Multiple fiber Bragg grating sensor system using code-division multiple access

We propose and demonstrate experimentally a multiple fiber Bragg grating sensor system that uses a LED as a light source and adapts the code-division multiple access method to separate individual sensor signals. We measured static and dynamic strains and cross-talk levels among sensor signals. With our proposed scheme, a low-cost sensor system can be designed with a fast response speed. Cross-talk levels between sensor signals are found to be below the -30-dB level.     

Identical-dual-bandpass sampled fiber Bragg grating and its application to ultranarrow filters

We have theoretically proposed and experimentally demonstrated a new kind of ultranarrow identical-dual-bandpass sampled fiber Bragg gratings (SFBGs) with a pi phase shift technique. The spacing of two bandpasses of the proposed grating can be flexibly adjusted by changing the sampled period, and any desired spacing can be achieved in principle. An experimental example shows that the transmission peaks of two narrow transmission-band are near 1549.1 and 1550.1 nm. Based on the proposed SFBG, an ultranarrow identical-dual-channel filter is designed. Two channels of the proposed filter have an equal bandwidth, an even strength, and the same group delay. The bandwidth of each channel of our filter is as small as 1 pm and up to 10(-3) pm (corresponding to approximately 0.1 MHz), which is less than the bandwidth of the conventional SFBG filters by a factor of 10(2)-10(4). The proposed grating and filter can find potential applications with slow light and dual-wavelength single-longitudinal-mode fiber lasers.     

取样光纤布拉格光栅特性的研究

用传输矩阵法从理论上计算了取样光纤布拉格光栅的反射谱特性。这种方法将光栅视为多层均匀薄膜的叠加,利用每一层的传输矩阵相乘获得了光栅的反射谱特性。研究表明,随着光栅长度的增加和采样率、折射率调制深度的减少,反射峰的均匀性得到了改善,旁瓣的反射率变小,带宽明显变窄,而反射峰间隔保持不变。反射峰的间隔由光栅周期决定,与采样率无关,而某些文献则要求采样率小于10%。这与频谱分析所得结论相吻合。     

TFBG的Fabry-Perot干涉型纵向微位移传感研究

提出了一种利用光纤Fabry-Perot(F-P)干涉结构的微位移传感器,该F-P干涉结构是由双倾斜光纤光栅(tilted fiber Bragg grating,TFBG)沿轴向错开一定距离构成空气腔形成的.入射光经双TFBG及其高反射端面往返传播,从而构成F-P干涉结构.光纤沿纵(z轴)向移动时,F-P腔的腔长发生变化,致使干涉光谱的自由光谱范围随之产生变化.实验验证,该传感器在0~115μm的测量范围内获得了高达0.475nm/μm的灵敏度,较之前提出的光纤光栅型传感器灵敏度提高了近3倍.     

Design and fabrication of thin film optical filters to replace fiber Bragg grating in Yb-doped fiber laser

Thin film optical filters were designed and fabricated on the end-side of the fiber as the end pumping and out coupler devices to replace the fiber Bragg grating in Yb-doped fiber laser system. It was found that the average transmittance of the end pumping device in the laser pumping wavelength (900-985 nm) is around 95.2%, and the average reflectance in the laser irradiation wavelength (1065-1085 nm) is 99.72%. The average reflectance of the out coupler device is 99.7% (900-1035 nm), and the average transmittance is 20% (1065-1085 nm), respectively.     

Design of binary long-period fiber grating filters by the inverse-scattering method with genetic algorithm optimization

An approach is presented to the design of binary long-period fiber grating (LPFG) filters based on the Gel'fand-Levitan-Marchenko (GLM) inverse-scattering method and genetic algorithm optimization. The nonuniform coupling strength of the binary grating can be realized by varying the local duty ratio. A coupled-mode theory combined with the Poisson sum formula for treating the binary index perturbation is developed for the application of the GLM synthesis method. Since the coupled-mode theory, which smears out the discrete coupling nature, can be regarded only as an approximation to the modeling of a binary LPFG, we use instead the transfer-matrix model to analyze the coupling behavior of a nonuniform binary LPFG. Based on the synthesized grating patterns from the GLM method, a real-coded genetic algorithm with the transfer-matrix model is used to compensate for the discrepancies resulting from use of the coupled-mode theory and to optimize the design. We exemplify the above procedure by designing a flatband LPFG filter and a high-visibility all-fiber Mach-Zehnder filter.     

Fiber-optic Fabry-Perot sensors based on a combination of spatial-frequency division multiplexing and wavelength division multiplexing formed by chirped fiber Bragg grating pairs

Effective multiplexing for a very large number of fiber-optic fiber-Bragg-grating-based Fabry-Perot (FBGFP) sensors is proposed that is based on wavelength division multiplexing (WDM) and spatial-frequency division multiplexing (SFDM). For WDM, FBGFP sensors are arranged in different wavelength domains formed by a series of chirped fiber Bragg gratings with different central wavelengths while the sensors with different cavity lengths within the same wavelength domain are multiplexed by use of SFDM because they have different spatial frequencies as a result of their different cavity lengths. In principle, a thousand FBGFP sensors could be multiplexed with such an approach. The experimental results show that a strain accuracy of better than +/-10 microepsilon has been achieved with little cross talk.     

Interference exploitation using adaptive code allocation for the downlink of precoded multiple carrier code division multiple access systems

A novel transmitter code optimisation technique based on adaptive code-to-user allocation is presented for interference exploitation on the time division duplex downlink of binary phase shift keying-based multiple carrier code division multiple access systems. The principle of the proposed technique is to exploit the dependency of multiple access interference on the instantaneous symbol values of the active users. The objective is to adaptively allocate the available spreading sequences to users on a symbol-by-symbol basis to enhance the ratio between constructive and destructive interference and thus optimise the decision variables at the downlink receivers. The resulting signal-to-interference plus noise improvement happens by making use of the energy inherent in the system so the performance benefit is attained with no additional power-per-user investment. It will be shown that when this optimisation technique is incorporated with common pre- and post-equalisation schemes, a significant bit error rate performance improvement is achieved while the associated adaptation overhead is kept less than 6% of the available bandwidth.     

利用WDM光纤耦合器的光纤光栅传感解调技术

根据 WDM 光纤耦合器波长解调方案的工作原理、偏振特性以及影响系统波长分辨力的因素,提出一种改进的利用 WDM 光纤耦合器的光纤光栅传感解调技术。该技术在原技术的基础上,采用偏振控制器控制入射光偏振状态,提高了解调的精度和稳定性。对 WDM 光纤耦合器的多次波长扫描结果表明,采用偏振控制器后,其波长误差可减小到 5pm 左右。实验采用 1540/1560nm的 WDM 光纤耦合器对单点光纤光栅应变传感器进行静态解调,结果表明:按此技术开发的解调系统具有 0.01nm 波长分辨力和 10nm 的波长线性解调范围。     

Performance of blind channel estimation algorithms for space-frequency block coded multi-carrier code division multiple access systems

The problem of blind channel estimation for downlink space-frequency block coded multi-carrier code division multiple access (SFBC MC-CDMA) schemes is considered. For these schemes, the authors first develop a system model for complex modulated signals, which reduces the multichannel estimation problem to a single-input single-output problem. Then, they present an intuitive subspace-based channel estimation method along with the corresponding necessary and sufficient conditions under which the channel estimate is unique (within a complex scalar). Their studies highlight two interesting properties of SFBC MC-CDMA systems: (i) there is no antenna order ambiguity (also known as permutation ambiguity) even though only one spreading code is assigned to each user; (ii) channel identifiability is guaranteed, regardless of the channel zeros location. They also establish the unbiasedness of the channel estimates and derive closed-form expressions for the mean-square-error of the estimates as well as the corresponding Cramer-Rao bound (CRB). In the derivation of the CRB, they suggest a novel approach which assumes the knowledge of only the spreading code of desired user. This approach results in a tighter bound than the CRB derived based on the knowledge of all users' signatures.     

Simple high-resolution wavelength monitor based on a fiber Bragg grating

A compact and low-cost device for monitoring the peak wavelength of single-peak spectral distributions is presented. The system is based on the transmission properties of a fiber Bragg grating when its period is modulated. Different types of optical signal, such as the emission of distributed-feedback lasers and the reflection of a broadband optical source produced by fiber gratings used in sensor systems, can be measured with this device. We demonstrate that a high wavelength resolution of micro 1 pm can be achieved and that our proposal can be used for real-time monitoring.     

基于双折射效应的光纤光栅称重方法研究

与通常的光纤光栅传感器原理不同,本文基于光纤在外界径向载荷作用下产生的双折射效应为原理,建立了一套简单的称重实验装置,并对其检测原理进行了详细的分析。实验表明,这种光纤光栅称重传感器适合大载荷情况的重量检测问题,特别适合于公路计重收费系统中对车辆载重的监测领域,其称重灵敏度可达5kg。     

Precoding in downlink multi-carrier code division multiple access systems using expectation maximisation algorithm

The presence of multiple access interference and hardware complexity of the mobile terminal are two major burdens for multi-carrier code division multiple access schemes. Both burdens can be overcome in downlink by precoding the transmitted signal using the knowledge of the channel state information. In multiuser precoding techniques, the transmitter is optimised to combat channel impairments through the use of new spreading sequences that are obtained by solving an optimisation problem based on some criterion. Among these optimisation problems, the problem based on maximum likelihood is hard and complex to solve. In the proposed precoding scheme, the well-suited expectation maximisation algorithm is used to solve this problem. The proposed precoding technique is simulated and its performance is analysed and compared with some other precoding and detecting techniques. The results show that the proposed scheme considerably outperforms the previous precoding schemes.     

光纤光栅肘关节角度测量方法

为解决现有关节角度测量装置的不足,提出一种基于多光纤布拉格光栅(fiber Bragg grating, FBG)的关节角度测量方法。该方法将多光纤光栅固定在套袖上,套袖戴在肘关节上,辅以自主设计的基于LabVIEW关节角度计算及数据处理的程序和可视化界面,实现对肘关节角度实时测量并输出可视化数据。该套袖具有柔性化程度高、不受电磁信号干扰、测量效率高、结构简单的优点。对所制作套袖进行测量实验,实验结果表明,该测量方法的误差约为2.88%,重复性误差在4%以下,具有实际的应用价值。     

基于FBG轴向拉力的液位测量传感器

根据光纤布喇格光栅(fiber Bragg grating,FBG)的传感原理,提出了一种基于波纹管结构的FBG液位传感器,利用补偿FBG(FBG1)与液位FBG(FBG2)具有相同的温度灵敏度进行温度补偿,将FBG2固定在波纹管内,液压产生的张力可以使波纹管的自由端产生位移,从而使固定在里面的FBG2产生轴向应变进而影响布喇格中心波长的改变.通过布喇格中心波长的改变来监测液位的变化,在40~340mm的液位变化范围内进行了实验研究.结果表明,光纤布喇格光栅中心反射波长漂移对液位呈现良好的线性关系,线性拟合度大于0.998,灵敏度为15.7pm/cm,理论测量范围可达到3m.说明通过改变波纹管的波纹数目和其他参数,可实现对传感器测量范围和灵敏度的调整,以满足各种应用场合.     

Design and characteristics of refractive index sensor based on thinned and microstructure fiber Bragg grating

A refractive index sensor based on the thinned and microstructure fiber Bragg grating (ThMs-FBG) was proposed and realized as a chemical sensing. The numerical simulation for the reflectance spectrum of the ThMs-FBG was calculated and the phase shift down-peak could be observed from the reflectance spectrum. Many factors influencing the reflectance spectrum were considered in detail for simulation, including the etched depth, length, and position. The sandwich-solution etching method was utilized to realize the microstructure of the ThMs-FBG, and the photographs of the microstructure were obtained. Experimental results demonstrated that the reflectance spectrum, phase shift down-peak wavelength, and reflected optical intensity of the ThMs-FBG all depended on the surrounding refractive index. However, only the down-peak wavelength of the ThMs-FBG changed with the surrounding temperature. Under the condition that the length and cladding diameter of the ThMs-FBG microstructure were 800 and 14 mum, respectively, and the position of the microstructure of the ThMs-FBG is in the middle of grating region, the refractive index sensitivity of the ThMs-FBG was 0.79 nm/refractive index unit with the wide range of 1.33-1.457 and a high resolution of 1.2 x 10(-3). The temperature sensitivity was 0.0103 nm/ degrees C, which was approximately equal to that of common FBG.     

Design of short fiber Bragg gratings by use of optimization

We demonstrate an optimization approach for designing fiber Bragg gratings. A layer-peeling inverse-scattering algorithm is used to produce an initial solution, which is optimized numerically with an iterative optimization method. To avoid problems with local minima, we use merit functions that are zero for wavelengths for which the predefined demands (acceptance limits) are fulfilled, making it possible to alter the local minima under the optimization process without disturbing the global minimum. Because short gratings are difficult to design with inverse scattering, and because the time consume of the optimization increases rapidly with the grating length, the method is particularly useful for designing short gratings. The method is also useful when the demands are complex and difficult to handle with inverse-scattering methods. Design examples are given, including a dispersionless bandpass filter suitable for dense wavelength-division multiplexing and a filter with linear reflectivity.     

单光纤光栅实现窄带全光纤反射器的分析

提出了一种由单个光纤光栅和一个光纤方向耦合器组成的新型全光纤反射器,推导出了当光栅为均匀 Bragg 光栅、器件任意端口输入时,任何一端口的输出解析式。分析表明器件具有法布里-珀罗腔干涉仪的特点,耦合器的耦合比系数类似于法布里-珀罗腔的反射率, 耦合比系数越大,输出光谱半高全宽度(FWHM)越窄, 消光比越好。当耦合比系数大于 0.8 时,FWHM 可以窄到0.02nm,消光比大于 0.9。如果光栅是“强”耦合,器件具有均匀分布的多通道梳状输出特性;光栅为“弱”耦合时,则能实现 FWHM 小于 0.02nm 的单频输出。器件只需单个光栅,克服了制作两个完全相同光栅的困难。     

Mechanical characterization of bone cement using fiber Bragg grating sensors

The aim of this work was the study and understanding of the behavior and linearity of an optical fiber Bragg grating (FBG) sensor embedded in bone cement. Test its ability to monitor strains inside bone cement during different mechanical tests, at real-time. Bone cement is a biomaterials based on polymethacrylate used as fixation method in artificial joints. Work as a bonding, load transfer and optimal stress/strain distribution inside the complex human body environment. Bone cement is the weakest element in a joint implant, being considered the main reason of prosthesis loosening.Inside the bone cement, its temperature, longitudinal strain and load were measured using fiber Bragg gratings. All the measurements report a linear response showing a good adaptation and optimization of the load transfer between the biomaterial and the embedded optical sensor.     

Multiple-wavelength reference based on interleaved, sampled fiber Bragg gratings and molecular absorption

We present a wavelength calibration reference based on interleaved, sampled fiber Bragg gratings stabilized to a molecular absorption line. Such a hybrid reference can provide multiple stable calibration peaks over a wide range of wavelengths. We demonstrate a wavelength reference that has at least 20 peaks suitable for use as calibration references in each of three wavelength regions: 850, 1300, and 1550 nm. We monitored the stability of a 1300-nm reflection peak and found that the standard deviation of the peak wavelength was 0.7 pm over a 70-day period.