Single-carrier versus sub-carrier bandwidth considerations for coherent optical systems

40 and 100 Gb/s Ethernet services have been recently defined and 400G and 1 Tb/s services are anticipated in the future. Optical transport networks are capable of separating the services provided and the underlying optical transport; service bandwidths may be larger or smaller than the bandwidth carried on a single optical 'wave'. In this work, we compare different arrangements of optical transport systems using polarization division multiplexed (PDM) coherent modulation formats of 4 to 8 bits/symbol for baud rates spanning 8 - 91 GHz. A nonlinear threshold has been defined and nonlinear performances are compared over 20 uncompensated spans of SMF and NZDSF for various modulation formats. The finding is that lower baud rate equates to slightly more reach at equal capacity. For example, for 100G using single carrier vs. dual carrier on a 50 GHz BW, it is observed that for the EDFA-only case, using dual-carrier transmission yields a reach improvement of 5%, whereas in the Raman-assisted EDFA case, a reach improvement of 4% in favor of dual-carrier transmission. This shows that one can achieve the same or better optical performance without having to drive up the baud rate and the speed of associated electro-optics.