Abstract
Long Period Fibre Gratings (LPFGs) were fabricated by femtosecond (fs) laser direct writing in a standard single-mode fibre (SMF-28e) to measure variations in the surrounding refractive index (SRI). The sensing sensitivity of these structures was optimized with the deposition of homogeneous thin layers of titanium dioxide (TiO2) by physical vapour deposition (PVD) process. A set of LPFGs were coated with different thickness layers of TiO2, and the spectral features were monitored for different SRI solutions. The wavelength shift and the optical power variation of the LPFG minimum attenuation band were measured achieving sensitivities of similar to 570 nm/RIU at using SRI near to 1.3600 in the case of the LPFG coated with 60 nm of TiO2, a 10-fold increase over the corresponding for a bare LPFG. For SRI values higher than the cladding refractive index, a sensitivity over similar to 3000 nm/RIU was determined for 30 nm of TiO2 thick film, a region where the bare LPFGs are useless. For 30 nm of TiO2, the optical power variation follows a quasi-linear function of the SRI, with a range of similar to 10 dB. Moreover, values as high as 50 and 120 dB/RIU at 1.3200 and 1.4200, respectively, can be obtained by choosing the proper film thickness. Preliminary studies revealed that coating fs-laser direct writing LPFGs with titanium dioxide improves their performance.

Abstract
Optical microbubble resonators are among the highest sensitivity optical sensors. In the context of its application in the detection of water micro contaminants, in portable systems, their interrogation must be made by tracking the resonant wavelength peak position with the highest accuracy possible, at a reasonable cost. In this work different laser sources and scanning methods were tested and compared, aiming the development of a portable prototype. Each tunable laser source, was evaluated using a C2H2 Gas cell, which provided an absolute wavelength reference. Light transmitted through the cell was recorded using a photodetector and a software controlled feedback loop, enabling locking into selected reference peaks. Three distinct scanning methods were tested and compared for each laser source: large and short-range laser scanning and external waveform dithering, from which minimum standard deviations of 20, 0.18, and 0.07 pm, were obtained, respectively.

Abstract
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Abstract
Long period fiber gratings (LPFGs) were fabricated in a standard single mode fiber (SMF-28e) through femtosecond (fs) laser direct writing. LPFGs with longer and shorter periods were fabricated, which allows coupling from the fundamental core mode to lower and higher order asymmetric cladding modes (LP(1,6)and LP1,12, respectively). For the grating periods of 182.7 and 192.5 mu m, it was verified that the LP(1,12)mode exhibits a TAP at approximately 1380 and 1448 nm in air and water, respectively. Characterization of the LPFGs subjected to high-temperature thermal treatment was accomplished. Fine-tuning of the resonance band's position and thermal stability up to 600 degrees C was shown. The temperature sensitivity was characterized for the gratings with different periods and for different temperature ranges. A maximum sensitivity of -180.73, and 179.29 pm/degrees C was obtained for the two resonances of the 182.7 mu m TAP LPFG, in the range between 250 and 600 degrees C.

Abstract
Biogenic amines, such as putrescine are potential indicators of food storage condition and deterioration. The real time measurement of their concentration in food may become an important method of food control. It was found that putrescine diffuses through a thin layer made from a solution of Poly(ethylene-co-vinyl acetate) (PEVA) and maleic anhydride. Poly(ethylene-co-vinyl acetate) is a common non-chlorinated vinyl capable to adsorb specific analytes as putrescine which upon diffusion, reversibly binds to the maleic anhydride causing the polymer swelling resulting in spectral changes from the optical point of view. Long Period Fiber Gratings coated with 30 nm titanium dioxide, a high refractive index material used to increase the intrinsic sensitivity to the external refractive index, were overcoated with a thin layer of maleic anhydride doped Poly(ethylene-co-vinyl acetate). When exposed to solutions containing small concentrations of putrescine the resonant band corresponding to the LP1,6 cladding mode was found to move to shorter wavelengths. The observed blue shift corresponds to the increasing concentration of putrescine in the fiber sensor structure. Further work is being carried out to improve the sensitivity and the limit of detection of the sensing system as well as to increase range of operation, which is presently limited to 0.3 to 0.5 M.

Abstract
In this work, a long-period fiber grating (LPG) based sensor was evaluated as a sensing device for micro-force measurement, in the order of micro Newtons. It was used an LPG fabricated by arc-inducted technique in a SMF-28 standard optical fiber. The optical fiber was fixed between two clamps with a separation of 150 mm with the middle of the LPG located at the center. Characterizations were performed in terms of temperature, curvature and strain. The grating was then used as a micro-force sensor by means of both curvature and strain, induced by a hung mass in a stretched fiber. Furthermore, the evaluation of a precurvature LPG was performed to assess if an increase of sensitivity is achieved. Micro-force sensitivity achieved with the stretched LPG was 1.41 nm/mN and it was demonstrated that its sensitivity can be enhanced to 5.14 nm/mN with a pre-curvature of 2.2 m–1 applied to the LPG, achieving a spectral resolution of at least 15.6 µN.