Low-loss grating coupler based on inter-layer mode interference in a hybrid silicon nitride platform.

Area of Science:

• Photonics
• Integrated Optics
• Quantum Technologies

Background:

• Surface grating couplers are essential for connecting photonic integrated circuits (PICs) with optical fibers.
• Conventional designs suffer from low directionality and poor fiber-to-grating field overlap, limiting performance.
• Non-uniform gratings improve efficiency but complicate fabrication; uniform gratings offer simpler fabrication but have limited theoretical overlap (80%).

Purpose of the Study:

• To propose a novel uniform grating coupler that overcomes the field overlap limitation of conventional designs.
• To enhance the efficiency and performance of interfacing PICs with optical fibers.
• To enable efficient light coupling for hybrid quantum photonic integrated circuits.

Main Methods:

• Leveraging inter-layer mode interference via a virtual directional coupler effect.
• Utilizing a hybrid amorphous-silicon (α-Si) on silicon nitride (Si3N4) platform.
• Optimizing inter-layer gap and grating geometry, validated by 3D finite-difference time-domain (FDTD) simulations.

Main Results:

• Achieved an unprecedented grating-to-fiber overlap of 96% with a near-Gaussian output beam profile.
• Demonstrated high directionality (84%) and a record low coupling loss of -1.27 dB.
• Obtained a 1-dB bandwidth of 20 nm for the uniform grating coupler at 950 nm.

Conclusions:

• The proposed uniform grating coupler significantly enhances light coupling efficiency.
• This design offers a practical solution for high-performance PIC-fiber interfacing, especially for quantum applications.
• The device is suitable for hybrid quantum photonic integrated circuits utilizing III-V quantum dot single photon sources.