Wavelength division multiplexing devices, and methods of forming the same, include a coupling lens and a waveguide, the lens being positioned over a mirror formed in a transmission path of the waveguide. The mirror reflects incoming light signals out of the transmission path through the lens and further reflects light signals coming from the lens and into the transmission path. An optical chip is positioned near a focal length of the lens. The optical chip has an optical filter configured to transmit a light signal at a first wavelength and to reflect received light signals at wavelengths other than the first wavelength.