  Microresonator-based frequency combs are strong contenders as light sources
for wavelength-division multiplexing (WDM). Recent demonstrations have shown
the potential of microresonator combs for replacing tens of WDM lasers with a
single laser-pumped device. These experiments relied on microresonators
displaying anomalous dispersion. Devices operating in the normal dispersion
offer the prospect of attaining high power conversion efficiency - an aspect
that will be crucial in the future for enabling energy-efficient coherent
communications with higher order modulation formats or lighting several spatial
channels in space-division multiplexing. Here we report the experimental
demonstration of coherent communications using normal dispersion microresonator
combs. With polarization multiplexed (PM) quadrature phase-shift keying, we
transmitted data over more than 6300 km in single-mode fiber. In a second
experiment, we reached beyond 700 km with PM 16 quadrature amplitude modulation
format and an aggregate data rate above 900 Gbit/s assuming 6% error correction
overhead. These results represent the longest fiber transmission ever achieved
using an integrated comb source.
