A multimode optical fiber comprises a central core having an alpha profile, a depressed cladding having a portion in continuity with the alpha profile of the central core and a stepped portion, and an outer cladding. The alpha profile is obtained by co-doping at least two dopants. The variation in concentration of each dopant and its derivative in relation to the fiber radius are continuous. A multimode fiber for Ethernet optical system with an improved bandwidth is thus obtained.