Recent advances in biomedical photonic sensors: a focus on optical-fibre-based sensing

In this invited review, we provide an overview of the recent advances in biomedical pho tonic sensors within the last five years. This review is focused on works using optical-fibre technology, employing diverse optical fibres, sensing techniques, and configurations applied in several medical fields. We identified technical innovations and advancements with increased implementations of optical-fibre sensors, multiparameter sensors, and control systems in real applications. Examples of outstanding optical-fibre sensor performances for physical and biochemical parameters are covered, including diverse sensing strategies and fibre-optical probes for integration into medical instruments such as catheters, needles, or endoscopes.This work was supported by Ministerio de Ciencia e Innovación and Agencia Estatal de Investigación (PID2019-107270RB-C21/AEI/10.13039/501100011033), and TeDFeS Project (RTC-2017- 6321-1) co-funded by European FEDER funds. M.O. and J.F.A. received funding from Ministerio de Ciencia, Innovación y Universidades of Spain under Juan de la Cierva-Formación and Juan de la Cierva-Incorporación grants, respectively. P.R-V. received funding from Ministerio de Educación, Cultura y Deporte of Spain under PhD grant FPU2018/02797

Novel Specialty Optical Fibers and Applications

Novel Specialty Optical Fibers and Applications focuses on the latest developments in specialty fiber technology and its applications. The aim of this reprint is to provide an overview of specialty optical fibers in terms of their technological developments and applications. Contributions include:1. Specialty fibers composed of special materials for new functionalities and applications in new spectral windows.2. Hollow-core fiber-based applications.3. Functionalized fibers.4. Structurally engineered fibers.5. Specialty fibers for distributed fiber sensors.6. Specialty fibers for communications

Fibre optic hydrogen sensing for long term use in explosive environments

Hydrogen is an explosive and flammable gas with a lower explosive limit of just 4% volume in air. It is important to monitor the concentration of hydrogen in a potentially hazardous environment where hydrogen may be released as a by-product in a reaction or used as a principal gas/liquid. A fibre optic based hydrogen sensor offers an intrinsically safe method of monitoring hydrogen concentration. Previous research studies have demonstrated a variety of fibre optic based techniques for hydrogen detection. However the long-term stability of the hydrogen sensor and interrogation system has not yet been assessed and is the focus of this study. In the case of sensor heads being permanently installed in-situ, they cannot be removed for regular replacement, making long-term stability and reliability of results an important feature of the hydrogen sensor. This thesis describes the investigation and characterisation of palladium coated fibre optic sensor heads using two designs of self-referenced refractometer systems with the aim of finding a system that is stable in the long term (~6 months). Palladium was the chosen sensing material owing to its selective affinity for absorbing hydrogen. Upon hydrogen absorption, palladium forms a palladium- hydride compound that has a lower refractive index and lower reflectivity than pure palladium. The refractometers measured the changes in the reflectivity to enable calculation of the concentration of hydrogen present. A low detection limit of 10ppm H2 in air was demonstrated, with a response time of 40s for 1000ppm H2 in air. A further aspect to sensor stability was investigated in the form of sensor heads that had a larger area for palladium coverage. Hydrogen induced cracking in palladium is a common failure mechanism. A hypothesis is presented that a larger sensor area can reduce the probability of catastrophic failure resulting from cracks, which may improve the predictability of the sensor’s performance. Two sensor head designs have been proposed – fibre with a ball lens at the tip and fibre with a GRIN lens at the tip, both of which potentially offer a larger area than the core of a singlemode optical fibre. The limit of detection and response times of the sensor heads were characterised in hydrogen. For long term stability assessment of the sensor head and the interrogation unit, the system was left running for a period of 1 to 4 weeks and the noise and drift in the system was quantified using an Allan deviation plot

Femtosecond Laser Micromachining of Advanced Fiber Optic Sensors and Devices

Research and development in photonic micro/nano structures functioned as sensors and devices have experienced significant growth in recent years, fueled by their broad applications in the fields of physical, chemical and biological quantities. Compared with conventional sensors with bulky assemblies, recent process in femtosecond (fs) laser three-dimensional (3D) micro- and even nano-scale micromachining technique has been proven an effective and flexible way for one-step fabrication of assembly-free micro devices and structures in various transparent materials, such as fused silica and single crystal sapphire materials. When used for fabrication, fs laser has many unique characteristics, such as negligible cracks, minimal heat-affected-zone, low recast, high precision, and the capability of embedded 3D fabrication, compared with conventional long pulse lasers. The merits of this advanced manufacturing technique enable the unique opportunity to fabricate integrated sensors with improved robustness, enriched functionality, enhanced intelligence, and unprecedented performance. Recently, fiber optic sensors have been widely used for energy, defense, environmental, biomedical and industry sensing applications. In addition to the well-known advantages of miniaturized in size, high sensitivity, simple to fabricate, immunity to electromagnetic interference (EMI) and resistance to corrosion, all-optical fiber sensors are becoming more and more desirable when designed with characteristics of assembly free and operation in the reflection configuration. In particular, all-optical fiber sensor is a good candidate to address the monitoring needs within extreme environment conditions, such as high temperature, high pressure, toxic/corrosive/erosive atmosphere, and large strain/stress. In addition, assembly-free, advanced fiber optic sensors and devices are also needed in optofluidic systems for chemical/biomedical sensing applications and polarization manipulation in optical systems. Different fs laser micromachining techniques were investigated for different purposes, such as fs laser direct ablating, fs laser irradiation with chemical etching (FLICE) and laser induced stresses. A series of high performance assembly-free, all-optical fiber sensor probes operated in a reflection configuration were proposed and fabricated. Meanwhile, several significant sensing measurements (e.g., high temperature, high pressure, refractive index variation, and molecule identification) of the proposed sensors were demonstrated in this dissertation as well. In addition to the probe based fiber optic sensors, stress induced birefringence was also created in the commercial optical fibers using fs laser induced stresses technique, resulting in several advanced polarization dependent devices, including a fiber inline quarter waveplate and a fiber inline polarizer based on the long period fiber grating (LPFG) structure

Development of optical fibre curvateure sensors for subsea instrumentation

The experimental results presented in this thesis were part of a bigger research project (LAkHsMI) funded by the European Union (Horizon 2020). The overall project’s aim is to monitor sea flow speeds for different applications with electrical and optical fibre sensors. Here, the fabrication of the sensors which exploit Fibre Bragg Grating (FBG) using conventional, as well as novel optical fibres is presented. Two different curvature sensors were produced for the purposes of the project. The first one was fabricated using four single mode fibres (SMFs) bundled together, while the second used one Multicore fibre (MCF) with four optical cores. The structure of both permitted their curvature to be determined in two dimensions. The first type of optical fibre curvature sensor was also used for the fabrication of a FBG Attitude sensor, where the orientation of a platform in two planes could be determined with accuracy of ±2°. The main priority of the studies presented here was the fabrication and performance of the MCF curvature sensors. The transmittance and reflectance (using inscribed FBGs) of the light into the MCF is explored, where an interface device between the MCF to SMF is required. Two different cases are reported, the first uses a silica inscribed waveguide fan-out device, while the second uses a tapered MCF fan-out device. In the first case the temperature sensitivity of the silica waveguide fan-out device is detected. This sensitivity can be diminished with the use of the tapered MCF fan-out device and discussed. Moreover, in the second case coupling of light from one core of the MCF to all four cores is required. Hence, inscription of different FBGs into the cores of the MCF was achieved so that the overlap of the FBGs spectra would be avoided. The challenges that arose during the fabrication and performance of these sensors are reported. Moreover, their temperature sensitivity as well as the strengths and weaknesses over several aspects are reported. Finally, a comparison between the curvature sensors is included and states which sensor can be used for subsea flow measurements and which has the potentials for further development

Integrated multicore fibre devices for optical trapping

The work described in this thesis details the development of a multicore fibre device that can be used to optically trap multiple cells and particles. The optical trapping of multiple cells at close proximity allows for cell-to-cell interactions to be studied. Current methods available for creating arrays of traps are free space optical systems that use diffractive optics, laser scanning techniques or the interference of multiple beams to create the multiple traps. A fully integrated, fibre optic based, multiple particles, optical trapping device could be used in non-optical research facilities such as biological laboratories to aid with their research into cellular processes. In order to create the multiple traps, the distal end of the multicore fibre needs to be modified to induce a lensing effect. The multicore fibre device presented in this thesis was lensed in a fusion splicer; this refracts the outputs from the four cores to a common point in the far field where interference fringes are formed. The initial investigation demonstrated one-dimensional interferometric optical trapping through coupling light into two of the diagonal cores of the lensed multicore fibre. This produced linear interference fringes approximately 250 ± 25 μm from the end of the fibre with a fringe spacing of 2 ± 0.3 μm. The linear interference fringes were used to optically trap polystyrene microspheres with diameters of 1.3 μm, 2 μm and 3 μm in the high intensity regions of the fringes. Coupling into all four cores using a diffractive optical element produced an array of intensity peaks across the interference pattern with high visibility fringes greater than 80 %. Each intensity peak, spaced 2.75 μm apart could trap a single particle in two dimensions. The optical trapping of multiple microspheres and Escherichia coli bacterial cells was demonstrated proving that the lensed multicore fibre has the potential to be used to trap cells in biological experiments. The active manipulation of trapped 2 μm microspheres was also demonstrated through the rotation of the input polarisation to the multicore fibre. Finally, work towards creating a “turn-key” optical trapping device was demonstrated through the fabrication of a fully integrated multicore fibre device using an ultrafast laser-inscribed fan-out to couple light into each core. Single mode operation of the device was demonstrated at 1550 nm, using a weaker lensed MCF device. The two dimensional trapping of 4.5 μm polystyrene microspheres was shown in an array of peaks spaced 11.2 μm apart at a distance of 400 ± 25 μm from the end of the fibre

Glassy Materials Based Microdevices

Microtechnology has changed our world since the last century, when silicon microelectronics revolutionized sensor, control and communication areas, with applications extending from domotics to automotive, and from security to biomedicine. The present century, however, is also seeing an accelerating pace of innovation in glassy materials; as an example, glass-ceramics, which successfully combine the properties of an amorphous matrix with those of micro- or nano-crystals, offer a very high flexibility of design to chemists, physicists and engineers, who can conceive and implement advanced microdevices. In a very similar way, the synthesis of glassy polymers in a very wide range of chemical structures offers unprecedented potential of applications. The contemporary availability of microfabrication technologies, such as direct laser writing or 3D printing, which add to the most common processes (deposition, lithography and etching), facilitates the development of novel or advanced microdevices based on glassy materials. Biochemical and biomedical sensors, especially with the lab-on-a-chip target, are one of the most evident proofs of the success of this material platform. Other applications have also emerged in environment, food, and chemical industries. The present Special Issue of Micromachines aims at reviewing the current state-of-the-art and presenting perspectives of further development. Contributions related to the technologies, glassy materials, design and fabrication processes, characterization, and, eventually, applications are welcome

Optical biosensors

Continuous glucose monitoring facilitates the stringent control of blood glucose concentration in diabetic and intensive care patients. Low-cost, robust, and reusable continuous glucose monitoring systems that can provide quantitative measurements at point-of-care settings is an unmet medical need. Phenylboronic acids (PBAs) have emerged as synthetic receptors that can reversibly bind to cis-diols such as glucose molecules. The incorporation of phenylboronic acids in hydrogels offer exclusive attributes as the binding process with glucose induces Donnan potential that leads to osmotic pressure, resulting in volumetric changes in the hydrogel matrix. Optical glucose sensors based on PBA-functionalized hydrogels have emerged as strong candidates for commercialization; however, their complex and time-consuming fabrication process, and their bulky and expensive readouts methods made them undesirable for quantitative analyses. In this dissertation, optical hydrogel sensors have been developed and attached to contact lenses for continuous glucose detection in physiological conditions. A simple fabrication method was utilized, and smartphone technology was employed for recording the output signals. A 1D photonic structure was replicated on a PBA-functionalized hydrogel to function as a transducer and to improve the sensor performance. Upon binding glucose with boronate anions immobilized in the hydrogel matrix, a positive volumetric shift occurred modifying the periodic constant of the photonic structure, consequently its diffraction properties altered. A correlation has been established between the sensor’s periodic constant and glucose concentration in the range of 0-50 mM. The hydrogel sensor was attached to a soft commercial contact lens (ACUVUE) and was interrogated for glucose detection in artificial tears. The ambient light sensor of a smartphone captured the intensity of the laser diffracted signals and was correlated with glucose concentrations. The smart contact lens showed very short response time (3 s), and a saturation time of near 4 minutes in continuous monitoring conditions. However, a laser beam was necessary to interrogate the contact lens which is uncomfortable, and the frequent exposure might be harmful to the eye cornea. Alternatively, a novel transducer has been introduced to enable interrogating the smart contact lens by using a white light beam. Light diffusing microstructures (LDMs) have been introduced for the first time for the sensing applications. The LDMs can be considered as densely-packed microparticles of different shapes and dimensions which have the capacity to diffuse the polychromatic and monochromatic light in the forward and backward directions. The LDMs were imprinted on the glucose -responsive hydrogel to monitor the volumetric shift due to glucose complexation. The volumetric modification of the hydrogel upon glucose complexation induced a change in the dimensions and refractive index of the LDMs, resulting in a variation in the diffusion efficiency. The glucose sensor was attached to a commercial contact lens and a smartphone measured the optical output signals. The alternative transducer enabled interrogating the smart contact lens by a white light beam and retained on the simplicity of the fabrication and the readout methodology; however, the response time of the senor increased significantly. The proposed smart contact lenses can be considered as a non-invasive way for continuous glucose monitoring, and can detect many other biomarkers that are beneficial for medical diagnostic applications. For implantable and remote monitoring of glucose concentration, fiber optic probes have been developed. Fiber optics have inherent advantages such as immunity to electromagnetic interference, miniaturization, and small volumes of samples. These merits candidate them for biosensing applications; however, complexity of the manufacturing process, poor mechanical properties, unpracticality of the readout methodology have hindered their practical applications. We have developed fiber optic probes for glucose detection that overcome the limitations mentioned above. Capability of the LDMs to scatter/diffuse the incident light beam in the forward and backward directions was exploited. The glucose responsive hydrogel imprinted with the LDMs was attached to the tip of a multimode silica fiber. Swelling of the attached hydrogel led to a decrease in the refractive index of the LDMs, inducing a decrease in the light scattering angles. Whereas the numerical aperture of the optical fiber indicates the range of the angles of the incident rays those satisfy the total internal reflection condition. Accordingly, swelling the hydrogel attached to the fiber result in more incident rays fall within the accepted range of angles to be guided in the optical fiber. Hence, the optical power guided in the fiber increased with glucose concentration. The fabricated fiber probe was interrogated for glucose detection in transmission configuration and the smartphone was utilized to pick up the fiber’s signals. Also, the probe was tested in reflection configuration which is a more practical mode for implantable biosensing applications. The probe overcame some limitations of the existing probes such as interferometric, SPR, and fluorescent probes in terms of ease of the fabrication and the interrogation processes. Additionally, the probe showed high sensitivity, rapid response, and selectivity for glucose over lactate. The lactate interference was found to be ~ 0.1% in the physiological condition. Furthermore, biocompatible hydrogel fibers were introduced to prevent or reduce the immune reaction in the implantation sites. The probes were tested for glucose detection and showed similar response to that of silica fiber probes; however, they presented lower sensitivity which might be the result of a higher light loss in the hydrogel fiber. In order to emphasize the variety of applications of these novel fiber optic probes that we developed, two more probes were fabricated for alcohol detection and pH measurements. The alcohol probe showed real-time sensing of ethanol, propanol, and dimethyl sulfoxide with a response time in seconds and a saturation time around 60 s. Also, the pH probe showed high sensitivity and rapid response in the acidic region with a sensitivity near 20% pH-1. For medical applications, the pH sensor was attached to a biocompatible fiber optic and was tested for pH sensing in reflection configuration. The probe can be recommended for gastric pH detection. The fabricated optical fiber sensors may also have applications in wearable and implantable point-of-care and intensive-care continuous monitoring systems

Hybrid Optical Fibers – An Innovative Platform for In‐Fiber Photonic Devices

The field of hybrid optical fibers is one of the most active research areas in current fiber optics and has the vision of integrating sophisticated materials inside fibers, which are not traditionally used in fiber optics. Novel in-fiber devices with unique properties have been developed, opening up new directions for fiber optics in fields of critical interest in modern research, such as biophotonics, environmental science, optoelectronics, metamaterials, remote sensing, medicine, or quantum optics. Here the recent progress in the field of hybrid optical fibers is reviewed from an application perspective, focusing on fiber-integrated devices enabled by including novel materials inside polymer and glass fibers. The topics discussed range from nanowire-based plasmonics and hyperlenses, to integrated semiconductor devices such as optoelectronic detectors, and intense light generation unlocked by highly nonlinear hybrid waveguides

Fiber Optic Sensors for Energy Applications under Harsh Environmental Conditions

Real-time monitoring physical and chemical parameters in next generation energy-production system is of significant importance to improve the efficiency and reduce the emission for a wide range of applications. Traditional electrical point sensors have limited utilities for direct measurements at high temperature or in highly reactive and corrosive environment. Given the resilience at high temperatures, immunity to electromagnetic interference and intrinsic explosion proof in combustion gas, fiber optic sensors open up opportunity to perform various measurements in energy applications under harsh environments. In this thesis, both chemical and physical sensors were demonstrated to explore the potential of fiber optic sensors in energy industry. The first scheme is fiber optic chemical gas sensing enabled by nanostructured functional metal oxides. A scalable manufacturing approach was developed to produce nano-porous metal oxides with the refractive index tailored to match the optical fiber material. Combined with this functional semiconducting metal oxides, fiber optic chemical sensors with high selectivity and sensitivity was developed using both D-shaped fiber and single crystal sapphire fiber. The sensors performed accurate hydrogen measurement at a record-high temperature of 800 deg C. The second scheme covers a high temperature distributed sensing using Rayleigh backscatter based optical frequency domain reflectometry. Ultrafast laser direct writing method was used to enhance the in-fiber scattering signal and high-temperature stability. Due to the high signal-to-noise ratio and thermal stability of the inscribed nanogratings in the fiber, real-time monitoring of temperature distribution in the operational solid oxide fuel cell was achieved with 5-mm spatial resolution at 800 deg C. In the third scheme, a multi-point sensing system for thermal dynamics monitoring of lithium-ion battery assembly was demonstrated using multimode random air hole fibers infiltrated with quantum dots. The photoluminescence intensity dependence on the ambient temperatures were used to gauge the local operational temperature of lithium-ion batteries. Multi-point temperature sensing systems were developed by bundling quantum dots infiltrated random air hole fibers together. The temperature of the batteries can be real-time monitored using a low-cost UV diode laser as light source and a cellular phone CCD camera as detector