Analysis of the Characteristics of PVA-Coated LPG-Based Sensors to Coating Thickness and Changes in the External Refractive Index

Recent research has shown that the transmission spectrum of a long period grating (LPG) written into an optical fiber is sensitive to the thickness and the refractive index (RI) of a thin layer deposited on it, a concept which forms the basis of a number of optical fiber sensors. The research reported herein is focused, in particular, on sensors of this type to create a design with an optimized thickness of the deposited layers of polyvinyl alcohol (PVA) on LPGs forming the basis of a highly sensitive probe. In creating such sensors, the dip-coating technique is used to minimize deleterious effects in the attenuation bands resulting from the inhomogeneity of the coated surface. In this paper, LPGs with different coating thicknesses are uniformly coated with thin layers of PVA and submerged in oils of known RI over the range from 1.30 to 1.70, to create effective RI probes. It is observed that when the coating thickness reaches a particular value that enables a substantial redistribution of the optical power within the overlay, a maximum sensitivity of the sensor can be achieved, even when the overlay has a RI higher than that of the cladding. The experimental results obtained through the characterization of the devices developed are shown to be in good agreement with the results of a theoretical model.