RONAR-SMITH^® LASER OPTICS & IR IMAGING

Using Laser Beam Expanders

Laser Beam expanders are designed to decrease the laser's beam spot size at large distances. There are two main types of beam expanders: Keplerian and Galilean. In its simplest form, the Galilean type consists of a positive and a negative focal length lens whereas the Keplerian type consists of two positive focal length lenses. Both designs provide a certain angular magnification, called the Expander Power. The beam diameter is first increased in size by this power and then the beam divergence is reduced by the same power. This combination yields a beam that is not only larger, but one that is also highly collimated. The result is a smaller beam at a large distance when compared to the laser alone (Equation 1.0).

Note that the beam divergence specifications in our catalogs are for the full angular spread of the beam.

In addition, an expanded beam can yield smaller spot sizes when used in combination with additional focusing optics. This is very useful in focusing optimization. however, many applications simply require a larger continuous beam.

The beam expander power (MP) is equal to the ratio of the effective focal length (EFL) of the objective lens to the effective focal length of the entrance optic. The physical separation between the objective lens and entrance optic is equal to the sum of their back focal lengths (BFL).

1. The Galilean design contains no internal focal point and
2. The overall length of the Galilean as compared to a similar power Keplerian design is smaller.