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ZnSe Positive Meniscus Lenses


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ZnSe Positive Meniscus Lenses

Specifications
MaterialLaser Grade Zinc Selenide
Wavelength Range0.6 - 16.0 μm
AR Coating Range8-12 μm
Reflectance over Coating
Range (Avg.)
<1.5%
Damage Thresholda5 J/cm2 (10.6 µm, 100 ns, 1 Hz, Ø0.478 mm)
Diameter Tolerance+0.00/-0.10
Thickness Tolerance±0.2 mm
Focal Length Tolerance±1%
Surface Quality60-40 Scratch-Dig
Spherical Surface Powerb3λ/2
Spherical Surface Irregularity
(Peak to Valley)
λ/2
Centration≤3 arcmin
Clear Aperture80% of Diameter
Design Wavelength10.6 μm
  • Limited by the antireflection coating.
  • Much like surface flatness for flat optics, spherical surface power is a measure of the deviation between the surface of the curved optic and a calibrated reference gauge, typically for a 633 nm source, unless otherwise stated. This specification is also commonly referred to as surface fit.

Features

  • ZnSe Substrate
  • Broadband AR Coating for the 8-12 μm Range
  • Focal Lengths from 15 - 1000 mm Available

Thorlabs' Ø1/2" and Ø1" Zinc Selenide (ZnSe) positive meniscus lenses offer high transmission from 0.6-16 μm, and are available with a broadband AR coating optimized for the 8-12 μm spectral range deposited on both surfaces. This coating greatly reduces the high surface reflectivity of the substrate, yielding an average transmission in excess of 97% over the entire AR coating range. See the Graphs tab for detailed information. ZnSe lenses are particularly well suited for use with high-power CO2 lasers.

When handling optics, one should always wear gloves. This is especially true when working with zinc selenide, as it is a hazardous material. For your safety, please follow all proper precautions, including wearing gloves when handling these lenses and thoroughly washing your hands afterward. Due to the low hardness of ZnSe, additional care should be taken to not damage these lenses.

Best Form Design
Due to the high refractive index of ZnSe, the spherical best form design for a ZnSe lens is the positive meniscus design. Therefore, these lenses induce small aberrations, spot sizes, and wavefront errors comparable to best form lenses constructed of other materials. See the graphs tab for a comparison of the aberrations induced by different lens shapes.

Positive meniscus (convex-concave) lenses, which are thicker in the middle than at the edges and cause light rays to converge, are designed to minimize third-order spherical aberration. When used to focus a collimated beam, the convex side of the lens should face the source to minimze spherical aberration. They are often used in conjunction with other lenses to decrease the focal length, and therefore increase the numerical aperture (NA), of an optical assembly. Since a positive meniscus lens has a greater radius of curvature on the concave side of the lens than on the convex side, real images can be formed.

Thorlabs will accept all ZnSe lenses back for proper disposal. Please contact Tech Support to make arrangements for this service.

Zemax Files
Click on the red Document icon next to the item numbers below to access the Zemax file download.
Optic Cleaning Tutorial
Optical Coatings and Substrates
Selection Guide
Other MIR Lenses
Calcium Fluoride
(0.18 - 8.0 µm)
Plano-ConvexUncoated
D Coated (1.8 - 2.4 µm)
E Coated (3 - 5 µm)
Bi-ConvexUncoated
E Coated (3 - 5 µm)
Plano-ConcaveUncoated
E Coated (3 - 5 µm)
Bi-ConcaveUncoated/AR Coated
Positive MeniscusUncoated
E Coated (3 - 5 µm)
Negative MeniscusUncoated
E Coated (3 - 5 µm)
Germanium (2.0 - 16.0 µm)Plano-Convex
Silicon (1.2 - 8.0 µm)Plano-Convex
Barium Fluoride (0.2 - 11.0 µm)Plano-Convex
Magnesium Fluoride (200 nm - 6 µm)Plano-Convex

Transmission of BBAR Coating


Shown above is a graph of the theoretical percent reflectivity of the AR coating as a function of wavelength. The average reflectivity in the 8 - 12 μm range is <1.5%. The blue shading indicates the region for which the AR coating is optimized.

Total Transmission of ZnSe


Shown above is a graph of the theoretical transmission of the AR-coated zinc selenide Bi-Convex lens. The blue shaded region denotes the 8 - 12 μm spectral range where the AR coating is optimized. For this wavelength range, the measured transmission is in excess of 97%.

Total Transmission of Optic (ZnSe Substrate + AR Coating)

The table below gives the approximate transmission of these optics for a few select wavelengths in the 0.6 - 16 μm range. To see an excel file that lists all measured transmission values for this wavelength range, please click here. Please note that the transmission values stated for wavelengths outside of the AR coating range are approximate and can vary significantly by coating lot.

Wavelength (μm)Total Transmission
0.60.117
1.00.312
1.40.674
1.80.875
2.20.810
2.60.803
3.00.859
3.40.880
3.80.962
4.20.733
Wavelength
(μm)
Total Transmission
4.60.740
5.00.880
5.40.874
5.80.810
6.20.802
6.60.845
7.00.904
7.40.950
7.80.973
8.20.979
Wavelength
(μm)
Total Transmission
8.60.978
9.00.975
9.40.975
9.80.976
10.20.978
10.60.979
11.00.979
11.40.977
11.80.973
12.20.968
Wavelength
(μm)
Total Transmission
12.60.961
13.00.953
13.40.945
13.80.936
14.20.928
14.60.920
15.00.913
15.40.906
15.80.900
 

Using Positive Meniscus Lenses

  • Achieve Tighter Focusing by Combining a Meniscus Lens with Plano-Convex Lenses
  • Build Multi-Element Lens Systems to Achieve Higher NA without Significant Increases in Aberrations

Positive meniscus lenses are designed to minimize spherical aberration. They have one convex and one concave surface. When used in combination with another lens, a positive meniscus lens will shorten the focal length and increase the NA of the system. Figure 1c shows a meniscus lens being used to shorten the focal length of a 100 mm focal length plano-convex lens. In addition, the transverse and lateral aberrations are greatly reduced. The convex surface of both lenses should be facing away from the image.

Figure 1. These figures illustrate the performance gains that can be achieved by using multi-element imaging systems. The combination of a meniscus lens and a plano-convex lens yields a 21 µm focused spot versus a 240 µm spot from the single plano-convex lens.

Damage Threshold Specifications
Coating Designation
(Item # Suffix)
Damage Threshold
-F5 J/cm2 (10.6 µm, 100 ns, 1 Hz, Ø0.478 mm)

Damage Threshold Data for Thorlabs' F-Coated ZnSe Lenses

The specifications to the right are measured data for Thorlabs' F-coated, ZnSe lenses. Damage threshold specifications are constant for all F-coated, ZnSe lenses, regardless of the size or focal length of the lens.

 

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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Ø1/2" ZnSe Positive Meniscus Lenses
Item #DiameterFocal
Length
Radius of
Curvature 1
Radius of
Curvature 2
Center
Thickness
Edge
Thickness*
Back
Focal Length**
Reference
Drawing
LE7246-F 1/2" 15.0 mm 7.5 mm 9.0 mm 2.9 mm 2.0 mm 11.6 mm Positive Meniscus Lens Drawing
LE7276-F 1/2" 20.0 mm 7.5 mm 7.8 mm 3.0 mm 2.7 mm 15.3 mm
LE7963-F 1/2" 40.0 mm 10.9 mm 11.3 mm 3.0 mm 2.9 mm 33.6 mm

*Edge thickness given before 0.2 mm at 45o typical chamfer.
**Measured at the design wavelength, 10.6 µm.

Suggested Fixed Lens Mount: LMR05

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal/Imperial Price Available / Ships
LE7246-F Support Documentation
LE7246-F Ø1/2" ZnSe Positive Meniscus Lens, f = 15.0 mm, ARC: 8-12 µm
$350.00
Today
LE7276-F Support Documentation
LE7276-F Ø1/2" ZnSe Positive Meniscus Lens, f = 20.0 mm, ARC: 8-12 µm
$350.00
Today
LE7963-F Support Documentation
LE7963-F Ø1/2" ZnSe Positive Meniscus Lens, f = 40.0 mm, ARC: 8-12 µm
$350.00
Today
Ø1" ZnSe Positive Meniscus Lenses
Item #DiameterFocal
Length
Radius of
Curvature 1
Radius of
Curvature 2
Center
Thickness
Edge
Thickness*
Back
Focal Length**
Reference
Drawing
LE7185-F 1" 25.4 mm 15.5 mm 22.6 mm 4.7 2.0 20.9 Positive Meniscus Lens Drawing
LE7981-F 1" 50.0 mm 15.5 mm 16.9 mm 4.0 mm 3.1 mm 42.5 mm
LE7996-F 1" 75.0 mm 21.0 mm 23.3 mm 4.0 mm 3.5 mm 66.6 mm
LE7031-F 1" 100.0 mm 28.4 mm 32.6 mm 4.0 mm 3.6 mm 91.8 mm
LE7667-F 1" 150.0 mm 42.7 mm 50.7 mm 4.0 mm 3.7 mm 141.8 mm
LE7898-F 1" 200.0 mm 57.7 mm 69.7 mm 4.0 mm 3.8 mm 191.9 mm
LE7495-F 1" 500.0 mm 146.1 mm 181.6 mm 4.0 mm 3.9 mm 492.0 mm
LE7117-F 1" 750.0 mm 219.6 mm 274.5 mm 4.0 mm 3.9 mm 742.0 mm
LE7199-F 1" 1000.0 mm 293.0 mm 367.5 mm 4.0 mm 3.9 mm 992.0 mm

*Edge thickness given before 0.2 mm at 45o typical chamfer.
**Measured at the design wavelength, 10.6 µm.

Suggested Fixed Lens Mount: LMR1

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal/Imperial Price Available / Ships
LE7185-F Support Documentation
LE7185-F Ø1" ZnSe Positive Meniscus Lens, f = 25.4 mm, ARC: 8-12 µm
$505.00
Today
LE7981-F Support Documentation
LE7981-F Ø1" ZnSe Positive Meniscus Lens, f = 50.0 mm, ARC: 8-12 µm
$505.00
Today
LE7996-F Support Documentation
LE7996-F Ø1" ZnSe Positive Meniscus Lens, f = 75.0 mm, ARC: 8-12 µm
$505.00
Today
LE7031-F Support Documentation
LE7031-F Ø1" ZnSe Positive Meniscus Lens, f = 100.0 mm, ARC: 8-12 µm
$505.00
Today
LE7667-F Support Documentation
LE7667-F Ø1" ZnSe Positive Meniscus Lens, f = 150.0 mm, ARC: 8-12 µm
$505.00
Today
LE7898-F Support Documentation
LE7898-F Ø1" ZnSe Positive Meniscus Lens, f = 200.0 mm, ARC: 8-12 µm
$505.00
Today
LE7495-F Support Documentation
LE7495-F Ø1" ZnSe Positive Meniscus Lens, f = 500.0 mm, ARC: 8-12 µm
$505.00
Today
LE7117-F Support Documentation
LE7117-F Ø1" ZnSe Positive Meniscus Lens, f = 750.0 mm, ARC: 8-12 µm
$505.00
Today
LE7199-F Support Documentation
LE7199-F Ø1" ZnSe Positive Meniscus Lens, f = 1000.0 mm, ARC: 8-12 µm
$505.00
Today
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