两路固体激光相干合成的实验研究

相干合成技术是获得高功率、高亮度激光输出的有效途径之一。现提出了基于角锥棱镜的两路固体激光相干合成方案;在理论分析的基础上,开展了两路固体激光器相干合成的实验研究;得到了不同占空比和出射光束直径条件下的远场光强分布。远场光强分布和条纹可见度主要受到占空比和出射光束直径的影响。随着激光光束间距的减小,相干合成后的光斑数量减少,光斑的宽度增大,相干度明显提高;出射光束直径增大时,高阶模增多,光束质量变差但合成功率增大。     

多次函数拟合法计算光纤模场直径

光纤的模场直径是评价光纤性能的一个重要参数。测量单模光纤模场直径的常用方法是远场可变孔径法。通过测量给定单模光纤透过不同尺寸孔径光阑的多组远场辐射光功率数据并对实测数据进行处理,可得到被测光纤的模场直径。在实际测量过程中,为了消除被测光纤模场中心与孔径光阑中心偏离引起的测量误差,应用多次函数拟合法对远场可变孔径法测得的数据进行处理,以此自动识别并排除测量数据中的个别不合理的误差数据,从而有效提高模场直径的测量精度。     

大光斑离轴式光纤旋转连接器的耦合性能

针对直接对准型离轴式光纤旋转连接器中自聚焦透镜准直器出射光斑小、光纤耦合损耗大和有效对准时间短的问题,本文提出了一种由双合透镜和热扩芯光纤构成的大光斑光纤准直器。基于出射光斑在相对旋转过程中重叠面积的周期性变化规律,分析了准直器间存在径向误差和角向误差时的耦合损耗变化。根据转子圆环和出射光斑尺寸得到耦合损耗为3 dB时的理论有效对准时间。为此搭建了光纤耦合实验平台测试其耦合性能及有效对准时间。实验结果表明,光纤准直器采用的单模光纤模场直径越大耦合损耗越低,其中模场直径为28μm时耦合损耗低至0.22 dB。在转速为60 r/min时,通过光电转化和函数拟合得到四种模场直径光纤准直器的有效对准时间。模场直径为17μm时最大有效对准时间为7.05 ms,与理论值7.1672 ms基本一致。最后应用光纤布拉格光栅传感实现无接触式传输光信号。这种设计为大光斑离轴式光纤旋转连接器的研究及其在光纤传感领域的应用提供了理论和技术支撑。     

基于动态权重PSO算法的眼科OCT设备横向分辨率检测

眼科光学相干层析成像(OCT)设备的横向分辨率检测易受光斑噪声和干涉条纹影响,为提高横向分辨率检测精度,提出一种基于动态权重粒子群优化(DWPSO)算法的眼科OCT设备横向分辨率检测方法。通过构造动态权重因子对局部PSO算法进行改进,构建了DWPSO算法;建立了光束光强分布模型,采用DWPSO算法辨识模型参数并获得光束宽度,应用最小二乘算法拟合光束宽度得到数值孔径,用于实现眼科OCT设备横向分辨率检测。实验结果表明:DWPSO算法与其他相关算法相比较能够快速得到光束光强分布模型的全局最优解;被测眼科OCT设备在近焦点位置的横向分辨率为18. 21μm,远焦点位置的横向分辨率为49. 91μm,该横向分辨率检测方法使光斑噪声和干涉条纹的影响有效降低,具有较好的噪声鲁棒性。     

纳米分辨微小光斑光强分布检测系统设计

介绍了现有的微小光斑测量技术,针对纳米分辨率检测精度要求,对纳米分辨微小光斑光强分布检测技术进行深入研究,设计和构建了具有纳米分辨的微小光斑光强分布检测系统。利用该检测系统进行了低数值孔径弱聚焦下所形成的微光场光强分布检测实验,得到较好的光强分布图。实验结果表明,该检测系统具有可靠性高、稳定性好、便于操作等优点。     

多模光纤数值孔径测试仪的研制

数值孔径是多模光纤的重要参数,它表征光纤端面接收光的能力,其取值的大小要兼顾光纤接收光的能力和对模式色散的影响,对光纤的传输带宽影响也较大。本文设计一种基于远场光分布法作为测试原理的用于通信光纤数值孔径的测试仪。该测试仪包括:光注入装置、检测装置、数据采集和计算机处理系统。实验表明该装置测试准确迅速,性能稳定可靠。     

半导体激光器光束的准直设计

利用计算机仿真,设计了相互垂直的椭圆截面柱透镜组来准直半导体激光束。两个椭圆截面柱透镜分别对半导体激光器两个方向的光束进行准直,采用自适应加权和法进行优化计算,结果可达到50μrad量级的发散角,同时也可在远场形成圆形光斑。     

基于旁瓣光束衍射反演的强激光远场焦斑测量方法

针对大型激光装置使用纹影法无法实现旁瓣光束弱信号区域光强分布精确测量的问题,提出了基于旁瓣光束衍射反演的纹影法强激光远场焦斑测量方法.采用逆向推演间接测量的研究方法,沿光路传播逆方向推导,以旁瓣光束衍射光强图像和相位图像作为输入,通过计算获得未遮挡前旁瓣光束远场焦斑分布.相比传统基于纹影的远场焦斑测量方法,本文的主要改...     

一种测量大尺寸孔同轴度的新方法

提出一种精密测量大孔同轴度的新方法,采用单模光纤的尾纤半导体激光器的准直光束作为测量基准线,单模光纤对光束模式的严格限制可使准直激光束的方向稳定性达到1.5×10-6/小时。分析了高斯光斑中心坐标的几种数值计算方法,指出重心法对光斑的光强分布缺陷不敏感,在实际应用中更为稳定。通过对孔的圆周进行数据采样,计算出孔的多个截面的光斑中心坐标并拟合出孔的中心线,即可计算出孔的同轴度。     

半导体激光鼓膜造孔术的光学系统设计

设计并研制了一套可在视频监控和红光指示下实施半导体激光鼓膜造孔术的光学系统.首先,利用光束整形和波长合束技术将半导体激光单管出射的650 nm激光和半导体激光列阵出射的810 nm激光耦合到芯径为200 μm,数值孔径为0.22的光纤中;然后,利用消色差透镜准直光纤出射的双波长激光,再利用另一个消色差透镜将光束聚焦到耳鼓膜上,该聚焦镜可通过机械部件沿光轴方向移动,从而调节鼓膜上光斑的大小;成像部分则直接使用商用视频耳镜;热反射镜用于使激光和成像光同轴.手术时,根据显示器上的红色指示光斑确定造孔位置.测量结果显示:该系统出光孔处的激光功率在0～13.3 W间连续可调,造孔直径在1～3mm内连续可调.使用本系统可缩短手术时间,减少并发症;儿童患者手术时无需全身麻醉.另外,该系统还具有整机体积小、重量轻、电光转换效率高等优点.     

Probing highly confined optical fields in the focal region of a high NA parabolic mirror with subwavelength spatial resolution

Parabolic mirrors with a high numerical aperture can be conveniently used to produce highly confined optical fields in the focal region. Furthermore, these fields can have interesting polarization behaviour due to the high numerical aperture. In particular, if the mirror is illuminated with a size matched radially polarized or azimuthally polarized doughnut mode, the electric field has in the focal region almost exclusively a longitudinal or a transverse polarization component. Such field distributions are interesting for applications in confocal or near‐field optical microscopy. Here we present experimental results where we have probed some of these field distributions by raster scanning a fine gold tip in nanometer steps through the focal region and detecting the scattered light intensity. The measured intensity patterns are compared with corresponding vector‐field calculations.     

Nano-patterning photosensitive polymers using local field enhancement at the end of apertureless SNOM tips

We show experimentally that local optical field enhancement can occur at the end of an apertureless SNOM tip illuminated by an external light source. Our approach consists in the use of a photosensitive polymer, placed in the tip near‐field, to record intensity distribution in the vicinity of the tip end. The excited nanometre‐size light source permits us to produce nano‐patterns on the polymer surface which are then characterized by atomic force microscopy. Experimental images show the influence, on the field enhancement, of three important experimental parameters: the polarization state of the incident light, the geometry of the external illumination and the radius of curvature of the tip apex. These results are shown to be in good agreement with two‐dimensional numerical calculations based on the finite‐difference time‐domain method. We show preliminary nanometre‐size patterns created by this nano‐source excited at a metallic tip extremity and discuss the potential of this approach for near‐field optical lithography.     

马赫-曾德尔干涉集成化的全光纤磁场与温度传感器

为了简化光纤磁场与温度传感器的结构并提高传感器灵敏度,设计并制作了马赫-曾德尔干涉集成化的全光纤磁场与温度传感器。将单根光纤的马赫-曾德尔模间干涉结构和双臂马赫-曾德尔干涉结构结合:将总长度为1.2m的单模光纤部分制备成长度为2.7cm、锥腰直径为30.1μm的锥形微纳光纤,并得到了拉锥时间与锥腰直径的关系。将锥形微纳光纤放置尼龙槽内并包覆磁凝胶构成传感头,实现模间干涉的马赫-曾德尔磁场传感器;将磁场传感器通过两耦合比为50%∶50%的耦合器并联带有可调谐光衰减器的单模光纤形成马赫-曾德尔干涉的温度传感器。从理论上分析了光谱漂移对磁场和温度传感的特性关系,实验测得室温下磁场强度在25~50mT时,磁场传感的灵敏度为0.301 14nm/mT;在磁场强度为0,温度由25℃升高到30℃时,温度传感的灵敏度为0.518 86nm/℃。该传感器可广泛应用于电力系统放电检测、材料加工、安全监控等领域。     

压力作用于LiNbO3脊形波导的模场分析

为了实现基于光弹效应的光波导压力和加速度传感器,需要了解光弹波导中的模场变化。根据晶体的光弹效应理论,分析了在压力作用下M-Z结构LiNbO_3脊形波导中折射率的改变,采用三维FD-BPM法对光弹波导中的导模场进行仿真,并运用光波导的电磁理论对仿真结果进行分析。由仿真及分析结果得出:在压力的作用下,波导的折射率增加,波导中导模光场增大,并且波导对光波能量的约束作用增强。     

受控等效光速减缓及其在磁场强度测量中的应用

提出一种基于等效光速减缓的光纤磁场强度传感方法,可用于细长型沿轴线呈线性分布的线圈的场强测量。工作时,多束激光可在光折变光纤内写入固定和移动两种光栅。将光纤置于特定磁场中,磁光效应导致部分光栅失效,从而改变了光速的等效减缓量。实测减缓量即可间接测得磁场强度值。     

Electric field depolarization in high aperture focusing with emphasis on annular apertures

Electromagnetic focusing theory of light predicts that at high apertures field components arise that are polarized perpendicular to the initial polarization. Although vectorial depolarization has received considerable attention in focusing theory, no evidence has been presented as to its relevance in experiments. We measure the intensity of the perpendicularly orientated field in the focal region by utilizing monomolecular, fluorescent polydiacetylene layers whose transition dipoles are orientated in a single direction. For a 1.4 numerical aperture oil objective lens illuminated with linearly x -polarized light, we find that the integral of the modulus squared of the y -polarized focal field amounts to 1.5% of its x -polarized counterpart. In particular, we show here that the depolarization increases when using annular apertures. Annuli formed by a central obstruction with a diameter of 89% of that of the entrance pupil raise the integral to 5.5%. This compares well with the value of 5.8% predicted by electromagnetic focusing theory; however, the depolarization is also due to imperfections connected with focusing by refraction. Besides fluorescence microscopy and single molecule spectroscopy, the measured intensity of the depolarized component in the focal plane is relevant to all forms of light spectroscopy combining strong focusing with polarization analysis.     

Surface plasmon-enhanced micro-cylinder mode in photonic quasi-crystal

Two-dimensional micro-cylinder patterns of dodecagonal photonic quasi-crystal (12-PQC) and tetragonal photonic crystal (2-PC) were fabricated on the top surface of a GaN light-emitting diode (LED). The patterns were evaporated with a 10-nm-thick Ag film. Spatially resolved surface emission was recorded and analysed by scanning near-field optical microscopy. Electromagnetic energy was confined and enhanced at the top surface when the surface plasmon (SP) resonated. The enhancement factor for 12-PQC was 1.9 times that of 2-PC, 8.6 times that of non-patterned LED in the near field and 6.7 times in the far field, respectively. Finite-difference time-domain simulations are consistent with the experimental data. The results show that with a patterned structure on the top surface of an LED, the light emission can be greatly enhanced due to SP resonance.     

PRINCIPLES OF STRUCTURAL SLOW LIGHT AND ITS APPLICATIONS FOR OPTICAL FIBER SENSORS: A REVIEW

Generation methods of structural slow light and the recent optical fiber sensing applications are summarized in this paper. Five kinds of different structures for slow light are discussed: in fiber Bragg grating, fiber Fabry-Perot cavity, circled coupled resonator optical waveguides, photonic crystal waveguides, and microspheres. The corresponding applications for optical fiber sensors of the different slow light technologies are also introduced. Developments of structural slow light and the characteristics of the slow light sensing methods are analyzed, and improvement trends and potential applications in the fiber-sensing field are forecasted. It is concluded that the technology of structural slow light will increase in fiber sensing and other fields.