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In this paper, the types and principles of operation of fiber sensors based on fiber Bragg gratings (FBGs) are investigated. The influence of strain and temperature on the characteristics of FBGs is considered, and a method for the simultaneous measurement of these parameters is presented. Laboratory studies were carried out in the temperature range from +18 °C to +135 °C with an incremental step of 5 °C, with the actual temperature not deviating by more than ±0.5 °C. From the data obtained, the Bragg wavelength–temperature relationships were plotted, which showed a linear increase in wavelength with increasing temperature. This study shows that the use of two FBGs with a different sensitivity to temperature and strain allowed for the simultaneous measurement of both parameters. Numerical models created in the MATLAB R2022b environment confirmed the high accuracy and precision of the measurements. The FBG-based sensors demonstrated a robust performance in harsh environments, withstanding temperatures of up to 160 °C and high humidity, making them applicable in various industries and sciences. This work confirms that FBGs are a promising tool for accurate temperature and strain measurements, providing reliable results in harsh environments. [ABSTRACT FROM AUTHOR]

Fiber SPR biosensors have low sensitivity and accuracy in detecting biomolecules. In this study, the TRIZ is applied to molecules and optimization. By defining the prototype system, clarifying the components and interacting objects, and adopting a functional model analysis and causal analysis, we deeply explored the key points and root causes of leakage and used a variety of tools, such as technological contradiction, object-field analysis, the HOW-TO model and knowledge effect library, physical contradiction, and the villain method, to come up with different solutions. A novel multimode–single-mode–multimode fiber SPR sensor based on the structure of gold (Au)–molybdenum disulfide (MoS2)–gold nanoparticles was designed and manufactured by using the TRIZ systematically for the first time. Under the effective guidance of the TRIZ, the sensitivity of the novel sensor is significantly enhanced after the optimization and experimental verification. The new sensor provides an effective reduction in the cost of detection and production and at the same time has excellent stability, ease of preparation, and high reproducibility, which is of significant value and significance for practical applications. [ABSTRACT FROM AUTHOR]

Bridges are widely used for train transportation. Some bridges must be constructed close to geologic faults or across them due to the constraints of travel route alignment and the geographical environment. Taiwan is located at the junction of the Eurasian Plate and the Philippine Plate, where geological joints are present and earthquakes are frequent. In Taiwan, the monitoring and early warning of structural displacements is increasingly important, especially in the mutual control and monitoring of bridges and railways. This study utilizes fiber as a continuous sensor to monitor the safety of railway bridges in a near-fault region. This research builds upon the theory of Brillouin frequency shift (BFS) and applies it to a practical scenario of a fault-crossing railway bridge. BFS is related to the strain and temperature change in a single-mode fiber. Distributed fiber optic sensing (DFOS) systems enable us to detect shifts in frequency on the sensing fiber. A systemic approach to installing DFOS systems will be discussed. Data from a DFOS system are collected, and through data processing, they are converted into strain with regard to the deformations (bending, tension, compression) of a box girder bridge. Changes in the geometric structure of the box girder bridge throughout the year are measured and processed into graphical data. This system can be effectively applied to the structural safety monitoring of railway bridges. Through this research, several functions have been achieved, including continuous displacement, automatic monitoring, and real-time automatic alarm functions, without the need for human intervention. [ABSTRACT FROM AUTHOR]

In this work, a high performance surface plasmon resonance (SPR) fiber sensor based on metallic elliptical nanohole and circular nanodisk binary array has been theoretically designed and investigated. The binary array structure is integrated on the end facet of optical fiber to excite SPR and generate three optical resonance dips in reflection spectra in the near-infrared region. The first resonance dip not only fits the characteristics of narrow linewidth, but also has the advantages of high peak-to-dip signal ratio and sensitivity. When the external environment changes in the refractive index (RI) range of 1.33 ~ 1.40, a high RI sensitivity of 669 nm/RIU, a narrow FWHM of 5.8 nm, and a high FOM value of 115.3 are yielded simultaneously for the first dip. Compared to unitary nanostructure arrays, the composite plasmonic structure of nanodisk and elliptical nanohole array demonstrated here can generate SPR modes with sharper spectral feature which is crucial to high sensitivity and precision detection technology. These features make our proposed plasmonic fiber tip sensor highly promising in terms of high-performance biosensing applications. [ABSTRACT FROM AUTHOR]

Due to their contactless and fast measuring capabilities, laser interferometers represent an interesting alternative to tactile stylus instruments for surface profilometry. In addition to these outstanding attributes, acquisition costs play a major role in industry, limiting the frequent use of optical profilometers, which are significantly more expensive than tactile profilometers. We present a low-cost laser distance-measuring interferometer featuring axial repeatability below 1 nm at acquisition rates of 38,000 height values per second. The sensor's performance is validated on several surface standards, achieving lateral scan velocities up to 160 mm/s. Further to high scan velocities, the high acquisition rate enables improved measurement accuracy by averaging measured height values. For example, the standard deviation of 625 pm for repeated measurements can be reduced to 265 pm at the expense of the data rate. However, the sensor concept provides the potential for further improvements in data rate and measurement repeatability. [ABSTRACT FROM AUTHOR]

In this paper, we propose a novel side polished PCF-based SPR sensor with shifted core region employing gold as a plasmonic layer for sensing analytes of refractive index ranging from 1.33 to 1.39. For the ease of plasmonic excitation, we shift the core region towards the edge of the polished cladding. Upon optimizing the sensor structure by varying the geometrical parameters for the sensing range of 1.33–1.39, the proposed PCF sensor structure exhibits a maximum wavelength sensitivity of 24,200 nm/RIU with the resolution of 4.13 × 10−6 RIU. Also, this sensor exhibits a maximum amplitude sensitivity of 432 RIU−1 with a resolution of 2.3 × 10−6 RIU. The proposed sensor exhibits linearity with the R2 value of 0.98791 for the refractive index ranging between 1.33 and 1.39. Furthermore, we carry out perturbation analysis to ensure that the proposed sensor withstands the fabrication tolerance. Therefore, the proposed sensor holds good as a potential contender for sensing in chemical and biological applications. [ABSTRACT FROM AUTHOR]

Kidneys are one of the most important organs in the body. They cleanse the blood and convert waste and excess water into urine. If the kidneys are unable to perform their functions, dialysis is prescribed to assist the patient. Clean and healthy water, as well as accurate evaluation of dissolved substances in water, play a key role in the dialysis process to ensure the patient's safety and well-being. As the dissolved substances change the refractive index of pure water, a refractive index biosensor can be a useful system. In this research, a fiber optic sensor based on surface plasmon resonance with a coating of nanocomposite layers has been studied theoretically. In the numerical simulation, water is used as the sensing medium. The aim of this simulation is to test the sensors that detect impurities in water for dialysis. The simulation is conducted for nanocomposites of nickel-tellurium oxide, platinum-tellurium oxide, and cobalt-tellurium oxide with thicknesses of 50 nm, 70 nm, 90 nm, 110 nm, and 130 nm. The simulation also considers various volume filling factors, including 0.55, 0.65, 0.75, and 0.85. The comparison of the results showed that Ni-TeO2 (with a thickness of 110 nm and a volume filling factor of 0.85) has the highest quality and is considered the winning structure. The graphene layers are tested on the nanocomposites to enhance the quality of the final structure. The results of the simulations show that adding 50 layers of graphene can improve the sensor figure of merit from 80.603 (RIU−1) to 85.522 (RIU−1), while the refractive index of the aqueous media changes from 1.33 to 1.34. [ABSTRACT FROM AUTHOR]

A greenhouse gas sensor has been developed to simultaneously detect multiple gas species within a hollow-core photonic bandgap fiber (HC-PBF) structure entirely composed of fibers. To enhance sensitivity, the gas cell consists of HC-PBF enclosed between two single-mode fibers fused with a reflective end surface to double the absorption length. The incorporation of side holes for gas diffusion allows for analysis of the relationship between gas diffusion speed, number of drilled side holes, and energy loss. As the number of drilled holes increases, the response time decreases to less than 3 min at the expense of energy loss. Gas experiments demonstrated detection limits of 0.1 ppm for methane and 2 ppm for carbon dioxide, with an average time of 50 s. In-situ testing conducted in rice fields validates the effectiveness of the developed gas detection system using HC-PBF cells, establishing all-fiber sensors with high sensitivity and rapid response. [ABSTRACT FROM AUTHOR]

Long-period fiber gratings (LPFGs) functioning as band-reject filters have played a pivotal role in the realm of optical communication. Since their initial documentation in 1996, LPFGs have witnessed rapid advancements in areas such as optical sensing, the equalization of optical amplification, and optical band-pass filtering, etc. The unique attributes of optical fiber-based grating, including their miniaturized size, cost-effectiveness, and immunity to electromagnetic interference, have contributed significantly to various sectors over the last two decades. This paper presents a review of the evolution of LPFGs, with a specific focus on the progression and current trends of mechanically induced long-period fiber gratings. It offers a concise overview of coupled-mode theory, the fabrication processes, the merits, and the limitations associated with mechanically induced LPFGs. Moreover, this review elucidates the application methodologies of mechanically induced LPFGs and anticipates future directions in this field. [ABSTRACT FROM AUTHOR]

In this paper, the types and principles of operation of fiber sensors based on fiber Bragg gratings (FBGs) are investigated. The influence of strain and temperature on the characteristics of FBGs is considered, and a method for the simultaneous measurement of these parameters is presented. Laboratory studies were carried out in the temperature range from +18 °C to +135 °C with an incremental step of 5 °C, with the actual temperature not deviating by more than ±0.5 °C. From the data obtained, the Bragg wavelength–temperature relationships were plotted, which showed a linear increase in wavelength with increasing temperature. This study shows that the use of two FBGs with a different sensitivity to temperature and strain allowed for the simultaneous measurement of both parameters. Numerical models created in the MATLAB R2022b environment confirmed the high accuracy and precision of the measurements. The FBG-based sensors demonstrated a robust performance in harsh environments, withstanding temperatures of up to 160 °C and high humidity, making them applicable in various industries and sciences. This work confirms that FBGs are a promising tool for accurate temperature and strain measurements, providing reliable results in harsh environments.

Fiber SPR biosensors have low sensitivity and accuracy in detecting biomolecules. In this study, the TRIZ is applied to molecules and optimization. By defining the prototype system, clarifying the components and interacting objects, and adopting a functional model analysis and causal analysis, we deeply explored the key points and root causes of leakage and used a variety of tools, such as technological contradiction, object-field analysis, the HOW-TO model and knowledge effect library, physical contradiction, and the villain method, to come up with different solutions. A novel multimode–single-mode–multimode fiber SPR sensor based on the structure of gold (Au)–molybdenum disulfide (MoS2)–gold nanoparticles was designed and manufactured by using the TRIZ systematically for the first time. Under the effective guidance of the TRIZ, the sensitivity of the novel sensor is significantly enhanced after the optimization and experimental verification. The new sensor provides an effective reduction in the cost of detection and production and at the same time has excellent stability, ease of preparation, and high reproducibility, which is of significant value and significance for practical applications.

Bridges are widely used for train transportation. Some bridges must be constructed close to geologic faults or across them due to the constraints of travel route alignment and the geographical environment. Taiwan is located at the junction of the Eurasian Plate and the Philippine Plate, where geological joints are present and earthquakes are frequent. In Taiwan, the monitoring and early warning of structural displacements is increasingly important, especially in the mutual control and monitoring of bridges and railways. This study utilizes fiber as a continuous sensor to monitor the safety of railway bridges in a near-fault region. This research builds upon the theory of Brillouin frequency shift (BFS) and applies it to a practical scenario of a fault-crossing railway bridge. BFS is related to the strain and temperature change in a single-mode fiber. Distributed fiber optic sensing (DFOS) systems enable us to detect shifts in frequency on the sensing fiber. A systemic approach to installing DFOS systems will be discussed. Data from a DFOS system are collected, and through data processing, they are converted into strain with regard to the deformations (bending, tension, compression) of a box girder bridge. Changes in the geometric structure of the box girder bridge throughout the year are measured and processed into graphical data. This system can be effectively applied to the structural safety monitoring of railway bridges. Through this research, several functions have been achieved, including continuous displacement, automatic monitoring, and real-time automatic alarm functions, without the need for human intervention.

In this paper, we investigate a simple arc shaped Au coated photonic crystal fiber-based refractive index sensor that works on principle of surface plasmon resonance. We analyze the performance of the proposed sensor using the finite element method. Further, we analyze the effect of the arc shaped metal coating on different orientations of air holes in the cladding region. For the sensing range of 1.32–1.37, the proposed PCF sensor structure exhibits a maximum wavelength sensitivity of 14,100 nm/RIU with the resolution of 7.09 × 10–6 RIU. Also, this sensor exhibits a maximum amplitude sensitivity of 109 RIU−1 with a resolution of 9.17 × 10–6 RIU. Further, from the linearity of the proposed sensor, R2, is 0.96181 for the refractive index ranging between 1.32 and 1.37. Furthermore, we carry out perturbation analysis to ensure that the proposed sensor withstands the fabrication tolerance. Therefore, the proposed sensor holds good for a potential contender for sensing in chemical and biological applications. [ABSTRACT FROM AUTHOR]

Fiber-optic biosensors have garnered significant attention and witnessed rapid development in recent years owing to their remarkable attributes such as high sensitivity, immunity to electromagnetic interference, and real-time monitoring. They have emerged as a potential tool in the realm of biomarker detection for low-concentration and small molecules. In this paper, a portable and cost-effective optical fiber biosensor based on surface plasmon resonance for the early detection of breast cancer is demonstrated. By utilizing the aptamer human epidermal growth factor receptor 2 (HER2) as a specific biomarker for breast cancer, the presence of the HER2 protein can be detected through an antigen-antibody binding technique. The detection method was accomplished by modifying a layer of HER2 aptamer on the flat surface of a gold-coated D-shaped polymer optical fiber (core/cladding diameter 120/490 μm), of which the residual thickness after side-polishing was about 245 μm, the thickness of the coated gold layer was 50 nm, and the initial wavelength in pure water was around 1200 nm. For low-concentration detection of the HER2 protein, the device exhibited a wavelength shift of ~1.37 nm with a concentration of 1 μg/mL (e.g., 5.5 nM), which corresponded to a limit of detection of ~5.28 nM. Notably, the response time of the biosensor was measured to be as fast as 5 s. The proposed biosensor exhibits the potential for early detection of HER2 protein in initial cancer serum and offers a pathway to early prevention of breast cancer. [ABSTRACT FROM AUTHOR]

Due to their contactless and fast measuring capabilities, laser interferometers represent an interesting alternative to tactile stylus instruments for surface profilometry. In addition to these outstanding attributes, acquisition costs play a major role in industry, limiting the frequent use of optical profilometers, which are significantly more expensive than tactile profilometers. We present a low-cost laser distance-measuring interferometer featuring axial repeatability below 1 nm at acquisition rates of 38,000 height values per second. The sensor’s performance is validated on several surface standards, achieving lateral scan velocities up to 160 mm/s. Further to high scan velocities, the high acquisition rate enables improved measurement accuracy by averaging measured height values. For example, the standard deviation of 625 pm for repeated measurements can be reduced to 265 pm at the expense of the data rate. However, the sensor concept provides the potential for further improvements in data rate and measurement repeatability.

A greenhouse gas sensor has been developed to simultaneously detect multiple gas species within a hollow-core photonic bandgap fiber (HC-PBF) structure entirely composed of fibers. To enhance sensitivity, the gas cell consists of HC-PBF enclosed between two single-mode fibers fused with a reflective end surface to double the absorption length. The incorporation of side holes for gas diffusion allows for analysis of the relationship between gas diffusion speed, number of drilled side holes, and energy loss. As the number of drilled holes increases, the response time decreases to less than 3 min at the expense of energy loss. Gas experiments demonstrated detection limits of 0.1 ppm for methane and 2 ppm for carbon dioxide, with an average time of 50 s. In-situ testing conducted in rice fields validates the effectiveness of the developed gas detection system using HC-PBF cells, establishing all-fiber sensors with high sensitivity and rapid response.

Long-period fiber gratings (LPFGs) functioning as band-reject filters have played a pivotal role in the realm of optical communication. Since their initial documentation in 1996, LPFGs have witnessed rapid advancements in areas such as optical sensing, the equalization of optical amplification, and optical band-pass filtering, etc. The unique attributes of optical fiber-based grating, including their miniaturized size, cost-effectiveness, and immunity to electromagnetic interference, have contributed significantly to various sectors over the last two decades. This paper presents a review of the evolution of LPFGs, with a specific focus on the progression and current trends of mechanically induced long-period fiber gratings. It offers a concise overview of coupled-mode theory, the fabrication processes, the merits, and the limitations associated with mechanically induced LPFGs. Moreover, this review elucidates the application methodologies of mechanically induced LPFGs and anticipates future directions in this field.

We demonstrate a method for fast and precise Brillouin frequency shift measurement based on searching for the zero-crossing point of a virtually composed spectrum of Brillouin gain and loss obtained by a dual-frequency probe beam. Simulations and experiments show that searching for the zero-crossing point of a virtually synthesized Brillouin gain spectrum can be easily done without a large error compared with peak search of the Brillouin gain spectrum in the conventional method.

In sensors utilizing the traditional harmonic Vernier effect, it is imperative to isolate the reference interferometer to mitigate the impact of external environmental fluctuations on the reference spectrum, thereby averting measurement inaccuracies. However, achieving such isolation between interferometers in compact fiber optic sensors poses significant challenges. We propose a fiber optic strain sensor based on the enhanced harmonic Vernier effect. In the enhanced harmonic Vernier effect, two interferometers are simultaneously involved in sensing. Hence, the isolation of interferometers is no longer a concern. Furthermore, the two interferometers exhibit responses with opposite signs, leading to a greater displacement of the envelope. Consequently, the sensitivity of the sensor is further enhanced without altering the harmonic order. Firstly, we conducted numerical simulations to analyze the enhanced harmonic Vernier effect. Secondly, we fabricated a Fabry–Perot interferometer (FPI) and a Mach–Zehnder interferometer (MZI) with strain sensitivity of opposite signs, generating a first-order harmonic. The experimental results indicate that the sensor's sensitivity is 15.1 pm/ μ ϵ , which is 1.41 and 2.80 times higher than the traditional harmonic Vernier effect using FPI and MZI as sensing interferometers, respectively. The proposed sensor and enhancement method hold promising potential for applications in high-sensitivity measurement fields. • High sensitivity fiber optic strain sensor. • Enhanced Harmonic Vernier Effect for improved sensitivity. • Hybrid interferometer design, eliminating the need for a reference interferometer. • Mitigation of external environmental fluctuations without the need for isolation. • Sensitivity enhancement without altering the harmonic order. [ABSTRACT FROM AUTHOR]

Fiber optic intrusion detection is recognized as one of the most effective perimeter defense strategies against intruders and terrorists. Most current infiltration detection systems gather information via optical cables and trace invasions using classification techniques. This research introduces two intrusion detection algorithms and a hybrid approach to address the challenges of unstable and hard-to-extract fiber optic infiltration features, especially during rainy weather. The first method, known as the phase parameter-matching algorithm, calculates the system's disturbance covariance at any position and uses the Frobenius norm to identify intrusions by measuring the difference between disturbance covariances at distinct sites. The second method, the cross-rebuilding algorithm, employs the Kalman filter to reconstruct the output of each point based on the non-infiltration state parameters, detecting intrusions through cross-reconstruction error. Experimental tests were conducted to evaluate both methods. The results indicate that both methods are viable, with the phase parameter-matching algorithm being more robust but slower, and the cross-rebuilding algorithm being faster but less reliable. The hybrid algorithm combines the strengths of both approaches, offering a balanced solution with moderate computation time and enhanced resistance to environmental interference, leading to improved overall detection accuracy and reduced false positive rates. [ABSTRACT FROM AUTHOR]

In this paper, a D-shaped photonic crystal fiber (PCF) sensor based on a Sagnac interferometer is proposed, and it can achieve ultrahigh refractive index (RI) and temperature sensitivity when operating around the turning point of group birefringence (B g). We undertake a theoretical analysis on B g and a simulation calculation to study the sensing characteristics and obtain the optimized structure parameters of the D-shaped PCF sensor. The simulation results show that the maximum average RI sensitivities can reach 3253.33 and 15500 nm/RIU in the RI range of 1.33 to 1.35 and 1.40 to 1.42, respectively. When the temperature changes from -50 to 0 °C and 0 to 50 °C, the maximum average temperature sensitivities are up to 10.11 and 10.67 nm/°C, respectively. The proposed D-shaped PCF sensor can achieve dual-parameter sensing and has great potential for practical applications in biochemical and environmental science. • A novel D-shaped photonic crystal fiber sensor based on a Sagnac interferometer is proposed. • The proposed sensor can achieve ultrahigh sensitivity when operating around the turning point of group birefringence. • The proposed sensor can achieve the dual-parameters sensing, and has great potential for practical applications. [ABSTRACT FROM AUTHOR]

In this paper, a gold/graphene/Ti3C2Tx-MXene hybrid layered D-type photonic crystal fiber (PCF) design based on surface plasmon resonance (SPR) sensors is proposed for cancer cell detection. This design uniquely combines gold, graphene, and Ti3C2Tx-MXene materials to achieve a synergistic effect, significantly enhancing the sensitivity and specificity of the sensor. The full vector finite element method (FVFEM) is used for the entire numerical analysis of the proposed biosensor. The cladding of the D-type PCF has a hexagonal arrangement of air holes. In the first cladding, the two air holes closest to the metal layer are narrowed down to enhance the plasma wave and provide an efficient leakage channel. The last two air holes closest to the metal layer in the same layer are enlarged to limit light scattering and couple more energy to the surface plasmon polariton (SPP) mode. The sensitivity of the sensor improves by using these different diameter air holes and coating the D-type PCF surface with a hybrid gold/graphene/Ti3C2Tx-MXene layer. The geometrical parameters are optimized to obtain higher sensor sensitivity. The corresponding wavelength sensitivities are 3000 nm/RIU for Basal cells, 5000 nm/RIU for HeLa cells (Henrietta Lacks cells), 5714 nm/RIU for Jurkat cells (Human T lymphocyte cells), 7143 nm/RIU for PC12 cells (Pheochromocytoma cells), 8571 nm/RIU for MDA-MB-231 cells (Breast cancer cells), and 9286 nm/RIU for MCF-7 cells (Michigan Cancer Foundation-7, a breast cancer cell line), respectively, confirming the excellent performance of the proposed sensor. The sensor proposed paves the way for efficient, simple, low-cost, and highly sensitive cancer detection techniques that could replace surgical and chemical techniques. [ABSTRACT FROM AUTHOR]

In this work, a simple design and cost-effective displacement-curvature sensor based on macro-bend single-mode fiber (SMF) is planned and demonstrated. The optical fiber sensor encompasses a bending configuration and the macro-bending losses were exploited as the sensing process for the capturing of angular displacement motion in an angle span between 0° and 90° (that corresponds to curvature displacement between 0 and 49.5 cm). A straight SMF shape and other four bent sensor structures (U-like, drop-like, Knot-like, and eight-like) have been fabricated from sections of 90 cm length SMF. These five sensor configurations will provide high maneuvering for versatile applications. In these adjustable sensing schemes, the fiber sensors were described by monitoring the power loss corresponding to the diverse angular displacement at various bending structures in terms of the shape of the bend, the bending diameter, and the number of wrapping turns. Then, these measurement schemes based on the macro-bending losses were employed to detect the human motion elbow joint, and a proper sensing scheme was adopted to examine the flexion/extension joint motion in detail. The proposed sensors showed enhanced sensitivity to flexion/extension joint motion range between −0.25 and −0.477 dB/° and good sensitivity to curvature ranging between −0.552 and −1.052 dB/cm. The demonstrated and planned sensing techniques are adjustable and accessible for different types of human and mechanical movements. [Display omitted] • Wearable Optical Fiber Sensor based macro-bending loss for Biomechanical Motion Evaluation. • A simple design and cost-effective displacement-curvature sensor based on macro-bend single-mode fiber (SMF) has been integrated into elbow joint phantoms and measuring its response to various elbow joint movements in the range of 0°-90°. • A straight SMF shape and other four bent sensor structures (U-like, drop-like, Knot-like, and eight-like) have been exploited as the sensing process for the capturing of angular displacement motion in a span between 0 and 49.5 cm. • Adjustable sensing measurement schemes based on the macro-bending losses have been employed to detect the human motion elbow joint, and a proper sensing scheme has been adopted to examine the flexion/extension joint motion in detail. [ABSTRACT FROM AUTHOR]

• PEDOT:PSS is a promising material for pH detection. • The incorporation of poly(vinyl alcohol) with PEDOT:PSS enhances the usability of device. • PEDOT:PSS/PVA is resistant to measurements in highly acidc conditions. • The sensor exhibits temporal stability. In this work, a pH sensitive transducer element attached to a multimode optical fiber section is proposed. The sensitive element is a PEDOT:PSS/PVA composite film that exhibited sensitivity at low pH values in the range of 1–7 pH. The addition of PVA was important for the adhesion process to the optical fiber surface. The morphology of the PEDOT:PSS polymers allowed characterization at pH stimuli that were based on the optical power transmission generated from the interaction of the light propagating in the optical fiber core and the optical fiber section coated with the sensitive film. A sensitivity of −0.05 dB/pH and a linear fit R2 of 0.93 was obtained, the response time was 3 s, while the sensor recovery time was 50 s maintaining stability higher than 10 min. [ABSTRACT FROM AUTHOR]

• A high sensitivity and low temperature cross-talk magnetic field sensor with small dimension is proposed and investigated. • It is easily fabricated by sandwiching a tiny ferrofluid-filled capillary tube between two SMFs via a short MMF at the capillary tube's both ends. • The magnetic field sensitivity achieves up to 8.966 nm/mT and 4.264 nm/mT for weak longitudinal and vertical magnetic field, respectively, as well as good stability and accuracy. • The temperature cross-talk is very low. A ferrofluid-filled in-fiber Mach-Zehnder interferometer (MZI) is presented and validated for magnetic field sensing. It is constructed by embedding a short capillary tube filled with ferrofluid which is sandwiched between two SMFs through a short MMF at its both ends, and the device is experimentally proved having extreme high refractive index (RI) sensitivity. When the sensor is placed in an external magnetic field, the RI of the ferrofluid changes, causing a shift in the transmission spectrum of the interferometer. Experimental results show that the proposed sensor exhibits a high sensitivity of 8.9657 nm/mT and 4.2642 nm/mT for longitudinal and vertical magnetic field within the range of 0–1.5 mT and 0.8–2.2 mT, respectively. Furthermore, the sensor exhibits excellent low temperature cross-talk in longitudinal and transverse magnetic fields as low as 0.014 mT/°C and 0.030 mT/°C, respectively. The sensor offers advantages such as stable operation, compact size, and robust mechanical structure. [ABSTRACT FROM AUTHOR]

• A concise all-fiber hydrostatic pressure sensor is proposed. • High hydrostatic pressure sensitivity of −0.2358 nm/MPa in the range of 0.1 MPa to 16.1 MPa is obtained. • Ultra-low temperature–pressure cross-sensitivity of 0.04368 MPa/℃ proves our sensor more competitive in harsh environment applications. With the rapid development of physical ocean observation, high-performance pressure sensor integrated with stable mechanical strength, sufficient pressure sensitivity as well as negligible temperature crosstalk is highly desirable but still lack of research. This paper proposes an all-fiber optical sensor based on a Mach-Zehnder interferometer (MZI) for hydrostatic pressure measurement, utilizing a multi-mode fiber (MMF)-single-hole-dual-core fiber (SHDCF)-MMF structure. The MMF sections act as a beam splitter and coupler to enhance the Mach-Zehnder interference. The SHDCF serves as the pressure-sensing section. Under the action of the enclosed air hole in SHDCF, hydraulic variations cause changes in the structural parameters of the fiber, especially the refractive index change caused by the elasto-optical effect, which leads to the shift of the interference dips. The experimental results prove that the SHDCF-based MZI sensor has a higher hydrostatic pressure sensitivity than other hole-less silica fiber-based sensors. The max pressure sensitivity is −0.2358 nm/MPa in the range of 0.1 MPa to 16.1 MPa with good linearity and reliability. Moreover, the low temperature–pressure cross-sensitivity of 0.04368 MPa/℃ also proves our sensor more competitive in harsh environment applications. [ABSTRACT FROM AUTHOR]

• We designed a fiber sensor system comprising fused silica fiber Bragg grating and silicon core fiber-based fiber Bragg grating. • A hybrid fiber Bragg grating sensor system was proposed, employing only two sensing devices to detect three different environmental targets. • Our focus was solely on addressing the connection between two distinct fibers, namely fused silica and silicon core fiber. We discussed the associated loss and spectral distortion issues. In this article, we introduce a multi-parameter fiber sensor system that utilizes a hybrid FBG design. The design combines a fused silica fiber Bragg grating and a fiber Bragg grating based on a silicon core fiber. This combination enables the sensor to detect surrounding temperature, water level, and refractive index. The sensor successfully measured temperature within the range of 10 to 40 °C, water level within the range of 40 to 140 cm, and refractive index within the range of 1.2 to 1.4. These ranges exhibit a high level of linearity within the proposed sensing range. Additionally, we discuss the challenges associated with connecting the fused silica fiber and the silicon core fiber. [ABSTRACT FROM AUTHOR]

A fast interrogation scheme for fiber Bragg grating (FBG) using Kalman filter (KF) algorithm is presented. The resolution of the system using KF algorithm is tested, and the dynamic interrogation of the system is verified using vibration signals. Meanwhile, different degrees of static pressure were applied to the FBG, and the static interrogation of the system is verified. Experimental verification shows that the Bragg wavelength resolution of the proposed algorithm can reach 0.3 pm and is relatively stable as the wavelength resolution of the system decreases. In addition, the antinoise performance of KF algorithm is compared with that of direct peak search algorithm. Compared with other commonly used FBG peak search algorithms, the innovative algorithm has the characteristics of low computational cost, high sensitivity, and stability. [ABSTRACT FROM AUTHOR]

• Formed a F-P interferometer on a 45° polished side hole eccentric core fiber. • Can be used as an RI sensing probe for solutions, which is easy to operate. • Can be used as sensors for other parameters, such as temperature by filling suitable materials. A Fabry-Pérot interferometer on the tip of a microstructured fiber and its sensing application was demonstrated. The microstructured side hole eccentric core fiber was fabricated using the drilling hole method. The fiber end face was polished to 45° which could reflect the light transverse to the fiber, utilizing the side wall of the air hole as the reflection surface. Sensing abilities of the interferometer were investigated in the experiment. It can work as a probe for liquid RI sensing by simply plunging the fiber tip into the solution being tested, in addition, it can become sensors for different parameters by filling functional materials into the air hole. An example of filling UV curable adhesive into the fiber to be a temperature sensor was demonstrated in the experiment. The sensing structure on the fiber tip shows the advantages of high linearity, easy fabrication and good reproducibility, as a sensor it is also convenient to operate. [ABSTRACT FROM AUTHOR]

Diseases in pregnancy endanger millions of fetuses worldwide every year. The onset of these diseases can be early warned by the dynamic abnormalities of biochemicals in amniotic fluid, thus requiring real-time monitoring. However, when continuously penetrated by detection devices, the amnion is prone to loss of robustness and rupture, which is difficult to regenerate. Here, an interface-stabilized fiber sensor is presented for real-time monitoring of biochemical dynamics in amniotic fluid during pregnancy. The sensor is seamlessly integrated into the amnion through tissue adhesion, amniotic regeneration, and uniform stress distribution, posing no risk to the amniotic fluid environment. The sensor demonstrates a response performance of less than 0.3% fluctuation under complex dynamic conditions and an accuracy of more than 98% from the second to the third trimester. By applying it to early warning of diseases such as intrauterine hypoxia, intrauterine infection, and fetal growth restriction, fetal survival increases to 95% with timely intervention., (© 2023 Wiley-VCH GmbH.)

Significance: Photoacoustic (PA) imaging is an emerging biomedical imaging modality that can map optical absorption contrast in biological tissues by detecting ultrasound signal. Piezoelectric transducers are commonly used in PA imaging to detect the ultrasound signals. However, piezoelectric transducers suffer from low sensitivity when the dimensions are reduced and are easily influenced by electromagnetic interference. To avoid these limitations, various optical ultrasound sensors have been developed and shown their great potential in PA imaging., Aim: Our study aims to summarize recent progress in optical ultrasound sensor technologies and their applications in PA imaging., Approach: The commonly used optical ultrasound sensing techniques and their applications in PA systems are reviewed. The technical advances of different optical ultrasound sensors are summarized., Results: Optical ultrasound sensors can provide wide bandwidth and improved sensitivity with miniatured size, which enables their applications in PA imaging., Conclusions: The optical ultrasound sensors are promising transducers in PA imaging to provide higher-resolution images and can be used in new applications with their unique advantages., (© 2024 The Authors.)