Polarization-insensitive multimode interference coupler based on a subwavelength grating structure

A multimode interference (MMI) coupler is one of the basic components for photonic integrated circuits. However, MMI couplers realized by conventional waveguides are polarization sensitive, which is undesired for many applications, such as optical switches and communications. In this Letter, we propose a polarization-insensitive MMI coupler on a 220-nm silicon-on-insulator platform by constructing different effective interference lengths for TE and TM modes assisted with subwavelength grating structures. The designed MMI coupler shows an excess loss of <0.24(0.43) dB and a power imbalance of <0.6(0.5) dB for the TE(TM) mode over the wavelength range of 1.5–1.6 µm in theory. Experimentally, the fabricated MMI exhibits low excess loss <0.64(0.53) dB and power imbalance <1(0.85) dB for the TE(TM) mode over a wavelength range of 1.55–1.61 µm.