Long-period Grating Sensor Research Articles

Long Period Fiber Grating for Refractive Index Sensing

Refractive index (RI) sensors are very valuable in first level detection of changes to the environment. In this project an optical fiber-based sensor is proposed to detect changes in surrounding RI. Most optical fiber sensors require tapering to be done to enhance the light interaction with the surrounding. This causes the fiber to become fragile and difficult to handle. In this research, optical fibers with long period gratings (LPG) are proposed to overcome this issue. Zinc Oxide (ZnO) nanomaterial was deposited over the LPG region using seeding method to enhance the performance of the sensor. The LPG fiber sensor was then used to investigate RI changes in the environment. A broadband laser source was used as the input and an optical spectrum analyser was used to observe the output light spectrum of the LPG sensor for different refractive index mediums. The ZnO coated LPG showed a sensitivity of 428.57 nm/RIU over an RI range of 1 – 1.3578.

Delafossite AgAlO2 modified long-period grating for highly-sensitive ammonia sensor

Owing to the unique merits of high sensitivity and large range detection, long-period grating (LPG) optical fiber sensor modified with functional coatings has attracted significant attention recently. As a refractive index sensor, the appropriate choice of the coating properties is critical for exploring the high-performance LPG sensor. In this study, p-type delafossite AgAlO2 (AAO) nanoparticles were proposed as a novel sensitive coating material for the cladding modification of LPG develop a high-sensitivity fiber-optic ammonia sensor. As the resistance/dielectric constant of AAO cladding layer is sensitive to concentration variation of ammonia vapor even at room temperature. Sensing performances of the LPG optical fiber sensor could be substantially enhanced with the presence of AAO cladding layer. A linear sensitivity of 2.07 pm/ppm to ammonia from 25 ppm to 400 ppm was achieved at room temperature. Moreover, the LPG sensor exhibits short response (38 s) and recovery time (50 s), and high selectivity to ammonia compared to several possible interfering vapors.

Label-Free Creatinine Optical Sensing Using Molecularly Imprinted Titanium Dioxide-Polycarboxylic Acid Hybrid Thin Films: A Preliminary Study for Urine Sample Analysis

Creatinine (CR) is a representative metabolic byproduct of muscles, and its sensitive and selective detection has become critical in the diagnosis of kidney diseases. In this study, poly(acrylic acid) (PAA)-assisted molecularly imprinted (MI) TiO2 nanothin films fabricated via liquid phase deposition (LPD) were employed for CR detection. The molecular recognition properties of the fabricated films were evaluated using fiber optic long period grating (LPG) and quartz crystal microbalance sensors. Imprinting effects were examined compared with nonimprinted (NI) pure TiO2 and PAA-assisted TiO2 films fabricated similarly without a template. In addition, the surface modification of the optical fiber section containing the LPG with a mesoporous base coating of silica nanoparticles, which was conducted before LPD-based TiO2 film deposition, contributed to the improvement of the sensitivity of the MI LPG sensor. The sensitivity and selectivity of LPGs coated with MI films were tested using CR solutions dissolved in different pH waters and artificial urine (near pH 7). The CR binding constants of the MI and NI films, which were calculated from the Benesi–Hildebrand plots of the wavelength shifts of the second LPG band recorded in water at pH 4.6, were estimated to be 67 and 7.8 M−1, respectively, showing an almost ninefold higher sensitivity in the MI film. The mechanism of the interaction between the template and the TiO2 matrix and the film composition was investigated via ultraviolet–visible and attenuated total reflectance Fourier-transform infrared spectroscopy along with X-ray photoelectron spectroscopy analysis. In addition, morphological studies using a scanning electron microscope and atomic force microscope were conducted. The proposed system has the potential for practical use to determine CR levels in urine samples. This LPG-based label-free CR biosensor is innovative and expected to be a new tool to identify complex biomolecules in terms of its easy fabrication and simplicity in methodology.

Numerical simulation of long-period grating sensors (LPGS) transmission spectrum behavior under strain and temperature effects

PurposeThe purpose of this study aims to simulate the long-period fiber grating sensor pulse peak position against the transmission range. The long-period fiber grating sensor pulse peak position against the transmission range is simulated clearly where the pulse peak value at zero position is 0.972655 with the ripple factor of unity. It is demonstrated that the long-period fiber grating sensor bandwidth can be estimated to be 50 µm. Wavelength shift of the long-period grating sensor (LPGS) is reported against grating wavelength, applied temperatures and applied micro strain.Design/methodology/approachThis work has reported the numerical simulation of LPGS transmission spectrum behavior characteristics under the strain and temperature effects by using OptiGrating simulation software. The sensor fabrication material is silica-doped germanium. The transmittivity/reflectivity and input spectrum pulse intensity of long-period Bragg sensor variations are simulated against the grating wavelength variations. Input/output pulse intensity of LPGS variations is simulated against the timespan variations with the Gaussian input pulse from 100 to 500 km link length.FindingsTemperature variation and strain variation of the LPGS are outlined against both applied temperatures and micro-strain variations at the central grating wavelength of 1,550 nm.Originality/valueIt is demonstrated that the long period fiber grating sensor bandwidth can be estimated to be 50 µm. Wavelength shift of the long period grating sensor is reported against both grating wavelength, applied temperatures and applied micro strain. Temperature variation and strain variation of the long period grating sensor are outlined against both applied temperatures and micro strain variations at the central grating wavelength of 1550 nm.

Long-period grating fiber-optic sensors exploiting molecularly imprinted TiO2 nanothin films with photocatalytic self-cleaning ability.

Highly sensitive and selective long-period grating (LPG) fiber-optic sensors modified with molecularly imprinted TiO2 nanothin films were fabricated. The films were deposited onto the surface of the optical fiber via liquid-phase deposition (LPD), using tetrakis(N-methylpyridinium-4-yl)porphyrin (TMPyP) as a template. Three LPG resonance bands were monitored during film deposition, which was of duration 4.5h. Prior to template removal, heat treatment at 60°C under high-humidity conditions led to an increase in refractive index of the TiO2 film, evidenced by changes in the central wavelengths of the attenuation bands. After template removal using HCl solution (0.01M), the TMPyP-imprinted film-modified LPG sensor showed higher sensitivity to the template molecule than to structurally related guest molecules. This was measured at the 1st and 2nd resonance bands, with wavelengths ranging from 690 to 738nm and 815 to 905nm, respectively. No selective binding of the template was observed with a non-imprinted TiO2 film prepared in the same manner. Furthermore, the heat-treated imprinted films exhibited a substantial enhancement of photocatalytic activity for template irradiation. In particular, the self-cleaning property of the imprinted film-modified LPG sensor under ultraviolet irradiation led to highly efficient and selective binding to the template. The mechanism of the interaction between the template and the TiO2 matrix was investigated by UV-vis and Fourier-transform infrared (FTIR) spectroscopies. Additionally, morphological studies using scanning electron microscopy (SEM) were conducted. Graphical abstract.

Theoretical Investigation of a Long Period Grating Sensor Employing Multilayer Fiber Geometry

A theoretical formulation of the long-period grating sensor is implemented with respect to efficient refractive indices of core and cladding. The obtained result is utilized further for determination of ERIs of core and cladding using graphical analytical method and multilayer fiber geometry. ERI values of primary guided core, bi and tri layer fiber geometry using MATLAB software. In the present paper effective refractive index of core and cladding for the two and three layers have been investigated. The cladding modes have also been calculated using Erdogan’s proposed three-layer geometry as well as two-layer graphical solution method. We have compared both the techniques, discussed the limitations of graphical solution method and the benefits of three-layer geometry.

Combined Long-Period Fiber Grating and Microcavity In-Line Mach-Zehnder Interferometer for Refractive Index Measurements with Limited Cross-Sensitivity.

This work discusses sensing properties of a long-period grating (LPG) and microcavity in-line Mach–Zehnder interferometer (µIMZI) when both are induced in the same single-mode optical fiber. LPGs were either etched or nanocoated with aluminum oxide (Al2O3) to increase its refractive index (RI) sensitivity up to ≈2000 and 9000 nm/RIU, respectively. The µIMZI was machined using a femtosecond laser as a cylindrical cavity (d = 60 μm) in the center of the LPG. In transmission measurements for various RI in the cavity and around the LPG we observed two effects coming from the two independently working sensors. This dual operation had no significant impact on either of the devices in terms of their functional properties, especially in a lower RI range. Moreover, due to the properties of combined sensors two major effects can be distinguished—sensitivity to the RI of the volume and sensitivity to the RI at the surface. Considering also the negligible temperature sensitivity of the µIMZI, it makes the combination of LPG and µIMZI sensors a promising approach to limit cross-sensitivity or tackle simultaneous measurements of multiple effects with high efficiency and reliability.

Ultra-Sensitive Fiber Optic Gas Sensor Using Graphene Oxide Coated Long Period Gratings

In this letter, we develop two types of refractive index based sensors using single-mode fiber (SMF-28) to sense hazardous toxic gases in underground mines. The first sensor design is simply a graphene-oxide (GO) deposited long period grating (LPG) and second sensor consist of two LPGs with an interspacing distance of 1 cm ( $\Delta \text{l}$ ) acting as Mach-Zehnder interferometer (MZI). In addition, we also confirm GO deposition on the SMF using ultraviolet–visible spectroscopy, atomic force microscopy (AFM), and field emission scanning electron micrograph (FESEM). Our experiment results show that the sensitivity of two-LPGs based fiber optic sensor (SLPG−2 = 76.32 dB/RIU) is higher as compared to the single LPG sensor (SLPG−1 = 54.65 dB/RIU). Hence, reported graphene oxide deposited long period grating sensor possess better sensitivity and can detect maximum $1.52\times 10^{-5}$ index variation in the optical telecommunication band.

Comparison of regression methods for transverse load sensor based on optical fiber long-period grating

In this work, we report the comparison of regression methods in a long-period grating (LPG) for transverse strain measurement. We analyze the transverse strain sensing characteristics, such as load intensity and azimuthal angle, based on the birefringence effect induced in LPG sensor. Therefore, we employ the different orthogonal responses of the grating to develop regression methods, which allow the estimation of the strain behavior of the LPG sensor. The predictive performances of these interrogation models are compared in terms of square correlation coefficient (R2) and root mean square error (RMSE). Finally, the results indicate that the best method to predict load intensity is the Fourth-Degree Polynomial Fit, whereas the artificial neural network (ANN) model could be successfully employed to predict the azimuthal angle.

Functionalized fiber-optic long-period grating with reduced cladding size for humidity sensing

Relative humidity (RH) is an important factor in the field of structural health monitoring, especially during the early stage of corrosion. Many methods have been proposed for humidity sensing, and one of the attractive sensors is fiber-optic long-period grating (LPG) sensors. Unfortunately, the current sensing sensitivity of this kind of sensors is limited. A thin fiber-optic LPG sensor with a self-assembled thin film of PAH + / PAA − is proposed and demonstrated for humidity measurements. The LPG sensor is inscribed in a single-mode fiber using a CO2 laser, and the cladding size is reduced to about 27 μm. It shows that an LPG sensor with a reduced cladding size has an enhanced refractive index sensing characteristic, compared to those with normal cladding size due to the enhancement of the evanescent field. In a next step, selectivity is added to the LPG sensor coated with a film of PAH + / PAA − for functionalization to be sensitive to humidity. The resonance spectral responses of LPGs are experimentally investigated with respect to its sensitivity to a change in humidity that modifies the index of the nanolayer and the cladding, leading to a resonant wavelength shift. The experimental results show that the coated thin LPG has a highly sensitive resonance wavelength shift of −220.75 pm / % RH for an RH variation from 25% to 80%, of which the sensitivity is enhanced thrice compared to those with a normal cladding size. The proposed sensing setup opens LPG structures for a variety of sensing and detection applications.

Ultrasensitive long-period gratings sensor works near dispersion turning point and mode transition region by optimally designing a photonic crystal fiber

A novel and ultrasensitive long-period gratings (LPGs) sensor based on photonic crystal fiber (PCF) has been proposed and theoretically investigated. The surrounding refractive index (SRI) sensitivity can be improved enormously by optimizing structure parameters of PCF to ensure LPGs work near dispersion turning point (DTP). Based on this, mode transition (MT) has been also designed and realized in LPGs based on PCF for the first time to further explore the theoretical limit of sensitivity. The simulation results illustrated that the optimized SRI sensitivity is 67,600 nm/RIU in low RI measurement region (1.334–1.344), which is much higher than that of other reported LPGs sensor based on PCF in previous literatures. The competitive advantages make the proposed sensor has a great application prospect in biosensing field, such as biomolecules detection and label-free immune sensing.

Highly sensitive label-free antibody detection using a long period fibre grating sensor

An optical fibre long period grating (LPG) biosensor is appealing in the detection of biomolecules because of the high sensitivity, label-free and real-time measurement. The miniaturized size, ability of remote sensing and immunity to electromagnetic interference of the LPG biosensor provide various possibility of single-point sensing in situations such as point of care diagnostics and in vivo measurement. Two optical fibre LPG based biosensors are reported for detection of streptavidin (SV) and immunoglobulin M (IgM) respectively. The LPG is coated with a film containing three layers of Poly(allylamine hydrochloride)/gold coated silica nanoparticles via the layer-by-layer method. Biotin is covalently bonded to the surface of the gold shell by means of the formation of an amide bonds for detection of streptavidin. The concentration of SV in water for detection varied from 1.25 nM to 2.7 μM. The LPG sensor, operating close to the phase matching condition shows a high sensitivity of 3.88 (ng/mm2)−1 and a detection limit of 0.86 pg/mm2 for the detection of SV. The limit of detection is 22 times lower than previously demonstrated with this type of sensor. The developed IgM sensor has the same configuration of film but has anti-IgM embedded on the LPG instead of biotin, demonstrating versatility of the sensing platform. This was used for the detection of human IgM with concentrations from 15.6 μg/ml to 1 mg/ml. The LPG sensor exhibits a sensitivity of 11 nm (ng/mm2)−1 for the detection of IgM with a detection limit of 15 pg/mm2. The developed highly sensitive IgM sensor shows the potential application of clinical point of care for detection of lower concentration of IgM in vitro. The proposed biosensor exhibits high sensitivity and rapid detection of low concentrations biomolecules from the small size of SV to the large size of IgM.

Vibration and Magnetic Field Sensing Using a Long-Period Grating

A long-period grating (LPG) written on a standard single mode fiber is investigated as a fiber optic sensor for vibration and magnetic field sensing. It is demonstrated the high sensitivity of the device to applied curvature and the possibility to monitor vibration in a wide range of frequencies from 30 Hz to 2000 Hz. The system was tested using intensity-based interrogation scheme, providing a frequency discrimination of 913 mHz. The goal of these tests was to evaluate the sensor as a passive vibration monitor in the detection of changes in resonant vibration frequencies of support infrastructures can provide information on its degradation. Furthermore, taking advantage of the intrinsic sensitivity to micro strain, alternating magnetic fields were also measured using an intensity-based interrogation scheme by coupling a Terfenol-D magnetostrictive rod to a pre-strained LPG sensor, providing a resolution below 5.61 $\mu \text{T}_{\mathrm {\mathbf {rms}}}/\surd $ Hz from 1.22 mTrms up to 2.53 mTrms.

Multi-measurement scheme for a fiber-optic sensor based on a single long-period grating

In this paper, we present and discuss a scheme to perform simultaneous measurement of multi-perturbations using a single Long-Period Grating (LPG). It was shown that different spectral shifts in the resonant bands of the transmitted spectrum of an LPG sensor can be used for measuring concurrent perturbations. The functioning mechanism of this multi-parameter sensing scheme is discussed and used to perform simultaneous sensing of temperature, mechanical strain and refractive index variations.

Volatile Organic Compounds Sensing Using Optical Fibre Long Period Grating with Mesoporous Nano-Scale Coating.

A long period grating (LPG) modified with a mesoporous film infused with a calixarene as a functional compound was employed for the detection of individual volatile organic compounds (VOCs) and their mixtures. The mesoporous film consisted of an inorganic part, SiO2 nanoparticles (NPs), along with an organic moiety of poly(allylamine hydrochloride) polycation PAH, which was finally infused with the functional compound, p-sulphanato calix[4]arene (CA[4]) or p-sulphanato calix[8]arene (CA[8]). The LPG sensor was designed to operate at the phase matching turning point to provide the highest sensitivity. The sensing mechanism is based on the measurement of the refractive index (RI) change induced by a complex of the VOCs with calixarene. The LPG, modified with a coating of 5 cycles of (SiO2 NPs/PAH) and infused with CA[4] or CA[8], was exposed to chloroform, benzene, toluene and acetone vapours. The British Standards test of the VOCs emissions from material (BS EN ISO 16000-9:2006) was used to test the LPG sensor performance.

Highly sensitive fiber grating chemical sensors: An effective alternative to atomic absorption spectroscopy

Accuracy in quantitative determination of trace elements like Zinc, present in drinking water in ppm level, is a big challenge and optical fiber gratings as chemical sensors may provide a promising solution to overcome the same. This paper presents design of two simple chemical sensors based on the principle of shift in characteristic wavelength of gratings with change in their effective refractive index, to measure the concentration of Zinc in drinking water using etched short period grating (FBG) and Long period grating (LPG) respectively. Three samples of drinking water from different places have been examined for presence of Zinc. Further, the results obtained by our sensors have also been verified with the results obtained by a standard method, Atomic absorption spectroscopy (AAS). The whole experiment has been performed by fixing the fibers in a horizontal position with the sensor regions at the center of the fibers, making it less prone to disturbance and breaking. The sensitivity of LPG sensor is about 205 times that of the FBG sensor. A few advantages of Fiber grating sensors, besides their regular features, over AAS have also been discussed, that make our sensors potential alternatives for existing techniques in determination of trace elements in drinking water.

Multi-parameter measurements using optical fibre long period gratings for indoor air quality monitoring

An array of three long period gratings (LPGs) fabricated in a single optical fibre and multiplexed in the wavelength domain was used to measure simultaneously temperature, relative humidity (RH) and volatile organic compounds (VOCs), which are key indoor air quality (IAQ) indicators. Each LPG sensor was designed with optimised response to a particular measurand. The first, with no surface modification, was used to measure temperature. The second, modified by a mesoporous coating of silica nanoparticles (SiO2 NPs), was used to measure RH and the third, modified with a coating of SiO2 NPs infused with a functional material, p-sulphanatocalix[8]arene (CA[8]), was employed to monitor VOC concentration. The LPGs were fabricated with periods such that they operated at or near the phase matching turning point. The sensors were calibrated in the laboratory and the simultaneous measurement of the key indoor air quality parameters was undertaken in laboratory and office environments. It was demonstrated successfully that the data produced by the LPG sensor array under real conditions was in a good agreement with that produced by commercially available sensors. The average differences between values obtained by the optical fibre sensor and standard temperature and RH sensors were better than 0.5°C and 5% respectively. Further, the potential application of fibre optic sensors for VOC detection at high concentrations has been demonstrated.

Bacteriophage Adhesin-Coated Long-Period Grating-Based Sensor: Bacteria Detection Specificity

In this paper, we study a label-free detection specificity of an optical fiber long-period grating (LPG) biosensor working near the dispersion turning point of higher order cladding modes. The LPG sensor is functionalized with bacteriophage adhesin and tested with specific and nonspecific bacteria dry weights. It is shown that that such a biosensor is able to selectively bind to specific bacterial strains. In the experiments bacteria dry weights of E. coli B were used as a positive test and E. coli K12 and Salmonella enterica as negative tests. The resonance wavelength shift induced by E. coli B reaches over 90 nm, while for E. coli K12 and Salmonella enterica approximately 40 and 20 nm, respectively. Additionally, whole-cell-ELISA tests were made to confirm LPG-based biosensor outcomes, and numerical simulations were conducted to estimate biolayer impact on sensor's performance.

Titanium oxide thin films obtained with physical and chemical vapour deposition methods for optical biosensing purposes

This work discusses an application of titanium oxide (TiOx) thin films deposited using physical (reactive magnetron sputtering, RMS) and chemical (atomic layer deposition, ALD) vapour deposition methods as a functional coating for label-free optical biosensors. The films were applied as a coating for two types of sensors based on the localised surface plasmon resonance (LSPR) of gold nanoparticles deposited on a glass plate and on a long-period grating (LPG) induced in an optical fibre. Optical and structural properties of the TiOx thin films were investigated and discussed. It has been found that deposition method has a significant influence on optical properties and composition of the films, but negligible impact on TiOx surface silanization effectiveness. A higher content of oxygen with lower Ti content in the ALD films leads to the formation of layers with higher refractive index and slightly higher extinction coefficient than for the RMS TiOx. Moreover, application of the TiOx film independently on deposition method enables not only for tuning of the spectral response of the investigated biosensors, but also in case of LSPR for enhancing the ability for biofunctionalization, i.e., TiOx film mechanically protects the nanoparticles and induces change in the biofunctionalization procedure to the one typical for oxides. TiOx coated LSPR and LPG sensors with refractive index sensitivity of close to 30 and 3400nm/RIU, respectively, were investigated. The ability for molecular recognition was evaluated with the well-known complex formation between avidin and biotin as a model system. The shift in resonance wavelength reached 3 and 13.2nm in case of LSPR and LPG sensors, respectively. Any modification in TiOx properties resulting from the biofunctionalization process can be also clearly detected.

Overwrite fabrication and tuning of long period gratings.

The central wavelengths of the resonance bands are critical aspect of the performance of long period gratings (LPGs) as sensors, particularly for devices designed to operate near the phase matching turning point (PMTP), where the sensitivity to measurements can vary rapidly. Generally, LPGs are characterized by their period, but the amplitude of the amplitude of the index modulation is also an important factor in determining the wavelengths of the resonance bands. Variations in fabrication between LPG sensors can increase or decrease the sensitivity of the LPG to strain, temperature or surrounding refractive index. Here, the technique of overwritten UV laser fabrication is demonstrated. It is shown that, on repeated overwriting, the resonance bands of an LPG exhibit significant wavelength shift, which can be monitored and which can be used to tune the resonance bands to the desired wavelengths. This technique is applied to periods in the range 100 to 200 µm, showing the cycle-to-cycle evolution of the resonance bands near the PMTPs of a number of cladding modes. The use of online monitoring is shown to reduce the resonance band sensor-to-sensor central wavelength variation from 10 nm to 3 nm.