Tilted optical fiber Bragg grating with fluorinated graphene-like overlayer for ammonia detection

We describe an optical fiber sensor for detection of ammonia vapors employing a fluorinated graphene-like overlayer on a tilted Bragg grating. Exploiting the laser-mediated explosive synthesis and transfer (LEST) of graphene (Gr) flakes a thin film of few-layer turbostratic graphene flakes doped with F atoms (~ 3.3 at. %) are deposited on the fiber at the location of the grating. The response of the sensor was investigated for NH4OH vapors while for reference, the effect of H2O was also monitored at identical conditions. Under increasing vapor pressure of NH4OH, wavelength shift is recorded not only in the cladding modes but also for the fundamental Bragg mode, indicating that the effect is not solely due to changes in the optical parameters of the overlayer. The monitored wavelength shift is initially negative turning to positive when vapor saturation is reached. Furthermore, there is a distinct difference in the magnitude of the monitored shifts with the higher order mode exhibiting 2.5x higher values compared to the Bragg mode. The study is ongoing and will also include overlayers of pure LEST Gr and LEST Gr decorated with Six nanoparticles.