MIMO-Free WDM/MDM Optical Wireless System With 120-GBaud PAM4 and Linear Equalization

A free-space multi-plane light conversion module is employed for MIMO-free mode division multiplexing, achieving inter-mode crosstalk lower than -21.2 dB and insertion loss below 11.5 dB across the entire C-band. A net 5.4 Tb/s (240 Gb/s/λ × 8λ × 3 modes) PAM4 WDM-MDM IM/DD transmission system is experimentally demonstrated over a 1.8-m free-space link. All spatial and wavelength channels achieve Q-factors above the 6.7% HD-FEC threshold using only a 217-tap linear feed-forward equalizer, eliminating the need for complex MIMO digital signal processing. To the best of our knowledge, this represents the first demonstration of MIMO-free MDM transmission at baud rates exceeding 100 GBaud.