Abstract

Extremely-low-noise operation of in-line amplifiers in transoceanic submarine systems imposes conflicting requirements. Because signal loss at the erbium-doped fiber amplifier (EDFA) input adds directly to the noise figure, counterpropagation of pump and signal is necessary. On the other hand, it is the inversion at the EDEA input that is most important in determining the noise figure. For 980-nm-pumped in-line EDFAs in long-haul submarine systems (which for reasons of system robustness will be operated with gain compression of at least 6 dB), pump depletion in counterpumped amplifiers seriously degrades the noise figure. We show that the use of a two-stage 980-nm-pumped amplifier with a counterpumped first stage and a midamplifier filter to suppress the buildup of amplified spontaneous emission (ASE) around the 1530-nm gain peak allows noise figures as low as 3.4 dB. Previously, the behavior of two-stage amplifiers has been considered primarily for preamplifier applications and only for copropagating pump and signal.^2,3

© 1994 Optical Society of America