Fiber Optic Sensor Research Articles

High generalization identification method based on MI-SI distributed optical fiber sensor and video signals

A pattern recognition method for intrusion signals is introduced, leveraging distributed optical fiber sensors and convolutional neural networks. To enhance the model's generalization capabilities, two strategies are employed. Firstly, the sensor structure is refined, incorporating the strengths of both the Sagnac and Michelson interferometers to create a hybrid design with a wide frequency response. Comparative experiments confirm that this exceptional frequency response significantly boosts the model's generalization power. Secondly, to address the limitations of distributed fiber optic sensors, a multimodal approach is proposed, integrating video frame difference and fiber optic pattern recognition. The trained convolutional neural network model exhibits low computational overhead, robust generalization, and practical applicability. When applied to a dataset collected in diverse scenarios not included in the model training, the model achieved a CNR of 0% and a FPR of only 0.26% for the intrusion behavior of climbing fences.

Sodium-based no-core fiber surface plasmon resonance sensor with high sensitivity and narrow FWHM

Since fiber-optic sensors using noble metal-excited surface plasmon resonance (SPR) effects have encountered bottlenecks in improving performance, we propose a fiber-optic sensor using sodium in combination with a no-core fiber (NCF) to measure both refractive index (RI) and temperature. We deposited sodium thin films on the surface of NCF optical fibers and protected them with polymethyl methacrylate (PMMA) or polydimethylsiloxane (PDMS) for RI sensing or temperature sensing. We performed computational simulations and performance analyses of the sensors using the finite element method, and the results show that the sodium-based SPR sensors have higher sensitivity, wider detection range, and narrower full width at half-maximum (FWHM) than the noble metal SPR sensors. SPR sensors with different sodium film thicknesses have different sensing characteristics, so we can get optical fiber sensors with more flexible transmission characteristics, which helps us arrange sensors more conveniently in practical applications. The simulation and numerical results show that when the sensor is used to measure RI, the average sensitivity of the sensor can reach 7977 nm RIU−1, the maximum sensitivity can reach 23100 nm RIU−1, the narrowest FWHM is 14.23 nm, and the maximum figure of merit (FOM) is 719.42 RIU−1 under different thicknesses of sodium film. The corresponding RI ranges from 1.32 to 1.41. When the temperature measurement range is 0 °C ∼ 100 °C, the average sensitivity can reach 7.86 nm °C−1, the maximum temperature sensitivity can reach 21.1 nm °C−1, and the narrowest FWHM is 17.84 nm. In summary, the proposed sodium-based SPR sensor has flexible and high-performance sensing characteristics, and our research work provides more powerful theoretical support for the application of sodium-based plasma devices.

Structural Health Monitoring for Prefabricated Building Envelope under Stress Tests

This paper details the comprehensive activities conducted in a laboratory setting to assess the structural health monitoring (SHM) of prefabricated building envelopes. Integrating sensors into building components like curtain wall facades poses challenges but offers opportunities for monitoring structural health, requiring compliance with regulatory standards. The research investigates the possibility of defining a kit of conventional and multi-parameter sensors integrated within the building envelope to monitor its behavior during the performance test conducted. The kit of sensors also includes Fiber Optic Sensors for effectively monitoring building envelope behavior and structural integrity. In this context, the European project InComEss (H2020-GA862597) aims to define a stand-alone solution for SHM using Piezoelectric Energy Harvesting Systems (PE-EHS) for façade monitoring through FBG/FOS system. After analyzing the main façade structural stress, a series of FBGs, accelerometers, and force washers were integrated within a 1:1 scale façade prototype and tested in a laboratory following the test sequence parameters required by the curtain wall standard EN 13830. The data collected were analyzed with the aim of monitoring the façade behavior before and after the tests. The results show that the façade’s performance passed the assessing test criteria without reporting any damages. In addition, the outcomes demonstrated the effectiveness of the defined kit of multi-parameter sensors for the building envelope’s SHM.

Porphyrin based-optical fiber sensor for pattern recognition of chlorides and nitrates

The paper introduces a novel approach to develop a photonic device for chemical applications using an evanescent wave absorption technique-based optical fiber probe layered with a Tetraamine porphyrin (TAPP) monolayer exploiting the optical properties of TAPP. The interactions between the TAPP and nitrate and chloride ions in the solution phase were monitored and the changes in the absorption spectra of TAPP molecules were recorded. Distinct optical patterns emerge, and ion-dependent shifts in the Soret and Q-bands are analyzed for eight samples of nitrates and chlorides. Principal Component Analysis (PCA) is employed to discern ion groups based on the spectral pattern recognition. These fiber probes exhibit sensitivity to nitrate and chloride ions, with enhanced selectivity, robustness, and a rapid response time of 2–3 seconds. The study demonstrates the emergence of porphyrin-based optical fiber sensors as a useful sensor technology in environmental monitoring.

Ultra-sensitive fiber-optic temperature sensor based on UV glue-based FPI and Vernier effect

In this paper, compact cascaded Fabry-Perot interferometers (FPI) for fiber-optic temperature sensors are proposed and verified. The sensors are prepared by combining the properties of temperature-sensitive material ultraviolet glue (UV glue) and the basic principle of the traditional Vernier effect (TVE), first-order harmonic Vernier effect (HVE), and second-order HVE. The sensing cavity FPIs is made up of UV glue filling the gap between two single-mode fibers (SMF) with flat cut ends. The reference cavity FPIr is composed of SMF-capillary-SMF structure. The temperature sensitivity of the FPIs is up to 1.01 nm/°C by the high thermal expansion coefficient of the UV glue. By changing the free spectral range (FSR) of the FPIr, high detection sensitivities achieved are −10.14 nm/°C, 15.22 nm/°C, and 22.24 nm/°C, corresponding the situation of TVE, first-order HVE, and second-order HVE, respectively. The experimental results indicate that temperature sensors based on UV glue possess a simple and compact structure and strong resistance to electromagnetic interference, making them suitable for temperature measurement in various environments.

Enhancing Microwave Photonic Interrogation Accuracy for Fiber-Optic Temperature Sensors via Artificial Neural Network Integration

In this paper, an application of an artificial neural network algorithm is proposed to enhance the accuracy of temperature measurement using a fiber-optic sensor based on a Fabry–Perot interferometer (FPI). It is assumed that the interrogation of the FPI is carried out using an optical comb generator realizing a microwave photonic approach. Firstly, modelling of the reflection spectrum of a Fabry–Perot interferometer is implemented. Secondly, probing of the obtained spectrum using a comb-generator model is performed. The resulting electrical signal of the photodetector is processed and is used to create a sample for artificial neural network training aimed at temperature detection. It is demonstrated that the artificial neural network implementation can predict temperature variations with an accuracy equal to 0.018 °C in the range from −10 to +10 °C and 0.147 in the range from −15 to +15 °C.

Magnetic Field Sensing via Acoustic Sensing Fiber with Metglas® 2605SC Cladding Wires

Magnetic field sensing has the potential to become necessary as a critical tool for long-term subsurface geophysical monitoring. The success of distributed fiber optic sensing for geophysical characterization provides a template for the development of next generation downhole magnetic sensors. In this study, Sentek Instrument’s picoDAS is coupled with a multi-material single mode optical fiber with Metglas® 2605SC cladding wire inclusions for magnetic field detection. The response of acoustic sensing fibers with one and two Metglas® 2605SC cladding wires was evaluated upon exposure to lateral AC magnetic fields. An improved response was demonstrated for a sensing fiber with in-cladding wire following thermal magnetic annealing (~400 °C) under a constant static transverse magnetic field (~200 μT). A minimal detectable magnetic field of ~500 nT was confirmed for a sensing fiber with two 10 μm cladding wires. The successful demonstration of a magnetic field sensing fiber with Metglas® cladding wires fabricated via traditional draw processes sets the stage for distributed measurements and joint inversion as a compliment to distributed fiber optic acoustic sensors.

Humidity sensing characteristics of compact coupled optical fiber structure by microsphere and tapered capillary tube

In this paper, an optical fiber humidity sensor has been proposed based on the coupling structure using BaTiO3 microsphere and conical cavity of capillary pigtail. The sensitive material polyvinyl alcohol (PVA) was coated near the coupling region of microsphere and conical cavity to improve its sensing performance. The humidity sensitivity has been demonstrated to be 0.09 dBm/%RH based on intensity demodulation technique. The morphological parameters of the coupling structure are further optimized to obtain the maximum sensitivity of −0.297 dBm/%RH. Compared with other microsphere based optical fiber sensors, the sensor has the advantages of compact structure, simple manufacture, low production cost, high sensitivity and good stability.

Integration of Piezoelectric Energy Harvesting Systems into Building Envelopes for Structural Health Monitoring with Fiber Optic Sensing Technology

This paper presents a study about the integration of Piezoelectric Energy Harvesting Systems (PE-EHSs) into building envelopes for powering Fiber Bragg Grating (FBG) sensors, enabling efficient and low-consumption monitoring with the objective of leveraging structural health monitoring (SHM). The research includes preliminary tests conducted in a real environment to validate the PE-EHS when fully integrated into a ventilated façade, capturing mechanical vibrations generated mainly by wind loads. Based on these activities, the final configuration of PE-EHSs is defined to provide a complete system for façade monitoring. This integrated system includes the piezoelectric generator (PEG), supercapacitor (SC), Power Conditioner Circuit (PCC), Fiber Optic Sensing (FOS) interrogator, and the IoT gateway transmitting measurement data within an Internet of Things (IoT) monitoring platform. This configuration is tailored to address the challenges related to the structural integrity of building envelopes. Results demonstrate a potential for a stand-alone solution in the façade sector but raise issues for certain limitations, requiring further investigation. In particular, the study emphasizes constraints related to the energy production of PE-EHSs for façade integration. It highlights the necessity to carefully consider these limitations within the broader context of their applicability, providing insights for the informed deployment of piezoelectric energy harvesting technology in building envelope monitoring.

Reflection based silicon incorporated silver coated fiber optic SPR sensor for refractive index and temperature measurement

Reflection based silicon incorporated silver coated fiber optic SPR sensor for refractive index and temperature measurement

Generalized method for distributed detection and quantification of cracks in bridges

The development of a generalized machine learning approach based on distributed detection and quantification of cracks by optical fibers is described in this article. A Brillouin scattering optical fiber sensor system was employed to develop, test, and verify the method. The main components of the approach described herein consist of an unsupervised crack identification module based on the iForest algorithm and a crack quantification component by the one-dimensional convolutional neural network method. The main attribute of this model is the versatility for application in various types of structures. The proposed method does not require further application-dependent training or calibration as long as the structural applications employ the same optical fiber type and installation adhesives. The effectiveness of the proposed method was verified by two experiments involving a 15-m steel beam in the laboratory and monitoring a twin set of 332-m-long, five-span continuous box girder concrete bridges. Regarding crack detection capabilities, it was possible to detect 107 out of 112 cracks in the laboratory beam and 20 out of the 21 in the bridges. The resolution of crack opening displacements for the steel beam and concrete bridges were 20.6 and 21.7 µm, respectively. The verification experiments further indicated the generality of the approach in applications to various types of structures and materials.

Strain Monitoring and Installation Adjustment of Satellite–Rocket Connection Device Based on Distributed Optical Fibers

In this study, a distributed optical fiber sensor was used for strain monitoring and installation adjustment of a satellite–rocket connection device under a preload. The distributed optical fiber sensor was installed on both the tape and the satellite docking frame, utilizing OFDR (Optical Frequency Domain Reflectometry) based on Backward Rayleigh scattering strain demodulation methods to precisely measure the strain distribution of both components when subjected to a preload. In order to deal with the uneven stress of the belt in the process of preloading, a finite element analysis was performed to obtain the strain distribution of the belt under preloading. The strain monitoring results of the optical fiber and strain gauge were compared, and the strain trend of the finite element simulation results was verified. Finally, the measured strain data were adopted to assist the installation and adjustment of the satellite–rocket connection device to achieve a uniform distribution of the preload. The experimental results showed that the standard deviation of strain at each position of the tape was reduced after adjustment. This study provides guidance for the installation of satellite–rocket connection devices.

All-sapphire fiber-optic sensor for the simultaneous measurement of ultra-high temperature and high pressure

An all-sapphire fiber-optic extrinsic Fabry-Perot interferometric (EFPI) sensor for the simultaneous measurement of ultra-high temperature and high pressure is proposed and experimentally demonstrated. The sensor is fabricated based on all-sapphire, including a sapphire fiber, a sapphire capillary and a sapphire wafer. A femtosecond (fs) laser is employed to drill a through hole at the side wall of the sapphire capillary to allow gas flow. The sapphire fiber is inserted from one side of the sapphire capillary. The sapphire wafer is fixed at the other side of the sapphire capillary. The first Fabry-Perot (FP) cavity, composed of the end face of the sapphire fiber and the front surface of the sapphire wafer, is used for measuring pressure, while the second FP cavity, composed of the two surfaces of the sapphire wafer, is used for measuring temperature. Experimental results show that the sensor can simultaneously measure ultra-high temperature and gas pressure within the temperature range of 20 - 1400 °C and the pressure range of 0 - 5 MPa. The temperature sensitivity is 0.0033 µm/°C, and the pressure sensitivity decreases as the temperature increases, reaching 1.8016 µm/MPa and 0.3253 µm/MPa at temperatures of 20 °C and 1400 °C, 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.

Wide-range pH sensor based on a cascaded MZI up-down tapered hetero-core structure

In this paper, we describe the first demonstration of an optical fiber sensor based on two cascaded architectures of the Mach–Zehnder interferometer (MZI) with an up-down-tapered (UDT) hetero-core fiber structure for simultaneous pH measurement at two different spatial locations. The two fiber structures, namely structures I and II, were fabricated by sandwiching a 45 mm and 60 mm long piece of no-core fiber (NCF) between two single-mode fibers (SMFs), respectively. By inserting a down-taper between two adjacent up-tapers in the NCF section using the over-fusion splicing technique, the UDT hetero-core fiber structure was achieved. The down-taper works as an optical attenuator, while the two up-tapers each function as a fiber splitter/combiner. By cascading the two sensor structures I and II, two distinct interference dips were obtained, resulting in dual sensing points (so-called sensing points I and II). According to experimental results, it was shown that the proposed sensing points I and II are pH-sensitive, and exhibit sensitivities of 1.0428 nm/pH and −1.7857nm/pH for simultaneous measurement for each point as the pH ranged from 8 to 14 and from 1 to 7 pH, respectively. The obtained results show that the proposed dual point pH sensor has the potential to be used for the simultaneous detection of pH parameters in any environment and at various places.

Inverse finite element method with energy-based regularization for deformation reconstruction and structural health monitoring

Distributed Fiber Optical Sensing (DFOS) can be used to perform compact, low-invasive and large-scale strain measurements, providing an important tool for strain-based deformation reconstruction and Structural Health Monitoring (SHM). However, most current deformation reconstruction methods are difficult to apply to the sparse uniaxial strain measurement conditions associated with DFOS, especially for structures with complex geometries. This work introduces an energy-based regularization method, i.e., a priori assumption that the actual deformation state is close to the state with the minimum deformation energy under the premise of satisfying the strain constraints, into inverse finite element method to perform load-independently deformation reconstruction under the condition of sparse uniaxial strain measurement. The Inverse Finite Element Method with Energy-based Regularization (iFEM-EBR) can naturally construct the DFOS-based deformation reconstruction model with quasi-forward finite element analysis without introducing additional mathematical processing. The sensitivity of this method to structural characteristics, such as material parameters, makes it a unique prospect in SHM.

Four-wavelength laser interferometry for the demodulation of dual-cavity fiber-optic extrinsic Fabry-Perot interferometric sensors

A four-wavelength passive demodulation algorithm is proposed and experimentally demonstrated for the interrogation of the one cavity in a dual-cavity extrinsic Fabry-Perot interferometric (EFPI) sensor. The lengths of two cavities are adjusted to generate four quadrature signals for each individual cavity. Both simulation and experimental results are presented to validate the performance of this technique. The experimental results demonstrate that dynamic signals at frequencies of 100 Hz, 200 Hz, and 300 Hz with varying amplitude are successfully extracted from a dual-cavity EFPI sensor with initial lengths of 93.4803 µm and 94.0091 µm. The technique shows the potential application to measure dynamic signals in dual-cavity fiber-optic EFPI sensors.

Nonlinear error elimination using the fusion of PGC-DCM, geometric fitting, and Atan algorithms.

A phase generated carrier (PGC) demodulation scheme is always accompanied by nonlinear errors. We propose a fusion of PGC differential and cross multiplying (PGC-DCM), geometric fitting, and arctangent (Atan) algorithms for fiber optic interferometric sensors to eliminate nonlinear errors. The output amplitude of the PGC-DCM algorithm is used to judge whether the Lissajous figure of quadrature signals is larger than 1/2 ellipse arc. When the Lissajous figure exceeds 1/2 ellipse arc, the contaminated quadrature signals are corrected by the ellipse correction parameters calculated from the geometric fitting; otherwise, the previous fitting parameters are employed for correction. Geometric fitting is realized by minimizing the Sampson error, and its failure problem under small signals is solved by using the temporary stability of fitting results. Finally, desired signals are extracted from the corrected quadrature signals by the Atan algorithm. Experimental results show that the fusion combines the merits of the three algorithms and expands the application of the geometric fitting in PGC demodulation schemes.

Soft Polymer Optical Fiber Sensors for Intelligent Recognition of Elastomer Deformations and Wearable Applications.

In recent years, soft robotic sensors have rapidly advanced to endow robots with the ability to interact with the external environment. Here, we propose a polymer optical fiber (POF) sensor with sensitive and stable detection performance for strain, bending, twisting, and pressing. Thus, we can map the real-time output light intensity of POF sensors to the spatial morphology of the elastomer. By leveraging the intrinsic correlations of neighboring sensors and machine learning algorithms, we realize the spatially resolved detection of the pressing and multi-dimensional deformation of elastomers. Specifically, the developed intelligent sensing system can effectively recognize the two-dimensional indentation position with a prediction accuracy as large as ~99.17%. The average prediction accuracy of combined strain and twist is ~98.4% using the random forest algorithm. In addition, we demonstrate an integrated intelligent glove for the recognition of hand gestures with a high recognition accuracy of 99.38%. Our work holds promise for applications in soft robots for interactive tasks in complex environments, providing robots with multidimensional proprioceptive perception. And it also can be applied in smart wearable sensing, human prosthetics, and human-machine interaction interfaces.

Experimental study on progressive interfacial mechanical behaviors using fiber optic sensing cable in frozen soil

Frost heave and thaw settlement in cold regions pose a significant threat to engineering construction. Optical frequency domain reflectometry (OFDR) based on Rayleigh scattering can be applied to monitor ground deformation in frozen soil areas, where the interface behavior of soil-embedded fiber optic sensors governs the monitoring accuracy. In this paper, a series of pullout tests were conducted on fiber optic (FO) cables embedded in the frozen soil to investigate the cable‒soil interface behavior. An experimental study was performed on interaction effects, particularly focused on the water content of unfrozen soil, freezing duration, and differential distribution of water content in frozen soil. The high-resolution axial strains of FO cables were obtained using a sensing interrogator, and were used to calculate the interface shear stress. The interfacial mechanical response was analytically modeled using the ideal elasto‒plastic and softening constitutive models. Three freezing periods, correlating with the phase change process between ice and water, were analyzed. The results shows that the freezing effect can amplify the peak shear stress at the cable-soil interface by eight times. A criterion for the interface coupling states was proposed by normalizing the pullout force‒displacement information. Additionally, the applicability of existing theoretical models was discussed by comparing the results of theoretical back‒calculations with experimental measurements. This study provides new insights into the progressive interfacial failure behavior between strain sensing cable and frozen soil, which can be used to assist the interpretation of FO monitoring results of frozen soil deformation.