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We propose and theoretically demonstrate the extinction ratio (ER) enhanced all-optical tunable wavelength conversion based on cascaded second-harmonic generation and difference-frequency generation (cSHG/DFG) in a periodically poled lithium niobate (PPLN) waveguide by setting the signal at the quasi-phase matching (QPM) wavelength. The conversion performance including extinction ratio enhancement, conversion efficiency, and tunable bandwidth are analyzed systematically. Both analytical solutions with clear physical insights and detailed simulation results are presented. The proposed cSHG/DFG is compared with the conventional one on the extinction ratio enhancement effect. The analytical expression predicts that the suggested cSHG/DFG can effectively enhance the extinction ratio by two times in dB with a short waveguide. The simulation results show that the extinction ratio enhancement drops off with the increase of the waveguide length due to propagation loss and power depletion. It is easy to perform tunable operation, i.e. the converted idler can be tuned in a wide wavelength range simply by changing the pump wavelength. There is a tradeoff between tunable bandwidth, extinction ratio enhancement, and conversion efficiency. In addition, wavelength multicasting with extinction ratio enhancement is also demonstrated. Moreover, the potential of performance improvement in terms of optical signal-to-noise ratio (OSNR) and Q-factor is also investigated.