Laser Beam Spot Size

Spot Size: The spot size of a Coherent Monochromatic Photon Laser Beam, 
varies from 15 μm to 170 μm – 0.003” to 0.007”, depending upon the 'OPTICAL' lens used.

The shorter the focal length projected from the OPTICAL lens, the smaller the spot diameter. 
The further the focal point is projected from the OPTICAL lens, then the larger the spot diameter, 
both effect the Kerf width. also known as the 'waist diameter’

Fibre Laser cutting of thick materials requires a wide Kerf, as a large volume of molten or vaporised material is expelled during the cutting process. therefore, focusing a small spot on the material surface when cutting thicker material is not suitable.
To create this large beam spot, the Laser beam is focused either above or below the surface of the material, the actual focal beam’s position depends on the assist gas.

For Fiber Laser cutting of thinner material, a small spot, a narrow kerf, focused on the material’s surface is much more effective than a larger spot / Kerf simply because less material removal occurs.

The actual focal beam’s required spot size depends on 2 other factors, these being :

Spot size for Exothermic Reactions
Exothermic reactions are created by using a assist gas, such as Oxygen, that offers accelerant properties, creating increased heat, with a wide Kerf, The focus position, i.e., the focal spot is placed above the material

Spot size for Endothermic Reactions
Endothermic reactions, are created when you use a gas that has inert, or non-reactive, properties such as Nitrogen, that does not produce increased heat, such gas is primarily used to protect the cut edge from oxidization, plus removal of the molten, vaporized material, through the Kerf, i.e., the focal spot is placed slightly into the material.