Specifications

Diamond™ GEM, G, & K, Integrator’s Guide

Focusing a Beam

Most laser processing applications call for focusing the laser beam

to a small spot so that the high power density can accomplish the

desired work. This is true for applications involving cutting, drilling,

scribing, welding, and others on a wide range of material. The

typical question is what is the spot size that will be achieved for this

application. In order to achieve the smallest spot size, the beam must

be focused with a lens that transmits the laser wavelength. In order

to achieve the desired spot size, one must size the clear aperture for

the diameter of the beam at that point using the guidelines covered

in the section on beam diameters. The approximate spot size of the

focused laser beam using a lens with focal length f is:

Where D

is the beam diameter at the focusing lens and D

is the

focused beam diameter. Calculating for the same beam in the beam

propagation example with a 5 inch (127 mm) focal length lens for a

beam a 2 meters from the beam waist

= (4 x 127 mm x 0.0106 mm x 1.2)/ (3.14 x 8.4 mm)

= 0.245 mm = 245 µ

One important note that to reduce the focused spot size the designer

reduce the focal length of the lens or increase the spot size on the

focusing lens. The danger either approach is that the aberrations and

non-ideal behavior in the optics can lead to larger spot sizes that

calculated here. As a general guide, the equation above will give

valid answer if the lens focal length divided by the beam diameter

on the lens (f/D

) is greater than 10. The beam waist will be located

near the focal length of the lens. The location of the focused beam

waist also depends on the distance from the beam waist to the

focusing lens.

Rayleigh Range

and Depth of

Focus

When processing material it is important to have knowledge of the

work range where the process will function properly. The major

issue is the acceptable range in the distance between a focusing lens

and the work surface. A convenient model for this is to calculate the

Rayleigh range for the focused beam as an initial evaluation of the

4fλM

πD

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