Laser system designers have a broad range of beam positioning architectures available to them to solve various applications.This paper explores the design, benefits, consequences, anduse of the 3-axis scan head technology as compared to 2-axisscan head architecture.In a typical two axis laser scanning system, a collimated beam is reflected by the X and Y axis scanning mirrors before enteringthe focusing lens. The lens focuses the beam on the work surface.
Rotation of the X and Y mirrors causes movement of the focusedspot within a flat field. The size of the spot and the size of the fieldare determined by the lens (and other factors). F-theta lenses areintended for this purpose. This configuration is known as apre-objective scanning system because the laser strikes thescanning mirrors before the focusing (objective) lens.
The architecture works well as long as the laser beam’s diameterand field size are relatively small. For example, applications usingbeam diameters less than 20mm and field size less than 300mmare well suited to Z-axis pre-objective scanning.
As the field size requirements grow, larger scan mirrors and laser beam diameters are needed to maintain a numerical aperture (NA) consistent with a small focused spot. F-theta scan lenses forthese large laser beams would be big, costly, and impractical.
For this reason, a 3-axis scanning solution should be considered.In a 3-axis scanning system, the XY mirrors are placedafter the final focusing lens, hence they are referred to as apost-objective scanning system. Since the laser beam doesnot move on the objective lens, the lens does not need to be very large; however, this arrangement does not create a flat field.
To achieve a flat field, a third axis (Z-axis) of motion is introducedin the form of a linear lens translator.
The typical laser system uses a telescope to expand the laser beam to a diameter consistent with the required NA. The distance between the telescope input lens(s) and objective lens(s) determines the focus distance of the system. By mounting the input lens(s) on a linear lens translator (the third axis), we gain dynamic control over the focus distance. (See figure 2)
By coordinating the motion of the linear lens translator with the rotation of the X and Y scanning mirrors, we achieve a focused laser spot throughout a flat field.