Fiber Optic Sensors Embedded in Textile-Reinforced Concrete for Smart Structural Health Monitoring: A Review.

The last decade has seen rapid developments in the areas of carbon fiber technology, additive manufacturing technology, sensor engineering, i.e., wearables, and new structural reinforcement techniques. These developments, although from different areas, have collectively paved way for concrete structures with non-corrosive reinforcement and in-built sensors. Therefore, the purpose of this effort is to bridge the gap between civil engineering and sensor engineering communities through an overview on the up-to-date technological advances in both sectors, with a special focus on textile reinforced concrete embedded with fiber optic sensors. The introduction section highlights the importance of reducing the carbon footprint resulting from the building industry and how this could be effectively achieved by the use of state-of-the-art reinforcement techniques. Added to these benefits would be the implementations on infrastructure monitoring for the safe operation of structures through their entire lifespan by utilizing sensors, specifically, fiber optic sensors. The paper presents an extensive description on fiber optic sensor engineering that enables the incorporation of sensors into the reinforcement mechanism of a structure at its manufacturing stage, enabling effective monitoring and a wider range of capabilities when compared to conventional means of structural health monitoring. In future, these developments, when combined with artificial intelligence concepts, will lead to distributed sensor networks for smart monitoring applications, particularly enabling such distributed networks to be implemented/embedded at their manufacturing stage.

All-Optical Planar Polymer Waveguide-Based Biosensor Chip Designed for Smartphone-Assisted Detection of Vitamin D.

An all-optical plasmonic sensor platform designed for smartphones based on planar-optical waveguide structures integrated in a polymer chip is reported for the first time. To demonstrate the applicability of the sensor system for biosensing purposes, the detection of 25-hydroxyvitamin D (25OHD) in human serum samples using an AuNP-enhanced aptamer-based assay was demonstrated. With the aid of the developed assay sensitivity of 0.752 pixel/nM was achieved for 25OHD concentrations ranging from 0-100 nM. The waveguide structure of the sensor enables miniaturisation and parallelisation, thus, demonstrates the potential for simultaneous detection of various analytes including biomarkers. The entire optical arrangement can be integrated into a single polymer chip which allows for large scale and cost-efficient sensor fabrication. The broad utilization and access of smartphone electronics make the proposed design most attractive for its wider use in lab-on-chip applications.

SPR Biosensor Based on Polymer Multi-Mode Optical Waveguide and Nanoparticle Signal Enhancement.

We present a surface plasmon resonance (SPR) biosensor that is based on a planar-optical multi-mode (MM) polymer waveguide structure applied for the detection of biomolecules in the lower nano-molar (nM) range. The basic sensor shows a sensitivity of 608.6 nm/RIU when exposed to refractive index changes with a measurement resolution of 4.3 × 10-3 RIU. By combining the SPR sensor with an aptamer-functionalized, gold-nanoparticle (AuNP)-enhanced sandwich assay, the detection of C-reactive protein (CRP) in a buffer solution was achieved with a response of 0.118 nm/nM. Due to the multi-mode polymer waveguide structure and the simple concept, the reported biosensor is well suited for low-cost disposable lab-on-a-chip applications and can be used with rather simple and economic devices. In particular, the sensor offers the potential for fast and multiplexed detection of several biomarkers on a single integrated platform.

Grating assisted glass fiber coupler for mode selective co-directional coupling.

To keep pace with the increasing demand of transmission capacity, space division multiplexing technologies are currently intensively investigated. In this context, mode selective glass fiber couplers are of great interest due to their compatibility with existing glass fiber networks. In this work, we present a novel type of mode selective glass fiber coupler for co-directional coupling based on fiber gratings and fused asymmetric fibers. The achieved mode selective coupling efficiency agrees well with numerical simulations performed for comparison. The benefits of the grating approach are a lower mode crosstalk and a simple adaption of the propagation constants through changing of the grating-period.

Investigating the Strain, Temperature and Humidity Sensitivity of a Multimode Graded-Index Perfluorinated Polymer Optical Fiber with Bragg Grating.

In this work we investigate the strain, temperature and humidity sensitivity of a Fiber Bragg Grating (FBG) inscribed in a near infrared low-loss multimode perfluorinated polymer optical fiber based on cyclic transparent optical polymer (CYTOP). For this purpose, FBGs were inscribed into the multimode CYTOP fiber with a core diameter of 50 µm by using a krypton fluoride (KrF) excimer laser and the phase mask method. The evolution of the reflection spectrum of the FBG detected with a multimode interrogation technique revealed a single reflection peak with a full width at half maximum (FHWM) bandwidth of about 9 nm. Furthermore, the spectral envelope of the single FBG reflection peak can be optimized depending on the KrF excimer laser irradiation time. A linear shift of the Bragg wavelength due to applied strain, temperature and humidity was measured. Furthermore, depending on irradiation time of the KrF excimer laser, both the failure strain and strain sensitivity of the multimode fiber with FBG can be controlled. The inherent low light attenuation in the near infrared wavelength range (telecommunication window) of the multimode CYTOP fiber and the single FBG reflection peak when applying the multimode interrogation set-up will allow for new applications in the area of telecommunication and optical sensing.

Evaluating the Performance of Functionalized Carbon Structures with Integrated Optical Fiber Sensors under Practical Conditions.

An Optical Frequency Domain Reflectometry (OFDR) based fiber optic sensor scheme "embedded" in concrete for the purpose of structural health monitoring (SHM) of carbon concrete composites (C³) is presented. The design, while strengthening the concrete structure, also aims to monitor common SHM parameters such as strain and cracks. This was achieved by weaving the carbon fiber together with optical fiber, based on a specialized technique that uses an embroidery setup where both the carbon and optical fiber are woven on a water dissolvable polymer substrate. The performance of the sensing scheme was characterized in-situ utilizing the OFDR based technique and the results presented. The sensors embedded on a custom made concrete block were subjected to varying strain via a three point bending test to destruction and the results discussed. The intended dual-achievement of the scheme thus proposed in SHM and strengthening the C³ is demonstrated. The suitability of the OFDR scheme for C³ is combined with a fibre Bragg grating (FBG)-based approach, and discussed in detail.

Long-Period Gratings in Highly Germanium-Doped, Single-Mode Optical Fibers for Sensing Applications.

Long-period fiber gratings (LPGs) are well known for their sensitivity to external influences, which make them interesting for a large number of sensing applications. For these applications, fibers with a high numerical aperture (i.e., fibers with highly germanium (Ge)-doped fused silica fiber cores) are more attractive since they are intrinsically photosensitive, as well as less sensitive to bend- and microbend-induced light attenuations. In this work, we introduce a novel method to inscribe LPGs into highly Ge-doped, single-mode fibers. By tapering the optical fiber, and thus, tailoring the effective indices of the core and cladding modes, for the first time, an LPG was inscribed into such fibers using the amplitude mask technique and a KrF excimer laser. Based on this novel method, sensitive LPG-based fiber optic sensors only a few millimeters in length can be incorporated in bend-insensitive fibers for use in various monitoring applications. Moreover, by applying the described inscription method, the LPG spectrum can be influenced and tailored according to the specific demands of a particular application.

Structural Health Monitoring Using Textile Reinforcement Structures with Integrated Optical Fiber Sensors.

Optical fiber-based sensors "embedded" in functionalized carbon structures (FCSs) and textile net structures (TNSs) based on alkaline-resistant glass are introduced for the purpose of structural health monitoring (SHM) of concrete-based structures. The design aims to monitor common SHM parameters such as strain and cracks while at the same time acting as a structural strengthening mechanism. The sensor performances of the two systems are characterized in situ using Mach-Zehnder interferometric (MZI) and optical attenuation measurement techniques, respectively. For this purpose, different FCS samples were subjected to varying elongation using a tensile testing machine by carefully incrementing the applied force, and good correlation between the applied force and measured length change was observed. For crack detection, the functionalized TNSs were embedded into a concrete block which was then exposed to varying load using the three-point flexural test until destruction. Promising results were observed, identifying that the location of the crack can be determined using the conventional optical time domain reflectometry (OTDR) technique. The embedded sensors thus evaluated show the value of the dual achievement of the schemes proposed in obtaining strain/crack measurement while being utilized as strengthening agents as well.

Fibre optic surface plasmon resonance sensor system designed for smartphones.

A fibre optic surface plasmon resonance (SPR) sensor system for smartphones is reported, for the first time. The sensor was fabricated by using an easy-to-implement silver coating technique and by polishing both ends of a 400 µm optical fibre to obtain 45° end-faces. For excitation and interrogation of the SPR sensor system the flash-light and camera at the back side of the smartphone were employed, respectively. Consequently, no external electrical components are required for the operation of the sensor system developed. In a first application example a refractive index sensor was realised. The performance of the SPR sensor system was demonstrated by using different volume concentrations of glycerol solution. A sensitivity of 5.96·10(-4) refractive index units (RIU)/pixel was obtained for a refractive index (RI) range from 1.33 to 1.36. In future implementations the reported sensor system could be integrated in a cover of a smartphone or used as a low-cost, portable point-of-care diagnostic platform. Consequently it offers the potential of monitoring a large variety of environmental or point-of-care parameters in combination with smartphones.

"All-fiber" tunable laser in the 2 µm region, designed for CO2 detection.

A stable and tunable thulium-doped "all-fiber" laser offering a narrow linewidth has been created specifically to act as a compact and simple laser source for gaseous CO(2) detection. This has been done through a careful design to match the laser output wavelengths to the CO(2) absorption lines at 1.875 and 1.997 µm, respectively. A sustainable output power of 11 mW over a tuning range of 7 nm has been obtained by using a combination of a high-reflective fiber Bragg grating with a low-reflective broadband mirror, fabricated at the end of the fiber through silver film deposition. The tuning was achieved using the relaxation-compression mechanism of the fiber Bragg grating, which formed an integral part of the laser resonant cavity. A fiber Bragg grating at 1.548 µm was utilized as a wavelength reference to monitor the tuning of the laser output over the 2 µm wavelength range with a simple and inexpensive interrogator, to avoid the use of an expensive optical spectrum analyzer and to facilitate "in-the-field" operation. This "all-fiber" laser resonator has been shown to be superior in terms of laser tuning range, output power, and linewidth compared to that created with a fiber Bragg grating pair, which was limited by the nonuniform strain transfer to both fiber Bragg gratings.

Characterization of an Aptamer Directed against 25-Hydroxyvitamin D for the Development of a Competitive Aptamer-Based Assay.

Detection of the small molecule 25-hydroxyvitamin D (25(OH)D) as the most relevant marker for vitamin D supply suffers from a high variability of results using the current detection methods, such as high-performance liquid chromatography (HPLC) and immunoassays. A new detection approach using a highly specific aptamer directed against 25(OH)D was established in this study based on the target-induced dissociation (TID) sensing approach. In this work, the aptamer was investigated regarding its structural properties as well as its binding affinity by using microscale thermophoresis (MST). Moreover, complementary oligonucleotides were designed based on the aptamer structure and were evaluated in MST experiments. Binding experiments of immobilized aptamers were conducted in microarray experiments. It could be shown that the aptamer exhibited the usual B-DNA structure and did not form any G-quadruplexes. The design of complementary oligonucleotides for the TID assay identified a putative 25(OH)D binding site within the aptamer. The limit of detection of the established competitive assay was determined to be 5.4 nM, which sets the stage for the development of a biosensor system.