Submicron-resonator-based add-drop optical filter with an ultra-large free spectral range

Area of Science:

• Photonics and optical engineering
• Integrated optics
• Waveguide design

Background:

• Microring resonators (MRRs) are fundamental components in integrated photonics for wavelength-selective filtering.
• Achieving a large free spectral range (FSR) in MRRs is crucial for applications requiring wide channel spacing, but often limited by bending losses and multimode interference.
• Existing designs struggle to balance low loss, small footprint, and a wide FSR.

Purpose of the Study:

• To demonstrate a novel low-loss add-drop microring resonator (MRR) with an ultra-large free spectral range (FSR).
• To overcome the limitations of conventional MRRs by employing innovative waveguide and coupler designs.
• To achieve high performance metrics including low insertion loss and narrow bandwidth.

Main Methods:

• Introduction of an ultra-sharp multimode waveguide bend to minimize bending losses at small radii (R = 0.8 μm).
• Utilization of bent asymmetrical directional couplers (ADCs) to effectively suppress higher-order mode resonances.
• Fabrication and characterization of the proposed add-drop MRR device.

Main Results:

• The fabricated device exhibits a low excess loss of 1.8 dB.
• A narrow 3 dB-bandwidth of 0.8 nm was achieved at the drop port.
• A record-breaking FSR of 93 nm was demonstrated, significantly larger than conventional designs.
• Low bending loss was confirmed due to the multimode waveguide structure.

Conclusions:

• The proposed design successfully integrates low loss and an ultra-large FSR in a compact microring resonator.
• The use of multimode waveguide bends and bent ADCs represents an effective strategy for enhancing MRR performance.
• This advancement holds significant potential for applications in optical communication and signal processing requiring dense wavelength-division multiplexing (DWDM) with wide channel spacing.