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    Area of Science:

    • Optics
    • Materials Science
    • Electrical Engineering

    Background:

    • Grating-based coupling is established for single-mode waveguides and fibers.
    • Multimode waveguide coupling is complex and less understood, yet crucial for incoherent light.
    • Applications include exciting guided modes from incoherent sources and extracting radiation.

    Purpose of the Study:

    • To design a grating for coupling multiple modes between a 2D slab waveguide and a multimode fiber.
    • To identify and discuss challenges in multimode coupling.
    • To demonstrate a rudimentary design for multimode fiber-to-waveguide-to-fiber coupling.

    Main Methods:

    • Design of a grating coupler for multimode waveguides.
    • Modeling and simulation of the coupler's performance.
    • Experimental validation of the modeled geometry.

    Main Results:

    • A rudimentary design demonstrated coupling from a multimode fiber into a waveguide slab and back into another fiber.
    • Achieved a total efficiency of -34 dB for the fiber-waveguide-fiber system.
    • Good agreement between modeling and experimental results explained the coupler's physics.

    Conclusions:

    • Multimode waveguide coupling requires careful matching of mode numbers and scattering angles.
    • Coupling efficiency can significantly decrease due to these factors.
    • The study provides insights for improving directivity and maximizing coupling efficiency in future designs.