In this work is reported the experimental and numerical results of the refractive index response of etched fibre Bragg gratings written in a graded index multimode fibre. The responses of the modes coupled by the grating inscribed in a multimode fibre are compared with the mode coupled by a grating inscribed in single mode fibre. The results of this study show that the refractive index sensitivity and the dynamical range of etched fibre Bragg gratings written in multimode fibres are higher than the ones verified in single-mode fibres. The determination of oil-biodiesel blend concentrations are also compared as an example of practical applications. It is shown that a greater core diameter of the multimode fibre enables the Bragg gratings to exhibit enhanced sensitivity without requiring further fibre diameter reduction.
In this work we demonstrate the potential of the polyaniline to the development of a gas sensor exploring the optical properties of a polymer which was verified by UV-Vis spectroscopy measurements. This polymer used as active material, changes its optical properties when exposed to the vapors of different alcohols and acetic acid. The transmittance spectrum of the samples was measured in the range of 350 nm to 1000 nm and results show that is it possible to made a selective detection among the isopropanol, ethanol and the acetic acid. Analyses of transmittance measurements as a function of exposure time show an exponential behavior. The fit parameters obtained to the different contaminants are used to prove the response time and sensitivity of the polyaniline film and these can be a indicative to develop a sensor functionalized with this organic material.
In this paper, an induced birefringence was observed on an ammonia sensor based on an etched fiber Bragg grating (EFBG) coated with an organic film. The film was prepared with a solution of the perylene derivative, along with the copolymer PCDTBT in chlorobenzene. The EFBG was coated by drop cast technique, and a split in the reflection band, characteristic of birefringence, was observed after deposition. The response of the two observed peaks as a function of time was modeled by a pair of exponentials that could represent two processes: adsorption followed by diffusion of the gas molecules in the microstructured film. Such phenomena alter the boundary conditions of the optical fiber propagating mode, which allows an indirect ammonia detection by monitoring the EFBG wavelength shift of the reflected peaks. The sensor was experimentally tested in a concentration range from 3 ppm to 41,700 ppm, and the calibrations curves were determined in a range from 27 ppm to 6960 ppm. The two observed peaks showed different sensitivities, which presents an intrinsic potential for multiparameter measurements. The split in the reflection band was hypothesized to be birefringence induced by the formation of an asymmetric film. This hypothesis was numerically evaluated and compared with the experimental results.
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