Disclosed is an optical fiber having a core of SiO.sub.2 doped with fluorine and an alkali metal oxide dopant. The alkali metal oxide is selected from the group consisting of K, Na, Li, Cs and Rb and is provided in amount of at least 20 ppm wt. %. The fiber has an inner cladding surrounding the core, which also includes fluorine. A relative refractive index of the inner cladding (.DELTA..sub.2%), measured relative to pure silica, is preferably between -0.39% and -0.7%. The fiber preferably exhibits attenuation at 1550 nm of less than or equal to 0.178 dB/km.