## The Fiber Laser Learning Lab Series with Russ Sadler

In this Series, Lotus Laser have lent Russ a MOPA 20 watt fiber laser to “play with”. Although Russ has a moderate understanding of laser technology (his words) and how constant power glass tube systems work, pulsing fiber laser marking machines are shrouded in a deeper mystery than the glass tube machines. So let’s learn more about Fiber Laser MOPA

They have been designed for high speed marking and the technology has been well tried and proven. There are limited “tricks” that the pulsing laser technology can perform. You enter predefined parameters for each marking “trick” you wish the machine to deliver , then stand back in amazement. Most correspondents tell Russ that they have bought their machine direct from China and received a machine and EZCAD software, preloaded with a few default parameters. No other instructions beyond the EZCAD manual are forthcoming.

Russ states “I am neither a teacher or expert in this field so you join me in my learning adventure with the warning that I have a simple but inquisitive mind and will probably make mistakes on my way to discovering the truth. I WILL oversimplify and maybe distort the scientific detail in my quest to build a simple picture of why and how this technology works. I am not trying to reverse engineer anything, just to break through the seemingly impenetrable ‘techno cotton wool’ that surrounds this amazing piece of science.”

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## Transcript for Fiber Laser MOPA Matrix Test Leads to Understanding

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0:00welcome to another five ELISA learning

0:03laughs today we’re going to carry on a

0:05little bit more with the mobile Acer

0:07designed because I had a problem last

0:12time if you remember and I couldn’t find

0:14out very much information about the

0:17actual pumped laces that were used with

0:20this system and to be honest I wasn’t

0:22particularly worried because I didn’t

0:24think it was relevant

0:25now as I mentioned to you before I was

0:28presented with this list of numbers here

0:30and these charts for all the 16 variable

0:34pulses that this machine can work with

0:37now this is a 20 watt machine and some

0:41of these pulses can deliver up to

0:44something like about 12 kilowatts or

0:46down here around about 7 or 8 kilowatts

0:49peak power sounds amazing

0:53I’ve got no concept of what damage these

0:57shapes these pulses can make to material

1:00because look we’ve got a pulse here

1:02which is only 2 nanoseconds wide that’s

1:05to billions of a second what damage can

1:0912 kilowatts for 2 billionths of a

1:11second to do this machine does not

1:14produce 12 kilowatts continuously it can

1:17only deal with it in dot dot dots spots

1:20and those spots are only going to be as

1:24good as the lens can produce and the

1:27spot size on here is 0.065 of a

1:30millimeter 65 microns I thought I would

1:33do a very quick test to get some sort of

1:38concept of what these numbers actually

1:40mean so I jumped onto this locust mark

1:42software otherwise known as easy cared

1:45and produce this matrix of the 16 pulses

1:50across here and down here I’ve chosen 14

1:52different frequencies from a thousand

1:55two kilohertz down to 5 kilohertz so

1:59first of all I ran it a couple of times

Transcript for Fiber Laser MOPA Matrix Test Leads to Understanding (Cont…)

2:01on a piece of anodized black anodized

2:04aluminium and I think from where you’re

2:07looking

2:07you’ll quite clearly see that there is a

2:10lot more damage down here

2:13it’s difficult to see on aluminium the

2:16problem with aluminium is is a what I

2:19call a binary material it’s either black

2:22or white there’s nothing in between I’ve

2:27generated these shades of grey on here

2:30by virtue of some patterning that I’ve

2:33done and I will talk about that in a

2:35future session but instead of aluminium

2:37I chose to use a piece of stainless

2:40steel

2:40it might look from your point of view at

2:44the moment as though it’s it’s pretty

2:47brand we’ve got some stuff down here and

2:48then nothing else here’s an enlarged

2:50picture of the stainless steel obviously

2:52the background has now removed a

2:53reflectivity of it so it’s much easier

2:55to see now the thing about stainless

2:57steel is it is not a binary material as

3:01we heat it up it produces various oxide

3:04colors on the surface so there are

3:06shades that we can pick up now what you

3:09can’t see with some of these colors here

3:11is the variation in depth that you can

3:14probably see much clearer under a

3:16microscope but trust me basically what

3:18we’ve got is very little power here

3:20decreasing to almost nothing over here

3:23and no power at all down in this corner

3:27hang about I’ve got nothing down here

3:30what’s happened down here now this was

3:33my very simplified model of this machine

3:35the Mopar and in essence what we have

3:38we’ve got two fibers we’ve got one thin

3:42one and one thick one this one here is

3:44I’d call a signal conditioning fiber

3:46that produces the signal the pulses that

3:49we’re looking for and then they drop

3:51through to another fiber which amplifies

3:53the shape of that pulse to get an output

3:57now we went through clearly last time

4:00the way in which the laser actists this

Transcript for Fiber Laser MOPA Matrix Test Leads to Understanding (Cont…)

4:02type of laser the fiber laser works so

4:04I’ve drawn these pump lasers in blue

4:07these are laser diodes which are

4:09injecting photons into the cladding here

4:13and the photons are whizzing around in

4:16here and they’re colliding with these

4:18special ytterbium electrons which are

4:21sitting in this very thin core and so

4:24here we’ve got the seed

4:26leisa which is a laser diode which

4:29injects coherent beam and runs through

4:32here collecting as it goes more and more

4:34photons so the beam then passes through

4:37to cross to this amplification stage

4:39where these laser diodes have already

4:41injected photons into here and excited

4:44this core up to its maximum energy

4:47potential now that was the way that

4:50which I was expecting it to work I did

4:53have a problem as you can remember

4:54saying I couldn’t quite work out how

4:57these laser diodes were working if you

5:00assume that these laser diodes are on

5:02all the time which is what I did so

5:06let’s take this example here of eight

5:07nanoseconds and 252 kilohertz so every

5:13cycle takes four microseconds and four

5:16microseconds is 4000 nanoseconds so we

5:23want an 8 nanosecond pulse but I’ve got

5:264000 nanoseconds ahead of that pulse in

5:30which to charge up these in a cause I

5:34was expected to see pulses all the way

5:36down to 0 almost where the 8 nanoseconds

5:40is that line and the 250 I haven’t got

5:46250 because between 200 and 300 so it’s

5:49some way across there all right so it’s

5:52between these two so basically it’s

5:55there is my maximum power and if we take

6:00a look at the colors we can clearly see

Transcript for Fiber Laser MOPA Matrix Test Leads to Understanding (Cont…)

6:03that we’ve got maximum power there

6:05because it’s fainter there and it drops

6:07off there but hang about it drops off

6:09there and then there’s world is just a

6:11hint of something there and then there’s

6:13nothing why is there nothing because

6:15this this model that I’d produced

6:18predicted that I would have this density

6:22all the way down to the bottom that was

6:24my problem all these are missing down

6:27here for a very good reason and the

6:30missing part of the puzzle was all to do

6:32with these blue pomp diodes how they

6:35work

6:36and I had to go back to Lotus later

6:39where their expertise was able to

6:43furnish me with the details

6:45the secret is these do not run at

6:51constant power they run at variable

6:55powers wind and just before the seed

6:59laser is injected into the system

7:02obviously the less power that we feed to

7:04these diodes and the less photons and

7:07the slower the excitation rate of the

7:09electrons in that core because it turns

7:12out that to put too much power in here

7:14for too long is not good for the

7:18lifetime of the core material okay this

7:21diagram will help rather than hinder

7:22your understanding of the process here

7:25we’re showing the frequency scale from

7:26zero to a thousand kilohertz the period

7:28between the pulses is getting longer and

7:30longer and longer as the frequency gets

7:33lower and lower and that’s quite

7:35important because here we can see the

7:37blue pomp diode intensity with this line

7:40and along the top here and the intensity

7:43is increasing with frequency and then it

7:47stays level

7:48my assumption with my model was that it

7:51was level from the start and that was

7:53where my failure occurred it takes but a

7:57few nanoseconds to promote enough

7:59electrons in here up to their excited

Transcript for Fiber Laser MOPA Matrix Test Leads to Understanding (Cont…)

8:01level so we certainly don’t need 4000

8:06nanoseconds here to build up energy for

8:11an 8 nanosecond pulse hence the reason

8:13why they start off at 0 and gradually

8:16build you up towards the point where we

8:19are going to discharge a 8 nanosecond

8:21pulse and at that point there we’ve got

8:2320 watts of energy stored in the fiber

8:26it’s been arranged such that we get our

8:29peak power at 8 nanoseconds and after 8

8:32nanoseconds the power starts to drop off

8:35now why does it drop off well the power

8:37doesn’t drop off at all there’s still 20

8:39watts all the way down this line is just

8:43that the pulse repetition rate is

8:45increasing and what that basically means

8:48is let’s assume we had a cake

8:49one cake equals 20 watts and we’re going

8:53to cut the cake into 250 slices when we

8:57get to this end we’re going to cut the

8:59cake into a thousand slices so the

9:02slices at this end are much smaller than

9:04they are at this end we’ve still got a

9:06cake but what basically that means we’ve

9:10got less power

9:11per pulse as we get to the higher

9:14frequencies when we look at our eight

9:17known a second pulse here at 250

9:19kilohertz we can see that we’ve got peak

9:23power but they’re both black just either

9:25side of the ideal which is in the middle

9:29there so there we are at a hundred

9:31kilohertz and at 100 kilohertz we come

9:35up here and we find that we’ve only got

9:37a very small amount of power power

9:40roughly let’s just guess the same as we

9:43would have if we were at 700 and 700 is

9:48about here and yeah there’s not a lot of

9:53difference between those two the point

9:55being that we’ve got the decreasing

9:58power down here and the further we come

### Transcript for Fiber Laser MOPA Matrix Test Leads to Understanding (Cont…)

10:02down of the frequency range the less

10:04power we shall have to recharge the

10:07fiber and less power means we shall have

10:09no pulses now this graph showing its

10:12decrease in recharging ability as we get

10:15to the lower frequencies clearly

10:17explains why we haven’t got anything

10:21down here and conversely when we start

10:25increasing the frequency up here the

10:27power starts dropping off so as we go up

10:30the frequency chart the power is clearly

10:33dropping off so all of a sudden

10:36everything is explained it’s all nice

10:39and neat and tidy and we understand how

10:43the motor laser works so now we found

10:47another parameter which we can play with

10:49ie

10:50frequency which will allow us to control

10:52the power in the pulse beyond the peak

10:56power values that are set

10:58I think we could also play with this but

11:01this is a much more uncontrollable

11:03section down below that peak power I

11:07think we stand much more chance of using

11:10this part of the curve it does add to

11:13the pool of variables yet something else

11:15that we can use to change the energy

11:18density at the surface of a piece of

11:21material this machine only produces dots

11:24and we only damage the surface by

11:27converting the light energy into heat

11:30energy within the atomic structure of

11:32the material we’ve got all these

11:33variables to play with to caused that

11:36heating effect so within the program the

11:39software program we have the opportunity

11:41to change the percentage power well I

11:44don’t know quite what that’s doing I’ve

11:46got an idea that and that might actually

11:48be changing the power of the pump basis

11:50so that we can regulate the if you like

11:54the amount of energy it might be doing

11:56exactly the same as this when we

11:58increase the frequency if it is then

Transcript for Fiber Laser MOPA Matrix Test Leads to Understanding (Cont…)

12:01we’ve got two ways of changing the power

12:04per pulse or the energy per pulse one of

12:07them is with a percent power button and

12:09the other one is with thee if you like

12:11the pulse repetition rate which is in

12:13fact the frequency and although we’re

12:15changing this frequency we’ve also

12:17changing the frequency of a specific

12:20pulse that we’ve decided to use now

12:24after percent power we’ve got 16 pulse

12:27durations there the question I don’t

12:28know is whether or not if I choose a 15

12:32nanosecond pull will it either drop back

12:35to a 13 nanosecond pulse or will it go

12:37out to a 20 nanosecond pulse or will

12:40actually respond to something in between

12:42maybe I’ll find out directly from Lotus

12:45later so for each one of these specified

12:48pulses there is a maximum power or a

12:50maximum amount of energy but in addition

12:53to that we’ve also got other

12:55opportunities as we’re finding out by

12:57playing with the power to change the

12:58energy per pulse so we’ve got the

13:01opportunity of playing with the pulse

13:03energy now the other thing that can

13:05dramatically change things is speed now

13:09unlike the continuous

13:12beam laser we are pulsing on this

13:14machine and that gives us a completely

13:17different set of circumstances I’ve got

13:20a line which is not 0.1 millimeters long

13:23and I’m going to run a thousand

13:26millimeters a second

13:28along that line so that’s going to take

13:32roughly a hundred microseconds to scan

13:36that at a thousand millimeters a second

13:39if I’m using the two nanosecond pulse at

13:43eight hundred and fifty kilohertz which

13:47is the top one on this chart then

13:51basically here’s what I’m going to get

13:54I’m going to get a huge number of pulses

13:57along that point one of a millimetre

Transcript for Fiber Laser MOPA Matrix Test Leads to Understanding (Cont…)

14:00line I’m going to get a huge energy

14:03density in the area because I’m going

14:06over the same spot again and again and

14:09again and again now this is where

14:12something else comes into play there

14:16haven’t got my decent piece of paper

14:17here at the moment but if we take a

14:19quick look at the relationship between

14:21the pulse length and the repetition rate

14:24which is the frequency if I just round

14:28that up to a thousand for the sake of

14:31argument I’ve got one microsecond per

14:34cycle and I’ve got two nanoseconds of

14:39power within that cycle so that’s two

14:43nanoseconds per thousand nanoseconds

14:47which is a ratio of 500 to one or

14:51basically 498 of the power being off and

14:55one of it being on so there’s a heating

14:58to cooling ratio of almost 500 to one so

15:02there’s a 500 delay time to allow the

15:06heat to cool down and if we take a look

15:09down this set of numbers here we shall

15:11find that it drops down to as little as

15:14114 to 1 at the bottom here for these

15:17longer pulses so there’s another factor

15:20there which is could be a very important

15:22factor the fact that we haven’t got as

15:24much cooling to

15:25down here in other words we likely to

15:27build up the heat in the surface quicker

15:29the whole way in which his machine works

15:31is by creating heat in the surface so

15:34this I believe is a very important

15:36factor when it comes to assessing which

15:38of these we should choose to do certain

15:40types of job now I think probably people

15:43have been round and they’ve trolled all

15:44these numbers and they’ve put them in

15:46thousands of hours of testing and

15:48they’ve found things that work I haven’t

15:51got thousands of hours left in my

15:52lifetime so I’m not certainly going to

15:54do that I want to try and choose some

15:56factors which will give me the

15:58information quickly so speed I mean if I

Transcript for Fiber Laser MOPA Matrix Test Leads to Understanding (Cont…)

16:01run the same thing thousand millimetres

16:03a second but this time we’ll go to 350

16:07nanoseconds which is this one 350

16:11nanoseconds okay the heating and cooling

16:13ratio is different we’re not going to

16:16cool as quickly as the previous one and

16:18we’re going to probably have put more

16:19energy into it as well so the net effect

16:22should be greater but if we carry on

16:25running that at a thousand millimeters

16:27per second and we also run it at 25

16:30kilohertz because the pulse rate is so

16:33much slower what’s going to happen is

16:35we’re going to space these out and we’re

16:37not going to get as much of a heating

16:38effect so we’re certainly probably not

16:40going to get as much power into the

16:42surface this way as if we were to do it

16:45this way I don’t think but I don’t know

16:48that’s what we’re gonna have to try and

16:49find out so that’s how speed is going to

16:53affect us so as well as speed we’ve got

16:57scan patterns now there are several

17:00ideas for the scan pattern number one is

17:04we could just scan scan scan scan and

17:09put the lines beside each other and

17:12accept that is the heating effect for

17:14there and now we’ve got another heating

17:16effect here and another heating effect

17:18here or we can overlap them so for

17:21instance if this is a point O three spot

17:23on here if I shift it down by 0.01 five

17:27pitch then I will double up on that half

17:31of the heating and put a new set here

17:34and then when I put another line down

17:35that will get doubled up so they

17:38a different energy density if I choose

17:41to use a line spacing of 0.03 or 0.015

17:46there are other things in scaled pattern

17:49which the software allows us to do as

17:52well as this overlapping pattern we can

17:57scan in one direction only the other way

17:59is to scan and come back scan and you go

Transcript for Fiber Laser MOPA Matrix Test Leads to Understanding (Cont…)

18:04back scan like that so we’ve got

18:07uni-directional

18:08or bi-directional scanning there’s

18:10another option as well and that is to

18:12send the dot in a circular motion in

18:16other words we start off with our dot

18:18there but we can do this with it and we

18:22can drive the dot round in a circular

18:24pattern to produce this overlapping

18:29high-density picture where we want high

18:32energy density into the surface to get

18:35the temperature up to do certain amount

18:37of damage then we’ve got the beam

18:44profile now this is an interesting one

18:46because they make a big thing about this

18:49machine having a Gaussian energy profile

18:56to the beam which means we’ve got a lot

18:58more energy in the center of the beam

18:59then we have at the outside edge of the

19:02beam which means that we should be

19:04cutting grooves in the material

19:08effectively which look like that so

19:10there’s the surface of material if we

19:12decide to scan across we’re likely to be

19:15putting grooves in which look like this

19:17now there are other beam patterns that

19:21could be chosen that would produce a

19:23much flatter more uniform shape rather

19:28than a Gaussian you could have something

19:30like that which which would produce a

19:32much shallower more even heating effect

19:35I don’t know whether that’s what we need

19:37or not but it is another variable

19:41so that’s beam profile and then of

19:44course finally we’ve got things like

19:46focus

19:48that’s a very emotive subject in some

19:52people’s eyes focus is focus and you

19:55must stay with the focus but I’m here to

19:59break the rules I’m very happy to run

### Transcript for Fiber Laser MOPA Matrix Test Leads to Understanding (Cont…)

20:01this machine at whatever is required

20:03because as you change the focus you’re

20:05actually going to change the energy

20:07density in the in the little pool as we

20:10as we Inc as we change the focus and I

20:13going to change the size of the beam

20:17that’s a subject for a future session

20:19and then of course finally you’ve got

20:22material every material will have a

20:28different set of parameters is a year

20:31going to be long enough to try and find

20:32out how this machine works well on that

20:35note it sounds as anger to be spending

20:36more time with this machine than I am

20:38with my wife and I think that just about

20:40rounds off this second session and gives

20:43us a bit of a handle now a real handle

20:45on what this machine can do and the

20:48possible problems that we’re going to

20:49encounter in the future with all these

20:51variables so thank you very much for

20:54your time and I’ll catch up with you in

20:56the next session