Design, Fabrication, and Characterization of Graphene-Silicon Nitride Integrated Mode Filters

We present the design, fabrication, and characterization of broadband graphene-silicon nitride integrated mode filters working in the optical C-band, centered at a wavelength of 1.55 μm. The devices presented here prevent modal crosstalk, thus avoiding signal degradation in multimode communication systems. In particular, the fabricated filters are based on a dual-mode silicon nitride waveguide, partially covered by a centered graphene nanoribbon that induces a stronger absorption for the TE₀ mode than for the TE₁ mode. The geometry of the design has been optimized to minimize the length and insertion losses of the device. A complete fabrication process, including the transfer and lithography of commercial graphene, has been developed. A novel approach was introduced during the etching step, which entailed the simultaneous curing of the resist to encapsulate the graphene nanoribbons prior to the deposition of the upper cladding. Finally, transmission through an array of fully fabricated filters of varying lengths was characterized with a measurement setup employing optical fiber coupling. The maximum experimentally measured contrast between the TE₀ and TE₁ modes is 123 dB/cm, achieved at a wavelength of 1569 nm, simultaneously with a minimum loss of -38 dB/cm for the TE₁ mode. Overall, we fully demonstrate an integrated mode filter based on commercial graphene that paves the way for the implementation of integrated multimode optical communication systems.