Introducing a periodically poled LiNbO3 (PPLN) waveguide in fiber ring lasers, tunable wavelength conversion has been realized by the nonlinear interactions between the launched signals and the pump and control waves generated through the fiber ring lasers. The principle of all-optical wavelength conversion based on the cascaded sum-and difference_frequency generation in the quasi-phase-matched PPLN waveguide is introduced in the article. The evolution of the power of the pump, the signal, the control and the converted waves along the PPLN waveguide has been numerically simulated respcctively. The dependence of the conversion efficiency on the wavelength of the converted wave was calculated. Finally, the tunability of the wavelength conversion was experimentally verified.