A laser beam is an electromagnetic wave and therefore has properties similar to water waves or sound waves. One consequence of this wave-like nature is that a laser beam cannot be focussed to a sharp point. Instead the focus has a spot size which can be calculated as follows:
df = (4/Π) M2 λ FL / din
df = focus diameter, M2 = beam quality, λ= laser wavelength, FL = focal length of focussing lens, din = diameter of incoming beam
Examples: ( λ = 10.6 µm, M1 = 2)
din = 20 mm, FL = 7.5" >> df = 0.13 mm din = 20 mm, FL = 3.75" >> df = 0.065 mm
First of all, the example shows that focus diameters are much larger than the values calculated in the section above. This means that in most cutting applications, spherical aberration can be neglected. Diffraction is therefore the most important effect concerning focus diameters. In general, the formula shows that by decreasing the focal length, the focus diameter is decreased as well, with the consequence that the intensity of the laser beam is increased. As high laser intensity is useful in most cutting applications, focal length should be as short as possible. However, a short focal length has the disadvantage that the beam diameter increases rapidly above and below the focus. Therefore, maximal thickness of materials which can be cut efficiently is very limited, and the optimal focal length increases with increasing thickness of material.
During laser operation with several kilowatts, the focusing lens is heated because it absorbs a small portion of the laser power. Absorption takes place mainly in the AR coatings and at dirt on the lens. At a new clean lens with standard AR coating, absorption is typically 0.2% of the incoming laser power. A CO2 laser lens such as Ophir's BLACK Magic is a low absorption lens (lower than 0.15%). During use in a CO2 laser cutting machine, absorption increases gradually due to increasing amounts of dirt on the lower surface. When the lens needs to be replaced, absorption usually is in the range 0.3 to 0.4%. Heating of the lens causes additional surface curvature due to thermal expansion and increases the refractive index of the lens material (ZnSe lens). As a consequence of these effects, the lens focal length becomes shorter, and the focus position cannot be predicted exactly because it depends on many parameters like laser power, intervals laser on/off, cleanliness of lens, and others. Therefore, use of low absorption CO2 laser lenses with can make the focal length more stable and therefore improve reliability of the cutting process. If there are dirt particles on the lens, the lens material is not heated uniformly, but mainly at the areas close to these dirt particles. As a consequence, focusing properties become worse; focus diameter increases, and cutting quality decreases. So if a certain "critical" amount of dirt has accumulated on the lens, it needs to be replaced. However, it might still work fine at reduced laser power.