Commercial Fiber Bragg Grating Research Articles

Linear Fiber Laser Configurations for Optical Concentration Sensing in Liquid Solutions

In this study, different configurations based on linear fiber lasers were proposed and experimentally demonstrated to measure the concentration of liquid solutions. Samples of paracetamol liquid solutions with different concentrations, in the range from 52.61 to 201.33 g/kg, were used as a case-study. The optical gain was provided by a commercial bidirectional Erbium-Doped Fiber Amplifier (EDFA) and the linear cavity was obtained using two commercial Fiber Bragg Gratings (FBGs). The main difference of each configuration was the coupling ratio of the optical coupler used to extract the system signal. The sensing head corresponded to a Single-Mode Fiber (SMF) tip that worked as an intensity sensor. The results reveal that, despite the optical coupler used (50:50, 60:40, 70:30 or 80:20), all the configurations reached the laser condition, however, the concentration sensing was only possible using a laser drive current near to the threshold value. The configurations using a 70:30 and an 80:20 optical coupler allowed paracetamol concentration measurements with a higher sensitivity of (−3.00 ± 0.24) pW/(g/kg) to be performed. In terms of resolution, the highest value obtained was 1.75 g/kg, when it was extracted at 20% of the output power to the linear cavity fiber laser configuration.

Effect of spectrum broadening on photon-counting fiber Bragg grating sensing.

In a photon-counting fiber Bragg grating (FBG) sensing system, a shorter probe pulse width reaches a higher spatial resolution, which inevitably causes a spectrum broadening according to the Fourier transform theory, thus affecting the sensitivity of the sensing system. In this work, we investigate the effect of spectrum broadening on a photon-counting FBG sensing system with a dual-wavelength differential detection method. A theoretical model is developed, and a proof-of-principle experimental demonstration is realized. Our results give a numerical relationship between the sensitivity and spatial resolution at the different spectral widths of FBG. In our experiment, for a commercial FBG with a spectral width of 0.6 nm, an optimal spatial resolution of 3 mm and a corresponding sensitivity of 2.03 nm-1 can be achieved.

Simultaneous Implementation of the High Reliability and Accuracy in a Fiber Bragg Grating Sensing System for Space Application

The complex space environment puts forward more stringent requirements for the reliability and accuracy of sensors and instrumentation. In this paper, a dual light sources multiplexing fiber Bragg grating (FBG) sensing system with high reliability and accuracy for space application is proposed. The proposed FBG sensing system has two independently driven light sources, which emit scanning laser light into the FBG sensors. The proposed system eliminates single-point failures (SPOF) compared with other system configurations. The five-year single channel reliability and the five-year all channels reliability are 0.9802 and 0.9349 respectively, which are the highest among the five systems. The reliability of the proposed system is improved by 14.1% compared with the common commercial FBG sensing system. Moreover, the experimental systems were established and two FBGs were tested six times on a constant temperature heating stage. The average coefficient of determination (COD) R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of FBG1 and FBG2 are 0.9112 and 0.9058 in the proposed system, which are 4.9% and 8.4% higher than those in the common commercial FBG sensing system owing to higher wavelength resolution. Therefore, the proposed dual light sources multiplexing FBG sensing system will provide higher reliability and accuracy performance in space applications.

Temperature-independent multi-parameter sensor based on polarization maintaining fiber Bragg grating

Dynamic multi-parameter detection is of great significance in predicting fatigue damage to structures such as tunnels, bridges, and pipelines. Developing a high-sensitivity, environmentally friendly, low-cost, and easy-to-operate multi-parameter dynamic detection technology has always been the goal of the industry. The polarization-maintaining fiber Bragg grating (PM-FBG) has a special grating structure composed of fiber Bragg grating (FBG) directly written into high birefringence and polarization-maintaining fiber, and it supports two distinct polarization eigenmodes with two effective refractive indices. The PM-FBG couples the light beams polarized along the two principal axes corresponding to slow axis and fast axis at two different Bragg wavelengths. The two peaks of PM-FBG have different responses to external changes, which may be used to solve the cross-sensitivity problem of FBG sensor and realize the simultaneous multi-parameter measurement of the temperature, longitudinal strain, transverse strain, or twist. In order to solve the problems of complex structure and principle and high production cost of FBG-based multi-parameter sensors, a novel multi-parameter fiber-optic sensor with high sensitivity and temperature independence is designed based on PM-FBG in this work. The PM-FBG sensor proposed can simultaneously measure the changes of displacement and twist in two vertical directions at a certain point and has the function of temperature self-compensation. The external structure of the sensor is fabricated by using three-dimensional printing technology through the fused deposition method and the raw material for creating different components through using polylactic acid. Experimental results show that the fast axis and slow axis of the sensor have different temperature responses, with linear sensitivities of 11.4 pm/℃ and 10.6 pm/℃, respectively, and the temperature compensation coefficient and average torsional sensitivity of the PM-FBG sensor are 0.8 pm/℃ and 0.20 dB/(°), respectively. The fast axis and slow axis of the PM-FBG sensor have the same response to displacement, with a sensitivity of 31.5 pm/mm and an adjustable range of 0–20 mm. The sensitivity to displacement, torsion, and temperature sensitivities of the sensor are all superior over those of commercial FBG sensors. By changing the temperature field around the sensor, its displacement- and torsion-sensing performances are not affected, thereby realizing the temperature self-compensation. Consequently, the proposed sensor has potential applications in the multi-parameter dynamic detection due to its simple structure, high sensitivity, good mechanical strength, and low cost.

Optical Magnetostrictive Current Sensor Based on In-Fiber Fabry–Pérot Cavity

In this article, a compact fiber-optic current sensor (FOCS) based on an in-fiber Fabry–Pérot interferometer (FPI) and a magnetostrictive transducer is presented. The in-fiber FPI sensor employed in the proposed FOCS was fabricated with a 273- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> Section of a capillary fiber spliced between two pieces of a standard single-mode fiber (SMF). The in-fiber FPI sensor and a commercial fiber Bragg grating (FBG), which was used as a reference sensor for sensitivity comparison, were bonded to a small magnetostrictive actuator (Terfenol-D) and assembled in the proposed FOCS for electric current monitoring up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$800 {A}_{\text {rms}}$ </tex-math></inline-formula> using a commercial FBG interrogator. The in-fiber FPI sensor presented a sensitivity 84.4% higher than that of the FBG sensor. An alternative interrogation system based on an edge-filter technique with temperature compensation was also developed and employed for the characterization of the proposed FOCS under the same range of electric current. Using the edge-filter interrogation system developed, the in-fiber FPI sensor presented a sensitivity 169.8% higher than that shown by the FBG sensor. The results show that the proposed FOCS using an in-fiber FPI sensor presented high sensitivity to electric current sensing and, therefore, is a potential candidate for current monitoring in complement to conventional current transformers (CTs) as well as to conventional optical CTs (OCTs).

High-Precision Magnetic Field Sensor Based on Fiber Bragg Grating and Dual-Loop Optoelectronic Oscillator

A novel fiber-optic magnetic field sensor with high interrogation speed and resolution by using an etched fiber Bragg grating (FBG) in conjunction with a dual-loop optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. A commercial FBG is firstly dipped into mixed hydrofluoric acid solution to remove the cladding layer and then is embedded with the magnetic fluid (MF) as a sensing element. The central wavelength reflected from the FBG is related to the overall time delay of the dual-loop OEO, which determines the oscillating frequency of the OEO. Therefore, the magnetic field can be estimated by measuring the oscillating frequency shift of OEO. The experimental results show that the oscillating frequency linearly increases with the increment of the magnetic field, achieving the sensitivity of 16.3 Hz/Oe with a R-square of 0.991 in the range of 5 mT–10 mT. In addition, the maximum error is within ±0.05 mT in the range of 7 mT–8 mT, which offers potentials in many fields where the high-precision magnetic field measurement is required.

Highly sensitive bending sensor based on Mach-Zehnder interferometer built in Sagnac loop with specified offset

A high sensitivity bending sensor based on Mach-Zehnder interferometer (MZI) built in Sagnac loop is proposed and experimentally demonstrated. One stress zone in Panda type polarization maintaining fiber (PMF) was intentionally collimated to the core of single mode fiber (SMF) and then the discharge was performed by fusion splicer. A commercial fiber Bragg grating (FBG) acting as phase shift monitor was fusion spliced to the PMF. The prepared fiber optical configuration was connected to a 3 dB coupler to construct a Sagnac loop. The spectral characteristics of the proposed bending sensor as a function of curvature were investigated experimentally. The bending sensitivity of 12.5 nm/m−1 was obtained experimentally. Furthermore, the π phase shift was detected when the curvature was 0.587 m−1. Such bending sensor with simple fabrication and cost-efficient could be a good candidate in the field of curvature measurements.

Fibre Bragg Grating Based Acoustic Emission Measurement System for Structural Health Monitoring Applications.

Fiber Bragg grating (FBG)-based acoustic emission (AE) detection and monitoring is considered as a potential and emerging technology for structural health monitoring (SHM) applications. In this paper, an overview of the FBG-based AE monitoring system is presented, and various technologies and methods used for FBG AE interrogation systems are reviewed and discussed. Various commercial FBG AE sensing systems, SHM applications of FBG AE monitoring, and market potential and recent trends are also discussed.

High sensitivity strain sensor based enhanced polarized mode coupling and FBG sensitization

A highly sensitive strain sensor based on enhanced polarized mode coupling is proposed and demonstrated experimentally. The proposed Sagnac interferometric strain sensor comprises a segment of Panda-type polarization maintaining fiber (PMF) serving as a sensing component and a commercial fiber Bragg gratings (FBG) serving as a sensitizing component. The sensitization effect of FBG has been analyzed and demonstrated experimentally. The core of single mode fiber is intentionally collimated and fusion spliced to one stress applied zone built-in Panda type PMF to excite effectively the polarized mode. The spectra shift towards long wavelength with increasing strain. The measured strain sensitivity reaches up to 176 pm/με. The proposed fiber strain sensor characterizing simple fabrication and high sensitivity can be a good candidate in the field of strain measurements.

Cold Crack Monitoring and Localization in Welding Using Fiber Bragg Grating Sensors

A study on the design, framework, experimental arrangement, data acquisition, and subsequent analysis of cold crack detection using fiber Bragg grating (FBG) sensors during welding is presented. Cold crack induced acoustic emission (AE) events are detected by multiple FBG sensor configurations and using a commercial FBG AE interrogation system. The crack locations are theoretically predicted using a method of linear localization based on the differences in the time of arrival of the AE events at different FBG sensors. The approach is experimentally validated by employing packaged and unpackaged FBG sensor arrays in a welding experiment with gas tungsten arc welding on ultrahigh toughness high hardness armor steel. Frequency analysis of the AE events is also carried out to understand the cold crack characteristics. The weld region of the steel plate is diced into 1-mm cross-sectional pieces, and the positions of the cold cracks are identified using scanning electron microscope imaging and then cross correlated with the measured AE events, and thereby confirming the correct identification of cold crack location using the FBG sensing method.

All optical fiber thermal vacuum gauge

A new concept of an all optical, dual-fiber-based Pirani thermal vacuum gauge is proposed and demonstrated. The configuration utilizes two fibers: one that produces heat, and one that responds to the resultant thermal exchange. The temperature of the latter fiber is a function of the heat transfer through the gas in which it resides. The active heat-generating fiber is a luminescence-quenched, heavily Yb3+-doped optical fiber that efficiently produces thermal energy when optically pumped. The temperature sensor is implemented with a conventional commercial fiber Bragg grating. Both fibers are inserted into a custom vacuum chamber whose internal pressure can be carefully controlled. Performance of the system is characterized with pressures ranging from 20 mTorr to Standard Pressure. The proposed system may also be used as a sensitive flow rate sensor.

Cross-sectional refractive-index variations in fiber Bragg gratings measured by quantitative phase imaging

Unexpected micrometer-scale patterns in the induced refractive index of various commercial fiber Bragg gratings (FBGs) are observed in the cross-sectional fiber directions, which are in addition to the expected periodic variations along the fiber axis. These measurements were made using 3D tomographic deconvolution phase microscopy, a type of quantitative phase imaging. The cross-sectional patterns observed are shown to exhibit a variety of appearances, including fringes normal to the fiber axis and radial blades, the details apparently depending on the FBG writing method.

Research on FBG-Based Sensor Networks and Their Coexistence with Fiber Optical Transmission Systems

Market forecasts and trends for the usage of fiber optical sensors confirm that demand for them will continue to increase in the near future. This article focuses on the research of fiber Bragg grating (FBG) sensor network, their applications in IoT and structural health monitoring (SHM), and especially their coexistence with existing fiber optical communication system infrastructure. Firstly, the spectrum of available commercial optical FBG temperature sensor was experimentally measured and amplitude-frequency response data was acquired to further develop the simulation model in the environment of RSoft OptSim software. The simulation model included optical sensor network, which is combined with 8-channel intensity-modulated wavelength division multiplexed (WDM) fiber optical data transmission system, where one shared 20 km long ITU-TG.652 single-mode optical fiber was used for transmission of both sensor and data signals. Secondly, research on a minimal allowable channel spacing between sensors’ channels was investigated by using MathWorks MATLAB software, and a new effective and more precise determination algorithm of the exact center of the sensor signal’s peak was proposed. Finally, we experimentally show successfully operating coexistence concept of the spectrum-sliced fiber optical transmission system with embedded scalable FBG sensor network over one shared optical fiber, where the whole system is feed by only one broadband light source.

Critical Issues of Double-Metal Layer Coating on FBG for Applications at High Temperatures

Use of fiber Bragg gratings (FBGs) to monitor high temperature (HT) applications is of great interest to the research community. Standard commercial FBGs can operate up to 600 C. For applications beyond that value, specific processing of the FBGs must be adopted to allow the grating not to deteriorate. The most common technique used to process FBGs for HT applications is the regeneration procedure (RP), which typically extends their use up to 1000 C. RP involves a long-term annealing of the FBGs, to be done at a temperature ranging from 550 to 950 C. As at that temperature, the original coating of the FBGs would burn out, they shall stay uncoated, and their brittleness is a serious concern to deal with. Depositing a metal coating on the FBGs prior to process them for RP offers an effective solution to provide them with the necessary mechanical strengthening. In this paper, a procedure to provide the FBG with a bimetallic coating made by copper and nickel electrodeposition (ED) is proposed, discussing issues related to the coating morphology, adherence to the fiber, and effects on the grating spectral response. To define the processing parameters of the proposed procedure, production tests were performed on dummy samples which were used for destructive SEM–EDS analysis. As a critical step, the proposed procedure was shown to necessitate a heat treatment after the nickel ED, to remove the absorbed hydrogen. The spectral response of the FBG samples was monitored along the various steps of the proposed procedure and, as a final proof test for adherence stability of the bimetallic coating, along a heating/cooling cycle from room temperature to 1010 C. The results suggest that, given the emergence of Kirkendall voids at the copper–nickel interface, occurring at the highest temperatures (700–1010 C), the bimetallic layer could be employed as FBG coating up to 700 C.

Perrogator: A Portable Energy-Efficient Interrogator for Dynamic Monitoring of Wavelength-Based Sensors in Wearable Applications.

In this paper, we report the development of a portable energy-efficient interrogator (Perrogator) for wavelength-based optical sensors. The interrogator is based on a compact solution encompassing a white light source and the spectral convolution between the sensor and a tunable filter, which is acquired by a photodetector, where a microcontroller has two functions: (i) To control the filter tuning and to (ii) acquire the photodetector signal. Then, the data is sent to a single-board computer for further signal processing. Furthermore, the employed single-board computer has a Wi-Fi module, which can be used to send the sensors data to the cloud. The proposed approach resulted in an interrogator with a resolution as high as 3.82 pm (for 15.64 nm sweeping range) and maximum acquisition frequency of about 210 Hz (with lower resolution ~15.30 pm). Perrogator was compared with a commercial fiber Bragg grating (FBG) interrogator for strain measurements and good agreement between both devices was found (1.226 pm/µε for the commercial interrogator and 1.201 pm/µε for the proposed approach with root mean square error of 0.0144 and 0.0153, respectively), where the Perrogator has the additional advantages of lower cost, higher portability and lower energy consumption. In order to demonstrate such advantages in conjunction with the high acquisition frequency allowed us to demonstrate two wearable applications using the proposed interrogation device over FBG and Fabry-Perot interferometer (FPI) sensors. In the first application, an FBG-embedded smart textile for knee angle assessment was used to analyze the gait of a healthy person. Due to the capability of reconstructing the FBG spectra, it was possible to employ a technique based on the FBG wavelength shift and reflectivity to decouple the effects of the bending angle and axial strain on the FBG response. The measurement of the knee angle as well as the estimation of the angular and axial displacements on the grating that can be correlated to the variations of the knee center of rotation were performed. In the second application, a FPI was embedded in a chest band for simultaneous measurement of breath and heart rates, where good agreement (error below 5%) was found with the reference sensors in all analyzed cases.

Integrated Dynamic Wavelength Division Multiplexed FBG Sensor Interrogator on a Silicon Photonic Chip

A wavelength division multiplexed fiber Bragg grating (FBG) sensor interrogator using an integrated unbalanced Mach–Zehnder interferometer followed by an arrayed waveguide grating on silicon-on-insulator platform is presented. The Bragg wavelength shift is determined through the phase-generated carrier demodulation technique, allowing accurate dynamic FBG interrogation. The performance of the proposed device was validated by comparing the measurements with a commercial FBG readout unit based on a spectrometer, serving as a reference. Experimental results demonstrate a dynamic strain resolution of 4.56 nϵ/√Hz and minimum detectable strain down to ∼0.49 μ ϵ at 1-kHz bandwidth.

Temperature Monitoring With Fiber Bragg Grating Sensors in Nonuniform Conditions

Fiber optics is the elective technology for sensing temperatures in harsh environments. Among the possible exploitable working principles, fiber Bragg gratings (FBGs) are the most widespread implementation for their excellent balance between system complexity and performance. However, despite they are well known and established, so far, metrological investigations have been limited to cases in which the sensor is used in uniform temperature conditions. This paper analyzes the response of commercial FBGs employed as temperature sensors in applications that imply large temperature gradients, such as in laser-based thermal treatments of solid tumors. A theoretical model of the sensor is implemented first and then used to evaluate its response in nonuniform temperature distributions. The model results are experimentally validated by means of a liver phantom and comparisons between different grating configurations, such as single or multiplexed with different lengths, are made.

Real-time estimation of delamination occurrence induced by low-velocity impact in composite plates using optical fiber sensing system

Low-velocity impact is the one of the most critical events to reduce the reliability of composite structures, because it can generate hidden damages such as delamination inside the structures. In order to address this problem, impact monitoring systems have been suggested using various types of built-in sensors. Generally, such impact monitoring systems are consisted of impact localization and damage assessment. In this paper, a methodology of impact-induced delamination assessment was studied using fiber Bragg grating (FBG) sensors. Because of its multiplexing capability, FBG sensors are advantageous of monitoring large areas of target structures. However, it is hard to be applied in impact monitoring systems due to its low sensitivity and narrow bandwidth. In this study, a quantification method of delamination was examined with maintaining the merits of FBG sensors. For the acquisition of impact-generated acoustic emission (AE) signals, the commercial FBG system was adopted with a sampling frequency of 100 kHz. Then, the acquired AE signals during the low-velocity impact fracture tests were analyzed with the wavelet transform (WT) method for measuring the delamination areas due to different energies. Finally, it can be concluded that the quantities of WT detailed portions are linearly correlated with the delamination areas.

Study of Optical Fiber Sensors for Cryogenic Temperature Measurements.

In this work, the performance of five different fiber optic sensors at cryogenic temperatures has been analyzed. A photonic crystal fiber Fabry-Pérot interferometer, two Sagnac interferometers, a commercial fiber Bragg grating (FBG), and a π-phase shifted fiber Bragg grating interrogated in a random distributed feedback fiber laser have been studied. Their sensitivities and resolutions as sensors for cryogenic temperatures have been compared regarding their advantages and disadvantages. Additionally, the results have been compared with the given by a commercial optical backscatter reflectometer that allowed for distributed temperature measurements of a single mode fiber.

Fiber Optic Measurement System for Fresnel Reflection Sensing: Calibration, Uncertainty, and Exemplary Application in Temperature-Modulated Isothermal Polymer Curing

Fiber Bragg grating (FBG) sensors and Fresnel reflectometer sensors have emerged as promising tools for monitoring process state parameters during composite manufacturing. However, multifunctional fiber optic sensor systems exploiting the versatile nature of these sensors are not commercially available. In principle, most FBG measurement systems are equipped with the required hardware to measure the refractive index (RI) by interrogating Fresnel reflectometer sensors. However, the implications of using an FBG system for this purpose have not been previously studied. Therefore, we calibrated a commercial FBG measurement system in the refractive index range between 1.3 and 1.6 at 1550 nm, based on data comparison with an independent RI measurement technique, and determined the measurement uncertainty. Additionally, we investigate the suitability of the proposed Fresnel reflectometer sensor for use in temperature-modulated isothermal epoxy curing experiments for detecting the glass transition by exploiting a concept that resembles temperature modulated optical refractometry. This paper describes in detail the calibration routine, the determination of the measurement uncertainty, and the application of Fresnel reflectometer sensors for glass transition detection during isothermal curing. We observed an offset between the measured RI and the theoretical value in the range of $10^{{{\hbox{--}} 3}}$ . The measurement system uncertainty was found to be of the order of $10^{{{\hbox{--}} 4}}$ . That is comparable to the accuracy of most commercial refractometers. Furthermore, we showed that Fresnel reflectometer sensors are suitable for glass transition detection during isothermal curing. The results show that commercial FBG measurement systems are viable options for multifunctionality sensing in online process monitoring applications.