In the paper, we describe in detail a highly efficient S-band thulium-doped fiber amplifier (TDFA) employing a high-thulium-concentration doping technique. A TDFA with a dual-pass configuration is superior to a TDFA with a single-pass configuration with regard to power-conversion efficiency, whereas it is inferior with regard to noise figure (NF). We propose a cascaded configuration consisting of a single- and double-pass configuration, which provides high efficiency and a low NF. The configuration generated no interference noise, which can result from multipass interference (MPI) with a double-pass configuration. We achieved gains exceeding 26 dB and NFs of less than 7 dB in the 1480–1510-nm wavelength region (30-nm bandwidth). Furthermore, an 8 × 10-Gb/s transmission experiment also confirmed that there was no MPI-induced excess noise.
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