Long-period Fiber Grating Sensor Research Articles

Application of Long-Period Fiber Grating Sensors in Structural Health Monitoring: A Review

Structural health monitoring (SHM) is crucial for preventing and detecting corrosion, leaks, and other risks in reinforced concrete (RC) structures, ensuring environmental safety and structural integrity. Optical fiber sensors (OFS), particularly long-period fiber gratings (LPFG), have emerged as a promising method for SHM. Various LPFG sensors have been widely used in SHM due to their high sensitivity, durability, immunity to electromagnetic interference (EMI) and compact size. This review explores recent advancements in LPFG sensors and offers insights into their potential applications in SHM.

Simultaneous measurement of the surrounding refractive index and temperature using a local-micro-structured long-period fiber grating sensor operating near the phase-matching turning point

We have proposed a local-micro-structured long-period fiber grating (LMSLPFG) sensor near the phase-matching turning point (PMTP) for simultaneous measurement of the surrounding refractive index (SRI) and temperature. By using the HF solution or a femtosecond laser to partially change the cladding radius of LPFG, the LMSLPFG is realized. Such change affects the effective refractive index of cladding mode and further causes a new resonant peak in the spectrum. By using the transfer matrix method, the formation mechanisms of the peak in the middle of the LMSLPFG transmission spectrum corresponding to a little off-resonant state (ALOR) near PMTP and the other two peaks on both sides corresponding to the dual-peak resonance are studied. On this basis, the sensing characteristics of such three resonant peaks to SRI and temperature are analyzed. When the SRI and temperature change, the transmissivity of the peak in ALOR changes without a response occurring at resonant wavelength, while the wavelength of the other two dual peaks will respond proportionately. The numerical simulation results demonstrate that the sensitivities of the distance between the dual peaks to SRI and temperature are 777.14 nm/RIU and −0.22nm/∘C, respectively, and the sensitivities of single peak transmissivity to SRI and temperature are −394.06dB/RIU and 0.05 dB/°C, respectively. Therefore, the sensitivity matrix can be used to measure SRI and temperature simultaneously with a single LMSLPFG.

Probability of detection for corrosion-induced steel mass loss using Fe–C coated LPFG sensors

The traditional probability of detection (POD) method, as described in the Department of Defense Handbook MIL-HDBK-1823A for nondestructive evaluation systems, does not take the time dependency of data collection into account. When applied to in situ sensors for the measurement of flaw sizes, such as fatigue-induced crack length and corrosion-induced mass loss, the validity and reliability of the traditional method is unknown. In this paper, the POD for in situ sensors and their associated reliability assessment for detectable flaw sizes are evaluated using a size-of-damage-at-detection (SODAD) method and a random parameter model (RPM). Although applicable to other sensors, this study is focused on long-period fiber gratings (LPFG) corrosion sensors with thin Fe–C coating. The SODAD method uses corrosion-induced mass losses when successfully detected from different sensors for the first time, while the RPM model considers the randomness and difference between mass loss datasets from different sensors. The Fe–C coated LPFG sensors were tested in 3.5 wt.% NaCl solution until the wavelength of transmission spectra did not change. The wavelength shift of 70% of the tested sensors ranged from 6 to 10 nm. Given a detection threshold of 2 nm in wavelength, the mass losses at 90% POD are 31.87%, 37.57%, and 34.00%, which are relatively consistent, and the upper-bound mass losses at 95% confidence level are 33.20%, 47.30%, and 40.83% from the traditional, SODAD, and RPM methods, respectively. In comparison with the SODAD method, the RPM method is more robust to any departure from model assumptions since significantly more data are used. For the 90% POD at 95% confidence level, the traditional method underestimated the mass loss by approximately 19%, which is unconservative in engineering applications.

Long period fiber grating sensor coated with MXene (Ti3C2TX) functionalized by poly-levodopa for rapid detection of Cu2+

An optical sensor based on long period fiber grating (LPFG) modified by MXene-polymer composites was proposed to detect Cu2+. Ti3C2Tx is prepared by etching aluminum with hydrofluoric acid (HF) from the Ti3AlC2. MXene-based materials demonstrate the potential to adsorb multi-pollutants, but the adsorption performance is limited. Mussel-inspired poly-levodopa (PDOPA) modified Ti3C2Tx, with a simple and eco-friendly reaction process, has been proven to enhance the adsorption capacity for Cu2+. The resonance spectrum of LPFG is sensitive to changes of refractive index (RI) aroused by Cu2+ adsorption on the sensing film. Thus, the Cu2+ sensing performance of the LPFG sensor coated with Ti3C2Tx and PDOPA-Ti3C2TX was separately investigated. The best sensitivity of the PDOPA-Ti3C2TX-LPFG was measured to be 3.48 × 10−4 nm/ppb (0–103 ppb) and the detection limit was determined to be 10.34 ppb (0.162 μM), which was obviously better than the results of 1.80 × 10−4 nm/ppb and 25 ppb (0.393 μM) with only Ti3C2Tx coated. Moreover, the stability and reusability of the sensor were also evaluated. This sensor provides a new approach for metal ions detection in the water environment.

Exoskeleton type long-period fiber grating for cross-talk-resistance single-parameter measurement.

This paper proposes a new, to the best of our knowledge, design framework of long-period fiber grating (LPFG) sensors resistant to multi-parameter cross talk. A section of hollow quartz capillary (HQC), which acts as an exoskeleton, is periodically merged with a single-mode fiber (SMF) by the arc-discharge method. The mechanical stress in the SMF is released while the thermal stress is enhanced after a high-temperature fusion process. Under the influence of the elastic-optical effect, the refractive index of the core is periodically modulated along the axial direction to form an exoskeleton long-period fiber grating (Es-LPFG). The unique exoskeleton structure not only induces mode coupling but also enables the proposed device to resist cross talk among the strain, ambient refractive index, and vector bending. The temperature is able to be measured independently with a sensitivity of 74 pm/ ∘C. The novel Es-LPFG is promising in single-parameter sensing, mode-locked lasers, and frequency-locked gain flattening.

High Sensitivity Refractometer Based on Tapering Two-Mode Fiber Embedded Long-Period Fiber Grating

A novel tapered two-mode fiber embedded long-period fiber grating (TTMF-LPFG) sensor is proposed. The long-period fiber grating (LPFG) is fabricated by periodically splicing the two-mode fiber (TMF) and single-mode fiber (SMF), and then the LPFG is tapered to make a refractive index (RI) sensor. By comparing the sensors with different waist diameters, we found that the smaller the diameter of the sensor is, the higher the sensitivity is. When the taper waist diameter is 36 <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> , the maximum sensitivity reaches 1360.43 nm/RIU. TTMF-LPFG shows low sensitivity to the temperature, avoiding the influence of temperature. For the traditional taper fiber, a very thin taper waist is required to excite strong evanescent field (EF) in order to obtain a large RI sensitivity. This situation creates a sensor fragile. For our structure, through the taper, LPFG can be formed and the sensitivity can be improved at the same time. The sensor has great potential in chemical composition analysis and biomedical applications.

A label-free biosensor for pepsin detection based on graphene oxide functionalized micro-tapered long period fiber grating

The rapid and sensitive detection of pepsin plays an important role in clinical and medical practice. A label-free and sensitive micro-tapered long-period fiber grating (MTLPFG) sensor that functionalized by graphene oxide (GO) was proposed for pepsin detection. MTLPFG was fabricated with CO2 laser heating and tapered to form a series of periods, and the GO that activated oxygen groups by EDC/NHS was deposited onto MTLPFG surface. On account of large specific surface area and oxygen containing groups of GO, the biomolecules can be adsorbed on the GO surface through amide groups and π-π stacking. The spectrum variation trend during sensing process obeys Langmuir absorption mode, which illustrates biomolecule adsorption on GO layer. The bare MTLPFG and GO-MTLPFG separately demonstrate the limit of detection (LOD) of 104.6 ng/ml and 25.79 ng/ml, which corresponding to the effectively detection range of 1–1000 ng/ml. Combination of GO and optical fibers exhibit great adaptive capacity to the biosensors, and provides inspirations for biochemical sensing applications.

Long-Period Fiber Grating Based on Side-Polished Optical Fiber and Its Sensing Application

A novel side-polished long-period fiber grating (LPFG) sensor was proposed and experimentally validated. Side-polished can provide a stronger evanescent field than traditional grating and bring superior sensitivity. The greater the side-polished depth, the higher the refractive index (RI) sensitivity. When <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$d = 44\,\,\mu \text{m}$ </tex-math></inline-formula> , the RI sensitivity reached 466.85 nm/RIU in the range of 1.3330–1.3580, which is fourfold higher than the LPFG prepared by the electric-arc discharge (EAD) method. A graphene oxide (GO) nano-film is coated on the LPFG to make it realize high sensitivity relative humidity (RH) sensing. Humidity sensitivity reached −0.193 nm/%RH in the range of 40%–80% RH. In addition, side-polished breaks the symmetry of the distribution of the cross-sectional light field, which determines the ability to achieve vector curvature measurement. It shows good sensing performance in the same/opposite bending direction as the side polished surface. When the input light polarization is 90°, the average sensitivity reaches 5.03 and −5.9 nm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{-1}$ </tex-math></inline-formula> in the range of 0–19.67 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{-1}$ </tex-math></inline-formula> , respectively. This strongly indicates that the fabricated sensors show high sensitivity, low-cost materials, and robust performance and break the limitations of the EDA method to prepare gratings, which have good application potential for biomedicine and the field of construction.

Rapid and sensitive detection of ammonia in water by a long period fiber grating sensor coated with sol-gel silica.

A sensitive ammonia sensor based on long-period fiber grating (LPFG) is designed and manufactured for the detection of ammonia concentration in water. Femtosecond laser direct writing technology is used to write LPFGs on standard single-mode silica fiber. A thin layer doped with basic dyes is coated on the optical fiber for sensing by using the sol-gel method. The thicknesses of sol-gel layers, which play a key role in the sensitivity of the LPFG sensor, were carefully studied. Experimental results show that LPFG with a functional layer of ∼340 nm has the best sensing performance, and the detection limit is 0.08 ppm. The response time of the sensor is less than one minute, and the sensor has good repeatability with a short recovery time. Compared with other organic molecules and ions in water, the proposed LPFG sensor has not only good reusability, but also selectivity for the detection of ammonia.

A Long-Period Fiber Grating Sensor Based on a Core-Cladding Misalignment Structure

In this article, a novel long-period fiber grating (LPFG) sensor based on a core-cladding misalignment structure is described. The LPFG consists of multiple short-length singlemode fiber (SMF) segments joined with core-cladding misalignment splicing. Due to the refractive index (RI) difference between the core and the cladding, periodic RI modulation along fiber axis is achieved. The resonance wavelength responses of the core-cladding misalignment structure based LPFG to RI, strain and temperature are investigated experimentally, and a maximum RI sensitivity of 992.8 nm/RIU, a strain sensitivity of −7.4 pm/μϵ, and a temperature sensitivity of −31 pm/°C are obtained, respectively. Such a core-cladding misalignment structure provides a low-cost fabrication route for LPFGs while offering high sensitivities, which has potential applications for accurate measurement and sensing in industrial and harsh environments.

Long-period fiber grating sensor interrogation with single strain modulated FBG and harmonic analysis

Optical fiber sensors have been used in several applications due to their unique advantages, such as immunity to electromagnetic interference, small size, light-weight, and robustness, for example. Long-period fiber grating (LPFG) is a type of fiber sensor with great sensitivity to several parameters, including the surrounding refractive index. However, the challenge in using LPFGs lies in their interrogation, which relates the change in optical characteristics with the measurand. This paper presents a novel method for the interrogation of LPFG sensors. This method uses a single FBG attached to a vibrating piezoelectric component and harmonic analysis of the optical power reflected by the FBG filter to estimate the resonant wavelength of an LPFG. The method was simulated to process several arc-induced LPFG spectra and achieved results comparable to the baseline method for half of the test set and improved by roughly 5 times the accuracy and precision by limiting the dynamic range.

Design of a coated thinly clad chalcogenide long-period fiber grating refractive index sensor based on dual-peak resonance near the phase matching turning point

A novel method for designing chalcogenide long-period fiber grating (LPFG) sensors based on the dual-peak resonance effect of the LPFG near the phase matching turning point (PMTP) is presented. Refractive index sensing in a high-refractive-index chalcogenide fiber is achieved with a coated thinly clad film. The dual-peak resonant characteristics near the PMTP and the refractive index sensing properties of the LPFG are analyzed first by the phase-matching condition of the LPFG. The effects of film parameters and cladding radius on the sensitivity of refractive index sensing are further discussed. The sensor is optimized by selecting the appropriate film parameters and cladding radius. Simulation results show that the ambient refractive index sensitivity of a dual-peak coated thinly clad chalcogenide LPFG at the PMTP can be 2400 nm/RIU, which is significantly higher than that of non-optimized gratings. It has great application potential in the field of chemical sensing and biosensors.

A high sensitivity sensor based on novel CLPFG with wavelength and intensity modulation for simultaneous measurement of SRI and temperature

In this paper, a novel cascaded long-period fiber grating (CLPFG) sensor based on the modulation of both wavelength and intensity is demonstrated, which is able to simultaneously detect surrounding refractive index (SRI) and temperature. The appropriate grating periods are designed by theoretical simulations. One grating operate in single-peak resonance state and the other one operates at slight deviation of resonance (SDOR) state. When the SRI and temperature change, the wavelength of resonant peak in the single-peak resonance state shifts, and the intensity of the resonant peak at the SDOR state changes accordingly. In experiments, the CLPFG is inscribed based on the theoretical design and the responses of the sensor to SRI and temperature are investigated. The experimental results show that the SRI sensitivities are −177.6 nm/RIU and −202.7 dB/RIU, and the temperature sensitivities are 0.1175 nm/℃ and 0.0997 dB/℃. The SRI and temperature sensitivities of our proposed sensor are 30 times and 10 times higher than those of other sensors with dual-parameter modulation of wavelength and intensity. The proposed sensor with simplified structure, good linearity, and high sensitivity can be expanded to the area of multi-parameter sensing.

A Fuzzy Approach to LPFG-Based Optical Sensor Processing and Interrogation

In this work, we report on a new interrogation method for long-period fiber-grating (LPFG) sensors. This type of sensor has been gaining increasing attention due its several interesting practical and metrological advantages. However, the LPFG optical signal is complex and could be difficult to process. We present a novel approach to the LPFG resonant wavelength detection and tracking using a multifilter LPFG interrogator. Multifilter LPFG interrogators present a great solution for LPFG sensor interrogation because there is no need for a priori knowledge concerning the LPFG sensor spectrum. We present a novel source-compensating preprocessing and a fuzzy inference system (FIS) to track the LPFG resonant wavelength. Our proposal was compared to two baseline models: a simple linear regression and an artificial neural network. Our results showed the new approach could reduce resonant wavelength uncertainty by almost three times comparing to results found in the literature.

Rapid and sensitive detection of Staphylococcus aureus by using a long-period fiber grating immunosensor coated with egg yolk antibody

The rapid and reliable detection of bacteria plays an important role in clinical and veterinary practice. A stable, label free, compact, and sensitive long-period fiber grating (LPFG) sensor based on egg yolk antibody (IgY) was proposed for the detection of Staphylococcus aureus (S. aureus). LPFG was fabricated with laser writing technology, and specific IgY was further immobilized on the grating region and then combined with the corresponding bacteria. S. aureus was detected by tracking the change of resonance wavelength in the LPFG transmission spectrum caused by bacteria-antibody interaction induced by the increase of biological cover thickness and density after the immune reaction. The testing results showed that the proposed sensor was selective and sensitive to S. aureus measurement, and the detection limit is approximately 33CFU/ml. The proposed antibody immobilization method is very simple, and the optical fiber can be manufactured in batch to reduce the cost. The detection time of the sensor is around 20min, which is fast and suitable for detection. The assay was successfully applied for the quantitative analysis of S. aureus in natural waters and met the needs of on-site screening trace pathogenic bacteria in food safety control.

3d-printed force sensitive structure using embedded long-period fiber grating

In this work, we present a low-cost tactile sensor based on arc-induced long–period fiber grating (LPG) embedded in polylactic acid (PLA) by fused deposition modeling (FDM). The 3d printing process used to embed the LPG, offers easy customization, flexibility, fast prototyping, and low-cost fabrication. At the same time, optical fiber sensors such as fiber Bragg gratings (FBGs) and LPGs are compact, resistant, and immune to electromagnetic interference. Therefore, the easy fabrication and low cost of arc-induced LPGs coupled to the 3d printing advantages offer low-cost custom force sensing elements with all the advantages of optical fiber sensors. This way, the proposed tactile sensor is a lightweight, small-size, and cost-effective tactile sensing scheme that offers easy implementation and tailoring to specific applications. In this study, we fabricated, embedded, calibrated, and evaluated the proposed LPG sensor tactile sensor. We showed this approach provided high force sensitivity (11.723 nm.N−1) with high linearity (99.28%), sub-decimal measurement error, and no drift.

Design of High-sensitivity π-PS-LPFG sensor based on SDOR for simultaneous measurement of SRI and temperature

In this paper, a π-phase-shifted long-period fiber grating (π-PS-LPFG) sensor structure operating near phase-match turning point (PMTP) is proposed, which is able to simultaneously detect surrounding refractive index (SRI) and temperature. First, the parameters of film, grating period and grating length are designed by theoretical simulations. Then, a π-phase-shift is introduced to make the single peak of the 25th cladding mode disappear, and two loss peaks are formed in both sides of the original single peak. When the external environment changes, the distance between the center wavelengths of the two loss peaks changes, while a single peak is formed again at the wavelength of the original 25th cladding mode with the change of intensity. Finally, the wavelength spacing of dual-peak and the intensity of the 25th cladding mode resonance peak with SRI and temperature are analyzed, and the results show a good fitting. The simulation results show that the SRI sensitivities based on wavelength modulation and intensity modulation are 9093.3 nm/RIU and −2870 dB/RIU respectively, and the temperature sensitivities based on wavelength modulation and intensity modulation are −0.3 nm/℃ and 0.217 dB/℃ respectively. Compared with other wavelength and intensity modulation based dual-parameter LPFG sensors, the SRI and temperature sensitivities of our proposed sensor structure are at least 10 times and 2 times higher than that of those LPFG sensors that have been reported. Its low fabrication cost, simple configuration, good fitting and high sensitivity make this sensor have potential applications in multi-parameter measurement.

A corrosion threshold-controllable sensing system of Fe-C coated long period fiber gratings for life-cycle mass loss measurement of steel bars with strain and temperature compensation

In this study, a corrosion threshold-controllable sensing system of long period fiber gratings (LPFG) is developed and validated for life-cycle monitoring of steel bars in corrosive environments. Three Fe-C coated LPFG sensors with two bare LPFG sensors in LP06 and LP07 modes for strain and temperature compensation were multiplexed and deployed inside three miniature, coaxial steel tubes to measure three (long-term in years) critical mass losses through the penetration of tube walls and their corresponding (short-term in hours) corrosion rates in the life span of steel bars. The strain/temperature and mass loss measurements are based on the changes in grating period and refractive index of surrounding medium, respectively. Thermal/mechanical loading and accelerated corrosion tests were conducted to validate the functionality, sensitivity, accuracy, and robustness of the proposed sensing system. Since both the steel tube and Fe-C layer represent the material composition of steel bars in the context of corrosion, the mass loss correlation among any two of the steel tube, Fe-C layer and steel bar is independent of the test conditions such as the current density and sample length, and thus applicable to engineering practices. The outer tube can notably delay and decelerate the corrosion process of its inner steel tube due to the reduced current effect.

Glutathione-functionalized long-period fiber gratings sensor based on surface plasmon resonance for detection of As3+ ions

Development of simple and accurate methods for the detection of As3+ is highly desirable and technically important. In this work, a highly sensitive and selective long-period fiber gratings sensor based on surface plasmon resonance was developed for As3+ detection by designing glutathione-functionalized Au nanoparticles as a signal amplification tag. Based on the chemical interaction between As3+ and glutathione, the self-assembling glutathione on the surface of the gold film combines selectively with As3+, and then anchors the glutathione-functionalized Au nanoparticles, which changes the refractive index of the surrounding environment, resulting in a shift of the transmission spectrum. Results show that the sensor could detect As3+ with concentrations ranging from 0.02 to 2 ppb. The sensor exhibited excellent specificity for As3+ against other metal ions, such as Na+, Fe3+, Mg2+, Cu2+, Pb2+, Ni2+, Ba2+, and Co3+. The fiber sensor was successfully employed to detect As3+ in pond water samples, demonstrating that it has the potential for As3+ detection with the advantages of low cost, high sensitivity, and a simple structure.

Black phosphorus thin film integrated long-period fiber grating for highly refractive index and biomolecules detection

We have developed a sensing platform using a long-period fiber grating (LPFG) integrated black phosphorus (BP) thin film from the numerical simulation analysis and experimental process verification. Due to the high optical absorption of BP, the attenuated depth of the cladding mode in the BP coated LPFG (named BP-LPFG) sensor can be varied by the change of external environment. Benefited from the BP film, the sensing system shows a superior enhancement in the refractive index (RI) detection compared to the use of bare LPFG. The sensitivity can reach 122.1 dB/RIU, 139.5 dB/RIU, and 931.9 dB/RIU in the RI regions of 1.33–1.38, 1.39–1.44, and near 1.455, respectively. Moreover, this sensing system has been applied to implement the highly sensitive and label-free detection for the bovine hemoglobin (BHb) molecules, with a sensitivity of 5.6 dB/(mg/mL) and a detection limit of 0.05 mg/mL achieved. The detection limit is 2600 times lower than the hemoglobin threshold for the confirmation of men’s anemia illness, defined by the World Health Organization (WHO). This newly sensing platform is promising in the early clinical diagnose for anemia illness.