Theoretical Characterization of the Ultra-Broadband Gain Spectra at $\sim$1600–2100 nm From Thulium-Doped Fiber Amplifiers

In this paper, we present the theoretical modeling and characterization of the gain spectra at ~1600-2100 nm from thulium-doped fiber amplifiers (TDFAs) for the first time. Our theoretical model was validated through the comparison with the experiments on the widely tunable TDFAs and thulium-doped fiber lasers using in-band pumping (³H₆ →³F₄) and 79 χ-nm pumping (³H₆ →³H₄) schemes. Different emission and absorption cross sections of the commercial thulium-doped fibers were employed for comparison and comprehensive demonstration. We then calculated the performance of the in-band-pumped TDFAs under different spectral cross section profiles and amplifier configurations. It shows, without the restriction of the loss induced by the fiber-optic components, that high optical gains (> 20 dB in small signal regime and > 3 dB in saturation regime) can be achieved over an ultrabroad spectrum ranging from ~1580 to ~2080 nm. We also demonstrate, although it is rarely used in the experiments of the broadband TDFAs, that the 79x-nm cladding pumping scheme can effectively avoid the strong reabsorption at short wavelengths when long gain fibers are used.