Ultra-compact X-shaped waveguide crossings with flexible angles based on inverse design

Area of Science:

• Photonics Integrated Circuits (PICs)
• Silicon Photonics
• Waveguide Technology

Background:

• Current 2x2 waveguide crossings limit the area utilization of large-scale PICs due to their fixed 90° intersection angle.
• Smaller intersection angles are desirable for improved area efficiency but have been underexplored.
• Need for compact and high-performance waveguide crossings with flexible angles.

Purpose of the Study:

• To design and demonstrate ultra-compact X-shaped waveguide crossings with flexible intersection angles.
• To achieve high optical performance (low insertion loss and crosstalk) for transverse electric (TE0) mode.
• To explore inverse design and novel processing techniques for further miniaturization and performance enhancement.

Main Methods:

• Utilized finite-difference frequency-domain (FDFD) numerical analysis and global optimization.
• Employed inverse design principles to achieve compact footprints and desired intersection angles (30°, 45°, 60°, 80°, 90°).
• Proposed a primary structure processing technique involving electric field distribution for optimization.

Main Results:

• Achieved compact footprints as small as 4.5 µm² for X-shaped waveguide crossings.
• Simulated and measured low insertion losses (0.1–0.3 dB simulated, 0.2–0.4 dB measured) and excellent crosstalk (-20–-50 dB simulated, -20–-32 dB measured) across C-band.
• Demonstrated a 25.5% footprint reduction for a 90° crossing using the novel processing technique, with improved simulated performance (IL ~0.29 dB, CT ~-37 dB).

Conclusions:

• Compact X-shaped waveguide crossings with flexible angles are feasible and offer high performance on SOI platforms.
• Inverse design is effective for creating ultra-compact photonic components.
• The proposed primary structure processing technique shows potential for further device miniaturization and performance enhancement in PICs.