High Finesse Transversely-Coupled Fabry-Pérot Resonators on CMOS-Compatible SiN

Silicon nitride has greatly improved the versatility of integrated photonics technologies due to its wide transparency window, compatibility with existing CMOS-foundry infrastructure, and its potential for extremely low propagation loss. However, its low refractive index reduces mode confinement, increasing the minimum device footprint of components such as the widely adopted ring resonator filter, resulting in a heightened need for reduced-bending architecture to maintain high integration density. This work presents CMOS-manufactured transversely-coupled Fabry-Pérot (TCFP) resonators, a reduced-bending substitute for the add-drop ring resonator filter. The cavity mirrors are composed of distributed Bragg reflectors (DBRs), and the resonators exhibit a maximum intrinsic Q of >2 million and a maximum finesse of >400. Device performance is found to be limited by mirror scattering losses, with extracted losses of 1.3-2.2 dB/cm dependent on grating strength.