In this work we study all-optical multi-channel return-to-zero (RZ)–on-off keying (OOK) to nonreturn-to-zero (NRZ)–OOK format conversion in single uniform fiber Bragg grating (FBG) for mixed line-rate dense wavelength-division multiplexing systems using mathematical simulations. Forty and 20 Gbit/s RZ–OOK signals with 33% and 50% duty cycles are converted to NRZ–OOK signals in single uniform FBG with 21% reflectivity. Impact of amplitude noise from FBG contrast profile on modulation format conversion efficiency is also studied.