OPTICAL FIBER AND METHOD FOR MAKING SUCH FIBER

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According to one example of the invention an optical fiber comprises : (i) silica based, rare earth doped core having a first index of refraction n1; (ii) at least one silica based cladding surrounding the core and having a second index of refraction n2, such that n1> n2 with the following features, alone or in combination : said cladding includes 0.5 to 5 wt% F and 0.5 to 20 wt% B, said optical fiber has less than 8dB/km core background loss at a wavelength of 1280 nm. at least one of the core or cladding is doped with AI203 concentration is less than 2 : 1.