  We report an on-chip single mode microlaser with low-threshold fabricated on
Erbium doped lithium niobate on insulator (LNOI). The single mode laser
emission at 1550.5 nm wavelength is generated in a coupled photonic molecule,
which is facilitated by Vernier effect when pumping the photonic molecule at
970 nm. A threshold pump power as low as 200 uW is demonstrated thanks to the
high quality factor above 10^6. Moreover, the linewidth of the microlaser
reaches 4 kHz, which is the best result in LNOI microlasers. Such single mode
micro-laser lithographically fabricated on chip is highly in demand by photonic
community.
