Moving a focused laser spot within a two or three-dimensional space can be accomplished using several different laser positioning schemes. Common examples are the 2-axis, pre-objective scan head, the 3-axis, post-objective scan head and X, Y plotters. Selecting an appropriate beam positioning and scanning mechanism is an important consideration in any laser-based system. Frequently, a laser system designer has more than one solution available for a given application. A number of technical issues should be considered when selecting an optimized solution.

Definition and Comparisons
A post-objective scanning architecture is one in which a two-dimensional x-y scanning mechanism is placed after a movable expander lens and a focusing objective lens. The scanning mechanisms used are most often galvanometers with reflective mirrors attached, which can address a specific field size in two or three dimensions.

The X, Y plotter has the advantage of an on-axis optical system. In these systems a very small spot size can be produced and moved over very large fields. There are few optical aberrations, which are present to a small extent in both two and three-axis scan heads. However, these systems are often large and suffer from slow scan speeds. Although used in a variety of applications, these factors do limit the number of applications that can be addressed.

Post-objective scanning with a 3-axis scan head offers the designer much of the speed advantage of a 2-axis system. It can produce a small spot size and scan large field sizes like the X, Y plotter. It can address three-dimensional objects. It is appropriate for a wide range of laser wavelengths and some models offer the flexibility to adjust field size and spot size. These heads are used in marking, cutting, drilling, rapid prototyping and imaging applications on a variety of materials including; glass, wood, textiles, metals, tiles, stone, paper, photo-sensitive plates and other materials. An emerging application for these heads is to process a three-dimensional work surface.

The table below highlights some attributes of three beam positioning architectures.

In a scan head without focus correction, the focused laser spot at the center of the field describes an arc when moved in either axis, creating a sphere of focused points above the scan field (figure 2). At locations away from the center of the scan field, the laser beam is not focused. This is due to the increased length from the objective lens to the work piece, as the scanners direct the beam away from the center of the field. In a pre-objective 2-axis scan head "field flattening" or focus correction is accomplished using a specially designed objective lens after the scanners. In a post objective, 3-axis scan head, "field flattening" or focus compensation is accomplished by slight adjustments in the distance between the expander lens and the objective lens, in real-time, as the scanners direct the beam across the scan field.

The distance between the expander lens and the objective lens is adjusted real-time by a third moving "Z" axis, thus the name "3-axis scan head". The expander lens may be moved by a galvanometer based rotary-to-linear actuator or by a linear voice coil. This linear lens translator requires reasonable bandwidth to keep up with the X, Y scanning. As the expander lens moves closer and further from the objective lens, the focal point of the laser is adjusted to keep the focused spot on the scan field at all X, Y locations within that field (figure 2).

Control of the third axis is typically accomplished using software, which addresses a look up table containing focus correction values or by using a polynomial equation that commands the linear lens translator to a location proportional to the X, Y distance from the center of the scan field. The result is continuous focus correction throughout the scan field.

In the 3-axis scan head, the post objective scan mirrors can be larger than in typical 2-axis scan heads, thus the aperture can be significantly larger too. This allows for the use of larger numerical apertures and thus smaller spot sizes (chart 1).

Another advantage found in some 3-axis scan heads, is the ability to offset the nominal distance between the expander lens and the objective lens by repositioning the linear lens translator (figure 1). This allows the user to change the working distance, field and spot size with the same scan head. This feature is an added advantage if you're running different applications with the same system in a job shop setting or if the user wants to experiment with different field and laser spot sizes for a new application. In general, the laser spot size will increase linearly with increased field size. If the field size is doubled, the spot size will double too.

Optical Considerations
Along with the advantages of a 3-axis scan head, there are some issues, which also need to be considered. Most 3-axis scan heads can scan +/- 20 degrees optical across the field in two dimensions. As the beam is steered away from the center of the scan field, the angle of incidence between the beam and the work piece departs from normal. This causes two effects; an increasing elliptical laser spot on the work piece and increasing angles of incidence as the scan angle increases away from the center of the field. An elliptical laser spot may be intolerable when drilling holes. A non-normal incident angle of the beam to the work piece will cause beveled edges in cutting applications. The system designer must decide to what degree these aberrations may affect the application and make trade-offs with other advantages such as speed. Another aberration is small laser spot size changes between the center of the field and the outside corners of the field. This is caused by the slight variations in distance between the expander lens and the objective lens used to correct for focusing throughout the field. The spot size can grow up to around 15% larger from the center to the corner of the scan field causing differences in energy density of the laser spot.

Conclusions
Post-objective, 3-axis scan heads are used in applications requiring larger field sizes and smaller spot sizes than can be achieved with a 2-axis scan head. The 3-axis scan head has a speed advantage over X, Y plotter systems but does present some optical considerations, which may or may not affect a given application.

Some 3-axis scan heads offer the advantage of a variable field and spot size adjustment, while others only offer one fixed field size. Some scan heads offer a custom expander lens matched specifically for a given laser and application. This eliminates the need for an additional costly beam expansion element before the scan head.

An emerging application for 3-axis scan heads is the ability to address three-dimensional work pieces. This involves more sophisticated optical configurations and software.

When considering a beam positioning solution, make a list of all the parameters that are important to your application. Prioritize your list and weigh it against the several solutions offered. Once you have thought through your options, you can look at the various products offered within the category selected to make your final determination.