  Frequency comb generation in microresonators at visible wavelengths has found
applications in a variety of areas such as metrology, sensing, and imaging. To
achieve Kerr combs based on four-wave mixing in a microresonator, dispersion
must be in the anomalous regime. In this work, we demonstrate dispersion
engineering in a microbubble resonator (MBR) fabricated by a two-CO$_2$ laser
beam technique. By decreasing the wall thickness of the MBR down to 1.4 $\mu$m,
the zero dispersion wavelength shifts to values shorter than 764 nm, making
phase matching possible around 765 nm. With the optical \textit{Q}-factor of
the MBR modes being greater than $10^7$, four-wave mixing is observed at 765 nm
for a pump power of 3 mW. By increasing the pump power, parametric oscillation
is achieved, and a frequency comb with 14 comb lines is generated at visible
wavelengths.
