Alcohols detection by a polymeric sensing film deposited on a long-period fiber grating

In this paper, we report the construction and characterization of a sensor to detect volatile organic compounds, particularly alcohols. The sensor was fabricated by depositing a polydimethylsiloxane sensing film on the surface of a long-period fiber grating (LPFG). The LPFG spectrum, which was taken as the sensor response, was measured in a Teflon chamber, where different concentrations of methanol, ethanol, propanol, and butanol samples were injected. Spectral shifts to lower wavelength were observed for the last three samples. However, a shift to higher wavelengths for the case of methanol was observed, which was attributed to the fact that its refractive index is smaller than the one for PDMS. Therefore, curves of the sensor response in function of concentration were performed using different analysis techniques: analysis in a fixed wavelength, analysis in a fixed spectrum amplitude and spectral analysis trough principal components analysis (PCA) method. The results showed a regular behavior of the responses according to the molecular weight of the samples. A discussion of the use of each technique was performed.