Frequency comb generation via cascaded second-order nonlinearities in microresonators

Abstract: Optical frequency combs are revolutionising modern time and frequency metrology. In the past years, their range of applications has increased substantially, driven by their miniaturisation through microresonator-based solutions. The combs in such devices are typically generated using the third-order $\chi^{(3)}$-nonlinearity of the resonator material. An alternative approach is making use of second-order $\chi^{(2)}$-nonlinearities. While the idea of generating combs this way has been around for almost two decades, so far only few demonstrations are known, based either on bulky bow-tie cavities or on relatively low-$Q$ waveguide resonators. Here, we present the first such comb that is based on a millimetre-sized microresonator made of lithium niobate, that allows for cascaded second-order nonlinearities. This proof-of-concept device comes already with pump thresholds as small as 2 mW, generating repetition-rate-locked combs around 1064 nm and 532 nm. From the nonlinear dynamics point of view, the observed combs correspond to the Turing roll patterns.