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  • DCV-Lenses-ZnSe-1

Zinc Selenide (ZnSe)
Bi-Concave Lenses

Bi-concave or Double-concave (DCV) lenses have a negative focal length. These diverging lenses can be used to diverge a collimated beam to a virtual focus and are commonly used in a Galilean-type beam expander. They are also often used to diverge or increase the divergence of a converging beam. In optical systems, it is common for researchers to choose their optics carefully so that the aberrations introduced by the positive- and negative-focal-length lenses approximately cancel. Others use these lenses in pairs to increase the effective focal length of a converging lens just like negative meniscus lens.

When deciding between a plano-concave lens and a bi-concave lens, both of which cause the incident light to diverge, it is usually more suitable to choose a bi-concave lens if the absolute conjugate ratio (object distance divided by image distance) is close to 1. When the desired absolute magnification is either less than 0.2 or greater than 5, the tendency is to choose a plano-concave lens instead.

ZnSe lenses are particularly well suited for use with high-power CO2 lasers. Paralight Optics offers Zinc Selenide (ZnSe) Bi-Concave or Double-Concave (DCV) Lenses 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% across the entire AR coating range. For more information on coatings, please check the following Graphs for your references.

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Features:

Material:

Zinc Selenide (ZnSe)

Coating Options:

Available Uncoated or with Antireflection Coatings

Focal Lengths:

Available from -25.4mm to -200 mm

Applications:

Ideal for CO2 Laser Applications Due to Low Absorption Coefficient

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Common Specifications:

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Reference Drawing for

Double-Concave (DCV) Lens

f: Focal Length
fb: Back Focal Length
ff: Front Focal Length
R: Radius of Curvature
tc: Center Thickness
te: Edge Thickness
H”: Back Principal Plane

Note: The focal length is determined from the back principal plane, which does not necessarily line up with the edge thickness.

 

Parameters

Ranges & Tolerances

  • Substrate Material

    Laser-Grade Zinc Selenide (ZnSe)

  • Type

    Double-Convave (DCV) Lens

  • Index of Refraction

    2.403 @ 10.6μm

  • Abbe Number (Vd)

    Not defined

  • Thermal Expansion Coefficient (CTE)

    7.1x10-6/℃ at 273K

  • Diameter Tolerance

    Presicion: +0.00/-0.10mm | High Precison: +0.00/-0.02mm

  • Thickness Tolerance

    Presicion: +/-0.10 mm | High Precison: +/-0.02 mm

  • Focal Length Tolerance

    +/- 1%

  • Surface Quality (scratch-dig)

    Presicion: 60-40 | High Precison: 40-20

  • Spherical Surface Power

    3 λ/4

  • Surface Irregularity (Peak to Valley)

    λ/4 @633 nm

  • Centration

    Precison: < 3 arcmin | High Precision < 30 arcsec

  • Clear Aperture

    80% of Diameter

  • AR Coating Range

    8 - 12 μm

  • Reflectance over Coating Range (@ 0° AOI)

    Ravg < 1.0%, Rabs < 2.0%

  • Transmission over Coating Range (@ 0° AOI)

    Tavg > 97%, Tabs > 92%

  • Design Wavelength

    10.6 μm

  • Laser Damage Threshold

    5 J/cm2 (100 ns, 1 Hz, @10.6μm)

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Graphs

♦ Transmission curve of 5 mm thick, uncoated ZnSe substrate: high transmission from 0.16 to 16 μm
♦ Transmission curve of 5 mm AR-coated ZnSe substrate: Tavg > 97% over the 8 - 12 μm range

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Transmission Curve of 5 mm Thick AR-coated (8 µm - 12 μm) ZnSe Substrate at 0° AOI

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