A gain-switched laser diode (LD) was used as an external pulse injection source to reduce the timing jitter of gain-switched Fabry-Perot and DFB LDs. The low j itter and frequency-independent optical pulse can be achieved using this techniq ue. Phase noise measurement shows that with external pulse injection, the timing jitter of gain-switched FP LD was reduced from 1.2ps to 830fs and the timing ji tter of gain-switched DFB was reduced from 12ps to 1.2ps.