Chirped Fiber Bragg Grating Sensors Research Articles

Chirped Fiber Bragg Grating Sensors for Force Intensity and Location Assessment in Occlusal Splints: A Proof-of-Concept.

Fiber Bragg Grating (FBG) sensors are strain and temperature sensors with advantages such as small diameter, multiplexing capability, intrinsic safety and biocompatibility. In dentistry, FBG sensors were tested in appliances, implants and splints. In addition to the study of dental biomechanics, these sensors are used in the development of new techniques and products or in the comparison of established treatments. The working principle of measurement of bite forces applied to occlusal splints and the distributed sensing capability along the Chirped FBG (CFBG) are demonstrated in the present proof-of-concept using a PDMS silicone splint. The CFBG exhibits different spectral shapes for each of the four positions of the one-point force application. The average wavelength and the full width at half maximum (FWHM) of the spectra reflected by the CFBG are able to estimate the intensities and the positions, respectively, of the compression forces of the tooth. The instrumentation of occlusal splints with CFBG sensors can help in the distinction between different bite patterns in occlusal stabilization splints during bruxism episodes, along with the understanding of the effects and efficacy of these appliances in treatments and the comparison of different 3D-printed and traditional occlusal splints.

Damage detection in Aerospace structures using Chirped fiber Bragg grating

In the aviation industry, there is a need to detect even the smallest damage in order to avoid a fatal accident. Aerospace structures undergo various kinds of damages due to the impact force of objects during flight. In particular, the forward- facing components like a wing leading edge are subjected to impact forces caused by natural incidents like bird strikes. This work investigates the damage detection on the wing’s leading edge when hit by a bird during flight. The damage detection method proposed in this work uses Chirped Fiber Bragg Grating (CFBG) as a sensing element, whose reflection spectrum is analyzed to verify the presence of damage. The already existing methods make use of the Fiber Bragg Grating (FBG) sensor which has lesser impact than the CFBG sensor. The aircraft wing has been designed in SOLIDWORKS and strain simulation is carried out using COMSOL software. The parameters such as stress, strain and wave length shift are obtained which can be used for damage detection in aircraft structures.

Shallow-Tapered Chirped Fiber Bragg Grating Sensors for Dual Refractive Index and Temperature Sensing.

In this work, we present a gold-coated shallow-tapered chirped fiber Bragg grating (stCFBG) for dual refractive index (RI) and temperature sensing. The stCFBG has been fabricated on a 15-mm long chirped FBG, by tapering a 7.29-mm region with a waist of 39 μm. The spectral analysis shows two distinct regions: a pre-taper region, in which the stCFBG is RI-independent and can be used to detect thermal changes, and a post-taper region, in which the reflectivity increases significantly when the RI increments. We estimate the RI and thermal sensitivities as 382.83 dB/RIU and 9.893 pm/°C, respectively. The cross-talk values are low (−1.54 × 10−3 dB/°C and 568.1 pm/RIU), which allows an almost ideal separation between RI and thermal characteristics. The stCFBG is a compact probe, suitable for long-term and temperature-compensated biosensing and detection of chemical analytes.

Evaluation of a Novel Spectral Pair of a Chirp FBG Embedded in a Cantilever Beam for Simultaneous Temperature and Transverse Forces Measurement

An evaluation of a novel spectrum features combining the distant longer and shorter wavelengths of a single chirped fiber Bragg grating (CFBG) for temperature and transverse forces discrimination is experimentally demonstrated. The shift of the two distant wavelengths’ pair is compared with the conventional pair combining the bandwidth modulation and center wavelength shift. The CFBG sensor is simply bonded to a cantilever beam and subjected to transverse loading (four times) and a heating-cooling cycle. The transverse forces calibration results show a repeatability of 3.9 pm and 1.7 pm for the bandwidth’s and center wavelength’s responses, respectively, while the distant wavelengths’ show a repeatability of 2.37 pm and 3.01 pm, respectively. The cantilever CFBG sensor exhibits high correlation coefficients of 0.9 between the two heating and cooling data sets, except for the bandwidth, which only had a lower coefficient of 0.75. The linear model of both pairs for calculating temperature and transverse forces can provide an accurate estimate, with the longer-shorter wavelengths’ pair having an advantage over the pair of the bandwidth-centre-wavelength. The study has demonstrated the feasibility of the method proposed by our group in a previous work, by which three physical quantities can be measured with a single custom FBG.

A Review of Recent Results on Simultaneous Interrogation of Multiple Fiber Bragg Grating-Based Sensors Using Microwave Photonics

We review recent results on exploiting microwave photonics to enable simultaneous interrogation of multiple fiber Bragg grating (FBG)-based sensors. In particular, we describe the use of (1) microwave photonic filtering and (2) chirped microwave pulse generation and compression as a means to map the wavelength (spectral) changes in the response of FBG-based sensors (specifically, an in-fiber Fabry-Pérot cavity sensor based on FBGs, FBG sensors directly, and a linearly chirped FBG sensor) to applied temperature (or strain) to the power of a radio-frequency signal (i.e., a wavelength-to-power mapping) or to the correlation peak of the compressed microwave signal. The approaches support high-resolution and high-speed interrogation and can be suitable for large scale sensing networks.

Review of Chirped Fiber Bragg Grating (CFBG) Fiber-Optic Sensors and Their Applications.

Fiber Bragg Gratings (FBGs) are one of the most popular technology within fiber-optic sensors, and they allow the measurement of mechanical, thermal, and physical parameters. In recent years, a strong emphasis has been placed on the fabrication and application of chirped FBGs (CFBGs), which are characterized by a non-uniform modulation of the refractive index within the core of an optical fiber. A CFBG behaves as a cascade of FBGs, each one reflecting a narrow spectrum that depends on temperature and/or strain. The key characteristic of CFBGs is that their reflection spectrum depends on the strain/temperature observed in each section of the grating; thus, they enable a short-length distributed sensing, whereas it is possible to detect spatially resolved variations of temperature or strain with resolution on the order of a millimeter over the grating length. Based on this premise, CFBGs have found important applications in healthcare, mechanical engineering, and shock waves analysis, among others. This work reviews the present and emerging trends in CFBG sensors, focusing on all aspects of the sensing element and outlining the application case scenarios for which CFBG sensors have been demonstrated.

Ultrafast Fiber Bragg Grating Interrogation for Sensing in Detonation and Shock Wave Experiments

Chirped fiber Bragg grating (CFBG) sensors coupled to high speed interrogation systems are described as robust diagnostic approaches to monitoring shock wave and detonation front propagation tracking events for use in high energy density shock physics applications. Taking advantage of the linear distributed spatial encoding of the spectral band in single-mode CFBGs, embedded fiber systems and associated photonic interrogation methodologies are shown as an effective approach to sensing shock and detonation-driven loading processes along the CFBG length. Two approaches, one that detects spectral changes in the integrated spectrum of the CFBG and another coherent pulse interrogation approach that fully resolves its spectral response, shows that 100-MHz–1-GHz interrogation rates are possible with spatial resolution along the CFBG in the 50 m to sub-millimeter range depending on the combination of CFBG parameters (i.e., length, chirp rate, spectrum) and interrogator design specifics. Results from several dynamic tests are used to demonstrate the performance of these high speed systems for shock and detonation propagation tracking under strong and weak shock pressure loading: (1) linear detonation front tracking in the plastic bonded explosive (PBX) PBX-9501; (2) tracking of radial decaying shock with crossover to non-destructive CFBG response; (3) shock wave tracking along an aluminum cylinder wall under weak loading accompanied by dynamic strain effects in the CFBG sensor.

Use of a surface-mounted chirped fibre Bragg grating sensor to monitor delamination growth in a double-cantilever beam test

A surface-mounted chirped fibre Bragg grating (CFBG) sensor has been used for the first time to monitor delamination growth within a composite material (a transparent, unidirectionally reinforced glass fibre/epoxy resin double-cantilever beam (DCB) specimen). The specimens were tested using a constant displacement rate, with the delamination length being measured using complementary techniques: (i) in situ photography, (ii) surface-mounted strain gauges, and (iii) the surface-mounted CFBG sensors. The unidirectionally reinforced DCB specimens showed characteristics typical of such material which complicate the curvature of the beams, i.e. the development of extensive fibre bridging and pronounced R-curve behaviour. To validate the interpretation of the CFBG reflected spectrum, the experimentally determined strains from the surface-mounted strain gauges have been used, together with in situ photographs of the position of the delamination front. Using the CFBG sensor technique, the delamination length was measured to within about 4mm over the 60mm sensor length.

外贴啁啾光栅监测带双缺口复合材料静态拉伸损伤

外贴啁啾光栅监测带双缺口复合材料静态拉伸损伤

Real-Time Interrogation of a Linearly Chirped Fiber Bragg Grating Sensor for Simultaneous Measurement of Strain and Temperature

We propose and demonstrate a novel approach to real-time interrogation of a high-birefringence linearly chirped fiber Bragg grating (Hi-Bi LCFBG) for simultaneous measurement of strain and temperature based on chirped microwave pulse compression with increased resolution and signal-to-noise ratio (SNR). A theoretical analysis is developed, which is validated by an experiment. The experimental results show that the proposed system can provide a resolution better than ± 1.2 °C and ± 13.3 με at an interrogation speed of 48.6 MHz.

Real-Time Interrogation of a Linearly Chirped Fiber Bragg Grating Sensor Based on Chirped Pulse Compression With Improved Resolution and Signal-to-Noise Ratio

A novel approach to interrogating in real time a linearly chirped fiber Bragg grating (LCFBG) sensor based on spectral-shaping and wavelength-to-time (SS-WTT) mapping with improved interrogation resolution and signal-to-noise (SNR) ratio is proposed and experimentally demonstrated. The proposed system consists of a mode-locked laser source, an optical interferometer incorporating an LCFBG, and a dispersive element. The optical interferometer has a spectral response with an increasing free spectral range (FSR). The incorporation of the LCFBG in the interferometer would encode the sensing information in the spectral response as a change in the FSR. After SS-WTT mapping, a linearly chirped microwave waveform is obtained. The correlation of the linearly chirped microwave waveform with a chirped reference waveform would provide a sharp correlation peak with its position indicating the wavelength shift of the LCFBG. A theoretical analysis is carried out, which is validated by a numerical simulation and an experiment. The experimental results show that the proposed system can provide an interrogation resolution as high as 0.25 με at a speed of 48.6 MHz.

Effect of Disbond Propagation on the Reflected Spectra of CFBG Sensors Embedded within the Bondline of Composite Bonded Joints

Chirped fiber Bragg grating (CFBG) sensors were embedded within the adhesive bondline of single-lap CFRP-GFRP bonded composite joints. The effect of disbond propagation (as a consequence of fatigue loading) on the reflected spectra from the CFBG sensor has been studied. As the disbond propagates, thermal strains generated during the bonding of the joint at elevated temperature are released and, as a consequence, a peak in the reflected spectra of the CFBG sensor can be seen. Using a transparent GFRP adherend, it has been possible to demonstrate that there is reasonable agreement between the position of the peak in the reflected spectrum and the disbond front position in the bonded joint.

Spectrally-overlapped chirped fibre Bragg grating sensor system for simultaneous two-parameter sensing

A spectrally-overlapped chirped fibre Bragg grating (FBG) sensor system for simultaneous two-parameter sensing is presented. The chirped FBGs are amplitude-modulated in such a way that they can be spectrally overlapped with similar spectral characteristics, namely the centre Bragg wavelength and bandwidth. That is, the chirped FBGs are multiplexed via the spectral-overlap multiplexing scheme. Because of its compatibility with the wavelength division multiplexing scheme, the number of gratings to be multiplexed can be increased by several times. The multiplexed sensor signal is demultiplexed and demodulated using the discrete wavelet transform. The resolution, accuracy and crosstalk of the sensor system are investigated through strain measurements. The application of this sensor system is demonstrated through the simultaneous measurement of strain and temperature of a metal alloy plate.

The use of an embedded chirped fibre Bragg grating sensor to monitor disbond initiation and growth in adhesively bonded composite/metal single lap joints

Chirped fibre Bragg grating (CFBG) sensors embedded within glass fibre reinforced plastic (GFRP) adherends have been used to monitor disbond initiation and growth in an adhesively bonded GFRP/aluminium alloy single lap joint. The elevated temperature curing of the adhesive used in the manufacture of the joint leads to thermal strains being generated within the GFRP and aluminium adherends. Disbond initiation and growth between the adherends during fatigue cycling causes a relaxation of the residual thermal strains within the composite adherend and perturbations (peaks or dips) in the reflection spectra from the CFBG sensor. These perturbations can be used, when the joint is unloaded, to monitor both the initiation of a disbond in the joint and to monitor the growth of the disbond with fatigue cycling.

Monitoring of Delamination Growth in CFRP Laminates using Chirped FBG Sensors

Chirped fiber Bragg grating (FBG) sensors are applied to the monitoring of the propagation direction and the size of a delamination in CFRP cross-ply laminates, since it has been found previously that the reflection spectrum from chirped FBG sensors is expressed as a function of their positions along the grating. The reflection spectra are measured at various delamination lengths under a four-point bending test. Two chirped FBG sensors are embedded in two opposite directions in order to investigate the relationship between the deformation of the spectrum and the propagation direction of the delamination. The spectrum changes sensitively depending on both the size and the propagation direction of the delamination. For confirmation of the measured results, the spectra are simulated using FEA computed strain distributions of the chirped FBG sensors. In the simulation, a birefringence effect of the optical fiber on the spectrum is observed in the case of chirped FBG sensors. The relationship between the measured spectral change and the delamination growth is illustrated by the calculated spectra, the strain distributions, and the properties of the chirped FBG sensors. As a result, it is found that not only the delamination size, but also the propagation direction of the delamination can be evaluated by using chirped FBG sensors.

Real-time Detection of Debonding between Honeycomb Core and Facesheet using a Small-diameter FBG Sensor Embedded in Adhesive Layer

The authors have developed a novel technique to detect debonding in honeycomb sandwich structures using small-diameter fiber Bragg grating (FBG) sensors. A small-diameter FBG sensor is embedded in the adhesive layer between the core and the facesheet during the curing process of the adhesive layer. After the curing process, the reflection spectrum from the small-diameter FBG sensor is distorted because the formation of fillets induces a non-uniform strain distribution in the adhesive layer. However, after debonding, the core with the fillets peels off the facesheet, and, as a result, the reflection spectrum recovers its original shape due to the release of the non-uniform strain. Debonding can thus be detected with high sensitivity in real time from the recovery in the shape of the reflection spectrum. In addition, a more reliable debonding detection technique using a small-diameter chirped FBG sensor is proposed, in which two indicators in the reflection spectrum have been successfully used for a more reliable judgment of debonding.

Damage identification in a holed CFRP laminate using a chirped fiber Bragg grating sensor

This study proposes damage identification for holed CFRP cross-ply laminates using an embedded chirped fiber Bragg grating (FBG) sensor. We investigated changes in the reflection spectrum of the chirped FBG sensor due to damage near the hole. The reflection spectrum of the chirped FBG sensor was further studied by estimating the strain distribution along the gage section as an inverse problem based on the spectrum shape. We experimentally confirmed that the reflection spectrum of the chirped FBG sensor changed distinctively within certain wavelengths when each type of damage (transverse cracks or delamination) occurred and extended. Local strain changes were estimated from the reflection spectrum of the chirped FBG sensor; their locations coincided with those for the observed transverse cracks. We also demonstrate damage identification for the holed specimen and conclude that this feature of the chirped FBG sensor provides successful identification of a damage pattern near the hole.

Monitoring damage in holed CFRP laminates using embedded chirped FBG sensors

This study proposes a new approach to monitoring the damage process in holed CFRP laminates using an embedded chirped fiber Bragg grating (FBG) sensor. To this end, we experimentally and numerically investigated the damage process and the damage-induced changes in the spectrum shape. It was experimentally confirmed that multiple types of damage (e.g., splits, transverse cracks and delamination) appeared near a hole, and that the spectrum shape of the embedded chirped FBG sensor changed as the damage extended. Our proposed simulation for the reflection spectrum considering the damage agreed with the experiments. Furthermore, this study investigated the effect of each damage pattern on the changes in the spectrum shape. Finally, based on these discussions, we present simple damage identifications with the embedded chirped FBG for the holed CFRP laminates under completely unloaded conditions.

Application of chirped fiber Bragg grating sensors for identification of crack locations in composites

Chirped fiber Bragg grating (FBG) sensors were applied for the identification of crack locations in carbon fiber reinforced plastic (CFRP) laminates since the reflection spectrum from a chirped FBG was expressed as a function of the position along the grating. For the crack identification in the 90° ply of a cross-ply laminate, a chirped FBG sensor, whose grating length and spectrum width were 50 mm and about 5 nm respectively, was embedded into the 0° ply of the CFRP cross-ply laminate, and the reflection spectra were measured after tensile loadings were applied to the laminate. As a result, the spectra had dips corresponding to locations of transverse cracks. This change in the form of the spectrum was also confirmed by a theoretical calculation. Hence, the crack locations could be identified from the form of the spectrum. Furthermore, it was demonstrated that a small-diameter chirped FBG sensor could also be applied for the identification of the crack locations.