<p>According to one example of the invention an optical fiber comprises: (i) a core (12) consisting of Al doped silica having a first index of refraction n1; (ii) at least one silica based cladding (14) surrounding the core and having a second index of refraction n2, such that n1>n2; (iii) a hermetic carbon based coating (16) surrounding said cladding, said hermetic coating being 300nm -1000 nm thick; and (iv) a second coating (18) surrounding said hermetic coating, said second coating being 5 µm to 80 µm thick.</p>
