【作者】 陈爽；

【导师】 冯莹；

【作者基本信息】 国防科学技术大学 ， 光学工程， 2009， 博士

【摘要】 与传统线性腔光纤激光器相比,高功率主振荡功率放大（MOPA）光纤激光技术具有结构简单、设计灵活等优势,可进一步提高光纤激光系统输出特性,有着较高的应用和学术价值。近年来,研究工作主要集中在如何抑制主放大级系统中的非线性效应,从而进一步提高输出功率,以及具有较大模场面积和一定高阶模抑制特性的增益介质光纤的设计。本论文围绕高功率MOPA光纤激光系统主放大级的几个关键技术展开实验和理论研究,旨在探索其设计、构建和优化方法。主要内容包括:1.从经典光纤放大器物理模型和稳态速率方程出发,通过对信号激光沿光纤放大过程的分析,给出了放大器内各个模式信号和泵浦激光的功率传输方程;引入功率填充因子、光纤截面模场分布函数和模式耦合系数,对纤芯模式竞争的物理机制进行了研究,分析了空间激光束在光纤端面的激励特性,给出了光纤输出光束质量与出射端模式特性的关系,建立了包层泵浦大模面积光纤放大器理论模型。2.根据所建立的包层泵浦大模面积光纤放大器理论模型,在考虑到模式之间的增益、损耗和相互耦合的情况下,理论模拟了多模运转光纤放大器的输出特性,并研究了不同端面激励条件下,光纤长度、泵浦构型、泵浦功率和耦合系数等结构参数对输出激光特性的影响。研究结果表明:多模运转的光纤放大器中,模式之间竞争激烈,从而导致输出激光光束质量下降;降低耦合系数、减小光纤长度和优化端面激励条件可以有效改善输出光束质量。由此,提出了利用大芯径多模纤芯双包层光纤构建主放大器的思路,通过对其进行模式控制,抑制高阶模式,可改善输出激光光束质量特性。3.分析了高功率MOPA光纤激光系统主放大级对增益介质光纤的需求,研究了大模面积增益光纤结构参数对光纤性能的影响。从光纤模场理论和所建立的大模面积光纤放大器理论模型出发,给出了任意折射率分布和掺杂分布增益介质光纤有效模场面积和高阶模抑制特性的计算方法,并分析了弯曲所引入的模场畸变对增益介质光纤特性的影响,说明了复合折射率设计的光纤较之其它折射率分布光纤表现出较强的抗弯曲特性,具有较大的模场面积,在芯径为数十微米量级的大模面积光纤中更具实用性,光纤中稀土离子向纤芯中心集中,可以抑制高阶模,从而改善系统输出光束质量。由此,对模场面积1000μm的增益介质光纤进行了优化设计。4.通过对大模面积光纤放大器特性的数值模拟,分析了系统结构参数对放大器输出特性的影响,并对端面泵浦掺Yb³⁺双包层光纤放大器的各个重要部分和关键技术进行了研究。针对放大器系统中ASE和自激现象抑制问题,对双包层光纤端面研抛角度对其光学特性的影响进行了理论和实验研究,并探索了双包层光纤端面研磨抛光工艺技术。最后,提出了高功率光纤放大器系统的设计、优化和构建方法。5.基于所构建的高功率包层泵浦光纤放大器平台,进行了单频、连续激光和ns级脉冲激光放大实验研究。获得了2.3W的1083nm单频激光输出;重频7kHz、平均功率73mW、脉宽<1ns的脉冲通过放大器后,输出平均功率为832mW,峰值功率约为120W。在此基础上,对多模运转光纤放大器系统弯曲选模方法进行了实验研究,设计并构建了光束质量测试系统,通过测量增益光纤在不同缠绕半径下的输出激光光束质量,发现缠绕光纤能有效改善放大器输出光束质量,实验结果与理论计算结果具有较好的一致性。更多还原

【Abstract】 Master-Oscillator Power-Amplifiers (MOPA) have advantages over typical high power fiber laser with linear cavity in conversion efficiency, output beam quality, availability of high power and compactness; besides, they can further improve the output features of fiber laser systems. Thus, it enjoys a good application and academic value. Recent researches on MOPA focus on the control of the nonlinear optical effects in fiber core of power amplifier to scale the output power and the design of large mode area active fiber which can suppress high-order modes. Therefore, in this thesis, theoretical and experimental study on the key techniques of the power-amplifier-stage in MOPA system is presented in an attempt to construct a better MOPA fiber laser system with high power and high beam quality. The main work included as follows:1. Based on the classic theoretical model and stable-state equations of fiber amplifier, the theoretical model of large-mode-area clad-pumped fiber amplifier is built up. The power equations of multimode signal and pump lights are presented by analyzing the transmission; power flowing, amplification and competition of signal transverse modes. The overlap factors, intensity distribution functions and coupling coefficients of modes are introduced, then the launching condition of free beam on the fiber facet and beam quality of the multimode fiber are studied.2. On the basis of the model mentioned above, the output features of multimode large-mode-area fiber amplifier are simulated by computing the gain, loss and coupling effect of transverse modes. Moreover, fiber length, pump configuration, pump power and coupling, which may influence the output features of amplifier, are considered. It is indicated that in large-mode-area fiber amplifier mode competition could result in degeneracy of the output beam quality, which may otherwise be improved by reducing coupling coefficients, shortenning fiber length and optimizing launching condition. Therefore, high power output which is produced in main-amplifier-stage of MOPA system can be obtained by using large core multimode double-clad active fiber, in which the high-order modes are suppressed by using mode control technology.3. The large-mode-area active fibers in different design are studied respectively to find out the most suitable fiber for the high power MOPA fiber laser system. An analytical method of effective mode area of fundamental mode and high-order mode suppressing for large-mode-area active fibers with arbitrary refractive-index profile and dopant distributions is presented. Moreover, the mode bend distortion induced by fiber coiling, which could influence the performace of active fiber, is numerical simulated. It is shown that the fiber with bybrid profiles, which demonstrate better bending resistance and larger mode area than other fibers, is comparatively applicable in the design of active fiber with core diameter of tens of microns, for the confined dopant profile can improve the output beam quality. Based on the research results, the optimizing design of large-mode-area active fiber with effective mode area 1000μm is presented.4. Key parameters of the amplifier system which may influence the output features are analyzed by numerically simulating the large-mode-area fiber amplifier. Design method and key techniques of yetterbium-doped double-clad fiber amplifier are introduced in detail. The reflection of fiber ends is estimated by discussing the techniques of fiber end polishing and lapping so as to explore a way to suppress the amplified spontaneous emission and restrain the self-oscillations. With the help of the finding above, an optimized high power fiber amplifier system is constructed.5. Based on the high power double-clad fiber amplifier system, a series of experiments are conducted to test the single-frequaency, continuous-wave and ns-regime pulse of Yb3+-doped fiber amplifiers. During the experiments, 2.3W single-frequaency output laser with 1083nm is produced, and average power 832mW and peak power 120W are obtained when seeded by a duration<1ns, 73mW-average power and 7kHz repetition rate pulse. It is worth mentioning that no nonlinear effect is observed throughout the experiments. Moreover, additional experiments are carried out to study the output characteristics of multimode fiber amplifier with coiling, thereby constructing a system to examine the beam quality. By using the system to measure the beam quality factors of amplifiers with different coiling radius, it is indicated that the beam quality can be improved by bending the active fiber, which is consistent with the calculation.更多还原