An apparatus for estimating at least one parameter in a downhole environment includes: an optical fiber configured to be disposed in a borehole, the optical fiber including a core having a first index of refraction and a cladding surrounding the core and having a second index of refraction that is lower than the first index of refraction, at least a portion of the core being made from a hydrogen resistant material; at least one fiber Bragg grating (FBG) formed within the hydrogen resistant material; a light source configured to send an optical signal into the optical fiber; and a detector configured to receive a return signal generated by the at least one FBG and generate data representative of the at least one parameter.