Abstract:

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 (f_p), signal (f_s), and idler (f_i) 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.