  On-chip optical resonators have proven to be a promising platform for
generating Kerr frequency combs. Whispering gallery mode resonators are
particularly attractive because of their small footprint as well as low
threshold and power consumption. This performance can be attributed to two
characteristics: the cavity quality factor (Q) and the cavity dispersion. The
input optical field into the cavity is amplified by the cavity Q, enabling
nonlinear processes to occur with low input powers. In addition, the total span
of the optical comb is governed by the dispersion. In an optical cavity-based
comb, the dispersion is governed by the geometric dispersion of the cavity and
the material dispersion. While many different materials have been explored, the
realization of ultra-high Q (UHQ) on-chip frequency combs sources is still
challenging for most materials. One exception is the family of integrated
silica devices. However, because the silica attracts water molecules from the
ambient environment, the lifetime of the device performance is fundamentally
limited, unless packaged in a controlled atmosphere. Here, we demonstrate the
generation of environmentally-stable frequency combs fabricated from
SiO1.7N0.13 microtoroidal resonators. Due to the UHQ factors of the cavities,
parametric oscillations with sub-microwatt thresholds are demonstrated. Based
on their geometry and material properties, the cavities have normal dispersion.
However, due to avoided mode crossing, frequency combs are generated. Finally,
unlike SiO2, the SiO1.7N0.13 inhibits the formation of a water monolayer,
allowing sub-microwatt performance to be maintained for over a week in devices
stored with minimal environmental controls.
