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The impact of physical parameters of uniform fiber Bragg grating (U-FBG) like grating period, length of grating, and width of grating on the performance of U-FBG fiber by using finite differences time domain (FDTD) based on surface plasmon polaritons (SPP) is evaluated. An FBG is similar to a distributed Bragg reflector created in a small segment of optical fiber that reflects some particular wavelengths of light and transmits the other wavelengths. It is observed that the maximum received optical power at the reflected port achieved is −1.67×10-6 w/m2 with silver (Ag) profile material of U-FBG at 0.1 w/m2 input transmission power and wavelength of 1.55 μm with 0.9 μm grating length and 0.2 μm grating width. The result shows that the received optical power is changing by optimizing the physical parameters of U-FBG.

Micro-structured fibers are important devices that have drawn intensive attentions and proved to be powerful platforms for various applications over the past decades due to their remarkable merits and advantages, such as small footprint, immunity to electromagnetic interferences, light weight, high physical flexibility, and low cost. Modifications in optical fibers can be used as light-steering elements to excite and couple back different core and cladding modes and form various in-fiber structures, including in-line fiber interferometer, fiber micro-cantilever, fiber random gratings, and so on. These micro-structures, when applied as fiber-optic sensors in the presence of external disturbances, show high sensitivity in terms of the significant changes in the guided light features. Novel micro-structured bend-insensitive fiber-based in-line fiber interferometer and micro-cantilever have been proposed to realize both static and dynamic parameter measurements, including temperature, axial strain, surrounding refractive index, and vibration. We have also developed a novel fiber random grating along with a spectral correlation algorithm for simultaneous measurement of three static measurands. To move a step forward, random fiber lasers based on fiber random grating are achieved for either improving the laser performances or sensing applications of temperature, strain, and ultrasound measurements with high sensitivity.

A directional bending sensor based on thin-core fiber Bragg grating is proposed and demonstrated experimentally. It is inscribed by off-center technique and exposed by 193 nm ArF excimer laser through a phase mask. A series of cladding modes are excited and their intensities are enhanced to about 10 dB. The formation mechanism of those cladding modes is discussed and analyzed. The intensities of these cladding mode resonances is detected for bending and direction with maximum sensitivity 1.93 dB/m 1 at 0 ° to − 1 . 95 dB/m 1 at 180 ° under the curvature varied from 0 m−1to 2.5 m−1. The sensitivity of surrounding temperature is 11.3pm/°C ranging from 25 °C to 60 °C. This all-fiber structure has a great advantage for fiber orientation identification sensor with more convenient manufacture and needless de-localize FBGs.

This work investigates two types of multiwavelength Brillouin fiber laser configurations. These configurations are ring-cavity multi-wavelength Brillouin fiber laser employs fiber Bragg grating and S-shaped multiwavelength Brillouin fiber laser. The two arrangements comprise of 10 km long single mode fiber as a gain medium and operate in the C-band region. Replacing the S-Shaped with Bragg grating fiber has led to a loss reduction in the ring-cavity multiwavelength Brillouin fiber laser and subsequently generated higher number Brillouin Stokes (BS) signals. The maximum of 33 BS signals with ∼0.08 nm (∼10 GHz) spacing is obtained. Optical signal to noise ratio of about 15 dB is observed at output coupling ratio of 70%, when 1550 nm of Brillouin pump is launched into the ring-cavity multiwavelength Brillouin fiber laser employs fiber Bragg grating. In contrast, due to the limitation of high cavity losses, only 17 BS signals with ∼0.16 (20 GHz) spacing are generated from the S-shaped multi-wavelength Brillouin fiber laser. These findings contribute new insights and profiles of the multi-wavelength Brillouin fiber laser employs fiber Bragg grating that has the capability to reduce cavity losses while generating a high number of Brillouin Stokes signals.

We propose a vector mode conversion approach based on tilted fiber Bragg grating (TFBG) written in ring-core fiber with effective separation of eigenmodes. The mode coupling properties of TFBG are numerically investigated. It is shown that under the constraint of phase matching, the conversion of high-order vector modes could be achieved at specific wavelengths. Moreover, the polarization of incident light and tilt angle of TFBG play critical roles in mode coupling process. The proposed TFBG provides an efficient method to realize high-order vector mode conversion, and it shows great potential for fibers based OAM beam generation and fiber lasers with vortex beams output.

Soft glass fibers can transmit the wavelengths longer than 2 μm, which can realize the sensors in mid-infrared waveband. Compared to the FBG sensors based on silica fibers, FBG sensors based on soft glass fibers have the advantages of high sensitivity. For the first time, the sensing characteristics of FBGs based on different soft glass fibers are analyzed and compared. The sensing characteristics of temperature, strain and pressure of FBGs based on tellurite, chalcogenide and fluoride fibers are numerically simulated and compared. By analyzing and comparing a series of sensitivity curves of FBGs, we get some conclusions. For temperature sensing, the FBG based on the As2S3 fiber has the highest sensitivity coefficient of 175.7 pm/°C, while the FBG based on the silica fiber has the lowest sensitivity coefficient of 11.3 pm/°C. The Bragg wavelengths are red-shifted with the increase of temperature. For strain sensing, the FBG based on the β-ZnS (CVD) fiber has the highest strain sensitivity coefficient of 2.76 pm/μe, while the FBG based on the silica fiber has the lowest strain sensitivity coefficient of 1.22 pm/μe. The Bragg wavelengths are red-shifted with the increase of strain. For pressure sensing, the pressure sensitivity of the FBG based on the As2S3 fiber is the highest and the Bragg wavelength has is red-shifted; the pressure sensitivity of the FBG based on the silica fiber is the lowest and the Bragg wavelength is blue-shifted. These results will be significant for optical sensing and fiber lasers in mid-infrared waveband.

Z-domain model of fiber Bragg grating is developed, and its performance analysis is presented in this letter. The developed mathematical model is used for the evaluation of reflection and transmission response. Spectral characteristics in terms of bandwidth, peak transmittance, and side lobes formation is determined for a model of length 0.84 mm and is found to be in very close agreement with previously published fabrication result. A similar structure with different parameters is considered. The transmission response provides a wide full width half maximum (FWHM) bandwidth of 9.52 nm and a peak transmission of 0.067. The proposed model is extended to determine the transmission response for a very small incidence angle variation. Performance parameters, such as FWHM, stopband width, and optical density, are analyzed. Transmission response of the proposed model for different length combinations is evaluated.

This paper reports on the development and evaluation of fiber optic hydrogen sensors based on fiber Bragg gratings. The sensors were tested in a newly developed measurement system for long term experiments of fiber optic gas sensors. Two types of palladium metal sensors were manufactured; sputter coated and a new concept using palladium foil. The sputter coated sensor has a 1600 nm palladium coating with a 30 nm titanium layer between the etched fiber and the palladium serving as an adhesion layer. The foil sensors have either a 20 μ m or 100 μ m palladium foil attached to the fiber. The sensors were tested for different hydrogen concentration in a newly developed gas sensing test setup. The maximum wavelength change (at 90 °C and 1060 mbar with both 1% and 5% hydrogen in nitrogen) of the coated sensor, the 20 μ m and the 100 μ m foil sensor was found to be 10, 160, and 80 pm respectively to 1% hydrogen and 25, 480, and 225 pm respectively to 5% hydrogen. A method is presented for calculating the wavelength change of the coated and foil sensors to show the current limitations and the theoretical future potential of each sensor.

Plane-by-plane fabrication of fiber Bragg gratings in optical fibers using short-pulse femtosecond IR laser is proposed and demonstrated. By incorporating a cylindrical lens in the fabrication setup, a plane of index modification can be directly inscribed in fiber core by a single laser pulse through the proposed method. This plane-by-plane method simplifies the grating inscription process and allows for the fabrication of complicated grating structures., 25th International Conference on Optical Fiber Sensors 2017, 24-28 April 2017, Jeju, South Korea, Series: Proceedings of SPIE; no. 10323

This paper proposes a displacement measurement system using two-photon absorption (TPA) process in a Si-avalanche photodiode and fiber Bragg gratings (FBGs). The use of TPA process eliminates high-frequency and complicated RF circuits usually required for intensity correlation measurements. By controlling the phase difference between the modulation signals for probe and reference light, the displacements of FBGs are precisely measured. Small displacement of an FBG by 0.4 mm was successfully measured in the proof-of-concept experiment.

Recent studies demonstrated the potential of increasing the Lamb wave detection sensitivity of fiber Bragg grating (FBG) sensors by bonding the optical fiber away from the grating location, instead of the conventional method of bonding the FBG directly. The FBG located at a remote location further along the optical fiber collects the guided traveling wave in the optical fiber generated from the Lamb wave signal. This remote bonding method could potentially be extended to a series of multiplexed FBGs. However, previous experiments also detected coupling to guided traveling waves in both directions in the optical fiber, which could have significant effects on multiplexed signals. In this paper, we measure the coupled signal amplitudes in both forward and backward directions, when ultrasonic waves couple from a thin plate to an optical fiber and from an optical fiber to a thin plate. The forward- and backward-induced modes are measured in both the optical fiber and the plate. The same experiment is then performed for the case when ultrasonic signal is coupled from the optical fiber to the plate. In addition, two different types of bonding, cyanoacrylate adhesive and frictional bond, are explored to investigate how the signal conversion depends on the bonding method. The results demonstrate that the coupling of ultrasonic waves from a thin structure to an optical fiber and from an optical fiber to the structure is complex. The coupling does not only occur in the direction of the wave propagation, but can be coupled into both forward and backward modes, depending on the bonding configuration used.

An ultrafast high-sensitivity refractive index (RI) and temperature-sensing system based on an optoelectronic oscillator (OEO) is proposed and demonstrated in this paper. A Fabry-Perot fiber Bragg grating (FP-FBG), which combines a gap with two FBGs in a silica V-shaped slot and characterizes a narrow notch in the reflection spectrum, is incorporated in the OEO to implement a microwave photonic filter and perform oscillating frequency selection. A microwave signal is generated by the OEO, whose oscillating frequency is determined by the center frequency of the FP-FBG notch, which varies with the surrounding environments. The RI or the temperature change can be accordingly measured by monitoring the frequency shift of the microwave signal using an electrical spectrum analyzer or a digital signal processor. An experiment is performed to verify the proposal. An RI sensitivity of 413.8 MHz/0.001RIU and a temperature sensitivity of 2516 MHz/°C are successively demonstrated.

We demonstrate an apodization technique by tapering the corrugations of spiral Bragg grating waveguides on the silicon-on-insulator platform, for the fundamental transverse magnetic mode. The transmission and reflection responses of uniform and apodized spiral Bragg grating waveguides are experimentally compared to show higher sidelobe suppression ratios by the proposed apodization scheme. We also present an apodized, period-chirped spiral Bragg grating waveguide, and the group delay of the proposed device has been measured; the results show an efficient suppression in the ripples of the group delay response.

A process for femtosecond laser-based manufacturing was proposed to fabricate a long-period fiber grating (LPFG) with a screw shape. The screw-shaped LPFG was continuously inscribed by single laser scanning, which results in the improvement of fabrication time. For the single laser scanning, an optical fiber was traveled along fiber axis and rotated about the fiber axis. The 44.65-mm-long LPFG with a screw shape and the period of 450 $\mu \text{m}$ was fabricated about 17 min, and its sensitivity was 48 ~ 51 nm/RIU for the glycerin solutions. The proposed process enables us to produce the screw-shaped LPFGs with fast manufacturing time and reliable performance compared with the process with multi-path scanning.

We demonstrated a simple single-exposure method of fabricating phase-shifted fiber Bragg gratings (PSFBGs), in which a non-photosensitive SMF-28 fiber was irradiated by a femtosecond laser beam passing through a uniform period phase mask. The phase-shifted structure in the FBG was formed by shielding a small region of the incident femtosecond laser beam during its irradiation. The fabricated PSFBG exhibited a 0.2 nm narrow resonant frequency band, 88% peak reflectance, and 77% extinction ratio. All-optical switching based on cross-phase modulation was demonstrated in the PSFBG at 1549.625 nm. A switching extinction ratio of more than 40% was achieved for 0.6 kW pulse peak power.

We investigate the hydrostatic pressure dependence of the Bragg wavelength of a fiber Bragg grating (FBG) inscribed in a perfluorinated graded-index (PFGI-) polymer optical fiber (POF) at 1550 nm. At 0.5 MPa, the Bragg wavelength increased with time and became almost constant ~150 min later. Such a long time constant probably originates from the unique structure of the PFGI-POF, which has a thick overcladding around its core and cladding. The pressure-dependence coefficient without considering the time constant was estimated to be 1.3 nm/MPa; this is over five times the values of other types of POF-FBGs. This indicates that by removing the overcladding of the PFGI-POF, fast-response high-sensitivity pressure sensing will be feasible. Once the Bragg wavelength became constant at 0.5 MPa, the pressure-dependence coefficient of the Bragg wavelength was measured to be −0.13 nm/MPa, the absolute value of which was comparable with those of other POF-FBGs, but with an opposite sign.

This paper has proposed a novel fiber Bragg grating (FBG)-based hybrid displacement and force sensor with a compact structure and excellent resolution by using the transverse property of a tightly suspended and slightly bent optical fiber. The optical fiber, embedded with an FBG element, has been suspended with its ends fixed on the sensor frame and implemented with a pre-tension force by the displacement loading along its vertical direction to form a bent shape. A conversion mechanism has been designed to convert the displacement and force inputs into the transverse movement of different points along the suspended fiber. Experimental results show that the displacement sensitivity and force sensitivity are −219.69 pm/mm within the range of 0–2.5 mm and −345.2 pm/N with a high calculated resolution of 2.9 mN, respectively. Results from both the displacement and force experiments have illustrated a close agreement with values from commercial sensors.

In this work, a theoretical study was conducted on temperature sensing in Ge–Sb–Se multimode fibre Bragg grating (MM-FBG). The sensing characteristics of the designed MM-FBGs with different...

For the optical current sensor that combines FBG and magnetostrictive material, a key problem is that the performance of FBG and magnetostrictive material is influenced by the operating temperature. In this paper, in order to overcome this problem, we proposed a method of temperature compensation based on the dual FBG configuration, which can make the measuring result of magnetic field be essentially temperature independent. In this method, two FBGs with the same type are bonded on two giant magnetostrictive materials, respectively. The two giant magnetostrictive materials have the identical shape and come from the same bulk material, while they have the orthogonal magnetostriction directions. We perform the experiment to investigate the performance of this method at different temperatures and at different magnetic fields, in order to verify the feasibility of this method. The experiment results demonstrate that this method significantly decreases the influence of temperature, and thus it can maintain a relative good performance in the temperature range of 20 °C–70 °C.

Gold-coated tilted fiber Bragg gratings can probe surface Plasmon polaritons with high resolution and sensitivity. In this work, we report two configurations to interrogate such plasmonic biosensors, with the aim of providing more efficient alternatives to the widespread spectrometer-based techniques. To this aim, the interrogation is based on measuring the optical power evolution of the cladding modes with respect to surrounding refractive index changes instead of computing their wavelength shift. Both setups are composed of a broadband source and a photodiode and enable a narrowband interrogation around the cladding mode that excites the surface Plasmon resonance. The first configuration makes use of a uniform fiber Bragg grating to filter the broadband response of the source in a way that the final interrogation is based on an intensity modulation measured in transmission. The second setup uses a uniform fiber grating too, but located beyond the sensor and acting as a selective optical mirror, so the interrogation is carried out in reflection. Both configurations are compared, showing interesting differential features. The first one exhibits a very high sensitivity while the second one has an almost temperature-insensitive behavior. Hence, the choice of the most appropriate method will be driven by the requirements of the target application.

We demonstrated femtosecond laser inscription of fiber Bragg gratings (FBGs) in a twin-core few-mode fiber (TC-FMF) for directional bend sensing. An FBG was selectively inscribed in one core of the TC-FMF by using an 800 nm femtosecond laser through a phase mask. Three resonance peaks at the wavelengths of 1549.05, 1547.65, and 1546.08 nm were observed in the reflection spectrum of the TC-FM FBG, and were generated by the LP01 mode resonance, LP01–LP11 mode cross-coupling resonance, and LP11 mode resonance, respectively. Moreover, the TC-FM FBG exhibited the capability of directional bend sensing and achieved a maximum bend sensitivity of −37.41 pm/m−1. Hence, the proposed TC-FM FBG directional bend sensors could further be developed as promising solutions for detecting vectorial seismic or acoustic waves and 3-D shape sensing.

An all-fiber CW cylindrical vector beam (CVB) fiber laser based on a few-mode fiber Bragg grating (FW-FBG) with switchable radially and azimuthally polarized beam generation has been demonstrated. The CVB fiber laser operates at a wavelength of 1053.95 nm with a 3 dB line width of 0.1 nm, a signal-to-background ratio of more than 50 dB. The CVB output power can reach 75 mW, and the mode purity is measured to be >95.5%. This compact CVB fiber laser has potential applications in many areas such as optical tweezers, optical trapping and optical sensing systems.

Fiber Bragg grating (FBG) is the most researched and implemented sensor of all the available fiber optic sensors till date. Despite being highly sensitive and accurate, the applications of FBG sensors are limited. The main reasons for their limited applications are the high cost and bulkiness of the interrogation system, optical spectrum analyzer (OSA). In this paper, a wavelength shifted chirped FBG (CFBG) interrogation system is presented. It is an optical intensity based FBG interrogation system which can measure positive/negative strain and temperature changes simultaneously. This system is simple, cost effective, and compact with a strain resolution of around 1 μe. A chirp bandwidth of 5 nm can easily provide a strain measurement range of about ±4000 μe. Essentially, this sensor design abrogates the need for OSA and makes FBG more suitable for field applications.

Subject of Research.The paper deals with mutlipulse inscription comparative results of the type IBragg gratings in the birefringent optical fiber with elliptical stress cladding and increased GeO2 concentration at different birefringence axes positions. Method. The gratings were inscribed by the phase mask method. The excimer laser Coherent COMPexPro 102F, working with the gas mixture KrF (248 nm), was used as the radiation source. The phase mask Ibsen Photonics with a period of1065.3 nm was used. Main Results.The results have shown that the multipusle inscription is more effective and has better dynamics when the plane containing the fiber axis and its slow birefringence axis is parallel to the writing radiation incidence plane containing the fiber axis. Practical Relevance. The research results give the possibility to make recommendations for the multipulse Bragg gratings inscription efficiency enhancement in the specialty birefringent fibers. The preinscription positioning of birefringence axes also makes it possible to reduce the polarization fading that is the noise source in phase interferometric sensors.

Multicore and microstructured fibers open a new door for designing all-fiber telecom components. In this article we propose a design of an optical power splitter based on the phenomenon of power coupling in the tapered splice between a single-core (SMF-28) and a seven core fiber (MCF-7), which was originally developed for spatial division multiplexing telecommunication systems. Comprehensive numerical analysis is presented and backed up with an experimental demonstration.

In this work, we investigate the time-dependent variation of both the reflectivity and resonance wavelength of microstructured polymer optical fiber Bragg grating (mPOFBG) array sensors embedded in silicone rubber and polyurethane resin diaphragms in contact with water and aircraft fuel, respectively. The array sensors were inscribed using two different phase masks with pitches of 557.5 and 580 nm and the thermal annealing of the inscribed fiber was used to change the Bragg wavelengths. Both the reflection and the resonance wavelength shift were monitored over 90 days submerged in liquid and two studies were investigated. In the first study, in addition to the mPOFBGs coated with the diaphragm, also the rest of the fiber is totally protected between the sensors with the same material used for diaphragms. On the other hand, in the second study, the fiber between sensors is unprotected – in direct contact with liquid. PMMA and TOPAS fibers were used and this study suggests that TOPAS fiber should be a good option for long-term liquid monitoring applications.

Fiber Bragg gratings (FBGs) are inherently sensitive to temperature, axial strain, and pressure, which can be easily measured by a shift of the Bragg wavelength in their reflected/transmitted power spectrum. FBG sensors acquire many more additional sensing modalities and applications when the polarization of the interrogating light is controlled. For the polarization to have an effect, the cylindrical symmetry of the fiber must be broken, either by the structure of the fiber itself, by that of the FBG, or by the perturbation to be measured. Polarization control allows for sensing parameters that are spatially oriented, such as lateral force, bending or twist, and also for measurements of the properties of anisotropic media. Most importantly, polarization control enables high quality all-fiber surface plasmon resonance (SPR) FBG sensors and localized SPR-assisted sensing. This tutorial will cover the theory of polarized measurements in fiber gratings, their experimental implementation, and review a selection of the most important applications.

In this paper, a review of microstructured optical fiber (MOF) sensors is given. Various kinds of MOFs are described and their sensing applications are summarized. Two main types of MOF sensors in terms of grating based and interferometry based are reviewed. In particular, several types of microstructured optical fibers designed for sensing applications in our works are demonstrated, including the measurement of physical parameters, e.g., pressure, strain, torsion, temperature, and the detection of biomedical parameters such as refractive index and microfluidic flow rate. The basic design principles of MOF sensors based on fiber Bragg grating and interferometers are given. Comparisons of the performances of the fibers and sensors are also conducted according to the relevant topics.

We propose a high-linearity refractive index (RI) sensor based on an analyte-filled defect hollow core Bragg fiber (DHCBF) for large dynamic ranges. A resonant wavelength induced by the transverse coupling between core modes and defect modes is used as the characteristic wavelength for realizing a high-linearity RI sensor. The numerical results show that the linear correlation between characteristic wavelength and analyte RI can be significantly improved by optimizing structural parameters of a defect layer, which would be beneficial to the enlargement of dynamic measurement range. Furthermore, we numerically demonstrated an RI sensor with the sensitivity of 2062 nm/RIU and the adjusted R-Square value of 0.99967 in the measurement range of 1.324-1.432, by using an analyte-filled DHCBF with a low RI-contrast cladding. Compared with that based on a standard HCBF, our proposed high-linearity RI sensor can achieve high sensitivity in a larger dynamic range.

Recently developed ultrahigh resolution optical fiber grating sensors provide powerful tools for crustal deformation monitoring thanks to their unique advantages such as high resolution, low cost, easy deployment, and capability of remote and multiplexed sensing. This paper reviews the development of several types of fiber-optic sensors with ultrahigh resolution in quasi-static domain, including a fiber Bragg grating sensor interrogated with a narrow-linewidth wavelength-sweeping laser, a fiber grating based Fabry–Perot interferometer sensor by using Pound–Drever–Hall technique, sensors interrogated with sideband interrogation method, and the realization of time division multiplexed ultrahigh resolution fiber sensors. The implementation of fiber grating sensors for in situ measurement of crustal deformation are also introduced.

A single-longitudinal-mode fiber laser is presented, which is composed of a ring cavity laser and a π phase-shifted fiber Bragg grating. The ring cavity structure can reduce the spatial hole burning, but the mode hopping and competition are still existing due to the long fiber ring cavity length. The π phase-shifted fiber Bragg grating has very narrow transmittance spectrum width as a band-pass filter. Combined with a wavelength-matching fiber Bragg grating, it is able to efficiently suppress the mode hopping and competition in the ring cavity. The single longitudinal mode lasing is verified using a scanning F-P interferometer. Its frequency noise is measured by the self-homodyne technology with a 3×3 optical fiber coupler. The calculated linewidth from the frequency noise is about 21 kHz when the measurement time is 0.2 s.

A short fiber Bragg grating (FBG) is successfully written in a multimode fiber (MMF) with core and cladding diameters of 50 μm and 125 μm using 800 nm femtosecond laser side-illumination technique. Three-type grating inscriptions can be realized at the different positions over the core of MMF by moving the focal-line position of laser beam. Both fundamental mode and higher-order modes of MMF are excited at the core-mismatch junction, resulting in two well-defined resonances in transmission. The strain measurement with a single core resonance mode is demonstrated experimentally at the ultra-high temperature. The result indicates that the strain sensitivity improved to 5.24 pm/µe at the temperature of 600–900 °C, making it as a good candidate for the strain measurement at the high temperature environments.

A stress sensitivity analysis of optical fiber-based Fabry-Perot (FP) sensors has been performed, taking into account the influence of fiber loss on the interferometric sensitivity. The sensitivity increases proportionally with the interferometric cavity length, but it decreases with the attenuation present along the optical path. In addition, fibers constructed from materials with lower Young's modulus provide higher intrinsic stress sensitivity. In this study, three fiber materials have been studied: fused silica, poly (methyl methacrylate) (PMMA), and cyclic transparent optical polymer (CYTOP), which have different Young's modulus and optical attenuation. In general, PMMA-based FPs are best for the shortest cavity lengths due to their lower Young's modulus, the performance of silica dominates for long cavities due to the lowest fiber loss and CYTOP is the best choice for medium length cavities. The boundaries between the three length regimes depend on the operating wavelength (which strongly influences the fiber loss) and the reflectivity of the mirrors forming the interferometer. Where the mirrors take the form of fiber Bragg gratings, the grating length also impacts the system performance. The presented model can be used to identify which material offers the highest stress sensitivity when the physical and optical characteristics of a FP stress sensor have been predefined.

results in visualization of fiber Bragg gratings of type II recorded in birefringent single-mode optical fibers with an elliptical stress cladding with a high concentration of germanium dioxide are shown. Periodic structures of the induced refractive index are inscribed by a single pulse of an excimer laser with a working- gas mixture of ArF using the phase mask. Fiber Bragg gratings into anisotropic optical fiber were investigated by optical and scanning-probe microscopes. It is shown that single-pulse recording by ArF excimer laser creates in the fiber, both inside and on the surface of the quartz cladding, periodic structures, the spatial period of which corresponds to the period of the phase mask, optimized for the +1/–1 diffraction order.

Fibre Bragg Grating (FBG) sensors are frequently being used for Structural Health Monitoring (SHM) of aerospace structures. One of the most important advantages of using FBG sensors is that it is possible to embed them into composites. In this paper, manufacturing methods of composite specimens with embedded FBG sensors are given. To avoid stress concentrations at ingress/egress regions of fibre optic wires, PTFE (Teflon) tubes were used during manufacturing. Moreover, FBG connectors melt at high curing temperatures. Therefore, those connectors were cut and after manufacturing, these connectors were spliced back to the FBG sensors. Embedded FBG’s were then checked and the correct wavelength data were taken. All the sensors were observed as intact and ready for bending tests. Procedure for bending tests is also explained including applied loads, boundary conditions, test setup and the peripheral equipment. Results of bending tests show that the system is an appropriate one for SHM purposes.

The paper presents some results obtained within a project of the “NUCLEU” Program financed by the Ministry of Research and Innovation-ANCS. The project supposes, among others, the design and the realization of a demonstrator for strain and stress measurements made with Fiber Bragg Gratings optical sensors. The paper details the construction of the demonstrator. The strain measurements induced in a cantilevered aluminum plate are compared with the analytical values provided by a mathematical model, and with the numerical values obtained by FEM analysis. The consistency of these comparative data indicates the achievement within the project of a level of competence necessary for later use of FBG sensors in the applicative researches involving the aerospace structures monitoring.

Optical fiber gratings (OFGs), especially long-period gratings (LPGs) and etched or tilted fiber Bragg gratings (FBGs), are playing an increasing role in the chemical and biochemical sensing based on the measurement of a surface refractive index (RI) change through a label-free configuration. In these devices, the electric field evanescent wave at the fiber/surrounding medium interface changes its optical properties (i.e. intensity and wavelength) as a result of the RI variation due to the interaction between a biological recognition layer deposited over the fiber and the analyte under investigation. The use of OFG-based technology platforms takes the advantages of optical fiber peculiarities, which are hardly offered by the other sensing systems, such as compactness, lightness, high compatibility with optoelectronic devices (both sources and detectors), and multiplexing and remote measurement capability as the signal is spectrally modulated. During the last decade, the growing request in practical applications pushed the technology behind the OFG-based sensors over its limits by means of the deposition of thin film overlays, nanocoatings, and nanostructures, in general. Here, we review efforts toward utilizing these nanomaterials as coatings for high-performance and low-detection limit devices. Moreover, we review the recent development in OFG-based biosensing and identify some of the key challenges for practical applications. While high-performance metrics are starting to be achieved experimentally, there are still open questions pertaining to an effective and reliable detection of small molecules, possibly up to single molecule, sensing in vivo and multi-target detection using OFG-based technology platforms.

Large-scale distributed fiber Bragg grating (FBG) sensor networks have a wide range of applications. However, serious crosstalk could occur in a near-identical serial FBG sensor network because of multiple reflections. Since crosstalk cannot be observed directly and separately from signal, experimental investigation of crosstalk is challenging. In this work, we point out the similarity and symmetry between crosstalk and ghost gratings, and by way of measuring the spectrum of ghost gratings, we experimentally study the characteristics of crosstalk in a serial array of 3010 near-identical FBGs using a commercial FBG sensor interrogation system. We show that ghost gratings drop much more rapidly in power as one goes downstream, and they are more homogeneous in spectral distribution than real gratings. The peak wavelengths of real gratings vary within ±100 pm while the peak wavelengths of the ghost gratings they generate vary within ±20 pm only. Spectral ripples are observed for ghost gratings and real gratings severely affected by crosstalk. These ripples reduce the FBG wavelength reading accuracy to ±5 pm. The work constitutes the first experimental study on crosstalk in large-scale FBG arrays.

The conventional single-angle tilted fiber Bragg grating (TFBG) can only excite a certain range of cladding modes, limiting it for refractive index (RI) measurement in a wide range. In this letter, we fabricate and demonstrate a multi-angle TFBG, in which five individual TFBGs with tilt angles ranging from 5° to 25° are sequentially inscribed along the core of a single mode fiber within a length of 20 mm. The multi-angle TFBG excites a continuous spectral comb of narrowband-cladding modes over a much wider wavelength range (>170 nm) than a single-angle TFBG, making it suitable for dynamic RI measurement over a wide range (1.15–1.45). We have experimentally measured aqueous solutions with RI ranging from 1.30 to 1.45 using the uncoated (by monitoring the cut-off mode) and gold-coated (by monitoring the surface Plasmon resonance) multi-angle TFBGs, and both methods show linear responses, with RI sensitivities about 500 nm/RIU.