Optical Fiber Bragg Grating Research Articles

In-situ characterization of 3D printed continuous fiber reinforced composites: A methodological study using fiber Bragg grating sensors

This work demonstrates the applicability of optical fiber Bragg grating (FBG) sensors for in-situ characterization of continuous fiber reinforced thermoplastic composites (CFRTPCs) fabricated through the Fused Deposition Modeling (FDM) technique. Prismatic carbon and glass fiber reinforced Nylon composite specimens were built with various fiber orientations using a MarkTwo desktop 3D printer. FBGs were embedded along with thermocouples (TCs) into the middle-plane of the composite structures for simultaneous monitoring of both residual strains and temperature profiles developed during the building procedure. Additionally, the residual strain magnitudes were further investigated at a post-fabrication state. The samples were then subjected to thermal loading for measuring the thermally induced strains as a function of applied temperature and consequently, the composites' coefficients of thermal expansion (CTEs), based on the recorded FBG wavelengths. The results indicate that the in-process generated temperature profiles and the calculated CTE values are vigorously affected by the fiber type and orientation. It is also shown that the developed strain fields are strongly dependent on the fabrication induced temperature profiles as well as the composites' expansion behavior under applied temperature.

General Expression of Poly(Methyl Methacrylate) Optical Fiber Bragg Grating Sensing Response

Poly(methyl methacrylate) (PMMA) optical fiber inherently responds to the changes of temperature and relative humidity. PMMA optical fiber Bragg grating (POFBG) has been reported with diverse temperature and relative humidity sensitivities. These results cannot present the full performance of a POFBG sensor, as they are measured at limited environmental conditions. In this letter, we systematically investigated the POFBG temperature and relative humidity sensing responses. We found that POFBG temperature/relative humidity sensitivity is the function of both temperature and relative humidity. In the experiments, POFBG sensing responses vary linearly with temperature/relative humidity at fixed relative humidity/temperature. Based on this linearity, a general expression of POFBG sensing response to temperature and relative humidity has been obtained and verified for the first time to describe the POFBG performance in the desired operation range. The anisotropy in POF induced by drawing history could induce uncertainty in POFBG response but pre-straining POFBG overcomes this problem.

Temperature Sensing of Stepped-Metal Coated Optical Fiber Bragg Grating with the Restructured Dual-Peak Resonance

This paper demonstrates a method for the development of stepped-metal coating on optical fiber Bragg grating. The paper also analyzes the dual-peak resonance restructured by the nickel/copper stepped-metal coating. According to the coefficients of linear thermal expansion of the coatings, the modeling and experimental analysis is categorized into three types: Type A, Type B, and Type C. We denote the temperature sensitivity difference between the two peaks as ΔKT for Type A, ΔK′T for Type B, and ΔK″T for Type C. The experimental results show that ΔKT, ΔK′T, and ΔK″T are 2.1 pm/°C, 6.5 pm/°C, and 0.8 pm/°C, respectively. The model analysis and the experimental results all show the Type B stepped-metal coating causes the most obvious temperature sensitivity difference between the two resonance peaks. The stepped-metal coating on the same one Bragg grating can restructure the single resonance into dual-peak resonance with different temperature sensing, and Type B can be used to develop a dual-parameter optical fiber Bragg grating sensor at one location which can measure two physical parameters simultaneously.

Regenerated Fiber Bragg Grating Sensing System for Ultrasonic Detection in a 900 °C Environment

Heat-resistant composites, such as ceramic matrix composites and heat-resistant carbon fiber reinforced plastics (CFRPs), are expected to be used for aircraft engine parts. The development of reliable heat-resistant composite materials requires the use of nondestructive test techniques for evaluating the progression of damage during material testing at elevated temperatures. Furthermore, structural health monitoring (SHM) technologies that operate under harsh environments are expected to be realized for monitoring heat-resistant composite structures. To provide potential solutions for the establishment of such technologies, this research developed a heat-resistant ultrasonic sensor based on a regenerated fiber-optic Bragg grating (RFBG). First, we fabricated an RFBG by annealing a normal fiber-optic Bragg gratings (FBG) sensor. Because the RFBG exhibits high heat resistance at temperatures of 1000 °C, the sensor achieved stable ultrasonic detection at an elevated temperature. In addition, we attempted to use a π-phase-shifted FBG (PSFBG) as the seed grating to construct an ultrasonic sensor with enhanced performance. As a result, the regenerated phase-shifted fiber-optic Bragg grating (R(PS)FBG) sensor possessed a very short effective gauge length and achieved a broad frequency response to ultrasonic waves with frequencies greater than 1.5 MHz. The broadband detectability enables the R(PS)FBG sensor to acquire an accurate response to ultrasonic waves. Hence, we believe the regenerated Bragg grating-based ultrasonic sensors can contribute to establishing an effective nondestructive evaluation method for composite materials, thereby enabling a structural health monitoring system for a composite-made structure operating under extreme high-temperature environments.

光纤布拉格光栅滑觉感知单元

光纤布拉格光栅滑觉感知单元

Manufacturing and reliability of a fiber Bragg grating strain sensor designed for uniaxial compression test of rocks

In this work we present an optical Fiber Bragg Grating (FBG) strain sensor appropriate for uniaxial compression test of rocks. The rocks are a complex aggregate of materials and may exhibit in some cases mineral grains of size bigger than the conventional electrical strain gages used. Furthermore, commercial surface mountable optical FBG strain sensors are packaged in planar configuration, which is not appropriate for the cylindrical rock cores in uniaxial compression test. The optical FBG sensors were designed and manufactured manually fitted for cylindrical rock samples. A calibration process was carried out in order to stablish the mechanical performance of the FBG packaging; later, a compression test was conducted on hard rock sample and the strain was recorded by the traditional electrical strain gage (ESG) and FBG system. Under this comparison, the FBG sensors reaches a larger recording area, sensitivity under 100 microstrain and a reliable strain transfer, all of which is appropriate to measure strain for heterogeneous rock composition and minerals grain size.

Calculation of Light Scattering on a Bragg Grating by Recursion of Transfer Matrices on a Nonuniform Grid

The direct problem of light scattering for a fiber optic Bragg grating is considered. The formulation and solution of the problem based on the transfer-matrix method are discussed. A modification of the method is proposed which reduces it to a computationally convenient universal recursive algorithm similar to the Thomas algorithm. Using the finite volume method in the coupled-mode approximation, the elements of transfer matrices were calculated with local third-order accuracy in coordinate on a nonuniform computational grid. Numerical calculations for the direct scattering problem for a Bragg grating with apodization and nonlinear chirp were performed using the recursive algorithm. Numerical simulations confirmed the significant increase in the accuracy of calculations when solving the scattering problem on a nonuniform grid.

Solitons in optical fiber Bragg gratings with dispersive reflectivity

This paper retrieves bright, dark and singular solitons of both types for Bragg gratings with dispersive reflectivity. The method of undetermined coefficients is the integration algorithm adopted in this paper. Several constraints guarantee the existence of such solitons.

Solitons in optical fiber Bragg gratings with dispersive reflectivity by extended trial function method

This paper retrieves optical solitons in fiber Bragg gratings, with dispersive reflectivity, by extended trial function approach. Bright and singular soliton solutions are listed and the corresponding integrability conditions are also enumerated.

Hot water-assisted fabrication of chirped polymer optical fiber Bragg gratings.

We obtained chirped gratings by performing hot water gradient thermal annealing of uniform poly (methylmethacrylate) (PMMA) microstructured polymer optical fiber Bragg gratings (POFBGs). The proposed method's simplicity is one of its main advantages because no special phase mask or additional etching are needed. It not only enables easy control tuning of the central wavelength and chirp characteristics, but it also leads to obtain flexible grating response, compared with tapered chirped POFBGs. Therefore, a flexible and low-cost chirped POFBG devices fabrication technique has been presented by using a single uniform phase mask.

Toward Commercial Polymer Fiber Bragg Grating Sensors: Review and Applications

Interest in polymer optical fiber Bragg gratings (POFBGs) arises from the different material properties and sensing modalities brought by polymers relative to silica. Polymer fibers typically offer twice the sensitivity to temperature of conventional silica fiber and increased sensitivity to strain overall. In addition, polymer fibers have higher elastic limits and as a result a larger range of operation for physical constraints. While some polymers are effectively humidity insensitive, others present inherent humidity sensitivity. Their organic properties also allow a variety of chemical processes to create (bio)chemical sensors, with the consequences of fiber breakage in situ being less hazardous than silica. These attributes have led to the use of POFBGs for applications that remain complex using silica fibers. This review paper covers the progress toward commercialization and the increasing number of specific applications.

Femtosecond writing of refractive index structures in multimode and multicore optical fibres

The possibility of writing fibre Bragg gratings (FBGs) in optical fibres with a complex spatial structure using femtosecond IR laser pulses is considered. In particular, selective writing of uniform FBGs in individual cores of a seven-core fibre and writing of serial and parallel FBG arrays in the core of a multimode fibre with a gradient refractive index profile are demonstrated.

Acoustic Fiber Bragg Grating and Its Application in High Temperature Sensing

A novel concept of acoustic fiber Bragg grating (AFBG) was conceived and experimentally validated as an effective technique for high-temperature sensing. Similar to the well-known optical fiber Bragg grating, the AFBG employs interactions between waves and periodic structures on an elongated waveguide. Acoustic waves reflected from periodic structures interfere with each other and result in the emergence of resonance frequency when acoustic wavelength matches with pitch length of the periodic structures. When the single-mode operation condition is satisfied, the resonance frequency location can be accurately resolved and thus be used as an effective indicator for temperature sensing. To demonstrate this concept, an AFBG sample was fabricated on a telecommunication optical fiber (125- $\mu \text{m}$ diameter) with femtosecond laser micromachining setup, and it demonstrated sensing capability up to 700 °C, which was limited by the fiber material (fused silica) and could be over 1400 °C if other material, such as sapphire, was used as the waveguide. With the features of various applicable material, low system cost, and potential multiplexing capability, this AFBG technology is a promising candidate for high-temperature sensing, even in a distributed manner.

Water content detection in aviation fuel by using PMMA based optical fiber grating

Water in aviation fuel is a destructive contaminant and can cause serious problems that compromise aircraft’s safe operation and reduce its efficiency and lifetime. Online monitoring of water content in aviation fuel would permit the control of water content before it builds up to dangerous level. Optical fibers made of PMMA have water affinity. In a PMMA based optical fiber Bragg grating (POFBG) its refractive index and volume vary with the water content. This feature is used to detect tiny water content in aviation fuel in this work. The sensing mechanism of POFBG is analyzed. POFBG wavelength is found to be the function of both temperature and equilibrium relative humidity (ERH). POFBG response to water content in fuel can be determined by the ERH. The sensor is experimented at different environmental conditions to identify its sensitivity. As a result, a general expression of POFBG response is achieved. Water content in Jet-A1 is measured by using POFBG sensor calibrated with both environmental chamber and coulometric titration. POFBG sensor is finally tested in a simulation fuel tank, demonstrating a better performance than coulometric titration. A sensitivity of POFBG wavelength change to water content of 33 pm/ppm is achieved at room temperature, indicating detectable water content of 0.03 ppm.

Simultaneous Vector Bend and Temperature Sensing Based on a Polymer and Silica Optical Fibre Grating Pair.

The bending response of polymer optical fibre Bragg grating (POFBG) and silica optical fibre Bragg grating (SOFBG) mounted on a brass beam have been systematically studied and compared. The results indicate that POFBG has higher (almost twice as much) bend sensitivity than SOFBG. Based on the difference between the bend and temperature sensitivity of POFBG and SOFBG, a new method of measuring vector bend and temperature simultaneously was proposed by using a hybrid sensor head with series connection of one POFBG and one SOFBG with different Bragg wavelengths. It provides high sensitivity and resolution for sensing bend and temperature changes simultaneously and independently. The proposed sensor can find some applications in the fields where high sensitivity for both bend and temperature measurements are required.

Birefringent Bragg Grating in C-Shaped Optical Fiber as a Temperature-Insensitive Refractometer.

We demonstrate a simple-to-fabricate refractometer based on the inscription of fiber Bragg gratings in a special C-shaped optical fiber. The C-shaped fiber was drawn into shape using a quarter cladding removed preform of a commercial standard single-mode fiber by simple machining. The sensor did not suffer from cross-sensitivity of the refractive index with ambient temperature fluctuations, commonly occurring with many optical fiber refractometers. A refractive index sensitivity of 1300 pm per refractive index unit (RIU) was achieved without employing any additional sensitization techniques such as tapering or etching.

Miniature all-fibre microflown directional acoustic sensor based on crossed self-heated micro-Co2+-doped optical fibre Bragg gratings

The large size and electromagnetic interference of microphone arrays are long standing challenges for sound source localization. Here, an all-fibre vector acoustic sensor based on crossed microfiber Bragg gratings (micro-FBGs) has been proposed and experimentally demonstrated that enables the two-dimensional sound source localization with a size less than 1.5 mm. Two micro-FBGs inscribed in the Co2+-doped fibre are fabricated through a hydrofluoric acid solution, which are placed parallel to each other to form a micro-FBG pair. Based on the self-heating and asymmetric temperature distribution of two Co2+-doped fibres, the crossed micro-FBGs provide a direct two-dimensional measurement of the acoustic particle velocity. The experimental results show that an orientation sensitivity of 1.57 mV/deg for the micro-FBGs is achieved with a figure-of-eight response of the acoustic source direction. Meanwhile, the direction responses of two crossed pairs of micro-FBGs are exactly orthogonal to each other, which could detect the sound source localization. The all-fibre vector acoustic sensor is immune to electromagnetic interference, which appears to have a variety of potential applications, including research in acoustic communication, pipeline monitoring and navigation, etc.

Direct inscription and evaluation of fiber Bragg gratings in carbon-coated optical sensor glass fibers for harsh environment oil and gas applications.

In this research work, we show the successful inscription of fiber Bragg gratings into carbon-coated pure silica as well as germanium-doped glass fibers by applying the pulsed laser point-by-point manufacturing technique. First, the parameters used for the Ti:sapphire femtosecond laser process are demonstrated. Without removing the polymeric carbon coating, destruction-free formation of highly reflective Bragg gratings is performed with selected types of hermetically enclosed fibers. We demonstrate the advantage of the carbon coating by long-term exposure to a pure hydrogen atmosphere at an elevated temperature. Such harsh conditions exist in the oil and gas industry, which means there is high application potential for technologically advanced optical sensors. Compared to the also examined standard glass fibers with a distinct signal attenuation, carbon-coated fibers show no significant degradation. Finally, we analyze the mechanical stability of the processed fibers via standardized tensile tests. No substantial decrease in strength occurs among the sensor-integrated samples.

Thermal Treatments and Compensation Techniques for the Improved Response of FBG Sensors in POFs

The influence of annealing under different humidity conditions on temperature, strain, and transverse force performance of polymer optical fiber Bragg gratings (POFBG) inscribed in cyclic transparent amorphous fluoropolymers is presented. The POFBG are imprinted using direct-write, plane-by-plane femtosecond laser inscription method. Further improvement to the sensor's performance, with respect to sensitivity, linearity, and hysteresis, is obtained with a simple technique based on polymer viscoelastic response applied to the annealed POFBGs. The sensors were characterized for temperature, strain, and transverse force after annealing and results show that sensors annealed under water have the lowest hysteresis. Additionally, the temperature sensitivity increased when the sensor was annealed in low humidity conditions. Finally, for strain and transverse force, the highest sensitivity was obtained in POFBG sensors annealed under water. The compensation technique did not show a substantial decrease in hysteresis response for applied strain tests; however, for temperature and transverse force tests, the hysteresis decreased by more than two and four times, respectively.

An Experimental Investigation on Flexural Behavior of Reinforced Concrete Beams Strengthened by an Intelligent CFRP Plate with Built-In Optical Fiber Bragg Grating Sensors

The difficulties induced from high-durability package, large-scale measurement, and sensor installation make it a big challenge but an urgent need to predict the mechanical behavior of strengthened reinforced concrete structure with the externally bonded fiber-reinforced polymer (FRP) plate under harsh environments and rough construction process. In an effort to solve this challenge, this paper proposes a novel end anchored self-sensing CFRP plate assembly with built-in optical fiber Bragg grating (FBG) sensor. The self-sensing principle is presented and a demonstration test was carried out in the laboratory in order to investigate the self-sensing properties of the intelligent CFRP plate. Three-point flexural tests of a total of ten RC beams were carried on: six strengthened with the bonded or unbonded posttensioned intelligent CFRP plates, one strengthened with the ordinary CFRP plate, and three strengthened with the control beams. The full-range CFRP plate strain at the maximum moment was measured utilizing the FBG interrogator. Using this monitoring data, the short-term prestress loss attributed to anchorage set and the relationship between the external load and the CFRP strain at the middle span associated with the integral response of the strengthened member were investigated. Besides, the objective of the tests was also to gain a better understanding of the failure mode and the flexural behavior of RC beams strengthened with posttensioned CFRP plates taking into account the different strengthening methods, initial damage, and the dead load before strengthening. The experimental results showed that the developed CFRP plate with built-in FBG sensors not only can be the reinforcement of the RC structures, but also provide an effective way to monitor the full-range behavior of the CFRP plate with the excellent self-sensing property shown in the demonstration tests.