Optimizing the spectral purity of photon sources from a nonlinear microring resonator by pump engineering

Cavity-enhanced spontaneous four-wave mixing (SFWM) in microring resonators (MRRs) provides an effcient route to produce integrated photon sources. Despite the largely improved spectral purity without lossy filtering, standard MRR SFWM sources pumped by conventional single-pulses are still limited to 92% purity because all modes share the same Lorentzian lineshape, hindering their use in applications requiring near-unity purity. Here, we address this problem by proposing a pump-engineering scheme that uses a single Fabry–Pérot cavity to shape the pump by reflection. We show that the pump linewidth in the MRR is broadened well beyond the signal and idler linewidths and the spectral purity can approach unity. Our scheme is robust to MRR parameter deviations and can be applied to both all-pass and add-drop MRRs, offering a practical route to high-purity integrated photon sources.