Uneven power

[Lee DeRaud]

Something that has always puzzled me is that the power varies with distance from the tube: it's lower in the upper left corner (where the laser comes out) than in the opposite corner. It's not a lot, maybe 10-15% difference, but enough that some settings that work in one corner don't quite cut through in the other. Focus appears to be very consistent across the table, so it's not that. I would expect a beam divergence issue to work the other way around, not to mention that I had the same symptom with the old tube also. It's not enough to be a real problem, just something I have to watch for when I've got the settings right on the edge of "optimal".

Is this normal? And does anyone have an explanation for it that makes sense?

[Mike Mackenzie]

Lee,

One of two things Beam window or alignment I would check the alignment first and then if that is OK remove the beam window and run a test. Do not run the system for long periods without the beam window in place, however you can test it with out any problems.

[Lee DeRaud]

Mike, this is a VL200: how do I check the alignment, and if it's off, what adjustments are there?

I'm a bit puzzled by the suggestion to remove the beam window...isn't the beam always going to hit it at the same spot anyway?

[Glenn Corser]

Lee, I've noticed that also with my V-460. It has puzzled me also. I did notice that the corner mirror, the one that makes the right angle to the cutting mirror/lens, is screwed on and comes loose sometimes. If I screw it tight it seemse to reduce the problem - why I'm not sure but it does.

[Zvi Grinberg]

For basic alignemt review, tape the left side of the lens carriage (the round hole), and view the red dot at all 4 corners.
Dot should be about the center of the round hole, and more inportant - even if not exactly on the center, deviation should be consistent
This is a preliminary test. If you have indications about misalignment, I suggest you call tech support and they will guide you and provide documented procedures.

If you would like to get more info during the weekend, drop a line and I wil try to assist.

[Lee DeRaud]

For basic alignemt review, tape the left side of the lens carriage (the round hole), and view the red dot at all 4 corners.
Dot should be about the center of the round hole, and more inportant - even if not exactly on the center, deviation should be consistent
Quick check using a Mk1, Mod 0 eyeball says the dot is maybe 1mm off-center, with no discernable movement over the entire field.

Maybe I'm missing something obvious, but I'm having trouble wrapping my head around the idea that an alignment problem can make the laser cut better as the path length gets longer.

[Larry Bratton]

I have always understood that you have more power in the upper right side of the table than you do on the left because of being closer to the beam source. My machine has always been that way. If I have something I want to cut and absolutely insure I get through it on the first pass, I try and put it in the upper right. My table is a 36x24. Maybe I'm missing something ? I'm in perfect alignment.

[Lee DeRaud]

I have always understood that you have more power in the upper right side of the table than you do on the left because of being closer to the beam source.You'd think so, wouldn't you? OTOH the energy loss in a laser beam through a couple feet of (relatively) clean air should be almost unmeasurable.

Mine is the exact opposite of that, hence this thread. (Beam comes out of the back-left on mine.)

[Susan Dennis]

I'm thinking maybe your table is not completely level to your laser head. I'm a complete newbie but that was the issue I was having weak power near the output and stronger the further out. Turns out my table is off by 1/16 inch at the far end throwing the focus off, but I had to determine this by measuring the distance between the laser head and the table.

[Lee DeRaud]

I'm thinking maybe your table is not completely level to your laser head. I'm a complete newbie but that was the issue I was having weak power near the output and stronger the further out. Turns out my table is off by 1/16 inch at the far end throwing the focus off, but I had to determine this by measuring the distance between the laser head and the table.It's a good thought, but I'm measuring less than 0.015" variance corner-to-corner.

I may try down-focusing a few hundredths to get the focus "into" the substrate and see if that helps any.

[Susan Dennis]

Ah and I see you stated the focus is consistent across the whole area. My issue was loss of focus and seemingly power.

[Glenn Corser]

We were talking this over and my son had a thought. Basically, the assumption is that where the beam hits the corner mirror varies a little as it moves around the table. The upper left would be the place of highest energy transfer to the mirror - could it be that the mirror gets degraded enough in this position to cause cutting power to go down? The laser I use is about 9 years old, we bought it used 6 years ago and have never changed that mirror. I'm waiting for a new tube from ULS and am using their loaner (nominal 50 watts measuring 57) and still see the same phenomenon.

[Lee DeRaud]

We were talking this over and my son had a thought. Basically, the assumption is that where the beam hits the corner mirror varies a little as it moves around the table. The upper left would be the place of highest energy transfer to the mirror - could it be that the mirror gets degraded enough in this position to cause cutting power to go down?It could, if the beam hits the mirror in a substantially different position as the carraige moves, but that kind of gross misalignment would also show up during an alignment check...see post #6.

For that matter, what percentage of the beam energy would have to be tranferred to the mirror for this effect to be detectable? (Not to mention that "energy transfer to the mirror" equates to "heating of the mirror", which has all sorts of ugly implications for mirror life.)

It also gets back to something I brought up earlier: just how much difference (if any) is there in the laser beam energy between (1) just after it leaves the beam window at the back-left corner and (2) after it's gone through a couple feet of air (28" in my case) to get to the front-right corner?

[Richard Rumancik]

. . .It also gets back to something I brought up earlier: just how much difference (if any) is there in the laser beam energy between (1) just after it leaves the beam window at the back-left corner and (2) after it's gone through a couple feet of air (28" in my case) to get to the front-right corner?

Lee, I'll take an educated guess and say there will be a 10-15% difference. (I based this on your first post.)

I really think your laser is working normally and what you are seeing is the way most lasers with simple optics tend to work. The air is "relatively" clean in the laser enclosure, although there may be some smoke particles at times. But humidity in the air will also attenuate the laser beam resulting in lower energy at the front-right location (for lasers with back-left beam entry.) In addition, the beam has divergence that means that the optics optimized in the back left may not be ideal in the front right corner.

I agree that the 10-15% difference can be a nuisance - I usually do my test cuts near the origin (back left) but if you try to cut a sheet of parts with that setting you may find the front-right doesn't cut through. If rastering gets a bit lighter in the front right it may not be a problem but incomplete cuts sure are. I have struggled with this. One solution was to adjust settings so that parts in the front-right get a hotter beam - by using color mapping. Rather crude but sorta works. Time consuming to set up and tricky to manage. But mostly for small jobs I just cut on the left half of the table. I'd rather just flip the sheet than waste material or try to salvage uncut parts. I've spent too much time trying to rescue a part only to give up and scrap it.

A power meter would tell you more. I don't have one but I borrowed one some time ago, and the power was definitely less in the front right corner. I can't give you an number offhand though.

I've often thought that the laser manufacturers should allow for firmware compensation for the power-loss phenomenon. That is, I set the power/speed for cutting in the back left corner and I select "energy compensate over table" and it tweaks the laser power on-the-fly to compensate for the losses depending on beam travel. Nobody has taken me up on the idea, however.

The divergence problem, as I understand, will cause variation in the depth of field, which could result in incomplete cutting. A larger beam reduces spot size, but it decreases DOF (bad) and increases spherical aberration (bad). (If you try to use the full diameter of your lens you will not focus on a single point. Rather like having many differing focal points occuring at the same time. So you won't get a pin-point hot spot on the material.)

Divergence can be somewhat addressed by using a beam collimator. Here is what Synrad says on the subject:

"In flying optics systems the focusing optics must focus a beam of varying beam diameter, which in turn causes changes in the depth of focus and focus spot size ultimately leading to inconsistent processing. In these instances, a beam expander/collimator inserted into the optical path substantially reduces the divergence of the beam and any variance caused by changing optical path length."

[Lee DeRaud]

Lee, I'll take an educated guess and say there will be a 10-15% difference. (I based this on your first post.)

I really think your laser is working normally and what you are seeing is the way most lasers with simple optics tend to work. The air is "relatively" clean in the laser enclosure, although there may be some smoke particles at times. But humidity in the air will also attenuate the laser beam resulting in lower energy at the front-right location (for lasers with back-left beam entry.) In addition, the beam has divergence that means that the optics optimized in the back left may not be ideal in the front right corner.I agree with everything you just said. Problem is, it's not applicable to the situation at hand.

Reread my first post: what I'm seeing is that the apparent power at the front-right corner is 10-15% higher than at the left-rear, contrary to what any reasonable intuition and/or analysis would predict.

[Glenn Corser]

Lee, I agree with what you said above. As I was assembling some lamps today I was thinking about it and came to the same conclusion. But, something else occurred to me. I think the beam basically illuminates the mirror with a two dimensional distribution that has a long tail in the direction of the cutting assembly and may look like a gaussian in the direction normal to that. As the corner mirror moves farther from the upper left, the illuminated area becomes bigger due to beam dispersion. So if the spot where the middle of the beam hits were to become less reflective, then as you approached the upper left hand corner and the beam became more focused on the "bad" spot, you would reflect less power. As you moved away from the corner, less of the beam's energy would be reflected by the "bad" spot and you would, paradoxically, get more of the beam reflected to the cutting assembly. That and a buck will buy you a cup of coffee.

[Lee DeRaud]

I think the beam basically illuminates the mirror with a two dimensional distribution that has a long tail in the direction of the cutting assembly and may look like a gaussian in the direction normal to that. As the corner mirror moves farther from the upper left, the illuminated area becomes bigger due to beam dispersion. So if the spot where the middle of the beam hits were to become less reflective, then as you approached the upper left hand corner and the beam became more focused on the "bad" spot, you would reflect less power. As you moved away from the corner, less of the beam's energy would be reflected by the "bad" spot and you would, paradoxically, get more of the beam reflected to the cutting assembly. That and a buck will buy you a cup of coffee.This is the point, in conversations at work, when people look at each other, shrug, and say, "Well, it sounds good.":D:cool:

It brings up yet another gap in my knowledge about this machine: just how fast does the beam diverge? Again, we're only talking about (in my case) a maximum of 28" of beam length...I wouldn't expect significant (or even measurable) divergence over that distance.

As an aside, the whole "bad spot on the mirror" thing strikes me as one of those positive-feedback deals where, once one develops, decreased reflectance equals increased heating, which makes the bad spot bigger, lather-rinse-repeat, very quickly accelerating until the mirror is effectively destroyed.

[Rob Bosworth]

Lee, as hard as it is to believe, the larger the beam diameter on a laser, the smaller the focus spot size. So you might be seeing the effect of beam divergence. As the beam gets further from the source, the larger the diameter of the raw beam. The large the diameter of the beam, the smaller the spot size will be at focus. The smaller the focus spot size, the more energy density of the spot. The greater the energy density at the focus point, the more effective your laser processing should be. It is very hard to wrap your mind around, but it is true.

[Richard Rumancik]

I agree with everything you just said. Problem is, it's not applicable to the situation at hand.

Reread my first post: what I'm seeing is that the apparent power at the front-right corner is 10-15% higher than at the left-rear, contrary to what any reasonable intuition and/or analysis would predict.

Ok, I'm a bit embarrassed as I read that first post wrong. I try to read the questions carefully but I guess it was a case of "knowing" the answer but assuming I understood the question. Your laser is operating exactly opposite to what I see. So now I am really puzzled . . . and I don't have an answer either.

[Dan Hintz]

The large the diameter of the beam, the smaller the spot size will be at focus. The smaller the focus spot size, the more energy density of the spot.
Within limits... as you approach the edges of the lens, the focal point isn't kept as tight, so you can actually lose power density.




Lee,

Beam divergence is in the single-digit millirad range. Over a large table, the beam may only grow by a couple of millimeters in diameter.

[Lee DeRaud]

Lee, as hard as it is to believe, the larger the beam diameter on a laser, the smaller the focus spot size. So you might be seeing the effect of beam divergence. As the beam gets further from the source, the larger the diameter of the raw beam. The large the diameter of the beam, the smaller the spot size will be at focus. The smaller the focus spot size, the more energy density of the spot. The greater the energy density at the focus point, the more effective your laser processing should be. It is very hard to wrap your mind around, but it is true.Actually, that's the first suggested explanation for the phenomenon that I can wrap my mind around.
(I still find it hard to believe that there is that much beam divergence over the distances involved, but there ya go.)

[Lee DeRaud]

Beam divergence is in the single-digit millirad range. Over a large table, the beam may only grow by a couple of millimeters in diameter.(looks for envelope to scribble on the back of...)
Call it 5 millirad, or a tick over 0.25 degrees...sin(0.25) is a little under 0.5% times max path length of 700mm = 0.35mm growth. A bit larger than I expected, but not eyeball-detectable. If I remember correctly, the nominal beam size on my 25W system is around 5mm, so that's about 7.5% diameter growth or about 15% area growth...
Hmmm...assuming spot area is the driver here, we're at least in the right order-of-magnitude for the kinds of power variation I'm seeing.

Given that it goes up as the square of beam length, I'm a bit surprised that the guys with bigger machines don't complain about this more. Do the higher-power tubes have bigger beams and/or lower divergence values? (Or perhaps it's partially offset by beam absorbtion/dispersion effects?)

[Rob Patterson]

What is your bed size?

[Richard Rumancik]

I'd like to know if other members tend to see the same phenomenon as Lee is seeing on their tables.

Rob, I don't disagree with what you are saying, but do you see what Lee is seeing on every laser that you have used? Or just some? I had it in my head that laser "effectiveness" would generally drop off as to get farther from the source.

Rob, your explanation would suggest that cutting should be better the farther you go from the source. But working against this is (a) the spherical aberration of the lens, and (b) that if you have a larger incoming beam, your depth of field reduces. The spherical aberration causes loss which increase as the cube of the beam diameter, the depth of field is reduced by the square of the beam diameter. So on one hand you get benefit (processing efficiency) from the increased incoming beam diameter but a couple other things work can against the improvement. Ideally the effects would all cancel out and you'd have a constant "processing efficiency" over the table but I'm sure this is not likely to happen.

My explanation assumed the losses outweighed the potential improvement. Maybe it is opposite in Lee's case? (My explanation describes what I see on my system, but was 100% wrong for what Lee is seeing.)

[Lee DeRaud]

What is your bed size?12"x16"...max path length around 28" or 700mm.

[Lee DeRaud]

Rob, your explanation would suggest that cutting should be better the farther you go from the source. But working against this is (a) the spherical aberration of the lens, and (b) that if you have a larger incoming beam, your depth of field reduces. The spherical aberration causes loss which increase as the cube of the beam diameter, the depth of field is reduced by the square of the beam diameter. So on one hand you get benefit (processing efficiency) from the increased incoming beam diameter but a couple other things work can against the improvement. Ideally the effects would all cancel out and you'd have a constant "processing efficiency" over the table but I'm sure this is not likely to happen.I agree that the various effect tend to cancel each other out, with the caveat that they are all slightly variable machine-to-machine and also variable with respect to environment (dust/humidity/etc). That cancellation, to whatever degree it occurs, is a good thing.

[Dan Hintz]

(looks for envelope to scribble on the back of...)
Call it 5 millirad, or a tick over 0.25 degrees...sin(0.25) is a little under 0.5% times max path length of 700mm = 0.35mm growth. A bit larger than I expected, but not eyeball-detectable. If I remember correctly, the nominal beam size on my 25W system is around 5mm, so that's about 7.5% diameter growth or about 15% area growth...
Your math appears to be a bit bodged... 5 mrad (which, incidentally, is the spec given to the average-powered ULS tubes, +/1 mrad... good guess) = 0.9 degrees, which is roughly 1.6% beam divergence. Over a 700mm path, that's a divergence of 11mm.

Wow, that value surprises even me! Did I do my math right?

EDIT: Math was good, I missed pi. 5 mrad --> 0.0286 degrees, leading to a roughly 0.5% divergence. Over 700mm, that's 3.5mm... more in line with what I initially expected.

[Richard Rumancik]

. . . Do the higher-power tubes have bigger beams and/or lower divergence values? . . .
Well, if you go by the Synrad series, I would say you are right on both counts. Take a look at this pdf for the Synrad product line. The higher power tubes have larger beam diameters and lower divergence. I don't quite understand the 1/e**2 part, but the "beam waist diameter" is roughly the minimum diameter of the beam which will occur near the exit of the laser tube assembly. So for our discussion we can call it the initial beam diameter.

Regarding the math - shouldn't the growth be more like 3 or 3.5mm in your calculation? Dan, there seems to be something wrong with the .9 degree estimate . . .


Synrad says that a full-angle beam divergence of 4 milliradians means that the diameter will increase 4mm/meter. So if you started off with 3.5mm initial (waist) diameter (Synrad 48 series tube) and it increased 700/1000 x 4mm you would end up with about a 6.5mm beam at the end of travel. I know your tube is not a Synrad but the numbers are probably in the same order of magnitude. So the beam diameter could roughly double over a 700mm path. That reduces the spot size by 1/2 and reduces the depth of field by 1/4. Smaller spot size can be good due to increased power density, but smaller depth of field is bad for cutting effectiveness especially for thicker materials. How it all plays out with all the other variables seems to be uncertain.

[Lee DeRaud]

5 mR (which, incidentally, is the spec given to the average-powered ULS tubes, +/1 mR... good guess) = 0.9 degrees, which is roughly 1.6% beam divergence.180/pi * 0.005 = 0.28 degrees, right? The divergence would be the sine of that: sin(0.28 degrees) = 0.005 (or 0.5%). But yeah, that's more like 3.5mm beam growth, which seems high.

Again, that's using a 5mm initial beam diameter figure, which is what I dimly recall from a discussion with the vendor. (The mirrors and lens are about 15mm, which makes 11mm beam growth, um, problematic.)

[Dan Hintz]

Yep, missed the pi in my initial calculation (must have fat-fingered the calculator keyboard)... I knew that sounded huge. 0.286 degrees.

5 mrad --> 0.0286 degrees, leading to a roughly 0.5% divergence. Over 700mm, that's 3.5mm... more in line with what I initially expected.

[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?