Optical duobinary signals have been applied to dense wavelength division multiplexing (WDM) systems with high-spectral efficiency to fully utilize a limited gain bandwidth of about 35 nm (4.4 THz) for an erbium-doped fiber amplifier (EDFA). These signals are one type of partial response signals and have narrower bandwidth than conventional intensity modulation (IM) signals. Thus, the use of these signals should make possible to attain ultradense WDM systems. In this paper, characteristics of optical duobinary and IM signals in ultradense WDM systems are compared through experimental evaluations at 20 Gb/s. The measured minimum channel spacing for 20 Gb/s optical duobinary and IM signals were 32 and 45 GHz, respectively. The performance limitations for both signals were clarified by numerical simulations considering a coherent crosstalk. The calculated results agree well with experimental results and indicate that optical duobinary signals allow us to set narrower channel spacing than that for IM signals optimized both transmitter bandwidth and {\alpha}-parameter. Dispersion tolerances were also evaluated. The high and symmetrical dispersion tolerance of 600 ps/nm observed in experimental results for the optical duobinary signals is twice the tolerance of IM signals. The optical duobinary signals can achieve superior characteristics for both coherent crosstalk suppression and dispersion tolerance simultaneously. To the contrary, there is a tradeoff involved in both characteristics of the IM signals and the potential applications of these signals are more limited than those for optical duobinary signals. By using optical duobinary signals which have compact spectrum and high dispersion tolerance, a 2.64 Tb/s (20 Gb/s132-channel) signal has been successfully transmitted over 120 km standard fiber with only common dispersion compensation fiber at the receiver. High-spectral efficiency of 0.6 b/s/Hz was reached in this demonstration.


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