Fiber-chip edge coupler with large mode size for silicon photonic wire waveguides

Area of Science:

• Integrated optics
• Silicon photonics
• Waveguide devices

Background:

• Fiber-chip edge couplers are crucial for connecting optical fibers to planar waveguide circuits.
• Existing couplers often require lensed fibers for efficient light coupling.
• Silicon photonic wire waveguides present challenges due to their small mode size.

Purpose of the Study:

• To introduce a novel fiber-chip edge coupler concept for silicon photonic wire waveguides.
• To enable direct coupling with conventional optical fibers, including large mode size fibers.
• To achieve high coupling efficiency without specialized fiber preparation.

Main Methods:

• Design and simulation of a new edge coupler on a 220 nm silicon-on-insulator (SOI) platform.
• Utilized 3D Finite-Difference Time-Domain (FDTD) and fully vectorial 3D Eigenmode Expansion (EME) for analysis.
• Employed subwavelength refractive index engineering with Si3N4 layers in the SiO2 cladding for mode expansion.
• Optimized mode overlap at the chip facet using a vectorial mode solver.

Main Results:

• Achieved overall coupling efficiency exceeding 90% for both 6 µm and 10.4 µm MFD fibers.
• Demonstrated effective mode expansion at the chip edge with minimal substrate leakage (3 µm BOX).
• Successfully implemented adiabatic mode transformation using Si3N4 layers and inverse tapering of the Si-wire waveguide.

Conclusions:

• The proposed fiber-chip edge coupler offers a high-efficiency, direct coupling solution for silicon photonics.
• The design circumvents the need for lensed fibers, simplifying integration.
• The use of Si3N4 layers provides a novel method for mode size management and efficient light coupling.