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Molded Glass Aspheric Lenses: Uncoated

Aspheric lenses focus or collimate light without introducing spherical aberration into the transmitted wavefront. For monochromatic sources, spherical aberration is often what prevents a single spherical lens from achieving diffraction limited performance when focusing or collimating light. Thus, an aspheric lens is often the best single element solution for many applications including collimating the output of a fiber or laser diode, coupling light into a fiber, spatial filtering, or imaging light onto a detector.

All molded glass aspheres are also available premounted in non-magnetic 303 stainless steel lens cells that are engraved with the part number for easy identification. These mounted aspheres have a metric thread that makes them easy to integrate into an optical setup or OEM application. The mounted aspheres are readily adapted to our SM1 series of lens tubes by using our Aspheric Lens Adapters. Mounted aspheres can be used as a drop-in replacement for multi-element microscope objective by combining the lens with our Microscope Objective Adapter Extension Tube.

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 (i.e., the flatter surface) 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.

Molded glass aspheres are manufactured from a variety of optical glasses to yield the indicated performance. The molding process will cause the properties of the glass (e.g., Abbe number) to deviate slightly from those given by glass manufacturers. Specific material properties for each lens can be found by clicking on the Glass link in the tables below.

Choosing a Lens

Aspheric lenses are commonly chosen to couple incident light with a diameter of 1 - 5 mm into a single mode fiber. A simple example will illustrate the key specifications to consider when trying to choose the correct lens.

Example:
Fiber: P1-630A-FC-2
Collimated Beam Diameter Prior to Lens: Ø3 mm

The specifications for the P1-630A-FC-2, 630 nm, FC single mode patch cable indicate that the mode field diameter (MFD) is 4.3 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

Thorlabs offers a large selection of mounted and unmounted aspheric lenses to choose from. Scanning through the list of options under the Selection Guide tab, you'll note that the aspheric lens with a focal length that is closest to 16 mm has a focal length of 15.29 mm (Item# 352260-B). This lens also has a clear aperture that is larger than the collimated beam diameter. Therefore, this aspheric lens is the best option given the initial parameters (i.e., a P1-630A-FC-2 single mode fiber and a collimated beam diameter of 3 mm). Remember, for optimum coupling the spot size of the focused beam must be less than the MFD of the single mode fiber. As a result, if an aspheric lens is not available that provides an exact match, then choose the aspheric lens with a focal length that is shorter than the calculation above yields. Alternatively, if the clear aperture of the aspheric lens is large enough, the beam can be expanded before the aspheric lens, which has the result of reducing the spot size of the focus beam.

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 Drawings and Documents tab or by clicking on the part number in the price table below.

Choosing a Collimation Lens for Your Laser Diode

Since the output of a laser diode is highly divergent, collimating optics are necessary. Since aspheric lenses do not introduce spherical aberration, they are commonly chosen when the collimated laser beam is to be between one and five millimeters. A simple example will illustrate the key specifications to consider when choosing the correct lens for a given application.

Example:
Laser Diode to be Used: L780P010
Desired Collimated Beam Diameter: Ø3 mm (Major Axis)

The specifications for the L780P010 laser diode indicate that the typical parallel and perpendicular beam divergences are 10^o and 30^o, respectively. Therefore, as the light diverges, an elliptical beam will result. To collect as much light as possible during the collimation process, consider the larger of these two divergence angles in any calculations (i.e., in this case use 30^o). If you wish to convert your elliptical beam in to a round one, we suggest using an Anamorphic Prism Pair, which magnifies one axis of your beam.

From the information above, the focal length of the lens can be determined:

With this information known, it is now time to choose the appropriate collimating lens. Thorlabs offers a large selection of aspheric lenses to choose from. For this application the ideal lens is an -B AR coated molded glass aspheric lens with focal length near 5.6 mm. The C170TME-B (mounted) or 352170-B (unmounted) aspheric lenses have a focal length of 6.16 mm, which will result in a collimated beam diameter (major axis) of 3.3 mm. Next, check to see if the numerical aperture (NA) of the diode is smaller than the NA of the lenses so that the light emitted from the laser diode is not clipped by the lens:

0.30 = NA_Lens > NA_Diode ~ sin(15) = 0.26