High power lasers are a growing industry with numerous applications. As 1 μm fiber laser technology advances and more demanding applications develop, the optics used in such systems must provide superior levels of performance.
During laser operation with several kilowatts, the focusing lens is heated because it absorbs a small portion of the laser power. The anti-reflection (AR) coating was developed for CO2 lenses many years ago when lasers were lower powered than they are now. They were the best coatings available for many years. Now, of course, the average laser machine is no longer 1K to 2.5K, but can be up to 5 or 6K or more. These equipment improvements required that new optical coatings be developed to handle the thermal demands of higher power.
There are a number of articles that you can find on the web that discuss the comparisons of the CO2 performance to the fiber laser. It has been covered by a number of well-respected industry magazines. There is no longer a discussion about if fiber will be able to compete with CO2. Some of the OEMs have already reached the tipping point and are selling more fiber lasers than CO2.
The focusing lens is the last optic in the laser path, before it hits the workpiece. Its main role is to focus the laser beam to a specific focal length (FL) - depending on the application. Therefore, the focal length - that is dictated by the radiuses and curvatures of the lens - is its most important feature.
The focusing lens is normally made of Zinc Selenide (ZnSe) using an anti-reflective coating. Focusing lenses are either Plano-Convex or Meniscus.
Focal length and mounting distance
In general, there are two types of focusing lenses: planoconvex lenses which have one convex surface (convex = dome-like curvature) and one flat surface, and meniscus lenses which have one convex surface and one concave surface (concave = hollow curvature). In most laser cutting machines, meniscus lenses are used because they produce a smaller focus diameter (see next section). In some machines, plano-convex lenses are used because their production costs are a little bit lower.
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 (Anti Reflection) 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 lens with Ophir's Black MagicTM coating initial absorption is guaranteed to be less 0.15%.
In our newest type of lenses the Clear Magic, the Max absorption is lower than 0.13% and the typical absorption is 0.10% and lower
In the cavity of a CO2 laser, CO2 molecules are excited by a gas discharge. This excitation energy is fed into a laser beam if it has sufficient intensity. For building up this intensity, mirrors are placed at both ends of the discharge such that the laser beam is reflected back and forth many times. Such an arrangement is called a laser cavity. In real cavities, both mirrors have some transmittance: one of them is the output coupler where the transmitted beam constitutes the useable laser beam; the other one is the rear mirror where the transmitted beam has very low intensity and is used for controlling purposes. - For building up a laser cavity with output laser power of several kW, the total length of the discharge needs to be several meters. Covering this distance with one discharge is very problematic. Therefore, it is split up into several discharges working in line. In order to make the mechanical setup as compact as possible, the path of the laser beam within the laser cavity is "folded" several times by using suitable mirrors which are called total reflectors
In the beam delivery section of a laser working machine, the laser beam is transferred from the laser cavity to the working head. In principle, two moveable mirrors would be sufficient in a 2D-machine for guiding the laser beam to any point on the worksheet. In modern 2D-machines and especially in 3Dmachines, however, the beam delivery section has additional functions which require additional mirrors with specific properties. In order to optimize function of these mirrors, different substrate materials are used - the most common ones are silicon(Si) and copper (Cu). Silicon mirrors have light weight and are therefore preferred in flying optics where high accelerations are needed. Copper has high thermal conductivity, and channels for cooling water can be included directly into the mirrors. Therefore, copper mirrors are preferred if best-possible cooling is important, for example in machines with very high laser power. - The optical properties of a mirror (reflectance, phase shift, etc) are determined by its coating. So in order to realize different mirror functions, different coatings are needed.