A WDM multiplexing/demultiplexing system includes a de-multiplexer configured to separate and guide light beams from an incident ray having a plurality of wavelengths to corresponding lenses on an optical device, a multiplexer configured to guide light beams from optical transmitters having various wavelengths through the corresponding lenses on the optical device and combine the light beams, a lens array including the corresponding lenses to receive and/or transmit the light beams from or to the de-multiplexer and multiplexer, and a light beam collimator configured to function with the multiplexer and de-multiplexer. The light beams received or transmitted by the light beam collimator and the light beams transmitted or received from or to the multiplexer and de-multiplexer are collinear. The light beam collimator and multiplexer/de-multiplexer can be easily positioned to predetermined or designed positions, thereby providing light beams output through the lenses in a plastic optical device. The WDM system advantageously reduces optical signal loss, while increasing the assembly yield.