Ag-coated Long-period Fiber Grating

Based on coupled-mode theo ry , the eigenvalue equation of five-layered long-period fiber grating(LPFG) sens or with Ag film and gas-sensitive film overlays are firstly studied. The probl em of resolving complex eigenvalue equation on five-layered LPFG is analyzed, a nd the method of resolution is also given. Then the eigenvalue equation of three -layered metal cladding LPFG is analyzed, and the complex transcendental equati on is also discussed. The computing result shows that the coupling between the l ow-order EH modes and the core mode is much stronger than that between the low -order HE modes and the core mode.

Fabrication of long-period fiber gratings by CO_2 laser in fiber under tension

We demonstrate experimentally fabrication of CO2 laser written long-period fiber gratings （LPFGs） in single-mode fiber （SMF） under tension. New transmission dips can be generated due to the frozen-in stress in the fiber under tension. Dynamics analysis of laser writing process is performed to study the mechanism of the grating inscription. Experimental results show that the wavelength shift of new generated resonance dips can shift towards the longer or shorter wavelength with the increase of laser scanning cycles for gratings with different pitches.

Convention of Optical Vortices in Two-Helix Long-Period Fiber Gratings

An effective method to fabricate two-helix long-period fiber gratings （TH-LFGs） is presented. Based on the coupling mode theory, the conversion of optical vortices （OVs） in TH-LFGs are analyzed in detail. The conversions of OVs with different topological charges： 0 → ±2 and 1 → 3 are simulated as three examples and the conversion efficiency higher than 98% can be realized.

Hybrid Long-Period Fiber Grating with Multimode Fiber Core for Refractive Index Measurement

A refractive index （RI） sensor based on hybrid long-period fiber grating （LPFG） with multimode fiber core （MMFC） is proposed and demonstrated. The surrounding RI can be determined by monitoring the separation between the resonant wavelengths of the LPFG and MMFC since the resonant wavelengths of the LPFG and MMFC will shift in opposite directions when the surrounding RI changes. Experimental results show that the sensor possesses an enhanced sensitivity of 526.92nm/RIU in the RI range of 1.387-1.394 RIU. The response to the temperature is also discussed.

Sensing characteristics of a metal film coated long-period fiber grating

To obtain the influence rules of the coating parameters of a long-period fiber grating(LPFG)with respect to temperature,strain and refractive index sensing properties,based on the mode coupling theory,a strict four-layer theorietical model of a metal film coated LPFG is established,and these parameters that affect the spectral characteristics of the metal film coated LPFG are studied.The simulation results show that there is an optimal metal film thickness on the surface of the LPFG that will induce the surface-plasmon resonance(SP R)effect,which results in higher sensitivity to the environmental temperature and refractive index but has little influence on the strain There is theoretical evidence that when the silver thickness is between0.8and1.2nm,the refractive index sensitivity will reach the peak point of42.4026,at which the refractive index sensor sensitivity is increased by4.S%.The theoretical results of coating a long-period fiber grating provide a good theoretical basis and guidance for LPFG design and parameters optimization

Perimeter Security System Based on Ultra-Weak Fiber Bragg Grating Array

A perimeter security system based on ultra-weak fiber Bragg grating high-speed wavelength demodulation was proposed. The demodulation system for signal acquisition and high-speed wavelength calculation was designed based on field programmable gate array (FPGA) platform. The principle of ultra-weak fiber Bragg grating high-speed demodulation and signal recognition method were analyzed theoretically, and the Support Vector Machine model was introduced to optimize the event recognition accuracy of the system. A perimeter security experimental system containing 1000 ultra-weak fiber Bragg gratings, ultra-weak fiber Bragg grating sense optical cables with a diameter of 2.0 mm and a reflectivity of 0.01%, steel space frames and demodulation equipments was built to recognize four typical events such as knocking, shaking, wind blowing and rainfall. The experimental resulted show that the system has a spatial resolution of 1m and an acquisition frequency of 200 Hz. The joint time-frequency domain detection method is used to achieve 99.2% alarm accuracy, and 98% recognition accuracy of two intrusion events, which has good anti-interference performance.

大跨越架空输电导线铝部应力分布试验

为了掌握大跨越架空输电导线铝部应力空间分布特性,以JLHA1/G6A-500/280型特强钢芯铝合金导线为研究对象,采用铝股丝表面激光刻槽,内嵌高灵敏、超大复用容量超弱光纤光栅,搭建了大跨越输电导线铝部应力试验平台,研究了不同张力条件下大跨越架空输电导线铝部应力分布规律,建立了大跨越导线实体有限元模型,分析了铝股应力分布特征,并验证了试验结果的正确性。结果表明:导线承受张力作用时,外层铝股和次外层铝股应力不同,外层铝股应力小于次外层铝股应力,导线同层铝股应力基本相同;铝股应力随导线张力的增大呈线性增大趋势,张力每增大1%,铝股应力增大约10%。不同导线张力作用时钢股和铝股应力均呈现环状分层特性,铝股平均应力小于钢股平均应力,铝部和钢股应力比约为3∶7;外层和次外层铝合金股丝应力沿圆周向分布不均匀,建议大跨越导线线型设计时考虑铝股丝分层特性。

基于超弱光纤光栅的声波传感探头增敏方法研究

针对光纤直接探测声波时灵敏度低的问题,提出一种超弱光纤光栅缠绕式薄壁圆筒的声波传感探头增敏方法。理论分析了薄壁圆筒半径、壁厚、弹性模量等参数对光纤探头声增敏的影响,仿真分析了光纤缠绕方式对探头谐振频率的影响,优化设计了弹性管式探头的结构。搭建了基于超弱光纤光栅声波传感系统,并对探头的声敏特性进行了测试。实验结果显示,探头的声压灵敏度最高可达到6.39746 rad/Pa(-103.8798 dB re rad/μPa),在1000~2000 Hz的平均声压灵敏度为3.30341 rad/Pa(-109.6208 dB re rad/μPa);相较于未增敏的裸光纤,探头的平均声压灵敏度提高了约31 dB。

基于超弱光纤布拉格光栅的声速测量方法

利用超弱光纤布拉格光栅对振动的敏感特性,提出了一种基于分布式光纤声学传感的声速测量方法。通过测量声速传递过程中不同超弱光纤布拉格光栅传感器接收到声音信号的时差来计算声速。该方法可以测量不同温度下的声速,其测量值与理论值误差小于0.5%。为声速测量提供了新的方法。

Highly sensitive torsion sensor based on triangular-prism-shaped long-period fiber gratings

We propose and investigate a compact optical fiber sensor that aims to measure the torsion in both amount and direction with high sensitivity.This sensor is configured by a triangular-prism-shaped long-period fiber grating,which is fabricated by the high frequency CO_(2) laser polished method.The unique design of the triangular-shaped structure breaks the rotational symmetry of the optical fiber and provides high sensitivity for torsion measurement.In preliminary experiments,the torsion response of the sensor achieves a good stability and linearity.The torsion sensitivity is 0.54 nm/(rad/m),which renders the proposed structure a highly sensitive torsion sensor.

Utilizing phase-shifted long-period fiber grating to suppress spectral broadening of a high-power fiber MOPA laser system

Suppressing nonlinear effects in high-power fiber lasers based on fiber gratings has become a hotspot.At present,research is mainly focused on suppressing stimulated Raman scattering in a high-power fiber laser.However,the suppression of spectral broadening,caused by self-phase modulation or four-wave mixing,is still a challenging attribute to the close distance between the broadened laser and signal laser.If using a traditional fiber grating with only one stopband to suppress the spectral broadening,the signal power will be stripped simultaneously.Confronting this challenge,we propose a novel method based on phase-shifted long-period fiber grating(PS-LPFG)to suppress spectral broadening in a high-power fiber master oscillator power amplifier(MOPA)laser system in this paper.A PS-LPFG is designed and fabricated on 10/130 passive fiber utilizing a point-by-point scanning technique.The resonant wavelength of the fabricated PS-LPFG is 1080 nm,the full width at half maximum of the passband is 5.48 nm,and stopband extinction exceeds 90%.To evaluate the performance of the PS-LPFG,the grating is inserted into the seed of a kilowattlevel continuous-wave MOPA system.Experiment results show that the 30 dB linewidth of the output spectrum is narrowed by approximately 37.97%,providing an effective and flexible way for optimizing the output linewidth of highpower fiber MOPA laser systems.

Characterization of external refractive index sensitivity of a photonic crystal fiber long-period grating

Long-period gratings （LPGs） are fabricated in a photonic crystal fiber （PCF） using the symmetric point- by-point CO2 laser irradiation method to explore the sensitivity characterization of PCF-LPG. Numerical sim- ulation is used to guide the investigation. It is found that the refractive index （RI） sensitivity of PCF-LPG depends on the coupled cladding modes as well as the coupled resonance wavelength （RW） of the LPG. Experimental studies show that the longer the RW, the higher the RI sensitivity for the same cladding mode. At similar RWs, the lower the cladding mode, the higher the RI sensitivity of PCF-LPG.

Intensity detecting refractive index sensors with long-period fiber gratings written in STS structure

A refractive index intensity detecting sensor with long-period grating written in the single-mode–thin-core–single-mode fiber(STS) structure is proposed and optimized theoretically.The sensor is composed of two single-mode fibers connected by a section of long-period fiber grating fabricated on thin-core fiber.After optimization and benefitting from the phase matching point, the loss peak of the structure can reach 62.8dB theoretically.The wavelength of the characteristic peak is fixed at the phase matching point, so intensity detection can be achieved.The sensitivity can reach 272.5dB/RIU.The structural optimization in this Letter provides a reference for the fabrication of an easy-made all-fiber sensor without extra cladding.

A new phase-shifted long-period fiber grating for simultaneous measurement of torsion and temperature

A novel phase-shifted long-period fiber grating(PS-LPFG)for the simultaneous measurement of torsion and temperature is described and experimentally demonstrated.The PS-LPFG is fabricated by inserting a pretwisted structure into the long-period fiber grating(LPFG)written in single-mode fiber(SMF).Experimental results show that the torsion sensitivities of the two dips are?0.114 nm/(rad/m)and?0.069 nm/(rad/m)in the clockwise direction,and?0.087 nm/(rad/m)and?0.048 nm/(rad/m)in the counterclockwise direction,respectively.The temperature sensitivities of the two dips are 0.057 nm/℃ and 0.051 nm/℃,respectively.The two dips of the PS-LPFG exhibit different responses to torsion and temperature.Simultaneous measurement of torsion and temperature can be implemented using a sensor.The feasibility and stabilization of simultaneous torsion and temperature measurement have been confirmed,and hence this novel PS-LPFG demonstrates potential for fiber sensing and engineering applications.

Optical Sensing Based on Light Coupling Between Two Parallel Long-Period Fiber Gratings

A new optical intensity-based sensing mechanism for the measurement of refractive index and minute displacement is proposed in this paper, which is based on modifying the amount of light coupled between two parallel long-period fiber gratings. The characteristics of this sensing mechanism with experiments and simulation results are demonstrated.

Novelty Design in Gain Flattening Filter of ASE Source Based on Fat Ultra-Long Period Fiber Grating

A new type of gain flattening filter for amplified spontaneous emission （ASE） source based on erbium doped fiber （EDF） is proposed and demonstrated by fabricating and writing two series ultra-long period fiber grating （ULPFG） on single mode fiber （SMF-28）. The novelty method in this research is based on writing the two ULPFGs as fat gratings. The LPG is written by a simple and available arc-discharge method. The pump power based on single-pass backward pump configuration is around 100mW, and the average wavelength is near to 974nm. The gain flattening profile is obtained by 3.4 （±1.7）dB ripple in the wavelength range between 1524nm and 1565nm with 41-nm band width.

Dual-channel microfluidic sensor based on side-hole fiber with two long-period fiber gratings

We propose and demonstrate a dual-channel microfluidic sensor based on a side-hole fiber(SHF)with two longperiod fiber grating(LPFG)structures.There are two air holes in the SHF,which are natural microfluidic channels.We fabricate two LPFGs(long-period gratings LPG-A and LPG-B)in the SHF with the resonance wavelengths of 1268.7 nm and 1385.8 nm,respectively.Results show that the refractive index sensitivities of LPG-A and LPG-B are?76.0 nm/RIU and?71.1 nm/RIU,respectively.One can measure the refractive index of liquid samples in two channels simultaneously.The proposed dual-channel microfluidic sensor has advantages of good linearity response,fluidic technology compatibility,and easy light input/output coupling and system integration,which helps the sensor to have a potential application in environmental detection and food safety detection.

Effects Of Average Index Variation in Apodized Long-Period Fiber Gratings

We analyze the effects of average index variation on the transmission characteristics of an index-apodized long-period fiber grating （LPFG） by the transfer matrix method and study how these effects depend on the grating length, the grating profile, the modal dispersion factor, and the duty cycle of the index modulation. Apart from shifting the resonance wavelength and modifying the rejection band, average index variation can give rise to significant side lobes that may appear on the short-wavelength or long-wavelength side of the rejection band, depending on the signs of the average index change and the modal dispersion factor. Our results provide general guidance for the writing of LPFGs for the minimization of side lobes. Our analysis compares well with published experimental results and should be useful for the design and fabrication of LPFGs.

Coupled Mode Characteristics From the Perturbation of 3D Printed Long-Period Fiber Grating Devices

Characteristics of electric field from a coupled mode inside an optical fiber under perturbation by three-dimensional(3D)printed long-period fiber grating(LPFG)device have been observed in this work by the experiment and simulation.The various periodic index differences referring to the weights of perturbation by 3D printed LPFG device are applied on the single-mode fiber.The experimental results show that the resonant wavelength shift is a linear function of the grating period with the maximum coefficient of determination R2 of 0.9995.Some of resonant wavelengths are chosen to run simulations to investigate the electric field distribution.The scattering direction of the electric field states the magnitude of leaking optical power when the light transmits through the grating region applied to the single-mode fiber.Both the experimental and simulation results demonstrate that our proposed scheme can usefully be applied to selective tunable filters,intruder sensors,etc.

基于超弱光纤光栅的LAMOST内部温度监测方法

针对大天区面积多目标光纤光谱天文望远镜(LAMOST)观测过程中内部空间温度场扰动的问题,提出了一种基于超弱光纤光栅的空间温度监测方法。理论分析了单涂层超弱光纤光栅传感器的感温原理,设计了一种免应力的超弱光纤光栅感温光缆,对光缆的温度特性进行标定,灵敏度约为10.8 pm/℃,在-20~50℃范围内线性度优于0.9997。将23个间距1 m的铠装光栅光缆沿LAMOST的M B反射镜及廊道布设,实时监测内部温度的变化,监测结果反映了M B反射镜前温度梯度的变化过程,以及观测行为及外部环境对镜筒温度分布的影响,为分析大型光学望远镜内部的温度场提供了参考。