How to choose the correct focus lens for laser cutting machine?

The focus lens is an indispensable part of laser cutting machines. When you choose the focus lens, raw materials, diameter, focal length, and coating are the core factors

In order to achieve high-quality engraving and precise cutting with your laser cutter, the laser beam is directed through a series of mirrors and through the focus lens.

The focus lens narrows or “focuses” the laser beam to a very small, precise spot that will enable graphic images to be engraved or cut with extremely accurate results.

In terms of raw materials

Common materials for CO2 focusing lenses are divided into zinc selenide, gallium arsenide and germanium.

 Zinc selenide (ZnSe) focus lenses generally have two types of coatings: CVD (chemical vapor deposition) and PVD (physical vapor deposition).

The CVD ZnSe focus lens is a little more expensive than the PVD ZnSe lens, and its light transmittance is slightly higher, about 99.8%, while the PVD is only 99.5%. This is also because it has better high temperature resistance and can cope with higher power. At the same time, PVD only recommends about 80W.

Focus lenses made of GaAs and Ge are not commonly used.
Ge focus lenses are susceptible to thermal runaway and are only suitable for low-power machines. And the transmittance is also lower than ZnSe.

The GaAs lens is a single crystal military-grade material, durable and high temperature resistant, with fine focus, thin and deep engraving lines, and the surface coating is not easy to fall off, and can accept power below 500W. But It’s opaque to visible light.

Lens Shapes

Meniscus and plano-convex is the regular shapes of focus lens.
The meniscus lens produces a slightly sharper spot, but its cutting performance is slightly worse. Cut details can be better represented with plano-convex lenses.

Focal Length

For CVD ZnSe focusing lens reference only

1.5" Lens

Recommended for small font or fine detail engraving;
High resolution engraving, recommended for raster engraving above 600 DPI resolution;
Best for thin (less than 1/16 inch) materials;
The thickness is about 38.5mm;

2" Lens

Standard lens on most laser systems;
Recommended for raster engraving at 300 DPI to 600 DPI resolutions;

2.5" Lens

Graphics with medium detail and resolution
The thickness is about 63.5mm

4" Lens

Produces a focused beam over a longer vertical distance, suitable for engraving in recessed areas of the product;
For cutting thicker materials;
The thickness is about 101.6mm;