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This article presents the refractive index (RI) sensitivity study of cladding reduced uniform long-period fiber grating. The displacement of resonant wavelength with the change in ambient RI is observed to inspect the sensitivity. Here, the grating period is fixed at a value (550 μm) to analyze RI sensitivity of cladding modes (HE13-HE14). However, it varies in earlier mathematical studies to adjust the resonant wavelength of all modes at a specific wavelength. The three-layer fiber geometry-based mathematical approach is employed to estimate the core and cladding modes. The obtained results support decline in quantity of cladding modes sustained by a single-mode fiber. The decrease in cladding radius has shown enhancement in RI sensitivity. For a reduction of 22 μm in cladding radius, the highest ordered HE14 cladding mode has shown 5.8 times improvement in its sensitivity. [ABSTRACT FROM AUTHOR]

This article presents a long-period fiber-grating sensor based on a congruent quasi-helical structure (CQH-LPFG) with the two-parameter measurement of both temperature and curvature. The CQH-LPFG sensor was manufactured using a high-frequency CO2 laser, and an innovative quasi-helical structure was introduced into the two-parameter measurement of the temperature and curvature of the optical fiber sensor with excellent results. The experiment and analysis demonstrate that the curvature sensitivities of the three resonance peaks in the 1440 nm to 1540 nm transmission spectrum were 11.88 nm/m−1, 8.05 nm/m−1, and 11.11 nm/m−1, and the curvature varied ranging from 0.156 m−1 to 0.494 m−1. The three resonance peaks showed temperature responsivities of 29.87 pm/°C, 24.65 pm/°C, and 36.85 pm/°C, respectively, and the linear fit was of excellent quality. In the case of measuring both curvature and temperature changes simultaneously, the resonant peak wavelength of the CQH-LPFG sensor was demodulated through matrix analysis, with dip A and dip C providing superior simultaneous measurements. These features make it a promising candidate for applications such as engineering machinery and the health inspection of buildings. [ABSTRACT FROM AUTHOR]

We propose and demonstrate the simulation and fabrication of an all-fiber orbital angular momentum (OAM) mode converter capable of generating first- to fourth-order modes simultaneously, which is realized by inscribing a cascaded preset-twist long-period fiber grating (CPT-LPFG) in a six-mode fiber utilizing a CO2 laser. A new segmented Runge–Kutta method is proposed to simulate the preset-twist long-period fiber gratings. By calculating the twist angle and relative coupling coefficient for each pitch and then solving the coupled mode equations utilizing the Runge–Kutta algorithm. The simulation illustrates that the preset-twist method significantly improves the coupling coefficient of higher-order modes, thereby reducing coupling difficulty. In the experiment, by twisting the fiber at an angle of 1080° and fabricating cascaded gratings with periods of 745 μm, 310 μm, 204 μm, and 146 μm, it is feasible to generate first- to fourth-order OAM modes simultaneously, at wavelengths of 1635 nm, 1548 nm, 1460 nm, and 1334 nm, respectively. The insertion loss is less than 1 dB, and the mode purity is over 90 %. To the best of our knowledge, this is the first time that first- to fourth-order OAM modes are simultaneously generated utilizing a single long-period fiber grating. [ABSTRACT FROM AUTHOR]

The curing of epoxy resin is a complex thermo-chemical process that is difficult to monitor using existing sensing systems. We monitored the curing process of an epoxy resin by using long-period fiber gratings. The refractive index of the epoxy resin increases during the curing process and can be measured to determine the degree of curing. We employed long-period fiber gratings that are sensitive to the refractive index of an external medium for the measurement of refractive index changes in the resin. We observed that the resonances of long-period fiber gratings increased their depth with the increased refractive index of the resin, which was well described by our simulation taking the coupling to radiation modes into account. We demonstrated that the degree of cure can be estimated from the depth of the grating resonances using a phenomenological model. At the same time, long-period fiber gratings are sensitive to temperature variations and internal strains that are induced during curing. These factors may affect the measurements of curing degree and should also be addressed. [ABSTRACT FROM AUTHOR]

An optical fiber pH sensor based on a long-period fiber grating (LPFG) is reported. Two oppositely charged polymers, polyethylenimine (PEI) and polyacrylic acid (PAA), were alternately deposited on the sensing structure through a layer-by-layer (LbL) electrostatic self-assembly technique. Since the polymers are pH sensitive, their refractive index (RI) varies when the pH of the solution changes due to swelling/deswelling phenomena. The fabricated multilayer coating retained a similar property, enabling its use in pH-sensing applications. The pH of the PAA dipping solution was tuned so that a coated LPFG achieved a pH sensitivity of (6.3 ± 0.2) nm/pH in the 5.92–9.23 pH range. Only two bilayers of PEI/PAA were used as an overlay, which reduces the fabrication time and increases the reproducibility of the sensor, and its reversibility and repeatability were demonstrated by tracking the resonance band position throughout multiple cycles between different pH solutions. With simulation work and experimental results from a low-finesse Fabry–Perot (FP) cavity on a fiber tip, the coating properties were estimated. When saturated at low pH, it has a thickness of 200 nm and 1.53 ± 0.01 RI, expanding up to 310 nm with a 1.35 ± 0.01 RI at higher pH values, mostly due to the structural changes in the PAA. [ABSTRACT FROM AUTHOR]

Long-period fiber gratings (LPFGs) functioning as band-reject filters have played a pivotal role in the realm of optical communication. Since their initial documentation in 1996, LPFGs have witnessed rapid advancements in areas such as optical sensing, the equalization of optical amplification, and optical band-pass filtering, etc. The unique attributes of optical fiber-based grating, including their miniaturized size, cost-effectiveness, and immunity to electromagnetic interference, have contributed significantly to various sectors over the last two decades. This paper presents a review of the evolution of LPFGs, with a specific focus on the progression and current trends of mechanically induced long-period fiber gratings. It offers a concise overview of coupled-mode theory, the fabrication processes, the merits, and the limitations associated with mechanically induced LPFGs. Moreover, this review elucidates the application methodologies of mechanically induced LPFGs and anticipates future directions in this field. [ABSTRACT FROM AUTHOR]

Mode division multiplexing (MDM) is currently one of the most attractive multiplexing techniques in optical communications, as it allows for an increase in the number of channels available for data transmission. Optical modal converters are one of the main devices used in this technique. Therefore, the characterization and improvement of these devices are of great current interest. In this work, we present a dataset of 49,736 near-field intensity images of a modal converter based on a long-period fiber grating (LPFG) written on a few-mode fiber (FMF). This characterization was performed experimentally at various wavelengths, polarizations, and temperature conditions when the device converted from LP 01 mode to LP 11 mode. The results show that the modal converter can be tuned by adjusting these parameters, and that its operation is optimal under specific circumstances which have a great impact on its performance. Additionally, the potential application of the database is validated in this work. A modal decomposition technique based on the particle swarm algorithm (PSO) was employed as a tool for determining the most effective combinations of modal weights and relative phases from the spatial distributions collected in the dataset. The proposed dataset can open up new opportunities for researchers working on image segmentation, detection, and classification problems related to MDM technology. In addition, we implement novel artificial intelligence techniques that can help in finding the optimal operating conditions for this type of device. [ABSTRACT FROM AUTHOR]

This article presents a long-period fiber-grating sensor based on a congruent quasi-helical structure (CQH-LPFG) with the two-parameter measurement of both temperature and curvature. The CQH-LPFG sensor was manufactured using a high-frequency CO2 laser, and an innovative quasi-helical structure was introduced into the two-parameter measurement of the temperature and curvature of the optical fiber sensor with excellent results. The experiment and analysis demonstrate that the curvature sensitivities of the three resonance peaks in the 1440 nm to 1540 nm transmission spectrum were 11.88 nm/m−1, 8.05 nm/m−1, and 11.11 nm/m−1, and the curvature varied ranging from 0.156 m−1 to 0.494 m−1. The three resonance peaks showed temperature responsivities of 29.87 pm/°C, 24.65 pm/°C, and 36.85 pm/°C, respectively, and the linear fit was of excellent quality. In the case of measuring both curvature and temperature changes simultaneously, the resonant peak wavelength of the CQH-LPFG sensor was demodulated through matrix analysis, with dip A and dip C providing superior simultaneous measurements. These features make it a promising candidate for applications such as engineering machinery and the health inspection of buildings.

Concrete is the most commonly used construction material nowadays. With emerging cutting-edge technologies such as nanomaterials (graphene, carbon nanotubes, etc.), advanced sensing (fiber optics, computer tomography, etc.), and artificial intelligence, concrete can now achieve self-sensing, self-healing, and ultrahigh performance. The concept and functions of smart concrete have thus been partially realized. However, due to the wider application location (coastal areas, cold regions, offshore, and deep ocean scenarios) and changing climate (temperature increase, more CO2 emissions, higher moisture, etc.), durability monitoring (pH, ion penetration, carbonation, corrosion, etc.) becomes an essential component for smart concrete. Fiber optic sensors (FOS) have been widely explored in recent years for concrete durability monitoring due to their advantages of high sensitivity, immunity to harsh environments, small size, and superior sensitivity. The purpose of this review is to summarize FOS development and its application in concrete durability monitoring in recent years. The objectives of this study are to (1) introduce the working principle of FOS, including fiber Bragg grating (FBG), long-period fiber grating (LPFG), surface plasmon resonance (SPR), fluorescence-based sensors, and distributed fiber optic sensors (DFOS); (2) compare the sensitivity, resolution, and application scenarios of each sensor; and (3) discuss the advantages and disadvantages of FOS in concrete durability monitoring. This review is expected to promote technical development and provide potential research paths in the future for FOS in durability monitoring in smart concrete. [ABSTRACT FROM AUTHOR]

The curing of epoxy resin is a complex thermo-chemical process that is difficult to monitor using existing sensing systems. We monitored the curing process of an epoxy resin by using long-period fiber gratings. The refractive index of the epoxy resin increases during the curing process and can be measured to determine the degree of curing. We employed long-period fiber gratings that are sensitive to the refractive index of an external medium for the measurement of refractive index changes in the resin. We observed that the resonances of long-period fiber gratings increased their depth with the increased refractive index of the resin, which was well described by our simulation taking the coupling to radiation modes into account. We demonstrated that the degree of cure can be estimated from the depth of the grating resonances using a phenomenological model. At the same time, long-period fiber gratings are sensitive to temperature variations and internal strains that are induced during curing. These factors may affect the measurements of curing degree and should also be addressed.

An optical fiber pH sensor based on a long-period fiber grating (LPFG) is reported. Two oppositely charged polymers, polyethylenimine (PEI) and polyacrylic acid (PAA), were alternately deposited on the sensing structure through a layer-by-layer (LbL) electrostatic self-assembly technique. Since the polymers are pH sensitive, their refractive index (RI) varies when the pH of the solution changes due to swelling/deswelling phenomena. The fabricated multilayer coating retained a similar property, enabling its use in pH-sensing applications. The pH of the PAA dipping solution was tuned so that a coated LPFG achieved a pH sensitivity of (6.3 ± 0.2) nm/pH in the 5.92–9.23 pH range. Only two bilayers of PEI/PAA were used as an overlay, which reduces the fabrication time and increases the reproducibility of the sensor, and its reversibility and repeatability were demonstrated by tracking the resonance band position throughout multiple cycles between different pH solutions. With simulation work and experimental results from a low-finesse Fabry–Perot (FP) cavity on a fiber tip, the coating properties were estimated. When saturated at low pH, it has a thickness of 200 nm and 1.53 ± 0.01 RI, expanding up to 310 nm with a 1.35 ± 0.01 RI at higher pH values, mostly due to the structural changes in the PAA.

A methodology to enhance the sensitivity of long-period fiber gratings (LPFGs) based on the combination of three different enhancement approaches is presented; the methods here adopted are the working near mode transition (MT) of a cladding mode (CM), working near the turn-around point of a CM and the enhancement of the evanescent field of CMs by reducing the cladding diameter or by increasing the order number of CMs. In order to combine these enhancement methodologies, an electrostatic self-assembly (ESA) process was used to deposit a polymeric overlay, with a chosen thickness, onto the etched fiber. The add-layer sensitivity of the sensor was theoretically calculated, and the demonstration of the real applicability of the developed LPFG as a biosensor was performed by means of an IgG/anti-IgG immunoassay in human serum in a thermostated microfluidic system. The limits of detection (LODs) calculated by following different procedures (three times the standard deviation of the blank and the mean value of the residuals) were 6.9 × 10−8 µg/mL and 4.5 × 10−6 µg/mL, respectively. The calculated LODs demonstrate the effectiveness of the applied methodology for sensitivity enhancement. [ABSTRACT FROM AUTHOR]

The monitoring and conversion of photonic orbital angular momentum (OAM) play fundamental and important roles for both classic and quantum technologies, especially in low-loss transmission media such as ring-core fibers (RCFs), which make many OAM applications practical or vastly more flexible. However, in a RCF, the modes associated with different OAM states are highly overlapping due to the circular refractive index distribution structure, which makes it difficult to distinguish and monitor the OAM modes and in turn limits its inline conversion. Here, we report the first experimental realization of mode monitoring in a RCF using mode filters (MFs), which takes advantage of the difference in the mode adiabatic evolution and the higher-order mode cutoff conditions in tapered RCFs. Different-order OAM can be filtered using MFs with different geometric parameters, as demonstrated by the linearly polarized mode intensity. Combined the mode manipulations in RCF and single-mode fiber, the fundamental mode coupling efficiency can reach 90%, the RCF mode conversion monitoring through inline transmission spectrum evolution can be realized, and the inline fabrication of RCF grating, which couples one mode to a desired mode, can be demonstrated by the fabricating process of three long-period fiber gratings. The mode conversion efficiency between 0-order and 1, 2- or 3-order OAM modes exceeds 96%. Our work provides an efficient approach to monitor and convert OAM modes in higher-order mode supporting RCFs and even other special fibers and further promotes the improvement of the capacity of OAM transmission in RCFs.

In this study, a new temperature sensor with high sensitivity was achieved by four-layer Ge and B co-doped long-period fiber grating (LPFG) based on the mode coupling principle. By analyzing the mode conversion, the influence of the surrounding refractive index (SRI), the thickness and the refractive index of the film on the sensitivity of the sensor is studied. When 10 nm-thick titanium dioxide (TiO2) film is coated on the surface of the bare LPFG, the refractive index sensitivity of the sensor can be initially improved. Packaging PC452 UV-curable adhesive with a high-thermoluminescence coefficient for temperature sensitization can realize high-sensitivity temperature sensing and meet the requirements of ocean temperature detection. Finally, the effects of salt and protein attachment on the sensitivity are analyzed, which provides a reference for the subsequent application. The sensitivity of 3.8 nm/°C in the range of 5–30 °C was achieved for this new sensor, and the resolution is about 0.00026 °C, which is over 20 times higher than ordinary temperature sensors. This new sensor meets the accuracy and range of general ocean temperature measurements and could be used in various marine monitoring and environmental protection applications. [ABSTRACT FROM AUTHOR]

A high-sensitivity axial force sensor with a large measurement range based on a dual-peak long-period fiber grating (LPFG) is proposed and experimentally demonstrated. Previously, the relationship between the grating period and the dual-peak wavelengths has been investigated based on the coupled-mode theory. In our experiment, the LPFG was fabricated in our laboratory by illuminating the fiber core with the aid of a 213 nm UV laser. The sensitivity of the proposed axial force sensor can reach −14.047 nm/N in the force range from 0.490 N to 4.508 N. Taking the advantages of a compact size, low cost, and large measurement range, our force sensor has more applicable abilities in harsh environments. [ABSTRACT FROM AUTHOR]

Long-period fiber gratings (LPFGs) functioning as band-reject filters have played a pivotal role in the realm of optical communication. Since their initial documentation in 1996, LPFGs have witnessed rapid advancements in areas such as optical sensing, the equalization of optical amplification, and optical band-pass filtering, etc. The unique attributes of optical fiber-based grating, including their miniaturized size, cost-effectiveness, and immunity to electromagnetic interference, have contributed significantly to various sectors over the last two decades. This paper presents a review of the evolution of LPFGs, with a specific focus on the progression and current trends of mechanically induced long-period fiber gratings. It offers a concise overview of coupled-mode theory, the fabrication processes, the merits, and the limitations associated with mechanically induced LPFGs. Moreover, this review elucidates the application methodologies of mechanically induced LPFGs and anticipates future directions in this field.

Mode division multiplexing (MDM) is currently one of the most attractive multiplexing techniques in optical communications, as it allows for an increase in the number of channels available for data transmission. Optical modal converters are one of the main devices used in this technique. Therefore, the characterization and improvement of these devices are of great current interest. In this work, we present a dataset of 49,736 near-field intensity images of a modal converter based on a long-period fiber grating (LPFG) written on a few-mode fiber (FMF). This characterization was performed experimentally at various wavelengths, polarizations, and temperature conditions when the device converted from LP01 mode to LP11 mode. The results show that the modal converter can be tuned by adjusting these parameters, and that its operation is optimal under specific circumstances which have a great impact on its performance. Additionally, the potential application of the database is validated in this work. A modal decomposition technique based on the particle swarm algorithm (PSO) was employed as a tool for determining the most effective combinations of modal weights and relative phases from the spatial distributions collected in the dataset. The proposed dataset can open up new opportunities for researchers working on image segmentation, detection, and classification problems related to MDM technology. In addition, we implement novel artificial intelligence techniques that can help in finding the optimal operating conditions for this type of device.

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Acetone detection in human breath is important since it is a diabetes mellitus biomarker frequently found in concentrations in the order of 1.8 ppm for human patients. On the other hand the use of new data analysis techniques in Mach–Zehnder interferometer sensor systems have been increasing since they allow to explore the behavior of all range of transmission or reflection spectrum, providing more information that is not evident or sometimes perceptible on such spectra, hence other kind of techniques for characteristics extraction and data analysis such as principal component analysis, and principal components regression are used. They are methods that reduce dimensionality and can find some relationship or pattern from the data under analysis. In this work we show the performance of these techniques on the measured responses of three optical fiber sensors that are based on a Mach–Zehnder interferometer in reflection mode. The limit of detection found using principal component regression was approximately 1.5 ppm for acetone, which could be used to detect diabetes mellitus using breath analysis. • Limit of detection improvement for acetone detection. • Sensor response analysis using principal components regression. • Sensor sensitivity improvement by Mach–Zehnder interferometer in reflection mode. • Acetone sensor fabrication by coating optical fiber devices with polydimethylsiloxane. [ABSTRACT FROM AUTHOR]

Concrete is the most commonly used construction material nowadays. With emerging cutting-edge technologies such as nanomaterials (graphene, carbon nanotubes, etc.), advanced sensing (fiber optics, computer tomography, etc.), and artificial intelligence, concrete can now achieve self-sensing, self-healing, and ultrahigh performance. The concept and functions of smart concrete have thus been partially realized. However, due to the wider application location (coastal areas, cold regions, offshore, and deep ocean scenarios) and changing climate (temperature increase, more CO2 emissions, higher moisture, etc.), durability monitoring (pH, ion penetration, carbonation, corrosion, etc.) becomes an essential component for smart concrete. Fiber optic sensors (FOS) have been widely explored in recent years for concrete durability monitoring due to their advantages of high sensitivity, immunity to harsh environments, small size, and superior sensitivity. The purpose of this review is to summarize FOS development and its application in concrete durability monitoring in recent years. The objectives of this study are to (1) introduce the working principle of FOS, including fiber Bragg grating (FBG), long-period fiber grating (LPFG), surface plasmon resonance (SPR), fluorescence-based sensors, and distributed fiber optic sensors (DFOS); (2) compare the sensitivity, resolution, and application scenarios of each sensor; and (3) discuss the advantages and disadvantages of FOS in concrete durability monitoring. This review is expected to promote technical development and provide potential research paths in the future for FOS in durability monitoring in smart concrete.

The influences on the tuning range of a wavelength-tunable L-band mode-locked fiber lasers incorporating a long-period fiber grating (LPFG) are investigated. Through controlling both resonance wavelength and bandwidth of the LPFG, the net gain of the laser can be designed appropriately, hence leading to an optimized tuning range of the L-band mode-locked lasers. A wide spectral tuning range of 23.98 nm is obtained when using the LPFG with 21.60 nm bandwidth. Benefiting from the narrow band optical attenuation characteristic of LPFG, the lowest power variation of the laser is merely 4.15 µW/nm. Moreover, theoretical gain analysis corresponds to the experimental observation well.

In this paper, we investigated the evolution of the dispersion curves of long-period fiber gratings (LPFGs) from room temperature down to 0 K. We considered gratings arc-induced in the SMF28 fiber and in two B/Ge co-doped fibers. Computer simulations were performed based on previously published experimental data. We found that the dispersion curves belonging to the lowest-order cladding modes are the most affected by the temperature changes, but those changes are minute when considering cladding modes with dispersion turning points (DTP) in the telecommunication windows. The temperature sensitivity is higher for gratings inscribed in the B/Ge co-doped fibers near DTP and the optimum grating period can be chosen at room temperature. A temperature sensitivity as high as −850 pm/K can be obtained in the 100–200 K temperature range, while a value of −170 pm/K is reachable at 20 K. [ABSTRACT FROM AUTHOR]

A methodology to enhance the sensitivity of long-period fiber gratings (LPFGs) based on the combination of three different enhancement approaches is presented; the methods here adopted are the working near mode transition (MT) of a cladding mode (CM), working near the turn-around point of a CM and the enhancement of the evanescent field of CMs by reducing the cladding diameter or by increasing the order number of CMs. In order to combine these enhancement methodologies, an electrostatic self-assembly (ESA) process was used to deposit a polymeric overlay, with a chosen thickness, onto the etched fiber. The add-layer sensitivity of the sensor was theoretically calculated, and the demonstration of the real applicability of the developed LPFG as a biosensor was performed by means of an IgG/anti-IgG immunoassay in human serum in a thermostated microfluidic system. The limits of detection (LODs) calculated by following different procedures (three times the standard deviation of the blank and the mean value of the residuals) were 6.9 × 10−8 µg/mL and 4.5 × 10−6 µg/mL, respectively. The calculated LODs demonstrate the effectiveness of the applied methodology for sensitivity enhancement.

We demonstrate the fabrication of a high-sensitivity magnetic field sensor based on the packaged long-period fiber grating (LPFG) with magnetic fluid. The transmission spectra and the surrounding refractive index (SRI) characteristics of the LPFGs near the dispersion turning point (DTP) were investigated theoretically and experimentally. We have studied the influence of mode order and wavelength separation at DTP on the SRI sensitivity of the DTP-LPFG. The experimental results are consistent with the simulation. The LP0,10 mode has the highest SRI sensitivity of 1956.4 nm/RIU in the range of 1.333-1.435 when the wavelength separation at DTP is 89.1 nm. The sensor achieves a magnetic field sensitivity of 1.9 nm/mT in the magnetic field intensity range of 0-20.5 mT, which indicates that the sensor can be used as a high-sensitivity magnetic field sensor. [ABSTRACT FROM AUTHOR]

There are great challenges in the generation of higher-order modes for long period fiber grating (LPFG). The cross-coupling coefficient between the fundamental mode and the higher-order azimuthal modes is extremely small due to the difficulty of angular modulation with traditional methods. Therefore, how to generate higher-order orbital angular momentum (OAM) modes with a high efficiency and low insertion loss is an urgent problem to be solved. In this paper, an efficient all-fiber approach to generate fourth-order OAM mode by employing a preset twisted LPFG (PT-LPFG) in a few-mode fiber (FMF) is presented. The difficulty of generating fourth-order mode utilizing traditional single-side exposure carbon dioxide laser manufacturing method and the advantages of the PT-LPFG in enhancing the coupling coefficient are theoretically analyzed in detail. In experiment, the conversion from the fundamental mode to the fourth-order mode is achieved, and the fourth-order interference patterns demonstrated the successful generation of the OAM±4 modes. The conversion efficiency of the OAM modes is over 99% and the purity is measured to be more than 90%. To the best of our knowledge, this is the first time to generate the fourth-order OAM mode in an all-fiber system using single long-period fiber grating. [ABSTRACT FROM AUTHOR]

This paper presents a design method of a small chirped long-period fiber grating (LPFG) for high sensitivity refractive index sensor. By means of the cladding layer, the LPFG operates in the mode barrier region, in which it is sensitive to the refractive index of the external environment. By introducing a small chirped grating, the LPFG sensor can realize the periods changing, from the period at phase-matching turning point (PMTP) to the period near the PMTP. This would solve the problem of large bandwidth of the loss peak of an LPFG when working directly at the PMTP. According to the transfer matrix method based on the coupled mode theory, by selecting the appropriate grating parameters and calculating the transmission spectrum, it is found that the distance between bimodal peaks has a good linear match with the surrounding refractive index (SRI). The simulation results show that the average sensitivity of SRI is as high as 9650 nm/RIU, which is 10.5 times higher than that uncoated and non-chirped and 2.7 times higher than that coated and non-chirped. Compared with the traditional LPFG bimodal sensors, the SRI sensitivity is approximately 3.6 times higher than that of those LPFG sensors that have been reported and has a smaller bandwidth. Given its excellent characteristics, the sensor can be used in marine water quality monitoring, bio-security monitoring and other fields. [ABSTRACT FROM AUTHOR]

A novel bidirectional high-sensitivity bending sensor based on the $\upsilon $ -shaped long-period fiber grating (LPFG) is designed and demonstrated. The $\upsilon $ -shaped LPFG is composed of an array of asymmetrical $\upsilon $ -shaped grating planes, which can provide stronger refractive index modulation than conventional planar grids. Therefore, the proposed sensor provides a more sensitive result. The experimental results show that the maximum bending sensitivity of $\upsilon $ -shaped LPFG reaches −24.08 nm/ $\text{m}^{-{1}}$ within the bending range of 0.22 $\text{m}^{-{1}}\,\,\sim \,\,2.21\,\,\text{m}^{-{1}}$ , enhancing the bending sensitivity while improving sensitivity to the micro-bending area. The sensitivity of $\upsilon $ -shaped LPFG can be potentially improved by optimizing the structural parameters of the grid. [ABSTRACT FROM AUTHOR]

• The holes enclosing core fiber can block the diffusion of dopants from the core to the cladding. • The thermal blocking effect of holes enclosing core fiber is analyzed in simulation and experiment. • Long-period fiber grating devices based on holes enclosing core fiber have a more powerful potential for application in high-temperature and high-pressure environments. We have developed a variety of optical fibers with holes enclosing the core to increase the service life of fiber optic devices under high-temperature and high-pressure environments by blocking the diffusion of dopants in the core with the help of air holes. Simulation results show that the fiber with uniformly distributed holes enclosing the core, the more the hole number, the smaller the core hole distance, and the larger the hole radius, the more significant the blocking effect on thermal diffusion. By comparing the refractive index distribution of three kinds of holes enclosing core fibers and single-mode fibers after prolonged high-temperature heating treatment, the thermal diffusion blocking effect of the holes enclosing core fiber is verified. Long-period fiber gratings fabricated by periodic refractive index modulation of holes enclosing core fibers can realize the measurement of various physical parameters. The thermal diffusion blocking effect of the holes enclosing core fiber enables the long-period fiber grating devices to work for a long time under high-temperature and high-pressure environments, showing the potential for practical applications in various industrial processes. [ABSTRACT FROM AUTHOR]

A high-sensitivity axial force sensor with a large measurement range based on a dual-peak long-period fiber grating (LPFG) is proposed and experimentally demonstrated. Previously, the relationship between the grating period and the dual-peak wavelengths has been investigated based on the coupled-mode theory. In our experiment, the LPFG was fabricated in our laboratory by illuminating the fiber core with the aid of a 213 nm UV laser. The sensitivity of the proposed axial force sensor can reach −14.047 nm/N in the force range from 0.490 N to 4.508 N. Taking the advantages of a compact size, low cost, and large measurement range, our force sensor has more applicable abilities in harsh environments.

In this study, a new temperature sensor with high sensitivity was achieved by four-layer Ge and B co-doped long-period fiber grating (LPFG) based on the mode coupling principle. By analyzing the mode conversion, the influence of the surrounding refractive index (SRI), the thickness and the refractive index of the film on the sensitivity of the sensor is studied. When 10 nm-thick titanium dioxide (TiO2) film is coated on the surface of the bare LPFG, the refractive index sensitivity of the sensor can be initially improved. Packaging PC452 UV-curable adhesive with a high-thermoluminescence coefficient for temperature sensitization can realize high-sensitivity temperature sensing and meet the requirements of ocean temperature detection. Finally, the effects of salt and protein attachment on the sensitivity are analyzed, which provides a reference for the subsequent application. The sensitivity of 3.8 nm/°C in the range of 5–30 °C was achieved for this new sensor, and the resolution is about 0.00026 °C, which is over 20 times higher than ordinary temperature sensors. This new sensor meets the accuracy and range of general ocean temperature measurements and could be used in various marine monitoring and environmental protection applications.

Gastric cancer significantly contributes to global cancer mortality, often leading to inoperable stages and high recurrence rates post-surgery. Elevated levels of G-17 and G-gly have been identified as potential risk factors, particularly in patients with duodenal ulcers. This study introduces an innovative D-shaped grinding long-period fiber grating sensor (D-LLPFGs) designed for non-invasive detection of the gastrin G-17 antigen, employing a layer-by-layer chemical self-assembly to bond G-17 antibodies onto the fiber surface through hydrogen bonding. The D-LLPFGs sensor demonstrated significant spectral shifts within 1 min of antigen-antibody interaction, highlighting its rapid detection capability. At an optimized antibody concentration of 4 μg/ml, antigen testing across different concentrations (10, 12.5, 20, 50 μg/ml) showed peak changes at 12.5 μg/ml antigen, with a 1.186 nm shift and 0.503 dB loss. The sensor exhibited a wavelength sensitivity of 0.095 nm/μg/ml, indicating its high sensitivity and potential in gastric cancer classification, diagnosis, and treatment. This research concludes that the D-shaped fiber sensor is an effective and sensitive tool for detecting G-17 antigen levels, presenting a significant advancement in non-invasive gastric cancer diagnosis. Its quick response time and high sensitivity highlight its potential for broad biomedical applications, offering a new avenue for early cancer detection and improving patient prognosis. The success of this study opens the door to further exploration and implementation of fiber optic sensors in clinical settings, marking a significant step forward in the fight against gastric cancer. [Display omitted] • Quick response time highlights the sensor's potential for rapid clinical diagnostics and timely patient care. • D-LLPFGs sensor enables non-invasive gastrin G-17 antigen detection, advancing biomedical sensing significantly. • Optimal antibody concentration of 4 µg/ml ensures high sensitivity and specificity for G-17 antigen detection. [ABSTRACT FROM AUTHOR]

• A dual-wavelength erbium-doped fiber laser (EDFL) for strain sensing with compound filter of a Sagnac loop and a long period fiber grating (LPFG) is proposed and experimentally proved. • The proposed strain-sensing system combines fiber-laser technology, sensing technology, interference technology, and filtering technology. • The proposed EDFL enables flexible laser outputs of single-wavelength and dual-wavelength through adjusting the compound filter. The wavelength tuning range of the single-wavelength output laser exceeds 10 nm. • The strain is measured through monitoring the output wavelength shift in the EDFL. Owing to the sensitivity of the output wavelength to strain and the sharp spectrum in EDFL, the strain measurement sensitivity reaches 8.33 pm/με with a high resolution of 2.4 με. A dual-wavelength erbium-doped fiber laser (EDFL) strain sensing system composed of compound filter of a long period fiber grating (LPFG) and a Sagnac loop is proposed. Polarization maintaining fiber (PMF) in Sagnac loop acts both an interference filter and a strain sensing probe with high sensitivity, low cost, and low loss. LPFG is used to modulate the transmission wavelength and bandwidth of EDFL through adjusting its bending curvature. Flexible laser output with single wavelength and dual-wavelength tunable wavelength band of 10 nm is achieved in EDFL. The measurement sensitivity of strain reaches 8.33 pm/με, while the 3 dB linewidth is less than 2 nm and the signal-to-noise ratio (SNR) exceed 36 dB. This sensor has a high resolution (R) of 0.11 nm and a high quality factor (Q) of 38.13. The experimental results suggest that the system is suitable for precise strain measurement and has significant potential in fields like biomedicine, environmental monitoring, health monitoring, and aerospace. [ABSTRACT FROM AUTHOR]

Long-period fiber grating (LPFG) is one of the most important passive fiber devices. They work by coupling light from the fundamental core mode to the forward-propagating cladding mode, creating a series of resonant peaks in the transmission spectrum. In this paper, a series of no-core fiber (NCF) sections were periodically misaligned and embed in the single-mode fiber (SMF), to form a LPFG through the periodic refractive index (RI) modulation along the fiber axis. This fabricating method enhances the RI modulation to shorten the length of the proposed sensor to 2.82 mm. Experimental result shows that the highest RI sensitivity for the sensor reaches −4460.83 nm/RIU in a measuring range of 1.33–1.40 RIU at room temperature. Hopefully, the proposed study can lay some implementation methods for the design of the all-fiber compact RI sensor. [Display omitted] • Offset splicing by highly precise fiber cutting technology. • Periodic RI modulation along the fiber axis is achieved in this misaligned core cladding structure to form the LPFG. • The highest refractive index sensitivity of the OC-LPFG reaches to 4460.83 nm/RIU. • Compact size, low cost and high sensitivity. [ABSTRACT FROM AUTHOR]

A long period fiber grating prepared by multimode fibers (MMF-LPFG) is proposed and experimentally demonstrated as a sensitivity-enhanced strain sensor. The core diameter mismatch between the MMFs excites the high order cladding mode to obtain a compact sensing structure (3.6 mm). The strain sensitivity of the MMF-LPFG structure is enhanced to −15.06 pm/ $\mu \varepsilon $ when the grating period is closer to the dispersion turning point (DTP). Meanwhile, the novel fiber structure also has an excellent mechanical property to achieve a large dynamic range ($\text{0-4508} ~\mu \varepsilon $) in strain measurement. [ABSTRACT FROM AUTHOR]

Torsion sensing is a promising technique in a variety of different scientific and industrial applications. In this paper, we present a novel torsion sensor based on the cascaded long-period fiber grating inscribed in a four-mode fiber (CLPFG-FMF). The change in rotatory vector makes resonant wavelength shift, for the twisted CLPFG-FMF. The key factor, which severely impacts on the properties and performance of torsion sensor, is also investigated and discussed. Experimental results show that the resonant wavelength linearly shifts with the torsion rate and the cascaded angle plays an important role in enhancing torsion sensitivity. The proposed torsion sensor achieves optimal sensitivities of -0.519 nm/(rad/m) in counterclockwise and 0.501 nm/(rad/m) in clockwise. The repeatability and stability of the CLPFG-FMF with cascaded angle of 0° are studied as well. The refractive index sensitivity of the CLPFG-FMF with cascaded angle of 0° is -5.20 nm/RIU from 1.3431 to 1.4374 refractive index ranges and the temperature sensitivity is -0.0173 nm/°C from 25 °C to 90 °C. The high sensitivity of torsion sensing is expected to be useful for broad applications across engineering fields.

Refractive index (RI) sensing is a promising technique in a variety of different scientific and industrial applications. In this paper, we theoretically and experimentally propose a newly-designed RI sensor. The proposed sensor consists of a particular long-period fiber grating inscribed in a two-mode fiber (LPFG-TMF) coated with tunable graphene oxide (GO), and the special structure allows an extremely high sensitivity in the transition region. The mode coupling between the LP11 core mode and the sixth-order cladding mode in the LPFG-TMF offers an apparently higher sensitivity feature. And the sensitivity is higher than that of a single-mode fiber-based LPFG (LPFG-SMF), which originates from the coupling between the core mode and the sixth-order cladding mode. Furthermore, the ultrasonic treatment time of GO, which impacts on the properties and performance of RI sensing, is also studied and investigated. Proof-of-concept results demonstrate significantly that comparing with the bare LPFG-TMF (621.66 nm/RIU) and LPFG-SMF coated with GO ultra-sonicated 5 hours (323.68nm/RIU), the LPFG-TMF coated with GO ultra-sonicated 5 hours performs optimized sensitivity of 11605.79 nm/RIU when surrounding RI ranges from 1.4558 to 1.4577. With its premium ability to operate high-sensitivity measurements, the RI sensing is expected to be useful for broad applications across the chemical fields.

We proposed and experimentally demonstrated a vector bending sensor based on a core-offset long period fiber grating (CO-LPFG). The CO-LPFG is configured by CO2 laser polishing on three mutually orthogonal planes of a single mode fiber (SMF) to different depths and then heating it by the electrical arc-discharge of a fusion splicer. The asymmetrical cladding structure leads to an uneven density distribution of the fiber in the molten state. Under the effects of releasing the residual stress and fiber surface tension, the fiber core deviates toward the polished deeper directions to form a core-offset structure in two dimensions. The experimental results demonstrate that the bending sensitivities of CO-LPFG reach 37.06 nm/m −1 −5.85 nm/m−1-14.59 nm/m−1 and −12.45 nm/m−1 for four bending directions within the range of 1.12-2.10 m−1 The temperature sensitivity of CO-LPFG is 57.90 pm/°C in the range of 40-160°C. [ABSTRACT FROM AUTHOR]

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In this paper, we investigated the evolution of the dispersion curves of long-period fiber gratings (LPFGs) from room temperature down to 0 K. We considered gratings arc-induced in the SMF28 fiber and in two B/Ge co-doped fibers. Computer simulations were performed based on previously published experimental data. We found that the dispersion curves belonging to the lowest-order cladding modes are the most affected by the temperature changes, but those changes are minute when considering cladding modes with dispersion turning points (DTP) in the telecommunication windows. The temperature sensitivity is higher for gratings inscribed in the B/Ge co-doped fibers near DTP and the optimum grating period can be chosen at room temperature. A temperature sensitivity as high as −850 pm/K can be obtained in the 100–200 K temperature range, while a value of −170 pm/K is reachable at 20 K.

For ambient refractive index (ARI) higher than cladding region, the refractive index (RI) sensitivity of long-period fiber grating (LPFG) measured in terms of the shift in the cladding mode resonance wavelength (CMRW) is very low. This may be due to the occurrence of a small CMRW shift with a change in ARI. This paper emphasizes the effect of cladding radius reduction as well as cladding mode-order over the spectral behavior, RI response and figure of merit of LPFG for such ARIs. Therefore, mathematical approaches designed on three-layer and two-layer fiber geometries are employed on single-mode optical fiber (SMF28e) to estimate optical modes. The influence of cladding radius reduction (from 62.5 to 32.5 µm) on spectral profile and RI sensitivity is analyzed for three cladding modes (HE13, HE14 and HE15). The spectral behavior of all three modes for the ARI range (1.458–1.738) has described the shifting of CMRW towards longer wavelengths. From the analysis, the larger shifts in CMRW with ARIs have been shown by HE15 cladding mode at a cladding radius of 32.5 µm as compared to normal cladding LPFG. Moreover, the spectral bands of ARIs near to cladding index have shown very low differentiation in attenuation spectrum depths at reduced cladding radius. [ABSTRACT FROM AUTHOR]

We proposed a high sensitivity long-period fiber grating strain and temperature sensor. The sensor is composed of input single mode fiber (SMF), helical capillary optical fiber (HCOF), and output SMF in cascade. By using a fusion junction structure between SMF and HCOF, almost all the light from input SMF can be coupled to the silica cladding of HCOF, and then modulated by HCOF. Because the light propagates in the same channel as the silica cladding of HCOF, the sensor is insensitive to refractive index (RI) change but has a high sensitivity to stress and temperature. The experiment results show that the strain sensitivity can reach −9.29 pm/μϵ when the hollow air capillary diameter and twisting pitch of HCOF are 10 μm and 1.2 mm, respectively. And the temperature sensitivity can reach 17.14 pm/°C in this condition. The sensing characteristics of the sensor are related to the diameter of the hollow air capillary and the twisting pitch of HCOF. Benefiting from eliminating the cross-influence of strain and temperature, the sensor may have the potential to realize stress detection in a harsh environment. [ABSTRACT FROM AUTHOR]

Mode exchange (ME) can effectively improve the utilization of network resources in short-distance mode division multiplexing (MDM) communication systems. In this paper, a mechanical long-period fiber grating (LPFG) mode converter is used to realize the channel exchange of LP 01 - and LP 11 -mode dual-polarization (DP) signals, with the channel crosstalk of −14 dB or less. We experimentally tested the receiver sensitivities of two DP channels, which are dependent on channel crosstalk and polarization-dependent loss (PDL), and demonstrated the practicability of the real-time data transmission and exchange for 100 Gb/s dual-polarization quadrature phase shift keying (DP-QPSK) in multiple-input multiple-output free (MIMO-free) MDM system. The experimental results show that the ME system can provide the optical power margin of about 25 dB, capable of effectively supporting the metropolitan transmission and exchange between the data centers. • The exchange between LP01 and LP11 modes is achieved through a mechanical long-period fiber grating (LPFG) mode converter, each of which carries 100 Gb/s dual-polarization quadrature phase shift keying (DP-QPSK) signal. • The modal crosstalk of two DP channels is less than -14 dB, and the mode conversion efficiencies of the LPFG are 61.1% and 84.7%, respectively. • The real-time data transmission and exchange for 100 Gb/s DP-QPSK signals in multiple-input multiple-output free (MIMO-free) mode division multiplexing (MDM) system is demonstrated, with the optical power margin of about 25 dB. [ABSTRACT FROM AUTHOR]

We propose and demonstrate an ultra-broadband mode converter based on a cascade chirped long-period fiber grating (CLPFG) written in a two-mode fiber. The mode converter can convert fundamental mode (HE11even and HE11odd modes) into the first order cylindrical vector (CV) modes (TE01, TM01, HE21even and HE21odd modes). We design and analyze the mode conversion characteristics of this kind of grating in theory. The simulation results show a 10 dB bandwidth of 170 nm and 20 dB bandwidth of 145 nm can be achieved by optimizing the parameters of the CLPFG. In the terms of experiment, we achieve the broadband mode converter with 10 dB bandwidth of 170 nm from 1472 nm to 1642 nm.

The response of a long‐period fiber grating (LPFG) based refractometric sensor is directly related to the optical modes' effective index of refraction (EIR). In this paper, the quantitative behavior of EIR is investigated with LPFG surrounded dielectric medium's refractive index (RI) higher than cladding RI and cladding radius. To analyze the impact on EIR, the 2‐layer and 3‐layer fiber structure‐based mathematical models have been introduced. In the past literature, the interaction of the ambient medium with the core mode field is rarely investigated. Therefore, such studies have been considered and as a result, a significant impact on core mode EIR has been observed. The influence of ambient‐medium RI (ARI = 1.458–1.738) on EIR is found ascended for its value near to cladding RI at normal cladding radius. However, the range of influencing ARIs has been extended with mode order and cladding radius reduction. This study encourages the measuring of LPFG RI response for the aforesaid ARI range in terms of shift in coupled mode resonance wavelength and improves the precision in sensing ARI. [ABSTRACT FROM AUTHOR]

Here, we report a novel optical fiber grating sensor capable of simultaneous measurement of torsion and temperature by utilizing a long-period fiber grating (LPFG) written on a polarization-maintaining photonic crystal fiber (PMPCF) and a Faraday rotator mirror (FRM) concatenated in series with the LPFG. The LPFG written on the PMPCF, referred to as a PMPC-LPFG, which is connected in series with the FRM, exhibits a polarization-independent wavelength-dependent loss spectrum in its reflection spectrum, which has two attenuation dips (ADs) designated as AD 1 and AD 2. When these two ADs have different cladding-mode orders, they have different torsion responses as well as temperature responses. For the two ADs with resonance wavelengths of λ1 = ~1508.02 nm and λ2 = ~1533.48 nm, torsion and temperature responses were investigated in a twist angle range from -70 to 70° (step: 10°) and an ambient temperature range from 25 to 95 °C (step: 10 °C), respectively. The torsion sensitivities of AD 1 and AD 2 were measured as ~157.0 and ~272.7 pm/rad at room temperature, and their temperature sensitivities were measured as ~5.8 and ~8.2 pm/°C, respectively. Owing to their linear and independent responses to torsion and temperature, torsion and temperature changes applied to the PMPC-LPFG can be simultaneously evaluated from the measured wavelength shifts of AD 1 and AD 2 using their predetermined torsion and temperature sensitivities. [ABSTRACT FROM AUTHOR]

We propose and demonstrate an optical fiber vector curvature sensor based on a long-period fiber grating (LPFG) inscribed in Panda type polarization-maintaining fiber (PMF) using CO2-laser heating and tapering system. The uniform heating approach removes the necessity of CO2-laser exposure alignment along the axis of PMFs during grating fabrication. A serial of PMF-LPFGs with grating period from 350 to 490 μm with a step of 20 μm have been fabricated experimentally. The curvature response of the fast and slow axis resonance dips for the LPFG with a period of 450 μm was investigated in the curvature range from 0 to 4.137 m−1 at four bending orientations. The maximum curvature sensitivities were measured to be 9.26 and −6.89 nm/m−1 for the fast and slow axis LP15 modes, respectively. The sensor can achieve 2-dimensional bending direction recognition. The proposed grating has potential application as a vector curvature sensor. [ABSTRACT FROM AUTHOR]

We demonstrated an optical fiber sensor capable of performing strain-insensitive simultaneous measurement of bending and temperature using cascaded long-period fiber gratings (LPFG’s) inscribed on double-clad fiber with a CO2 laser at $10.6~\mu \text{m}$. In the interference fringe spectrum of the fabricated cascaded LPFG’s, we selected three transmission dips as sensor indicators for the simultaneous measurement of bending and temperature. The measured bending sensitivities of the three indicator dips were −427.63, −911.29, and 233.33 pm/ $\text{m}^{-1}$ , and their measured temperature sensitivities were 60.49, 56.44, and 74.61 pm/°C. Owing to their linear and uncorrelated responses to bending and temperature, we could simultaneously estimate applied bending and temperature with two of the three indicators by converting the measured wavelength shifts of the two selected dips with their pre-determined bending and temperature sensitivities. In particular, our sensor showed a very low strain sensitivity less than 0.05 pm/ $\mu \varepsilon $ , 25 times less than that of a conventional LPFG, which can significantly reduce strain-induced measurement uncertainty. With two optimal indicator dips chosen considering the measured perturbation responses of the three indicators, we carried out the simultaneous measurements of bending and temperature for arbitrary sixteen points of applied bending and temperature. The standard deviations of the measured bending and temperature values were $\sim 0.059\,\,\text{m}^{-1}$ and ~0.85 °C, respectively. [ABSTRACT FROM AUTHOR]

In this paper, we propose a sensor with high torsion sensitivity. This sensor has the ability to distinguish the torsion direction. A long-period fiber grating (LPFG) with a unique geometric structure is used to configure this sensor. The proposed LPFG is prepared by CO2 laser polishing method and etching method. First, we periodically polish the two opposite planes which are parallel to the axis direction of the fiber to get sheets structure. Then, the V-shaped grooves are etched on the surface orthogonal to the sheets to fabricate the proposed LPFG. Because the V-shaped grooves are arranged along the axis direction of the optical fiber, we name it by V-LPFG. This unique geometric structure results in the sensor with high torsion sensitivity. Experimental results prove that the torsion sensitivity of the proposed sensor reaches 0.399 nm/(rad/m) in the range of–10.5 rad/m to 10.5 rad/m. More importantly, the torsion sensitivity is significantly increased from 0.041 nm/(rad/m) to 0.399 nm/(rad/m) after the V-shaped grooves are etched. The temperature sensitivity of the proposed sensor is 50 pm/°C at the temperature range of 30–150 °C. Due to the high torsion sensitivity, relatively easy fabrication process and high price to quality ratio, the proposed sensor has the potential of carrying out torsion measurement with high sensitivity. [ABSTRACT FROM AUTHOR]

In this study we present a two‐dimensional force sensor based on a single long‐period fiber grating (LPG) to be applied to robotic manipulators grasping control. The measuring force is applied to the fiber transverse section and this lateral compression induces birefringence on the fiber, which splits the LPG resonant dip into two. We track these dips using polarized controlled light and process the LPG spectra using artificial neural networks to determine the force intensity and direction. Finally, our single sensor proposal for 2D forces measurement can estimate an applied force with 0.2148 N and 5.977 deg. error. [ABSTRACT FROM AUTHOR]