Ultra-Powerful Laser To Be Built In Romania

cripkd writes "The 3rd pillar from the ELI program was given the go ahead Tuesday: 'In Romania, Magurele, the ELI pillar will focus on laser-based nuclear physics. For this purpose, an intense gamma-ray source is foreseen by coupling a high-energy particle accelerator to a high-power laser.' Here are some specs and details about why this is not your regular key-chain laser."

What is it to "coherently add" laser beams?

[impossiblefork]

A week ago or so I wanted to build some sort of passive optical arrangement to combine two images additively. One can't do it with mirrors and if one tries to do it with lenses, or tries to use internal total reflection somehow it will turn out that one can never get the internally reflected ray and the ray the ray entering through the surface of the lens to coincide. In fact I ended up strongly suspecting that it couldn't be done.

But "coherently adding" beams sounds exactly like this. Does anyone know if it really is the same thing, or how it works?