Completely CMOS compatible SiN-waveguide-based fiber coupling structure for Si wire waveguides.

Area of Science:

• Photonics and Optical Engineering
• Materials Science for Integrated Circuits

Background:

• Silicon wire waveguides are crucial for integrated photonics but face challenges in efficient fiber coupling.
• Achieving low-loss and polarization-independent mode conversion is essential for practical silicon photonic devices.

Purpose of the Study:

• To design and analyze a highly efficient fiber coupling structure for silicon wire waveguides.
• To achieve low-loss and polarization-independent mode conversion between silicon and silicon nitride waveguides.

Main Methods:

• Designed a fiber coupling structure using a Si inverted taper waveguide and a thin SiN waveguide with an SiO2 spacer.
• Utilized a 310 nm-square SiN waveguide core to expand the optical field for fiber matching (4.0-μm mode field diameter).
• Analyzed effective refractive indices to understand mode conversion and derive design guidelines.

Main Results:

• Achieved efficient fiber coupling with a SiN waveguide, expanding the optical field to match standard fibers.
• Demonstrated low-loss (≤1 dB) and low-polarization-dependent mode conversion between SiN and Si taper waveguides.
• Maintained performance over a wide wavelength bandwidth (1.36 μm to 1.65 μm).

Conclusions:

• The designed structure enables highly efficient fiber coupling and mode conversion for silicon wire waveguides.
• The use of SiN waveguides and an SiO2 spacer provides a CMOS-compatible solution for low-loss, polarization-independent operation.
• The analysis provides guidelines for optimizing device length and reducing polarization dependence in future designs.