A relative humidity sensor using a hydrogel-coated long period grating

A long period grating ( LPG) coated with hydrogel has been developed for monitoring the relative humidity ( RH) levels. The response wavelength of the LPG varies with the changes of the relative humidity. The four- layered step index waveguide model is presented to explain this observation, which is based on the effect of the hydrogel thin film, the index of which increases with the increasing RH. Experiment shows that the hydrogel- coated LPG sensor is highly sensitive between 38.9% and 100% RH, and behaves linearly with humidity with an accuracy of +/- 2.3% RH. The small- size and low- cost LPG fibre sensor will have extensive applications in various areas, such as remotely measuring RH, building health monitoring, medical facilities and goods' preservation.