  Microcavity lasers based on erbium-doped lithium niobate on insulator (LNOI),
which are key devices for LNOI integrated photonics, have attracted much
attention recently. In this Letter, we report the realization of a C-band
single-mode laser using Vernier effect in two coupled Erbium-doped LNOI
microrings with different radii under the pump of a 980-nm continuous laser.
The laser, operating stably over a large range of pumping power, has a pump
threshold of ~200 {\mu}W and a side-mode suppression ratio exceeding 26 dB. The
high-performance LNOI single-mode laser will promote the development of lithium
niobate integrated photonics.
