Since the success of the Er-doped fiber amplifier, the rare earth doped glasses have attracted much attention as a candidate of optical amplifiers for broad-band wavelength-division-multiplexed (WDM) telecommunication as a result of growing demand for transmission capacity. In order to improve the amplifier performance, the development of novel host materials is important. To broaden the emission spectra of doped rare earth ions in glasses, it is important to understand the relationship between the local ligand field and various optical properties due to related 4f-transitions, such as the radiative transition probability, the nonradiative decay rate, which dominate the spectral linewidth and quantum efficiency of amplification transitions. For the Er3+: 1.55 μm transition, the role of Judd-Ofelt Ω6 parameters is presented on the emission bandwidth, which is correlated to the Er-ligand bond covalency in glasses. The Tm3+: 1.46 μm transition shows quantum efficiency over 90%, high enough for the S-band application, in heavy metal oxide glasses with moderate phonon energy, in which wider emission spectra are obtained than in fluorides. Novel glass materials have potential to vary the radiative cross section, quantum efficiency, and gain bandwidth, which are important for amplifiers in the future WDM system.