An optical multiplexer and demultiplexer for dense wavelength division multiplexed ("DWDM") fiber optic communication systems is disclosed. As a multiplexer, the device functions to spatially combine the optical signals from several laser sources (each of which is a different wavelength) and launch the spatially combined laser beams into a single optical fiber. As a demultiplexer, the device functions to spatially separate the different wavelengths of WDM optical link and launch each of the different wavelengths into a different optical fiber. The device includes both bulk optic and integrated optic components. The spatial separation or spatial combination of laser beams of different wavelength is achieved with the use of bulk diffraction gratings. Also, bulk optical components are used to collimate and shape (or steer) the free space propagating laser beams to enable efficient coupling of light into single mode optical fibers, or integrated optic waveguides, and to reduce optical crosstalk.