An improved optical fiber achieves both reduced bending and microbending losses, as well as a much higher Brillouin threshold, as compared to standard transmission fibers. The optical fiber comprises a core including at least two dopants and having a refractive index difference .DELTA.n.sub.1 with an outer optical cladding, a first inner cladding having a refractive index difference .DELTA.n.sub.2 with the outer cladding, and a depressed, second inner cladding having a refractive index difference .DELTA.n.sub.3 with the outer cladding of less than -3.times.10.sup.-3. The radial concentration of at least one of the core dopants varies continuously over the entire core region of the optical fiber.