Frequency comb generation in quadratic nonlinear media

Abstract

We experimentally demonstrate and theoretically explain the onset of optical frequency combs in a simple cavity-enhanced second-harmonic-generation system, exploiting second-order nonlinear interactions. Two combs are simultaneously generated around the fundamental pump frequency, with a spectral bandwidth up to about 10 nm, and its second harmonic. We observe different regimes of generation, depending on the phase-matching condition for second-harmonic-generation. Moreover, we develop an elemental model which provides a deep physical insight into the observed dynamics. Despite the different underlying physical mechanism, the proposed model is remarkably similar to the description of third-order effects in microresonators, revealing a potential variety of new effects to be explored and laying the groundwork for a novel class of highly efficient and versatile frequency comb synthesizers based on second-order nonlinear materials.

Numerical modelling of frequency comb generation in nonlinear resonators

“…However, if a particular second-order process dominates the dynamics, the problem can be greatly simplified by introducing a set of distinct field envelopes with widelyspaced carrier frequencies. For the particular case of secondharmonic generation (SHG) in high-finesse cavities, where frequency combs have been experimentally observed [14], the second-harmonic field is found to be dynamically slaved to the fundamental field, with the latter evolving according to the following mean-field equation [15], [16]:…”

“…Although all microresonator frequency comb studies to date have revolved around third-order Kerr nonlinear effects, recent studies in macroscopic, free-space cavities have shown that similar phenomenologies can also manifest themselves in quadratically nonlinear cavities [13], [14]. In particular, experiments have shown that, similarly as for Kerr cavities, a cw laser coupled into a quadratically nonlinear cavity can transform into a frequency comb [14].…”

“…In particular, experiments have shown that, similarly as for Kerr cavities, a cw laser coupled into a quadratically nonlinear cavity can transform into a frequency comb [14]. The ability to leverage second-order effects for frequency comb generation holds great promise to enable sources at spectral regions that are difficult to access with alternative comb technologies, such as the visible and mid-infrared spectral ranges [15], [16].…”

Frequency comb generation in a continuously pumped optical parametric oscillator

“…In order to generate optical frequency combs (OFCs), the use of passive nonlinear cavities has been shown to represent an attractive alternative to traditional techniques based on femtosecond mode-locked lasers. Indeed, comb generation has been demonstrated to occur in continuosly pumped resonators with third-or second-order nonlinearities [1,2]. In cavities dominated by the second-order nonlinerity, OFCs can be generated when the nonlinear crystal is phase-matched for second harmonic generation and the cavity is pumped above the threshold of the so-called internally-pumped optical parametric oscillation [2,3].…”

“…Indeed, comb generation has been demonstrated to occur in continuosly pumped resonators with third-or second-order nonlinearities [1,2]. In cavities dominated by the second-order nonlinerity, OFCs can be generated when the nonlinear crystal is phase-matched for second harmonic generation and the cavity is pumped above the threshold of the so-called internally-pumped optical parametric oscillation [2,3]. Here, we experimentally demonstrate, for the first time to the best of our knowledge, that quadratic frequency combs can be directly generated in a cw-pumped, nearly degenerate, doubly resonant optical parametric oscillator (OPO).…”