1. Introduction
Glasses doped with rare-earth ions (REn+) are proving to be luminescence materials as they have high emission
efficiencies. These emissions correspond to 4f–4f and 4f–5d electronic transitions in the REn+. The 4f–4f transition
gives an especially sharp fluorescence pattern from the ultraviolet (UV) to the infrared region. This is due to
shielding effects of the outer 5s and 5p orbitals on the 4f electrons. In recent years, glasses doped with rare-earth
ions have drawn much attention due to their potential applications in solid-state lasers, optical amplifiers and three-dimensional displays [1]–[4].