NRZ to RZ conversion was achieved base d on the nonlinear optical loop mirror(NOLM).The gain-switched distributed feed back diode laser(GS DFB-LD) was used as the control optical source,the modelock ed laser was used as the signal source.The experimental results showed that the pulse width of the converted signal was narrower than that of the control pulse. The optical pulse with the repetitive frequency of 20 GHz was achieved using the technology of the time multiplexer.