Nonlinear Differential Coding for Spectral Shaping of PAM Signal in High-Baudrate Short-Reach Optical Transmission

Rapid increase of data-center traffic due to the growing use of cloud services and mobile broadband services requires a higher capacity Ethernet link based on an economical approach and high-performance short-reach optical transmission techniques based on intensity-modulation and direct-detection have been attracting huge interest. In the next-generation Ethernet such as 800GbE or 1.6TbE, a higher-baudrate signal will be necessary and the transmission performance will be limited by the transceiver bandwidth. In this article, we show a spectral-shaping technique for PAM signals based on a nonlinear differential coding which realizes the narrower signal spectrum and enhances the tolerance to bandwidth limitation. We theoretically investigate the characteristics of the proposed scheme and demonstrate 186-Gb/s 10-km transmission by means of an O-band experiment with a 3-dB bandwidth of 14 GHz in which the proposed technique is applied to a 93-Gbaud PAM4 signal. Furthermore, we propose a combination of nonlinear differential coding and LLR-based decoding and show that the combined scheme realizes 4-dB improvement in the required optical power. We also investigate the performance dependency on the parameters such as the number of taps in the equalizers utilized in the decoder.