With the rapid growth of internet business, optical networks will require better performance and higher capacity. Future optical networks are likely a hybrid of wavelength division multiplexing (WDM) and optical time division multiplexing (OTDM) by combining the advantages of both technologies. All-optical non-return-to-zero (NRZ) to return to zero (RZ) format conversion, which can enable data with different modulation format to transmit in different region of network freely, is one of the key interface techniques of constructing this hybrid network. The research progress of all-optical NRZ to RZ format conversion is introduced. And the operation principle, characteristics, performance are analyzed in detail. Also the advantages and disadvantages of these methods are listed. Finally, the future development is prospected.