This paper describes in detail the amplification characteristics of gain-shifted thulium-doped fiber amplifiers (GS-TDFAs) operating in the 1480-to 1510-nm wavelength region (1.49-µm S-band) for use in wavelength-division-multiplexing (WDM) systems. Gain shifting of a TDFA, which normally has a gain band at 1.47 µm (S+-band),is achieved by two types of dual-wavelength pumping: (1) 1.05 and 1.56 µm or (2) 1.4 and 1.56 µm. The main pump source at 1.05 or 1.4 µm creates population inversion between 3F4 (upper laser level) and 3H4 (lower laser level), while the auxiliary pump source at 1.56 µm reduces the average fractional inversion down to approximately 0.4, which is a desired level for gain shifting. We show experimentally that the former provides a low internal noise figure (< 4~dB) due to high fractional inversion at the input end of a thulium fiber, while the latter provides a very high optical efficiency but a higher internal noise figure (∼ 5~dB) due to the lower fractional inversion at the input end. These characteristics were verified by numerical simulation based on a comprehensive rate equation modeling. We demonstrated a 1.4-and 1.56-µm laser-diode-pumped GS-TDFA with an optical efficiency of 29.3% and high output power of +21.5~dBm. Gain flatness and tilt control were also investigated. These results strongly confirm the feasibility of using GS-TDFAs in practical ultralarge-capacity WDM networks.
[IEEE ]PDF Article