This patent document discloses, among others, wavelength-selective switches (WSS) for redirecting optical WDM signals or channels based on a combination of spatially separating light in different optical polarizations in an optical birefringent material and using diffractive optics for separating light at different optical WDM wavelengths into spatially separated optical beam to perform wavelength-selective optical switching in optical WDM applications. Notably, the optics for processing the optical WDM signals in the disclosed optical WSS devices is designed to provide scalable optical WSS devices where different WDM signals share optical components to reduce designed optical components for different WDM signals.