Webpage Features
	Click for complete specifications, documents, and drawings.
Performance Hyperlink	Click to view item-specific focal length shift data and spot diagrams at various wavelengths.

Features

• Focus or Collimate Light without Introducing Spherical Aberration
• Ø4.00 mm, Ø5.00 mm, or Ø7.80 mm Unmounted Clear Aperture
• AR Coated for 1.8 - 3 µm (-D), 3 - 5 µm (-E), or 8 - 12 µm (-F)
• Available Unmounted or Mounted in a Threaded, Engraved Stainless Steel Housing
• Black Diamond Substrate Provides Stable Operation up to 130 °C

Spherical aberration often prevents a spherical lens from achieving diffraction-limited performance. The surfaces of an aspheric lens are corrected for spherical aberration, thereby providing a robust single element solution for many applications, such as collimating the output of a fiber or laser diode, coupling light into a fiber, spatial filtering, or imaging light onto a detector. In particular, our IR aspheric lenses are ideal for collimating light from mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) sources, including Quantum Cascade Lasers (QCLs).

Our molded glass aspheres are available unmounted or premounted in stainless steel lens housings that are engraved with the part number for easy identification. These housings have a metric external threading that makes them easy to integrate into an optical setup or OEM application. For example, they are readily adapted to our SM1 (1.035"-40) Lens Tubes by using our Aspheric Lens Adapters. Mounted aspheres can also be used as a drop-in replacement for multi-element microscope objectives in conjunction with our RMS-threaded Objective Replacement Adapters.

Black Diamond
Black Diamond-2 (BD-2), a chalcogenide made of an amorphous mixture of germanium (28%), antimony (12%), and selenium (60%), has several advantages over germanium, which is traditionally used to fabricate IR optics. BD-2's thermally stable refractive index (see the Refractive Index tab) and low coefficient of thermal expansion (13.5 x 10^-6 / °C) result in a smaller change in focal length as a function of temperature than for germanium. Additionally, germanium suffers from transmission loss as temperature increases, while BD-2 aspheric lenses can be used in environments up to 130 °C. This material performs particularly well over the 1.7 - 2.2 µm spectral range, providing >99% transmission and a flat dispersion curve.

If an unmounted aspheric lens is being used to collimate the light from a point source or laser diode, the side with the greater radius of curvature should face the point source or laser diode. To collimate light using one of our mounted aspheric lenses, orient the housing so that the externally threaded end of the mount faces the source.

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The shaded region in each graph indicates the range for which the coating is specified.

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The refractive index of Black Diamond-2 (BD-2) as a function of wavelength, shown above, was calculated using the Herzberger Equation, an infrared-specific analog of the Sellmeier Equation. The Herzberger coefficients for BD-2 are given to the table to the right.

Herzberger Equation (for λ in µm)

Choosing a Lens for Fiber Coupling

Aspheric lenses are commonly used to couple incident light with a spot size of 1 - 5 mm into a single mode fiber. The following simple example illustrates the key specifications to consider when trying to choose the correct lens.

Example

• Wavelength: 2 µm
• Fiber: P1-2000-FC-1
• Collimated Beam Diameter Prior to Lens: Ø2 mm

At 2 µm, Thorlabs' P1-2000-FC-1 single mode patch cable is specified with a mode field diameter (MFD) of 13 μm. This specification should be matched to the diffraction-limited spot size given by the following equation:

Here, f is the focal length of the lens, λ is the wavelength of the input light, and D is the diameter of collimated beam incident on the lens. Solving for the desired focal length of the collimating lens yields

The mounted aspheric lens that is AR coated for our 2 µm wavelength and most closely matches the desired focal length of 10.2 mm is our C021TME-D (f = 11.00 mm), shown below. Its clear aperture of 4.00 mm is easily larger than the collimated beam diameter of 2 mm. It therefore meets the requirements of the example setup.

For optimal coupling, the spot size of the focused beam should be smaller than the MFD of the single mode fiber. Therefore, if an aspheric lens is not available that provides an exact match, choose an aspheric lens with a focal length that is shorter than that yielded by the calculation above. Alternatively, assuming the clear aperture of the aspheric lens is sufficiently large, the beam can be expanded before the aspheric lens to allow the focused beam to have a tighter spot.

Lens Design Formula

• Positive Radius Indicates that the Vertex is Located Left of the Center
• Negative Radius Indicates that the Vertex is Located Right of the Center

Aspheric Lens Coefficients

The aspheric lens coefficients are listed on the product page that is loaded by clicking on the part number in the price table below and in the .pdf and .dxf files available for each lens. Links to the files can be found under the Documents & Drawings tab or by clicking on the part number in the price table below.