基于包层模的光纤布拉格光栅折射率传感特性

提出了基于光纤布拉格光栅(FBG)包层模式的折射率传感方案。实验中,利用不同浓度的丙三醇水溶液作为外界折射率传感溶液,采用氢氟酸溶液化学腐蚀的方法来减小光纤包层的直径以增大包层模式对外界折射率的敏感度,研究了腐蚀后光纤布拉格光栅包层模式的耦合波长对外部折射率的变化关系。实验结果表明在1.3300~1.4584的折射率范围内,包层模式耦合波长随外界折射率增大而增大,在接近光纤包层折射率处具有很高的折射率灵敏度,最大达到了172 nm/riu(refractive index unit)。而且,包层模谐振的光谱半峰全宽(约0.07 nm)仅为布拉格纤芯模谐振光谱半峰全宽的1/4,能够获得更好的传感精度。     

长周期光纤光栅模式与耦合的研究

长周期光纤光栅是在光纤纤芯中沿轴向形成折射率周期性调制的带阻型光纤器件。与光纤布拉格光栅不同，它表现为前向传播的纤芯基模与同向传播的各阶次包层模式在特定波长的耦合。研究了纤芯基模、包层模式（HE1t/EH1t）及其有效折射率随波长的变化，研究了纤芯基模与包层模式（HE1t/EH1t）的耦合系数。研究表明，纤芯基模与一阶低次包层模式的有效折射率随波长增大而减小，纤芯基模与HE1t产生的耦合系数远大于与EH1t的耦合系数，并且包层模式次数较低时，耦合系数随次数的升高而增加。这与以前的研究结果有所不同。     

有限包层半径光纤Bragg光栅的理论研究

采用光纤波导三层模型,对有限包层半径光纤Bragg光栅导模有效折射率的改变进行了理论分析,结果表明:当包层直径小于16μm时,单模光纤Bragg光栅(纤芯直径为8.3μm)的导模有效折射率才开始发生明显变化.在包层外添加外包层,通过改变外包层的折射率可以实现对光栅Bragg反射波长的调谐,同时对不同芯子直径的光栅Bragg波长移动进行了数值计算.在保证光纤归一化频率不变的前提下,芯径越小Bragg波长调谐范围越大,当包层厚度为1μm时,芯径为a＝2.2μm的光栅Bragg波长调谐范围约为3.9μm.     

Influence of uniaxial crystal material cladding on reflectivity and dispersion of uniform fiber Bragg grating

The fiber Bragg grating with cladding made of uniaxial crystal material whose optical axis is parallel to the axis of fiber Bragg grating proposed in the paper published in 2003 was investigated again and an error was corrected in the calculation, its effective index, reflectivity and dispersion were examined using coupled-mode theory and numeric solution. The calculated results indicate that no low frequency cutoff phenomenon exists in the HE₁₁ mode, more power is transmitted by the core of the fiber with cladding made of isotropic material, the reflectivity of the fiber Bragg grating with cladding made of uniaxial crystal material is much higher than that with cladding made of isotropic material, the parameter K_cl, i.e., the ratio of the extraordinary to the ordinary ray refractive index, has a stronger impact on the reflectivity, Bragg wavelength and the dispersion of this kind of fiber Bragg grating when it varied from 1.00 to 1.01 than in other regions. This means that the characteristics of the fiber Bragg grating with uniaxial crystal cladding can be changed through adjusting K_cl while keeping its length, periodicity and the other parameters as constants.     

少模光纤布拉格光栅折射率传感的分析与测量

理论分析和模拟计算了少模光纤布拉格光栅基模及高阶模的耦合与传输特性,得到在相同外部折射率变化情况下,少模光纤基模与高阶模耦合对应的布拉格波长变化,比正、反向基模之间耦合对应的布拉格波长变化显著增大.实验上制作了少模光纤布拉格光栅,测量了基模之间以及基模与高阶模之间对应的布拉格波长随外部折射率、温度变化的情况,得到与理论分析相符的结果.而对于温度变化对折射率测量结果干扰的问题,提出了通过计算布拉格波长差来克服温度影响的方法.这些结果为采用布拉格光纤光栅测量外部折射率变化提供了一种新的途径.     

不同包层直径的倾斜光纤光栅折射率传感特性

倾斜光纤光栅的透射谱中有纤芯模和大量的包层模,它们具有与布拉格光栅相同的温度特性.利用HF酸腐蚀的方法得到具有不同包层直径的倾斜光纤光栅,研究了其对外界折射率的传感特性.结果表明,外界环境折射率在1.333～1.4532之间变化时,同一直径倾斜光纤光栅的高阶包层模的敏感性要比低阶包层模强;随着包层直径的减小,包层模的敏感性增强,且在折射率比较高的环境中有更高的敏感性.因此,利用倾斜光纤光栅的温度特性不仅可以解决温度交叉敏感问题,而且通过小同的腐蚀程度能定制所需要的灵敏度,以实现对环境折射率的高灵敏度测量.该办法可应用于对生物和化学等高灵敏度传感领域的各种溶液进行实时监控.     

一种同时测量液体折射率和温度的方法

由长周期光纤光栅的相位匹配条件与光纤纤芯基模LP01和光纤包层模LP0P的模式方程,利用长周期光纤光栅透射谱的谐振峰随温度与外围介质折射率变化而移动的特性,提出了同时测量水的折射率和温度的方法,并分析了其工作原理.对温度与谐振波长的关系进行了计算机仿真.用该方法进行了实验研究,实验结果与理论研究结果基本吻合.     

模式耦合理论在圆周对称长周期光纤光栅建模中的应用

长周期光纤光栅是不同于光纤Bragg光栅的一种光纤光栅器件 ,根据模式耦合理论 ,长周期光纤光栅表现为前向传播的纤芯导模和同向的各阶次包层模式之间的耦合。分析研究了长周期光纤光栅轴向的模场变化。忽略轴向的模式耦合以及包层模式之间的相互耦合作用 ,并认为折射率指数的调制只存在于纤芯中 ,建立了简化的长周期光纤光栅数学模型。对圆周对称轴向均匀型长周期光纤光栅谱特性进行了仿真 ,其结果与实验结果基本吻合 ,表明了简化的数学模型的合理性     

Fiber refractometer based on a fiber Bragg grating and single-mode-multimode-single-mode fiber structure

A refractive index (RI) sensor based on a novel fiber structure that consists of a single-mode-multimode-single-mode (SMS) fiber structure followed by a fiber Bragg grating was demonstrated. The multimode fiber in the SMS structure excites cladding modes within output single-mode fiber (SMF) and recouple the reflected cladding Bragg wavelength to the input SMF core. By measuring the relative Bragg wavelength shift between core and cladding Bragg wavelengths, the RI can be determined. Experimentally we have achieved a maximum sensitivity of 7.33 nm/RIU (RI unit) at RI range from 1.324 to 1.439.     

双带通啁啾光纤光栅光谱特性仿真及分析

针对啁啾光纤光栅的带通滤波特性,设计了纤芯/包层复合结构的啁啾光纤光栅滤波器.基于光纤耦合模和传输矩阵理论及坐标变换方法,对其光谱特性进行仿真分析,验证了该光栅的双带通滤波特性;对周期为536、537、538nm,长度为8、10、12mm,折射率调试深度为0.0002、0.0003、0.0004,啁啾系数为0.05、0...     

Tilted fiber Bragg grating sensors

Optical fiber gratings have developed into a mature technology with a wide range of applications in various areas, including physical sensing for temperature, strain, acoustic waves and pressure. All of these applications rely on the perturbation of the period or refractive index of a grating inscribed in the fiber core as a transducing mechanism between a quantity to be measured and the optical spectral response of the fiber grating. This paper presents a relatively recent variant of the fiber grating concept, whereby a small tilt of the grating fringes causes coupling of the optical power from the core mode into a multitude of cladding modes, each with its own wavevector and mode field shape. The main consequence of doing so is that the differential response of the modes can then be used to multiply the sensing modalities available for a single fiber grating and also to increase the sensor resolution by taking advantage of the large amount of data available. In particular, the temperature cross‐sensitivity and power source fluctuation noise inherent in all fiber grating designs can be completely eliminated by referencing all the spectral measurements to the wavelength and power level of the core mode back‐reflection. The mode resonances have a quality factor of 10⁵, and they can be observed in reflection or transmission. A thorough review of experimental and theoretical results will show that tilted fiber Bragg gratings can be used for high resolution refractometry, surface plasmon resonance applications, and multiparameter physical sensing (strain, vibration, curvature, and temperature).     

倾斜光纤光栅溶液折射率传感器的实验研究

根据倾斜光纤光栅(TFBG)和表面镀金的TFBG传感器测量折射率的基本原理,通过OptiGrating软件模拟了不同浓度溶液下TFBG的透射谱和芯层模与某阶包层模耦合引起的谐振峰,初步得出了TFBG各阶包层模随着外界折射率的增大而向右偏移、在一定的传感范围内中心波长与外界折射率呈线性关系的结论。用小型离子溅射仪对TFBG镀45 nm厚度左右的金膜,并用扫描电镜在微观上观察镀膜效果。通过不同浓度下的NaCl溶液、MgCl₂溶液、CaCl₂溶液实验,对比研究了裸TFBG和镀金TFBG传感器对溶液折射率的传感特性。从而验证了模拟仿真得出的结论并定量分析得知：镀金后具有表面等离子体共振的TFBG溶液折射率灵敏度大于500 nm·RIU-1,而裸TFBG为2 nm·RIU-1左右,大约提高了200～300倍,且在一定范围内中心波长与溶液折射率的线性拟合度都在0.99以上。     

Reflective fiber-optic refractometer based on a thin-core fiber tailored Bragg grating reflection

A novel reflective refractometer based on a thin-core fiber (TCF) sandwiched between a leading single-mode fiber (SMF) and a fiber Bragg grating (FBG) imprinted SMF stub was demonstrated. The reflection from the fiber stub occurs in two well-defined wavelength bands, corresponding to the Bragg core mode and cladding modes. The TCF section functions as a tailorable bridge between the FBG core mode reflection and the surrounding refractive index (SRI). Linear response with enhanced sensitivity of 133.26 dB/refractive index unit for temperature-immune SRI measurement within the biologically desirable sensing range of 1.33-1.41 has been achieved via cost-effective power detection.     

光纤纤芯及包层模有效折射率计算及仿真

光纤结构设计、模间色散求解、光纤光栅模式耦合等问题的研究, 都需要对光纤纤芯及包层模的有效折射率进行精确计算. 本文以光纤三层结构模型为基础, 结合该模型下的模式本征方程, 使用截弦法求解了纤芯模有效折射率, 并将计算结果与COMSOL软件模拟的对应纤芯模的传输光场进行对比, 验证了计算结果正确.使用区间遍历算法对包层模有效折射率进行了求解, 与已有的传统方法相比, 该方法可以有效防止求解过程中根的遗漏、避免特征方程产生的奇点, 并能保证模式的正规性.本文采用Mathematica软件对求解过程进行仿真, 获得了纤芯模和包层模有效折射率与波长关系曲线. 关键词： 光纤传输模式 有效折射率 截弦法 区间遍历算法     

包层模谐振特种光纤及传感特性研究

研究了一种具有双包层结构的包层模谐振特种光纤,该特种光纤具有一个低折射率内包层,基于耦合模原理,纤芯模与包层模之间发生谐振耦合,从而获得具有带阻特性的传输光谱.系统介绍了包层模谐振光纤的制备、传输原理及其弯曲、溶液折射率、折射率/温度双参量等传感特性.实验和理论研究结果表明,包层模谐振特种光纤对于弯曲、溶液折射率参量具...     

Performance study of a liquid-core Bragg fiber sensor in presence of a defect layer

Performance parameter of a Bragg fiber waveguide based resonant sensor in presence of a defect layer in cladding regions is theoretically studied. The Bragg fiber waveguide consists of a liquid-core surrounded by alternate high and low refractive indices materials in cladding regions. Reflectivity of the proposed waveguide based resonant sensor is formulated using transfer matrix method for a non-homogeneous multilayer cylindrical system. The waveguide shows a band gap region with a narrow defect mode in the band gap region under the considered wavelength range. Instead of taking a whole band gap as a sensing signal, here the defect peak is taken as the sensing signal. It is observed that the intensity of defect mode is more sensitive for core refractive index than the intensity of traditional band gap region (lobe). This study shows that the higher sensitivity can be achieved by creating the defect at a position in cladding region where the intensity of transmitted light lies between 40% and 90%. Presence of a defect layer is able to increase the detection accuracy of the sensor and, hence increase the overall performance of this sensor.     

侧面研磨光纤Bragg光栅的外部折射率敏感特性研究

光纤Bragg光栅对外界折射率的变化不灵敏,无法直接作为气体、液体等的敏感元件.侧面研磨的光纤Bragg光栅,一侧包层减小至几微米,其辐射场可通过包层透射到被传感物质,因而被传感物质的折射率的变化可以改变光纤的有效折射率,从而使光栅的Bragg波长产生位移.实验中利用不同浓度的NaCl溶液来改变光纤光栅的外部折射率,Bragg中心波长的偏移量和浓度近似为线性关系.同时,透射功率的变化量和浓度也近似为线性关系.     

Optimal design of temperature-insensitive long period fiber gratings for athermal refractive-index sensor

Based on coupled-mode theory of long-period fiber grating (LPFG), a theoretical analysis and simulations for the optimal design of a temperature-insensitive LPFG is presented in order to achieve an athermal condition for sensing the refractive index of the external medium. Effects of the variation of the cladding radius and grating period on the temperature sensitivity of the LPFG are discussed. Both of these parameters are found to be important to control the temperature sensitivity when the thermo-optic coefficients of core and cladding materials are of the same order. Other grating parameters are also optimized in order to achieve a good contrast of the grating period with resonance wavelength in the 1.5 μm region and to sense the external medium refractive index over a wide range. Variation of external medium refractive index from 1.0 to 1.45 results a red-shift in the LPFG resonance wavelength by 86 nm with its temperature sensitivity as low as 0.008 nm/°C over a temperature range of 0–80 °C for this optimally designed LPFG.     

Design of novel DS-OCDMA encoders/decoders using phase sampled long period grating

A novel long period grating structure with phase sampled technique is proposed to act as encoders/decoders in the direct sequence optical code division multiple access (DS-OCDMA) system. The structure with address code of 31 chips is designed and the relation between the sample period, the input pulse width and the refractive index difference is given. The encoded and decoded results are demonstrated to verify the feasibility of utilizing the coupling ratio to the cladding mode. The whole structure is expected to move on the photonic crystal fiber, for it can support the large refractive index difference between the core mode and cladding mode, as well as minimize the wavelength shift caused by the temperature change.     

侧边抛磨区材料折射率对光纤光栅波长的影响

针对轮式光纤侧边抛磨法,研究了在侧边抛磨光纤光栅抛磨区覆盖不同折射率的材料时,侧边抛磨光纤光栅Bragg波长随外界折射率的改变而变化的特性.理论计算与实验结果都表明,侧边抛磨光纤光栅Bragg波长会随抛磨区覆盖材料折射率的增大向长波长方向偏移；侧边抛磨面离光栅区纤芯表面越近,覆盖材料折射率对波长偏移的影响越大.实验指出,当侧边抛磨区覆盖材料的折射率从1.389 7变到1.447 9时,Bragg波长将会发生1.402 nm的偏移.用轮式光纤侧边抛磨法制备的侧边抛磨光纤光栅可应用于光纤光栅的波长调谐或传感器.