28-Gbaud PAM4 and 56-Gb/s NRZ Performance Comparison Using 1310-nm Al-BH DFB Laser

We have developed a high-speed 1310-nm Al-MQW buried-hetero laser having 29-GHz bandwidth (BW). The laser was used to compare 28-Gbaud four-level pulse-amplitude-modulation (PAM4) and 56-Gb/s nonreturn to zero (NRZ) transmission performance. In both cases, it was possible to meet the 10-km link budget, however, 56-Gb/s NRZ operation achieved a 2-dB better sensitivity, attributable to the wide BW of the directly modulated laser and the larger eye amplitude for the NRZ format. On the other hand, the advantages for 28-Gbaud PAM4 were the reduced BW requirement for both the transmitter and the receiver PIN diode, which enabled us to use a lower bias to the laser and a PIN with a higher responsivity, or conversely enable the possibility of high temperature operation with lower power consumption. Both formats showed a negative dispersion penalty compared to back-to-back sensitivity using a negative fiber dispersion of -60 ps/nm, which was expected from the observed chirp characteristics of the laser. The reliability study up to 11 600 h at 85 °C under accelerated conditions showed no decrease in the output power at a constant bias of 60 mA.