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Fiber Bragg gratings (FBGs) in cyclic transparent fluoropolymer (CYTOP) optical fiber are the subject of a lot of research as they can be of interest for many applications, such as temperature, humidity, strain, and radiation sensing. We report here a new technique to produce high quality FBGs in CYTOP fiber. It uses a femtosecond laser system operating at 400 nm and a phase mask. In contrast to previously reported results, the gratings are obtained in a few seconds with a writing power as low as 80 μW. With this setup, 2 mm-long gratings with reflectivity up to 92 % and full width at half maximum bandwidth around 0.5 nm were obtained in less than 10 s. The resonance wavelengths of the FBGs are confirmed by numerical computation in the graded-index multimode CYTOP fiber, and the mode selection characteristic of FBGs in CYTOP is investigated. Finally, the temperature sensitivity of CYTOP FBG is measured in different mode groups for heating up and cooling down, showing values independent of the mode group measured, but with a small hysteresis.

Bare and gold-coated tilted fiber Bragg gratings (TFBGs) can nowadays be considered as a mature technology for volume and surface refractometric sensing, respectively. As for other technologies, a continuous effort is made towards the production of even more sensitive sensors, thereby enabling a high-resolution screening of the surroundings and the possible detection of rare events. To this aim, we study in this work the development of TFBG refractometers in 4-core fibers. In particular, we show that the refractometric sensitivity of the cut-off mode can reach 100 nm/RIU for a bare grating. Using another demodulation method, a tenfold sensitivity increase is obtained when tracking the extremum of the SPR (surface plasmon resonance) envelope for a gold-coated TFBG configuration. Immobilization of DNA probes was performed as a proof-of-concept to assess the high surface sensitivity of the device.

Type-I fiber Bragg gratings photo-inscribed in hydrogen-loaded B/Ge co-doped silica single-mode optical fibers have been regenerated efficiently at 450°C, which is the lowest temperature reported so far. The mechanical strength of the annealed fiber is preserved while ensuring temperature sensing of the regenerated gratings up to 900°C. Unlike low temperature cycles (≤600°C), an annealing process at higher temperatures revealed faster regeneration for strong gratings. Changes in grating strength were also measured before the regeneration cycle. These behaviors suggest the contribution of different mechanisms to the regeneration process with different relative dynamics.

Tilted fiber Bragg gratings (TFBGs) are now a well-established technology in the scientific literature, bringing numerous advantages, especially for biodetection. Significant sensitivity improvements are achieved by exciting plasmon waves on their metal-coated surface. Nowadays, a large part of advances in this topic relies on new strategies aimed at providing sensitivity enhancements. In this work, TFBGs are produced in both single-mode and multimode telecommunication-grade optical fibers, and their relative performances are evaluated for refractometry and biosensing purposes. TFBGs are biofunctionalized with aptamers oriented against HER2 (Human Epidermal Growth Factor Receptor-2), a relevant protein biomarker for breast cancer diagnosis. In vitro assays confirm that the sensing performances of TFBGs in multimode fiber are higher or identical to those of their counterparts in single-mode fiber, respectively, when bulk refractometry or surface biosensing is considered. These observations are confirmed by numerical simulations. TFBGs in multimode fiber bring valuable practical assets, featuring a reduced spectral bandwidth for improved multiplexing possibilities enabling the detection of several biomarkers.

It is common belief that photonic crystals behave similarly to isotropic and transparent media onlywhen their feature sizes are much smaller than the wavelength of light. Here, we counter that belief andwe report on photonic crystals that are transparent for anomalously high normalized frequencies up to0.9, where the crystal’s feature sizes are comparable with the free space wavelength. Using traditionalphotonic band theory, we demonstrate that the isofrequency curves can be circular in the region abovethe first stop band for triangular lattice photonic crystals. In addition, by simulating how efficiently atightly focused Gaussian beam propagates through the photonic crystal slab, we judge on the photoniccrystal’s transparency rather than on isotropy only. Using this approach, we identified a wide range ofphotonic crystal parameters that provide anomalous transparency. Our findings indicate the possibilityto scale up the features of photonic crystals and to extend their operational wavelength range forapplications including optical cloaking and graded index guiding. We applied our result in the domainof femtosecond laser micromachining, by demonstrating what we believe to be the first point-by-pointgrating inscribed in a multi-ring photonic crystal fiber.

In this Letter, we report the fast growth of high quality uniform Bragg gratings in trans-4-stilbenemethanol (TS)-doped poly(methyl methacrylate) (PMMA) step-index optical fibers. Grating manufacturing was obtained using a 400 nm femtosecond pulsed laser and a 1060-nm-period uniform phase mask. For 20 mW mean laser beam power, the grating reflectivity reaches 98% in ∼60 s.

Fiber Bragg gratings (FBGs) are well-known to be essential elements in optical communications, optical fiber based sensors and fiber lasers [1]. One of the most recent methods for fabricating FBGs is referred to as point-by-point inscription and uses femtosecond duration pulses of an infrared laser tightly focused to the fiber core region, such that every pulse forms a single grating period [2]. The periodicity of the grating is controlled by moving the fiber along its axis using translation stages at a pre-defined speed for a known repetition rate of the laser. A multi-photon absorption process is responsible for the local refractive index modification induced by a single pulse. Gratings obtained using point-by-point inscription can for example be used at much higher temperatures than those fabricated using conventional UV laser based inscription.

New dual temperature and strain sensor has been designed using eccentric second-order fiber Bragg gratings produced in standard single-mode optical fiber by point-by-point direct writing technique with tight focusing of 800 nm femtosecond laser pulses. With thin gold coating at the grating location, we experimentally show that such gratings exhibit a transmitted amplitude spectrum composed by the Bragg and cladding modes resonances that extend in a wide spectral range exceeding one octave. An overlapping of the first order and second order spectrum is then observed. High-order cladding modes belonging to the first order Bragg resonance coupling are close to the second order Bragg resonance, they show a negative axial strain sensitivity (−0.55 pm/με) compared to the Bragg resonance (1.20 pm/με) and the same temperature sensitivity (10.6 pm/°C). With this well conditioned system, temperature and strain can be determined independently with high sensitivity, in a wavelength range limited to a few nanometers.

In this work, eccentric Bragg gratings are photoinscribed in telecommunication-grade optical fibers. They are localized close to the core-cladding interface, yielding strong cladding mode resonance couplings and high photoinduced birefringence. Their transmitted amplitude spectrum is measured with polarized light while they are exposed to temperature changes up to 900°C. Despite the gratings' overall good thermal stability that confirms their robustness for high-temperature refractometry, we report an interesting polarization effect depending on both the cladding mode resonance family and mode order. While the core mode birefringence decreases with growing temperatures, certain cladding mode resonances show an increase in wavelength splitting between their orthogonally polarized components. This differential behavior is of high interest in developing high-resolution, multiparametric sensing platforms.

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

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

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

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

We have numerically identified hexagonal lattice parameters for PCF cladding that allow point-by-point grating writing. We designed and fabricated a PCF with suitable parameters and successfully inscribed a femtosecond point-by-point Bragg grating.

International audience; In this article, we study the possibility to use a pair of specifically designed microstructured optical fiber Bragg gratings (MOFBGs) as a multi-component strain sensor when embedded within composite materials. The dependence on the orientation of the transverse sensitivity of the MOFBGs is exploited to build a sensing device able to measure the strain field along the three principal mechanical directions of a laminate composite. We developed an analytical and numerical model of such a sensor and benchmarked it with experiments performed on laminated composite coupons equipped with this sensor. We report on a theoretical strain resolution of about 5 μ in the transverse directions of the composite material, which is a six-fold improvement over results reported in literature. © 2015 IOP Publishing Ltd.

When a short-length fiber Bragg grating is inscribed into a polarization-maintaining fiber, the resonant peaks produced by birefringence can overlap. In this case, the discrimination between strain and temperature effects is only possible by means of polarization control devices, i.e. a linear polarizer and a polarization controller. To avoid this complexity and the measurement errors that it involves, we propose a novel demodulation technique based on the numerical reconstruction of the grating physical parameters. Our technique evaluates the grating parameters from the amplitude spectrum by means of the coupled-mode equations. The convergence tool used to search the solutions is the Nelder–Mead simplex algorithm. The method is tested on uniform and apodized Bragg gratings and a very good accuracy is reported. The method is finally applied to a 2 mm-long uniform polarization-maintaining grating in order to simultaneously measure longitudinal strain and temperature.

In this paper, we analyze the polarization properties of uniform Bragg gratings written into polarization maintaining fibers by studying the evolution with wavelength of the degree of polarization for both the reflected and transmitted signals. We present numerical simulations realized by means of the coupled mode equations and the Stokes formalism. We also present experimental results performed on a uniform Bragg grating inscribed into hydrogen-loaded bow-tie fiber. We then report a good agreement between theoretical and experimental evolutions. Finally, we investigate the behavior of the reflected degree of polarization when the grating is subjected to a change of external temperature.

In this paper, we present a very reliable simulating algorithm to synthesize the physical parameters of a fiber Bragg grating structure from its reflection spectrum. The knowledge of the gratings parameters allows the determination of the maximum wavelength. The algorithm is then tested to monitor the shift of the central wavelength in response to a change of temperature. Our numerical program uses the transfer matrix method and the Nelder–Mead simplex algorithm. It can be easily implemented in the case of twin Bragg gratings. A twin grating is composed of two identical gratings separated by a short length of fiber. The demodulation technique has been tested experimentally with temperature sensors. It is very accurate and provides absolute measurements.

We present fiber Bragg grating (FBG)-based hydrostatic pressure sensing with highly birefringent microstructured optical fibers. Since small deformations of the microstructure can have a large influence on the material birefringence and pressure sensitivity of the fiber, we have evaluated two microstructured fibers that were made from comparable fiber preforms, but fabricated using different temperature and pressure conditions. The magnitude and sign of the pressure sensitivity are found to be different for both fibers. We have simulated the corresponding change of the Bragg peak separation with finite-element models and experimentally verified our results. We achieve very high experimental sensitivities of -15 and 33 pm/MPa for both sensors. To our knowledge, these are the highest sensitivities ever reported for birefringent FBG-based hydrostatic pressure sensing.

We present the use of microstructured optical fiber Bragg grating-based sensors for strain and temperature monitoring inside the concrete buffer of the Belgian supercontainer concept, demonstrated in a half-scale test in 2013. This test incorporated several optical fiber sensors inside the concrete buffer for production and condition monitoring. The optical fiber sensors presented here consist of small carbon-reinforced composite plates in which highly birefringent Butterfly microstructured optical fibers, equipped with fiber Bragg gratings, were embedded. The double reflection spectrum of these MOFGBs allows to simultaneously monitor strain and temperature, as confirmed by comparison with data obtained from thermocouples and vibrating-wire sensors installed near the MOFBGs.

We present a vibration sensor based on highly birefringent fiber Bragg gratings written in standard single mode optical fiber and realized with UV femtosecond pulses. This vibration sensor takes advantage of the stress-induced phase shift between the two orthogonally polarized fiber eigenmodes which induces intensity distribution changes in the two fiber Bragg grating reflection modes. The gratings are inscribed with the femtosecond line by line technique and have a birefringence value of 6 10 -4 . We demonstrate that theses gratings are temperature birefringence insensitive and ideal for vibration measurements.

Fiber optic sensors outperform traditional sensor technologies in fields such as structural health monitoring, vibration and seismic activity monitoring, intrusion detection, and many other applications. Their key advantages include electromagnetic interference immunity, lightweight, small size, multiplexing capabilities, low power consumption, corrosion and high temperature resistance. To meet the demand of more and more challenging optical sensors a new generation of optical fibers, the so-called microstructured optical fibers (MOFs), has appeared. These fibers are composed of a structure of holes surrounding a solid core, which offers a unique design flexibility to optimize their waveguide properties for specific applications. In particular, the design can be optimized to strongly reduce the crosssensitivity of a sensor to parasitic physical parameters like temperature variations, as is the case for the sensor presented here. Our sensor is based on a Bragg grating inside a temperature independent highly birefringent MOF with a high transverse strain sensitivity, to evaluate vibrations by a polarimetric measurement of the reflection spectrum. This technique takes advantage of the stress-induced phase shift between the two orthogonally polarized fiber eigenmodes. It consists in coupling linearly polarized light through one arm of an optical coupler (50:50) in the sensing optical fiber in which a highly reflective fiber Bragg grating is inscribed. The reflected signal is analysed through a linear polarizer. The optical fiber is crushed by a mechanical transducer designed to transform the vibration into a mechanical stress transversal to the fiber's axis. The vibration therefore induces a change of the phase modal birefringence that varies in time at the vibration frequency. In this study we show that using standard single-mode fibers to realize the sensor do not provide stable measurements and that using conventional polarization-maintaining fibers lead to a significant crosssensitivity to temperature. We then show that the use of a specific type of highly birefringent microstructured optical fiber allows temperature independent (up to 120 degrees C) and repeatable vibration measurements.

Photo-induced birefringence changes are investigated on several undoped LiNbO 3 crystals as a function of laser-irradiation time and intensity. Comparison points out that the changes are the smallest in the crystals grown from high temperature top seeded solution (TSS). It is concluded that the optical damage resistance in undoped LiNbO 3 crystals is increased with a decrease of the amount of intrinsic defects related to the non-stoichiometry. The necessity to carefully control the true crystal composition is stressed.

We report the electro-optic properties in iron-doped LiNbO 3 crystals. We investigated samples with various crystal compositions and the same doping concentration. The value of the electro-optic coefficient r c monotonously decreases with respect to the crystal composition. To separate the influence of the intrinsic defects related to the non-stoichiometry and the effect of the introduction of iron, we compared the results with data obtained in pure LiNbO 3 crystals. The effect of Fe ions on the composition of the crystal and the electro-optic properties is discussed from their site occupation in LiNbO 3 lattice.

We present a dedicated fiber Bragg grating (FBG) inscription experiment to investigate the compatibility of a microstructured optical fiber (MOF) with conventional FBG inscription setups. For the studied MOF, the angular orientation of the fiber in the interferometric excimer laser setup was found to have no significant influence on the final reflection of the inscribed FBGs. We also show that an array of multiplexed FBGs can be inscribed in a single MOF with a repeatability and quality that match fiber sensing requirements.

We demonstrate shear stress sensing with a Bragg grating-based microstructured optical fiber sensor embedded in a single lap adhesive joint. We achieved an unprecedented shear stress sensitivity of 59.8 pm/MPa when the joint is loaded in tension. This corresponds to a shear strain sensitivity of 0.01 pm/mu epsilon. We verified these results with 2D and 3D finite element modeling. A comparative FEM study with conventional highly birefringent side-hole and bow-tie fibers shows that our dedicated fiber design yields a fourfold sensitivity improvement. (C) 2013 Optical Society of America

In this paper, we report photo-inscription of uniform Bragg gratings in Trans-4-stilbenemethanol-doped photosensitive step-index polymer optical fiber characterized by a core diameter of 8.2 μm. Single-mode gratings were produced at ~1550 nm by the phase mask technique with a Helium-Cadmium emitting at 325 nm with an average power of 30 mW. The grating growth was monitored during the manufacturing process, showing that the reflected band is blue shifted by a few hundreds of picometers. Finally, the gratings were characterized in temperature in the range 25 − 50 °C. Their sensitivity has been computed equal to − 47 pm/°C.

In this letter, we describe a very accurate and simple demodulation technique for fiber Bragg grating sensors. The technique is suitable for both single and twin Bragg gratings. A twin grating is composed of two identical gratings located at different positions in the same single-mode fiber. Our demodulation technique evaluates the wavelength position of the reflection spectrum with respect to the spectrum of an undisturbed sensor. To calculate the spectrum shift, it computes the autocorrelation product between the two reflection spectra. The demodulation method, which is very fast, has been tested experimentally with temperature sensors. It gives absolute measurements and provides high accuracy compared to a conventional temperature probe.

We investigate the use of a bent fiber as a vibration sensor. In particular, the linear behavior is analyzed to define a range in which the mechanical excitation spectrum is recovered in a reliable manner.

Optical fiber sensors bring to measurement systems all the advantages inherent to the optical fibers technology. In this frame, fiber Bragg gratings (FBGs) are particularly interesting, thanks to their wavelength-encoded response and inherent multiplexing capability. As many FBGs can be interrogated using a single device, the cost per sensor unit can be drastically decreased, above all when an efficient and cost-effective interrogation set-up is developed. Such a development constitutes the main objective pursued in the work presented here.

Combining the functionalities of fiber Bragg gratings (FBGs) and microstructured optical fibers (MOFs) offers promising technological perspectives in the field of optical fiber sensors. Indeed, MOFs could overcome some of the limitations of FBGs in conventional fibers for sensor applications. The added value of MOFs stems from the ability to design an optical fiber in which an FBG acts as a sensor with a selective sensitivity, e.g. a sensor that is sensitive to directional strain but not to temperature. For this purpose we use a MOF with a phase modal birefringence on the order of 8×10-3. A FBG in this MOF yields two Bragg peak wavelengths, with a wavelength separation that depends on the phase modal birefringence of the MOF. We characterize these FBGs for transversal loads on a bare fiber and compare the results with simulated sensitivities. Then, we embed the sensor in a composite coupon and measure the response of the Bragg peak wavelengths as a function of the applied transversal pressure on the composite material. This allows drawing conclusions on the advantages of FBGs in MOFs for sensing applications.

In this paper, we proposed a sensor based on the measurement of the first Stokes parameter of the transmitted signal by a short length PM-FBG. Since it is important to characterize the polarization properties of PM-FBG for a lot of applications, we also present theoretical and experimental studies of the reflected and transmitted Stokes parameters. Experimental evolutions are obtained with a PM-FBG by means of a tunable laser source and a polarimeter. It is concluded that the technique is compatible with frequency multiplexing and can be used in the case of quasi-distributed sensors.

Tilted fiber Bragg gratings and long period gratings enhance the coupling to the cladding modes and constitute excellent refractometers. In this paper, we study how the cladding diameter influences the coupling to the cladding modes for these two types of gratings. Our experiments are made on different tilted Bragg gratings, characterized by different tilt angles. For every grating, we study the evolution of the sensitivity to the surrounding refractive index as function of the cladding radius. We also investigate two ways to correlate the spectral evolution of a tilted fiber Bragg grating with respect to the external refractive index. The first method is based on the local monitoring of the transmitted spectrum while the second one makes use of the correlation between a reference spectrum and a perturbed spectrum. These methods offer both very good accuracy and repeatability.

We analyze the evolution of the cladding modes couplings in tilted fiber Bragg gratings when the fiber diameter and the refractive index of the surrounding medium are modified. To decrease the fiber diameter, we make use of hydrofluoric acid and to modify the external refractive index, we use a set of index oils. We also investigate a new way to correlate the spectral evolution of tilted Bragg gratings with respect to a change of the external refractive index.

We present an efficient numerical method to evaluate the induced birefringence generated during the inscription process of an FBG. We use the experimental transmitted spectrum and the wavelength evolution of the corresponding first normalized Stokes parameter to obtain the value of the birefringence. Our reconstruction technique is based on the Nelder-Mead simplex algorithm.

We will demonstrate the potential use of a 2 cm long 6° tilted Bragg grating for simultaneous refractive index and temperature sensing during the production process of sugar from sugarbeets. This sensor is easy to use and allows realtime measurements. Its accuracy falls within ± 0.1 % and ± 1°C for sugar concentration and temperature, respectively.

When a short Bragg grating is written into a polarization maintaining fiber, the resonant peaks produced by birefringence can overlap. In this case, the simultaneous measurement of strain and temperature is only possible by means of polarization control devices. To avoid this complexity, we propose a demodulation technique allowing to accurately estimate the resonant wavelengths from the reflected spectrum measured using an unpolarized light. We use this technique to discriminate between strain and temperature effects with a 2 mm long grating written into PANDA fiber. Our method allows real-time functioning. Its accuracy falls within ±2°C and ±12 µe in the ranges 25-90°C and 0-1800 µe, respectively.

We propose a simple, very reliable and fast optical sensor based on a tilted fiber Bragg grating for the simultaneous measurement of temperature and macro bending. The transmitted spectrum of a tilted Bragg grating is composed of numerous discrete dips which have two distinct origins: the dip at the longest wavelength comes from the self coupling of the core mode while the others are due to the backward coupling with the cladding modes. We apply a different demodulation technique to each of these two contributions in order to realize a dual sensor. This sensor allows the detection of small curvatures and provides a good accuracy.

A simple, accurate, and fast method to synthesize the physical parameters of a fiber Bragg grating numerically from its reflectivity is proposed and demonstrated. Our program uses the transfer matrix method and is based on a Nelder-Mead simplex optimization algorithm. It can be applied to both uniform and nonuniform (apodized and chirped) fiber Bragg gratings. The method is then used to synthesize a uniform Bragg grating from its reflectivity taken at different temperatures. It gives a good estimate of the thermal expansion coefficient and the thermo-optic coefficient of the fiber.

We analyze the polarization properties of uniform fiber Bragg gratings written into polarization maintaining fibers by studying the evolution with wavelength of the normalized Stokes parameters and the degree of polarization for both the reflected and transmitted signals. Theoretical expressions are derived for the Stokes parameters. Numerical simulations and experimental results are also reported. We demonstrate the possible realization of a sensor using the information contained in the evolution of teh degree of polarization in reflection. This new demodulation technique has been experimentally tested with temperature sensors.

Twin gratings are important devices for sensor application. They are formed by two gratings inscribed in cascade in a single fiber. We present here a detailed study of twin Bragg gratings. We first show the evolution of twin Bragg gratings spectra versus the spacing between the gratings and their length. Owing to the coupled mode theory, we explain the particular shape of the various spectra. Then we demonstrate the influence of the reflectivity of each Fiber Bragg Grating on the global spectral response of the two cascaded FBGs. All the gratings we have inscribed are simulated using the Transfer Matrix Method in order to retrieve their parameters (grating length, period, index modulation and distance between gratings)from experimental reflection spectra. Finally we characterize these FBGs for temperature sensing. We discuss the design suitable to sensor applications and describe the interrogation technique used in this case. We use a digital algorithm based on Fast Fourier Transform. It offers precise, fast and absolute measurements.

Highly localized refractive index modulations are photo-written in the core of pure silica fiber using point-by-point focused UV femtosecond pulses. These specific gratings exhibit a comb-like transmitted amplitude spectrum, with polarization-dependent narrowband cladding mode resonances. In this work, eccentric gratings are surrounded by a gold sheath, allowing the excitation of surface plasmon polaritons (SPP) for radially-polarized light modes. The spectral response is studied as a function of the surrounding refractive index and a maximum sensitivity of 50 nm/RIU (refractive index unit) is reported for a well-defined cladding-mode resonance among the spectral comb. This novel kind of plasmonic fiber grating sensor offers rapidity of production, design flexibility, and high temperature stability.

A new type of highly birefringent microstructured optical fiber has been tested for vibration measurements using a polarimetric technique. This technique takes advantage of the stress-induced phase shift between the two orthogonally polarized fiber eigenmodes. Comparison of three different fiber types shows that standard single-mode fibers do not provide stable measurements and that conventional polarization-maintaining fibers lead to a significant cross-sensitivity to temperature. However, for highly birefringent microstructured fibers specifically designed to provide a temperature-independent birefringence, our experiments show repeatable vibration measurements over a frequency range extending from 50 Hz to 1 kHz that are unaffected by temperature variations (up to 120 degrees C). (c) 2012 Optical Society of America

The combination of the functionalities of Fiber Bragg Gratings (FBGs) and Photonic Crystal Fibers (PCFs) has unveiled new potential for FBG based sensors. The fabrication of FBGs in PCFs has been reported in literature. However, using dedicated PCFs to improve the sensitivity of FBG-based sensors has received only limited attention. In this report we therefore show how to eliminate some of the drawbacks of FBGs in conventional step-index fibers for sensor applications by exploiting the design flexibility of PCFs. The added value of PCFs stems from the ability to design an optical fiber in which an FBG acts as a sensor with a selective sensitivity, e.g. a sensor that is sensitive to strain but not to temperature. For this purpose we use a PCF with a birefringence on the order of 10 -3 , which is one order of magnitude larger than for conventional birefringent fibers. The two FBG reflection peaks are therefore significantly separated from each other, e.g. 2 nm, which makes these FBGs suited for sensing purposes since both peaks can be unambiguously and accurately identified. As a conclusion we summarize the advantages and disadvantages of our approach to design and fabricate selective FBG-based sensors.

Fiber Bragg gratings constitute excellent sensor elements, able to measure static and dynamic fields, such as temperature and pressure. We demonstrate that superimposed Bragg gratings written into hydrogen-loaded polarization maintaining fiber can be used to quantitatively monitor micro-strains or characterize the shrinkage of different cements. In fact, since the fiber Bragg gratings are written with a sufficient wavelength spacing between their Bragg wavelength, they exhibit different sensitivities and consequently, they allow the simultaneous measurement of temperature, strain, pressure and transversal force, which is sufficient to completely characterize shrinkage. We discuss the calibration of these superimposed gratings making use of a completely automated system allowing to load the fiber both in the axial and transversal direction and to apply controlled temperature changes.

We present fiber Bragg grating based transversal load sensing with a highly birefringent microstructured optical fiber. For the bare fiber, the change of the Bragg peak separation under a transverse line load was simulated with a finite-element model and experimentally verified. We also show that microstructured optical fibers with fiber Bragg gratings can be successfully embedded in a carbon fiber reinforced composite material. The linear dependence of the Bragg peak separation to a transversal stress in the composite sample was measured to be 15.3 pm/MPa.

We first shortly review the state-of-the-art of microstructured optical fiber Bragg grating (MOFBG) sensors for transverse load and pressure monitoring applications. We then focus on a specific microstructured fiber design to which we refer as ‘Butterfly’ MOF. This fiber is highly birefringent and has a large mechanical asymmetry. Its microstructure was optimized to encode loads perpendicular to the fiber's axis into a change of the spectral distance between the two Bragg peaks reflected by a fiber Bragg grating in this fiber. Such MOFBG sensors allow measuring transverse strains and hydrostatic pressures with an order of magnitude increase of the sensitivity over conventional birefringent fibers. We then discuss the potential of our MOFBG sensors for structural health monitoring related applications, including high-pressure sensing, three-dimensional strain measurements inside composite materials and disbond monitoring in adhesive joints based on shear strain measurements.

We present the measurements of the refractive indices and the electro-optic coefficients in lithium niobate crystals doped with a small amount of chromium. It is found that the coefficient r c has a significant dependence on the Cr concentration. Electrical moments and deformations, created by Cr 3+ at non-isovalent replacement of host lattice ions, can be responsible for this effect.