High throughput optical satellite communication (SATCOM) systems need to rely on effective and robust technology to enable wavelength-division multiplexing (WDM) in a commercially viable way. The main challenge to implement WDM in optical feeder links deals with the multiplexing of high power channels. Currently the levels of power required for communication, tens of watts per channel, make unfeasible to multiplex several channels in a waveguiding device. A free space architecture is devised to mitigate this issue. The paper describes the architectural choices made, the optical and mechanical design for a multiplexer to be employed in a Optical Feeder Link terminal combining 13 channels, each carrying 50W of optical power. Within the TOmCAT (Terabit Optical communiCation Adaptive Terminal) project a demonstrator of the full system has been realized. The demonstrator multiplexer supports 5 channels, each carrying up to 2W of optical power, with an optical bandwidth of 25 GHz, centered on the 200 GHz ITU grid. The design and the experimental results obtained during the integration of the multiplexer demonstrator are here presented and discussed.