An optical fiber having a given refractive index profile comprises a core region and a cladding region. The core region includes at least two co-dopants and the concentration of at least one of said core dopant varies continuously over the entire core region. The optical fiber has, at a wavelength of 1550 nm, a spontaneous Brillouin spectrum width equal or larger to 100 MHz.The optical fiber of the invention achieves a much higher Brillouin threshold compared to standard transmission fibers with limited fiber loss, less than 0.3 dB/km at a wavelength of 1550 nm, and without change in the optical transmission parameters of the fiber.