A single mode optical transmission fiber comprises a depressed core having at least 0.41 weight percent fluorine and an index difference (|.DELTA.n.sub.1|) with pure silica greater than 1.5.times.10.sup.-3, a depressed cladding having at least 1.2 weight percent fluorine and an index difference (|.DELTA.n.sub.2|) with pure silica greater than 4.5.times.10.sup.-3 and an index difference (|.DELTA.n.sub.2|-|.DELTA.n.sub.1|) with the depressed core greater than or equal to 3.times.10.sup.-3.