【作者】 朱坤；

【导师】 赵修建；

【作者基本信息】 武汉理工大学 ， 材料学， 2002， 硕士

【摘要】 随着红外激光技术的不断发展，特别是Er：YAG（2.94μm），CO（5.0μm），CO₂（10.6μm）等中红外激光器在激光加工、激光医疗中的应用，使得人们对红外激光传输系统的需求越来越迫切。目前研究传输红外激光光纤一般分为两类：红外实芯光纤和红外空芯波导。红外实芯光纤的研究已经深入开展了很久，也开发了一系列样品，但后来的实验研究证明，传统的实芯光纤结构存在着很大的缺点：一是芯层材料的折射率比较大，因而耦合时光纤端面的反射损失比较大；二是实芯结构所存在的结构缺陷大大降低了光纤的能量传输阈值，所以其性能还远不能达到应用的要求。因而在中红外激光传输系统中，空芯光纤因其诸多优点仍然是较为理想的一种传输手段。 本论文研究的重点是CO₂激光的传输，包括传输用空芯光纤和输入端耦合器件两个方面。而两方面的研究都是建立在一定的理论设计基础上的。 在空芯光纤方面，从红外空芯波导的传输特性出发，讨论了电介质／金属型空芯波导的导光机理以及电介质层对HE11模传输性能的改善。通过空芯波导结构设计理论的分析，设计出了电介质／金属型空芯波导的合适的波导结构，对材料的选择和膜厚的确定都给出了明确的指导。单层电介质的理论最佳折射率为，最佳厚度为，金属膜为银膜，要求厚度为0.1μm以上。空芯波导较为合适波导半径为0.3-1.0mm。 因材料性能的限制，通过传输性能以及工艺可控性综合分析，确定制备的空芯光纤为单层电介质涂覆金属膜结构，电介质膜定为碘化银，理论厚度为0.77μm左右，由银膜直接碘化得到。碘化银膜的厚度主要由碘化的浓度和时间来控制。在碘化浓度为0.01M（I₂的CCl₄溶液），碘化时间为60S左右，碘化银的晶型主要为β和γ型，其中β型较多，膜厚约为0.7μm，此时 空芯光纤的传输率最高。截断法测得在10．6tim处的损耗为0．sdB／m，且弯 曲附加损耗较小，在弯曲角度为90度时，附加损耗小于ldB／m。空芯光纤 的内径为1刀m－m，制备长度为12m。 在输入耦合器方面，借助于波导耦合和外腔式波导激光器的相关理论， 设计了工艺上可行的锥型耦合器，确定了透镜、耦合器的尺寸以及耦合器与 透镜的相对位置：耦合透镜的焦距为65mm，直径为srnm，耦合器的长度为 15mm，输入端直径为17mm，输出端直径为0．65mm，耦合器与耦合透镜的 设计距离为44mm。 耦合系统为光纤插拔式结构，避兔了透镜耦合时每一次调整的麻烦。测 试结果表明：耦合器的能量传输率达90％以上。但由于耦合时在空芯光纤中 造成一定的高阶模式损耗，整个激光传输系统的传输效率不是很高，仅65％ 左右。输出激光能量可以稳定在 13w以上，基本上可以满足激光医疗的需 要。更多还原

【Abstract】 The requirements of Mid-infrared transmission system have been greatly stimulated by the advance in the field of infrared lasers for surgery,cutting,welding,heat treatment,etc. But efficient transmission instruments are still unavailable until now. Available band of traditional fused silica optical fibers usually does not exceed 2um because of molecule vibrancy absorption,though the preparation methods are very mature. Generally speaking,there are two types of optical fibers for Mid-infrared laser transmission. One is solid-core optical fiber and the other is hollow waveguide. For solid-core optical fibers,great endeavor has been made and several kinds of fibers have been obtained. But the following research indicates that traditional solid-core structure has some deadly disadvantages:one is large reflective loss in input ends because of large reflective index of the solid core;the other is low transmission threshold because of the structure disfigurements of solid-core. So their performances are far from the applied requirements. Accordingly,hollow waveguides are more attractive transmission mediums for their great advantages.The emphasis of this paper is the delivery of CO2 laser radiation,including hollow waveguides and coupling devices for input ends,and both of them are based on special theoretic design. According to the mode structure and transmission characteristics of general hollow infrared waveguides,we discuss the transmission mechanism of dielectric-coated metallic hollow waveguides,analyze the low-loss transmission conditions of the HEn mode. Based on these,the most advisable structure of dielectric-coated metallic hollow waveguides is designed and specific guidance for the selection of materials and inner multi-layers are provided.The optimum refractive index of the coated dielectric is,and the optimum thickness is The best choice of themetallic film is silver,with the thickness beyond 0.1 jam. And the relatively appropriate radius of the waveguides should range between 0.3-1.0mm.Because of the restriction of the performances of the infrared materials,we decide that the hollow waveguide should have the structure of single layer dielectric coated metallic film according to the general analysis of process controllability and transmission capability. The dielectric we choose is silver iodide,made from silver and iodine,and the optimum thickness is about 0.77um. The thickness of the silver iodide film is controlled by the concentration of the iodine and the reaction time. While the concentration of the iodine is O.OIM and the reaction time is 60s,the thickness of silver iodide film is about 0.7um. In thiscondition,the silver iodide has two crystal types,the main type and few ytype,and the transmission efficiency reaches the maximum. The transmission loss at 10.6um band is 0.5dB/m,tested by Cut-off method. The accessional loss due to bend is also very small. While the bend angle is 90 degree,the accessional loss is less than IdB/m. The tested hollow waveguides have the radius of l.Oum and the length of l-2m.As far as input coupling devices concerned,we design a feasible taper coupler by means of the theory of waveguide coupling and outside cavity waveguide laser and make decision of the sizes of the lens and the taper coupler,and the relative distance of them. The focus of the lens is 65mm and the radius is 5mm. The length of the taper coupler is 15mm,and the radiuses of the input and output ends separately are 17mm and 0.65mm. The design distance between the lens and the coupler is 44mm.The coupling system has a fiber insert-pullout structure,avoiding the trouble of adjusting the fiber each time. The tested results indicate that transmission rate of the taper coupler reaches 90%. But because of the higher modules loss caused by the coupling condition,the total transmission rate of the system can only reaches 65%. The output energy can be stable at 13W,which can satisfy the requirements of laser survey by and large.更多还原