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 close to the core-cladding interface, yielding strong cladding 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.

Behavior of femtosecond laser-induced eccentric fiber Bragg gratings at very high temperatures.

In this work, eccentric Bragg gratings are photoinscribed in telecommunication-grade optical fibers. They are localized close to the core-cladding interface, yielding strong cladding mode resonance couplings and high photoinduced birefringence. Their transmitted amplitude spectrum is measured with polarized light while they are exposed to temperature changes up to 900°C. Despite the gratings' overall good thermal stability that confirms their robustness for high-temperature refractometry, we report an interesting polarization effect depending on both the cladding mode resonance family and mode order. While the core mode birefringence decreases with growing temperatures, certain cladding mode resonances show an increase in wavelength splitting between their orthogonally polarized components. This differential behavior is of high interest in developing high-resolution, multiparametric sensing platforms.