Good Mode Quality, It's Not Rocket Science

Scott Kiser, Technical Consultant, Laser Maintenance Group

It doesn’t matter how good your operators and maintenance staff are. It doesn’t matter how new or powerful your laser center is. It doesn’t matter that you use the best consumables, or have the best programming software. Sound true? Of course not, except in one case. If your resonator’s mode quality is poor none of the above will get you to cut good parts on your laser center. Well then what determines if your mode is good or bad?

Before we start discussing mode quality we should explain what we mean by “mode.” The term mode as it applies to lasers is short for Transverse Electromagnetic Mode. Light traveling through free space will make a TEM mode since it has no electric or magnetic field in the direction of propagation. It’s used to basically describe the shape of your laser beam. Lower order modes work best for laser cutting applications since they tend to focus better. This is due to the fact they have less complexity or fewer points to focus than do the higher order modes. The lowest order mode or TEM00 is called a Gaussian mode. It’s from here that the other modes will build from.

Beam Delivery Optics

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.