  Synchronization is a ubiquitous phenomenon in nature that manifests as the
spectral or temporal locking of coupled nonlinear oscillators. In the field of
photonics, synchronization has been implemented in various laser and oscillator
systems, enabling applications including coherent beam combining and high
precision pump-probe measurements. Recent experiments have also shown
time-domain synchronization of Kerr frequency combs via coupling of two
separate oscillators operating in the dissipative soliton [i.e., anomalous
group-velocity dispersion (GVD)] regime. Here, we demonstrate all-optical
synchronization of Kerr combs in the non-solitonic, normal-GVD regime in which
phase-locked combs with high pump-to-comb conversion efficiencies and
relatively flat spectral profiles are generated. Our results reveal the
universality of Kerr comb synchronization and extend its scope beyond the
soliton regime, opening a promising path towards coherently combined normal-GVD
Kerr combs with spectrally flat profiles and high comb-line powers in an
efficient microresonator platform.
