  Quantum-dash (QD) mode-locked laser diodes (MLLD) lend themselves as
chip-scale frequency comb generators for highly scalable wavelength-division
multiplexing (WDM) links in future data-center, campus-area, or metropolitan
networks. Driven by a simple DC current, the devices generate flat broadband
frequency combs, containing tens of equidistant optical tones with line
spacings of tens of GHz. Here we show that QD-MLLDs can not only be used as
multi-wavelength light sources at a WDM transmitter, but also as
multi-wavelength local oscillators (LO) for parallel coherent reception. In our
experiments, we demonstrate transmission of an aggregate data rate of 4.1
Tbit/s (23x45 GBd PDM-QPSK) over 75 km standard single-mode fiber (SSMF). To
the best of our knowledge, this represents the first demonstration of a
coherent WDM link that relies on QD-MLLD both at the transmitter and the
receiver.
