Variable Optical Beam Expanders

  • Continuously Variable Beam Expansion
  • 2X to 5X Zoom
  • AR-Coated Optics


1050 - 1620 nm AR Coating


400 - 650 nm AR Coating


650 - 1050 nm AR Coating

Related Items

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Nominal Values for Peak-to-Valley Wavefront Error

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Diagram of BE02-05 Beam Expanders


  • Max Beam Diameter (1/e2): Ø4.0 mm
  • Surface Quality: 20-10 (Scratch-Dig)
  • Wavefront Error: <λ/4
  • AR Coating: Ravg < 0.5% Over Coating Range
  • Input Aperture Threading: SM1 (1.035"-40)
  • Output Aperture Threading: SM2 (2.035"-40)
  • Post-Mountable via 8-32 and M4 Taps
  • Housing Length (L):
    • -A and -B: 188 - 195 mm
    • -C: 174 - 181 mm

Thorlabs' Variable Magnification Galilean Beam Expanders have a 2X - 5X continuously variable beam expansion ratio. The expansion ratio can be adjusted from the minimum to the maximum value via one full rotation of the zoom control. Once the desired magnification is obtained, a setscrew can be tightened to lock the magnification. If the beam exiting the expander is collimated and the magnification is changed, the focus should be readjusted to maintain collimation. Please note that the induced wavefront error varies with the magnification ratio. This wavefront error can be seen in the graph to the right.

Our variable beam expanders are offered with a broadband antireflection coating for the 400 - 650 nm (-A), 650 - 1050 nm (-B), or 1050 - 1620 nm (-C) range. These units also feature achromatic operation over the specified AR coating range. The input lens can also be rotated to adjust the distance between the input and output lenses in order to compensate for the divergence or convergence of the output beam. Each beam expander features SM-threaded input and output ports (see the diagram above for details), making them compatible with a number of Thorlabs components such as lens tubes. The housing features both 8-32 and M4 taps for mounting. Additionally, this housing has a Ø2" portion so that it can be mounted in our KS2 Ø2" mirror mount.

Thorlabs offers variable magnification beam expanders that use a sliding lens to achieve expansions from 0.5X to 2X. Other beam expander options available are our UV fused silica, achromatic, and ZnSe fixed beam expanders, UV fused silica and achromatic zoom beam expanders, and reflective beam expanders. For more information on our extensive line of beam expanders, please see the Beam Expanders tab.

Item # Expansion Input
Max Input Beam
Diameter (1/e2)
AR Coating Range Lens Substrates
BE02-05-A 2X - 5X Ø8.0 mm Ø4.0 mm SM1 (1.035"-40) SM2 (2.035"-40) 8-32 (M4) 400 - 650 nm N-BK7, SF2, N-FK5, SF5
BE02-05-B 2X - 5X Ø8.0 mm Ø4.0 mm SM1 (1.035"-40) SM2 (2.035"-40) 8-32 (M4) 650 - 1050 nm BAFN10, SFL6, N-FK5, LAKN22
BE02-05-C 2X - 5X Ø8.0 mm Ø4.0 mm SM1 (1.035"-40) SM2 (2.035"-40) 8-32 (M4) 1050 - 1620 nm BAFN10, SFL6, N-FK5, LAKN22

Damage Threshold Specifications
Item # Suffix Laser Type Damage Threshold
-A Pulsed 0.5 J/cm² (532 nm, 10 ns Pulse, 10 Hz, Ø0.566 mm)
CWa 600 W/cm (532 nm, Ø0.020 mm)
-B Pulsed 5.0 J/cm² (810 nm, 10 ns Pulse, 10 Hz, Ø0.155 mm)
CWa 9,000 W/cm (1064 nm, Ø0.025 mm)
-C Pulsed 5.0 J/cm² (1542 nm, 10 ns Pulse, 10 Hz, Ø0.181 mm)
CWa 350 W/cm (1550 nm, Ø0.194 mm)
  • The power density of your beam should be calculated in terms of W/cm. For an explanation of why the linear power density provides the best metric for long pulse and CW sources, please see the "Continuous Wave and Long-Pulse Lasers" section below.

Damage Threshold Data for Thorlabs' Variable Beam Expanders

The specifications to the right are the damage thresholds for Thorlabs' variable beam expanders.


Laser Induced Damage Threshold Tutorial

The following is a general overview of how laser induced damage thresholds are measured and how the values may be utilized in determining the appropriateness of an optic for a given application. When choosing optics, it is important to understand the Laser Induced Damage Threshold (LIDT) of the optics being used. The LIDT for an optic greatly depends on the type of laser you are using. Continuous wave (CW) lasers typically cause damage from thermal effects (absorption either in the coating or in the substrate). Pulsed lasers, on the other hand, often strip electrons from the lattice structure of an optic before causing thermal damage. Note that the guideline presented here assumes room temperature operation and optics in new condition (i.e., within scratch-dig spec, surface free of contamination, etc.). Because dust or other particles on the surface of an optic can cause damage at lower thresholds, we recommend keeping surfaces clean and free of debris. For more information on cleaning optics, please see our Optics Cleaning tutorial.

Testing Method

Thorlabs' LIDT testing is done in compliance with ISO/DIS 11254 and ISO 21254 specifications.

First, a low-power/energy beam is directed to the optic under test. The optic is exposed in 10 locations to this laser beam for 30 seconds (CW) or for a number of pulses (pulse repetition frequency specified). After exposure, the optic is examined by a microscope (~100X magnification) for any visible damage. The number of locations that are damaged at a particular power/energy level is recorded. Next, the power/energy is either increased or decreased and the optic is exposed at 10 new locations. This process is repeated until damage is observed. The damage threshold is then assigned to be the highest power/energy that the optic can withstand without causing damage. A histogram such as that below represents the testing of one BB1-E02 mirror.

LIDT metallic mirror
The photograph above is a protected aluminum-coated mirror after LIDT testing. In this particular test, it handled 0.43 J/cm2 (1064 nm, 10 ns pulse, 10 Hz, Ø1.000 mm) before damage.
Example Test Data
Fluence # of Tested Locations Locations with Damage Locations Without Damage
1.50 J/cm2 10 0 10
1.75 J/cm2 10 0 10
2.00 J/cm2 10 0 10
2.25 J/cm2 10 1 9
3.00 J/cm2 10 1 9
5.00 J/cm2 10 9 1

According to the test, the damage threshold of the mirror was 2.00 J/cm2 (532 nm, 10 ns pulse, 10 Hz, Ø0.803 mm). Please keep in mind that these tests are performed on clean optics, as dirt and contamination can significantly lower the damage threshold of a component. While the test results are only representative of one coating run, Thorlabs specifies damage threshold values that account for coating variances.

Continuous Wave and Long-Pulse Lasers

When an optic is damaged by a continuous wave (CW) laser, it is usually due to the melting of the surface as a result of absorbing the laser's energy or damage to the optical coating (antireflection) [1]. Pulsed lasers with pulse lengths longer than 1 µs can be treated as CW lasers for LIDT discussions.

When pulse lengths are between 1 ns and 1 µs, laser-induced damage can occur either because of absorption or a dielectric breakdown (therefore, a user must check both CW and pulsed LIDT). Absorption is either due to an intrinsic property of the optic or due to surface irregularities; thus LIDT values are only valid for optics meeting or exceeding the surface quality specifications given by a manufacturer. While many optics can handle high power CW lasers, cemented (e.g., achromatic doublets) or highly absorptive (e.g., ND filters) optics tend to have lower CW damage thresholds. These lower thresholds are due to absorption or scattering in the cement or metal coating.

Linear Power Density Scaling

LIDT in linear power density vs. pulse length and spot size. For long pulses to CW, linear power density becomes a constant with spot size. This graph was obtained from [1].

Intensity Distribution

Pulsed lasers with high pulse repetition frequencies (PRF) may behave similarly to CW beams. Unfortunately, this is highly dependent on factors such as absorption and thermal diffusivity, so there is no reliable method for determining when a high PRF laser will damage an optic due to thermal effects. For beams with a high PRF both the average and peak powers must be compared to the equivalent CW power. Additionally, for highly transparent materials, there is little to no drop in the LIDT with increasing PRF.

In order to use the specified CW damage threshold of an optic, it is necessary to know the following:

  1. Wavelength of your laser
  2. Beam diameter of your beam (1/e2)
  3. Approximate intensity profile of your beam (e.g., Gaussian)
  4. Linear power density of your beam (total power divided by 1/e2 beam diameter)

Thorlabs expresses LIDT for CW lasers as a linear power density measured in W/cm. In this regime, the LIDT given as a linear power density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size, as demonstrated by the graph to the right. Average linear power density can be calculated using the equation below. 

The calculation above assumes a uniform beam intensity profile. You must now consider hotspots in the beam or other non-uniform intensity profiles and roughly calculate a maximum power density. For reference, a Gaussian beam typically has a maximum power density that is twice that of the uniform beam (see lower right).

Now compare the maximum power density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately. A good rule of thumb is that the damage threshold has a linear relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 10 W/cm at 1310 nm scales to 5 W/cm at 655 nm):

CW Wavelength Scaling

While this rule of thumb provides a general trend, it is not a quantitative analysis of LIDT vs wavelength. In CW applications, for instance, damage scales more strongly with absorption in the coating and substrate, which does not necessarily scale well with wavelength. While the above procedure provides a good rule of thumb for LIDT values, please contact Tech Support if your wavelength is different from the specified LIDT wavelength. If your power density is less than the adjusted LIDT of the optic, then the optic should work for your application. 

Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. The damage analysis will be carried out on a similar optic (customer's optic will not be damaged). Testing may result in additional costs or lead times. Contact Tech Support for more information.

Pulsed Lasers

As previously stated, pulsed lasers typically induce a different type of damage to the optic than CW lasers. Pulsed lasers often do not heat the optic enough to damage it; instead, pulsed lasers produce strong electric fields capable of inducing dielectric breakdown in the material. Unfortunately, it can be very difficult to compare the LIDT specification of an optic to your laser. There are multiple regimes in which a pulsed laser can damage an optic and this is based on the laser's pulse length. The highlighted columns in the table below outline the relevant pulse lengths for our specified LIDT values.

Pulses shorter than 10-9 s cannot be compared to our specified LIDT values with much reliability. In this ultra-short-pulse regime various mechanics, such as multiphoton-avalanche ionization, take over as the predominate damage mechanism [2]. In contrast, pulses between 10-7 s and 10-4 s may cause damage to an optic either because of dielectric breakdown or thermal effects. This means that both CW and pulsed damage thresholds must be compared to the laser beam to determine whether the optic is suitable for your application.

Pulse Duration t < 10-9 s 10-9 < t < 10-7 s 10-7 < t < 10-4 s t > 10-4 s
Damage Mechanism Avalanche Ionization Dielectric Breakdown Dielectric Breakdown or Thermal Thermal
Relevant Damage Specification No Comparison (See Above) Pulsed Pulsed and CW CW

When comparing an LIDT specified for a pulsed laser to your laser, it is essential to know the following:

Energy Density Scaling

LIDT in energy density vs. pulse length and spot size. For short pulses, energy density becomes a constant with spot size. This graph was obtained from [1].

  1. Wavelength of your laser
  2. Energy density of your beam (total energy divided by 1/e2 area)
  3. Pulse length of your laser
  4. Pulse repetition frequency (prf) of your laser
  5. Beam diameter of your laser (1/e2 )
  6. Approximate intensity profile of your beam (e.g., Gaussian)

The energy density of your beam should be calculated in terms of J/cm2. The graph to the right shows why expressing the LIDT as an energy density provides the best metric for short pulse sources. In this regime, the LIDT given as an energy density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size. This calculation assumes a uniform beam intensity profile. You must now adjust this energy density to account for hotspots or other nonuniform intensity profiles and roughly calculate a maximum energy density. For reference a Gaussian beam typically has a maximum energy density that is twice that of the 1/e2 beam.

Now compare the maximum energy density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately [3]. A good rule of thumb is that the damage threshold has an inverse square root relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 1 J/cm2 at 1064 nm scales to 0.7 J/cm2 at 532 nm):

Pulse Wavelength Scaling

You now have a wavelength-adjusted energy density, which you will use in the following step.

Beam diameter is also important to know when comparing damage thresholds. While the LIDT, when expressed in units of J/cm², scales independently of spot size; large beam sizes are more likely to illuminate a larger number of defects which can lead to greater variances in the LIDT [4]. For data presented here, a <1 mm beam size was used to measure the LIDT. For beams sizes greater than 5 mm, the LIDT (J/cm2) will not scale independently of beam diameter due to the larger size beam exposing more defects.

The pulse length must now be compensated for. The longer the pulse duration, the more energy the optic can handle. For pulse widths between 1 - 100 ns, an approximation is as follows:

Pulse Length Scaling

Use this formula to calculate the Adjusted LIDT for an optic based on your pulse length. If your maximum energy density is less than this adjusted LIDT maximum energy density, then the optic should be suitable for your application. Keep in mind that this calculation is only used for pulses between 10-9 s and 10-7 s. For pulses between 10-7 s and 10-4 s, the CW LIDT must also be checked before deeming the optic appropriate for your application.

Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. Contact Tech Support for more information.

[1] R. M. Wood, Optics and Laser Tech. 29, 517 (1998).
[2] Roger M. Wood, Laser-Induced Damage of Optical Materials (Institute of Physics Publishing, Philadelphia, PA, 2003).
[3] C. W. Carr et al., Phys. Rev. Lett. 91, 127402 (2003).
[4] N. Bloembergen, Appl. Opt. 12, 661 (1973).

Posted Comments:
user  (posted 2021-12-13 06:35:16.743)
Hello, it would valuable to provide: - details on what knurled ring to turn for magnification, and for focus. And what not to touch to avoid disassembly when that can be a risk. - considerations on their respective position (if the focus ring is at its deepest, is it supposed to correspond to min & max magnification focus compensation ?) - general advice or example on centering, beam collimation & refocusing with these products. We have observed significant beam shifting when turning the focus ring (supposed to be the last adjustment). The advised KS2 mount is only tilting so that may not be enough to correct the positioning in that case.
cdolbashian  (posted 2022-01-05 04:53:50.0)
Thank you for reaching out to us Gregory. In general, this should be handled as any other optical device with regards to alignment. As for the specifics for the performance and usage for this device, I have contacted you directly to answer your questions specifically.
Naoki Oda  (posted 2021-03-24 17:30:01.34)
Could you let me know transmission property of BE02-05-A beam expander, especially at 532nm?
YLohia  (posted 2021-04-08 10:49:27.0)
Hello, the transmission of the BE02-05-A at 532nm is ~97.4% according to supplied Zemax file.
user  (posted 2019-10-21 05:16:31.88)
Dear Thorlabs, We purchased one of these beam expanders for a system that will be shipped to a customer. There seems to be an incredible amount of play in the magnification adjustment, causing the beam to deviate a huge amount when changing. Is this a faulty product or standard. Please contact me ASAP as I will have to find an alternative if this cannot be sorted. We envision purchasing up to 50 of these over some years. I also noticed the mounted beamsplitter cubes do not seem to be orientated particularly well with regards to the baseplates.
DJayasuriya  (posted 2019-10-24 10:08:52.0)
Response from Dinuka @ thorlabs: thank you for your query I have got in touch with you directly to resolve your issue.
simon.neves  (posted 2018-10-12 16:17:27.357)
Hello, Can these beams expanders be used from both side ? I mean, can we reverse input and output in order to get a beam "reducer" ? Thank you very much.
YLohia  (posted 2018-10-12 11:29:32.0)
Hello, thank you for contacting Thorlabs. Yes, these can indeed be used as beam "reducers".
michael.baumgartner  (posted 2016-02-17 10:58:12.19)
kann der Strahlaufweiter auch 'rückwärts' genutzt werden (um den Strahl zu verkleinern)?
shallwig  (posted 2016-02-19 05:03:26.0)
Stefan von Thorlabs: Vielen Dank für ihre Anfrage. Sie können den BE02-05-A auch in umgekehrter Richtung zur Strahlverkleinerung benutzen. Ich habe Sie direkt kontaktiert um weitere Fragen zu klären.
f.gaertner  (posted 2014-10-31 13:32:49.15)
The input beam diameter is declared to be 2.8mm for 1/e². I assume this is the value for a normal gaussian beam, getting some resonable profile at the output of the expander. Since we have a flat-top intensity distribution i would like to know what is the max input diameter in this case. Thanks!
jlow  (posted 2014-10-31 09:39:31.0)
Response from Jeremy at Thorlabs: The max. input of Ø2.3mm is for diffraction-limited performance at 10X. For 5X, it would be about Ø4mm. You can use the same numbers for a flat-top profile.
cdaly  (posted 2012-11-30 16:24:02.953)
Response from Chris at Thorlabs: Thank you for using our feedback feature. We are unable to provide 2010 SolidWorks files directly as the version we use for design (2012) restricts us from creating backwards compatible files, but we do have Step files available for download as well. These files can be opened in SolidWorks 2010 and then saved as an .sldprt file, which will give you the format you require.
xiaoqiang026403  (posted 2012-11-29 02:15:44.87)
I use SolidWorks 2010,but some of your products' Drawing and Documents (SolidWorks) are opened by SolidWorks 2012. Could you send me the BE02-05-A SolidWorks Documents in version 2010?
jlow  (posted 2012-11-06 09:39:00.0)
Response from Jeremy at Thorlabs: We ran a simulation in Zemax and found that the performance is pretty much the same for the BE05-10-C and using doublet. If you do not need the zoom function, I would recommend using a fixed magnification because the cost is generally lower.
neil.troy  (posted 2012-10-18 22:55:46.787)
What is likely to be the performance for these systems with broad light sources, say 100 nm wide? If I could live with a fixed lens magnification would I be better off with using a pair of appropriately chosen achromatic doublets?
bdada  (posted 2012-02-24 14:52:00.0)
Response from Buki at to omertzang: Thank you for your feedback. We do not have any specific data on the damage threshold for pulsed light but we expect the beam expander to withstand 100 mJ/cm2 if the pulse is 10 ns. Pleae contact if you have any questions.
omertzang  (posted 2012-02-05 06:04:13.0)
I could not find the damage threshold for Pulsed laser. please send me the damage threshold specification for pluses. I intend to use a 800nm femto-second Ti-Sapphire and I am sure your costumers have experience in a beam expansion setup for such lasers. Thank you!
Thorlabs offers fixed and variable magnification beam expanders, as well as zoom beam expanders that do not need to be refocused when the magnification is adjusted since the collimation remains constant. The table below provides a direct comparison of the options we offer. Please contact Tech Support if you would like help choosing the best beam expander for your specific application.
Beam Expander
Fixed Magnification
Laser Line,
Sliding Lens
Fixed Magnification Achromatic,
Sliding Lens
Fixed Magnification
Sliding Lens
Sliding Lens
Zoom Achromatic,
Sliding Lens
Expansions Available 2X, 3X, 5X, 10X, 20Xa 2X, 3X, 5X, 10X,
15X, 20X
2X, 5X, 10X 0.5X - 2.5X, 1X - 4X 0.5X - 2.5X, 1X - 4X,
2X - 8X, 4X - 16X
AR Coating Range(s)
(Item # Suffix)
240 - 360 nm (-UVB)
248 - 287 nm (-266)
325 - 380 nm (-355)
488 - 580 nm (-532)
960 - 1064 nm (-1064)
400 - 650 nm (-A)
650 - 1050 nm (-B)
1050 - 1650 nm (-C)
7 - 12 μm (-E3) 240 - 360 nm (UVB)
330 - 370 nm (3)
495 - 570 nm (2)
980 - 1130 nm (1)
400 - 650 nm (A)
650 - 1050 nm (B)
1050 - 1650 nm (C)
Mirror Coating (Range) N/A
Reflectance (per Surface) Ravg < 0.2%
(RMax < 1.5% for -UVB)
RMax < 0.5% Ravg < 1.0% Ravg < 0.2%
(RMax < 1.5% for UVB)
RMax < 0.5%
Max Input Beam Diameter 2X: 8.5 mm
3X: 9.0 mm
5X: 4.3 mm
10X: 2.8 mm
20X: 2.0 mm
2X: 8.5 mm
3X: 9.0 mm
5X: 5.0 mm
10X: 3.0 mm
15X: 2.5 mm
20X: 2.0 mm
2X: 9.5 mm
5X: 6.7 mm
10X: 3.5 mm
0.5X - 2.5X: 10.9 to 8.0 mm
1X - 4X: 10.9 to 8.8 mm
0.5X - 2.5X: 10.9 to 8.0 mm
1X - 4X: 10.9 to 8.8 mm
2X - 8X: 6.0 to 4.4 mm
4X - 16X: 6.0 to 2.7 mm
Wavefront Error <λ/4 (Peak to Valley)
Surface Quality 10-5 Scratch-Dig 20-10 Scratch-Dig 80-50 Scratch-Dig 10-5 Scratch-Dig 20-10 Scratch-Dig
  • These 20X beam expanders are only available with V coatings for 355 nm, 532 nm, or 1064 nm.

Beam Expander
Variable Magnification,
Rotating Lens
Variable Magnification,
Sliding Lens
Reflective Beam Expander
Fixed Magnification
Expansions Available 2X - 5X 0.5X - 2X 2X, 4X, 6X
AR Coating Range(s)
(Item # Suffix)
400 - 650 nm (-A)
650 - 1050 nm (-B)
1050 - 1620 nm (-C)
400 - 650 nm (-A)
650 - 1050 nm (-B)
Mirror Coating (Range) N/A Protected Silver
(450 nm - 20 μm)
Reflectance (per Surface) Ravg < 0.5% Ravg < 0.5% Ravg > 96%
Max Input Beam Diameter 2X - 5X: 4.0 mm 0.5X - 2X: 6.0 mm to 3.0 mm 3 mm
Wavefront Error <λ/4 <λ/10a (RMS)
Surface Quality 20-10 Scratch-Dig 40-20 Scratch-Dig
  • For a Ø1.5 mm Input Beam at 2X magnification, Ø1.0 mm Input Beam at 4X magnification, or Ø0.5 mm Input Beam at 6X magnification.

2X - 5X Variable Zoom Galilean Beam Expanders

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available
BE02-05-A Support Documentation
BE02-05-AOptical Beam Expander, 2X - 5X Zoom, AR Coated: 400 - 650 nm
Lead Time
BE02-05-B Support Documentation
BE02-05-BOptical Beam Expander, 2X - 5X Zoom, AR Coated: 650 - 1050 nm
Lead Time
BE02-05-C Support Documentation
BE02-05-COptical Beam Expander, 2X - 5X Zoom, AR Coated: 1050 - 1620 nm
7-10 Days