beam expanders (original) (raw)

Definition: optical devices for modifying the beam radius of a collimated beam

Category: general optics

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Related: laser beamscollimated beamsbeam radiusmagnificationlensesGaussian beamsanamorphic prism pairstelescopes

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📦 For purchasing beam expanders, use the RP Photonics Buyer's Guide — an expert-curated directory for finding all relevant suppliers, which also offers advanced purchasing assistance.

Contents

What are Beam Expanders?

In laser technology and general optics, one often works with collimated beams, by definition having a roughly constant beam radius over some length. Sometimes, it is necessary to substantially modify a beam radius, for example to achieve a reduced beam divergence for transmitting the beam over a larger distance. For that purpose, beam expanders can be built and are also available as fixed optical components.

In most cases, a beam expander is realized as an optical telescope consisting of two lenses (or in some cases of two curved mirrors). Two different configurations are common:

• A Keplerian telescope consists of two focusing lenses, where the distance between the two lenses is the sum of their focal lengths. There is then a possibly tight focus between the lenses (which might cause laser-induced breakdown in air for high peak powers). The beam radius after that telescope is modified if the two focal length values are different. For example, a doubled beam radius is achieved if the second lens has twice the focal length of the first one.
• A Galilean telescope consists of a focusing and a defocusing lens. Again, the distance between the lenses equals the sum of the focal lengths — where however one focal length is negative (that of the defocusing lens). The advantage of this type of telescope is that it can be more compact.

Fig. 1 shows the calculated evolution of beam radius for a Keplerian telescope for a 2 × beam expansion.

Figure 1: Beam radius versus position in a 2 × Keplerian beam expander.

For achieving a given magnifying power (expansion ratio, ratio of beam radii), one may use different values of focal lengths. Most compact solutions are possible with small focal lengths, but there are limitations. In particular, one may then require lenses with very high numerical aperture, if at the same time a large output beam radius is required. Therefore, beam expanders for operation with large beams are generally longer.

Of course, a beam expander can also be operated “in reverse”, i.e., as a beam reducer.

Variable Beam Expanders

There are variable beam expanders (zoom expanders), i.e., devices where the magnification can be adjusted in a certain range (e.g. from 2× to 5× or from 5× to 10×). (See also the article on zoom lenses.) Those contain at least three lenses and some fine mechanics to adjust the position of at least one of them.

Figure 2: A beam expander with adjustable magnification. Source: Excelitas Technologies

Beam Expanders for One Direction Only

Using cylindrical lenses, one can realize beam expanders which work in one transverse direction only. For that purpose, one may also use anamorphic prism pairs.

Various Aspects

Inappropriate Input Beams

Beam expanders are generally not designed for use with divergent beams, but only for collimated beams, and only within a certain range of beam radii. Otherwise, one may obtain clipping effects and/or not get a collimated beam out. Obviously, a beam can be collimated over a certain length only if its beam waist is large enough. As an example, Figure 3 shows the evolution of beam radius in the same beam expander as considered in Figure 1, but with a five times smaller initial beam radius. Here, the beams can no longer be considered as collimated beams.

A good familiarity with Gaussian beams is a good basis for understanding the operation of beam expanders and similar devices.

Figure 3: Beam radius versus position for a too small input beam radius.

Wavelength Range

For minimum losses of optical power, the lenses are usually equipped with anti-reflection coatings. These, however, work only within a limited wavelength range.

Optical Damage

For application with pulsed lasers, the used lens coatings should also have a sufficiently high optical damage threshold. Further, one should avoid operation with misaligned high-power beams, which could lead to overheating of some parts.

For very high laser powers, purely reflective beam expanders (with mirrors instead of lenses) are used. This is because thermal effects such as thermal lensing are weaker on mirrors. Also, that way one can avoid any parasitic reflections. A disadvantage, however, is that some amount of astigmatism is generally introduced by the mirrors.

Beam Pointing Angles

When modifying the beam radius, one also modifies the strength of beam pointing deviations. For example, doubling the beam radius implies that angular changes in the output beam are only half as strong as those of the input beam.

Frequently Asked Questions

What is an optical beam expander?

A beam expander is an optical device, typically a telescope, that increases the diameter of a collimated beam of light. A key consequence of this is that the beam divergence is reduced.

What are the two main types of beam expanders?

The two common configurations are the Keplerian telescope, which uses two focusing lenses and has an internal focal point, and the Galilean telescope, which uses one focusing and one defocusing lens and can be more compact.

Can a beam expander also reduce a beam's size?

Yes, when operated in reverse, a beam expander functions as a beam reducer, decreasing the diameter of the incoming collimated beam.

What is a variable beam expander?

A variable or zoom beam expander is a device containing at least three lenses where the magnification ratio can be adjusted by changing the positions of the lenses.

Why are mirrors sometimes used instead of lenses in beam expanders?

For very high laser powers, reflective beam expanders with mirrors are used because mirrors are less susceptible to thermal effects like thermal lensing and have a higher optical damage threshold than lenses.

How does a beam expander affect beam pointing stability?

Expanding a beam reduces the effect of angular pointing errors. For example, doubling the beam radius will halve the angular deviation of the output beam compared to the input beam.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 55 suppliers for beam expanders. Among them:

⚙ hardware

Shanghai Optics Inc's custom beam expanders are used in many applications such as laser ranging, laser illumination, interferometry, etc. In high-power laser systems, beam expanders are used increase the beam area without significantly affecting the total laser energy. This results in a reduction of the laser power density which reduce the risk of damaging the coatings and optical materials of optical components. In a laser ranging system, a beam expander is used to minimize the laser divergence, resulting in a smaller collimated beam at a long distance.

⚙ hardware

Beam expanders are optical devices which increases the diameter of the input beam to produce a larger output. Used with collimated light, they have important applications in remote sensing, interferometry, and laser scanning.

⚙ hardware

Photonic Devices' precision Galilean beam expanders have been sold globally for over 20 years and include an extensive number of standard models. These multi-element beam expanders have been computer-designed to provide excellent wavefront quality with minimum beam deformation. All lens elements are air spaced and the majority of models have variable air spacing for collimation adjustment. All lens surfaces have multilayer antireflection coatings, with power handling capabilities over 1 kW (> 200 W for beam expanders operating at 10.6 microns).

⚙ hardware

Edmund Optics has a variety of beam expanders, including devices for different wavelength regions (e.g. Nd:YAG and CO2), rotating or non-rotating optics for divergence adjustment and research-grade devices.

⚙ hardware

Available in singlet, doublet (positive and negative focal lengths) and triplet varieties, Knight Optical can supply customised beam expanders for a range of laser-based, cutting-edge projects. Whether your application works in the ultraviolet (UV), visible, near-infrared (NIR), mid-infrared (MIR) or far-infrared (FIR) spectrum, opting for our custom-made components allows you to specify suitable substrates for your required wavelength.

⚙ hardware

Optogama offers a variety of beam expanders designed to manipulate laser beam parameters with high precision. Our product lineup includes:​

MEX and MEX-V2 Series: Compact motorized laser beam expanders for adjustable beam diameter control.​

MEX-HP Series: High-power motorized beam expanders suitable for demanding applications.​

FEX Series: Fixed ratio beam expanders for consistent beam expansion needs.​

VEX and VRE Series: Variable beam expanders and reducers for versatile beam shaping requirements.​

These systems are engineered for high reliability and performance across UV, visible, and NIR spectral ranges, making them ideal for industrial and research applications.​

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