A reliable return-zero(RZ) formats light source scheme could be realized by modulating a pulse train of chirp-free or low chirp with data signals,where the key component is a stable light source that generates transform-limited pulses.Superior to the conventional fiber laser,a regeneratively mode-locked fiber laser(RMLFL) operates steadily against environmental changes,and it could output high repetition pulse train with tunability both in wavelength and duration,which makes it competitive within the field of RZ light sources.In this paper,the application in RZ light source of RMLFL was demonstrated.By using Mach-Zehnder modulator and pseudo-random code modulation,the backto-back eye-diagrams with good performances were obtained,and RZ format light sources with pulse duration of 5 ps based on RMLFL were realized,which meet the requirements of 10 Gbit/s long haul transission.Furthermore,such a 10 GHz RMLFL could be upgraded to support 40 Gbit/s RZ light sources simply by 4 fold OTDM.At the same time,we compared this RZ light source with the result using two-stage modulation.It is found that the former has more advantages in the aspect of pulse duration and signal-noise-ration.