Low-Loss Multimode Waveguide Bends with Direct Laser Writing in Polymer.

Area of Science:

• Integrated photonics
• Nanofabrication
• Computational electromagnetics

Background:

• Waveguide bends are essential for routing in integrated photonic circuits.
• Designing compact bends in air-clad polymer waveguides via two-photon polymerization direct laser writing (2PP-DLW) is challenging due to multimode effects.

Purpose of the Study:

• To systematically model and optimize Bézier-shaped 90° and S-bends for 2PP-DLW polymer waveguides.
• To establish design guidelines for achieving low loss and high compactness in 3D photonic circuits.

Main Methods:

• Utilized a variational Finite-Difference Time-Domain (FDTD) solver for systematic modeling.
• Investigated the impact of bend span, curvature, and waveguide dimensions (widths 2-4 µm) on optical performance.
• Explored Bézier parameters (B = 0-0.2) to control bend shape.

Main Results:

• Smaller waveguides (2-4 µm) and lower Bézier parameters (B=0-0.2) significantly reduce bend losses.
• Achieved near-unity transmission for 90° bends with spans of 20-30 µm.
• Demonstrated feasibility of aspect ratios below 1 for compact S-bends.
• Identified a 2 µm waveguide width as an optimal balance between performance and fabrication limits.

Conclusions:

• Optimized design rules enable low-loss, space-efficient 3D photonic circuits using 2PP-DLW.
• Simulation-driven optimization is crucial for maximizing the potential of 2PP-DLW technology.
• Findings provide insights for future integrated photonic device architectures.