Optical Frequency Combs in Quadratically Nonlinear Resonators

Abstract

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 quadratic combs and the physical mechanism that underlies their formation. We mainly review our recent experimental and theoretical work on formation and dynamics of quadratic frequency combs. We experimentally demonstrated comb generation in two configurations: a cavity for second harmonic generation, where combs are generated both around the pump frequency and its second harmonic and a degenerate optical parametric oscillator, where combs are generated around the pump frequency and its subharmonic. The experiments have been supported by a thorough theoretical analysis, aimed at modelling the dynamics of quadratic combs, both in frequency and time domains, providing useful insights into the physics of this new class of optical frequency comb synthesizers. Quadratic combs establish a new class of efficient frequency comb synthesizers, with unique features, which could enable straightforward access to new spectral regions and stimulate novel applications.

Kerr Frequency Comb Interaction with Raman, Brillouin, and Second Order Nonlinear Effects

“…Quadratic frequency combs primarily based on second‐order nonlinearity have been extensively investigated in recent years. [ 187,188 ] Theoretical modeling of quadratic combs in singly and doubly resonant SHG systems reveals rich nonlinear behaviors in such quadratic resonator platforms. [ 154,155,189–193 ] Experimental investigation on microcavity quadratic combs have also been carried out using mechanically polished WGMRs made from MgO doped LiNbO 3 .…”

Infrared Comb Spectroscopy of Buffer-Gas-Cooled Molecules: Toward Absolute Frequency Metrology of Cold Acetylene

“…In parallel, with regard to spectral regions not addressable directly, nonlinear frequency conversion processes starting from available MLL systems began to be extensively used. Over time, many other effective comb platforms, either MLL-based or not, have also emerged [ 15 , 16 ].…”

Optical-parametric-oscillation-based $χ^{(2)}$ frequency comb in a lithium niobate microresonator

“…After the rapid growth of the activities dedicated to Kerr frequency combs, based on the χ (3) nonlinearity, the possibility of microcomb generation using the χ (2) nonlinearity has received more attention in the recent years, as it can provide new opportunities and advantages. Among them, there are the possibilities to use the linear electro-optic effect for stabilization and to work on more challeging wavelength regimes like the UV or the mid-IR [4]. The first realizations of χ (2) frequency combs were done in bow-tie mirror cavities, started via second harmonic generation (SHG) and via optical parametric oscillation (OPO) [5][6][7][8][9].…”