Responses of a long-period grating fabricated in a graded-index optical fiber to temperature and refractive-index changes

Sensing experiments with a long-period grating (LPG) inscribed by a CO₂ laser in a single-mode fiber with an inverted-parabolic index profile in the fiber cladding were done. A single LPG with a period of 500 μm was used in experiments and spectral changes of the attenuation band centered at ≈1520 nm were measured. Responses of the LPG to temperature changes and temperature-induced refractive-index changes were investigated. An average shift of 0.56 nm/°C was obtained for the bare LPG and of 0.86 nm/°C for the LPG recoated with a polymeric layer. An overall shift of the central wavelengths was observed in experiments repeated after two weeks that can be attributed to post-curing processes in the polymeric layer.