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Analysis of optical response of long period fiber gratings to nm-thick thin-film coating.

We theoretically and experimentally demonstrated that the resonant wavelength of long period fiber gratings (LPG) could be shifted in a large magnitude by coating only nm-thick thin-film whose refractive index is higher than that of the glass cladding. The resonant wavelength shift results from either the variation of the thickness of the film and/or the variation of the refractive index of the film. These results demonstrate the sensitivity of LPG-based sensors could be enhanced by using a sensing thin film with an allowed large thickness and high refractive index. This coating schematic offers an efficient platform for achieving high-performance index-modulating fiber devices and high-performance index/thickness-sensing LPG-based fiber sensors for detecting optical property variations of the sensing thin-film coating.