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Mounted Calcite Beam Displacers


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Mounted Calcite Beam Displacers

Item #BD27BD40
MaterialHigh Optical Grade Calcite
Spectral Range350 nm to 2.3 µm
Damage Threshold20 J/cm2 (1064 nm, 10 ns, 10 Hz, Ø0.433 mm)
Beam Separation (@ 1500 nm)
(Click for Graph)
2.7 mm4.0 mm
Length (L)1.10" (2.8 cm)1.60" (4.1 cm)
Clear Aperture10 mm x 10 mm
Beam Deviation<30 arcsec
Wavefront Distortion<λ/4
Surface Quality40-20 Scratch-Dig

Features

  • 2 Orthogonally Polarized Outputs
  • 2.7 mm, 4.0 mm, or Custom Displacements Available
  • Outputs Parallel to Input
  • 350 nm to 2.3 μm Operation
  • Highest Grade Optical Calcite
  • Ø1" Housing

Beam displacing prisms are used to separate an input beam into two orthogonally polarized output beams and can be used as polarizing beamsplitters in applications where the 90° separation of the beams is not possible. Fabricated from a single piece of the highest quality optical grade calcite, our beam displacers can be used with wavelengths from 350 nm up to 2.3 μm. Since calcite is a soft crystal that can be easily damaged, our calcite polarizers are housed in a Ø1" metal housing that can be mounted into our optomechanical rotation mounts. For further information on the properties of calcite, please see the Calcite tab above. Our mounted beam displacers are offered in two versions, which provide a beam displacement of either 2.7 mm or 4.0 mm.

In both versions both output beams are parallel to the input beam to within 30 arcsec. In addition Thorlabs also offers a complete line of Calcite Polarizers,Polarization, Optics Components and Polarization Measurement Equipment.

Zemax Files
Click on the red Document icon next to the item numbers below to access the Zemax file download.

Beam Displacer Mechanical Dimensions

Calcite Beam Displacer
Polarization-Dependent Refraction - Calcite Beam Displacer

Our calcite polarization-dependent beam displacers are all based on high-grade, birefringent, calcite crystals. Due to the birefringent structure of calcite, a differential delay is created between two orthogonally polarized waves traveling in the crystal. As shown in the image to the right, this birefringent structure creates a polarization-dependent refraction that effectively steers the polarization planes in two angles. With the incident waves normal to the entry face, the ordinary polarization plane will travel straight through the crystal, while the extraordinary plane will exit the crystal displaced by a distance proportional to the wavelength as well as the length of the crystal.

A calcite beam displacing polarizer is used to separate the orthogonally-polarized components of a beam. Our calcite beam displacers are composed of single rectangular mounted calcite prisms (shown in the drawing to the right). The optical axis of these prisms has been designed to separate beams by 2.7 mm or 4.0 mm. Since calcite is a soft crystal that is easily damaged, almost all of our calcite polarizers are offered in metal housings. With convenient threadings and adapters, these housings can easily be mounted into our opto-mechanical products.

Thorlabs' Calcite Polarizers
Glan-Laser PolarizersGlan-Taylor PolarizersWollaston Polarizer
Mounted Glan-Thompson Polarizers
Unmounted Glan-Thompson Polarizers
Double Glan-Taylor PolarizerBeam Displacers
Frame B
lens tube mounted to PRM1
Frame A
inserting lens tube in PRM1
secure polarizer's position by using spanner wrench
Frame D
inserting polarizer in lens tube
Frame C

Beam Displacer Mounting

Thorlabs Calcite Beam Displacers can be mounted to a PRM1 rotation mount by using a SM1L20 lens tube:

  1. Remove the SM1RR retaining ring that comes with the PRM1 rotation mount.
  2. Screw the threaded end of the lens tube into the rotation mount as shown in Frame A.
  3. Once the lens tube is threaded (Frame B), insert the mounted beam displacer into the other end of the lens tube (see Frame C)
  4. Secure the beam displacer into place using the retaining ring included with the lens tube.
  5. A SPW602 spanner wrench can be used to tighten the retaining ring (Frame D).

Note: The SM1P1 Ø1" to SM1 adapter can be used in place of the SM1L20 lens tube when space is not available for the extra length of the lens tube.

Laser Induced Damage Threshold Tutorial

This 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/DIS11254 specifications. A standard 1-on-1 testing regime is performed to test the damage threshold.

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 a set duration of time (CW) or number of pulses (prf 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.
LIDT BB1-E02
Example Test Data
Fluence# of Tested LocationsLocations with DamageLocations Without Damage
1.50 J/cm210010
1.75 J/cm210010
2.00 J/cm210010
2.25 J/cm21019
3.00 J/cm21019
5.00 J/cm21091

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 it is only representative of one coating run and that Thorlabs' specified damage thresholds 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. Additionally, when pulse lengths are between 1 ns and 1 µs, LIDT can occur either because of absorption or a dielectric breakdown (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 large 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. Linear power density of your beam (total power divided by 1/e2 spot size)
  3. Beam diameter of your beam (1/e2)
  4. Approximate intensity profile of your beam (e.g., Gaussian)

The power density of your beam should be calculated in terms of W/cm. The graph to the right shows why the linear power density provides the best metric for long pulse and CW sources. Under these conditions, linear power density scales independently of spot size; 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 consider hotspots in the beam or other nonuniform 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). 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 pulse lengths that our specified LIDT values are relevant for.

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 Durationt < 10-9 s10-9 < t < 10-7 s10-7 < t < 10-4 st > 10-4 s
Damage MechanismAvalanche IonizationDielectric BreakdownDielectric Breakdown or ThermalThermal
Relevant Damage SpecificationN/APulsedPulsed and CWCW

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 the energy density provides the best metric for short pulse sources. Under these conditions, energy density scales independently of spot size, 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 power 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 (1997).
[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).

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Posted Comments:
Poster:
Posted Date: 2013-07-18 09:37:28.1
I'd like to know the approx. damage threshold of BD27 & Bd40. Is it comparable to GL10???
Poster: pbui
Posted Date: 2013-07-18 16:30:00.0
Response from Phong at Thorlabs: Yes, the damage threshold of BD27 and BD40 would be similar to the GL10. Uncoated calcite has a damage threshold of about 500W/cm^2. We will contact you directly to discuss this further.
Poster: jlow
Posted Date: 2013-04-16 12:11:00.0
Response from Jeremy at Thorlabs: At 1.5µm, the ordinary refractive index is around 1.63457 and for the extraordinary, it is around 1.47743.
Poster:
Posted Date: 2013-04-15 18:08:02.91
Can you please tell me the refractive index of the BD27 prism?
Poster: tcohen
Posted Date: 2012-05-04 15:59:00.0
Response from Tim at Thorlabs: Thank you for your feedback! There was a discrepancy between the specified wavefront distortion found on our Wollaston Prism family page and the individual product drawings. The correct specification is lambda/4, the same as these calcite beam displacers. We will update the web presentation to reflect this information.
Poster: ahambi
Posted Date: 2012-05-02 08:02:55.0
Dear Sirs, We are interested in receiving info on your microlens arrays. Two of the most important data we need to know is the concentration of the microlenses and their focal length. I look forward to receiving your feedback. With best regards,
Poster: tcohen
Posted Date: 2012-02-29 10:55:00.0
Response from Tim at Thorlabs: Thank you for your feedback. The calcite crystal in the BD27 is 26.8mm +/- .12mm. In the BD40 it is 39.7mm +/- .12mm.
Poster:
Posted Date: 2012-02-29 06:50:24.0
What are the tolerances for the length of the calcite crystals?
Poster: bdada
Posted Date: 2011-03-04 19:06:00.0
Response from Buki: Yes, the beam displacers are bidirectional and can be used as either a polarization splitter/combiner or as a polarizer element that removes the angled, orthogonally polarized component of a beam. However, the minimal beam separation we offer is 2.7mm for BD27. The beam displacers are not AR coated, but we can offer custom versions with a coating.
Poster: shrotriy
Posted Date: 2011-03-04 12:55:06.0
Are the beam displacer bidirectional i. e. can these also be used to combine the beams. we are interested in a beam displacer with 2mm or 1mm beam separation to used with HeNe laser (633 nm). Is there anti-reflective coating.
Poster: Thorlabs
Posted Date: 2010-07-26 10:54:23.0
Response from Javier at Thorlabs to acer.su: Thank you for your feedback. We can quote a shorter version of these beam displacers that yields a beam separation of 1.5 mm. I will contact you directly for quoatation details.
Poster: acer.su
Posted Date: 2010-07-26 05:52:14.0
My special requirement is output beam "D" is under 1.5 mm , would you pls check the possibility solution .
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