This PDF file contains the front matter associated with SPIE Proceedings Volume 12643 including the Title Page, Copyright information, Table of Contents, and Conference Committee lists.

This paper reports the results of a theoretical and experimental feasibility study of a generic force or strain sensor, incorporating a suspended vibrating optical fiber, incorporating a Fiber Bragg Grating (FBG) as the sensing element. The operational principle is based on monitoring the dependence of the mechanical resonant frequency of a vibrating clamped fiber segment on the tensile force stretching the fiber. It is shown that this frequency change is proportional to the applied force and can be measured using the slope of the reflection spectrum of an inscribed FBG. Furthermore, the fractional frequency change can be made much larger than the change in the strain. Thus, the proposed technique makes possible force measurements in cases, where the strains are too small to be sensed directly by an FBG. Also, the suggested FBG-based approach potentially allows for multiplexed sensing at multiple locations along the host structure. The expected scale factor, Frequency/Strain, is calculated using a simple double-clamped beam model and favorably compared to direct strain measurements. In the framework of the custom-built setup, fiber vibrations were excited by an external shaker, driven by a sweep of frequencies, and a slope-assisted FBG reading method was employed for the measurements of the frequencies in the kHz range. Experimental results, which are consistent with the model predictions, demonstrate the feasibility of using FBG-based measurements of the fiber’s mechanical resonant frequency to the indirect measurement of strains, aiming at eventually achieving sensitivities better than direct strain methods.

The work presented demonstrates that key parameters in aerodynamic structural characterisation of pressure, strain, and structural dynamics, can be all measured via optical fibre sensors interrogated using the principles of range-resolved interferometry (RRI). When used to interrogate sensors simultaneously deployed on a high lift wind in a wind tunnel, the approach yielded resolutions of 31 μPa/ √ Hz and 1 nε/ √ Hz at a bandwidth of 1526 Hz for pressure and strain, respectively, demonstrating the accuracy and versatility of the RRI signal processing technique.

We report on the development and field test of fiber optic hydrophones (FOHs). In details, we focused on the development of acoustic hydrophones for towed arrays applications and seismic hydrophones for seismological monitoring applications. In both cases, the sensing configuration is based on a Michelson interferometer where a fiber coil is wrapped around a compliant mandrel acting as a sensitive element. For the first application, acoustic hydrophones were characterized in an instrumented tank at Leonardo Finmeccanica premises. The hydrophone exhibited a responsivity of about 19 nm/Pa in the frequency range 3÷10 kHz with a resolution down to 300μPa /√Hz. The same FOHs were used as a basic building block to develop a towed array with five elements. By using the same enabling technology, but tailoring the physical and geometrical properties of hydrophones, we developed seismic hydrophones. The sensing system was integrated in the seismologic monitoring system and installed at Campi-Flegrei caldera. During the field trials, we detected several earthquakes occurred in the area and compared the results with a reference piezoelectric hydrophone. The seismic sequence was used to retrieve the sensor responsivity in the frequency range 1-80Hz. The sensing system exhibited a responsivity of about 300nm/Pa and an average noise floor level down to 100μPa/√Hz. The reported field trials demonstrated the capability of FOHs to operate in relevant environments and realistic scenarios.

Polymer fiber Bragg grating (FBGs) demonstrate a wider strain range and stronger temperature sensitivity compared to standard silica FBGs. Besides, their advantageous feature is sensitivity to humidity that enables FBG-based relative humidity (RH) sensing. However, practical realization of RH sensors requires temperature cross-sensitivity elimination. A certain optimal fiber pre-strain and gamma irradiation of perfluorinated (CYTOP) FBGs up to certain optimal dose are potential recently proposed solutions for this problem. In this work, we investigate temperature and RH response of FBGs inscribed line-by-line in a few-mode polymer fiber with 20-μm CYTOP core and 250-μm XYLEX overclad. We compare the cases of the pristine FBG sample and the sample received 200 kGy irradiation dose. The 200-kGy dose was previously confirmed to provide temperature sensitivity minimization at 40%RH. Here, we show the close-to-zero temperature sensitivity (≈1pm/°C) for 200-kGy dose at high RH value of 89%. Besides, we briefly analyze the stabilization process of FBGs response to strong and quick RH changes.

The capabilities of optical fiber sensing based on forward Brillouin scattering (FBS) can be extended by exploiting the differential response to fiber perturbations of radial and torsional-radial acoustic modes. Radial and a subfamily of torsional-radial modes present a different sensitivity to changes of temperature and strain. By combining experimental measurements of the resonance frequencies of the different acoustic modes, we obtain (1) the Poison ratio of the optical fiber with an accuracy better than 1‰, and its temperature and strain responses, and (2), we demonstrate simultaneous and discriminative measurements of strain and temperature with accuracy better than 25 με and 0.2 °C, respectively.

In this paper a multiplexed multi-parameter marinized sensory array is described. This was deployed on the continental slope off the Keri Island marine observatory in the Gulf of Finland (Estonia). The sensor array is made up of 4 measurement stations which are connected in series. Across these measurement stations, a total of 16 temperature sensors, 4 attitude sensors (each consisting of 3 individual fiber sensors) and 16 flow sensors were successfully deployed. They were addressed over 1.1 km via 20 single-mode (SMF-28e) optical fibers contained in a single marine compliant ruggedized umbilical. The bio-inspired fiber optic flow sensors are designed to mimic the behavior of the superficial neuromasts naturally found as part of the lateral line sensory organ present in fish. The sensor is composed of optical fibers inscribed with Fiber Bragg Gratings glued together in a polymer matrix which are then encapsulated in a polyurethane shell. The sensors response has been tested in DC flows in a tow tank and have demonstrated the ability of measuring flow speed from 0.05 ms^-1 to 2.5 ms^-1. The main aim of the deployment was to demonstrate the capabilities of fiber sensor technology for long-term oceanographic applications. The measurement period described lasted over two months and the sensor system performed well in comparison with data was gathered from commercial instrumentation available.

Compression therapy is a widespread clinical treatment requiring the application of therapeutic pressures to the lower limbs of patients using bandages or hosiery. The amount of pressure applied to the limb is critical in promoting patient recovery. Initial results from a fibre optic cantilever pressure sensor incorporating Fibre Bragg Gratings (FBGs) are reported. Calibration and bandaging experiments performed on a phantom leg model are presented, alongside a comparison to a reference sub-bandage pressure monitor. The proposed sensor shows increased pressure sensitivity in comparison to a previously reported design in which an FBG is encapsulated in a polymer and shows potential for application in the context of healthcare wearables.

Current composite structures used in aircraft can suffer from barely visible impact damage (BVID) that can compromise the load-bearing function of these structures. Especially damage-prone regions, such as the feet of a skin-stiffener structure, must therefore be frequently inspected for such damage. This increases aircraft downtimes and associated costs. A permanently installed structural health monitoring (SHM) network based on optical fiber sensors is an ideal candidate for performing condition-based maintenance (CBM) on such a structure. Individual FBG sensors have a known potential to detect the presence of BVIDs. In this work we propose a Global Damage Index (GDI) for quantifying the health of a composite component in manner of seconds, based on a network of 60 FBG sensors. We first establish a damage detection threshold and then carry out temperature compensated BVID detection with the GDI.

The investigation of impact of ionizing radiation on photonic waveguides and devices is of a large interest due to new demanding applications in harsh environments, such as space or high-energy-physics experiments and more. Thus, the effects of gamma radiation on refractive index inducing propagation loss different optical fibers are investigated by means of several approaches including fiber Bragg gratings and long period gratings (LPG). In this work, we report the results on exposure to gamma irradiation up to tens of kGy of LPGs written in single-mode optical fibers with unconventional dopants, such as B and P. The LPGs in a reflective configuration have been written using the electric arc discharge technique. The attention is focused on the real-time measurement of LPG resonance, i.e., wavelength shift and transmission power changes during irradiation, as well as the recovery effects after the irradiation. It has been found that the impact of gamma is significantly dependent on the fiber type and the LPG properties. The LPGs in B/Ge co-doped fiber show both the higher resonant wavelength shift and low power losses. Such results are useful for those working with optical fibers and related sensors in environments exposed to radiations.

We report the best noise and drift ever achieved by a laser-driven FOG, namely an angular random walk of 368 μdeg/√h and a drift of 6.66 mdeg/h. This improvement was achieved by interrogating the 3-km Sagnac interferometer of the FOG with a low-coherence light source consisting of three lasers broadened by the same noise-driven phase modulator, which further reduces the temporal coherence compared to a single broadened laser. Proper optical gating is also applied to suppress the residual drift due to the Kerr effect. The experimental results agree well with our prediction that both the noise and the drift improve as the square root of the number of lasers. Using multiple lasers also improves the mean-wavelength stability of the light source compared to a single laser. Thanks to the low cost of semiconductor lasers, this technique is a promising and cost-effective solution that can be easily extended to a larger number of lasers for further reduction of the noise and drift in high-accuracy FOGs.

Various systems are used to monitor the technical conditions of railway lines and railway vehicles during operation. This contribution presents an optical fiber monitoring system based on a Fabry-Perot interferometer. The sensing optical fiber placed on the foot of the rail is elongated due to the bending of the rail during the passage of the train. The optimized signal demodulation algorithm allows determining not only the presence of a train in a particular area, the number of axles, and the speed of the train, but also some defects related to the technical conditions of railway vehicles. The detection method is shown in the passage of a passenger train consisting of a locomotive and four wagons moving at a speed of 98.08 km/h. The advantage of the system is the possibility to determine different useful parameters of train passage with the technical conditions of vehicles during normal operation.

Optical fiber-based sensors are non-invasive and suitable instrumentations for physical sensing. In this technology, signals are transmitted through pulses of light and are not affected by electromagnetic interference or electrical noise. Although optical fibers perform well at typical operating temperatures, their properties at cryogenic temperatures down to 1.6 K under ultrahigh vacuum remain largely unknown. Future applications in quantum computing, superconducting research, and aerospace will all require cryogenic technologies. For such cold applications, a solid understanding of optical fiber performance and losses is crucial. This study evaluates the optical fiber losses for discrete and distributed strain sensing down to cryogenic temperatures (1.6 K).

Radiation impact on the strain transfer for a bonded fiber Bragg grating (FBG) has been investigated. The main goal of this preliminary study is to evaluate which adhesive and which fiber are the most appropriate to design a very sensitive strain sensor able to operate in nuclear environment. We performed static and dynamic experiments on different kinds of adhesives and different optical fibers, before and after a 1 MGy(SiO₂) X-ray irradiation. The results reveal that the FBG strain sensors can be as much sensitive as classic strain-gauges and that the radiation effects on adhesives can have a positive impact on the strain transfer efficiency.

Composites have added new dimensions to the design and fabrication of various structures. These structures are usually used to withstand hefty loads, so their integrity must be guaranteed. Due to optical sensors' advantages and FBGs' unique position among optical fibre sensors, we propose in this paper the integration of FBG sensors for the monitorisation of the curing temperature and strain of multidirectional carbon-reinforced polymer cured by microwave radiation heating method and for strain monitoring of a fibre reinforced polymer pultrusion beam.

We investigate the performances accessible in terms of strain and temperature discrimination using Brillouin Optical Time Domain Analysis combined with the LEAF fiber from Corning, AllWave fiber (AW) from Lucent and TrueWave (TW) fiber from OFS when exposed to γ rays and X-rays up to 1 MGy(SiO₂). All these fibers present a multipeak Brillouin Gain Spectrum (BGS) with unique dependencies of each of its peaks over temperature (T) and strain (ε). The evolution of their T and ε discrimination capability is investigated to evaluate how radiation affects the sensing performances. High dose irradiation changes the sensor performances through two main effects. First, the Radiation Induced Attenuation (RIA) limits the BGS amplitude, the sensing range and discrimination capability techniques relying on BGS amplitude. Second, radiations modify the Brillouin scattering properties by slightly changing the refractive indices and the acoustic velocities of the silica-based leading to small changes in T and ε dependencies and also to a limited Radiation Induced Brillouin Frequency Shift (RI-BFS) that causes a direct measurement error. Results exhibit an overall decrease of T and ε uncertainties through discrimination process after 1 MGy reaching 0.9°C and 29 με for 0.1 MHz frequency uncertainty for sensors based on the LEAF fiber.

A novel prototype sensing device based on polymer (CYTOP-XYLEX) optical fibre Bragg grating sensors was developed to monitor temperature and relative humidity levels of reinforced concrete from its initial curing phase to a prolonged period. The prototype can be used for concrete quality control offering numerous advantages, such as small size, robustness, high sensitivity, and low-cost continuous in-situ measurements.

This paper demonstrates a portable optical sensor for human gait monitoring. The device is based on a smartphone and POF sensor specifically designed for use in shoe insoles. The monitoring of multiple sensors by a single smart device is achieved by image segmentation based on Voronoi tessellation, as this work describes in detail. Experimental tests performed with the sensor have demonstrated its ability to provide information on spatial and temporal parameters of gait as well as pressure at different plantar loci.

In this paper, we proposed an in-fiber Mach-Zehnder temperature sensor based on a dual-core fiber (DCF) with one core as the sensing arm suspended in the centre fluidic channel of the DCF and the other core as the reference arm running eccentrically along the fiber. The fluidic channel was infiltrated with silicone oil. Temperature variations would change the refractive index of silicone oil and thus the effective index of the guided mode in the suspended core, thus shifting the interference spectra. The sensitivity of the sensor using a DCF infiltrated with ∼20 cm-long silicone oil was found to be as high as −1.42 nm/°C, comparable to those of the SPR fiber sensors and other interferometric sensors. The measuring range of the sensor was more than 120°C. The proposed sensor could be easily fabricated with good robustness and stability, which makes the sensor suitable for applications such as environment and architecture monitoring.

In this work, two new interferometric sensors based on multicore optical fibers for the measurement of strain with the ultimate goal of traffic monitoring are presented. The operating principle of each sensor relied on the monitoring of the phase shift difference accumulated between the supermodes of the structure of the multicore segment in a full round trip. The strain characterization for both sensors resulted in a linear response, with sensitivities of -4.073·10^-3 rad/με and - 4.389·10^-3 rad/με for the aligned and V-shaped cases respectively, and one-hour instabilities below 4.6·10^-3 rad with a 95% confidence level. These results suggest its feasibility in applications requiring high sensitivities over very wide strain ranges, such as heavy-vehicle traffic monitoring. To corroborate the hypothesis, both sensors were integrated into the pavement and their response to the traffic was analyzed.

We present a temperature sensor based on a polymer exhibiting a Lower Critical Solution Temperature (LCST) in aqueous solution encapsulated in a capillary. Parameters are chosen such that the solution exhibits a cloud point in a temperature range of 30° C to 39°C. The characteristic of thermoresponsive polymers with an LCST, is that above that temperature phase separation of the polymer takes place which leads to a temperature-dependent formation of a cloudy suspension. An optical intensity measurement over the desired temperature range is established by an increase of optical attenuation inside the polymer solution caused by a rising temperature. For our purpose, the polymer capillary is connected to transmitter and receiver via a Polymer Optical Fibre (POF). Our intensity measurement is, to the best of our knowledge, a novel method and can be considered simple when compared to existing fibre-based temperature measurement techniques. Due to the lack of electrical components at the probe, this sensor is suitable for measurements in strong electromagnetic fields and environments for which flying sparks are hazardous, i.e., inflammable fluids or gases. Furthermore, all manufactured sensors share the same temperature dependence and, therefore, are well-suited for comparative measurement, e.g., flow measurement systems. With the given temperature range, a body temperature measurement is also suitable.

This paper proposes an approach for lateral force sensing based on fibre Bragg gratings (FBGs) and Gaussian process regression (GPR). The monitoring system is based on FBG sensors embedded in a glass structure (load cell), which converts the external lateral force into axial strain along the fibre. Whilst the use of conventional peak detection techniques to retrieve the Bragg wavelength of the embedded FBG sensor can easily fail due to the FBG spectral distortions induced by the lateral force, the proposed GPR approach permits a direct mapping of the FBG spectral shape (including its peak wavelength and any observed distortion) into the applied lateral force. Experimental results validate the proposed approach, demonstrating highly accurate lateral force monitoring even in conditions with distorted FBG reflection spectra.

Monitoring the temperature of lithium-ion batteries (LiB) is crucial for safe operation. The degradation of a LiB is highly dependent on temperature. It is desirable to keep the LiB within a temperature window between 15 °C and 35 °C. Otherwise, enhanced degradation will take place. So far, only the surface temperature of a LiB at a single point is measured, but the internal temperature can be significantly higher during operation. In this work, we integrated fiber Bragg grating (FBG) temperature sensors into lithium-ion pouch cells and evaluated the stability of the sensitivity of the FBG sensors over four months. Two prototype pouch cells were manufactured each with two fibers. Each fiber has three FBGs inscribed, hence there are six temperature measurement points per pouch cell. The FBG temperature sensors comprise a polyimide-coated fiber with the three inscribed FBGs and a polyimide-coated fused silica capillary. The capillary mitigates the influence of external strain on the FBGs because the fiber can move freely in the capillary due to the single mounting point on one side of the capillary. The sensitivity was measured once a month for a period of four months. Although, the Bragg wavelength changed over time, and baseline correction is required for reliable temperature calculation. The measurements show comparable stability of sensitivity over time with another publication.

This paper reports experimental results obtained on a fiber Bragg gratings (FBG) system able to sense the deformations of a vertical axis wind turbine (VAWT) tower. Cost and time optimization is paramount in the industry. Therefore, structural health monitoring (SHM) is needed to prevent machines from failure. In this project, three FBG strain sensors are placed vertically along a VAWT tower, each spaced by 1.5 meter. A comparison of every sensor revealed that height has no significant influence on the sensor sensitivity for the first 4.5 meters of the tower. By means of a power spectral density (PSD) applied on the measured signal, three sources of deformation can be retrieved: wind force, blades unbalance and 1st tower mode resonance. Each deformation source is characterized by a specific frequency. The wind force and the blades unbalance induce mechanical stresses at a frequency that depends on the rotational rate. The 1st tower mode only depends on the system geometry, so not on the rotational rate. A qualitative analysis of the deformation amplitude is performed for different rotational rates within the VAWT operational range (10-35 rpm). The results show the deformation amplitude due to the wind force depends on the wind speed which is naturally not predictable. The amplitude for the resonance depends on how close the rotational rate is to the resonant frequency (22 rpm) and on the duration lasted at this rate. For the blades unbalance, the deformation amplitude increases with the rotational rate, due to the centrifugal effect.

This paper reports on the development of a smart elastic textile band containing pre-strained fiber Bragg gratings (FBG) that was specifically designed with the ambition to dynamically measure the position of the backbone. To this aim, the textile band is 700 mm long and 60 mm wide. A piece of standard single-mode optical fiber, in which four fiber Bragg gratings were inscribed, is sewn on the band. Each FBG is glued on a 3D-printed pad in a pre-strained way, allowing the detection of FBG compression in addition to elongation. Measurements were performed on this sensing elastic band and the resulting sensitivity is a Bragg wavelength shift of 12 pm per mm of textile elongation. Validation tests were also carried out to highlight the sensitivity to compression and to show that the sensing system is capable of repeatability in a dynamic environment.

A probe-type all-fiber ultraviolet photodetector is proposed in this paper. A ZnO microwire is fixed on the end facet of a single-mode fiber through a glass tube with specific diameter to form a Fabry-Pérot interferometer. With this all-fiber structure, fast-response ultraviolet detection can be realized in an all-optical scheme. Since the refractive index of ZnO microwire increases under the illumination of ultraviolet, interference wavelengths of abovementioned device redshifts with the increase of ultraviolet light intensity. By employing a continuous 266-nm laser beam and chopping method, the sensitivity is obtained to be 0.268 nm/(W·cm^-2 ) and the response time is only 0.56 ms. To be more specifically, the response speed of the device is further explored by a 266-nm pulsed laser, and the response time of the device is measured to be only 13 μs. The proposed device provides a new idea for the next generation of high-performance ultraviolet photodetectors and may find potential applications in the future.

This study presents a novel approach to detect partial discharges (PD) in a medium-voltage (MV) cable termination using fiber optic-based acoustic PD sensors. The sensing system is designed by the company Optics11. It is a commercial off-the-shelf product under the trademark name of OptiFender, which uses non-metallic, passive fiber opticbased acoustic sensors, which are galvanically isolated. Given these properties, OptiFender sensors can be installed directly on the cable accessories. The sensors can withstand the curing conditions of the filling compound used in the cable accessories, making them suitable for both retrofitting and embedding applications. In this study, defect terminations with PD were investigated. Several OptiFender sensors were installed all around the defect termination, and all of them could detect partial discharge with a sufficient signal-to-noise ratio (SNR). The termination was operated at medium voltages, with PD levels of from a few pC up to around 100 pC, but the application of OptiFender is not limited to only medium voltages, and there have been examples of using the same sensors at voltages of up to 400 kV. All the sensors picked up PD with a high SNR. Acoustic PD sensors provide an indirect measure of the PD activity by measuring the released energy of the partial discharge in the acoustic domain, rather than the traditional direct measurement of the displaced charge. The OptiFender system can provide uninterrupted, continuous, and unsupervised monitoring of electrical assets for both indoor and outdoor applications.

The noise conversion relationships of relative intensity noise (RIN) and shot noise in a broadband source sensing system using a 3×3 coupler are studied. RIN and shot noise are converted into the phase noise floor through the arctangent demodulation scheme and the performances of the 3×3 coupler. To quantify the contribution of RIN and shot noise, we analyzed the equivalent phase noise induced by RIN and shot noise. The theory and experiment demonstrate that the equivalent phase noise of RIN is related to the correlation of three RINs and the initial phase, and the shot noise is related to the initial phase.

High-pressure resin transfer moulding is a novel process to manufacture composite materials, having the advantages of allowing fast moulding speed and strong part performance. One of the most significant difficulties of the process is that the flow of resin used inside a closed mould cannot be directly monitored. In this paper, a fibre Bragg grating (FBG) sensor is embedded into the glass fibre layers placed inside the mould with the purpose of monitoring the resin flow in the process. This is achieved by heating the mould and measuring how the cold resin flows through the mould using a series of FBG sensors. Results find out that the FBG peak wavelength sharply decreases when the resin reaches the position of the sensor, allowing the monitoring of the flow front through a closed mould. The method is validating comparing FBG measurements with camara recording to visualise the actual position of the fluid. Although FBG sensors respond to both temperature and strain changes, the sharp thermal impact can be easily detected, making unnecessary any temperature-strain discrimination.

In this paper, a method based on fibre Bragg gratings (FBGs) is proposed to detect ice in vibrating beam-like mechanical structures. For this, two points of an optical fibre, with a pretensioned FBG sensor in the middle, are glued on the upper surface of a cantilever beam. The fibre is placed in a way that the central fibre section containing the FBG remain unglued and loose when the cantilever is placed at the most upward bending position. This way, the vibrating beam and dangling FBG are constructed into a bowstring structure. To validate the method, ice is induced on the upper surface of the cantilever beam by applying a freezing spray in laboratory. Results point out that when no ice is present in the vibrating beam, the FBG response shows a quasi-sine wave response with a flat-bottom section, which indicates the condition of the loose FBG. On the contrary, when ice cumulated on top of the cantilever, the ice induces a gluing effect that bonds the FBG to the vibrating beam, so that its strain response corresponds to an almost undistorted sine wave. By analysing the shape of the time-domain and frequency-domain strain response of the FBG, an effective method for ice detection is demonstrated.

Esophageal pressure, bile content and pH are important parameters in gastroesophageal diseases. An all-optical technology is described to perform simultaneously oesophageal manometry, pH-metry and bilimetry. The pressure measurement along the oesophagus is performed with 11 fibre optic gratings (FBGs) within a single mode fibre, which ensures monitoring every 2.5 cm along the oesophagus. The bile measurement is based on the direct absorption of bilirubin, the main biliary pigment, using a bundle of plastic optical fibres (POFs) carrying the signal. Regarding the detection of pH, an acid-base indicator that modifies its absorption as a function of pH is immobilized on glass particles with controlled porosity immobilized at the end of two POFs. The sensors are integrated in the same catheter for the simultaneous measurement of the three parameters. The prototype for the catheter interrogation was designed and developed, constituted by the integration of two optoelectronic modules for the interrogation of the pressure sensor and for the bile and pH sensors, respectively.

A temperature-compensated sensor architecture for a fiber optic hydrogen sensor consisting of a partly palladium-coated pi-shifted fiber Bragg grating was modeled and compared with measurements. The transfer matrix formalism was used to calculate the spectral line shape of the pi-shifted FBG with a hydrogen-induced, non-homogeneous strain distribution along the grating axis. The temperature response of the grating itself can be compensated by referencing the notch to the flank wavelength. In addition, the hydrogen solubility in Pd shows a non-linear temperature dependence that was also included in the sensor performance calculations. For the investigated H₂ concentration range of 200 ppm to 20000 ppm and between 15 °C and 40 °C, measurement data fit well to the simulation above 3000 ppm but become diffuse below, indicating deviations from the expected dependence according to Sieverts’ square root law.

This work describes an experimental study towards label-free sensing of C-reactive protein (CRP) – a protein recognized as a inflammation marker. A multimode optical fiber with a section of its core coated with indium tin oxide (ITO) thin film was used as a sensor. ITO film allows for guiding lossy modes and can simultaneously be used as a transparent electrode for electrochemical measurements. Therefore, optical and electrochemical detection based on a single sensor was possible. Such a dual-domain approach is practical, especially when the results in one of the domains are not accurate enough, which was the case in this work. A case of different functionalization methods of ITO surface was also pointed out. The proposed sensor allows for recognition as low as ng/mL.

In this work, the classification and the identification of two different types of solvent solutions (i.e., isopropanol:water and ethanol:water) were achieved by exploiting the solvent solution properties combined with Artificial Intelligence. More in detail, a Surface Plasmon Resonance (SPR) sensor based on D-shaped plastic optical fibers (POFs) acted as an optical transducer to build a proper dataset. The plasmonic probe was used to monitor the bulk refractive index variations of the solvent solutions due to the evaporation phenomenon over time. The collected experimental data were used to train a machine learning-based algorithm useful for building a prediction model. In such a way, it was made possible to determine the presence of the solvent in the solution under test (water or alcoholic solutions) and, in addition, to recognize the type of solvent. Finally, the results obtained from the testing of unknown solutions testified to the goodness and suitability of the proposed simple sensing approach.

Graphene oxide (GO) thin films fabricated by the vacuum filtration method were deposited on bare tilted fiber Bragg grating for refractometry measurements. Two different layer thicknesses (100 nm and 200nm) were used to prepare the samples. The amplitude spectra of the GO-coated TFBGs (GO-TFBGs) were measured with linearly polarized light for different refractive index values of LiCl solutions (1.3333-1.3342). We show that when polarized light is used, the 200 nm GO-TFBGs achieve similar behavior as plasmonic gold-coated TFBGs (Au-TFBGs). This latter exhibits a characteristic attenuation in the amplitude spectrum when P-polarized light excites a surface plasmon resonance (SPR). This behavior suggests that GO is present as a mix of discontinuous and stratified flakes favorable for plasmon-plasmon hybridization, which can be generated for both P and/or S-polarization of the light.

We developed a miniaturized optical probe, based on a customized Fiber Bragg Grating, for mechanical characterization of biological tissues with sub-millimeter spatial resolution. The probe is integrated inside a metallic cannula (16 gauge) used for clinical applications, and it is driven by a robotic arm (KUKA LBR Med 7). The optical sensor has a resolution of less than 1 mN and measures the force on controlled tissue indentations. The functionality of the sensor was assessed by means of different tests carried out on real prostates obtained from radically surgeries of patients at different stages of the carcinoma (Gleason score from 6 to 8). Specifically, in this work, we demonstrate that our system provides results that are on line with to the biopsy analysis performed before and the surgery. Our findings lay the foundation for the development of compact optical fiber probes, with size compatible with needle/catheter, able to perform in vivo mechanical characterizations of the prostatic tissue with high sensitivity and spatial resolution.

The inscription of a tilted Bragg grating in the core of a standard telecommunication-grade fiber grants the latter a sensitivity to the refractive index (RI) of the fluid in which it is immersed. A gold-coated tilted fiber Bragg grating (Au-TFBG) is a transposition of the Kretschmann prism configuration onto a cylindrical structure. Taking advantage of the surface plasmon resonance (SPR), the RI sensitivity is further increased. In this work, we extract relative phase difference spectra from two orthogonal polarization states using the Jones formalism and use them to experimentally detect the insulin hormone at concentrations ranging from 0.1 ng/mL to 100 ng/mL.

Nowadays, multi-domain systems are crucial for accurate measurements since they improve error avoidance in empirical observations by supplementing one domain with the other. Such an approach also yields more useful information about the sample under study. In this work, we present the results obtained from a sensing system composed of a microcavity inline Mach-Zehnder interferometer (μIMZI) combined with indium tin oxide (ITO) electrodes, which is the first approach to two-domain, real-time, and label-free observation of cell behavior. The µIMZI structure was manufactured with a femtosecond laser ablation process in the side surface of single-mode optical fiber. It is susceptible to refractive index (RI) change within its volume, i.e., hundreds of picolitres. The μIMZI sensitivity to RI reaches over 15,000 nm/RIU. The μIMZI was attached to the glass plate with eight ITO electrodes formed using laser irradiation. This optical and electrochemical domain system was used for cell medium measurements, followed by one hour of HepG2 cells monitoring. Finally, trypsin was added to the solution, and its effects on the HepG2 cells were investigated optically and electrochemically. The presented monitoring setup and obtained results are proof of concept for a multi-domain cell monitoring system.

In this work, we designed a fused silica lab-on-chip that combines optical techniques such as absorption, Raman scattering and fluorescence to quantify phytoplankton type and concentration in water. In the absorption stage of the chip, a Fabry-Perot resonator significantly enhances the spectral response. Scattering and fluorescence spectroscopy are considered with light focusing on sample channel. The design process, and more specifically the integrated in-chip aspheric lenses, is carried out by ray-tracing simulations.

We propose a novel cryogenic liquid level sensor based on a long-period grating inscribed in one of the external cores of a multicore optical fibre. The proposed device has a small grating period to improve the sensitivity to external refractive index changes while reducing the temperature sensitivity. We compare the results obtained with an LPG inscribed in a single-core fibre, demonstrating that the optical spectrum of our device is not divided into two different resonances and shifts smoothly towards shorter wavelengths as the relative liquid level increases. The sensitivity of the proposed level sensor when measuring liquid nitrogen is -0.772 nm/cm.

The work discusses the possible impact of the electric charge of biological material on the properties of label-free biosensors, in particular those operating in dual domains, i.e., optical and electrochemical. Optical fiber lossy-mode resonance (LMR) sensors based on indium tin oxide (ITO) were investigated as label-free biosensors with a model biological receptor-target pair, i.e., biotin-avidin. Each of the used biological materials shows different properties, i.e., size, isoelectric point, and, therefore, also charge. The investigations were performed in two electrolytes with differently charged redox couples to better identify the possible influence of chargé of biological material on the optical readout. The obtained results clearly indicate that in designing label-free biosensing solutions, consideration of a broader range of biological materials properties than just refractive index, such as their charge, is required.

We describe an optical fiber sensor for detection of ammonia vapors employing a fluorinated graphene-like overlayer on a tilted Bragg grating. Exploiting the laser-mediated explosive synthesis and transfer (LEST) of graphene (Gr) flakes a thin film of few-layer turbostratic graphene flakes doped with F atoms (~ 3.3 at. %) are deposited on the fiber at the location of the grating. The response of the sensor was investigated for NH₄OH vapors while for reference, the effect of H₂O was also monitored at identical conditions. Under increasing vapor pressure of NH₄OH, wavelength shift is recorded not only in the cladding modes but also for the fundamental Bragg mode, indicating that the effect is not solely due to changes in the optical parameters of the overlayer. The monitored wavelength shift is initially negative turning to positive when vapor saturation is reached. Furthermore, there is a distinct difference in the magnitude of the monitored shifts with the higher order mode exhibiting 2.5x higher values compared to the Bragg mode. The study is ongoing and will also include overlayers of pure LEST Gr and LEST Gr decorated with Six nanoparticles.

Plasmonic tilted fiber Bragg gratings (TFBGs) are well-suited for accurate, rapid and minimally-invasive biosensing. They present a very dense transmitted amplitude spectrum of narrowband cladding mode resonances (full width at half maximum < 1 nm). This response is commonly demodulated using highly-resolved interrogators (wavelength resolution < 10 pm). This work investigates the possibility of reading-out the amplitude spectrum of a gold-coated TFBG by using a coarsely resolved spectrometer (166 pm). A refractometric sensitivity of 2656 nm/RIU has been observed thanks to a refined analysis of the spectral content which has led to the development a more efficient signal processing. This value represents a fivefold enhancement compared to previously reported read-out methods. Biosensing has been successfully achieved with gold-coated TFBGs used in reflection mode for the detection of insulin, with specific antibodies grafted on the gold surface. Our experimental study is an advance towards an industrialization of the FBG technology, as it opens the way to rapid parallel biodetection, benefiting from the multiple sensing channels (up to 64) of the interrogator as well as its high processing speed (repetition rate up to 3 kHz).

Plasmonic optical fiber gratings are usually relying on centimeter-long sections of fibers locally modified with a thin metal film to enhance their sensitivity to the surrounding refractive index through surface plasmonic resonance (SPR). They are considered as a transposition of the Kretschmann prism configuration (cf. commercial SPR devices) and enable the development of lab-on-fiber tools for versatile applications, from biomedical diagnosis to environmental sensing where they can bring unique features. In this paper, we provide an overview of the main achievements in label-free biosensing with gold-coated tilted fiber Bragg gratings (TFBGs), from in vitro bioassays to the ex vivo detection of biomarkers expressed at the surface of cancer tissues. We discuss their performances and draw the lines for the next improvements and their implementation in the real world.

Plasmonic optical fiber sensors have been investigated for biosensing purposes as they provide miniaturized devices with fast, label-free and highly sensitive responses. This study explores gold-coated optical fiber tip sensors, based on multimode fibers of 200 and 600 μm core diameters, for the detection of the heart failure biomarker N-terminal pro-Brain natriuretic peptide (NT-proBNP). The obtained results demonstrated that the 200 μm core tip reached the highest refractometric sensitivity, between 1158.54 and 2750.34 nm/RIU, and that both sensors responded to NT-proBNP concentrations in a range that included the values of clinical interest for the diagnosis of this cardiovascular disease, which correspond to 0.125 ng/mL and 0.300 ng/mL.

In this work, a comparison between the temperature and strain decoupling performed by a bare polarization-maintaining FBG (PM-FBG) sensor and by a hybrid sensing setup, involving the slow peak of the PM-FBG and the fringe shift of the fiber loop mirror (FLM) interferometer (FLM+FBG_slow), was studied and discussed. To promote such comparison, the sensors were submitted to temperature and strain variations at the surface of a cylindrical Li-ion battery (LiB), cycling between 3.0 V and 4.2 V. The data spread of the FLM+FBG_slow indicates a root mean square deviation of ± 0.1 °C and ± 7.8 με, against ± 0.5 °C and ± 9.7 με in PM-FBG, for temperature and strain, respectively. The matrixial method through the sensitivities values of each sensor was used for data processing. Although both sensing configurations showed similar trend behavior of strain and temperature variations during all the experiment, the FLM+FBGslow setup presented better accuracy when compared to the PM-FBGs, however, it could only perform measurements in all the LiB extension, while the PM-FBG could provide data regarding a specific spot of the LiB. The highest temperature and strain variations were achieved in the end of the discharge processes. Keywords: Fiber loop mirror, polarization-maintaining

A unique side-polished balloon shaped heterocore structure plastic optical fibre (POF) sensor for real-time measurement of very low to high ethanol concentration in water is reported. The sensor is designed as a large core-small core-large core heterocore structure where small core fibre (SCF) acts as a sensing region, whereas large core fibre (LCFs) are used as input and output light waveguide s as well as to introduce the light leakage in the cladding of SCF at the heterocore structure’s input interface and hence generate the significant evanescent field. The principle of operation of the sensor is based on evanescent field interaction at the interface of modif ied SCF and the liquid boundary. The sensor is characterized for ethanol-water solutions in the ethanol concentration ranges of 20 %v/v to 80 %v/v, 1 %v/v to 10 %v/v, and 0.1 %v/v to 1 %v/v, demonstrating a maximum sensitivity of 54673 %/RIU. The experimentally evaluated high sensitivity of this sensor design for real-time measurement of ethanol concentration in water at different ranges makes it a potential candidate for implementation in the industry as a low-cost and real-time solution for ethanol sensing as well as other RI sensing applications.

State-of-the-art optical fiber pressure sensors use displacement diaphragms and mechanical transducers to enhance pressure sensitivity, however, due to their bulkiness and large size they can’t be easily integrated inside pressure guide wire for intravital monitoring. Fiber Bragg Gratings (FBGs) due to their inherent advantages can be designed in a way that is suitable for monitoring Intracranial Pressure (ICP) and Instantaneous Wave-Free Ratio (iFR) pressure indices. The main disadvantage of FBG is that it has a low-pressure sensitivity of 3.04pm/MPa, which is insufficient for these applications and is made worse by the cross-sensitivity caused by temperature. We hereby present a two-pronged strategy to tackle this issue. The first step in improving sensitivity is to modify FBGs, and the second is to use signal processing methods to recover minor wavelength shifts. A frequency-selective detection technique can be used to measure sub-pm wavelength shifts for small modulated pressure signals. This technique was used to establish a test bench for measuring the pressure sensitivity of standard acrylate and polyimide coated FBGs as well as to confirm a linear relationship between the pressure range of interest and Bragg wavelength shift.

This paper presents the design, cost optimization and performance analysis of a distributed humidity and temperature fibre optic sensor for environmental monitoring. The sensor utilises a 1-metre spatial resolution phasesensitive Optical Time Domain Reflectometry (OTDR) interrogator and employs a pair of fibre optic cables as sensing elements. One cable is coated with polyimide for humidity sensing and the other is coated with acrylic for temperature sensing. The sensor is designed to be reliable, accurate and cost-effective, enabling its use in various industrial environments. New software was developed for fast data acquisition and processing, and the hardware was assembled to allow measurements to be taken at thousands of different locations over the same fibre optic. The cost of the current version and the acquisition time have been reduced by half compared to the reference version. The sensor’s performance was evaluated in both a controlled laboratory environment and in a real-world deployment. The results indicate that the sensor effectively measures relative humidity (RH) and temperature across a broad range of conditions while preserving the precision of the previous version. Additionally, it utilizes cost-effective hardware and has a significantly faster response time.

In this work, an erbium-doped fiber ring cavity based on a 3dB optical coupler for refractive index measurements is presented and experimentally verified. This interrogation system, based on a 1 x 2 optical coupler, uses one of these output ports to increase the reflected optical power by means of a fiber Bragg Grating (FBG), used as a reflector. Moreover, the other output port is used as a refractive index sensing head. A spectral analysis of this interrogator system as well as a fiber cavity ring refractive index sensor characterization are carried out. Finally, an experimental comparison of the refractive index sensor range when the 3dB coupler is replace by a 1x3 one is also presented.

A sensing probe is presented for the detection of 2,2,2-trifluoroethanol in the vapor phase, while using poly(vinylidene fluoride (PVDF) thin films, overlaid onto tilted optical fiber Bragg gratings. The 2,2,2-trifluoroethanol sensor operates in the 1.5 μm band, in transmission mode, where the signal of both the core and cladding modes is monitored. Best detectivities obtained are 2 ppm for 2,2,2-trifluoroethanol vapors in ambient atmosphere, for typical response times of 50 min. The sensing probe presented - based on PVDF transductor - shows limited reversibility after being used in the tracing of 2,2,2-trifluoroethanol vapors; its subsequent exposure to nitrogen flow, partly reverses its spectral behavior back to the starting point, denoting the involvement of mechanisms other than physisorption into the underlying transduction. The actual sensing mechanism of 2,2,2-trifluoroethanol vapors while using thin PVDF films is currently under investigation.

The need for miniaturized biological sensors which can be easily integrated into medical needles and catheters for in vivo liquid biopsies with ever-increasing performances has stimulated the interest of researchers in Lab-on-Fiber (LOF) technology. In this framework, the integration of Metasurfaces (MSs) on the tip of the optical fiber (Optical Fiber Meta- Tip, OFMT) has represented a major breakthrough. Indeed, we showed that a suitably designed plasmonic OFMT biosensor significantly outperforms standard plasmonic ones due to the advanced light wave manipulation of MSs. Here, to further improve the sensing performances, we propose a novel class of LOF optrodes for labelled biosensing based on dielectric fluorescence enhancing OFMT. We envision a single fiber probe with integrated a Silicon MS on its tip as a light coupled substrate that illuminates the sample and simultaneously collects the enhanced emission from the dye molecules labeling the biological target. We present a numerical environment to compute the fluorescence enhancement factor collected by a multi-mode-fiber, when on its tip a Silicon MS is laid, consisting of an array of cylindrical nanoantennas. According to the numerical results, a suitable design of the dielectric MS allows for a fluorescence enhancement up to three orders of magnitudes. Moreover, a feasibility study is carried out to verify the possibility to fabricate the designed MSs on the termination of multimode optical fibers using electron beam lithography followed by reactive ion etching. This work provides the main guidelines for the development of advanced LOF devices based on the fluorescence enhancement for labeled biosensing.

We propose and experimentally demonstrate a fiber refractometer based on a C-shaped fiber and the Vernier effect. The sensor is fabricated by cascading a single mode fiber (SMF) pigtail together with a C-shaped fiber segment and another SMF segment. Thus, the C-shaped fiber would constitute an open cavity (sensing cavity) in which test analytes could be filled, while the SMF segment would constitute another reference cavity. Due to the similar optical path length of these two cavities, Vernier effect would be activated, thus forming spectral envelops in the reflection spectrum of the sensor. Variations in the refractive index (RI) of analytes would result in the shifts of the spectral envelops. Experiments are carried out in the characterization of the sensor measuring gaseous analytes. The sensitivity of the sensor is found to be ~37238 nm/RIU for gas RI measurement. The proposed sensor features the advantages such as ease of fabrication, extremely high sensitivity, capability of sensing of both gaseous and liquid analytes, small footprint, and good mechanical strength.

The demand for highly sensitive, fast and low-cost biosensors for reliable quantification of small biomolecules or cancer biomarkers is leading to the development of a new class of devices able to change the techniques currently used for diagnosis in oncology. Lab‐on‐fiber (LoF) optrodes offer several advantages over conventional techniques for point‐of‐care platforms aimed at real‐time and label‐free detection of clinically relevant biomarkers. Moreover, the easy integration of LoF platforms in medical needles, catheters and nano-endoscopes offers unique potentials for in vivo biopsies and tumor microenvironment assessment. Here, we demonstrate the capability to improve the immobilization strategies through the use of hinge carbohydrates by involving homemade antibodies that demonstrated a significantly improved recognition of the antigen with ultra‐low detection limits. In order to create an effective pipeline for the improvement of biofunctionalization protocols to be used in connection with the LoF platform, here we first, optimized the protocol using a microfluidic Surface Plasmon Resonance device. Then we transferred the optimized strategy on LoF platform, based on Optical Fiber Meta‐tip (OFMT), for the final validation. As a clinically relevant scenario, we focused on a serological biomarker, Cripto‐1, for its ability to promote tumorigenesis in breast and liver cancer. Reported results demonstrate that the proposed approach based on oriented antibody immobilization is able to significantly improve Cripto‐1 detection with a ten‐fold enhancement versus the random approach. Therefore, our work opens new avenues in the development of high‐sensitivity LoF biosensors for the detection of clinically relevant biomarkers in the sub‐ng/mL range.

This study investigates the use of U-shaped cladded plastic optical fiber (POF) and glass optical fiber (GOF) probes for refractive index (RI) sensing as they allow a simple one-step fabrication process. The RI sensitivity of the U-shaped cladded POF probes were evaluated in narrow (1.333 to 1.348, increment of 0.003 RI) and intermediate RI ranges (1.34 to 1.39, increment of 0.01 RI). No considerable improvement or drop was observed. However, the U-shaped silica cladded GOF showed 1.3 and 1.54 -fold improvement in the RI sensitivity in comparison to the decladded probes in the narrow and intermediate RI range respectively. The highest sensitivities for cladded POF and GOF probes in the intermediate RI range were 4.7 and 10.8 ΔA_{530 nm}/ΔRIU respectively.

This proof-of-concept study demonstrates a seed-mediated growth technique to synthesize gold film on a U-bent fiber optic sensor probe for plasmonic sensing applications. Here, gold seed nanoparticles were physisorbed on the surface of a U-bent silica probe. Later the seed-immobilized probe was incubated in growth solution for gold film growth. The newly fabricated gold film-coated probe exhibited a surface plasmon peak at 655 nm wavelength and sensitivity of 2271 nm/RIU and a figure of merit of 22.

In this work, we propose a novel immunoassay platform for the detection of human Thyroglobulin (Tg) to be integrated with fine-needle aspiration biopsy for early identification of lymph node metastases in thyroid cancer patients. The sensing platform detects Tg (a well-known biomarker for the classification of metastatic lymph nodes related to thyroid cancer) by a sandwich immunoassay involving a self-assembled surface-enhanced Raman scattering (SERS) substrate assisted and empowered by functionalized gold nanoparticles enabling additional Raman signal amplification and improved molecular specificity. The sandwich assay platform was preliminary validated in a planar configuration and a detection limit as low as 7 pg/mL was successfully achieved. The sandwich assay was successfully demonstrated on washout fluids of fine needle aspiration biopsies from cancer patients and confirmed the high specificity of the proposed methodology when complex biological matrices are considered. Finally, optical fiber SERS optrodes were fabricated and successfully used to detect Tg concentration by applying the same bio-recognition strategy. This opens the possibility of transferring the Tg detection approach to the optical fiber tip to develop point-of-care platforms that can be directly integrated into fine needle aspiration biopsies.

Surface Plasmon Resonance (SPR) based Optical fiber sensors are ubiquitously used for varied applications in biosensing, food monitoring, water quality, gas sensing due to their simple, low cost, and miniaturised setup. The improvement in the sensitivity of these sensors is a critical challenge. 2-D MXenes are found to provide enhanced sensitivity for plasmonic biosensing because of high hydrophilicity, metallic conductivity, abundance of functional groups for bio conjugation, and wide band optical absorption. Here, we demonstrate the enhancement in the sensitivity of MXene (Ti₃C₂) coated multimode SPR optical fiber sensor by resolving the response of two different Refractive indices values. The effect of MXene concentration and the number of layers has been investigated. The introduction of MXene has caused a noticeable enhancement in the sensor’s sensitivity. Further, Ti₃C₂ has also been combined with Terbium based metal-organic framework (Tb-BTC) to prepare a sensor for haemoglobin. Tb-BTC binds to the iron fragment of haemoglobin assisting a selective response from the above developed SPR sensor. The response of the sensor for haemoglobin has been studied from 100 – 500 μg/mL. The sensor is also specific even in co-presence of other analytes such as immunoglobin, glucose, uric acid, and bovine serum albumin.

Optical fibers have been explored as sensors for application in different fields, including diagnostics, food security, and environmental monitoring, due to their low size and weight, flexibility and electrical safety. Particularly, optical fiber biosensors based on surface plasmon resonance (SPR) are broadly reported given their rapid, label-free and highly sensitive sensing ability. Among them, plasmonic unclad tips are a cost-effective tool, in which light is reflected by the metal at the fiber end-face, eliciting SPR and doubling the optical path. Tips are recognized for their sensitivity, portability and mechanical strength, being more suitable for in-situ sensing compared to fragile or less compact sensors. In this work, a numerical model was developed to simulate the SPR curve of Au-coated tips (Au-tips) to readily predict their optimized design parameters and therefore enhance their sensitivity. The algorithm determined the reflected power using the three-layer Fresnel equation for p-polarization, with the SPR response being simulated for solutions with different refractive index (RI) surrounding the sensing area. The performance was then assessed by determination of the sensitivity. This way, a relationship was established between the sensor’s performance and its parameters, namely, the ratio between sensing region length and core diameter, numerical aperture and Au thickness. This simple method provided the idealized design parameters for an Au-tip sensor, which might have application for RI monitoring and as a highly sensitive biosensor.

It is shown here that measurements of a tilted fiber Bragg grating with a single-sided gold coating using an unpolarized light source and no polarization control in the interrogation path can be used instead, thereby considerably facilitating both the fabrication of the grating sensor and simplifying the interrogation system requirements. A 10 degree tilt, 1 cm-long grating with Bragg wavelength near 1610 nm and a single-sided deposition of a 50 nm gold layer results in well-separated TE-HE and TM-EH mode groups with minimum and maximum sensitivity to surrounding refractive index changes, respectively. In these conditions, a well-defined SPR resonance is observed in the transmission spectrum as well as the position of the cladding mode cutoff. The differential sensitivities of mode group resonances in spectrum slice provide clear signatures of surrounding index change, both from surface effects on the gold layer and from cutoff wavelength shifts, thereby providing multi-resonant data and more accurate sensing results.

In this work, we report on the refractometric sensing properties of an interferometer produced by fusion-splicing a segment of a seven coupled-core fiber to a standard single-mode fiber. We demonstrate that for a ~10 nm-thick gold layer deposited on the multicore facet, the interferometer amplitude and response to refractive index changes is polarization dependent. We thereby show that refractometric sensitivity is maximum for the polarization-induced minimum amplitude spectrum.

Ultrasensitive nanomechanical instruments, e.g., atomic force microscopy (AFM), can be used to perform delicate biomechanical measurements and reveal the complex mechanical environment of biological processes. However, these instruments are limited because of their size and complex feedback system. Here, we demonstrate a miniature fiber optical nanomechanical probe (FONP) that can be used to detect the mechanical properties of single cells. The stiffness matching of the FONP and sample can be realized by customizing the microcantilever’s spring constant. As a proof-of concept, three FONPs with spring constants varying from 0.421 N/m to 52.6 N/m by more than two orders of magnitude were prepared. The Young's modulus of heterogeneous soft materials, such as polydimethylsiloxane, onion cells and MCF-7 cells, were successfully measured. FONP has made substantial progress in realizing basic biological discoveries, and our strategy provides a universal protocol for directly programming fiber-optic AFMs.

We demonstrate Distributed Acoustic Sensing (DAS) with extended distance range utilising repeaterless all-optical amplification and single-side interrogation schemes. This work addresses the need for over 200 km range of distributed fibre-optic sensing in applications where very long assets need to be monitored, e.g., subsea power cable monito ring. A commercially available DAS interrogator and an advanced prototype interrogator setup with increased system performance are used. Both are based on coherent OTDR to detect Rayleigh backscattering. Range extension is achieved by compensating optical losses with amplifying the pulse traveling down the fibre as well as the Rayleigh backscattered signal coming back to the interrogator. We do this by launching CW pump light into the sensing fibre to create a combination of distributed Raman amplification and a remote optically pumped amplifier in an erbium doped fibre. We analyse the DAS interrogator’s ability to detect acoustic events at distances between 170 km and 200 km of ultra-low loss telecom fibres in a quantitative and linear way. To this end, events are simulated by periodically modulating the length of a short fibre at those distances with a piezo fibre stretcher. Results show that the thereby created optical phase shift is correctly measured and that amplitude and frequency of the applied signal are successfully reconstructed. It is thereby proven that singleended DAS is possible with the presented interrogators and all optical amplification schemes to achieve at least 200 km range.

Measuring acoustic waves propagation in solid or fluid media is an important task in applications such as Structural Health Monitoring (SHM), seismology, oceanography, underwater acoustic communications and more. While there are quite a few acoustic sensors that are considered to be highly sensitive and broadband, such as geophones for seismic applications or hydrophones for underwater applications, they are all point sensors. Point sensors are limited since they cannot provide spatiotemporal measurement of propagating acoustic waves. In addition, their coverage volume is limited due to the attenuation of the acoustic waves in the medium. These limitations can be alleviated by using an array of acoustic sensors which can provide the required spatiotemporal measurement capability in addition to extended detection volume. This work describes the implementation of an underwater fiber-optic sensor array for ultrasonic (US) waves. To overcome the well-known trade-off between update rate and sensing fiber length a Coded Array Matched Interrogation (C-AMI) method was implemented. The method enabled an enhancement of the theoretical sampling rate by a factor of 54. The system successfully measured the propagation of an ultrasonic pulse with a carrier of 95kHz along a 20m long test pool.

We demonstrate the potential of Brillouin distributed fiber sensor for the measurement of groundwater flow in an experimental site of Port Douvot close to the city of Besan¸con. The flow measurement is obtained by using active heating method based on heat pulse instrument. An industrial sensor cable with single mode fiber and multimode fiber was immersed on ground. We compare distributed Brillouin sensor reflectometry (BOTDR) and Analysis (BOTDA) on single mode fiber (SMF) and multimode optical fiber (MMF) with a spatial resolution of 1 m, a temperature resolution of 0.2 °C and an acquisition time of 1 min. These parameters are compatible with hydrology application. Active heating of borehole water in conjunction with fiber optic distributed temperature sensor measurements are realized. Contrary to Raman based distributed temperature sensor, Brillouin instrument allows measuring absolute temperature measurement and simplify the implementation on the setup. We demonstrate in this paper that Brillouin scattering based temperature sensor can be used for hydrogeology application.

A low-noise φ-OTDR sensor is developed by adding a 2-stage pre-amplification system mixing linear and nonlinear amplification in a 3x3 Michelson interferometric configuration. Its performance is evaluated by deploying it as a distributed acoustic sensing system in a survey well (sampling rate: 10 kHz), showing noise floor levels below 1 nε

In this work, it is presented an analysis of FBG arrays installed in a public road. The arrays were installed in a newly paved urban road and were monitored for more than one year. The study evidences the permanent deformation of the wearing course and the degradation of the reflected spectra of the sensors.

This paper introduces a novel method to simultaneously measure the cores of a multi-core fiber, enabling higher acquisition rates in shape sensing. The two-dimensional shape of the optical fiber is determined from the distributed strain measurements performed with the optical frequency domain reflectometry technique.

This paper reports on the experimental characterization by means of optical frequency-domain reflectometry of a White-type multipass gas cell used for trace gas spectroscopy. The fractional Lambertian reflections inevitably arising from the three high reflectivity mirrors of this multipass cell is precisely detected due to the high sensitivity of the reflectometer. Each bounce of light on the mirror surface generates backscattered light, which returns to the sensing system. Then, using the measured distribution of multiple back-reflections as a function of distance the position of the 3mm-thick CaF₂ entrance window is clearly identified, thanks to the spatial resolution of 731μm. In addition, the physical distance between mirrors at both sides of the cavity is accurately assessed to be 40.72cm, delivering the exact optical path length of light inside the multipass cell of 30.9853m, which is an important parameter for improving the accuracy of the computation to retrieve the gas concentration from the measured light absorption spectrum.

Modern process industry, particularly (petro)chemical industry faces, many challenges pertaining to sustainability. In this respect, more stringent regulations on reducing emissions are motivating plant and process owners to implement condition monitoring and predictive maintenance strategies. Bolted flange connections equipped with sealing gaskets, for example, can be a significant source of emissions and their performance remains often ambiguous under modern standards. Gasket stress is a key performance indicator of a bolted flange connection, which is typically approximated using methods that rely on many simplifications and assumptions. This study investigates the potential of using fiber-optic sensors, more specifically fiber Bragg gratings, as strain sensors to estimate gasket stress in bolted flange connections with gaskets. To the best of our knowledge, it is the first time that said gaskets are instrumented with fiber Bragg gratings. For our experiments, we submit these gaskets to relevant mechanical loads, both in a laboratory setting and in a realistic industrial environment. We analyze the relation between the fiber Bragg grating response and the applied mechanical load to define transfer functions that allow estimating the gasket stress and hence the sealing performance of the flange connection.

ITER is a tokamak-based fusion device where the knowledge of plasma current is crucial for its safe and successful operation. A polarimetric optical fibre sensor, installed around a section of the ITER Vacuum Vessel (VV), can provide a measure of the plasma current, by exploiting the Faraday effect-induced state of polarization (SOP) rotation of the light launched into the sensing fibre. In the system discussed here, spun fibre is used as the sensing fibre and a polarization-sensitive reflectometer (PSR) is used as the interrogator. In this paper, we analyse the impact of the ITER VV wall ambient temperature on the sensor’s plasma current measurement accuracy, when the other inevitable perturbation effects, namely fibre bending, and twisting are also considered. As ITER is not yet operational and there is no practical way of imitating the ITER operating conditions, we resort to the Jones formalism-based simulation approach to estimate the minimum required L_B/ S_P ratio of the spun sensing fibre that satisfies the ITER plasma current measurement specifications, under the considered perturbation effects.

We present a novel amplified space-time coding technique which combines cyclic-Simplex and Simplex binary codes to overcome the main limitations of conventional coding for OTDR based ultra-long distance distributed temperature sensing applications. The decoding process is performed in two successive steps, addressing the main issue related to the computational complexity of conventional codes, which increases quadratically with the code lenght, seriously affecting their performance when dealing with extremely long code-words. A link control technique is also proposed to suppress gain transients induced by the EDFA dynamics, avoiding performance degradation due to nonlinear effects and codewords distortion. The proposed scheme provides significant coding gain enhancement and stable operations below the stimulated Raman scattering threshold, pushing the performance of Raman based distributed temperature sensors close to their physical limit using commercial off-the-shelf components.

We highlight the importance of the laser source phase noise in sensing applications and show that the standard Lorentzian linewidth criterion is not sufficient to characterize the performance of a sensing system. We then derive a laser linewidth related to the phase noise spectral region of interest, according to the length of the fiber to sense. This is illustrated in a setup based on coded interrogation and with two sensing dedicated laser sources.

Optical fiber sensors are becoming a prominent solution to perform shape sensing thanks to their interesting and well-known advantages such as flexibility, lightweight, sensitivity, etc. However, the different approaches commonly used to date, which include fiber Bragg gratings, OFDR or OTDR architectures, are not able to achieve a fully distributed and fast performance over relatively long ranges (e.g., tens of meters) and with fine (cm-scale) resolution. Here, we present a novel scheme to perform curvature sensing attaining all the previously mentioned features by application of time-expanded phase-sensitive (TE-Φ)OTDR technology. TE-ΦOTDR is a promising distributed sensing technique that delivers a performance ranging between that of OFDR and ΦOTDR. As a proof-of-concept, we interrogate three cores of a multicore fiber (MCF) using TE-ΦOTDR, attaining curvature sensing with 10 cm resolution over a maximum measurable range of 125 m and a sampling rate of 50 Hz. The implementation of shape sensing schemes with the performance provided by TE-ΦOTDR technology may open the door to new and interesting applications in civil engineering, medicine and seismology.

From Distributed Acoustic Sensing (DAS) measurements over deployed Multi-Core Fiber (MCF), we discuss several signal processing options to enhance the sensing sensitivity, namely core combination and longitudinal averaging.

Fibre-optic based sensing technologies are becoming popular in the field of geophysics since enable long range and high spatial resolution acoustic measurements. In this work, we present preliminary results obtained using quasi-distributed Fibre-Bragg grating sensing and Distributed Acoustic Sensing (DAS) to monitor seismic activities in an operational underground mine. 12 FBGs and 800 metres of fiber optic cable was installed in the tunnel lining an operational mine and recorded mine seismicity such as production blasts and a small seismic activity of magnitude 1.41 in September 2022.

In this work we utilize multimode optical fibers for the detection of simulated errors or failures in underground power cables. It is known that in cases of failure the underground transmission cables overheat locally, they become a hot-spot, and it is extremely difficult to detect and locate the problem. The proposed methodology is as follows, having an underground electric cable we simulate various temperature profiles whilst the optical fiber was placed in selected distances away from our simulated fault to examine the detection performance of our fiber. In this way we aim to stabilize the operation of the underground cable damage detection system that is placed by the Electricity Authority of Cyprus. The EAC has certain locations where the existing single-mode optical fibres are collocated with the underground power cables, although relative spacing may not be constant. Our data will give an indication of how important is uniform spacing between power and optical cables. We examine if any change in the temperature of the power cable is also reflected in the optical fibre cable. The real-time and continuous monitoring of the temperature of the optical cables through the distributed sensing systems may help identifying abnormal cable behaviour (hot spots) and possible future network failures in the power network.

We report on the development of a field-proven distributed fiber optic sensing system for structural health monitoring in road construction and civil engineering. The system is based on a cost-efficient digital incoherent optical frequency domain reflectometry (I-OFDR) for distributed strain detection along a polymer optical fiber. In this method, the strain-induced backscatter increase in a graded-index multimode perfluorinated polymer optical fiber (PF-POF) is determined by measuring the complex transfer function of the sensing fiber using a compact digital data acquisition unit. This unit replaced an oversized vector network analyzer (VNA) usually used in the I-OFDR technique making the measurement system more robust and more suitable for its use in the field, whilst at the same time providing a significant reduction of the total sensor system costs. This paper presents a successful implementation of the entire sensor concept in a real construction project for the soil reinforcement and fiber optic monitoring of a road embankment using geosynthetics with incorporated PF-POF. The presented research includes development and installation in the field of the sensors integrated geosynthetics used for ground stabilization and additionally providing fiber optic POF-based monitoring function. The publication shows first measurement results of the further developed measurement method I-OFDR conducted along the PF-POF integrated in the geosynthetics embedded into a road embankment of the federal road B91 south from Leipzig.

A fiber optic health-monitoring system for refractory lining in steel-making processes is presented. Its applicability as an early-warning system for lining damage is demonstrated by the results obtained in a field trial, in which 240 m of fiber was embedded in the lining of an electric arc furnace. The system is based on Raman distributed sensing and polyimide coated fibers in metal tube. The results presented from temperature cycling and calibration at temperatures up to 600 °C show that adequate accuracy and stability for the application can be attained.

Wavelength scanning coherent optical time domain reflectometer (WS-COTDR) is a good candidate to spatially resolve the environmental information at comparatively low frequency. This paper reveals it can also work as a traditional optical time domain reflectometer (OTDR) to identify Fresnel reflection by averaging the obtained signal over the wavelength scanning range. Simultaneous distributed vibration sensing and a traditional OTDR measurement are experimentally demonstrated using the WS-COTDR system.

In this study, an optical fiber, buried in the trackbed during track renewal, is used to measure the vertical support and pressure distribution of the track. The pressure distribution was obtained using a distributed acoustic sensor (DAS) system by measuring the pressure-induced strain changes on the sensing fiber. The experimental results obtained by the DAS system show a good agreement with the numerical model of the trackbed.

We present a long-range Brillouin optical time domain reflectometer (BOTDR) based on photon counting technology using single-mode fibres. We use the slope of a fiber Bragg grating (FBG) as a frequency discriminator, in order to convert count rate variation into a frequency shift. We demonstrate experimentally the ability to perform a distributed temperature measurement, by detecting a hot spot in a thermal bath and the possibility to achieve measurement above 100 km with a spatial resolution of 10 m. A performance study of our distributed sensor as a function of the photon counter efficiency is also presented.

In this paper, we make use of a phase-sensitive time domain reflectometry (phi-OTDR) sensor with 60-cm spatial resolution to detect the Lamb waves generated by a piezo-ceramic actuator in an aluminum plate. Furthermore, a machine learning algorithm based on Support Vector Machine (SVM) classifiers was employed for damage localization. We show that SVMs are able to identify the characteristics in Lamb wave signals that may be linked to damage location. This study makes full use of the rich information provided by the phi-OTDR sensor, extracting damaged data from diverse damage spots. The results indicate that the proposed technique has the potential to identify and locate damages in thin-plate structures.

This paper presents a novel technique for the distributed measurement of the modal birefringence in a few-mode fiber (FMF). The method exploits two different phenomena observed in distributed Brillouin measurements: the dependence of the Brillouin frequency shift (BFS) on the effective refractive index (ERI) of the interacting optical beams, and the spatial oscillations of the Brillouin gain deriving from multimodal interference. Using both phenomena, a wide range of ERI separations can be measured, from ≈ 10^-7 to 10^-2 or more. The measurements have been carried out over a two-mode graded-index FMF, using two photonic lanterns to selectively excite the desired spatial modes. We use the BFS measurements to derive the ERI difference between the LP01 and LP11 mode groups, while the spatial oscillations of the Brillouin gain reveal the birefringence between the vector components (TE₀₁, TM₀₁ and HE₂₁) of the LP₁₁ mode group. The experimental measurements are partly supported by full-vector finite-element-method (FEM) simulations. The reported method may also find application in the field of distributed sensing, by taking advantage of the dependence of modal birefringence from physical parameters such as strain and temperature.

We present the results of an experimental campaign aimed at demonstrating the use of a distributed optical fiber sensor based on Brillouin scattering in static and dynamic temperature measurements at cryogenic temperatures (≈ 84 K). The experimental results, obtained through Brillouin Optical Frequency-Domain Analysis (BOFDA) at a spatial resolution of 16 mm, are compared with temperature measurements using thermocouples and fiber Bragg gratings. The distributed sensor is able to capture local temperature variations of ≈ 2 °C at an acquisition rate of 1 Hz.

Distributed Temperature Sensing (DTS) has been widely used for infrastructure monitoring. Most common applications are pipeline leak detection prevention and geohazard mitigation as well as power cable thermal rating. The study of the soil-atmosphere thermal interaction reveals that natural phenomenon can be monitored with DTS and buried communication Optical Fiber Cables (OFC). The current article discusses the application of DTS to the monitoring of the effect of Soil-atmosphere thermal interaction showing annual and daily variations. DTS data from over 10 years is analyzed, allowing for the observation of the El Niño 2014-2016 event, which is among the strongest of the recent El Niño Southern Oscillation (ENSO) occurrences. It illustrates how DTS technology and communication backbone can provide data to study environmental effects at a global level.

Fiber distributed sensing based on Rayleigh, Brillouin or Raman backscattering is just over 40 years old. However, it took almost half of that time to transform physical concepts into measuring instruments and another 10 years to achieve permanent and reliable deployment in the field. Through this lengthy but classical process, Rayleigh, Brillouin and Raman became Distributed Acoustic Sensing (DAS), Distributed Temperature Sensing (DTS) or Distributed Strain Sensing (DSS), standards were written, and multiple software tools were developed to handle the ever-growing amount of measured data. In this paper, we first illustrate some of the fundamental steps required to go from the physics to the interrogators, in particular the importance of standardization with the associated common language and reference tests. Then we describe how to move from the interrogator to the field with the use of fully automated, reliable and self-diagnosed interrogators, including the needs for communication and data management. Eventually, we show how to obtain meaningful data from the field through recent deployment examples in the power cable industry, together with some of the typical software tools.

Distributed optical fiber vibration sensors (DOFSs) such as those based on phase-sensitive optical time-domain reflectometry (φ-OTDR) have contributed to improve structural health monitoring (SHM). φ-OTDR allows distributed vibration sensing by analysing the interference properties of the backscattered/reflected signal when an optical pulse is launched into the sensing fiber. As the Rayleigh backscattered light is relatively weak, fiber Bragg grating (FBG) arrays can be inscribed in the sensing fiber to increase the signal-to-noise ratio. However, the interference of the signals reflected by two consecutive FBGs (containing information about the locally applied vibration) is subject to a polarization fading effect as the sensing fiber presents some birefringence. Therefore the states of polarization (SOP) of the interfering reflected signals are no longer fully aligned. The present work proposes a measurement setup to quantify the polarization effects in a direct detection phase-OTDR scheme through the polarization fading sensitivity (PFS) parameter.

Geometric phase measured per beat period in a Φ-OTDR based on coherent heterodyne detection is used to measure strain. Proposed method is robust to polarisation mismatch fading as a polarisation mismatch between interfering beams is not a hindrance to the measurement of the geometric phase. The Geometric phase is a function of the intensities of the interfering beams as well as the envelope of the beat signal. Its calculation does not require phase unwrapping and accordingly does not suffer the phase unwrapping errors. It is required to be equated with the traditionally measured phase by applying a scaling factor. The spatial resolution of the measured strain is reduced as it is calculated per beat period. Results are verified using a piezo-electric transducer inline a fiber-under-test.

Time-expanded phase-sensitive (TE-φ )OTDR is a distributed optical fiber sensing (DOFS) technique that takes advantage of the dual-frequency comb technology to offer distributed, dynamic, and high-spatial resolution measurements. The performance delivered by this recent approach is unmatched by any other DOFS, combining the high resolution of OFDR with the potential for long range and fast sampling of φ OTDR. In this contribution, we present an optimized TE-φ OTDR scheme with important improvements with respect to the traditional one. In particular, the new architecture uses electrooptical phase modulation instead of intensity modulation, increasing the energy-efficiency. Additionally, it employs an optical hybrid to double the spectral efficiency of the system, which in practical terms results in doubling the spatial resolution for the same interrogating comb bandwidth. The proposed architecture has been experimentally validated through a scheme providing 5 mm of spatial resolution, 80 m of range and 70 Hz sampling rate with a simple, compact and low-cost setup using field-programmable gate arrays (FPGA) and relatively low bandwidth photodetection (2 MHz).

In this contribution, the method of Code-division Multiplexing (CDM) is investigated for its dynamic measurement capabilities. In earlier publications, this technique has already been shown to be capable of measuring thousands of Draw Tower Grating® (DTGs®) in a single fiber, where many FBGs with identical wavelengths are used. The basics of CDM are explained. In addition, the ability to do dynamic measurements is investigated theoretically and experimentally and the results are presented. Good correspondence between theory and experiment could be found. Possible system improvements are proposed to find a suitable compromise between detection accuracy and system speed for massive optical sensor networks.

Chirped-pulse phase sensitive (CP-Φ) OTDR is a distributed sensing technology that allows for quantitative measurement of strain and temperature along an optical fiber by simply direct detection of the Rayleigh backscattering. Typically, chirped pulses have a linear frequency modulation covering few GHz. Backscattered traces must be amplified before detection, which introduces noise and limits the signal-to-noise ratio (SNR) and, therefore, the maximum measurable range. To increase the SNR, an optical filter is usually placed before photodetection aimed at reducing broadband optical noise caused by amplified spontaneous emission. However, narrow-band filters (e.g., 10 GHz bandwidth) are not easily compatible with multi-wavelength approaches, used to improve the long-term stability. Furthermore, in practice it is not straightforward to find narrowband optical filters that continuously match the central frequency of the laser, considering laser wavelength drifts. In this study, the influence of the optical filter bandwidth on the range in CP-ΦOTDR is theoretically investigated for two types of photodetection: direct and coherent. The results show that when using coherent detection, the SNR does not depend on the filter bandwidth. Therefore, it is possible to achieve an equivalent measurement range by using a wide optical filter (e.g., 100 GHz) as compared to that obtained when using direct detection with a narrowband filter. This finding suggests that coherent detection can be used to increase the range in CP-ΦOTDR and could be compatible with the use of multi-wavelength techniques to improve the long-term stability for applications such as civil engineering and seismology.

Although ground anchors are widely used in fields of civil engineering, as in soil and rock stabilization or anchoring structures, such as excavation pits, retaining walls or tunnel constructions, the load transfer behaviour has not been entirely investigated yet. Ground anchors are usually monitored by load cells at the anchor head or by strain gauges at selected points along the anchor, which both do not deliver reliable information along the entire anchor. Distributed strain sensing provides an opportunity to get the strain information along the entire length of the anchor, on the tendons as well as in the grout, as we have already shown in a preliminary anchor test. However, there were still lots of issues, which did not allow a general conclusion for all anchor types and ground conditions. Thus we investigated further ground anchors in different soil conditions (clay, rocklike material and gravel), within a research project. This paper gives an overview of the sensor systems used, the obtained results of the anchor pullout tests, gathered experiences and finally gives a brief concept of a monitoring anchor for long term monitoring.

The backscattering process in hollow core fibres shows a large similarity with Rayleigh scattering, offering the potential to be exploited for distributed sensing. A classical Φ-OTDR implementation is used to observe the backscattering signal from the surface roughness at the silica-air interface in hollow-core photonic bandgap fibres. In contrast with standard single mode silica-core fibres, the hollow core photonic bandgap fibre shows a chaotic response when the temperature is slightly changed, but stable results under strictly constant temperature conditions. Another temperature-dependent effect is highly perturbing the coherent scattering response, and it is believed that higher-order guided modes cause detrimental interferences totally jamming the response. By using single-mode hollow core fibres it should be in principle possible to obtain the relevant temperature measurement pattern, though as anticipated the extreme weakness of the signal certainly represents an insurmountable challenge.

An extrinsic fiber Fabry-Perot interferometer (EFFPI) is proposed and implemented for the characterization of waveguides inscribed by the femtosecond-laser direct writing technique. Various waveguides of 7.08 mm in length were inscribed in a soda-lime glass substrate by varying the laser scanning velocity in order to induce different refractive index (RI) changes. The measurement of the RI of the waveguide was carried out by means of an EFFPI formed between the end face of a multicore fiber (MCF) with seven coupled cores and the two polished surfaces of the inscribed waveguide. The end face of the MCF tip provided a broad beam and a wide effective area to ensure a large collection of the reflected light. The optical spectrum obtained by the interference of the multiple reflected beams was transformed to the Fourier domain and visualized in real-time. In the Fourier domain, the waveguides’ optical path length (OPL) was obtained and used to calculate its refractive index since its physical length was fixed and known. To obtain the value of the RI of the medium surrounding the waveguide, the fiber tip was displaced parallel to the polished surface, about 40 μm far from the center of the waveguide. The relative refractive index difference (Δn), which defines the light propagation and the insertion loss in a waveguide, was calculated for each waveguide inscribed at different velocities.

A fiber Bragg grating has been inscribed in a 100-µm diameter sapphire optical fiber with the phase mask technique and a fs-laser emitting at 800 nm. The grating was placed inside a sealed alumina capillary to protect the fiber from the environment. Then the fiber was set inside an oven and cycled up 7 times to a maximum temperature of 1500°C during 2 h. We observed that after two cycles, the grating is stabilized and no more hysteresis on the Bragg wavelength is observed. However, the temperature uncertainty is as high as 15°C and is principally due to modal interference. Then the grating is submitted to a 3-day annealing and two annealing successive 4-day annealing – for a total of eleven days – at a temperature of 1500°C. During this treatment, the grating amplitude remained constant and the Bragg wavelength showed no significant drift. As a conclusion, the packaged grating did not exhibit any erasure during these annealing experiments and perform reliable temperature measurement up to 1500°C.

A simplified model describing the polarisation characteristics of spun fibres is proposed, aiming at determining how close to a circularly birefringent medium such a fibre is. This is of crucial importance regarding the interest of such a medium for magneto-optic sensing using optical fibres, mostly exploited in fibre optics current sensors (FOCS). The spun fibre is modelled as a stack of thin linearly birefringent plates, each experiencing a slight incremental rotation. The eigenvalues and eigenvectors of the full stack of plates are determined, enabling us to assess how far a given spun fibre is from an ideal circularly birefringent medium for any spinning rate and linear birefringence. Experimental validation is carried out to compare the model results with real fibres.

Pairs of arrays of point-by-point fiber Bragg Gratings written with a femtosecond laser were anchored with K-type thermocouple within an asymmetrical titanium substrate. Three anchoring methods were used, including silica based, graphite based glue commercially available as well as Yttria Stabilized Zirconia deposited by Atmospheric Plasma Spraying. The optical fibers were first recoated with an inorganic coating, specifically developed to resist to high temperatures above 800°C. The titanium plate was exposed to an intense heat flow delivered by tungsten halogen lamps to reach temperature of around 800°C. High temperature and strain measurements was performed in situ with a relative error of less than 10% at 800°C.

A long period grating (LPG) operating in the visible wavelength range functionalised with sensitive dyes is reported. Two different sensing mechanisms transmission based, using intensity ratio; and refractive index change based, using the PMTP attenuation bands shift are presented. The visible range LPG’s with phase matching turning point (PMTP) ~700 nm was fabricated using amplitude mask laser inscribing approach on a hydrogen loaded SM600 fibre. Ammonia sensitive thin film based on diazonium resin (DAR) and tetrakis-(4-sulfophenyl)porphine (TSPP) dye was deposited via layer-by-layer deposition on LPG fibre and glass substrate. Glass substrate coated by DAR-TSPP was used to calculate the complex refractive index (RI) difference. The response of the visible LPG was compared to an infrared LPG with a same coating.

We demonstrate a polarization analyser based on processing of speckle patterns generated by a scattering medium. Each speckle pattern at a given wavelength and polarization state is unique and deterministic, and thus the polarization angle alters the speckle pattern motif. The polarization state of a given input light is obtained using reconstructive linear algebra methods. The system consists of a femtosecond laser written scattering chip and a CMOS sensor and contains no moving parts, making the proposed solution is low-cost and compact. The linear polarization angle was accurately reconstructed over a 0-20° test range, with 6 arcminutes (1/10° ) standard error. To demonstrate an application as a polarimeter, we used the system to measure Faraday rotation in a SF59 lead silicate glass within an electromagnet. The magnetic field was successfully traced by determining the induced changes in the input beam’s linear polarization angle in the range 0-80 mT with 10 mT standard error.

Sapphire fiber Bragg gratings (SFBG) were used for temperature diagnosis of a commercial inductive heated furnace of a fiber draw tower. In a fiber preform temperature gradients > 10K/mm were detected. In the temperature range from 1000°C to 1900°C the measured temperature inside the furnace is about 140K below the control temperature detected by a pyrometer on a spot of the heating element. The dependence of applied temperature on the furnace heating power was derived that can allow furnace control without pyrometer and therefore extending operation range below 1000°C. No degradation of SFBG was observed.

We propose and demonstrate a vector vibration sensor with high-temperature resistance, consisting of a ring cavity laser and fiber Bragg gratings (FBGs) inscribed in seven-core fiber (SCF) by using femtosecond laser auto-positioning pointby- point technology. A vibration sensing probe is composed of three FBGs inscribed in the outer cores of SCF. Note that they have 120° angular separation in the SCF. Moreover, the FBG inscribed in the central core of SCF is employed as a narrow band reflector in the ring cavity laser, enabling the laser to achieve the ability of temperature and strain compensation. Such a proposed sensor can be used to measure vibration orientation and acceleration simultaneously. The sensing performance of this device was demonstrated and discussed. The results show that it has a working frequency bandwidth ranging from 4 to 68 Hz, a maximum sensitivity of 54.2 mV/g, and the azimuthal angle accuracy of 0.21°. Furthermore, we investigated the vibration responses at high temperature of this device, and the results demonstrate that the proposed vector vibration sensor can operate at 550 °C. Therefore, such a proposed vector vibration sensor can be applied in harsh environments, such as aerospace and nuclear reactor.

This article presents a novel approach for fabricating a temperature sensor by depositing nanoparticles on the surface of a plastic optical fiber (POF). The sensor utilizes the principle of surface plasmon resonance (SPR) and exploits the temperature-dependent changes in refractive index (RI) of the surrounding medium. The nanoparticles are deposited over the fiber surface using a micropipette, and the resulting sensor is characterized using a custom-built experimental setup. The sensor showed a linear response to temperature changes in the range of 10°C to 35°C, with a sensitivity of 0.457 nm/°C. The proposed sensor has several advantages over existing temperature sensors, such as low cost, ease of fabrication, and compatibility with a wide range of optical fibers. Overall, this work demonstrates the potential of using SPR-based sensors for temperature sensing applications, and paves the way for future research in this area.

In this work, we reported 2-mm-long fiber Bragg gratings (FBGs) in benzyl dimethyl ketal (BDK)-doped poly(methyl methacrylate) (PMMA) optical fibers by means of a 520 nm femtosecond laser and point-by-point FBG inscription technique. The highest reflectivity of ~99% is obtained with a pulse energy of 11.2 nJ, showing a large refractive index modulation amplitude of 7.2×10^-4 .

In this work, we reported highly reflective polymer optical fiber Bragg gratings (POFBGs) in poly(methyl methacrylate) (PMMA)-based polymer optical fibers (POFs) by means of a 266 nm pulsed laser and phase mask technique. Fiber Bragg gratings (FBGs) were inscribed with a single pulse up to 3.7 mJ. After post-annealing, a stable refractive index change up to 4.2×10^-4 was obtained. The behavior may mainly be attributed to the movement of initiating radicals arising from BDK under UV irradiation.

This work reports as a novelty the excitation of lossy mode resonance (LMR) phenomena in uncoated double cladding fiber (DCF), without the need of an external high refractive index thin film. The configuration chosen for this purpose is a DCF having W-type refractive index (RI) profile spliced between multimode fibers. Here, the outer cladding of the DCF acts as a high RI overlay permitting the generation of LMR. The tuning of the phenomenon is performed by changing the thickness of the outer cladding, e.g., through chemical etching. Resulting LMR device was characterized towards surrounding refractive index demonstrating a sensitivity around 300 nm/RIU in the range of 1.33-1.39, which makes this fiber sensor suitable for bio-chemical sensing applications. Such device exhibits advantages in terms of simplicity and cost with respect to traditional LMR devices.

This work proposes an in-fiber Mach-Zehnder interferometer (MZI) device which is fabricated by embedding a short section of double cladding fiber (DCF) in between two standard fibers utilising the core-offset splicing approach. Moreover, the DCF has a W-shaped refractive index (RI) profile, where the outer cladding has RI higher than inner one and core. Consequently, by modifying the thickness of the outer cladding, the mode transition of cladding modes from outer to inner cladding can be induced, with the possibility to tune the sensitivity to surrounding refractive index. Specifically, the outer cladding diameter was decreased by means of chemical etching down to a diameter of 112 µm and a sensitivity of -200 nm/RIU was achieved in the range 1.33-1.39, with a 2.5 gain in comparison to unetched fiber. The proposed sensing device has considerable compactness, low manufacturing cost and simplicity, as well as high sensitivity for future applications in chemical and biosensing domain and other related fields.

In this work, the potential of using general-purpose paints to fabricate highly reflective, low-cost optical mirrors is evaluated. The study shows that high-reflectivity mirrors can be created in standard single-mode fiber in a reliable, simple and economic manner using the appropriate metallic paint. Preliminary results confirm that the grain size of the metallic particles is a crucial factor in the reflective behavior, together with the substrate in which the particles are suspended. Moreover, interferometric patterns have been observed in some cases, which could lead to the creation of simple and economic fiber optic sensors.

We investigate the behavior and stability of fiber Bragg gratings written by femtosecond laser pulses in Ge-doped fused silica optical fibers, using both the phase mask and point-by-point techniques, during their annealing at 1200°C for 30 min and subsequent aging at 1000°C during 43 hours. Bragg wavelength drifts and reflected peak amplitude variations were shown to drastically differ depending on the writing scheme and thermal history. Particularly, we show that amplitude decay of point-by-point gratings at 1200°C may be easily mitigated by tuning the writing pulse energy. Future work may be pursued in order to finely unravel the high temperature mechanisms regarding the stability of fs-written fiber Bragg gratings used as temperature sensors in order to improve measurement stability and accuracy.

The proposed work has the aim to investigate a full analog electrical circuitry to convert the wavelength-encoded signal coming from a pair of Fiber Bragg Grating (FBG) sensors into a single monotonic electrical signal. The latter can be used either to be read from a PLC system (or directly by a switch) if a 4-20 mA signal is needed (e.g. for safety application) or to have an instantly conversion without employing the classical interrogation system with a post-processing by means of a digital unit. Since its peculiarities (robust, reliable and completely free from any digital processing section) the proposed system has the aim to overcome the classical interrogator, with the aim to pave the way to a wider employment of FGB sensor in that environment where the reliability given by the interrogator based on multiple digital processing unit, handled by an operative system, may be subjected to failure. In the proposed manuscript, the system was studied analytically and numerically, taking advantage of its characteristic to behaves linearly in a range of 200pm Bragg wavelength shifting, due to the Arrayed Waveguide Grating (AWG) device, used as optical filter. As results, the capability to perform compensated measurement, by means of 2 FBG subjected to different physical quantities, was investigated. The obtained formula comprises FBGs linear coefficient in function of the physical phenomenon to measure and the system output.

An approach for directional bending monitoring based on a multimode fiber and a machine learning algorithm is presented. The sensor if formed by splicing a single mode fiber to a multimode elliptical-core fiber. Using this elliptical-core fiber, multimode interference generates an interferogram with non-uniform amplitude and non-periodic shape. These characteristics are important to process the sensing signal using a machine learning algorithm. The machine learning algorithm implemented is the well-known random forest algorithm. In the reported experiments, the fiber is bended in different directions and different magnitudes of bending, generating a specific interferogram in each position, then each bending position is identified by the random forest algorithm. Once the position is identified, the trajectory of the sensor can be calculated. Experimental demonstration for directional bending monitoring, based on a machine learning algorithm, is presented.

A smartphone has been characterized as colorimeter, using both the built-in camera and a clip-on dispersive grating. Certified colorimetric samples provided by Labsphere® were considered as test samples. Color measurements directly performed by means of the smartphone camera only were obtained using the “RGB Detector” App, downloaded from the Google Play Store. More precise measurements were obtained by means of the commercially available GoSpectro grating and related App. In both cases, in order to quantify the reliability and sensitivity of smartphone-based color measurements, the CIELab color difference ΔE between the certified and smartphone-measured colors were calculated, and are reported in this paper. Also, as an example of practical application of interest for the textile industry, several samples of cloth fabrics with a palette of the most common colors were analyzed, and the comparison with the certified color values are presented.

Polymer microtips manufactured at the end face of standard optical fibers have been used for effectively coupling with a selected antiresonant fibers (ARFs). Four tested ARFs had similar geometry, 7 capillaries placed around the central air core. It was shown how such coupling structure modified the ARFs spectral characteristics. Applying polymer microtip successfully excited additional bands. Illumination of the ARF with a microtip that operates as a microlens causes a change in numerical aperture and mode number.

A water flow and velocity aluminum-coated Fiber Bragg Grating sensor system for open channels was designed, simulated and tested. The sensing head was designed, ruggedized and customized to measure velocities at different depths, in order to calculate the discharge in open channels. This paper shows, for the first time to our knowledge, the simulation of such kind of fiber sensors in open channels.

A new tunable fiber laser structure based on an erbium-doped fiber ring laser (FRL) and a polymer-based microbottle resonator (PMBR) as the wavelength selective filter is proposed and demonstrated. The tunability of the laser output in response to axial strain of up to 253.6 με applied to the PMBR is demonstrated experimentally. When the strain was applied to the PMBR’s long axis, the central lasing wavelength shifted towards shorter wavelengths in a linear fashion. The laser's strain sensitivity was determined to be 0.69 pm/με. The proposed strain-tunable PMBR laser offers the advantages of simple structure, low cost, robust performance, and has the potential for applications in sensing and tunable micro lasers.

A Fabry Perot (FP) based fiber sensor for multiparameter measurement is proposed. The sensor is constituted by a short section of a hollow square core fiber (HSCF) spliced between a single mode fiber and a long section of a silica capillary tube. In a reflection scheme, several FP cavities are enhanced in different areas of the HSCF. In a single 439 μm long sensing head, three FP cavities are excited. Using the Fourier band-pass filter method, each cavity was individually monitored towards variations of pressure, temperature, and curvature. The maximum sensitivities of (3.23 ± 0.04) nm/MPa, (9.6 ± 0.3) pm/°C, and (-32 ± 1) pm/m^-1 were obtained for pressure, temperature, and curvature, respectively within a measurement range of 0.4 MPa, 110°C, and 9 m^-1. The distinct responses of the FP cavities to the measurands allow for a triple-hybrid application of the sensor towards simultaneous measurement of pressure, temperature, and curvature. The proposed sensor is robust with simple fabrication and small dimensions, revealing promising to be employed in a wide range of applications where the measurement of several physical parameters is required.

This paper introduces and numerically investigates a special optical fiber cable with zero temperature-induced phase shift. The cable structure consists of stacked layers of two materials with opportune mechanical, thermal, and geometrical properties. This structure allows adjusting the thermal-induced strain to the fiber, resulting in a broad tunability of the bare thermal expansion, including the negative range. By a proper choice of materials, the thickness of each layer, and the radius of the cable, the induced thermal strain can fully compensate for the thermo-optic effect, resulting in a complete temperature insensitivity of the phase shift. This cable may be of great interest in the sensing fields in all those applications where the temperature compensation is critical, such as in low-frequency distributed acoustic sensing. Moreover, it could be relevant for a wide range of telecom applications that require precise thermal control.

Silica-based U-bent fiber optic sensor (U-FOS) probes exhibit excellent absorbance and refractive index sensitivity. They have been typically fabricated by manual means with the help of a butane flame, which is plagued by high probe-to-probe variations in the geometry, leading to rejection rates as high as 50% - 70%. In particular, fibers with 200 μm core and bend diameter as small as 1 mm pose a severe challenge. To overcome these limitations, we have developed an automated fiber bending machine (FBM) that consists of a CO₂ laser as heating source with a mechanism to automate laser beam deflection for precise control of heating zones on fiber and an automated articulating arm mechanism that holds both the ends of fiber and bends them after reaching glass transition temperature of about 1200 °C. FBM is capable of fabrication of U-FOS probes as many as 60-80 probes in an hour with bend diameter down to 0.55 mm and minimal geometric deviations. The proposed design is highly rugged, and more than ten thousand probes have been fabricated with this FBM so far.

This paper presents a machine learning (ML) solution to detect the peak wavelength of fibre Bragg grating (FBG) sensors multiplexed with overlapped reflection spectra, and using a serial topology. ML solutions generally require high-quality, high-volume datasets, which can be difficult to obtain in some scenarios. In contrast, here the proposed model is a sparse autoencoder convolutional neural network that can be trained using only the joint reflection spectrum containing all multiplexed FBGs, without information on the spectral position of each sensor. The technique is first verified through simulations and then with experimental data using two wavelengthmultiplexed FBG sensors in series. Comparing with existing methods, results verify that the proposed model has promising adaptation capability under multiple simulation scenarios, outperforming existing methods, whilst the model matches one of the best existing approaches when using experimental data.

Optical fibers are often used as a medium for nonlinear optical processes, especially for frequency doubling or second harmonic generation (SHG). Obtaining efficient SHG requires phase matching, which is challenging to achieve with ultrashort laser pulses. For that purpose we study how specific optical fiber designs can facilitate said phase matching. In this report we show that multi-step index fibers, which can be considered as an approximation of graded index fibers, can provide for simultaneous modal phase matching (MPM) and group velocity matching (GVM). That leads to an efficient pulsed second harmonic generation in optical fiber, which will open new opportunities for fiber sensing domain as well, where bio-sensing, medical sensing and strain sensing can be in the target.

Femtosecond laser pulses are increasingly utilized for the micro/nano-machining of a wide range of materials. They have been effectively employed in the production of fiber Bragg gratings (FBGs) through the implementation of point-by-point, line-by-line, and plane-by-plane processes. This study reports on the use of such lasers for the manufacture of Bragg gratings in pure fused silica planar substrates. In particular, the commercial system known as FEMTOprint was employed. This machine enabled the efficient production of Bragg gratings from bulk silica through several steps. Initially, a waveguide was engraved into the glass substrate through precise control of laser pulses and paths. Subsequently, an access point was created at one edge of the substrate to facilitate the easy connection of a standard optical fiber for light injection and collection. This was accomplished through the use of femtosecond laser pulses, followed by an etching process utilizing KOH to selectively ablate some material and create the necessary open spaces in the substrate. Finally, a third femtosecond laser process was utilized to inscribe a Bragg grating within the waveguide. The reflected amplitude spectrum of the grating was characterized with an FBG interrogator, and the obtained experimental results will be presented in this paper.

A novel flowmeter composed of a liquid crystal-filled nested capillary is proposed and experimentally demonstrated. Whispering gallery modes (WGMs) in the nested capillary are excited by a tapered fiber coupled perpendicularly to the nested capillary. The WGM transmission spectrum of the fiber taper was optimized to achieve the highest possible quality (Q) factor by moving the capillary along the axis of the fiber taper. The air flowing through the capillary cools it down, which leads to a temperature-induced change of the refractive index of the nematic liquid crystal. This change in turn leads to a spectral shift of the WGM resonances, which can be linked to the airflow speed in capillary. A sensitivity of 0.242 nm/sccm has been demonstrated in our experiment. The proposed sensor provides a new platform for WGM flowmeters and offers the advantages of high sensitivity and miniature size.

Optical fiber-sensors based on biodegradable and biocompatible optical fibers can be considered for implantation and invivo biosensing applications. We report on the fabrication and characterization of microstructured biodegradable and biocompatible polymer optical fibers (mbioPOF) from poly(D,L-lactic acid) (PDLLA), which is a commercially available polyester regulated by the U.S. FDA. We manufactured the optical fiber preforms by means of a novel technique based on transfer molding and subsequently we fabricated microstructured optical fibers using a standard heat-draw tower. The attenuation coefficient of our mbioPOF is as low as 0.065 dB∕cm at 898 nm for a microstructured fiber with a diameter of 219±27 μm. Prolonged immersion of mbioPOFs in PBS at 37°C leads to an increase of the optical loss with only 0.4 dB/cm after 6h and with 0.8 dB/cm after 17h, as measured at a wavelength of 950 nm.

High-temperature sensing is in great demand in the aviation, nuclear power and petroleum industries. Single-crystal sapphire fiber is a promising candidate for the fabrication of ultra-high temperature sensor due to its high melting temperature of 2045 °C. However, sapphire fiber usually exhibits multimode operation owing to it having no cladding. We demonstrate a new method for fabricating single-mode helical Bragg grating waveguides (HBGWs) in a multimode sapphire fiber based on femtosecond laser direct writing technique. Such a helical Bragg waveguide can be obtained by using merely one fabrication step. The negative refractive index changes region works as a depressed cladding waveguide, and the periodical structure yields Bragg resonance. And hence, a single-mode HBGW created in sapphire fiber was successfully fabricated by using the proper parameters, such as a diameter of 10 μm and a single-pulse energy of 29.9 nJ, and the bandwidth of its reflection spectrum was merely 0.68 nm. Subsequently, the temperature response of the fabricated HBGW created in sapphire fiber was tested and it could withstand the high temperature of 1800 °C and its temperature sensitivity was 41.2 pm/°C.

Several different fiber Bragg gratings (FBGs) were exposed to accumulated high doses of gamma rays (up to 80 MGy) and neutrons (5*10¹⁸/cm³) in a research grade nuclear reactor. The FBG peak wavelengths were measured continuously in order to monitor radiation induced shifts. Gratings inscribed with IR femtosecond pulses through a phase mask showed the smallest shifts (around 20 pm), while under identical conditions point-wise inscribed femtosecond gratings and a UV inscribed grating showed shifts of around 100 pm and 400 pm respectively. The different responses to irradiation are attributed to the various inscription techniques inducing gratings whose refractive index modulation is derived from different physical modifications of the fiber material.