Compact all-optical half adder based on topology optimization

We proposed an inverse-designed compact half adder on a silicon-on-insulator platform with a footprint of 2µm×2µm. The optical power of SUM and CARRY is controlled by different input combinations, according to the truth table of a half adder. Topology optimization is applied to cope with multiple objective functions in such a combinational logic circuit. The transmittance at 1550 nm for CARRY with 11 input is 170.2%, with extinction ratios (ERs) of 27.1 and 5.8 dB for SUM and CARRY, respectively. The SUM and CARRY outputs have ERs over 22.0 dB and 5.7 dB from 1515 nm to 1600 nm. Phase condition and morphology analysis show that the device has high tolerance on phase fluctuation and fabrication. The proposed device with compact footprint, low insertion loss, and large bandwidth presents a novel, to the best of our knowledge, approach to achieve all-optical combinational logic circuits with inverse design.