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

On the Validity of the Effective Index Method for Long Period Grating Photonic Crystal Fibers

This paper focuses on the investigation of modal characteristics and sensing properties of long period grating photonic crystal fibers (LPG-PCFs). An improved effective index method is employed with an objective to study its limitations for various designs of LPG-PCFs. Results so obtained with the above method are compared with the corresponding values of multiple multipole (MMP) method results which points the range of validity and applicability of the improved effective index method to LPG-PCFs. It is shown that this method is excellent when the surrounding media is assumed to be air. However, it becomes less accurate when the fiber is immersed into a liquid with a refractive index close to that of the cladding.

长周期光纤光栅(LPG)均衡滤波器在光纤放大器中的应用

根据耦合模理论 ,给出了两级关联和非关联长周期光纤光栅的透射率表达式 ,从而指出在进行LPG均衡滤波器设计时 ,应采用非关联的光栅 ,可以构成灵活多变的透射谱 ,非常适合光纤放大器的不同增益谱对均衡滤波器传输特性 (透射谱 )的不同要求。以此为基础 ,设计和模拟计算了 LPG均衡光纤宽带放大器的增益及噪声谱。结果表明 。

级联LPG的原理与应用

因为特有的滤波特性,级联长周期光纤光栅已被设计为优异的多波长滤波和传感器件,为此介绍了级联长周期光纤光栅的工作原理及其应用。

分步迭代算法在LPG折射率特性研究中的应用

采用分步迭代算法对简化的包层模式本征方程进行数值求解,这种方法由机器选取初值,整个过程无需人工干预,核心代码只需几十行且计算速度快,可求解任意环境折射率下各次包层模式的有效折射率(包括实数解和复数解),从而可在很宽的范围内对长周期光纤光栅(LPG)的折射率特性进行数值仿真。与实验结果进行了对比,证明了该算法对求解拥有多个复数根的超复杂超越方程是一种比较有效的方法,这对于应用该算法研究更多包层光栅及镀膜光纤光栅的其他特性具有一定的意义。

Vector bending sensor based on chirped long-period grating with off-axis micro-helix taper

Vector bending sensing has been consistently growing in many fields.A low-cost and high sensitivity vector bending sensor based on a chirped long-period fiber grating[LPFG]with an off-axis micro helix taper is proposed and experimentally demonstrated.The grating is composed of several sections of single-mode fiber with gradually larger lengths,and the off-axis micro helix tapers with fixed lengths when they are fabricated by using the arc discharge technology.The large refractive index modulation in the micro-helix taper greatly reduces the sensor size.The total length of the sensor is only 4.67 mm.The micro-helix taper-based LPFG can identify the bending direction due to the asymmetric structure introduced by the micro helix.The experimental results show that the transmission spectra of the sensor have distinct responses for different bending directions,and the maximum bending sensitivity is 14.08 nm/m^(-1)in the range from 0.128 m^(-1)to 1.28 m^(-1).The proposed bending sensor possesses pronounced advantages,such as high sensitivity,small size,low cost,and orientation identification,and offers a very promising method for bend measurement.

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.

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.

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.

A robust salinity sensor based on encapsulated long-period grating in microfiber

A robust fiber sensor for salinity measurement based on encapsulated long-period grating in microfiber is proposed. The long-period grating is fabricated in microfiber by inducing periodical deformation with CO2 laser, which is then encapsulated in a holey capillary tube. The encapsulation tube is designed to effectively protect the microfiber from external interference, but does not change the optical properties of the fiber and the response speed of the sensor, which makes the sensor more robust for real applications. Experimental results show that the sensor can achieve a sensitivity of 2.16 nm/% with a good linearity for concentration from 0% to 20%. It is theoretically proved that the sensitivity can be further improved by optimizing the diameter parameters. Such structure may be used as low loss evanescent-wave-coupled optical absorption, fluorescent and gain cells, photoacoustic cells, and etc.

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.