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Beam Expander Theory

Beam Expander Theory

A laser beam expander is designed to either decrease the laser's beam spot size at large distances or produce a larger collimated output laser beam.  Laser beam expanders come in two main types: Keplerian laser beam expander or a Galilean laser beam expander. In its simplest form a Galilean beam expander consists of a postive and a negative focal length lens whereas the Keplerian consists of two postive focal length lenses.  More advanced forms of laser beam expander utilise more than two lens elements to correct for spherical abberation. Both beam expander designs provide a certain anular magnifaction, called 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 highly collimated.  The result is a smaller beam at large distances when compared to the laser alone.  The laser beam expander is also useful when a larger collimated beam is required over a given range. See the below equation:

BL = B + OL(0.3048)

where

BL = Beam Diameter

B = Increase in Beam Diameter = Beam Diameter (mm) x Expander Power

O = 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 from a laser beam expander.

In addition, an expanded collimated laser beam from a beam expander can produce smaller spot sizes when used in combination with additional focussing optics. This is very useful in focus optimization, however many applications just require a larger continuous beam.  Simply put 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 seperation between the objective lens and the entrance optic is the equal to the sum of their back focal lengths (BFL).

DiOptika's beam expander range are of the Galilean type and utilize a three element lens design to minimise spherical abberation and optimise the beam expander performance.