Dealing with the increase in data workloads and network complexity requires efficient selective manipulation of any channels in hybrid mode-/wavelength-division multiplexing (MDM/WDM) systems. A reconfigurable optical add-drop multiplexer (ROADM) using special modal field redistribution is proposed and demonstrated to enable the selective access of any mode-/wavelength-channels. With the assistance of the subwavelength grating structures, the launched modes are redistributed to be the supermodes localized at different regions of the multimode bus waveguide. Microring resonators are placed at the corresponding side of the bus waveguide to have specific evanescent coupling of the redistributed supermodes, so that any mode-/wavelength-channel can be added/dropped by thermally tuning the resonant wavelength. As an example, a ROADM for the case with three mode-channels is designed with low excess losses of <0.6, 0.7, and 1.3 dB as well as low cross talks of < − 26.3, −28.5, and −39.3 dB for the TE0, TE1, and TE2 modes, respectively, around the central wavelength of 1550 nm. The data transmission of 30 Gbps / channel is also demonstrated successfully. The present ROADM provides a promising route for data switching/routing in hybrid MDM/WDM systems.
Compact passive silicon photonic devices with high performance are always desired for future large-scale photonic integration. Inverse design provides a promising approach to realize new-generation photonic devices, while it is still very challenging to realize complex photonic devices for most inverse designs reported previously due to the limits of computational resources. Here, we present the realization of several representative advanced passive silicon photonic devices with complex optimization, including a six-channel mode (de)multiplexer, a broadband 90 deg hybrid, and a flat-top wavelength demultiplexer. These devices are designed inversely by optimizing a subwavelength grating (SWG) region and the multimode excitation and the multimode interference are manipulated. Particularly, such SWG structures are more fabrication-friendly than those random nanostructures introduced in previous inverse designs. The realized photonic devices have decent performances in a broad bandwidth with a low excess loss of <1 dB, which is much lower than that of previous inverse-designed devices. The present inverse design strategy shows great effectiveness for designing advanced photonic devices with complex requirements (which is beyond the capability of previous inverse designs) by using affordable computational resources.
A graphene photodetector based on ultra-thin silicon waveguide at 1.55μm is proposed. By reducing the silicon core thickness, the fundamental TE waveguide mode is less confined and light-graphene interaction is enhanced. Benefiting from the ultrathin silicon waveguide and reflector structure, the graphene absorption coefficient reaches 0.36 dB/μm. A 10nm-thick CVD-grown hexagonal boron nitride is covered on the graphene to improve the device performance. With the help of metal-graphene-metal structure, the contact resistance is reduced dramatically. The devices have shown a responsivity of 1.4 mA/W at 0 V bias and 23.1 mA/W at 0.3 V bias with 0.24 mW input optical power. The measured 3-dB bandwidth is 17GHz under 0V bias voltage at 1550 nm.
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