Temperature sensitivity of long-period gratings inscribed with a CO2 laser in optical fiber with graded-index cladding

Long-period gratings (LPGs) inscribed in a novel fiber, in which the smoothed refractive index of the cladding increases approximately quadratically with radius (IGI fiber), are investigated for temperature sensing. Fabrication of the fiber and LPGs is briefly described and results of a theoretical analysis are given. The analysis yields fields and effective indices of the LP01 core mode and LP0i cladding modes of the IGI fiber. Wavelength-dependent effective indices, computed for 11 lowest-order cladding modes and an external index of 1.33, are incorporated into a diagram of the phase-matching condition covering also coupling through higher harmonics of the grating.Temperature sensitivities of LPGs submerged in water and in a liquid with a refractive index that decreases with temperature and is close to that of the pure-silica fiber cladding are determined. Average sensitivities of 0.60 and 0.56 nm/°C are obtained for the LPGs with periods of 1000 and 500 μm in water. For the same LPGs in the above liquid, average sensitivities of 5.40 (3.95) and 3.76 (3.11) nm/°C are obtained in a 10 °C (20 °C) temperature interval with a lower limit of sensing at about 50 °C. A significant improvement in sensitivity is observed after slightly bending an LPG in the liquid.