35 km amplifier-less four-level pulse amplitude modulation signals enabled by a 23 GHz ultrabroadband directly modulated laser

The 4-level pulse amplitude modulation（PAM4） based on an 23 GHz ultrabroadband directly modulated laser（DML）was proposed. We have experimentally demonstrated that based on intensity modulation and direct detection（IMDD） 56 Gbps per wavelength PAM4 signals transferred over 35 km standard single mode fiber（SSMF） without any optical amplification and we have achieved the bit error rate（BER） of the PAM4 transmission was under 2.9 × 10–4 by using feed forward equalization（FFE）.