支持向量回归算法在光纤光栅非均匀应变重构中的应用

当布拉格光栅轴向存在大的应变梯度时,其反射光谱的形状会被扭曲.甚至出现多峰,发展非均匀应变分布重构方法对于结构健康监测技术具有重要意义.但采集反射光谱时的测试噪声会显著影响应变分布重构的精度.为此,提出了采用支持向量机对含噪的反射光谱进行回归预处理,并运用适应度排序改进的遗传优化算法结合传输矩阵反射光谱构建方法识别布拉格光栅轴向非均匀应变分布的方法.该方法将反射光谱视为时间序列.利用支持向量回归的全局优化和泛化能力进行噪声抑制.从而回归出有效的反射光谱;通过传输矩阵方法将光栅轴向应变分段均匀化,利用改进的遗传算法进行并行重构.对多种应变分布形式下的应变重构进行了仿真研究,结果表明,支持向量机方法可以有效地进行反射光谱同归,提高非均匀应变分布重构的精度.     

Simple grating synthesis algorithm

Many authors (including this one) have suggested that the problem of synthesizing a grating from its reflection spectrum is significantly more difficult than the direct problem of obtaining the spectrum from the grating. Fortunately, this view is mistaken. One can synthesize a grating from any reflection spectrum by direct solution of the standard coupled-mode equations while simultaneously evaluating a simple integral to obtain the grating strength. This approach has the advantage of being based on a well-understood set of equations, making it amenable to physical interpretation, mathematical approximation, and generalization.     

三角形大带宽反射光谱光纤光栅的设计和制备

反射谱具有三角形形状的光纤光栅在光纤传感等领域中具有重要的应用前景。利用遗传算法设计出了产生三角形光谱分布的光纤光栅的耦合系数沿光纤的分布。计算表明,三角形光谱光纤光栅是可以实现的,其反射带宽可以通过光栅的啁啾量进行调节,常周期的三角光栅反射带宽小,变周期的三角光栅反射带宽大,其带宽同其啁啾量大致相等。实验中,采用光束扫描法制备了反射底宽为0．77nm三角形光谱的光纤光栅。     

Proposal of metal bellows-based fiber Bragg grating accelerometer

A metal bellows-based fiber Bragg grating(FBG) accelerometer is proposed and experimentally demonstrated. The optical fiber(containing the FBG) is pre-tensioned,and the two ends of the optical fiber are fixed directly from the shell to the inertial mass. In this design,the FBG is uniformly tensioned to obtain a constant strain distribution over it. By employing this configuration,the FBG always has a sharp reflection characteristic with no broadening in its reflection spectrum during wavelength shifting. Dy...     

基于叠栅的Bragg光纤光栅综合方法的研究

提出一种新的光纤光栅综合方法，将Bragg光纤光栅的层剥离综合算法与光纤叠栅理论结合起来，层剥离综合算法根据需要的反射谱计算出光纤光栅的折射率分布，用多个带切趾函数的光纤叠栅来拟合这个折射率分布，通过改变各叠栅中的相关参数，可以得到适合需要的反射谱，相对误差一般小于4.0%。该方法具有简单、快速、易于实现的特点。     

Arbitrary chirped fiber Bragg gratings on application of filtering for optical communications

This paper presents an analysis of how to obtain narrow transmission band in the centre of the spectrum of the arbitrary chirped fiber Bragg grating (ACFBG). Grating chirp coefficient and refractive index profile are the critical parameters in contributing to performance of fiber Bragg grating. The reflection spectra and transmission band were analyzed with different refractive index profile. Apodization techniques are used to get optimized reflection spectra. The simulations are based on solving coupled mode equations by transfer matrix method that describes the interaction of guided modes.     

柚子型光子晶体光纤啁啾光栅的反射谱特性

利用有限元法和传输矩阵法,对柚子型结构的光子晶体光纤啁啾光栅的反射谱进行了理论分析。得到了啁啾光栅参数和光纤参数对光子晶体光纤啁啾光栅反射谱的影响规律。结果表明随着啁啾系数和折变量的增加,光栅反射谱呈规律变化,当啁啾系数增大到一定程度时几个反射峰会连到一起,形成一个大的反射带宽。光纤的内包层直径和纤芯直径的改变能够影响各反射峰之间的间距。同时研究了切趾变迹对光子晶体光纤啁啾光栅反射谱的影响,研究表明,所用切趾函数会同时对多个峰进行切趾,每个峰的切趾效果都很好。对反射谱波动较大的几个峰相连的啁啾光栅经过柯西切趾后反射谱曲线平滑,适合于实际应用。     

Experimental observation of the slowdown of optical beams by a volume-index grating in a photorefractive LiNbO3 crystal

We investigate the group velocity of light in a one-dimensional volume-index grating inside a photorefractive LiNbO>(3) crystal. The slowdown of the group electromagnetic propagation is observed experimentally by tuning of the wave number of the optical beam close to the outside edge of the forbidden bandgap. We obtain a large group index of up to 7.5 in a 3.5-cm crystal sample. The group index is compared with the result of a theoretical derivation. The results are presented and discussed.     

相位修正的耦合模理论用于计算光纤Bragg光栅法布里-珀罗腔透射谱

基于光纤Bragg光栅(FBG)反射复振幅相移对FBG法布里-珀罗腔透射谱的影响,分析了传统耦合模理论计算均匀FBG反射复振幅相移产生误差的原因.引入折射率分布初始相位参数描述FBG折射率分布纵向的微小偏移,用真实的反透射系数代替简明形式的反透射系数,对传统耦合模理论进行了修正,增加了与折射率分布初始相位参数有关而与波长无关的相位因子.在此基础上进一步对计算非均匀FBG的传输矩阵法的相位进行了修正.修正后的快速计算结果用于FBG法布里-珀罗腔透射谱的计算,可反映折射率分布初始相位参数对透射峰波长位置的影响,与Rouard 算法及实验值均有较好的一致性. 关键词： 光纤Bragg 光栅 法布里-珀罗腔 耦合模理论     

Rewritable self-assembled long-period gratings in photonic bandgap fibers using microparticles

We report the demonstration of rewritable self-assembled long-period gratings in air-core photonic bandgap fibers. The long-period gratings are written by filling the air-core region of the fiber with a solution containing polystyrene microspheres. The microspheres are self-assembled into a periodic structure as the liquid inside the fiber evaporates, forming the long-period grating. The formed grating can then be easily erased for rewriting purposes by connecting the end of the fiber to a microfluidic pump through a microfluidic channel and washing out the microparticles with an appropriate liquid. The same microfluidic system can also be used to fine control the writing process of the gratings. This technique creates possibilities for writing various different filtering functions in air-core photonic bandgap fibers by varying the composition and/or the shape of the used microparticles. The presented technique is potentially applicable to solid-core photonic crystal fibers as well.     

带宽调制型单光纤光栅温变无补偿位移传感

报道了利用反射谱带宽调制和光强差分探测技术实现单一光纤光栅温变无补偿位移精确测量的新方法。设计了一种结构新颖的曲臂梁位移传感装置,结合光波导理论与材料力学原理分析了光纤光栅在高斯应变作用下光栅反射谱侧向梯度展宽的成因,理论推导了特殊结构梁在外力作用下光栅反射谱带宽/反射光强与压力之间的响应关系。光栅反射谱侧向梯度展宽的同时反射光强线性增加,利用光强差分检测方法消除光源出光抖动的影响,提高了位移测量精度。基于带宽调制的光纤光栅位移传感方法免受温度变化的影响,在-10℃~80℃的温度变化范围内,测量误差小于1.2%,实现了单光纤光栅温变无补偿位移测量。     

Linearly polarized fiber laser using a point-by-point Bragg grating in a single-polarization photonic bandgap fiber

We present a narrow-linewidth, linearly polarized neodymium-doped fiber laser that incorporates a point-by-point Bragg grating inscribed into the core of a single-polarization all-solid photonic bandgap fiber. The Bragg grating was written within a single-polarization wavelength band of the fiber; thus, the Bragg reflection was polarized. This all-fiber laser produced 7.2 W, linearly polarized output with 25 pm FWHM and 19.6 dB polarization extinction ratio.     

Temperature-insensitive strain measurement using differential double Bragg grating technique

A novel differential double Bragg grating sensor for temperature-insensitive strain measurement is presented. The sensor consists of two identical weak measuring and reference gratings separated by some distance. The reference grating is placed inside the silica capillary that made it almost strain insensitive. The basic idea in differential double Bragg grating sensor is to measure the energy of the oscillating term in the reflection spectrum of double Bragg grating structure. The normalized energy of the interference term depends on the pitch difference of the two gratings and does not change when pitch variations of both gratings are equal with temperature variation. Therefore, the normalized energy of the interference term can be used for the temperature-insensitive strain measurement.     

Slow Light Excitation in Tapered 1D Photonic Crystals: Theory

Slow light (SL) states corresponding to wavelength regions near the bandgap edge of grated structures are known to show strong field enhancement. Such states may be excited efficiently by well-optimized adiabatic transitions in grated structures, e.g., by slowly turning on the modulation depth. To study adiabatic excitations, a detailed research in 1D is performed to obtain insight into the relation between the device parameters and properties like enhancement and modal reflection. The results enable the design of an adiabatic device for efficient excitation of SL states in 1D. The effect of small wavelength variations as well as small fluctuations in the modulation depth of the grating has been investigated.     

Self-talk of Reflection Hologram During its construction

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Distributed temperature measurement using a Fabry-Perot effect based chirped fiber Bragg grating

A temperature distribution sensing system based on the reflection spectrum of a 70 mm apodized chirped fiber Bragg grating has been demonstrated. Reflection variations created by the Fabry-Perot effect due to the intra-grating wavelength shift are presented. By fitting a parametric transfer matrix model of the grating response to measured spectra, the temperature change, position and width of a localized temperature change are obtained. This system is particularly attractive of its simplicity, high spatial resolution and ability in measuring a non-monotonic distribution. Experiments to measure two localized heating regions along the grating indicate that the technique is promising for measuring more complex temperature profiles.     

用于光纤布拉格光栅法布里-珀罗腔的改进的Rouard算法

比较了光纤布拉格光栅法布里一珀罗腔的极限法、平衡法及传输矩阵法3种算法,证明了它们是一致的,指出了光纤布拉格光栅复振幅传输矩阵的相移特性对法布里一珀罗腔透射率的影响.对计算任意折射率调制的光纤布拉格光栅的Rouard算法进行了改进,在分层方法中考虑了折射率分布初始相位的影响,获得了更为准确的反射复振幅相位特性,将该结果应用于计算光纤布拉格光栅法布里一珀罗腔,得到了光纤布拉格光栅法布里一珀罗腔具有多峰结构的透射谱,并经过实验验证了该理论的正确性.     

Diffraction from angular multiplexing slanted volume hologram gratings

The diffraction characteristics of the volume holographic gratings made by multiexposures with angular multiplexing during its construction is investigated. Because of the reflection by the interface between the emulsion and the substrate or the air there is an extra unslanted periodic structure inside a slanted grating, it will affect properties of the slanted volume holographic gratings. When the angle between the surface of the substrate and the grating plane of the slanted grating is less than a certain value, an extra peak accompanying the main peak will appear in the diffraction spectrum. But, when the angle is larger than the certain value, one designed and expected main peak will disappeared while the extra peak is kept and observed. This phenomenon limits the capacity of the volume hologram for the application in wavelength-division multiplexing (WDM).     

Polarization-independent guided-mode resonance filter with cross-integrated waveguide resonators

A cavity-resonator-integrated guided-mode resonance filter (CRIGF) has been proposed and investigated in order to realize high-efficiency narrowband reflection with a small aperture. The CRIGF consists of a grating coupler integrated in a cavity resonator constructed by a pair of distributed Bragg reflectors on a thin-film waveguide. This time, orthogonally crossed integration of two CRIGFs was demonstrated in order to obtain polarization-independent reflection spectrum. An SiO₂-based device with 10?μm aperture was designed and fabricated for around 850?nm wavelength operation, and narrowband polarization-independent reflection was confirmed experimentally.     

倾斜Bragg光纤光栅反射谱特性理论和实验研究

光纤Bragg光栅在通信和传感领域有着广泛的应用。根据倾斜光纤Bragg光栅的结构特点，利用耦合模原理，运用传输矩阵法，模拟分析了倾斜角度对光纤Bragg光栅反射特性的影响，并用实验对模拟结果进行了验证。实验和模拟结果表明：随着倾斜角的增大，中心反射波长向长波方向移动，反射带宽变窄，反射率减小。这个结论对倾斜Bragg光纤光栅的生产和应用有一定的指导作用。