All-Fiber Spectroscope with Second-Order Fiber Bragg Grating for Rotational Raman Lidar

Owing to the second-order fiber Bragg grating possessing narrower reflectivity bandwidth than first-order, an all-fiber spectroscopic filter with several second-order fiber Bragg gratings and optical fiber couplers in the visible region is designed to extract interest rotational Raman spectra. Considering the inference fringe visibility of inscribing fiber Bragg grating and the fiber core index dispersion effect, numerical simulations are made by the improved mathematical model based on an exponential photorefractive dynamic, and its simulative results show better agreement with the experimental data. We propose an all-fiber spectroscopic configuration, which could balance lidar sensitivity against signal-to-noise ratio for optimizing remote sensing performance by fiber Bragg gratings serially. Simulative results show that the thumb principle for fabricating second-order fiber Bragg grating is the increasing gratings length, and that this all-fiber spectroscope can achieve >70 dB suppression to elastic scattering and efficiently extract the rotational Raman signal for profiling atmospheric temperature.     

Random period arc-induced long-period fiber gratings

We report the fabrication of arc-induced long-period fiber gratings with strong random variations in the period. Long-period fiber gratings with standard deviations in the period from 8.50 to 36.98 μm were fabricated. The spectral position of the resonant bands is determined by the average period value, being similar to that observed in a long-period fiber grating with a fixed period equal to the average period of the random grating. Moreover the notch bands keep the shape characteristics like wideband and depth compared with a long-period grating with a constant period. In addition, their sensitivity to external parameters such as ambient refractive index is not too different with that of fixed period long-period gratings.     

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.     

Reduction of Bragg Grating-Induced Coupling to Cladding Modes

We discuss fiber designs that have been suggested for the reduction of Bragg-grating-induced coupling to cladding modes. The discussion is based on a theoretical approach that includes the effect of asymmetry in the UV-induced index grating, made by UV-side writing. Experimental results from gratings in a depressed-cladding fiber are compared with simulations. The model gives good agreement with the measured transmission spectrum and accounts for the pronounced coupling to asymmetrical cladding modes, even when the grating is written with smallest possible blaze. The asymmetry causing this is accounted for by the unavoidable attenuation of the UV light. It is found for the considered fiber designs that a high numerical-aperture fiber increases the spectral separation between the Bragg resonance and the onset of cladding-mode losses. A depressed-cladding fiber reduces the coupling strength to the lower order cladding modes, and the UV-sensitive cladding design reduces the cladding-mode coupling loss. The analysis suggests that the UV-sensitive cladding design is the most effective in reducing the cladding-mode-coupling losses.     

Grating resonances in air-silica microstructured optical fibers

We report what is believed to be the first demonstration of optical fiber gratings written in photonic crystal fibers. The fiber consists of a germanium-doped photosensitive core surrounded by a hexagonal periodic air-hole lattice in a silica matrix. The spectra of these gratings allow for a detailed characterization of the fiber. In particular, the gratings facilitate coupling to higher-order leaky modes. We show that the spatial distribution and the effective index of these modes are determined largely by the design of the lattice and that the grating spectra are unaffected by the refractive index surrounding the fiber. We describe these measurements and corresponding simulations and discuss their implications for the understanding of such air-hole structures.     

Analysis of designing multichannel fiber Bragg gratings with different inverse design algorithms

We have analyzed different inverse design algorithms for the synthesis problems of fiber gratings. An example of dense wavelength-division-multiplexing (DWDM) multichannel fiber Bragg grating (MCFBG) for optical fiber communication was synthesized and simulation results were analyzed and compared with use of the discrete layer-peeling (DLP) inverse scattering algorithm and Lagrange-multiplier constrained optimization (LMCO) algorithm. Simulation results show the LMCO method was more flexible than the DLP method.     

Hybrid FBG–LPG sensor for surrounding refractive index and temperature simultaneous discrimination

In this work, we propose a configuration of a hybrid fiber Bragg grating together with a long period grating sensor used for simultaneously detect surrounding refractive index and temperature to be applied in aqueous environment and to reveal pollution. We present the simulation of such sensor and analyze the reflected wavelength and amplitude variations of the fiber Bragg grating spectrum to obtain the temperature and the external refractive index variations, respectively. The results show that the fiber Bragg grating reflected amplitude change depends on the variation of the long period grating transmission spectrum with the surrounding refractive index modification and the reflected wavelength shift depends on the temperature of the aqueous solution.     

用相位掩模法制作光纤光栅的技术

准分子激光照射相位掩模后,在近场形成周期的条纹分布,利用近场光强对光敏光纤进行曝光,可以在纤芯中写人光栅。对光纤光栅折射率分布进行合理假设,运用耦合模理论计算了光纤光栅的反射率,对其反射率与相关参数的关系进行了讨论,并且分析了用相位掩模技术制造光纤光栅的影响因素,得出要获得窄带宽高反射率光栅需有适当的折射率调制和大的光纤光栅长度的结论。另外还导出了掩模板后表面的近场光强分布和光纤光栅的周期公式。     

光纤光栅温度与轴向应变响应机制分析

本文对温度和轴向应变对长周期光纤光栅和Bragg光纤光栅耦合波长的影响机制作出分析,通过有代表性的举例计算数值给出了各因素对耦合波长的影响,指出轴向应变对耦合波长的影响除了来自光弹性效应带来的光纤折射率的改变外,光纤半径的变化,特别是光栅周期的改变也起重要作用.对比分析找到了长周期光纤光栅与Bragg光纤光栅温度,应变响应系数数量级异同的原因.     

Two dimensional silicon waveguide chirped grating couplers for vertical optical fibers

Shallow etched two dimensional gratings for coupling light between silicon-on-insulator nanophotonic waveguides and vertical optical fibers were designed and experimentally characterized. We show that the large second order back reflection could be suppressed effectively by applying a linear chirp in the grating period for two dimensional grating couplers. The total coupling efficiency from an optical fiber to two orthogonal silicon waveguides is independent of the input polarization. An almost linear relationship between the average effective index of the grating region and the fill factor for 1D grating is obtained and verified. We also show that the average effective index for the two dimensional grating is similar to the one dimensional grating with the same fill factor in the light propagation direction, when the fill factor of the etched holes was larger than 0.5.     

Observation of type I and type II gratings behavior in polymer optical fiber

We report the first observation of type I and type II fiber grating behaviors in making polymer optical fiber Bragg gratings. Our observation reveals that there are two distinctive stages in the fabrication of polymer optical fiber Bragg gratings. Notably the two stages of grating formation correspond to low and high-index modulation gratings, which match well with those in silica fiber grating fabrication. Thus we refer them as type I and type II polymer fiber gratings, following the same nomenclatures for different types of silica fiber gratings. In addition, the characteristics and formation mechanism for type I and type II polymer fiber gratings are also examined.     

Slow and fast light using fiber Bragg grating F-P cavity

In this paper, we analyzed the properties of fiber Bragg grating F-P cavity with different grating parameters at two ends. The features of transmission spectra and group velocity were investigated under different parameters including slight increase for the differences of the lengths of the two gratings, the periods of the two gratings, the refractive index modulation depths of the two gratings. The characteristics of the slow light in fiber Bragg grating and fiber Bragg grating F-P cavity were also compared. The results of this study can be useful for actual experiments and applications in using the fiber Bragg grating F-P cavity in the slow light.     

A method of designing photonic crystal grating slow-wave circuit for Ribbon--Beam microwave travelling wave amplifiers

A method of designing a photonic crystal grating slow-wave circuit in which the cylinders of the 2D photonic crystals dot on a cross-sectional plane is established by calculating the band structures of the 2D photonic crystals, and the eigenfrequency of the equivalent waveguide grating. For calculating the band structures, the eigenvalue equations of the photonic crystals in the system of photonic crystal grating slow-wave circuit are derived in a special polarization mode. Two examples are taken to show the method. The design result is validated by the scattering parameters of the same circuit. The result indicates that there exists no photonic band gap if the metal gratings do not extend into the photonic crystals; the design of the circuit without the metal gratings extending into the photonic crystals is less flexible than that with the metal gratings extending into the photonic crystals.     

Electrically pumped two-dimensional Bragg grating lasers

We demonstrate electrically pumped InGaAsP two-dimensional Bragg grating (2DBG) lasers with two line defects. The 2DBG structure uses a weak 2D index perturbation surface grating to control the optical modes in the plane of the wafer. Measurements of the 2DBG lasers show that modal control in both the longitudinal and transverse directions is due to the gratings and defects. The 2DBG lasers are promising candidates for single-mode, high power, and high efficiency large-area lasers.     

掺铒光纤光栅的开关动力学特征研究

研究在连续光入射下掺饵光纤光栅的开关动力学特征，发展其开关过程有类似于相变的特征。数值结果表明，主动光纤光栅在连续光条件下更适合于脉冲串产生。     

双面曝光长周期光纤光栅偏振相关损耗

研究了采用高频CO2激光脉冲通过单面和双面曝光制作出的长周期光纤光栅(LPFG)的偏振相关损耗特性。单面曝光制作的长周期光纤光栅由于光纤横截面折射率改变不均匀,对入射光偏振态有较强的依赖性,主要表现为偏振相关损耗较大;为了降低偏振相关损耗,在制作长周期光纤光栅的过程中采用双面曝光的方法,以使其横截面折射率调制受高频CO2激光曝光改变均匀。实验数据显示,高频CO2激光脉冲制作的普通单面曝光长周期光纤光栅最大偏振相关损耗高达1.24 dB,而采用双面曝光的相同谐振峰值长周期光纤光栅的偏振相关损耗则可降至0.42 dB。因此,双面曝光制作的长周期光纤光栅能较好地克服入射光偏振态的不良影响,从而满足对偏振敏感的光纤通信和传感系统的应用要求。     

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。     

基于单个光纤光栅的Sagnac干涉仪的理论与实验研究

提出了一种结构新颖的Sagnac干涉仪,分析计算了所含光纤光栅分别为均匀Bragg光栅和线 性啁啾光栅时的透射响应,得到了均匀光栅Sagnac环的解析解,发现含均匀光栅时可得到间隔均匀的通带结构,而含啁啾光纤光栅时只能得到间隔非均匀的通带结构.这两种结构都可 能有应用价值,前者可用在密集波分复用系统中,后者可用在光纤传感领域中.实验制作了 两种通带间隔不同的、通带数分别为8和16的多通带滤波器,实验证实了理论计算得到的结果. 关键词： 光纤光栅 Sagnac干涉仪 光学滤波器     

横向衍射技术测量光纤光栅的特性

提出了一种用Ｈｅ－Ｎｅ激光侧面照射光纤光栅，从另一侧探测光栅的布拉格衍射效率来测量光纤光栅光谱特性的技术，并用该技术测量了光栅的折射率变化的分布。     

一种简易的光纤光栅涂覆装置

阐述了对裸光纤光栅进行涂覆的原理和必要性。设计了一种简易可行的光纤光栅涂覆装置。该装置简单灵活,成本较低,尤其适合于长度较长的光纤光栅的一次性涂覆,涂层厚度灵活可调。以啁啾光纤光栅为例进行了涂覆实验,涂层光滑均匀,涂覆后的光栅强度得到了增强,抵抗机械应力的能力得到了提高,使用起来更方便。对光栅在涂覆前后分别作了特性参数测试。结果表明,光栅特性基本上不发生变化,性能未劣化,光纤光栅的涂覆简单易行。