A multimode step-index polymer optical fiber has been developed based on a highly crosslinked polycarbosiloxane as the core material and a fluorinated polyolefin (poly(fluoroethylene-co-fluoropropylene)-FEP) cladding layer. The combination offered a system with a high thermal stability, both optically and dimensionally, up to at least 200 degrees C. The polymer optical fiber was produced by injection of a poly(phenylmethylvinylhydro)siloxane crosslinkable prepolymer in an FEP tubing, after which the prepolymer was cured in situ. From attenuation measurements losses as low as 0.98 dB/m at 780 nm were found. The refractive indices of the core material and the cladding are 1.48 and 1.30, respectively. The large difference in refractive index results in a high value for the numerical aperture of 0.70. The prepolymer is a single-component, self-crosslinking, and conveniently processable system. The combination of a low attenuation and easy processability makes these materials very suitable not only for polymer optical fibers, but for all kinds of optical components.