Gute Laserleistung ist keine Hexerei

Article: Good Mode Quality, It's Not Rocket Science
By 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.

Mode quality is defined differently by almost every laser OEM. They each have their own ways to determine what a good mode is. Most will place a block of acrylic a specified distance from the resonator and fire the raw beam into the block for a set period of time. Others will use thermal blocks to observe the shape of the beam. Still yet a few others tune for peak power and graph out all sorts of mirror combinations. In any case measurements are taken, calculations made, tolerances are taken into account and a decision is made whether it’s good or not. Every time I tune a mode, I am reminded of a quote from an optical engineer at one of the biggest laser manufacturers. I was a brand new service tech, and the mode we shot was a TEM10. No matter what I did I could not get a TEM 01* like I was trained for. I called for help, and after much abuse I was informed that, “All that matters is symmetry! It’s not rocket science you know!” Turned out he was right. When I finished, the laser center cut parts at the rated feed rates and with superb edge quality.

Anytime you change optics in your resonator your mode will have to be re-evaluated, and most likely corrected. Start off by using only high quality optics. Refurbs, reconditioned, or low quality optics will result in disappointing or short lived mode quality. When you’re done tuning make sure your mode is symmetrical. If it leans, has interference rings, or any asymmetry, fix it now before you move on. Symmetry is critical since once the lens focuses the beam it needs to consistently present itself to the material no matter which direction you are cutting. If your mode leans it will cut well in one direction, and poorly in another. No amount of focusing or parameter adjustments will solve this problem.
Settling for a marginal mode will result in wasted time. Part quality and production will suffer until it is done correctly. Next time you have your service tech adjust the mode, go stand with him and look at it together. Just like any other aspect of the laser maintenance, quality is paramount. Make sure your mode is good. Remember ‘symmetry’ and you will be fine.

Before I wrote this article I reviewed some of the information on modes in an effort to be technically accurate, and to explain it in terms the rest of us normal people use.

 

Here are a few of the calculations the physicists use to measure and predict the different orders of modes in cylindrical cavities: I’m starting to now wonder if it’s not “Rocket Science” it must be close. I’m just glad tuning a good mode is not.