Low-Cost Online Monitoring System for the Etching Process in Fiber Optic Sensors by Computer Vision.

The present research exposes a novel methodology to manufacture fiber optic sensors following the etching process by Hydrofluoric Acid deposition through a real-time monitoring diameter measurement by computer vision. This is based on virtual instrumentation developed with the National Instruments® technology and a conventional digital microscope. Here, the system has been tested proving its feasibility by the SMS structure diameter reduction from its original diameter of 125 µ until approximately 42.5 µm. The results obtained have allowed us to demonstrate a stable state behavior of the developed system during the etching process through diameter measurement at three different structure sections. Therefore, this proposal will contribute to the etched fiber optic sensor development that requires reaching an enhanced sensitivity. Finally, to demonstrate the previously mentioned SMS without chemical corrosion, and the etched manufactured SMS, both have been applied as glucose concentration sensors.

Enhancement of lossy mode resonance sensing properties by the introduction of an intermediate low-refractive-index layer.

Devices based on the lossy mode resonance (LMR) effect have found numerous sensing applications. Herein, the enhancement of the sensing properties by the introduction of an intermediate layer between the substrate and the LMR-supporting film is discussed. Experimental results for a silicon oxide (SiO2) layer of tuned thickness between a glass slide substrate and a thin film of titanium oxide (TiO2) prove the possibility of significantly increasing the LMR depth and the figure of merit (FoM) for refractive index sensing applications, which is supported by a numerical analysis using the plane wave method for a one-dimensional multilayer waveguide. The application of the intermediate layer allows the introduction of a new, to the best of our knowledge, degree of freedom into the design of LMR-based sensors, resulting in improved performance for demanding fields such as chemical sensing or biosensing.

Gas Sensor Based on Lossy Mode Resonances by Means of Thin Graphene Oxide Films Fabricated onto Planar Coverslips.

The use of planar waveguides has recently shown great success in the field of optical sensors based on the Lossy Mode Resonance (LMR) phenomenon. The properties of Graphene Oxide (GO) have been widely exploited in various sectors of science and technology, with promising results for gas sensing applications. This work combines both, the LMR-based sensing technology on planar waveguides and the use of a GO thin film as a sensitive coating, to monitor ethanol, water, and acetone. Experimental results on the fabrication and performance of the sensor are presented. The obtained results showed a sensitivity of 3.1, 2.0, and 0.6 pm/ppm for ethanol, water, and acetone respectively, with a linearity factor R2 > 0.95 in all cases.

Spectral measurements with hybrid LMR and SAW platform for dual parameter sensing.

Lossy mode resonance (LMR)-based optical sensors change their wavelength upon contact with substances or gases. This allows developing applications to detect the refractive index of the surrounding medium and even the thickness of the biolayers deposited on the waveguide. In the same way, when acoustic sensors are in contact with a liquid, it is possible to determine parameters, especially mechanical ones such as shape of the particle or molecule, mass load, elastic constants and viscosity of the liquid. This work reports the development of a system that combines LMR with surface acoustic wave (SAW) technologies to characterize a liquid in terms of its refractive index and viscosity simultaneously. Conveniently prepared glucose solutions are used for sensor calibration. The refractive index of the solutions ranges from 1.33 to 1.41 and its viscosity ranges from 1.005 mPa·s to 9 mPa·s, respectively. A sensitivity of 332 nm per RIU has been achieved with the optical sensor while the acoustic sensor has shown a sensitivity of -1.5 dB/(mPa·s). This new combinational concept could be expanded to the development of more demanding applications such as chemical sensors or biosensors.

Design and On-Field Validation of an Embedded System for Monitoring Second-Life Electric Vehicle Lithium-Ion Batteries.

In the last few years, the growing demand for electric vehicles (EVs) in the transportation sector has contributed to the increased use of electric rechargeable batteries. At present, lithium-ion (Li-ion) batteries are the most commonly used in electric vehicles. Although once their storage capacity has dropped to below 80-70% it is no longer possible to use these batteries in EVs, it is feasible to use them in second-life applications as stationary energy storage systems. The purpose of this study is to present an embedded system that allows a Nissan® LEAF Li-ion battery to communicate with an Ingecon® Sun Storage 1Play inverter, for control and monitoring purposes. The prototype was developed using an Arduino® microcontroller and a graphical user interface (GUI) on LabVIEW®. The experimental tests have allowed us to determine the feasibility of using Li-ion battery packs (BPs) coming from the automotive sector with an inverter with no need for a prior disassembly and rebuilding process. Furthermore, this research presents a programming and hardware methodology for the development of the embedded systems focused on second-life electric vehicle Li-ion batteries. One second-life battery pack coming from a Nissan® Leaf and aged under real driving conditions was integrated into a residential microgrid serving as an energy storage system (ESS).

Lossy Mode Resonance Based Microfluidic Platform Developed on Planar Waveguide for Biosensing Applications.

The development of resonance phenomena-based optical biosensors has gained relevance in recent years due to the excellent optical fiber properties and progress in the research on materials and techniques that allow resonance generation. However, for lossy mode resonance (LMR)-based sensors, the optical fiber presents disadvantages, such as the need for splicing the sensor head and the complex polarization control. To avoid these issues, planar waveguides such as coverslips are easier to handle, cost-effective, and more robust structures. In this work, a microfluidic LMR-based planar waveguide platform was proposed, and its use for biosensing applications was evaluated by detecting anti-immunoglobulin G (anti-IgG). In order to generate the wavelength resonance, the sensor surface was coated with a titanium dioxide (TiO2) thin-film. IgG antibodies were immobilized by covalent binding, and the detection assay was carried out by injecting anti-IgG in PBS buffer solutions from 5 to 20 µg/mL. The LMR wavelength shifted to higher values when increasing the analyte concentration, which means that the proposed system was able to detect the IgG/anti-IgG binding. The calibration curve was built from the experimental data obtained in three repetitions of the assay. In this way, a prototype of an LMR-based biosensing microfluidic platform developed on planar substrates was obtained for the first time.

Fault Detection of Planetary Gears Based on Signal Space Constellations.

A new method to process the vibration signal acquired by an accelerometer placed in a planetary gearbox housing is proposed, which is useful to detect potential faults. The method is based on the phenomenological model and consists of the projection of the healthy vibration signals onto an orthonormal basis. Low pass components representation and Gram-Schmidt's method are conveniently used to obtain such a basis. Thus, the measured signals can be represented by a set of scalars that provide information on the gear state. If these scalars are within a predefined range, then the gear can be diagnosed as correct; in the opposite case, it will require further evaluation. The method is validated using measured vibration signals obtained from a laboratory test bench.

Optimization of Fiber Bragg Gratings Inscribed in Thin Films Deposited on D-Shaped Optical Fibers.

A fiber Bragg grating patterned on a SnO2 thin film deposited on the flat surface of a D-shaped polished optical fiber is studied in this work. The fabrication parameters of this structure were optimized to achieve a trade-off among reflected power, full width half maximum (FWHM), sensitivity to the surrounding refractive index (SRI), and figure of merit (FOM). In the first place, the influence of the thin film thickness, the cladding thickness between the core and the flat surface of the D-shaped fiber (neck), and the length of the D-shaped zone over the reflected power and the FWHM were assessed. Reflected peak powers in the range from -2 dB to -10 dB can be easily achieved with FWHM below 100 pm. In the second place, the sensitivity to the SRI, the FWHM, and the FOM were analyzed for variations of the SRI in the 1.33-1.4 range, the neck, and the thin-film thickness. The best sensitivities theoretically achieved for this device are next to 40 nm/RIU, while the best FOM has a value of 114 RIU-1.

Trends in the Design of Intensity-Based Optical Fiber Biosensors (2010-2020).

There exists an increasing interest in monitoring low concentrations of biochemical species, as they allow the early-stage detection of illnesses or the monitoring of the environment quality. Thus, both companies and research groups are focused on the development of accurate, fast and highly sensitive biosensors. Optical fiber sensors have been widely employed for these purposes because they provide several advantages for their use in point-of-care and real-time applications. In particular, this review is focused on optical fiber biosensors based on luminescence and absorption. Apart from the key parameters that determine the performance of a sensor (limit of detection, sensibility, cross-sensibility, etc.), other features are analyzed, such as the optical fiber dimensions, the sensing set ups and the fiber functionalization. The aim of this review is to have a comprehensive insight of the different aspects that must be taken into account when working with this kind of sensors.

Interdigital concept in photonic sensors based on an array of lossy mode resonances.

Multi-parameter detection is key in the domain of sensors. Here it is demonstrated that an indium tin oxide (ITO) nanocoating can be used to generate multiple lossy mode resonances (LMRs) in the optical spectrum. To achieve this, a nanocoating with a gradient in thickness is generated on the surface of a planar waveguide, permitting broadening of the LMR because the position of an LMR in the optical spectrum is directly related to the nanocoating thickness. The nanocoating with a gradient in thickness contributes multiple LMRs, each one centred at a different wavelength. With a further etching or deposition using a mask, a pattern of deposited and non-deposited regions can be created, resulting in isolation of the LMRs by preventing LMR overlap. This enables tracking of each central wavelength separately, which can be tuned through control of the gradient or nanocoating pattern. The array of LMR-based sensors is a photonics analogue to the interdigital concept in electronics, enabling multiple resonances to be used for multiparameter sensing.

Dually nanocoated planar waveguides towards multi-parameter sensing.

The incidence of light on the edge of a glass coverslip for a microscope slide, deposited with a thin film on both faces, permits exciting two resonances in each polarisation state of the input light, TE and TM. This dually nanocoated waveguide can be used for detecting simultaneously two different parameters on the basis of a further deposition of suitable materials on each face. As an example, the possibility of detecting temperature and humidity by using polydimethylsiloxane and agarose coatings, respectively, was demonstrated, which opens the path for the development of other dual-parameter sensors, and for even more parameters in cases in which each face of the coverslip is patterned. Moreover, the device was optimised in order to position two resonances in the near infrared (NIR) and two resonances in the visible region, with sensitivities of 0.34 nm/°C and 0.23 nm/%RH in the visible region and 1.16 nm/°C and 0.34 nm/%RH in the NIR, respectively, demonstrating the possibility of using the device in both spectral ranges and opening the path for the development of sensors based on multiple resonances, each one related to a different parameter to be detected.

Fiber Optic Gas Sensors Based on Lossy Mode Resonances and Sensing Materials Used Therefor: A Comprehensive Review.

Pollution in cities induces harmful effects on human health, which continuously increases the global demand of gas sensors for air quality control and monitoring. In the same manner, the industrial sector requests new gas sensors for their productive processes. Moreover, the association between exhaled gases and a wide range of diseases or health conditions opens the door for new diagnostic applications. The large number of applications for gas sensors has permitted the development of multiple sensing technologies. Among them, optical fiber gas sensors enable their utilization in remote locations, confined spaces or hostile environments as well as corrosive or explosive atmospheres. Particularly, Lossy Mode Resonance (LMR)-based optical fiber sensors employ the traditional metal oxides used for gas sensing purposes for the generation of the resonances. Some research has been conducted on the development of LMR-based optical fiber gas sensors; however, they have not been fully exploited yet and offer optimal possibilities for improvement. This review gives the reader a complete overview of the works focused on the utilization of LMR-based optical fiber sensors for gas sensing applications, summarizing the materials used for the development of these sensors as well as the fabrication procedures and the performance of these devices.

Author Correction: Lossy mode resonance sensors based on lateral light incidence in nanocoated planar waveguides.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Generation of lossy mode resonances with different nanocoatings deposited on coverslips.

The generation of lossy mode resonances (LMRs) with a setup based on lateral incidence of light in coverslips is a simple platform that can be used for sensing. Here the versatility of this platform is proved by studying the deposition of different coating materials. The devices were characterized with both SEM and AFM microscopy, as well as ellipsometry, which allowed obtaining the main parameters of the coatings (thickness, refractive index and extinction coefficient) and relating them with the different sensitivities to refractive index attained with each material. In this way it was possible to confirm and complete the basic rules observed with lossy mode resonance based optical fiber sensors towards the design of simpler and more compact applications in domains such as chemical sensors or biosensors.

Lossy mode resonance sensors based on lateral light incidence in nanocoated planar waveguides.

The deposition of an indium oxide (In2O3) thin film on conventional planar waveguides (a coverslip and a glass slide) allows generating lossy mode resonances (LMR) by lateral incidence of light on the waveguide and by registering the optical spectrum in a spectrometer. This novel sensing system becomes an alternative to optical fibre, the substrate where LMR-based sensors have been developed so far, since it is easier to handle and more robust. An additional advantage is that cost effective waveguides, such as slides or coverslips, can be used in a platform that resembles surface plasmon resonance-based sensors in the Kretschmann configuration but without the need for a coupling prism and with the advantage of being able to generate TE and TM LMR resonances with metallic oxide or polymer thin films. The results are corroborated with simulations, which provide in-depth understanding of the phenomena involved in the sensing system. As a proof-of-concept for the optical platform, two refractometers were developed, one with low sensitivity and for a wide range of refractive indices, and the other with higher sensitivity but for a narrower refractive index range. The sensors presented here open up the path for the development of LMR-based chemical sensors, environmental sensors, biosensors, or even the generation of other optical phenomena with the deposition of multilayer structures, gratings or nanostructures, which is much easier in a planar waveguide than in an optical fibre.

Fabrication of Long Period Gratings by Periodically Removing the Coating of Cladding-Etched Single Mode Optical Fiber Towards Optical Fiber Sensor Development.

Here, we present a novel method to fabricate long period gratings using standard single mode optical fibers (SMF). These optical devices were fabricated in a three-step process, which consisted of etching the SMF, then coating it with a thin-film and, the final step, which involved removing sections of the coating periodically by laser ablation. Tin dioxide was chosen as the material for this study and it was sputtered using a pulsed DC sputtering system. Theoretical simulations were performed in order to select the appropriate parameters for the experiments. The responses of two different devices to different external refractive indices was studied, and the maximum sensitivity obtained was 6430 nm/RIU for external refractive indices ranging from 1.37 to 1.39.

Femtomolar Detection by Nanocoated Fiber Label-Free Biosensors.

The advent of optical fiber-based biosensors combined with that of nanotechnologies has provided an opportunity for developing in situ, portable, lightweight, versatile, and high-performance optical sensing platforms. We report on the generation of lossy mode resonances by the deposition of nanometer-thick metal oxide films on optical fibers, which makes it possible to measure precisely and accurately the changes in optical properties of the fiber-surrounding medium with very high sensitivity compared to other technology platforms, such as long period gratings or surface plasmon resonances, the gold standard in label-free and real-time biomolecular interaction analysis. This property, combined with the application of specialty structures such as D-shaped fibers, permits enhancing the light-matter interaction. SEM and TEM imaging together with X-EDS tool have been utilized to characterize the two films used, i.e., indium tin oxide and tin dioxide. Moreover, the experimental transmission spectra obtained after the deposition of the nanocoatings have been numerically corroborated by means of wave propagation methods. With the use of a conventional wavelength interrogation system and ad hoc developed microfluidics, the shift of the lossy mode resonance can be reliably recorded in response to very low analyte concentrations. Repeated experiments confirm a big leap in performance thanks to the capability to detect femtomolar concentrations in human serum, improving the detection limit by 3 orders of magnitude when compared with other fiber-based configurations. The biosensor has been regenerated several times by injecting sodium dodecyl sulfate, which proves the capability of sensor to be reused.

Micro and Nanostructured Materials for the Development of Optical Fibre Sensors.

The measurement of chemical and biomedical parameters can take advantage of the features exclusively offered by optical fibre: passive nature, electromagnetic immunity and chemical stability are some of the most relevant ones. The small dimensions of the fibre generally require that the sensing material be loaded into a supporting matrix whose morphology is adjusted at a nanometric scale. Thanks to the advances in nanotechnology new deposition methods have been developed: they allow reagents from different chemical nature to be embedded into films with a thickness always below a few microns that also show a relevant aspect ratio to ensure a high transduction interface. This review reveals some of the main techniques that are currently been employed to develop this kind of sensors, describing in detail both the resulting supporting matrices as well as the sensing materials used. The main objective is to offer a general view of the state of the art to expose the main challenges and chances that this technology is facing currently.

Optimization in nanocoated D-shaped optical fiber sensors.

Nanocoated D-shaped optical fibers have been proven as effective sensors. Here, we show that the full width at half minimum (FWHM) of lossy mode resonance can be reduced by optimizing the nanocoating width, thickness and refractive index. As a counterpart, several resonances are observed in the optical spectrum for specific conditions. These resonances are caused by multiple modes guided in the nanocoating. By optimizing the width of the coating and the imaginary part of its refractive index, it is possible to isolate one of these resonances, which allows one to reduce the full width at half minimum of the device and, hence, to increase the figure of merit. Moreover, it is even possible to avoid the need of a polarizer by designing a device where the resonance bands for TE and TM polarization are centered at the same wavelength. This is interesting for the development of optical filters and sensors with a high figure of merit.

Humidity Sensor Based on Bragg Gratings Developed on the End Facet of an Optical Fiber by Sputtering of One Single Material.

The refractive index of sputtered indium oxide nanocoatings has been altered just by changing the sputtering parameters, such as pressure. These induced changes have been exploited for the generation of a grating on the end facet of an optical fiber towards the development of wavelength-modulated optical fiber humidity sensors. A theoretical analysis has also been performed in order to study the different parameters involved in the fabrication of this optical structure and how they would affect the sensitivity of these devices. Experimental and theoretical results are in good agreement. A sensitivity of 150 pm/%RH was obtained for relative humidity changes from 20% to 60%. This kind of humidity sensors shows a maximum hysteresis of 1.3% relative humidity.