Ultra-Compact Silicon Mode (De) Multiplexer Using Inverse-Designed Adiabatic Coupler

Silicon-based mode (de)multiplexer [(De)MUX] is a key component in on-chip mode-division multiplexing (MDM) systems. Here, we propose an ultra-compact mode (De)MUX based on the inverse-designed adiabatic coupler with a footprint of only 4 μm × 1.25 μm. By combining the mode-evolution based adiabatic coupler and inverse-designed subunit array, the strength of mode interaction can be greatly enhanced, and an ultra-compact device length can be achieved. Using the direct binary search (DBS) algorithm, two types of mode (De)MUXs are inverse-designed based on circular and square subunit arrays, respectively. Experimental results show that for the circular subunit array based structure, the insertion loss of TE
₀
mode within a 140-nm bandwidth is measured to be <2.16 dB, and the 3-dB bandwidth of the TE
₁
mode is over 111 nm. For the crosstalk of <−15 dB, the operating bandwidths of TE
₀
and TE
₁
modes are 112 and 91 nm, respectively. For the square subunit array based structure, the TE
₀
and TE
₁
modes have insertion losses of less than 5.65 and 2.96 dB, respectively within a 140-nm wavelength range. The bandwidths of two modes are 87 and 134 nm for the crosstalk of <−11 dB.