Nowadays the channel spacing of DWDM systems is becoming more and more dense to accommodate the exploded traffic demand. Interleaver, which separates the incoming spectra of the optical signals into two compleraeiuary sets of periodic spectra, could provide a solution for DWDM devices and systems. In this paper an interleaver based on crystal waveplate (Yttrium Vanadate) is introduced. The performance of the output spectra is optimizing by use of Fourier expansion. Experimental results give excellent agreements with the theoretical analysis.