基于长周期光纤光栅的折射率传感器

长周期光纤光栅具有制作成本低、无背向反射、结构紧凑等优点,且对外界折射率变化有较高的敏感性,特别适用于折射率传感．简要概括了长周期光纤光栅折射率传感的基本原理,介绍了基于长周期光纤光栅的折射率传感器的研究进展,目前的研究重点集中在克服温度交叉干扰、提高传感精度以及拓宽传感范围的方面,比较了各种方案的优缺点．     

长周期光纤光栅

长周期光纤光栅作为一种重要的光无源器件,与所有光纤传感器一样具有抗电磁干扰、耐腐蚀、灵敏度高、体积小、与光纤系统兼容等优点,在光纤传感与光纤通信领域都有广泛的应用。本文对长周期光纤光栅的模式耦合原理、理论分析方法、光栅制备技术及其在光纤传感与光纤通信领域的主要应用进行了总结分析,其中制备技术主要总结了激光制备技术（包括紫外激光、二氧化碳激光、飞秒激光制备技术）与非激光制备技术（包括电弧放电、机械微弯、包层腐蚀、熔融拉锥、离子注入、超声调制技术）,分别介绍了长周期光纤光栅在温度、应变、弯曲、扭转和环境折射率、生化传感等方面的应用,对于光纤通信领域的应用则侧重于全光纤滤波器、模式转换器、起偏器、模式耦合器等方面。本文是按照教程论文的形式撰写的,旨在为相关领域的学生、科研工作者提供一个系统的介绍。     

基于模式转换的镀膜长周期光纤光栅传感器的结构优化设计

基于耦合模理论,根据镀膜长周期光纤光栅(LPFG)包层模有效折射率随膜层参数的变化,指出了模式转换区的划分及特点,考察了模式转换区及其附近LPFG透射谱折射率的响应特性。从传感器设计角度,进一步针对波长偏移和透射率两种检测类型,给出了两类灵敏度的定义,讨论了模式转换区及其附近灵敏度随薄膜参数与光栅参数变化的关系,通过优化设计,给出了高灵敏度对应的最佳设计参数带及区域。结果表明,选择合适的膜层参数与光栅参数,利用波长偏移和透射率两种检测方法,该传感器对薄膜折射率的分辨率均可达10-7以上,且模式转换区适合于制作波长偏移型的传感器,模式转换区附近则适合用于制作透射率型传感器。     

基于长周期光纤光栅的光纤液位传感器

为了测量液位在警戒值附近变化的情况, 采用新款光纤熔接机制作了一种基于锥形结构的长周期光纤光栅测量液位的光纤传感器, 对传感器进行了理论分析, 搭建了液位传感实验系统, 根据传感器对外界环境的折射率灵敏度, 测量浸没在液体中的光纤长度。结果表明, 在0 mm~12 mm的液位测量范围内, 光纤液位传感器的峰值波长灵敏度和透射功率灵敏度分别是0.700 nm/mm和1.377 dB/nm。该传感器对液位变化测量较为准确, 且采用刻栅方式可有效解决传统长周期光纤光栅中存在的非对称模耦合和偏振依赖性高等问题, 同时具有制作简单、成本低和应用前景广泛等优点。     

长周期光栅——新兴的光纤光栅传感器

近两、三年来，一类新兴的光纤光栅传感器－长周期光栅型传感器从众多的光栅传感器中脱颖而出。文章重点介绍了ＬＰＧｓ的传感原理、应用及目前发展状况。     

光子晶体光纤长周期光栅的特性

将修正的矢量等效折射率模型和耦合模理论相结合,系统地研究了光子晶体光纤长周期光栅(PCFLPG)的特性.验证了光子晶体光纤长周期光栅的双谐振峰特性;定量地研究了光子晶体光纤结构参数如空气孔间距、相对孔径比和光栅参数如光栅周期、光栅长度、折射率调制深度和光栅啁啾量对光栅谐振峰强度和谐振波长的影响.研究结果为光子晶体光纤长周期光栅在光纤通信、光纤传感等方面的应用提供了理论依据.     

小区域腐蚀的长周期光纤光栅迈克耳孙干涉仪传感特性研究

基于尾纤端面镀金属反膜的单根长周期光纤光栅,提出了一种包层部分腐蚀的长周期光纤光栅迈克耳孙干涉仪结构,提高了长周期光纤光栅(LPFG)的灵敏度。应用干涉原理,得出了长周期光纤光栅迈克耳孙干涉仪干涉波峰随外界环境折射率的变化关系。并从实验上对腐蚀前的长周期光纤光栅迈克耳孙干涉仪和带腐蚀结构的长周期光纤光栅迈克耳孙干涉仪的折射率传感灵敏度进行对比。结果表明,包层部分腐蚀的长周期光纤光栅迈克耳孙干涉仪反射谱中的干涉峰值对折射率传感的灵敏度得到了显著的提高,对于折射率为1.333和1.352的液体,干涉峰值的移动量从0.9 nm增加到3.4 nm。     

机械感生可调谐长周期光纤光栅的特性研究

在耦合模理论分析的基础上,利用机械线加工技术设计周期性压力槽,基于机械感生原理写制了可调谐长周期光纤光栅(LPFG)。设计周期性压力槽的周期为600μm,周期数60,长周期光纤光栅最大谐振峰值损耗可达18 dB,通过改变压力槽与光纤之间的夹角实现了谐振波长可调范围不小于12 nm,并实验研究了LPFG的压力和温度特性。制作的长周期光纤光栅具有谐振波长和峰值损耗均可调谐、成本低以及可擦除等优点。     

表面膜层参数对长周期光纤光栅特性的影响

采用严格的耦合模理论建立了表面涂覆弱吸收薄膜长周期光纤光栅（LPFG）的理论模型，用微扰法对复数超越特征方程进行了求解。研究发现，对应某一膜厚，存在一折射率范围，有效折射率的变化和谐振波长偏移特别明显；但消光系数对有效折射率和谐振波长的影响则很小。从功率密度分布曲线可看出低阶奇次包层模式与纤芯模式的耦合要强于低阶偶次包层模式与纤芯模式的耦合，消光系数引起的功率损耗较小。薄膜折射率对透射谱的变化（幅度和谐振峰偏移）影响很大；消光系数对谐振峰偏移影响很小，对谐振峰幅度的影响与具体的薄膜折射率有关。结果表明，敏感薄膜折射率和消光系数与传感器的灵敏度高低有直接关系，传感器的结构优化非常必要。     

长周期光纤光栅对外界折射率的敏感性

研究了长周期光纤光栅对外界折射率的敏感特性,包括耦合波长和耦合系数随外界折射率变化的情况。运用耦合模理论,通过理论分析计算和实验测试,给出了在外界折射率小于、接近和大于光栅包层折射率等不同情况下,耦合波长和耦合系数随外界折射率变化的规律。     

基于级联长周期光纤光栅的新颖温度传感器

设计了一种基于级联长周期光纤光栅(Long period fiber grating)的新颖温度传感器.理论上得到光栅间光纤的长度随温度增加变化时,级联LPG的干涉峰将线性地向长波方向漂移,其消光比亦将呈现线性变化.实验上制作出了基于级联LPG的光纤温度传感器.利用其某一干涉峰的中心波长和消光比测量温度变化时,测量灵敏度分别为0.0353 nm/℃和0.0684 dB/℃.基于级联长周期光纤光栅的温度传感器具有广泛的应用前景.     

Step-changed long-period fiber gratings

The spectral characteristics of a new type of long-period grating (LPG) structure, step-changed LPG, are investigated theoretically and experimentally. It is shown that the spectral profile of such gratings can be tailored by adjusting the step parameters. A good agreement between simulated results and measured spectra is obtained. This type of LPG has potential applications in gain flattening of erbium-doped fiber amplifiers     

High-sensitivity temperature sensor based on long-period fiber grating

Temperature sensitivity is greatly improved by taking the following three measures： proper long-period fiber grating （LPFG） whose strain coefficient of the core is larger than that of the cladding is employed, the LPFG is coated with a thin film of the material whose refractive index decreases with the temperature, and the sensor is encapsulated by metal material whose thermal expansion coefficient is large. By computer simulation, a measured temperature coefficient of 0.2375 nm/℃ and a temperature resolution less than 0.1 ℃ are obtained.     

Analysis of phase-shifted long-period fiber gratings

Phase-shifted long-period fiber gratings (LPFG's) are analyzed by the coupled-mode theory together with the fundamental-matrix method. The effects of introducing multiple phase shifts at various locations along a LPFG are highlighted. The results compare well with published experimental data     

Sensitivity characteristics of long-period fiber gratings

We present a detailed investigation into the sensitivity of long-period fiber gratings (LPFGs) as a function of temperature, strain, and surrounding refractive index, with particular attention to the higher order cladding modes and the possibilities for ultrasensitive sensors. From a general theoretical analysis, we identify a general sensitivity factor which offers new physical insight into LPFG behavior and represents a useful design aid in conjunction with a set of measurand-specific sensitivity factors. Our analysis reveals the existence of turning points in the mode dispersion characteristics at which ultrasensitive operation may be obtained. In an extensive set of coordinated experiments, we verify the theoretical predictions with close agreement and provide demonstrations of the device behavior close to the turning points. Alternative sensor schemes for temperature, strain, and refractive index based, respectively, on measurement of the dual resonance characteristic of the modes and on the transmission characteristics close to the turning points, utilizing higher order modes of the LPFG, are presented. For two variables at least, we record the highest LPFG sensitivities yet reported     

Multiwavelength Raman fiber-ring laser based on tunable cascaded long-period fiber gratings

Multiwavelength Raman fiber lasers using intracavity tunable cascaded long-period fiber gratings (LPFGs) are discussed. The application of the cascaded LPFGs as a multichannel fiber filter for the multiple-wavelength generation is proposed and experimentally demonstrated. The characteristics of multiwavelength fiber-ring lasers can be controlled by changing the physical parameters of cascaded LPFGs such as the separation distance between the gratings, grating length, and number of gratings. The multiwavelength Raman fiber-ring laser with nine wavelength-division-multiplexing channels with 100-GHz spacing and 19 channels with 50-GHz spacing has been achieved by varying the physical parameters of cascaded LPFGs.     

基于光子晶体光纤长周期光栅的传感器研究

详细研究了光子晶体光纤长周期光栅(LPG-PCF)的应力和温度特性,利用成功制作的温度不敏感PCF-LPG,实现了两种结构简单光纤传感器.在温度不敏感应力传感器中,PCF-LPG作为温度不敏感的应力传感头,当以波长为1553nm的分布反馈(DFB)激光器为入射光源、用分辨率为0.001dB的光功率计测量时,PCF-LP...     

Gain-flattening filter using long-period fiber gratings

We propose a novel structure for a gain-flattening filter, in which a conventional long-period fiber grating (LPFG) and a phase-shifted LPFG are written closely. The transmission characteristics are investigated in the case of a closely arranged configuration. For the fabrication of a phase-shifted LPFG, a UV-trimming method to control the amount of the phase shift is examined theoretically and experimentally. Finally, a 67-mm-long LPFG gain-flattening filter designed for an erbium-doped fiber amplifier is fabricated and the measured spectrum is shown     

Optical fiber gas sensor based on self-assembled gratings

The electrostatic self-assembled monolayer synthesis process is used to fabricate gas sensors by building up grating sensor elements on the ends of multimode optical fibers. These novel sensors can be designed to operate in the transmission windows of standard optical fibers and implemented using specific reference wavelengths to normalize the output signals. Experimental results for such sensors designed to detect dichloromethane gas are presented     

In-fiber spectrometer using tilted fiber gratings

We demonstrate a compact in-fiber low-resolution spectrometer. Wavelength resolution of 7 nm was achieved using the core radiation-mode coupling of a tilted grating in an optical fiber with a photosensitive cladding. We applied the device to measure the powers of a multiwavelength Raman pump source.