In-Focus Spot Magnification in Additive Manufacturing

QUICK FINDER 

• AT A GLANCE 
• AM IN SEARCH OF HIGHER PRODUCTIVITY 
• SPOT MAGNIFICATION FOR FAST AREA EXPOSURE 
  Defocus – Simple method for spot magnification exhibiting an ill-defined focus profile 
  Multicore and Ring Mode Lasers – Alternative Beam Shapes with Fixed Focus Diameters 
  Optical Zoom – Continous spot magnification with optimum beam quality 
• COMBINATION OF MULTICORE LASER AND ZOOM FOR MAXIMUM FLEXIBILITY 
• OPTIMAL BEAM GEOMETRY ENABLES EFFICIENT AM APPLICATIONS 

High productivity is the key in every AM application. You probably know this from your own experience. Only if you can produce the necessary quantities and also keep the costs per part low, additive production is a real alternative to conventional production. 

So, if you are in the process of redesigning or optimizing an AM system, you are probably also looking for ways to reduce the exposure time. Because especially with large part production, a more efficient exposure is an important way to increase the output.
 

In addition to the obvious increase in exposure dynamics, an enlargeable focus diameter is a good option for optimizing exposure time. Much like a master painter who uses different brushes for the large areas as opposed to the details, a larger spot size is also a good option for filling bulk material in additive manufacturing. After all, this allows you to generate more volume with each pass, significantly reducing the number of hatches and reversal points.

However, since you still need a spot as small as possible for the fine structures of your component, a dynamically adjustable focus diameter becomes an excellent option.

To achieve this, you have several options at your disposal:

• Increase of the spot diameter by defocusing
• Beam shaping with adaptable ring mode lasers
• Spot increase using a zoom function

Let’s take a closer look at the advantages and disadvantages of these options below. 

Defocus - Simple method for spot magnification exhibiting an ill-defined focus profile

The simplest way to dynamically increase the spot diameter is to defocus the laser. You move the focus position below the working plane via a z-axis. This results in an enlarged spot diameter in the powder.

Especially with pre-focusing deflection units, this possibility of spot enlargement is ideal, since the required optical z-axis is already part of the scan system. However, this leads to a poorly defined beam shape outside the focus. As a result, the defocused spot does not retain its original energy distribution, but becomes blurred (Fig.1). While with single mode lasers the beam profile remains “Gaussian-like”, the blurring is clearly visible with more specialized beam shapes such as flat-top or ring mode profiles. 
 

Since the power distribution in the spot depends strongly on the amount of defocus, you have to determine the distribution experimentally for each magnification factor and develop the associated process parameters specifically for this setup. This additional effort and the fact that the magnified spot is only accessible in a blurred state limit the possible applications of the defocus.

Multicore and Ring Mode Lasers - Alternative Beam Shapes with Fixed Focus Diameters

Another option to obtain larger spot diameters are multicore fibers or adaptable ring mode lasers. Here, the fiber core usually delivers a single mode spot, while the fiber ring creates a donut-shaped ring with a larger diameter. Typically, you can distribute the power of the laser between the core and the ring to create a variety of beam shapes. You can flexibly switch between single mode, ring mode, or flat top profile. Both the core and the ring are in focus, ensuring a clean beam profile.

Due to their larger diameter with the ring or flat top profile, the adjustable ring mode lasers are well suited to realize a speed advantage when filling large areas. However, the magnification ratio between core and ring is fixed. In addition, the large spot is only available as a ring or top hat. 

So if you want to keep the identical power distribution in focus even with a magnified spot or want more than one magnification factor, a customizable ring mode reaches its limits.

Optical Zoom function - continous spot magnification with optimum beam quality

However, continuous magnification cannot be implemented with a standard deflection unit. For this, you need an additional movable telescope in the beam path, which can vary the beam diameter as an extension for your pre-focusing deflection unit. This is because the minimum spot diameter in the focus also changes with the beam diameter. Therefore, such a zoom optic allows you to continuously adjust the focus diameter without leaving the focal plane. In this way, the original beam profile is still sharply imaged, which is particularly important for special beam shapes such as flat top and donut.
 

Limiting factors for the magnification factor include the free aperture in the beam path, the available space in the deflection unit and the power compatibility of the mirrors and optics. In the accessible magnification range, however, you can implement continuous magnification and flexibly adapt the spot diameter to your application.