This paper presents a wavelength division multiplexing (WDM) waveguide coupler that is composed of a pair of curved circular waveguides, where changing the radius of curvature and the spacing between the two waveguides controls the coupler operation. Based on coupled mode theory (CMT), a 4-channel WDM system is derived and designed using the curved waveguide couplers. The 3D beam propagation method (BPM) is used to verify the correctness of the design. In the comparison using CMT and BPM, the designed and simulated results agree quite well. Fabrication details and useful parameter variations are given for the curved coupler, and advantages of the curved coupler over a conventional straight coupler are discussed. © 2003 Elsevier B.V. All rights reserved.