Laser Beam Expander Theory
Laser beam expanders are designed to either decrease the laser's beam spot size at large distances or
produce a larger diameter collimated laser beam. The 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.  See the below equation:

BL

=

ß + ØL (0.3048)

BL

=

Beam Diameter (mm) at distance L

ß

=

Increase in Beam Diameter

=

Beam Diameter (mm) x Expander Power

Ø

=

Decrease in Beam Divergence

=

Beam Divergence (mrad) Expander Power

L

=

distance (ft.)

This equation is an approximation for the collimated output beam size at a given distance.

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).

DiOptika’s  beam expanders are of the Galilean type. The advantages of the Galilean design over the
Keplerian is that it contains no internal focal point therefore reducing the length.  In addition to improving
beam collimation, DiOptika’s beam expanders can also be used to focus laser beams.


 


                        
           

           
           
          





 




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