The present invention provides an improved wavelength division multiplexer (WDM). The WDM includes a birefringent plate; a reciprocal optical rotator optically coupled to the birefringent plate; and a diffraction grating optically coupled to the reciprocal optical rotator at a side opposite to the birefringent plate. This grating-based WDM provides very low polarization-dependent loss, and therefore, greatly reduces insertion loss. This increases the reliability of the data carried by the channels.