Efficient integrated tri-modal coupler for few-mode fibers

Area of Science:

• Photonics and Optical Engineering
• Integrated Optics
• Fiber Optic Communications

Background:

• Few-mode fibers (FMFs) support multiple spatial modes, offering higher bandwidth potential than single-mode fibers.
• Efficiently coupling light into and out of specific modes in FMFs is a critical challenge for realizing their capacity.
• Existing mode couplers often suffer from low efficiency or complexity, hindering practical applications.

Purpose of the Study:

• To demonstrate a high-efficiency vertical grating coupler capable of addressing multiple modes (LP01x, LP11ax, LP11bx) in a graded-index few-mode fiber.
• To design and fabricate a compact and efficient mode coupler compatible with complementary metal-oxide-semiconductor (CMOS) processes.
• To validate the performance of the coupler through experimental measurements of coupling efficiency for each targeted mode.

Main Methods:

• Utilized inverse design to create a non-uniform, straight, bidirectional grating coupler.
• Integrated two mode-selective directional couplers and two tapers to enhance mode selectivity and efficiency.
• Fabricated the device using electron-beam lithography with a minimum feature size of 100 nm.
• Experimentally measured coupling efficiencies for the LP01x, LP11ax, and LP11bx modes.

Main Results:

• Achieved high coupling efficiencies of -3.0 dB for LP01x, -3.6 dB for LP11ax, and -3.4 dB for LP11bx modes.
• Demonstrated a CMOS-compatible fabrication process suitable for mass production.
• The device design effectively addressed the specific modes of the graded-index few-mode fiber.

Conclusions:

• The developed vertical grating coupler offers high efficiency for multiple modes in few-mode fibers.
• Its CMOS compatibility and high performance position it as a key enabling device for future high-capacity optical communication networks.
• The technology supports space division multiplexing (SDM) strategies, crucial for meeting increasing data demands.