Measuring beam power

[Dan Hintz]

Just won a lightly-used Synrad Power Wizard from eBay for about half price, should be here before Christmas. If anyone local has an interest in measuring their machines, I'm sure we can work something out (Steve, if you have a get-together this season, I'll bring it along).

[James Stokes]

I would be interested in finding out exactly how much power I am getting.

[Rob Bosworth]

Dan, you could possibly be opening up a can of worms for yourself and others!

First, I have to say that I have never used a Synrad Power Wizard. I use an analogue power meter that is calibrated by the manufacture. It is a very simple design that measures average power over a certain period of time.

That being said, you just might be setting yourself up to have heartache and pain, for no real reason. There are many machines that are fairly hard to get an accurate reading of the actual laser output. Laser pulsing can change the overall output readings of some power meters. For example, Epilog used to make a system called a Summit. The Summit used Synrad 48-2 (25 watt) sealed lasers. These were extremely hard to get an accurate output from the laser. They would test out on most power meters at ~ 1/2 their rated output. So you would test the laser output while the laser was in the machine, see that the power was low, remove the laser from the system and re-test just the laser on the test bench, where it would show that the laser output power of the laser was actually 31 watts. Put it back into the machine and get the same low output reading you got before. This would tell you that there was a problem with the system. So you would spend hours trying to find out why you were not getting full power from the laser. Finally giving up, you would call the factory, and the Techs would tell you to engrave on some known material, like melamine, at a certain speed and power and send it to them. They would then look at the piece and tell you that your laser was outputting about what it was supposed to be outputting. It was not a super scientific way to check power, but it was more accurate than using a calibrated instrument for that particular machine.

(I happen to know this because a number of customers over the years would have a buddy come in who was a "laser expert". This expert would watch the system for a little bit, then pull out their calibrated gadget, and announce that the owner of the machine had been taken because the laser was too underpowered. This usually happened on a Friday night, or Saturday for some reason. The owner of the machine would get their undies in a bundle over being taken, and by Monday morning, they would be wild. 8:00 AM the phone would ring here and I would get more than an earful. It usually took a number of phone calls back and forth before we could debunk the "expert's" facts.)

I have also heard that the Synrad Wizard works well with most Synrad lasers, but not as accurate with other manufactures lasers. I do not understand why, but I have heard it a number of times over the years.

So Dan, enjoy your new instrument. I hope it works out that it works perfectly and that it will give you satisfaction in knowing how your laser is performing.

[Dan Hintz]

Edited for clarity...

Not sure what to tell you, Rob. If a laser is set to 100% power in the GUI/controller and it puts out 30W, but the company claims it's a 60W tube, the company is lying. 100% is 100%, whether it's measured by a meter or measured by a test sample (and the test sample would be less repeatable due to differences in material). Now, the meter may display only half of the true power, or 15W for a 30W tube, but that same meter had also better display 45W on a 90W tube, else the meter is faulty. If the meter is repeatable, one should only need to use a correction factor, and that same correction factor should apply to all measurements, not just on certain machines. Hopefully the meter is accurate, as well. Synrad claims +/-5% accuracy, and that's plenty for me.

In short, the manufacturer of the laser cartridge is irrelevant, as long as the wavelength is correct. If the meter says a cartridge is 30W but a company claims it's 60W, I'm crying foul on the company. If it's truly putting out 30W at 100% settings, then the company has dumbed down the cartridge through the user interface, but that still means the cartridge is as good as only a 30W. "Capable of 60W" and "Using 60W" are completely different, and most could care less about what the cartridge is capable of, only what it does.

[Rob Bosworth]

So Dan, your laser outputs power like your chainsaw. Full throttle means you are outputting full horsepower?

That is true if you are outputting CW (continuous wave). But most systems run by pulsing the laser. It might look like it is just turned on at the beginning and off at the end, but it is not in most cases. Pulsing and pulse shaping are huge factors in successful laser processing. You might set your PPI at 175 and get a smoother cut on certain materials than you would if you had it set at 1000. It has a lot to do with how the energy is concentrated and the duration of the energy during the life of the pulse. A lower PPI setting might allow a huge spike of power early in the pulse, then have the power drop down towards the end of the pulse. It might still only average 30 watts for the length of the pulse, but you might see 150 watts early in that pulse, and a significant drop in power of 7 watts for most of the duration of that pulse. If your power meter is measuring the energy over a period of time similar to the pulse duration, then you might see the real output power of the laser. If the "sampling" time of the digital electronics are not very similar to the output of the laser pulse, you might see almost any results on the digital display. That is one reason we use a thermal energy measuring device. Not too fast, but better for long term average in my opinion.

All that being said, we still haven't touched onto the dynamics of the mode quality of the beam. Will a dounut mode give you the same reading as a TEM 00 or high quality mode. A star shaped mode giving out the same results on a digital power meter as a multi-mode output? I know all things being equal power wise, I would rather engrave with a doughnut mode output from a laser than a perfect high order mode. And I would rather have a high order mode than a star shaped mode for cutting.

[Dan Hintz]

The measurement is obviously done at the highest power and at the highest ppi. For all intents and purposes, these are CW systems... pulsed mode operation is only really relevant in the case of fiber systems where peak power in time-limited bursts are the major mode of operation. Our cartridges are measured as average power, not peak. the meter measures over several seconds, so a 5kHz pulse train is irrelevant.

All of the lasers we're working with here (I'm ignoring the kW systems a rare few here have) at TEM00, not that that makes any difference whatsoever to what we're measuring considering the entire beam is encapsulated within the target plate, so I'm not even sure why you would mention it. None of it serves a purpose other than to confuse the issue at hand.

Doughnut profiles are typically created using a TEM00 profile, again, as if it matters. Why you would choose to use a doughnut profile that spreads out the power when a Gaussian TEM00 profile is generally preferable for the focusing needed with our typical product? If I were processing certain items (Cermark, for example), I would prefer a TEM01, and I won't even get into polarization of the beam here (but again, for most of the stuff we do, circular would be my choice if for no other reason than the kerf would be symmetrical on all cut lines, regardless of their orientation).

[doug king]

http://www.hayabusa.org/forum/images/smilies/ikf/fight.gif

[Kevin Groenke]

I'll leave the technical debate to the experts.

I ebay'd a power wizard last year and find it to be a very useful tool for troubleshooting problems. In ~ a minute I can simply determine whether or not the output is consistent with what is was last time I checked.

Thus far it has basically confirmed that our tubes are ok and that problems are associated with alignment, optics, focus, etc... issues.

If anybody in the twincities metro area ever wants to borrow, drop me a line.

kg

[Rodne Gold]

Our power wizard seems quite accurate on both synrad and coherent tubes
As per Kevin's case , Its pretty good to check alignment and optics as we take readings at differing points in the beam path and over the table.
It's more useful as a "change" tool than an absolute tool...much like a bathroom scale , it may not tell you your exact mass , but will let you know if you lost or gained weight.
We have 6 lasers so a power wizard is justified as we send the same file to each laser and have adjustable compensation factors re speed and power for each and thus each laser should engrave identically ...for a single machine , Im not sure whether I would bother as its more obvious when things go awry or when the tube is going.

[Dan Hintz]

For a single machine , Im not sure whether I would bother as its more obvious when things go awry or when the tube is going.
Just as you have compensation tables, I'm attempting to put together a more accurate power table for my machine. Things are slow, but steady, right now, and I will eventually be getting a second machine. Some materials I'm working with (e.g., anodized Al) are quite sensitive to power changes, so I'd like to keep track of how my power changes over the months to adjust accordingly. This would also allow me to (hopefully) add in my own compensation factor should I upgrade or recharge a tube down the line.

[Richard Rumancik]

I have used the Synrad power meter (on a Synrad tube) and it seems to work fine.


. . . A lower PPI setting might allow a huge spike of power early in the pulse, then have the power drop down towards the end of the pulse. . . . If your power meter is measuring the energy over a period of time similar to the pulse duration, then you might see the real output power of the laser. If the "sampling" time of the digital electronics are not very similar to the output of the laser pulse, you might see almost any results on the digital display. That is one reason we use a thermal energy measuring device. Not too fast, but better for long term average in my opinion.

Rob, I'm not sure what you are saying here - the Synrad device IS a thermal device. It does not measure laser power in "real time" i.e. display a power measurement on a millisecond-by-millisecond basis. I think it uses a 6 second interval or so to accumulate the beam energy - then it calculates laser power from the temperature rise of the metal disk.

The device is "smart" in that you don't have to cool it down to a specific temperature before the start of the test. It knows the start temp of the disk, and it's cooling rate. So on the second test, it accounts for the different initial disk temperature in determining the 2nd power measurement.

I don't own one, but as I recall I put the system into "test mode" as if I were doing a beam alignment. So I manually turned on the beam (CW) for x seconds and got a reading. Now if I wanted to get a reading with a specific PPI set, I don't know how I would actually do that. I am not sure if that is possible, or if the reading would even be very useful. I suppose you could cause the laser to draw a spiral on the metal disk at a specific ppi but Synrad says not to move the device or beam while data is being collected. (BTW the beam must be UNFOCUSSED off the collector when doing a measurement - so you don't burn a hole in it.)

Since it is a thermal system I can't think of any reason why it would not work the same with any laser (not just Synrad) - heat is heat.

I have read about a "real time" measurement system on a laser - of course, it still has to average the power over a reasonable length of time to make any sense. This method diverted some percentage of the beam into the measuring device. So unfortunately you lose that % of energy all the time. On low power systems we can't afford that. But I am waiting for a self-calibrating laser system where it can check the power output in the 4 corners of the table on start-up, and show the readings on the display.

Dan, whether the device is suitable for making calibration tables or not, I can't say. I wish that my settings were more repeatable for repeat jobs, but I usually have to play around to verify the settings and adjust as needed. There are lots of reasons - laser tube temperature, how long it has been on, alignment, mirror cleanliness, lens cleanliness and condition, humidity etc. It would be nice if we could dial in a power and speed, like the CNC router guys enter their speed & feed, and expect the same results as last time. But lasers are a lot more fickle unfortunately. The power meter will sure help you find out what has changed but I tend to agree with Rodne that it is more useful as a "relative" measurement than absolute.

I'd buy one if they weren't so expensive. . . .

[Dan Hintz]

The meter arrived yesterday. Tracking said it was here Friday afternoon, but the package was no where to be found. I spent some time talking to the postman yesterday, talking about the weather, workload, etc. and I asked if he had seen anything like what I described...nope. While he was filling in the other boxes and I was lamenting not receiving such an important package, he pulled a small package out of some else's box and asked if this was it? Seems the temp postmen they hired for the holidays aren't what they're cracked up to be.

My hand is the equivalent of a 2.7W laser :D

[Scott Shepherd]

My hand is the equivalent of a 2.7W laser :D

Careful where you put it :eek:

[Dan Hintz]

Careful where you put it :eek:
If I hold it on something long enough, I'm hoping I can get it to smolder... Amy just sizzles a little. The real trick is getting it to focus... I think it's a 2" :p