$4\times100$ -Gb/s PAM-4 FSO Transmission Based on Polarization Modulation and Direct Detection

For the first time, we experimentally demonstrate a 400-Gb/s [4×100-Gb/s wavelength-division multiplexing (WDM)] pulse-amplitude-modulation-4 (PAM-4) transmission based on free space optics (FSO) utilizing the polarization modulation (PolM) and direct detection (DD) scheme. Each WDM channel comprises a polarization modulator, a polarizing beam splitter (PBS), and two photodetectors (PDs). Four wavelengths each carrying a 100-Gb/s PAM-4 signal (aggregate 400 Gb/s) are transmitted over the free space. At the receiver side, the PBS separates the two orthogonal polarizations, and then, the two polarized signals are detected by two PDs. Then, the two detected signals are combined by a subtractor, followed by digital signal processing (DSP). The PolM-DD system can achieve higher data rate while maintaining the required forward-error-correction (FEC) threshold compared with intensity modulation and direct detection (IM-DD) scheme. Moreover, owing to the stable polarization state in the free space and the mitigation of the common mode noise by the subtraction, the PolM-DD system provides enhanced tolerance to the amplitude fluctuations attributable to weather turbulence, which makes it more reliable than the conventional IM-DD FSO system. To the best of our knowledge, this is the first experimental demonstration of a 400-Gb/s FSO system using the PolM-DD scheme with the transmission distance of 1.2 m.