Beam expanders are optical lens assemblies that are used to increase the diameter of a laser beam or other light beam. There are two types of products: Kepler and Galileo. Kepler beam expanders or Keplerian beam expanders have two positive lenses or groups of lenses. Galileo beam expanders or Galilean beam expanders have both negative and positive lenses or lens systems. In the Kepler-type arrangement, the intermediate focus produces high-grade reference wave fonts with a homogenous intensity. Consequently, Kepler laser beam expanders are used in interferometry and other applications that require an intermediate focal point with a pinhole for spatial filtering. Galileo laser beam expanders do not have an internal focal point and are usually shorter in length. They produce very high levels of energy at the focal point and are used in lasers for material processing applications.
Both Keplerian beam expanders and Galilean beam expanders provide a magnification type known as expander power. After this power increases the beam diameter in size, the beam divergence is then reduced by this same power. The combination produces a light beam or laser beam that is both larger in size and highly collimated. Typically, beam divergence specifications are given for the full angular spread of the beam. Although these beams are smaller over larger distances, additional focusing options can be used to yield even smaller spot sizes. As a rule, the beam’s expander power is equal to the ratio of the effective focal length (EFL) of the objective lens to the EFL of the entrance optic. The separation between these two optical components is equal to the sum of their back focal lengths (BFL).
Zoom systems and modular systems require special beam expanders. During the trial phase of a laser system, for example, zoom beam expanders are used. These optical components, which are also known as variable beam expanders, are Galileo-style beam expanders where the negative entrance optics are split into two subgroups. Varying the distance of the subgroups by their individual focal lengths yields adjustments to the total focal length. When the two subgroups are then adjusted with respect to the exit optics, the expansion factor can be varied. If a laser system’s spot size requires adjustment, then modular beam expanders are used. Focusing stability is possible because the optics are shifted without rotation.