  The recent advancement in lithium niobate on insulator (LNOI) technology is
revolutionizing the optoelectronic industry as devices of higher performance,
lower power consumption, and smaller footprint can be realized due to the high
optical confinement in the structures. The LNOI platform offers both large
\c{hi}(2) and \c{hi}(3) nonlinearities along with the power of dispersion
engineering, enabling brand new nonlinear photonic devices and applications
towards the next generation of integrated photonic circuits. However, the Raman
scattering, one of the most important nonlinear phenomena, have not been
extensively studied, neither was its influences in dispersion-engineered LNOI
nano-devices. In this work, we characterize the Raman radiation spectra in a
monolithic lithium niobate (LN) microresonator via selective excitation of
Raman-active phonon modes. Remarkably, the dominant mode for Raman oscillation
is observed in the backward direction for a continuous-wave pump threshold
power of 20 mW with a reportedly highest differential quantum efficiency of 46
%. In addition, we explore the effects of Raman scattering on Kerr optical
frequency combs generation. We achieve, for the first time, soliton modelocking
on a X-cut LNOI chip through sufficient suppression of the Raman effect via
cavity geometry control. Our analysis of the Raman effect provides guidance for
the development of future chip-based photonic devices on the LNOI platform.
