An optical fiber having at least a first central core segment, said central core segment comprising at least a first region having a width no more than 0.2 .mu.m over a core delta % of 0.1 or greater. The first core region may also over a delta height of at least 0.15 delta % exhibits a profile volume less than 0.1. Such core segments can facilitate optical fibers which exhibit an L01 acousto-optic effective area which is greater in magnitude than the L02 acousto optic effective area.