High-efficiency dual-layer grating coupler for vertical fiber-chip coupling in two polarizations.

Area of Science:

• Photonics and Nanotechnology
• Integrated Optics
• Semiconductor Devices

Background:

• Efficient optical fiber-to-waveguide coupling is critical for integrated nanophotonics.
• Existing methods often struggle with polarization diversity and vertical coupling.
• High-index-contrast silicon waveguides are key components in modern photonic integrated circuits.

Purpose of the Study:

• To demonstrate a high-efficiency dual-layer grating coupler for vertical polarization-diversity fiber-chip coupling.
• To optimize the grating coupler design for both y- and x-polarized light.
• To achieve high coupling efficiency and directionality while minimizing polarization-dependent loss.

Main Methods:

• Design and simulation of a dual-layer grating coupler structure.
• Orthogonal distribution of waveguide layers for distinct polarizations (y and x).
• Optimization using a particle swarm algorithm and 3D finite-difference time-domain (FDTD) simulations.

Main Results:

• Record simulation efficiencies of 92% (-0.38 dB) for y-polarization and 85% (-0.72 dB) for x-polarization.
• Polarization-dependent loss (PDL) below 2 dB over a 160 nm bandwidth.
• Cross-talk between polarizations below -24 dB.
• Tolerance to fabrication imperfections like etching width and lateral shifts.

Conclusions:

• The demonstrated dual-layer grating coupler is an effective solution for polarization diversity coupling in silicon photonics.
• It offers high directionality, low PDL, and efficient vertical coupling capabilities.
• The design is robust against typical fabrication variations, paving the way for practical applications.