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
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