Temperature Cross-sensitivity Research Articles

Trigger-Free and Low-Cross-Sensitivity Displacement Sensing System Using a Wavelength-Swept Laser and a Cascaded Balloon-like Interferometer

A wavelength-swept laser (WSL) demodulation system offers a unique time-domain analysis solution for high-sensitivity optical fiber sensors, providing a high-resolution and high-speed method compared to optical spectrum analysis. However, most traditional WSL-demodulated sensing systems require a synchronous trigger signal or an additional optical dispersion link for sensing analysis and typically use a fiber Bragg grating (FBG) as the sensing unit, which limits displacement sensitivity and increases fabrication costs. We present a novel displacement sensing system that combines a trigger-free WSL demodulation method with a cascaded balloon-like interferometer, featuring a simple structure, high sensitivity, and low temperature cross-sensitivity. The sensor is implemented by bending a short length of single-mode fiber with an optimal radius of around 4 mm to excite cladding modes, which form an interference spectral response with the core mode. Experimental findings reveal that the system achieves a high sensitivity of 397.6 pm/μm for displacement variation, corresponding to 19.88 ms/μm when demodulated using a WSL with a sweeping speed of 20 nm/s. At the same time, the temperature cross-sensitivity is as low as 5 pm/°C or 0.25 ms/°C, making it a strong candidate for displacement sensing in harsh environments with significant temperature interference.

Passive temperature-compensated fiber Fabry-Perot magnetic field sensor.

A passive temperature-compensated magnetic field sensor based on the material thermal-optics effect has been proposed. The proposed structure is easy to fabricate, consisting of only two single-mode optical fibers adhered to a magnetostrictive rod and the Fabry-Perot (FP) cavity filled with polydimethylsiloxane (PDMS). Benefitting from the negative thermal-optics coefficient of PDMS, the temperature cross-sensitivity of the sensor decreased from 351.02pm/°C to 12.59pm/°C, while exhibiting a high magnetic field sensitivity of 160.66pm/mT@38.47-61.52mT. The proposed sensor avoids complex control systems, composite structures, and bulky packaging. This feature effectively compensates for temperature influences during magnetic field detection, making it an ideal candidate for stable magnetic field sensors in practical applications.

Low-temperature-crosstalk humidity fiber sensor based on a thermal expansion-resistant Fabry-Perot interferometer

A low-temperature-crosstalk fiber-optic humidity sensor based on a thermal expansion-resistant Fabry-Pérot interferometer (FPI) has been proposed. This sensor is constructed by cascading a single-mode fiber, a hollow core fiber, an air-clad photonic crystal fiber (APCF), and a polyimide microrod. The two mirrors of FPI are composed of the end faces of single-mode optical fiber and polyimide microrod (located on the end face of APCF). Thus, hollow core fiber thermal expansion will offset the change in FPI length caused by polyimide microrod thermal expansion, thereby achieving low temperature sensitivity. In contrast, an increase in humidity causes the polyimide microrod to absorb moisture and swell, resulting in a shortening of the FPI. Experimentally, the sensor’s humidity sensitivity between 30 % RH and 80 % RH and temperature sensitivity between 20 °C and 200 °C are 203.1 pm/% RH and 0.95 pm/°C, respectively. With a temperature cross-sensitivity as low as 0.0047 % RH/°C. This article provided a novel solution for achieving low temperature cross sensitivity in humidity sensors.

GWO-elman based composite detection of transformer oil flow temperature

This paper addresses the issue of nonlinear coupling error caused by cross-sensitivity of strain and temperature in the composite detection of flow rate and temperature in transformer oil using a Fiber Bragg Grating (FBG) sensor. This paper focuses on the FBG sensor as the research object, elucidating the underlying principles of composite detection. Subsequently, a composite detection experimental platform is established for the purpose of analyzing the coupling error of flow rate and temperature. Ultimately, a nonlinear decoupling algorithm based on the Grey Wolf Optimizer (GWO) is proposed to enhance the nonlinear decoupling algorithm of the Elman neural network (simple recurrent neural network, Elman). The findings demonstrate that within the flow rate range of 0–5 m/s and the temperature range of 30 °C–150 °C, the maximum error is reduced by 72.0 % and 81.3 %, and the average error is reduced by 74.4 % and 79.4 %. This significantly enhances the precision and reliability of the sensor’s detection capabilities.

Simultaneous measurement of temperature and refractive index based on fiber optic Fabry–Pérot cavity in batteries

Abstract A Fabry-Pérot (FP) sensor for simultaneous measurement of refractive index and temperature of an all-fiber synchronous liquid based on the Vernier effect is proposed, special application for internal monitoring of lithium-ion battery electrolytes. The theoretical model of high sensitivity cascade Fabry-Pérot interferometer (PFPI) is established, the sensitivity amplification mechanism of PFPI is analyzed in detail, the perfect matching scheme for measuring the refractive index of the electrolyte inside the battery is designed according to the matching conditions of the optical path, and the corresponding sensors for simultaneous measurement of the temperature and the refractive index are fabricated. The experimental results show that the sensor has a maximum refractive index sensitivity of 15391.5033 nm/RIU, a good linear fit, and can effectively monitor the refractive index change of the electrolyte by 10-4 orders of magnitude during normal charging and discharging of lithium-ion batteries. Compared to traditional epoxy adhesive bonded or laser processed FP sensors, this all-fibre optic structure not only improves temperature and corrosion resistance, but also significantly reduces temperature cross-sensitivity. Its small size and excellent resistance to high temperature and corrosion ensure the sensor's excellent response and broad application prospects in monitoring the refractive index of the electrolyte in 18650 lithium-ion batteries, which provides effective technical support for monitoring the health status of batteries and detecting early safety issues.

Large Range Curvature Measurement Using FBGs in Two-Core Fiber with Protective Coating.

In this work, we propose a fiber Bragg grating (FBG)-based sensor for curvature measurements. Two gratings are inscribed through the protective coating in a specialty optical fiber using focused femtosecond laser pulses and point-by-point direct writing technology. One grating is inscribed on the central core adjacent to an air channel, while the other is inscribed on the eccentric core. The bending characteristics of the two-core fiber strongly depend on the bending direction due to the asymmetry of the fiber cores. A bending sensitivity of 58 pm/m-1 is achieved by the FBG in the eccentric fiber core over the curvature range of 0-50 m-1. Temperature and humidity cross-sensitivity could be significantly reduced by analyzing the differences in peak shifts between the two gratings. The sensor features a large sensing range and good robustness due to the presence of its protective buffer coating, which makes it a good candidate for curvature sensing in engineering fields.

Speckle-decoded temperature-insensitive strain identification in a multimode optical fiber.

Fiber-optic sensing systems are significant tools for measuring various physical or biochemical parameters. However, temperature cross-sensitivity prevents accurate recognition of the target input signal when optical sensors are applied in practical scenarios. Herein, leveraging a deep learning algorithm, a speckle-decoded temperature-insensitive strain sensor is proposed and experimentally demonstrated. Scattering patterns are utilized to estimate the axial strain since the external force could change the coherent superposition of the amplitudes of propagating modes. The experimental results show that the recognition accuracy of the sensing system based on a classification model can reach 99.28% within a wide strain range of 0-0.3 N in the presence of temperature cross talk. In addition, the strain prediction demonstrates an average root-mean-square error of 1.02 N%. Such an intelligent speckle sensing strategy has the potential to broaden the applications of fiber-optic sensors in various engineering applications.

Temperature-insensitive and cost-effective distributed NP-Doped optical fiber sensors

This paper presents the development of a cost-effective distributed optical fiber sensor for temperature-insensitive assessment of mechanical disturbances along an optical fiber cable. The proposed sensor system uses a nanoparticle (NP)-doped optical fiber with enhanced Rayleigh backscattering to provide higher sensitivity and spatial resolution using the transmission and reflection analysis (TRA) approach, where the transmitted and backscattered optical powers are analyzed as a function of the mechanical disturbance. In addition, Fiber Bragg Gratings (FBGs) are used as wavelength filters to provide the wavelength division multiplexing of the proposed device, which enable the use of 3 different NP-doped optical fiber sections for simultaneous detection of multiple curvature conditions in a cost-effective distributed sensing approach. The sensor characterization tests are performed by means of applying curvature angles from 360° to 1080° at different positions along NP-doped fibers (namely 25 mm, 100 mm and 175 mm) at 4 different temperatures of 25 °C, 30 °C, 40 °C and 50 °C. The results indicate the feasibility of the proposed approach, where the temperature variations lead only to a wavelength shift of the Bragg wavelength, whereas the mechanical disturbances (the curvatures) lead only to variations in the transmitted and reflected optical powers. Thus, by analyzing the transmitted and reflected optical powers in conjunction with the Bragg wavelength shift, it is possible to estimate both the mechanical disturbance amplitude (i.e., curvature angle) and the position along each NP-doped optical fiber section. Results indicate a relative error of around 3 % and 4 % for the mechanical disturbance location and absolute value, respectively. Moreover, the temperature cross-sensitivity in this case is below 2 % considering both amplitude and location of the mechanical disturbance. The proposed approach can be applied in structural health monitoring of different types of structures by integrating the fibers in the structures themselves with the possibility of measuring the strain distribution along the fibers (instead of in different points along the fiber) using a lower cost hardware when compared with similar distributed optical fiber sensing approaches.

A small-period long-period fiber grating biosensor based on immobilization of concanavalin A on polydopamine nanospheres for simultaneous detection of D-glucose and temperature

The accurate and sensitive monitoring of glucose levels plays a crucial role in diagnosing diabetes in clinical settings. In this work, a novel small-period long-period fiber grating (SP-LPG) biosensor was first developed for D-glucose detection based on the immobilization of concanavalin A (Con A) on polydopamine (PDA) nanospheres. SP-LPG was fabricated by using femtosecond laser direct writing technology and the obtained grating region was only 2.4 mm. The recognition element of D-glucose, Con A, was attached to the surface of PDA nanospheres. Due to the high specific surface area of PDA nanospheres, a large number of binding sites were available for Con A. The as-prepared sensor showed selective and sensitive to D-glucose measurement in the range of 10 nM to 10 mM and a low detection limit of 1.95 nM (S/N = 3) was achieved. Moreover, the small grating period of SP-LPG allows for the observation of Bragg reflection, which can be utilized for temperature sensing. This property eliminates the issue of temperature cross-sensitivity present in refractive index sensing. And the sensor exhibited a thermal sensitivity of 10 pm/°C within the range of 30 ∼ 100 °C. The developed sensor offers a convenient and efficient platform of simultaneously measuring multiple parameters, including target molecules and temperature, for real-time medical monitoring applications.

MZI-based high intensity responsive sandwiched multi-layer fiber optic humidity sensor

This paper presents a multi-layer optical fiber humidity sensor based on dual-beam Mach-Zehnder interference (MZI) for optical intensity sensitivity. The sensor is fabricated using a large-scale axial offset fusion technique between a multimode fiber (MMF) and an offset hole two-core fiber (OHTC), making it suitable for agricultural and health monitoring applications. The 27 μm axial offset enables tilted light incidence, enhancing environmental sensitivity. With a single-cycle Graphene oxide (GO) coating of approximately 22 nm, the MMF-OHTC structure achieves high humidity sensitivity (0.39 dB/%RH) over the 25 %-80 % RH range with excellent linearity (R2≈0.99). The sensor exhibits stable, rapid response (2.52 s/0.46 s) and low temperature cross-sensitivity (0.06 %RH/°C). When combined with a hollow-core fiber Fabry-Perot interferometer (HCF-FPI), the system allows zero cross-talk simultaneous humidity and temperature measurements, expanding the potential applications of multi-layered structures in multifunctional detection systems.

Femtosecond laser direct writing annular small-period long-period fiber gratings for simultaneous refractive index and temperature sensing.

A new type of small-period long-period fiber grating (SP-LPFG) consisting of a series of annuli inscribed by a femtosecond laser in a single-mode fiber is proposed and demonstrated. The effects of the annuli radii and the number of annuli in each period on the transmission spectrum are studied. The transmission spectrum of the annular SP-LPFG exhibits both strong Bragg resonances and cladding mode resonances, which have similar temperature sensitivities of 9.26 and 9.75 pm/°C, respectively. The Bragg resonances are insensitive to the change of environment refractive index (RI), while the cladding mode resonances show a RI sensitivity of 141.89 nm/RIU. The proposed sensor offers a potential solution for biomolecular sensing applications by effectively eliminating the issue of temperature cross-sensitivity. Its compact design and ease of spectral acquisition enhance its practicality and convenience.

High sensitivity Mach-Zehnder interferometric fiber-optic humidity sensor based on multimode interference enhancement

Variations in humidity can significantly disrupt the proper functioning of precision instruments and meters across various industrial, medical, and scientific applications, thereby necessitating real-time humidity monitoring. Here, a high sensitivity fiber-optic relative humidity (RH) sensor based on Mach-Zender interferometer (MZI) is proposed and demonstrated. The humidity sensor consists of single-mode fiber (SMF), peanut structure (PS), and multi-mode fiber (MMF). A multi-mode interference (MMI) enhanced fiber-optic MZI was constructed using a SMF-PS-SMF-MMF-SMF-PS-SMF cascade structure to detect small changes in the refractive index (RI) and expansion force of gelatin film caused by humidity variation. PS, as a beam splitter and combiner of MZI, provides the possibility for simple, economical, and efficient manufacturing of the fiber-based part of humidity sensors. The addition of MMF will excite higher-order core and cladding modes, laying the foundation for achieving high-sensitivity fiber-optic humidity sensors. The fiber-optic RH sensor prototype demonstrate a high sensitivity of about 0.633 nm/% RH in the RH range of 50 % RH to 80 % RH, and a low temperature cross-sensitivity of 0.165 % RH/°C. This sensor has the advantages of safe and easy manufacturing process, low manufacturing cost, and high sensitivity, providing continuous high accuracy humidity measurement results for humidity sensing in different fields.

Draw-tower-grating-based Fabry-Perot interferometers for simultaneous measurements of temperature and salinity.

In this paper, a fiber optic sensor with draw-tower-grating (DTG)-based Fabry-Perot interferometers (FPIs) are proposed to measure the temperature and salinity of seawater simultaneously. The sensing structure utilizes DTG as the reflector, and two adjacent gratings can be cascaded to compose an FPI. By employing moisture-sensitive materials to the surface of the optical fiber, the variation in salinity can be measured by the axial strain applied by the coating on the sensing optical fiber. The salinity altering causes swelling or shrinking actions of the coating through absorbing or releasing water. Two different moisture-sensitive materials are chosen to eliminate the cross-sensitivity of temperature and salinity. The experimental results show that the sensor exhibits linear response to temperature and salinity changes with good repeatability and stability, and the temperature and salinity sensitivities are 12308.65 rad/°C and 95.02 rad/‰, respectively. The optical path configuration of the DTG sensing array matching compensation interferometer provides the possibility for efficient distributed solutions, which has promising potential for application in marine engineering.

High-sensitivity strain sensor based on an asymmetric tapered air microbubble Fabry-Pérot interferometer with an ultrathin wall

A Fabry-Pérot interferometer (FPI) with an asymmetric tapered structure and air microbubble with an ultrathin wall is designed for high-sensitivity strain measurement. The sensor contains an air microbubble formed by two single-mode fibers (SMF) prepared by fusion splicer arc discharge, and a taper is applied to one side of the air microbubble with a wall thickness of 3.6 µm. In this unique asymmetric structure, the microbubble is more easily deformed under stress, and the strain sensitivity of the sensor is up to 15.89 pm/µɛ as evidenced by experiments.The temperature sensitivity and cross-sensitivity of the sensor are 1.09 pm/°C and 0.069 µɛ/°C in the temperature range of 25-200°C, respectively, thus reducing the measurement error arising from temperature variations. The sensor has notable virtues such as high strain sensitivity, low-temperature sensitivity, low-temperature cross-sensitivity, simple and safe process preparation, and low cost. Experiments confirm that the sensor has good stability and repeatability, and it has high commercial potential, especially strain measurements in complex environments.

Dual-parameter fiber sensor based on Fabry-Pérot cavity cascaded with Terfenol-D rod integrated fiber Bragg grating

A kind of dual-parameter fiber sensor based on the Terfenol-D rod combined with the cascaded Fabry-Pérot interferometer-fiber Bragg grating (FPI-FBG) structure is demonstrated, which integrates the FBG with the Fabry-Pérot cavity made by fiber and polydimethylsiloxane (PDMS). The problem of temperature cross-sensitivity in magnetic field measurement is simply and ingeniously avoided. The experimental results show that the sensing system's temperature and magnetic field sensitivities are 121.56 pm/°C and 21.6 pm/mT, respectively. By using the dual-parameter matrix, the resolutions of temperature and magnetic field measurement are obtained to be 1.003 °C and 0.069 mT, respectively.

Ultra-sensitive pressure sensor with low-temperature crosstalk based on the Vernier effect and helical structure

What we believe to be a novel high-sensitivity fiber-optic pressure sensor based on the vernier effect and helical structure is proposed and experimentally verified. The sensor utilizes the superposition of higher-order mode Mach-Zehnder interference and Sagnac fundamental mode polarization interference in a single fiber ring to achieve the vernier effect. In addition, a non-invasive encapsulation structure was fabricated to convert the rise and fall of the pressure value into the change in the twist angle of the optical fiber. This approach reduces the interference of the detecting medium on the sensor signal while simultaneously increasing the sensitivity of the pressure sensor. According to experimental data, the detection sensitivity of the sensor can reach −67277 nm/MPa, which is 65 times higher than the sensitivity of the conventional vernier effect pressure sensor. It also solves the issue of temperature interference with the Vernier-effect structured fiber optic sensor. The sensor has a measured temperature cross-sensitivity of 0.000065 kPa/°C, which is significantly lower than that of comparable sensors. This makes the sensor highly sensitive and ideal for low crosstalk pressure measurement.

Characterization of optical fibers doped with nanoparticles for distributed displacement sensing.

High-scattering optical fibers have emerged as a key component in distributed sensing systems, primarily due to their capacity to enhance signal-to-noise ratio. This paper presents an experimental characterization of optical fibers doped with oxide nanoparticles for displacement sensing. They were manufactured using the phase-separation technique and different doping compounds, including calcium, strontium, lanthanum and magnesium. The Rayleigh backscattering (RBS) signatures in time and frequency domains were acquired using an Optical Backscatter Reflectometer (OBR). The maximum representative length, backscattering gain and strain sensitivity were evaluated. The results indicate that the fiber co-doped with magnesium and erbium chlorides offered the best compromise between strain sensitivity (0.96 pm/μ ϵ) and maximum length (17 m). For conditions of single and multiple perturbations, strain saturation was reached at ≥7000 μm and <1500 μm, respectively. In addition, the results reveal that, under a condition of variable temperature (30-60 °C), the sensor response becomes significantly nonlinear over length, requiring a technique for temperature cross-sensitivity mitigation that accounts for nonlinearities in sensitivity and hysteresis.

Long-Period Grating with Asymmetrical Modulation for Curvature Sensing

We propose and demonstrate a curvature sensor based on long-period fiber grating (LPFG) with asymmetric index modulation. The LPFG is fabricated in single-mode fiber with femtosecond laser micromachining. The grating structure is not introduced in the central fiber core, but is located off-axis with a distance of a few micrometers. Experimental results indicate that the offset distance has direct influence on the grating spectra. By utilizing such an asymmetric structure, two-dimensional vector curvature sensing can be realized. For an LPFG with an offset distance of 6 μm, the curvature sensitivity is around 29 nm/m−1 in the 0° and 180° direction and about 20 nm/m−1 in the 90° and 270° direction. The difference in curvature sensitivity in different bending directions makes the sensor capable of distinguishing the curvature orientation. The temperature response of the sensor is also experimentally investigated, and results indicate that the sensor has a very low temperature cross-sensitivity of 0.003 m−1/°C. The characteristics of high curvature sensitivity, two-dimensional bending direction identification, and compact structure make the device an ideal candidate to be applied in the field of power grid health monitoring and intelligent robotics.

Ultra-sensitive air pressure sensor based on gelatin diaphragm-based Fabry-Peroy interferometers and Vernier effect

We proposed and experimentally demonstrated a gelatin diaphragm-based Fabry-Perot interferometer (FPI) structure ultra-sensitive fiber optic air pressure sensor. The Fabry-Perot (F–P) cavity of the sensing probe FPI1 is made of gelatin diaphragm, with a thickness of approximately 0.8 μm and the probe length of approximately 136 μm. The air pressure sensitivity of the probe obtained in the experiment is as high as 334 nm/MPa. In order to further improve the sensitivity of the probe, we fabricated an FPI2 with an air F–P cavity that is approximately the length of the FPI1 cavity, and then paralleled FPI2 with FPI1 to generate a Vernier effect. The average sensitivity of the Vernier probe obtained in the experiment reached −2650 nm/MPa, which magnified the sensitivity of a single FPI1 by about 7.9 times. The temperature cross-sensitivity is only 3.6 kPa/°C. This proposed air pressure sensor has the advantages of ultra-high sensitivity, simple manufacturing, low cost, good repeatability and stability, and has broad application prospects in high sensitivity pressure and acoustic detection.

A comparison of temperature compensation methods in a diaphragm-embedded FBG

This paper presents the comparison of temperature compensation techniques for Fiber Bragg Grating (FBG)-based pressure sensor. Such temperature compensation is obtained through a physical assembly using a continuous FBG pair embedded in a rubber diaphragm through a vulcanization method in conjunction with different signal processing methods. The so-called continuous FBG pair is based on two consecutive FBGs (with different Bragg wavelengths) inscribed with physical separation up to 10 mm. The sensor system is tested as a function of temperature and pressure, where the temperature sensitivities of both FBGs are around 50 pm/°C. Pressure estimation errors around 148.22 Pa/°C were observed when this variation in sensitivity was not taken into account by the compensation technique. Furthermore, there is a comparison between techniques that consider the pressure sensitivity variation, namely multiple linear regression using the response of both FBGs and exponential regression using the sum of FBGs. In this case, the exponential regression indicated a smaller temperature cross-sensitivity (3.07 Pa/°C), higher determination coefficient (0.97) and smaller root mean squared error (0.40 kPa). Therefore, the exponential regression using the response of both FBGs is the suitable technique for temperature compensation when the continuous FBG pair embedded in the diaphragm assembly is considered. The results obtained in this work provide a guideline for the development of diaphragm-embedded sensors with temperature compensation and suitable for large-scale production in practical applications.