Transmission of a 56 Gbit/s PAM4 signal with low-resolution DAC and pre-equalization only over 80 km fiber in C-band IM/DD systems for optical interconnects.

A low-cost intensity modulation and direct detection scheme using a 4-bit digital-to-analog converter for 56 Gbit/s pre-equalized PAM-4 generation at the transmitter and without receiver side post-equalization over 80 km single-mode fiber transmission in a C-band is experimentally demonstrated in this Letter. Pre-equalization includes pre-emphasis for channel response, a modified Gerchberg-Saxton algorithm with five iterations for chromatic dispersion compensation, and a noise shaping technique implemented by a five-tap finite impulse response filter for quantization noise suppression. Enabled by the pre-equalization techniques, the bit error rate of the received signal without post-equalization can reach the hard-decision forward error correction threshold with -2 receiver optical power. The experimental results indicate that our proposed scheme is a promising candidate for the standardization of 400 GbE wavelength division multiplexing-based long-haul distance data center interconnects.