Integrated waveguide coupled ultralow-loss multimode waveguides based on silicon nitride resonators.

Area of Science:

• Photonics
• Materials Science
• Integrated Optics

Background:

• Ultra-low-loss silicon nitride (Si3N4) waveguides are crucial for photonic devices like micro-ring resonators (MRRs).
• Standard multi-project wafer (MPW) processes often lead to high scattering losses in Si3N4 waveguides due to sidewall roughness.
• Achieving low loss and large free spectral ranges (FSRs) in MRRs typically requires specialized fabrication methods.

Purpose of the Study:

• To present an innovative approach for fabricating ultra-low-loss and compact Si3N4 waveguides using standard processes.
• To reduce optical field interaction with waveguide sidewalls, thereby minimizing scattering losses.
• To enhance the performance of on-chip micro-ring resonators.

Main Methods:

• Introduction of a compact and multi-mode waveguide structure.
• Utilization of modified Euler bends for smooth transitions.
• Incorporation of a weakly tapered gap directional coupler for adiabatic transmission.
• Fabrication using standard LIGENTEC-AN800 technology.

Main Results:

• Achieved an ultra-low propagation loss of 0.051 ± 0.003 dB/cm in 7-µm wide Si3N4 waveguides.
• Demonstrated an intrinsic quality factor (Qi) of (6.8 ± 0.4) × 10^6 for MRRs at 1550 nm, a fourfold improvement over previous results with the same process.
• Successfully achieved adiabatic transmission with simultaneous ultra-low loss and compact size.

Conclusions:

• The novel multi-mode structure effectively mitigates sidewall scattering losses in high-confinement Si3N4 waveguides fabricated with standard MPW processes.
• This work offers a viable pathway to ultra-high-Q integrated photonics without resorting to specialized laboratory fabrication.
• The findings provide significant insights into reducing propagation loss in Si3N4 waveguides, broadening prospects for advanced photonic devices.