High temperature sensing with fiber Bragg gratings in sapphire-derived all-glass optical fibers

Metallurgical Aspects of Ni-Coating and High Temperature Treatments for FBG Spectrum Regeneration

The structural integrity of mechanical components is assessed by FBG sensors in many industrial fields. The FBG sensor has a relevant application at very high or low temperatures. To avoid the variability of the reflected spectrum and the mechanical properties degradation of the FBG sensor, metal coatings have been used to guarantee the grating's integrity in extreme temperature environments. Particularly, at high temperatures, Ni could be a suitable selection as a coating to improve the features of FBG sensors. Furthermore, it was demonstrated that Ni coating and high-temperature treatments can recover a broken, seemingly unusable sensor. In this work, two main objectives were pursued: first, the determination of the best operative parameters to achieve the most compact, adherent, and homogeneous coating; second, the correlation between the obtained morphology and structure and the FBG spectrum modification, once Ni was deposited on the FBG sensor. The Ni coating was deposited from aqueous solutions. By performing heat treatments of the Ni-coated FBG sensor, it was investigated how the wavelength (WL) varied as a function of temperature and how that variation was caused by the structural or dimensional change of the Ni coating.

High Temperature Measurement with Low Cost, VCSEL-Based, Interrogation System Using Femtosecond Bragg Gratings

In this article, a cost-effective and fast interrogating system for wide temperature measurement with Fiber Bragg Gratings is presented. The system consists of a Vertical Cavity Surface Emitting Laser (VCSEL) with a High Contrast Grating (HCG)-based cavity that allows for the fast tuning of the output wavelength. The work focuses on methods of bypassing the limitations of the used VCSEL laser, especially its relatively narrow tuning range. Moreover, an error analysis is provided by means of the VCSEL temperature instability and its influence on the system performance. A simple proof of concept of the measurement system is shown, where two femtosecond Bragg gratings were used to measure temperature in the range of 25 to 800 °C. In addition, an exemplary simulation of a system with sapphire Bragg gratings is provided, where we propose multiplexation in the wavelength and reflectance domains. The presented concept can be further used to measure a wide range of temperatures with scanning frequencies up to hundreds of kHz.

Optical Fiber Sensors for High-Temperature Monitoring: A Review

High-temperature measurements above 1000 °C are critical in harsh environments such as aerospace, metallurgy, fossil fuel, and power production. Fiber-optic high-temperature sensors are gradually replacing traditional electronic sensors due to their small size, resistance to electromagnetic interference, remote detection, multiplexing, and distributed measurement advantages. This paper reviews the sensing principle, structural design, and temperature measurement performance of fiber-optic high-temperature sensors, as well as recent significant progress in the transition of sensing solutions from glass to crystal fiber. Finally, future prospects and challenges in developing fiber-optic high-temperature sensors are also discussed.

Disordered mullite grains in a sapphire-derived fiber for high-temperature sensing

In this study, a sapphire-derived fiber (SDF)-based Fabry-Pérot interferometer (FPI) is proposed and experimentally demonstrated as a high-temperature sensor using the arc discharge crystallization process, forming a region with disordered mullite grains. This shows that the disordered mullite grains are related to the gradual temperature distribution during the arc discharge process, which results in a larger refractive index (RI) modulation of the SDF near the fusing area, forming a reflection mirror. An FPI was obtained by combining the optical fiber end facet. Considering the high-temperature resistance of the fiber, the fabricated FPI was used for high-temperature sensing. This shows that the device can operate at temperatures of up to 1200 °C with a sensitivity of 15.47 pm/°C, demonstrating that the proposed devices have potential applications in high-temperature environments.

Sapphire-Derived Fiber Bragg Gratings for High Temperature Sensing

In this paper, a sapphire-derived fiber (SDF) with a core diameter of 10 μm and a cladding diameter of 125 μm is fabricated by the melt-in-tube method, and fiber Bragg gratings (FBGs) with reflectivity over 80% are prepared by the femtosecond laser point-by-point direct writing method. By analyzing the refractive index distribution and reflection spectral characteristics of the SDF, it can be seen that the SDF is a graded refractive index few-mode fiber. In order to study the element composition of the SDF core, the end-face element distribution of the SDF is analyzed, which indicates that element diffusion occurred between the core and the cladding materials. The temperature and stress of the SDF gratings are measured and the highest temperature is tested to 1000 °C. The temperature and strain sensitivities are 15.64 pm/°C and 1.33 pm/με, respectively, which are higher than the temperature sensitivity of the quartz single-mode fiber. As a kind of special fiber, the SDF expands the application range of sapphire fiber, and has important applications in the fields of high-temperature sensing and high-power lasers.

Compound Fabry-Pérot interferometer for simultaneous high-pressure and high-temperature measurement

We have proposed and experimentally demonstrated a sapphire-derived fiber (SDF) and silica capillary-based compound Fabry-Pérot interferometer (FPI) for high-pressure and high-temperature sensing. The SDF owns high alumina dopant concentration core, which can generate a mullite crystallization region during an arc discharge process. The crystallization region acts as a reflective interface to form one FPI in the SDF. The other FPI contains an air cavity constructed by the silica capillary and is used for high-pressure sensing. Both gas pressure within a range from 0 MPa to 4 MPa and temperature within a range from 20°C to 700°C are measured. Experimental results show that the wavelength shift of the FPI versus the applied pressure is linear at each tested temperature. The pressure sensitivity is measured to be 5.19 nm/MPa at a high temperature of 700°C, and the linear responses show excellent repeatability with linearity of 0.999. Meanwhile, the proposed FPI can stably function at a high temperature of 700°C with a temperature sensitivity of 0.013 nm/°C. The proposed FPI sensor provides a promising candidate for simultaneous measurement of high pressure and high temperature in extreme conditions.

A Strain-Transfer Model of Surface-Bonded Sapphire-Derived Fiber Bragg Grating Sensors

An improved strain-transfer model was developed for surface-bonded sapphire-derived fiber Bragg grating sensors. In the model, the core and cladding of the fiber are separated into individual layers, unlike in conventional treatment that regards the fiber as a unitive structure. The separation is because large shear deformation occurs in the cladding when the core of the sapphire-derived fiber is heavily doped with alumina, a material with a high Young’s modulus. Thus, the model was established to have four layers, namely, a core, a cladding, an adhesive, and a host material. A three-layer model could also be obtained from the regressed four-layer model when the core’s radius increased to that of the cladding, which treated the fiber as if it were still homogeneous material. The accuracy of both the four- and three-layer models was verified using a finite-element model and a tensile-strain experiment. Experiment results indicated that a larger core diameter and a higher alumina content resulted in a lower average strain-transfer rate. Error percentages were less than 1.8% when the four- and three-layer models were used to predict the transfer rates of sensors with high and low alumina content, respectively.

Enhancement of spectral response of Bragg gratings written in nanostructured and multi-stepped optical fibers with radially shaped GeO2 concentration

We present experimental results on fiber Bragg gratings inscription in nanostructured graded-index (nGRIN) and multi-step index (MSIN) optical fibers, both having non-uniform radial distribution of GeO₂ dopant in the fiber cores. In particular, the positive role of radial shaping the GeO₂ distribution in the fiber core on grating reflection efficiency is reported. We postulate that an appropriate spatial distribution of the germanium concentration that matches the fundamental mode profile improves grating spectral response due to more efficient grating-mode interaction, as compared with uniformly doped step-index optical fibers with the same overall doping level. Moreover, we show that radially shaped fibers exhibit moderately higher temperature responses than their step-index counterparts.

High temperature strain sensing with alumina ceramic derived fiber based Fabry-Perot interferometer

A Fabry-Perot interferometer (FPI) based on an alumina ceramic derived fiber (CDF) is proposed and demonstrated for high temperature strain sensing. The strain sensor is constructed by splicing a piece of CDF between two standard single-mode fibers (SMFs). The strain properties of the sensor are investigated from room temperature to 1200 °C. Experimental results show that the wavelength shift of the CDF-FPI presents a linear relationship with the tensile strain at both room temperature and high temperature with up to 1000 °C. The strain sensitivity is calculated to be 1.5 pm/µɛ at 900 °C, and the linear response is repeatable within 0-3000 µɛ. Moreover, for each applied force at 1000 °C, the wavelength shift versus time shows the stability of the developed CDF-FPI sensor within 0-2000 µɛ. The obtained results show that such a CDF-FPI has potential application in various engineering areas, such as aeronautics, metallurgy, and gas boiler.

Sapphire derived fiber based Fabry-Perot interferometer with an etched micro air cavity for strain measurement at high temperatures

A sapphire derived fiber (SDF) based Fabry-Perot interferometer (FPI) with an etched micro air cavity for strain measurement at high temperatures is proposed. The FPI is formed by splicing a section of SDF between an etched single mode fiber (ESMF) and a capillary. The SDF's core containing 51.3mol.% aluminum provides the intrinsic Fabry-Perot interferometer cavity with an enhanced fringe contrast through the narrow etched air cavity reflector. Because the different Poisson effects of the cladding and the core have different deformations under axial stress, the transverse strain imposed from the cladding to the core was introduced to the additive model. The strain sensitivity of the FPI was theoretically analyzed and experimentally demonstrated at room temperature. A thermal annealing process was performed to study the stability in high temperatures and to release the residual stress during the sensor's fabrication. The strain calibration was carried out subsequently from 20℃ to 1000℃. Benefiting from the doping in the core and diffusion in the cladding of the high temperature resistant material Al₂O₃, the proposed sensor was proved to operate well in 950℃ and was also characteristized by a sensitivity of 1.19 pm/µɛ and 1.06 pm/µɛ in the process of loading and unloading strain separately.

Fiber Bragg grating characterization using factorial design

The design of the experiment is a scientific approach that provides the maximum amount of information with the minimum number of experiments. It is applicable in scientific and industrial research. We represent a three-variable two-level factorial design to assess fiber Bragg grating properties under simultaneous temperature, humidity, and strain stimuli. Three uniform gratings were inscribed in a single-mode standard optical fiber using the interferometric technique. Two gratings were recoated, one with acrylate and another with a layer of polyimide, while the third grating remained bare. With only eight measuring points, the sensitivities of temperature, strain, and humidity were computed. Moreover, with this technique, the cross-sensitivities between temperature and strain, temperature and humidity, humidity and strain, and between all three factors could also be quantified. We have proven that the results of this design are comparable with those of the classic method. For all gratings, the temperature and strain sensitivities were obtained in the order of 10 pm/°C and 1.1 pm/με, respectively. The humidity sensitivity of the polyimide recoated grating was estimated to be in the order of 4.47 pm/%RH at room temperature.

Crystallization-induced refractive index modulation on sapphire-derived fiber for ultrahigh temperature sensing

We have demonstrated crystallization-induced refractive index (RI) modulation on sapphire-derived fiber (SDF) showing superheat resistance and developed the SDF based Fabry-Perot interferometers (FPIs) for ultrahigh temperature sensing. The SDF is a special fiber with high concentration of alumina to silica in the fiber core region. Reheating and cooling the SDF by arc discharge generates mullite particles in the core region, which achieves RI modulation up to ~0.015. Such crystallized region in the SDF is explored as mirrors for FPI, showing a good linear response to temperature with sensitivity of ~13.2 pm/°C. Benefiting from superheat resistance of the crystallized SDF being mirrors, the developed SDF-FPI sensor is capable to withstand high temperature up to 1600°C, which is the highest working temperature for amorphous fiber. Moreover, the SDF-FPI sensor exhibits 6-hour stability at 1200°C. The crystallized SDF-FPIs with compactness, wide temperature working range, high sensitivity, and robustness show great potential application in harsh environment such as turbine engines, power plants, petrochemical, gas industry, etc.

In-fiber integrated quasi-distributed high temperature sensor array

An in-fiber integrated quasi-distributed high temperature sensor array is proposed and demonstrated. The sensor array consists of some weakly reflective joint surfaces which are welded by single mode fiber (SMF) and double-clad fiber (DCF). The characteristics of the reflected signal of the sensor array are analyzed, and the relationship between the signal intensity and the number of sensors is simulated for evaluating sensor multiplex capacity. Due to its all-silica structure, the sensor array could test temperature up to 1000°C for a long time. This sensor array is flexible and easy to be fabricated only by splicing without any connector, which will be beneficial to space constrained quasi-distributed high temperature sensing applications.

Fiber Bragg Grating Sensors for the Oil Industry

With the oil and gas industry growing rapidly, increasing the yield and profit require advances in technology for cost-effective production in key areas of reservoir exploration and in oil-well production-management. In this paper we review our group’s research into fiber Bragg gratings (FBGs) and their applications in the oil industry, especially in the well-logging field. FBG sensors used for seismic exploration in the oil and gas industry need to be capable of measuring multiple physical parameters such as temperature, pressure, and acoustic waves in a hostile environment. This application requires that the FBG sensors display high sensitivity over the broad vibration frequency range of 5 Hz to 2.5 kHz, which contains the important geological information. We report the incorporation of mechanical transducers in the FBG sensors to enable enhance the sensors’ amplitude and frequency response. Whenever the FBG sensors are working within a well, they must withstand high temperatures and high pressures, up to 175 °C and 40 Mpa or more. We use femtosecond laser side-illumination to ensure that the FBGs themselves have the high temperature resistance up to 1100 °C. Using FBG sensors combined with suitable metal transducers, we have experimentally realized high- temperature and pressure measurements up to 400 °C and 100 Mpa. We introduce a novel technology of ultrasonic imaging of seismic physical models using FBG sensors, which is superior to conventional seismic exploration methods. Compared with piezoelectric transducers, FBG ultrasonic sensors demonstrate superior sensitivity, more compact structure, improved spatial resolution, high stability and immunity to electromagnetic interference (EMI). In the last section, we present a case study of a well-logging field to demonstrate the utility of FBG sensors in the oil and gas industry.

Dynamic Strain Measurements on Automotive and Aeronautic Composite Components by Means of Embedded Fiber Bragg Grating Sensors

The measurement of the internal deformations occurring in real-life composite components is a very challenging task, especially for those components that are rather difficult to access. Optical fiber sensors can overcome such a problem, since they can be embedded in the composite materials and serve as in situ sensors. In this article, embedded optical fiber Bragg grating (FBG) sensors are used to analyze the vibration characteristics of two real-life composite components. The first component is a carbon fiber-reinforced polymer automotive control arm; the second is a glass fiber-reinforced polymer aeronautic hinge arm. The modal parameters of both components were estimated by processing the FBG signals with two interrogation techniques: the maximum detection and fast phase correlation algorithms were employed for the demodulation of the FBG signals; the Peak-Picking and PolyMax techniques were instead used for the parameter estimation. To validate the FBG outcomes, reference measurements were performed by means of a laser Doppler vibrometer. Sensors 2015, 15 27175 The analysis of the results showed that the FBG sensing capabilities were enhanced when the recently-introduced fast phase correlation algorithm was combined with the state-of-the-art PolyMax estimator curve fitting method. In this case, the FBGs provided the most accurate results, i.e. it was possible to fully characterize the vibration behavior of both composite components. When using more traditional interrogation algorithms (maximum detection) and modal parameter estimation techniques (Peak-Picking), some of the modes were not successfully identified.