Bragg Wavelength Research Articles

Fiber-Optic Bragg Grating Sensor for Photothermally Examinating Moisture of Meat

The illegal water injection into meat not only breaks the market equity, but also deteriorates the meat quality and produces harmful substances. In this work, we proposed a fiber Bragg grating (FBG) sensor that enabled fast, quantitative, and in-situ detection of the moisture content of water-injected meat. The FBG was written in the erbium-ytterbium (Er/Yb) co-doped fiber, which could perform the self-photothermal effect by injecting the near infrared laser into the fiber. As the heated fiber sensor probe was inserted into the meat sample, the temperature decreased due to the heat dissipation mediated by moisture. The intracore Bragg grating could monitor the temperature loss by recording the Bragg wavelength shift, which reflected the water content quantitatively. The results revealed that the sensor could complete the detection within 15 s. The sensor’s sensitivity to detect changes in the pork water content was theoretically calculated to be 0.090847%. The proposed sensor is expected to provide a novel approach for examination of the meat moisture.

Highly birefringent side-hole fiber Bragg grating for high-temperature pressure sensing.

We demonstrate a novel, to the best of our knowledge, high-temperature pressure sensor based on a highly birefringent fiber Bragg grating (Hi-Bi FBG) fabricated in a dual side-hole fiber (DSHF). The Hi-Bi FBG is generated by a femtosecond laser directly written sawtooth structure in the DSHF cladding along the fiber core through the slow axis (i.e., the direction perpendicular to the dual-hole axis). The sawtooth structure serves as an in-fiber stressor and also generates Bragg resonance due to its periodicity. The DSHF was etched by hydrofluoric acid to increase its pressure sensitivity, and the diameter of two air holes was enlarged from 38.2 to 49.6 µm. A Hi-Bi FBG with a birefringence of up to 1.8 × 10-3 was successfully created in the etched DSHF. Two distinct reflection peaks could be observed by using a commercial FBG interrogator. Moreover, pressure measurement from 0 to 3 MPa at a high temperature of 700°C was conducted by monitoring the birefringence-induced peak splits and achieved a high-pressure sensitivity of -21.2 pm/MPa. The discrimination of the temperature and pressure could be realized by simultaneously measuring the Bragg wavelength shifts and peak splits. Furthermore, a wavelength-division-multiplexed (WDM) Hi-Bi FBG array was also constructed in the DSHF and was used for quasi-distributed high-pressure sensing up to 3 MPa. As such, the proposed femtosecond laser-inscribed Hi-Bi FBG is a promising tool for high-temperature pressure sensing in harsh environments, such as aerospace vehicles, nuclear reactors, and petrochemical industries.

A comparison of temperature compensation methods in a diaphragm-embedded FBG

This paper presents the comparison of temperature compensation techniques for Fiber Bragg Grating (FBG)-based pressure sensor. Such temperature compensation is obtained through a physical assembly using a continuous FBG pair embedded in a rubber diaphragm through a vulcanization method in conjunction with different signal processing methods. The so-called continuous FBG pair is based on two consecutive FBGs (with different Bragg wavelengths) inscribed with physical separation up to 10 mm. The sensor system is tested as a function of temperature and pressure, where the temperature sensitivities of both FBGs are around 50 pm/°C. Pressure estimation errors around 148.22 Pa/°C were observed when this variation in sensitivity was not taken into account by the compensation technique. Furthermore, there is a comparison between techniques that consider the pressure sensitivity variation, namely multiple linear regression using the response of both FBGs and exponential regression using the sum of FBGs. In this case, the exponential regression indicated a smaller temperature cross-sensitivity (3.07 Pa/°C), higher determination coefficient (0.97) and smaller root mean squared error (0.40 kPa). Therefore, the exponential regression using the response of both FBGs is the suitable technique for temperature compensation when the continuous FBG pair embedded in the diaphragm assembly is considered. The results obtained in this work provide a guideline for the development of diaphragm-embedded sensors with temperature compensation and suitable for large-scale production in practical applications.

High-temperature strain sensor based on sapphire fiber Bragg grating.

Sapphire fiber Bragg grating (SFBG) is a promising high-temperature strain sensor due to its melting point of 2045°C. However, the study on the long-term stability of SFBG under high temperature with an applied strain is still missing. In this paper, we reported for the first time to our knowledge on the critical temperature point of plastic deformation of the SFBG and demonstrated that the SFBG strain sensor can operate stably below 1200°C. At first, we experimentally investigated the topography and the spectral characteristics of the SFBG at different temperatures (i.e., 25°C, 1180°C, and 1600°C) with applied 650 µε. The reflection peak of the SFBG exhibits a redshift of about 15 nm and broadens gradually within 8 h at 1600°C, and the tensile force value decreases by 0.60 N in this process. After the test, the diameter of the SFBG region decreases from 100 to 88.6 µm, and the grating period is extended from 1.76 to 1.79 µm. This indicates that the plastic deformation of the SFBG happened indeed, and it was elongated irreversibly. Moreover, the stability of the Bragg wavelength of the SFBG under high temperature with the applied strain was evaluated. The result demonstrates the SFBG can be used to measure strain reliably below 1200°C. Furthermore, the strain experiments of SFBG at 25°C, 800°C, and 1100°C have been carried out. A linear fitting curve with high fitness (R2 > 0.99) and a lower strain measurement error (<15 µε) can be obtained. The aforementioned results make SFBG promising for high-temperature strain sensing in many fields, such as, power plants, gas turbines, and aerospace vehicles.

Investigation of the effects of grating length, Bragg wavelength and wavelength shift on temperature sensitivity in fiber Bragg grating-based sensing systems

Investigation of the effects of grating length, Bragg wavelength and wavelength shift on temperature sensitivity in fiber Bragg grating-based sensing systems

The Study on the Fundamental of Fiber Bragg Grating (FBG) Sensing Principle

Abstract: A single mode fibre core is laterally exposed to a periodic pattern of powerful UV laser light to create an optical sensor called an FBG. The exposure results in a long-lasting rise in the fiber's core's refractive index (ncore), producing fixed index modulation known as grating (Λ). A specific wavelength of input light, known as the Bragg wavelength or Bragg related to grating period, must be reflected by the grating inside the fibre optic core while transmitting all other wavelengths. FBG operates as intended by using the regular variations in the single mode fibre core's refractive index, a Bragg reflector can be constructed on an optical fibre. A specific wavelength of light will be reflected and all others will be transmitted as it passes through the FBG. A shift in the wavelength of the light reflected when the temperature or strain surrounding the grating changes is seen.

The Study on the Spectral Analysis of Fiber Bragg Grating (FBG) as a Strain Sensor

Abstract: By adjusting the grating length and refractive index change, parameters of the Fibre Bragg grating which are the effective refractive index, Bragg wavelength, grating period, and strain-optic constant are provided and discussed, along with the characterization of the grating, including strain, Bragg wavelength shift, spectral response, and bandwidth. The coupled mode equations form the basis for data analysis and acquisition. This research uses a high-level computer language, such as GNU Octave software, to simulate FBG spectral responses.

Microwave-photonic Vernier effect enabled high-sensitivity fiber Bragg grating sensors for point-wise and quasi-distributed sensing

This paper reports a sensitivity-improved fiber Bragg grating (FBG) sensor system based on microwave-photonic interferometry and the Vernier effect. An incoherent microwave photonics system based on a broadband light source is employed to interrogate the FBG sensor using the wavelength-to-delay mapping technique combined with interferometry. Specifically, the sensing FBG together with a reference FBG is used to construct a microwave photonics Michelson interferometer (MI). Changes in the Bragg wavelength of the sensing FBG subject to external perturbations are encoded into the spectral shifts of the microwave interferogram of the MI. A virtual interferometer is then generated from the sensing MI based on a computational Vernier effect modality. By superimposing the spectra of the sensing MI and the virtual interferometer, the Vernier effect is generated. By tracking the spectral shift of the Vernier envelope, it is shown that the measurement sensitivity of the sensing FBG is remarkably enhanced with an expected factor. Moreover, a quasi-distributed sensor system with enhanced sensitivity based on cascaded FBGs and the proposed virtual microwave-photonic Vernier effect technique is implemented, representing the first demonstration of a Vernier effect-enhanced FBG array sensor. Additionally, the possibility of employing the harmonic Vernier effect for further sensitivity enhancement is investigated, where a remarkable sensitivity enhancement factor up to 685 with a strain sensitivity of 94 MHz/µε is successfully demonstrated.

Long-term creep monitoring of composite wing leading edge using embedded fiber Bragg grating

The creep behavior of composite wing leading edges resulting from nonequilibrium residual stresses and material viscoelasticity needs to be evaluated comprehensively as it significantly affects assembly. In this study, long-term creep monitoring of a composite wing leading edge used in an actual airplane for 710 h is conducted using embedded fiber Bragg grating arrays and a creep extraction algorithm. The spectra and Bragg wavelength shifts of two embedded arrays, which involve temperature, thermal expansion, and creep, are recorded and analyzed. The creep curve of the composite wing leading edge is reconstructed and further predicted using the creep extraction algorithm based on multiparameter decoupling and the Burgers model. This study elucidates the enlargement of the opening size in the composite wing leading edge by measuring the tensile strain above the neutral axis. The predicted creep time serves as a valuable reference for determining the appropriate assembly timing.

Simultaneous Measurement of Microdisplacement and Temperature Based on Balloon-Shaped Structure.

An optical fiber sensor for the simultaneous measurement of microdisplacement and temperature based on balloon-shaped single-mode fibers cascaded with a fiber Bragg grating with two core-offset joints is proposed. The interference between the core mode and cladding mode is caused by the stimulation of the cladding mode by the core-offset joints' structure. The cladding of the core has a distinct refractive index, which causes optical path differences and interference. The balloon-shaped structure realizes mode selection by bending. As the displacement increases, the radius of the balloon-shaped interferometer changes, resulting in a change in the interference fringes of the interferometer, while the Bragg wavelength of the fiber grating remains unchanged. Temperature changes will cause the interference fringes of the interferometer and the Bragg wavelength of the fiber grating to shift. The proposed optical fiber sensor allows for the simultaneous measurement of microdisplacement and temperature. The results of the experiment indicate that the sensitivity of the interferometer to microdisplacement is 0.306 nm/µm in the sensing range of 0 to 200 μm and that the temperature sensitivity is 0.165 nm/°C, respectively. The proposed curvature sensor has the advantages of a compact structure, extensive spectrum of dynamic measurement, high sensitivity, and simple preparation, and has a wide range of potential applications in the fields of structural safety monitoring, aviation industry, and resource exploration.

Optical Reflection Induced by Flexural Acoustic Wave on Fiber Bragg Grating

Flexural acoustic waves can induce optical reflection peaks on one side of the Bragg wavelength in the reflection spectrum of a fiber Bragg grating, thus laying a cornerstone of active acoustic-optic fiber devices. The acoustically-induced refractive index distribution of FBG is deduced. The mechanism of forming acoustically-induced reflection peaks is investigated using mode coupling theory in vector mode presentation. A precise picture is depicted with the selection rules for mode coupling and observation of phase matching, such that the two-step and the three-step processes for acoustically-induced reflection peaks are identified with experimental confirmation. While the three-step coupling theory is discovered, our theoretical framework also refines the former scalar-mode two-step coupling theory. Empowering the theoretical prediction to perfectly match experiment, this work will facilitate the design and application of novel active fiber-optic devices based on acoustic-optic interactions.

Temperature Drift Characteristics Analysis of GMM-FBG Current Sensor Based on Finite-Element Multi-Physics Simulations

A composite current sensor is designed with soft ferrite as the magnetizer in combination with giant magnetostrictive material (GMM) and fiber Bragg grating (FBG). The temperature drift characteristics of the GMM, detecting performances under thermal strain caused by temperature variation in the GMM rod, are investigated by simulating the coupled fields of magnetostriction and thermal expansion with the finite-element multi-physics method to explore the temperature-drift mechanism of the ferrite–GMM current detector. The sensing characteristics of the GMM-FBG current sensor under quasi-static current excitation at various work temperatures are evaluated by simulating thermal stress between the GMM and FBG to analyze the temperature drift mechanism of the Bragg wavelength signal modulated by FBG. Even though temperature elevation suppresses GMM magnetization and thereby reduces the slopes of stress–strain curves, the steering magnetization of magnetic domains in the GMM rod tends to saturation without appreciable thermal inhibition in the high-stress region of large current excitation, while the magnetostrictive strain is still abated by the reduction in magnetic flux density caused by the thermal expansion of GMM rods. The temperature elevation can also produce thermal stress between the GMM and FBG, which will decrease the detection sensitivity and testing range of the GMM-FBG current sensor. The temperature drift characteristics of the GMM-FBG are generalized into a formula by fitting the wavelength shifting as a function of thermal strain, which will significantly facilitate designing the scale calibration for various ambient temperatures. The present researchers provide a theoretical basis and experimental guidance for developing GMM-FBG current sensors with high sensitivity and stability.

Fabrication and characterization of polymer optical waveguide Bragg grating for pulse signal sensing.

Polymer materials have the advantages of a low Young's modulus and low-cost preparation process. In this paper, a polymer-based optical waveguide pressure sensor based on a Bragg structure is proposed. The change in the Bragg wavelength in the output spectrum of the waveguide Bragg grating (WBG) is used to linearly characterize the change in pressure acting on the device. The polymer-based WBG was developed through a polymer film preparation process, and the experimental results show that the output signal of the device has a sensitivity of 1.275 nm/kPa with a measurement range of 0-12 kPa and an accuracy of 1 kPa. The experimental results indicate that the device already perfectly responds to a pulse signal. It has significant potential application value in medical diagnostics and health testing, such as blood pressure monitoring, sleep quality monitoring, and tactile sensing.

Characterization Study of Double Filtered Sensor Length Effect on Strain Sensitivity

In this simulation study, Optisystem 18 software is used to monitor and study the effectiveness of side strain on selected lengths of two virtual uniform fiber Bragg grating (FBG) sensors. The operational FBG sensor Bragg wavelength was 1550 nm, which is used to find the measured shift in deflected light source optical spectrum. This value is also supplied by the light source to offer the minimum absorption and attenuation during transmission inside the optical fiber. Reliability of the sensor and technique of transferring the signal under such effect are screened. The investigator is also used to observe the shift in wavelength with altered applied side strain. The influence of sensor active length on side strain sensitivity is studied where according to theory, the length of the FBG influences the sensitivity via reflectivity . The constructed sensor sensitivity is observed against length before and during the experiment. The sensing principle, in essence, depends on tracking the wavelength shift due to the variation of such strain. Results achieved in this study show an inverse relationship between sensor effective length and shift in the observed wavelength. The measured strain sensitivity is carried out for the active sensor length, which ranges from 0.05 to 15 cm, with corresponding sensitivity values of 1.19 pm/ OC to 0.9 pm/OC, respectively, under the same strain conditions. The empirical results also show the success of the suggested sensing system in measuring strain. The strain measurement, ε, is linearly increasing, identical to the increasing values of the wavelength shift of Bragg. It's also been observed that the wavelength of Bragg is shifting during small ratios in the length protraction of the FBGs.

Large-scale cascading of first-order FBG array in a highly multimode coreless fiber using femtosecond laser for distributed thermal sensing.

This research focuses on the performance analysis and characterization of a fiber Bragg gratings (FBGs) array, consisting of 10 first-order FBGs inscribed by a femtosecond (FS) laser in a highly multimode coreless fiber. The study evaluates the FBG array's ability to function as a distributed thermal sensing (DTS) platform, with the coreless fiber chosen as the sensing element due to its immunity to dopant migration at high temperatures. The design of a large cascaded first-order FBG array effectively eliminates unwanted harmonic peaks across a wide spectrum range. In contrast, higher-order FBGs introduce limitations due to the overlapping of Bragg peaks with harmonics. The FBG array's performance is evaluated by measuring the reflection spectrum of each grating at different temperatures, showing a high temperature sensitivity of 15.05 pm/°C at a Bragg wavelength of 1606.3 nm, with a linear response in the temperature range of 24 - 1100 °C. The FBG array was designed for a spatial resolution of 5 mm. A mode scrambler in the sensing network is employed, which suppresses multimodal interference, characterizes FBG peak visibility, and stabilizes the interference spectrum. The stability of the FBG array is also assessed over 24 hrs at 1100 °C, and it is observed to be stable during thermal treatment. Heat treatment at 1100°C improves the signal to noise ratio of the FBG array, demonstrating the robustness and suitability of the proposed FBG array on highly multimode coreless fiber as a potential sensing platform for DTS applications in harsh environmental conditions, overcoming the issues of dopant migration presented by dopes silica optical fibers at high temperatures.

Real-time monitoring of urea slow release through fiber Bragg grating sensors

This research aimed at using fiber Bragg grating (FBG) sensors for monitoring the slow release of urea. Urea granules were coated with ethyl cellulose for controlled-nitrogen release. The FBGs were fabricated using a 10 mm single-mode glass fiber optic, and silver nanoparticle coating was deposited on the partially removed cladding of the FBG using the electroless technique. The silver-coated FBGs at three operating wavelengths (FBG1: 1534 nm, FBG2: 1559 nm, and FBG3: 1565 nm) were used to monitor the slow release of urea in water and soil. The Bragg wavelength shift of FBG 1, 2, and 3 to the controlled-release rate in water was 0.028, 0.12, and 0.20 nm, respectively. In soil, the coated fertilizer responded to the FBG 1, 2, and 3 showing wavelength shifts of 0.028, 0.10, and 0.12 nm, respectively. This study demonstrates the potential of FBG sensors in agricultural applications.

Temperature-Insensitive and Sensitivity-Enhanced FBG Strain Sensor by Using Parallel MPF

A temperature-insensitive and sensitivity-enhanced FBG strain sensor with the Vernier effect is proposed and demonstrated. The FBG sensor consists of two FBGs with different Bragg wavelengths as the sensing heads and two parallel-installed fiber ring resonator (FRR)-based microwave photonic filters (MPF) as the interrogation devices. The measurements can be read from the frequency shift in the notch of the envelope signal. Thanks to the Vernier effect produced by the parallel-FRR-based MPF, the sensitivity of the proposed sensor to strain is improved about 23 times compared with the single FRR-based strain sensor, which is experimentally demonstrated to be −33.862 kHz/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> . And the sensitivity of the sensor to temperature was drastically decreased from 325.583 kHz/°C to 1.031 kHz/°C. The unique merits of the proposed sensor, such as high sensitivity, high resolution, and stability against random disturbances, give the sensor great potential for sensing applications in complex environments.

A New Biometric Identification System Based on Plantar Pressure

Biometric recognition based on physiological characteristics has been widely used for security reasons. In these systems, unique features such as fingerprints, iris patterns, face structure, and voice are used to recognize a person instead of codes that can be easily transferred. The biometric identification system based on plantar pressure proposed in this work applies a support vector machine procedure to classify footprint patterns detected by a platform instrumented with fiber Bragg gratings. Two sets of 7 in-series gratings, with Bragg wavelengths within 1526.96 nm and 1553.86 nm range, delimit two sensing regions corresponding to the left and right footprints. The coupled and non-linear responses provided by the sensors are pre-processed and organized in a matrix. An algorithm based on support vector machine was used to extract and recognize foot features. The pattern recognition system constructed in this way returned hit rates greater than 86% for the 12 classes, corresponding to different stepping patterns, with data not belonging to the training set.

Continuous Fiber Bragg Grating Optical Sensors for Superconducting Magnet Quench Detection: The Effects of Attenuation and Position in the Array

Quasi-continuous arrays of fiber Bragg gratings (FBGs) show great promise for quench protection in superconductors. The number of identical gratings and the reflectivity of the individual gratings are important parameters determining the sensitivity of the system to a local temperature or strain perturbation. We apply strain to shift the Bragg wavelength of a single FBG through the spectrum of an array of up to 1000 identical gratings, and map out the detection sensitivity for a given <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Delta \lambda$</tex-math></inline-formula> relative to the peak reflectance of the FBG array, and show that the position of the perturbed FBG matters only due to the broadband attenuation of the probe light down the array. Varying the number of gratings in the array shows the interplay between the number of gratings, the reflectivity of the gratings, and the sensitivity of the system. We demonstrate that the ability to model the spectrum of the FBG array is crucial to predicting system performance.

Temperature compensated magnetic field sensor based on magnetic fluid infiltrated cascaded structure of micro-tapered long-period fiber grating and fiber Bragg grating

Magnetic fluid (MF) infiltrated fiber-optic interference structures can be used to realize miniature magnetic field sensor for various applications needing sensitive magnetic field measurement, however, these sensors are typically also sensitive to temperature variations. To solve the problem of temperature cross sensitivity, an MF-infiltrated micro-tapered long-period fiber grating (MT-LPFG) and a femtosecond laser inscribed fiber Bragg grating (fsFBG) cascaded sensing structure is proposed and demonstrated. The MT-LPFG is fabricated by a fused tapering method under the illumination of a CO2 laser and is inserted into a glass capillary infiltrated with MF for magnetic field measurement, and the fsFBG is cascaded to the MT-LPFG for temperature measurement. Through the simultaneous monitoring of the resonant wavelength shift of the MT-LPFG and the Bragg wavelength shift of the fsFBG, temperature and magnetic field strength can be simultaneously extracted through a sensitivity coefficient matrix. As a result, the influence of the temperature on the MF-infiltrated MT-LPFG can be effectively compensated. Experimental results show that a magnetic field strength sensitivity of 21.86 pm/Oe can be achieved in a magnetic field strength range of 10-80 Oe and a temperature range of 30-60℃.