Thermooptical response of the graphene-coated FBG. (a) Schematic scheme of the measurement: same Bragg gratings are written in the core of the GCF and the silica fiber, and a heat probe is used to control the temperature. (b) Central wavelength of the in-core FBGs redshifts when increasing the temperature in fiber. The graphene coating does not influence the sensitivity (12.5 pm/K). (c) Time-dependent wavelength shift of FBGs in the GCF (blue dots) and in the typical silica fiber (red dots), driven by the temperature alteration between 24°C and 104°C. The blue and red dashed line marks that the wavelength shift approaches stability. (d) A higher environmental temperature enables lower delay of thermooptical response. Here, TD means optically measured thermal diffusivity. Error bar denotes the measurement uncertainty.