Ultra-compact Silicon 90° Optical Hybrid by Adjoint-based Inverse Design Method

We demonstrate an ultra-compact $2\times 490^{\circ}$ optical hybrid with adjoint-based inverse design method. The device footprint is 6.4 $\mu\mathrm{m}\times 4.4\mu \mathrm{m}$. For S and R-light input, the insertion losses are less than 0.78 dB.