Long-period gratings inscribed in air- and water-filled photonic crystal fiber for refractometric sensing of aqueous solution

We report a comparative study of inscription of long-period gratings (LPGs) in air- and water-filled photonic crystal fiber (PCF) via residual stress relaxation using a scanning CO2 laser for refractometric sensing. Under the same inscribing condition, LPGs fabricated following the two PCF schemes exhibit different resonance spectral features. Using NaCl aqueous solution of various concentrations, we demonstrate that both PCF-LPGs possess excellent sensitivity of ∼10−7 refractive index unit within the index range of 1.33 and 1.35, whereas LPGs inscribed in water-filled PCF show much narrower full width at half maximum in the resonance spectrum.