This paper presents a design approach for multistage gain-flattened fiber Raman amplifiers (FRAs) utilizing the multiwavelength-pumping scheme. To the authors' best knowledge, it is the first time that Raman amplifiers of more than one stage are considered in the design process so as to optimize multistage amplifier performance simultaneously. It is shown both theoretically and experimentally that optical power path integrals among several stages can, in principle, be arbitrarily redistributed while maintaining its gain performance, as long as the consolidated sums at different pump wavelengths are unaltered. The overall gain spectrum of a multistage FRA is thus very close to the cumulative spectrum of chained identical single-stage amplifiers with span numbers equal to its stages. However, the traditional prerequisite to minimize gain flatness in every stage is no longer required. It provides flexibilities to take full advantage of pump lasers with moderate launched powers and allows the reduction of the number of pump lasers and/or wavelengths in most stages. By arranging small positive net gain to the first stages and enhancing it at shorter signal wavelengths, significant reduction and flattening of total noise figure (NF) is achieved even if a pure backward pumping scheme is utilized. Finally, various pumping configurations for Raman amplifiers with hybrid dispersion-compensating fiber (DCF) and standard single-mode fiber (SMF) are discussed, with the objective of realizing flattened gain and noise performance simultaneously without using forward pumps.
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