According to one example of the invention an optical fiber comprises: (i) a silica based core, said core having a core diameter greater than 80 .mu.m and a numerical aperture NA.gtoreq.0.24; and (ii) a silica based cladding in contact with and surrounding the core and having a second index of refraction n.sub.2, such that n.sub.1>n.sub.2; wherein the cladding includes B and F. Preferably the numerical aperture NA is at least 0.3.