Enabling ROADM in mode division multiplexing networks with mode-selective switches

Mode division multiplexing (MDM) enhances optical communication systems by introducing an additional multiplexing dimension. We designed and validated a reconfigurable optical add/drop multiplexer (ROADM) prototype using MDM as an optical subsystem targeting data center interconnects and metro optical networks. Our MDM-ROADM employs a mode-selective switch at each node, selectively de/multiplexing the first three transverse-electric (TE) modes. Key components are based on 220 nm silicon photonics, using subwavelength grating structures and inverse design methodology. Machine learning-based fabrication correction, via our tool PreFab, improved mode selectivity by 57% for mode-selective thermo-optic phase shifters. We also developed mode filters for two modes with a crosstalk of ${-}{9}\;{\text{dB}}$ for ${{\text{TE}}_0}$ and ${-}{15}\;{\text{dB}}$ for ${{\text{TE}}_1}$ within a 35 nm wavelength range. Experimental validation using ${{\text{TE}}_0}$ and ${{\text{TE}}_1}$ modes at 1555 nm shows an aggregate payload transmission of 80 Gb/s NRZ and a PAM-4 transmission at 40 Gbaud with a bit error rate of $1.1 \times {10^{- 9}}$ and $3.8 \times {10^{- 3}}$, respectively.