This PDF file contains the front matter associated with SPIE Proceedings Volume 9916, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

In this work, we demonstrate the effect of detuning the resonant coupling between a long period grating and the plasmonic band of gold nanoparticles on the device sensitivity. In an intensity coded configuration, the sensitivity was measured at 568.12 nm and 598.62 nm, for surroundings refractive indexes ranging from 1.3629 to 1.4184. A comparison between the responses of the two dips centered at these wavelengths resulted in a sensitivity enhancement of about 17 times for the dip localized close to the center of the localized surface plasmon resonance.

A curvature sensor based on a Fabry-Perot interferometer is proposed. A capillary tube of silica is fusion spliced between two single mode fibers, producing a Fabry-Perot cavity. The light propagates in air, when passing through the capillary tube. Two different cavities are subjected to curvature and temperature. The cavity with shorter length shows insensitivity to both measurands. The larger cavity shows two operating regions for curvature measurement, where a linear response is shown, with a maximum sensitivity of 18.77pm/m^-1 for the high curvature radius range. When subjected to temperature, the sensing head produces a similar response for different curvature radius, with a sensitivity of 0.87pm/°C.

We present high temperature (~1100°C) stability tests of, Ge-doped and F-doped, optical fibre sensors. Our analysis includes the variation in their behaviours within high temperature environments and how the dopant diffusion affects their long term stability.

In this paper a novel patent pending high resolution optical fibre temperature sensor, based on an optical fibre pressure and temperature sensor (OFTPS), which is surrounded by an oil filled chamber, is presented. The OFPTS is based on a Fabry Perot interferometer (FPI) which has an embedded fibre Bragg grating (FBG). The high ratio between the volume of the oil filled outer cavity and the FPIs air filled cavity, results in a highly sensitive temperature sensor. The FBG element of the device can be used for wide range temperature measurements, and combining this capability with the high resolution capability of the FPI/oil cavity results in a wide range and high resolution temperature sensing device. The outer diameter of the sensor is less than 1mm in diameter and can be designed to be even smaller. The sensors temperature response was measured in a range of ΔT = 7K and resulted in a shift in the optical spectrum of Δλ_F = 61.42nm. Therefore the Q-point of the reflected optical FPI spectrum is shifting with a sensitivity of s^ot = 8.77 nm/K . The sensitivity can easily be further increased by changing the oil/air volumetric ratio and therefore adapt the sensor to a wide variety of applications.

In this work, a new application of random distributed feedback lasers to fiber optic sensing has been presented. The particular properties of these lasers, such as the lack of longitudinal modes and high stability, have been exploited to monitor transversal load using a phase-shifted fiber Bragg grating (PS-FBG), obtaining a resolution of 1g and a sensitivity of 3.95GHz/Kg. Due to the PS-FBG birefringence and the load-interrelated transmission lines generated by the PS-FBG along the orthogonal polarization directions, the beating of the two emission lines generated in the laser can be monitored in the electrical domain. As a result, transversal load applied on the sensor can be measured.

This work presents the application of a sensor based on quasi-distributed Fiber Bragg Gratings to monitor stator bars temperature of large electric generators. The applied FBG packaging method follows industrial standard procedures, and resulted in a robust and reliable sensing method, facilitating the future installation in the power plant. Experimental results are acquired in laboratory using the expected range of temperature values in the real machine. The measurement errors in the recorded results are within the calculated uncertainties and the time constant is shorter than what is obtained with conventional RTD for the same application.

This paper examines a technique that utilizes a Sagnac loop with a microfiber coupler (MFC) as a coupler which allows the MFC to operate effectively as a sensor but with larger than normal tapered fiber diameters. The proposed structure is found to be suitable for temperature and refractive index (RI) sensing. It is shown that a variation in the surrounding of the MFC RI results in a shift of the output spectrum, while a temperature variation leads to changes in the intensity of the interference dips. A decrease in the waist diameter of the MFC results in an increase in the sensitivity to temperature. For MFC structures based on a 5.6 μm and a 3 μm fiber waist diameter, the minimum transmission power level of a selected spectral dip decreases by 1.7 dB and 5.03 dB respectively, as the temperature changes from 18 °C to 44 °C. A change in the surrounding RI from 1.334 to 1.395 results in the spectral redshift of 8 nm using a 5.6 μm fiber waist diameter. By functionalizing the surface of the MFC with various materials, the structure could potentially be used for sensing of other parameters.

The thermal and deformation properties of fiber Bragg gratings (FBG) in the visible range were characterized for the first time in our knowledge. The FBG were written in silica single mode (cutoff in the visible and infrared range) and multimode fibers, using a phase-mask (460 nm period) illuminated by a 248 nm femtosecond laser.

The luminosity upgrade of the Large Hadron Collider (HL-LHC) planned at the European Organization for Nuclear Research (CERN) requires the development of a new generation of superconducting magnets based on Nb3Sn technology. The instrumentation required for the racetrack coils needs the development of reliable sensing systems able to monitor the magnet thermo-mechanical behavior during its service life, from the coil fabrication to the magnet operation. With this purpose, Fiber Bragg Grating (FBG) sensors have been embedded in the coils of the Short Model Coil (SMC) magnet fabricated at CERN. The FBG sensitivity to both temperature and strain required the development of a solution able to separate mechanical and temperature effects. This work presents for the first time a feasibility study devoted to the implementation of an embedded FBG sensor for the measurement of the "true" temperature in the impregnated Nb₃Sn coil during the fabrication process.

This work presents an experimental demonstration of a scheme based on an internally modulated fiber ring laser for high-sensitivity temperature sensing. The attained temperature resolution has been as low as ± 2pm even when a commercial FBG with a sensitivity of 10 pm/°C was used. Thus, a fivefold improvement in the temperature sensor resolution can be achieved when compared to a simple FBG interrogation scheme. In addition to this, the measuring range could be selected only by changing the frequency modulation of the fiber ring laser. This technology also allows to triple the photodiode bandwidth unambiguously when temperature or strain measurements are carried out, which is a remarkable achievement in term of cost reduction.

An optical fibre sensor for simultaneous temperature and humidity measurements consisting of one fibre Bragg grating (FBG) to measure temperature and a mesoporous film of bilayers of Poly(allylamine hydrochloride)(PAH) and silica (SiO₂) nanoparticles deposited onto the tip of the same fibre to measure humidity is reported. The hygroscopic film was created using the layer-by-layer (LbL) method and the optical reflection spectra were measured up to a maximum of 23 bilayers. The temperature sensitivity of the FBG was 10 pm/°C while the sensitivity to humidity was (-1.4x10^-12 W / %RH) using 23 bilayers. The developed sensor was tested in the mechanical ventilator and temperature and humidity of the delivered artificial air was simultaneously measured. Once calibrated, the optical fibre sensor has the potential to control the absolute humidity as an essential part of critical respiratory care.

An optical fiber gamma ray detector is presented in this work. It is based on a Terbium doped Gadolinium Oxysulfide (Gd2O2S:Tb) scintillating powder which cover a chemically etched polymer fiber tip. This etching improves the fluorescence gathering by the optical fiber. The final diameter has been selected to fulfill the trade-off between light gathering and mechanical strength. Powder has been encapsulated inside a microtube where the fiber tip is immersed. The sensor has been irradiated with different air Kerma doses up to 2 Gy/h with a ¹³⁷Cs source, and the spectral distribution of the fluorescence intensity has been recorded in a commercial grade CCD spectrometer. The obtained signal-to-noise ratio is good enough even for low doses, which has allowed to reduce the integration time in the spectrometer. The presented results show the feasibility for using low cost equipment to detect/measure ionizing radiation as gamma rays are.

Thermal annealing can be used to induce a permanent negative Bragg wavelength shift for polymer fibre grating sensors and it was originally used for multiplexing purposes. Recently, researchers showed that annealing can also provide additional benefits, such as strain and humidity sensitivity enhancement and augmented temperature operational range. The annealing process can change both the optical and mechanical properties of the fibre. In this paper, the annealing effects on the stress and force sensitivities of PMMA fibre Bragg grating sensors are investigated. The incentive for that investigation was an unexpected behaviour observed in an array of sensors which were used for liquid level monitoring. One sensor exhibited much lower pressure sensitivity and that was the only one that was not annealed. To further investigate the phenomenon, additional sensors were photo-inscribed and characterised with regard their stress and force sensitivities. Then, the fibres were annealed by placing them in hot water, controlling with that way the humidity factor. After annealing, stress and force sensitivities were measured again. The results show that the annealing can improve the stress and force sensitivity of the devices. This can provide better performing sensors for use in stress, force and pressure sensing applications.

The influence of the distance between the fiber end and the machined surface on temperature measurements in a two-color fiber-optic pyrometer is analyzed. The propose fiber-optic pyrometer is capable of measuring highly localized temperatures, while avoiding the use of lenses or fiber bundles, by using a standard graded index glass fiber OM1 with 62.5/125 core and cladding diameters. The fiber is placed very close to the target and below the tool insert. The output optical power at both wavelength bands is theoretically and experimentally analyzed for a temperature of 650°C at different fiber positions in a range of 2mm. The results show that there is no influence of the fiber position on the measured optical power and therefore, on the measured temperature.

We propose a high-resolution static-strain sensor based on a FBG Fabry-Perot interferometer (FBG-FP) and a wavelet domain cross-correlation algorithm. This sensor is used for crust deformation measurement, which plays an important role in earthquake precursor observation. The Pound-Drever-Hall (PDH) technique based on a narrow-linewidth tunable fiber laser is used to interrogate the FBG-FPs. A demodulation algorithm based on wavelet domain cross-correlation is used to calculate the wavelength difference. The FBG-FP sensor head is fixed on the two steel alloy rods which are installed in the bedrock. The reference FBG-FP is placed in a strain-free state closely to compensate the environment temperature fluctuation. A static-strain resolution of 1.6 n(epsilon) can be achieved. As a result, clear solid tide signals and seismic signals can be recorded, which suggests that the proposed strain sensor can be applied to earthquake precursor observation and earthquake monitoring.

The sensitivity of low-loss perfluorinated polymer optical fiber (PF-POF) to gamma radiation is investigated for on-line radiation monitoring purposes. The radiation-induced attenuation (RIA) of a commercial PF-POF based on Cytop material is measured in the visible spectral region. The fiber RIA shows strong wavelength dependence with rapid increase towards the blue side of the spectrum. The wide range of radiation sensitivities is available via careful selection of appropriate monitoring wavelength. The accessible sensitivities span from 1.6 ± 0.2 dBm^-1/kGy measured at 750 nm to 18.3 ± 0.7 dBm^-1/kGy measured at 420 nm. The fairly high radiation sensitivity as well as its wide tunability makes the fiber a promising candidate for a broad range of applications.

In this paper we report the field test of fiber optic ocean bottom seismograph (OOBS) which can be used in the active source seismic research. There are three fiber laser accelerometers (FLAs) and one fiber laser hydrophone (FLH), which is wavelength division multiplexed, in the OOBS. The interrogation system is put on shore and is connected with the OOBS with optical fiber cable. The field test of using an air gun is carried out under water with a depth of 30 m. The results show that the OOBS has similar performance as conventional electric OBS.

A smart metal component having the potential for high temperature strain sensing capability is reported. The stainless steel (SS316) structure is made by selective laser melting (SLM). A fiber Bragg grating (FBG) is embedded in to a 3D printed U-groove by high temperature brazing using a silver based alloy, achieving an axial FBG compression of 13 millistrain at room temperature. Initial results shows that the test component can be used for up to 700°C for sensing applications.

We present a novel and technically advanced system – Fibre-Optic System for Rotational Events & Phenomena Monitoring (FOSREM). It has been designed in order to register and monitor rotational events in seismological observatories, engineering constructions, mines and even on glaciers and in their vicinity. Its wide application field is a result of unique parameters and electronic solutions which give an opportunity to measure a component of rotation in the wide range of a signal amplitude from 10^-8 rad/s to 10 rad/s, as well as a frequency from 0 Hz to the upper frequency between 2.56 Hz to 328.12 Hz. Moreover, the numerical analysis and simulations indicate that it keeps the theoretical sensitivity equal to 2·10^-8 rad/s/Hz^1/2. FOSREM is equipped with an advanced communication module which gives the possibility for a remote detection parameter control, as well as the recorded data receiving. It enables the sensor to assemble in any chosen place. In the paper we present laboratory investigations and tests which confirm the wide application field and practical aspects of FOSREM.

In this article, we study the feasibility of using a fibre-optics current sensor (FOCS) for the measurement of plasma current in the future fusion reactor ITER. The sensor is based on a classical FOCS interrogator involving the measurement of the state of polarization rotation undergone by the light in presence of a magnetic field (Faraday effect) in an optical fibre surrounding the current and terminated by a Faraday mirror. We considered a uniformly spun optical fibre as the sensing element and we used the Stokes formalism to simulate the sensor. The objective of the simulations is to quantify the ratio L_B/S_P (beat length over the spun period of the spun fibre) enabling a measurement error in agreement with the ITER specifications. The simulator takes into account the temperature variations undergone by the measurement system under ITER operation. The simulation work showed that a L_B/S_P ratio of 19.2 is adequate.

Glue-induced stresses decrease the accuracy of surface-mounted fiber Bragg gratings (FBG). Significant temperature dependent glue-induced birefringence was verified when a thermally cured epoxy-based bonding technique had been used. Determining the peak separation of two azimuthally aligned FBGs in PM fibers combined with a polarization resolved measurement set-up in a temperature range between -30°C and 150°C revealed high glue-induced stresses at low temperatures. Peak separations of about 60 pm and a nonlinear temperature dependence of the glue-induced birefringence due to stress relaxation processes and a visco-elastic behavior of the used adhesive have been shown.

In this work a quasi-distributed optical fiber load sensor based on a semi-auxetic structure is presented. By concatenating sections with positive Poisson’s ratio to sections with negative one it is possible to precisely encode the distributed load into a strain exerted on a fiber. The sensor is described and a simple proof of concept is built and tested. The fiber is interrogated by means of optical frequency domain reflectometry. The proposed sensor represents just one example of the potential applications of auxetic and semi-auxetic structures and materials in optical fiber sensors development.

The linear matrix approach is the common method for multi-parameter FBG-based strain and temperature sensing. As it does not include non-linear temperature responses and hence lacks accuracy, the application of an iterative matrix inversion technique can be used to remedy this deficiency. Employing this method in a set-up using a multi-parameter sensor system that consists of two FBGs in fibers, which differ in cladding diameters, significantly reduced temperature uncertainties of ± 1°C could be achieved within a temperature range between -20°C and 150°C.

In this paper, an investigation into the suitability of using fibre Bragg gratings (FBGs) for monitoring the accelerated curing process of concrete in a microwave heating environment is presented. In this approach, the temperature data provided by the FBGs are used to regulate automatically the microwave power so that a pre-defined temperature profile is maintained to optimize the curing process, achieving early strength values comparable to those of conventional heat-curing techniques but with significantly reduced energy consumption. The immunity of the FBGs to interference from the microwave radiation used ensures stable readings in the targeted environment, unlike conventional electronic sensor probes.

In this paper we focus on refractive index (RI) sensing properties of a micro-size In-fiber Mach-Zehnder Interferometer (μIMZI). The μIMZI structure was fabricated as a precisely controlled side opening of a single-mode fiber using a femtosecond laser. The sensitivity to RI change in the micro-cavity has been measured and two RI sensitivity regions have been found for RI 1.33-1.36 and 1.37-1.40 RIU. The sensitivity in the first region is over 12,000 nm/RIU, and in the higher RI region is close to 50% higher. The obtained structures are an excellent solution for RI sensing with negligible temperature cross-sensitivity, especially where small amounts of liquid are available, e.g. in lab-on-chip, microfluidics.

Low temperature Pyrometry at temperatures beyond 150°C is limited in the measurement speed due to slow pyroelectric detectors. To detect the circumferential temperature distribution of fast rotating machines a novel Fiber Optical Pyrometer Type is presented here.

In this paper, we report a fiber-optic triaxial magnetic field sensor, based on Fiber Bragg Gratings (FBGs) integrated with giant magnetostrictive material, the Terfenol-D. The realized sensor has been designed and engineered for Magnetic Resonance Imaging (MRI) applications. A full magneto-optical characterization of the triaxial sensing probe has been carried out, providing the complex relationship among the FBGs wavelength shift and the applied magnetostatic field vector. Finally, the developed fiber optic sensors have been arranged in a sensor network composed of 20 triaxial sensors for mapping the magnetic field distribution in a MRI-room at a diagnostic center in Naples (SDN), equipped with Positron emission tomography/magnetic resonance (PET/MR) instrumentation. Experimental results reveal that the proposed sensor network can be efficiently used in MRI centers for performing quality assurance tests, paving the way for novel integrated tools to measure the magnetic dose accumulated day by day by MRI operators.

An optical fibre sensor for monitoring low dose radiation is presented. The sensor is based on a scintillation material embedded within the optical fibre core, which emits visible light when exposed to low level ionising radiation. The incident level of ionising radiation can be determined by analysing the optical emission. An optical fibre sensor is presented, based on radioluminescence whereby radiation sensitive scintillation material, terbium doped gadolinium oxysulphide (Gd₂O₂S:Tb), is embedded in a cavity of 250μm of a 500μm plastic optical fibre. The sensor is designed for in-vivo monitoring of the radiation dose during radio-active seed implantation for brachytherapy, in prostate cancer treatment, providing oncologists with real-time information of the radiation dose to the target area and/or nearby critical structures. The radiation from the brachytherapy seeds causes emission of visible light from the scintillation material through the process of radioluminescence, which penetrates the fibre, propagating along the optical fibre for remote detection using a multi-pixel photon counter. The sensor demonstrates a high sensitivity to Iodine-125, the radioactive source most commonly used in brachytherapy for treating prostate cancer.

In this paper we report on advances made in the installation and use of distributed fiber optic sensors to monitor reinforced concrete piles subjected to static load tests. Eight concrete test piles, at three construction sites in London, have recently been instrumented with embedded DFOS. The Brillouin optical time domain reflectometry (BOTDR) technique was used to measure the changes in internal strain and temperature of the piles, during concrete curing and during load testing. These data were used to assess the quality of the pile and derive the load capacity parameters to be used in the foundation design of tall buildings which are to be erected on these sites. The measurements obtained from the DFOS system agreed well with the measurements taken simultaneously using conventional point sensors embedded in the piles. Whereas the conventional sensors only provided measurements at a small number of locations within the piles, the DFOS system made it possible to record the complete strain / temperature profiles along the length of the piles.

In this work, a graphene oxide-coated long period fibre grating (GO-LPG) is proposed for chemical sensing application. Graphene oxide (GO) has been deposited on the surface of long period grating to form a sensing layer which significantly enhances the interaction between LPG propagating light and the surrounding-medium. The sensing mechanism of GO-LPG relies on the change of grating resonance intensity against surrounding-medium refractive index (SRI). The proposed GO-LPG has been used to measure the concentrations of sugar aqueous solutions. The refractive index sensitivities with 99.5 dB/RIU in low refractive index region (1.33-1.35) and 320.6 dB/RIU in high index region (1.42-1.44) have been achieved, showing an enhancement by a factor of 3.2 and 6.8 for low and high index regions, respectively. The proposed GO-LPG can be further extended to the development of optical biochemical sensor with advantages of high sensitivity, real-time and label-free sensing.

Deoxynivalenol (DON) is a mycotoxin frequently occurring in cereals and derived products, and regulated in many countries. Raman spectroscopy performed using optical fibers, with excitation at 1064 nm and a dispersive detection scheme, was utilized to analyze wheat bran samples naturally contaminated with DON. A multivariate processing of the spectroscopic data allowed to distinguish two classes of contamination, with DON below and above 400 μg/kg, respectively. Only one highly contaminated sample was misclassified. This preliminary result demonstrates the potential of Raman spectroscopy as a useful analytical tool for the non-destructive and rapid analysis of mycotoxins in food.

Long-term chemical stability of organometallic reagents incorporated into polydimethylsiloxane matrix of polymer-clad optical fibres by soaking method is investigated by means of VIS/NIR absorption spectroscopy, proton-induced X-ray emission and electron paramagnetic resonance spectroscopy. Based on the obtained experimental results, a modified procedure of sensing coatings preparation is proposed leading to significantly reduced decay of optical properties with time and allowing thus fabrication of practically applicable long sensing fibres.

A fast method for the fabrication of the long period gratings (LPG) optical fibres operating at or near the phase matching turning point (PMTP) with the period of 109.0, 109.5 and 110.0 μm based on an amplitude mask writing system is described. The proposed system allows fabricating 3 cm long LPG sensors operating at PMPT within 20 min that is approximately 8 times faster than point-by-point approach. The reproducibility of the fabrication process was thoroughly studied. The response of the fabricated LPGs to the external change of the refractive index was investigated using water and methanol.

This work presents an application of sodium hydroxide (NaOH) as an effective method for regeneration of titanium oxide (TiO_x) nano-coated long-period grating (LPG) biosensor. Below 100 nm in thickness TiO_x coating was deposited with atomic layer deposition (ALD) method on LPGs for enhancing their refractive index sensitivity up to 2912 nm/RIU in RI range 1.33-1.36 RIU. Next, the sensors were biofunctionalized in order to immobilize receptor (biotin) on their surface and used for selective avidin detection. After successful biofunctionalization process and avidin detection the sensors were washed in NaOH and biofunctionalized again. The proposed method for recovering the sensor does not cause decrease in its functional properties. As a result of the applied procedure the biosensor was fully regenerated.

In this work, we demonstrate an extrinsic pressure sensor realized on single mode fiber tip by means of simple fabrication steps and with low-cost instrumentations. The sensing element consists in a Fabry-Perot cavity: one reflecting surface is the end of the optical fiber, precisely cut, and the other one is a metallic diaphragm. Under the action of the external pressure, the metallic diaphragm bends changing the optical cavity length and, consequently, the characteristics of the reflected signal. The holder structure, which allows the alignment of the fiber tip and reflecting diaphragm, consists in a commercial zirconia ferule with external diameter of D_ex = 2.5 mm. Despite its simplicity and cost-effectiveness, the achieved results show performance comparable to more complex and expensive configurations. By using an aluminum plate as reflecting diaphragm. sensitivity ranging in the 70-130pm/mmHg is experimentally.

We report a practical study of the thermal decay of cladding mode resonances in tilted fiber Bragg gratings, establishing an analogy with the “power law” evolution previously observed on uniform gratings. We examine how this process contributes to a great thermal stability, even improving it by means of a second cycle slightly increasing the annealing temperature. In addition, we show an improvement of the grating spectrum after annealing, with respect to the one just after inscription, which suggests the application of this method to be employed to improve saturation issues during the photo-inscription process.

A surface plasmon based fibre-optic chemical sensor for the detection of cocaine has been developed using a molecularly imprinted polymer (MIP) film with embedded gold nanoparticles as the recognition element. The MIP was formed on the layer of gold thin film which was deposited on the surface of a fibre core. The sensing was based on swelling of the MIP film induced by analyte binding that shifted the resonance spectrum toward a shorter wavelength. The sensor exhibited a response to cocaine in the concentration range of 0 - 400 μM in aqueous acetonitrile mixtures. Selectivity for cocaine over other drugs has also been demonstrated.

Addressing temperature and strain induced cross-talks simultaneously, we propose an inherently strain and temperature insensitive fiber-optic bio-sensor. The insensitivity has been achieved by properly adjusting the dopants and their concentrations, and by optimizing the grating period and the strength of concatenated dual-resonance long-period-gratings.

Optical fibre sensors have been applied to perform biophysical measurement in ex-vivo laser ablation (LA), on pancreas animal phantom. Experiments have been performed using Fibre Bragg Grating (FBG) arrays for spatially resolved temperature detection, and an all-glass Extrinsic Fabry-Perot Interferometer (EFPI) for pressure measurement. Results using a Nd:YAG laser source as ablation device, are presented and discussed.

This work demonstrates the viability of using a cavity ring-down technique (CRD) for remote sensing. A conventional CRD configuration is used where and optical circulator is added inside the fibre loop to couple 20 km of optical fibre with a gold mirror at its end with the purpose of remote sensing. As a proof-of-concept, an intensity sensor based on an eight-figure configuration is used at the end of the 20 km of fibre for displacement sensing. In this case, a commercial OTDR is used as modulated light source to send impulses down to the fibre ring.

Fiber probe structures composed of two physical microcavities were created using focused ion beam technology. These structures have a tip-like shape as they were milled in preciously etched tapered fiber tips. The microprobes are then characterized for temperature and refractive index sensing using a signal filtering technique to discriminate signals from distinct microcavities. Using fast Fourier transforms combined with band-pass filters, it is possible to reconstruct the spectra of each cavity independently and thus measure their individual spectral shifts.

This paper presents an optical fibre pressure and temperature sensor (OFPTS) system, which is adapted for use as a urodynamic pressure measurement system (UPS) for differential pressure measurement with temperature compensation. The OFTPS is based on a Fabry Perot interferometer (FPI), which acts as a pressure sensor and includes an embedded fibre Bragg grating (FBG) for temperature measurement. The sensor system is evaluated in a lower urinary tract (LUT) simulator, which simulates the bladder, rectum and detrusor muscle. The system was benchmarked against a commercially available urodynamic system, at the University Hospital Limerick (UHL) Urology Clinic. Both systems demonstrate a high correlation with a relative pressure variation of less than ±2.8cmH₂O for abdominal and ±4cmH₂O for vesical pressure. The repetitive measurement of the OFPTS system in the LUT simulator against the commercial system demonstrated the high repeatability. Furthermore, the low fabrication cost makes the OFPTS a potentially interesting instrument for urodynamic and other medical applications.

The aim of the study is to demonstrate the potential of the fibre Braggs grating (FBG) in the measurement of different jaw movements that are performed for patients with occlusal parafunction using occlusal splints. Two silicon plates each 2mm are used, the fibre optic sensor is positioned in the maxillary left first molar region above the point of contact with opposing tooth after pressing the first plate on the model. Then the second silicon plate is pressed. The device has a final thickness of 2 mm. The occlusal splint is installed in the mouth of the patient who underwent different movements on occlusal splint. The maximum frequency bite is monitored. The results demonstrate that the bite shows a difference between grinding and clenching movements. The curves behaviour patterns are presented in order to show these different comparisons. Therefore, it is concluded that the fibre Braggs grating consists in an efficient method for monitoring the mechanical behaviour bite of patients with occlusal splints.

This paper presents a numerically investigation of the performance analysis of a conventional photonic crystal fiber (PCF) with a planar metamaterials structure for refractive index sensing, based on surface plasmon resonance (SPR), using the finite element method (FEM). We study the concentration metamaterials conformed by the aluminium oxide (Al₂O₃) and silver (Ag) and compared its performance with a single metal (Ag), assessing their impacts in the effective refractive index. Furthermore, we also use different types of mechanics to describe the effects of varying the structural parameters sensor on the evanescent field and the sensor performance.

We report on a contactless optical fiber refractive index (RI) sensor that can be used to measure the RI of solid or liquid samples. The sensor is simple to construct and consists of a Fabry-Perot interferometer (FPI) with long cavity. The cavity of our FPI consists of a tapered optical fiber tip and an external flat mirror. The output beam of the fiber tip is partially reflected from different interfaces of the sample present in the interferometer cavity. Each of such reflections interferes with the beam internally reflected by the fiber tip. Thus, a multiple-beam FPI is formed whose reflection spectrum is composed by the superposition of several two-beam interferences. The analysis of the multiple interference spectra was carried out in the Fourier domain. Several glass samples, water-sucrose and water-glycerol solutions were prepared and tested. Since the fiber tip is not in direct contact with the sample under test the measurement is simple and immediate. To our-knowledge, this is the first time that a fiber optic sensor can be used to measure the RI of solid and liquid samples without any modification.

A novel sensing platform based on thin metal bilayer for surface plasmon resonance (SPR) in a D–shaped plastic optical fiber (POF) has been designed, implemented and tested. The experimental results are congruent with the numerical studies. This platform has been properly optimized to work in the 1.38 -1.42 refractive index range and it exhibits excellent sensitivity. This refractive index range is very interesting for bio-chemical applications, where the polymer layer are used as receptors (e.g. molecularly imprinted polymer) or to immobilize the bio-receptor on the metal surface. The proposed metallic bilayer is based on palladium and gold films and replaces the traditional gold by exhibiting higher performances. Furthermore, the deposition of the thin bilayer is a single process and no further manufacturing step is required. In fact, in this case the photoresist buffer layer between the POF core and the metal layer, usually required to increase the refractive index range, is no longer necessary.

The first example of an optical sensor platform based on surface plasmon resonance (SPR) in a plastic optical fiber (POF) integrated into a thermo-stabilized flow cell for biochemical sensing applications is proposed. In this work, an IgG/anti-IgG assay was implemented as model bioassay, with the IgG biolayer deposited on the sensor gold surface and the biological target, anti-IgG, transported through a new thermo-stabilized flow cell. The experimental results show that the proposed device can be successfully used for label-free biochemical sensing. This complete optical sensor system can be used for the future reduction of the device cost and dimension, with the possibility of integrating the POF-SPR sensing platform with microfluidic and optoelectronic devices.

We report highly localized second-order fibre Bragg gratings at 1585 nm inscribed by point-by-point focused infrared femtosecond pulses. A thin gold coating deposited on the fibre outer surface at the grating location allows shielding the cladding mode resonances from the outer medium, so that they remain present in the transmitted amplitude spectrum. The Bragg resonance of the second-order grating is surrounded by high-order cladding mode resonances of the first-order grating. These cladding modes exhibit the same temperature sensitivity as the Bragg resonance (10.6 pm/°C) but high differential strain sensitivity (-0.55 pm/μepsilon versus 1.20 pm/μepsilon for the Bragg mode). Therefore, the conditioning of the matrix inversion as demodulation method is fully satisfied, yielding a new design of fibre sensor able to discriminate between temperature and strain, with an unprecedented sensitivity.

This work relies on the development of a sensorized medical needle with an all-optical guidance (Lab in a Needle) system for epidural space identification. The device is based on the judicious integration of a Fiber Bragg grating sensor inside the lumen of an epidural needle to discriminate between different types of tissue and thus providing continuous and real time measurements of the pressure experienced by the needle tip during its advancement. Experiments carried out on an epidural training phantom demonstrate the validity of our approach for the correct and effective identification of the epidural space.

A method for measuring the contact pressure between an endotracheal tube cuff and the trachea was designed and developed by using a fibre Bragg grating (FBG) based optical fibre sensor. The FBG sensor is encased in an epoxy based UV-cured cuboid patch and transduces the transversely loaded pressure into an axial strain that induces wavelength shift of the Bragg reflection. The polymer patch was created by using a PTFE based mould and increases tensile strength and sensitivity of the bare fibre FBG to pressure to 2.10×10-2 nm/kPa. The characteristics of the FBG patch allow for continuous measurement of contact pressure. The measurement of contact pressure was demonstrated by the use of a 3D printed model of a human trachea. The influence of temperature on the measurements is reduced significantly by the use of a second FBG sensor patch that is not in contact with the trachea. Intracuff pressure measurements performed using a commercial manometer agreed well with the FBG contact pressure measurements.

This work presents an optical fiber refractive index sensors based on lossy–mode resonance (LMR) effect supported by titanium oxide (TiO_x) thin overlay. The TiO_x overlays of different thickness were deposited on core of polymer-clad silica (PCS) fibers using atomic layer deposition (ALD) method. Based on numerical simulations, a number of structures differing in the location of exposed core area and the thickness of TiO_x coatings were designed. For fabricated structures the spectral response to external refractive index (n_ext) was measured. The maximum sensitivity reaches 634.2 nm/RIU (n_ext range: 1.357 - 1.402 RIU; TiO_x coating thickness: 260.9 nm; investigated spectral range: 500-800 nm) and it highly depends on the thin-film thickness.

The development of an ammonia sensor, formed by the deposition of a functionalised titanium dioxide film onto a tapered optical fibre is presented. The titanium dioxide coating allows the coupling of light from the fundamental core mode to a lossy mode supported by the coating, thus creating lossy mode resonance (LMR) in the transmission spectrum. The porphyrin compound that was used to functionalise the coating was removed from the titanium dioxide coating upon exposure to ammonia, causing a change in the refractive index of the coating and a concomitant shift in the central wavelength of the lossy mode resonance. Concentrations of ammonia as small as 1ppm was detected with a response time of less than 1min.

We report the development of a low cost plastic optical fibre (POF) sensor for ammonium detection using molecularly imprinted polymers (MIP’s). The cladding of a 1 mm diameter PMMA fiber is removed, in which is grafted a molecular imprinted polymer (MIP), by radical polymerization with thermal initiation, that act as a selective sensing layer. For the polymerization, 2,2’-Azobis(2-methylpropionamidine)dihydrochloride (AAPH) is used as initiator, methacrylic acid (MAA) as a monomer, ethylene glycol dimethacrylate (EDMA) as a cross-linker, ammonium chloride (NH₄Cl) as a template and 30% of ethanol in water as a solvent. The sensing method consists of an intensity based scheme. The response to different concentrations of ammonium solutions in water has been evaluated at room temperature. Solutions with (0 - 0.6) M concentration, with the corresponding refractive indexes varying between 1.3325 - 1.3387, at 25°C were used. The response of the fiber with the original cladding, and after cladding removal has been monitored and compared to the response given by the developed sensor. The response is very fast, less than 1 minute and reversible, which allows the continuum use of the sensor. Further developments are focused in optimization of MIP grafting procedure and sensor performance, in order to increase sensitivity.

The use of multicore optical fibres (MCF) in optical sensing applications has gained increasing interest over the past years due to the benefits directly brought from their inherent spatial diversity. This property allows measuring either multiple physical magnitudes at the same time or the same magnitude with slight differences in order to compensate the cross-sensitivities. We have inscribed Regenerated Fibre Bragg Gratings (RFBGs) in MCFs with the aim of implementing temperature sensors with an enhanced accuracy and for a very wide temperature range (up to 1000°C). The sensors have been made in 4-core and 7-core commercially available homogeneous MCFs. The fabrication process has been designed to create different temperature sensitivities among the identical cores of the MCF. We have obtained significant wavelength-shift differences up to 1.2 nm at 765°C, what has been used to at least double the temperature accuracy.

We report on an incoherent OFDR interrogator of FBG arrays based on the concept of dispersive wavelength to time delay mapping. The system is specifically designed to show stability to environmental thermal variations by the incorporation of a composite dispersive delay and weak broadband reflectors for delay and dispersion monitoring. Dispersion is imparted by the combination of a fiber coil and an athermally-packaged chirped fiber Bragg grating for dispersion compensation. Using differential measurements over a single acquisition trace, the values of Bragg wavelengths and dispersion are determined from the delays experienced by the FBGs and by additional reference wavelengths reflected in the broadband reflectors. The results show maximum deviations of 20 pm and 0.2 ps/nm with respect to OSA measurements of Bragg wavelengths and nominal dispersion values, respectively.

In this study, the possibility to excite Bloch surface waves (BSWs) on the tip of a single-mode optical fiber is explored for the first time. In particular, we first show the possibility to achieve an on-tip excitation of BSWs, with optimized characteristic of the arising resonances, via an “all-fiber” grating-coupled configuration. Furthermore, envisioning novel high-performance fiber tip nanoprobes for label-free biosensing, we introduce an ad hoc design aimed at maximizing the refractive-index sensitivity. Numerical results indicate that the estimated sensitivities are comparable with those exhibited by current plasmonic lab-on-tip bio-probes, but are accompanied by a higher spectral selectivity. Therefore, this preliminary work paves the way to the development of new classes of miniaturized surface-wave optical fiber devices for low-detection-limit label-free chemical and biological sensing.

We experimentally demonstrate a novel optical fiber label free optrode platform resulting from the integration between two rapidly emerging technologies such as Lab-on-Fiber Technology (LOFT) and Microgel Photonics (MPs). The device consists of a microgel (MG) layer painted on a metallic slabs supporting plasmonic resonances, directly integrated on the optical fiber tip. A molecular binding event induces significant changes in the MG layer thickness (and consequently in its 'equivalent' refractive index) resulting in an evident wavelength shift of the resonant feature. As a case of study, glucose-responsive MGs have been synthesized by incorporating into the gel matrix boronic acid moieties, whose interaction with glucose rules the driving forces for gel swelling. Our results pave the way for new technological routes aimed to develop advanced label free fiber optic nanoprobes.

In this work, spiral phase lenses and Fresnel zone lenses for beam tailoring, fabricated on the tip of optical fibers, are reported. The spiral phase lenses allow tailoring the fundamental guided mode, a Gaussian beam, into a Laguerre - Gaussian profile without using additional optical elements. Whereas, the Fresnel lenses are used as focusing systems. The lenses are fabricated using Focused Ion Beam milling, enabling high resolution in the manufacturing process. The output optical intensity profiles matching the numerical simulations are presented and analyzed.

Temperature-independent strain measurement is achieved resorting to a taper fabricated on a Bragg fibre using a CO₂ laser. The characteristic bimodal interference of an untapered Bragg fibre is rendered multimode after taper fabrication and the resulting transmission spectra are analysed as temperature and strain change. The intrinsic strain sensitivity exhibited by the Bragg fibre is increased 15 fold after tapering and reaches 22.68 pm/μepsilon. The difference in wavelength shift promoted by variations in temperature and strain for the two fringes studied is examined and strain sensing with little temperature sensitivity is achieved, presenting a sensitivity of 2.86 pm/μepsilon, for strain values up to 400 μepsilon.

A simple, interferometric force sensor based on a multicore optical fiber (MCF) that operates in reflection mode is presented. The device consists of a short segment of MCF inserted at the distal end of a conventional single mode optical fiber (SMF). To demonstrate the concept we used a mechanical piece with grooves to press the MCF. In this way the external force on the MCF is converted in localized pressure on the fiber which causes attenuation losses to the interfering modes and makes the interference pattern to shrink. The changes experienced by the interference pattern can be easily monitored. The sensor here proposed is highly sensitive since it can resolve forces down to 0.01 N.

In this paper, a Fiber Specklegram Sensor (FSS) based on Plastic Optical Fiber (POF) has been proposed with a reflection-based configuration. A HeNe laser is launched through a coupler, reflected by a mirrored fiber end and detect the specklegram sequence using a commercial camera. Different sensor performance metrics have been analyzed in terms of final sensitivity to external vibration applied to the fiber. The results confirm the performance reduction of the reflection-based scheme in comparison with the transmission-based approach but it can be also employed as sensing system where only a single fiber end is available.

We present a novel optical fiber consisting of a suspended-fiber with core and cladding diameter of ~ 5 and 30 μm and a supporting ring with thickness of ~ 9 μm. The outer diameter of the fiber was 125 μm and a fiber Bragg grating (FBG) with a length of 1-mm was inscribed on it. Hydrostatic pressure was measured by monitoring the Bragg wavelength shifts of 9-mm long single-ring suspended fiber. Pressure sensitivity was measured to be –18.92 pm/MPa, which is about five times higher than FBG on standard single-mode fiber.

In this work, we report a straightforward and cost-effective fabrication route for the development of nano-patterned optical fiber tips. The technique is based on self-assembling polystyrene microspheres at the air/water interface and on their successive transferring on the fiber tip of single mode optical fiber. By applying to the fiber further treatments like particle size reduction, metal coating and sphere removal, different periodic structures have been conveniently realized. The morphological analysis reveals indeed the successful creation on the optical fiber tip of regular metallic-dielectric spheres’ arrays as well as metallic patterns with dimensional features down to a submicron scale. Finally, as proof of concept, we demonstrated the capability of the realized patterns to work as efficient Surface Enhanced Raman Spectroscopy (SERS) fiber probes.

In this paper, a new sensor system for relative humidity measurements based on a SnO2 sputtering deposition on a microstructured optical fiber (MOF) low-finesse Fabry-Perot (FP) sensing head is presented and characterized. The interrogation of the sensing head is carried out by monitoring the Fast Fourier Transform phase variations of the FP interference frequency. This method is low-sensitive to signal amplitude variations and also avoids the necessity of tracking the evolution of peaks and valleys in the spectrum. The sensor is operated within a wide humidity range (20%-90% relative humidity) with a maximum sensitivity achieved of 0.14rad/%. The measurement method uses a commercial optical interrogator as the only active element, this compact solution allows real time analysis of the data.

An optical fiber in-line Mach-Zehnder interferometer based on a fiber internal mirror constructed by use of a hollow ellipsoid fabricated by femtosecond laser micromachining and fusion splicing technique is demonstrated. The interface of the hollow ellipsoid surface and air can act as an internal mirror. The device has been used for refractive index, bending and high temperature measurement and simultaneous multiple parameter sensing.

We realize the first optical-fiber “meta-tip” that integrates a metasurface on the tip of an optical fiber. In our proposed configuration a Babinet-inverted plasmonic metasurface is fabricated by patterning (via focused-ion-beam) an array of rectangular aperture nanoantennas in a thin gold film. Via spatial modulation of the nanoantennas size, we properly tune their resonances so as to impress abrupt arbitrary phase variations in the transmitted field wavefront. As a proof-of-principle, we fabricate and characterize several prototypes implementing in the near-infrared the beam-steering with various angles. We also explore the limit case where surface waves are excited, and its capability to work as refractive index sensors. Notably, its sensitivity overwhelms that of the corresponding gradient-free plasmonic array, thus paving the way to the use of metasurfaces for label-free chemical and biological sensing. Our experimental results, in fairly good agreement with numerical predictions, demonstrate the practical feasibility of the meta-tip concept, and set the stage for the integration of metasurfaces, and their exceptional capabilities to manipulate light, in fiber-optics technological platforms, within the emerging “lab-on-fiber” paradigm.

Using transformation optics formalism, we numerically analyzed the effect of resonant couplings between core and cladding modes in a helical core fiber with large core offset. Our study revealed much richer spectrum of resonant couplings than predicted by perturbative methods in fibers with small core offsets. Moreover, we demonstrated that the resonant couplings to lower order cladding modes predicted by perturbative methods are in fact prohibited for larger core offsets. Finally, we demonstrated the existence of spectrally broad polarization sensitive couplings to the cladding modes suggesting that helical core fibers with large core offsets may be used as broadband circular polarizers.

In this paper, we report photo-inscription of uniform Bragg gratings in trans-4-stilbenemethanol-doped photosensitive step-index polymer optical fiber. Gratings were produced at ~1575 nm by the phase mask technique with a femtosecond laser emitting at 400 nm with different average optical powers (8 mW, 13 mW and 20 mW). The grating growth dynamics in transmission were monitored during the manufacturing process, showing that the grating grows faster with higher power. Using 20 mW laser beam power, the reflectivity reaches 94 % (8 dB transmission loss) in 70 seconds. Finally, the gratings were characterized in temperature in the range 20 - 45 °C. The thermal sensitivity has been computed equal to – 86.6 pm/°C.

A new type of fiber optic sensor for the detection and quantification of ammonia (NH3) vapor levels is proposed and experimentally demonstrated. This sensor is based on a spherical silica micro resonator coated with porous silica gel. Whispering gallery modes (WGMs) in the micro resonator are excited by evanescent coupling to a tapered fiber with a 3.3 μm waist diameter. The optical properties of the porous silica layer change when it is exposed to ammonia vapor, leading to a spectral shift of the WGM resonant wavelengths. The sensitivity of the proposed sensor has been tested by exposing it to different low level concentrations of ammonia: 4 ppm, 8 ppm, 12 ppm and 30 ppm at a constant relative humidity (50% RH) and constant temperature (23°C). The detection limit is calculated from experimental results as 57 ppb of ammonia for a 282 μm diameter porous silica coated microsphere.

A novel speckle-based method for sensing frequency vibration is demonstrated in a reflective configuration. By employing a visible dual-wavelength approach it is also possible to determine the relative spatial location of the vibrations along a plastic optical fiber lead of 8 m in a distributed scheme.

In this paper, we report on an effective way to locally alter the refractive index of a low-loss polymer optical fibre (POF), in order to fabricate novel fibre optical sensors. Such refractive index modifications, if reproduced periodically, create fibre Bragg gratings (FBGs) that find diverse applications in telecommunications and sensing. With a femtosecond laser set-up, we were able to inscribe refractive index changes in the core of the fibre on an area as small as a μm². This technique can be effectively used to produce FBGs with a tailored length and strength and, so, with desired optical properties. The fibre used was a large core, graded index, multimode perfluorinated fibre. FBGs resonate at different wavelengths depending on the mode distribution in multimode fibres, because the effective refractive index depends on the spatial distribution of the light inside the core. Therefore, the reflection spectrum from the grating degenerates into multiple resonances, each associated with a different mode. The detection of the reflected modes was performed with a custom made software that was able to track a specific reflected mode even when the FBG underwent perturbation, such as temperature or strain changes. Moreover, a key advantage of low-loss fibre is the possibility to use long lengths of fibre and to be able to inscribe several FBGs in a single piece of fibre. With our detection system, we managed to track the perturbation of individual FBGs in a fibre array of multiple gratings. The combination of our femtosecond inscription setup and a mode detection system is encouraging for the development of low loss POF sensing devices.

A locally micro-structured fiber Bragg grating (LMFBG) was manufactured by forming a circumferential groove in the middle of a type I fiber Bragg grating (FBG). The groove was directly ablated using a fs-laser and had a length of 86μm, a depth of 27μm and steep side walls. Due to the precisely machined geometry of the structure the reflection spectra can be accurately described with a fairly simple theoretical model. At several constant temperatures in the range from 5°C to 45°C this structure was exposed to various compressive loads in the range from 0N to -1.42N. Here the force and temperature sensitivity of the LMFBG are presented. This structure can be used for miniaturized compressive force sensing at variable temperatures, which is of particular interest for many bio-medical applications.

In this work, we present long period gratings (LPGs) in two different Fluorine-doped fibers realized by electric arc discharge (EAD) technique. Firstly, we optimized the EAD fabrication procedure for standard Ge-doped fibers where we are able to fabricate relatively short LPGs with deep attenuation bands (up to 32 dB) and trivial power losses. Successively, for the first time to the best of our knowledge, we produced LPGs in F-doped fibers with maximum attenuation band depths in range 25-30 dB and trivial power losses. We also investigated the sensitivity of LPGs fabricated in such F-doped fibers, with surrounding refractive index (SRI) and temperature changes, and compared the results with those of LPGs fabricated in standard fiber. We found that SRI response of LPGs in F-doped fibers is significantly higher than in standard fiber and it strongly depends on the type of F-doped fiber considered, whereas they exhibit a slightly lower sensitivity to temperature compared to LPGs in standard fiber.

Optical fibre sensors have the potential to be incorporated into wound dressings to monitor moisture and predict healing without the need to remove the dressing. A low cost polymeric optical fibre humidity sensor based on evanescent wave absorption is demonstrated for skin humidity measurement. The sensor is fabricated by coating the fibre with a hydrophilic film based on bilayers of Poly(allylamine hydrochloride) (PAH) and SiO₂ mesoporous nanoparticles. The Layer-by-Layer method was used for the deposition of the layers. Multimode polymeric optical fibre with a cladding diameter of 250μm was covered by 7 layers of PAH/SiO₂ film on the central region of an unclad fibre with a diameter of 190μm. The length of the sensitive region is 30mm. Experiment results show a decrease in light intensity when relative humidity increases due to refractive index changes of the fibre coating. The sensitivity obtained was 200mV/%RH and the sensor was demonstrated to provide a faster response to changes in the humidity of the skin microenvironment than a commercial sensor.

We introduce the optical frequency domain reflectometry (OFDR) technique based on intensity modulation frequency sweep measurement for distributed disturbance measurement in optical fibres. By evaluating interferometric Rayleigh scattering changes along the fibre, strain and temperature changes are detected with 100 n(epsilon) sensitivity and 10 mK resolution. The vibration frequencies for low frequencies and up to the kHz-range can be obtained from power change evaluation in the spatial domain. This novel OFDR approach is a low-cost alternative for distributed disturbance measurement up to distances of several kilometres.

Measurement and data recording systems are important parts of a holistic Structural Health Monitoring (SHM) system. New sensor technologies such as fiber-optic sensors are often used; however, standards (or at least guidelines) are not yet available or internationally approved. This lack in standardization makes the acceptance of FOS technologies in complex SHM systems substantially difficult. A standard family for different FOS technologies is therefore being developed that should help to design SHM systems in an optimal way. International standardization activities take place in several standardization bodies such as IEC and ASTM, and within SHM societies such as ISHMII. The paper reports on activities in standardization of fiber-optic sensors, on results already achieved, and on newly started projects. Combined activities of fiber sensor experts and SHM experts from Civil Engineering are presented. These contributions should help owners of structures as well as developers of sensors and monitoring systems to select effective and validated sensing technologies. Using these standards, both parties find recommendations how to proceed in development of SHM systems to evaluate the structural behavior based on e.g. standardized fiber optic sensors, and to derive necessary measures, e.g. the optimal maintenance strategy.

We experimentally demonstrate the feasibility of Fiber Bragg Grating sensors interrogation using integrated unbalanced Mach-Zehnder Interferometers (MZI) and phase sensitive detection in silicon-on-insulator (SOI) platform. The Phase- Generated Carrier (PGC) demodulation technique is used to detect phase changes, avoiding signal fading. Signal processing allows us to extract the wavelength shift from the signal patterns, allowing accurate dynamic FBG interrogation. High resolution and low cost chips with multiple interrogators and photodetectors on board can be realized by exploiting the advantages of large scale fabrication capabilities of well-established silicon based industrial infrastructures. Simultaneous dynamic reading of a large number of FBG sensors can lead to large volume market applications of the technology in several strategic industrial fields. The performance of the proposed integrated FBG interrogator is validated by comparing with a commercial FBG readout based on a spectrometer and used as a reference.

In this paper, we describe a theoretical study on the interference created by a phase mask when a femtosecond laser is used. The limitations of the phase mask-to-fiber distance are discussed and the optimal inscription range is established. Femtosecond lasers have the unique feature of short coherence length and thus the diffraction orders do not interfere after a certain distance travelled from the phase mask even if the phase mask has a poor zero order suppression. The equation describing this behaviour is presented and simulations are included for validation. The intensity profile of the overlapping ±1 diffraction orders after the phase mask is also studied for 1st order (1070 nm pitch) and for 2nd order (2140 nm pitch) phase masks.

The Karhunen-Loeve transform is applied to the coarsely sampled impulse response generated by an FBG cascade in order to calculate the temperature change suffered by the FBGs. Thanks to a dispersive media, the wavelength change performed by the temperature change produces a delay shift in the sample generated by an FBG, delay shift which is recorded in the eigenvalues calculated by the KLT routine, letting to measure the temperature variation. Although the FBGs samples are represented only by four points, a continuous temperature measurement can be performed thanks to the KLT algorithm. This means a three order reduction in the number of points giving this method a low computational complexity. Simulations are performed to validate the interrogation technique and estimate performance and an experimental example is provided to demonstrate real operation.

An infrared femtosecond pulses laser is used to manufacture point-by-point gratings in telecommunication-grade optical fibres. The refractive index modulations are localized close to the core-cladding interface, yielding a strong coupling to cladding mode resonances together with an important photo-induced birefringence. Such gratings have been recently used for refractrometric measurements. In this work, their transmitted amplitude spectrum is measured with polarized light while they are exposed to temperature changes up to 900 °C. Despite an overall good thermal stability of the gratings that confirms their robustness for high-temperature refractometry, we report an interesting polarization effect depending on both the cladding mode resonance family (radially- and azimuthally-polarized modes) and mode order. While the birefringence of the core mode resonance decreases with the temperature, certain cladding mode resonances show an increase of the wavelength splitting between their orthogonally-polarized components. This differential behaviour can be of high interest to develop high-resolution multiparametric sensing platforms.

Image processing is proposed and experimentally demonstrated to improve the capabilities of Raman distributed optical fibre sensors. The here reported technique consists in stacking consecutive one-dimensional Raman Stokes and anti-Stokes traces in two-dimensional data arrays (one for each Raman component), which are then processed by an image denoising algorithm. Owing to the high level of correlation between consecutive measurements in conventional Raman sensing, it is experimentally demonstrated that this newly-proposed two-dimensional denoising approach provides a significant signal-to-noise ratio improvement, which in this case reaches 13.6 dB with no hardware modification to the conventional set-up. Experimental results demonstrate Raman distributed sensing with a remarkably enhanced temperature resolution of 4 mK at 9 km distance, which is obtained with 2 m spatial resolution and a short acquisition time of 35 s.

The monitoring of an array of fibre Bragg gratings (FBGs) strain sensors was performed through a single channel, single mode fibre optic rotary joint (FORJ) mounted on a geotechnical centrifuge. The array of three FBGs was attached to an aluminum plate that was anchored at the ends and placed on the model platform of the centrifuge. Acceleration forces of up to 50g were applied and the reflection signal of the monitored FBGs recorded dynamically using a 2.5kHz FBG interrogator placed outside the centrifuge. The use of a FORJ allowed the monitoring of the FBGs without submitting the FBG interrogator to the high g-forces experienced in the centrifuge.

There is great interest in the development of flexible wavelength filters and optical fibre sensors, such as Bragg and superstructure gratings, grating arrays and chirped gratings in glass and polymer optical fibres. A major hurdle is the development of an inscription method that should offer flexibility and reliability and be generally applicable to all optical fibre types. With this in mind we have developed a novel femtosecond laser inscription method; plane-by-plane inscription, whereby a 3D-index change of controlled length across the fibre core, width along the fibre axis and depth is written into the optical fibre. We apply this method for the inscription of various grating types in coated silica and low- loss CYTOP polymer optical fibres. The plane-by-plane method allows for multiple and overlapping gratings in the fibre core. Moreover, we demonstrate that this novel fibre Bragg grating inscription technique can be used to modify and add versatility to an existing, encapsulated optical fibre pressure sensor. The femtosecond laser is operated in the green or the near infra-red, based on the material properties under laser modification.

A long-period grating (LPG) written on a standard single mode fiber is investigated as a curvature and vibration sensor. It is demonstrated a high sensitivity to applied curvature and the possibility to monitor vibration in a wide range of frequencies from 30 Hz to 2000 Hz. The system was tested using an intensity based interrogation scheme with the LPG sensor operating in the curvature regime. Results have shown a reproducible frequency discrimination in the 30 Hz to 2000 Hz, with resolutions between 11 mHz and 913 mHz. Frequency retrieval could be performed independent of temperature up to 86 °C.

The first demonstration of a polymer optical fibre Bragg grating (POFBG) embedded in a 3-D printed structure is reported. Its cyclic strain performance and temperature characteristics are examined and discussed. The sensing patch has a repeatable strain sensitivity of 0.38 pm/μepsilon. Its temperature behaviour is unstable, with temperature sensitivity values varying between 30-40 pm/°C.

In this communication, a novel compact fibre Bragg grating-based thermometer for on-line temperature monitoring of drill bits is reported. Our proposed technique can potentially be used to optimize any drilling process, requiring the use of small drill bits, through direct temperature measurement at the drill bit instead of relying on indirect parameters (speed of rotation, applied force) in order to avoid an overheating as it is currently done nowadays.

For structure or material monitoring low impact serial fiber Bragg grating (FBG) networks have attracted increasing research interest. Common sensor networks using wavelength division multiplexing (WDM) for FBG interrogation are limited in their efficiency by the spectral width of their light source, the FBG tuning range and the spectral guard bands. Overlapping spectra are strictly forbidden in this case. Applying time division multiplexing (TDM) or active resonator schemes may overcome these restrictions. However, they introduce other substantial disadvantages like signal roundtrip dependency or sophisticated control of active resonating structures. Code division multiplexing (CDM) as a means of FBG interrogation by simple autocorrelation of appropriate codes has been shown to be superior in this respect. However, it came at the cost of a second spectrometer introducing additional equalization efforts. We demonstrate a new serial FBG sensor network utilizing CDM signal processing for efficient sensor interrogation without the need of a second spectrometer and additional state of polarization (SOP) controlling components. It allows overlapping spectra even when all sensing FBGs are positioned at the same centre wavelength and it shows a high degree of insensitivity to SOP. Sequence inversed keyed (SIK) serial signal processing utilizing quasi-orthogonal balanced codes ensures simple and quick sensor interrogation with high signal-to-interference/noise ratio.

High-coherence light is stringently demanded in high-accuracy interferometric optical fiber sensors, where the phase noise of the light source greatly affects the sensitivity of the whole system. Distributed-feedback laser diodes with a phase noise of -80 ~ -90 dB/Hz^1/2 at 1 kHz (with 1 m optical path difference) is now easily obtained, but the interferometric fiber sensors requires the laser source with the phase noise lower than -100 dB/Hz^1/2. Lasers with ultra-low-noise usually require complicated and sophisticated techniques. We propose a novel structure to realize high-coherence light extraction through a compact Brillouin/erbium fiber laser (BEFL) which uses a length of 4 m erbium-doped fiber as both the Brillouin and linear gain media. The phase noise of the Brillouin pump light is greatly smoothed and suppressed after being transferred to the Brillouin Stokes light. High-coherence light with the phase noise of about -104 dB/Hz^1/2 at 1 kHz is extracted through the compact BEFL from a commercialized laser diode with the phase noise of about -89 dB/Hz^1/2. The capability of phase noise suppression in the compact BEFL presents much importance especially in large-array interferometric fiber sensor systems.

By analyzing the operation of the slope assisted Brillouin Optical Time-Domain Analysis (BOTDA) method, it comes out that the acquisition rate is practically limited by two fundamental factors: the polarization scrambling frequency and the phase noise from the laser. As regards polarization scrambling, we show experimentally that the scrambling frequency poses a limit on the maximum acquisition rate for a given averaging factor. As regards phase noise, we show numerically and experimentally that the slope assisted method is particularly sensitive to the laser phase noise, due to the specific positioning of the pump-probe frequency shift on the Brillouin Gain Spectrum (BGS).

We have studied the influence of gamma rays on physical properties of different commercially available silica optical fibers stepwise irradiated up to a total dose of 100 kGy. The detection of radiation-induced changes in silica glass offers the possibility of using selected optical fibers as distributed radiation sensors. The measurements performed by us were based on optical backscatter reflectometry and Brillouin distributed sensing. The measurement methods enable an analysis of radiation-induced modification of the group refractive index and density of the optical fibers. The most distinct physical effect observed by us concerns the increase of the optical attenuation with rising total radiation doses. Quantitative measurement results indicate a crucial impact of fiber dopants on radiation-induced physical and sensory characteristics of silica optical fibers affected by differences in fiber fabrication techniques. Based on the obtained results, the suitability of distributed Brillouin sensing for dosimetry applications seems to be improved by modifying the refractive index profile of the fiber core.

The scanning-free, precise reconstruction of the local value of the Brillouin frequency shift in a section of optical fiber is proposed and demonstrated experimentally. The Brillouin shift is identified based on transient analysis of the step response of the amplified signal wave. The measurement protocol requires a single trace, taken at an arbitrary frequency offset between the central optical frequencies of the pump and signal waves. The frequency offset may be chosen anywhere within ±100 MHz of the Brillouin frequency shift, corresponding to a measurement range of ±100 °C or ±2,000 μepsilon. The experimental error between the estimated Brillouin shift and its true value is less than±1 MHz. The protocol is suitable to high-resolution Brillouin optical correlation domain analysis.

In this paper we propose the employment of sub-pixel algorithms for the estimation of the central frequency of the Brillouin Gain Spectrum in a Brillouin Optical Time Domain Analyzer. The experimental results will show that the proposed solution shows a good performance when the chosen frequency step for the required frequency sweep is high. If the improved computational efficiency in comparison to the traditional Lorentzian fitting is also considered, it can be concluded that this approach may be of great interest for dynamic measurement scenarios.

Standard optical fibers are successfully embedded within a model wing of an unmanned aerial vehicle, constructed of carbon fiber and epoxy, during its production. Time-gated Brillouin optical correlation domain analysis along the embedded optical fibers is performed with a spatial resolution of 4 cm. Tests were carried out using a portable measurement setup prototype. The results represent an important step towards applications of high-resolution Brillouin analysis outside the research laboratory.

The spectrum of the temporal traces obtained from a phase-sensitive optical time-domain reflectometer is theoretically and experimentally analysed, demonstrating its dependence on the incident optical pulse shape. Numerical simulations and theoretical results are validated experimentally, showing a good matching for rectangular optical pulses. The influence of the photodetector bandwidth on the temporal trace quality is also investigated by simulation and experiment. Results show that the photodetector bandwidth needs to be ~ 40 % wider than the pulse spectrum to acquire time-domain traces of the Rayleigh backscattered light with direct detection.

In several applications a temperature contrast between the sensing fibre and the environment is required to detect changes in the environmental heat capacity. For this purpose the process of electrical heating in metallic-coated fibres is theoretically analysed and modelled in steady-state conditions based on the thermal energy generated by resistive heating and the losses induced by convection and radiation. The impact of ambient temperature and pressure is investigated. The proposed model for the thermal exchange is experimentally validated using a high-resolution Brillouin distributed fibre sensor, which is used to measure the longitudinal profile of the temperature reached by electrical heating along an Alcoated optical fibre.

In civil engineering pile systems are used in unstable areas as a foundation of buildings or other structures. Among other parameters, the load capacity of the piles depends on their length. A better understanding of the mechanism of load-transfer to the soil would allow selective optimisation of the system. Thereby, the strain variations along the loaded pile are of major interest. In this paper, we report about a field trial using an optical backscatter reflectometer for distributed fibre-optic strain measurements along a driven pile. The most significant results gathered in a field trial with artificial pile loadings are presented. Calibration results show the performance of the fibre-optic system with variations in the strain-optic coefficient.

A new and simple distributed fiber sensor which allows for the dynamic (single-shot) and quantitative measurement of perturbations is presented. It is based on a phase-sensitive OTDR using direct detection and linearly chirped pulses. Perturbations result in longitudinal shifts of the fiber trace, which can be calculated using a local correlation. As a proof of concept, distributed temperature variations of up to 5 Kelvin with millikelvin temperature resolutions over several minutes are demonstrated. Since the technique does not require a frequency sweep, operation ranging from dynamic strain measurements at kHz rates to temperature monitoring over several hours is readily envisaged.

A method to evaluate distributed temperature gradients along an optical fiber using phase-sensitive optical time domain reflectometry (ΦOTDR) with direct detection is proposed and experimentally validated. The measurement principle derives from the perturbation response of a single-wavelength ΦOTDR signal, which is analyzed as a unidimensional speckle pattern. Our method can be implemented in real-time, relies solely on a low-cost post-processing of the standard ΦOTDR traces and requires no scanning of the laser frequency. This post-processing method can be implemented over a conventional ΦOTDR system used for distributed intrusion detection, without affecting its operation or requiring any additional hardware.

A repeater-less Brillouin optical time-domain analyzer (BOTDA) with 157.68km sensing range is demonstrated, using the combination of random fiber laser Raman pumping and low-noise laser-diode-Raman pumping. With optical pulse coding (OPC) and Non Local Means (NLM) image processing, temperature sensing with ±0.70°C uncertainty and 8m spatial resolution is experimentally demonstrated. The image processing approach has been proved to be compatible with OPC, and it further increases the figure-of-merit (FoM) of the system by 57%.

This paper presents a numerical study of the phase noise from the laser in Brillouin Optical Time-Domain analysis (BOTDA) sensors. Due to laser phase noise, the phase shift cumulated by pump and probe beams during interaction in a generic fiber position is a stochastic variable, with zero mean and variance increasing with pulse duration. For negligibly small pulse leakage, the induced noise is independent of fiber length; otherwise, it increases with fiber length as long as the laser coherence length is longer than fiber.

For high-voltage cables, the maximum temperature of the insulation must never be exceeded at any location and at any load condition. The local temperatures depend not only on the cable design and load history, but also on the local thermal environment of the cable. Therefore, distributed temperature monitoring of high-voltage cables is essential to ensure the integrity of the cable at high load. Especially, the load of the export cables of wind farms varies strongly in dependence on weather conditions. In this field study, we demonstrate the measurement performance of a new, robust Brillouin distributed temperature sensing system (Brillouin-DTS). The system is based on spontaneous Brillouin scattering and does not require a fibre loop. This is essential for long submarine high-voltage cables, where normally no loop can be formed in the seabed. It is completely passively cooled and does not contain any moving or wearing parts. The instrument is dedicated for use in industrial and other rough environments. With a measuring time below 10 min, the temperature resolution is better than 1 °C for distances up to 50 km. In the field study, the submarine export cable of an off-shore wind farm has been monitored. The temperature profile of the export cable shows several hot spots, mostly located at cable joints, and also several cold spots.

Recent advancements in cloud computing technologies in the context of optical and optical fibre based systems are reported. The proliferation of real time and multi-channel based sensor systems represents significant growth in data volume. This coupled with a growing need for security presents many challenges and presents a huge opportunity for an evolutionary step in the widespread application of these sensing technologies. A tiered infrastructural system approach is adopted that is designed to facilitate the delivery of Optical Fibre-based “SENsing as a Service- SENaaS". Within this infrastructure, novel optical sensing platforms, deployed within different environments, are interfaced with a Cloud-based backbone infrastructure which facilitates the secure collection, storage and analysis of real-time data. Feedback systems, which harness this data to affect a change within the monitored location/environment/condition, are also discussed. The cloud based system presented here can also be used with chemical and physical sensors that require real-time data analysis, processing and feedback.

In this paper we study the SNR associated with acoustic detection in Rayleigh-based Distributed Acoustic Sensing (DAS) systems. The study is focused on phase sensitive DAS due to its superiority in terms of linearity and sensitivity. Since DAS is based on coherent interference of backscattered light from multiple scatterers it is prone to signal fading. When left unresolved, the issue of signal fading renders the associated SNR randomly dependent on position and time. Hence, its proper measurement and characterization requires statistical tools. Here such tools are introduced and a methodology for finding the mean SNR and its distribution is implemented in both experiment and simulation. It is shown that the distribution of the DAS-SNR can be obtained from the distribution of backscattered power in OTDR and the mean DAS-SNR is proportional to the energy of the interrogation pulse.

The ability of Brillouin-based fiber-optic sensing to detect damage in a moving cantilever beam is demonstrated. A fully computerized, distributed and high spatial resolution (10cm) Fast-BOTDA interrogator (50 full-beam Brillouin-gain-spectra per second) successfully directly detected an abnormally stiffened (i.e., ‘damaged’) 20cm long segment in a 6m Aluminum beam, while the beam was in motion. Damage detection was based on monitoring deviations of the measured strain distribution along the beam from that expected in the undamaged case.

Industrial piping systems are particularly relevant to public safety and the continuous availability of infrastructure. However, condition monitoring systems based on many discrete sensors are generally not well-suited for widespread piping systems due to considerable installation effort, while use of distributed fibre-optic sensors would reduce this effort to a minimum. Specifically distributed acoustic sensing (DAS) is employed for detection of third-party threats and leaks in oil and gas pipelines in recent years and can in principle also be applied to industrial plants. Further possible detection routes amenable by DAS that could identify damage prior to emission of medium are subject of a current project at BAM, which aims at qualifying distributed fibre optic methods such as DAS as a means for spatially continuous monitoring of industrial piping systems. Here, first tests on a short pipe are presented, where optical fibres were applied directly to the surface. An artificial signal was used to define suitable parameters of the measurement system and compare different ways of applying the sensor.

We present a novel fiber-optic sensing technique based on the distributed measurement of Brillouin-induced phase-shift in the reflection from Brillouin dynamic gratings in polarization-maintaining fibers. Subject to signal to noise considerations, the strain sensitivity of the phase-shift in the reflection of a pulsed probe, orthogonally polarized to the gratings-generating pumps, is independent of the pulse width, suggesting the potential to achieve higher spatial resolutions than those offered by slope-assisted, phasorial Brillouin sensing techniques in standard single-mode fibers. We report the measurement of 500Hz strain vibrations (at a sampling rate of 1MHz) with a spatial resolution of 20cm.

Brillouin Optical Correlation Domain Analysis (BOCDA) offers high speed random accessibility along a sensing fiber, because it can localize stimulated Brillouin scattering at an arbitrary fiber position. By using this function, simultaneous dynamic strain measurement at arbitrary selected multiple points along the fiber was achieved. However, measurement accuracy was restricted due to performance limitation of lock-in-amplifier in the system. This paper reports a new system which uses I/Q demodulator instead of the lock-in-amplifier. Measurement accuracy was improved.

We evaluate the Brillouin frequency shift (BFS) determination error when using the Brillouin phase spectrum (BPS) instead of the Brillouin gain spectrum (BGS) in BOTDA setups. We compare the error obtained in the BFS determination in both cases, both with theoretical arguments and experimental data. In comparison to the gain, for an equal SNR and linewidth, the phase generally provides a better fit of the BFS for smaller frequency spans. This result opens a possible way to reduce the measurement time of certain BOTDA systems by using the phase feature.

A high-temperature distributed sensor solution based on a Brillouin Optical Time Domain Analyzer and a multimode gold-coated fiber is presented and experimentally validated in this paper. Distributed temperature measurements up to 600°C will be demonstrated.

A novel distributed fibre sensing technique is described and experimentally validated, based on birefringence measurements using coherent Rayleigh scattering. It natively provides distributed measurements of temperature and strain with more than an order of magnitude higher sensitivity than Brillouin sensing, and requiring access to a single fibre-end. Unlike the traditional Rayleigh-based coherent optical time-domain reflectometry, this new method provides absolute measurements of the measurand and may lead to a robust discrimination between temperature and strain in combination with another technique. Since birefringence is purposely induced in the fibre by design, large degrees of freedom are offered to optimize and scale the sensitivity to a given quantity. The technique has been validated in 2 radically different types of birefringent fibres – elliptical-core and Panda polarization-maintaining fibres – with a good repeatability.

We demonstrate a simple technique to provide conventional Brillouin optical time-domain analysis sensor with compensation for pump pulse attenuation and tolerance to non-local effects. The technique is based on operating the sensor in loss configuration so that energy is transferred from the probe wave to the pump pulse that becomes amplified as it counter-propagates with the probe wave. Furthermore, the optical frequency of the probe wave is modulated along the fiber so that the pump pulse experiences a flat gain spectrum that equally amplifies all the spectral components of the pulse, hence, preventing distortion. The method is experimentally demonstrated in a 100-km fiber link, obtaining a measurement uncertainty of 1 MHz at the worst-contrast position.

This paper reports on a distributed fiber sensing scheme to achieve sub-metric spatial resolution and long sensing ranges based on a differential pulse-width pair (DPP) Brillouin sensor. The scheme uses a scanning method in which the spectral separation between the two probe sidebands is kept constant, while the optical frequency of the pump is swept to scan the Brillouin spectral response. Experimental results show that the method avoids detrimental temporal distortions of the pulses, which in a standard implementation prevent the DPP method to operate over long distances. Here we demonstrate that this novel scanning method enables distributed sensing over 37.5 km with a spatial resolution of 20 cm and a frequency uncertainty of 1.9 MHz, obtained with 2k averages in a measurement time of a few minutes.

We report on the effects of large pump pulse powers on Brillouin optical time-domain analysis (BOTDA) sensors based on phase-modulated probe wave and coherent detection. It is found that the large Brillouin gain that comes from the use of high power pulses induces a narrowing of the RF phase-shift spectrum that is measured in these sensors. This narrowing leads to a Brillouin frequency shift measurement error when the sensor is configured for dynamic measurements. However, the effect has been found to be less significant than that observed in dynamic slope-assisted BOTDA sensors based on amplitude.