Regenerated Fiber Bragg Gratings Research Articles

Accurate high-temperature profile sensing with dense multipoint arrays of regenerated fiber Bragg gratings

A spectrally and spatially dense sensor array consisting of 15 regenerated fiber Bragg gratings (RFBGs) over a length of 30 mm is presented for precise and fast multipoint temperature sensing up to 700°C. For the first time, it could be shown that with a dense fiber Bragg grating (FBG)-based sensor array the accuracy requirements of Class 1 thermocouples could be achieved and even exceeded. This also represents the highest spatial density of FBG-based high-temperature multipoint sensing reported so far. The mitigation of broadband losses during the regeneration process was studied, revealing the advantages of the low broadband loss characteristics of the RFBGs, especially when larger numbers of measuring points are required. Low measurement uncertainties were achieved by a new, improved calibration methodology and by an analysis of interferences of an FBG with the side lobes of spectrally neighboring FBGs as well as their suppression by a suitable arrangement of the Bragg wavelengths within the array and corresponding data processing. The capabilities of this multipoint sensor technique were demonstrated by resolving the temperature profile within the calibration volume of a calibration furnace and by resolving the temporal and spatial temperature gradients within the flame of a Bunsen burner. The results are of great importance for fiber-optic sensing of high-temperature profiles in real-world applications.

Real-time distributed observing on regeneration rates of an apodized type I fiber Bragg grating

Regenerated fiber Bragg gratings (RFBGs) have garnered widespread attention owing to their fascinating high-temperature resistance. The flexible modulating refractive index of fiber Bragg gratings (FBGs) is conducive to boost the versatility of RFBG fabrication. Nevertheless, the impact of different refractive index modulation (RIM) amplitudes on regeneration rate remains ambiguous. Given this, an optical low-coherence reflectometry (OLCR) is employed for real-time monitoring of RIM during the regeneration process. Experimental results illustrate the regeneration rate remains consistent at different RIM amplitudes. Furthermore, a functional relationship between regeneration ratio and regeneration time is proposed. To enhance the OLCR signal intensity, a promising optimized strategy is raised by introducing a phase shift at the edges of FBG. This work deepens our understanding of the FBG regeneration process and immensely improves the manufacturing flexibility of RFBG.

Development and analysis of a fiber-optic temperature sensor based on a regenerated fiber Bragg grating

Subject of study. A fiber-optic temperature sensor based on regenerated fiber Bragg gratings is studied. Aim of study. A high-temperature sensor based on a regenerated fiber Bragg grating is developed, and a thermal study of the sensor up to a temperature of 1000°C is performed. Method. The regenerated fiber Bragg grating was produced by annealing a “seed” fiber Bragg grating recorded on SMF-28 hydrogen-loaded optical fiber in a high-temperature muffle furnace at a continuously rising temperature from room temperature to 920°C (the regeneration temperature in the case of SMF-28 fiber). The reflection coefficient of the “seed” grating was as close to 100% as possible, with a structure length of 15 mm. The heating rate was 500 deg/h. Main results. During the work and thermal studies described above, which were performed over a temperature range from +25∘C to +1000∘C with steps of 100°C, a fiber-optic temperature sensor was developed based on a regenerated fiber Bragg grating with a reflectance of about 50% and a temperature sensitivity of 14.9 pm/°C. Practical significance. The proposed method for manufacturing regenerated fiber Bragg gratings enables them to be used as the sensing element in a temperature sensor. The ability to operate at such high temperatures opens up broad potential for application to a wide range of industrial applications (such as gas turbine engines, power plants, steel mills, etc.). This method enables a fiber-optic temperature sensor to be constructed without using additional equipment or materials.

High Mechanical Strength Thermally Regenerated Fiber Bragg Gratings for High-Temperature Stress Monitoring

High Mechanical Strength Thermally Regenerated Fiber Bragg Gratings for High-Temperature Stress Monitoring

A metalized and regenerated fiber Bragg grating temperature sensor with spectral self-repair ranging from −50 ℃ to 600 ℃

In this study, we developed a metalized and regenerated fiber Bragg grating (RFBG) temperature sensor with a spectral self-repairing function, aiming to measure both high and low temperatures for aircraft engine exhausting jet. The designed sensor consists of a metal substrate with an S-shaped groove, one fiber Bragg grating (FBG), and two welded sheets connected by laser welding. The high-temperature regeneration process improves the high-temperature measurement range of the sensor. In addition, an algorithm of discrete wavelet transform and Gaussian model iteration (DWT-GMI) for solving spectral degradation at low temperatures is proposed. Simulation analyses of the spectral degradation are performed to verify the feasibility of the proposed algorithm. The simulation results show that the root mean square error (RMSE) between the repaired wavelength and the actual wavelength is 0.0436. The temperature calibration experiment is conducted for different bonding methods, including AB adhesive, tin welding, and laser welding. The linear correlation coefficient of the RFBG sensor of laser welding is 0.99919, the sensitivity is 15.64 pm/℃, and the measurement range is −50 ℃ to 600 ℃, which is larger than the other two bonding methods. The accelerated life experiment is completed to predict the service life of the designed sensor according to the Arrhenius model. The above advantages enable the designed sensor suitable for air jet temperature monitoring for aircraft engines.

Size-dependent viscosity of silica optical fiber under high temperature

Viscosity of optical fiber plays an important role in high temperature applications in harsh environments. A size-dependent viscosity phenomenon of silica optical fiber under high temperature is observed by the stretching method with in-fiber regenerated fiber Bragg gratings (RFBGs). Higher viscosity is derived from optical fiber with larger diameters. The mechanisms of this size-dependent relationship of high temperature viscosity of silica optical fiber are discussed, and the difference in fictive temperature of silica optical fibers is considered as the mainly contribution. An equivalent fictive temperature of optical fiber at a metastable structure state is used to explain the difference in equilibrium viscosity. Then the relationship between equilibrium viscosity and fictive temperature for silica optical fiber at 1000 °C is derived.

Regenerated Fiber Bragg Gratings in Large Mode Area Fibers for High-Temperature Sensing

Regenerated fiber Bragg gratings (RFBGs) have been created in large mode area (LMA) fibers, and their key properties for their application as high-temperature sensors were experimentally characterized for the first time. UV written Type-I seed gratings in standard single-mode fibers (SMF-28) and in low numerical aperture LMA fibers with 125 μm and 250 μm cladding diameters were regenerated, and their wavelength drift characteristics at high temperatures, their temperature sensitivities, and their mechanical strengths were tested and compared with each other. We found that low dopant concentrations in the fiber cores led to lower wavelength drift rates during and after regeneration. A temperature calibration from room temperature to 800 °C showed that LMA fibers have generally similar but slightly lower temperature sensitivities than SMF-28 fibers. The tensile strength tests showed that LMA fibers with 250 μm cladding diameter have similar tensile fracture stresses as SMF-28 fibers but four times larger tensile force tolerances because of their twofold fiber diameter. Our research shows that using LMA fibers for RFBGs offers significant advantages for high-temperature sensing than their counterparts in standard fibers, which are also applicable to other silica fiber-based high-temperature sensors.

In-situ laser-ultrasonic visualization with the use of regenerated fiber Bragg grating sensors at elevated temperatures

The authors have developed a high-temperature ultrasonic sensing system with a regenerated fiber Bragg grating (RFBG) sensor, that was fabricated by annealing a fiber-optic Bragg grating (FBG). Different from the common FBG, the RFBG ultrasonic sensor is resistant to temperature as high as 900 °C. In this study, taking advantage of the RFBG sensor, we attempted to establish a laser-ultrasonic visualization method for in-situ damage diagnosis in heat-resistant materials at elevated temperatures. In the first part of this paper, after reporting the fabrication of the RFBG in detail, we validated that the RFBG sensor has reasonable sensitivity to ultrasonic measurement over a broad frequency range at high temperatures. The RFBG was then incorporated into a laser-ultrasonic visualizing inspector to visualize the ultrasonic wave propagating in a SiC plate with an artificial defect at 800 °C. An improved data processing method was employed to verify the reliability of the visualization result. In the processing method, the visualization result was transformed to the spectrum in the wavenumber-frequency domain with the help of three-dimensional fast Fourier transform. Referring to theoretically calculated dispersion curves of the Lamb wave propagating in the SiC plate, we identified S0 and A0 modes from the spectral results. In addition, the agreement between the theoretical results and the experimental results also verified the feasibility of our proposed system to visualize the ultrasonic wavefield in high-temperature environment. Moreover, we developed a window function to extract the wave modes from the visualization result based on the dispersion curves. The artificial defect in the SiC plate could be then clearly localized from the visualization results for the separated individual modes.

Regeneration turn-around-point: A milestone on the way to optimizing regenerated fiber Bragg grating

The influence of isochronal and isothermal annealing on grating regeneration in single mode silica optical fibers has been investigated. By rapid annealing of Type-I grating under isothermal conditions we demonstrated that the underlying processes in the fiber that contribute toward grating regeneration work best around a certain temperature and there is a turn-around-point or roll-over of strength of grating regeneration. We validate the existence of “regeneration turn-around-point” with Type-I gratings inscribed using a 248 nm laser and a 213 nm laser as well. Attaining a “regeneration turn-around-point” may be set as a milestone on the way to optimizing regenerated fiber Bragg gratings (RFBG) and subsequently to develop high quality FBG sensors suitable for use at elevated temperatures.

Generalized and wavelength-dependent temperature calibration function for multipoint regenerated fiber Bragg grating sensors.

A new calibration methodology for regenerated fiber Bragg grating (RFBG) temperature sensors up to 700 °C is proposed and demonstrated. A generalized, wavelength-dependent temperature calibration function is experimentally determined that describes the temperature-induced wavelength shifts for all RFBG sensor elements that are manufactured with the same fabrication parameters in the wavelength range from 1465 nm to 1605 nm. Using this generalized calibration function for absolute temperature measurements, each RFBG sensor element only needs to be calibrated at one reference temperature, representing a considerable simplification of the conventional calibration procedure. The new calibration methodology was validated with 7 RFBGs, and uncertainties were found to be compliant with those of Class 1 thermocouples (< ±1.5 K or < ±0.4% of the measured temperature). The proposed calibration technique overcomes difficulties with the calibration of spatially extended multipoint RFBG sensor arrays, where setting up an adequate calibration facility for large sensor fibers is challenging and costly. We assume that this calibration method can also be adapted to other types of FBG temperature sensors besides RFBGs. An accurate and practical calibration approach is essential for the acceptance and dissemination of the fiber-optic multipoint temperature sensing technology.

RFBG Thermal Aging Evaluation Above Regeneration Temperature

Regenerated fiber Bragg grating (RFBG) sensors for thermal aging at high temperatures from 600 °C to 1000 °C are produced and analyzed in this work. Seed FBGs are regenerated at 850 °C–900 °C and thermal characterized at the increasing steps of 50 °C–100 °C. Signal decay is measured from the reflected spectra until imminent erasure. The objective is to analyze the RFBG sensors lifetime above the regeneration temperature for long periods after annealing cycles with increasing thermal levels. Lifetime results from RFBG were reported to be a few hours above their regeneration temperature. Our results are significantly better. The sensors resisted for the time intervals of one to eight days up to 100 °C above the regeneration temperature. In addition, some sensors remained stable in terms of the spectral amplitude for more than four days in that thermal condition.

Optical Fiber Sensor With Double Tubes for Accurate Strain and Temperature Measurement Under High Temperature up to 1000 °C

In this work, we present a discriminative optical fiber sensor for temperature and strain measurement. The sensor comprises of two cascaded thermal regenerated Fiber Bragg gratings (RFBGs) incorporated with two fused silica capillary tubes. The RFBG <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is loosely enclosed in a fine fused silica tube and made solely sensitivity to temperature whereas the RFBG <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> sensor still retains its sensitivity to both strain and temperature. These properties have made the discriminative measurement accurate and directive. The experimental results indicate that temperature response is linear in the range of 100 – 1000°C with the sensitivity of ~15.7 pm/°C. Besides, it presents good repeatability in strain detection at high temperatures (300 °C – 900 °C). The incorporation of the two fine glass tubes has enhanced the modified RFBG’s strain sensitivity to as high as ~5.46 pm/ <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> in the measurement range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0~\mu \varepsilon $ </tex-math></inline-formula> to 120 <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> at 600 °C, which is about five times higher than that of common RFBG strain sensors. The sensitivity can be further enhanced by manipulating the parameters of the sensor’s structure.

In-Situ High Temperature and Large Strain Monitoring During a Copper Casting Process Based on Regenerated Fiber Bragg Grating Sensors

In-situ temperature and strain during copper casting process were monitored for the first time based on regenerated fiber Bragg grating (RFBG) sensors, with the maximum temperature up to 1100 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> C and the largest compressive strain approximately −14 000 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\varepsilon$</tex-math></inline-formula> . A generalized fifth-order polynomial calibration function was used to convert the measured Bragg wavelengths from RFBG into temperature values. With an RFBG temperature sensor array, the temperature distribution as a function of time was obtained, revealing how the temperature gradient changed during the casting process. In-situ strain was measured by an RFBG strain sensor in direct contact with copper, showing the strain progression inside the copper at different temperatures during the casting. The RFBG-based in-situ monitoring method provides a new way for measuring multiple parameters in high temperature casting, which can be used for improving the design of the casting mold and upgrading the casting quality. Furthermore, our investigation lays a good foundation for fiber-embedded smart metal structures as it shows that fiber sensors can be embedded in copper casts without special surface treatment.

Hollow Core Bragg Fiber Integrated With Regenerate Fiber Bragg Grating for Simultaneous High Temperature and gas Pressure Sensing

We have proposed and experimentally demonstrated a novel optical fiber sensor for simultaneous measurement of high temperature and gas pressure based on a Regenerated Fiber Bragg grating (RFBG) cascaded with a homemade hollow core Bragg fiber (HCBF). The HCBF functions as an anti-resonant reflecting waveguide and acts as a pressure and temperature sensing unit. The sample was fabricated and tested in our lab, exhibiting a high linear pressure and temperature sensitivity of −3.747 nm/MPa and 25.925 pm/°C in a large-dynamic range of 2 MPa and 600 °C, respectively. The RFBG, as a temperature monitor, was integrated to compensate the temperature cross-correspondence of the HCBF. By utilizing a 2 × 2 matrix obtained with the experimental results, simultaneous measurement of temperature and gas pressure can be achieved. Besides, compact size, low cost and high sensitivity make the sensor more competitive in harsh environment application.

Preisach Elasto-Plastic Model for Mild Steel Hysteretic Behavior-Experimental and Theoretical Considerations.

The Preisach model already successfully implemented for axial and bending cyclic loading is applied for modeling of the plateau problem for mild steel. It is shown that after the first cycle plateau disappears an extension of the existing Preisach model is needed. Heat dissipation and locked-in energy is calculated due to plastic deformation using the Preisach model. Theoretical results are verified by experiments performed on mild steel S275. The comparison of theoretical and experimental results is evident, showing the capability of the Presicah model in predicting behavior of structures under cyclic loading in the elastoplastic region. The purpose of this paper is to establish a theoretical background for embedded sensors like regenerated fiber Bragg gratings (RFBG) for measurement of strains and temperature in real structures. In addition, the present paper brings a theoretical base for application of nested split-ring resonator (NSRR) probes in measurements of plastic strain in real structures.

Regeneration of Fiber Bragg Gratings and Their Responses Under X-Rays

Fiber Bragg gratings (FBGs)-based temperature sensors present numerous advantages such as small packaging, fast acquisition rate, and accuracy for structural health-monitoring applications in nuclear environments. Among the various classes of FBGs, Type I FBGs are inscribed with a UV continuous or pulsed laser on a photosensitive fiber or with femtosecond laser even in nonphotosensitive fibers. These gratings, however, cannot survive at temperatures exceeding 400 °C. Regenerated FBG (RFBG) gratings, instead, are derivative from type I grating: when a high thermal treatment (> 650 °C) is applied after the inscription on a prehydrogenated fiber, a new grating (RFBG) appears with different thermal properties. It withstands temperatures as high as 1000 °C, opening the way to new application fields such as the temperature monitoring of the nuclear reactor cores. This work investigates the radiation response of RFBGs originated from type I seed FBGs inscribed with an argon laser (244 nm) in two different fibers (SMF-28e fiber or in a B/Ge co-doped fiber), loaded with either H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> or D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> before the inscription to enhance the fiber photosensitivity. Regeneration was achieved at 650 °C or 900 °C depending on the fiber type. After this, the RFBGs were irradiated under 40-keV X-rays at two different dose rates [1 Gy(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> )/s or 10 Gy/s] and at two temperatures of irradiation-25 °C or 250 °C. At room-temperature (RT) irradiation, a Bragg Wavelength Shift (BWS) of about 35 pm was observed for the SMF-28e and more than 130 pm for the B/Ge fiber at 400-kGy dose, and the combined temperature and radiation constraints reveal that the RFBGs are radiation-tolerant. Indeed, no BWS is observed anymore, at the highest temperature of irradiation.

Fiber Bragg Sensors Embedded in Cast Aluminum Parts: Axial Strain and Temperature Response.

In this study, the response of fiber Bragg gratings (FBGs) embedded in cast aluminum parts under thermal and mechanical load were investigated. Several types of FBGs in different types of fibers were used in order to verify general applicability. To monitor a temperature-induced strain, an embedded regenerated FBG (RFBG) in a cast part was placed in a climatic chamber and heated up to 120 within several cycles. The results show good agreement with a theoretical model, which consists of a shrink-fit model and temperature-dependent material parameters. Several cast parts with different types of FBGs were machined into tensile test specimens and tensile tests were executed. For the tensile tests, a cyclic procedure was chosen, which allowed us to distinguish between the elastic and plastic deformation of the specimen. An analytical model, which described the elastic part of the tensile test, was introduced and showed good agreement with the measurements. Embedded FBGs - integrated during the casting process - showed under all mechanical and thermal load conditions no hysteresis, a reproducible sensor response, and a high reliable operation, which is very important to create metallic smart structures and packaged fiber optic sensors for harsh environments.

Regenerated Fibre Bragg Gratings: A critical assessment of more than 20 years of investigations

Regenerated fibre Bragg gratings are formed when specially pre-treated seed gratings are heated up to several hundred degrees centigrade. During this process, the fibre Bragg grating (FBG) reflectivity vanishes and regrows, forming a regenerated grating, which can survive temperatures of more than 1000 °C. Right from the beginning, it was clear that the extraordinary temperature stability of regenerated FBGs will open new fields of applications for FBGs. Multiple investigations have been made to explore the mechanisms of the regeneration effect, often presenting contradictory explanations of the processes leading to their formation. To date, there is no comprehensive theory which can explain all the observed phenomena regarding the regeneration effect. However, a broad base of knowledge exists, which can be used for tailored manufacturing and application of regenerated FBG. In this article, we present a summary of the breadth of regenerated FBG research, including a critical overview and discussion of the various competing theories published to date. We conclude with an outlook on present and future applications of regenerated FBGs and identify further research necessary to unambiguously identify and explain the key processes occurring during regeneration.

Cascaded Fabry-Perot interferometer-regenerated fiber Bragg grating structure for temperature-strain measurement under extreme temperature conditions.

We demonstrated an optical fiber sensor based on a cascaded fiber Fabry-Perot interferometer (FPI)-regenerated fiber Bragg grating (RFBG) for simultaneous measurement of temperature and strain under high temperature environments. The FPI is manufactured from a ∼74 µm long hollow core silica tube (HCST) sandwiched between two single mode fibers (SMFs). The RFBG is inscribed in one of the SMF arms which is embedded inside an alundum tube, making it insensitive to the applied strain on the entire fiber sensor, just in case the temperature and strain recovery process are described using the strain-free RFBG instead of a characteristic due-parameter matrix. This feature is intended for thermal compensation for the FPI structure that is sensitive to both temperature and strain. In the characterization tests, the proposed device has exhibited a temperature sensitivity ∼ 18.01 pm/°C in the range of 100 °C - 1000 °C and excellent linear response to strain in the range of 300 °C - 1000 °C. The measured strain sensitivity is as high as ∼ 2.17 pm/µɛ for a detection range from 0 µɛ to 450 µɛ at 800 °C, which is ∼ 1.5 times that of a FPI-RFBG without the alundum tube.

Resurgent regenerated fiber Bragg gratings and thermal annealing techniques for ultra-high temperature sensing beyond 1400°C.

We report for the first time the resurgence of regenerated fiber Bragg gratings (RFBGs) useful for ultra-high temperature measurements exceeding 1400 °C. A detailed study of the dynamics associated with grating regeneration in six-hole microstructured optical fibers (SHMOFs) and single mode fibers (SMFs) was conducted. Rapid heating and rapid cooling techniques appeared to have a significant impact on the thermal sustainability of the RFBGs in both types of optical fibers reaching temperature regimes exceeding 1400 °C. The presence of air holes sheds new light in understanding the thermal response of RFBGs and the stresses associated with them, which governs the variation in the Bragg wavelength.