【作者】 王颖；

【导师】 刘旭； 顾培夫； 章岳光；

【作者基本信息】 浙江大学 ， 光学工程， 2009， 博士

【摘要】 光学薄膜在中红外激光器、红外激光探测等系统中发挥着重要作用,它一方面是保证系统实现设计指标的基础组成部分,同时也是决定系统整体性能、使用寿命和环境稳定性的关键因素。随着中红外激光武器技术的进步,机载激光器和单兵作战武器逐渐向小型化、集成化方向发展,激光系统输出的功率密度、能量密度不断提高,因此需要光学薄膜对激光功率密度、能量密度有更高的承受能力。无论是OPO激光系统还是红外光谱探测系统,都对薄膜的光学性能提出了更为复杂的要求,如系统中常用的激光腔镜、分束镜、增透膜等薄膜元件多需要满足多波段光谱的透反要求,这给薄膜的设计和制备都带了较大难度。尤其红外薄膜多采用牢固性、稳定性较差的软膜材料制备,且厚度远高于同类型可见光波段薄膜,这不仅对薄膜本身的性能稳定性提出了挑战,更为提高薄膜的激光损伤阈值带了更大难题,故而研究中红外激光薄膜的设计和制备工艺具有重要意义。本文重点对中红外薄膜的破坏机理,红外材料性能,激光腔镜和分束镜的设计制备,以及薄膜损伤阈值测试开展了研究工作,主要涉及以下几个方面的内容:1.中红外薄膜热场计算。鉴于红外波段材料吸收是引起薄膜损伤的首要原因,建立和发展了薄膜的热场计算模型。将应用于光子晶体领域的高斯频谱展开法引入热场计算,可分析激光入射角度、偏振态、激光尺寸对不同膜系热场分布的影响,同先前的平面波模型相比,该方法更为接近实际激光入射的情况,也便于深入分析薄膜的热力耦合损伤机理。2.分析了红外薄膜内节瘤缺陷的电磁场增强效应。膜层内缺陷被认为是导致薄膜损伤最主要的因素,针对中红外薄膜内的节瘤缺陷建立了时域有限差分模型,重点研究了节瘤的深度、种子尺寸、激光入射角度、偏振态对场增强的影响。结合SEM试验观察,总结出限制膜层损伤阈值的缺陷特征:对于中心波长为3.8μm的高反膜,种子直径大于3μm的较浅节瘤对激光最为敏感,同时,相对于s偏振态,p偏振态激光能够形成更高的场强峰值。3.对红外薄膜材料特性与工艺关系开展了研究。针对YbF₃,ZnSe和ZnS三种材料的工艺参数、光学性能、缺陷密度进行了分析,重点研究了沉积温度对YbF₃水吸收带和ZnSe截止吸收带的影响,为下面章节多层薄膜的制备摸索最优工艺参数。4.薄膜应力研究。分析了YbF₃和ZnSe薄膜应力行为的特点,重点关注工艺参数对应力的影响,选择适当的工艺条件保证高低折射率层应力匹配。搭建了一套应力分析系统,该系统通过测量薄膜应力与外界温度的关系分析应力的组成,应力同工艺参数、基板种类的关系。5.中红外多层薄膜的设计和制备。分别设计了应用于OPO激光系统中的输入镜和输出镜,晶体增透膜,以及三光合一系统中的分束镜。对薄膜设计中遇到的问题和解决方案进行了详细描述,给出了薄膜的透、反射率测量曲线,并对薄膜元件进行了环境试验,分析薄膜破裂同膜系设计之间的关系,最终保证薄膜元件光学特性、环境稳定性均符合系统需要。6.薄膜激光损伤阈值测量。搭建了一套中红外薄膜激光损伤阈值测试系统,详细描述了系统的构成和工作原理,并在测试之前对系统的各项参数进行了测量和标定,包括激光稳定性、靶面处激光半径、激光能量调节标定等。采用该系统对薄膜的损伤阈值进行了测量。更多还原

【Abstract】 Mid-infrared laser system are widly used in state of art weapons, remate sensing and atmosphere detection. Different kinds of multilayer coatings, including high reflectance mirrors, beam splitters, polarizers, are required in infrared laser systems. The current laser technology for applications appeals for high output energy density, compact solid state and small size, which bright great challenges for the development of mid-infrared optical coatings. Fine optical property, longlife stabilities and high laser energy resistance are critical for the coatings used in these laser systems. Especially, laser-induced damages of thin films are a limiting factor for high energy laser applications. Multi-band optical properties are desired of coatings applied in resonant optical parametric oscillators employed in optical-pumped midinfrared lasers and infrared spectrum system. At mean time, the choices of coating materials are limited in florides, sulfide and slenide which are commonly called soft matericals with characteristic of poor durability and optical stabilities. The research in this thesis foucuses on the interaction between laser and mid-infrared coating, including intrinsic propertery and defect influence, optimum deposition parameters of soft coating material, as well as design and fabrication of multi-band thin-films such as input mirror, output mirror and beam spliters which are key components used in infrared laser and spectral imaging equipments. The detailed contents of this thesis are as followed:1. As laser induced absorption in coatings are a significant factor of coating breakdown, a developed thermal model for calculating temperature distribution is presented to analyze thermal process between incident laser and mid-infrared coatings. Gaussian angular spectrum method, which is first proposed to investigate negative refraction phenomenon, is employed to simulate laser energy distribution in the coating. Comapared with simplified plane wave theroy, this method is more consistent with practical conditions and provide a simpler way to understand the effect of laser size, incident angle and polarization to the local heating of coatings.2. Electric field modeling of nodular defects is performed to investigate the interaction between defective multilayer coatings and Gaussian profile laser beam. Light intensity is significantly enhanced as large as 6 times in areas adjacent to the defect boundary as well as within the defects. Different geometry of defects irradiated by a range of 0 to 45 deg incident laser is analyzed. Nodules with large but shallow seed or in the case of 40 deg p polarization irradiation tend to have the greatest enhancement effect.3.. Fabricate and characterize single layers of different infrared coating materials to explore optimum deposition parameters for multi-layer infrared coating device. The influence of depositon temperature and deposition rate on the refractive index and water absorption of YbF₃, as well as the fundamental absorption edge of ZnSe coatings are analyzed. Moreover, we also measured the dependence between deposition rate and defect density of YbF₃ and we try to minimize material sputtering during EB depositon processure.4. Measure and analyze the rasidul stress of infrared coating materials. We introduce a stress analysis system to characterize the stress components and propertities of YbF₃ thin films. Since the material choice and deposition conditions are critical for reducing total stress of multi-layser thin films, mechanical stress and the microstructure of zinc selenide （ZnSe） and YbF₃ single layers deposited by electron beam evaporation at various substrate temperatures and deposition rates have been studied. The results obtained demonstrate that the ZnSe samples present compressive stress, while YbF₃ samples present tensile stress, and the stress of ZnSe samples is maximum at substrate temperature of 200℃. When the temperature increases above 200℃, the mechanical stress decreases with temperature. So it is possible to minimize the total stess of ZnSe/YbF3 stacks through optimizing deposition parameters. Finaly, the optimum parameters are discripted.5. Design and fabricate mirrors and AR coating for OPO laser system and two kinds of beam spliters used in infrared spectral imaging systerm. We discuss the coating design procesure and measure the reflectance and transmittance of corresponding band. For the OPO mirrors, the half-wavelength hole phenomenon, which lowers the transmittance of pump light, appears in the input mirrors, as the transmittion band is located at the half wavelength of the reflection band. Materials’ dispersion and control errors of coating thickness are main reasons that are responsible for half-wavelength hole phenomenon. We eliminates this effect by accurately control the coating thickness and utilizing the refractive index of coating materials obtained through single layer tests, which can reduce the differences between design results and measured results. The transmittance of input mirrors at 2μm is above 96%.6. Laser induced damage threshold testing facility working at mid-infrared region is set up and used to measure LIDT of YbF₃ and ZnS single layers. Coating defects and particulates are the origin of damage and the main factor that decreases the LIDT.更多还原