On July 8, 2026, our research team, led by Professor Xingjun Wang and Researcher Haowen Shu, published a research article entitled “Ultra-compact high-Q silicon microring resonator for terahertz microwave photonics” in Optics Letters. By optimizing the bend structure of a silicon microring, the researchers realized an ultra-compact microring resonator with an intrinsic quality factor of 3 × 10⁶ and a free spectral range of 425 GHz in a standard silicon photonics foundry process. Microwave photonic filtering was experimentally demonstrated up to 110 GHz, providing a device basis for high-resolution spectral processing of high-frequency signals.
Microring resonators select optical signals at specific frequencies and are important components of integrated microwave photonic filtering circuits. A higher quality factor generally enables a narrower linewidth response, while a larger free spectral range helps extend the alias-free processing range. Increasing the free spectral range, however, requires a shorter optical path. The resulting smaller bend radius tends to introduce additional loss and reduce the quality factor. Balancing these two properties within a compact footprint is therefore a key challenge in the design of high-frequency microwave photonic devices.
Our research team proposed a multimode-interference-assisted generalized Euler–circular bend. By designing how the waveguide curvature varies along the propagation path, the structure provides a smoother bend transition, suppresses unwanted higher-order-mode excitation, and uses controlled modal evolution to reduce loss. The device has an effective bend radius of approximately 5.5 μm, retaining both a compact footprint and low loss in a standard silicon photonics process.
Figure 1. Microwave photonic filtering principle and bend-structure design of the ultra-compact silicon microring
The measured free spectral range was 425 GHz. Near 1546.04 nm, the optical resonance linewidth was approximately 150 MHz, corresponding to a loaded quality factor of about 1.26 × 10⁶. After accounting for the coupling condition, the extracted intrinsic quality factor reached 3 × 10⁶. These results show that the proposed bend structure can combine a large free spectral range with a high quality factor in a compact device.
To evaluate the device’s signal-processing capability, the researchers incorporated the microring into a microwave photonic filtering link consisting of a laser, a phase modulator, and a photodetector. By adjusting the detuning between the laser wavelength and the microring resonance, they obtained a tunable bandpass response. Measurements showed a consistent passband shape from 6 to 110 GHz. The 425 GHz free spectral range suggests a potential alias-free operating range above approximately 200 GHz, but filtering beyond 110 GHz remains to be verified experimentally.
The study provides an implementable device approach to narrowband filtering in compact silicon photonic circuits and can support further exploration of channel selection and interference suppression in high-frequency communications, sensing, and radar. Wencan Li is the first author. Zihan Tao, Ruixuan Chen, and Haowen Shu are the corresponding authors. The work was supported by the National Natural Science Foundation of China, the Jiangsu Provincial Frontier Technology Research and Development Program, and the Fundamental and Interdisciplinary Disciplines Breakthrough Plan of the Ministry of Education of China.
Original article: https://doi.org/10.1364/OL.603822
