The conversion from all-optical non-return-to-zero (NRZ) to return-to-zero (RZ) format is a crucial technology in interfacing WDM and OTDM of future transparent photonic network. The conversion from all-optical single-to-dual NRZ to RZ format conversion is presented and experimentally demonstrated based on four-wave mixing (FWM) in a 50 m dispersion-flattened highly-nonlinear photon crystal fiber (DF-HNL-PCF). The original NRZ format is converted into RZ format by injecting synchronized clock signal into the DF-HNL-PCF. The FWM effect generates two sideband components, which carry the same data information as the original NRZ signal with RZ format. The proposed format converter has a wide and tunable operation wavelength range of 19.3 nm. The optimum conversion efficiency, extinct ratio and Q factor are -21 dB, 11.9 dB and 7.2, respectively. The system is transparent to both bit rate and modulation format. The advantage of this scheme consists in the ability of bandwidth scalable due to the fact that the dispersion flattening of HNL - PCF is used. Furthermore, it is all optical fiber, compact and robust, which makes it more competitive as well as easily accessible for use in practical optical communication systems.