Highly efficient dual-level grating couplers for silicon nitride photonics.

Area of Science:

• Photonics
• Materials Science
• Optical Engineering

Background:

• Grating couplers are essential for interfacing optical fibers with photonic integrated circuits.
• Silicon nitride (SiN) photonics offers advantages like low propagation loss and broad transparency.
• Achieving high coupling efficiency in SiN grating couplers remains a challenge.

Purpose of the Study:

• To propose and demonstrate a versatile strategy for designing highly efficient dual-level grating couplers in SiN photonic platforms.
• To achieve high directionality and optimal grating-fiber mode matching simultaneously.
• To enable the design for arbitrary SiN film thicknesses.

Main Methods:

• Simultaneous optimization of two grating coupler levels using two linear apodizations with opposite signs.
• Particle swarm optimization (PSO) method to determine grating design parameters.
• Numerical simulations on SiN platforms with thicknesses of 150, 300, 400, and 500 nm.

Main Results:

• Demonstrated a versatile strategy applicable to various SiN film thicknesses.
• Achieved high coupling efficiency (exceeding -0.45 dB for 400 nm SiN) using Si-rich SiN for the top layer.
• Showcased relaxed fabrication tolerances with Si-rich SiN (refractive index 2.7-3.3).
• Attained state-of-the-art performance for SiN grating couplers without back-reflectors.

Conclusions:

• The proposed dual-level grating coupler design strategy is effective for SiN photonics.
• The use of Si-rich SiN in the top layer enhances performance and fabrication tolerance.
• This work sets a new benchmark for SiN grating coupler efficiency.