Innovative Photonic Sensors for Safety and Security, Part II: Aerospace and Submarine Applications.

The employability of photonics technology in the modern era's highly demanding and sophisticated domain of aerospace and submarines has been an appealing challenge for the scientific communities. In this paper, we review our main results achieved so far on the use of optical fiber sensors for safety and security in innovative aerospace and submarine applications. In particular, recent results of in-field applications of optical fiber sensors in aircraft monitoring, from a weight and balance analysis to vehicle Structural Health Monitoring (SHM) and Landing Gear (LG) monitoring, are presented and discussed. Moreover, underwater fiber-optic hydrophones are presented from the design to marine application.

Innovative Photonic Sensors for Safety and Security, Part I: Fundamentals, Infrastructural and Ground Transportations.

Our group, involving researchers from different universities in Campania, Italy, has been working for the last twenty years in the field of photonic sensors for safety and security in healthcare, industrial and environment applications. This is the first in a series of three companion papers. In this paper, we introduce the main concepts of the technologies employed for the realization of our photonic sensors. Then, we review our main results concerning the innovative applications for infrastructural and transportation monitoring.

Innovative Photonic Sensors for Safety and Security, Part III: Environment, Agriculture and Soil Monitoring.

In order to complete this set of three companion papers, in this last, we focus our attention on environmental monitoring by taking advantage of photonic technologies. After reporting on some configurations useful for high precision agriculture, we explore the problems connected with soil water content measurement and landslide early warning. Then, we concentrate on a new generation of seismic sensors useful in both terrestrial and under water contests. Finally, we discuss a number of optical fiber sensors for use in radiation environments.

Ultrasound waves in tumors via needle irradiation for precise medicine.

Grounded in the interdisciplinary crosstalk among physics and biological sciences, precision medicine-based diagnosis and treatment strategies have recently gained great attention for the actual applicability of new engineered approaches in many medical fields, particularly in oncology. Within this framework, the use of ultrasounds employed to attack cancer cells in tumors to induce possible mechanical damage at different scales has received growing attention from scholars and scientists worldwide. With these considerations in mind, on the basis of ad hoc elastodynamic solutions and numerical simulations, we propose a pilot study for in silico modeling of the propagation of ultrasound waves inside tissues, with the aim of selecting proper frequencies and powers to be irradiated locally through a new teragnostic platform based on Lab-on-Fiber technology, baptized as a hospital in the needle and already the object of a patent. It is felt that the outcomes and the related biophysical insights gained from the analyses could pave the way for envisaging new integrated diagnostic and therapeutic approaches that might play a central role in future applications of precise medicine, starting from the growing synergy among physics, engineering and biology.

Liquid Resin Infusion Process Validation through Fiber Optic Sensor Technology.

In the proposed work, a fiber-optic-based sensor network was employed for the monitoring of the liquid resin infusion process. The item under test was a panel composed by a skin and four stringers, sensorized in such a way that both the temperature and the resin arrival could be monitored. The network was arranged with 18 Fiber Bragg Gratings (FBGs) working as temperature sensors and 22 fiber optic probes with a modified front-end in order to detect the resin presence. After an in-depth study to find a better solution to install the sensors without affecting the measurements, the system was investigated using a commercial Micron Optics at 0.5 Hz, with a passive split-box connected in order to be able to sense all the sensors simultaneously. The obtained results in terms of resin arrival detection at different locations and the relative temperature trend allowed us to validate an infusion process numerical model, giving us better understanding of what the actual resin flow was and the time needed to dry preform filling during the infusion process.

Feasibility analysis of an ultrasound on line diagnostic approach for oral and bone surgery.

During implant surgery procedures, surgical precision is an essential prerequisite for the functional and aesthetic success of the prosthetic crown to be placed on the dental implant. A modern implant surgical approach should be standardized as much as possible to guarantee extreme precision in the insertion of the implant into the upper and lower bone jaws. Among the most common surgical errors during implant surgery there is the over-preparation of the surgical alveolus with possible damage to the contiguous anatomical structures. To avoid this problem, in the recent years, there has been an increasing attention to the development of new control techniques. In this paper, we describe an innovative ultrasound approach, which exploits the integration of an electro-acoustic transducer with the surgical drill used for realizing the alveolus in the bone that will host the implant. Specifically, he proposed approach is based on the "time-of-flight" detection technique for measuring the thickness of the residual bone subjected to the drilling. In order to demonstrate the feasibility of the proposed approach, here we report on a detailed numerical analysis aimed at studying the propagation of ultrasonic waves through the drill-bit and through the involved tissues. The obtained results confirm the validity of our approach, and enable for a future first prototype implementation of a hi-tech surgical drill-bit, which in general is suitable not only for dental implant surgery but also for other uses in oral surgery, maxillofacial surgery and for bone surgery.

A novel method for EBT3 Gafchromic films read-out at high dose levels.

In this work, we report on a novel approach for measuring the dose absorbed by the EBT3 Gafchromic™ films exposed to 1 MeV electron beam and 250 kV X-rays in the range 0.5-100 Gy. Although EBT3 is specifically designed to obtain best performance for applications where the maximum dose is less than 10 Gy, there are certain clinical applications requiring dose ranges well above this value. In order to cover wider dose ranges, further models characterized by a thinner sensitive layer and/or different chemical composition have been released. Another method exploiting the three-channel flatbed scanner to delay the saturation point of EBT3 has been also reported. The technique proposed here, aimed at extending the sensitivity of the EBT3 film to high doses up to 100 Gy while ensuring a low dose uncertainty, is based on a broadband analysis of the absorption spectrum of the film in response to irradiation. By combining a wavelength-based approach with the monitoring of two characteristic peaks of the EBT3 absorption spectrum, we demonstrated the capability of measuring the dose in the range 0.5-100 Gy with an experimental uncertainty below 4% for doses lower than 5.52 Gy and below 2% for higher dose levels. Finally, through a dynamic fitting procedure integrating the two aforesaid approaches, a total uncertainty lower than 4%, including both the experimental and fitting errors, was achieved in the whole range 0.5-100 Gy. These results are promising in view of a potential application of this technique in the field of clinical dosimetry at high dose levels.

Optical fiber technology enables smart needles for epidurals: an in-vivo swine study.

Nowadays, epidural space identification is made by using subjective and manual techniques characterized by failure rates up to 7%. In this work, we propose a fiber optic sensor technology based needle guidance system, that is directly inspired by the most common technique currently used for epidurals; through real-time strain measurements, the fiber Bragg grating integrated inside the needle lumen is able to effectively perceive the typical force drop occurring when the needle enters the epidural space. An in vivo swine study demonstrates the validity of our approach, paving the way for the development of lab-in-a-needle systems.

Real-time dosimetry with radiochromic films.

Radiochromic film dosimetry has been widely employed in most of the applications of radiation physics for over twenty years. This is due to a number of appealing features of radiochromic films, such as reliability, accuracy, ease of use and cost. However, current radiochromic film reading techniques, based on the use of commercial densitometers and scanners, provide values of dose only after the exposure of the films to radiation. In this work, an innovative methodology for the real-time reading of radiochromic films is proposed for some specific applications. The new methodology is based on opto-electronic instrumentation that makes use of an optical fiber probe for the determination of optical changes of the films induced by radiation and allows measurements of dose with high degree of precision and accuracy. Furthermore, it has been demonstrated that the dynamic range of some kinds of films, such as the EBT3 Gafchromic films (intensively used in medical physics), can be extended by more than one order of magnitude. Owing to the numerous advantages with respect to the commonly used reading techniques, a National Patent was filed in January 2018.

A novel Lab-on-Fiber Radiation Dosimeter for Ultra-high Dose Monitoring.

In this work, we report on the first demonstration of Lab on Fiber (LOF) dosimeter for ionizing radiation monitoring at ultra-high doses. The new dosimeter consists in a metallo-dielectric resonator at sub-wavelength scale supporting localized surface plasmon resonances realized on the optical fiber (OF) tip. The resonating structure involves two gold gratings separated by a templated dielectric layer of poly(methyl methacrylate) (PMMA). Two LOF prototypes have been manufactured and exposed at the IRRAD Proton Facility at CERN in Geneva to 23 GeV protons for a total fluence of 0.67 × 1016 protons/cm2, corresponding to an absorbed dose of 1.8 MGy. Experimental data demonstrated the "radiation resistance" feature of the LOF devices and a clear dependence of the reflected spectrum versus the total dose, expressed by a cumulative blue-shift of ~1.4 nm of the resonance combined with a slight increase of 0.16 dBm in the reflected spectrum. The numerical analysis carried out to correlate the experimental results with the dimensional and physical properties of the resonator, expected to be tightly connected to the absorbed dose, suggests that the main phenomenon induced by exposure to proton beam and able to explain the measured spectral behavior is the reduction of the PMMA thickness, which is also consistent with past literature in the field. Preliminary results demonstrated the potentiality of the proposed platform as dosimeter at MGy dose levels for high energy physics experiments.

Smart Optical Catheters for Epidurals.

Placing the needle inside the epidural space for locoregional anesthesia is a challenging procedure, which even today is left to the expertise of the operator. Recently, we have demonstrated that the use of optically sensorized needles significantly improves the effectiveness of this procedure. Here, we propose an optimized configuration, where the optical fiber strain sensor is directly integrated inside the epidural catheter. The new design allows the solving of the biocompatibility issues and increases the versatility of the former configuration. Through an in vivo study carried out on a porcine model, we confirm the reliability of our approach, which also opens the way to catheter monitoring during insertion inside biological spaces.

Liquefied Petroleum Gas Monitoring System Based on Polystyrene Coated Long Period Grating.

In this work, we report the in-field demonstration of a liquefied petroleum gas monitoring system based on optical fiber technology. Long-period grating coated with a thin layer of atactic polystyrene (aPS) was employed as a gas sensor, and an array comprising two different fiber Bragg gratings was set for the monitoring of environmental conditions such as temperature and humidity. A custom package was developed for the sensors, ensuring their suitable installation and operation in harsh conditions. The developed system was installed in a real railway location scenario (i.e., a southern Italian operative railway tunnel), and tests were performed to validate the system performances in operational mode. Daytime normal working operations of the railway line and controlled gas expositions, at very low concentrations, were the searched realistic conditions for an out-of-lab validation of the developed system. Encouraging results were obtained with a precise indication of the gas concentration and external conditioning of the sensor.

Opto-mechanical lab-on-fibre seismic sensors detected the Norcia earthquake.

We have designed and developed lab-on-fibre seismic sensors containing a micro-opto-mechanical cavity on the fibre tip. The mechanical cavity is designed as a double cantilever suspended on the fibre end facet and connected to a proof mass to tune its response. Ground acceleration leads to displacement of the cavity length, which in turn can be remotely detected using an interferometric interrogation technique. After the sensors characterization, an experimental validation was conducted at the Italian National Institute of Geophysics and Volcanology (INGV), which is responsible for seismic surveillance over the Italian country. The fabricated sensors have been continuously used for long periods to demonstrate their effectiveness as seismic accelerometer sensors. During the tests, fibre optic seismic accelerometers clearly detected the seismic sequence that culminated in the severe Mw6.5 Norcia earthquake that struck central Italy on October 30, 2016. The seismic data provided by the optical sensors were analysed by specialists at the INGV. The wave traces were compared with state-of-the-art traditional sensors typically incorporated into the INGV seismic networks. The comparison verifies the high fidelity of the optical sensors in seismic wave detection, indicating their suitability for a novel class of seismic sensors to be employed in practical scenarios.

Label-free fiber optic optrode for the detection of class C ß-lactamases expressed by drug resistant bacteria.

This paper reports the experimental assessment of an automated optical assay based on label free optical fiber optrodes for the fast detection of class C ß-lactamases (AmpC BLs), actually considered as one of the most important sources of resistance to ß-lactams antibiotics expressed by resistant bacteria. Reflection-type long period fiber gratings (RT-LPG) have been used as highly sensitive label free optrodes, while a higher affine boronic acid-based ligand was here selected to enhance the overall assay performances compared to those obtained in our first demonstration. In order to prove the feasibility analysis towards a fully automated optical assay, an engineered system was developed to simultaneously manipulate and interrogate multiple fiber optic optrodes in the different phases of the assay. The automated system tested in AmpC solutions at increasing concentrations demonstrated a limit of detection (LOD) of 6 nM, three times better when compared with the results obtained in our previous work. Moreover, the real effectiveness of the proposed optical assay has been also confirmed in complex matrices as the case of lysates of Escherichia coli overexpressing AmpC.

Fiber Optic Thermo-Hygrometers for Soil Moisture Monitoring.

This work deals with the fabrication, prototyping, and experimental validation of a fiber optic thermo-hygrometer-based soil moisture sensor, useful for rainfall-induced landslide prevention applications. In particular, we recently proposed a new generation of fiber Bragg grating (FBGs)-based soil moisture sensors for irrigation purposes. This device was realized by integrating, inside a customized aluminum protection package, a FBG thermo-hygrometer with a polymer micro-porous membrane. Here, we first verify the limitations, in terms of the volumetric water content (VWC) measuring range, of this first version of the soil moisture sensor for its exploitation in landslide prevention applications. Successively, we present the development, prototyping, and experimental validation of a novel, optimized version of a soil VWC sensor, still based on a FBG thermo-hygrometer, but able to reliably monitor, continuously and in real-time, VWC values up to 37% when buried in the soil.

Optical fiber meta-tips.

We report on the first demonstration of a proof-of-principle optical fiber 'meta-tip', which integrates a phase-gradient plasmonic metasurface on the fiber tip. For illustration and validation purposes, we present numerical and experimental results pertaining to various prototypes implementing generalized forms of the Snell's transmission/reflection laws at near-infrared wavelengths. In particular, we demonstrate several examples of beam steering and coupling with surface waves, in fairly good agreement with theory. Our results constitute a first step toward the integration of unprecedented (metasurface-enabled) light-manipulation capabilities in optical-fiber technology. By further enriching the emergent 'lab-on-fiber' framework, this may pave the way for the widespread diffusion of optical metasurfaces in real-world applications to communications, signal processing, imaging and sensing.

Nanosphere lithography for optical fiber tip nanoprobes.

This paper reports a simple and economical method for the fabrication of nanopatterned optical fiber nanotips. The proposed patterning approach relies on the use of the nanosphere lithography of the optical fiber end facet. Polystyrene (PS) nanospheres are initially self-assembled in a hexagonal array on the surface of water. The created pattern is then transferred onto an optical fiber tip (OFT). The PS monolayer colloidal crystal on the OFT is the basic building block that is used to obtain different periodic structures by applying further treatment to the fiber, such as metal coating, nanosphere size reduction and sphere removal. Ordered dielectric and metallo-dielectric sphere arrays, metallic nanoisland arrays and hole-patterned metallic films with feature sizes down to the submicron scale are achievable using this approach. Furthermore, the sizes and shapes of these periodic structures can be tailored by altering the fabrication conditions. The results indicate that the proposed self-assembly approach is a valuable route for the development of highly repeatable metallo-dielectric periodic patterns on OFTs with a high degree of order and low fabrication cost. The method can be easily extended to simultaneously produce multiple fibers, opening a new route to the development of fiber-optic nanoprobes. Finally, we demonstrate the effective application of the patterned OFTs as surface-enhanced Raman spectroscopy nanoprobes.

Long period fiber grating nano-optrode for cancer biomarker detection.

We report an innovative fiber optic nano-optrode based on Long Period Gratings (LPGs) working in reflection mode for the detection of human Thyroglobulin (TG), a protein marker of differentiated thyroid cancer. The reflection-type LPG (RT-LPG) biosensor, coated with a single layer of atactic polystyrene (aPS) onto which a specific, high affinity anti-Tg antibody was adsorbed, allowed the label-free detection of Tg in the needle washouts of fine-needle aspiration biopsies, at concentrations useful for pre- and post-operative assessment of the biomarker levels. Analyte recognition and capture were confirmed with a parallel on fiber ELISA-like assay using, in pilot tests, the biotinylated protein and HRP-labeled streptavidin for its detection. Dose-dependent experiments showed that the detection is linearly dependent on concentration within the range between 0 and 4 ng/mL, while antibody saturation occurs for higher protein levels. The system is characterized by a very high sensitivity and specificity allowing the ex-vivo detection of sub ng/ml concentrations of human Tg from needle washouts of fine-needle aspiration biopsies of thyroid nodule from different patients.

An intrusion detection system for the protection of railway assets using Fiber Bragg Grating sensors.

We demonstrate the ability of Fiber Bragg Gratings (FBGs) sensors to protect large areas from unauthorized activities in railway scenarios such as stations or tunnels. We report on the technological strategy adopted to protect a specific depot, representative of a common scenario for security applications in the railway environment. One of the concerns in the protection of a railway area centers on the presence of rail-tracks, which cannot be obstructed with physical barriers. We propose an integrated optical fiber system composed of FBG strain sensors that can detect human intrusion for protection of the perimeter combined with FBG accelerometer sensors for protection of rail-track access. Several trials were carried out in indoor and outdoor environments. The results demonstrate that FBG strain sensors bonded under a ribbed rubber mat enable the detection of intruder break-in via the pressure induced on the mat, whereas the FBG accelerometers installed under the rails enable the detection of intruders walking close to the railroad tracks via the acoustic surface waves generated by footsteps. Based on a single enabling technology, this integrated system represents a valuable intrusion detection system for railway security and could be integrated with other sensing functionalities in the railway field using fiber optic technology.

Lab-on-fiber technology: toward multifunctional optical nanoprobes.

We propose a reliable fabrication process enabling the integration of dielectric and metallic nanostructures on the tip of optical fibers, thus representing a further step in the "lab-on-fiber" technology roadmap. The proposed fabrication procedure involves conventional deposition and nanopatterning techniques, typically used for planar devices, but here adapted to directly operate on optical fiber tip. Following this approach, we demonstrate a first technological platform based on the integration onto the optical fiber tip of two-dimensional hybrid metallo-dielectric nanostructures supporting localized surface plasmon resonances. By means of experimental measurements and full-wave numerical simulations, we characterize these resonant phenomena and investigate the underlying physics. We show that resonances can be easily tuned by acting on the physical and geometrical parameters of the structure. Moreover, with a view toward possible applications, we present some preliminary results demonstrating how the proposed device can work effectively as an optical probe for label-free chemical and biological sensing as well as a microphone for acoustic wave detection.