An optical fiber comprising: a silica based core having a first index of refraction n.sub.1; and at least one silica based cladding surrounding the core, the at least one silica based cladding comprising index lowering non-periodic voids containing a gas, wherein at least 80% of said voids have a maximum cross-sectional dimension of less than 2000 nm, and the NA of the fiber layer situated immediately adjacent to and inside said at least one silica based cladding is at least 0.2.