This paper details an approach for shifting the gain of an S-band thulium-doped fiber amplifier (TDFA) that employs a high thulium concentration doping technique. The technique can shift the gain from the conventional gain band to the promising longer wavelength region by forming a suitable population inversion between the <sup>3</sup>H<sub>4</sub> and <sup>3</sup>F<sub>4</sub> levels that results from cross relaxation between thulium ions. It has been shown that, unlike with an erbium-doped fiber amplifier, the interaction between thulium ions caused by the high concentration doping increases the efficiency up to a thulium concentration of 6000 ppm. The authors also describe the particular characteristics of Tm ions that appear when the fiber length and pump power ratio between the forward and backward pump powers in bidirectional pumping are varied. Furthermore, the authors optimized the thulium concentration, fiber length, and pump power ratio between the forward and backward pump powers, so that a high gain and a low noise figure (NF) in the S-band are obtained. As a result, the TDFA achieved a gain of 22 dB and NFs of < 6 dB in the 1477- to 1507-nm wavelength region at a thulium concentration of 6000 ppm.
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