Bend Sensing with Temperature-insensitive Long-period Fiber Grating

The changes in wavelength and attenuation of temperature-insensitive long period fiber grating subjected to bends with curvatures from 0 to 4.4 m-1 are examined. The wavelength change with curvature is nonlinear and the minimum detectable curvature change is 0.03 m-1.     

基于错位和花生形结构的全光纤马赫-曾德干涉仪的研究

研究了一种基于错位和花生形结构的全光纤马赫-曾德干涉仪,进行了液位和曲率的测量实验,利用错位结构将纤芯模式激发到包层,包层模式经过花生形结构被耦合到纤芯与原有的纤芯模式发生干涉。包层模式对外界物理量如折射率、应力的变化敏感,导致透射光谱漂移。波谷波长的漂移量与液位和曲率的变化成线性关系,利用波谷的漂移实现液位和曲率的测量。在液位实验中,在水位变化范围为1.00～5.00 cm时,波谷向短波方向漂移,灵敏度最高为-0.68 nm·cm-1,线性拟合度为0.995 4。在曲率实验中,曲率的变化范围为0.3～1.2 m-1时,波谷向长波方向漂移,灵敏度最高为22.47 nm·m,线性拟合度为0.986 4,表现出较高的灵敏度。错位结构和花生形结构被用于组成马赫-曾德干涉仪,用普通的光纤熔接机和普通单模光纤即可熔接,结构和制作方法简单,灵敏度高,尤其在曲率的测量中表现出较高的灵敏度。     

基于花生形和光纤布喇格光栅结构的温度不敏感光纤曲率传感器

为了解决光纤传感器波长解调成本高以及交叉敏感的问题,提出一种温度不敏感光纤曲率传感器,该传感器是由花生形结构连接一个光纤布喇格光栅组成,对反射光采用强度解调.光进入花生形结构后激发出包层模式,通过光纤布喇格光栅反射后,反射的纤芯模再次在花生形结构产生不同阶次的包层模,然后和反射的纤芯模耦合,在反射光谱中除了光纤布喇格光栅的布喇格反射峰外,在短波长处出现若干个谐振峰,波长越小,包层模的阶次越高.实验结果表明曲率的变化范围为0.669 0~1.250 0m~(-1)时,测量到反射谐振峰平均光功率为2.260×10~(-7)~1.501×10~(-7) mW,灵敏度为-1.306×10~(-7) mW/m-1,在温度范围25℃~75℃内,反射包层模光功率基本保持不变.该传感器成本低且花生形结构制作简单、机械强度大.     

长周期光纤光栅弯曲特性的模式耦合理论分析

运用模式耦合理论分析了长周期光纤光栅的弯曲特性,得出了该特性与光纤归一化频率V和模式的阶数m有关的结论,即随着弯曲曲率的增大,其谐振波长可能向长波方向漂移也可能向短波方向漂移,光纤的归一化频率和所考察模式的阶数是其决定因素.并运用等效倾斜光栅理论合理解释了弯曲长周期光纤光栅耦合峰幅值的变化特性.     

Bending sensor based on intermodal interference properties of two-dimensional waveguide array fiber

We propose a highly sensitive bending sensor based on the intermodal interference properties of a strongly coupled two-dimentional waveguide array fiber (WAF). The interference resonance peaks formed by the SMF-WAF-SMF Mach-Zehnder interferometer are intrinsically the result of interference between the LP(01)-like supermode and other higher order supermodes, displaying supernormal sensitivity to bending in a wide curvature range. The bending sensitivity of the intermodal MZI is a quadratic function of curvature, and the resonance wavelength shift is up to 100 nm within a curvature range 0-10 m(-1). The fabrication reveals briefness, and temperature response shows little impact on the bend sensing precision. The high bending sensitivity and wide sensing range can make this device a candidate for bending discrimination and measurement in widespread areas.     

对弯曲不敏感的长周期光纤光栅传感器

发现了用高频CO₂激光写入的长周期光纤光栅的谐振波长对弯曲灵敏度在圆周的不同方向上呈现周期性,且在两个特定的对称方向上对弯曲不敏感(即弯曲曲率达到1.1m^-1,谐振波长仅漂移-0.018nm).据此提出新型对弯曲不敏感的传感器和可调灵敏度弯曲传感器的设计方案,可望从根本上解决长周期光纤光栅在测量中存在的温度、应变或折射率与弯曲之间的交叉敏感问题.     

基于光纤光栅光谱分析的混凝土结构钢筋锈蚀监测

混凝土中钢筋发生锈蚀会严重影响混凝土结构的使用功能,文章提出了通过观察长周期光纤光栅(LPFG)的透射谱变化对混凝土结构中的钢筋锈蚀进行监测。这种方法基于长周期光纤光栅的微弯特性,利用其透射光谱随弯曲度增加逐渐变浅、谐振波长处的光功率与LPFG的弯曲度成线性这一特性,将钢筋锈蚀引起的钢筋径向膨胀转化为光栅的弯曲,通过监测光栅的光谱变化获知光栅的弯曲度进而判断钢筋的锈蚀程度,该方法对钢筋直接进行监测,简单易行,并利用光栅的光谱特性从而避免了环境温度、应变、折射率的影响,实验将光栅埋入到混凝土构件中,从已取得的实验结果来看,这种方法测量钢筋锈蚀厚度的准确度可达1.2 μm,可监测到的锈蚀厚度达3 mm,完全能够实现混凝土结构中钢筋锈蚀的早期至中期监测。     

Polynomial fitting of DT-MRI fiber tracts allows accurate estimation of muscle architectural parameters

Fiber curvature is a functionally significant muscle structural property, but its estimation from diffusion-tensor magnetic resonance imaging fiber tracking data may be confounded by noise. The purpose of this study was to investigate the use of polynomial fitting of fiber tracts for improving the accuracy and precision of fiber curvature (κ) measurements. Simulated image data sets were created in order to provide data with known values for κ and pennation angle (θ). Simulations were designed to test the effects of increasing inherent fiber curvature (3.8, 7.9, 11.8 and 15.3 m(-1)), signal-to-noise ratio (50, 75, 100 and 150) and voxel geometry (13.8- and 27.0-mm(3) voxel volume with isotropic resolution; 13.5-mm(3) volume with an aspect ratio of 4.0) on κ and θ measurements. In the originally reconstructed tracts, θ was estimated accurately under most curvature and all imaging conditions studied; however, the estimates of κ were imprecise and inaccurate. Fitting the tracts to second-order polynomial functions provided accurate and precise estimates of κ for all conditions except very high curvature (κ=15.3 m(-1)), while preserving the accuracy of the θ estimates. Similarly, polynomial fitting of in vivo fiber tracking data reduced the κ values of fitted tracts from those of unfitted tracts and did not change the θ values. Polynomial fitting of fiber tracts allows accurate estimation of physiologically reasonable values of κ, while preserving the accuracy of θ estimation.     

A high-sensitive fiber curvature sensor using twin core fiber-based filter

A high-sensitive fiber curvature sensor is proposed and experimentally demonstrated in a large measurement range by using a twin core fiber (TCF)-based filter as sensor head. Applying the coupled-mode theory and equivalent refractive index model, we theoretically anticipate the “blue shift” of the transmission dips of the TCF-based filter when sensor head is bent. Experimentally, we fabricate an 86.9 mm TCF-based filter with a free spectral range of 49 nm and characterize its curvature performance by measuring the wavelength shift. A nonlinear “blue shift” of the wavelength is observed when we increase the curvature. The relationship between wavelength shift and curvature is a second-order polynomial function. In the range from 0 to 9.30 m^?1, the maximum sensitivity is up to ?14.7 nm/m^?1. The measurement range can be further increased by selecting a shorter TCF.     

基于LED的非相干宽带腔增强吸收光谱技术

介绍了基于可见光波段高功率LED作为光源的高灵敏度宽带腔增强吸收光谱技术,该系统的探测灵敏度通过测量NO₂在472.3～479.3 nm范围内的吸收得到验证。将中心波长为457 nm的高功率LED发出的宽带非相干光耦合进入92.5 cm长、由两片高反射率透镜组成的高精度光学谐振腔内,使用CCD光谱仪(HR2000)测量透过光学腔的光强信号。腔镜在472.3～479.3 nm波长范围内的反射率通过O₂-O₂聚合物的吸收确定,实验测量了一系列低浓度NO₂气体样品,采用差分光谱拟合技术在80 s的平均时间内NO₂浓度反演的统计不确定性约为3.1 ppb(ng·mL-1)。     

Intensity curvature sensor based on photonic crystal fiber with three coupled cores

An intensity curvature sensor using a Photonic Crystal Fiber (PCF) with three coupled cores is proposed. The three cores were aligned and there was an air hole between each two consecutive cores. The fiber had a low air filling fraction, which means that the cores remain coupled in the wavelength region studied. Due to this coupling, interference is obtained in the fiber output even if just a single core is illuminated. A configuration using reflection interrogation, which used a section fiber with 0.13 m as the sensing head, was characterized for curvature sensing. When the fiber is bended along the plane of the cores, one of the lateral cores will be stretched and the other compressed. This changes the coupling coefficient between the three cores, changing the output optical power intensity. The sensitivity of the sensing head was strongly dependent on the direction of bending, having its maximum when the bending direction was along the plane of the cores. A maximum curvature sensitivity of 2.0 dB/m⁻¹ was demonstrated between 0 m and 2.8 m.     

四芯光纤弯曲传感器

提出了一种新颖的基于四芯光纤的弯曲传感器,可用于弯曲测量。该传感器采用一段四芯光纤作为敏感单元,四芯光纤的四个纤芯作为一个四光束干涉仪,在光纤出射端远场形成周期分布的干涉格子。该四光束干涉仪的干涉相位差是光纤弯曲曲率半径的函数,因而,弯曲曲率半径的变化可导致格子光场的移动。波长为650 nm的低相干半导体激光二级管被用来做为光源,所形成的干涉格子光场由CCD探测器记录。理论上,建立了远场格子图案强度分布函数与曲率半径的关系,并得到了实验的验证。     

基于组合拟合法的冰晶粒子的光散射计算

利用组合拟合法计算了冰晶粒子的单次散射特性。给出了消光效率因子、单次散射反照度及非对称因子的拟合公式,利用拟合公式对有效粒子尺度为20μm和120μm的六种冰晶粒子的消光效率因子、单次散射反照度及非对称因子进行了计算。结果表明,粒子的消光效率因子、单次散射反照率和非对称因子随着入射波长的增加有着较大的起伏,后两者随着波长的增加而变化趋势基本一致;对于单次散射反照率来说,在可见光波段,反照率非常接近于1;在短波段,粒子的非对称因子变化较小,并且随着波长的增加,非对称因子会逐渐增大。     

激光在准球形边界卷云中传输的衰减和透过特性

高空卷云主要由各种不同形状的冰晶粒子组成,是地空链路上激光信号传输的重要影响因素。依据高空卷云中冰晶粒子的分布特征和散射特性,采用C版本的离散纵标法（CDISORT）,充分考虑地球球形曲率及云层冰晶粒子多次散射影响因素,研究准球形边界云层的激光透过率和衰减特性,并比较了太阳天顶角不同时平面平行模式和准球面模式下卷云大气激光透过率的差异,数值计算了三种激光波长（0.65,1.06和3.8 μm）在卷云中传输时的衰减和透过特性。计算结果表明：较小太阳天顶角（小于80°）入射时,两种模式下卷云大气激光透过率相对误差很小,其中0.65 μm激光波长入射时两种模式下的相对误差仅为1.72%,较大太阳天顶角（大于80°）入射时,两种模式下卷云大气激光透过率相对误差明显增大,0.65 μm激光波长入射时两种模式下的相对误差最大达到69%;卷云粒子单次散射时,激光在云层的衰减与卷云粒子有效半径、传输距离、光学厚度及激光波长等因素有关,随光学厚度的增加,云层的激光透过率减少,1.06 μm激光波长入射时透过率最大,3.8 μm激光波长入射时透过率最小;0.65和1.06 μm激光波长入射时,随云层粒子有效半径的增加激光透过率逐渐增加,而3.8 μm波长激光,随云层粒子有效半径的增加激光透过率逐渐减少,随相对方位角的增加,云层的激光透过率减少,且不同卷云传输模型对激光透过率也存在不同的影响。该研究工作将为开展地空链路星载、机载激光通信、激光雷达探测等工程系统中的激光信号云层传输特性的应用提供理论支持,同时也可进一步拓展为地空链路激光遥感、制导和预警等应用提供预先理论研究基础。     

可调谐光子晶体微环腔型光分叉复用器的设计

提出了一种基于二维光子晶体微环腔的新型光分叉复用器,并可以通过改变环区的硅介质柱的折射率实现波长调谐.运用经典的微环自由光谱范围FSR和微环半径R的关系,证实新型的光子晶体微环腔也满足该关系.利用二维时域有限差分法系统分析了下路信号波长及相应效率随折射率的变化关系.结果表明,只需改变环区折射率0.005,下路波长就可以漂移1 nm,而有效微环半径还不足1.4 μm.     

基于薄芯光纤模态干涉技术的曲率光纤传感器

提出了一种新的基于薄芯光纤模态干涉技术的光纤曲率传感器。在单模光纤的中间部分插入薄芯光纤用于传感光路,没有插入薄芯光纤的单模光纤用于参考光路,以消除环境对曲率测量的影响。由于插入的薄芯光纤和单模光纤纤芯失配,导致包层的高次模被激发,并与纤芯模在单模光纤内形成干涉。当改变薄芯光纤的曲率时,沿纤芯和包层传播的模态和光纤长度会发生改变,使得干涉谷峰发生平移。将传感光纤的两端固定的平移台上,当调节平移台距离时,薄芯光纤的曲率发生改变,导致干涉谷峰向短波方向平移。通过观察谷峰的平移距离可以实现曲率的传感测量。实验表明,该装置具有低损耗、低成本和高灵敏度的特点。     

1.315 μm波长附近实际大气高分辨率吸收光谱

 用窄线宽、脉冲可调谐光参量振荡器（OPO）作光源，使用光程长达1 097m的怀特池，采用单探测器分时复用的探测方法，首次在吸收池中精确测量了实际大气中1.315 μm波长附近高分辨率吸收光谱，实验验证了实际大气中水汽是该波段的主要吸收气体；得到了实际大气中吸收分子在氧碘激光波长（7 603.14cm^-11）处的吸收截面为 (1.05±0.09)×10^-24 cm²(标准大气条件下）以及在该波段主要吸收谱线的参数，包括吸收线的位置、线强度、压力加宽半宽度等。利用实测的线参数计算了在氧碘激光波长附近大气分子的吸收截面，发现吸收最小的波长分别位于7 603.31和7 603.93cm^-1，其值约为(8.9±0.8)×10^-25 cm²，比氧碘激光波长处的吸收截面约小15%。     

基于侧漏型光子晶体光纤高灵敏度宽线性范围弯曲传感器的研究

基于所研制的侧漏型光子晶体光纤, 提出并研制出一种Sagnac干涉仪型高灵敏度宽线性测量范围的弯曲传感器. 实验研究结果表明, 当侧漏型光子晶体光纤中的线性缺陷与弯曲方向一致时, 采用群双折射和波谷波长偏移量测量弯曲曲率均可获得高的弯曲灵敏度, 但线性测量范围小, 且不能进行小弯曲曲率的测量. 当线性缺陷与弯曲方向垂直时, 以波谷波长偏移量进行弯曲曲率检测, 可获得10.798 nm/m^-1高灵敏度的同时且可实现0–5.03 m^-1的宽线性测量范围, 结合测量矩阵的引入可实现温度和弯曲曲率的同时测量, 进而剔除环境温度变化对弯曲曲率检测的干扰, 实现了高灵敏度宽线性范围的弯曲传感; 而以群双折射进行弯曲曲率检测, 虽然检测灵敏度较低, 但可实现对环境温度不敏感的弯曲传感. 关键词： 弯曲传感器 侧漏型光子晶体光纤 高灵敏度 宽线性范围     

Temperature Dependence of Resonant Four Wave Mixing in Erbium-Doped Optical Fibers

Temperature dependence of resonant four-wave mixing (RFWM) in an erbium-doped optical fiber without amplification is investigated experimentally. A large change in the refractive index resulting from the transition absorption is induced in the erbium-doped fiber. The maximum reflectivity of RFWM at 0.3 m fiber length increases from 4.1% to 5.5% with change in temperature from 40°C to - 196°C. The temperature-dependent reflectivity characteristics correspond to the change in the absorption cross section at 1.53 μm wavelength.     

基于迈克尔逊干涉的光纤弯曲传感器

设计并研制了一种新型的基于迈克尔逊干涉的光纤弯曲传感器.该光纤弯曲传感器由两根光纤组成的光纤带固定于弹性弯曲结构上来产生迈克尔逊干涉信号,用光纤折射率匹配液填充微型腔,并施加音频振荡信号对相位干涉信号实现低频调制.采用相位生成载波技术对相位干涉信号进行调制和解调,实现了对曲率变化高精确度的检测.实验测试了该光纤弯曲传感器的相位干涉信号与光纤带弯曲曲率的关系,并与理论分析对比.结果表明该光纤弯曲传感器具有43.96 rad/m－1的灵敏度及0.004 m－1的高分辨率.