Fabrication of silicon nitride-based integrated microcavity optical frequency comb devices (Invited)

Microcavity optical frequency comb (also called the microcavity comb), a subversive technology, is an integrated light source produced from a four-wave mixing process in a nonlinear optical microcavity. As a precision device with excellent properties of optical frequency, microcavity combs can be extensively applied in many fields such as molecular spectroscopy, coherent communication, LiDAR, metrology, and lightweight equipment for airborne system. Here, the fabrication of integrated silicon nitride (Si₃N₄) microcavity optical frequency comb devices was reported. The balance between the stress, thickness and stoichiometry of Si₃N₄ was well controlled. A reliable method was proposed to fabricate Si₃N₄ optical film with enough thickness and stoichiometry to meet the requirements of anomalous dispersion and reducing light absorption. The modified technology of Damascene process with microstructures to decline the stress of thick Si₃N₄ film was developed to reduce defects. Furthermore, the mask via with a 30 nm thick alumina compensation layer was optimized and a practicable etching process was used for fabricating Si₃N₄ microresonators with sub-15 nm roughness of lateral walls of microring and waveguide. The experimental results show a high quality of Si₃N₄ microcavity. Additionally, a coherent Kerr optical frequency comb spectrum can be produced with a wide spectral range from 1480 nm to 1640 nm via dual light pumping.