【作者】 高昆；

【导师】 明海； 许立新；

【作者基本信息】 中国科学技术大学 ， 光学， 2009， 博士

【摘要】 本文工作是在国家973项目《角锥光纤激光器相干合成研究》、《光纤激光器特性分析》、国家自然科学基金《宽带掺镱光纤放大器噪声特性研究》等项目的支持下开展的。高功率激光器是现代激光光学中的一个重要研究方向。作为固体激光器的一种特殊形式,光纤激光器以其结构紧凑、转换效率高、光束质量好等优点成为一类十分有前景的激光光源,在医学、通讯、雷达、传感、工业加工、以及科研领域都有着广泛的应用。因光纤结构的特殊优势,高功率光纤激光器作为光纤激光器的主要发展方向之一受到特别重视。在高功率光纤激光器中,进一步提升功率的主要限制是热效应以及光纤中较强的非线性效应。为了在提高输出功率的同时避免这些问题,一般可以从两个方面入手:一是改进单个激光腔的结构,适当增加光纤中的模场面积来减小非线性的影响;二是使用多个光纤激光器合成输出。本文内容即围绕这两个当前的主要发展方向展开论述。文中详细分析了新型的大模场瓣状微结构光纤应用于高功率放大器及激光器时的特性,给出了相应的优化方向;对于激光器输出合成的方案,文中以相干合成方案为主分析了合成光束的性质,并探讨了全光纤化相干合束装置的可行性。本论文的研究工作及主要成果有:1.利用简化二能级结构的速率方程和有限元方法,建立掺镱大模场瓣状微结构光纤作为稳态高功率放大器时的计算模型,系统地研究了这种放大器的特性。重点研究了瓣状光纤的结构参数对放大倍数和噪声特性的影响。模拟计算显示:对于依靠泄漏损耗实现单模运转的瓣状光纤放大器,需要选择合适的结构参数,以实现放大率、噪声指数、以及单模性能等的优化,同时也给出了放大器的性能优化参数;2.在掺镱瓣状光纤放大器数值模型的基础上,建立掺镱大模场光纤激光器模型。计算了不同结构的掺镱大模场瓣状光纤激光器的阈值、效率、信噪比,并分析相应的激光腔优化参数。分析结论为:瓣状光纤激光器的效率、单模性能比同样结构参数的瓣状光纤放大器为优,且其谐振腔参数的选择范围也较宽,选择合适的参数可使其达到近量子极限的效率;3.基于Fraunhofer标量衍射理论计算了六角形排列的空间面阵相干合束的远场光斑特性。对于理想相干合束的情况,详细计算了合成光束特性随单元激光器参数变化的趋势,指出尽量减小单元激光器排列间距是提高合成光束能量集中度的有效途径;对于非理想相干合束的情况,计算了单元激光器的各种参数偏离理想位置时,其光束的主峰效率随之变化的规律,指出实际的合束装置中最应注意的是单元激光器间的固定相位差,应使其尽量接近于零方可得到较高的合成效率;4.利用空间阵列式相干合束的干涉本质,探讨了使用非全同单元激光器构成的空间阵列式相干合束输出特殊形状光束的应用,并以相控扫描和平顶光束的输出为例初步讨论了其输出的效果;5.基于光纤激光器中的注入锁定现象,对共腔式全光纤相干合束方案作了一定改进。设计了2路注入锁定共腔相干合束实验,通过外加窄线宽的DFB光纤激光器的注入锁定,使共腔式相干合束方案原本紊乱的光谱得到了稳定;设计了2路注入锁定环形腔放大方案,通过将MOPA结构中的放大器改为环形腔激光器,使两路输出的相干度得到大幅提高。这些思想对于设计窄线宽、稳定性好、自适应性强的全光纤相干合束装置有一定的参考意义。本文的主要创新点:1.分析了新型掺镱大模场瓣状光纤用作光纤激光器时的特点,指出其与瓣状光纤放大器的异同;指出瓣状光纤独特的单模运转方式对放大器和激光器的性能参数有较大影响,并给出了相应优化参数;2.分析了空间阵列式相干合束的单元激光器排列方式对合成光束特性的影响,指出空间阵列式相干合束的输出光束的能量聚集程度可以达到单个单元激光器的水平;指出在激光雷达等注重主峰效率的应用中,单元激光器间的固定相位差是影响主峰效率和斯特列尔比的主要因素;提出并分析了用空间阵列式相干合束装置进行相控扫描和输出平顶光束的可能性;3.利用注入锁定的方法对共腔相干合束方案的缺点进行了改进:使用光纤DFB激光器的注入锁定,稳定了共腔相干合束装置的光谱;使用环形腔代替MOPA结构中的放大器,增加了输出光束之间的相干度;其思想对于进一步制作性能优良的全光纤相干合束装置有参考意义。更多还原

【Abstract】 High-power lasers have long been an important research topic in laser optics. As a special kind of solid-state lasers,the fiber lasers find prospective applications in medical,optical communication,lidar,optical sensing,material processing,and scientific researching areas because of its compactness,high efficiency,and high beam quality.Due to the various advantages of fibers,the high-power fiber lasers are attracting more and more researching interests nowadays.The main obstacles in developing a high-power fiber laser are the heating effect and fiber nonlinearity.To avoid such problems,we can either improve the cavity structure of a single laser using fibers with large core areas,thus less fiber nonlinearity,or simply combine several lasers together to reach a higher power.The discussions in this thesis include an analysis of the characteristics of a novel ytterbium(Yb) doped large-core segmented-cladding fiber(SCF),both as a fiber amplifier and as a fiber laser.It also includes an analysis of laser combining schemes,which is mainly about the characteristics of the coherent beam combining(CBC),as well as the feasibility of devising an all-fiber CBC scheme with better performance.The major work and results of this thesis are as follows:1.Establish a numerical model for simulating an Yb-doped large-core SCF amplifier by combining the rate equations of a two-level model and the mode field distribution calculated by finite-element method.In particular,we have investigated the influence of the structural parameters on the signal gain and noise level.The results show the necessity of choosing appropriate structural parameters of the SCF to make a balance between the gain,the noise figure,and the single-mode performance,due to the single-mode operation principle of a SCF.We also give the optimal parameters of the amplifier.2.Simulate a steady-state Yb-doped large-core SCF laser by adding feedback conditions to the established numerical model of the fiber amplifiers.We present an analysis of the SCF lasers including the threshold,the efficiency,the signal to noise ratio,as well as the optimal cavity parameters.The results show that,being different from a SCF amplifier,a SCF laser has much higher efficiency and better single-mode performance within a broad range of cavity parameters.By appropriately choosing the cavity parameters,the Yb-doped SCF laser can operate with near-quantum-limit efficiency.3.Calculate the characteristics of the far-field spot of general hexagonally arrayed coherent beam combining devices based on the Fraunhofer scalar diffraction theory.For ideal coherent beam combining devices,the calculation gives some general considerations on the devices to obtain higher power convergence,in which a major conclusion is to arrange the unit lasers as tightly as possible.For combining devices with some deviations,we plot the variation of the power percentage of the main peak of the beam with the change of each parameter.The result leads to the conclusion that one must pay special attention to the phase difference between unit lasers to obtain higher main peak efficiency.We also propose configurations by which arrayed CBC devices can deliver a flat-top beam,as well as the feasibility of changing the angular direction of the main peak like phased array radars.4.Make some improvement on the co-cavity CBC scheme based on the injection locking theory.We have proposed an improved 2-way co-cavity CBC experiment in which the injection from a fiber DFB laser helps stabilize the spectrum of the CBC laser.We have also devised a 2-way ring-cavity amplifying experiment to verify the coherence improvement by substituting the amplifying fibers of original MOPA structures into ring lasers.The experimental results have confirmed the effects of injection locking,with which we can develop all-fiber CBC devices with the advantages of narrower linewidth,better stability,and the ability of self-organizing.The innovative work and results in this thesis are as follows:1.Analyzed the characteristics of the novel Yb-doped large-core SCF lasers and highlighted the difference between the SCF laser and the SCF amplifier,as well as the influence of SCF structural parameters on the laser performance.The optimal cavity parameters are also calculated.2.Analyzed the influence of the arrangement of unit lasers on the combined beam for arrayed CBC devices.The results show that the power convergence can exceed that of the unit laser with some geometry.For applications as a lidar,the calculation leads to the conclusion that the phase difference between the unit lasers has maximum influence on the main peak efficiency as well as on the Streh1 ratio.We also proposed the concept of devising CBC devices with the ability of angular scanning or flat-top beam output.3.Made some improvement on the co-cavity CBC scheme based on the theory of injection locking.The spectrum of a co-cavity CBC device is greatly stabilized with the help of the DFB injection.We also attempted substituting the fiber amplifier in a MOPA laser into ring-cavity lasers,by which the coherence between different arms greatly increased.The idea helps in fabricating all-fiber CBC devices with better performance.更多还原