Inverse-designed low-loss and wideband polarization-insensitive silicon waveguide crossing.

Waveguide crossings are an essential component for constructing complex and functional on-chip photonic networks. Polarization-insensitive waveguide crossings are desired because photonic networks usually involve light with different polarizations. Here, we propose a polarization-insensitive waveguide crossing on a 250-nm silicon-on-insulator platform by using an inverse design method. In simulation, the designed waveguide crossing can maintain insertion loss below 0.18 (0.25) dB in the wavelength range of 1440-1640 nm for the TE0 (TM0) mode and achieve minimal insertion loss as small as 0.08 (0.07) dB at the wavelength of 1550 nm. The cross talk maintains below -32  dB and -35  dB for the TE0 and TM0 modes, respectively. Experimentally, the fabricated waveguide crossing achieves measured insertion loss less than 0.20 (0.25) dB for the TE0 (TM0) mode with minimal insertion loss as small as 0.1 dB. The measured cross talk is below -28  dB and -31  dB for the TE0 and TM0 modes, respectively. Therefore, our proposed waveguide crossing can be widely applied in photonic integrated circuits to construct photonic systems with the capabilities of polarization control and mode (de)multiplexing.