  Materials with strong $\chi^{(2)}$ optical nonlinearity, especially lithium
niobate, play a critical role in building optical parametric oscillators
(OPOs). However, chip-scale integration of low-loss $\chi^{(2)}$ materials
remains challenging and limits the threshold power of on-chip $\chi^{(2)}$ OPO.
Here we report the first on-chip lithium niobate optical parametric oscillator
at the telecom wavelengths using a quasi-phase matched, high-quality microring
resonator, whose threshold power ($\sim$30 $\mu$W) is 400 times lower than that
in previous $\chi^{(2)}$ integrated photonics platforms. An on-chip power
conversion efficiency of 11% is obtained at a pump power of 93 $\mu$W. The OPO
wavelength tuning is achieved by varying the pump frequency and chip
temperature. With the lowest power threshold among all on-chip OPOs
demonstrated so far, as well as advantages including high conversion
efficiency, flexibility in quasi-phase matching and device scalability, the
thin-film lithium niobate OPO opens new opportunities for chip-based tunable
classical and quantum light sources and provides an potential platform for
realizing photonic neural networks.
