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Abstract: Quasi-distributed sensors based on Fibre Bragg Gratings (FBG) usually deal with the concatenation of FBGs of different Bragg wavelengths analyzed through wavelength-sensitive devices. In these sensors, a given wavelengths range is dedicated to one particular FBG. The number of sensing points is directly limited by the source and detector spectral ranges, and by the wavelength spacing between two gratings. This spacing is linked to the maximum possible excursion of the physical parameter to be measured-before superimposition of the reflection spectra of the FBGs. In this paper, an original interrogating device is presented, that allows a very large number of concatenated gratings to be addressed. In this scheme, identical FBGs (same Bragg wavelength and same low reflectivity) are interrogated by the Optical Time Domain Reflectometry technique, for which a commercial device has been extended to a wavelength-tunable system, within an automated experimental set-up. Detection and localization of an important amount of sensing points along a unique optical fibre is demonstrated. Repeatability measurements did exhibit the very good accuracy of the presented sensor. [Copyright &y& Elsevier]

Abstract: We have investigated the polarization properties of Bragg gratings fabricated in hydrogen-free photosensitive Ge/B codoped PS1250/1500 and hydrogenated telecom SMF-28 fibres by high-intensity femtosecond UV pulses. We have shown that, under given experimental conditions, the Bragg grating inscription proceeds in these fibres differently, via single-photon (PS1250/1500) or two-photon (SMF-28) absorption mechanisms. In relation to that, in the first fibre both the polarization-dependent loss (PDL) and the differential group delay (DGD) grow linearly with the irradiation fluence, in the second one the PDL and DGD values show a linear dependence only up to fluence value of about 0.7kJ/cm2 and then increase drastically. We found that PDL and DGD values for Bragg gratings recorded by high-intensity 264nm pulses overcome significantly (by 2–4 and 4–10 times, respectively) the corresponding values obtained in the same fibres under low-intensity CW 244nm irradiation. [Copyright &y& Elsevier]

Abstract: 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 2mm-long uniform polarization-maintaining grating in order to simultaneously measure longitudinal strain and temperature. [Copyright &y& Elsevier]

The effective indices of the cladding modes of optical fibers depend on the refractive index of the medium surrounding the fiber. We show experimentally and theoretically that while cladding modes with similar effective indices normally have similar refractometric sensitivities, the addition of a 50 nm thick gold sheath enhances the sensitivity of some EH modes by more than one order of magnitude while nearly completely suppressing the sensitivity of neighbouring HE modes (by three orders of magnitude, down to insignificant levels). A differential sensitivity of ∼1000 nm/(refractive index unit) is experimentally reported between adjacent EH and HE grating resonances. [ABSTRACT FROM AUTHOR]

The first measurements of polarisation properties of a long-period grating inscribed in an endlessly singlemode photonic crystal fibre are reported. Strong modulation in the spectra of polarisation-dependent loss and differential group delay with periods of 2.6 and 1.3 nm, respectively, were found. As such an effect has not been observed in standard optical fibres, it is believed that this is due to the specific mode structure of the holey fibre used for grating fabrication. [ABSTRACT FROM AUTHOR]

We have studied the gamma-radiation induced effect on the Bragg peak position in fiber gratings. For the first time, to the best of our knowledge, a short-wavelength shift was observed for the Type I gratings written in an intrinsically photosensitive fiber. Based on the kinetics of the peak shift observed under irradiation and during post-irradiation annealing, we propose that two different types of defects are generated by ionizing radiation. One type is responsible for the refractive index increase during irradiation, while the creation of the defects of another type results in the index decrease. The concurrence of the two types of defects can be a way to obtain high stability of the gratings under irradiation. [ABSTRACT FROM AUTHOR]

In this paper, we report an optical fiber pH sensor based on a polyaniline coating deposited on the surface of a tilted fiber Bragg grating. The pH-sensitive film was synthesized by in situ chemical oxidative polymerization keeping track of the deposition time in order to optimize the sensor response. As a result, the sensor reacts to pH changes in the range of 2–12 with a fast response and its sensitivity is directly related to the film thickness. The main advantages of this PAni-TFBG pH sensor are its biochemical compatibility, temperature independence, long-term stability and remote real-time multipoint sensing features. [ABSTRACT FROM AUTHOR]