On-chip mode-selective manipulation based on the modal-field redistribution assisted with subwavelength grating structures

Efficient mode-selective manipulation in multimode photonics has drawn much attention as a key technology for realizing scalable and flexible mode-division multiplexing (MDM) systems. A mode-selective manipulation scheme based on the modal-field redistribution assisted with subwavelength grating (SWG) structures is proposed and demonstrated for the first time. In particular, the proposed scheme focuses on manipulating the coupling coefficient κ as well as the ratio δ/κ for different mode channels. The SWG structures are used to engineer the refractive-index profile and redistribute the modal field distributions in the multimode bus waveguide, so that different modes are localized in different local regions. In this way, the undesired mode coupling can be suppressed significantly while the desired mode coupling can be enhanced. With such mode manipulation scheme, the fundamental and higher-order mode channels in the bus waveguide can be added/dropped independently and freely. As a proof of concept, a three-channel mode-selective add-drop coupler utilizing the proposed scheme is fabricated and demonstrated experimentally on silicon. The fabricated devices show low excess losses ranging from 0.1 to 1.9 dB over a wavelength range of 70 nm. The inter-mode crosstalks are lower than −19.4 dB in the wavelength range of 1525–1600 nm. The crosstalks for the drop and through ports (i.e., the residual power) are suppressed to be as low as −18 ∼ −30 dB in the wavelength range of ∼60 nm with the assistance of an additional coupler in cascade for performance improvement. The present concept of manipulating the evanescent coupling of the mode-channels paves the way for designing multimode silicon photonic devices with flexible mode-selective manipulation for MDM systems.