A fusion splice including a first optical fiber having a first MFD and a first MFD expansion rate. The splice further includes a second fiber having a second MFD and a second MFD expansion rate, wherein the second MFD is lower than the first MFD. The second fiber comprises a core, a cladding radially surrounding the core, and a zone of high-concentration of fluorine between the core and the cladding. The rate of MFD expansion of the first fiber is less than the rate of MFD expansion of the second fiber during the fusion splicing operation.