Abstract: An optical fiber for signal amplification comprises a core, doped with at least 15% phosphorus by weight and a rare earth ion sufficient to provide amplification of an optical signal. The fiber also comprises a clad layer surrounding the core with an inner portion and an outer portion. The refractive index of the inner portion of the clad layer is lower than that at the outer radius of the core region. This depressed inner clad portion serves to confine electromagnetic radiation within a selected wavelength range to substantially only the core.