Ultra-fast all-optical demultiplexer is required in future high-speed optical time-division-multiplexed (OTDM) transmission systems [1]. As an important nonlinear interaction, the cascaded second-harmonic generation (SHG) and difference frequency generation (DFG) wavelength conversion in quasi-phase-matched (QPM) periodically poled lithium niobate (PPLN) waveguide has many advantages, such as ultra-fast response, low noise, high efficiency, broad band width, high dynamic range, and integration compatibility [2–4]. Recently, by using the SHG-DFG-based wavelength conversion technique, all-optical demultiplexing from 40 Gb/s to 10 Gb/s [5], from 100 Gb/s to 10 Gb/s [6] and from 160 Gb/s to 20 Gb/s [7] has been experimentally demonstrated, and some numerical analyses have also been reported [8], [9]. In the OTDM demultiplexing based on the SHG-DFG-based wavelength conversion, two input pulse trains, i.e., multiplexed signal and demultiplexing clock, are injected into a QPM PPLN waveguide, and taken as the pump and control waves. In a typical demultiplexing case shown in Fig. 1, the converted wave from the PPLN waveguide has the same bit rate as the clock (10 Gb/s), and carries the code information demultiplexed from the required channel of the 160-Gb/s signal. As depicted in Fig. 2, there are two schemes to arrange 160- and 10-Gb/s pulses with respect to their wavelengths and the device QPM wavelength. In Scheme I, the 10-Gb/s clock is set at the QPM wavelength (pump wave), and the 160-Gb/s signal is regarded as the control wave. Vice versa in Scheme II. The demultiplexed signal is hence different in the two schemes. In this work, typical OTDM demultiplexing from 160 to 10 Gb/s in the two schemes are theoretically analyzed. In particular, the characteristics of conversion efficiency, pulse reshaping and time delay of the demultiplexed pulses are emphasized.