Fiber Optic Temperature Research Articles

Highly sensitive fiber-optic temperature sensor based on Lyot filter cascaded with Fabry-Perot interferometer

A highly sensitive fiber-optic temperature sensor based on a Lyot filter cascaded with a Fabry-Perot interferometer (FPI) is proposed and demonstrated in this study. The FPI consists of a segment of hollow-core photonic crystal fiber (HCPCF) spliced between two single-mode fibers (SMFs), which is insensitive to temperature and strain and is an ideal reference interferometer. The Lyot filter is composed of two polarizers with an uiltra-fine polarization maintaining fiber (UFPMF) of a certain length for temperature sensing. By adjusting the length of the UFPMF, the free spectral range (FSR) of the Lyot filter is close to FPI, resulting in an optical Vernier effect and improved sensitivity. Experimental results show that with the Vernier effect, the temperature sensitivity is enhanced from −1.145 nm/℃ to −59.31 nm/℃, and the sensitivity amplification factor is about 51.8. The proposed sensor with the Vernier effect has good performance in temperature sensing and is suitable for applications fields where high sensitivity temperature measurement is required.

Measuring height of poured concrete via fiber-optic temperature sensing

Currently, there is no automatic method available to measure the height of poured concrete during concreting of underground diaphragm walls (UDWs), and the conventional manual method interrupts the operation. To address this issue, a distributed fiber-optic sensing-based method for measuring the height of poured concrete during the concreting of UDWs was developed and successfully applied to the real-time measurement of the height of poured concrete. The proposed method was verified by performing a scale mode test, wherein an acrylic circular tube was used to simulate a UDW steel cage and a temperature-sensing fiber-optic cable was laid spirally to increase the spatial resolution of measurement to 0.1 m. The temperature distribution in the tube and real-time measurements of the height of the poured concrete during concreting were obtained. The proposed method was applied to the construction of a real UDW. A total of 3360 data points representing the spatiotemporal temperature distribution in the UDW steel cage were obtained. The results indicated that the temperature of the mud in the steel cage was approximately 25 °C–26 °C when no concreting operation was performed and that the temperature of the concrete layer increased to approximately 28 °C–31 °C during concreting. The height of the poured concrete was the boundary representing the transition from the temperature of the mud to that of the concrete, and the measurement precision reached ±0.5 m. The measurement results obtained via the proposed method were consistent with those obtained manually using a plumb bob, thus confirming the effectiveness of the proposed method for high-precision, real-time measurement of the height of poured concrete during the concreting of UDWs.

High-sensitivity and fast-response fiber optic temperature sensor using an anti-resonant reflecting optical waveguide mechanism

Temperature sensing is essential for human health monitoring. High-sensitivity (>1 nm/°C) fiber sensors always require long interference paths and temperature-sensitive materials, leading to a long sensor and thus slow response (6–14 s). To date, it is still challenging for a fiber optic temperature sensor to have an ultrafast (∼ms) response simultaneously with high sensitivity. Here, a side-polished single-mode/hollow/single-mode fiber (SP-SHSF) structure is proposed to meet the challenge by using the length-independent sensitivity of an anti-resonant reflecting optical waveguide mechanism. With a polydimethylsiloxane filled sub-nanoliter volume cavity in the SP-SHSF, the SP-SHSF exhibits a high temperature sensitivity of 4.223 nm/°C with a compact length of 1.6 mm, allowing an ultrafast response (16 ms) and fast recovery time (176 ms). The figure of merit (FOM), defined as the absolute ratio of sensitivity to response time, is proposed to assess the comprehensive performance of the sensor. The FOM of the proposed sensor reaches up to 263.94 (nm/°C)/s, which is more than two to three orders of magnitude higher than those of other temperature fiber optic sensors reported previously. Additionally, a three-month cycle test shows that the sensor is highly robust, with excellent reversibility and accuracy, allowing it to be incorporated with a wearable face mask for detecting temperature changes during human breathing. The high FOM and high stability of the proposed sensing fiber structure provide an excellent opportunity to develop both ultrafast and highly sensitive fiber optic sensors for wearable respiratory monitoring and contactless in vitro detection.

Long-range surface plasmon resonance-based hollow fiber temperature sensor with ultrahigh sensitivity and tunable detection range.

A dielectric/Ag-coated hollow fiber (HF) temperature sensor based on long-range surface plasmon resonance (LRSPR) is proposed and experimentally demonstrated. The structural parameters, including the dielectric material and layer thicknesses, are optimized through comprehensive theoretical analysis to achieve the best performance. By filling it with a high refractive index (RI) thermosensitive liquid, the GK570/Ag-coated HF temperature sensor with optimal structural parameters is fabricated. Due to the high sensitivity of the LRSPR sensor and the optimized design, the fabricated sensor achieves a temperature sensitivity of 3.6∼20.5 nm/°C, which is almost the highest among the optical fiber temperature sensors based on surface plasmon resonance reported experimentally. Moreover, the detection range of the proposed sensor can be easily tuned up to 170°C by varying the RI of the filled thermosensitive liquid, and the sensor performance remains stable. Considering that most temperature sensors using polydimethylsiloxane have a fixed detection range, this is an outstanding advantage that could expand the application field of the optical fiber temperature sensor.

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.

High-sensitivity fiber optic temperature sensor based on CTFBG-FPI and Vernier effect.

A novel high-sensitivity temperature sensor based on a chirped thin-core fiber Bragg grating Fabry-Perot interferometer (CTFBG-FPI) and the Vernier effect is proposed and demonstrated. With femtosecond laser direct writing technology, two CTFBG-FPIs with different interferometric cavity lengths are inscribed inside a thin-core fiber to form a Vernier effect system. The two FPIs consist of two pairs of CTFBGs with a full width at half maximum (FWHM) of 66.5 nm staggered in parallel. The interferometric cavity lengths of the two FPIs were designed to be 2 mm and 1.98 mm as the reference arm and sensing arm of the sensor, respectively. The temperature sensitivity of this sensor was measured to be -1.084 nm/°C in a range of 40-90°C. This sensor is expected to play a crucial role in precision temperature measurement applications.

MONITORING PERFUSION-BASED CONVECTION IN CANCER TUMOR TISSUE UNDERGOING NANOPARTICLE HEATING BY ANALYZING TEMPERATURE RESPONSES TO TRANSIENT PULSED HEATING.

The dynamic nature of perfusion in living tissues, such as solid tumors during thermal therapy, produces challenging spatiotemporal thermal boundary conditions. Changes in perfusion can manifest as changes in convective heat transfer that influence temperature changes during cyclic heating. Herein, we propose a method to actively monitor changes in local convection (perfusion) in vivo by using a transient thermal pulsing analysis. Syngeneic 4T1 tumor cells were injected subcutaneously into BALB/c mice and followed by caliper measurements. When tumor volumes measured 150-400 mm3, mice were randomly divided into one of two groups to receive intratumor injections of one of two iron oxide nanoparticle formulations for pulsed heating with an alternating magnetic field (AMF). The nanoparticles differed in both heating characteristics and coating. Intratumor temperature near the injection site as well as rectal temperature were measured with an optic fiber temperature probe. Following heating, mice were euthanized and tumors harvested and prepared for histological evaluation of nanoparticle distribution. To ascertain the heat transfer coefficient from heating and cooling pulses, we fit a lumped capacitance, Box-Lucas model to the time-temperature data assuming fixed tumor geometry and constant experimental conditions. For the first particle set, the injected nanoparticles dispersed evenly throughout the tumor with minimal aggregation, and with minimal change in convection. On the other hand, heating with the second particle generated a measurable decline in convective performance and histology analysis showed substantial aggregation near the injection site. We consider it likely that though the second nanoparticle type produced less heating per unit mass, its tendency to aggregate led to more intense local heating and tissue damage. Further analysis and experimentation is warranted to establish quantitative correlations between measured temperature changes, perfusion, and tissue damage responses. Implementing this type of analysis may stimulate development of robust and adaptive temperature controllers for medical device applications.

Fiber optic sensor for transformer temperature detection

AbstractThe safe and stable operation of the transformer is affected by the internal temperature. Therefore, it is very necessary to accurately and effectively detect the temperature of the transformer winding. To realize the effective detection of the internal temperature of the transformer, a fiber optic temperature sensor based on the structure of multimode fiber (MMF)–no core fiber (NCF)–multimode fiber (MMF) (i.e., MMF–NCF–MMF) is studied. The temperature sensitivity reaches 0.0313 nm/°C. Moreover, to enhance the accuracy of the test, the sensitive material polydimethylsiloxane (PDMS) is coated on the sensing area (the position of the NCF), and its temperature sensitivity reaches 0.123 nm/°C. By comparison, it was found that the temperature detection sensitivity increased by 3.93 times after coating PDMS. The sensor has the characteristics of simple structure, high sensitivity, and high integration, which provides research and application directions for temperature detection in the field of transformer health detection.

A probabilistic assessment of surface water-groundwater exchange flux at a PCE contaminated site using groundwater modelling

Polluted groundwater discharge at a chlorinated solvent contaminated site in Hagfors, Sweden, is affecting a nearby stream flowing through a sparsely populated area. Because of difficulties related to source zone remediation, decision makers recently changed the short-term site management objective to mitigating discharge of polluted groundwater to the stream. To help formulating targeted remediation strategies pertaining to the new objective, we developed a groundwater numerical decision-support model. To facilitate reproducibility, the modelling workflow was scripted. The model was designed to quantify and reduce the uncertainty of surface water-groundwater (SW-GW) exchange fluxes for the studied period (2016–2020) through the use of history-matching. In addition to classical observations, thermal anomalies detected in fiber optic distributed temperature sensing (FO-DTS) measurements were used to inform the model of groundwater discharge. After assessing SW-GW exchange fluxes, we used measurements of surface water chemistry to provide a probabilistic estimation of mass influx and spatio-temporal distributions of contaminated groundwater discharge. Results show 1) SW-GW exchange fluxes are likely to be significantly larger than previously estimated, and 2) prior estimations of mass influx are located near the center of the posterior probability distribution. Based on this, we recommend decision makers to focus remediation action on specific segments of the stream.

A real time monitoring method for well-kick and lost circulation based on distributed fiber optic temperature measurement

In the process of oil drilling, if the formation pressure does not match the pressure in the borehole, it may cause well-kick or lost circulation. The existing methods of mud level monitoring and micro-flow monitoring have some shortcomings. In these methods, information feedback is untimely and inaccurate. Well-kick or lost circulation position cannot be located in time. In order to monitor the well-kick and lost circulation situation more accurately and find the location of well-kick or lost circulation in time, the borehole temperature distribution models under well-kick and lost circulation conditions were established and solved. The distribution law of wellbore temperature when well-kick and lost circulation occur during drilling was analyzed. Based on the above research results, this work proposes a method for real-time monitoring of well-kick and lost circulation based on distributed optical fiber temperature measurement. This method compares and analyzes the real-time measured temperature with the theoretically calculated temperature, promptly determines whether well-kick or lost circulation has occurred, identifies the location of well-kick or lost circulation, and issues an alarm. This method effectively solves the problems of low monitoring accuracy and information delay, shortens the time required to search for the locations of well-kick or lost circulation, and has broad application prospects.

All-fiber temperature and humidity sensor based on photopolymer and polydimethylsiloxane with low-crosstalk and high-sensitivity

The ability to monitor temperature and humidity simultaneously is critical for biomedical and industrial production. We demonstrate a novel temperature and humidity optical fiber sensor based on a photopolymer and polydimethylsiloxane for dealing with temperature cross-sensitivity problem. The low-frequency signal and the high-frequency signal in the interference spectrum were obtained by filtering. Due to structural and material properties, the low-frequency signal is only temperature dependent, while the high-frequency signal is affected by temperature and humidity. Temperature and humidity signals rely on different interferometric mechanisms for measurement to achieve negligible crosstalk. The sensor exhibits a high temperature sensitivity of 1.88 nm/°C by measuring the shift of an interference dip wavelength in the low-frequency signal. The humidity sensitivity is up to 0.282 nm/%RH due to the excellent water absorption capacity of the photopolymer. High accuracy, good repeatability, and stability enable the sensor to measure ambient temperature and humidity. We believe the sensor has excellent potential in environmental monitoring, biomedicine, and human–machine interface.

Distributed Temperature Sensor Calibration Algorithm

The paper considers various calibrating algorithm approaches of a distributed fiber-optic temperature sensor in order to modify the two-point calibration algorithm and improve the accuracy of converting a reflectogram into a thermogram. As a modification, it is proposed to use an additional feature of the original signal based on its first derivative. A scheme for constructing a generalized heuristic calibration formula that takes into account multiple features of the reflectogram is proposed. The accuracy of the two-point and modified (three-point) algorithms was evaluated on thermostated regions of the optical line. It is shown that the modified calibration algorithm makes it possible to achieve greater accuracy compared to the two-point one. So, for the region thermostated at –50 °C, the standard deviation for the modified algorithm is 0.08 °C, for the two-point algorithm 0.26 °C.

Magnetic Field and Temperature Two-Parameter Sensor Based on Mach–Zehnder Interferometer and Faraday Rotation Effect

A novel combination of structural design and sensing theory is proposed for an optical fiber magnetic field and temperature two-parameter sensor based on the Mach-Zehnder interferometer (MZI) and Faraday rotation effect. The sensor structure includes a polydimethylsiloxane (PDMS) microcavity and a small section of offset spliced fiber. Numerical analysis shows that the sensor exhibits high temperature sensitivity, which has been experimentally verified. The sensor has a temperature sensitivity of -846 pm/°C in the 20 to 35°C and a magnetic field intensity sensitivity of 190 pm/Gs in the range of 0 to 150 Gs. The temperature and magnetic field intensity responses of the sensor are linear and have been optimized for sensitivity demodulation. The sensor is well-suited for standardized monitoring of temperature and magnetic field in applications requiring accurate measurements.

Breakthrough the physical barrier on spatial resolution in Raman distributed fiber sensing using chaotic correlation demodulation

Raman distributed optical fiber temperature sensors possess the unique capability of measuring spatial environmental temperatures, which can be of great interest in several fields of application. The key physics barrier to spatial resolution for most optical time-domain reflection (OTDR) systems is the positioning principle of pulse-time-flight. It obtains the spatial resolution of the existing Raman distributed optical fiber temperature sensor, with the kilometer-level sensing distance being limited to the meter-level. Here, we propose a chaotic laser Raman distributed optical fiber temperature sensing scheme that replaces the traditional OTDR positioning principle used for more than 40 years with the chaotic correlation positioning principle. The proposed scheme possesses the characteristics of the chaos Raman scattering light excited by the chaos signal along the sensing fiber. A novel measurement mechanism based on chaotic time-domain differential reconstruction and chaotic correlation demodulation is developed, and a relationship between the temperature variation information and the chaotic correlation peak is experimentally demonstrated. Importantly, the proposed optics mechanism scheme overcomes the physics limitation of the effect of a wide pulse width on sensing spatial resolution; its spatial resolution is optimized from 50 to 0.3 m under the modulation of a 500 ns pulse width. This scheme provides a new concept for chaos optics and fiber sensing research.

Highly sensitive temperature and strain sensor based on polarization mode interferometer with Vernier effect

A new cascaded optical fiber temperature and strain sensor based on polarization mode interferometer (PMI) and Fabry-Perot interferometer (FPI) is proposed and demonstrated experimentally. The PMI made by fusion splicing a segment of polarization maintaining fiber (PMF) after a polarizer is used as a sensing interferometer. Integrating a FPI as a reference interferometer cascaded after the PMI to generate the Vernier effect and improve sensitivity. The FPI is made from a small segment of hollow core photonic crystal fiber (HCPCF), which is fusion spliced to PMF and multi-mode fiber (MMF) at each end to form two reflective surfaces of FPI. Based on numerical simulations, a suitable free spectral range (FSR) of the sensor is produced. Experimental results show that the temperature sensitivity can be improved from −1.095 nm/℃ to −22.753 nm/℃ and the strain sensitivity can be improved from 2.17 pm/με to 38.89 pm/με by using the Vernier effect. The measured amplification factors are about 21 and 18,respectively. The high sensitivity of PMI and FPI cascaded sensors with the Vernier effect offers a new option for simultaneous measurements in the field of temperature and strain.

Experimental study of anchor bolt stress evaluation with hybrid optical fiber monitoring

The evaluation of mechanical properties of bolts is crucial for ensuring the long-term stability and continuous maintenance of underground engineering. Current research prioritizes the development of highly adaptable bolt mechanical behavior monitoring methods. By utilizing bolts with embedded sensors, the working status and surrounding environment can be monitored without affecting the original support structure, enabling effective disaster prevention monitoring. In this study, we introduce an innovative hybrid monitoring system based on strain gauges, Fiber Bragg Gratings (FBG), and Brillouin Optical Time Domain Analysis (BOTDA). A specialized dynamometer with temperature compensation is employed to collect axial force data under step loading conditions. We compare the sensing characteristics of various methods and derive the strain transmission coefficients of fiber sensors at different distances. The results demonstrate that the FBG and BOTDA accurately reflect the magnitude and change patterns of the monitored parameters, highlighting the potential application of this novel system for bolt monitoring. This work proposes a unique fiber optic temperature compensator and strain transmission coefficient analysis method to promote the evaluation quality of anchor bolt stress behaviors, which would provide realistic significance to enhance the accuracy and reliability of bolt mechanical property evaluation in underground engineering.

An optical fiber temperature sensor based on fluorescence intensity ratio in Er3+/Yb3+ co-doped Gd2O3 phosphors

A simple optical fiber temperature sensing system based on up-conversion luminescence of Er3+/Yb3+ co-doped Gd2O3 phosphors is proposed and demonstrated for the first time. The Gd2O3: 1 mol% Er3+, 1 mol% Yb3+ phosphors are synthesized by traditional sol-gel method. A sandwich structure fabricated by two fiber connectors, one of which is deposited the fluorescent material cured with ultraviolet adhesive on the surface, is used as the sensing probe. Temperature measurement is achieved using the two green emissions, generated by Gd2O3: 1 mol% Er3+, 1 mol% Yb3+ phosphors, based on the fluorescence intensity ratio (FIR) technique. When the Gd2O3: 1 mol% Er3+, 1 mol% Yb3+ phosphor is excited by the 980 nm laser with the output power of 73 mW, the intense green emissions are observed and change with temperature. The temperature sensing characteristics are experimentally studied in the temperature range from 253 to 413 K, with the maximum absolute sensitivity of 4.15% K−1 at 413 K, the maximum relative sensitivity of 2.28% K−1 at 253 K and the maximum relative error of 0.43%. The proposed optical fiber temperature sensor not only offers excellent reversibility and stability, but also significantly contributes to accurate and remote temperature measurement in harsh environments.

Experimental and Numerical Study of the Influence of Solar Radiation on the Surface Temperature Field of Low-Heat Concrete in a Pouring Block

With the influence of intense solar radiation heat and the greater temperature difference between day and night, surface concrete with a drastic temperature change can easily experience a great nonlinear temperature difference, which increases the risk of early-age concrete cracking. In this study, a distributed optical fiber temperature sensing (DTS) system is used to monitor the surface temperature gradient of concrete in real time, and a solar radiation heat monitoring test is also carried out based on the Baihetan project. Based on this, a solar radiation loading model and a finite element model of a typical pouring block considering solar radiation are established. Combined with the measured temperature data and different calculation conditions, the surface temperature changes of medium-heat and low-heat concrete experiencing solar radiation are analyzed, and the temperature control effect of surface concrete with different surface insulation measures is further analyzed. The results show that the temperature variation of medium-heat concrete at the same depth is more obvious than that of low-heat concrete. Additionally, the temperature variation of low-heat concrete is noticeable within 20 cm of the top surface. In addition, in an intense solar radiation environment, covering the concrete with a 4- or 5-centimeter-thick polyethylene coil can effectively control the surface temperature gradient and maximum daily amplitude of low-heat concrete, and surface concrete cured by running water has a significant temperature control effect. Therefore, it is suggested that 22–24 °C water temperatures be used for water curing during periods of intense solar radiation during the day and a 4-centimeter-thick polyethylene coil be used for coverage at night. These study results have been employed in the Baihetan project to optimize the temperature control scheme of the pouring blocks.

Method and Application of Spatial Positioning for Valid Temperature-measuring Optical Fibers in Concrete Dams

Spatial positioning of Valid Temperature-measuring Optical Fiber (VTOF) is of great significance for accurately grasping the real temperature distribution of a concrete dam. However, there is barely an effective way to accurately locate the measuring points of optical fiber in concrete dams. A comprehensive data symmetry analysis technique is presented for determining the optimal spatial positioning of valid temperature-measuring optical fibers (VTOFs) in concrete dams. This approach includes a mathematical model to confirm the center of symmetry in monitoring data, which is required to reconstruct the real spatial temperature distribution. This process can identify VTOF locations and verify the reliability of temperature predictions. Experimental validation was conducted using findings from research on a new mathematical model, site measurements from a concrete dam, and corresponding data from distributed optical fibers. These results were successfully applied to the construction of the Baihetan dam; the concrete temperature from 405 pouring blocks in 5 typical dam monoliths was accurately monitored, and the temperature-measuring points of VTOFs were more than 24711. Meanwhile, the optical fiber temperature measurement and temperature field analysis system of Baihetan dam located in southwest China was developed based on accurately acquired temperature measurement data, which provides data basis and engineering application platform support for temperature evolution and temperature field analysis and precise temperature control management of pouring blocks.

Double Optical Fiber Temperature Compensation Method for Measurement of Interface Pressure Between Simulated Cable and Accessory at High Temperatures

Reliable interface pressure between cables and accessories is closely associated with the safe operation of a power cable system. Using an external optical fiber Bragg grating temperature-compensating curvature sensor, a new method for online measurement of the interface pressure between a cable and an accessory at high temperatures is proposed. On the basis of the sensing principle of a fiber Bragg grating, a mathematical pure bending model, and a model of a thick-walled cylinder, a fiber Bragg grating temperature-compensating curvature sensor is fabricated using Dow Corning Sylgard-184 silicone rubber. The interface pressures of 10 kV cable accessories with different expansion rates at different temperatures are measured. The results of the measurements show that the temperature-compensated deviation of wavelength measurements made by the fiber grating curvature sensor ranges from -2.59% to 5.50%. The interface pressure between the cable and the accessory gradually increases with an increase in temperature from 30 to 60 °C. The results are consistent with the results of measurements made using a traditional piezoelectric sensor. This method is expected to evaluate the long-term reliability of the interface pressure of cable accessories in actual complex-temperature environments.