Highly reflective Bragg gratings in slightly etched polymer optical fibers and their application for sensing

During the last decade, a strong progress has been made in the photo-writing of fiber Bragg gratings (FBGs) in polymer optical fibers (POFs), driven by the continuous wish to enhance the grating reflectivity and improve the sensing performances. In this paper, we report the photo-inscription of highly reflective gratings in step-index POFs, obtained from a slight etching of the cladding. We demonstrate that a cladding diameter decrease of ~20% is an ideal trade-off to produce highly reflective gratings with enhanced axial strain sensitivity, while keeping almost intact their mechanical resistance. For this, we make use of Trans-4-stilbenemethanol-doped photosensitive step-index poly(methyl methacrylate) (PMMA) POFs. Both uniform and tilted FBGs are inscribed at ~1550 nm by the scanning phase mask technique in POFs. Reflectivity reaching 97% is achieved for 6 mm long FBGs. We also report that a cladding decrease enhances the FBG axial strain sensitivity while keeping unchanged temperature and surrounding refractive index sensitivities. Surrounding refractive index sensing and transverse force sensing are also reported with tilted Bragg gratings and polarization-assisted uniform gratings, respectively.