Silicon nitride-on-silicon bi-layer grating couplers designed by a global optimization method.

Area of Science:

• Photonics and Optical Engineering
• Materials Science
• Nanofabrication

Background:

• Grating couplers are essential for efficiently coupling light between optical fibers and chip-based photonic circuits.
• Achieving high coupling efficiency and broad bandwidth while maintaining robustness to fabrication variations is a key challenge in photonic device design.
• Silicon nitride (SiN) is a promising material for photonic integrated circuits due to its low loss and compatibility with CMOS fabrication.

Purpose of the Study:

• To design and fabricate high-performance silicon nitride-on-silicon bi-layer grating couplers for the O-band (1260-1675 nm).
• To ensure the grating coupler designs are robust to fabrication variations across a 200 mm wafer.
• To achieve efficient fiber-to-chip coupling with a wide operational bandwidth.

Main Methods:

• An optimization-based design procedure was employed, incorporating design rules for reliable fabrication.
• Bi-layer silicon nitride structures were fabricated on a 200 mm silicon wafer.
• Coupling efficiency and 1-dB bandwidth were experimentally measured using standard single-mode fiber.

Main Results:

• A peak fiber-to-chip coupling efficiency of -2.2 dB and a 1-dB bandwidth of 72.9 nm were achieved for a representative device.
• Median peak coupling efficiency of -2.1 dB and median 1-dB bandwidth of 70.8 nm were measured across multiple chips on the wafer.
• Experimental results showed good agreement with simulation, indicating the robustness of the design.

Conclusions:

• The developed silicon nitride-on-silicon bi-layer grating couplers demonstrate high performance and robustness for O-band applications.
• The fabrication process yields consistent results across a large wafer, suitable for mass production.
• These grating couplers are a viable solution for efficient light coupling in silicon nitride photonic integrated circuits.