Uneven power

[Rob Bosworth]

The theoretical explanation does not take into account size of any of the optics. It is a fact of beam dynamics, and can be limited or enhanced based on the real life environment that it is in. Some of the larger range of motion systems, and some of the systems that use different lasers, will use a collimator and apertures to minimize the beam divergence over distance. I believe Epilog has used what the market as Radiance optics on some of their systems. ULS uses a "telescope" or collimator on the HPDFO assy. to achieve a smaller focus diameter across the range of motion of their systems.

I don't remember Lee mentioning anyting about the thickness of the material he is cutting. He just stated that he seemed to get more efficient cut through, the further he got from the beam source. He also said his table was level to the motion system and alignment was pretty close. I offered up a theory that might help explain what he is seeing. It might be something as simple as he gets better air flow or air assist as his laser processing moves from area to area on his system. Without having the system in front of any of us, we are pretty much guessing, er, I mean hypothesizing!

[Richard Rumancik]

Rob, that helps . . . I was wondering if I was under a misconception here as I had always assumed that laser systems generally cut better in the "back left" corner (i.e. nearest the laser source.) I read Lee's post wrong as I just assumed his operated like mine but it doesn't.

I see now that there are so many factors coming into play that it would be hard to do a rigid calculation or even an educated guess as to what will really happen at the 2 extremes of beam travel. It is hard to quantify the effect of spherical aberration of the lens. The sperical aberration will mean there is not a single focal point and it increases spot size. Other factors are the lens diameter, the quality of the beam, mode distribution (% at TEM00) . . . probably more . . . I am starting to think that the balance could tip either way. Otherwise I have no explanation of why Lee's system is 100% opposite of mine.


Another point is that laser power in any particular position is not necessarily equivalent to "efficient cutting" or "processing efficiency". If I use a power meter at a particular point on the table, the thermal sink absorbs the energy emitted regardless of beam diameter and over a broad focal length. So it "collects" all the output energy and gives a single number. But this number will not necessarily give an indication of the cutting ability at that point for a particular material as it amalgamates a lot of factors together.

I'm trying to get a few members to tell me if their laser either behaves like mine (best processing nearest laser source) or like Lee's (best processing farthest away). But nobody is answering this question . . .

[Lee DeRaud]

It is hard to quantify the effect of spherical aberration of the lens. The sperical aberration will mean there is not a single focal point and it increases spot size.

And that's another thing that bugs me: they can put multi-element aspheric lenses into $300 cameras, but they use a single-element spherical plano-convex lens in a $10K+ laser? Seriously?

I'm old enough to remember when aspheric lenses were as rare as cloned T-Rexes...of course in those days, they didn't have sub-$1K computers capable of running the design algorithms. But it's the 21st century now.

[Lee DeRaud]

I don't remember Lee mentioning anyting about the thickness of the material he is cutting. He just stated that he seemed to get more efficient cut through, the further he got from the beam source.

I've noticed the effect (to some extent) on virtually everything, but it's mostly an issue with wood and MDF (typically 1/8"-1/4"): with my 25W system, I need to run on the razor's edge of barely cutting through both for time and cut-quality considerations. For most other materials, jacking the power up or speed down to compensate doesn't have much downside.

[Dan Hintz]

And that's another thing that bugs me: they can put multi-element aspheric lenses into $300 cameras, but they use a single-element spherical plano-convex lens in a $10K+ laser? Seriously?

I'm old enough to remember when aspheric lenses were as rare as cloned T-Rexes...of course in those days, they didn't have sub-$1K computers capable of running the design algorithms. But it's the 21st century now.

In cameras, the multi-lens setups are attempting to correct chromatic aberration (mostly) within a very short focal distance. What we're running into is monochromatic aberration, which is solely a function of the lens accuracy (I'm pretty sure I posted some useful info on aberration with our lenses about a year or so ago...try searching for it here). Getting the accuracy up at the edges of the lens will increase the cost significantly... we're better off using a larger lens and staying as far from the edge as possible.

[Lee DeRaud]

Getting the accuracy up at the edges of the lens will increase the cost significantly...

I guess my point was more that spherical aberration should not even be an issue in a system at this price level. What does "increase the cost significantly" for the lens really mean? +$25? +$50? +$100? Just to put it in perspective, I can get that much variation on the bottom line for a $15K system just by picking which local jurisdiction I pay the sales tax in.

[Glenn Corser]

OK, we finally got Rob to chime in. I hadn't thought about the amount of stuff in the air, I cut a lot of HD .25" (really, 5.2 mm, really anywhere from 4.7 mm to 5.4 mm) oak plywood. And I can state for certain that I have the most gunk in the air in the upper left hand corner of the table. I hadn't thought about that affecting things but it really makes sense in my case.

[Dan Hintz]

I guess my point was more that spherical aberration should not even be an issue in a system at this price level. What does "increase the cost significantly" for the lens really mean? +$25? +$50? +$100? Just to put it in perspective, I can get that much variation on the bottom line for a $15K system just by picking which local jurisdiction I pay the sales tax in.

Depending upon quality, figure on it adding anywhere from an extra $50 to several hundred $s, depending upon final spec. That said, it's simply easier (and much cheaper) to make the lens larger and stay away from the edge. I agree that some of these issues shouldn't be issues in machines of this level, but it is what it is. Have you seen the number of carriage lenses used in the HPDFO setup? It could have been done with less, but guess what they were trying to avoid?

[Glenn Corser]

A small addendum. I had lunch today with some old work colleagues. Included in the assembly were two laser physicists, both PhDs, one old (63) one young (30). I discussed this thread with them and they both confirmed Rob's first posting as far as the physics of what is going on. Who needs grad school when we've got this forum?