Dual wavelength waveplate is widely used on Third Harmonic Generation (THG) system. When you need a NLO crystal for type II SHG (o+e→e), and a NLO crystal for type II THG (o+e→e), the out put polarization from SHG can not be used for THG. So you must turn the polarization to get two perpendicular polarization for type II THG. Dual wavelength waveplate works like a polarizing rotator, it can rotate the polarization of one beam and remain another beam's polarization.