Molded Glass Aspheric Lenses: 350 - 700 nm or 400 - 600 nm AR Coating
Aspheric Lens in a Fiber Launch Application
Molded Glass Aspheric Lenses: 350 - 700 nm or 400 - 600 nm Broadband AR Coating
Aspheric lenses focus or collimate light without introducing spherical aberration into the transmitted wavefront. For monochromatic sources, spherical aberration often 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 of these molded glass lenses 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-threaded (1.035"-40) of lens tubes by using our Aspheric Lens Adapters. Mounted aspheres can be used as a drop-in replacement for multi-element microscope objectives 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.
The specifications for the P1-630A-FC-2, 630 nm, FC/PC 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. The aspheric lens with a focal length that is closest to 16 mm has a focal length of 15.29 mm (Item# 354260-B or A260-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.
Click to Enlarge
Aspheric Lens Design Formula
Aspheric Lens Equation
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.
The specifications for the L780P010 laser diode indicate that the typical parallel and perpendicular FWHM beam divergences are 10° and 30°, 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°). 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.
Ø = Beam Diameter
Θ = Divergence Angle
From the information above, the focal length of the lens can be determined, using the thin lens approximation:
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 a -B AR-coated molded glass aspheric lens with focal length near 5.6 mm. The C171TMD-B (mounted) or 354171-B (unmounted) aspheric lenses have a focal length of 6.20 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 lens:
0.30 = NALens > NADiode ≈ sin(15°) = 0.26
Up to this point, we have been using the FWHM beam diameter to characterize the beam. However, a better practice is to use the 1/e2 beam diameter. For a Gaussian beam profile, the 1/e2 diameter is almost equal to 1.7X the FWHM diameter. The 1/e2 beam diameter therefore captures more of the laser diode's output light (for greater power delivery) and minimizes far-field diffraction (by clipping less of the incident light).
A good rule of thumb is to pick a lens with an NA twice of the NA of the laser diode. For example, either the A390-B or the A390TM-B could be used as these lenses each have an NA of 0.53, which is more than twice the approximate NA of our laser diode (0.26). Note that these lenses each have a focal length of 4.6 mm, resulting in an approximate major beam diameter of 2.5 mm.
The table below contains all molded visible and near-IR aspheric lenses offered by Thorlabs. For our selection of IR molded aspheres, click here. The item # listed is that of the unmounted, uncoated lens. An "X" in any of the five AR Coating Columns indicates the lens is available with that coating (note that the V coating availability is indicated with the design wavelength). The table to the right defines each letter and lists the specified AR coating range. Click on the linked X's to purchase the specific lens, which is available mounted and unmounted.