Abstract

Erbium-doped fiber amplifiers (EDFAs) are revolutionizing both long-distance and distribution-based optical communications because they compensate for fiber attenuation in the former, and network splitting losses in the latter, respectively. Furthermore, optically amplified systems can have a manifold increase in the system capacity through the incorporatioon of wavelength-division multiplexing (WDM). However, one difficulty in implementing a WDM system incorporating EDFAs is that the EDFA gain spectrum is wavelength dependent, whereas the link loss between amplifiers is wavelength independent. Such a gain/loss wavelength discrepancy will result in a severe signal-to-noise ratio (SNR) differential among channels after they pass through a cascade of EDFAs. Several methods have been employed to solve this gain-differential problem, e.g., passive filters,¹ amplifier cooling,² and acousto-optic tunable filters (AOTF) with a single passband.³ However, multipassband AOTFs, which possess actively and dynamically multidimensional equalizing capabilities, provide a promising solution for equalizing the nonuniform EDFA gain in a WDM cascaded-amplifier system. In this paper, we analyze the multipassband AOTF-filtered WDM cascaded-amplifier system with respect to critical system variables, including the number of passbands, the spacing between passbands, the passband bandwidths, and the robustness to variations in the link loss between amplifiers. Although it may be obvious that an infinite number of dynamic passbands will equalize any EDFA gain, we determine (i) the minimum required number of passbands to achieve equalization in a cascade of amplifiers, (ii) the minimum number of AOTFs required along the chain, and (iii) the system robustness to variations in the interamplifier-link loss. We find that the SNR differential among nine WDM channels can be reduced from 18.2 dB to only 2.4 dB after 10000 km by using only four 2-nm-wide passbands and placing an AOTF after every 10 EDFAs.

© 1995 Optical Society of America