Ricerc@Sapienza

Optical frequency comb sources based on three-wave-mixing in quadratic nonlinear materials allow for reduced pump power threshold and extended spectral coverage. We review recent progress on quadratic optical frequency combs based on second-harmonic generation and optical parametric oscillation.

We analyze the formation of localized structures in cavity-enhanced second-harmonic generation. We focus on the phase-matched limit, and consider that fundamental and generated waves have opposite signs of group velocity dispersion. We show that these states form due to the locking of domain walls connecting two stable homogeneous states of the system, and undergo collapsed snaking. We study the impact of temporal walk-off on the stability and dynamics of these localized states.

We report a theoretical investigation of quadratic frequency combs in a dispersive second-harmonic generation cavity system. We identify different dynamical regimes and demonstrate that the same system can exhibit both bright and dark localized cavity solitons in the absence of a temporal walk-off.

Optical frequency comb sources based on quadratic nonlinearities provide an interesting alternative to Kerr combs in terms of reduced pump power requirements and extended spectral coverage. We review theory and recent experiments of quadratic optical frequency combs based on second-harmonic generation and optical parametric oscillation.

The formation of frequency combs (FCs) in high-Q microresonators with Kerr type of nonlinearity has attracted a lot of attention in the past decade [1]. Recently it has been shown that FCs can be also generated in dissipative dispersive cavities with quadratic nonlinearities [2,3], opening a new possibility of generating combs in previously unattainable spectral regions. Previous work has shown that modulational instability (MI) induces pattern and FC formation in degenerate optical parametric oscillators (OPOs) [4].

Continuously-driven microresonators, whose nonlinear response is dominated by the third-order Kerr nonlinearity, have proven to be valid alternatives to comb sources based on femtosecond mode-locked lasers [1]. More recenlty, the direct generation of optical frequency combs (OFCs) entirely through quadratic interactions has also been demonstrated in singly resonant cavity second harmonic generation (SHG) and in cw pumped nearly degenerate optical parametric oscillation [2,3].

Pumping a dispersive Kerr cavity with chirped pump pulses permits to control the trapping position of temporal cavity soliton combs. Stability of single-cavity soliton generation is greatly enhanced by pump chirping.

Optical frequency combs are one of the most remarkable inventions in recent decades.
Originally conceived as the spectral counterpart of the train of short pulses emitted by mode-locked
lasers, frequency combs have also been subsequently generated in continuously pumped
microresonators, through third-order parametric processes. Quite recently, direct generation of
optical frequency combs has been demonstrated in continuous-wave laser-pumped optical resonators
with a second-order nonlinear medium inside. Here, we present a concise introduction to such

Continuously pumped passive nonlinear cavities can be harnessed for the creation of novel optical frequency combs. While most research has focused on third-order "Kerr" nonlinear interactions, recent studies have shown that frequency comb formation can also occur via second-order nonlinear effects. Here, we report on the formation of quadratic combs in optical parametric oscillator (OPO) configurations. Specifically, we demonstrate that optical frequency combs can be generated in the parametric region around half of the pump frequency in a continuously driven OPO.