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The influence of bitumen coating on the development of unit shaft resistance along driven steel and precast concrete piles resulting from subsiding surrounding soft soil (gyttja) induced by fill placement at terrain was investigated. All piles were instrumented with conventional discrete-point vibrating wire strain gauges and distributed fibre optic sensors to achieve high-resolution strain measurements. The magnitude of the mobilised unit shaft resistance along uncoated piles was observed to be primarily related to an increase in effective stress resulting from the dissipation of excess pore water pressures. The unit shaft resistance along bitumen-coated piles was found to be primarily related to the rate of relative movement between pile and soil, which highlights the effectiveness of bitumen coating in reducing shaft resistance. [ABSTRACT FROM AUTHOR]

This paper proposes and implements a novel scheme for recording signals from fibre optic sensors based on tandem low-coherence interferometry with an integrated optical reference interferometer. The circuit allows precision control of the phase shift. Additionally, the paper illustrates the potential for detecting vibration and object deformation using fibre optic Fabry–Perot sensors connected to the registration system.

In any high-rise building construction based on piles, it is essential to correctly evaluate their response when subject to high loads to avoid oversizing and consequently high costs. This work falls within the framework of the FONDASILEX project, which studies the behaviour of the pile foundations and the soil within the 'Silex2' tower project built in the Part-Dieu district in Lyon, France. This paper presents the geotechnical instrumentation executed for the foundations and the soil to enable the real-time monitoring of their behaviour. It details its conception, execution and limits. Five different types of electrical and unconventional sensors, including the fibre optic technique, which is based on reflectometry by Rayleigh scattering, were employed. This instrumentation allowed us to measure the soil settlement, the applied load at the head of the pile and the induced deformation in the concrete. The obtained measurements showed good agreement between all the sensors and were compatible with the real applied load. The results of this monitoring completed with an additional specific study to characterize the soil will be used in a future study to calibrate numerical models simulating the behaviour of high-rise building foundations. [ABSTRACT FROM AUTHOR]

Abstract In this study, a surface plasmon resonance sensor for detecting haemoglobin concentration based on a D‐type optical fibre with a graphene‐gold surface architecture is proposed. The graphene‐gold surface architecture included a 50 nm thick gold film and monolayer graphene film, which were decorated on a 10 mm long D‐type sensing region. The proposed sensor worked in wavelength interrogation mode, with the light wavelength ranging from 400 to 1100 nm. The authors realised the D‐type optical fibre surface polishing process, the gold film vacuum coating process, and the chemical‐vapour‐deposited graphene's wet transfer process. Furthermore, the fabricated sensors were used to detect the refractive index (RI) of haemoglobin samples, which varied from 1.331 to 1.346. Experiment results show that the fitted RI sensitivity of the sensor decorated with gold and graphene reaches 1874.41 nm/RIU, 4.995% higher than that of the sensor decorated only with gold. The concentration sensitivity of the sensor coated with gold and graphene film is 4.96 nm/(g/dL), and the proposed sensor can provide a resolution of 20.2 mg/dL for haemoglobin concentration detection.

In civil engineering, the analysis and characterisation of structural phenomena is key for the deviation of mechanical models and strongly depends on experimental studies. For this reason, the development of new measurement techniques plays a crucial role in research to allow capturing the mechanical behaviour of structures. In this context, distributed fibre optic sensing (DFOS) gained attention during the last years. Advantageous properties such as minimal invasiveness and quasi-continuous strain measurement enable new possibilities in structural monitoring. DFOS is able to sense minimal strain variations, yet this often results in recording unwanted anomalies. Thus, to generate the best results from raw measurement data, it is crucial to use a robust and reliable post-processing procedure. In order to meet this requirement, the software solution FOS Evaluator was developed using the Python programming language, targeting the most important aspects that may occur when using DFOS. Those aspects comprise a high data volume, disturbances and anomalies in the measured data and the necessity to carry out calculations with acquired data sets. In this paper, functionality and background of FOS Evaluator are presented and discussed. Furthermore, several methods for post-processing and evaluating DFOS measurements are presented, consisting of multiple functions for reducing, cleaning, or smoothing strain data, and calculating various mechanical properties from filtered measurements. Finally, the functional scope of FOS Evaluator is illustrated by various application examples in structural concrete. [ABSTRACT FROM AUTHOR]

Investigation of the bond behaviour of prestressed CFRP strands in concrete with fibre optic sensors Due to their high tensile strength and corrosion resistance, carbon fibre reinforced polymers (CFRP) are suitable as reinforcement of concrete elements, especially as tendons. Despite compliance with the relevant recommendations for the concrete cover of pretensioned carbon concrete members, tensile splitting cracks can occur in the concrete even at moderate prestressing levels compared to steel tendons, as investigations at the TU Berlin with pretensioned CFRP strands have shown. In this context, the bond behaviour of steel and CFRP strands in concrete was investigated by means of tests on transfer length. For the first time in this type of experiment, distributed fibre optic sensors (DFOS) were used to reliably determine transfer lengths, providing a high spatial resolution strain measurement. The results show significantly higher bond stresses in the CFRP strands due to their higher tensile strength, which allows for shorter transfer lengths, but can lead to splitting cracking. In addition to the concrete cover, the prestressing level was determined as a significant influencing factor for splitting cracks. The tests carried out are described in detail and the test results as well as the new insights gained through DFOS are presented and discussed. [ABSTRACT FROM AUTHOR]

In this study, a surface plasmon resonance sensor for detecting haemoglobin concentration based on a D‐type optical fibre with a graphene‐gold surface architecture is proposed. The graphene‐gold surface architecture included a 50 nm thick gold film and monolayer graphene film, which were decorated on a 10 mm long D‐type sensing region. The proposed sensor worked in wavelength interrogation mode, with the light wavelength ranging from 400 to 1100 nm. The authors realised the D‐type optical fibre surface polishing process, the gold film vacuum coating process, and the chemical‐vapour‐deposited graphene's wet transfer process. Furthermore, the fabricated sensors were used to detect the refractive index (RI) of haemoglobin samples, which varied from 1.331 to 1.346. Experiment results show that the fitted RI sensitivity of the sensor decorated with gold and graphene reaches 1874.41 nm/RIU, 4.995% higher than that of the sensor decorated only with gold. The concentration sensitivity of the sensor coated with gold and graphene film is 4.96 nm/(g/dL), and the proposed sensor can provide a resolution of 20.2 mg/dL for haemoglobin concentration detection. [ABSTRACT FROM AUTHOR]

The use of sensors to detect or measure ripening changes in fruit is a growing area of interest to the scientific community. Colorimeters are commonly employed for color and shade identification; however, their usage to measure the color parameters of fruit rinds based on image analysis can be expensive. This article presents a non-invasive and low-cost optoelectronic system for detecting color changes in oranges to predict the ripening stage. The system utilises a 1 W white LED as a light source, an RGB photodiode array, and two plastic optical fibres bundled in parallel to form the head of an extrinsic sensor. A microcontroller is employed for model integration and data acquisition. The evolution of the skin color of the fruit was monitored until over-ripeness was evident. The sensor was designed to detect the color changes; the CIE L*a*b* color difference between the optoelectronic device results and those obtained by colorimetry was 2.6–4.5. To predict the ideal conditions for fruit handling and determine the maturity level, a multilevel perceptron ANN was trained, achieving an accuracy of 96.4%. In addition, an overall precision of 96.6% was achieved when classifying fruit into three maturity categories (under-ripe, ripe, and over-ripe), and the error was 3.4%. The combination of the optoelectronic device and ANN improves considerably this fruit color classification accuracy, can facilitate the determination of the optimal time for consumption, and optimize the postharvest process efficiency.

In recent decades, the urgency to protect and upgrade cultural heritage structures (CHS) has become of primary importance due to their unique value and potential areas of impact (economic, social, cultural, and environmental). Structural health monitoring (SHM) and the management of CHS are emerging as decisive safeguard measures aimed at assessing the actual state of the conservation and integrity of the structure. Moreover, the data collected from SHM are essential to plan cost-effective and sustainable maintenance solutions, in compliance with the basic preservation principles for historic buildings, such as minimum intervention. It is evident that, compared to new buildings, the application of SHM to CHS is even more challenging because of the uniqueness of each monitored structure and the need to respect its architectural and historical value. This paper aims to present a state-of-the-art evaluation of the current traditional and innovative SHM techniques adopted for CHS and to identify future research trends. First, a general introduction regarding the use of monitoring strategies and technologies for CHS is presented. Next, various traditional SHM techniques currently used in CHS are described. Then, attention is focused on the most recent technologies, such as fibre optic sensors and smart-sensing materials. Finally, an overview of innovative methods and tools for managing and analysing SHM data, including IoT-SHM systems and the integration of BIM in heritage structures, is provided. [ABSTRACT FROM AUTHOR]

Abstract It is shown experimentally a new digital optical decoding scheme based on the transmission or polarized light at p polarization planes using a K‐Nearest Neighbour (KNN) algorithm through a single‐mode optical fibre at 633 nm. The optical power signal is sent at p polarization planes which constitute p classes required for signal bit recognition. Results show that it is possible to recognize 32 polarizations planes, 5 bits, using 4 features corresponding to the measurement of optical power at 4 different angles at the photodetector side with an average assertiveness of 99.02%.

Abstract This letter proposes a novel dynamic range extension method using an instantaneous frequency (IF)‐tracking method for heterodyne fibre optic hydrophones (FOHs). IFs are used to compensate for large signal amplitudes in the demodulator. Theoretical analysis and simulation prove the effectiveness of the proposed algorithm. Simulation results achieve at least 19.76 dB dynamic range improvement, demonstrating great potential for extending FOHs for hydroacoustic detection and communication applications.

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Abstract Optical fibre sensors with modified cladding have been a topic of great interest for decades. Thanks to the new techniques of fabricating nanostructured films, this interest has been renewed. Although there have been many increases in published experimental research, no comprehensive theoretical formulation of the structure is reported in the literature. In this study, a general theory for transmissive modified cladding optical fibre sensors is presented. The model is formulated based on the electromagnetic fields and optical power in the core and cladding of a step‐index fibre. The model considers various parameters, especially the radius of the optical fibre core, modified cladding refractive index, and light wavelength. The sensitivity, response, and optical transmission of the sensor are analysed as a function of the parameters. The analyses show that optical transmission reduces dramatically when the modified cladding refractive index approaches the refractive index of the core of the fibre and results in higher sensor response and sensitivity. Increasing the core radius of the fibre will result in lower sensor response with higher sensitivity. Both parameters will increase for a light with a larger wavelength. The results are in good agreement with experimental evidence.

Abstract In order to improve the sensitivity and accuracy of oil‐paper insulation partial discharge detection by the fibre‐optic Fabry–Perot (FP) ultrasonic sensor, this work studied the ultrasonic signal’s frequency characteristics of typical oil‐paper insulation partial discharges and the vibration characteristics of the FP sensor’s diaphragm in the liquid environment. Based on the works above, a multifrequency FP sensor array is proposed, consisting of several FP sensors with different resonant frequencies to detect partial discharges. The experimental results show that the liquid environment has a significant effect on the vibration characteristics of the FP sensor’s diaphragm, and the sensitivity and accuracy of partial discharge detection can be improved based on the multifrequency FP sensing array, which can also be applied in the pattern recognition and localisation of partial discharges.

It is shown experimentally a new digital optical decoding scheme based on the transmission or polarized light at p polarization planes using a K‐Nearest Neighbour (KNN) algorithm through a single‐mode optical fibre at 633 nm. The optical power signal is sent at p polarization planes which constitute p classes required for signal bit recognition. Results show that it is possible to recognize 32 polarizations planes, 5 bits, using 4 features corresponding to the measurement of optical power at 4 different angles at the photodetector side with an average assertiveness of 99.02%. [ABSTRACT FROM AUTHOR]

This letter proposes a novel dynamic range extension method using an instantaneous frequency (IF)‐tracking method for heterodyne fibre optic hydrophones (FOHs). IFs are used to compensate for large signal amplitudes in the demodulator. Theoretical analysis and simulation prove the effectiveness of the proposed algorithm. Simulation results achieve at least 19.76 dB dynamic range improvement, demonstrating great potential for extending FOHs for hydroacoustic detection and communication applications.This letter proposes an instantaneous frequency‐tracking method to extend the dynamic range for heterodyne fibre optic hydrophones (FOHs). They are used directly to compensate for large signal amplitudes. The working principle is discussed, and simulations are conducted. The simulations achieve at least 19.76 dB dynamic range improvement. [ABSTRACT FROM AUTHOR]

Abstract A fibre loop mirror sensor is proposed and demonstrated for strain and temperature measurements in experiment. In these schemes, fibre loop mirrors are constructed with single‐mode‐multimode‐polarization‐maintaining‐multimode‐single‐mode optical fibre (SMPMS) structures. The strain and temperature characteristics of the sensor, depending on the lengths of multimode fibre (MMF) and polarization‐maintaining fibre (PMF), are studied in the experiment. The results indicate that PMF and MMF lengths have less impact on strain sensitivity but a remarkable impact on temperature sensitivity, which is consistent with the theoretical analysis. The best strain and temperature sensitivities of an SMPMS structure sensor can reach up to 39.0 pm/με and 2.366 nm/°C, respectively. The sensors have the merits of easy fabrication, cost‐efficiency and high temperature sensitivity and are quite suitable for fields requiring high‐precision measurement.

Abstract This paper presents a long‐range distributed fibre‐optic temperature sensor with high sensing accuracy to monitor temperature using Brillouin scattering mechanism. Fourier deconvolution, Fourier wavelet regularised deconvolution, dichotomised singular value decomposition and hybrid algorithms were employed to measure the temperature of proposed sensor. Optical time domain reflectometry technique is used to extract the temperature profile for 70, 60 and 50 km sensing ranges. The improvement of temperature accuracy using hybrid algorithm is achieved for 70 km sensing distance. In order to extract temperature information, the numerical simulation process has been incorporated in the proposed long‐range sensing system. Moreover, the temperature accuracy of the proposed system is obtained using Fourier deconvolution, Fourier wavelet regularised deconvolution, dichotomised singular value decomposition and hybrid algorithms for 70, 60 and 50 km sensing ranges. The simulation results show that the temperature accuracy is observed as 4.8°K using hybrid algorithm compared to Fourier deconvolution algorithm that shows temperature accuracy as 14°K at 70 km. In the proposed sensor, a spatial resolution of 20 m is observed for a sensing range of 70 km using hybrid algorithm. In addition, the denoising competence of the proposed hybrid algorithm is validated using 50 km optical time domain reflectometry backscattered trace observed experimentally. Further, a peak dynamic range improvement of optical time domain reflectometry backscattered trace as 3.91 dB is observed using hybrid algorithm.

Abstract We propose highly sensitive trichannel photonic crystal fibre (TCPCF) refractive index (RI) sensor based surface plasmon resonance (SPR). In this design, we have been integrated the sensing layer with thin gold metal deposited over the surface of the TCPCF for fabrication feasible and easy liquid infiltration. Then, Y‐shaped channel is formed through reducing the diametre of air holes for controlling the direction of light through photonic crystal fibre to attain high sensitivity. Also, effective coupling between surface plasmon polariton mode at the metal layer and the core‐guided mode is obtained by numerical simulation of finite element method. The optimized TCPCF sensor exhibits maximum amplitude sensitivity of 1938.5 RIU−1 and wavelength sensitivity of 11,000 nm.RIU−1 within the analyte range from 1.34 to 1.39. Further, we have achieved amplitude resolution of 5.2 × 10−6 RIU and wavelength resolution of 9.1 × 10−6 RIU. It is confirmed high detection accuracy for small changes in RI analyte. Moreover, high figure of merit of 623.9 RIU−1 is obtained for the proposed sensor. Therefore, the proposed TCPCF RI sensor can be utilized in the chemical, bio and gas sensing applications.

Abstract Online fault detection, isolation and recovery using smart sensors play an important role in intelligent manufacturing system. Fibre optic sensors are very interesting for condition monitoring applications due to the advantage of this technology. Here, the experimental demonstration of Cu‐based shape memory alloy (SMA) coated optical fibre for temperature‐based sensing applications is reported. The benefit of Cu‐based SMA coated optical fibre over conventional metallic coating has been evaluated in the study. For consistent coating, an in situ fixture with a rotary drive setup has been designed and developed. Thermo optic test bench has been developed to study the actuation characteristics of the SMA coated optical fibre for varying current and voltage. Experiments were performed to investigate the light intensity in the SMA coated optical fibre at different actuation conditions. The displacement that takes place in the optical fibre due to the external temperature stimuli will create proportional intensity and wavelength shifts. The maximum average displacement of 4.9 mm has been achieved for Cu‐Al‐Mn‐Ni coated optical fibre. Results show variation in the optical fibre signal due to heating and cooling of the fibre from the applied electrical stimulus on Cu‐based SMA coating in the form of austenite to martensite transformation.

Mineral-impregnated carbon fibre (MCF) is a new type of reinforcement that has superior mechanical properties at elevated temperatures to conventional polymer-impregnated composites. However, its specific manufacturing process results in an inherent looseness, which adversely affects its structural performance under loading. Therefore, chemical prestressing was used herein to enhance the structural performance of the MCF reinforcement by introducing tensile stress inside this reinforcement, which can transform it from a passive into an active state, as well as prestressing the concrete. For this aim, sixteen concrete plates of dimensions 700 x 100 x 20 m m 3 and reinforced with MCF reinforcement were cast. CSA (calcium sulfoaluminate) was used as an expansive additive with replacement ratios by weight of cement of 0 % (control), 10 %, 12.5 %, and 15 %. Based on this, the specimens were divided into four groups, each of which consisted of 4 specimens. Distributed fibre optic sensor (DFOS) technology was used to measure the concrete expansion and to track the development of stresses with time. All specimens were tested under a direct tension load to determine the influence of expansive concrete. The results showed that high expansion rates can be achieved when using the proposed expansive concrete mixture; however, such expansion may lead to a limited negative effect on the strength of the resulting concrete, especially at high CSA replacement ratios. Nevertheless, expansive concrete can significantly enhance the structural performance of the MCF-reinforced concrete plates. This includes higher stiffness before cracking and higher cracking stress, which was approximately doubled for specimens with 15 % CSA compared to the control specimens with 0 % CSA. Furthermore, enhanced cracking behavior was also observed for all the specimens made of expansive concrete • MCF reinforcement has better performance at elevated temperatures than CFRP reinforcement. • Chemical prestressing was used to enhance the poor structural performance of the MCF reinforcement. • The DFOS technology was used to measure the stresses resulting from the chemical prestressing. • Chemical prestressing can significantly enhance the structural performance of the MCF reinforcement. [ABSTRACT FROM AUTHOR]

In recent decades, the urgency to protect and upgrade cultural heritage structures (CHS) has become of primary importance due to their unique value and potential areas of impact (economic, social, cultural, and environmental). Structural health monitoring (SHM) and the management of CHS are emerging as decisive safeguard measures aimed at assessing the actual state of the conservation and integrity of the structure. Moreover, the data collected from SHM are essential to plan cost-effective and sustainable maintenance solutions, in compliance with the basic preservation principles for historic buildings, such as minimum intervention. It is evident that, compared to new buildings, the application of SHM to CHS is even more challenging because of the uniqueness of each monitored structure and the need to respect its architectural and historical value. This paper aims to present a state-of-the-art evaluation of the current traditional and innovative SHM techniques adopted for CHS and to identify future research trends. First, a general introduction regarding the use of monitoring strategies and technologies for CHS is presented. Next, various traditional SHM techniques currently used in CHS are described. Then, attention is focused on the most recent technologies, such as fibre optic sensors and smart-sensing materials. Finally, an overview of innovative methods and tools for managing and analysing SHM data, including IoT-SHM systems and the integration of BIM in heritage structures, is provided.

Abstract A high sensitivity, simple fabrication all‐fibre strain sensor is proposed and demonstrated experimentally. The sensor consists of a section of Panda‐type polarization maintaining fibre (PMF) built into a Sagnac interferometer. The Panda‐type PMF is intentionally spliced to the single‐mode fibre through enhanced fusion splicing to excite high‐order polarized modes. The enhanced fusion splicing characterizes a large magnitude of overlap. The measured strain sensitivity reaches up to 32 pm/με and the strain resolution of 0.63 με has been achieved. The proposed fibre strain sensor characterizing easy fabrication and high sensitivity can be a good candidate in the field of structural health monitoring.

Abstract Two methods to inscribe co‐located fibre Bragg gratings using the small spot direct UV writing technique are described and compared, namely: superimposed gratings, which are written one over the other at the same spatial location, and superstructured gratings, where a refractive index modulation is digitally designed and all spectral signatures are written in a single step writing process. In this letter, we report a record number of 45 superimposed and 39 superstructured gratings being inscribed with an interference spot of only 14 μm in diameter.

This work presents a methodology for the real-time structural deformation reconstruction of solid or thin-walled prismatic beams using discrete strains. Based on the 1D inverse Finite Element Method (iFEM), the approach combines the beam kinematics of Timoshenko theory, strain–displacement relations, and the finite element discretisation framework to reconstruct the full-field deformations of beams with any general cross-section. Although applicable for any general class of beams, the present work investigates its experimental application specifically to conventionally and additively manufactured thin-walled beams discretised using various low- and high-order inverse beam elements. The results demonstrate the method's accuracy and robustness, albeit influenced by the element discretisation scheme and the number of strain sensors used. • Beam deformation reconstruction from discrete strains using the 1D iFEM. • Bending, shear, and torsional deformation sensing for complex cross-sectional beams. • 1D inverse beam elements formulated based on Timoshenko beam theory. • Experimental tests on thin-walled beams instrumented with fibre optic sensors. • Experimental shape sensing of an additively manufactured airfoil beam. [ABSTRACT FROM AUTHOR]

GPS and more recently multi-GNSS carrier phase measurements have been used to measure the dynamic displacements of large structures, such as long-span bridges, in both the time and frequency domains. Such measurements can be used as part of a structural health monitoring system. Additionally, fibre optic-based systems have been used to measure long-term deformations of structures, such as tunnels and roads. The research presented in this paper brings together the ideas and technologies in the two aforementioned areas of research, resulting in dynamic displacement measurements of the hull of a ship at high frequencies, and indoor environments. Field trials using kinematic GPS and FBG sensors on the 138-m long Smyril passenger and vehicle ferry operating in the North Atlantic Ocean on the Faroe Islands are presented. FBG sensors were in the bow and engine room of the ship, gathering data at 1 kHz. The configuration of the surveys and the results from the FBG and GPS sensors are presented, in both the time and frequency domains. Various frequencies were measured, due to the movements of the ship in the ocean as well as the vibrations caused through the ship mainly due to the engine. One dominant frequency was of 12.25 Hz, measured at all the FBG locations, due to the engines' operating speed of 735 RPM. Common frequencies were evident for both the FBG and GPS results for lower frequency displacement, caused by the movements in the sea. Such measurements could be used to monitor the long-term displacement characteristics and changes, in both the time and frequency domains, and used to help understand the health characteristics of the ship. Further, such measurements could be used to analyse the noise characteristics of the ship for both operational and environmental reasons. [ABSTRACT FROM AUTHOR]

Understanding stresses within the knee joint is central to understanding knee function, and the etiology and progression of degenerative joint diseases such as post-traumatic osteoarthritis. In this study, in vivo gait kinematics of four ovine subjects were recorded using a highly accurate Instrumented Spatial Linkage (ISL) as each subject walked on a standard treadmill. The subjects were then sacrificed, and the right hind limbs removed. Ten purpose-built Fibre Bragg Grating (FBG) sensors were positioned within each stifle joint and used to measured contact stresses on the articulating surface of the tibial plateau as the recorded gait was replicated using a 6-degrees-of-freedom parallel robotic system. This study provides the first accurate, direct measurement of stress in a joint during in vivo gait replication. It was hypothesized that the results would indicate a direct link between gait kinematics and measured stress values. Contrary to this expectation no direct link was found between individualistic differences in kinematics and differences in stress magnitudes. This finding highlights the complex multifactorial nature of stress magnitudes and distribution patterns across articular joints. The results also indicate that stress magnitudes within the knee joint are highly position dependent with magnitudes varying substantially between points only a few mm apart. [ABSTRACT FROM AUTHOR]

This study is intended to develop long period fibre grating sensors for potential applications in environmental and durability monitoring of coastal structures. High-quality helical long period fibre gratings (HLPFGs) are inscribed in different types of small-core single mode fibre (SMF) by use of hydrogen-oxygen flame heating technique. A detailed investigation of the effect of core diameter on their transmission spectrum and optimum length of the HLPFG has been pursued. A longer length is required to achieve the same coupling attenuation in a smaller-core SMF than that of a larger-core fibre. The strain, torsion and refractive index (RI) properties of the HLPFG is investigated experimentally to develop a high-sensitivity sensor. The experimental results show that the strain sensitivity could be enhanced by means of employing a larger-core diameter SMF. Moreover, the HLFPGs are also sensitive to the torsion and external RI. Hence, such HLFPGs have great potential for sensing applications. [ABSTRACT FROM AUTHOR]

Accurate and efficient monitoring of electrical machine (EM) operating parameters, including temperature, mechanical vibration, torque and rotating speed and others that can indicate the EM health conditions is becoming ever more important in the world of electrical drives. The traditional methodology of one sensor per parameter can be theoretically replaced by a "one sensor measures all" technology, which can be achieved through the use of fibre-optic sensors (FOS). In this paper, several FOSs, which use different optical sensing principles for multiple physical parameter measurements of EMs, are reviewed. This paper also provides an insight into the major developments, and discusses the engineering challenges of FOS used for EM monitoring over the last few decades, and compares the advanced features of FOS with those of conventional sensors in use. Finally, a novel FOS-EM observer system scheme employing the Fibre Bragg Grating technique for multi-parameter monitoring of EM health is proposed, after discussion of the preceding industrial and academic FOS cases for EM applications. [ABSTRACT FROM AUTHOR]

Transformer oil is widely used in high‐voltage transformers as a cooling and insulation medium. Electrical and thermal stresses expose transformer oil to degradation, which may produce dissolved decay products (DDPs) that are partially adsorbed on the insulation paper and accelerate the degradation of oil and paper. An evanescent field D‐shaped plastic optical fibre (POF) sensor was fabricated using the side‐polishing technique to detect DDPs in transformer oil. A laser light source and power meter were used in the prototype to measure the output power of the POF sensor. The measurement of DDPs was conducted on seven samples of insulating oil collected from the field. The sensor performance was evaluated based on different sensing areas, and the sensor sensitivity was measured. Results show an excellent correlation between the output power and DDPs. A correlation analysis was conducted amongst sensor output power, transformer oil refractive index and area under the absorbance curve. The performance of the sensor demonstrates improved linearity and sensitivity of 97.49% and 26.6779 µW/RIU, respectively. The POF sensor is easily fabricated, low cost and can be used for real‐time monitoring of high‐voltage transformers. [ABSTRACT FROM AUTHOR]

The liquid‐infiltrated photonic crystal fibre (LI‐PCF) is proposed for guiding terahertz radiation. Geometrical asymmetry is achieved by introducing a large ellipse in the core. By filling the ellipse with liquid cocaine, the optical properties of the photonic crystal fibre (PCF) are theoretically examined using finite element method‐based COMSOL multiphysics software. At an operating frequency of 1 THz, the proposed LI‐PCF demonstrates a sensitivity of 87.02% and confinement loss in the order of 10−4 cm−1. The PCF also demonstrates extremely low effective material loss <0.01 cm−1, a birefringence of 0.018, large effective mode area of 1.11 × 105 µm2, a high numerical aperture of 0.45 and near‐zero ultra‐flattened chromatic dispersion of 1.4351 ± 0.5883 ps/THz/cm. The design simplicity and high sensitivity, strong confinement factor, low material losses and high birefringence of the fibre suggest that the proposed fibre may be convenient for PCF‐based cocaine sensing, for application in the security and defence industries. [ABSTRACT FROM AUTHOR]

This study presents the development and validation of a fibre Bragg gratings (FBGs)‐based sensor system for the assessment of strain in the midpalatal suture in subjects using rapid palatal expanders (RPEs). The ex‐vivo experiments were made by means of positioning two RPEs in a porcine palatal region. The RPEs used were the Hyrax, a tooth‐borne expander and MARPE (microimplant‐assisted rapid palatal expansion), a bone‐borne expander. In order to define the regions in the palatal region for the sensors positioning, a finite‐element analysis was performed in a porcine head subjected to the loadings caused by an RPE. In addition, a strain transfer model was used to obtain a correction coefficient that approximates the strain estimated by the FBG to the actual strain in the structure under shear and normal stress. Results show high linearity in the sensors characterisation tests with the advantages of compactness, intrinsic safe operation and multiplexing capabilities of FBGs. In the RPE analysis, a higher strain was estimated in the anterior region, which is in accordance with the simulation and previously reported results, where MARPE showed a higher strain (with an exponential pattern) than Hyrax as the number of activations increase. [ABSTRACT FROM AUTHOR]

A novel liquid‐core photonic crystal fibre (LC‐PCF) based on the Zeonex substrate has been proposed for sensing the varying concentration of kerosene in petrol and diesel fuels in terahertz frequencies. Fuel sample infiltration is achieved by designing a single hexagon in the porous core region. The LC‐PCF has been numerically studied using a finite element method. By filling the core hexagon with different petrol–kerosene and diesel–kerosene concentrations, sensitivity and transmission profiles of the proposed LC‐PCF have been examined. The impact of different design parameters of the fibre on its performance has been analysed, allowing the optimum design parameter set to be selected for further analysis. The proposed fibre shows relative sensitivities above 96% and effective material losses in the order of 10−3. With negligible confinement losses, the proposed fibre demonstrates near‐zero ultra‐flattened group velocity dispersion profiles within 1–1.6 THz. It is envisaged that the proposed fibre may be fabricated easily due to its simple design, and thus, serving as a significant step towards understanding THz pulse interaction with liquid petrochemical products. [ABSTRACT FROM AUTHOR]

A small portion of an optical fibre clad was removed and coated with methylamine hydroiodide by a slurry deposition process to develop an evanescent wave optical fibre ammonia sensor. The experiment was performed for a few volatile organic compounds in order to understand the selectivity and sensitivity. Prior to the gas sensing measurement, the methylamine hydroiodide sample was characterised using X‐ray diffraction (XRD), scanning electron microscope with energy dispersive X‐ray spectroscopy, Fourier transform infrared, Raman, diffuse reflectance and UV–Vis spectroscopy. In order to understand the sensor response, the samples were independently exposed to acetone, ammonia, ethanol and methanol gases and then analysed using the XRD technique. This analysis reveals that the structural variation is observed because of the ammonia exposure as compared to other gases. It has initiated to fabricate a clad‐modified optical fibre ammonia sensor that exhibits ∼28% of the sensitivity. These experimental results are discussed in detail. [ABSTRACT FROM AUTHOR]

Abstract This letter reports a proof‐of‐concept experiment conducted on a novel application of the multi‐core fibre (MCF), where image reconstruction is based on a single‐pixel imaging (SPI) technique and the diffraction pattern emitted from an MCF. The technique is intended to reduce the size of the SPI system, the applications of which can now be extended with the help of this study.

Performance-based concrete design can accelerate the acceptance of emerging lower carbon concretes to reduce the environmental impact. A semi-adiabatic symmetric concrete prism experimental and analytical approach is proposed as a pathway for concrete thermal characterisation. The prism design promotes 1-D heat flow conditions using a single insulated specimen. The prism dimensions (700 mm ×100 mm ×100 mm) are rationalised to suit usage as a characterisation specimen and mix compositions that replicate those of in-situ concretes can be assessed. The experimental spatial and temporal thermal profiles of a CEM I mix and a high GGBS replacement mix were measured during hydration and through a step change in temperature in the hardened state. Thermocouples and a distributed fibre optic sensor along the central axis of the symmetric prism were used for this purpose. Multi-physics modelling provided a platform to address thermal measurement data uncertainty, extract fundamental concrete thermal parameters and gain insight into the heat generation and heat flow physics in the concrete prism specimens. The complete heat generation and maturity time histories were accurately modelled and 'concrete thermal footprints' derived. • Performance-based thermal assessment of concrete relies on characterisation testing. • A semi-adiabatic symmetric prism is used to derive concrete thermal footprints. • Heat generation and thermal properties are obtained from a single specimen. • The symmetric prism approach suits on-site application and a range of concrete mixes. • Embedded fibre optic or thermocouple sensors reveal internal temperature profiles. [ABSTRACT FROM AUTHOR]