Why Should We Care About Spot Size in Fiber Laser Cutting?

Within the world of fiber laser cutting, spot size plays a pivotal role in achieving optimal results: from accuracy to efficiency, it significantly affects various aspects of the laser cutting process.

What is Spot Size?

Before delving into the reasons why spot size is so important, let us establish what spot size is. Spot size refers to the diameter of the laser beam at its focus point. In other words, it is the area where the laser's energy is concentrated onto the material surface. A portion of that energy is absorbed by the material leading to localized melting. In fiber laser cutting of metals, high pressure assist gas delivered by a cutting head is used to expel the liquid metal away, creating a cut as the beam moves across the part, with the ‘kerf width’ a function of spot size.

What Impacts Spot Size?

The spot size is influenced by several parameters including laser wavelength, beam size at the lens, and focal length of the focusing lens, all of which have a profound effect on the cutting process. Longer wavelengths lead to larger spot sizes for identical optical setups. Ytterbium fiber lasers with a wavelength of 1070 nm have smaller spot sizes than CO₂ lasers which operate at longer wavelengths around 10,600 nm and lead to correspondingly larger spot sizes.

When using fiber-based beam delivery, the spot size can be calculated based on the diameter of the fiber and collimator focal length (both defining the size of the beam at the focusing optic) and focal length of the focusing optic (see Figure 1). Increasing the focal length of the focusing objective leads to larger spot sizes, while increasing beam size at the lens results in smaller spot sizes (as long as the beam is maintained within a size at lens that allows diffraction limited performance).

Figure 1: Spot size calculation for fiber beam delivery

Cutting Accuracy: Smaller is Better

Smaller spot sizes in laser cutting operations offer enhanced accuracy and resolution due to the laser energy being tightly focused within a smaller region, facilitating precise material removal. This enables the realization of intricate designs, finer details, and tighter tolerances. Additionally, the smaller kerfs resulting from the narrower spot sizes enable closer nesting of parts, thereby optimizing material utilization and minimizing waste in the manufacturing process.

Cutting Speed and Quality: Finding the Balance

In the quest to strike the perfect balance between cutting speed and quality, the interaction between laser parameters becomes crucial. Energy density plays a pivotal role, with smaller spot sizes enabling the consolidation of laser energy into a smaller area. This allows for achieving the same energy density with lower power lasers.

However, the relationship between spot size and material thickness is intricate. While a minimum energy density is required to achieve melting across the entire material thickness, the spot size needs to be large enough to allow for the molten material to be efficiently expelled. As the thickness of the material increases, larger spot sizes are needed to maintain cut quality and speed. Furthermore, the optimum spot size also depends on material properties such as thermal conductivity and viscosity.

The depth of focus, correlated with spot size, also demands attention. Smaller spot sizes are correlated with a shorter depth of focus, which is typically defined as the distance from the beam waist at which the beam area doubles. At distances longer than the depth of focus, the cut quality will start to be impacted. Thus, finding the delicate equilibrium between spot size, energy density, and depth of focus is imperative to achieve the desired cutting results without compromising on speed or quality.

Striking a Balance for Optimal Laser Cutting

All things considered, manufacturers must strike a balance. For thicker materials, a larger spot size may be preferable to maintain cutting quality and speed. However, for intricate work or thin materials, smaller spot sizes are preferred. Manufacturers typically choose an optimal beam delivery configuration resulting in a spot size that is the most adequate for the laser power available and the materials and thicknesses intended to be cut, with the goal of achieving the perfect balance between accuracy, cut quality and throughput.