Strain-temperature Cross-sensitivity Research Articles

Unraveling quench dynamics and real-time continuous detection in HTS tapes through distributed fiber optic sensing

High-temperature superconducting (HTS) tapes, coils, and magnets often experiences intricate quench instabilities and failures during high current-carrying operations, posing challenges to their practical applications. This study addresses the need for a measurement approach capable of monitoring multi-field signals in superconducting structures within cryogenic and extreme electromagnetic environments. We explore the application of distributed fiber optic sensing (DFOS) technology, specifically employing the optical frequency domain reflectometry scheme, which offers distinct advantages over traditional point-type electrical testing methods, particularly for superconducting materials and magnets. In this experimental study, we continuously track the quench evolution process in superconducting tapes using both bonded and stress-free fibers for real-time monitoring. A comprehensive analysis of the acquired temperature and thermoelastic strain profiles provides essential insights into the dynamic behavior of quench events. The findings demonstrate the effectiveness of DFOS in identifying and characterizing the onset and propagation of quenches. By arranging bonded and stress-free fibers in parallel on the HTS tape’s surface, we successfully decouple the effects of strain-temperature cross-sensitivity, enabling the extraction of temperature and train profiles. The bonded fiber optic sensor demonstrates rapid sensitivity to the thermally quenched events. Temporal derivatives of voltage and thermal strain exhibit characteristic plateaus and slope changes during quenches, respectively. The voltage rate displays two plateaus corresponding to superconducting-to-normal transitions, while strain rates exhibit potential as criteria for identifying quench events in HTS materials. Moreover, DFOS outperforms traditional terminal average voltage measurement, capturing quench evolution tails from the initial point of quench until the complete transition of the segment into the normal state. This experiment provides a solid foundation for further exploration of the underlying quench mechanism.

Investigation of Strain-Temperature Cross-Sensitivity of FBG Strain Sensors Embedded Onto Different Substrates

The strain-temperature cross-sensitivity problem easily occurs in the engineering strain monitoring of the self-sensing embedded with fiber Bragg grating (FBG) sensors. In this work, a theoretical investigation of the strain-temperature cross-sensitivity has been performed using the temperature reference grating method. To experimentally observe and theoretically verify the problem, the substrate materials, the preloading technique, and the FBG initial central wavelength were taken as main parameters. And a series of sensitivity coefficients calibration tests and temperature compensation tests have been designed and carried out. It was found that when the FBG sensors were embedded on different substrates, their coefficients of the temperature sensitivity were significantly changed. Besides, the larger the coefficients of thermal expansion (CTE) of substrates were, the higher the temperature sensitivity coefficients would be. On the other hand, the effect of the preloading technique and FBG initial wavelength was negligible on both the strain monitoring and temperature compensation. In the case of similar substrates, we did not observe any difference between temperature sensitivity coefficients of the temperature compensation FBG with one free end or two free ends. The curves of the force along with temperature were almost overlapped with minor differences (less than 1%) gained by FBG sensors and pressure sensors, which verified the accuracy of the temperature compensation method. We suggest that this work can provide efficient solutions to the strain-temperature cross-sensitivity for engineering strain monitoring with the self-sensing element embedded with FBG sensors.

A Fabry–Pérot interferometer strain sensor composed of a rounded rectangular air cavity with a thin wall for high sensitivity and interference contrast

A Fabry–Pérot interferometer (FPI) strain sensor with high sensitivity and interference contrast is designed and fabricated by a simple, cost effective, and safe process which can be scaled up for commercial production. The excellent strain sensitivity of 8 pm/μɛ and interference contrast of 25 dB stem from the special structure with the unique air cavity, small wall thickness, and excellent reflection surface of the air cavity. The strain-temperature cross-sensitivity of the sensor is only 0.59 μɛ/°C in the temperature range of 20–80°C. The sensor has large potential in applications such as strain sensing.

Optical fiber flowmeter based on a single mode-multimode-single mode structure

Single mode-Multimode-Single mode (SMS) sensors have been attracted a relevant attention because of their simple manufacturing, their capability of sensing different quantities, and their enhanced sensitivity compared to the most common fiber optic sensor represented by Fiber Bragg Gratings (FBGs). Moreover, SMS sensors exhibit blue-shift sensitivity to strain, opposite to FBGs, making them suitable in applications where strain-temperature cross-sensitivity may be an issue. SMS sensors are made by splicing a short multimode, preferably a two mode or quasi two-mode, optical fiber jumper between single mode pigtails. The interference of the modes propagating at different phase velocities produces a spectral pattern that shifts with temperature, strain or any perturbation of the phase difference among the modes. In this paper we review the main features of SMSs as temperature sensors and we present a potential biomedical application in an all-fiber flowmeter based on the hot-wire principle: a fiber-coupled laser source at 980 nm is used as a controllable heating source of the SMS sensor that, when immersed in fluid flow, converts the temperature variation, caused by the heat removal, into a wavelength shift of the transmitted spectrum. Thermal characterization and proof-of-concept experiments show the feasibility and functionality of the sensor and provide an outlook on possible developments and potential applications.

Sensitivity-Enhanced Extrinsic Fabry-Perot Interferometric Fiber-Optic Microcavity Strain Sensor.

This study presents an extrinsic Fabry–Perot interferometric (EFPI) fiber-optic strain sensor with a very short cavity. The sensor consists of two vertically cut standard single-mode fibers (SMFs) and a glass capillary with a length of several centimeters. The two SMFs penetrate into the glass capillary and are fixed at its two ends with the use of ultraviolet (UV) curable adhesives. Based on the use of the lengthy glass capillary sensitive element, the strain sensitivity can be greatly enhanced. Experiments showed that the microcavity EPFI strain sensor with initial cavity lengths of 20 μm, 30 μm, and 40 μm, and a capillary length of 40 mm, can yield respective cavity length–strain sensitivities of 15.928 nm/με, 25.281 nm/με, and 40.178 nm/με, while its linearity was very close to unity for strain measurements spanning a range in excess of 3500 με. Furthermore, the strain–temperature cross-sensitivity was extremely low.

Reducing Residual Strain in Fiber Bragg Grating Temperature Sensors Embedded in Carbon Fiber Reinforced Polymers

The temperature response of fiber Bragg gratings (FBGs) embedded in carbon fiber reinforce polymers (CFRPs) is investigated in this paper. To provide strain-free temperature measurements, two pieces of anti-sticking materials are placed at both sides of the embedded optical fiber and between carbon fiber prepregs; thus, providing slippery surfaces that minimize the strain transfer to the FBG sensor. In particular, the impact of different anti-stick materials on the temperature and residual bending strain response of the embedded FBGs is experimentally investigated. Results demonstrate that although some materials can allow for minimum residual strain being transferred to the FBG, their thermal conductivity does not always fulfill the requirements for reliable temperature sensing. It is found out that, among the tested materials, aluminum and copper foils can provide both reliable temperature response (with negligible delay and bias) and minimum residual strain. Using such anti-stick materials, the error induced by the residual strain on FBG temperature measurements is also experimentally evaluated by applying temperature and bending loads (strain) simultaneously to the CFRP packaging. While the study is here performed for FBG-based point sensors, most of the results and conclusions are also expected to be valid for applications of embedded distributed optical fiber sensors being affected by strain-temperature cross-sensitivity issues.

Method Research of Coal Mine Roof Strain Detection Based on the Fiber Bragg Grating Sensor with Thermodynamic Couple Numerical Condition

In order to solve the problem that the measure precision of coal roof rock strain amount detection using traditional electric sensor is insufficient, a kind of High Sensitive Surface-Pasted Fiber Bragg Grating Strain Sensor based on rock bolting is designed for substitution. Furthermore, a kind of Reference Grating Zero-Strain method based on sensitivity matrix coefficient is proposed for solving FBG strain-temperature cross-sensitivity problem and realizing the better separation relevant to strain and temperature. It is proved that the sensitivity and measurement precision and anti-interference ability of FBG strain sensors are better than traditional electricity class sensors, being able to realize the real-time and online detection for coal roof surrounding rock strain.

Cross Effect of Strain and Temperature on Brillouin Frequency Shift in Polymer Optical Fibers

The strain–temperature cross-sensitivity effect on the Brillouin frequency shift (BFS) in polymer optical fibers (POFs) is fully investigated. First, we show that the strain coefficient of the BFS is dependent on the temperature. In the strain ranges of 0–1.2% and 4.0–9.0%, the temperature dependence is linear with coefficients of 1.5 and – 0.3 MHz/(%·°C), respectively. We then find that the temperature coefficient of the BFS is linearly dependent on the strain with a coefficient of 1.5 MHz/(%·°C) in the strain range of 0–1.2%. For strains of 4.0–9.0%, the BFS basically decreases with increasing temperature. These results indicate that temperature (and strain) compensation for the strain (and temperature) sensitivity of the BFS is required to correctly detect the magnitudes of the strain and temperature in POF-based Brillouin sensing. For strains >13%, we show that temperature sensing with no sensitivity to strain is potentially feasible using the BFS in POFs. The height of the Brillouin gain spectrum is also evaluated.

Miniaturized fiber Fabry-Pérot interferometer for strain sensing

A temperature-insensitive miniaturized all-fiber Fabry–Perot interferometer strain sensor was proposed and experimentally studied. This sensor with an air cavity was made by splicing a short section of hollow-core fiber with two standard single mode fibers. The experimental results showed that these sensors have an fringe contrast exceeding 22 and 13 dB for the air cavity with a length of 167.08 and 557.11 μm, and its reflection spectrum shifts linearly to long wavelength by increasing the strain from 0 to 3000 μe with a strain sensitivity of 1.8 and 1.7 pm/μe, respectively. This strain sensor also showed good repeatability and long time stability, while its temperature sensitivity was only about 0.95 pm/°C in the range of 0°C–180°C for the 557.11 μm air cavity, so there is less strain–temperature cross-sensitivity. Moreover, this sensor was easy to be fabricated and there was no requirement for chemical etching, careful cleaving, and special fusion. It may be potentially used for strain sensing applications with high resolution and miniaturized size. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:1510–1514, 2016

Study on Practical FBG Acceleration System

A micro vibration, high sensitivity FBG demodulation system with close loop control is presented. By introducing a close loop control, the difficulty in matched FBG making process is overcame. Further more, the FBG strain-temperature cross sensitivity problem is settled. And low sensitivity in high frequency of cantilever FBG accelerator is overcame either. An example of the system application in acceleration measurement is introduced.

Effect of strain on temperature measurements using the fluorescence intensity ratio technique (with Nd3+- and Yb3+-doped silica fibers)

The effect of strain upon the fluorescence intensity ratio technique of optical fiber based temperature sensing has been investigated using a series of different Yb3+- and Nd3+-doped optical fibers. Measurements were conducted with each of the doped fibers for temperatures between approximately 10 and 140 °C and for strain values between 50 and 2100 με. Results suggest that the strain-temperature cross sensitivity for the fluorescence intensity ratio method is minimal, with an applied strain of 1000 με inducing temperature errors of less than 0.4 °C.

Distributed-feedback fiber laser sensor for simultaneous strain and temperature measurements operating in the radio-frequency domain.

Radio-frequency (rf) beat frequencies between two longitudinal modes and two polarization modes of a birefringent dual-longitudinal-mode moiré distributed-feedback fiber laser are employed to measure strain and temperature simultaneously. Operating entirely in the rf domain, this approach potentially allows one to employ low-cost and precise rf measuring techniques. A strain-temperature cross sensitivity of the strain- and the thermo-optic coefficients, which can be neglected in wavelength-based grating sensors, has been observed. The achieved sensor accuracy was +/-15 microepsilon and +/-0.2 degrees C.