  On-chip frequency comb generations enable compact broadband sources for
spectroscopic sensing and precision spectroscopy. Recent microcomb studies
focus on infrared spectral regime and have difficulty in accessing visible
regime. Here, we demonstrate comb-like visible frequency line generation
through second, third harmonic, and sum frequency conversion of a Kerr comb
within a high Q aluminum nitride microring resonator pumped by a single telecom
laser. The strong power enhancement, in conjunction with the unique combination
of Pockels and Kerr optical nonlinearity of aluminum nitride, leads to cascaded
frequency conversions in the visible spectrum. High-resolution spectroscopic
study of the visible frequency lines indicates matched free spectrum range over
all the bands. This frequency doubling and tripling effect in a single
microcomb structure offers great potential for comb spectroscopy and
self-referencing comb.
