Szabados, JanJanSzabadosSturman, BorisBorisSturmanBreunig, IngoIngoBreunig2022-03-062022-03-062020https://publica.fraunhofer.de/handle/publica/26472110.1063/5.0021424We investigate the threshold of CH(2) frequency comb generation in lithium niobate whispering gallery microresonators theoretically and experimentally. When generating a frequency comb via second-harmonic excitation, also commonly known as second-harmonic generation, the threshold for the onset of cascaded second-order processes leading to a comb is found to be ∼85 µW. The second-harmonic generation efficiency up to this value is in excellent agreement with a previously known theoretical framework. This framework is extended here, showing that the onset of cascaded CH(2) processes and the maximum of the second-harmonic generation efficiency coincide. The model introduced here allows us to determine the frequency comb generation threshold analytically. Furthermore, we observe that the frequency distance between the comb lines is a function of the pump power. It changes from four free spectral ranges at the oscillation threshold to one free spectral range at 590 µW.ensecond harmonic generationoptical phase matchingOptical MicroresonatorsphotodetectorsFrequency CombsbirefringenceCrystalline SolidsSpectrum Analyzers621journal article