A multimode silica glass optical fiber, comprising: a central fluorine-doped core surrounded by an outer cladding, said central core having ( i) an outer radius r 1, ( ii) an alpha-index profile, ( iii) a maximum refractive index difference Δn 1 with respect to said outer cladding that is less than or equal to zero, and ( iv) at said outer radius r 1, a minimum refractive index difference Δn end with respect to said outer cladding; and an interior fluorine-doped cladding layer positioned between said central core and said outer cladding, said interior cladding layer having a negative refractive index difference Δn 2 with respect to said outer cladding, characterised in that: said central core has a maximum germanium concentration of 0.1 weight percent or less; said central core has an average chlorine concentration of 0.1 weight percent or less, and said optical fiber has a core average halogen ratio of 20 or greater and a core-innermost-cladding-layer-average halogen ratio of 20 or greater; wherein, at said outer radius r 1, said central core has a minimum fluorine concentration of 3 weight percent or more, and wherein said optical fiber has a halogen ratio at the central core's outer radius r 1 of between 50 and 500; wherein the halogen ratio refers to the ratio of the weight fraction of elemental fluorine to the weight fraction of elemental chlorine. a central fluorine-doped core surrounded by an outer cladding, said central core having ( i) an outer radius r 1, ( ii) an alpha-index profile, ( iii) a maximum refractive index difference Δn 1 with respect to said outer cladding that is less than or equal to zero, and ( iv) at said outer radius r 1, a minimum refractive index difference Δn end with respect to said outer cladding; and an interior fluorine-doped cladding layer positioned between said central core and said outer cladding, said interior cladding layer having a negative refractive index difference Δn 2 with respect to said outer cladding, characterised in that: said central core has a maximum germanium concentration of 0.1 weight percent or less; said central core has an average chlorine concentration of 0.1 weight percent or less, and said optical fiber has a core average halogen ratio of 20 or greater and a core-innermost-cladding-layer-average halogen ratio of 20 or greater; wherein, at said outer radius r 1, said central core has a minimum fluorine concentration of 3 weight percent or more, and wherein said optical fiber has a halogen ratio at the central core's outer radius r 1 of between 50 and 500; wherein the halogen ratio refers to the ratio of the weight fraction of elemental fluorine to the weight fraction of elemental chlorine. said central core has a maximum germanium concentration of 0.1 weight percent or less; said central core has an average chlorine concentration of 0.1 weight percent or less, and said optical fiber has a core average halogen ratio of 20 or greater and a core-innermost-cladding-layer-average halogen ratio of 20 or greater; wherein, at said outer radius r 1, said central core has a minimum fluorine concentration of 3 weight percent or more, and wherein said optical fiber has a halogen ratio at the central core's outer radius r 1 of between 50 and 500; wherein the halogen ratio refers to the ratio of the weight fraction of elemental fluorine to the weight fraction of elemental chlorine.