400-Gb/s PDM-4PAM WDM System Using a Monolithic 2x4 VCSEL Array and Coherent Detection

We generate a 400-Gb/s line rate signal using a directly modulated 2x4 monolithic vertical-cavity-surface- emitting-laser (VCSEL) array. The signal consists of four wavelength-division-multiplexed (WDM) channels at a 100-GHz channel spacing and each channel carries a 100-Gb/s polarization- division-multiplexed 4-level pulse-amplitude-modulation (PDM- 4PAM) signal. Using digital coherent detection, we successfully transmit the 400-Gb/s signal over 5x80-km standard-single-mode -fiber (SSMF) spans with erbium-doped-fiber amplifiers (EDFAs) at 20% overhead soft-decision forward-error-correction (SD-FEC), achieving a net information bit rate of 333 Gb/s.