Calcium Fluoride Positive Meniscus Lenses, Uncoated

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Features

• Vacuum-Grade Calcium Fluoride Substrate
• Ø1/2" and Ø1" Versions Available
• Uncoated for 0.18 - 8.0 µm Transmission
• Focal Lengths from 20.0 mm to 1000.0 mm

Thorlabs' Ø1/2" and Ø1" Calcium Fluoride (CaF₂) Positive Meniscus Lenses offer high transmission from 0.18 µm to 8.0 μm. The lenses on this page are uncoated, while coated versions for 2 µm to 5 µm are also available.

CaF₂ is commonly used for applications requiring high transmission in the infrared and ultraviolet spectral ranges. Its extremely high laser damage threshold makes it useful for use with excimer lasers. The material exhibits a low refractive index, varying from 1.35 to 1.51 within its usage range of 180 nm to 8.0 μm. Calcium fluoride is also fairly chemically inert and offers superior hardness compared to its barium fluoride, magnesium fluoride, and lithium fluoride cousins.

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 minimize 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.

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. Figures 3.1 through 3.3 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. Figure 3.3 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.

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Figure 3.1  A single plano-convex lens focuses the beam to a Ø240 µm spot.

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Figure 3.2  Two plano-convex lenses focus the beam to a Ø81 µm spot.

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Figure 3.3  A plano-convex lens used with a positive meniscus lens focuses the beam to a Ø21 µm spot.

Wavelength-Dependent Focal Length Shift

The paraxial focal length of a lens is wavelength dependent. The focal length listed below for a given lens corresponds to the value at the design wavelength (i.e., the focal length at 4 µm). Since CaF₂ offers high transmission from 0.18 - 8.0 µm, users may wish to use these lenses at other popular wavelengths. Click on the icons in Tables 4.1 and 4.2 to view theoretically-calculated focal length shifts for wavelengths within the 0.18 - 8.0 µm range.

Table 4.1  Ø1/2" Positive Meniscus Lenses
Item #	LE5838	LE5243	LE5234
Focal Length @ 4 µm	20.0 mm	50.0 mm	80.0 mm
Focal Length Shift (Click for Details)
Raw Data (Click to Download)	Data	Data	Data

Table 4.2  Ø1" Positive Meniscus Lenses
Item #	LE5183	LE5801	LE5802	LE5803	LE5382	LE5414	LE5990	LE5656	LE5714
Focal Length @ 4 µm	40.0 mm	50.0 mm	75.0 mm	100.0 mm	150.0 mm	200.0 mm	500.0 mm	750.0 mm	1000.0 mm
Focal Length Shift (Click for Details)
Raw Data (Click to Download)	Data	Data	Data	Data	Data	Data	Data	Data	Data