The invention relates to an optical fiber comprising a gain medium which is equipped with: a core which is formed from a transparent material and nanoparticles comprising a doping element and at least one element for enhancing the use of said doping element; and an outer cladding which surrounds the core. A longitudinal end portion of the hose base body is diametrically expanded, and a main hose body is formed with a small diameter main portion, a large diameter swaged portion and a tapered portion. The processing solution may be contained within a flexible bladder within the chamber.