  The erbium-doped Lithium niobate on insulator (Er:LNOI) platform has great
promise in the application of telecommunication, microwave photonics, and
quantum photonics due to its excellent electro-optic, piezo-electric, nonlinear
nature as well as the gain characteristics in the telecommunication C-band.
Here, we report a single-frequency Er:LNOI integrated laser based on
dual-cavity structure. Facilitated by the Vernier effect and gain competition,
the single-frequency laser can operate stably at 1531-nm wavelength with a
1484-nm pump laser. The output laser has a power of 0.31 uW, a linewidth of 1.2
MHz, and a side mode suppression ratio (SMSR) of 31 dB. Our work allows the
direct integration of this laser source with existing LNOI components and paves
the way for a fully integrated LNOI system.
