Period Fiber Research Articles

High-sensitivity humidity and temperature sensor by cascading a polyimide coated long period fiber grating and a polydimethylsiloxane wrapped Mach-Zehnder interferometer.

Two different structures, long period fiber grating (LPFG) and a Mach-Zehnder interferometer (MZI), were cascaded to fabricate a sensor for sensing humidity and temperature simultaneously. Due to the humidity sensitive characteristic of polyimide (PI) and the temperature sensitive characteristic of polydimethylsiloxane (PDMS), the LPFG was coated with PI and the MZI was wrapped in PDMS to improve the humidity and temperature sensitivities, respectively. Humidity and temperature experiments and the stability and repeatability of the proposed sensors were performed. The sensor shows humidity and temperature sensitivities of ∼164 pm/%RH and 183 pm/°C in the humidity and temperature increasing experiments, respectively. Combining increasing, decreasing, and repeatable experiments of temperature and humidity, the proposed sensor shows stable performance in humidity sensitivity of ∼159 pm/%RH and temperature sensitivity of ∼175 pm/°C, with good linearity of ∼0.994. The structure of the proposed sensor has the advantages of low implementation cost, easy fabrication, and good stability.

Curvature sensor based on D-shape fiber long period fiber grating inscribed and polished by CO2 laser

In this paper, a D-shape fiber long period fiber grating (DLPFG) curvature sensor has been theoretically and experimentally investigated. Different from previous reported long period fiber grating (LPFG) curvature sensor based on D-shape single mode fiber (SMF), the fabrication of DLPFG is divided into two steps: LPFG with a low coupling efficiency is firstly written on a SMF by high-frequency CO2 laser, and then the grating area is subsequently polished to induce deeper resonance peaks and D-shape structure simultaneously. The polishing greatly enhances the fiber asymmetric modulation and thus results in the improvement of curvature sensitivity. Experiments show that the DLPFG has a high curvature sensitivity of 17.6 nm/m−1 and −11.9 nm/m−1 in two opposite directions within the plane perpendicular to CO2 laser-irradiated direction, which has exceeded that of the other reported LPFGs based on D-shape SMF. Besides, the temperature sensitivity is 45 pm/°C in range of −20 °C to 50 °C, and the temperature cross sensitivity is 2.58 × 10-3 m−1/°C. Therefore, this proposed DLPFG sensor with advantages of low-cost, easy-fabrication, high-reproducibility and high-sensitivity is promising for curvature measurement.

Reusable aptamer sensors based on long period fiber gratings for detecting Bisphenol A at ultralow concentrations

Bisphenol A migrated from plastic products can function as an endocrine-disrupting compound, leading to serious diseases. The construction of rapid, cost-effective, and user-friendly sensors to accurately monitor BPA at ultra-low levels has great significance for protecting human health. Here, we proposed an aptasensor combined with long-period fiber gratings (LPFGs) for the label-free detection of BPA molecules. A long DNA aptamer against BPA molecules was selected and immobilized on the surface of the LPFG through chemical modification. When exposed to BPA solutions, the long aptamer provides a large conformation transition to amplify the perturbation of the refractive index near the fiber surface, which is conducive to producing a measurable shift of the transmission spectra and improving the sensitivity of the BPA detection. As a result, an ultralow detection limit of 0.232 ng/mL was achieved, which is lower than the standard limit (1.5 ng/mL) in drinking water set by European Union (EU). The proposed sensor offers an effective strategy for BPA detection, featuring the advantages of a simple structure, low cost, excellent reusability, high sensitivity, and reliability.

Effect of embedded periodic fiber placement gap defects on the microstructure and bistable behavior of thermoplastic composite laminates

AbstractBy inducing periodic gaps, conventional automated fiber placement avoids overlap defects, which are generally prohibited in composite component manufacturing. However, bistable laminates are highly sensitive to imperfections, and it is critical to explore the effect of periodic gaps on the performance of bistable laminates. The principal curvatures and load–displacement curves of thermoplastic composite laminates with manually embedded periodic gaps was studied using experimental and numerical methods in this paper. SEM microscopy was used to observe the gap areas of the cross section of laminates to assess the microstructure of gaps with width variations after curing. In addition, a numerical method was proposed based on the experimentally determined evolution of gap widths and thicknesses of laminate. Subsequently, the effects of gap width, single‐layer thickness, and total thickness on the principal curvature were investigated. Gaps embedded in the 0° and 90° layers of bistable laminates demonstrated a substantial decrease in the snap load of the laminates. Interestingly, by embedding different gap widths in the 0° layers of the laminates, the snap load discrepancy between the snap‐through and snap‐back processes increased as the gap width widened, and the same trend was also found in the curvatures in the two stable states.Highlights Bistable characteristics of laminates with periodic gap defects. Utilizing gaps embedding in 0° layer to tailor the mechanical properties.

Distributed curvature sensing using long period fiber grating and machine learning numerical analysis.

In this numerical study, we propose a fiber distributed curvature sensor based on the analysis of the spectral transmission of a long period fiber grating (LPG) with a neural network. A simulation of the optical transmissions of a proposed 6-cm LPG structure for different curvature profiles is first performed using EigenMode Expansion and a coupled-mode theory algorithm. Both fiber curvature profiles and their corresponding optical transmission spectra are then injected into a four dense layer neural network which, after training, leads to a 0.40% relative median estimation error in the bending profiles. This paper demonstrates the efficiency of neural network-based optical sensors to analyze non-uniform perturbations, while also revealing long-period gratings to be promising candidates for such systems.

Dual-dip long-period fiber gratings for directional bending measurement

A high-sensitivity bending sensor based on dual-dip long period fiber gratings (LPFGs) in a single-mode fiber is proposed. The length of the gratings is very short only 7.2 mm. The asymmetric perturbation in fiber is generated during the fabrication, which makes the dual-dip LPFGs show different bending sensing characteristics at opposite directions. Meanwhile, the highest bending sensitivity of −35.91 nm/m−1 and −20.74 dB/m−1 can be obtained at bending directions of 0° and 180°, respectively. Furthermore, the torsion and temperature sensing characteristics are investigated experimentally. The torsion sensitivities follow sine function showing potential for directional sensing. The temperature sensitivities are 41.9 p.m./°C and 45.1 p.m./°C are close, which can eliminate the cross-sensitivity to temperature.

Femtosecond laser direct writing of long period fiber grating sensor with high refractive index sensitivity

Long-period fiber grating sensors with high refractive index sensitivity were designed and validated using femtosecond laser direct writing. The device consists of a single-mode fiber and a 2 cm long thin-core fiber fused. The refractive index is changed periodically by direct writing of a femtosecond laser pulse at the core of the thin-core fiber. Fiber gratings with periods of 80 μm, 150 μm, and 300 μm were selected for the study and showed maximum refractive indices of 1.4054, 1.4215, and 1.4326, respectively. With an 80 μm period, the maximum refractive index sensitivity increases significantly with wavelength and reaches 1430 nm/RIU. The temperature sensitivity is 7.6 pm/°C between 30 °C–430 °C and 14.4 pm/°C between 430 °C–630 °C. The sensor is high-temperature resistant, robust, simple to fabricate, and has a wide range of measurable refractive indices. This thin-core long-period fiber grating can better reduce crosstalk due to temperature effects in refractive index testing.

Theoretical analysis based on angle cascaded long period fiber grating for surrounding refractive index and temperature measurement

An angularly cascaded long period fiber grating (AC-LPFG) sensor for simultaneously measuring refractive index (SRI) and temperature in the surrounding environment has been proposed and theoretically demonstrated. The sensor consists of an un-tilted LPFG and a tilted LPFG with the same grating period, which has a transmission spectrum with typical three peaks. Our simulation results show that the simultaneous measuring of SRI and temperature can be achieved by monitoring the transmissivity in the middle peak and the wavelength distance of the two other peaks. For the transmissivity of the middle peak, the SRI and temperature sensitivities are –180.53 dB/RIU and 0.04 dB/℃, respectively. For the wavelength distance between the other two peaks, the SRI and temperature sensitivities are 604.55 nm/RIU and –0.19 nm/℃, respectively. From the perspective of intensity response, the maximum SRI of our sensor is 5 times higher than the cascaded LPFG-based sensors with dual-parameter ability.

Highly sensitive refractive index and magnetic field sensor based on long-period fiber grating inscribed in tapered optical fiber

A refractive index (RI) and magnetic field sensor based on tapered long period fiber grating (LPFG) is proposed. We prepared two different periods of LPFGs (LPFG1 and LPFG2) using high-frequency CO2 laser point by point method on tapered single-mode fiber. Firstly, LPFG1 is used to measure the RI of sodium chloride solution. LPFG1 is immersed in sodium chloride solutions with different RI to measure the sensitivity of LPFG to RI. The experimental results indicate that LPFG1 is particularly sensitive to changes in environmental RI, with a sensitivity of 381.820 nm/RIU. Due to the special sensitivity of tapered LPFG to environmental RI, in order to eliminate the influence of residual sodium chloride on the surface of LPFG1 on magnetic field measurement, we use LPFG2 with similar parameters to measure the environmental magnetic field. LPFG2 was encapsulated in a glass capillary tube filled with the magnetic fluid (MF). After the environmental magnetic field changed, the RI of MF changed, causing the resonance peak wavelength of LPFG2 to drift. We measured the magnetic field using LPFG2 within the magnetic field range of 0.66 mT to 20 mT. Only in the magnetic field range of 1.95 mT to 4.6 mT, good linear fitting was achieved, with a magnetic field sensitivity of 1.242 nm/mT and a fitting coefficient of 0.9852. The experimental results indicate that LPFG inscribed in tapered optical fiber has high sensitivity to environmental RI and magnetic field. The sensor has a simple structure, small volume, easy production, and high sensitivity, which has certain practical application prospects.

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.

Tunable Sensitivity in Long Period Fiber Gratings During Mode Transition With Low Refractive Index Intermediate Layer

Double-clad fibers where the second cladding has a lower refractive index than the first cladding, prove to be ideal structures for potentiating and tuning the sensitivity in long-period fiber gratings (LPFGs) operating in mode transition. When a thin film is deposited on the optical fiber, the second cladding performs acts as a barrier that initially prevents the transition to guidance in the thin film of one of the modes guided in the first cladding. Finally, the transition to guidance occurs with a sensitivity increase, in analogy to the tunnel effect observed in semiconductors. This improvement has been demonstrated both as a function of the thin film thickness and the surrounding medium refractive index, with enhancement factors of 4 and 2, respectively. This idea reinforces the performance of LPFGs, adding a new degree of freedom to the mode transition and the dispersion turning point phenomena. Moreover, the control of the variation of the effective index of cladding modes could be applied in other structures, such as tilted-fiber gratings or evanescent wave sensors.

Low-Cost Wideband Interrogation System for Fiber Optic Sensors

The interrogation of optical fiber sensors (OFS) often relies on complex devices such as Optical Spectrum Analyzers (OSA) which are expensive with low portability and mainly suited to laboratory measurements or dedicated interrogation systems with limited spectral range. An interrogation unit was designed and fabricated using a photodetector combined with a Micro Electro-mechanical System and a Fabry-Pérot Interferometer (MEMS-FPI) working as a tunable filter with a response in the range 1350 to 1650nm. Deconvolution techniques were applied to mitigate the effect of the broadband response of the tunable filter on the measured signal. The performance of the unit was validated with the interrogation of Long Period Fiber Gratings (LPFG) as temperature, refractive index (RI) and relative humidity (RH) sensors. For the temperature a sensitivity of 0.135 ± 0.007nm/°C was obtained, which showed a 4.9% relative error when compared to the same measurement with an OSA. For the RI a sensitivity of 147 ± 11 nm/RIU was obtained which showed a relative error lower than 1% when compared to the OSA. For the humidity, sensitivities of 0.742 ± 0.005 nm/%RH and 0.056 ± 0.006 nm/%RH were obtained, with errors of 2.75% and 6.67% respectively, when compared to a commercial dedicated interrogation system. The low relative error obtained when compared to commercial alternatives, shows the potential of the system to be used in real-time applications that require portability, low-cost, energy efficiency and capacity for integration in dedicated systems.

(Invited) high speed densely dispersion-managed soliton and four-wave-mixing in dense periodical fibers and conventional dispersion-managed systems

I briefly reviewed the interesting densely dispersion-managed soliton (DDMS), and single channel high speed (40–160 Gbit/s) DDMS can transmit record distance at that time. I briefly reviewed the four-wave-mixing (FWM) of continous wave (CW) lasers in DPF when the EDFAs are located in the middle of dispersion section (z2 section), and our relative FWM efficiency formula was the first formula even for conventional dispersion-managed system (CDM) as well. We reached much lower FWM efficiency over a broad wavelength range in the well designed DPF than that in CDM and constant dispersion systems. We first found the FWM resonant condition for our dense periodical fiber (DPF) and CDM system. FWM strongly relates to the location of EDFAs. In this paper, I first report the analytical formula of relative FWM of CW lasers in DPF when the EDFAs are located right after the z1 section with dispersion D1, and our results are also suitable for CDM systems.

An Erbium-Doped Fiber Source with Near-Gaussian-Shaped Spectrum Based on Double-Stage Energy Matching and LPFGs

An Erbium-doped fiber source with a near-Gaussian-shaped spectrum consisting of only a single peak, based on the double-stage energy matching and LPFGs, is proposed and demonstrated. A double-stage Erbium-doped fiber source system is built. The first-stage structure adopts the single-pass forward pumping method with a 15 m Erbium-doped fiber, while the second-stage structure adopts the backward-pumping method with a 26.5 m Erbium-doped fiber. The energy of the output spectrum is concentrated near the long wavelength (1560 nm) through the double-pump energy matching of the two stages. Long period fiber gratings (LPFGs) are used to filter the excess light near the short wavelength (1530 nm) in order to obtain a near-Gaussian-shaped spectrum consisting of only a single peak. The output power, pump conversion efficiency, line width, 3 dB bandwidth and mean wavelength of this near-Gaussian-shaped spectrum are tested and analyzed. When the pump powers of the first and second stages are 50 mW and 360 mW, respectively, the results show that a near-Gaussian-shaped spectrum with a power of 10.17 mW, spectral line width of 19.767 nm, and mean wavelength stability of −0.978 ppm/mW can be obtained. This research provides a method for the generation of a near-Gaussian-shaped spectrum with high output power and excellent mean wavelength stability, and it can produce multiple forms and energies of near-Gaussian-shaped spectra via this Erbium-doped fiber source.

FBG-LPFG-Based Sensor to Monitor 3-D Strain in Ice During Freezing–Melting Processes

Three-dimensional strain (3D strain) and the principal strain in ice are very important for glacier movement research and glacier-related disaster prediction. The internal 3D strain of ice is sensitive to temperature, and it is very complex during the whole freezing and melting process including phase transformation. In this study, we present a 3D strain sensor (3D-S Sensor) based on fiber Bragg grating (FBG) and long period fiber grating (LPFG) that can measure the 3D strain in ice. The LPFG (the temperature sensitivity coefficient is 56.3 pm/°C) was fabricated by the laser etching method, which was used for the temperature compensation of FBGs to improve the accuracy of strain measurement. The 3D principal strain can be calculated by the value of 6 direction strains from FBGs. A freeze-melt experiment is designed to verify the performance of 3D-S Sensor. The experimental results show that the sensor can measure the complex 3D deformation of ice from a more precise angle. Among the 6 directions of strain measured by the sensor, the strain value in the Z direction is the largest (ε <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>z</i></sub> = 6232.9 με). The first principal strain in ice is close to the negative direction of the Z axis (θ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>Z</i></sub> = 160.7°), and the maximum value is 6638.2 με. The measured results are consistent with the actual deformation of ice, indicating that the sensor can effectively measure the 3D strain and principal strain inside the ice.

A Seawater Salinity Sensor Based on Optimized Long Period Fiber Grating in the Dispersion Turning Point.

Variations of seawater salinity often cause ocean internal waves, water masses and stratification, which affect the stability of the ocean environment. Therefore, the study of seawater salinity is significant for the prediction of changes in the ocean environment. However, existing methods for measuring seawater salinity generally have the disadvantages of low sensitivity and low accuracy. In this work, we proposed a seawater salinity sensor based on long period fiber grating (LPFG) in the dispersion turning point (DTP), which has demonstrated the possibility to fabricate LPFG with a shorter grating period by CO2 laser in a thin single mode fiber (SMF) of 80 μm cladding diameter without etching. For obtaining higher sensitivity that could meet the measurement requirement in practice, the proposed sensor was optimized by combining etching cladding and DTP. After the LPFG working near DTP was fabricated by a CO2 laser, the cladding diameter was reduced to 57.14 μm for making cladding mode LP1,7 work near DTP by hydrofluoric acid (HF) solutions. The experimental results have demonstrated that a sensitivity of 0.571 nm/‱ can be achieved when the salinity increases from 5.001‱ to 39.996‱, and the sensor shows good repeatability and stability. Based on its excellent performance, the optimized LPFG is a prospective sensor to monitor seawater salinity in real time. Meanwhile, a low-cost way was provided to make LPFG work near DTP instead of ultraviolet exposure and femtosecond laser writing.

Minimum potential method appropriate to generate 2D RVEs of composites with high fiber volume fraction

This work proposes a novel method, entitled minimum potential method, to generate Representative Volume Elements (RVEs) of unidirectional fibers composites with high fiber volume fractions (FVFs, generally ≥70.0%). In the proposed method, a given VF of random and periodic fibers are initially created within the matrix and the fiber overlap is allowed, and then the fiber overlap is gradually removed by adjusting the fiber centroids based on the principle of the minimum overall potential. In the process of fiber overlap removal, the fiber periodicity and the fibers completely or partly contained in the matrix are preserved. The overall potential of the fibers consists of fiber overlap potential, fiber position potential and fiber periodicity potential, and is minimized using the gradient descent method. The numerical implementation of the proposed method is detailed, and the RVEs of the composites with various FVFs are generated. Through the probability distribution function of the nearest neighbor distance, the cumulative probability distribution function of the nearest neighbor orientation angle, the second-order intensity and pair correlation functions, the fiber distribution in the generated RVEs is analyzed, and the results indicate a completely random distribution of the fibers in the generated RVEs. Comparing with the available experimental test, the literature and the Double-Inclusion model, the proposed method to generate RVEs of the composites with high FVFs is validated.

Stochastic analysis of a three phase composite using variational asymptotic based homogenization technique coupled with stochastic reduced order model

The current work describes a stochastic framework to investigate the static response characteristics of periodic and random fiber arrays of reinforced three-phase composites with aleatory mechanical and thermal constituent properties. These properties were randomly modeled via Gaussian distribution using the statistical quantities of first and second moments. The framework is based on Variational Asymptotic Method (VAM) based homogenization technique coupled with Stochastic Reduced Order Model (SROM). When compared against the framework of VAM coupled with the traditional Monte Carlo Simulation (MCS) method, the computational cost of the SROM model with VAM was significantly less. Good agreement with solutions from traditional MCS combined with VAM was also achieved. To demonstrate applicability of the framework, a multivariate stochastic analysis was performed to determine the effect of fiber arrangements on the randomness of mechanical and thermal properties in a three-phase composite. Additionally, univariate stochastic analysis was performed, and the sensitivity of each constituent parameter corresponding to the stochastic homogenized response variable was investigated using the bivariate correlation and linear regression analysis methods.

A chirped long period fiber grating sensor based on micro-helix taper

A sensor based on micro-helix taper chirped long period fiber grating (MHT-CLPG) is proposed and experimentally demonstrated. The MHT-CLPG is fabricated by periodically tapering and twisting a standard single-mode fiber (SMF) using arc discharge technology. A chirped structure is prepared with gradual increasing the distance between two adjacent microtapers. The measured results show that the MHT-CLPG can realize highly sensitive torsion, axial strain and bending measurements (-0.1575 nm/(rad/m), −0.006846 nm/με and −10.62 nm/m−1). The proposed sensor has a good potential for torsion, axial strain and bending measurement due to low cost, simple fabrication, high extinction ratio and good performance.

Ultra-Wideband OAM Mode Generator Based on a Helical Grating Written in a Graded-Index Few-Mode Fiber

An ultra-wideband orbital-angular-momentum (OAM) mode generator has been proposed and demonstrated both theoretically and experimentally, which is based on a helical long period fiber grating (HLPG) but written in a graded-index few-mode fiber (GIFMF). Unlike all the HLPG-based broadband OAM generators reported to date, where the utilized HLPG (fabricated only in step-index fibers) all were operated at wavelengths very close to the dispersion-turning point (DTP), the GIFMF-based HLPG proposed in this study is, however, operated at wavelengths far from its DTP, which provides a new and robust method enabling to produce a ultra-wideband OAM mode generator with an ultra-high conversion-efficiency and yet with a great flexibility in controlling the central wavelength of itself. As a typical example, the first-order OAM mode generator with a central wavelength of ∼1510 nm, a bandwidth of ∼235 nm, and a conversion-efficiency larger than 99% (∼20.7 dB) has been successfully obtained. To the best of our knowledge, this is the first report for practical demonstration of the GIFMF-based HLPG.