Low-threshold frequency comb generation using second-order nonlinearities in lithium niobate whispering gallery resonators

Optical frequency combs have become a key technology for optical precision measurements [1] and, moreover, bear potentials to boost the bandwidth of telecommunication networks [2], to be applied for molecular spectroscopy [3], and to be used in modern quantum information systems [4]. Owing to their feasibility for such a vast application range, frequency comb technology has seen major advances in terms of complexity, power requirements and size, leading to micro- to millimeter-sized frequency comb generators emitting continuous-wave-pumped Kerr combs and, more lately, soliton frequency combs based on third-order (χ⁽³⁾) nonlinearities [2].