A large mode field diameter optical fiber is disclosed. It is a single mode optical fiber which has a core and a cladding. The core of this fiber has two regions, an inner core region and an outer core region which has a refractive index lower than that of the inner core region. The inner core region is doped with a rare-earth element, such as Er2O3, and a co-dopant, such as Al2O3, and the outer core region is doped with a dopant such as GeO2. The types and amounts of the dopants and co-dopants are adjusted to obtain a fiber with a mode field diameter greater than 5.5 μm at a wavelength of 1550 nm.