The development of the solid-state fiber laser has given a boost to the possibility of destroying a target without the need for a projectile or blind the photoreceptive element of a heat-seeker missile with laser emission. The growing presence of drones on the battlefield since 2010 and the on-board intelligence on guided missiles have given new credibility to these research programs. Solid state fiber laser, which uses optical fiber as an amplifying medium, has many advantages including efficiency, thermal and opto-mechanical stability, the elimination of free space optics using fiber components, and above all good beam quality thanks to its waveguide. In addition, these coherent sources are available at interesting wavelengths in line with military issues. However, now it is difficult to obtain a high-power fiber source (> 1 kW) while maintaining high beam quality and good spectral coherence. Power scaling whilst preserving beam quality can be achieved through coherent beam combining. The principle is to interfere constructively N mutually coherent single-mode laser beams with proper phase alignment into a single good quality beam. Conventional coherent beam combining is typically based on tiled apertures, for which theoretical maximum efficiency is 67% (due to limited lenses fill factor and secondary lobes in the far field), and for which experimental efficiency is currently below 50%. We present here a novel technique for coherent beam combining based on Multi-Plane Light Conversion. The beam combiner is designed as a spatial multiplexer which output modes form a Gaussian beam when superimposed constructively, reaching theoretically 100% efficiency. Moreover, reflective free-space design allows for high power handling up to 500W. We measure an experimental mode purity above 85% with a total efficiency of 70% (including optical losses) combining 6 beams. This system can be altered to provide error signals for easier phase-locking of the inputs.