Compact high-Q multimode InGaAsP/InP microring resonators enabled by mode-selective excitation

InGaAsP/InP has emerged as a promising platform for photonic integration due to its strong optical nonlinearity and compatibility with active devices. However, small radii InGaAsP microring resonators (MRRs) face significant challenges from scattering loss and bending loss, which limit the quality factor (Q). In this work, we demonstrate compact high-Q InGaAsP/InP MRRs by utilizing mode-selective excitation in multimode resonators. By deliberately increasing the waveguide width, both sidewall scattering loss and bending loss are significantly suppressed in an 80-µm-radius resonator. Meanwhile, intermodal coupling is effectively mitigated through phase-matched coupling conditions, ensuring selective excitation of the fundamental transverse-electric mode. Experimental characterization reveals an intrinsic quality factor Q_int of 1.17 × 10⁶, corresponding to a propagation loss of approximately 0.6 dB/cm at telecom wavelengths. To the best of our knowledge, this represents the highest reported intrinsic Q factor for InGaAsP/InP MRRs with sub-100-µm radii. This work demonstrates a practical approach for realizing compact high-Q microring resonators on the InGaAsP/InP platform, paving the way for nonlinear photonic applications in III-V semiconductor systems.