In this contribution we present investigations developed in the last years by our multidisciplinary research group concerning the possibility to use fiber grating based sensors for relative humidity monitoring in the Compact Muon Solenoid (CMS) experiment at CERN, in Geneva. In particular our research, firstly focused on the development of relative humidity fiber Bragg grating (FBG) sensors coated with micrometer thin polyimide overlays, has been recently extended to long period grating (LPG) sensors, coated with a finely tuned titanium dioxide (TiO2) thin layer (~100 nm thick). Experimental tests in the range [0-75] %RH and at different temperatures were carried out to assess the FBG and LPG humidity sensors performances in real operative conditions required in experiments running at CERN. Progressive irradiation campaigns with γ,- ionizing radiations were also performed. Obtained results demonstrate the strong potentialities of the two proposed technologies in light of their future exploitation as robust and valid alternative to currently used commercial hygrometers in High Energy Physics (HEP) applications.