Tunable Coupler-Augmented Microrings: Reconfigurable Q-Factor Control for Foundry-Scale Quantum Light Sources.

High-brightness-integrated quantum light sources represent a fundamental element for large-scale photonic quantum technologies. Integrated microring resonator (MRR) in highly nonlinear and ultralow-loss material platforms enable entangled photon-pair generation, but device performance is affected by fabrication tolerances and can thus not be judiciously controlled. Redundant designs based on parameter variation are impractical for chip-scale circuits, as the sheer number of parameters would be too complex. To address this, passive photonic circuits must be combined with active control elements to optimize photon-pair emission and to match emission characteristics among sources. Here, we present a comprehensive study of an actively tunable, interferometrically coupled integrated MRR implemented in silicon nitride (SiN). By manipulating two thermally controlled phase shifters, the ring-bus coupling and the resonator phase can be dynamically adjusted, enabling post-fabrication control of its quality factor (Q-factor) between 105 and 106 to optimize the photon emission bandwidth and the pair generation rate. We directly demonstrate for the first time that the optimal coupling and emission characteristics do not occur at critical coupling and vary across frequency modes due to dispersion effects, in good agreement with our theoretical model. These findings highlight the role of adaptive integrated photon sources for single-batch fabrication of reconfigurable devices with application-specific spectral properties.