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During recent years, the optical-fiber-based simultaneous sensing of strain and temperature has attracted increased interest for different applications, e.g., in medicine, architecture, and aerospace. Specialized fiber layouts further enlarge the field of applications at much lower costs and with easier handling. Today, the performance of many sensors fabricated from conventional fibers suffers from cross-sensitivity (temperature and strain) and relatively high interrogation costs. In contrast, customized fiber architectures would make it possible to circumvent such sensor drawbacks. Here, we report on the development of a high-quality coupled-core fiber and its performance for sensors—from the initial fiber layout via elaboration of the preform and fiber up to the sensor evaluation. A compact, high-speed, and cost-effective interrogation unit using such a specialized coupled-core fiber has been designed to monitor reflectivity changes while even being able to distinguish the direction of the force or impact. Several fiber core material techniques and approaches were investigated, which made it possible to obtain a sufficient volume of material for the required fiber core number and a specialized fiber core geometry in terms of core distances and radial refractive index profile, whilst handling the non-symmetrical fiber architectures of such modeled, complex structures and balancing resources and efforts. [ABSTRACT FROM AUTHOR]

Modern spacecraft and launch vehicle design is more oriented towards reducing system-level design and assembly complexities. In order to maintain high overall system performance while reducing these complexities, the use of smart materials and smart structural components is a well-known practice and is currently of rising interest to space systems' designers. The paper discusses a concept of smart space structures, in particular, a carbon fiber composites structure embedded with Optical Fiber Sensors (OFS) for spacecraft and launch vehicle applications. This study highlights the operational requirements for such tank and the smart features enabled by the optical fiber sensors. For the latter aspect, a quantitative comparison between Fiber Bragg Grating sensors (FBGs) and Distributed Optical Fiber Sensors (DOFS) based on Optical Frequency Domain Reflectometry (OFDR) is presented to state their core performance parameters, such as the sensitivity, sensing range, dynamic measurement capability, and spatial resolution. The increased performance and reliability in harsh environments associated with fiber optic sensors come with a reduction in size, mass, and power consumption compared to the conventional electronic sensors. Optical fiber sensors embedded in carbon fiber structures have proven their capability in providing accurate real-time measurements of temperature and monitoring structural integrity while detecting precisely possible points of rupture and failure as discussed and demonstrated in the literature review. The applications of fiber optic sensing in smart propellant tanks may extend to detecting fluid leakage, also providing increased precision in propellant gauging through temperature mapping, and can be used in on-ground qualification, pre-flight testing, as well as in-orbit operation, condition, and structural health monitoring. The article presents a statement for an optimal FOS embedding approach in composite pressure vessels and discusses the related placement and orientation method for the fiber optic sensors, coupled with a one component simplified analytical stress-strain transfer model deriving the stress component along the maximum principal direction (i.e., σ Max P r i n c i p a l ). The novel approach is believed to serve the optimal employment of embedded FOS in composite structures, e.g., pressure vessels and light-weight structures in spacecraft, among other applications. The simplified model is believed to pave the way for effective data interpretation and processing, utilizing the available limited computational resources on-board the spacecraft. • Smart propellant tank embedding fiber optic sensors for structural health monitoring. • The opto-mechanical model defines the local stress in the maximum principal strain direction. • Quasi-distributed optic fiber sensors employing arrays of FBGs for spacecraft monitoring. • Distributed fiber optic sensing based on OFDR for spacecraft applications. [ABSTRACT FROM AUTHOR]

This article presents a comprehensive study on the impact of irradiation optical fiber cores with a femtosecond-pulsed laser, operating at a wavelength of 1030 nm, on the signal amplitude in Rayleigh scattering-based optical frequency domain reflectometry (OFDR). The experimental study involves two fibers with significantly different levels of germanium doping: the standard single-mode fiber (SMF-28) and the ultra-high numerical aperture fiber (UHNA7). The research findings reveal distinct characteristics of reflected and scattered light amplitudes as a function of pulse energy. Although different amplitude changes are observed for the examined fibers, both can yield an enhancement of amplitude. The paper further investigates the effect of fiber Bragg grating inscription on the overall amplitude of reflected light. The insights gained from this study could be beneficial for controlling the enhancement of light scattering amplitude in fibers with low or high levels of germanium doping. [ABSTRACT FROM AUTHOR]

Over the past decade, distributed acoustic sensing has been utilized for structural health monitoring in various applications, owing to its continuous measurement capability in both time and space and its ability to deliver extensive data on the conditions of large structures using just a single optical cable. This work aims to evaluate the performance of distributed acoustic sensing for monitoring a multilayer structure on a laboratory scale. The proposed structure comprises four layers: a medium-density fiberboard and three rigid polyurethane foam slabs. Three different damages were emulated in the structure: two in the first layer of rigid polyurethane foam and another in the medium-density fiberboard layer. The results include the detection of the mechanical wave, comparing the response with point sensors used for reference, and evaluating how the measured signal behaves in time and frequency in the face of different damages in the multilayer structure. The tests demonstrate that evaluating signals in both time and frequency domains presents different characteristics for each condition analyzed. The supervised support vector machine classifier was used to automate the classification of these damages, achieving an accuracy of 93%. The combination of distributed acoustic sensing with this learning algorithm creates the condition for developing a smart tool for monitoring multilayer structures. [ABSTRACT FROM AUTHOR]

Structural health monitoring (SHM) is crucial for preventing and detecting corrosion, leaks, and other risks in reinforced concrete (RC) structures, ensuring environmental safety and structural integrity. Optical fiber sensors (OFS), particularly long-period fiber gratings (LPFG), have emerged as a promising method for SHM. Various LPFG sensors have been widely used in SHM due to their high sensitivity, durability, immunity to electromagnetic interference (EMI) and compact size. This review explores recent advancements in LPFG sensors and offers insights into their potential applications in SHM. [ABSTRACT FROM AUTHOR]

Structural health monitoring (SHM) is crucial for preventing and detecting corrosion, leaks, and other risks in reinforced concrete (RC) structures, ensuring environmental safety and structural integrity. Optical fiber sensors (OFS), particularly long-period fiber gratings (LPFG), have emerged as a promising method for SHM. Various LPFG sensors have been widely used in SHM due to their high sensitivity, durability, immunity to electromagnetic interference (EMI) and compact size. This review explores recent advancements in LPFG sensors and offers insights into their potential applications in SHM.

Optical fiber sensors consist of multiple Mach–Zehnder (MZ) interferometers and are common in the protection of different compounds. These sensors are very sensitive to any intrusion or threat. However, the spatial resolution is proportional to the number of MZ interferometers along the sensor. Consequently, a long sensor with a high resolution can be costly. In this paper, we suggest replacing the cascaded MZ interferometers with a couple of adjacent fibers, each of which have a harmonically varying refractive index. In this theoretical study, it is shown that two fibers with varying refractive indices demonstrate a sensitivity equivalent to that of multiple MZ interferometers. Furthermore, when the coupling coefficient between the fibers is weak, an analytical expression can be derived for the transmission between the fibers. This transmission reveals a quantization rule for which the light coupling between the two fibers vanishes. [ABSTRACT FROM AUTHOR]

We present a slope-assisted configuration for fiber-optic temperature sensing using multimodal interference (MMI) in a single-mode-multimode-single-mode structure. By measuring the power in the slope of the MMI spectrum, this method overcomes conventional limitations, enabling high-speed temperature measurement without the need for wavelength sweeping. The operational principles of this technique are described in detail, and a thorough evaluation of its limitations is conducted. [ABSTRACT FROM AUTHOR]

To facilitate the sensor fabrication and sensing operation in microstructured optical fiber-based surface plasmon resonance (SPR) sensors for high refractive index (RI) detection, we propose a special hollow fiber-based SPR sensor that comprises an opening on its body side and a thin gold layer coated on its outer surface. The analyte is able to flow into the hollow core through the side-opening to form new fiber core, with the Gaussian-like mode propagating in it. We investigate the sensing performance of the proposed sensor in a higher RI range of 1.48 to 1.54 at two feasible schemes: one is to only fill the fiber core with analyte (Scheme A), and the other is to directly immerse the sensor in the analyte (Scheme B). The results demonstrate that our sensor exhibits higher wavelength sensitivity at Scheme A with a maximum wavelength sensitivity of 12,320 nm/RIU, while a greater amplitude sensitivity was found at Scheme B with a maximum amplitude sensitivity of 1146 RIU−1. Our proposed sensor features the advantages of simple fabrication, flexible operation, easy analyte filling and replacing, enhanced real-time detection capabilities, high RI detection, and very high wavelength sensitivity and amplitude sensitivity, which makes it more competitive in SPR sensing applications. [ABSTRACT FROM AUTHOR]

In this study, a surface plasmon resonance (SPR) sensor based on modified plastic optical fibers (POFs) was combined with a specific molecularly imprinted polymer (MIP), used as a synthetic receptor, for glyphosate (GLY) determination in aqueous solutions. Since GLY is a non-selective herbicide associated with severe environmental and health problems, detecting glyphosate in environmental and biological samples remains challenging. The selective interaction between the MIP layer and GLY is monitored by exploiting the SPR phenomenon at the POF's gold surface. Experimental results show that in about ten minutes and by dropping microliter volume samples, the presented optical–chemical sensor can quantify up to three orders of magnitude of GLY concentrations, from nanomolar to micromolar, due to a thin MIP layer over the SPR surface. The developed optical–chemical sensor presents a detection limit of about 1 nM and can be used for onsite GLY measurements. Moreover, the experimental analysis demonstrated the high selectivity of the proposed POF-based chemical sensor. [ABSTRACT FROM AUTHOR]

A highly sensitive D-shaped microfluidic channel (MFC) incorporated in a single-mode fiber (SMF) equipped with a tungsten trioxide ( W O 3 ) layer coated with gold (Au) or silver (Ag) on the base of the MFC is designed and investigated utilizing a finite element method (FEM) to obtain the sensing performance of the optical fiber surface plasmon resonance (SPR) structure. The proposed sensor is appropriate for both bimetallic layers and analytes. With the sizeable cladding diameter of SMF, the fluidic channel size can be changed following demand, making fabrication more accessible and applicable. The hollow D-section above the core deposited a bimetallic structure of silver (Ag) or gold (Au) as plasmonic materials, followed by a tungsten trioxide ( W O 3 ) nanolayer. This optical fiber biochemical monitors the changes in the refractive index (RI) by measuring the transmission spectral shifts of the fiber at their resonance wavelengths. The W O 3 layer protects the plasmonic material from oxidation and effectively enhances the surface plasmon wave (SPW) while interacting with free electrons. Upon achieving the optimum structural parameters, the numerical investigation results showed that the maximum wavelength sensitivity of 6.0 μ m//RIU and 5.0 μ m//RIU for Au/ W O 3 and Ag/ W O 3 were obtained, with the coefficient of performance of 50.12 R I U - 1 and 134.84 R I U - 1 , respectively, for ultra-wide sensing ranges from 1.28 to 1.38. Also, the simulation outcomes revealed that the tungsten trioxide-coated with Ag or Au layers significantly enhanced the sensor. Finally, the numerical results of this study showed that the proposed sensor has substantial possibilities for large sensing RI applications such as biology and chemistry analytes, and it is also valid for other lower RI analyte sensing requirements as a consequence of its features such as high sensitivity, simple design, cost-effective design, and promising linear sensing performance. [ABSTRACT FROM AUTHOR]

Due to the benefits of the high sensitivity, real-time response, no labeling requirement, and good selectivity, fiber optic sensors based on surface plasmon resonance (SPR) have gained popularity in biochemical sensing in recent years. The current research on such sensors is hot in enhancing sensitivity, improving detection accuracy, and achieving the detection of biochemical molecules. The goal of this work is to present a thorough overview of recent developments in the optical fiber SPR biosensor research. Firstly, it explores the basic principles and sensing structures of optical fiber SPR biosensors, focusing on four aspects. Subsequently, this paper introduces three fiber optic surface plasmon biosensors: SPR, localized surface plasmon resonance (LSPR), and long-range surface plasmon resonance (LRSPR). Each concept is explained from the perspective of the basic principles of fiber optic SPR biosensors. Furthermore, a classification of fiber optic SPR biosensors in health monitoring, food safety, environmental monitoring, marine detection, and other applications is introduced and analyzed. Eventually, this paper summarizes the current research directions of SPR biosensors. Meanwhile, it provides a prospective outlook on how fiber optic SPR sensors will develop in the future. [ABSTRACT FROM AUTHOR]

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We investigate a simple structure in which a polarization-maintaining fiber (PMF) is sandwiched between two single-mode fibers (SMFs). By injecting broadband light and observing the modal interference spectrum of the transmitted light in the 870 nm range, we identify a clear shift dependent on the strain applied to the PMF. In addition, we demonstrate that in the same wavelength range, the strain applied to the SMFs before and after the PMF does not influence the interference spectrum. This result confirms that only the PMF segment exhibits strain sensitivity. [ABSTRACT FROM AUTHOR]

Abstract Due to the benefits of the high sensitivity, real-time response, no labeling requirement, and good selectivity, fiber optic sensors based on surface plasmon resonance (SPR) have gained popularity in biochemical sensing in recent years. The current research on such sensors is hot in enhancing sensitivity, improving detection accuracy, and achieving the detection of biochemical molecules. The goal of this work is to present a thorough overview of recent developments in the optical fiber SPR biosensor research. Firstly, it explores the basic principles and sensing structures of optical fiber SPR biosensors, focusing on four aspects. Subsequently, this paper introduces three fiber optic surface plasmon biosensors: SPR, localized surface plasmon resonance (LSPR), and long-range surface plasmon resonance (LRSPR). Each concept is explained from the perspective of the basic principles of fiber optic SPR biosensors. Furthermore, a classification of fiber optic SPR biosensors in health monitoring, food safety, environmental monitoring, marine detection, and other applications is introduced and analyzed. Eventually, this paper summarizes the current research directions of SPR biosensors. Meanwhile, it provides a prospective outlook on how fiber optic SPR sensors will develop in the future.

A novel approach to the development of Distributed Temperature-Sensing (DTS) systems based on Raman Scattering in Multimode optical fibers operating at around 800 nm is presented, focusing on applications requiring temperature profile measurement in the range of a few hundreds of meters. In contrast to the standard Raman DTS systems, which aim to shorten the pulse space width as much as possible to improve the precision of measurement, the novel approach studied in this work is based on the use of pulses with a space width that is approximately equal to the distance covered by the fiber under test. The proposed technique relies on numerical post-processing to obtain the temperature profile measurement with a precision of about ± 3   ° C and a spatial resolution of 8 m, due to the transaction phases of the optical pulses. This solution simplifies the electronic circuit development, also minimizing the required laser peak power needed compared to the typical narrow pulse techniques. [ABSTRACT FROM AUTHOR]

Optical fiber sensors have been studied, developed, and already used in the industry for more than 50 years due to their multiplexing capabilities, lightweight design, compact form factors, and electromagnetic field immunity. The scientific community continuously studies new materials, schemes, and architectures aiming to improve existing technologies. Navigating through diverse sensor technologies, including interferometry, intensity variation, nonlinear effects, and grating-based sensors, fiber specklegram sensors (FSSs) emerge as promising alternatives due to their simplicity and low cost. This review paper, emphasizing the potential of FSSs, contributes insights to the present state and future prospects for FSSs, providing a holistic view of advancements propelling FSSs to new frontiers of innovation. Subsequent sections explore recent research, technological trends, and emerging applications, contributing to a deeper understanding of the intricacies shaping the future of FFS sensor technologies. [ABSTRACT FROM AUTHOR]

Abstract Human tactile perception involves the activation of mechanoreceptors located within the skin in response to external stimuli, along with the organization and processing within the brain. However, human sensations may be subject to the issues related to some physiological factors (such as skin injury or neurasthenia), resulting in inability to quantify tactile information. To address this challenge, a novel bio‐inspired artificial tactile (BAT) sensing system enabled by the integration of optical microfiber (OM) with full‐connected neural network (FCNN) in this paper is demonstrated, inspired by human physiological characteristics and tactile mechanisms. In this system, the BAT sensor mimics human skin, where the OM serves as the mechanoreceptor for sensing tactile stimuli, while the FCNN functions as a simulated human brain to train and extract the signal characteristics for intelligent object recognition. The experimental results indicate that the proposed BAT sensor can sensitively respond to both the contact force (static tactile stimuli), as well as the vibrotactile events (dynamic tactile stimuli) for the recognition of regular textures. Furthermore, by integrating the trained FCNN, the BAT sensing system accurately identifies various intricate surface textures with an exceptional accuracy of 95.7%, highlighting its potential in next‐generation human‐machine interaction and advanced robotics.

In this paper, an intra-cavity SPR sensor based on erbium-doped fiber (EDF) laser is proposed to achieve higher sensitivity for detecting analyte refractive indices. A liquid-core fiber-based SPR sensor is first designed to match the operating wavelength of 1530 nm in the fiber laser intra-cavity, and then placed into the cavity of the EDF laser to constitute the all-fiber intra-cavity SPR sensor. We theoretical investigate the influence of pump power, cavity loss and EDF length on the sensitivity of the proposed intra-cavity SPR sensor. The results indicate that the sensitivity of the intra-cavity SPR sensor can reach 1.051 × 109 RIU−1 (refractive index unit), which is enhanced 4 × 106 times compared with the sensitivity of 253 RIU−1 obtained by the SPR sensor without fiber laser cavity. In addition, the sensitivity of the proposed intra-cavity SPR sensor can be flexibly tuned by changing pump power, cavity loss or EDF length. This work demonstrates that as an alternative approach, the combination of the fiber laser intra-cavity spectroscopy and the fiber-based SPR sensor can achieve a remarkable enhancement in sensitivity.

This paper reports a novel Fabry-Perot fiber optic microphone based on Au/PI composite film, with an optimal F-P cavity length of $500 \, \mu \text{m}$ , and the maximum extinction ratio (ER) obtained is about 12 dB. Besides, the sensor has flat response performance in the audio frequency range of 200–10 kHz, with a maximum signal-to-noise ratio (SNR) of 76 dB and a sensitivity of approximately 170 mV/Pa at the sound pressure level of 1000 Hz/94 dB, which is much higher than Brüel & Kjær’s electric microphone 4939. Within a certain range of sound pressure, there is a fine linear relationship between the output response of the sensor and the input sound pressure, with a fitting coefficient of 0.9987. The new diaphragm-type fiber optic microphone proposed in our work has superb economy, sensitivity, frequency response and linearity within the audible sound range, which is expected to achieve large-scale, low-cost fabrication and long-term stable application in audio detection and other fields.

During recent years, the optical-fiber-based simultaneous sensing of strain and temperature has attracted increased interest for different applications, e.g., in medicine, architecture, and aerospace. Specialized fiber layouts further enlarge the field of applications at much lower costs and with easier handling. Today, the performance of many sensors fabricated from conventional fibers suffers from cross-sensitivity (temperature and strain) and relatively high interrogation costs. In contrast, customized fiber architectures would make it possible to circumvent such sensor drawbacks. Here, we report on the development of a high-quality coupled-core fiber and its performance for sensors—from the initial fiber layout via elaboration of the preform and fiber up to the sensor evaluation. A compact, high-speed, and cost-effective interrogation unit using such a specialized coupled-core fiber has been designed to monitor reflectivity changes while even being able to distinguish the direction of the force or impact. Several fiber core material techniques and approaches were investigated, which made it possible to obtain a sufficient volume of material for the required fiber core number and a specialized fiber core geometry in terms of core distances and radial refractive index profile, whilst handling the non-symmetrical fiber architectures of such modeled, complex structures and balancing resources and efforts.

Over the past decade, distributed acoustic sensing has been utilized for structural health monitoring in various applications, owing to its continuous measurement capability in both time and space and its ability to deliver extensive data on the conditions of large structures using just a single optical cable. This work aims to evaluate the performance of distributed acoustic sensing for monitoring a multilayer structure on a laboratory scale. The proposed structure comprises four layers: a medium-density fiberboard and three rigid polyurethane foam slabs. Three different damages were emulated in the structure: two in the first layer of rigid polyurethane foam and another in the medium-density fiberboard layer. The results include the detection of the mechanical wave, comparing the response with point sensors used for reference, and evaluating how the measured signal behaves in time and frequency in the face of different damages in the multilayer structure. The tests demonstrate that evaluating signals in both time and frequency domains presents different characteristics for each condition analyzed. The supervised support vector machine classifier was used to automate the classification of these damages, achieving an accuracy of 93%. The combination of distributed acoustic sensing with this learning algorithm creates the condition for developing a smart tool for monitoring multilayer structures.

Temperature and refractive index are two important parameters for many fields, where their accurate measurement is crucial. This review discusses the development of refractive index and temperature dual-parameter fiber sensors based on different interferometric structures in recent years, such as the Mach–Zehnder, Michelson or Fabry–Perot interferometers, which are composited by the grating reflectors, and multi-separated-region based on thin film layers and special structures. The working principle and performance of different types of sensors are analyzed. This review can provide the technical references for the development of novel dual-parameter sensors with practical potential and high performance. [ABSTRACT FROM AUTHOR]

Background: Subsea power cable failures in offshore wind farms result in significant financial losses. One common failure mode is submarine power cable bending. Objective: The primary objective of this study is to validate two analytical models using strain readings obtained from a novel 3-point bending setup designed for power cable specimens. The setup incorporates two types of optical fiber sensors for simultaneous strain measurement. Methods: A 3-point bending setup is constructed, integrating optical fiber sensors installed on the embedded fiber optic cable within the submarine power cable. One set of sensors is fixed to the fiber optic cable sheath, while a second set consists of loose tube fibers that are inside the fiber optic cable. The strain readings of the fixed sensors are compared to two analytical models. The first analytical model assumes a constant power cable curvature, while the second model considers variable curvature. Results: The analytical models both predict nearly flat strain profiles and are in line with each other. The strain data, however, approaches zero strain away from the cable center. Model assumptions such as perfect sensor positioning and zero slip of the fiber optic cable cause this discrepancy. The results of the constant curvature model agree well with strain averages of the fixed sensors around the central region of the power cable, and both scale linearly with amplitude. Finally, the strain readings from the loose tube fibers demonstrate high reproducibility, facilitating the development of a calibration curve for estimating power cable curvature. Conclusions: The analytical models surpass existing models by providing good agreement with the measured strain around the cable center. Moreover, the highly reproducible strain readings from the loose tube fibers allow estimating power cable curvature. [ABSTRACT FROM AUTHOR]

The advantages of optical fiber sensors include their miniaturization, strong anti-interference ability, high sensitivity, low cost, and fast response speed. They can be used for in situ detection in harsh environments, making them suitable for a wide range of applications such as blood detection and monitoring. This technology holds great potential for medical diagnosis and health monitoring, opening up new possibilities in the field. Coating technology plays a crucial role in enhancing the sensitivity and stability of optical fiber sensors, ultimately improving their measurement accuracy and reliability. This manuscript expounds the application status and progression of optical fiber sensors in the determination of blood glucose concentrations, blood pH, diverse proteins in blood, and physical properties of blood. The principle of optical fiber sensors and the application of coating technology for detecting varying targets are scrutinized in detail, with particular emphasis on the advantages and limitations of distinct design schemes. The adept amalgamation of optical fiber sensing technology and coating technology amplifies the adaptability of optical fiber sensors in diverse practical scenarios, thereby presenting novel instruments and methodologies for researchers in pertinent fields to augment their advancement and development. [ABSTRACT FROM AUTHOR]

This paper describes design, theoretical analysis, and experimental evaluation of a π-Phase-Shifted Fiber Bragg Grating (π-PSFBG) inscribed in the standard telecom fiber for slow light generation. At first, the grating was designed for its use in the reflection mode with a central wavelength of 1552 nm and a pass band width of less than 100 pm. The impact of fabrication imperfections was experimentally investigated and compared to model predictions. The optical spectra obtained experimentally show that the spectral region used for slow light generation is narrower (less than 10 pm), thus allowing for too-low levels of slow light optical-output power. In the next step, the optimization of the grating design was conducted to account for fabrication errors, to improve the grating's spectral behavior and its temporal performance, and to widen the spectral interval for slow light generation in the grating's transmission mode. The targeted central wavelength was 1553 nm. The π-PSFBG was then commercially fabricated, and the achieved parameters were experimentally investigated. For the region of (1551–1554) nm, a 15-fold increase in the grating's pass band width was achieved. We have shown that a pair of retarded optical pulses were generated. The measured group delay was found to be ~10.5 ps (compared to 19 ps predicted by the model). The π-PSFBG operating in its transmission mode has the potential to operate as tunable delay line for applications in RF photonics, ultra-fast signal processing, and optical communications, where tunable high precision delay lines are highly desirable. The π-PSFBG can be designed and used for the generation of variable group delays from tens to hundreds of ps, depending on application needs. [ABSTRACT FROM AUTHOR]

Elastomeric polymer optical fiber (EPOF) enabled flexible optical sensors have been attracting intensive research interest due to their excellent stretchability, sensing performance, and anti‐electromagnetic interference. However, a soft EPOF without effective protection tends to be damaged, which may undermine its long‐term stability, and thus, limit its practical applications. In this work, a strategy for packaging a section of EPOF is proposed, a light‐emitting diode (LED), and a photodiode (PD) into a stretchable and flexible silicone patch for both healthcare and human‐machine interaction applications. The silicone package not only protects the EPOF from environmental disturbance, providing a compact sensing approach for conformal attachment on flexible or curved surfaces but also offers an effective manner to adjust the sensitivity and working range of the EPOF sensors. Typically, an EPOF sensor is sensitive to pressure, strain, or bending. As a proof‐of‐concept demonstration, a laryngoscope equipped with a packaged EPOF sensor that can bear a pressure of 60 MPa is developed. It can effectively avoid teeth damage by real‐time monitoring the contact force during the operation. Furthermore, a data glove with five packaged EPOF sensors for hand gesture recognition and remote control of a micro‐vehicle is fabricated. [ABSTRACT FROM AUTHOR]

We propose an air gap fiber Bragg grating (g-FBG) sensor that can measure strain and temperature simultaneously. The sensor is made by aligning two fiber Bragg gratings (FBGs), and an air gap exists between these two sub-gratings. This sensor's architecture allows it to form a spectrum with phase-shifted fiber Bragg grating (PSFBG) spectroscopy and Fabry–Perot interference (FPI) spectroscopy. Since the sensitivity of PSFBG and FPI spectra is different for strain and temperature, it is possible to measure both strain and temperature by measuring one of the reflected dips of PSFBG and the interference dip of FPI. The experimental results show that the strain sensitivity is about 11.95 pm/με via the dip wavelength detection of FPI, and the temperature sensitivity is about 9.64 pm/°C via the dip wavelength detection of PSFBG. The g-FBG sensor demonstrates a resolution of approximately ±3.7 με within the strain range of 0 to 1000 με and about ±0.6 °C within the temperature range of 25 °C to 120 °C. The proposed g-FBG sensor, characterized by its simple structure, compact size, and cost-effectiveness, exhibits significant potential in the field of multi-parameter measurements. [ABSTRACT FROM AUTHOR]

This study provides a review of the literature and an overview of optical fibers, as it is considered the highest class of intelligent materials, and there are many varieties and divisions based on the requirements of the manufacturer, it will review Optic Fiber Sensors (OFS) for measuring temperature and humidity. It has been divided into four main sections: The end user, and the environment. OFS Application in Healthcare, Impact OFS Qatar, Effect of chemical and mechanical properties of OFS Effect of external temperature changes in Surface Plasmon Resonance (SPR) for OFS Due to its unique characteristics, such as small size, lack of electromagnetic interference, high sensitivity, weights that allow sensing, accuracy, and very high dynamic range, it generates a fiber-optic sensor that is easy to manufacture and operate at the lowest possible cost. In addition, the development of Dispersive Sensor Systems (DSS) and OFS has found applications in both healthcare settings and strategies in biomedical research and structural Health Care Monitoring (HCM) to enhance the adoption of fiber-based medical equipment. Optics are two areas in which OFS has yet to realize its huge potential fully. The most recent published studies (2010-2023) were our main focus except for a few cases. [ABSTRACT FROM AUTHOR]

Previous studies demonstrated coupling of acoustic guided waves from one optical fiber to another through a simple adhesive bond coupler. This paper experimentally utilizes such an adhesive bond coupler to easily extend an already existing sensor network. We experimentally demonstrate this concept for detecting simulated cracks growing from circular holes in a thin aluminum plate. A single, remotely bonded FBG sensor is used to detect the original crack growth, followed by the addition of other optical fiber segments using adhesive couplers to detect new crack growth locations on the plate. A laser Doppler vibrometer is also used to measure the guided wave propagation through the plate to verify that the changes in the FBG sensor measurements are due to the growth of the cracks. [ABSTRACT FROM AUTHOR]

This article proposes an unequivocal method of labeling and numbering the cladding modes propagating in single-mode opticalwaveguides with tilted periodic structures. The unambiguous determination of individual propagating modes in this type of optical fiber is crucial for their use in sensory systems. The selection of the appropriate spectral range and mode determines the sensitivity and measuring range of tilted fiber Bragg grating (TFBG) sensors. The measurement methods proposed by individual research teams using TFBGs as transducers are usually based on the selection of specific modes. Unification of the labeling of modes and their numbering enables comparison of the basic metrological parameters of individual measurement methods and reproduction and verification of the proposed sensors and methods in the laboratories of other scientific and research centers.

High-temperature superconductors such as REBa2Cu3O7-x (REBCO, RE = rare earth) can generate strong magnetic fields that are promising for applications in particle accelerators and compact fusion reactors. Traditionally, voltage taps are installed in superconducting magnets to measure the voltage signals due to resistive transitions. The voltage-tap-based diagnostics is important for the development of magnet technology as it can help pinpoint the locations in the magnet windings that limit the magnet performance. The architecture of the multi-tape REBCO cable such as CORC wires, however, makes it difficult to apply the voltage-tap-based diagnostics to identify the locations of resistive transitions. Distributed fiber optic sensing (DFOS) has the potential to address this issue. In this paper, we report the measurements of thermal strain along a CORC wire based on optical frequency domain reflectometry with a maximum spatial resolution of 0.65 mm and a temporal resolution of 10 Hz. The optical fiber is co-wound with the CORC wire that is epoxy impregnated. During the test, current was increased until a resistive transition occurred in the conductor. The spectrum shift of the reflected light along the fiber was recorded. The results suggested that with proper thermal isolation from the cryogen, DFOS can be used to identify the locations of resistive transitions in CORC wires and magnets. The results will allow a better understanding of the causes of resistive transitions in REBCO conductors and magnets, which will help improve the REBCO magnet technology.

Optical fiber sensors consist of multiple Mach–Zehnder (MZ) interferometers and are common in the protection of different compounds. These sensors are very sensitive to any intrusion or threat. However, the spatial resolution is proportional to the number of MZ interferometers along the sensor. Consequently, a long sensor with a high resolution can be costly. In this paper, we suggest replacing the cascaded MZ interferometers with a couple of adjacent fibers, each of which have a harmonically varying refractive index. In this theoretical study, it is shown that two fibers with varying refractive indices demonstrate a sensitivity equivalent to that of multiple MZ interferometers. Furthermore, when the coupling coefficient between the fibers is weak, an analytical expression can be derived for the transmission between the fibers. This transmission reveals a quantization rule for which the light coupling between the two fibers vanishes.

In this study, a surface plasmon resonance (SPR) sensor based on modified plastic optical fibers (POFs) was combined with a specific molecularly imprinted polymer (MIP), used as a synthetic receptor, for glyphosate (GLY) determination in aqueous solutions. Since GLY is a non-selective herbicide associated with severe environmental and health problems, detecting glyphosate in environmental and biological samples remains challenging. The selective interaction between the MIP layer and GLY is monitored by exploiting the SPR phenomenon at the POF’s gold surface. Experimental results show that in about ten minutes and by dropping microliter volume samples, the presented optical–chemical sensor can quantify up to three orders of magnitude of GLY concentrations, from nanomolar to micromolar, due to a thin MIP layer over the SPR surface. The developed optical–chemical sensor presents a detection limit of about 1 nM and can be used for onsite GLY measurements. Moreover, the experimental analysis demonstrated the high selectivity of the proposed POF-based chemical sensor.

To facilitate the sensor fabrication and sensing operation in microstructured optical fiber-based surface plasmon resonance (SPR) sensors for high refractive index (RI) detection, we propose a special hollow fiber-based SPR sensor that comprises an opening on its body side and a thin gold layer coated on its outer surface. The analyte is able to flow into the hollow core through the side-opening to form new fiber core, with the Gaussian-like mode propagating in it. We investigate the sensing performance of the proposed sensor in a higher RI range of 1.48 to 1.54 at two feasible schemes: one is to only fill the fiber core with analyte (Scheme A), and the other is to directly immerse the sensor in the analyte (Scheme B). The results demonstrate that our sensor exhibits higher wavelength sensitivity at Scheme A with a maximum wavelength sensitivity of 12,320 nm/RIU, while a greater amplitude sensitivity was found at Scheme B with a maximum amplitude sensitivity of 1146 RIU−1. Our proposed sensor features the advantages of simple fabrication, flexible operation, easy analyte filling and replacing, enhanced real-time detection capabilities, high RI detection, and very high wavelength sensitivity and amplitude sensitivity, which makes it more competitive in SPR sensing applications.

Neck and/or head movements play an important role in the control of assistive devices such as robotic wheelchairs, considering these systems allow the acquisition of intentionality information and conversion into control commands, which can improve the mobility and independence of people with severe disabilities. This work addresses a gap based on the development of Human–Machine Interfaces (HMIs) by introducing Roborueda, a robust Graphical User Interface (GUI) designed to facilitate the interaction of robotic wheelchair users, with a primary focus on enhancing mobility and preventing future diseases. The main objective of the system is to recognize head and neck movements using strategies based on inertial and Optical Fiber Sensors (OFSs) to generate commands that drive a robotic wheelchair. In addition, the system provides a Posture Recognition System that can help in the preventive treatment of pressure ulcers. The GUI was developed in Python programming language using Wi-Fi communication functionalities, which encourages open software distribution. The system, together with the GUI, was initially evaluated with healthy subjects, where the subsystem in charge of sensing and control obtained an accuracy close to 100% using fuzzy logic techniques, whereas posture recognition using OFS presented accuracy close to 97% using Machine Learning (ML) classifiers, highlighting the k-Nearest Neighbors. The system presented response time of less than 250 ms, which allows observing the feasibility of real-time implementation. Additionally, the subjects showed compliance and familiarity with the GUI, which allowed concluding that the software is user friendly and has potential use in the field of assistive technology and robotic rehabilitation. The implementation of this GUI marks a significant step towards integrating OFS, which contributes to the advancement of assistive technologies. To ensure the effectiveness and practicality of Roborueda, future work will focus on validating the system through extensive testing with wheelchair users, thereby obtaining valuable feedback and making further improvements related to posture control to prevent pressure ulcers and more personalized controllers, such as speed control.

Muscle fatigue is defined as a reduced ability to maintain maximal strength during voluntary contraction. It is associated with musculoskeletal disorders that affect workers performing repetitive activities, affecting their performance and well-being. Although electromyography remains the gold standard for measuring muscle fatigue, its limitations in long-term work motivate the use of wearable devices. This article proposes a computational model for estimating muscle fatigue using wearable and non-invasive devices, such as Optical Fiber Sensors (OFSs) and Inertial Measurement Units (IMUs) along the subjective Borg scale. Electromyography (EMG) sensors are used to observe their importance in estimating muscle fatigue and comparing performance in different sensor combinations. This study involves 30 subjects performing a repetitive lifting activity with their dominant arm until reaching muscle fatigue. Muscle activity, elbow angles, and angular and linear velocities, among others, are measured to extract multiple features. Different machine learning algorithms obtain a model that estimates three fatigue states (low, moderate and high). Results showed that between the machine learning classifiers, the LightGBM presented an accuracy of 96.2% in the classification task using all of the sensors with 33 features and 95.4% using only OFS and IMU sensors with 13 features. This demonstrates that elbow angles, wrist velocities, acceleration variations, and compensatory neck movements are essential for estimating muscle fatigue. In conclusion, the resulting model can be used to estimate fatigue during heavy lifting in work environments, having the potential to monitor and prevent muscle fatigue during long working shifts. [ABSTRACT FROM AUTHOR]

Laser spectroscopy has shown great potential as a cost‐effective method for trace gas detection with high sensitivity and specificity. However, it still falls short in applications such as the measurement of stable isotope ratios, which require high precision and stability. Here, ultrasensitive gas detection with remarkable precision and stability as well as better immunity to isobaric interference is demonstrated by use of low‐coherence photothermal interferometry. With a 10 cm long hollow‐core fiber, acetylene detection is achieved with a noise‐equivalent concentration of 0.7 part‐per‐billion and measurement precision of 0.025%. The instability of the detection over a period of 3 h is ±0.038%, 10 times better than the state‐of‐the‐art photothermal spectroscopy. The measurement of the 13C/12C isotope ratio of acetylene is demonstrated with measurement precision of ≈0.01%. This study highlights the potential of low‐coherence photothermal interferometry as an alternative to mass spectrometers, offering reliable gas detection in a compact form. [ABSTRACT FROM AUTHOR]

This study investigated the use of distributed optical fiber sensing to measure temperature and strain during thermomechanical processes in printed circuit board (PCB) manufacturing. An optical fiber (OF) was bonded to a PCB for simultaneous measurement of temperature and strain. Optical frequency-domain reflectometry was used to interrogate the fiber optic sensor. As the optical fiber is sensitive to both temperature and strain, a demodulation technique is required to separate both effects. Several demodulation techniques were compared to find the best one, highlighting their main limitations. The importance of good estimations of the temperature sensitivity coefficient of the OF and the coefficient of thermal expansion of the PCB was highlighted for accurate results. Furthermore, the temperature sensitivity of the bonded OF should not be neglected for accurate estimations of strains. The two-sensor combination model provided the best results, with a 2.3% error of temperature values and expected strain values. Based on this decoupling model, a methodology for measuring strain and temperature variations in PCB thermomechanical processes using a single and simple OF was developed and tested, and then applied to a trial in an industrial environment using a dynamic oven with similar characteristics to those of a reflow oven. This approach allows the measurement of the temperature profile on the PCB during oven travel and its strain state (warpage). [ABSTRACT FROM AUTHOR]

Under conventional temperature regimes, optical fiber sensors (OFS) have been extensively utilized for real-time monitoring of strain and temperature responses in transportation, energy, and civil engineering structures. However, in low-temperature environments, OFS applications confront challenges such as poor adhesion, diminished sensitivity, weak signal transmission, and complex compensation requirements. Therefore, it is crucial to conduct in-depth research on the high-precision calibration characteristics of strain and temperature in cryogenic conditions. In this study, we propose an enhanced calibration technique for fiber Bragg grating (FBG) strain/temperature sensitivity, covering a low-temperature range of 77–296 K. Employing a custom-designed low-temperature testing apparatus, we calibrated the thermo-optic coefficients, temperature, and strain sensitivity coefficients. Subsequently, this improved method was applied to measure the coefficient of thermal expansion of various materials. The results confirm the accuracy of the proposed method and present the strain/temperature measurement properties of FBG. This research provides guidance for the precise use of FBG in low-temperature environments. [ABSTRACT FROM AUTHOR]

This paper presents a detailed analysis of a plasmonic sensor in which a plasmonic wire-grid structure is integrated into a hollow core fiber (HCF). The sensor has the capability to measure refractive indices (RIs) greater than that of the fiber material. The use of the HCF enhances the sensitivity and precision of the sensor as it allows for the efficient interaction of light with the sensing region of the wire-grid-based plasmonic structure, in comparison to a single plasmonic wire placed inside an HCF. We numerically investigate the optical properties of the sensor using the finite element method (FEM).The sensor exhibits high sensitivity, with an amplitude sensitivity of 673.97 RIU−1, a wavelength sensitivity of 15000 nm/RIU, and a resolution of 6.67 × 10−06 RIU, enabling precise detection of even small alterations in the RI of analyte. The sensor shows RI sensing range of 1.45 to 1.66, which is well-suited for a broad spectrum of uses, including the monitoring of biological and environmental samples. [ABSTRACT FROM AUTHOR]

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In this paper, a tapered fiber bioprobe based on Mach–Zehnder interference (MZI) is proposed. To retain the highly sensitive straight-tapered fiber MZI sensing structure, we designed a U-shaped transmission fiber structure for the collection of optical sensing signals to achieve a miniature-insert-probe design. The spectrum responses from the conventional straight-tapered fiber MZI sensor and our proposed sensor were compared and analyzed, and experimental results showed that our proposed sensor not only has the same sensing capability as the straight-tapered fiber sensor, but also has the advantages of being flexible, convenient, and less liquid-consuming, which are attributed to the inserted probe design. The tapered fiber bioprobe obtained a sensitivity of 1611.27 nm/RIU in the refractive index detection range of 1.3326–1.3414. Finally, immunoassays for different concentrations of human immunoglobulin G were achieved with the tapered fiber bioprobe through surface functionalization, and the detection limit was 45 ng/mL. Our tapered fiber bioprobe has the insert-probe advantages of simpleness, convenience, and fast operation. Simultaneously, it is low-cost, highly sensitive, and has a low detection limit, which means it has potential applications in immunoassays and early medical diagnosis. [ABSTRACT FROM AUTHOR]