beam expander / laser
The fixed ration beam expander comes in two types known as the Galilean and Keplerian. Using two converging lenses is how the Keplerian beam expanders are assembled. The benefit is real intermediate focus with spatial filtering where the pinhole can be placed. At the output, this provides a better beam wave front. At high power densities, the focus can cause air breakdown. The Galilean beam expander is assembled using one converging and one diverging lens. Inside the beam expander there is not a focal point so it is usable in high powers. The Galilean beam expander is preferred in low expansion ratios. It is also preferred due to it's small size and simplicity. The beam expanders can be made from two, three, or more spherical lenses.