【作者】 赵海川；

【导师】 赵伊君； 许晓军；

【作者基本信息】 国防科学技术大学 ， 光学工程， 2012， 博士

【摘要】 光纤激光器具有转换效率高、光束质量好、性能稳定、结构紧凑等优点，在工业加工、医疗美容、国防科研等领域有着广阔的应用前景。但是受非线性效应、热效应、光纤端面损伤等物理因素的限制，单根光纤激光器输出功率有限；增大光纤纤芯直径能有效提高激光器输出功率，但由此带来的光束质量退化反而会降低激光亮度。基于自适应光学的光束净化技术已广泛应用于提高大功率固体激光器的光束质量，如果能将光束净化技术用于改善大功率光纤激光器的光束质量，会进一步拓展高亮度光纤激光的发展方向及应用领域。论文提出了大功率光纤激光模场自适应优化控制的概念，并对此进行了系统的分析与研究，主要包括以下内容：1、对用于提高激光亮度的各种大模场光纤进行了总结与分析，指出依靠特殊光纤设计等手段很难实现高亮度光纤激光输出；对比大功率光纤激光器与大功率固体激光器发展历程，类比提出将基于自适应光学的光束净化技术用于提高大功率光纤激光器的光束质量，不再只单纯关注光纤激光器设计，而是用主振荡功率放大器（MOPA）结构实现光纤激光器的高功率，用光束净化实现光纤激光器的高光束质量，通过两个系统解决目前光纤激光器无法同时实现高功率和高光束质量的问题。2、针对大功率光纤激光的光束净化，使用光束传播法和模式分析法计算了单MOPA结构的大模场光纤激光器和多MOPA光纤合束激光器的出射光场，分析了光场特性，指出这两种结构出射激光光场模式的一致性，即可使用光束净化技术改善光束质量，提高激光亮度。建立了大功率光纤激光模场自适应优化控制模型，对应用于光纤激光的光束净化系统进行了分析，确定了优化式自适应系统更适合用于光纤激光光束净化；光束净化系统采用随机并行梯度下降算法（SPGD），选择桶中功率（PIB）为算法的性能评价函数；建立了图形化用户界面仿真环境，对大功率光纤激光光场的模式控制进行系统的仿真。3、对单MOPA大模场光纤激光模场自适应优化控制进行了理论研究和实验验证。从单个高阶模入手，研究了光束净化系统空间分辨率、相位噪声、光强起伏等对光场净化效果的影响，得到了算法收敛对相位噪声等影响因素的容忍度。仿真结果表明，使用PIB作为性能评价函数能有效减少光强起伏对算法收敛性的影响；当相位噪声频率与算法迭代频率相近时，光束净化可以容忍均方差在π2rad范围内的相位噪声。实现了单个高阶模（LP11）的模式控制，远场PIB值由控制前的0.2增大至0.75，得到近衍射极限的远场光强分布；进行了多模光纤激光光束实验，将多模光场的PIB值由净化前的0.08提高至0.7，光强远场能量集中度提高9倍，验证了多模光纤激光光束净化的可行性。4、对多MOPA光纤合束激光器多模光场的自适应优化控制进行了理论研究和实验验证。分析了多MOPA光纤合束光场的锁相原理，证明了光纤合束的多模光场能够实现相位锁定和光束净化。以参数为105/125μm，NA=0.22的多模传能光纤为例，分析了耦合时光轴的离轴、倾斜等误差对能量耦合效率及光场模式的影响。实现了2路激光合束的多模光场相位锁定和光束净化，将多模光场的PIB值由净化前的0.2提高至0.71，光强远场能量集中度提高3.5倍，验证了多MOPA光纤合束结构的大功率光纤激光器光束净化的可行性。更多还原

【Abstract】 Fiber lasers have wide potential for industrial and military applications due to their essential characters such as high conversion efficiency, excellent beam quality, robust performance and compact configuration. However, the ultimate output power of single fiber is limited by nonlinear effect, thermal effect and so on. Beam cleanup technique has been widely applied in high-power solid state laser systems to improve beam quality; therefore it would be an effective approach for achieving high brightness lasers if the beam cleanup technique could be employed in high power laser systems. In the present paper, the new method that control the mode field of high power fiber lases based on adaptive optics is presented and analyzed systematically, and some meaningful results are achieved as follows.1、Reviewed and analysis on various theniques of large mode area(LMA) fibers is presented, the result indicates that it is hard to achieve high brightness by means of fiber design. The idea that applies beam cleanup technique to increase high power fiber laser beam quality is proposed while the history of high power solid state laser technology and high power fiber laser technology are compared. The new method actualizes high power and high beam qualitr by master oscillator power amplifier (MOPA) and beam cleanup respectively, which get out of the traditionary thought in high brightness fiber laser technoly.2、Using beam propagation method and mode analysis method calculate the beam field of LMA fiber lasers based on the configuration of MOPA and multi-MOPA combine respectively. The characteristic of two kind beam fields are analyzed and the essential consistency of beam fields illuminates the feasibility of fiber laser beam cleanup. The system model of mode field control of high power fiber laser based on adaptive optics is build up and is analyzed in detail. Optimization algorithm based on adaptive optics (OABAO) is confirmed for the beam cleanup system. OABAO employs Stochastic Parallel Gradient Descent (SPGD) algorithm and Power in the bucket (PIB) is selected as the quality evaluation function. Simulation software with graphics-user-interface used for modeling and simulation of mode field control of high power fiber laser based on adaptive optics is developed.3、Beam cleanup of single LMA fiber is investigated in theory and experiment. Starting with single high order mode, the influence of system resolution、phase noise、 power fluctuation on the result of beam cleanup is theoretically analyzed. The conclusion indicates that PIB, as the quality evaluation function can eliminate the the influence of power fluctuation and the toleration of standard deviation of phase noise could be π/2when the frequency of noise and algorithm updating rate is approximate. A LP11mode transform is demonstrate in experiment. The system transforms the LP11 mode into a nearly Gaussian beam and the power in the diffraction-limited bucket in the far-field is increased by more than a factor of3.6. The beam cleanup of multimode fiber laser is demonstrated in experiment and the power in the diffraction-limited bucket in the far-field is increased from0.08to0.7that the feasibility of multimode fiber laser cleanup is proved.4、Beam cleanup of multi-MOPA combine fiber laser is investigated in theory and experiment. The theory of phase-locking and beam cleanup multi-MOPA combine fiber laser is analyzed. The influence of error axial on enegy coupling and mode composing is analyzed with the parameters of multimode fiber is105/125μm, NA=0.22. A two-channel fiber combine is demonstrated in experiment. After phase-lockingl and cleanup, the power in the diffraction-limited bucket in the far-field is increased from0.2to0.71, which the feasibility of multi-MOPA combine fiber laser beam cleanup is proved.更多还原