Nonlinear Differential Coding in DML-based IM/DD Systems Combined with VNLE in High-baudrate Short-reach Optical Transmission

Intensity modulation and direct detection (IM/DD) schemes with pulse amplitude modulation (PAM) is utilized to economically realize short reach transmission systems, such as data center (DC) applications. In the next-generation Ethernet, the transmission rates are expected to reach over 800 Gb/s, a higher baudrate is desirable due to the current demand on DC data traffic. Digital signal processing (DSP) technology is used to increase tolerance against inter-symbol interference (ISI) caused by narrow band components to achieve high speed data rates. The signal-signal beating terms produced by the square-law detection at the receiver side contribute most significantly to the system performance, which are the 2nd-order nonlinear distortions. Volterra nonlinear equalizers (VNLEs) have been proposed for the nonlinear impairments compensation.In this paper, a nonlinear differential coding based spectral-shaping technology and quadratic VNLE are used in directly modulated laser (DML)-based IM/DD systems. The tolerance to bandwidth limitations (BWL) can be increased with nonlinear differential coding PAM (NLDCP), the signal spectrum can also be narrowed. We verified this method in a 10 km system, and the results show that in DML-based IM/DD systems, NLDCP in combination with VNLE used can provide more than 3.8 dB and 1.7 dB BER gain when baud rate is 90 G at O-band and C-band, respectively. The optimal extinction ratio (ER) of the systems with NLDCP is about 0.3 dB and 0.1 dB lager than that in systems without NLDCP at O-band and C-band, respectively. We also investigate the performance dependency on the number of taps in the nonlinear equalizers.