In this contribution we present an in house-developed fused single mode wavelength division multiplexer (WDM) capable of multiplexing two wavelengths up to a power of 26 W without, and 16 W with integrating it in a core-pumped fiber amplifier. For the development of the high power WDM we used fiber with a core diameter of 10 μm and a numerical aperture of 0.075. The insertion loss and the isolation of the WDM were <; 0.3 dB and >; 17 dB. To verify the maximum power handling of the developed high power WDM in an amplifier setup, we integrated it in an all-fiber core pumped amplifier.