We demonstrate high speed optical transmission systems using digital coherent detection at all-electronically multiplexed symbol rates of 80 and 107 Gbaud. At 107 Gbaud, we demonstrate a single-carrier polarization division multiplexed quadrature phase shift keyed (PDM-QPSK) line rate of 428 Gb/s. At 80 Gbaud, we achieve a single-carrier line rate of 640 Gb/s using PDM 16-ary quadrature amplitude modulation (16-QAM). Using two optical subcarriers, we demonstrate a 1-Tb/s optical interface and conduct long-haul wavelength-division multiplexed (WDM) transmission on a 200-GHz grid over 3200 km of ultra-large effective area fiber.