Frequency comb generation via cascaded second-order nonlinearities in microresonators

TL;DR

This paper demonstrates a novel microresonator-based optical frequency comb generated via cascaded second-order nonlinearities in lithium niobate, achieving low pump thresholds and stable repetition-rate-locked combs.

Contribution

First demonstration of a microresonator-based frequency comb using cascaded second-order nonlinearities in lithium niobate.

Findings

Achieved comb generation with pump thresholds as low as 2 mW.

Generated stable repetition-rate-locked combs around 1064 nm and 532 nm.

Observed combs correspond to Turing roll patterns.

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.