An all-optical format conversion from non-return-to-zero (NRZ) to return-to-zero (RZ) is presented based on cross-phase modulation (XPM) in a silicon waveguide with a detuned optical bandpass filter (OBPF). The simulation results show that the tunable bandwidth of the OBPF leads to RZ signals with tunable pulse width. The conversion efficiency (CE) and the pattern effect of the RZ signal are attributed to the parameters of the pump pulse and the OBPF. The converted RZ signal exhibits lower timing jitter than the NRZ signal.