Characterization of Long-period Grating Refractive Index Sensors and Their Applications

All-fiber sensor for simultaneous measurement of refractive index and temperature based on hole-assisted three-core fiber

A hole-assisted three-core fiber (HATCF) has been proposed as a sensor for simultaneous measurement of refractive index (RI) and temperature. An 8 mm long HATCF is fused between two single mode fibers (SMFs). One air hole of the HATCF is opened by femtosecond laser ablation technique to expose a suspended core to the external environment. Due to the same diameters of the two suspended cores, the resonance couplings between the center core and the two suspended cores occur at the same wavelength, which leads to a strong resonance dip. When the solution is filled into the open air hole, the resonance dip is split in two dips because the phase matching wavelength between center core and the suspended core in the open air hole is changed. Simultaneous measurement of RI and temperature can be achieved by monitoring the wavelengths of the two dips. The measured RI and temperature sensitivities are 1369 nm/RIU in the range of 1.333-1.388 and 83.48 pm/°C in the range of 25-70 °C. The proposed sensor has outstanding advantages such as simple structure, high integration and dual parameter measurement, making it a potential application in the field of biological detection.

Hybrid sensor based on a hollow square core fiber for temperature independent refractive index detection

In this work, a hybrid sensor based on a section of hollow square core fiber (HSCF) spliced between two single mode fibers is proposed for the measurement of refractive index of liquids. The sensor, with a length of a few millimeters, operates in a transmission configuration. Due to the HSCF inner geometry, two different interferometers are generated. The first, a Mach-Zehnder interferometer, is insensitive to the external refractive index, and presents a sensitivity to temperature of (29.2 ± 1.1) pm/°C. The second one, a cladding modal interferometer, is highly sensitive to the external refractive index. An experimental resolution of 1.0 × 10^-4 was achieved for this component. Due to the different responses of each interferometer to the parameters under study, a compensation method was developed to attain refractive index measurements that are temperature independent. The proposed sensor can find applications in areas where refractive index measurements are required and the control of room temperature is a challenge, such as in the food and beverage industry, as well as in biochemical or biomedical industries.

Refractive Index Sensors Based on Long-Period Grating in a Negative Curvature Hollow-Core Fiber

Long-period optical fiber gratings (LPGs) are one of the widely used concepts for the sensing of refractive index (RI) changes. Negative curvature hollow-core fibers (NCHCFs), with their relatively large internal diameters that are easy to fill with liquids, appear as a very interesting medium to combine with the idea of LPGs and use for RI sensing. However, to date, there has been no investigation of the RI sensing capabilities of the NCHCF-based LPGs. The results presented in the paper do not only address this matter, but also compare the RI sensitivities of the NCHCFs alone and the gratings. By modeling two revolver-type fibers, with their internal diameters reflecting the results of the possible LPG-inscription process, the authors show that the fibers' transmission windows shift in response to the RI change, resulting in changes in RI sensitivities as high as -4411 nm/RIU. On the contrary, the shift in the transmission dip of the NCHCF-based LPGs corresponds to a sensitivity of -658 nm/RIU. A general confirmation of these results was ensured by comparing the analytical formulas describing the sensitivities of the NCHCFs and the NCHCF-based LPGs.

Enhancing Temperature Sensitivity of the Fabry-Perot Interferometer Sensor with Optimization of the Coating Thickness of Polystyrene

The exploration of novel polymers for temperature sensing with high sensitivity has attracted tremendous research interest. Hence, we report a polystyrene-coated optical fiber temperature sensor with high sensitivity. To enhance the temperature sensitivity, flat, thin, smooth, and air bubble-free polystyrene was coated on the edge surface of a single-mode optical fiber, where the coating thickness was varied based on the solution concentration. Three thicknesses of the polystyrene layer were obtained as 2.0, 4.1, and 8.0 μm. The temperature sensor with 2.0 μm thick polystyrene exhibited the highest temperature sensitivity of 439.89 pm °C^-1 in the temperature range of 25-100 °C. This could be attributed to the very uniform and thin coating of polystyrene, along with the reasonable coefficient of thermal expansion and thermo-optic coefficient of polystyrene. Overall, the experimental results proved the effectiveness of the proposed polystyrene-coated temperature sensor for accurate temperature measurement.

Corrosion Sensors for Structural Health Monitoring of Oil and Natural Gas Infrastructure: A Review

Corrosion has been a great concern in the oil and natural gas industry costing billions of dollars annually in the U.S. The ability to monitor corrosion online before structural integrity is compromised can have a significant impact on preventing catastrophic events resulting from corrosion. This article critically reviews conventional corrosion sensors and emerging sensor technologies in terms of sensing principles, sensor designs, advantages, and limitations. Conventional corrosion sensors encompass corrosion coupons, electrical resistance probes, electrochemical sensors, ultrasonic testing sensors, magnetic flux leakage sensors, electromagnetic sensors, and in-line inspection tools. Emerging sensor technologies highlight optical fiber sensors (point, quasi-distributed, distributed) and passive wireless sensors such as passive radio-frequency identification sensors and surface acoustic wave sensors. Emerging sensors show great potential in continuous real-time in-situ monitoring of oil and natural gas infrastructure. Distributed chemical sensing is emphasized based on recent studies as a promising method to detect early corrosion onset and monitor corrosive environments for corrosion mitigation management. Additionally, challenges are discussed including durability and stability in extreme and harsh conditions such as high temperature high pressure in subsurface wellbores.

Fabrication and Interrogation of Refractive Index Biosensors Based on Etched Fiber Bragg Grating (EFBG)

We discuss the fabrication and interrogation of a fiber-optic refractive index sensors based on wet-etched fiber Bragg grating (EFBG). The fabrication is based on chemical etching of an FBG through an HF-free etching solution (ammonium fluoride and sulfuric acid), which progressively depletes the fiber cladding exposing the core to the outer medium. Microscope inspection of the fiber and real-time detection of the Bragg wavelength allow controlling the sensitivity. The proposed interrogation method is based on a spline interpolation, that measures the change of Bragg wavelength when the FBG is exposed to variations of the refractive index in the surrounding medium. An experimental validation has been carried out, for small refractive index variations (up to $1.85\times 10^{-3}$ RIU), in order to verify the progressive change of sensitivity through fiber etching. The proposed EFBG sensing unit is a building block for functionalized fiber optic biosensors.

Ambient Refractive-Index Measurement with Simultaneous Temperature Monitoring Based on a Dual-Resonance Long-Period Grating Inside a Fiber Loop Mirror Structure

In this work, we report the experimental results on optimizing the optical structure for ambient refractive index measuring with temperature changes monitoring. The presented optical structure is based on a dual-resonance long-period grating embedded inside a fiber loop mirror, where the long-period grating acts as the head of the refractive-index sensor, whereas the section of polarization maintaining fiber in the loop mirror ensures suitable temperature sensing. The optimization process was comprised of tuning the resonance and interferometric peaks by changing the state of polarization of propagating beams. Experimental results establish that the response of the proposed sensor structure is linear and goes in opposite directions: an increase in the ambient refractive index reduces the signal response, whereas a temperature increase produces an increased response. This enables us to distinguish between the signals from changes in the refractive index and temperature. Due to the filtering properties of the interferometric structure, it is possible to monitor variation in these physical parameters by observing optical power changes instead of wavelength shifts. Hence, the refractive index sensitivity has been established up to 2375.8 dB/RIU in the narrow RI range (1.333⁻1.341 RIU) and temperature sensitivities up to 1.1 dBm/°C in the range of 23⁻41 °C. The proposed sensor is dedicated to advanced chemical and biological sensor applications.

Plasmonic Optical Fiber-Grating Immunosensing: A Review

Plasmonic immunosensors are usually made of a noble metal (in the form of a film or nanoparticles) on which bioreceptors are grafted to sense analytes based on the antibody/antigen or other affinity mechanism. Optical fiber configurations are a miniaturized counterpart to the bulky Kretschmann prism and allow easy light injection and remote operation. To excite a surface plasmon (SP), the core-guided light is locally outcoupled. Unclad optical fibers were the first configurations reported to this end. Among the different architectures able to bring light in contact with the surrounding medium, a great quantity of research is today being conducted on metal-coated fiber gratings photo-imprinted in the fiber core, as they provide modal features that enable SP generation at any wavelength, especially in the telecommunication window. They are perfectly suited for use with cost-effective high-resolution interrogators, allowing both a high sensitivity and a low limit of detection to be reached in immunosensing. This paper will review recent progress made in this field with different kinds of gratings: uniform, tilted and eccentric short-period gratings as well as long-period fiber gratings. Practical cases will be reported, showing that such sensors can be used in very small volumes of analytes and even possibly applied to in vivo diagnosis.

Manufacturing and Spectral Features of Different Types of Long Period Fiber Gratings: Phase-Shifted, Turn-Around Point, Internally Tilted, and Pseudo-Random

The manufacturing and spectral features of different types of long period fiber gratings (LPFGs), ranging from phase-shifted, turn-around point, and internally tilted gratings, to pseudo-random gratings, are described and discussed in detail. LPFGs were manufactured on boron-germanium co-doped photosensitive optical fibers with the point-by-point technique using an excimer KrF laser operating at 248 nm. The developed experimental setup to manufacture high-quality LPFGs was designed to totally customize any type of gratings with the possibility of setting different parameters, such as the grating period (or pitch), the number of grating planes, the number of laser shots for each plane, etc. Some important spectral features of the LPFGs’ spectra were taken into account. This allows realizing homemade devices useful in several fiber-based applications, such as optical filtering, coupling systems, random lasers, physical and chemical sensing, and biosensing.

An Optical Fiber Sensor and Its Application in UAVs for Current Measurements

In this paper, we propose and experimentally investigate an optical sensor based on a novel combination of a long-period fiber grating (LPFG) with a permanent magnet to measure electrical current in unmanned aerial vehicles (UAVs). The proposed device uses a neodymium magnet attached to the grating structure, which suffers from an electromagnetic force produced when the current flows in the wire of the UAV engine. Therefore, it causes deformation on the sensor and thus, different shifts occur in the resonant bands of the transmission spectrum of the LPFG. Finally, the results show that it is possible to monitor electrical current throughout the entire operating range of the UAV engine from 0 A to 10 A in an effective and practical way with good linearity, reliability and response time, which are desirable characteristics in electrical current sensing.

Investigation of a Fiberoptic Device Based on a Long Period Grating in a Ring Resonator

A fiberoptic architecture based on a ring resonator (RR) including a typical long period grating (LPG) was investigated. The interactions between the fundamental core mode (LP₀₁ or HE₁₁) coupled to the RR and the cladding mode (LP₀₈), excited into the cavity by means of the LPG, allow a peculiar spectral response characterized by two splitting resonances to be achieved. The new LPGRR architecture is investigated theoretically and a mathematical modelling based on the transfer matrix method (TMM) is proposed. The theoretical results are compared with the experiments measured by an open-loop LPG, while the performance of the relative LPGRR was estimated by a theoretical parametric analysis. Finally, an overview of the possible LPGRR sensing applications is provided by investigating the features of a strain sensor operating in different environmental conditions.

Effect of induced inner curvature on refractive index sensitivity in internally tilted long-period gratings

A new complete analysis of the effect of induced inner curvature on refractive index (RI) sensitivity in internally tilted long-period gratings (ITLPGs) is presented. The responses in terms of RI sensitivity of a standard LPG and different ITLPGs with curvature values between 15 and 19  m^-1 were compared. The analysis suggests first, that the larger the induced curvature, the greater the RI sensitivity; and second, that the RI sensitivity exponentially increases with both the curvature and cladding mode order. RI sensitivity greater than 100 nm RIU^-1 can be attained with curvature greater than 25  m^-1 for LP₀₆ mode. Conversely, the temperature sensitivity of ITLPGs is comparable to standard LPGs for the considered cladding mode order.

A review of refractometric sensors based on long period fibre gratings

In the last decade refractometric sensors have attracted an increasing interest by the scientific community due to their ability to perform ambient monitoring, to assess food quality and safety, and also to the fact that they enable the development of label free sensors in the biomedical area. These advances result, namely, from the use of long period fibre gratings in the turning points and/or with thin films in the transition region that allows resolutions of 10⁻⁶ to changes in the refractive index of the surrounding medium. Resolutions exceeding 10⁻⁸ can also be achieved when long period fibre gratings are combined with evanescent field based devices. This paper reviews the recent path towards the development of ultrahigh sensitive optical fibre refractometric sensors.

Reliable fiber sensor system with star-ring-bus architecture

This work presents a novel star-ring-bus sensor system and demonstrates its effectiveness. The main trunk of the proposed sensor system is a star topology and the sensing branches comprise a series of bus subnets. Any weakness in the reliability of the sensor system is overcome by adding remote nodes and switches to the ring and bus subnets. To construct the proposed star-ring-bus sensor system, a fiber ring laser scheme is used to improve the signal-to-noise ratio of the sensor system. The proposed system increases the reliability and capacity of fiber sensor systems.