Abstract

It is well known that distributed Raman amplification (DRA) is advantageous compared to EDFA-amplification in terms of amplified spontaneous emission (ASE) noise accumulation. The closer to the input end of the span the signal is amplified, the more attenuation the ASE noise will experience, and the lower the resulting noise figure.¹ However, there is a trade-off, since such ‘early’ amplification will result in higher path-averaged signal power P_av in the span, i.e., in larger nonlinear penalties. The optimal solution would be the one that maximizes the output OSNR (not the one minimizing the noise figure) for a fixed nonlinear weight, i.e., for a fixed P_av (given the transmission distance).² In this report, for the general case of multipump DRA for ultra-broadband WDM systems, we derive the fundamental limit for the ratio OSNR/P_av, and analyze the ways to approach this limit in practice. We show that OSNR/P_av is maximized when the signal power remains close to P_av along the fiber. Also, we show that the EDFA-post-amplification in DRA-systems always decreases this ratio.

© 2002 Optical Society of America