Beam Expanders

Beam expansion or reduction is a common requirement in many applications using lasers or other light sources. While beam expansion is discussed here, the optical system can simply be reversed to serve as a beam reducer. Simple beam expanders are essentially telescopes which, in their most basic forms, consist of two lenses. The input beam is assumed to be collimated. The first lens must have a diameter larger than the diameter of the input beam to avoid clipping the beam. Similarly, the diameter of the output lens should be larger than the expected output beam diameter. The size of the output beam is the product of the magnification of the system (or expansion ratio) and the input beam diameter. The magnification of a two lens system is equal to the ratio of the focal length of the second lens to that of the first lens. The spacing between the two lenses is equal to the sum of the focal lengths of the lenses. One common beam expander geometry is based on a Galilean telescope (see Figure 1) where the first lens has a negative focal length and the second lens has a positive focal length. Because of the difference in signs of the focal lengths there is no focal point between the lenses and the distance between the lenses is the difference between the magnitudes of these focal lengths. This can be contrasted with another expander geometry based on a Keplerian telescope, which uses two positive focal length lenses. In this case, the design is necessarily longer due to the addition of the focal lengths. Furthermore, the presence of a focal point within the system means that care should be taken when using lasers with high pulse energies, as energy densities can be large enough to ionize air. For the Galilean system, it is best to use a plano-concave lens for the negative lens and a plano-convex lens for the positive lens with the plano surfaces facing each other. To further reduce aberrations, only the central portion of the lens should be illuminated, so choosing oversized lenses is often recommended. An example of a Galilean-based beam expander on an optical rail system is shown in Figure 1.