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Wavelength-division multiplexing (WDM) systems based on 40-Gb/s channel bit rate appear as likely successors of current widespread N×10 Gb/s systems. However, they will provide larger throughputs than N×10 Gb/s systems only if some specific techniques are implemented, so as to improve the utilization ratio of the optical bandwidth, namely, the information spectral density (ISD). Narrow optical filtering is one such technique. Based on a review of some of our multiterabit/second transmission experiments in terrestrial configurations, we highlight the benefits of optical filtering to reshape the channel spectra. First, we show that vestigial sideband (VSB) narrow filtering of nonreturn-to-zero (NRZ) data makes 0.64-bit/s/Hz ISD possible over long-haul and ultra-long distances, provided that a specific wavelength allocation scheme is implemented. Using this scheme, a record 10-Tb/s capacity is demonstrated over 300 km by applying different data along each of the two polarization axes. However, along a single polarization axis and when channels are packed closer with NRZ-VSB filtering, at 0.8-bit/s/Hz ISD, nonlinear interactions between channels affect system performance and reduce the maximum error-free distance. We show that these interactions can be contained, still using narrow optical filtering, but by resorting to an alternative modulation format, namely, phase-shaped binary transmission.