In this thesis, we theoretically investigate optimum configurations of high conversion efficiency and gain-flattened L-band (1570-1600 nm) erbium-doped fiber amplifier (EDFA) by employing the 1480 nm and 980 nm bi-directional pumping configuration. The design criterion of L-band EDFA is to achieve highest channel output power while keeping the differential channel gain to be ¡Õ0.7 dB among 32 channels with low channel noise figure of ¡Õ6 dB. The nine L-band EDFA configurations are examined and compared. These configurations considered include the dual-forward, dual-backward, and different bi-directional pumping schemes, each with and without the midway optical isolator. Among all configurations, we find that the pump-pass case in forward-and-backward pumping scheme by employing the 1480 nm pumping wavelength is the best configuration to offer the highest channel output power with good channel gain uniformity and moderate low noise figure. Then we verified the simulation results through experiments. We also theoretically investigate gain-flattened C-band (1530-1560 nm) EDFA. Using L-band and C-band EDFAs in parallel, we can greatly expand the amplification wavelength region.