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All-optical time-domain demultiplexing based on cascaded second-order nonlinearities of second-harmonic generation and difference-frequency generation in quasi-phase-matched periodically poled lithium niobate waveguides is discussed in order to maximize conversion efficiency and control crosstalk under the given level. The three parameters, including the waveguide length, clock pulsewidth, and clock offset, are investigated carefully and comprehensively. The concepts of maximum waveguide length, maximum clock pulsewidth, and optimal clock offset are defined, and their theoretical expressions are derived for the demultiplexing from 160 to 10 Gb/s. The results calculated from the theoretical expressions are well-consistent with those from the numerical simulation. For two kinds of practical situations, the waveguide length or the clock offset being given, an optimum scheme is proposed to determine the other two parameters readily. The maximum conversion efficiency can therefore be achieved while the quality of the converted wave is sufficient for the practical situations.