Characterization of 1050-nm Pumped $S$-Band TDFA With Different Dopant Concentrations by Coherent Optical Frequency-Domain Reflectometry

For thulium-doped fiber amplifier (TDFA) design, knowledge of the gain distribution within the active fiber is of great value. For the first time, the distributed gain along highly thulium-doped fibers was measured with high-resolution reflectometry. The technique of coherent optical frequency-domain reflectometry is a nondestructive and noninvasive method well matched to this task due to its dynamic range, spatial resolution, and measurement range. Using thulium-doped fibers with different Tm
³⁺
-dopant concentrations, we show precise measurements of Rayleigh backscattering levels for obtaining the optimum gain-length ratio for S-band TDFA pumped by 1050-nm laser diodes