Prof. John Donegan 
Trinity College Dublin


Multi-color frequency combs in a single Si3N4 microresonator 

Kerr frequency combs, commonly referred to as microcombs, are generated from optical microresonators, offering the potential of a chip-scale multi-wavelength light source. These microcombs find applications in optical frequency synthesis, coherent communication and spectroscopy [1]. Having two microcombs co-existing in the same resonator holds potential for a cost-effective and miniaturized dual-comb photonic device [2]. In this talk, we present an investigation on dynamics within a single Si3N4 microring resonator and a single pump, where we observe the coexistence of various nonlinear effects, including four-wave mixing (FWM), OPO, and cross-phase modulation (XPM).

A Si3N4 microring resonator (MRR), with a radius of 115 µm and a cross-section of 3075 nm × 800 nm (width × height) is used. To induce the Optical Parametric Oscillation (OPO) effect, the resonator was intentionally designed to be strongly over-coupled [3]. Figure 1(a) shows the measured transmission curve of the MRR in transverse-magnetic (TM) polarization, with nearly resonant dual modes (TM00, denoted by circles) and the first-order mode (TM10, denoted by squares). Figure 1(b) displays the free spectral range (FSR) of the TM00 modes. Figure 1(c) depicts the frequency mismatch for the pump (fp), signal (fs), and idler (fi) in the OPO process.

We observe the comb dynamics and plotted them as (i) to (vi) in Figure 1(d). OPO first occurs, generating the signal and idler sidebands at ~137.1 and ~245.1 THz, respectively, which align well with the simulation results. Further increasing the pump wavelength, a primary comb is excited from the signal band instead of the pump [state (ii)]. Further tuning into the resonance, we observe a series of combs around the signal, pump and idler positions. We will discuss these results and the prospects for obtaining soliton generation with these multiple coloured combs. 

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