An apparatus for combining laser beams into a single high power output beam. The apparatus includes a self-imaging waveguide (SIWG) with parallel reflective elements that have a length that is an integer multiple of the self-imaging or Talbot length of the waveguide or an integer fraction of this self-imaging length. The apparatus also includes a system for controlling the phase of each of laser beams provided to an inlet of the waveguide. The phases are controlled, e.g., with phase shifters using active feedback or control signals from the inlet to the waveguide, such that a coherently combined beam is produced at the outlet of the waveguide; and this beam is typically a high power beam of 1 kW or higher. The phase shifters are often controlled to phase-lock the input beams and may be operated to steer or aim the output beam from the waveguide.