Abstract

The combination of erbium-doped fiber amplifiers (EDFAs) and wavelength-division-multiplexing (WDM) offers a dramatic increase in the system capacity for point-to-point ultralong-distance transmission and for local and widearea network,^1,2 However, one key difficulty in implementing such systems is that the gain of EDFA cascades is not uniform with wavelength.^2,3 Passive optical filtering for gain equalization can provide performance improvement for a static link, but such filtering is not satisfactory for a dynamically recomfigurable network. This is because: (i) the individual channel powers may vary significantly over time; and (ii) the gain nonuniformity varies with dynamic input load. Because of its ability to function as a multiple-passband filter with independent control of each wavelength passband, the acousto-optic tunable filter (AOTF)^4,5 with active feedback can be used to dynamically equalize the EDFA gain. Previous work uses the AOTF as notch filters to actively equalize the WDM channels over a 1000-km transmission distance.⁵ However, amplified spontaneous emission (ASE) accumulates between channels due to the narrow AOTF passband as compared to the channel spacing, thereby limiting the transmission distance. In this paper, we demonstrate the active equalization of the nonuniform EDFA gain and ASE suppression by using acoustic-optic tunable filters as transmission filters with active feedback. After 2500 km, the power differential is reduced from over 20 dB down to 2 dB for three 2.5-Gbit/s WDM channels covering an ~8-nm wavelength range. Furthermore, the active feedback fully compensates for dynamic variations in individual channel powers.

© 1996 Optical Society of America