This chapter presents the results obtained in collaboration with the civil engineering department of EPFL on the development of a FBG based humidity sensor. The first section, based on a conference paper presented in 2001, reports the influence of temperature and relative humidity on polyimide coated FBG. The following section presents the published joined paper appeared in Optics letters in 2002. A theoretical approach of diffusion based on Fick's law has also been developed and numerically implemented to study the swelling properties of the polyimide used for fiber recoating. The results are not presented hereafter but can be found in the thesis work of Pascal Kronenberg (Thesis work, EPFL, 2002).
