  Soliton microcombs are helping to advance the miniaturization of a range of
comb systems. These combs mode lock through the formation of short temporal
pulses in anomalous dispersion resonators. Here, a new microcomb is
demonstrated that mode locks through the formation of pulse pairs in
normal-dispersion coupled-ring resonators. Unlike conventional microcombs,
pulses in this system cannot exist alone, and instead must phase lock in pairs
to form a bright soliton comb. Also, the pulses can form at recurring spectral
windows and the pulses in each pair feature different optical spectra. This
pairwise mode-locking modality extends to higher dimensions and we demonstrate
3-ring systems in which 3 pulses mode lock through alternating pairwise pulse
coupling. The results are demonstrated using the new CMOS-foundry platform that
has not previously produced bright solitons on account of its inherent normal
dispersion. The ability to generate multi-color pulse pairs over multiple rings
is an important new feature for microcombs. It can extend the concept of
all-optical soliton buffers and memories to multiple storage rings that
multiplex pulses with respect to soliton color and that are spatially
addressable. The results also suggest a new platform for the study of quantum
combs and topological photonics.
