Cascaded erbium-doped fiber amplifiers (EDFAs) have enabled ultralong-distance transmission systems. By incorporating wavdength-division multiplexing (WDM), the system capacity can be dramatically enhanced.1 However, a key difficulty in implementing a WDM transmission system, which incorporates an amplifier cascade, is that the gain is nonuniform with wavelength, the gain peak is shifted from 1531 nm to near 1560 run along the cascade due to property of the absorption and emission cross-section of erbium ion and the saturation of amplifiers.2 Therefore, the different wavelength channels experience a different gain, resulting in a severe power differential among the channels after a cascade of EDFAs.3 Previous reported work has focused primarily on the equalization of the gain peak located at 1531 nm for a single amplifier.4,5 In this work, we experimentally demonstrate passive equalization of the nonuniform gain of cascaded EDFAs for which a newly-generated gain peak appears near 1560 nm. The passive equalization is accomplished over a 7.4-nm wavelength range by periodically using notch filters along a 1000-km transmission link. Our results show that the signal power differential among the four WDM channels is reduced from 19 dB to 4 dB and that the 10−9 BER receiver sensitivity differential among the channels is decreased from 7.5 dB to only 3 dB.

© 1996 Optical Society of America

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