Ultrawideband (UWB) is defined by the Federal Communications Commission (FCC) for short-range high-capacity wireless communications. We propose and numerically demonstrate a novel scheme of all-optical UWB pulse generation based on parametric processes in cascaded periodically poled lithium niobate (PPLN) waveguides. The commonly used UWB pulses known as derivatives of the Gaussian pulse are considered to be the combination of overshoots and undershoots. As parametric amplification produces overshoots and parametric attenuation creates undershoots, it is expected that UWB pulses can be generated in the optical domain based on parametric processes. By use of sum-frequency generation (SFG) providing parametric attenuation and cascaded second-harmonic generation and difference-frequency generation (SHG+DFG) offering parametric amplification, PPLN-based UWB pulse generation is suggested and implemented. In addition to UWB monocycle and doublet, UWB triplet, quadruplet, and quintuplet are also obtained. The waveforms and radio frequency (RF) spectra of all generated UWB pulses are calculated, with the results showing that all obtained RF spectra meet the UWB definition (FCC, part 15). The operation performance is studied, indicating that the proposed PPLN-based UWB pulse generation has good stability to the UWB pulse wavelength.