According to one example of the invention an optical fiber comprises: (i) a core comprising Al doped silica but essentially no Er or Yb, and having a first index of refraction n1; (ii) at least one F doped silica based cladding surrounding the core and having a second index of refraction n2, such that n1>n2, wherein the cladding comprises essentially of SiO2 and 0.2-5 wt % F; (iii) a hermetic carbon based coating surrounding said cladding, said hermetic coating being 200 to 1000 Angstroms thick; and (iv) a second coating surrounding said hermetic coating, said second coating being 5 μm to 80 μm thick.