The 200 Gbit/s passive optical network (PON) is most likely to be the next-generation scheme following 50G PON. The cost-effective direct detection (DD) system is the economical choice. However, larger-capacity DD systems will face much more serious power fading caused by chromatic dispersion (CD) combined with square-law DD and thereby significantly increases the complexity of equalization algorithms. In this paper, a 200 Gbit/s Nyquist 4-level pulse amplitude modulation (PAM4) single side-band (SSB) modulation-DD downlink scheme is designed, and a low complexity quadratic-nonlinear equalizer is proposed for this system. The computational complexity of the quadratic nonlinear equalizer is about 28% of that of the conventional Volterra nonlinear equalizer, while still exhibiting excellent nonlinear equalization ability. Simulation results for the 200 Gbit/s system with 20 km fiber transmission show that it can achieve a power budget of 29 dB, while a 30.4 dB power budget is obtained in the 50 Gbit/s experimental transmission.

Nonlinearity impairments and distortions have been bothering the bandwidth constrained passive optical network (PON) system for a long time and limiting the development of capacity in the PON system. Unlike other works concentrating on the exploration of the complex equalization algorithm, we investigate the potential of constellation shaping joint support vector machine (SVM) classification scheme. At the transmitter side, the 8 quadrature amplitude modulation (8QAM) constellation is shaped into three designs to mitigate the influence of noise and distortions in the PON channel. On the receiver side, simple multi-class linear SVM classifiers are utilized to replace complex equalization methods. Simulation results show that with the bandwidth of 25 GHz and overall bitrate of 50 Gbit/s, at 10 dBm input optical power of a 20 km standard single mode fiber (SSMF), and under a hard-decision forward error correction (FEC) threshold, transmission can be realized by employing Circular (4, 4) shaped 8QAM joint SVM classifier at the maximal power budget of 37.5 dB.