Faced with surging datacenter traffic demand, system designers are turning to multi-level optical modulation with direct
detection as the means of reaching 100 Gb/s in a single optical lane; a further upgrade to 400 Gb/s is envisaged through
wavelength-multiplexing of multiple 100 Gb/s strands. In terms of modulation formats, PAM-4 and PAM-8 are
considered the front-runners, striking a good balance between bandwidth-efficiency and implementation complexity. In
addition, the emergence of energy-efficient, high-speed CMOS digital-to-analog converters (DACs) opens up new
possibilities: Spectral shaping through digital filtering will allow squeezing even more data through low-cost, low-bandwidth
In this work we demonstrate an optical interconnect based on an EAM that is driven directly with sub-volt electrical
swing by a 65 GSa/s arbitrary waveform generator (AWG). Low-voltage drive is particularly attractive since it allows
direct interfacing with the switch/server ASIC, eliminating the need for dedicated, power-hungry and expensive
electrical drivers. Single-wavelength throughputs of 180 and 120 Gb/s are experimentally demonstrated with 60 Gbaud
optical PAM-8 and PAM-4 respectively. Successful transmission over 1250 m SMF is achieved with direct-detection,
using linear equalization via offline digital signal processing in order to overcome the strong bandwidth limitation of the
overall link (~20 GHz). The suitability of Nyquist pulse shaping for optical interconnects is also investigated
experimentally with PAM-4 and PAM-8, at a lower symbol rate of 40 Gbaud (limited by the sampling rate of the AWG).
To the best of our knowledge, the rates achieved are the highest ever using optical PAM-M formats.