Ultralow-loss silicon waveguide crossing using Bloch modes in index-engineered cascaded multimode-interference couplers

We investigate the loss mechanism in three-moded multimode-interference couplers that are the building blocks of a compact and low-loss waveguide crossing structure. Broadband silicon waveguide crossing arrays with $<0.01\mathrm{dB}$ insertion loss per crossing are proposed using cascaded multimode-interference couplers, where lateral subwavelength nanostructures are used to reduce the insertions loss. We design and fabricate a $101\times 101$ waveguide crossing array with a pitch of 3.08 μm. Insertion loss of $\sim 0.02\mathrm{dB}$ per crossing and crosstalk $<-40\mathrm{dB}$ at 1550 nm operating wavelength and a broad transmission spectrum ranging from 1520 to 1610 nm are experimentally demonstrated.