Since the laser diode's output light is highly divergent, it requires the use of collimating optics. Aspheric lenses do not introduce spherical aberration and generally choose aspheric lenses if the collimated beam is between 1 and 5 mm. Here's an example of the major specifications to consider for choosing the right lens for a given application.
Example:
Laser diode used: L780P010
Required collimated beam diameter: Ø3 mm (long axis)
Knowing from the L780P010 laser diode specification sheet, the typical horizontal and vertical divergence angles are 10 ° and 30 ° (FWHM), respectively, so an elliptical beam is formed as the light diverges. In order to collect as much light as possible during collimation, a larger divergence angle (30 ° in this case) should be used in the calculation.If you want to turn an elliptical beam into a circular beam, The direction of deformation of the light beam prism.
Ø = beam diameter
Θ = divergence angle
Based on the above information, use the following formula to approximate the lens focal length:
Based on this calculation, select the appropriate collimator lens. Thorlabs offers a large number of aspherical lenses. For this application, a molded glass aspherical lens with a B-coated AR coating and a focal length of 5.6 mm is the ideal choice.
The focal length of the C171TMD-B (installed) or 354171-B (not mounted) aspheric lens is 6.20 mm so the collimated beam diameter (long axis) is 3.3 mm. Next determine if the numerical aperture (NA) of the laser diode is less than Lens NA:
0.30 = NALens> NADiode ≈ sin (15 °) = 0.26
We are using FWHM beam diameter to characterize the beam here. However, the actual use of 1 / e2 beam diameter is a better choice. For a Gaussian beam profile, the 1 / e2 diameter is about 1.7 times the FWHM diameter. As a result, 1 / e2 diameter can collect more laser diode output light (deliver greater power) as well as minimize far field diffraction (block less incident light).
As a rule of thumb, NA is generally chosen to be twice as large as the laser diode NA. For example, the NA of the A390-B or A390TM-B is 0.53, which is more than twice the approximation of the NA of a laser diode (0.26) so it can be used for this laser diode. Note that the focal length of these lenses is 4.6 mm, so the collimated long axis beam diameter is about 2.5 mm.
