We present the inscription and characterization of fiber Bragg gratings (FBGs) in polypropylene coreless cylindrical fibers. Polypropylene material offers several advantages, such as strong chemical resistance, biocompatibility, and high tensibility. Therefore, polypropylene FBGs can be useful for sensing in chemically aggressive environments and in biomedical applications. The coreless, cylindrical polypropylene waveguides used in these experiments had a diameter of 150 μm, typical length up to 20 cm, and a refractive index of 1.49. The inscription was performed in the 1550 nm transparency window by using a phase mask technique and 193 nm excimer laser radiation. Inscribed FBGs demonstrated complex multi-peak reflection spectra due to highly multi-mode nature of the polypropylene waveguides. Due to a high attenuation of the polypropylene, the maximum waveguide FBG interrogation length -in reflection- was 6 cm. Gratings characterization demonstrated a strain sensitivity of 0.9 pm/με, a temperature sensitivity of -60.4 pm/°C and humidity-insensitive behavior.
A sensing probe is presented for the detection of 2,2,2-trifluoroethanol in the vapor phase, while using poly(vinylidene fluoride (PVDF) thin films, overlaid onto tilted optical fiber Bragg gratings. The 2,2,2-trifluoroethanol sensor operates in the 1.5 μm band, in transmission mode, where the signal of both the core and cladding modes is monitored. Best detectivities obtained are 2 ppm for 2,2,2-trifluoroethanol vapors in ambient atmosphere, for typical response times of 50 min. The sensing probe presented - based on PVDF transductor - shows limited reversibility after being used in the tracing of 2,2,2-trifluoroethanol vapors; its subsequent exposure to nitrogen flow, partly reverses its spectral behavior back to the starting point, denoting the involvement of mechanisms other than physisorption into the underlying transduction. The actual sensing mechanism of 2,2,2-trifluoroethanol vapors while using thin PVDF films is currently under investigation.
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