For the EU DEMO conductor testing, a temperature sensor based on Fiber Bragg Grating (FBG) optical fiber is studied at the EPFL Swiss Plasma Center. The SULTAN test facility has been upgraded to use fiber optic sensor for temperature monitoring and quench detection. Up to four optical fibers can be interrogated simultaneously (eight fibers separately). In order to qualify the system and improve fiber integration in complex superconducting structure, one 2.6 m-long REBCO conductor of a SULTAN sample has been instrumented with FBG sensors. Two fibers, each containing ten FBG sensors along their length and separated by 60 mm, are installed within the conductor. The first fiber is soldered together with the REBCO tapes and thus subjected to the thermal strain of the whole conductor. The second fiber is inserted inside a stainless steel capillary tube that insures strain free condition to the FBG sensor. A 4-FBG sensors fiber is also placed at the outer surface of the stainless steel jacket in correspondence with CERNOX sensors. The procedure for fiber routing and protection is described. In a separated experiment in a small cryostat, different coating materials are studied and used to calibrate the FBGs response from room temperature to 4.5 K over several cycles.