Topic Beam shaping

Speckle is the grainy interference pattern coherent light forms when it scatters from a rough surface. It cannot be filtered out, because it is real interference. It is removed by averaging over many independent patterns faster than the detector integrates.

An afocal beam expander enlarges a collimated beam and, in doing so, reduces its divergence by the same factor. That reduced divergence is usually the real reason to fit one: a larger beam stays collimated further and focuses to a smaller spot.