Coated Fiber Bragg Grating Research Articles

Groundwater salinity sensing using PI-FBG with improved capillary coating method

Polyimide (PI)-coated fiber Bragg grating (FBG) technology has certain potential for groundwater salinity measurement. However, this technology is immature for the field salinity monitoring due to the limitation of PI coating technology. Here, we fabricated the PI-FBG salinity sensing elements by improved capillary coating method, and encapsulated it with stainless steel tube to improve the robustness. The results show that different capillaries coated PI-FBGs were all linear with salinity ranging from 0 to 8%. Pre-treatment methods of soaking in the coupling agent and reducing the fiber diameter before coating can effectively improve the salinity sensitivity, and this coating method had good repeatability. The measurement results of the PI-FBG sensor in different gravel aquifers were highly consistent with those of the electronic salinometer. Our results show that the capillary coated PI-FBG has excellent salinity sensing performance, which provides a new means for in-situ monitoring of groundwater salinity.

Selective ultrasensitive FBG-SBL chemosensor for detection of copper ions in water resources based on plasmon resonance energy transfer

In this study, the gold (Au) coated fiber Bragg grating (FBG) probe decorated with Schiff base ligands (SBL) with variable denticity is proposed as a susceptible sensor for monitoring the copper ions (Cu2+) concentration in an aqueous solution. The sensing principle can be attributed to the interaction of the SBL coated on Au/FBG, which serves as anionic chelates with Cu2+, altering the environment refractive index and changing the surface plasmon resonance (SPR) condition. The sensitivity and limit of detection (LOD) through a Bragg wavelength shift are measured to be 23.1 pm/µM and 0.07 µM respectively, with a dynamic range of up to 4 µM (µM, 2.5 ppm), which is within the acceptable limit specified by the World Health Organization (WHO) for safe drinking water (1.3 ppm). The probe shows an assertive coordination behaviour towards Cu2+ for selective detection compared to other heavy metals. Excellent reusability (6 trial times) and specificity and portability make FBG-SBL chemosensor suitable for online monitoring of water resources.

Highly Efficient Fiber Optic Thermal Heating Device Based on Turn-Around-Point Long Period Gratings

The use of in-fiber core-to-cladding coupling components for thermal heating purposes has been well assessed in the last decades within the development of fiber optic devices for flow measurements and water thermal conductivity calculation. In these devices, light travelling in the fiber core is transferred into the cladding and absorbed by a metallic layer surrounding the fiber, with the consequent resistive heating generation. Here we demonstrate the use of a Turn-Around-Point (TAP) Long Period Grating (LPG) as resonant core-to-cladding light coupling mechanism for the fabrication of a highly efficient heating device based on metallic coated Fiber Bragg Grating (FBG). A properly designed TAP LPG was fabricated by means of point-to-point UV laser and spliced to a 150 nm thick Au-coated FBG. The heating efficiency characterization of the final device was analyzed, in both air and water, by evaluating the temperature increase in the gold layer surrounding the FBG at incremental values of the injected power. Collected results confirm that the use of LPGs involving the excitation of higher order cladding modes provides an excellent transferring mechanism of the fiber core light into the cladding, which in turn guarantees very high thermal heating efficiency to the final device. Moreover, by comparing such results with the performance of other in-fiber core-to-cladding coupling components already presented in literature, it was found that the TAP LPG-based device exhibits an actuation efficiency 2.5 times greater, thus resulting the most effective and highly performing solution for energy transfer to the metallic overcoat.

Fiber-optic airflow velocity sensing method based on a 45° tilt fiber grating combined with a single-walled carbon nanotube coated fiber.

We propose and experimentally demonstrate an airflow velocity sensing method based on a 45° tilt fiber grating (TFG) that is combined with a single-walled carbon nanotube (SWCNT) coated fiber Bragg grating (FBG). The principle behind which is to produce a dynamic thermal equilibrium between the light heating and the airflow cooling. For the first time, to the best of our knowledge, a 45°-TFG is used as the heating element for the hot-wire anemometer. By diagnosing the Bragg wavelength of the SWCNT coated FBG, the temperature variations of the sensing fiber are measured with respect to the airflow velocities, which vary from 0 to 1m/s. Moreover, under low light power consumption of 20 mW, the proposed sensor is shown to have good performance. Experimental results reveal that the sensitivity of the sensor increases with the heating pump. Due to the advantages of its simplicity and reliability, alongside its high photo-thermal conversion efficiency, this technique has excellent potential for future use in remote monitoring with airflow velocity sensing.

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.

Influence of Coating Thickness of Polyimide for Moisture Sensing

This work focused on the effect of existence of polyimide coated fibre Bragg grating (FBG) for moisture sensing in stingless bee. The moisture sensitive material namely polyimide (PI) was used to improve the sensitivity of the fibre optic sensor. By taking the benefits from amine groups in 3-(aminopropyl) triethoxysilane (APTES), PI is coated onto the segment of FBG through covalent interaction. Different thickness of PI coated FBG as well as determination and optimization of the designated sensor has been done. 3 layers of polyimide coating were used to improve the sensitivity of moisture sensing, and the sensitivity of coated fibre been analysed.

An optical fiber sensor based on polyimide coated fiber Bragg grating for measurement of relative humidity

Humidity is an important parameter in many industry fields and accurate humidity measurement is of great significance to improve production efficiency and ensure product quality. This paper presents a sensing model and scheme for utilizing thermoplastic polyimide (PI) coated fiber Bragg grating (FBG). The humidity sensitive PI layer was firstly prepared and coated on the FBG. The humidity calibration experiment was designed to verify the sensor performance based the saturated salt solution method. The experimental results show this FBG humidity sensor can work steady in the relative humidity (RH) condition ranging from 11%RH to 83% RH, with the sensitivity of the sensor less than 2 pm/% RH and measurement uncertainty of ±1% RH in the temperature range of 30 °C to 70 °C. In order to solve the problem that the PI-coated FBG is both sensitive to the humidity and temperature (cross sensitivity problem), a novel method of temperature compensation has been proposed. The method uses the character that the coefficients of expansion of the PI-coated layer for humidity and temperature are linearly superimposed and obtains the temperature compensation coefficient by linear interpolation of the function relationship between the measured temperature and the FBG central wavelength shift. The analysis of experimental data shows that within the temperature variation range of 20 °C, this temperature compensation method can effectively guarantee the average wavelength errors within 0.03 nm without reference grating. In this way, it can extend the FBG humidity measurement under environmental temperature up to 90 °C making it possible for application in the measurement the humidity in the high temperature environment.

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.

Development of Highly-Sensitive and Reliable Fiber Bragg Grating Temperature Sensors With Gradient Metallic Coatings for Cryogenic Temperature Applications

Temperature monitoring is an essential task in cryogenic engineering to ensure the safety of equipment, where highly sensitive and reliable sensors are required to provide real-time and accurate information about temperature. Titanium (Ti)-copper (Cu)-coated fiber Bragg grating (FBG) temperature sensors with a gradient of material properties to minimize thermal stresses are hence fabricated by a combination of Ti/Cu magnetron sputtering and Cu electroplating. The thermal characteristics and thermal fatigue behavior of the sensors are investigated. The sensors exhibit significantly higher sensitivity than that of the bare FBGs at cryogenic temperatures down to 79 K, with good repeatability and stability. Their response to the temperature changes can be precisely described by a quadratic-polynomial equation. No obvious variations in the zero-point, the reflection spectrum, and the temperature sensitivity of the sensors exposed to thermal fatigue from 77 K to room temperature are observed before fatigue failure which could be attributed to the formation and growth of subcritical cracks within the fiber rather than that of fatigue cracks within the metallic coatings, as a result of the cyclic thermal stresses induced by the large differences in coefficient of thermal expansion between the silica optical fiber and the metallic coating materials. The results demonstrate that the multilayer metal-coated FBG sensors provide great potential for temperature measurements or temperature compensation in cryogenic engineering by proper selection of metallic coatings with a gradient of material properties.

Development and Characterization of UV-Resin Coated Fiber Bragg Gratings

We report the development and characterizations of a fiber Bragg grating (FBG) sensor coated with different ultraviolet (UV) curable resins. The UV-curable resins were applied on the fiber after the FBG inscription and cured with an UV lamp. One set of samples used the NOA 68 resin and the other used NOA 88. The samples were characterized with respect to the temperature, moisture absorption and strain response. Furthermore, in order to understand the influence of the resin coating on the optical fiber’s mechanical properties, tensile tests were performed with the samples. Results show that all samples presented negligible sensitivity to moisture absorption in the 50-min long tests with the fibers immersed in a container filled with distillated water. Regarding the temperature responses, the coated FBGs presented higher sensitivity (13.84 pm/°C for NOA 88 and 12.76 pm/°C for NOA 68) than the uncoated FBGs due to the thermal expansion of the coatings. In the strain tests, all coated and uncoated samples presented similar sensitivities, but with a larger strain range applied for the coated samples (strains higher than 5500 µε) when compared with the uncoated samples (3500 µε). Moreover, the stress-strain curves of the coated samples indicated a Young’s modulus one order with magnitude lower than the one of the uncoated silica fiber, where the lowest Young’s modulus is 3.84 GPa and was obtained with the NOA 68 coating, which indicates the possibility of obtaining highly sensitive pressure and force sensors.

Diphenylalanine Nanotube Coated Fiber Bragg Grating for Methanol Vapor Detection

Etched fiber Bragg grating (EFBG) was coated with diphenilalanine nanotubes (DNT) to produce sensors capable of detecting methanol in methanol-ethanol blend vapors. The DNT was studied in two phases, hexagonal and orthorhombic, which were observed by Raman spectroscopy and scanning electron microscopy (SEM). The temperature transition between the phases was determined through small angle x-ray scattering (SAXS). Considering the methanol concentration in the vapor, the orthorhombic phase showed a sensitivity of -7.3 ± 0.8pm / (%v/v), which was 33 times greater than that observed for the hexagonal phase, with approximately the same response time. The maximum wavelength shift measured was -0.98 ± 0.02nm. Additionally, we proposed a mathematical model to describe the sensor's response, and we hypothesized the mechanisms that could lead to its nonlinear behavior.

Graphene-oxide-coated fiber Bragg grating sensor for ethanol detection in petrol

In this work, a study on a graphene oxide (GO)-coated fiber Bragg grating (FBG) sensor for ethanol detection in petrol is reported. The fiber Bragg gratings (FBGs) used in the present experiment were fabricated in-house using a high repetition rate (~5.5 kHz), with frequency-doubled copper vapour laser output radiation. The sensor head was made by partially removing the cladding of the FBG region using hydrofluoric acid (HF) and then coating a GO layer onto it. This GO-coated etched FBG (EFBG) shows a significant enhancement (by a factor of ten) in the detection of ethanol in comparison with un-coated EFBG. The present sensor is based on the measurements of change in intensity of the reflection spectra with the variation of concentration of ethanol in petrol. Ethanol detection using this technique is almost independent of the sample temperature within ±5 °C. A minimum of 0.5% ethanol in petrol was detected with GO-coated EFBG compared to 5% using un-coated EFBG. The sensitivity obtained with the GO-coated sensor was about 0.18 dBm/%.

Computational analysis of thermally induced stress in corrosion-resistant metal coated fiber optic sensors for oceanographic application

Sensors deployed for ocean observation are highly corroded by seawater. This study deals with the analysis of corrosion-resistant metal coated Fiber Bragg Grating (FBG) sensors for ocean temperature sensing. Firstly, the effect of thermally induced strain on FBG sensors coated with corrosion-resistant metal/alloy was computationally analyzed using Hookes law that reveals thermal strain sensitivity ranging from 28.6 μϵ/°C to 38.3 μϵ/°C for Stainless Steel 316L, Nickel and Nickel–Chromium alloy. This was followed by determining temperature sensing coefficient of FBG sensors coated with above mentioned metals/alloys that showed sensitivity ranging from 56.3 pm/°C to 75.3 pm/°C for two types of photosensitive fibers such as Germanium doped and Boron co-doped silica fibers for ocean temperature varying from 20 °C to 30 °C.

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.

Fiber-Optic Thermal Sensor for TiN Film Crack Monitoring.

The study focuses on the thermal and temperature sensitivity behavior of an optical fiber sensor device. In this article, a titanium nitride (TiN)-coated fiber Bragg grating (FBG) sensor fabricated using an ion beam sputtering system was investigated. The reflection spectra of the FBG sensor were tested using R-soft optical software to simulate the refractive index sensitivity. In these experiments, the temperature sensitivity of the TiN FBG was measured at temperatures ranging from 100 to 500 °C using an optical spectrum analyzer (OSA). The results showed that the temperature sensitivity of the proposed TiN FBG sensor reached 12.8 pm/°C for the temperature range of 100 to 300 °C and 20.8 pm/°C for the temperature range of 300 to 500 °C. Additionally, we found that the produced oxidation at temperatures of 400–500 °C caused a crack, with the crack becoming more and more obvious at higher and higher temperatures.

Detection of trace amounts of chromium(VI) using hydrogel coated Fiber Bragg grating

The present article investigates a hydrogel Coated Fiber Bragg grating (FBG) based sensor capable of detecting trace amounts of carcinogenic chromium(VI) metal ion. A stimulus responsive hydrogel encompassed with tetraalkylammonium salts which is specifically sensitive to Cr(VI) ion, is synthesized and coated on FBG. When the proposed sensor system is exposed to various concentrations of Cr(VI) ion solutions, FBG peak is shifted due to the mechanical strain induced by the stimulus swelling property of the hydrogel. The peak shift is correlated with the concentration of the Cr(VI) metal ion. Thus a Chemo-mechanical-optical sensing approach for the detection of Cr(VI) is studied. Trace amounts of chromium(VI) ion as low as 10ppb can be estimated by the method. The resolution of the sensor system is found to be 0.83 ppb. The sensor has shown good sensitivity, selectivity, and repeatability.

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.

Effects of the Dopant Ratio on Polyaniline Coated Fiber Bragg Grating for pH detection

Polyaniline (PAni) coated Fiber Bragg Grating (FBG) for pH detection is presented in this study. PAni was synthesized at low temperature through chemical oxidation by using aniline (Ani) as monomer, ammonium persulphate (APS) as oxidant and dioctyl sodium sulfosuccinate (AOT) as dopant. The immersion of PAni-coated FBG in different pH solutions could produce different Bragg wavelength shift (∼1557nm) in the optical spectrum analyzer (OSA). Fourier transformed infrared (FTIR) spectroscopy, ultraviolet–visible (UV–vis) spectroscopy and conductivity measurements were carried out to investigate the chemical interaction between protonated nitrogen atom of PAni backbone and pH analytes. The sensitivity of PAni-coated FBG was adjusted by changing the ratio of Ani:APS:AOT during the polymerization reaction. PAni-coated FBG with molar ratio of Ani:APS:AOT=5:5:5 achieved the highest sensitivity with response time of 10s. Besides, the PAni-coated FBG also showed good recyclability up to 10 times.

A study on intermediate buffer layer of coated Fiber Bragg Grating cryogenic temperature sensors

The sensor characteristics of a coated Fiber Bragg grating (FBG) thermal sensor for cryogenic temperatures depends mainly on the coating materials. The sensitivity of the coated FBG can be improved by enhancing the effective thermal strain transfer between the different layers and the bare FBG. The dual coated FBG's has a primary layer and the secondary layer. The primary coating acts as an intermediate buffer between the secondary coating and the bare FBG. The outer secondary coating is normally made of metals with high thermal expansion coefficient. In this work, a detailed study is carried out on chromium and titanium intermediate buffer layers with various coating thicknesses and combinations. To improve the sensitivity, the secondary coating layer was tested with Indium, Lead and Tin. The sensors were then calibrated in a cryogenic temperature calibration facility at Institute of Technical Physics (ITEP), Karlsruhe Institute of Technology. The sensors were subjected to several thermal cycles between 4.2 and 80 K to study the sensor performance and its thermal characteristics. The sensor exhibits a Bragg wavelength shift of 13pm at 20K. The commercially available detection equipment with a resolution of 1pm can result in a temperature resolution of 0.076 K at 20K.

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.