400-Gb/s PDM-4PAM WDM System Using a Monolithic 2×4 VCSEL Array and Coherent Detection

We generate a 400-Gb/s line rate signal using a directly modulated 2 × 4 monolithic vertical-cavity-surface-emitting-laser array. The signal consists of four wavelength-division-multiplexed channels at a 100-GHz channel spacing and each channel carries a 100-Gb/s polarization-division-multiplexed four-level pulse-amplitude-modulation signal. Using digital coherent detection, we successfully transmit the 400-Gb/s signal over 5 × 80-km standard-single-mode-fiber spans with erbium-doped fiber amplifiers at 20% overhead soft-decision forward-error correction, achieving a net information bit rate of 333 Gb/s.