Ultrahigh-Q silicon racetrack resonators

Abstract

An ultrahigh-Q silicon racetrack resonator is proposed and demonstrated with uniform multimode silicon photonic waveguides. It consists of two multimode straight waveguides connected by two multimode waveguide bends (MWBs). In particular, the MWBs are based on modified Euler curves, and a bent directional coupler is used to achieve the selective mode coupling for the fundamental mode and not exciting the higher-order mode in the racetrack. In this way, the fundamental mode is excited and propagates in the multimode racetrack resonator with ultralow loss and low intermode coupling. Meanwhile, it helps achieve a compact 180° bend to make a compact resonator with a maximized free spectral range (FSR). In this paper, for the chosen 1.6 μm wide silicon photonic waveguide, the effective radius R eff of the designed 180° bend is as small as 29 μm. The corresponding FSR is about 0.9 nm when choosing 260 μm long straight waveguides in the racetrack. The present high-Q resonator is realized with a simple standard single-etching process provided by a multiproject wafer foundry. The fabricated device, which has a measured intrinsic Q-factor as high as 2.3 × 10 6 , is the smallest silicon resonator with a > 10 6 Q-factor.

Exploiting Ultralow Loss Multimode Waveguides for Broadband Frequency Combs

“…We engineer the microresonator by leveraging nonlinear functions for minimizing rapid changes in curvature, which have been well studied in mathematics and applied in photonics. [ 21–24 ] We design the bending radius to change nonlinearly to ensure adiabaticity while maintaining a small footprint. For comparison, a design with a bending radius that changes linearly—using the same slope as the one in the coupling section of the nonlinear case to ensure adiabaticity—would require a ten times larger footprint.…”

High-Q-Factor Silica-Based Racetrack Microring Resonators

“…Though 3.70 × 10 7 Q-factor is acquired, the method is intended for SiN waveguides. On the other hand, bend couplers [34] and large radius [35] are used. In Reference [34], the Euler bend couplers with the effective radius R of 29 µm achieve a Q-factor as high as 1.30 × 10 6 and 0.9 nm FSR.…”

Ultra-Broadband Polarization Beam Splitter Based on Cascaded Mach-Zehnder Interferometers Assisted by Effectively Anisotropic Structures

“…5(a). Bends based on Euler curves have varying radium of curvature, which can reduce the generation of highorder modes compared to traditional bends [26], [27]. Here, we set Rmin=0.72 μm to make sure that the loss of these bends is large for the TM polarization but small for the TE polarization.…”