Long period grating fibre operating in visible range coated with porphyrin based thin film as an ammonia aqueous sensor

A long period grating (LPG) operating in the visible wavelength range functionalised with sensitive dyes is reported. Two different sensing mechanisms transmission based, using intensity ratio; and refractive index change based, using the PMTP attenuation bands shift are presented. The visible range LPG’s with phase matching turning point (PMTP) ~700 nm was fabricated using amplitude mask laser inscribing approach on a hydrogen loaded SM600 fibre. Ammonia sensitive thin film based on diazonium resin (DAR) and tetrakis-(4-sulfophenyl)porphine (TSPP) dye was deposited via layer-by-layer deposition on LPG fibre and glass substrate. Glass substrate coated by DAR-TSPP was used to calculate the complex refractive index (RI) difference. The response of the visible LPG was compared to an infrared LPG with a same coating.