Technical Guide: Beam expanders

Beam expanders are optical systems for increasing or decreasing the diameter of a laser beam. A beam expander can enlarge an input beam by the factor M, but it can also reduce it by the factor 1/M with a reversed optical beam path.

Usually beam expanders are used to increase the diameter of laser beams.  Stating that the product of beam diameter and divergence is constant for one wavelength, the divergence decreases with increasing beam diameter by the same factor.

This physical correlation shall apply for beam expanders with a fixed magnification as well as for zoom beam expanders with a variable magnification.

On principal a beam expander can be realized by two different types.

On the one hand there is the Kepler principle which consists of two focusing lens groups. The first one focusses the beam. The second lens group is positioned behind the common focus point and collimates the divergent light again.

The Galilei principle enables beam expanders with short overall lengths. In contrast to the Kepler structure there is no internal focus point, which can result in a critical rise of temperature inside the lens if the distance to the neighbored lenses is too small. Therefor this type is suitable for short pulsed lasers. Because of these crucial advantages over the Kepler principle the Galilei principle is often the first choice.

On the other hand there is the Galilei setup which consists of a dispersing and a collecting lens group. The group at the front side makes the beam divergent while the group behind collimates so that the input and the output beam are both parallel to the optical axis.

Every beam expander from Sill Optics has a mechanical or electrical divergence adjustment available. This enables a compensation of the inherent laser divergence and an optimal collimation. Furthermore the beam can be focused (convergent beam path), defocused (divergent beam path) systematically.

A decreasing distance between the first and second lens group makes the output beam more divergent (see middle of the figure). On the other hand an increasing distance makes the beam more convergent (see lower part of the figure).

The divergence adjustment is very sensible for a high magnification. Reversely, standard divergence adjustments in combination with small magnifications do not suffice in some cases.

The scale for the divergence adjustment of Sill lenses specifies the lens distance change [mm]. Normally it can be variated in a region of ± 3 mm. In case of custom specific special lenses the adjustmant range can even exceed 3 mm.

A tunable magnification is the main advantage of zoom beam expanders. A manual or motorized variable adjustment enables a 1 to 3 times or respectively a 1  to 8 times variation of the magnification. For that zoom beam expanders are more flexible than normal ones. A disadvantage is a lower beam quality. Furthermore most of the zoom beam expanders have a short-term internal beam reduction. This can result in high power or rather energy densities on top of the lens elements. Because of that reasons it is only recommended to use zoom beam expanders if it is really necessary. The suitability for high power and short pulsed lasers should be cleared up before.

The zoom beam expanders from Sill Optics also have a separate divergence adjustment which can be variated independent of the magnification.

Depending on your wishes an electrical adjustment of the divergence or the magnification or both by a computer is possible. Instructions and manuals are available on request.

Especially for zoom beam expanders the positioning inside the whole optical setup has an important role to play. A tilt between the optical axis and the expander can result in a high pointing default. Pointing means the lateral aberration of the laser beam or the change of direction after the beam expander depending on the zoom adjustment. Exact positioning or alternatively a setup which is able to compensate such position variations is necessary for an optimal light ray path through the whole setup.

Sill Optics offers two adjustable mounts (S5SET0125 and S5SET0150) with these adjustment possibilities as an accessories.

Furthermore for a horizontal setup it is better to fix the long zoom beam expanders with a central clamping ring instead of using the lateral C-Mount. For more details we are looking forward to supporting you.