CHARACTERISTICS OF OPTICAL DUOBINARY SIGNALS IN TERABIT S CAPACITY, HIGH-SPECTRAL EFFICIENCY WDM SYSTEMS/

Optical duobinary signals have been applied to dense wavelength divisi on multiplexing (WDM) systems with high-spectral efficiency to fully u tilize a limited gain bandwidth of about 35 mn (4.4 THz) for an erbium -doped fiber amplifier (EDFA). These signals are one type of partial r esponse signals and hale narrower bandwidth than conventional intensit y modulation (IM) signals. Thus? the use of these signals should make possible to attain ultradense WDM systems. In this paper, characterist ics of optical duobinary and IM signals in ultradense WDM systems are compared through experimental evaluations at 20 Gb/s, The measured min imum channel spacing for 20 Gb/s optical duobinary and IM signals were 32 and 45 GHz, respectively. The performance limitations for both sig nals were clarified by numerical simulations considering a coherent cr osstalk. The calculated results agree well with experimental results a nd indicate that optical duobinary signals allow us to set narrower ch annel spacing than that for IM signals optimized both transmitter band width 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 th e tolerance of IM signals, The optical duobinary signals can achieve s uperior characteristics for both coherent crosstalk suppression and di spersion tolerance simultaneously. To the contrary, there is a tradeof f involved in both characteristics of the IM signals and the potential applications of these signals are more limited than those for optical duobinary signals, BS using optical duobinary signals which have comp act spectrum and high dispersion tolerances a 2.64 Tb/s (20 Gb/s x 132 -channel) signal has been successfully transmitted over 120 km standar d fiber with only common dispersion compensation fiber at the receiver . High-spectral efficiency of 0.6 b/s/Hz was reached in this demonstra tion.