Femtosecond laser direct writing annular small-period long-period fiber gratings for simultaneous refractive index and temperature sensing.

A new type of small-period long-period fiber grating (SP-LPFG) consisting of a series of annuli inscribed by a femtosecond laser in a single-mode fiber is proposed and demonstrated. The effects of the annuli radii and the number of annuli in each period on the transmission spectrum are studied. The transmission spectrum of the annular SP-LPFG exhibits both strong Bragg resonances and cladding mode resonances, which have similar temperature sensitivities of 9.26 and 9.75 pm/°C, respectively. The Bragg resonances are insensitive to the change of environment refractive index (RI), while the cladding mode resonances show a RI sensitivity of 141.89 nm/RIU. The proposed sensor offers a potential solution for biomolecular sensing applications by effectively eliminating the issue of temperature cross-sensitivity. Its compact design and ease of spectral acquisition enhance its practicality and convenience.