Ground state absorption in thulium-doped fiber amplifier: experiment and modeling

S-band ground state absorption in a thulium-doped fiber amplifier (TDFA) has been studied in detail experimentally and theoretically. While traditionally considered difficult to measure, its presence is shown to have significant impact on amplifier power excursion during dynamic wavelength add-drop, due to the energy transitions inherent to the Tm/sup 3+/ ion. For specific pumping scheme and operating conditions, gain increases with increasing signal input, a behavior significantly different from conventional three-level optical amplifiers. Numerical models are presented that correctly account for these phenomena. This promises to provide insight for gain control algorithms for TDFAs under bursty traffic conditions in a future reconfigurable wavelength division multiplexed environment.