04 – Fiber Laser Colour Marking (25:47)

0:00well welcome to another fiber laser learning lab

0:02as I promised you last time

0:05we’re going to look into one of the what

0:09I would call tricks of this type of

0:11laser machine which is a mopa fiber laser machine

0:15now I’m told that you

0:18cannot perform this trick with a

0:20q-switch laser and certainly can’t

0:22perform it with a constant wave laser so

0:25we’re going to look into the secrets

0:27behind this trick as I will call it that

0:31this machine can perform to see if we

0:35can understand what the mechanism is and

0:38whether or not you might be able to do

0:41it with the q-switch laser now I suspect

0:42that many people ahead of me have tried

0:44to do it on a q-switched laser or even

0:46on a constant power laser like that but

0:48I think you’ll get to understand that I

0:50don’t accept that the round world is the

0:52best wheel for instance a square wheel

0:54it’s got some great properties you don’t

0:57need a handbrake in your car and it

1:00discourages theft so I’m gonna go out

1:03and explore all the possibilities

1:05although we want to get to a conclusion

1:07on this subject

1:08I’m afraid the conclusion will not be in

1:10this or any session soon because I can’t

1:14foresee that as I start progressing

1:17through this I shall get pulled off into

1:19all sorts of different side directions

1:21because I need to understand various

1:23other features that will go to finish up

1:26building the final picture so let’s get

1:30started now here I’ve got a piece of

1:34very thin stainless steel it’s

1:35point-five of a millimeter thick and

1:37it’s got a nice shiny surface to it and

1:40just here we’ve got blowing

1:44throw them behind there first of all

1:48you’ll see the metal distorting

1:51[Music]

1:55and then hopefully if I can control on

1:57the temperature right don’t get it too

1:59close

2:00I want it happen slowly can you see the

Transcript for Fiber Laser Colour Marking (Cont…)

2:03way the color is changing on the surface

2:05there are now hold it a little bit

2:07closer and keep it there

2:13[Music]

2:18now you can see look we’ve got it so hot

2:21is glowing red-hot

2:27if you watch carefully we’ll see that

2:28the colors are growing around the

2:32outside

2:34about those color since I was a very

2:37very young engineer because that was one

2:39of the first things they teach you when

2:40you’re in the training school how to

2:42harden metal using these colors as what

2:45they call tempering colors now these are

2:48basically colors of oxide that are

2:51formed when you heat metal up but

2:54they’re not colors in the sense that you

2:56normally see colors now when we look at

2:59that red fire extinguisher there we see

3:03it’s red because we’ve got white

3:06daylight shining on it and white light

3:12is composed of all the colors of the

3:14rainbow but basically what’s happening

3:18here is all the colors that are not red

3:21are being absorbed by the paint pigment

3:26that’s on the outside of that container

3:28and it’s only the red that’s being

3:31reflected like a mirror back at us so we

3:35normally see color by what I would class

3:38is subtractive absorption we only see

3:42the colors that are reflected off the

3:43surface we don’t see the colors that

3:45have been absorbed by the surface of the

3:47material now these are always nice clean

3:50crisp clear colors that we see so when

3:53we look at these colors here they’re not

3:55they’re nothing like normal absorption

3:59colors because these are created in a

Transcript for Fiber Laser Colour Marking (Cont…)

4:02completely different way but you can

4:05never get a pure rate a pure blue or

4:08appear green from this method of

4:11producing colors and this is the method

4:14that the mopa laser uses to produce

4:17colors it’s this technique here of

4:19oxidation on the surface of stainless

4:23steel particularly it can be done as I

4:25understand it on titanium I can’t prove

4:27on titanium because I don’t have any no

4:30in daylight out here virtually however I

4:33hold this whichever angle I hold this at

4:36we’ve got a nice mix of colors there are

4:39no pure colors there there are just

4:41these mixture of hmm interesting colors

4:45sort of

4:47and bright teal blue and golden yellow

4:51and a honey color now look if I twist

4:54this what we’re looking at there is an

4:58image of daylight from outside and

5:02that’s the image from inside under

5:04artificial LED light these are not real

5:07colors these are strange colors that are

5:10caused by some very interesting

5:13phenomena of light now here is a sample

5:16test piece given to me by Lotus later to

5:19show me what the Mopa type of laser can

5:22do the q-switch laser cannot do produce

5:26all these lovely colors look at that

5:28lovely green there and in daylight hello

5:33where’s the green gun we have all the

5:36colors gone so provided we get the light

5:43in the right direction we can have the

5:47colors so here we’ve got under our LED

5:51light and if I get the angle of the

5:53light right maybe some of the colors

5:56well you can just about get the green

5:58they’re provided look I provided those

Transcript for Fiber Laser Colour Marking (Cont…)

6:00shine the light on it right I can get

6:02the green to show otherwise it’s not

6:05green at all although this is one of the

6:07big selling points that they point out

6:10to you for a mopa laser hmm so first

6:15off let’s go and see how these pseudo or

6:17false colors are produced well we’re

6:20gonna start off with the electromagnetic

6:22spectrum

6:22courtesy of Penn State College of earth

6:25and mineral sciences which owns you

6:27which is where I found this very nice

6:29diagram running from cosmic rays very

6:32very short wavelength through the

6:34ultraviolet then the bit that we can see

6:38and then the infrared where we’re

6:40working normally we work with our ten

6:42point six somewhere here but now we’re

6:44working pretty close to the visible

6:47spectrum this is only 750 nanometers and

6:51we are at about 1060 so we’re not much

6:56further up the scale than the visible

6:59red end of the spectrum but my real

7:01point of showing you this drawing is you

7:04can see that we’ve got a visible

7:06sensitivity in our eyes between 350

7:11nanometer wavelength light up to 750

7:15nanometer wavelength light and you can

7:16see the way in which the color spectrum

7:18changes now the colors that we’ve been

7:23seeing up to now are really probably not

7:26going to be found on this scale because

7:29they’re not pure colors now you’ll

7:33recall this image of me heating up the

7:35back side of a piece of stainless steel

7:39and here we’ve overheated it to such an

7:43extent that we’ve made it red red hot

7:47you’ll notice that the spectrum of

7:50colors that we’re creating are always on

7:54the outside of this area I mean it’s an

7:57indication very much like the the energy

Transcript for Fiber Laser Colour Marking (Cont…)

8:00density in the laser beam it’s probably

8:03following some sort of Gaussian

8:04distribution where we’ve got very little

8:07heat on the outside here because you’ve

8:10got conductivity of the material you’ve

8:13got a large radiation area down to air

8:15temperature normal air temperature and

8:18so the heat within this material is very

8:22quickly dissipating and there’s it’ll be

8:24warm out here but not hot now what sort

8:29of temperatures are we talking about

8:30here that’s really what we’re interested

8:32in what about the red hotness to start

8:35with there’s a scale of color which has

8:40been devised and I would guess that we

8:43not right in the middle there that

8:46possibly could be classed as bright

8:48cherry red around this area here it’s

8:51red to dull red so possibly we might be

8:56up at 800 degrees C in the middle here

9:00let’s see what happens when we take the

9:02heat away

9:05the answer is that so

9:08that was the boundary almost about red

9:11hotness and within that boundary a very

9:14high heat

9:15look when we look at the oxide that’s

9:18been left behind on the surface it’s

9:21this tail blue going to a darker blue

9:25there and then merging to a sort of a

9:27move and then a brown and then a light

9:30straw

9:30well there are temperature definitions

9:32for each one of these colors so for

9:34instance the outside edge they’re called

9:36faint yellow which is right out here 176

9:40degrees C light straw which is this

9:44secondary phase in here 205 degrees C

9:49it’s only 29 degrees C hotter but it

9:53changes from faint yellow to light straw

9:55and then we’ve got this area here which

9:57is classed as light blue blue and then

Transcript for Fiber Laser Colour Marking (Cont…)

10:00all this section in the middle here has

10:02classed as grey blue and we’re talking

10:05about 371 degrees C so let’s just put

10:10that on the chart there that’s somewhere

10:12around about there 371 degrees they’re

10:19interesting because look we’ve heated

10:21that all the way up to 800 degrees C and

10:23500 degrees C in the middle there as it

10:26cools down to 371 degrees C it creates

10:29this color so but the thing I want to

10:31emphasize here is we’ve created these

10:33colors with a blow lamp a very crude

10:36instrument but it does require these

10:41temperatures here up to 371 degrees C to

10:46produce these colors now as I explained

10:49to in the previous session temperature

10:51is only a function of the vibrational

10:55level of the material so 371 is not very

11:02hot the question is how do we control it

11:08how do we make it happen

11:10do we need long pulses I mean it takes

11:13takes a lot of energy to put that much

11:15into the surface now we might not be

11:20able to see any accurate colors on here

11:22but what we can see is that we’ve got

11:24very accurate control of the shapes so

11:26we haven’t got a lot of overheating and

11:29heat flow away from our marking area now

11:34in the previous session we talked about

11:37this material which is our iminium if

11:39you remember I said to you oxygen and

11:42metal will automatically form a an oxide

11:46layer on the surface it’s incredibly

11:48thin and it’s completely transparent as

11:50you can see here because this looks like

11:52raw aluminium but I know that it’s not

11:56that there is an oxide layer on that

11:58surface there now this shiny surface on

Transcript for Fiber Laser Colour Marking (Cont…)

12:04the stainless steel is even more

12:06sensitive to the advances of oxygen

12:10let’s just have a quick look and I’ll

12:12explain why stainless steel has got a

12:16very high chromium content 18 to 20

12:20percent oxygen and chromium are

12:22absolutely in love with each other so

12:25this has got a very thin layer of oxide

12:29on the surface it’s much thicker than

12:30the aluminium it’s probably something in

12:33the region of about two nanometers thick

12:35and you know what that is maybe six

12:38eight ten atoms thick and literally as

12:43soon as you damage the surface within a

12:46few nano seconds so I’m told it will

12:49heal itself which is why this material

12:52is stainless its self-healing and it’s

12:55continuously protecting the surface with

12:58this chromium oxide this very thin layer

13:00of chrome oxide on the surface as you

13:02can see it is transparent because we can

13:06see through to the shiny material

13:07underneath and that leads us to a

13:11phenomenon that you have seen many times

13:15before I give you the soap bubble or an

13:19oil film on a puddle yeah I think this

13:21is a very interesting composite

13:23that’s not real because if you take a

13:25look here we’ve got these three trees

13:27here and we’ve got these three trees

13:30here which are one of the same and

13:31they’ve just rotated the image to give

13:33you a nice general impression but what

13:35you have got here is all the typical

13:37colors that we’re seeing in here but

13:41these have had no heat in them at all so

13:45where do those colors come from now you

13:48might want to go and do your own

13:50research on this but I’m very quickly

13:53going to go through the thin film

13:56interference theory which is what we’re

Transcript for Fiber Laser Colour Marking (Cont…)

14:00talking about it’s a phenomenon

14:02associated with light and light waves

14:05very very crudely we have a light wave

14:08that comes in and strikes the surface of

14:12the film that’s on the top there and

14:16then we’ve got a certain film thickness

14:18here let’s call it T I’m not going to

14:21get too technical about this you can go

14:23and do your own reading on this subject

14:25thin film interference but generally not

14:28always but generally there is something

14:30called a phase reversal as the light

14:33hits the surface so it comes on for

14:36example let’s just call these +100 is

14:42obviously these point here and minus one

14:43we’ve got a complete mirror image of

14:46what came in so that’s that’s the first

14:49thing that happens we get this light

14:51coming in and reflected off the surface

14:54and then we’ve got this thin film here

14:57and at last as we can see we can see

15:00through that thin film to the material

15:02underneath so the light travels in to

15:05that thin film okay so it’s only one or

15:07two nanometers thick but that means the

15:11light is traveling further the colour

15:14remains the same but the speed of the

15:16light actually slows down and as it

15:18slows down so it changes its angle and

15:21instead of coming through and hitting

15:24the bottom just here as you might expect

15:26it hits the bottom here and then it

15:29bounces off the bottom again probably

15:31where the phase reversal comes out at

15:33the same

15:33as the reflection these light beams are

15:36no longer in phase look the top of this

15:39one is now no longer in line with the

15:42top of that one so the color that came

15:45in and the color that came out will be

15:49the same but it might not be as intense

15:52so you can get something called additive

15:55or destructive interference between

15:58these two beams now I’ve drawn them a

Transcript for Fiber Laser Colour Marking (Cont…)

16:00long way apart they’re not going to be a

16:02long way apart so the simple story that

16:04I want to get over to you here as we

16:05send one wave in and we get two waves

16:10out one off the top surface and one off

16:12the bottom surface and those waves

16:13interfere with each other and I’m now

16:17going to show you the effect of those

16:18two waves interfering with each other

16:20now I’m indebted to the guys that

16:22created this piece of software because

16:25it makes my life a lot easier in trying

16:27to explain light waves to you now waves

16:31are waves whether they’re light or sound

16:33they all look the same so what we should

16:36be doing is hearing color rather than

16:39seeing color in this demonstration now

16:43the annoying buzz in the background is a

16:45color because this is what a color looks

16:48like now the only difference between

16:50colors is this

16:55the length of the waves now that’s the

17:00read long wavelength frequency there’s

17:04there’s one wave on the screen that’s a

17:07long wave now if we double its frequency

17:11and we put two waves on the screen

17:14that’s a different color but even here

17:17it’s a different color but you can hear

17:18that it’s a lovely smooth color now that

17:22relates to all those colors that you see

17:24on the color chart the color range that

17:28our eyes can see

17:30every pure color has got this lovely

17:33sine wave shape to it now when we get

17:37two different colored lights mixing

17:39together like this

17:42they produce a single wave which is

17:45completely different it’s the sum of all

17:48these bits and pieces here

17:51so if we look here we’re going to get

17:53addition of the red and the blue and

17:55here we’re going to get subtraction of

17:57the red from the blue so we’re going to

Transcript for Fiber Laser Colour Marking (Cont…)

18:00get around a strange waveform and if we

18:02use this little slide here it’ll show us

18:04what happens when we mix the two

18:07together now in the thin-film picture I

18:11showed you that the first wave came out

18:13let’s call it the blue wave and then the

18:16second wave came out at a different

18:18point in a different phase so there was

18:20a shift between the two waves and that

18:23shift basically is going to cause a an

18:28interference pattern between the two now

18:31it’s not going to cause a color

18:32interference because they’re both the

18:34same color what it’s actually going to

18:37do if we listen very carefully

18:42at that point there they are 180 degrees

18:45out of phase the pluses of one wave

18:49cancel the minuses of another wave and

18:52the net result as we like move on this

18:54slider and have a look is zero so we

18:57have no color at all now we’ve got

18:59blackness so the phase shift and this is

19:06called destructive interference actually

19:08cancels the light out and as we get

19:12further and further interphase we get

19:15back to the original intensity of light

19:18so we’ve got phase shift to think about

19:21and we’ve got this mixing of different

19:24types of waves now there’s a further

19:27mixing effect which takes place and

19:29that’s when we get several colors

19:31nowhere we start changing the frequency

19:33of these

19:35[Music]

19:45now say what they net out us

19:54see that’s a really weird signal and

19:57that’s the sort of light you can hear

19:59that it’s not a pure tone and you can

Transcript for Fiber Laser Colour Marking (Cont…)

20:02see that it’s not a pure sine-wave it

20:04sounds strange waveform which gives

20:07these weird colors that we’re talking

20:09about it’s an interference effect

20:15and that interference effect changes as

20:18the thickness of the film changes

20:21because as the film thickness changes so

20:24the relationship between the wave going

20:26in and the wave coming out changes but

20:30of course we’re not just talking about a

20:31single color wave but much more complex

20:35than that we’ve got thousands of light

20:38waves of different frequencies entering

20:41and thousands of light waves of

20:43different frequencies exiting so we get

20:45a massive combination of all these

20:47frequencies and colors that are inter

20:49playing with each other hence these very

20:52strange

20:53non-standard colors I will call them

20:55they’re not colors that you’re likely to

20:57find find in the standard color chart

21:00when you look at that straight on now

21:03everything looks pretty dull and black

21:05but if I move the camera and move this

21:10into the light so you’re still looking

21:11at it straight on at a certain angle of

21:14incidence of light the colors appear so

21:19you’re still looking at it straight on

21:20but the light angle into the surface has

21:24changed so we’ve got light coming in

21:27here and we’ve got your eye there and

21:33depending on the relative difference

21:36between these two angles depends on the

21:37color that you see because it depends on

21:40the distance of T the thickness of the

21:43film and the lower the angle the longer

21:48the distance the more the interference

21:52between the two so all these little

21:56black patches and black things here when

21:58you catch them in the light right you

Transcript for Fiber Laser Colour Marking (Cont…)

22:00can make them either disappear or turn

22:02into pale gray now I suspect that some

22:06of these have been done with scans

22:08across this way and some of them been

22:10done with scans across that way so that

22:12depending on how the light catches them

22:13maybe they do or they don’t disappear so

22:18somewhat none of these colors on here

22:20are pigment colors they’re all

22:22refraction colors

22:24caused by this thin film interference

22:27effect yes we used a lot of heat to

22:32create these colors but we didn’t use

22:35any heat there we have to draw the

22:38conclusion that the only thing that the

22:40heat is doing is changing the thickness

22:43of this film so I’m afraid there is no

22:46magic pen that this machine has which

22:49paints different colors on the surface

22:51of stainless steel it doesn’t even

22:53require heat to produce these colors it

22:57produces it needs a thin transparent

23:00film we’re going to have to manipulate

23:03the thickness of this film to produce

23:05these non pigment colors but when we

23:10come to modifying the film thickness of

23:13stainless steel here it appears that we

23:18need to have certain temperatures to

23:20create these colors I’m not sure that we

23:23have to locally heat up to these

23:26temperatures to produce these colors it

23:28could well be that if we stimulate the

23:30surface we might be able to promote

23:32growth in stages

23:35sometimes you can’t crack the problem

23:37with logic and in this particular

23:39instance I felt hey it’s very simple if

23:43we fire the mopa laser at this surface

23:45here the Mopa has got the ability to

23:49dispense very very small precise amounts

23:52of light energy and that light energy

23:56will be stimulating the molecules in

Transcript for Fiber Laser Colour Marking (Cont…)

24:00here to higher and higher temperatures

24:02and those higher temperatures just local

24:04temperatures in this area here are going

24:07to if you like percolate out into the

24:10background material and heat the

24:12background material up by conduction

24:15okay before it dissipates away to

24:18nothing but having hated the background

24:20material up

24:21that’s an improved environment for the

24:25combination of oxygen and chromium

24:28because adding heat to any chemical

24:31reaction speeds it up but hey the film

24:35is chromium oxide so it’s already got

24:37its oxygen

24:38so I’m a little bit puzzled as back as

24:41to whether or not we’re going to be able

24:44to promote growth on to the surface of

24:46the thickness or whether we’re going to

24:49be trying to promote growth into the

24:52material maybe just heating this little

24:55area up throws everything into confusion

24:57and we can actually get below the film I

25:01say maybe I told you before I’m not a

25:03chemist and I’m not a physicist so I

25:05don’t understand exactly how this

25:07mechanism works but what I do know is

25:09we’re going to have to change the film

25:11thickness to change the interference

25:13patterns between the incoming and the

25:15outgoing light so in the next session

25:18we’re going to get dragged off onto one

25:20of these tangents that I mentioned

25:21earlier and we’re going to have to

25:23investigate the actual power of pulses

25:26that we have on this machine we’re gonna

25:29have to look at ways in which we might

25:31be able to find a strategy for

25:34thickening this film I could go and ask

25:37Lotus laser but I’m not going to so on

25:41that stubborn note I’ll say cheerio to

25:44you and we’ll catch up in the next

25:45session

Transcript for Fiber Laser Colour Marking