Extending the amplification bandwidth of erbium-doped fiber amplifiers (EDFAs) is one of the most cost-effective means of expanding the fiber transmission capacity. In conventional aluminosilicate EDFAs, gain drops sharply beyond 1605nm. Several new or modified erbium host materials have been used to extend the amplification band to beyond the conventional L-band, such as tellurite, bismuth-oxide, antimony silicate, P-doped aluminosilicate, and phosphosilicate EDFAs. Although tellurite and bismuth-oxide based EDFAs have wider bandwidths compared to that of demonstrated phosphosilicate (P-Si) EDFAs, P-Si EDFAs have been shown to provide better noise performance when the gain bandwidth is extended to 1620 nm. In addition, unlike tellurite or bismuth-oxide fibers, P-Si EDF does not exhibit increased nonlinearity or weakened reliability, compared to conventional aluminosilicate EDFs. Furthermore, phosphosilicate erbium fiber is compatible with other silica fibers and can be fusion spliced to the standard SMF silica fiber with high return loss and extremely low splice loss. These properties make the P-Si EDF a top contender for commercial extended L-band EDFAs.
One key issue in the design of the extended L-band amplifiers is the optimization of power conversion efficiency while keeping the noise figure low. In this paper, we explore various amplifier configurations and compare their performances experimentally. We report high-power P-Si EDFAs with simultaneous improvement of PCE and NF by a combination of a single-pass low-noise stage with one or two double-pass high-efficiency stages. The best configuration yields high power (22dBm), low NF (5.5dB maximum over 1570-1620 nm band), and high efficiency (27% overall PCE after gain flattening).