beam size

[Ernie Balch]

When we aligned our new rabbit 80W laser I saw that the beam size is about 1/4" and the optics were about 1" lots of room for alignment. Last week we went to Sign World in Atlantic City, where I was able to look at three of the major brands. I was shocked at how tiny the machines are (except price) the optics are much smaller. Is the beam correspondingly small? I wonder if the small beam is limiting the spot size? When I asked about direct marking Stainless Steel I was told that it was not possible without Cermark. When I showed them my pocket knife engraved on one side and vectored on the other they were amazed.

Are the Chinese lasers actually better in this regard?

ernie

[Rodne Gold]

go here http://buildlog.net/cnc_laser/laser_calcs.htm to the spot size and energy density calculator , you will see if you put in wider laser beam sizes , the energy density and spot size gets "better" - an expanded beam diameter out the laser itself (assuming its good quality) will give better results thn a "thin" one. The extra beam diameter of the chinese tube will give you much higher energy density and thats why you can engrave on SS without cerdec , the energy of the spot is so high and the spot itself is smaller

[Ernie Balch]

Thanks, great website!Your summary is exactly what I was thinking. I am so happy that you have been promoting Chinese lasers. They are much better for my sign shop than the tiny expensive units.thanks,ernie

[Martin Boekers]

Rodne, not knowing much about the physics of lasers, how do they get a rating for power?
From what is said here I am reading that all 80 watt lasers that put out 80 watts are not
necessarly equal in performance is that correct? If so why do the not (or maybe they do) have a
efficiency rating to give a better idea of the "focused" power. It seems that would be a more accurate
way to judge power and performance than just a power rating.

Any thoughts on this?

[Rodne Gold]

Power meters see how much an anodised ally sensor can be heated up by the defocussed beam in a set time , as far as I know. (power = work/energy over time)The determinant of a lasers efficiency forprocessing a particular material is its spot size and the spots energy density (and by implication beam quality as this affects spot size) and this depends on optics so its difficult to set a "standard" test here.
You can really tell by thruput what your "power" is..if you cut 3mm acrylic at full power 2x faster than another laser at full power , you have 2x the power. Could be double the watts , could be better optics and beam quality , but you could describe it as double the watts instead of getting all complicated.

What you read about cheaper alternatives is often scaremongering from those with vested interests.

Ernie , I'm not really promoting the chinese lasers .. Just wanted to give a warts and all account of my foray into those machines and perhaps confirm or debunk some "myths" surrounding em. I still think mainstream with hand holding and support is the way to go for a cold start newbie.

[Dan Hintz]

Rodne, not knowing much about the physics of lasers, how do they get a rating for power?
From what is said here I am reading that all 80 watt lasers that put out 80 watts are not
necessarly equal in performance is that correct? If so why do the not (or maybe they do) have a
efficiency rating to give a better idea of the "focused" power. It seems that would be a more accurate
way to judge power and performance than just a power rating.

Any thoughts on this?
80W is 80W, no matter how you look at it, so the ratings are within reasonable statistical bounds from lot to lot. What you do with that power, however, is another story. Rodney already gave the reason why a wider beam is better when it comes to power density (though this only goes so far... as you get closer to the edge of the lens, interference effects come into play and the power curve starts dipping again). The same effect is accomplished with the "Western" lasers using the beam collimators. The beam out of the cartridge is increased in width before entering the cabinet. Once it hits the carriage, the larger beam is focused and gives you a higher power density.

[Martin Boekers]

80W is 80W, no matter how you look at it, so the ratings are within reasonable statistical bounds from lot to lot. What you do with that power, however, is another story. Rodney already gave the reason why a wider beam is better when it comes to power density (though this only goes so far... as you get closer to the edge of the lens, interference effects come into play and the power curve starts dipping again). The same effect is accomplished with the "Western" lasers using the beam collimators. The beam out of the cartridge is increased in width before entering the cabinet. Once it hits the carriage, the larger beam is focused and gives you a higher power density.

So 2 lasers all things equal, power, lenses etc if one has a larger beam it will actually have a higher performance than the other.
Is this correct?

[Dan Hintz]

So 2 lasers all things equal, power, lenses etc if one has a larger beam it will actually have a higher performance than the other.
Is this correct?
That's a bit too vague of a description, but the idea is correct. If the lens on both systems is appropriately sized to remove fringe effects from the equation, then yes, the system with the larger beam will be capable of a higher power density.

This does not take into account, however, beam quality (denoted as M2 -- "M squared", can't do subscripts here). Glass tubes will have a lower beam quality (interference effects mentioned earlier start to take precedence) than metal tubes overall, and the higher the power goes the stronger that statement holds. Even metal tubes start to lose beam quality as they get up into the several hundred Watt range, so other methods are required.

[Jon Colley]

It's also worth mentioning that a larger beam diameter causes a reduction in the depth of field, assuming that everything else stays the same. This means that you are more out of focus at the bottom when cutting thicker substrates. This makes it more difficult for cutting thicker pieces with good results.

It will give a smaller spot size for engraving, but this comes with a drawback. "Higher performance" really depends on your intended application.

[Ernie Balch]

Beam quality is important and M2 was developed as a shorthand way of comparing lasers with gaussian (bulls eye) energy profiles. A value of 1 is for a perfect diffraction limited beam, but often lasers have numbers like 1.2 or higher.

Poor quality laser beams may not be gaussian across both the horizontal and vertical cross sections. So you get a bulls eye pattern that is oval and is hard to focus. The gaussian beam is called a Mode of TEM00 other worse behaved modes may have 2 hotspots sometimes 4 or more and may be unstable.

You can think of a larger beam as one that uses more of the lens, thus focusing to a smaller spot.

Another factor to describe a laser beam is the divergence, how fast the laser beam is spreading out. You really want a parallel beam that does not spread so the focused spot size does not change across your table. I suspect that low divergence is much more difficult to achieve in a short laser tube hence collimators to fix the problem.

Average Power in a CW (continuous wave) laser is measured in Watts usually with a thermopile type meter. As Rodne said you are measuring the the temperature rise caused by the beam and converting it to watts. You can lay your finger on the sensor and get a reading of a couple watts just from the heat of your finger.

Pulsed lasers like fiber lasers and YAG lasers are entirely different from our CO2 lasers, Every pulse is only a few nanoseconds long so the instant power can be hundreds or thousands of watts even though the average power reading might be milliwatts.

Sorry to carry on, but I spent 20 years doing semiconductor processing and micro machining with lasers with systems I designed.
ernie

[Dan Hintz]

Any of the lasers used here are going to be TEM00 (Gaussian) beams. That said, I would trust the lens/reflector alignment of metal tubes to be more accurate compared to glass tubes. The divergence of a metal tube is also going to be better than a glass tube, but as stated, a collimator will take care of that easily enough.

[matthew knott]

You would really want to start with a small beam form the o/c optic, this would them allow you to have a higher expansion ration (if you have a beam expander and colimator) I dont think the m2 value is that great on wave guide co2 laser, dont know if the glass tubes are better or worse tbh, the longer the resonator (tube length) the better the quality of the laser beam. But it does all so deepend on what you are doing with the laser. Im still shocked at how slow (just got epliog ext36), im surprised how far the laser head needs to overshot the actual engraving area (raster mode), did a job the other day that was taking 3 mins, i had 50 to do so decided to fire up the galvo laser at the mark was only 5'x5' and i reduced the cycle time down to < 8 seconds. The epilog is very handy for cutting jigs out

[Dan Hintz]

You would really want to start with a small beam form the o/c optic, this would them allow you to have a higher expansion ration (if you have a beam expander and colimator) I dont think the m2 value is that great on wave guide co2 laser, dont know if the glass tubes are better or worse tbh, the longer the resonator (tube length) the better the quality of the laser beam. But it does all so deepend on what you are doing with the laser. Im still shocked at how slow (just got epliog ext36), im surprised how far the laser head needs to overshot the actual engraving area (raster mode), did a job the other day that was taking 3 mins, i had 50 to do so decided to fire up the galvo laser at the mark was only 5'x5' and i reduced the cycle time down to < 8 seconds. The epilog is very handy for cutting jigs out
The M2 value on metal tubes is pretty darn good... in the 1.1 area, give or take (depending upon model). Same for Synrad... some of their more powerful models are in the 1.2 range. Glass tubes will be slightly worse, just by the less-perfect nature of their design.

The carriage needs to overshoot due to mechanical resonance issues and have nothing to do with the laser. For example, Trotec goes twice as fast because they use powerful motors, high current, and ultra-low-weight carriage components. A galvo works so fast because it's only moving a lightweight mirror.

[Ernie Balch]

You are so right about galvo systems, that is the way to go for speed.My thoughts on divergence come from the fact that when the tube mirrors are further apart they have to be aligned more accurately to make the system lase. Of course the mirror shape ( flat or curved) makes a difference. Probably the aperture size at the exit mirror makes a difference as well.ernie

[Dan Hintz]

You are so right about galvo systems, that is the way to go for speed.My thoughts on divergence come from the fact that when the tube mirrors are further apart they have to be aligned more accurately to make the system lase. Of course the mirror shape ( flat or curved) makes a difference. Probably the aperture size at the exit mirror makes a difference as well.ernie
The exit mirror will define the beam diameter at the output, but the quality of the full-reflector's grind/polish will determine the divergence. Here is one case where you (as a laser manufacturer) have to decide on where to spend your money... use a high-accuracy grind/polish on the full-reflector mirror, or save money there and use a collimator somewhere else.

Alignment of the mirrors helps determines total power (or loss of it), and once you're past the "it will lase now" angle, it's all about how much you want to lose/gain again. Of course, the longer the travel the more accurate you need to be.

[David Fairfield]

Can beam shape/diameter vary or degrade as the tube age increases?

[Dan Hintz]

Can beam shape/diameter vary or degrade as the tube age increases?
If the mirrors move or become contaminated, yes... but generally, no.

[Martin Boekers]

I guess in a simplistic manner this could be compared to water.

If a water coming through say a 6" pipe gets constricted to a 2" pipe the
more powerful the pressure becomes.

[Dan Hintz]

I guess in a simplistic manner this could be compared to water.

If a water coming through say a 6" pipe gets constricted to a 2" pipe the
more powerful the pressure becomes.
Not sure I see the analogy...

[Rich Harman]

Water velocity increases when the pipe gets constricted not the pressure.

I have noticed that the beam shape can be distorted if the power is too low. At low power, like 20%, it fires just fine but the pattern is something like the infinity symbol. Increase the power a small amount and the beam is good. You can also see the difference in the tube itself by the pattern that the plasma makes when it does this.

[Rodne Gold]

Rich, are you getting a double lobed "spot" when firing at low power? like this oo ?

[Rich Harman]

Yes, that's correct.

[Rodne Gold]

Shouldnt do that at all , thats most likely nothing to do with beam shape but most likely something to do with triggering or timing or something else like something in the optics splitting the beam. Do you get that shape if you just pulse the "fire" button at a low power setting with no motion?
Does it do that on both machines?

[Rich Harman]

The oo shape of the beam was determined while aligning the first mirror, so that is how the beam appeared a few cm after exiting the tube @20% power - before the first mirror. It is the pattern it made on the tape.

[Rodne Gold]

Check the output optic on the tube itself for a dust or smudge , try your spare tube , the patterns we get with all our tubes is O - if you are using the beam combiner for the red dot pointer after the tube , remove it - on our machines it was the source of some problems.