Lenses are used in CO2 laser systems to focus all the beam’s power into a very small spot. Small, focused spot size is important for a couple of reasons: cutting and welding call for high power density, and power density is a strong function of spot size (reduce the spot diameter by 50% and the power density increases by 200%). The variables that affect the performance of a CO2 laser lens are focal length, diameter, shape, material and coating.
The focal length affects both spot size and depth of focus. In general, a shorter focal length will produce a smaller focused spot and a shorter depth of focus. Usually, the specified focal length is a compromise between desired spot size, penetration depth and work piece clearance.
The lens diameter is important for two reasons. Higher power lasers require larger diameter lenses to prevent thermal overload. Also, at any given focal length, a larger diameter lens will yield a smaller focused spot if the incoming beam is expanded to fill the larger lens.