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We investigated the transmission of 4 WDM channels at 40 Gb/s, through different optical links of approximate equal lengths, around 50 km, but with different residual dispersions produced by various types of fibers, G. 652 (STD) + DCF (dispersion compensating fiber), G. 653 (DS) and G. 655 (NZD+). The experimental setup includes components usually employed in laboratory characterizations of WDM transmission systems at 10 Gb/s. The optical system behavior is analyzed theoretically and experimentally with bit-error-rate (BER) measurements at 40 Gb/s, in the back to back configuration and transmitted trough different optical fiber links. The results show the generation of BER floor by spontaneous emission and phase-intensity noises, as well as by the chirp of the external modulator at the transmitter. Different approaches are discussed to upgrade the 10 Gb/s legacy, and to prevent the combination of chromatic dispersion effects, that produce severe penalties in WDM transmissions at 40 Gb/s.