  We demonstrate an on-chip single-mode Er3+-doped thin film lithium niobate
(Er: TFLN) laser which consists of a Fabry-P\'erot (FP) resonator based on
Sagnac loop reflectors (SLRs). The fabricated Er: TFLN laser has a footprint of
6.5 mmx1.5 mm with a loaded quality (Q) factor of 1.6x105 and a free spectral
range (FSR) of 63 pm. We generate the single-mode laser around 1550-nm
wavelength with a maximum output power of 44.7 {\mu}W and a slope efficiency of
0.18 %.
