In recent years, railway infrastructures and systems have played a significant role as a highly efficient transportation mode to meet the growing demand in transporting both cargo and passengers. Application of these structures in extreme environmental situation under severe working and loading conditions, caused by the traffic growth, heavier axles and vehicles and increase in speed makes it extremely susceptible to degradation and failure. In the last two decades, a significant number of innovative sensing technologies based on fiber optic sensors (FOS) have been utilized for structural health monitoring (SHM) due to their inherent distinctive advantages, such as small size, light weight, immunity to electromagnetic interference (EMI) and corrosion, and embedding capability. Fiber optic-based monitoring systems use quasi-distributed and continuously distributed sensing techniques for real time measurement and long term assessment of structural properties. This allows for early stage damage detection and characterization, leading to timely remediation and prevention of catastrophic failures. In this scenario, FOS have been proved to be a powerful tool for meticulous assessment of railway systems including train and track behavior by enabling real-time data collection, inspection and detection of structural degradation. This article reviews the current state-of-the-art of fiber optic sensing/monitoring technologies, including the basic principles of various optical fiber sensors, novel sensing and computational methodologies, and practical applications for railway infrastructure monitoring. Additionally, application of these technologies to monitor temperature, stresses, displacements, strain measurements, train speed, mass and location, axle counting, wheel imperfections, rail settlements, wear and tear and health assessment of railway bridges and tunnels will be thoroughly discussed.

A review of railway infrastructure monitoring using fiber optic sensors

Photoacoustic imaging with fiber optic technology: A review

In-fiber interferometric-based sensors are a rapidly growing field, as these sensors exhibit many desirable characteristics compared to their regular fiber-optic counterparts and are being implemented in many promising devices. These sensors have the capability to make extremely accurate measurements on a variety of physical or chemical quantities such as refractive index, temperature, pressure, curvature, concentration, etc. This article is a comprehensive overview of the different types of in-fiber interferometric sensors that presents and discusses recent developments in the field. Basic configurations, a brief approach of the operating principle and recent applications are introduced for each interferometric architecture, making it easy to compare them and select the most appropriate one for the application at hand.

https://www.mdpi.com/2304-6732/8/7/265/pdf

In-Fiber Interferometric-Based Sensors: Overview and Recent Advances

Smart railway sleepers - a review of recent developments, challenges, and future prospects

An ultra-short high-temperature fiber-optic sensor based on a silicon-microcap created by a single-mode fiber (SMF) and simple fusion splicing technology is proposed and experimentally demonstrated. A section of the SMF with a silicon-microcap at one end is connected to the “peanut” structure to build the microcap-based optical fiber improved Michelson interferometer (MI). The optimal discharge parameters of microcap and length of SMF has been investigated to achieve the best extinction ratio of 6.61 dB. The size of this microcap-based improved MI sensor is 560 µm and about 18 times shorter compared to the current fiber tip interferometers (about 10 mm). Meanwhile, it showed good robustness during the two heating-cooling cycles and the duration period stability test at 900 °C. This microcap-based improved MI sensor with the smaller size, simple fabrication, low cost, high reliability, and good linearity within a large dynamic range is beneficial to practical temperature measurement and massive production.

Ultra-compact silicon-microcap based improved Michelson interferometer high-temperature sensor.

This letter presents the design, fabrication, and testing of a diaphragm-based, all-silica fiber tip pressure sensor. A piece of 1.2- $\mu \text{m}$ -thick silica diaphragm was thermally bonded to the end face of an etched fiber with an outer diameter (OD) of 125 $\mu \text{m}$ to form a Fabry–Perot (FP) interferometer. The diameter of the etched FP cavity is $105~\mu \text{m}$ , enabling the sensing area to be large, and thus, a static pressure sensitivity of 12.4 nm/kPa (85.3 nm/psi) is accomplished without the need for any post polishing or etching. The sensor also shows a low-temperature cross sensitivity during tests, owing to its all-silica structure. It is suitable for extensive production, and it has a great potential to work in applications, where miniature size and high sensitivity are essential such as medical applications.

Highly Sensitive Miniature All-Silica Fiber Tip Fabry–Perot Pressure Sensor

Monitoring and manipulating the physical and chemical behavior of single molecules is an important development direction of molecular electronics that aids in understanding the molecular world at the single-molecule level. The electrical detection platform based on single-molecule junctions can monitor physical and chemical processes at the single-molecule level with a high temporal resolution, stability, and signal-to-noise ratio. Recently, the combination of single-molecule junctions with different multimodal control systems has been widely used to explore significant physical and chemical phenomena because of its powerful monitoring and control capabilities. In this review, we focus on the applications of single-molecule junctions in monitoring molecular physical and chemical processes. The methods developed for characterizing single-molecule charge transfer and spin characteristics as well as revealing the corresponding intrinsic mechanisms are introduced. Dynamic detection and regulation of single-molecule conformational isomerization, intermolecular interactions, and chemical reactions are also discussed in detail. In addition to these dynamic investigations, this review discusses the open challenges of single-molecule detection in the fields of physics and chemistry and proposes some potential applications in this field.

Single-Molecule Junction: A Reliable Platform for Monitoring Molecular Physical and Chemical Processes.

An all-optical ultrasound sensing system based on photoacoustic principle is developed to monitor and investigate the initiation of early stage steel rebar corrosion in real time. The ultrasound generator was developed by small size gold nanocomposites coated optical fibers to generate 8-MHz bandwidth signals. The ultrasound receiver was applied by a fiber Bragg grating sensor. This system was employed for the first time as a disruptive approach in order to replace the application of conventional electrical-based sensors due to their unique advantages including miniature size, lightweight, immunity to electromagnetic interference, resistance to corrosion, and distributed sensing capability. Accelerated corrosion tests were conducted on steel rebars to validate the developed sensing system. The obtained results indicated that the mass loss of corroded steel rebars was characterized by the centroid frequency shift of measured ultrasound signals. The correlation between the centroid frequency shift and the mass loss depicted an exponential relationship.

Fiber Optic Sensors Based on Photoacoustic Effect for Rebar Corrosion Measurement

This article presents an application of an active all-optical photoacoustic sensing system with four elements for steel rebar corrosion monitoring. The sensor utilized a photoacoustic mechanism of gold nanocomposites to generate 8 MHz broadband ultrasound pulses in 0.4 mm compact space. A nanosecond 532 nm pulsed laser and 400 μm multimode fiber were employed to incite an ultrasound reaction. The fiber Bragg gratings were used as distributed ultrasound detectors. Accelerated corrosion testing was applied to four sections of a single steel rebar with four different corrosion degrees. Our results demonstrated that the mass loss of steel rebar displayed an exponential growth with ultrasound frequency shifts. The sensitivity of the sensing system was such that 0.175 MHz central frequency reduction corresponded to 0.02 g mass loss of steel rebar corrosion. It was proved that the all-optical photoacoustic sensing system can actively evaluate the corrosion of steel rebar via ultrasound spectrum. This multipoint all-optical photoacoustic method is promising for embedment into a concrete structure for distributed corrosion monitoring.

All-Optical Photoacoustic Sensors for Steel Rebar Corrosion Monitoring.

Single-molecule junctions (SMJs) offer a novel strategy for miniaturization of electronic devices. In this work, we realize a graphene-porphyrin-graphene SMJ driven by electric field and proton transfer in two configurations. In the transistor configuration with ionic liquid gating, an unprecedented field-effect performance is achieved with a maximum on/off ratio of ~4800 and a gate efficiency as high as ~179 mV/decade in consistence with the theoretical prediction. In the other configuration, controllable proton transfer, tautomerization switching, is directly observed with bias dependence. Room temperature proton transfer leads to a two-state conductance switching, and more precise tautomerization is detected, showing a four-state conductance switching at high bias voltages and low temperatures. Such an SMJ in two configurations provides new insights into not only building multifunctional molecular nanocircuits toward real applications but also deciphering the intrinsic properties of matters at the molecular scale.

Xu Guo

Papers

Highly Sensitive Miniature All-Silica Fiber Tip Fabry–Perot Pressure Sensor

Single-Molecule Junction: A Reliable Platform for Monitoring Molecular Physical and Chemical Processes.

Fiber Optic Sensors Based on Photoacoustic Effect for Rebar Corrosion Measurement

All-Optical Photoacoustic Sensors for Steel Rebar Corrosion Monitoring.

Single-molecule field effect and conductance switching driven by electric field and proton transfer