Fluoride glass-based optical fiber for an optical amplifier which contains rare earth metal ions in the core glass. The fluoride glass core is doped with rare earth metal ions, and part of the fluorine in the fluoride glass is substituted by at least one halogen. Pr 3+ , Pr 3+ -Yb 3+ -Nd 3+ , or Pr 3+ -Er 3+ can be doped as the rare earth metal ions. Chlorine, bromine or iodine may be used as the halogen.