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On-chip resonators are promising candidates for applications in a wide range of integrated photonic fields, such as communications, spectroscopy, biosensing, and optical filters, due to their compact size, wavelength selectivity, tunability, and flexible structure. The high quality (Q) factor is a main positive attribute of on-chip resonators that makes it possible for them to provide high sensitivity, narrow bandpass, and low power consumption. In this Tutorial, we discuss methods to achieve ultra-high Q factor on-chip resonators on a silicon nitride (Si3N4) platform. We outline the microfabrication processes, including detailed descriptions and recipes for steps such as deposition, lithography, etch, cladding, and etch facet, and then describe the measurement of the Q factor and methods to improve it. We also discuss how to extract the basic loss limit and determine the contribution of each loss source in the waveguide and resonator. We present a modified model for calculating scattering losses, which successfully relates the measured roughness of the waveguide interface to the overall performance of the device. We conclude with a summary of work done to date with low pressure chemical vapor deposition Si3N4 resonator devices, confinement, cross-sectional dimensions, bend radius, Q factor, and propagation loss. 

Abstract Low propagation loss in high confinement waveguides is critical for chip‐based nonlinear photonics applications. Sophisticated fabrication processes which yield sub‐nm roughness are generally needed to reduce scattering points at the waveguide interfaces to achieve ultralow propagation loss. Here, ultralow propagation loss is shown by shaping the mode using a highly multimode structure to reduce its overlap with the waveguide interfaces, thus relaxing the fabrication processing requirements. Microresonators with intrinsic quality factors (Q) of 31.8 ± 4.4 million are experimentally demonstrated. Although the microresonators support ten transverse modes only the fundamental mode is excited and no higher order modes are observed when using nonlinear adiabatic bends. A record‐low threshold pump power of 73 µW for parametric oscillation is measured and a broadband, almost octave spanning single‐soliton frequency comb without any signatures of higher order modes in the spectrum spanning from 1097 to 2040 nm (126 THz) is generated in the multimode microresonator. This work provides a design method that can be applied to different material platforms to achieve and use ultrahigh‐Qmultimode microresonators.