Optical fiber for optical amplifier and process for manufacturing thereof

An optical fiber for an optical amplifier having a clad and a core, the core comprising:

• a first core layer doped with first impurities comprising Tm3+ ions and metal ions selected from the group consisting of Al, La, Te and mixture thereof;
• and
• a second core layer surrounding the outer surface of the first core layer and containing no doped impurities comprising Tm ions and metal ions selected from the group consisting of Al, La, Te and mixture thereof;
• wherein each of said clad, first core layer and second core layer of the optical fiber is a silica-based glass fiber which contains fluoride of a constant concentration, the optical fiber being used for the optical amplifier in the signal band of 1430 nm to 1530 nm.