Abstract: An optical fiber for transmitting high optical power and of the type having a central core portion adapted to conduct radiation, a cladding layer surrounding the core and at least one covering surrounding the cladding adapted to give an improved mechanical stability to the fiber and to damp scattered radiation wherein, in order to prevent the radiation level in any volume part of the covering from exceeding the limit at which the material of the covering is damaged, the fiber is, at least at its entrance side over a part of its length, provided with radiation resistant and radiation absorbing materials comprising a radiation resistant transmitting material with a real refractive index exceeding or close to that of the cladding, and one, or several, additional material layers of which the outermost are of heat-conducting materials, the radiation being absorbed in the outermost heat-conducting layer(s).