Cutting applications require focusing a laser beam to a minimum spot size. This is necessary to maximize the energy density and produce precision cuts. Many factors affect spot size. The most important are:
Lens shape and focal length determine the latter two factors. Of course, laser mode is determined by the laser and beam delivery system. II-VI offers plano-convex, meniscus, and aspheric lenses in a wide variety of standard focal lengths and diameters. The following images show how these three factors affect spot size, and how to calculate spot size for plano-convex, meniscus, and aspheric lenses. The notes outline a simple procedure for picking the right lens for a given application.
Diffraction Diffraction, a natural and inescapable result of the wave nature of light, is present in all optical systems, and determines the ultimate theoretical limit on their performance. Diffraction causes light beams to spread transversely as they propagate. If a “perfect” lens is used to focus a collimated laser beam, the spot size is limited only by diffraction. Spot size formula:
|Spot Size Formula|
This equation is used to determine the spot size produced by an aspheric lens. Diffraction’s most important factor is that the spot size increases linearly with focal length but is inversely proportional to beam diameter. Thus, as the input laser beam diameter increases for a given lens, spot size decreases due to lower diffraction. Also, as focal length decreases for a given laser beam diameter, spot size again decreases.
M2 - Laser Mode Parameter
As seen in the previous formula for diffraction, focal spot size is directly proportional to the laser mode parameter, M2. M2 expresses how quickly a given beam diverges while propagating; a perfect TEM00 laser beam has M2=1. This parameter is measured by advanced instruments, or is obtained from laser manufacturers’ specifications.