The use of optical fibre Bragg gratings (FBGs) and direct fibre optic shape sensing (DFOSS) in the ground vibration testing of a full size helicopter rotor blade has been evaluated, with the performance benchmarked against measurements made using accelerometers. An array of FBGs was used to monitor the amplitudes of the mode frequencies at specific locations along the blade and DFOSS was used to measure directly the shape of the blade and to characterise its modal frequencies. While it was possible to measure modal frequencies using both approaches, DFOSS proved capable of detecting modal frequencies using a single sensor array located along the longitudinal axis of the blade.

A novel sensitive and fast technique is described for monitoring pm scale shifts of narrow Bragg grating features. The technique is based on a grating pair and heterodyne measurement, and offers inherent compensation of temperature variations under strain measurement.

Optical current sensors or so called optical current transformers (OCT) based on the Faraday effect have an intrinsic temperature dependency of the current sensitivity caused by the natural drift of the Verdet constant as well as birefringence in the sensing medium. In order to reach high class accuracy of up to 0.2%, comparable to inductive current transformers, these effects have to be compensated.

A temperature-insensitive 2-D inclinometer by incorporating two fiber Bragg gratings (FBGs) with a homogeneous pendulum transducer is proposed and experimentally demonstrated. The two FBGs are located in two orthogonal planes of the pendulum and only half of each FBG is glued on the surface of the columnar rod. Due to the strain difference between glued- and non-glued part of each FBG, the reflection spectrum of each FBG splits into two peaks. The direction and magnitude of inclination can be readily determined by measuring the wavelength separations of the split peak in two FBGs, whilst the temperature-induced cross-sensitivity is eliminated. A sensitivity of 0.047 nm/° with an accuracy of 0.23° is achieved within the tilt angle range of -20°–+20°.

The temperature dependence of the force sensitivity of a regenerated fiber Bragg grating (RFBG) in a smf28 fiber was experimentally determined up to temperatures of 400°C for the first time. The force sensitivity was found to decline linearly with increasing temperature. The RFBG showed a pure elastic response to the applied forces and no wavelength drift occurred during the measurement. This demonstrates that RFBG based sensors are suited for force monitoring at temperatures up to 400°C.

The study presented in this article was aimed to construct an appropriate set of interferometric optical fiber sensors for recording rotational events with their parameter investigation. Presented systems apply the principle of the gyroscope in an open loop configuration, as well as an electronic system, involving specific electronic solutions. They have a wide measuring range both in domain of the signal amplitude (3∙10^-8 – 10 rad/s) and frequency [from DC to 328.12/n Hz (n=1, …, 128)]. They are portable and equipped with an additional independent power supply, as well as they are remotely controlled. To determine parameters of random errors, we applied the Allan variance analysis with Angle Random Walk and Bias Instability determination. Nevertheless, in this paper, we present results which indicate that the appropriate method of error estimating is significant, but environmental conditions are crucial.

An optical fibre temperature sensor modified with coating a thermochromic liquid crystal (TLC) film on the tip of fibre is reported. The interrogation is conducted in the wavelength domain using a CCD spectrometer and halogen light source. The TLC sensor shows a reversible wavelength shift of the reflective light peak in the detected temperature range (28 – 46°C). The response of the sensor to temperature was linear with a sensitivity of -4.52 nm/°C for temperature increasing and -4.60 nm/°C for decreasing. There is no measurable hysteresis in the temperature calibration experiment for TLC sensor.

Abstract: We report on experimental studies of polarimetric sensitivity to torsion and temperature in a series of spun highly birefringent side-hole fibers with spin pitches ranging from 5 to 200 mm. The polarimetric sensitivities to torsion and temperature were measured by monitoring a displacement of the spectral interference fringes arising in the output signal because of interference of ellipticaly polarized modes. The experimental results show that the sensitivity to torsion normalized to the fringe width in the spun highly birefringent fibers increases asymptotically with the twist rate to the value of 1/π rad^-1 . In contrast to the torsional sensitivity, the temperature sensitivity decays asymptotically to zero with increasing fiber twist rate. Therefore, the spun fibers with short spin pitches are especially well suited for torsion measurements because the torsional sensitivity is enhanced in such fibers while at the same time the cross-sensitivity to temperature is reduced.

A concept for fiber optic measurement of strain in rotating structures where the fiber cannot access the central rotation axis is described. Various interrogation techniques are considered, and the use of a fast spectrometer-based interrogator is preferred. An automated algorithm for optical alignment while the structure is rotating is described.

Here, we report a comparative experimental investigation about the radiation sensitivity of Long Period Gratings (LPGs), fabricated in several optical fibers (from standard to radiation hardened ones) by electric arc discharge technique. For the purpose, similar set of gratings were tested under two different conditions. One is gamma radiation produced by a ⁶⁰Co source using a 0.2 kGy/h dose rate and up to 25-35 kGy total doses. The other is mixed neutron-gamma exposure in a nuclear reactor at 9 Gy/s gamma-rate and 1.25∙10¹² n/(cm² ∙s) neutron flux, reaching a total gamma dose of about 65 kGy and neutron fluence of 9.18∙10¹⁵ n/cm² . The analysis was focused on the real-time radiation-induced wavelength shift exhibited by the gratings. As an outcome, the responses of the LPGs subjected to the different radiations were compared and correlated with fiber compositions.

This paper proposes and demonstrates a practical temperature-insensitive pressure sensor based on the simple reflective birefringence fiber interferometer which consists of only a polarization beam splitter (PBS), two segments of solid core polarization-maintaining photonic crystal fibers (PM-PCFs) with film reflector deposited on the far end of one of the PM-PCFs. We derived its spectrum response equation and the practical temperature insensitive lateral pressure sensor was designed and demonstrated experimentally.

This study describes the development and qualification of strain sensors and thermal compensator for monitoring of ITER vacuum vessel. The operating conditions require 20000h at 200°C and gamma radiation doses up-to 10MGy under high vacuum. A sensor concept was designed based on two spot weldable sensing elements: one weldable strain sensor and one weldable temperature compensator. The developed elements were subjected to qualification tests including optical, thermal cycling, thermal aging, mechanical and radiation. The results validated the solution and proved that the elements comply with requested vacuum vessel environment, withstanding 10MGy radiation, ±1000μm/m for 10E+5 cycles at 100°C, 500 cycles from 100°C to 200°C, 100°C for 120000h, 200°C for 20000h and being fully operational after 80h at 250°C.

This study reports on high temperature behaviour and stability tests of type I and type IA FBGs. The gratings temperature is firstly increased monotonously up to 650 °C, then fixed at this temperature for several hours. By monitoring the reflection spectra of the gratings we show that standard type I grating disappears and then regenerates while type IA grating decreases monotonously and shifts to lower wavelength before it disappears without regenerating.

Fibre Bragg gratings (FBGs) can be used as strain, temperature and external refractive index sensor in various application domains and more particularly in harsh environments such as nuclear plants, radioactive waste storages or in a space environment inside artificial satellites. In this study, we report the wavelength and temperature sensitivity radiation response of FBGs written in different optical fibres and thanks to different lasers and methods of inscription. We show that the Bragg wavelength radiation-induced shift depends only on the core doping and not of the type of laser, but in all cases, the temperature sensitivity is modified with gamma irradiation.

In this paper, we propose the use of a series of fibre Bragg grating (FBG) sensors encapsulated in dedicated packaging made of carbon fibre reinforced polymers (CFRP) for monitoring the structural health of wind turbine blades. The manufactured CFRP packages are experimentally tested in a real-field fatigue test carried out on a 56.85 m-long composite wind blade over 16 days, in which alternating loads (i.e. tensile and compressive strain) are applied to the entire wind blade. Experimental results indicate the CFRP-packaged FBG sensors provide dynamic strain measurements with high precision and with no degradation over time, as shown to happen with conventional strain gauges.

In this paper, we developed Lab On Fiber (LOF) accelerometers based on micro-opto-mechanical cavities on the optical fiber tip. We designed the mechanical structures with different performances features specialized for target applications. In particular, a LOF accelerometer was designed for seismic surveillance applications. The sensor response was first characterized in laboratory, exhibiting a resolution down to 0.44μg/√Hz over a 3 dB frequency band of 60 Hz. To demonstrate the sensor capability to operate in a realistic seismic surveillance system, the developed sensors were continuously used in combination with a commercial seismic sensing network. During the field trial, the LOF sensor registered the ground acceleration associated with the seismic sequence that struck central Italy on October 30, 2016. Overall, the comparison, with traditional sensors incorporated into the geophysical network, demonstrated that the LOF sensors exhibit competitive performance with commercial seismic accelerometers. Ongoing activities are devoted to the design and realization of LOF accelerometers, based either on cantilever structures or on membranes, featuring different mechanical features and thus leading to different performances of the final accelerometer, by retaining the same principle of operation.

We report on a innovative 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 0.67x10¹⁶ protons/cm² , corresponding to an absorbed dose of 1.8 MGy. Experimental data demonstrate the "radiation resistance" feature of the LOF devices and a clear dependence of the reflected spectrum on 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. According to the numerical analysis and the literature, the main phenomenon induced by exposure to proton beam and able to explain the measured spectral behavior is the reduction of the PMMA thickness. Preliminary results demonstrated the potentiality of the proposed platform as dosimeter at MGy dose levels for High Energy Physics (HEP) experiments.

We present the design and field test of a rugged FBG sensor prototype for high-sensitivity measurement of underground water level. Pressure sensors have many fields of application, ranging from environmental monitoring to the oil and gas industry. In particular, pressure sensors can be used to monitor the stability of dikes and embankments by measuring the inner phreatic level at their foot to detect anomalous filtration and excess of pore pressures. For this application, rather high sensitivity at an affordable cost is required. Fiber optic pressure sensors have been explored with different solutions, but the technologies proposed so far have either small sensitivity, and hence are befitted for large pressure ranges, or are based on interferometry, and hence require rather expensive laser sources. The sensor described in this paper exploits a 3D-printed mechanical transducer to convert external pressure in longitudinal strain along the fiber. A second FBG, embedded in the sensor, is used to compensate for temperature cross-sensitivity. The structure is enclosed in an aluminum alloy case to withstand harsh environments and installation procedures. Pressure and temperature sensitivities of the sensor are about 20 pm/cm H₂O and 17 pm/°C respectively. Three sensors of this kind have been successfully tested in a large scale dike at the Flood Proof Holland facility, in Delft, Netherlands.

In this paper, we propose a method that allows to estimate the vertical displacement of a two-dimensional structure by means of strain measurements. Fifteen fiber Bragg gratings (FBGs) 10 mm long have been selected as strain sensors and embedded in a rectangular multilayer panel in order to test the proposed method. Only few optical fibers have been employed avoiding the complex wiring typical of the electrical sensors. The deflection tests have been carried out loading the panel in different configurations. Once the deflection maps were obtained, the values in the center were compared with the displacements directly measured by means of a mechanical comparator finding a very good agreement.

We report on the use of Fiber Bragg Grating (FBG) sensors integrated onto an aircraft landing gear for remote and realtime load monitoring. Several FBGs strain sensors, both in a linear and tri-axial configuration, have been integrated on different locations of true landing gears (both Main and Nose gears) based on their load condition derived from FEM numerical analysis and exposed to numerous qualification lab tests where the load applied to the gears was varied in the range 0-20kN. To this aim, the gears were mounted on a 25kN hydraulic press, that changed the shock absorber route from 0 mm up to 200 mm (corresponding to the maximum take-off weight,~4600 kg). Obtained results are in good agreement with those provided by reference electrical strain gauges located very close to their optical counterparts, and demonstrate the great potentialities of FBG sensors technology to be employed for remote and real time load measurements on aircraft landing gears.

Power transformers are at the core of power transmission systems. The occurrence of system failure in power transformers can lead to damage of adjacent equipment and cause service disruptions. Structural and electrical integrity assessment in real time is of utter importance. Conventional techniques, typically electrical sensors or chemical analysis, present major drawbacks for real-time measurements due to high electromagnetic interference or for being time-consuming. Optical fiber sensors can be used in power transformers, as they are compact and immune to electromagnetic interferences. In this work, an optical fiber sensor composed by 2 fiber Bragg gratings, attached in a cantilever structure was explored. The prototype was developed with a 3D printer using a typical filament (ABS) that enable a fast and low-cost prototyping. The response of the sensor to vibration was tested using two different vibration axes for frequencies between 10 and 500 Hz. Oil compatibility was also studied using thermal aging and electrical tests. The studies shown that ABS is compatible with the power transformer mineral oil, but the high working temperatures may lead to material creeping, resulting in permanent structural deformation.

Improvements in the detection accuracy of miniature optical fibre real time X-Ray radiation sensor based on plastic optical fibre coupled with a fast decay time (10s ns) trivalent cerium activated yttrium aluminum garnet Y3Al5O12 (YAG:Ce) scintillator material is reported. The YAG material is tested only in its powder form. The sensor was irradiated on a Siemens Artiste linear accelerator (linac) and tested for their percentage dose depth (PDD) response whilst being immersed in a standard water test tank in the clinic. The accumulated dose during each test was simultaneously recorded using a standard ionization chamber (PTW Semiflex) which allowed direct comparison and calibration of the PDD response as measured using the Optical Fibre Sensor. Initial experimental results corresponding to the PDD are reported and these demonstrate increased accuracy from previously reported data which is primarily achieved through improvement of exposure conditions for the multi-pixel photon counting (MPPC) detector and monitoring of its real time analogue voltage output.

A critical aspect in the design of marine propellers is their hydrodynamic performance which, when evaluated experimentally, requires a number of parameters to be monitored at the same time, i.e. the thrust and torque a propeller generates as well as the propeller shaft and vessel speed. In this investigation, three of those parameters are measured using Fibre Bragg Grating-based sensors, thus allowing for computationally derived performance values to be verified. For that purpose, open water tests were carried out where an instrumented propeller shaft was installed into a research vessel and measurements taken, evaluated and the results compared favorably with advanced computer-based simulations.

A CYTOP multimode gradient index perfluorinated polymer fiber is experimentally investigated as a sensing element of a multimode fiber interferometer. Speckle patterns, formed at the output of CYTOP fiber, were measured and analyzed at two wavelengths for different excitation conditions, and for fiber lengths 20, 80, and 400 m. The influence of fiber length on selective mode excitation ability and the formation of speckle patterns were revealed. The interferometric response was investigated, when the CYTOP fiber was subjected to temperature and strain perturbations. The dependences of the interferometer’s properties on wavelength and excitation conditions were analyzed by the measuring of interferometer’s averaged transfer function.

A multimode fiber interferometer, consisting of two fiber segments and a long period grating (LPG) between them, is investigated. We observe that low order excited modes in the first segment were converted to the broad modal distribution in the second segment utilizing the LPG. The interferometer’s signal was measured, when the first and the second fiber segments were subjected to temperature and strain perturbations. On calculating the averaged transfer functions we demonstrated that the interferometer’s sensitivity depends on the fiber segment that was subjected to external perturbations. These results confirm the potential of using LPGs in distributed multimode fiber interferometers having external perturbation localization capability.

This paper presents an optical fiber embedded intelligent carpet for gait analysis, where the system is able of measuring the ground reaction force and the foot position on the carpet (which can be used on the gait spatiotemporal parameters assessment). The system comprises of fiber Bragg gratings (FBGs) inscribed in silica fibers to detect the foot positioning and intensity variation-based sensors on polymer optical fibers (POFs) to estimate the ground reaction forces. The proposed system was characterized as function of foot impact positioning as well as applied forces and a high correlation between the sensors responses and the applied parameters was obtained on each case. Then, a proof-of-concept test was performed, where a volunteer placed his foot in different positions on the carpet, which show the feasibility of the system in the proposed application.

In the paper we present smart textiles for application in critical infrastructures based on fiber Bragg grating sensors. The suitability of the selected polyester fabric was evaluated by experimental measurement. Low elasticity caused an irreversible deformation of the fabric and thus change of the reference value when stretching the textile. With a 4.5 Newton tensile, the reference value change was up to 0.14 pm. Therefore, a combination of two layers of different textiles was tested. The second layer caused less deformation of the first layer by up to 50 percent.

This review summarizes the status of three current efforts to develop optical gyroscopes with improved performance over state-of-the-art fiber optic gyroscopes (FOGs) in terms of accuracy, size, and/or cost. The first approach consists in replacing the temporally incoherent Er-doped fiber source used in FOGs withy a low-coherence laser whose linewidth is broadened to tens of GHz by an external phase modulator driven by noise. A FOG with a 3.24-km coil interrogated by such a source has recently produced a noise and a drift approaching strategic-grade performance, and exhibits a source mean-wavelength stability below 1 ppm. The second approach is the use of a hollow-core fiber (HCF) in the sensing coil of a FOG to reduce thermal drift. A FOG utilizing 250 m of polarization-maintaining HCF and interrogated with a broadened laser is shown to exhibit a noise of 0.135 deg/√h limited by backscattering arising from surface modes in the fiber, and a drift of 1.2 deg/h dominated by polarization coupling. The third investigation is an optical gyroscope made of two coupled ring resonators, one exhibiting loss and the other one gain, operated at or near an exceptional point. Time-domain simulations predict that when operated below threshold and interrogated with a conventional biasing and read-out scheme, this gyroscope exhibits a rotation sensitivity at least 170 times larger than an optimized single-ring resonator with the same radius (5 mm) and loss (0.5 dB). Such systems have a great potential for producing a new generation of gyroscopes with significantly smaller footprints than FOGs.

Fibre Bragg grating sensors have gained a lot of attention in damage detection and strain measurement applications in the past few decades. These applications include matrix crack detection and delamination tip monitoring in composite structures, crack detection in concrete and civil engineering structures and etc. The damage localisation accuracy of such methods, directly depends on precise knowledge on the position of the FBG sensor. However, this information is not commonly provided by manufacturing companies with such accuracy. In this paper, we propose a novel approach to accurately determine the position of an FBG sensor with a low complexity setup. The proposed method offers an accuracy of below 10 μm, and can consequently increase the spatial resolution of damage detection methods.

In this paper, we report on an offshore field validation of a FBG based optical fiber sensor for simultaneous monitoring of hydrostatic pressure and temperature. The sensor consists of a femtosecond laser induced grating written in a Butterfly microstructured fiber. The sensor has an extremely low cross-sensitivity between temperature and pressure which makes it ideal for monitoring large transients in pressure or temperature, like is the case in wireline intervention. Pressure and temperature readings from the FBG based optical fiber sensor are compared with the readings from a battery powered electrical quartz gauge during an offshore wireline intervention job in an oil well. Good agreement was found between both measurements

We proposed biomedical optoacoustics/photoacoustics more than 25 years ago and developed this technology for imaging, monitoring, and sensing applications [1-35]. Optoacoustic diagnostic modality is based on thermoelastic generation of optoacoustic waves and combines high optical contrast and ultrasound spatial resolution. We developed optoacoustic systems (including highly-compact, multi-wavelength, fibercoupled, FDA-compliant laser diode systems) with fiber-optic, ultra-sensitive, wide-band optoacoustic probes. We tested them in small and large animal studies and in clinical studies in healthy volunteers and patients with traumatic brain injury and circulatory shock as well as in neonatal and fetal patients. The high sensitivity and bandwidth of the fiber-optic optoacoustic probes allowed for: 1) signal detection from tissues at depths well beyond the optical diffusion limit (up to several centimeters); 2) detection of microscopic tissue volumes; 3) optoacoustic imaging in large tissue phantoms and tissues with high resolution; 4) monitoring of tissue thermotherapy; 5) noninvasive probing of cerebral tissues in large animals and in humans (both neonates and adults); 6) accurate oxygenation measurements in humans in tissues and in specific blood vessels; and 7) optoacoustic waves therapeutic effects that can be used for noninvasive optoacoustic theranostics. The obtained data indicated that the systems were capable of optoacoustic measurements and mapping of cerebral blood oxygenation in adults and in neonates, detection of intracranial hematomas, oxygenation measurements both from cerebral and central blood vessels and from cerebral tissues.

A fast indentation system for 2D stiffness mapping of cartilage is presented. The goal of this technique, which has the potential to be deployed endoscopically, is to localize and to distinguish between regions of healthy and degenerated cartilage. In the system presented here, forces are measured by a fiber Bragg grating (FBG), which is located at the end of a fiber optic indenter probe. This approach enables small indenter dimensions, which are advantageous for endoscopic use. Differences between the stiffness parameters measured on healthy cartilage and on the same sample after artificial degeneration are observed. It is shown that the fast indentation system has the capability to distinguish healthy bovine cartilage from artificially degenerated one. The evaluation of this capability will make the conclusions of the work.

We propose a novel extrinsic optical fiber sensor, based on two plastic optical fibers (POFs) combined with a bacterial cellulose (BC) optical waveguide. The POFs connect the green disposable optical sensor with a white light source and with a spectrometer. We have deposited a thin gold film on this innovative slab waveguide for obtaining a Localized Surface Plasmon Resonance (LSPR) sensor. In fact, the deposited thin gold layer is not formed by a continue film and the preliminary experimental results confirm the possibility of using the BC based composite as an environmental friendly optical sensor platform with LSPR capabilities. This approach could be exploited for realizing disposable biosensors, when the green slab waveguide is fixed in a special support. The device has been tested by measuring the refractive index of different water-glycerin solutions.

A tip-based fiber-optic localized surface plasmon resonance (LSPR) sensor is reported for sensing of acetone. It is designed by coating the tip of multi-mode optical fiber with gold nanoparticles (size: ~ 40 nm) via a chemisorption process and further functionalization with a metal-organic framework (MOF) HKUST-1 via a layer-by-layer process. Two sensors with a different number of layers (80 and 120) corresponding to different thicknesses are reported. Both sensors show a redshift of resonance wavelength to acetone as a result of an increase in local refractive index induced by acetone adsorption into the HKUST-1 thin film. Sensors gradually saturate as acetone concentration increases and are fully reversible when the concentration decreases. The sensor with a thicker film exhibits slightly higher sensitivity to acetone than the thinner film with a wavelength shift of 5.27 nm for the concentration of 3.4 %.

Photopolymerization process is present in the optical fiber technology since its beginning. Organic coatings preventing degradation of an optical fiber was the most important implementation of this process which ensured its practical industry application in telecommunication. However, this process can be implemented to design specialty transducers related to both optical and chemical sensors, as well. Simple optical transducer can be a microtip being an extinction of the optical fiber core as a polymer microelement. A chemical transducer can be a thin polymer layer manufactured on the tapered part of optical fiber. The second type of transducer can be applied to chromatography measurements as a solid phase microextraction fiber. In the paper, technology of the tapered optical fiber and photopolymer sensing layer formations were described. In the first step a standard heat and pull technique was used to taper an optical fiber which, then, was cleaved in two symmetric pieces. Light was launched to such a single element and output optical power from tapered part of the fiber was measured to optimize the photopolymerization process. Placing this element in a photopolymerizable monomer mixture and using a specially selected holder allow for manufacturing a polymer sorption layer applicable as a solid phase microextraction fiber. Extended description of technology of this chemical transducer type and preliminary experimental results displaying its feasibility were presented.

Continuous patient monitoring has been evidenced as very beneficious for reducing degeneration¹. Due to this, a POF specklegram sensor has been developed based on a previous work². This work presents a comparative between analysis methods of the specklegram signal for achieving a precise and robust non-contact monitor system. Two different techniques have been used: one based on the Fast Fourier Transform (FFT) and the other based on the Hilbert Transform (HT). Each technique has been employed with two different methods, for heart rate and breath rhythm. The different algorithms are tested on 10 volunteers of different ages and sex.

Control systems for automotive applications have rapidly evolved introducing intelligence to address the increasing demand for pollution and oil consumption reduction. Fiber Bragg Grating (FBG) sensors are used in this work for monitoring Gasoline Direct Injectors (GDI) in order to optimize the engine performance and reduce the emissions. Several fast-acting solenoid injectors have been instrumented with FBG sensors and mounted in a test bench at the testing department of CPT Italy S.r.l to simulate the injector's behavior during the actuation phase. The FBG sensors, installed on the stem of the GDI, provide dynamic measurement of the strain variation during the injection process, pointing out the unwanted effects of the reopening, leading to injector tip wetting and consequent increased polluting emissions. The acquired data allows one to fully understand the GDI process and to optimize the injector design in order to reduce emissions, as required by recent European directives for the emission standards.

The work presents the design and fabrication of a fiber optic biosensor based on a Long Period Grating (LPG) with multi-layer coating. An LPG working in single-ended configuration was developed, which is very convenient in biological applications. The transducer was optimized in terms of sensitivity by means of a nanometric layer of Polycarbonate, casted by dip coating technique, which was designed to favorite cladding mode transition. Subsequently, a Graphene Oxide thin film was selected as outer layer, which provides an excellent bio-functionalization of the sensor surface due to its carboxylic functional groups. The performance of the sensor was tested towards the detection of the well assessed binding process of biotin to streptavidin. As a result, the detection of biotinylated Bovine Serum Albumin (bBSA) in phosphate buffer solution was achieved at very low concentrations in the atto-molar range.

The cement industry is facing pressure to find technological solutions in reducing greenhouse gas emissions owing to the large amount of process emissions originating from the calcination of limestone. In this communication, an all-fibre gas monitoring system based on anti-resonant hollow-core fibres is proposed. An on-field test was performed in the harsh environment of a cement factory and it demonstrated the feasibility of using this system for low-concentration carbon dioxide and carbon monoxide monitoring in exhaust fumes.

A detailed knowledge of the internal flow distribution inside a zinc-nickel flow battery is of critical importance to ensure smooth flow of the electrolyte through the battery cell and better operation of the device. Information of this type can be used as a useful means of early detection of zinc deposition and dendrite formation inside the cell, negative factors which affect the flow and thus which can lead to internal short circuiting, this being a primary failure mode of these types of batteries. This deposition occurs at low pH levels when zinc reacts with the electrolyte to form solid zinc oxide hydroxides. Traditionally, manual inspection is conducted, but this is time consuming and costly, only providing what are often inaccurate results – overall it is an impractical solution especially with the wider use of batteries in the very near future. Fibre Bragg grating (FBG) sensors integrated inside the flow cell offer the advantage of measuring flow changes at multiple locations using a single fibre and that then can be used as an indicator of the correlation between the internal flow distribution and the deposition characteristics. This work presents an initial study, where two networks of FBGs have been installed and used for flow change detection in an active zinc-nickel flow battery. Data have been obtained from the sensor networks and information of battery performance completed and summarized in this paper. The approach shows promising results and thus scope for the future research into the development of this type of sensor system.

The combination of fiber-optic–based platforms for biosensing with nanotechnologies is opening up the chance for the development of in situ, portable, lightweight, versatile, reliable and high-performance optical sensing devices. The route consists of the generation of lossy mode resonances (LMRs) by means of the deposition of nm-thick absorbing metaloxide films on special geometric-modified fibers. This allows measuring precisely and accurately the changes in surface refractive index due to the binding interaction between a biological recognition element and the analyte, with very high sensitivity compared to other optical technology platforms, such as fiber gratings or surface plasmon resonance. The proposed methodology, mixed with the use of specialty fiber structures such as D-shaped fibers, allows improving the light-matter interaction in a strong way. The shift of the LMR has been used to monitor in real-time the biomolecule interactions thanks to a conventional wavelength-interrogation system and an ad-hoc developed microfluidics. A big leap in performance has been attained by detecting femtomolar concentrations in real samples of human serum. The biosensor regeneration has been also studied by using a solution of sodium dodecyl sulphate (SDS), proving the device reusability. Therefore, this technology possibly represents a paradigm shift in the development of a simple, high-specificity and label-free biosensing platform, which can be applied to speed up diagnostic healthcare processes of different diseases toward an early diagnostic and personalized treatment system.

We analyse the polarimetric response of fiber Bragg gratings involved in a fiber-based resonant microscanner designed to produce polarimetric images of biological tissues through an endoscope for medical diagnosis. The intrinsic birefringence of the grating is measured and its detrimental consequences on the measured values are discussed. Solutions able to overcome this problem are proposed, paving the way for the fabrication of the very first polarimetric endomicroscope.

An ultra-sensitive plasmonic fiber-optic photothermal anemometer is proposed and demonstrated. The device consists of a highly-tilted fiber Bragg grating (TFBG) sensor, which is coated with a gold layer exciting surface plasmon resonance (SPR) and then carbon nanotubes as the photothermal conversion element. The carbon nanotubes deposited on the sensor surface efficiently convert light from the heating laser, which is wavelength matched to the SPR signature, into heat. Air flow draws away the surface heat, thus inducing both a strong SPR wavelength shift and a changed of the power modulation. Using this approach, the proposed anemometer accounts for a dynamic range from 0.05 m/s to 0.65 m/s for wind speed measurement. In addition, the real-time monitoring of wind speed has been proved by measuring the intensity of the heating laser source. This device is a valuable candidate for a wide range of potential applications in both scientific research and industrial production, given its simplicity and robustness in structure.

This work presents a metal-dielectric colloidal crystal (MDCC) structures used as the sensing layer of an optical fiber SERS probe working in optrode configuration. The MDCC permits to combine localized surface plasmon resonance (LSPR) of noble metallic nanoparticles and photonic bandgap (PBG) of colloidal type photonic crystals (PhCs); while MDCCs received attention to develop enhanced SERS substrates, the integration with optical fiber technology still represents open challenges. The present paper reports recent results about MDCCs fabricated directly onto fiber optic tip surface by successive depositions of PS-based Colloidal Crystal and Au-NanoParticles. Our goal is the fabrication of miniaturized fiber optic devices for SERS sensing applications. For the sensing test, the fiber probes were immersed in a 100μM aqueous solution of Rhodamine (R6G). Then, preliminary SERS results were retrieved by a Labspec Aramis confocal Raman spectrometer.

Air pollution by particles of aerodynamic diameter less than 2.5 μm (PM_2.5) is currently an important environmental issue. As standard methods for monitoring and characterization are time-consuming and expensive, there is need for a simple, effective and inexpensive device for real-time PM_2.5 monitoring. Although optical methods based on the scattering of laser beams exist, they do not provide independent measurement of particle size and refractive index. In systems based on detection in an evanescent optical field, particles tend to adhere to the surface, strongly limiting the device lifetime. Here we report a new approach to airborne particle characterization that makes use of hollow-core photonic crystal fibre (HC-PCF). Employing optical gradient and radiation forces to trap and propel airborne particles along the hollow core, it provides in situ particle counting, sizing and refractive index measurement in real-time with effectively unlimited device lifetime. We show that the transmission drop and time-of-flight can be unambiguously mapped to particle size and refractive index.

A reflective fiber optic sensor based on a Fabry-Perot cavity made by splicing two sections of multimode fiber is demonstrated to measure the needle curvature. The sensing structure was incorporated into a medical needle and characterized for curvature and temperature measurements. The maximum sensitivity of -0.152dB/m^-1 was obtained to the curvature measurements, with a resolution of 0.089m^-1. When subjected to temperature, the sensing head presented a low temperature sensitivity, which resulted in a small cross-sensitivity.

We report on our activities related to the development of surface enhanced Raman scattering (SERS) probes realized onto the optical fiber tip (OFT) through nanosphere lithography. In the first stage of our research, we adapted the nanosphere lithography to operate on the optical fiber tip, by assessing the process and demonstrating either the potentiality or the repeatability of the proposed nanopatterning approach. Successively, we investigated the ability of the manufactured structures on the fiber tip to act as SERS probes by measuring the SERS spectra in presence of a Biphenyl Thiol (BPT) monolayer. Firstly, we focused the attention on the samples shaped as closed packed array of nanospheres covered by gold. The analysis allowed us to identify the most promising SERS platform, exhibiting an Enhancement Factor (EF) of 4×10⁵ and a SERS measurements variability lower than 10%. We addressed also the limitations related to the use of the same optical fiber for both illumination and light collection by selecting a commercial optical fiber exhibiting a suitable trade-off in terms of high excitation/collection efficiency and low silica background. Current activities are devoted to the investigation of other nanopatterns on the optical fiber tip (namely, Sparse Array of metallodielectric spheres) and the analysis of the probes response against different molecules.

Refractive index measurement is important for evaluation of liquid materials, optical components, and bio sensing. One promising approach for such measurement is use of optical fiber sensors such as surface plasmonic resonance or multimode interference (MMI), which measure the change of optical spectrum resulting from the refractive index change. However, the precision of refractive index measurement is limited by the performance of optical spectrum analyzer. If such the refractive index measurement can be performed in radio frequency (RF) region in place of optical region, the measurement precision will be further improved by the frequency-standard-based RF measurement. To this end, we focus on the disturbance-to-RF conversion in a fiber optical frequency comb (OFC) cavity. Since frequency spacing f_rep of OFC depends on an optical cavity length nL, f_rep sensitively reflects the external disturbance interacted with nL. Although we previously demonstrated the precise strain measurement based on the f_rep measurement, the measurable physical quantity is limited to strain or temperature, which directly interacts with the fiber cavity itself. If a functional fiber sensor can be installed into the fiber OFC cavity, the measurable physical quantity will be largely expanded. In this paper, we introduce a MMI fiber sensor into a ring-type fiber OFC cavity for refractive index measurement. We confirmed the refractive-indexdependent shift of optical spectrum event though the MMI fiber sensor is included in the cavity. Such the spectral shift was converted into refractive-index-dependent shift of f_rep via the wavelength dispersion of the cavity fiber.

This work discuses indium tin oxide (ITO) coated optical fiber lossy-mode resonance (LMR) sensor working in an electrochemical setup for monitoring of protein binding to the sensor’s surface. The binding mechanism has been observed simultaneously in optical and electrochemical domain. The combined measurement was enabled by the electrically conductive ITO overlay. In the experiment, biotin molecules have been used to test the collective optical and electrochemical setup and to illustrate the binding effect. It has been shown that the effect is observed in the investigated domains at applied potential, and the qualitative comparison shows high resemblance of the outcome.

Microgel assisted lab-on-fiber optrodes, arising from the integration of multiresponsive microgels onto nanostructured optical fibers tip, are emerging as intriguing multifunctional devices exploitable in biomedical applications, especially for label-free molecule detection. We have recently demonstrated that the sensitivity range of these devices is strictly related to the microgel films characteristics, which can be controlled during the microgel deposition procedure, based on the dip coating technique. With the aim of optimizing the deposition procedure in terms of fabrication throughput, we evaluate each fabrication step (fiber dipping, rinsing, and drying), demonstrating that the overall deposition duration can be significantly reduced from 960 min (of the ‘standard’ procedure) to 31 min, without affecting the microgel film characteristics. The analysis has been carried out by means of both optical and morphological characterization, and validated through repeatability tests. Overall, our results set the stage for engineering microgel assisted miniaturized optrodes, enabling their possible exploitation in industrial applications.

It is necessary that real-time and continuous monitoring of heavy metal ions in solution. Traditional electrochemical methods have security issues and background interference. To solve these problems, a new method based on surface plasmon resonance (SPR) electrochemical optical fiber sensor has been proposed. Combined with anodic stripping voltammetry for electrochemical measurement, SPR is used for detection of heavy metal ions simultaneous by coating metal ion lead onto the surface of the optical fiber sensor while recording the electrochemical signal, the characteristic is that SPR is highly sensitive and excludes electrochemical background interference. The gold-coated film optical fiber works as both a sensor and a working electrode, allowing real-time monitoring of heavy metal ions in solution. The advantages of optical fiber sensor, such as compact size, flexible shape and remote operation capabilities, opens a new way for high sensitive electrochemical detection.

Raman spectroscopy (RS) is a non-destructive analytical technique, that provides a unique fingerprint of molecules with high accuracy. It proves to be a reliable and practical alternative to chemical analysis, allowing sample identification without the use of reagents. This label-free technique finds applications in quality control and in-line process monitoring, however, like any other technique RS also presents its challenges such as expensive and delicate instrumentation and complex design, which often confines the technique to the laboratory. In order to address these challenges, a 3D printed Lab-On-Chip (LOC) was fabricated and assembled with four channel optical fibres, which will collect the Raman scattering. The performance of our Raman Probe on Chip is evaluated using Isopropanol alcohol (IPA) as a validation sample.

A simple fused silica capillary interferometric (FSCI) sensor has been proposed and investigated for the detection and analysis of multiple chemical compounds content in aqueous solutions. The sensor was fabricated by splicing a commercially available fused silica capillary (FSC) with two single mode fibers to create a 0.7 cm long air cavity. The fiber surface was functionalized with two different polymers: poly (allylamine hydrochloride) (PAH) and sol-gel silica in sequence using a layer-by-layer deposition method. The operating principle of the sensor relies on light interference in the fused silica capillary cavity due to adhesion of the different chemical compounds on the functional coating surface. Studies of the sensors response to the presence of five different compounds in water solutions at different concentrations have been carried out and the results have been analyzed using the principal component analysis (PCA). This work is a preliminary investigation towards the development of a novel method for assessment of content and quality of alcoholic beverages in real time using functionalized FSCIs.

In this paper we present an exploration of the stability and repeatability of a hollow core microstructured fibre (HCMOF) Raman gas sensor. Raman gas detection using HC-MOFs is an exciting technique as it enables high sensitivity, multi-species detection using a small gas volume and within a small physical space. Several previous works have demonstrated the utility of HC-MOF fibres as Raman gas cells for the detection of a wide range of gas species such as methane and hydrogen. Here we take a first look at the Raman signal stability (in a single fibre) and signal reproducibility (from fibre-to-fibre). We show that a HC-MOF Raman system can achieve low within-day variability of 0.3 %CV and fibre-to-fibre variability of 7.6 %CV. Understanding the error within systems such as the one presented is critical in the development of HC-MOF-based gas sensors for practical applications.

A wearable speckle-based heart-rate monitoring device is presented. It uses a lead of plastic optical fiber (POF), as the sensing element. The system includes a microprocessor-powered portable system that controls a laser and a CMOS camera to take real-time measurements. A simple optical system is considered to provide portable capabilities. Several processing algorithm alternatives are assessed, using a device under test for emulating the heart-rate mechanical vibrations. Errors below 1% are achieved for some filter and processing techniques.

This article proposes a process to manufacture optical fiber shape sensors for biomedical applications. By using polymer extrusion on three optical fibers, a triplet with a diameter below 600 μm is obtained which can be inserted into surgical needles or catheters. Furthermore, the fiber triplet position within the coating and the angle of the triplet are parameters that need to be controlled, to enable the best performance. The radial and angular positions of the optical fibers in the triplet are measured with an accuracy of 3μm and 4 degrees, respectively. The sensor incorporating our recently developed ROGUEs (random optical fiber gratings written by UV or ultrafast laser exposure) backscatter enhanced fibers, is then used for shape sensing demonstration with an OFDR technique.

Optical fibre Bragg grating sensors inscribed in polymer optical fibre have been shown to be sensitive to the water content of the medium surrounding the fibre and this property has been applied, for example, to the monitoring of humidity and the water content of aviation fuel. In this work we assess the feasibility of using such sensors for monitoring the saturation of soils, which is important for civil engineering applications. We find a very non-linear response, with a rapid increase in Bragg wavelength as the water content increases from 0 to 0.5%, with a much more gradual, but still monotonic, increase thereafter. We speculate on the causes of this response.

A stable, compact and portable optical fibre sensor for the real time detection of cadmium ions in dilute aqueous media has been designed, developed and evaluated. The sensing mechanism was based on fluorescence turn-on of a coumarin (acting as the fluorophore) bearing a dipicolylamine moiety (acting as the metal ion receptor) in the presence of metal ions via photoinduced electron transfer (PET). The fluorophore was covalently immobilized onto the fibre surface by polymerisation and exhibited a significant increase in fluorescence intensity in response to Cd²⁺ in the μM concentration range. A referencing scheme allowing for corrections due to fluctuations of the excitation light and transmission properties of the optical fibre or environmental perturbation to the sensor system has been introduced using a highly stable perylene red based material as the internal reference. A high selectivity, excellent photo-stability and reversibility have also been demonstrated, making this type of sensor potentially well suited for in-situ monitoring of the metal in the environment.

This work discusses optical fiber sensors based on lossy-mode resonance (LMR) effect and their potential for simultaneous sensing in multiple domains, i.e., optical and electrochemical. As electrically conductive materials able to guide lossy modes, two doped tin oxides, i.e., fluorine doped tin oxide (FTO) and indium tin oxide (ITO) thin films were employed. Since the ITO-LMR sensor has already been discussed broader, this work focuses on properties of the FTO-LMR sensor and brief comparison of devices based on the two materials. In optical domain the sensitivity to surrounding medium refractive index was determined by immersing the sensors in solutions of different refractive index. Both the sensors showed sensitivity of 300 nm/RIU in a refractive index range of approx. 1.33-1.39 RIU. Electrochemical measurements were performed in 0.01 M phosphate-buffered saline (PBS, pH 7.0) to identify the influence of the applied potential on the optical response of both sensors. In applied potential from -1.0 V to 1.0 V the FTO-LMR sensor reached LMR shift of 31.3 nm compared to 23.8 nm of the ITO-LMR one.

The development of renewable energies is a fundamental need to cover the ambitious energy targets demanded by industry and the European Union in the coming years. Wind energy is one of the most promising sources of renewable energy today. But, in the case of offshore wind energy, it has certain limitations due to its relative youth and associated costs, especially in the maintenance, operation and repair operations. Within the project, that includes this work, has been developed a new multi-material component with high structural requirements for the offshore wind sector. A combination of steel and fiberglass (GF) composite material, manufactured by filament winding, with a protection paint from to biofouling and corrosion, will be developed and validated in a real conditions test in the experimental zone of INEGA (A Coruña, Spain), by a demonstrator to scale 1:5. For real-time monitoring of the multi-material structure, a monitoring system based on fiber optics sensors type Bragg Grating (FBG) has been developed and integrated in this multi-material structure. This monitoring system includes sensors of corrosion, temperature and strain. This paper shows the development and characterization of these sensors at the laboratory level against mechanical, thermal tests and a durability study in a marine environment. In addition, it describes the integration of the monitoring system in the demonstrator and its response during the validation phase.

The sensing properties of zinc oxide (ZnO) combined with tilted fiber Bragg gratings have been studied in this work for the development of organic vapor optical fiber sensors. ZnO has been deposited onto tilted optical fiber Bragg gratings by drop casting technique to obtain coatings with thickness below 100nm. For a title angle of 4°, cladding modes up to 4 dB peak to peak amplitude were obtained and, as a consequence, the fundamental mode strength dropped below 0.5dB. The sensing features were evaluated by exposing the sensor to saturated atmospheres of three different alcohols and acetone. A negligible change was registered for methanol but in the case of ethanol and isopropanol a 24pm red shift was observed; in the case of acetone, the shift was 96pm which is of the order of spectral width of certain cladding modes. For all cases, the shifts were reversed once the organic vapors were removed. Ongoing studies are focused on the optimization of the sensing layer as well as the tilted angle to enhance the spectral response of the sensor.

This paper presents and experimentally demonstrates a new approach for the noise reduction and measurement accuracy enhancement in Brillouin optical time domain analyzers (BOTDA) by applying low pass filtering to the detected radio frequency (RF) signal. The simulation and experimental results indicate that the noise level of the BOTDA traces is reduced by using RF filtering. The corresponding measurement accuracy improvement depends on the cut-off frequency of the employed low pass filter. RF filtering is more efficient than other post-processing methods since it overcomes the long processing time. However, the results also imply that RF filters with too low bandwidths distort the trace signals and lead to detection errors.

We present for the first time, to the best of our knowledge, a novel method to improve the measurement accuracy of Brillouin optical time-domain analyzers (BOTDA) by engineering the gain spectrum of the Brillouin interaction. As will be shown, the engineered spectrum shape is more robust against noise and leads to a doubled frequency accuracy of the sensor. Since with the proposed method the frequency error is less susceptible to the intrinsic fiber loss, the sensing range can be extended by up to 60%. This work might open a new way to improve the BOTDA sensing performance with an engineered spectrum shape.

A distributed clad mode optical fiber sensor is reported for the first time. Random-access coupling of light to a clad mode of a standard single-mode fiber is achieved using Brillouin dynamic gratings. Coupling is restricted to a single, few-centimeters-long section that is scanned along a fiber under test. No permanent gratings are required, and all optical fields are launched and detected in the core mode. The coupling spectrum is affected by the local refractive index of the substance outside the cladding. Distributed mapping of surrounding media is reported over 2 meters of fiber with 8 centimeters resolution. The sensitivity of outside index measurements is between 4e-4 and 4e-3 refractive index units.

Rayleigh Backscatter has been used for many years as the source signal for fibre optic distributed acoustic sensing (DAS) in many industrial and civilian activities such as situational information monitoring and down-hole hydrocarbon exploitation. The signal from a DRS system is affected by temperature, strain and acoustics/vibrations. The very low frequency content representing temperature and strain contributions has historically been overlooked in favour of the higher frequency acoustic content. In this paper we describe the potential of a quantitative DRS system to deliver low frequency strain monitoring together with an understanding of the degree of thermal coupling to the system which allows useful results. A long term (50 day) measurement using a quantitative DRS system was carried out to investigate how much drift there was in the system and demonstrate a compensation approach. An example of strain monitoring in a novel pipeline system is illustrated to show the degree of measurement resolution deliverable as compared to conventional strain gauges.

In this paper, an investigation on the working point of slope-assisted dynamic distributed Brillouin sensing is presented. A comparison has been carried out between the sensing performances achieved at the inflection point and the 3 dB point of the Brillouin gain spectrum. Besides the intrinsic 13.1% frequency-to-amplitude sensitivity enhancement and a higher signal level, the dynamic sensing at the inflection point can achieve a doubled in maximum and in average a 36.8% wider dynamic range with much better working point symmetry. Simulations with strain signals also demonstrate that, compared to the 3 dB point, the average error at the inflection point can be significantly reduced to only 27.7%. As shown in this work, by a simple shift of the working point from the 3 dB to the inflection point, slope-assisted dynamic sensing can be well enhanced.

Chirped pulse phase-sensitive optical time-domain reflectometry (chirped pulse Φ-OTDR) allows the interrogation of tens of kilometers of optical fiber with high sensitivity and linearity, but typically with spatial resolution limited to ten meters to ensure proper processing and signal-to-noise ratio (SNR). In this paper, we propose a method to increase the spatial resolution of a chirped pulse Φ-OTDR without reducing the pulse width. The improvement is achieved by adding an optical carrier to the input chirped pulse and by applying digital filtering to the measured backscatter traces. Experimental results validate the method, demonstrating a 10-fold resolution improvement with minimum impact on the measurement SNR.

Humidity is one of principal environmental parameters that plays an important role in various application areas. Using measurement of strain induced in an optical fiber by a water swellable coating represents a promising approach for realization of distributed humidity sensing (DHS). In this work, humidity and temperature response of four different commercial PI-coated fibers and four tight-buffered (TB) fibers is investigated with the aim of evaluating their potential for development of DHS in context of water ingress sensor for high-voltage power cable splices. PI-coated fibers exhibited close-to-linear humidity and temperature response. While the temperature response is relatively coating-independent, magnitude of humidity response was broadly correlated to the relative fiber-to-coating thickness ratio. In contrast, both humidity and temperature response of TB fibers is strongly influenced by buffer type, with Leoni TB900L fiber with Hytrel buffer exhibiting largest humidity and temperature sensitivity. While the response of tight-buffered fibers is generally nonlinear, roughly three-times higher humidity response can be achieved with TB900L compared to the most sensitive PI-coated fiber. Using the TB fiber can be, therefore, advantageous for simpler water detection applications, such as one targeted in this study, when larger sensitivity is more important than the linear response of the sensor.

In this paper, we describe an optical fibre cable for distributed pressure sensing. The cable structure encodes the local pressure into strain exerted onto a standard optical fibre, embedded inside the cable following a meandering path. The cable has been designed and preliminarily tested by means of optical frequency domain reflectometry. Nonetheless, the cable can be interrogated by any optical fibre distributed strain sensing technique. Up to our knowledge, the proposed cable is the first real distributed fibre optic pressure sensing cable embedding standard fibres, capable of providing high- pressure sensitivity.

Both Rayleigh and Brillouin scatterings measurements in optical fibers are able to provide distributed strain and temperature profiles. Acquisition parameters, measurement quality, impact of the environment over time, effective spatial resolution obtained on field are some of the driving specifications. We report here some studies on Brillouin and Rayleigh scattering techniques, related to their performances concerning measurement quality, distance range and spatial resolution. Even if Rayleigh-based measurements are more accurate with 6 µε uncertainty (versus 17 με), Brillouin-based methods are more reliable than Rayleigh-based on cross-correlation for strain difference over 500 με; while with the same parameters Rayleigh-based systems are able to provide a higher effective spatial resolution.

We present a technique for distributed temperature gradient sensing in real-time along an optical fiber utilizing simple amplitude-based direct-detection coherent optical time domain reflectometry (C-OTDR) and a special sensing fiber. Our technique enables us to determine phase changes or low-frequency variations of the C-OTDR signal stemming from temperature variations. The distinct feature of the used sensing fiber is its structuring with equidistant strongly scattering dots. Consecutive pairs of these scatterers form the dominant local interferometers, effectively overwriting the otherwise highly nonlinear transfer function of common optical fiber. This enables a quasi-phase-resolved evaluation of perturbation responses originating from temperature changes at sensor positions between the scatterers. Using our method, we show the measurement of a nonlinear temperature transient from a heating process with a maximum temperature gradient of 0.8°C over 20 s and a total temperature increase of 28.4°C. This method requires almost no post-processing and can be used for simultaneous distributed vibration sensing (DVS) and quantification of local temperature gradients in a single fiber, e.g., for the use in condition monitoring of infrastructure or industrial installations.

Monitoring of seismic activity around the word is a topic of high interest for the analysis and understanding of deep Earth dynamics. However, the deployment of a homogeneous network of seismic stations both onshore and offshore poses a strong economic challenge that makes this solution practically inviable. Using the pre-existing fiber optical network for seismic monitoring arises as an excellent solution with important advantages in terms of ubiquity and cost. In this communication, we present the detection of an M8.2 earthquake occurred in Fiji Island using distributed acoustic sensing based on chirped-pulse φOTDR. Two sensors were placed simultaneously at two different locations at >9,000 km from the earthquake epicenter: a metropolitan area and a submarine environment. The recorded data is postprocessed using a 2D linear filter to cancel out environmental noise. The resulting signals are compared with the signals acquired by nearby seismometers. The attained good matching between the recorded data and the seismometer data shows the strong potential of the use of the already-deployed communication fiber network for teleseism monitoring.

Fibre optics sensors have been identified as very good candidates for environmental monitoring inside the silicon detectors operated at CERN’s Large Hadron Collider. In this study, we present the results from the first highly sensitive relative humidity distributed sensor with kilometres sensing range. The setup is a 70 cm spatial resolution phase-sensitive Optical Time Domain Reflectometry (OTDR) and is able to monitor fibre lengths up to 10 km. The coating effect is also evaluated, analysing different coating thicknesses, number of coating layers, different manufacturing and different materials. Relative humidity tests were performed at two different temperatures (25°C and 42°C). Polyimide coated fibres show in general a higher humidity sensitivity then a standard acrylate coated fibre, while acrylate fibres offer the fastest response and settling time. The system is able to resolve 0.1% RH and all tested fibres proved to be good candidates to be employed in a distributed relative humidity sensor. If the requirements are a fast time response and short settling time at room temperature, the standard acrylate coated fibres are the best candidates. However, if the requirements are high sensitivity and measurement stability at different temperatures, the polyimide-coated fibres offer advantages on several aspects.

Brillouin optical time-domain reflectometry is used to perform distributed forward stimulated Brillouin scattering (FSBS) measurements. This configuration suppresses the need for an additional frequency scanning to evaluate the local Brillouin peak gain when probing the FSBS resonance. The use of a broad pass-band filter makes the system insensitive to moderate temperature or strain fluctuations, but enables to accurately retrieve any change in intensity due to FSBS.

In this work, we present an enhanced design for a Brillouin Ring Laser (BRL) based on a doubly-resonant cavity (DRC) with short fiber length, paired with a heterodyne-based wavelength-locking system, to be employed as pump-probe source in Brillouin sensing applications. The enhanced source is compared with the long-cavity (LC) (~ 2 km) BRL pump-probe source that we recently demonstrated, showing a significantly lower relative intensity noise (~ -145 dB/Hz in the 0-800 MHz range), a narrower linewidth (10 kHz), combined with large tunability features and an excellent pumpprobe frequency stability (~200 Hz) which is uncommon for fiber lasers. The measurement of intensity noise on the novel BRL signal yielded an improved signal-to-noise ratio (SNR) of about 22 dB with respect to LC-BRL schemes that is expected to lead to a temperature/strain resolution enhancement in BOTDA applications up to 5.5 dB.

In this work we demonstrate the multiplexing capability of new optical fiber Fabry-Perot interferometers based on airmicrocavities using a commercial FBG interrogator. Three optimized air-microcavity interferometer sensors have been multiplexed in a single network and have been monitored using the commercial FBGs interrogator in combination with FFT calculations. Results show a sensitivity of 2.18 π rad/mε and a crosstalk-free operation.

Brillouin distributed optical fiber sensors have proved to be a powerful technology for real-time detection of strain and temperature. In such sensors the optical fiber is interrogated along its full length with a resolution down to decimeters and a frequency-encoding of the measurement data that is not affected by variation of the optical attenuation. The fiber sensing cable plays a key role, since it must provide accurate strain transfer, robustness and durability. In this paper, a novel, suitably designed fiber cable for achieving optimal strain measurement performance is presented. The concept and development phases are illustrated, together with results on the strain transfer capability.

In this paper, we demonstrate the use of single-mode Cobalt-doped fibers for active distributed temperature sensing with optical heating. A high-power EDFA at 1550nm is used for optically heating the fiber, while a Brillouin Optical FrequencyDomain Analysis (BOFDA) set-up operating at 850nm is employed to monitor the corresponding temperature changes. Owing to the use of a dual wavelength scheme, longer sensing distances can be reached even with relatively high doping concentrations. As a proof of concept, we demonstrate the possibility to distinguish two 1-m fiber sections, one immersed into water and another one surrounded by air.

Temperature/strain cross sensitivity is a long-standing issue in Brillouin-based distributed sensors, impairing the reliability of such sensors. So far, all the proposed methods perform the discrimination by measuring two quantities showing distinct responses to temperature and strain. Distributed Raman sensing enables temperature measurement without strain sensitivity. However, due to its weak signal intensity resulting from the principle based on spontaneous scattering, the spatial resolution is typically limited to ∼ 1 m. Here, for the first time, we use stimulated Brillouin scattering in gas-filled hollow-core photonic crystal fibers for distributed temperature sensing and we demonstrate ∼ 1 cm spatial resolution and 0.3°C temperature resolution fully free of strain cross sensitivity. Substantially higher performance is obtained thanks to the higher Brillouin gain, narrower gain linewidth and relaxed optical power restrictions when compared to solid silica single-mode fiber. This opens a new avenue in high performance distributed fiber sensing based on gas nonlinear optics.

Monitoring the presence of external electric fields over large distances and detecting losses along power transmission networks, is of extreme importance nowadays due to concerns with environment, efficiency, cost and safety. In this work, we evaluate a method to achieve distributed measurements of the quadratic electro-optic Kerr effect in silica fibers in a distributed way. For this purpose, we integrate a twin-hole fiber filled with BiSn alloy electrodes and monitor its electric-field induced refractive index (RI) change Δn by using a chirped-pulse phase-sensitive OTDR (CP-ΦOTDR). By exploiting its high sensitivity (RI changes of the order of 10^-9 ), we demonstrate that the proposed system is able to detect the intrinsic quadratic electrooptic nonlinearity (the electric Kerr effect) in the fiber, an effect that is usually considered to be too weak to be exploited for practical applications. Additionally, we show that the CP-ΦOTDR is sufficiently sensitive to measure the electric Kerr effect with untreated standard telecommunication fibers under realistic fields.

Any temperature change or strain acting on a section of the fiber induces a local variation of the refractive index. If the fiber is monitored by a chirped pulse φ-OTDR system, the variation of the refractive index causes a local shift of the backscattering trace and produces a change in the round trip time of the light coming from any further position of the fiber. While usually negligible, due to the high sensitivity of the chirped pulse φ-OTDR, in extreme occasions the distributed round trip time change may appear in the measurement as an undesired “virtual” perturbation. In this paper, we discuss and experimentally validate a mathematical model to account for (and eventually correct) the “virtual” perturbation.

A novel technique is proposed to obtain a flexible and variable spatial resolution from a conventional Brillouin optical time-domain analyzers using a fast post-processing algorithm. The approach is very attractive since a fine spatial resolution can be obtained from a coarsely resolved measurement obtained using a pulse longer than the acoustic settling time, leading to a better overall sensing performance, in particular for sub-metric spatial resolutions, with no compromises on sensing range and measurement time.

The signal-to-noise ratio (SNR) of the measurement for direct-detection Brillouin optical time-domain analyzers is modelled and experimentally validated, with and without the use of optical pre-amplification. Results indicate that preamplification associated with a good-quality photo-detector improves considerably the actual SNR, with only 1.5 dB penalty compared to the ideal shot noise limit.

Phase-sensitive optical time domain reflectometry measurements using chirped pulses and direct detection are a common option for cost-effective distributed acoustic sensing, able to reach remarkable sensitivities across long ranges in standard fibers. One of the main sources of noise of the technique originates from frequency drift and phase noise of the light source. A previous work has detailed the effects of laser phase noise in strain measurements and described a simple post-processing strategy to mitigate its first order effects. This, however, was restricted to the case where phase noise is the most dominant noise contribution to measurements. In this work, we expand the analysis for the cases of low and high phase noise lasers, and describe the considerations that should be taken in each case. We show that, when relying on the phase-noise cancellation strategy, a portion of the additive noise is converted to spatially correlated noise across the whole fiber, so whenever phase noise is not the dominant noise contribution, there is a minimum number of compensated windows that needs to be used in order to ensure an improvement.

We present fundamental limits on the strain resolution of phase sensitive distributed acoustic sensing based on considerations of thermal noise. These limits suggest that, despite ongoing advances, distributed sensors are unlikely to approach the sensing performance of traditional geophones and hydrophones.

Quasi-distributed sensing, e.g. Quasi-Distributed Acoustic Sensing (Q-DAS), with optical fibers is commonly used for various applications. Its excellent performance is well known, however, it necessitates high sampling rates and expensive hardware for acquisition and processing. In this paper, we introduce a technique, based on Compressed Sensing (CS) theory, to locate discrete reflectors' along a sensing fiber with a smaller number of samples than required according to Nyquist criterion. The technique is based on the fact that the fiber profile consists of a limited number of discrete reflectors and significantly weaker reflections of Rayleigh back-scatterers, and thus can be approximated as a sparse signal. The task of reconstructing a sparse signal from a sub-Nyquist sampled signal using Orthogonal Matching Pursuit (OMP) is presented and tested experimentally.

This work presents an on-field validation of an in-house built real-time phase-OTDR for monitoring the status of roller bearings. The acoustic sensor prototype was designed and assembled at RISE and evaluated on a 1 m diameter bearing at SKF AB facilities in Göteborg, Sweden. A 0.24 numerical aperture single-mode optical fiber was installed in the bearing lubrication groove, which is 50 mm large and 5 mm deep. Tests were performed to verify the response of the phaseOTDR to acoustic emissions in the bearing such as hammer hits and running the rollers at different loads. The fiber optic sensor results agree with the measurements performed by a standard industrial high sensitivity electronic accelerometer used for comparison. Moreover, as opposed to the reference electronic sensor, the phase-OTDR proved to be insensitive to electrical disturbances present on the environment.

The paper presents the first experimental results on operation of an optical frequency domain reflectometer (OFDR) based on a self-sweeping fiber laser. Frequency tuning in this laser is performed without actively tunable elements. Its intensity dynamics consists of regular microsecond pulses. The self-sweeping laser has high linearity of frequency tuning, which allows us to measure the reflectograms without additional spectral correction. The OFDR demonstrates capability of operation with spatial sampling of ~ 200 μm and sensitivity down to ~ -80 dB with line length of up to 9 meters.

We propose a fully distributed optical fiber sensor capable of performing spectrally-resolved detection of visible light radiation. The sensor is based on monitoring the temperature change between two optical fibers with different coating colors. In our implementation, the temperature is simultaneously monitored in a black-coated fiber (which is highly sensitive to all input wavelengths) and a color-coated fiber, which basically acts as an optical stop-band filter for a certain input color. By comparing the temperature behavior attained for each fiber, it is possible to obtain information of the wavelength/color of a given optical radiation present in the environment. Suitable calibration could lead to distributed colorimetry measurements.

The performance of the differential pulse-width pair Brillouin optical time domain analysis (DPP-BOTDA) is evaluated experimentally using either the gain from log normalization or linear normalization for the subtraction of traces collected with pump pulses of slightly different pulse widths. Using pump pulses widths of 43 ns and 40 ns, amplified Brillouin time domain probe traces were obtained for 10 km of standard single mode fiber. Two hotspots of length 30 cm and 6 m, separated by more than the spatial resolutions of the individual pulses and kept in a temperature controlled hot bath facility, were interrogated with temperature variations from 5 to 70°C, having probe signal gain of ~ 40% at the Brillouin Frequency Shift (BFS). This research work demonstrates, for the first time, that the use of linear gains for the subtraction step in creating the Brillouin gain spectrum, produces results for small to medium Brillouin frequency shifts (≤30 MHz), that deviate from the results of the subtraction of the logarithmic gains by as much as 2 MHz (~ 2°C), particularly for hotspots of the order of the spatial resolution of the DPP-BOTDA. For hotspots longer than the spatial resolution of the technique, the difference between results of the two processing methods show BFS deviations only at the end of the hotspots.

Smart textiles, also known as smart fabrics, in combination with automated intelligence (AI) technology are revolutionizing our world because of their ability to sense, communicate, conduct energy, and self-transform providing a new sense of intelligence reality to our daily lives similar to the human body’s nerve system capability to sense and react to its local environment. Over the past years our research group at Redondo Optics and support collaborators have been working on developing novel methods for weaving fiber optic sensors within high performance synthetic textile materials used in the manufacturing of “Smart” fabrics, ropes, and cables. Specifically, our group has investigated and developed automated manufacturing techniques for weaving and braiding glass and plastic optical fiber sensors within the strands and yarns of the produced textiles. The weaved fiber sensors – Microbend and FBGs Strain and Temperature, DCS and DBS Chemical and Biological – and support wearable electronics and power are used to monitor stress, strain, fatigue, load and movement, temperature, pressure, respiration and hearth rates, and body sweat chemical and biological constituents, and to transmit this information in real time to wearable communication devices for global dissemination to key intelligence sources. In the paper we provide on-going results on the use of the DIFOS technology for the global measurement of strain in high performance applications such as supersonic parachute canopies and strength member ropes, to jet fighter arresting gear cables in aircraft carriers.

This paper presents the embedding of calibrated fiber optic cables into a simplified concrete structure to obtain spatially dense internal strain measurements under a 3-point bending load. Taking advantage of the millimeter-level spatial resolution of the measurement system, it is possible to identify the onset of cracking within the concrete and compare the behavior of both unreinforced and steel rebar-reinforced structures. Integrating these sensors during new construction, or retrofitting into existing structures, will enable the rapid and cost-efficient inspection and lifetime monitoring, identifying impending failure before a catastrophic event occurs. This is exemplified by two applications in France: instrumenting a tower’s foundation and measuring settlement of a pillar built using a new design.

In this paper, we present research on the use of femtosecond lasers to develop a two-dimensional bending sensor by inscribing a 4 mm fiber Bragg grating (FBG) in each of the four cores of a multicore fiber (MCF) Fibercore SM-4C1500. The sensor located at the end of the fiber is spliced to a 50/125 multimode fiber (MMF). Due to the geometry of the MCF, part of its cores do not directly attach to the core of the multimode fiber, so that different curvatures cause variations in the reflected power. In this way, a reflection configuration and a commercial spectrometer are used to study its power response, simplifying the sensing, since it is not necessary to have WDM elements for the handling of wavelengths that vary tenths of nm in this type of sensors. Likewise, by carefully controlling the laser parameters and the motor stage position we are able to inscribe the FBGs by means of the point-by-point (PbP) method.

In this paper, a 6 mm hybrid Mach-Zehnder Interferometer (MZI) has been manufactured within a standard optical fiber using multiscan inscription with femtosecond laser. This technique allows the employ of cladding waveguides (CWG) as sensing arms for the interferometer. Refracted Near Field (RNF) profilometry and Quantitative Phase Microscopy (QPM) consistently suggest that CWG exhibit a smooth Type I refractive index change (RIC) that increases with the number of scans. This makes the scan number a potential way to control the coupling and Free Spectral Range (FSR) of the manufactured MZI. Its combination with a fiber Bragg grating (FBG) inscribed in the core makes possible to discriminate between different parameters.

FBG fabrication and thermal characterization of a silica/silicone hybrid microstructured optical fiber is demonstrated. The thin film of PDMS created on the inner surface of the microstructured optical fiber acts as the cladding. The measured temperature sensitivity is -110.9 pm/°C. Bragg wavelength of the FBG is tuned through UV irradiation and the thermal effect is further explored.

We report an orientation-sensitive omnidirectional vibration sensor based on fiber Bragg gratings (FBGs) inscribed in a seven-core optical fiber. By monitoring the central wavelength shifts of three FBGs in the central core and two outer cores, orientation information in 0-180° range as well as the acceleration value are obtained. The performance of the vibration sensor is characterized at different frequencies, orientations and accelerations. Comparison results for orientation discrimination using different groups of outer cores are investigated to enhance the reliability of the vibration sensor. The experimental results demonstrate an accuracy as high as 0.01° for orientation discrimination. The compact size and simple structure make the vibration sensor potentially superior in industrial applications where precise monitoring of the orientation is a necessity.

Fiber optic shape sensing is innovative technology, which enables distributed structural health monitoring, providing real-time feedback on shape and position, based on smart sensors arrays, which consists of optical fiber bundles or multicore fibers with embedded strain sensors. This paper describes a numerical analysis carried out to identify the effects of uncertainty in strain measurement and core position on the accuracy of fiber optic shape sensors, taking into consideration one of the most utilized geometry for fiber optic sensing applications, the seven-core Multicore Fiber, and distinct values of core spacings (distance between outer cores and sensor axis). The Monte Carlo Method was employed to simulate the real measurement process and one million of iterations were performed for each simulation with the aim of defining the laws of uncertainty propagation. The results of this study demonstrate the influence of core position uncertainty, strain measurement resolution and core spacing on optical shape sensors accuracy and can support the design of new sensors, predicting the achievable performance.

In this work, we present an experimental measurement of temperature and strain sensitivities of a micro-drilled optical fiber (MDOF). The MDOF consisted of a quasi-randomly distributed reflector along a single mode fiber (SMF). A fiber cavity laser based on MDOF was experimentally studied, attaining a single-wavelength laser emission centered at 1568.6nm. The output power level obtained from this single-laser oscillation when pumped at 140mW was around - 9.6dBm, and an optical signal to noise ratio (OSNR) of around 45dB was measured. Although temperature sensitivities of fiber Bragg gratings used as sensors are similar to our MDOF, strain sensitivity is enhanced around one order of magnitude when the MDOF was used.

We present a new approach to optical fiber photoacoustic spectroscopy for gas trace detection. After an explanation of its working principle, we show the results of the measurements obtained with our first prototype. We then analyze the advantages that this approach may provide, and further discuss how the expertise developed by the optical fiber sensor community may contribute to the field of photoacoustic gas sensing.

The cylindrical geometry of optical fibers produces an astigmatic distortion in a wavefront focused within it. In the case of femtosecond lasers, this produces a fluence loss that decreases its processing performance. In this work, the phase change produced by an astigmatic femtosecond laser beam (direct exposition to the) and a corrected beam (applying a simple adaptive optics process) is compared. The astigmatic correction decreases the modification threshold by approximately a magnitude order and changes the sign of the refractive index change at low pulse energies.

In this work, we report on a simple, but highly sensitive sensor based on two intrinsic Fabry-Pérot interferometers (FPI) inscribed in a standard optical fiber. A brief theoretical study on the Vernier effect is presented, followed by a simulation of the FPIs physical characteristics to achieve the desired sensitivity enhanced factor. Based on the simulation results, the FPIs were fabricated using a custom micromachining setup based on a near-infrared femtosecond laser and a motorized XYZ platform. A real-time monitoring was performed throughout the entire process, by visualizing the inscription with a CCD camera and recording the resulted spectra. High-resolution was demonstrated for the manufactured devices, achieving, in the best-case, values of ~7 nε and 0.001°C for strain and temperature, respectively. The sensor’s performance allied with its versatile and customizable configuration allows an operando and in-situ strain and temperature monitoring under harsh environments.

Photonic liquid crystal fibers (PLCFs) have been studied for over a decade as an emerging field of sensing and telecommunication devices. Exciting properties of liquid crystals (LCs) infiltrating photonic crystal fibers (PCFs) can be additionally tuned by doping with various materials that are sensitive to external influences, such as an electric field or temperature. Among them, metallic nanoparticles (NPs) are gathering a great interest, due to their potential applications. NPs can be used to highly influence material properties of LCs as dielectric anisotropy, elastic constants, and viscosity. This may lead to many desirable effects, such as a decrease of the Fredericks threshold voltage or reduction of LC switching times. In this paper we doped a nematic LC with 2-nm gold (Au) and 8-nm silver (Ag) NPs, and infiltrated the prepared mixtures into photonic crystal fibers. We examined the influence of this doping in two different electric field systems, one with two flat copper electrodes, and second with four copper microelectrodes. Our results indicate that metallic NP (both Au and Ag) doping enhances sensitivity of the PLCF-based sensors to an electric field and decreases the threshold voltage. Additionally, due to smaller distances between the electrodes, the proposed four microelectrode system requires lower voltages to effectively tune the PLCF.

Here we report about mode transition phenomenon occurring in uncoated Long Period Grating (LPG) and the subsequent possibility of enhancing the surrounding refractive index sensitivity. For this purpose, we have considered a grating realized in double cladding fiber with W-type refractive index profile. This fiber presents an outer cladding with refractive index higher than inner cladding one; thus, through the chemical etching of the outer diameter, the transition phenomenon from outer- to inner-cladding of some cladding modes can be induced in controllable manner. The effect is illustrated by numerical analysis and it is experimentally verified using an LPG written in pure-silica core with F-doped inner cladding and silica outer cladding fiber, by means of the electric arc discharge technique.

The aim of this work was to design a miniature size optical fiber sensors` transducer which is sensitive to refractive index changes. A polymer microtip manufactured by photopolymerization at the end of a multi-mode optical fiber was used as a transducer. Proper geometry parameters of the microptips allow to use these elements as transducers for reflective types of the refractive index sensors. Two various multi-mode optical fibers were used in the experiments, i.e. a gradient-index with a 62.5 μm core diameter and a step-index with a 105 μm core diameter. They were selected to test influence of light propagation properties on the optical fiber transducer reflective parameters. Optical Backscatter Reflectometer was applied to validate mentioned above transducers’ parameters. The principle of sensors’ work was based on measurement of an amplitude of back scattered signal when the refractive index of liquid around the microtip was changed. Experimental tests were made when the selected microtip was immersed into liquids with various refractive indices in the range of 1.30 – 1.70. Minimal amplitude of the reflected signal occurred when the refractive index of the liquid was equal to the refractive index of the microtip. For both tested optical fiber types, changes of the liquid refractive index caused proportional changes of the signal reflected from the microtip. Maximal measured sensitivity of such optical fiber transducer of 180 dB/RIU with a dynamic range of 32 dB was achieved. Therefore, it is possible to use these micro-elements as transducers for the refractive index optical fiber sensor.

The paper develops an analytical apparatus to describe the resolution limits of optical fiber sensors, based on high-finesse Fabry-Perot interferometer, interrogated by means of spectral interferometry. The properties of interferometer spectral function, required for the analysis of resolution, are developed; in order to analyze the influence of noises on the resolution, Cramer-Rao formalism is utilized. An estimation of achievable resolution and optimal interferometer parameters in case of dominant photo detector noise is done.

We present the inscription of a fibre Bragg grating (FBG) array in non-transparent carbon coated optical fibre using the direct write, plane-by-plane femtosecond laser inscription method. Our application is targeting high temperature measurements in harsh environments. The grating spectra were characterized in reflection and the temperature response of the array was measured up to 160°C.

We present an extremely flexible femtosecond (fs) laser inscription method, applicable to the development of critical filtering and wave-guiding components in optical fibres. We inscribe in-fibre devices, such as cladding waveguides (CWGs), cladding Mach-Zehnder interferometers (MZIs) and embedded waveguide Bragg gratings (WBGs) using the same key femtosecond laser parameters, via an “inscribe and step”, plane-by-plane (Pl-by-Pl) approach, applied as necessary on two orthogonal axes. This leads to femtosecond laser-inscribed cladding waveguides and ultra-compact MZIs that can support functional, integrated fibre Bragg gratings (FBGs); the unique sensing characteristics of the filters are maintained and provide complementary measurand information. The flexibility and control in waveguide/grating fabrication leads to sensing device customization, e.g. tailored bend sensing. We characterize CWG-WBG devices for their bend response, whereas the MZI-WBG is exposed to temperature and humidity excursions, confirming the unique sensor responses are maintained for this compact, compound sensor. The MZI exhibits response to external refractive index, a large, negative wavelength response with temperature and high sensitivity to humidity, whereas the MZI-located WBG displays a similar sensitivity to conventional core-based Bragg grating sensors to temperature and no response to relative humidity. We consider that this research is an important step in developing compact, smart optical fibre sensors.

Femtosecond (fs) lasers are well suited for high-resolution inscription in transparent materials of all types, and in particular optical fibres. We use the versatility afforded by combining fs lasers and optical fibres to realise cladding integrated circuits that we term “bridging waveguides”; in this case we consider coupling between the displaced cores of different types of optical fibres. In particular, this new, direct, and efficient coupling technique is used to couple light from one core to multi-core optical fibres, once the two fibre types have been fusion spliced. We add further versatility by incorporating several fibre Bragg grating (FBG) arrays that are inscribed into three of the four cores of a four-core fibre. The bridging waveguides direct light from the single core fibre (SMF28) to the four-core fibre and recover the reflection spectra, so that the single core fibre can be connected to a conventional spectrometer, allowing for standard FBG characterization and demodulation. We can selectively inscribe the bridging waveguides and monitor the growth of the FBG arrays or first inscribe the FBGs and assess the quality of the waveguides in recovering the FBG spectra. The response of the FBGs after the inscription were measured and calibrated for bend and shape sensing.

Fiber Bragg gratings (FBGs) in a poled silicate fiber are used to detect external voltage applied to the fiber’s internal electrodes. This work shows a basic proof-of-concept of a single-ended, fiber-based voltage sensor that can be used to measure periodic high-voltage signals. The setup can be extended to a multiplexed e-field interrogation system and used in the electric power industry for remote sensing of transmission lines and power plants.

We exploit backward stimulated Brillouin scattering in gases to achieve unprecedented nonlinear optical amplification in fibers. The gain coefficient is 10 times larger than any reported nonlinear gain in gas-filled HC-PCF and 6 times larger than the largest nonlinear gain in standard silica single-mode fiber (SMF). Furthermore, our system can work at any wavelength from vacuum ultraviolet to mid-infrared thanks to the nature of stimulated Brillouin scattering. This massive gain enables us to realize a single-pass gas Brillouin laser with 140 mW threshold power over 50 m of fibre. This platform opens new avenues in gas Brillouin lasing, all-optical signal processing and high performance distributed fiber sensing.

We present the results on fabrication of 3D fiber Bragg grating (FBG) arrays in specialty 7-core optical fibers with straight or twisted (spun) cores. Femtosecond laser inscription technology allowed us to modify the fibers through the acrylate or polyimide protective coatings that significantly increases the durability of the FBG sensors as compared to conventional UV inscription approach, requiring the coating removal. Custom-made 7-core fiber with polyimide coating opens up new prospects for shape sensors operating in high-temperature environment. Twisted-core fiber makes it possible to measure not only the shape, but also the direction of fiber torsion that is essential for a free-standing sensors. A novel method enabling core-selective FBGs inscription in a 7-core spun optical fiber is presented in this work. By using the created sensors bending radii down to several millimeters can be measured with a high precision. Separation of different core FBGs by wavelength makes it possible to combine several cores during their interrogation, which allows for sensor measurements through a single optical port.

One gateway to understanding physical, chemical, and biological processes at the nanoscale level is high-speed tracking of single nano-objects. Here I will present our recent results on tracking individual nano-objects inside optofluidic microstructured fibers via elastic light scattering. Conceptually, the nano-objects (e.g., gold nano-spheres, polymer beads, sub-20nm viruses) are located within a liquid environment inside a well-selected channel of respective microstructured fiber. Light elastically scattered by the freely diffusing nano-objects is transversely detected by a microscopic setup continuously imaging the liquid-filled channel from the side at very high frame rates. Here I will discuss (i) tracking of unlabeled virions at rates of over 2 kHz for durations of tens of seconds in nanobore optical fibers, (ii) retrieving full 3D information about the nano-object’s trajectory using modified step index fibers, and (iii) simultaneous detection and identification of hundreds of nano-objects in anti-resonant hollow core fibers. Any of the mentioned approaches addresses light scattering intensities and diffusion constants, allowing us to determine key properties of the nano-objects such as size or hydrodynamic radius. Together with the compatibility with fiber circuitry and microfluidics, the presented approach defines a new platform for fiber sensors and nano-scale physics with applications in a multitude of fields such as bioanalytics and soft matter material science.

This paper shows that fiber Bragg gratings written in standard single mode optical fiber with IR femtosecond pulses and point-by-point technique are high temperature resistant (<1000°C). Moreover, after calibration process, these gratings can be used as a reference to study and discriminate between different high temperature annealing mechanisms involved in other types of gratings and/or fibers. Here we have considered the regeneration process of gratings written by UV laser in boron/germanium co-doped single mode optical fiber. Hence, the monitoring of grating strength and differential wavelength shift between femtosecond and type-I gratings during annealing cycle yields the wavelength shift due to the annealing of doping (mainly boron) and UV-related defects and their relative contributions to the regeneration mechanism.

A novel method for high-sensitive measurements of optical attenuation in metal-coated optical fibers in a wide range of wavelengths is demonstrated. Some part of the radiation transmitted through the waveguiding core of the metallized silica fiber is scattered and eventually absorbed in the carbon layer or the metal coating, thus heating it. Absorption in the silica core also contributes to the overall fiber heating. The method used for determination of optical attenuation coefficients of metallized fibers is based on the measurement of the change of temperature-dependent electrical resistance of metal coating induced by transmitted laser radiation. A number of single-mode and multimode metallized fibers with different geometry were investigated using several laser sources operating in visible and near infrared ranges. Experimentally obtained spectral dependence of optical losses of copper-coated fibers was analyzed. Possible explanations for optical attenuation mechanisms at different wavelengths and in different fibers were proposed. The obtained results can help to optimize various devices based on metal-coated fibers, such as laser radiation power fiber sensors or high-power laser sources.

In this paper, the design and implementation of a cost-effective interrogation architecture, for dynamic strain monitoring of in-line Fabry-Perot interferometric (FPI) optical fiber sensors is presented. The common interrogation techniques for this type of sensors are based in the full spectrum analysis, which render them not adequate for dynamic/high frequency monitoring applications. In this work, we propose an alternative cost-effective solution, based on a simplified edge-filter technique, for the dynamic monitoring of FPI sensing devices. The FPI based sensor was produced from the recycling of optical fiber previously damaged by the catastrophic fuse effect using precise splicing techniques. A characterization was performed with two different devices, an optical spectrum analyzer (OSA) and the proposed device leading to similar behavior and sensitivity values.

In this paper we introduce a novel approach for the measurement of the intensity profile of high-power laser radiation, which does not require any preliminary attenuation. It is based on the application of the matrix made of multimode passive copper-coated optical fibers. It is well known, that laser radiation experiences Rayleigh and Mie scattering while transmitting along an optical fiber. Radiation scattered inside the fiber core is completely absorbed by the outer copper layer leading to its heating. The temperature change of the metal coating proportional to the transmitted optical power is determined directly by measuring its electrical resistance. A matrix sensor was assembled for measuring the transverse intensity distribution of the laser beams. It comprised nineteen 660 μm (core diameter 600 μm) multimode copper-coated optical fibers. Intensity profile measurements were carried out for the 67 W single-mode Yb-doped fiber laser and 72 W multimode laser diode sources. The laser radiation was directed into the polished end faces of the fiber matrix elements. Optical power transmitted through each fiber was proportional to the incident optical intensity at corresponding location of the fiber end face. The transverse intensity profile of the laser beam was evaluated by measuring the resistance change of the copper coating of each fiber. Preliminary the calibration of the resistance dependence on the incident optical power was separately conducted for all 19 fibers. An introduced technique can be also applied for the determination of the radiation beam quality factors such as M² and BPP.

We proposed a novel structure of short-cavity random fiber laser (RFL) with a stable narrow linewidth output. A random-spaced Bragg grating array is used for random feedback. A ring optical path and the grating array form a short half-open cavity, and a high-precision π-phase-shifted grating (π-FBG) is placed in the ring path for filtering and modelocking to ensure a stable single-mode random laser operation. The linewidth of the laser is 257 Hz with 50 dB sidemode-suppression-ratio (SMSR). The laser wavelength drift measured in the laboratory is less than 1 pm within 20 min. The RFL has a simple structure and can achieve a stable narrow linewidth single mode output, which provides a new choice for high resolution optical fiber sensing.

In this manuscript, we propose a new technique to suppress the noise level in an interferometric optical fiber sensing system. Previous work shows that laser frequency noise is the main factor that limits the resolution in a phase generated carrier (PGC) demodulation system. Using a bidirectional pumped random distributed feedback fiber laser (RDFL), the laser frequency noise can be suppressed from 30 Hz/√Hz to 4 Hz/√Hz above 1 kHz. Therefore, the wavelength resolution is improved from 5×10^-7 pm/√Hz@ 1 kHz to 5×10^-8 pm/√Hz@ 1 kHz.

A high sensitivity optical fibre strain sensor based on a hollow-core fibre embedded single-mode-multimode-singlemode (HESMS) fibre structure is described. The HESMS fibre structure is fabricated by connecting a length of hollowcore fibre (HCF) between two sections of multimode fibre (MMF). The embedded HCF acts as a Fabry-Perot cavity, and therefore a composite interference pattern based on the inherent multimode interference (MMI) and the introduced Fabry-Perot interferometer (FPI) is established. This resultant composite interference greatly improves the performance of the SMS fibre structure for strain sensing. A maximum strain sensitivity of -2.71 pm/με over the strain range of 700-1200 με is achieved experimentally. Benefiting from the simple fabrication process, this low-cost, high sensitivity strain sensor can be realistically applied in many areas where high-accuracy strain measurement is required.

We investigate the properties of the ROGUE (Random Optical Grating by Ultraviolet or ultrafast laser Exposure), a type of grating we demonstrated recently, to better understand the origins of its enhanced reflectivity and broad bandwidth. Through this study, we are able to distinguish which proportion of the signal comes from random defects originating from the laser exposure, and which proportion comes from the random grating written in the fiber core. Through the understanding of the cause and the parameters responsible for the signal enhancement, we can optimize ROGUE fabrication to minimize scattering losses due to the random generation of defects and optimize the signal backscatter. Additionally, we investigate the origins of the ROGUE’s large bandwidth, and derive the parameters defining it allowing us to tune the bandwidth to the optimal value for sensing applications.

Thermal annealing was initially introduced for multiplexing purposes, since it can induce a permanent negative Bragg wavelength shift for polymer fibre grating sensors. At a later stage, it is shown that annealing can also provide additional benefits, such as strain and humidity sensitivity enhancement and augmented temperature operational range. In this paper, we report additional usage of thermal annealing on PMMA fibre Bragg grating sensors. We show the possibility to tune Bragg wavelengths to longer wavelengths permanently by stretching the polymer optical fibre during the thermal annealing process. An array of sensors fabricated with only one phase-mask, demonstrates the concept by having Bragg wavelengths below and above the original inscribed spectral position. In addition, we report that thermal annealing can be also used to enhance the performance of sensors when used for stress and force monitoring.

Tensile tests of optical fibers and vibration tests of optical fiber rings have been conducted to measure the exact physical properties of optical fibers and optical fiber sensors. However, there have been few studies using optical fibers themselves as measurement sensors. In this study, we propose a free vibration test using an FBG sensor embedded in an optical fiber and a method to estimate the mechanical properties of the optical fiber sensor from the measured natural frequency. The proposed method does not require additional equipment such as tensile tester and can easily evaluate the mechanical characteristics even with the FBG sensor strain measuring equipment already provided. Therefore, it is expected to be used for checking the physical properties of the optical fiber itself and the properties of the recoated optical fiber. In order to analyze the natural frequency of the optical fiber, we compared the experimental results with the finite element method and evaluated the elastic modulus from the natural frequency of the optical fiber.

The detection and quantification of the presence of certain chemical species is of central importance regarding permanent structural health monitoring of key industrial fields and civil infrastructures such as oil extraction boreholes or radioactive waste repositories, where H₂ is released. With this work we propose and test a competitive technique able to measure the concentration of hydrogen and deuterium thanks to their diffusion into the silica glass of a standard optical fiber, already employed for the distributed monitoring of large infrastructures. The proposed technique, based on Chirped-Pulse Phasesensitive Reflectometry (CP-φOTDR), could represent a novel solution for this problem, thanks to its ability to provide dynamical measurements of refractive index change, with great linearity and sensitivities of 10^-8 refractive index units, featuring spatial resolutions of a few meters and kilometric sensing ranges.

SCiO is a smartphone-connected pocket spectrometer operating in the 700-1100 nm band. Together with a learning machine algorithm, it already demonstrated the effectiveness for distinguishing extra virgin from non extra virgin olive oils and for the multi-analysis of nutraceutical indicators. This paper shows a new experiment for the assessment of water residue at the end of the olive oil production process. Principal Component Analysis and Linear Discriminant Analysis were used to demonstrate a qualitative screening with a threshold of 0.5% v/v of water content and an accuracy of 93%. Also, a model for predicting the water concentration was created by means of the Partial Least Square regression, providing a regression coefficient R² =0.92, and an error of 0.26%.

In this work, we present an enhanced design for a Brillouin Ring Laser (BRL) based on a doubly-resonant cavity (DRC) with short fiber length, paired with a heterodyne-based wavelength-locking system, to be employed as pump-probe source in Brillouin sensing applications. The enhanced source is compared with the long-cavity (LC) (~ 2 km) BRL pump-probe source that we have recently demonstrated, showing a significantly lower relative intensity noise (~-145 dB/Hz in the whole 0-800 MHz range), a narrower linewidth (10 kHz), combined with large tunability features and an excellent pumpprobe frequency stability (~200 Hz) which is uncommon for fiber lasers. The measurement of intensity noise on the novel BRL signal yielded an improved signal-to-noise ratio (SNR) of about 22 dB with respect to LC-BRL schemes that is expected to lead to a temperature/strain resolution enhancement in BOTDA applications up to 5.5 dB.

Biosensing using optical fibers allows the detection of low concentrated analyte, bringing point-of-care and remote analyses. Tilted fiber Bragg gratings (TFBGs) are permanent structures photo-inscribed inside the core of telecommunication-grade optical fibers and are known to be highly sensitive refractometers. In this paper, we present a hybrid gold deposition method to monitor thin depositions in real time though the inherent properties of the spectra. This yield Surface Plasmon Resonance (SPR) enhancements that is of interest for the detection of low concentrations of analytes. We show how to functionalize our sensors against proteic biomarkers. This strategy is one of a series to manufacture TFBGs platforms adapted for biosensing.

In this paper we study de response time and sensitivity of a previous reported fiber optic sensor network, multiplexed in the time and spatial domain. As functions of ring resonator coupling coefficient (K) and fiber optic sensor attenuation (A). The network is studied without a frequency based self-reference as previously reported. The response time is defined as the time needed to reach the 95% of the steady state total output power different to the previous work. We find better sensitivities for K and A of 0.99 and 1 respectively. And bigger response times for K and A of 0.02 and 1 respectively.

An optical fiber sensor consisting of short sample of experimental Hi-Bi optical fiber integrated between two single mode (SM) optical fibers is proposed and experimentally demonstrated. The experimental fiber consists of core with diameter equals 8.5 μm and the two side Stress Applied Part (SAP) of diameter 25 μm. The SAPs are made of Al-Ladoped silica. The birefringence of the fiber is 3.19·10^-4. We measured spectral distribution of intensity at the output of the SM fiber as function of the analyzer's adjustment angle and the suitable position of the analyzer was determined. Then we applied the bodies with weights up to 100 g on the fiber for two significant positions of SAPs: SAPs were next to each other and above each other. Evaluation of the measurements were done by the peak-to-peak amplitude of optical power difference (in dB) of two measurement for which we determine the applied force (weight). The sensitivities were determined to be 0.081 dB/g for SAPs next to each other and 0.037 dB/g for second position of SAPs, respectively. From the results it follows that the sensor could be used not only for determination of lateral force but also for determination of applied force position.

We present our first results of ionizing radiation measurement using optical fibers and LYSO scintillator. The LYSO scintillator was used to detect the radiation and the silica optical fibers with 1 mm fiber core was used to deliver scintillation radiation from LYSO to single photon counter. We present our setups and measurement results of several ionizing sources Co-60, Cs-134, Cs-137 and Am-241 with different level of the activities. Focus to measurement gamma radiation of these nuclides was motivated by the research of measurement possibilities in the nuclear power plants by optical fibers. Used nuclides have different level of radiation energies and can be used to design and optimize of the measurement setup. We used one and four optical fibers with different types of collimation at both ends of the optical fibers to deliver scintillation radiation to the single photon counter. The comparison of these setups of the measurement is presented. Our measurement confirmed the optical fibers can be used to measure radiation of these nuclides with activities from tenths up to hundreds kBq. Next will be verified the measurement of higher level of radiation activities.

In this work, a deep convolutional adaptive filter is proposed to enhance the performance of a Raman based distributed temperature sensor system by the application of domain randomization methods for its training. The improvement of the signal-to-noise ratio in the Raman backscattered signals in the training process and translation to a real scenario is demonstrated. The ability of the proposed technique to reduce signal noise effectively is proved independently of the sensor configuration and without degradation of temperature accuracy or spatial resolution of these systems. Moreover, using single trace to noise reduction in the ROTDR signals accelerates the system response avoiding the employment of many averages in a unique measurement