Waveguide grating couplers with bandwidth beyond 200 nm.

Area of Science:

• Photonics
• Integrated Optics
• Nanophotonics

Background:

• Waveguide grating couplers (GC) are essential components in integrated photonic circuits.
• Achieving wideband operation in GCs while maintaining high coupling efficiency remains a challenge.
• Conventional GCs often require complex fabrication processes or additional structures like metal reflectors.

Purpose of the Study:

• To propose and validate a new approach for wideband waveguide grating couplers.
• To enable flexible tuning of grating strength, mode effective index, and dispersion for enhanced performance.
• To achieve unprecedented optical bandwidth without relying on bottom metal reflectors or etching on the substrate material.

Main Methods:

• Utilizing a slot waveguide grating structure positioned above a conventional channel waveguide.
• Employing 3D Finite-Difference Time-Domain (FDTD) simulations to predict performance.
• Experimental validation using standard single-mode fiber and a lithium niobate waveguide platform.

Main Results:

• 3D FDTD simulations predicted a coupling efficiency of -4.08 dB with a 1 dB bandwidth of 229 nm.
• Experimental results demonstrated a coupling efficiency of -4.47 dB in the C-band.
• Experimental results showed a 1 dB bandwidth of 171 nm and a 3 dB bandwidth exceeding 200 nm.

Conclusions:

• The proposed slot waveguide grating structure enables flexible tuning for high coupling efficiency and wideband operation.
• The approach achieves unprecedented optical bandwidth without complex fabrication steps.
• This technology offers a promising solution for broadband integrated photonic applications.