Refractive index sensor in eccentric fiber Bragg gratings using a point-by-point IR femtosecond laser

In this paper, we demonstrate point-by-point inscription of eccentric fiber Bragg grating (EFBG) in conventional single-mode fiber (Corning SMF-28) with second-order Bragg resonances within the C-band using an IR femtosecond laser. Highly localized fiber Bragg gratings can be inscribed point-by-point with focused ultrashort pulses. EFBGs are localized dose to the core-cladding interface, yielding strong dadding mode resonance couplings and high photoinduced birefringence. The inscription conditions to optimize the grating fabrication have been explored to achieve second-order resonances of good spectral quality. Potential applications of EFBG in refractometry sensing are further illustrated. Two interrogation techniques for EFBG in refractometry are reported and experimentally tested. (C) 2019 Optical Society of America