  Photonic integrated resonators stand out as reliable frequency converters due
to their compactness and stability, with second-harmonic generation (SHG)
efficiencies of up to 17000%/W reported recently in aluminum nitride
microrings. In this work, a sufficiently strong second-harmonic (SH) signal up
to microwatts was generated by a photonic integrated frequency doubler using a
milliwatt infrared (IR) laser source. Furthermore, increased SHG bandwidth
covering $^{85}$Rb and $^{87}$Rb D$_2$ transition lines as well as saturated
absorption spectroscopy (SAS) were demonstrated by tuning the pump power and
chip temperature. Here, we present, to the best of our knowledge, the first
successful locking of an IR laser to Rb saturated absorption lines via a
photonic chip frequency doubler.
