Laser Beam Expanders: Keplerian vs. Galilean Beam Expanders and Other Configurations

Author: Bryan Ng – Marketing Manager

Editor: Qu Yingli – R&D Director

Published on:

Last edited:

1. Introduction to Laser Beam Expanders

Laser Beam Expanders Article
Wavelength Opto-Electronic Ronar-Smith® Laser Beam Expanders

Laser beam expanders are critical optical devices that manipulate and enhance laser beams in various applications. They are pivotal in extending the beam’s width, collimation, and divergence, making them invaluable tools in research, manufacturing, telecommunications, and medical fields.

There are several configurations of laser beam expanders and they can be grouped differently. For instance, the Keplerian and Galilean beam expander design is one way of classifying laser beam expanders. Let’s first look at the difference between these two designs before we delve into other configurations.

2. Galilean Beam Expanders vs. Keplerian Beam Expanders

Keplerian beam expander and Galilean beam expander
Figure 1: Keplerian Beam Expander and Galilean Beam Expander Design

. The Keplerian and Galilean beam expanders classification is based on the lens or lens group combination structure.

2.1 Keplerian Beam Expander Design and Applications

As shown in Figure 1, the Keplerian beam expander design involves a positive lens pair with focal positions coincident with each other. The total length between the lenses is the sum of the two focal lengths. The applications for the Keplerian beam expander design are:

  1. Interferometry and other applications that require using spatial filtering in the intermediate focal point. This filter will block out any light that is not in the desired spatial mode.
  2. Low-power laser applications. Due to the intermediate focal point, Keplerian beam expander can only be used in lower-power laser applications such as alignment, measurements, scanning, size gauging, etc.

2.2 Galilean Beam Expander Design and Applications

As shown in figure 1, the Galilean beam expander design makes use of a negative and a positive lens pair with their focal position coincident with each other and the total length between the lenses is the sum of the two focal lengths.

Galilean beam expanders can be used for a majority of laser applications, including high-power laser applications such as laser marking, soldering, cutting, etc.

3. Other Laser Beam Expanders Configurations

Now we understand the difference between Keplerian and Galilean beam expander design, let’s look into other different configurations. A laser beam expander can also be classified based on the viewing angle, whether the magnification is fixed or varying. For instance, a laser beam expander is either in a fixed or zoomed magnification. Additionally, based on its tuning method, the zoomed laser beam expanders can also be classified as manual or motorized zoomed configurations.

Another type of classification is based on the refraction or reflection being applied. So, a laser beam expander can be classified as refractive (in most cases) or a reflective beam expander. There are laser beam expanders designed for CO2 laser applications.

4. Fixed and Zoomed Laser Beam Expanders

4.1 Fixed Laser Beam Expanders

Figure 2: Fixed Beam Expander Diagram

In a fixed beam expander, lenses can only be tuned to adjust the divergence angle of the laser beam. Compared with a zoomed beam expander, the fixed magnification beam expander features a more compact size and lighter weight.

We offer fixed beam expanders (BEX series) with the following specifications:

Magnification: 1x – 50x
Beam Divergence: Adjustable
Design Type: Galilean/Keplerian
Pointing Stability: < 1 mrad
Wavelength: 266 / 355 / 405 / 532 / 633 / 1064 / 1550 / 2000 / 9400 / 10600nm

4.2 Zoomed Laser Beam Expanders

Figure 3: Zoomed Beam Expander Structures

The concept of zoomed laser beam expanders works on the internal translation stages and focusing mechanisms to adjust and account for the changes in magnification continuously. It also considers laser divergence and performs relative adjustments without affecting the overall housing length.

Manual/motorized zoomed laser beam expanders have variable magnification in a certain range. This flexibility in magnification makes the beam expanders useful for applications that need controllable focused beam size. We offer zoomed beam expanders (BXZ and BXZ-MOT series) with the following specifications:

Magnification: 0.25x – 10x
Divergence Adjustability: Yes
Angle of Incidence: 0 ± 0.06°
Pointing Stability: < 1 mrad
Wavelength: 257 / 355 / 532 / 1064 / 1550 / 1940 / 2800 / 9400 / 10600nm

4.2.1 Manual Zoomed Laser Beam Expanders

Manual Zoom Beam Expander Outline
Figure 4: Manual Zoomed Beam Expander Outline

Figure 4 shows a typical manual adjusted zoomed beam expander. There are two adjusting rings, one is to set magnifications whereas another is to set the focusing and divergence angle of the laser beam.

4.2.2 Motorized Zoomed Laser Beam Expanders

Auto Motorized Zoom Beam Expander and Control Board
Figure 5: Motorized Zoomed Laser Beam Expander and In-House Developed Control Board

Following the global trend in automation, Wavelength Opto-Electronic came up with a new series of automatic/motorized laser beam expanders. The unique design of an integrated printed circuit board provides an integrated communication port, a memory-based calibration function, and a high accuracy of 10μm.

5. Reflective Laser Beam Expanders

Figure 6: Design of Reflective Laser Beam Expander

To reduce the thermal effects in high-power lasers as well as to eliminate the chromatic aberration in ultrafast lasers, we came up with new modules based on reflective focusing optics. It could be used as a stand-alone module, or integrated with other transmission-based beam expanders.

6. CO2 Laser Beam Expanders

CO2 laser beam expanders are developed to work with higher power (>100w) CO2 laser. We have developed below CO2 laser beam expanders for various CO2 laser applications:

CO2 Laser Beam Expander (UniBet Series)

Collimation of COLaser: >100W
Fixed Magnifications: 1.5x – 10x
Design: Galilean
Adjustable: Divergence

CO2 Laser Beam Expander (MiniBet Series)

Collimation of COLaser: >100W
Fixed Magnifications: 1.5x – 6x
Design: Galilean
Adjustable: Divergence

CO2 Laser Water Cool Beam Expander (BET-WC Series)

Collimation of COLaser: >200W
Fixed Magnifications: 1.5x – 10x
Design: Galilean
Adjustable: Divergence

7. Conclusion

Wavelength Opto-Electronic designs and manufactures our in-house Ronar-Smith® brand of laser beam expanders, available in different magnification configurations and types of Galilean as well as Keplerian. With our state-of-the-art production facilities and design capabilities, you can be assured that our Ronar-Smith® laser beam expanders demand the highest quality in beam broadening and collimation without compromising on the expanded beam quality.

We value safety as much as we do and you can be assured that all our beam expanders are subjected to rigorous testing before market sales. All our fixed and zoom laser beam expanders are telescopic by nature and require a collimated beam input while delivering an expanded collimated beam at the output. All designs come with a detailed specification table and operation manual.

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Can you widen a laser beam?

Yes, a laser beam can become wider or narrower by a laser beam expander The magnification of the laser beam expander is decided by the focal length ratio of the divergent and collimated lens inside. When the laser beam in the exit side of the beam expander, the laser beam size will be decreased.

How do laser beam expanders work?

A Laser beam expander is made up of two groups of lenses: a divergent lens and a converging lens. The diverging lens expands the input beam and the converging lens then collimates the divergent beam to be parallel. Thus the input laser beam size can be increased.  The magnification of a laser beam expander is equal to the focal length ratio of the converging/collimating lens and the diverging lens.
 There are two main types of beam expanders: Galilean and Keplerian. The main difference between these two types of laser beam expander is in the diverging lens: A Galilean type uses a negative lens as the diverging lens whereas a Keplerian type applies a positive lens as the diverging lens. For both types, the focal points position of the diverging and converging lens are coincident.

What is the difference between Galilean and Keplerian laser beam expanders?

Galilean beam expanders do not have an internal focus, which makes them safer for high-power applications. Whereas Keplerian beam expanders have internal focus which can be used for spatial filtering.

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