Strain Cross-sensitivity Research Articles

Strain-insensitive fiber Bragg grating composite structure for wide-range temperature sensing

This paper reports on the design, fabrication, and temperature strain sensing performance of a fiber Bragg grating composite structure for surface mounted temperature measurements over a wide temperature range, with highly reduced strain cross-sensitivity. The fiber Bragg grating sensor is encapsulated in a polyimide tube filled with epoxy resin, forming an arc-shaped cavity. This assembly is then placed between two layers of glass fiber prepreg with a flexible pad in between and cured into shape. Experimental results, supported by finite element simulations, demonstrate an enhanced temperature sensitivity is 26.3 pm/ºC over a wide temperature range of -30℃ to 70℃, and high strain transfer isolation of about 99.65%. Conflict of Interest Statement The authors declare no conflicts of interest.

An improved sensitivity temperature sensor based on tapered Hollow-core Bragg fiber

A Mach-Zehnder interferometer (MZI) based on tapered Hollow-core Bragg fiber (HCBF) for temperature measurement is proposed and experimentally investigated. The MZI is fabricated by sandwiching the tapered HCBF between two single-mode fibers (SMFs). The tapered HCBF excites strong evanescent wave and forms a sensitive periodic envelope in the spectrum when the ambient temperature changes. Experimental results show that the temperature sensitivity of the sensor is 9.12 pm/°C, 18.90 pm/°C, 24.90 pm/°C, 27.12 pm/°C, and the strain sensitivity is −0.241 pm/με, −0.270 pm/με, −0.427 pm/με, −0.545 pm/με, when the tapered region is not stretched and when it is stretched by 2 mm, 2.4 mm, and 2.8 mm, respectively. The proposed sensor is characterized by high temperature sensitivity and low strain cross-sensitivity as well as simplicity of fabrication. It has good prospects in the field of biochemical and physical sensing.

Bending-Loss-Resistant Distributed Temperature and Strain Discriminative Brillouin Sensor Based on 98 mol% Germania-Doped Few-Mode Fiber

Temperature–strain cross-sensitivity and bending-induced optical loss are two major limitations that challenge the in-situ application of distributed optical fiber sensors. Researchers have attempted to solve these two issues but seldom can overcome both. This paper proposes a novel bending-loss-resistant distributed temperature and strain discriminative sensor that is based on a 98 mol% germania-doped few-mode fiber and a standard Brillouin optical time-domain analysis setup. In a proof-of-concept demonstration, distributed temperature and strain discriminative measurements were conducted under a bending radius of 0.6 cm, where the maximum temperature and strain deviation were 2.78 ℃ and 33.2 μϵ , respectively.

High Spatial Resolution Sensing of Temperature and Strain Without Cross Sensitivity Based on Chaotic Brillouin Dynamic Grating

A distributed fiber-optic sensing system based on chaotic Brillouin dynamic grating (BDG) without cross-sensitivity of temperature and strain is proposed and experimentally demonstrated. The chaotic BDG is stimulated by interaction of two chaotic pump beams with a certain frequency difference along the slow axis of polarization-maintaining fiber (PMF) and detected by the continuous probe light along the fast axis. The generated chaotic BDG has the characteristic of single, permanent, and localized grating. The grating length is determined by the low coherence length of the chaotic laser, which ensures that the spatial resolution of the chaotic BDG sensing system can reach the centimeter scale. Two parameters, i.e., Brillouin frequency shift (BFS) and birefringence frequency shift (BireFS) are measured to achieve the temperature and strain simultaneous discrimination. In the experiment, a 2.3-cm spatial resolution non-cross-sensitivity measurement was demonstrated over a 100-m long PMF, where temperature and strain discriminative accuracy can reach 0.3 °C and 11 με, respectively.

Machine learning assisted BOFDA for simultaneous temperature and strain sensing in a standard optical fiber.

We report, to our knowledge for the first time on simultaneous distributed temperature and strain sensing in a standard telecom optical fiber using a machine learning assisted Brillouin frequency domain analysis (BOFDA) system. The well-known temperature and strain cross-sensitivity problem is addressed by developing a BOFDA system with a high signal-to-noise ratio and applying machine learning. The spectrum consists of four highly resolved peaks, whose Brillouin frequency shifts are extracted and serve as features for the machine learning algorithms. The spectra result from a 450-m standard SMF-28 optical fiber, and particularly from a segment of 30 m. This fiber segment is coiled around a stretcher and placed in a climate chamber. The applied temperature and strain values range from 20 °C to 40 °C and from 0 µɛ to 1380 µɛ, respectively. The total measurement time to achieve a high SNR and resolve four peaks with a spatial resolution of 6 m is 16 min. To discriminate temperature and strain effects, simple frequentist and more sophisticated Bayesian-based algorithms are employed with the powerful Gaussian process regression (GPR) delivering the best performance in terms of temperature and strain errors, which are found to be 2 °C and 45 µɛ, respectively. These errors are calculated using leave-one-out cross-validation, so that an unbiased estimation of the sensor's performance is provided.

The early warning for overcharge thermal runaway of lithium-ion batteries based on a composite parameter

The wider application of lithium-ion batteries(LIBs)is restricted by electric abuse. Moreover, overcharging is an important reason for thermal runaway. In order to prevent the occurrence of overcharge thermal runaway (OTR) events, a new three-level warning method is presented. By comparing Fiber Bragg Grating (FBG) and thermocouple real-time thermal monitoring, FBG shows a better temperature response. The OTR experiment is carried out, and the whole process of lithium battery thermal runaway under different overcharge rates (1 C, 1.5 C, and 2 C) is divided into three stages. Based on the cross-sensitivity of strain and temperature for FBG, a composite parameter (virtual temperature) affected by temperature and strain is proposed as an early warning index. Combining with the changes in virtual temperature, surface temperature, and the early voltage variation of thermal runaway, a three-level early warning model for thermal runaway is proposed. The first-level warning: the temperature difference between virtual temperature and surface temperature reaches 5 °C. The second-level warning: the virtual temperature reaches 70 °C. The third-level warning: the surface temperature reaches 80 °C. This method can provide a reference for monitoring and early warning of LIBs in energy storage power stations.

Ultra-simple fiber sensor with ultra-low strain cross-sensitivity for embedded temperature detection

An ultra-simple fiber temperature sensor with ultra-low strain cross-sensitivity was proposed. It was fabricated by splicing a section of hollow core fiber (HCF) between two single-mode fibers with an offset at the lead-in interface. The transmission spectrum is the superposition of anti-resonant (AR) valleys and Mach–Zehnder (MZ) interference fringes, which show opposite strain sensitivities based on the wavelength shift. The actual superposed sensitivity of the sensor can be compensated (less than 0.08 pm μɛ−1) by changing the extinction ratios of two fringes through optimizing the offset value. When it is used for embedded temperature detection, the strain influence of the solid material can be ignored owing to its ultra-low strain cross-sensitivity. We chose the copper block to test. The result showed the deviation of temperature sensitivity caused by copper expansion was only 4%. Compared with the 50% deviation of fiber Bragg grating (FBG) temperature sensor, our structure is more competitive in embedded detection.

An Optimized Self-Compensated Solution for Temperature and Strain Cross-Sensitivity in FBG Interrogators Based on Edge Filter.

Optical fiber sensors based on fiber Bragg gratings (FBGs) are prone to measurement errors if the cross-sensitivity between temperature and strain is not properly considered. This paper describes a self-compensated technique for canceling the undesired influence of temperature in strain measurement. An edge-filter-based interrogator is proposed and the central peaks of two FBGs (sensor and reference) are matched with the positive and negative slopes of a Fabry–Perot interferometer that acts as an optical filter. A tuning process performed by the grey wolf optimizer (GWO) algorithm is required to determine the optimal spectral characteristics of each FBG. The interrogation range is not compromised by the proposed technique, being determined by the spectral characteristics of the optical filter in accordance with the traditional edge-filtering interrogation. Simulations show that, by employing FBGs with optimal characteristics, temperature variations of 30 °C led to an average relative error of 3.4% for strain measurements up to 700μϵ. The proposed technique was experimentally tested under non-ideal conditions: two FBGs with spectral characteristics different from the optimized results were used. The temperature sensibility decreased by 50.8% as compared to a temperature uncompensated interrogation system based on an edge filter. The non-ideal experimental conditions were simulated and the maximum error between theoretical and experimental data was 5.79%, proving that the results from simulation and experimentation are compatible.

Simultaneous Strain and Temperature Measurement Based on Chaotic Brillouin Optical Correlation-Domain Analysis in Large-Effective-Area Fibers

Chaotic Brillouin optical correlation domain analysis (BOCDA) has been proposed and experimentally demonstrated with the advantage of high spatial resolution. However, it faces the same issue of the temperature and strain cross-sensitivity. In this paper, the simultaneous measurement of temperature and strain can be preliminarily achieved by analyzing the two Brillouin frequencies of the chaotic laser in a large-effective-area fiber (LEAF). A temperature resolution of 1 °C and a strain resolution of 20 µε can be obtained with a spatial resolution of 3.9 cm. The actual temperature and strain measurement errors are 0.37 °C and 10 µε, respectively, which are within the maximum measurement errors.

Tilted fiber Bragg grating design for a simultaneous measurement of temperature and strain

Bragg wavelength is sensitive to both temperature and strain changes. Therefore, in sensors that are designed using a fiber Bragg grating (FBG), it is not possible to discriminate the cross-sensitivity of temperature and strain. The design of tilted fiber Bragg gratings (TFBG), which is a family of short-period gratings, has been one of the solutions to this problem. The core mode resonance (LP01) and cladding resonances (LPmn) appear simultaneously in the transmission spectrum. It is possible to perform the simultaneous, independent measurement of temperature and strain using only a TFBG. In this study, the design of a TFBG sensor with a tilt angle of 5° were performed by using the Optigrating 4.2.2 software in order to measure the temperature and strain simultaneously. While the varying temperature was applied to the TFBG in the first stage, the varying strain was applied to it in the second stage, and simultaneously varying temperature and strain were applied to it in the third stage, and linear shifts occurring in wavelengths were calculated using Optigrating. In the design stage of the temperature sensor, research was conducted with different thermal expansion and thermo-optic coefficients, the amounts of shifts caused by these coefficients in the wavelength were examined. It was observed that the change in the wavelength caused by the simultaneous application of temperature and strain was equal to the total change in wavelength that occurred in the non-simultaneous application of temperature and strain.

Stabilization of high sensitivity optical fiber AE sensing for long-term detection

The optical fiber acoustic emission (AE) sensing system with high sensitivity is stabilized for long-term detection in this paper. The high linear slope window of the phase-shifted fiber Bragg grating (PS-FBG) spectrum is tracked by the tunable laser source (TLS) to maintain high AE detection sensitivity. The experimental results show that the proposed method successfully solves the problem of PS-FBG spectrum shift caused by cross sensitivity of temperature and strain. In the high linear slope window of the spectrum, the stabilized sensing system can guarantee the AE strain sensitivity of 0.63nε/Hz1/2 at 200 kHz under the DC voltage of 10 mV. The stability of the AE sensing system is verified by stretching a carbon fibre reinforced plastic (CFRP) sample. The practical application demonstrates that the stabilized system can accurately capture the lowest order modes in the plate wave during long-term AE detection.

Intense Brillouin amplification in gas using hollow-core waveguides.

Among all the nonlinear effects stimulated Brillouin scattering offers the highest gain in solid materials and has demonstrated advanced photonics functionalities in waveguides. The large compressibility of gases suggests that stimulated Brillouin scattering may gain in efficiency with respect to condensed materials. Here, by using a gas-filled hollow-core fibre at high pressure, we achieve a strong Brillouin amplification per unit length, exceeding by six times the gain observed in fibres with a solid silica core. This large amplification benefits from a higher molecular density and a lower acoustic attenuation at higher pressure, combined with a tight light confinement. Using this approach, we demonstrate the capability to perform large optical amplifications in hollow-core waveguides. The implementations of a low-threshold gas Brillouin fibre laser and a high-performance distributed temperature sensor, intrinsically free of strain cross-sensitivity, illustrate the potential for hollow-core fibres, paving the way to their integration into lasing, sensing and signal processing.

Highly Sensitive Torsion Sensor Based on Side-Hole-Fiber Sagnac Interferometer

A highly sensitive torsion sensor based on side-hole fiber (SHF) Sagnac interferometer is proposed and experimentally demonstrated in this paper. The theoretical analyses on sensing performances of the proposed interferometric sensor are in good accordance with their experimental measurement counterparts. Due to photo-elastic effect, the birefringence of SHF in the loop would vary with the torsional rate, giving rise to linear wavelength shift and sinusoidal power variation responses to torsion applied on the SHF. Based on the wavelength and intensity interrogations of transmission dips, the torsional sensitivities reach up to about 2.2 nm/(rad/m) and ±10 dB/(rad/m), respectively. Opposite wavelength shift responses for clockwise and counterclockwise torsion states equip the proposed sensor with good twisting direction distinguishability. Besides, the experimental results indicate that our proposed sensor has a low axial strain cross sensitivity of 4.3 pm/με. The side-hole-fiber-based interferometric torsion sensor possesses such desirable merits as high sensitivity, low axial strain cross sensitivity, good torsional direction distinguishability, and good mechanical robustness, ensuring its viability for practical use on torsion sensing occasions.

Multiplexing of anti-resonant reflecting optical waveguides for temperature sensing based on quartz capillary.

A novel multiplexing scheme of anti-resonant reflecting optical waveguides (ARROW) for temperature sensing based on quartz capillary is firstly proposed and experimentally demonstrated. Each ARROW-based sensor is manufactured by fusion splicing a short section of capillary between two standard single-mode fiber (SMF). Experiments were demonstrated on a three cascaded ARROW-based sensing network chopped from a fiber array with multiple sensors. Simulation and experimental results demonstrate that the positions of sharp periodic transmission dips are independent of the capillary length, but dependent on the refractive indexes and thickness of the silica cladding. The temperature sensitivities of 18.77 pm/°C, 18.19 pm/°C, and 17.94 pm/°C are achieved by wavelength demodulation of selected dips of Gaussian fitting algorithm, and the tunability of sensitivity can be realized by selecting different dips. Furthermore, the merits of low strain cross-sensitivity (-0.71 pm/με), easy fabrication capability and high spectral extinction ratio (about 30dB) make the proposed multiplexing scheme an excellent candidate for performing multi-point and multi-parameter measurements in harsh environments.

Temperature and Strain Discrimination in BOTDA Fiber Sensor by Utilizing Dispersion Compensating Fiber

Standard single mode fiber that normally used in Brillouin optical time domain analysis fiber sensors typically has only one Brilllouin gain peak (BGP), and its central frequency [i.e., Brillouin frequency shift (BFS)] can be affected by both temperature and strain, which gives rise to temperature and strain cross sensitivity. In this paper, we propose a new method to realize temperature and strain discrimination, for the first time to the best of our knowledge, by using commercial single mode dispersion compensating fiber (DCF) that has two BGPs located around 9.536 and 10.052 GHz, respectively. The two peaks have different temperature coefficients but nearly the same strain coefficients, which permits simultaneous temperature and strain sensing by combing the BFSs obtained from the two BGPs. The biggest advantage of the DCF is that the frequency space between its two BGPs is >100 MHz wider than some other fibers, ensuring much wider temperature and strain sensing range. In a proof of concept experiment, temperature and strain are fully discriminated with a temperature resolution of ~2.6 °C and a strain resolution of $\sim 64.6~\mu \varepsilon $ at the end of a 1-km sensing fiber with 2-m spatial resolution.

Highly sensitive torsion sensor with femtosecond laser-induced low birefringence single-mode fiber based Sagnac interferometer.

A highly sensitive optical fiber torsion sensor with femtosecond laser-induced low birefringence SMF-based Sagnac interferometer (SI) is proposed and experimentally demonstrated in this paper. A straight-line waveguide positioned horizontally with respect to the fiber core is inscribed by the femtosecond laser in the cladding of the SMF, which leads to the asymmetry stress distribution in the SMF, and then gives rise to the low birefringence in the SMF. Compared with most of the previous reported SI based torsion sensors, there is no splicing joint in the femtosecond laser-induced low birefringence SMF-based SI, which lowers the transmission loss and makes the SI based torsion sensor more robust simultaneously. The experiment result shows that the proposed torsion sensor exhibits a torsion sensitivity of up to 3.2562 nm/degree, with the high torsion resolution of 0.003 degree. In contrast, the temperature cross-sensitivity and strain cross-sensitivity of the proposed torsion sensor are low, to -0.000055 degree/°C and 0.000013 degree/με, respectively, thus overcoming the cross-sensitivity problem resulting from temperature and strain. Moreover, theoretical analysis are carried out to compare with the experimental results to demonstrate the feasibility and good consistency.

Enhanced selfintegration algorithm for fiber torsion sensor based acoustically-induced fiber grating

Mechanical parameter monitoring based on optical mode detection benefits from its low cross sensitivity and inexpensive instrument. The key to improving detection accuracy is to generate high-quality detection light and use efficient algorithms. We present a strain-independent torsion sensor based on acoustically-induced fiber grating (AIFG) in the dual-mode fiber (DMF) and use the enhanced self-integration algorithm to improve the sensing accuracy. By tuning the radio frequency of driving signal, the LP11 mode generated by the AIFG can be exploited to measure the dynamic torsion variations. Without the complex device such as fiber interferometers and photonic crystal fibers (PCFs), the simple structure built by mode converter and charge coupled device (CCD) can track the dynamic variations and has less cross sensitivity of strain along the transmission direction. The AIFG driven by a radio frequency as a mode converter at specific wavelength does not participate in sensing but generates the high-purity LP11 mode that accounts for more than 90% of total power. With the twist from the rotator stage, the DMF keeps rotating and CCD records the spatial distribution of mode profiles. The features of optical mode is enhanced based on matrix analysis and then the relationship between twist angle and mode features is obtained. Based on image processing, the dynamic variation of spatial beam detected by CCD can be easily tracked and quantified. In experiment, the rotation angle can be obtained by calculating the feature value of the optical mode. Our image detection algorithm is specially designed for the optical fiber mode. Compared with traditional image recognition based on feature learning, it is simple and fast because it is needless to use image segmentation and stochastic processing. Through a series of experiments on angle rotation and parallel strain, we verify the correctness of the enhanced self-integration model and analyse the computational uncertainties that influence the stability of experiment. In the 0 to 180 measurement range, the maximum range of measurement error is less than 11%. When the axial strain is between 100 and 1500 , the sensor is strain-independent. Thus, it is verified that the torsion sensor based on AIFG has high sensitivity and can overcome the cross sensitivity of strain along a certain direction. The pertinent results have significant guidance in designing the multi-parameter sensor. The optical mode detection, instead of the traditional spectrum measurement, enables the whole structure to have the potential to be rebuilt by inexpensive devices that work in visible wavelengths.

考虑温度和应变交叉敏感的光纤布拉格光栅测量研究

考虑温度和应变交叉敏感的光纤布拉格光栅测量研究

ANSYS-based birefringence property analysis of side-hole fiber induced by pressure and temperature

In this paper, we theoretically investigate the influences of pressure and temperature on the birefringence property of side-hole fibers with different shapes of holes using the finite element analysis method. A physical mechanism of the birefringence of the side-hole fiber is discussed with the presence of different external pressures and temperatures. The strain field distribution and birefringence values of circular-core, rectangular-core, and triangular-core side-hole fibers are presented. Our analysis shows the triangular-core side-hole fiber has low temperature sensitivity which weakens the cross sensitivity of temperature and strain. Additionally, an optimized structure design of the side-hole fiber is presented which can be used for the sensing application.

Cascaded-cavity Fabry–Perot interferometer for simultaneous measurement of temperature and strain with cross-sensitivity compensation

We propose and demonstrate a fiber sensor for simultaneous temperature and strain measurements. The proposed sensor is implemented by a cascaded-cavity Fabry–Perot (FP) fiber interferometer. The two cascaded FP cavities comprise a micro-air-cavity in a hollow-core tube fiber and a micro-silica-cavity in a standard single-mode fiber. To separate the interference spectrum of each FP cavity, the total spectrum is filtered in the frequency domain through band-pass filters, whose central frequencies were predesigned based on the relationship between the spatial frequency and free spectral range of each FP cavity. The different cross-sectional areas and thermal-optic coefficients of the two FP cavities confer different sensitivities to temperature and strain. Both parameters were measured simultaneously by tracking the wavelength shifts in the filtered interference spectra of the FP cavities. Moreover, the temperature–strain cross-sensitivity was compensated by solving a sensitivity-coefficient matrix equation for the two cavities, using the calibrated temperatures and strains. Other advantages of the proposed sensor are simple fabrication and an all-fiber structure. Owing to these properties, the proposed sensor is potentially applicable to real sensing applications.