Event

Unintentional perturbations in integrated photonic devices may lead to the coupling of counter-propagating modes, resulting in a weakened standing wave pattern. In high-quality (Q) factor resonators, such coupling manifests as a splitting of the Lorentzian resonance in the transmission spectrum. However, for moderate-Q resonators or weak perturbations, backscattering remains hidden in conventional transmission measurements, highlighting the need for techniques capable of resolving the spatial mode distribution. This study introduces an innovative approach to visualize and probe such a mode profile in Si3N4 microring resonators using a near-field optical microscopy (NSOM) technique. In contrast to conventional approaches that rely on the scattered-out signal, our findings show that the transmission signal can potentially reflect the true nature of the optical signal if the evanescent field is perturbed by a metal-coated tip. After addressing the accurate characterization induced by the tip in NSOM, a coupled mode theory approach has been employed to support the experimental data at different wavelengths. The study additionally reveals an exceptional method for determining the coupling condition of a waveguide-coupled resonator. Typically, the phase profile of a microring resonator inverts its sign at the resonance wavelength. This research indicates that the introduction of a minor perturbation by the tip results in the phase undergoing a sign flip twice, a characteristic exclusive to the undercoupling region. Both theoretical and experimental investigations consistently align to formulate a more thorough conclusion regarding the entire system.