Integrated Resonators in an Ultralow Loss Si3N4/SiO2 Platform for Multifunction Applications

Integrated optical resonators are key building blocks for an ever-increasing range of applications including optical communications, sensing, and navigation. A challenge to today's photonics integration is realizing circuits and functions that require ultralow loss waveguides on-chip while balancing the waveguide loss with device function and footprint. Incorporating Si₃N₄/SiO₂ waveguides into a photonic circuit requires tradeoffs between waveguide loss, device footprint, and desired device specifications. In this paper, we focus on the design of resonator based circuits in the silicon nitride platform and the balancing of desired properties like quality factor <inline-formula><tex-math notation="LaTeX">${\text{Q}}$</tex-math></inline-formula>, free spectral range, finesse, transmission shape with waveguide design, and footprint. The design, fabrication, and characterization of two resonator-based circuit examples operating at 1550 nm are described in detail. The first design is a thin core, large mode-volume bus-coupled resonator, with a 2.72 GHz free spectral range and a measured intrinsic <inline-formula><tex-math notation="LaTeX">${\text{Q}}$</tex-math></inline-formula> of 60 million and loaded <inline-formula><tex-math notation="LaTeX">${\text{Q}}$</tex-math></inline-formula> on the order of 30 Million, representing the highest reported loaded <inline-formula><tex-math notation="LaTeX">${\text{Q}}$</tex-math> </inline-formula> for a large mode volume resonator with a deposited upper cladding. The second circuit is a thicker core, smaller footprint, low loss flat passband third-order resonator filter with an ultrahigh extinction ratio of 80 dB tunable over 100% of the free spectral range and insertion loss under 1.3 dB.