Application of Sapphire-Fiber-Bragg-Grating-Based Multi-Point Temperature Sensor in Boilers at a Commercial Power Plant

In-Bulk Temperature Profile Mapping Using Fiber Bragg Grating in Fluids

The capabilities of Fiber Bragg Grating (FBG) sensors to measure temperature variations in the bulk of liquid flows were considered. In the first step of our research project, reported in this paper, we investigated to what extent the use of thin glass fibers without encapsulation, which only minimally disturb a flow, can fulfill the requirements for robustness and measurement accuracy. Experimental tests were performed in a benchmark setup containing 24 FBG measuring positions, which were instrumented in parallel with thermocouples for validation. We suggest a special assembly procedure in which the fiber is placed under a defined tension to improve its stiffness and immobility for certain flow conditions. This approach uses a single FBG sensor as a reference that measures the strain effect in real time, allowing accurate relative temperature measurements to be made at the other FBG sensor points, taking into account an appropriate correction term. Absolute temperature readings can be obtained by installing another well-calibrated, strain-independent thermometer on the reference FBG. We demonstrated this method in two test cases: (i) a temperature gradient with stable density stratification in the liquid metal GaInSn and (ii) the heating of a water column using a local heat source. In these measurements, we succeeded in recording both spatial and temporal changes in the linear temperature distribution along the fiber. We present the corresponding results from the tests and, against this background, we discuss the capabilities and limitations of this measurement technique with respect to the detection of temperature fields in liquid flows.

Performance evaluation of a time-division multiplexed fiber Bragg grating sensor based on heterodyne detection

Using a technique to observe reflection spectra, the signal-to-noise ratio can be improved for time-division multiplexed interrogation of three fiber Bragg gratings (FBGs) based on heterodyne detection methods. To calculate peak reflection wavelengths of the FBG reflections, absorption lines of ¹² C ₂ H ₂ are used as wavelength markers, and temperature dependence of the peak wavelength is measured for one FBG. Positioning the FBGs at a distance of 20 km from the control port demonstrates the applicability of this method to a long sensor network.

High-speed and high-resolution YAG fiber based distributed high temperature sensing system empowered by a 2D image restoration algorithm

High temperature monitoring is critical to the health and performance of vital pieces of infrastructure such as jet engine, fuel cells, coal gasifiers, and nuclear reactor core. However, it remains a big challenge to realize reliable distributed high temperature sensing system with high speed, high spatial and temperature resolution simultaneously. In this work, a Raman distributed high temperature sensing system with high temperature resolution and high spatial resolution was realized in a single-crystal YAG fiber. The sensing system demonstrated operation from room temperature up to 1400°C with a spatial resolution of 7 cm and response time of 1 millisecond in a 1m long YAG fiber. The average temperature sensitivity of the system is about 7.95 × 10^-4/°C. To the best of our knowledge, this is the best spatial resolution and response time reported in literature. In this system, a 2D image restoration was used to boost the signal to noise ratio of sensor. Empowered by the algorithm, the average temperature standard deviation along the sensing fiber of 7.89 °C was obtained based on a single frame data in 1 millisecond. A new record of temperature resolution of 0.62 °C was demonstrated in only 1 second frame data traces, which enables a fast response capacity.

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.

Point-by-point inscribed sapphire parallel fiber Bragg gratings in a fully multimode system for multiplexed high-temperature sensing

We study the point-by-point inscription of sapphire parallel fiber Bragg gratings (sapphire pFBGs) in a fully multimode system. A parallel FBG is shown to be critical in enabling detectable and reliable high-order grating signals. The impacts of modal volume, spatial coherence, and grating location on reflectivity are examined. Three cascaded seventh-order pFBGs are fabricated in one sapphire fiber for wavelength multiplexed temperature sensing. Using a low-cost, fully multimode 850-nm interrogator, reliable measurement up to 1500°C is demonstrated.

Femtosecond Laser-Based Micromachining of Rotational-Symmetric Sapphire Workpieces

Sapphire is a robust and wear-resistant material. However, efficient and high-quality micromachining is still a challenge. This contribution demonstrates and discusses two novels, previously unreported approaches for femtosecond laser-based micromachining of rotational-symmetric sapphire workpieces, whereas both methods are in principal hybrids of laser scanning and laser turning or laser lathe. The first process, a combination of a sequential linear hatch pattern in parallel to the workpiece's main axis with a defined incremental workpiece rotation, enables the fabrication of sapphire fibers with diameters of 50 μm over a length of 4.5 mm. Furthermore, sapphire specimens with a diameter of 25 μm over a length of 2 mm can be fabricated whereas an arithmetical mean height, i.e., S_a parameter, of 281 nm is achieved. The second process combines a constant workpiece feed and orthogonal scanning with incremental workpiece rotation. With this approach, workpiece length limitations of the first process are overcome and sapphire fibers with an average diameter of 90 µm over a length of 20 cm are manufactured. Again, the sapphire specimen exhibits a comparable surface roughness with an average S_a value of 249 nm over 20 cm. Based on the obtained results, the proposed manufacturing method paves an innovative and flexible, all laser-based way towards the fabrication or microstructuring of sapphire optical devices, and thus, a promising alternative to chemical processes.

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.

Sapphire Photonic Crystal Waveguides with Integrated Bragg Grating Structure

This contribution demonstrates photonic crystal waveguides generated within bulk planar sapphire substrates. A femtosecond laser is used to modify the refractive index in a hexagonal pattern around the pristine waveguide core. Near-field measurements reveal single-mode behavior at a wavelength of 1550 nm and the possibility to adapt the mode-field diameter. Based on far-field examinations, the effective refractive index contrast between the pristine waveguide core and depressed cladding is estimated to 3·10−4. Additionally, Bragg gratings are generated within the waveguide core. Due to the inherent birefringence of Al2O3, the gratings exhibit two distinct wavelengths of main reflection. Each reflection peak exhibits a narrow spectral full width at a half maximum of 130 pm and can be selectively addressed by exciting the birefringent waveguide with appropriately polarized light. Furthermore, a waveguide attenuation of 1 dB cm−1 is determined.

Stabilized Ultra-High-Temperature Sensors Based on Inert Gas-Sealed Sapphire Fiber Bragg Gratings

In situ measurement of high temperature is critical in aerospace, petrochemical, metallurgical, and power industries. The single-crystal sapphire fiber is a promising material for high-temperature measurement owing to its high melting point of ∼2045 °C. Sapphire fiber Bragg gratings (SFBGs), which could be inscribed in sapphire fibers with a femtosecond laser, are widely used as high-temperature sensors. However, conventional SFBGs typically exhibit a significant deterioration in their spectra after long-term operation at ultra-high temperatures, resulting from the formation of some unwanted microstructural features, that is, lossy spots and micro-etched lines, on the surface of the sapphire fiber. Here, we report for the first time, to the best of our knowledge, a thermally stabilized ultra-high-temperature sensor based on an SFBG created by femtosecond laser inscription, inert gas-sealed packaging, and gradient temperature-elevated annealing. The results indicate that the lossy spots are essentially aluminum hydroxide induced by high-temperature oxidation, and the inert gas-sealed packaging process can effectively insulate the sapphire fiber from the ambient air. Moreover, the formation of micro-etched lines was suppressed successfully by using the gradient temperature-elevated annealing process. As a result, the surface topography of the SFBG after operating at high temperatures was improved obviously. The long-term thermal stability of such an SFBG was greatly enhanced, showing a stable operation at 1600 °C for up to 20 h. In addition, it could withstand an even higher temperature of 1800 °C with a sensitivity of 41.2 pm/°C. The aforementioned results make it promising for high-temperature sensing in chemical, aviation, smelting, and power industries.

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.

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 ∘C 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.

Strain Transfer Characteristics of Multi-Layer Optical Fiber Sensors with Temperature-Dependent Properties at Low Temperature

Optical fiber sensors have been potentially expected to apply in the extreme environment for their advantages of measurement in a large temperature range. The packaging measure which makes the strain sensing fiber survive in these harsh conditions will commonly introduce inevitable strain transfer errors. In this paper, the strain transfer characteristics of a multi-layer optical fiber sensing structure working at cryogenic environment with temperature gradients have been investigated theoretically. A generalized three-layer shear lag model incorporating with temperature-dependent properties of layers was developed. The strain transfer relationship between the optical fiber core and the matrix has been derived in form of a second-order ordinary differential equation (ODE) with variable coefficients, where the Young’s modulus and the coefficients of thermal expansion (CTE) are considered as functions of temperature. The strain transfer characteristics of the optical sensing structure were captured by solving the ODE boundary problems for cryogenic temperature loads. Case studies of the cooling process from room temperature to some certain low temperatures and gradient temperature loads for different low-temperature zones were addressed. The results showed that different temperature load configurations cause different strain transfer error features which can be described by the proposed model. The protective layer always plays a main role, and the optimization geometrical parameters should be carefully designed. To verify the theoretical predictions, an experiment study on the thermal strain measurement of an aluminum bar with optical fiber sensors was conducted. LUNA ODiSI 6100 integrator was used to measure the Rayleigh backscattering spectra shift of the optical fiber at a uniform temperature and a gradient temperature under liquid nitrogen temperature zone, and a reasonable agreement with the theory was presented.