Ultra-compact polarization-independent directional couplers utilizing a subwavelength structure

Ultra-compact polarization-independent directional couplers are proposed and demonstrated on the silicon-on-insulator (SOI) platform. By using the subwavelength structure in the coupling region, the coupling strength is greatly enhanced only for TE polarization, so that the coupling strength could be equivalent between TE and TM polarizations. Both complete coupling and 3 dB splitting have been demonstrated. The coupling length could be as short as ∼3.75  μm. The measured excess losses and polarization dependence losses are <∼1 and <∼0.5  dB.