Designing of Optical Fiber Sensing Probe

Evanescent wave configuration has been extensively used in the development of fiber-optic sensor for different biomedical applications. In the present investigation we have theoretically proposed the designing of single-mode and multimode optical fiber sensing probes, which can be prepared by removing a few centimeters of cladding near the distal end of the step index optical fiber. In this theoretical study, we found that the removal of cladding causes V-number mismatching and this has led to the loss of signal acquisition from the sensing region. Therefore, to minimize these losses we have proposed to reduce the radius by step etch and tapering technique. It was found that step etching and tapering enhances the strength and penetration depth of the evanescent wave significantly.     

An Efficient Dispersion Calculation Method for Axially Symmetric Optical Fibers

An efficient numerical modal calculation method for an axially symmetric optical fiber is presented using a numerical Galerkin's method. By decomposing the transverse electric field of a guided wave with Bessel functions of the first kind in cylindrical polar coordinates, we have shown that dispersion characteristics of an optical fiber can be obtained very easily. In order to illustrate the effectiveness of this suggested method, numerical examples are given for a step index fiber, a W-shape depressed inner cladding fiber, and a dispersion-flattened fiber.     

A multipoint quasi-distributed optical fiber pH sensor

This paper reports the development and characterisation of a multipoint quasi-distributed optical fiber sensor for pH measurement. The system is based on a 170 m length of 200 μm core diameter plastic cladding silica fiber where sections of cladding have been removed and replaced with dye immobilised sol-gel glass to form sensing points. Evanescent wave excitation of a dye, immobilised within 2 mm long sections of cladding, enables the pH value of any spillage material to be determined by optical time domain reflectometry along the length of the fiber. The results suggest a spatial resolution of better than 2.5 meters for this fiber system and indicate that this arrangement could form the basis of a practical sensor/actuator system for chemical spillage, provided that suitable dye/analyte combinations are available.     

Theoretical analysis of novel fiber grating pair

A novel fiber grating pair that consists of a conventional long-period fiber grating and a fiber Bragg cladding grating (FBCG) is proposed. The FBCG is a new type of fiber grating in which refractive index modulation is formed in the cladding. Through the coupled-mode theory, we accurately calculate the coupling coefficients between modes supported in the fibers. And some other mode coupling features in the fiber cladding gratings are analyzed in detail. The calculation of the modes involved in this paper is based on a model of three-layer step-index fiber geometry. Then, we have investigated the sensitivity characteristics for variation of the modulation strengths of the fiber Bragg cladding gratings’ resonance peaks and the long-period cladding gratings’ (LPCGs) dual resonant peaks. Finally, the modulation strength sensitivity of the grating pair’s three resonant peaks is demonstrated, and the results indicate that these grating pairs may find potential applications in optical fiber sensing.     

Investigation of cladding and coating stripping methods for specialty optical fibers

Fiber optic sensing technology is used extensively in several engineering fields, including smart structures, health and usage monitoring, non-destructive testing, minimum invasive sensing, safety monitoring, and other advanced measurement fields. A general optical fiber consists of a core, cladding, and coating layers. Many sensing principles require that the cladding or coating layer should be removed or modified. In addition, since different sensing systems are needed for different types of optical fibers, it is very important to find and sort out the suitable cladding or coating removal method for a particular fiber. This study focuses on finding the cladding and coating stripping methods for four recent specialty optical fibers, namely: hard polymer-clad fiber, graded-index plastic optical fiber, copper/carbon-coated optical fiber, and aluminum-coated optical fiber. Several methods, including novel laser stripping and conventional chemical and mechanical stripping, were tried to determine the most suitable and efficient technique. Microscopic investigation of the fiber surfaces was used to visually evaluate the mechanical reliability. Optical time domain reflectometric signals of the successful removal cases were investigated to further examine the optical reliability. Based on our results, we describe and summarize the successful and unsuccessful methods.     

Microbend fiber optic detection of continuously varying refractive index of chlorinated water

In this paper we report a simple microbend fiber optic sensor using a plastic fiber, LED and photodetector assembly for sensing minute variation of refractive index of chlorinated water. Chlorinated water has slightly higher refractive index and gradual release of chlorine from bleaching powder treated water causes a temporal decrease in refractive index of the solution. The evaporation of chlorine from water and the change in refractive index is found to follow a first order exponential decay function of time. Introducing microbends in an optical fiber is easier and less complex than etching of fiber cladding for traditional optical fiber sensors.     

基于腐蚀光纤的温度及葡萄糖溶液浓度传感器

提出并制备了一种基于本征倏逝波原理的温度及葡萄糖溶液浓度传感器.通过研究腐蚀包层厚度与透射光谱之间的关系,确定较为合适的腐蚀厚度.将标准单模光纤包层腐蚀至2.4μm,利用光纤倏逝波对外界介质变化敏感的原理,通过测量输出光功率的变化量实现温度及葡萄糖溶液浓度传感.实验结果表明:传感器在1070℃的温度范围内具有9.58×10~(-3) dBm/℃的灵敏度,线性度达到99.36%;在葡萄糖溶液03%的浓度范围内具有0.126dBm/(g/L)的灵敏度,线性度达到97.95%.该传感器的响应时间小于30s,具有操作简便、测量准确度高、重复性好、适用范围广等优点,具备良好的应用价值.     

空芯带隙型光子晶体光纤整体空气孔包层导光特性研究

 传统光纤包层中仅存在泄漏的倏逝波,能量较小,不利于包层传感的应用。增大包层中的能量,实现整体包层导光是提高光纤传感灵敏度的有效途径。从理论上分析了利用空芯带隙型光子晶体光纤(HC-PCF)包层进行导光的机理。在实验上选用带隙外的冷光源和激光对一种典型结构的HC-PCF进行了空气孔包层的导光实验,并利用折射率引导型光子晶体光纤和单模光纤进行对比实验。结果表明,带隙范围外光波在HC-PCF中传输时将不受禁带效应的约束泄露至包层中重新分布。包层中SiO2与空气孔的周期型结构将光波约束在高折射率介质中,实现HC-PCF整体空气孔包层中光波的稳定传输。PBG-PCF包层的整体导光在传感上有提高灵敏度的潜在价值。     

Modeling and simulation of polarization errors in Sagnac fiber optic current sensor

Sagnac fiber optic current sensor (S-FOCS) is a kind of optical interferometer based on Sagnac structure, optical polarization states of sensing light wave in Sagnac fiber optic current sensor are limited. However, several factors induce optical polarization error, and non-ideal polarized light waves cause the interference signal crosstalk in sensor, including polarizer, quarter-wave retarder, splice angular, birefringence and so on. With these errors, linearly polarized light wave in PM fiber and circularly polarized light wave in sensing fiber become elliptically polarized light wave, then, nonreciprocal phase shift induced by magnetic field of the current is interrupted by wrong polarization state. To clarify characteristics of optical polarization error in fiber optic current sensor, we analyze the evolution process of random optical polarization state, linear optical polarization state and circular optical polarization state in Sagnac fiber optic current sensor by using Poincare sphere, then, build optical polarization error models by using Jones matrix. Based on models of polarization state in Sagnac fiber optic current sensor, we investigate the influence of several main error factors on optical polarization error characteristics theoretically, including extinction ratio in polarizer, phase delay in quarter-wave retarder, splice angular between quarter-wave retarder and polarization maintaining fiber. Finally, we simulate and quantify nonreciprocal phase shift to be detected in fiber optic current sensor related with optical polarization errors. In the end, we demonstrate S-FOCS in test. The results show that transfer matrix errors are induced by inaccurate polarization properties during polarization state conversion, then, the stability and accuracy of the S-FOCS are affected, and it is important to control the polarization properties at each step of the polarization state conversion precisely.     

CdSe/ZnS量子点在倏逝波光纤pH传感中的应用

将新型量子点荧光传感技术与光纤倏逝波传感技术相结合,发展了一种基于量子点荧光效应并结合倏逝波传导进行溶液酸碱度检测的新型传感技术,具有灵敏度高、检测速度快、便于微环境检测、可实现远程探测、实时监测和原位分析等特点。详细介绍了用于倏逝波传感的锥柱组合型光纤荧光探头的制备方法,量子点在光纤探头表面的修饰流程,光谱与强度两种光纤pH传感平台的建立,并分别从响应范围、线性度、重复性和稳定性等方面对CdSe/ZnS量子点应用于光纤pH传感进行了评价。结果表明,在pH值为2~12的范围内,CdSe/ZnS量子点的荧光光谱信号的峰位在强酸和强碱的情况下都会产生红移,且红移量随pH值的变化呈线性关系,其量子点荧光强度信号随pH值的减小呈线性降低关系,通过在强酸和强碱下交替测试的实验表明其具有较好的重复性,利用荧光强度传感平台进行实时监测的实验表明其具有较好的稳定性。因此,将CdSe/ZnS量子点用于倏逝波光纤pH传感具有可行性,在生物化学、环境监测、医学临床、食品安全等领域的pH值测量方面有着广泛的应用前景。     

Optical fiber modulator derivates from hollow optical fiber with suspended core

A fiber optic integrated modulation-depth-tunable modulator based on a type of hollow optical fiber with suspended core is proposed and investigated. We synthesized magnetic fluid containing superparamagnetic Fe(3)O(4) nanoparticles and encapsulated it in the hollow optical fiber as the cladding layer of the suspended core by fusing the hollow optical fiber with the multimode optical fibers. The light with a wavelength of 632.8 nm is coupled in and out of the modulating element by a tapering technique. Experimental results show that the light attenuation in the system can be greatly influenced by only 2.0×10(-2) μL of the magnetic fluid under different magnetic field strengths. The saturated modulation depth is 43% when the magnetic field strength is 489 Oe. The response time of the system is <120 ms. Significantly, this work presents information for the development of all-fiber modulators, including other integrated electro-optic devices, such as optical switch, optical fiber filter, and magnetic sensors utilizing the special structure of this hollow optical fiber with suspended core and superparamagnetic magnetic fluid.     

Particle size effect on porous Sol–Gel doped cladding U-shaped optical fiber critical micelle concentration (CMC) sensor

In this paper, a sol–gel derived coating U-shaped optical fiber sensors for measurement of critical micelle concentration of surfactants has been studied. The doped sol–gel cladding fiber was used to construct an active cladding U-shaped optical fiber CMC measurement sensor. The porous sol–gel film was deposited a 1000 m core plastic clad silica fiber after removing the cladding of the fiber at the middle portion nearly 6 cm and bending in U-shaped design. CMC detection is based on an adsorption effect in sample solution of sodium dodecylbenzenesulfonate. Three different types of particles size of sol–gel porous cladding were tested in our lab. Test results show that the sensitivity of the small particle size porous sol–gel cladding U-shaped optical fiber in sensing region is higher than that of large particle size sol–gel cladding based U-shaped. PACS 42.81.Bm; 42.81.Pa     

微纳尺度光纤布拉格光栅折射率传感的理论研究

亚波长直径微纳光纤强倏逝场传输的光学特性,使其对周围介质折射率的变化具有极高的灵敏度.本文提出一种基于微纳尺度光纤布拉格光栅(MNFBG)的折射率传感器,结合微纳光纤倏逝场传输和光纤布拉格光栅(FBG)强波长选择的特性来实现高精度折射率传感,对其制备可行性进行了讨论.论文中对MNFBG折射率传感机理进行了深入的理论分析,并使用OptiGrating软件进行了数值模拟,模拟数据显示MNFBG折射率测量的灵敏度随着光纤半径的减小而增加,其中光纤半径为400 nm的MNFBG灵敏度可达到993 nm/RIU,相比于包层蚀刻的FBG灵敏度增加了170倍,说明MNFBG对发展微型化、高灵敏度折射率传感器具有良好的应用前景. 关键词： 微纳光纤 光纤布拉格光栅 折射率传感     

Design theory and experiment of acousto-optical tunable filter by use of flexural waves applied to thin optical fiber

Spectral response of acoustically induced microbending through thin optical fiber is discussed from mode-coupling of core and cladding modes. The thin fiber is analyzed in three-layered structure (core-cladding-air) to gain insights into acousto-optic modulation. We explained the dependence of core and/or cladding diameters on acoustic source parameters from numerical calculations. According to the calculations, we successfully fabricated all-optical tunable filter using this thin fiber that yields an efficient mode-coupling at flexural wave frequencies less than 1MHz. To increase the acousto-optic effect, we used a specially designed thin optical fiber (80 μm of cladding diameter) in the section where flexural wave is produced, and spliced both ends of the thin fiber to the tapered 125 μm fibers. The frequency and voltage tuning of fabricated filter is also confirmed by changing the driven frequency and applied voltage of the PZT, respectively. This result suggests a possibility of fiber-optic device application as all-optical tunable filter at 1.55 μm.     

一种单模-多模-单模结构的干涉型光纤温度传感器

SMS光纤结构是一种近年来新兴的光纤器件,该结构光纤具有结构简单、易于实现、成本低廉、灵敏度高和无温度应力交叉敏感等特性.在光纤传感技术领域是一种继光纤光栅之后的具有广泛发展前景的光纤器件.从理论上分析了SMS结构和SMS级联结构的光纤传感原理,并从实验上研究了SMS结构和级联SMS结构的温度传感特性.实验表明:对于SMS结构光纤传感器,随着温度的增加,特征波长移动量逐渐增加,向长波方向漂移了4.05 nm,其传感器的温度灵敏度为15.76 pm/℃;对于级联SMS结构的光纤传感器,级联后不改变原来SMS结构的中心波长的位置,两个SMS结构仍然是独立的,不相互影响.所以说将两个SMS结构串联是可行的,可以用于温度和应力的同时测量,也可以用于分布式光纤传感器.     

基于光纤粗锥型马赫-曾德尔干涉仪的高灵敏度温度传感器的研制

光纤传感是现代光纤技术的重要应用之一。制作了一种基于两个单模光纤粗锥串接的全光纤型马赫-曾德尔高温高灵敏温度传感器。纤芯中传输的光通过第一个光纤锥耦合, 一部分进入纤芯传输,另一部分进入包层形成包层模,纤芯模和包层模具有不同的有效折射率,经过干涉臂的传输产生了光程差。纤芯和包层传输的光再经过第二个光纤锥耦合,形成干涉进入输出光纤传输。对不同长度的传感器进行实验研究,得出传感臂长度与干涉周期之间的关系。研究了传感器温度响应特性,给出了温度响应灵敏度。实验结果表明,在30～400 ℃温度范围内,长度为35 mm的传感器可以得到较高的温度响应灵敏度,其响应灵敏度为0.115 nm·℃-1。利用傅里叶变换对传感器透射谱进行了分析,可以确定在长度为35 mm的传感器中仅有基模LP₀₁和高阶模LP₀₈两种模式,透射谱就是由这两种模式干涉形成的。该传感器体积小、精度高、抗电磁干扰,具有易于制作、对比度大、质轻、灵敏度高、耐高温等优点。可用于高温气体温度测量及油气井测井等领域的高灵敏度温度传感测量。     

Terahertz-wave generation in a conventional optical fiber

We theoretically propose surface-emitted and collinear phase-matched terahertz (THz)-wave generation in a conventional optical fiber. The third-order nonlinear effect, four-wave mixing (FWM), is used to generate THz waves in an optical fiber. Surface-emitted THz-wave generation via FWM is realized using a single-mode fiber. Perfect phase matching is obtained at ~800 nm and 1.5 microm pumping, and it follows that third-order polarization in an optical fiber has the same phase at any point. In this situation, the optical fiber acts like a phased array antenna of the THz wave. Collinear phase-matching THz waves are obtained under the same conditions as for surface-emitted THz waves, and the THz wave is propagated in the silica cladding of the optical fiber. This is a promising method for realizing a reasonable THz-wave source.     

CO2激光脉冲边缘写入的长周期光栅折射率特性研究

报道了一种用高频CO2激光脉冲在普通通信光纤包层边缘单侧写入的新型长周期光纤光栅。研究发现,这种长周期光纤光栅的折射率变化主要发生在光纤包层区域,而纤芯的折射率变化较小;同时该光栅的附加损耗低于0.5 dB。进一步折射率特性实验研究表明,由于其特殊的折变结构,这种光栅具有较高的外界环境灵敏度,当外界折射率在1.41~1.45范围内变化时,其谐振波长漂移量高达15.52 nm,比实验测得的用传统方法写入的长周期光纤光栅谐振波长漂移量高出近3倍,这种光栅结构在光纤传感中将具有重要的应用。     

全光纤干涉式结构中传感模式仿真分析

全光纤干涉式传感结构中包层模场与外界物理量作用,携带被感测信息,因此对包层模的研究是设计制作和提高该类传感器传感性能的关键.利用有限差分光束传播法获得传感光纤不同长度和不同芯径比时传感器对应的光谱,通过傅里叶变换获得其干涉频谱,计算出各主要参与干涉的包层模组的有效折射率,利用色散方程确定对应包层模.理论仿真结果显示,随着传感部分光纤长度增加,参与干涉的包层模式随之增加,并且向高阶模式变化,光谱变得稠密,是多阶包层模干涉的叠加,传感器输出干涉谱的自由光谱范围变小.随着输入光纤与传感光纤芯径比变化,会明显改变纤芯包层功率分布,同时,芯径比增大也会增加参与干涉的包层模种类和阶数.     

In-line fiber optic structures for environmental sensing applications

Fiber optic sensors based on the interaction of surface plasmons or evanescent waves with the surrounding environment are usually obtained by tapering an optical fiber, which significantly weakens the structure, or by use of just the end of the optical fiber. A fiber optic structure that maintains the structural integrity of the optical fiber with a long environmental interaction length is presented. Graded-index optical fiber elements are used as lenses, and a coreless optical fiber acts as the environmental interaction area. These elements are fused by an optical fiber splicer and result in a continuous fiber optic sensing system.