Ultrahigh-speed (>50 Gb/s) short-reach fiber-optic links can be implemented cost-effectively by using directly
modulated lasers (DMLs). For demonstrations, we successfully transmit 56-Gb/s PAM-4 signal over 20 km of SSMF by using 1.5-µm DML and 117-Gb/s DMT signal over 30 km of SSMF by using 1.3-µm DML.