  Polarization splitter-rotators (PSRs) are the key elements to realize on-chip
polarization manipulation. Current PSRs on thin film lithium niobate (TFLN)
rely on sub-micron gaps to realize modes separation, which increase the
difficulties of lithography and etching. In this paper, a polarization
splitter-rotator on TFLN based on multimode interference (MMI) is demonstrated.
Mode division is achieved by an MMI-based mode demultiplexer. The feature size
of the PSR is 1.5 {\mu}m, which can be fabricated with low priced i-line
contact aligners. Experimental results show a polarization extinction ratio
(PER) > 20 dB and insertion loss (IL) <1.5 dB are achieved in a wavelength
range of 1542-1600 nm for TE-polarized light. And a PER > 9.5 dB and an IL <3.0
dB are achieved in a wavelength range of 1561-1600 nm for TM-polarized light.
This PSR could find application in the low-cost fabrication of
dual-polarization TFLN integrated photonic devices.
