We demonstrate the application of distributed fiber optic strain sensing based on optical frequency-domain reflectometry for the early identification and location of fatigue cracks in welds in steel tubular test specimens. This involved subjecting two welded tubular specimens, equipped with surface-mounted optical fiber sensors, to a resonant bending load. We continuously measured the strain distributions in the test specimens without any interruption throughout the entire duration of the test. The measurements were carried out with a spatial resolution of 2.6 mm and a strain resolution of 1 mu epsilon. The test samples were designed so that the fatigue cracks initiated from the inner surface of the pipe at the weld root and propagated through the wall thickness towards the outer surface. The fatigue tests were performed under two stress ranges. In Sample 1 (higher stress), we successfully detected and located the strain concentration region at approximately 5% of the specimen's lifetime before the breakthrough. For Sample 2 (lower stress) the initiation of the hotspot was detected around 27% of the specimen's lifetime before the breakthrough. We demonstrate that our method allows for on-the-fly detection and location of fatigue cracks originating from the inner surface of the specimens' wall.

We demonstrate the application of distributed fiber optic strain sensing based on optical frequency-domain reflectometry for early detection and location of fatigue cracks in welds in steel tubular test specimens. To do so, we have subjected welded tubular specimens instrumented with surface-mounted optical fiber sensors to resonant bending load, and we have measured the strain distributions in the test samples continuously and without any interruption of the test throughout its whole duration, with a 2.6 mm spatial resolution and a 1 & mu;& epsilon; strain resolution. We show that the fatigue cracks, which initiate from the inner surface of the wall of the specimens, can be successfully detected and located in real time using our measurement method. We conclude that the detection of the crack initiation may provide relevant information serving the estimation of the remaining lifetime of the component.

We have fabricated and characterized microstructured biodegradable and biocompatible polymer optical fibers using commercially available poly(D,L-lactic acid) (PDLLA). We first report on the preparation of the preforms by means of a novel technique based on transfer molding and on the fiber manufacturing using a regular heat-drawing process. We address the influence of the polymer processing on the decrease of the molar mass of PDLLA following the preform fabrication and the fiber optic drawing process. We investigate the in vitro degradation of the fabricated fibers in phosphate buffered saline (PBS) that reveals 21, 25 and 43% molar mass loss over a period of 105 days for fibers with diameters of 400, 200 and 100 μm, respectively. Cutback measurements return an attenuation coefficient as low as 0.065 dB/cm at 898 nm for a microstructured fiber with a diameter of 219 ± 27 μm. Due to immersion in PBS at 37 °C, the optical loss increases by 0.4 dB/cm at 950 nm after 6 h and by 0.8 dB/cm after 17 h.

Spectral multiplexing of biosensors in a single optical fiber has been a long-standing challenge, which we address here for the first time by combining photonic crystal fibers (PCF) with fiber Bragg grating technology. We exploit the features of the optical transmission spectrum of a straight fiber Bragg grating written in a PCF that allows exciting cladding mode resonances within a spectral span of about 60 nm, which is significantly narrower than the width of the transmission spectra of tilted gratings in standard single-mode step-index fibers. More specifically, we consider the cladding mode resonances that feature effective index values close to the refractive index of phosphate buffered saline, and we demonstrate plasmonic label-free biodetection of HER2 (human epidermal growth factor receptor 2) protein. We report on the simultaneous monitoring of the wavelength shifts of said cladding mode resonances from two spatially separated biofunctionalized Bragg gratings and we find that the PCF sensor is able to detect the protein concentration of 8.62 nM with high reproducibility.

Historia del artículo: Recibido 5 de Mayo 2017 En la versión revisada 5 de Mayo 2017 Aceptado 31 de Mayo 2017 Accesible online 21 de Junio 2017 La monitorización y gestión del estado de las aeroestructuras (Structural Health Monitoring o SHM) mediante el uso de sensores basados en fibra óptica (FO) embebidos puede ser capaz de realizar la diagnosis y prognosis de daños y defectos estructurales. El sistema SHM monitoriza el estado de la estructura basándose en los datos adquiridos que resultan de la detección y caracterización de daños sin necesidad de que la aeronave esté en tierra. Los sensores de fibra óptica se instalan permanentemente en la estructura y activan las acciones de mantenimiento solo cuando un daño o defecto, de determinado tamaño, es detectado.

Fiber Bragg grating (FBG) feedback has initially been investigated as a promising approach to conceal the time-delay signature in optical chaos generation. It has been shown that the laser dynamics vary greatly with respect to the FBG properties, especially to the frequency detuning between the laser emission and the Bragg wavelength. As a result, adjusting the FBG features will lead to significantly different behaviour. Here, we theoretically study the response of FBGs with different lengths but with similar reflectivity: this way, the impulse response is stretched over a longer period of time while its overall shape is maintained. This leads to a broadening of the FBG bandwidth and, thus, to a longer distribution of the feedback over time. In this work, we analyse the effects of the time-distribution variations for long gratings by simply tracking the first Hopf bifurcation and the feedback rate needed to destabilize the laser. The numerical results are generated using a modified version of the well-known Lang and Kobayashi equations. Our results show that the time-distribution of the feedback seem to have little effect in itself on the overall dynamics though it obviously affects the FBG spectra properties. We report stability oscillations of the laser behavior when long, narrow-bandwidth gratings are considered. The influence of the grating length on the specific dynamic details is investigated through the time delay signature (TDS) focusing especially on the implication of the stability oscillations on the TDS. We report that although variation of the TDS for long grating are observed the better TDS suppression is achieved with relatively short gratings.

We designed, fabricated and characterized low loss and polarization independent waveguide Bezier-shape S-bends. We optimized the shape parameter to obtain the shortest length for a given S-bend offset.

Multiplexing of plasmonic biosensors in a single optical fiber offering parallel readout has been a long-standing challenge, which we address here for the first time with an optrode at the intersection of fiber Bragg grating technology and photonic crystal fibers (PCF). We exploit the advantage of the cladding mode resonances excited in the optical transmission spectrum of straight fiber Bragg gratings in the selected PCF. We focus on the modes with amplitudes of up to 10 dB occupying the spectral span of only ~20 nm and featuring effective index values close to the refractive index of the buffer (phosphate buffered saline) containing the target analyte. As a proof-of-principle, we show label-free biodetection of HER2 (human epidermal growth factor receptor 2) protein used as a biomarker for breast cancer diagnosis. We simultaneously monitor the wavelength shifts of plasmon-coupled cladding mode resonances from two spatially separated functionalized Bragg gratings showing a minimum detection threshold of 8.62 nM of HER2 protein.

Modern sealing components are used in a wide range of industrial processes. The recent global attention for reducing emission of environmentally harmful substances motivates gasket manufacturers to quantify the performance of sealing gaskets. We investigate the use of fiber Bragg grating sensors for measuring seating stress on a spiral wound gasket. The fiber-optic sensors are integrated in the gasket, following a particular instrumentation strategy. The instrumented gaskets are first installed in a standardized flange following a state-of-the-art installation protocol. Then, the gaskets are unloaded with a particular unloading strategy to simulate sealing performance loss. We compare the load in the mounting bolts during the (un)loading procedure with the response of the FBG sensors in the gasket and find a high correlation. We thus evidence the potential of fiber-optic sensors for measurements during installation as well as monitoring of sealing gaskets serving the accountability of these components.

We demonstrate distributed optical fiber-based pressure measurements with sub-bar pressure resolution and 1 m spatial resolution over a ∼100 m distance using a phase-sensitive optical time-domain reflectometry technique. To do so, we have designed a novel highly birefringent microstructured optical fiber that features a high pressure to temperature sensitivity ratio, a high birefringence and a mode field diameter that is comparable to that of conventional step-index single mode fibers. Our experiments with two fibers fabricated according to the design confirm the high polarimetric pressure sensitivities (-62.4 rad×MPa-1×m-1 and -40.1 rad×MPa-1×m-1) and simultaneously low polarimetric temperature sensitivities (0.09 rad×K-1×m-1 and 0.2 rad×K-1×m-1), at a wavelength of 1550 nm. The fiber features a sufficiently uniform birefringence over its entire length (2.17×10-4 ± 7.65×10-6) and low propagation loss (∼3 dB/km), which allows envisaging pressure measurements along distances up to several kilometers.

Modern sealing components are essential in today’s industry. The recent global focus on environment, sustainability and safety is encouraging gasket manufacturers to innovate and think of the gasket of tomorrow. Not only are performance expectations increasing rapidly, but advancements in gasket manufacturing and material technologies are producing gaskets that — in both theory and practice — generate tighter seals and reduce harmful emissions. The major impediment to attaining these results in the field is the lack of advancement in assembly accuracy and real-time monitoring of gasket stress. Currently the user relies on industrial calculation standards that are designed to determine the required bolt load to maintain a leak free seal over the required timeframe. Such calculations often yield a proper approximation of the situation, but they inherently rely on simplifications and assumptions. Furthermore, the correct execution of installation protocols is difficult to verify. The state-of-play does not allow in-situ measurements of the seating stress in the gasket. This study addresses state-of-the-art shortcomings in bolted flange connections and proposes a solution to mitigate them by means of sensors. We successfully integrate optical fiber sensors inside semi-metallic gaskets and experimentally demonstrate the direct measurement of seating stress. Such in-situ seating stress quantification enables installation and condition monitoring serving an optimal lifecycle prediction and failure prevention. As such, this approach contributes to increased sustainability of bolted flange connections.

Optical fibers provide a favorable medium for nonlinear optical processes owing to the small mode field size and concurrently high optical intensity combined with the extended interaction lengths. Second harmonic generation (SHG) is one of those processes that has been demonstrated in silica glass optical fibers. Since silica is centrosymmetric, generating SHG in an optical fiber requires poling of the glass. In addition and when one wants to use ultrashort pulses for SHG, achieving both phase and group velocity matching is crucial. Although fibers that feature either modal phase velocity or group velocity matching for SHG have been reported, the possibility of simultaneous modal phase and group velocity matching was never reported before. In this paper we address this challenge, and for the first time to our knowledge, we show that it is feasible to do so with silica microstructured optical fibers featuring at least one ring of air holes in the cladding and a heavily Germanium doped core (above 25 mol.%) by exploiting the LP01(ω) and LP02(2ω) modes at 1.06 µm pump and 0.53 µm second harmonic wavelengths. This finding can greatly impact applications requiring waveguide based SHG generation with ultrashort pulses, including microscopy, material characterization and nonlinear imaging.

Applications associated with complex, structured optical fibers having no circular symmetry often require information about their specific angular orientation. Such cases include, for example, splicing, sophisticated mode coupling, selective fiber Bragg gratings inscription, or direction‐sensitive sensing. Besides the external angular orientation of the fiber, additionally an intrinsic twist of the internal fiber structure has been identified as a critical parameter for applications. Such an intrinsic fiber twist might occur as a side effect of the fiber drawing process, or it may have been introduced intentionally as a special fiber property. In all such cases, a measurement of the local orientation along the fiber is highly desirable. A method for such measurements based on the analysis of the refractive–diffractive scattering patterns is presented. By comparing such measured patterns with reference patterns from modeling or from reference measurements, the orientation of the fiber can be identified with high accuracy and without measuring or reconstructing the detailed cross section of a fiber. The method, therefore, provides an approach for quick identification of a local angular fiber orientation status.

We show for the first time that it is feasible to achieve simultaneous modal phase- and group-velocity matching for second harmonic generation in heavily Ge-doped microstructured optical fiber (MOF) and step-index fiber configurations.

Fiber Bragg gratings (FBGs) have been advantageously used to improve the chaotic properties of semiconductor lasers. Though these components are known to be highly sensitive to environmental conditions, feedback phase fluctuations are often neglected. In this work, we experimentally demonstrate that the small variations of the propagation time induced by a simple thermal tuning of the FBG are sufficient to induce significant changes of the laser behavior. We report periodic stability enhancements linked with phase variations and highlight that both phase variation and phase offsets play an important role. Last, we show a good qualitative agreement with simulations based on an expanded version of the Lang-Kobayashi model.

We present a miniature freeform lightguide for sensing applications, designed according to the principles of the flow-line method from Nonimaging Optics. The optic is obtained by combining two 2D flow-line concentrators in a curved monolithic piece, achieving 45° half-acceptance angle and 40° beam steering in a very compact volume (about 1.3 x 2.0 x 20 mm3). We show how the initial design has been adjusted after a thorough tolerance analysis and describe its fabrication through plastic injection molding. The design of the mold involves a non-standard 3D-puzzle approach, which allows uniform high optical quality and minimizes the fillet radius on the optic.

We give an overview of our recent progress on interfacing components for short-reach optical interconnects fabricated through two-photon polymerization-based laser direct writing. We show mode field conversion tapers printed on single-mode optical fibers for easy and efficient interfacing to various photonic integrated circuits, circular and square planar waveguide structures with V-groove inspired alignment structures for easy coupling to fibers, and fan-out diffractive optical elements. For all these components, we present the process flow from optical design and simulation over laser direct writing fabrication and metrology to proof-of-concept demonstration.

We report on highly reflective fiber Bragg gratings in photonic crystal fibers (PCF) that excite two types of cladding mode resonances. We consider two hexagonal lattice PCF structures with a similar air-filling fraction (0.4) but different lattice pitch values (2.5 and 3.6 μm). We demonstrate both experimentally and numerically that the lattice parameters of the microstructure influence the spectral location of the resonances and the spectral span that they occupy. For the PCF with the lattice pitch of 2.5 μm, we demonstrate its application for surface plasmon-enhanced refractometry of waterbased solutions with potential for biosensing applications.

We report on an extrinsic surface plasmon-enhanced refractometer based on cladding mode resonance excitation in a photonic crystal fiber (PCF) equipped with a straight fiber Bragg grating (FBG). First, we show that the lattice pitch and the air hole diameter of the PCF microstructure define the spectral location of the excited cladding mode resonances. Second, we demonstrate that if the PCF parameters are properly selected, those resonances are sensitive to increases in steps of 1 x 10(-4) refractive index units (RIU) of the refractive index value close to that of water. To the best of our knowledge, this is the first time that the sensitivity of PCF cladding mode resonances to refractive index changes in water-based solutions is reported. We achieved experimental values of 40.3 nm/RIU in terms of wavelength sensitivity and -801 dB/RIU in terms of amplitude sensitivity. The performance of our sensor is therefore comparable to that of tilted FBGs in step-index fibers used for water refractometry, which indicates the potential of our PCF sensor for biosensing. In addition, the sensor fabrication does not require any post-processing such as etching or polishing, which allows preserving the integrity of the fiber probe. Finally, the narrow spectrum within which the PCF operates, allows envisaging multi-target detection with a single fiber probe by using cascaded wavelength-multiplexed gratings.

We demonstrate that the feedback phase resulting from a wavelength-detuned fiber Bragg grating affects the laser behavior. Highly detailed experimental and theoretical maps of the laser’s dynamics indicate that the border between dynamical states fluctuates in the same manner as the phase change.