With rapid development of cloud services, real-time video, and online social networks, there occurs an urgent requirement of transmission capacity for optical metro and access networks. The network operating at 100 Gb/s or 400 Gb/s has drawn significant research efforts as well as standardization activities, such as the IEEE 802.3bs 400G Ethernet Task. Compared with other modulation formats, PAM-4 has advantages of low cost, free of digital to analog converter (DAC), and easy implementation. In this work, we propose nonlinear digital signal processing in metro and access networks. We experimentally demonstrated 4x28Gb/s C-band directed modulated laser (DML)-based metro network over 160-km fiber transmission. We also experimentally demonstrate, for the first time, 2 × 64 Gb/s O-band PAM-4 signal transmission over 70 km SMF using DMLs with a bandwidth of 18 GHz. The sparse Volterra filter (SVF) is also proposed to compensate the transmission impairments with substantial computational complexity reduction.