Optimising apodized grating couplers in a pure SOI platform to -0.5 dB coupling efficiency

Area of Science:

• Photonics
• Materials Science
• Optical Engineering

Background:

• Silicon-on-insulator (SOI) technology is crucial for integrated photonics.
• Grating couplers are essential for coupling light into and out of SOI chips.
• Optimizing grating couplers for higher efficiency and manufacturability is an ongoing challenge.

Purpose of the Study:

• To theoretically optimize 1D apodized grating couplers in a pure SOI architecture (without bottom reflectors).
• To investigate the impact of silicon layer thickness on coupling efficiency.
• To assess the feasibility of these designs under deep-UV lithography constraints.

Main Methods:

• Theoretical optimization using a general mutative method.
• Comprehensive 2D Finite Difference Time Domain (FDTD) simulations.
• Analysis of grating couplers in 220 nm and 340 nm SOI platforms.
• Inclusion of 193 nm UV-lithographic fabrication constraints.

Main Results:

• A maximum coupling efficiency of 65% (-1.9 dB) was found for 220 nm SOI.
• A record coupling efficiency of 89% (-0.5 dB) was achieved with a 340 nm SOI design.
• Further increases in silicon layer thickness did not improve efficiency, suggesting -0.5 dB as a potential maximum.
• The 340 nm design maintained -0.7 dB efficiency even with fabrication constraints.

Conclusions:

• The study presents the first pure SOI grating couplers compatible with deep-UV lithography exceeding -1 dB insertion loss.
• The optimized 340 nm SOI design offers record-breaking efficiency and manufacturability.
• These designs are readily compatible with existing silicon photonics foundry processes.