Pattern conversion is the key technology for all-optical clock recovery from non-return-to-zero (NRZ)signal. A novel scheme for all-optical NRZ to pseudo-return-to-zero (PRZ)conversion using a folded ultrafast nonlinear interferometer (UNI) has been put forward and demonstrated for the first time. Based on the segmented semiconductor optical amplifier model and the transmittance of the UNI, theoretical model for this scheme is established and the output characteristics are calculated theoretically. Stable format conversion from NRZ data to PRZ data has been achieved at 10, 20 and 40 Gbit/s. The influences of the input signal characteristics on the performance of the format conversion are investigated. Experimental results are in agreement with the numerical simulation results.