  We propose an efficient method to determine the effective refractive index of
step-index optical fibers from the visible to the mid-IR and thus allowing to
infer their dispersive properties over a broad spectral range. The validity of
the method, based on the writing of an array of fiber Bragg gratings (FBGs)
with known periods using the fs scanning phase mask technique, is first
confirmed with a standard silica fiber, then applied to various fluoride glass
fibers to determine their effective refractive index and dispersion over more
than three octaves, i.e. from 550 to 4800 nm.
