsand the number of the arrays is also caculated.A sheme for an all-fiber active coherent combining via a fiber combiner isillustrated. The feasibility of a single MMF laser cleanup is proved in experiment. Anexperimental system of the active combining and contol of the fiber combiner is built.The system carried on the mode control for the coherent output by beam cleanup systemafter phase-locking the fiber combiner. The beam cleanup of a home-made21fibercombiner are investigated in experiment under incoherent combining, coherentcombining without phase locking and coherent combining with phase locking. Theresluts proves the feasibility for high brightness output of the fiber combiner.更多还原