Increasing the spectral bandwidth of WDM optical transmission is a solution to improve bit-rate carried in one fiber. Future transmitted wavelengths should be optically amplified, like in current 1.55-μm systems. To increase total throughput of WDM transport networks, addition of a new band at the short wavelengths (S-band) in addition to the C-band and the L-band is likely. Since the band located around λ=1510 nm±10 nm is used for the optical supervisory channel, a location for this S-band could be between λ=1470 nm and λ=1500 nm, thus minimizing loss of the link fiber. Thulium doped fluoride fiber amplifier (TDFA) is a promising candidate for the next generation of optical amplifier because its amplification bandwidth (centered arround λ=1.47 μm) is located in a low-loss region for silica line fibers. However, its power conversion efficiency and its spectral gain characteristics are crucial issues to address.
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