Coherent coupling of fibre lasers. Left: two lasers, right: four coupled lasers.
The coupling of several lasers is a concept for dealing with the power limitations of single sources. In the geometric coupling of independent laser sources the beam quality worsens markedly in comparison with the single source. However, if the laser sources are coupled in such a way that the phase relationships of the single sources are constant, this is called coherent coupling. The coupled laser sources then behave as if they were sub apertures of a sole laser source and their beam quality is that of the single sources.