Residual strain monitoring during composite manufacturing

This work explores a new possibility in accurate measurement of multi-axial residual strains during the production of composite structures. It investigates the usability of the polarization dependent loss (PDL) of an optical fibre Bragg grating as a sensitive indicator of multi-axial residual strains. The experimental work presented is done on a cross-ply carbon fibre reinforced polymer fabricated using an autoclave cycle

Hybrid fiber grating cavity for multi-parametric sensing.

We propose an all-fiber hybrid cavity involving two unbalanced uniform fiber Bragg gratings (FBGs) written at both sides of a tilted FBG (TFBG) to form an all-fiber interferometer. This configuration provides a wavelength gated reflection signal with interference fringes depending on the cavity features modulated by spectral dips associated to the wavelength dependent optical losses due to cladding mode coupling occurring along the TFBG. Such a robust structure preserves the advantages of uniform FBGs in terms of interrogation methods and allows the possibility of simultaneous physical and chemical sensing

Single and Multiple Phase Shifts Tilted Fiber Bragg Gratings

The spectral behavior of single and multiple phase shifts tilted fiber Bragg gratings has been experimentally investigated. To this aim, a simple and cost-effective postprocessing technique based on local thermal treatment was used to create arbitrary phase shifts along the tilted grating structure. In particular, UV written tilted fiber Bragg gratings were treated by the electric arc discharge to erase the refractive index modulation in well-defined regions. We demonstrate that these defects give rise to interference pattern for all modes, and thus defect states can be achieved within all the attenuation bands, enabling a simple wavelength independent spectral tailoring of this class of devices

Temperature insensitive cure cycle monitoring of cross-ply composite laminates using the polarization dependent loss property of FBG

very important aspect of the composite manufacturing process is the appearance of residual strains and stresses during the curing cycle. Composites exhibit large residual strains after curing. Therefore, in this paper, we propose to follow the evolution of the polarization depend loss peaks (amplitude and wavelength) of fibre Bragg gratings during the manufacturing of the composite material to highlight the residual strains appearanc

Finding Hydrogen Leaks by Means of the Fiber Bragg Gratings Technology

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L-band CYTOP Bragg gratings for ultrasound sensing

Polymer optical fibre (POF) has been receiving increasing attention for sensing applications. The fundamental properties of POF such as PMMA deliver at least an order of magnitude in improvements over silica fibres, though practical difficulties create additional complexity. POF has the potential to deliver lower acoustic impedance, a reduced Young's Modulus and a higher acoustic sensitivity within the megahertz region. In contrast, existing piezo-electric transducers have an inherent narrow acoustic bandwidth and a proportionality to size that causes difficulties for applications such as endoscopy within the biomedical domain. POF generally suffers high attenuation per kilometre at telecommunications wavelengths, limiting fibre lengths to mere centimetres. However, CYTOP, a graded index perfluorinated polymer, is a commercially certified product allowing the use of telecoms region technology and tens of meters of fibre without exceeding acceptable losses. With an effective refractive index between 1.32 and 1.33, it is fundamentally better placed for applications using water or a similar media for acoustic coupling. We demonstrate ultrasonic detection at 5,10 and 15 MHz using a TFBG within a CYTOP fibre in the telecoms region and the latest knowledge in POF handling and connectorisation. This first step in the use of CYTOP demonstrates the viability of the sensor and paves the way towards further advances towards its eventual application

Thermal annealing of tilted fiber Bragg gratings

We report a practical study of the thermal decay of cladding mode resonances in tilted fiber Bragg gratings, establishing an analogy with the “power law” evolution previously observed on uniform gratings. We examine how this process contributes to a great thermal stability, even improving it by means of a second cycle slightly increasing the annealing temperature. In addition, we show an improvement of the grating spectrum after annealing, with respect to the one just after inscription, which suggests the application of this method to be employed to improve saturation issues during the photo-inscription process.Á. González-Vila is supported by the F.R.S.-FNRS through a FRIA grant. C. Caucheteur is supported by the F.R.S.-FNRS. The authors would also like to thank the financial support from the ERC (European Research Council) Starting Independent Grant PROSPER (grant agreement No. 280161 – http://hosting.umons.ac.be/aspnet/erc-prosper/) and from the project TEC2013-47264-C2-1-R of the Spanish government

Near-infrared grating-assisted SPR optical fiber sensors : design rules for ultimate refractometric sensitivity

Plasmonic optical fiber sensors are continuously developed for (bio)chemical sensing purposes. Recently, surface plasmon resonance (SPR) generation was achieved in gold-coated tilted fiber Bragg gratings (TFBGs). These sensors probe the surrounding medium with near-infrared narrowband resonances, which enhances both the penetration depth of the evanescent field in the external medium and the wavelength resolution of the interrogation. They constitute a unique configuration to probe all the fiber cladding modes individually. We use them to analyze the modal distribution of gold-coated telecommunication-grade optical fibers immersed in aqueous solutions. Theoretical investigations with a finite-difference complex mode solver are confirmed by experimental data obtained on TFBGs. We show that the refractometric sensitivity varies with the mode order and that the global SPR envelope shift in response to surrounding refractive index (SRI) changes higher than 1e-2 RIU (refractive index unit) can be ~25% bigger than the local SPR mode shift arising from SRI changes limited to 1e-4 RIU. We bring clear evidence that the optimum gold thickness for SPR generation lies in the range between 50 and 70 nm while a cladding diameter decrease from 125 µm to 80 µm enhances the refractometric sensitivity by ~20%. Finally, we demonstrate that the ultimate refractometric sensitivity of cladding modes is ~550 nm/RIU when they are probed by gold-coated TFBGs

Microstructured PMMA POF chirped Bragg gratings for strain sensing

[EN] We demonstrate a chirped microstructured polymer fiber Bragg grating based on taper technology for strain sensing application. The effective bandwidth of the grating is dependent on strain and remains practically constant with respect to temperature and humidity changes. We report a sensitivity of 0.90 pm/mu e for the central wavelength under stable temperature and humidity values. The 3-dB bandwidth of the grating has been measured under different temperature and humidity conditions.The authors acknowledge the financial support from FCT through the fellowship SFRH/BPD/109458/2015, program UID/EEA/50008/2013 by the National Funds through the Fundacao para a Ciencia e a Tecnologia/Ministerio da Educacao e Ciencia, and the European Regional Development Fund under the PT2020 Partnership Agreement. This work was also supported by the Research Excellence Award Programme GVA PROMETEO 2017/103 and the Science Foundation of Heilongjiang Province of China (2018026).Min, R.; Ortega Tamarit, B.; Broadway, C.; Hu, X.; Caucheteur, C.; Bang, O.; Antunes, P.... (2018). Microstructured PMMA POF chirped Bragg gratings for strain sensing. Optical Fiber Technology. 45:330-335. https://doi.org/10.1016/j.yofte.2018.08.016S3303354