Multiwavelength oscillation is demonstrated with a novel laser scheme, in which cascaded second-order nonlinearity in a periodically poled LiNbO3 (PPLN) waveguide is used to double the lasing lines. Twenty-two wavelengths approaching S- and L-bands are simultaneously achieved with a channel spacing of 1.6 nm, and the band separation is changeable. The number of wavelengths and channel spacing can be controlled by the adjustment of the tunable Fabry-Perot (FP) etalon and light polarization states in the laser cavity.