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A special issue of Sensors (ISSN 1424-8220). This special issue belongs to the section "Optical Sensors".

Deadline for manuscript submissions: closed (31 October 2020) | Viewed by 39835

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Dr. Marcel Fajkus

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Department of Telecommunications, Faculty of Electrical Engineering and Computer Science, VSB-Technical University of Ostrava, 17. Listopadu 2172/15, 70800 Ostrava, Czech Republic
Interests: fiber bragg sensor; multipoint measurement; distributed sensing; biomedical application; car monitoring; railway monitoring; structural health monitoring
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Special Issue Information

Dear Colleagues,

Fiber-optic sensing technologies offer a high potential for utilization due to their properties compared to conventional sensor types. This Special Issue will explore new designs and improved sensing approach architectures to achieve higher measurement accuracy and resolution, and increase the functionality of sensors for measuring multiple quantities at once. This Special Issue will focus on the following fiber optic sensing technologies:

Fiber-optic and polymer Bragg gratings
Distributed systems based on Rayleigh, Raman, and Brillouin scattering
Fiber-optic interferometric and polarimetric systems
Intensity sensors
Micro- and nano-structured fiber sensors
Fiber specklegram sensors
New concepts for photonic sensing

Another goal is to extend the usability of these technologies and improve their principles for sensing applications in the following areas:

Biomedical applications
Structural health monitoring
Automobile, train, and air transport
Measurement of electrical and magnetic quantities
Very high-temperature measurement
Measurement in the chemical industry
Perimetric applications
Use of smart materials for fiber-optic sensors

Dr. Marcel Fajkus
Guest Editor

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Keywords

Fiber Bragg gratings
Brillouin scattering
Raman scattering
Rayleigh scattering
Mach–Zehnder interferometer
Fabry–Perot interferometer
Sagnac interferometer
Distributed sensing

Published Papers (13 papers)

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Research

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16 pages, 2546 KiB

Open AccessArticle

Continuous Hue-Based Self-Calibration of a Smartphone Spectrometer Applied to Optical Fiber Fabry-Perot Sensor Interrogation

by Aleksandr Markvart, Leonid Liokumovich, Iurii Medvedev and Nikolai Ushakov

Sensors 2020, 20(21), 6304; https://0-doi-org.brum.beds.ac.uk/10.3390/s20216304 - 05 Nov 2020

Cited by 16 | Viewed by 2651

Abstract

Smartphone-based optical spectrometers allow the development of a new generation of portable and cost-effective optical sensing solutions that can be easily integrated into sensor networks. However, most commonly the spectral calibration relies on the external reference light sources which have known narrow spectral lines. Such calibration must be repeated each time the fiber and diffraction grating holders are removed from the smartphone and reattached. Moreover, the spectrometer wavelength scale can drift during the measurement because of the smartphone temperature fluctuations. The present work reports on a novel spectral self-calibration approach, based on the correspondence between the light wavelength and the hue features of the spectrum measured using a color RGB camera. These features are caused by the nonuniformity of camera RGB filters’ responses and their finite overlap, which is a typical situation for RGB cameras. Thus, the wavelength scale should be externally calibrated only once for each smartphone spectrometer and can further be continuously verified and corrected using the proposed self-calibration approach. An ability of the plug-and play operation and the temperature drift elimination of the smartphone spectrometer was experimentally demonstrated. Conducted experiments involved interrogation of optical fiber Fabry-Perot interferometric sensor and demonstrated a nanometer-level optical path difference resolution. Full article

(This article belongs to the Special Issue Fiber Optic Sensing Technology)

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19 pages, 3806 KiB

Open AccessEditor’s ChoiceArticle

Strain Transfer in Surface-Bonded Optical Fiber Sensors

by Francesco Falcetelli, Leonardo Rossi, Raffaella Di Sante and Gabriele Bolognini

Sensors 2020, 20(11), 3100; https://0-doi-org.brum.beds.ac.uk/10.3390/s20113100 - 30 May 2020

Cited by 53 | Viewed by 4444

Abstract

Fiber optic sensors represent one of the most promising technologies for the monitoring of various engineering structures. A major challenge in the field is to analyze and predict the strain transfer to the fiber core reliably. Many authors developed analytical models of a coated optical fiber, assuming null strain at the ends of the bonding length. However, this configuration only partially reflects real experimental setups in which the cable structure can be more complex and the strains do not drastically reduce to zero. In this study, a novel strain transfer model for surface-bonded sensing cables with multilayered structure was developed. The analytical model was validated both experimentally and numerically, considering two surface-mounted cable prototypes with three different bonding lengths and five load cases. The results demonstrated the capability of the model to predict the strain profile and, differently from the available strain transfer models, that the strain values at the extremities of the bonded fiber length are not null. Full article

(This article belongs to the Special Issue Fiber Optic Sensing Technology)

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15 pages, 3047 KiB

Open AccessArticle

Iteration Bayesian Reweighed Algorithm for Optical Carrier-Based Microwave Interferometry Sensing

by Yuxiao Li, Ciming Zhou, Dian Fan, Sijing Liang and Li Qian

Sensors 2020, 20(11), 3079; https://0-doi-org.brum.beds.ac.uk/10.3390/s20113079 - 29 May 2020

Cited by 4 | Viewed by 1953

Abstract

This paper proposes a novel iteration Bayesian reweighed (IBR) algorithm to obtain accurate estimates of a measurement parameter that uses only a few noisy measurement data. The method is applied to optimize the frequency fluctuation in an optical carrier-based microwave interferometry (OCMI) system. The algorithm iteratively estimates the frequency of the S-parameter valley point by collecting training samples to rebalance the weights between prior samples, which reduces the impact of noise in the system. Simulation shows that the estimated result of this algorithm is closer to the true value than that of the maximum likelihood estimation (MLE) using the same amount of measured data. Under the influence of system noise, this algorithm optimizes the frequency fluctuation of the S-parameter and reduces the impact of individual measured data. In this study, we applied the algorithm in the strain sensing experiment and compared it with the MLE. When axial strain changes 240 με, the IBR algorithm yields a deviation of 36 με, which is a significant reduction from 138 με (using the MLE method). Moreover, the average error rate decreases from 25% to 3% (with the MLE method), suggesting that the linear fitting degree of the estimated results and accuracy of the system are improved. Moreover, the algorithm has a wide range of applicability, for it can handle different application models in the OCMI system and the systems with frequency fluctuation problems. Full article

(This article belongs to the Special Issue Fiber Optic Sensing Technology)

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15 pages, 5375 KiB

Open AccessArticle

Discerning Localized Thermal Heating from Mechanical Strain Using an Embedded Distributed Optical Fiber Sensor Network

by R. Brian Jenkins, Peter Joyce, Adam Kong and Charles Nelson

Sensors 2020, 20(9), 2583; https://0-doi-org.brum.beds.ac.uk/10.3390/s20092583 - 01 May 2020

Cited by 3 | Viewed by 2022

Abstract

Prior research has demonstrated that distributed optical fiber sensors (DOFS) based on Rayleigh scattering can be embedded in carbon fiber/epoxy composite structures to rapidly detect temperature changes approaching 1000 °C, such as would be experienced during a high energy laser strike. However, composite structures often experience mechanical strains that are also detected during DOFS interrogation. Hence, the combined temperature and strain response in the composite can interfere with rapid detection and measurement of a localized thermal impulse. In this research, initial testing has demonstrated the simultaneous response of the DOFS to both temperature and strain. An embedded DOFS network was designed and used to isolate and measure a localized thermal response of a carbon fiber/epoxy composite to a low energy laser strike under cyclic bending strain. The sensor interrogation scheme uses a simple signal processing technique to enhance the thermal response, while mitigating the strain response due to bending. While our ultimate goal is rapid detection of directed energy on the surface of the composite, the technique could be generalized to structural health monitoring of temperature sensitive components or smart structures. Full article

(This article belongs to the Special Issue Fiber Optic Sensing Technology)

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13 pages, 5943 KiB

Open AccessArticle

Transformer Winding Deformation Detection Based on BOTDR and ROTDR

by Shuguo Gao, Yunpeng Liu, Huan Li, Lu Sun, Hongliang Liu, Qun Rao and Xiaozhou Fan

Sensors 2020, 20(7), 2062; https://0-doi-org.brum.beds.ac.uk/10.3390/s20072062 - 07 Apr 2020

Cited by 10 | Viewed by 2812

Abstract

In order to realize distributed measurement of transformer winding temperature and deformation, a transformer winding modification scheme with a built-in distributed optical fiber was designed. By laying a single-mode fiber and a multi-mode fiber on the transformer winding, the Brillouin optical time domain reflection technique (BOTDR) and the Raman optical time domain reflection technique (ROTDR) are used to measure the strain and temperature of the winding to complete the more accurate winding deformation detection. The accuracy of strain and temperature sensing of this scheme was verified by simulation. Then, according to the scheme, a winding model was actually wound, and the deformation and temperature rise tests were carried out. The test results show that this scheme can not only realize the deformation detection and positioning of the winding, but can also realize the measurement of the winding temperature; the temperature measurement accuracy reached ±0.5 °C, the strain measurement accuracy was 200 με, and spatial resolution was up to 5 m. In this experiment, the deformation location with the precision of 2 turns was realized on the experimental winding. Full article

(This article belongs to the Special Issue Fiber Optic Sensing Technology)

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15 pages, 3762 KiB

Open AccessArticle

A Micro Absolute Distance Measurement Method Based on Dispersion Compensated Polarized Low-Coherence Interferometry

by Xun Sun, Kunpeng Feng, Jiwen Cui, Hong Dang, Yizhao Niu and Xuping Zhang

Sensors 2020, 20(4), 1168; https://0-doi-org.brum.beds.ac.uk/10.3390/s20041168 - 20 Feb 2020

Cited by 4 | Viewed by 2010

Abstract

Micro absolute distance measurement (MADM) is widely used in industrial and military fields. To achieve high accuracy and frequency response, a polarized low-coherence interferometry (PLCI)-based method for MADM is proposed. The nearly linear relationship between the envelope center and m-order PLCI fringe (PLCIF) peak center is found and verified. Dispersion compensation is achieved by fringe peak position estimation and polynomial fitting to get rid of the dependence on an a priori model and birefringence parameters, and make this method very robust. Meanwhile, the zero-order PLCIF center is estimated and located to demodulate the measured displacement. Then, the measurement accuracy is raised by polynomial fittings. In comparison to conventional methods, the proposed method can effectively avoid jump errors and has a higher accuracy. Experimental results indicate that the measurement accuracy is higher than 19.51 nm, the resolution is better than 2 nm, and its processing data rate can reach 35 kHz. Full article

(This article belongs to the Special Issue Fiber Optic Sensing Technology)

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10 pages, 5304 KiB

Open AccessArticle

A Refractive Index Sensor Based on H-Shaped Photonic Crystal Fibers Coated with Ag-Graphene Layers

by Tianshu Li, Lianqing Zhu, Xianchao Yang, Xiaoping Lou and Liandong Yu

Sensors 2020, 20(3), 741; https://0-doi-org.brum.beds.ac.uk/10.3390/s20030741 - 29 Jan 2020

Cited by 101 | Viewed by 4337

Abstract

An Ag-graphene layers-coated H-shaped photonic crystal fiber (PCF) surface plasmon resonance (SPR) sensor with a U-shaped grooves open structure for refractive index (RI) sensing is proposed and numerically simulated by the finite element method (FEM). The designed sensor could solve the problems of air-holes material coating and analyte filling in PCF. Two big air-holes in the x-axis produce a birefringence phenomenon leading to the confinement loss and sensitivity of x-polarized light being much stronger than y-polarized. Graphene is deposited on the layer of silver in the grooves; its high surface to volume ratio and rich π conjugation make it a suitable dielectric layer for sensing. The effect of structure parameters such as air-holes size, U-shaped grooves depth, thickness of the silver layer and number of graphene layers on the sensing performance of the proposed sensor are numerical simulated. A large analyte RI range from 1.33 to 1.41 is calculated and the highest wavelength sensitivity is 12,600 nm/RIU. In the linear RI sensing region of 1.33 to 1.36; the average wavelength sensitivity we obtained can reach 2770 nm/RIU with a resolution of 3.61 × 10⁻⁵ RIU. This work provides a reference for developing a high-sensitivity; multi-parameter measurement sensor potentially useful for water pollution monitoring and biosensing in the future. Full article

(This article belongs to the Special Issue Fiber Optic Sensing Technology)

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11 pages, 1028 KiB

Open AccessArticle

Comparative Study on Noise Reduction Effect of Fiber Optic Hydrophone Based on LMS and NLMS Algorithm

by Zhihua Yu, Yunfei Cai and Daili Mo

Sensors 2020, 20(1), 301; https://0-doi-org.brum.beds.ac.uk/10.3390/s20010301 - 05 Jan 2020

Cited by 12 | Viewed by 3497

Abstract

Adaptive filtering has the advantages of real-time processing, small computational complexity, and good adaptability and robustness. It has been widely used in communication, navigation, signal processing, optical fiber sensing, and other fields. In this paper, by adding an interferometer with the same parameters as the signal interferometer as the reference channel, the sensing signal of the interferometric fiber-optic hydrophone is denoised by two adaptive filtering schemes based on the least mean square (LMS) algorithm and the normalized least mean square (NLMS) algorithm respectively. The results show that the LMS algorithm is superior to the NLMS algorithm in reducing total harmonic distortion, improving the signal-to-noise ratio and filtering effect. Full article

(This article belongs to the Special Issue Fiber Optic Sensing Technology)

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16 pages, 1823 KiB

Open AccessArticle

Towards Improved Field Application of Using Distributed Temperature Sensing for Soil Moisture Estimation: A Laboratory Experiment

by Benjamin Apperl, Matthias Bernhardt and Karsten Schulz

Sensors 2020, 20(1), 29; https://0-doi-org.brum.beds.ac.uk/10.3390/s20010029 - 19 Dec 2019

Cited by 6 | Viewed by 2514

Abstract

The “dual probe heat pulse” (DPHP) method using actively heated fiber optic (AHFO) cables combined with distributed temperate sensing (DTS) technology has been developed for monitoring thermal properties and soil water content at the field scale. Field scale application, however, requires the use of robust and thicker fiber optic cables, corroborating the assumption of an infinite thin heat source in the evaluation process. We therefore included a semi-analytical solution of the heat transport equation into the evaluation procedure in order to consider the finite thermal properties of the heating cable without a calibration procedure to estimate effective thermal properties of the soil. To test this new evaluation procedure, we conducted a laboratory experiment and tested different heating scenarios to infer soil moisture from volumetric heat capacity. Estimates were made by analyzing the shift of the temperature amplitude at the sensing cable and the characteristics of the response heating curve. The results were compared with results from the calibrated infinite line source solution and in situ water content point measurements and showed a good approximation of thermal properties for strong and short heat pulses. Volumetric water content estimates are similarly accurate to the results of the calibrated infinite line source solution. Problems arose with the cable spacing and the resettlement process after burying the cable. Full article

(This article belongs to the Special Issue Fiber Optic Sensing Technology)

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10 pages, 2584 KiB

Open AccessArticle

Turbidity and RI Dependency of a Polymer Optical Fiber-Based Chromatic Sensor

by Daniel P. Duarte, Rogério N. Nogueira and Lucia Bilro

Sensors 2020, 20(1), 19; https://0-doi-org.brum.beds.ac.uk/10.3390/s20010019 - 19 Dec 2019

Cited by 3 | Viewed by 3139

Abstract

An in-line and real time chromatic sensor for liquids based on plastic optical fiber was developed. It uses an air gap, fiber to fiber, transmission principle. Its dependency to turbidity and refractive index is studied and characterized. This information will provide the necessary knowledge for future implementation of more complex auto-compensations routines. Due to the predictable behavior of the sensor to variations of turbidity and refractive index, it is shown that a posterior compensation could be applied for the discrimination of color. The real-time color sensor can be used in different turbid liquids and contain different refractive indices. Full article

(This article belongs to the Special Issue Fiber Optic Sensing Technology)

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14 pages, 4221 KiB

Open AccessArticle

Fiber-Optic Multipoint Sensor System with Low Drift for the Long-Term Monitoring of High-Temperature Distributions in Chemical Reactors

by Franz J. Dutz, Andreas Heinrich, Rolf Bank, Alexander W. Koch and Johannes Roths

Sensors 2019, 19(24), 5476; https://0-doi-org.brum.beds.ac.uk/10.3390/s19245476 - 12 Dec 2019

Cited by 19 | Viewed by 3196

Abstract

A low-drift fiber-optic sensor system, consisting of 24 regenerated fiber Bragg gratings (RFBG), equally distributed over a length of 2.3 m, is presented here. The sensor system can monitor spatially extended temperature profiles with a time resolution of 1 Hz at temperatures of up to 500 °C. The system is intended to be used in chemical reactors for both the control of the production ramp-up, where a fast time response is needed, as well as for production surveillance, where low sensor drifts over several years are required. The fiber-optic sensor system was installed in a pilot test reactor and was exposed to a constant temperature profile, with temperatures in the range of 150–500 °C for more than two years. During this period, the temperature profile was measured every three to five months and the fiber-optic temperature data were compared with data from a three-point thermocouple array and a calibrated single-point thermocouple. A very good agreement between all temperature measurements was found. The drift rates of the 24 RFBG sensor elements were determined by comparing the Bragg wavelengths at a precisely defined reference temperature near room temperature before and after the two-year deployment. They were found to be in the range of 0.0 K/a to 2.3 K/a, with an average value of 1.0 K/a. These low drift rates were achieved by a dedicated temperature treatment of the RFBGs during fabrication. Here, the demonstrated robustness, accuracy, and low drift characteristics show the potential of fiber-optic sensors for future industrial applications. Full article

(This article belongs to the Special Issue Fiber Optic Sensing Technology)

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12 pages, 2630 KiB

Open AccessLetter

Simultaneous Measurement of Temperature and Mechanical Strain Using a Fiber Bragg Grating Sensor

by Shiuh-Chuan Her and Wei-Nan Lin

Sensors 2020, 20(15), 4223; https://0-doi-org.brum.beds.ac.uk/10.3390/s20154223 - 29 Jul 2020

Cited by 14 | Viewed by 3008

Abstract

Based on the shift of the Bragg wavelength, fiber Bragg grating (FBG) sensors have been employed to measure a variety of physical parameters such as stress, strain, displacement, temperature, vibration and pressure. In this work, a simple and easy way to be implemented FBG sensing methodology was proposed to measure the temperature and strain simultaneously. Half of the FBG was bonded on the host structure, while the other half of the FBG was left free. The host structure was an aluminum test specimen with dimensions of 20 × 3.8 × 0.5

{cm}^{3}

. As the host structure subjected to mechanical and thermal loadings, the Bragg wavelengths reflected from the bonded and unbonded FBGs are different. Theoretical predictions of the Bragg wavelength shifts of the bonded and unbonded FBGs were presented. Utilizing the Bragg wavelength shift of unbonded FBG, the temperature can be determined and is independent of mechanical strain. The Bragg wavelength shift of the bonded FBG allows the determination of the mechanical strain. The temperature measured by FBG sensor was compared with the result from a thermocouple, while the mechanical strain was validated with the theoretical prediction. Good agreement between the experimental measurement and theoretical prediction demonstrates that temperature-strain discrimination can be realized using the proposed method with one single FBG sensor. Full article

(This article belongs to the Special Issue Fiber Optic Sensing Technology)

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9 pages, 3225 KiB

Open AccessLetter

A Novel Approach to Double the Sensitivity of Polarization Maintaining Interferometric Fiber Optic Gyroscope

by Dengwei Zhang, Cui Liang and Nan Li

Sensors 2020, 20(13), 3762; https://0-doi-org.brum.beds.ac.uk/10.3390/s20133762 - 05 Jul 2020

Cited by 11 | Viewed by 2978

Abstract

In this paper, a novel optical approach to double the sensitivity to angular rate of interferometric fiber optic gyroscope (IFOG) is proposed. Two fiber polarization combiner/splitters (FPCSs), as the key components, are added in the traditional IFOG light path. The FPCSs are able to either combine two orthogonal polarizations transmitting at two different polarization-maintaining fibers (PMFs) into the two orthogonal axes of one PMF, respectively, or split two polarizations transmitting at the two orthogonal axes of one PMF into two polarizations to transmit at two different PMFs, respectively. Through the specific placement and coupling of these two FPCSs, the incident light can transmit twice along the polarization-maintaining fiber coil (PMFC). The novel approach is verified experimentally and the experimental results show consistency with the theoretical analysis. The proposed approach is able to double the sensitivity of IFOGs and can increase the signal-to-noise ratio (SNR) without increasing the length of PMFC, which is very susceptible to environmental influences and is of great significance in the technical improvement of IFOGs, as well as the miniaturization of IFOGs. Full article

(This article belongs to the Special Issue Fiber Optic Sensing Technology)

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