Coated Fiber Bragg Grating Sensor Research Articles

Study of the transverse strain effect on the Fiber Bragg Grating Sensor (FBGS) response with polyimide coating under experimental biaxial tests

The aim of this research work is to study the behaviour of Fiber Bragg Grating (FBG) sensors to interpret their response more accurately in a structural monitoring system. In the presence of a complex stress state, a structure is subjected to strain fields in multiple directions simultaneously and the sensors used for its structural monitoring are affected as well. However, the traditional procedure to calibrate FBG sensors consists of uniaxial tensile tests to calculate a strain sensitivity factor (K) that relates the sensor response to the strain measured during the test. In this study, a polyimide coated FBG sensor is embedded in a cruciform composite specimen to analyse its response to biaxial strain states. The experimental methodology has consisted of carrying out a campaign of biaxial tests on this specimen in which the longitudinal strain (εx) has been kept constant and the transverse strain (εy) has been varied by means of a triaxial testing machine to reproduce different plane stress states. Biaxial tests have allowed to study experimentally the influence of the εy on the longitudinal measurement of the FBG sensor and therefore on the calibration procedure. Finally, the experimental results obtained in both uniaxial and biaxial tests have been compared to the strain-optic theory.

Detection of chromium using polymer coated etched FBG

A chemo-mechanical-optical sensing technique is presented in this paper to detect carcinogenic Chromium Cr(VI) metal ion utilizing etched FBG (Fiber Bragg Grating) coated with stimulus responsive hydrogel. The Hydrogel is formed with blends of p(AMPSNa-co-APTMACl) which are greatly responsive to Cr (chromium) ions and exhibit a volume variation whenever dipped in the Cr solution. When the sensor head is exposed to different hexavalent Chromium (VI) ion concentration solutions, it exhibits a peak shifts because of the induced mechanical strain through the gel swelling. Chromium (VI) ion concentration is correlated with the peak shift. The sensor’s sensitivity is observed to be 15.1 pm/ppb which is 10 times the sensitivity of an unetched gel coated FBG sensor. The Cr (VI) ions trace amount is as low as 10 ppb will be sensed through technique. The proposed sensor has exhibited better repeatability, selectivity and sensitivity and the significant features of the proposed sensor are less weight, adoptability and compact size to monitor remotely.

An investigation on temperature sensitivity of conductive carbon coated fiber Bragg grating

We propose a conductive carbon coated fiber Bragg grating (FBG) temperature sensor for enhancing temperature sensitivity. The temperature sensitivity of the proposed sensor is effectively enhanced by approximately 4 times higher than those of the bare-type FBG sensors due to thermal expansion and physical stress possessed by the conductive carbon coating, with the operating temperature ranging from 40 °C to 260 °C. The proposed sensor is much more convenient to fabricate compared to other similarly coated FBG sensors, giving a leap advantage of easy reformation of the coating to fit various housings. This proposed sensor is significantly ideal for various applications such as optoelectronic circuit and waveguide temperature measurement.

Gold Nanoparticles Coated FBG Sensor Based on Localized SPR for Adulterated Honey Classification

Two types of fiber optics, namely macro etched silica single mode fiber (SMF) and Fiber Bragg Grating (FBG) (Bragg wavelength of 1550 nm and 1554 nm) had been used to detect various types of honey samples, such as Apis Dorsata Honey, Trigona Honey and Capilano Australian Honey. To study the effect of exposure period in open environment at room temperature, all honey samples were exposed in open environment at room temperature from 2 to 10 days. In comparison with macro etched SMF and FBG (λB=1554 nm), the FBG (λB=1550 nm) portrays an excellent sensing properties with sensitivity and selectivity of 33.56 dB/RIU and 24.07 dB/RIU respectively. The output of this work concludes that the quality of honey based on optical output power reduces up to 0.35% as period of exposure to the open environment increased.

A novel low-cost gas sensor for CO2 detection using polymer-coated fiber Bragg grating

The realization of detection of carbon dioxide (CO2) is important for environmental protection. A critical challenge is how to fabricate low-cost fiber Bragg grating (FBG) sensor. Here, a new type of polyether sulfone (PES)-coated FBG sensor for CO2 detection is reported to address this challenge. The PES coating exhibits volume dilation in CO2 exposure and can transfer the stress to the grating, causing changes of the period and the refractive index of the grating. The uniform and smooth PES coating is critical for the repeatability and reliability of the sensor, which is realized through the low-temperature spin coating and high-temperature curing. The influence factors and the performance of the FBG sensor were investigated. The maximum Bragg wavelength shift is positively related to coating thickness and negatively related to temperature. The PES-coated FBG sensor possesses good selectivity to CO2 and the minimum response time is 3.27 min. The limit of detection (LOD) can be down to 0.78 % for CO2 detection. Finally, an over-concentration alarm system for on-line CO2 monitoring was developed. The outstanding features of the PES-coated FBG sensor with low-cost and simple fabrication process provide widely prospect in applications.

Characterization of an ORMOCER®-coated FBG sensor for relative humidity sensing

In this paper, an ORMOCER® (OR)-coated fiber Bragg grating (FBG) sensor encapsulated with stainless steel was proposed for relative humidity (RH) sensing. Various sensing characteristics of the sensor, including RH and temperature sensitivity, hysteresis, repeatability, stability, and water resistance, were investigated systematically and compared with a polyimide (PI)-coated FBG humidity sensor. The results show that the OR-coated FBG humidity sensor can measure simultaneously the humidity and temperature, with sensitivity coefficients of 2.4 pm/%RH and 12.9 pm/°C, respectively. The OR-coated sensor has obvious advantages over the PI-coated sensor in terms of sensitivity, response speed, accuracy, and mechanical strength. Furthermore, two practical encapsulation methods were proposed for the sensor for applications in complex and harsh environments. Combined, the developed OR-coated FBG sensor provides an innovative means for RH monitoring.

Study of fiber Bragg gratings with TiN-coated for cryogenic temperature measurement

In this study, single metal-coated Fiber Bragg grating (FBG) sensors and bare FBG sensors were placed in an aluminum alloy mold, which were in turn packaged in a thermal insulation container to monitor the temperature response, and used as cryogenic temperature sensors. Specifically, the performance of titanium nitride (TiN)-coated FBG sensors in cryogenic temperature environments was evaluated. Using an optical spectrum analyzer (OSA), the reflection spectra of the TiN-coated FBG sensors were measured and compared with those of bare FBG for temperatures ranging from 25 to −195 °C. The results showed that the TiN-coated FBG exhibited a temperature sensitivity of 10.713 pm/°C and R-squared value of 0.995. The TiN-coated on the FBG altered the non-linear characteristics of the thermo-optic and thermal expansion coefficients of the FBG. Through this research to get thermo-optic coefficient (ξ = 5.12 × 10−6) and thermal expansion coefficient (α = 1.81 × 10−6) and resulted in more precise measurements at cryogenic temperature.

A novel modified fiber Bragg grating (FBG) based ammonia sensor coated with polyaniline/graphite nanofibers nanocomposites

Abstract The sensing of ammonia leakage at room temperature has potential applications in many areas such as industry. A sensitive etched-tapered fiber Bragg grating (FBG) based ammonia sensor is developed and investigated towards different concentrations of ammonia. The investigations are performed at room temperature. FBGs are etched with 50 µm diameter and tapered with 15 and 30 µm waist diameters. These FBGs are coated with polyaniline/graphite nanofibers (PANI/GNF) nanostructures as a sensing layer using spray method. The developed FBG sensors are based on Bragg wavelength shift on exposure to different ammonia concentrations due to change in refractive index of the sensing layers as interact with ammonia molecules. FBG sensors proved a linear Bragg wavelength shift with ammonia concentrations. The developed sensors exhibited linear, stable response, high sensitivity and good repeatability. Furthermore, FBGs sensors proved strong response towards NH3 by exhibiting high Bragg wavelength shift. This is because of porous structures of PANI\GNF nanostructures which improves the capability to adsorb additional NH3 molecules. This is in turn eventually enhances the interaction between NH3 and PANI\GNF nanocomposites. PANI\GNF coated FBG sensor with 15 µm waist diameter shows the highest sensitivity and good selectivity towards NH3.

Investigation of a Bragg Grating-Based Fabry–Perot Structure Inscribed Using Femtosecond Laser Micromachining in an Adiabatic Fiber Taper

This paper presents the fabrication of a fiber Bragg grating (FBG)-based Fabry–Perot (FP) structure (7 mm total length) in an adiabatic fiber taper, investigates its strain and temperature characteristics, and compares the sensing characteristics with a standard polyimide coated FBG sensor. Firstly, a simulation of the said structure is presented, followed by the fabrication of an adiabatic fiber taper having the outer diameter reduced to 70 μ m (core diameter to 4.7 μ m). Next, the sensing structure, composed of two identical uniform FBG spaced apart by a small gap, is directly inscribed point-by-point using infrared femtosecond laser (fs-laser) micromachining. Lastly, the strain and temperature behavior for a range up to 3400 μ ε and 225 ° C, respectively, are investigated for the fabricated sensor and the FBG, and compared. The fabricated sensor attains a higher strain sensitivity (2.32 pm/ μ ε ) than the FBG (0.73 pm/ μ ε ), while both the sensors experience similar sensitivity to temperature (8.85 pm/ ° C). The potential applications of such sensors include continuous health monitoring where precise strain detection is required.

Aerospace-grade surface mounted optical fibre strain sensor for structural health monitoring on composite structures evaluated against in-flight conditions

Optical fibre sensors are being investigated since many years as candidates of choice for supporting structural health monitoring (SHM) in aerospace applications. Fibre Bragg grating (FBG) sensors, more specifically, can provide for accurate strain measurements and therefore return useful data about the mechanical strain state of the structure to which they are attached. This functionality can serve the detection of damage in an aircraft structure. However, very few solutions for protecting and bonding optical fibres to a state-of-the-art aircraft composite material have been reported. Most proof-of-principle demonstrations using optical fibre sensors for aerospace SHM-related applications reported in literature indeed rely on unpackaged fibre sensors bonded to isotropic metallic surfaces in a mostly unspecified manner. Neither the operation of the sensor, nor the adhesive material and bonding procedure are tested for their endurance against a full set of standardized in-flight conditions. In this work we propose a specialty coated FBG sensor and its permanent installation on aerospace-grade composite materials, and we demonstrate the compatibility with aerospace in-flight conditions. To do so we thoroughly evaluate the quality of the operation of the FBG sensor by correlating the reflection spectra of the installed sensors before and after exposure to a full set of realistic in-flight conditions. We also evaluate the difference in strain measured by the FBG, since any damage in the adhesive bond line would lead to strain release. The applied test conditions are based on aerospace standards and include temperature cycling, pressure cycling, exposure to humidity and hydraulic fluid and fatigue loading. We show that both the bond line and the quality of the sensor signal were negligibly affected by the applied environmental and mechanical loads representing in-flight conditions and therefore conclude that it can be considered for SHM of aerospace-grade composite materials.

Characterization of metallic-packaging fiber Bragg grating sensors with coated and bare fibers.

Metallic packaging of fiber Bragg grating (FBG) sensors is developed using the ultrasonic welding method. Both polyimide-coated fiber and bare fiber could be bonded well to aluminum alloy substrate using Sn-Bi alloy. Two kinds of metal-packaged FBG sensors, coated FBG and bare FBG, are characterized for studying the thermal sensitivity, strain response, short-term creep, and temporal temperature response. Both FBG sensors showed increasing sensitivity with temperature from -40°C to 80°C. The metal-packaged coated FBG sensor displayed relative strengths in strain stability, repeatability, creep, spectra shape, and temperature response when compared with the bare one. Moreover, the boundaries between optical fiber and metal alloy are intact, and cross-sectional scanning electron microscope micrographs clearly illustrated that metal alloy coated well with the coated and bare fiber.

Effect of Polymerization Temperatures on Polyaniline Coated Fiber Bragg Grating Sensor for Chloroform Detection

A new fiber Bragg grating (FBG) sensor based on polyaniline (PAni) has been developed for chloroform detection. PAni was synthesized through chemical oxidation method by using equimolar amount of aniline monomer (Ani), dioctyl sodium sulfosuccinate dopant (AOT) at different three polymerization temperatures (−5, 0 and 25 °C). The chemical structure and conducting behavior of PAni were confirmed using Fourier transform infrared (FTIR) spectroscopy, ultraviolet‐visible (UV‐vis) spectroscopy and conductivity measurements. In this study, the FBG sensor was produced by removing the cladding layer and replaced with PAni coating. The immersion of sensor in varying concentration of chloroform (from 10 to 100 ppm) produced different Bragg wavelength shift at ∼1557 nm in the optical spectrum analyzer (OSA). The effect of PAni polymerized at different polymerization temperatures affect the sensor performance. It was found that the FBG sensor coated with PAni polymerized at 0 °C exhibited the highest sensitivity for chloroform detection and good recyclability up to 10 cycles with fast response time of 7 s and recovery time of 8 s. The FTIR, UV‐vis and conductivity analyses supported the proposed mechanism for the interaction between PAni and chloroform being the dipole‐dipole interaction (physical interaction) between the partial negative charge (Clδ−) of chloroform and the partial positive charge (NHδ+) of PAni.

Selecting of FBG Coatings for Quench Detection in HTS Coils

Fiber Bragg grating (FBG) temperature sensors are now regarded as a relatively mature sensing technology for temperature measurement in ambient conditions, and may offer potential as a cryogenic temperature sensor for quench detection. In this paper, the temperature response of three different coated FBG sensors from 77 to 293 K was studied; bare (silica) FBG, acrylic coated, and polyimide coated FBG sensors. We examine the effect of fiber coatings on the temperature sensitivity of an embedded FBG sensor down to cryogenic temperatures. Measurements were repeated with sensors from two FBG suppliers. The measured temperature response was then compared and contrasted with the predicted sensor response. In order to assess the sensor sensitivity, repeatability, and any hysteresis, these sensors were then subjected to repeated cool-down / warm-up cycles (77–293 K). The effect of epoxy resin impregnation on the temperature response characteristics was analyzed by comparing the sensor response characteristics before and after embedding into a 10 mm × 2 mm × 4 mm epoxy block. We compare this to the uncoated fiber for embedment in HTS coils. Finally, we recommend the optimum low-cost sensor construction for use in quench detection in coils.

Theoretical analysis and coating thickness determination of a dual layer metal coated FBG sensor for sensitivity enhancement at cryogenic temperatures

Use of Fiber Bragg Grating (FBG) sensor is very appealing for sensing low temperature and strain in superconducting magnets because of their miniature size and the possibility of accommodating many sensors in a single fiber. The main drawback is their low intrinsic thermal sensitivity at low temperatures below 120 K. Approaching cryogenic temperatures, temperature changes lower than a few degrees Kelvin cannot be resolved, since they do not cause an appreciable shift of the wavelength diffracted by a bare FBG sensor. To improve the thermal sensitivity and thermal inertia below 77 K, the Bare FBG (BFBG) sensor can be coated with high thermal expansion coefficient materials. In this work, different metal were considered for coating the FBG sensor. For theoretical investigation, a double layered circular thick wall tube model has been considered to study the effect on sensitivity due to the mechanical properties like Young’s modulus, Thermal expansion coefficient, Poisson’s ratio of selected materials at a various cryogenic temperatures. The primary and the secondary coating thickness for a dual layer metal coated FBG sensor have been determined from the above study. The sensor was then fabricated and tested at cryogenic temperature range from 4-300 K. The cryogenic temperature characteristics of the tested sensors are reported.

Metal-coated fiber Bragg grating for dynamic temperature sensor

This paper focuses on the performance of a metal-coated fiber Bragg grating (FBG) used as a high temperature sensor. Specifically, the performance of a chromium nitride (CrN)-coated FBG sensor was evaluated in extreme temperature environments. Using an optical spectrum analyzer (OSA), the reflection spectra of the CrN-coated FBG sensor were measured and compared with those of bare FBG for temperatures ranging from 100°C to 650°C. In this way, the Bragg wavelength shift and temperature sensitivity of both types of FBG were obtained. The results indicated that the CrN-coated FBG sensor exhibited a 14.0 pm/°C greater temperature sensitivity than the bare FBG.

Azobenzene-Based Gel Coated Fibre Bragg Grating Sensor for Moisture Measurement

A fibre Bragg grating sensor is coated with a novel polymer gel in order to investigate its suitability for nondestructive measurement of moisture in materials that can potentially lose their integrity due to moisture ingress. Absorption and desorption of moisture lead to swelling/shrinkage of an azobenzene-based gel, which induces a strain in the Bragg grating resulting in wavelength shifts. The results demonstrated that the amount of wavelength shift is linearly dependent on the amount of water ingress by the gel. The performance of the proposed optical fibre moisture sensor was found to be repeatable with no detectable hysteresis and has the potential to offer a low-cost route for monitoring moisture content.

Improvement in temperature sensitivity of FBG by coating of different materials

This paper presents the simulation results of coated Fiber Bragg Grating (FBG) as temperature sensor. As we know that bare FBG cannot be used as temperature sensor because they are made of silica and the thermal expansion coefficient of silica is very less. Hence bare FBG is less sensitive to temperature variation. To enhance the temperature sensitivity of the FBG sensor different types of materials are coated on FBGs whose thermal expansion coefficients are greater than that of silica. In this simulation Lead, Indium, Copper, Aluminium, and PMMA are coated on FBG and the simulation results are compared with the previous experimental work for the temperature range 50K – 300K. PMMA coated FBG sensor showed best sensitivity value among above coating materials.

Effect of Thermal Residual Stress on FBG Sensors Embedded in Composite T-Stiffened Panels

The reflection spectrum from the Fiber Bragg Grating (FBG) sensors embedded in the composite was disturbed because of the thermal residual stress. In this study, two types of the FBG sensors, uncoated and UV-cured resin coated FBG sensors, were embedded in the triangle area of the T-stiffened composite panels. The strains during the curing process were measured by these two kinds of FBG sensors. Through the comparison of the results, it was found that the effect of thermal residual stress on the reflection spectrum could be attenuated when the FBG sensor was coated with UV-cured resin. Furthermore, the strain responses during the low-velocity impact test were observed by both two FBG sensors. The maximum-strain measured by the coated FBG sensors was more accurately than that measured by the uncoated one.

Polymer‐coated fiber Bragg Grating Sensor for cryogenic temperature measurements

AbstractCryogenic temperature sensing has been demonstrated using Polymer coated Fiber Bragg Grating Sensor, and the experimental results are presented in this article. Teflon coating thicknesses of 20 and 40 μ applied on Fiber Bragg Gratings, inscribed in silica fiber (TFBG) are used for these studies. It was observed that the sensitivity of Teflon‐coated FBG has been enhanced three times compared to uncoated regular FBGs. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:1154–1157, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25914

Design parameter evaluation of a metal recoated Fiber Bragg Grating sensors for measurement of cryogenic temperature or stress in superconducting devices

There are plenty of complex physical phenomena which remain to be studied and verified experimentally for building an optimized superconducting magnet. The main problem for experimental validations is due to the unavailability of suitable sensors. This paper proposes a Fiber Bragg Gratings (FBG) sensor for this purpose which allows access to the local temperature/stress state. To measure the low temperature (20 K), FBG can be recoated with materials having high thermal expansion coefficient (HTCE). This can induce a thermal stress for a temperature change, which in turn increases the sensitivity of the sensor. The performance of such sensors has been experimentally studied and reported in earlier paper [Rajinikumar R, Suesser M, Narayankhedkar KG, Krieg G, Atrey MD. Performance evaluation of metallic coated Fiber Bragg Grating sensors for sensing cryogenic temperature. Cryogenics 2008;48:142–7]. This paper aims at evaluation and determination of different design parameters like coating materials, coating thickness, grating period and the grating length for design of better performance FBG sensor for low temperature/stress measurements.