Successful operation of ITER will rely on the use of a large set of magnetic diagnostics. Fundamental parameters such as plasma position, shape, and current are required for real-time plasma control and machine protection. For operating tokamaks such measurements are successfully performed using inductive sensors. In ITER and later in DEMO, the presence of strong radiations combined with steady-state operation creates a difficult problem: the useful signal is affected by the integration of radiation-induced noise. An attractive alternative for plasma current measurement consists in using Fiber Optic Current Sensor (FOCS). However, combined effects of radiation, elevated temperatures, vibrations together with the requirement of vacuum compatibility and installation constrains present a significant challenge for the ITER FOCS system. This paper describes recent results of the ITER FOCS R&D intended to demonstrate that the system can be installed on the tokamak and its performance can satisfy the required criteria. We emphasize that the choice of appropriate fibers is critical. Our simulations show that the spun fibers allow satisfying the target performance provided that the fiber beat length over spun period ratio is above a given value. This conclusion is confirmed by the experimental results obtained on JET.