Amplifier-free 4×96 Gb/s PAM8 transmission enabled by modified Volterra equalizer for short-reach applications using directly modulated lasers.

Abstract

By using four O-band directly modulated lasers (DMLs), for the first time a 384 Gb/s (4 × 96 Gbit/s) 8-level pulse amplitude modulation (PAM8) signal is successfully transmitted over a 15 km standard single mode fiber (SSMF) with no optical amplifier. The nonlinear Volterra equalizer is usually used to cope with the distortions induced by the nonlinearity of DML and the bandwidth-limited components. However, the Volterra equalizer would also enhance the noise at high frequency, which is harmful, especially to PAM8 signal because it is more sensitive to noise. Thus, the Volterra equalizer is modified in our scheme by adding a decision feedback process behind. With the help of the modified Volterra equalizer, the enhanced noise at high frequency is effectively eliminated, and a power gain of 0.5 dB and 3.3 dB for 4 × 64 Gbit/s PAM4 signal transmission over 30 km SSMF and 4 × 96 Gbit/s PAM8 signal transmission over 15 km SSMF at the HD-FEC limit can be obtained, respectively. Moreover, the computation complexity of the modified Volterra equalizer could be reduced by 38% compared with the conventional Volterra equalizer.