Number 02 – RDWorks Learning Lab 215 – AT LAST!!! I understand How Laser Lenses Cut.

0:00welcome to another rdworks learning lab today we’re going to carry on with our

0:07session about gallium arsenide benzes now in the last session we saw that

0:12a one and a half inch gallium arsenide lens had some really strange properties it looked as though

0:18there was a focal point buried underneath the existing focal point maybe five millimeters down

0:25below the focal point are we going to see the same sort of characteristic or properties when we test a two inch

0:32and a two and a half inch now as i was editing the last video it did cross my mind that i was using

0:38mdf as a means of finding the focal point for this lens i wondered whether

0:46in fact i should be using acrylic because hey that was the material i was

0:52testing so what i’ve done is to very quickly run the same sorts of tests again with

0:59acrylic and let’s just have a quick look last session we were using this 38 millimeter lens

1:05and we set it up like this so it had a 28 millimeter gap

1:10i changed it to 24 millimeters so that we can carry out this focus test and sure enough 24 25

1:1726 27 28 is the thinnest line so the focus came out absolutely spot-on

1:23but when we did the penetration tests maybe if i’d have done my focus test on acrylic

1:29it would come out differently did i get it wrong with the focus we’re going to carry out this focus test

1:35at various speeds and powers to see if we can

1:41get an idea of what’s happening to the focus the first test is what we were using

1:47originally which is 15 power this time we’re going to speed up to 200 millimeters a second which is

1:53engraving speed and i’m purposely overlapping

Transcript for How Laser Lenses Cut (Cont…)

2:01the test with the edge so that i can see the depth of cut as it runs onto the

2:08edge we’ll look at these under the microscope in a minute 15

2:14200 millimeters a second

2:22so that’s 15 [Music] at 20 millimeters a second

2:32so that’s 99 at 20 millimeters a second

2:38right so we’re going to go really slow now with 100 power i don’t think we should burn right

2:44through but we might get pretty close

2:53[Applause]

3:01we need to go and have a look at those results under the microscope to see if we can work out what on earth is going

3:07on perhaps this will give us some clues as to where the focal point is on this lens

3:13so here’s our standard test at 15 power and 20 millimeters a second so if

3:18you remember we set it up to 24 24 25 26

3:2327 plus the 10 that was buried inside the lens which is 37 millimeter focus

3:30and then it start going out of focus again okay now let’s see what happens when we

3:35run at 200 millimeters a second you can hardly see it here but there is a very faint line here

3:43because the beam is four millimeters out of focus into the material so that’s 24 25

3:5026 we’ve just got a little bit of a hump there and then 27 looks as though we’ve got

3:56more and let’s see what 28 is like [Music]

Transcript for How Laser Lenses Cut (Cont…)

4:0628 looks as though it’s getting shallower again and 29 has nearly disappeared so again at 200 millimeters a second

4:14at 27 millimeters we’ve still got the same intensity focus point i mean the actual

4:20intensity is a lot less because we’ve run faster and the power has been spread out more

4:26this is 20 millimeters a second but a hundred percent power maximum power now you can see here

4:33we’ve now achieved the best focus we first of all we’ve got a very narrow

4:40cut and secondly we’ve got quite a deep cut you can see that that cut is deeper than

4:45this one 24 25 26 27 the actual position

4:53where the intensity of the focus has not changed it’s remained at 27 millimeters

5:00and then it starts disappearing again and getting bigger and bigger and bigger as we go

5:08past the focal point and start drawing the beam out this is what’s happening after the focal

5:15point you can see it’s growing pretty quickly so now we’ve slowed the beam right down

5:20there’s no obvious focus detectable by the depth of cut

5:25we’re running very very slowly now at five millimeters a second with a hundred percent power and we’re seeing the same sort of effect

5:33that we saw with our spike test look we’ve got powerful beams coming in here

5:40which are passing through what looks like a focal point here at maybe two or three millimeters into the job

5:46now as we raise the focal point up we can see that we’re also raising this focal point up as well to

5:53probably close to one millimeter and we’re getting a wider neck

5:58we’re getting slightly less erosion on the inside here and then here we

Transcript for How Laser Lenses Cut (Cont…)

6:05nearly got the focus right out to the surface again and as we put the focus out to the

6:12surface we’ve got a little bit less erosion on the inside minimum intensity at the surface is

6:18occurring just here that’s the smallest net beam intensity

6:26and here we’re starting to get bigger again as the beam moves out this is what’s happening below

6:32the focal point it’s getting very messy 24 25 26 27 millimeters

6:40is still the smallest entry point so it appears that speed or power

6:48does not change the effective intensity focus it remains at

6:54approximately the same distance away from the lens well time is passing

7:00and i’m sure i’m getting grayer since the last time i spoke to you a few days have passed i’ve been

7:07puzzling over these little test pieces that we made now since we checked

7:13that the focal point for this material appears to be 37 millimeters and not 38 millimeters

7:21as per the lens focal point i’ve been back and i’ve redone

7:28these little tests setting the focal point on the surface at 37 millimeters

7:35and also five millimeters into the surface as well so that i’ve got some

7:40comparisons because i’ve got tremendous faith in this material acrylic

7:47just like a young child that doesn’t know how to lie this is exactly the same it’s going to

7:53tell me what’s going on although this stuff doesn’t tell lies sometimes it’s difficult to read what

Transcript for How Laser Lenses Cut (Cont…)

8:00it’s actually saying one of the puzzles that i’m trying to solve is what is this ballooning

8:08effect that happens inside here it doesn’t happen inside a four inch

8:14lens but it is definitely happening inside this one and a half inch lens several

8:20possibilities exist i think that it’s most likely

8:25sideways beams coming in and crossing over and scouring out

8:30beams that are very very slightly off axis but on the other hand it’s been

8:36suggested to me that it’s probably scouring

8:41caused by heated gases inside the enclosed tube

8:47if the gas is getting that hot inside there then it’s certainly a viable proposition

8:54that we could be heating the gas up and that gas the longer it exists in

9:00that tube could be scouring away the inside of the tube away from what the effect of the laser

9:07beam actually are and i think i found a way of testing and solving that problem now i’m

9:14not going to bore you with all the details i’ve set all the machine up manually so

9:20that i’ve got a 300 millisecond pulse and it’s been set 37 millimeters above

9:26the surface so each one of these little marks along here has been done at the focal point on the surface okay

9:33and then what we’re trying to do is to analyze what’s happened to each one of these you said but they’re all the same well

9:40no they’re not because what i’ve done i’ve started off

9:46maybe two millimeters inside the edge of the material and i’ve skewed the material so that as

9:53i step across the material i’m going to start scanning down and at some stage and i didn’t know at

Transcript for How Laser Lenses Cut (Cont…)

10:00which stage i’m going to start producing half a hole and that will tell me

10:05whether or not what we’re seeing is a trapped gas situation and we need to go and look at

10:11that under the microscope to get the answer to that question now there’s the view that the laser beam

10:17sees and as you can see we’ve pierced into the top there and we’re gradually creeping towards the

10:23edge of the material because i set the piece of material across at an angle well we can clearly see that all these

10:29are exactly the same and then if i change the light a little bit

10:34what we’ve got the neck just in here is within the material and then it

10:40breaks out of the material just here so there is no possibility of

10:46captive gases in this column here and yet it’s still ballooned and this

10:53last one which is a perfect section still showing all the signs of that

10:58ballooning so it must be rays that are almost parallel to the axes of the beam

11:05that are coming in here and scouring away the edge of this so that’s one problem solved but now

11:10what i’ve got to do is to try and decode what the rest

11:15of these pipes are telling me so i’ve got a lot of thinking and some i think some cad work to do

11:22some ray tracing to see if i can establish what might be happening i’ve done the focus test and on acrylic

11:29it says the best focus is 37 mil we’ve got the gap set to 22

11:35which is equivalent to 32 millimeter focal distance so the focus is actually 5 millimeters into the material at the

11:41start of the test

11:59the flat side down is consistently cutting shallower than the flat side up

Transcript for How Laser Lenses Cut (Cont…)

12:06next test that we’re going to do is something i call a speed test because this is an important test to try and

12:12find out how deep and how fast a lens can cut

12:18at the moment we’ve been interested in mainly depth but here what we’re doing we’re using

12:23two milliseconds four milliseconds six milliseconds etc

12:29to see how quickly we can pulse into this material in other words

12:35this will give us an idea of the cutting speed performance of a lens

12:40[Music] now before i’ve tried to reverse

12:47engineer what’s happening to the rays i’ve done

12:52eight tests two for each of the lenses that i’ve got

12:59one has been with the lens flat side up and the other with the lens flat side

13:06down now while i had the lenses in position i did another test for every lens that

13:13was to try and determine the speed of cutting of each lens

13:19the speed of penetration is in fact the speed of cutting so what we’ve got

13:25here is 2 milliseconds 4 6 8 10 all the way up here we’ve got a little bit of a trace

13:31of how quickly the cutting develops in each one of these lenses whether it be face down or face

13:38up they’re all down the side here flats up flat down again something that we’ve proved before is

13:44that this orange which is the two and a half inch is definitely one of those that’s going

13:51to carry on going up there and is responsible for deep cutting this

13:56pink one here which is a 50.82 inch flat side up may also perform pretty well

Transcript for How Laser Lenses Cut (Cont…)

14:04according to this prediction the other advantage of this two inch lens is if you look here the rate at which it

14:11climbs extremely quickly here at somewhere in the region of about

14:17three milliseconds for a three millimeter thick piece of acrylic

14:23so what that really means is that if we’ve got a 0.2 curve

14:28the diameter of the beam is say point two that means that in

14:35a one millimeter length of cut i’ve assumed that i’ve got these all

14:40joined up and i’ve got one two three four five cuts per millimeter in other words

14:49i’ve got five cuts at four milliseconds a cut which gives me 20 milliseconds to cut

14:57one millimeter now there’s a thousand milliseconds in a second

15:02and 20 milliseconds to cut one millimeter divide those two and what you get is a

15:09cutting speed of 50 millimeters a second can i cut three millimeter acrylic at 50

15:16millimeters a second i don’t know that’s what the facts tell me so

15:21it may will be marginally less than that i think we experimented like this before and we found that maybe we had something like about 0.8 of

15:28that value but it’s still a way of trying to assess how efficient

15:33you can cut with different lenses well the cutting speed test was just an interesting diversion because i had the

15:39lenses in place and it was easy enough to do the test but the real task for this session was to try and see if

15:46there’s any way that i can decode what’s happening in these tubes so we’ve

15:52got five millimeters into the material four three two one that’s supposedly the focal point

Transcript for How Laser Lenses Cut (Cont…)

16:00and then it’s one two three four five six seven eight millimeters below the focal point

16:06and the further we drop below the focal point you see the beam is becoming very blunt and it does not penetrate very much into

16:13the into the material so decoding these is my next task now as i said to you maybe in the last

16:20session the biggest problem i’ve got is trying to decode and decide whether or not these are the result of a focus

16:28which is below the focal point or whether these are the result of a mode burn

16:35i must remind yourself and me of something that i did several months ago and that was this

16:42mode burn test under two different circumstances the first circumstance

16:47was a parallel beam straight out the laser and burning into this material this is

16:54not a focus point this point happens because i’ve got high intensity

17:01at the center of the beam and low intensity at the edge of the beam and if i give it

17:08enough time and in this case it was 10 seconds the center of the beam

17:13burns in faster than the edge of the beam hence i get this conical shape now

17:20this more slender conical shape happened in three seconds

17:27as opposed to 10 seconds because i had interposed a long focus lens

17:35into the beam and reduced the beam diameter before it hit the edge of the block

17:42now that basically increased the i suppose the energy density or what i prefer to call the intensity of

17:49the light before it hit this piece of acrylic and higher intensity

17:54light means faster damage so if you allow intense light to hit this

Transcript for How Laser Lenses Cut (Cont…)

18:02it will penetrate faster than the less intense light

18:08so we finish up with the same result a pointed beam but look this pointed beam has got

18:14nothing to do with the focus of the actual lens path itself so we mustn’t be confused between

18:22these two issues that is not a focus point that is a point caused by very high intensity

18:30and an exposure time the low intensity here is doing very little damage

18:36and the high intensity is doing much more damage what we are now trying to do with the

18:42analysis of these tubes we’re trying to move further and further

18:47down the beam and we’re taking samples at various points across the beam and we’re getting

18:53different more intense mode burns if i go past the focal point

19:00then the mode burn is going to change dramatically the only problem is these lenses do not have a single

19:08focal point and that causes all sorts of problems in trying to

19:14analyze it i feel as though i’m reading tea leaves trying to analyze or reverse engineer

19:21these rays so i need some additional information and i’m doing that with this

19:27piece of card so i’ve got my four inch or 101 millimeter lens in here and here i’ve got a 61

19:36millimeter spacer i’m 40 millimeters below the focal point at the moment

19:42[Music] and what i’m going to do is i’m going to burn some marks

19:48in this paper at five millimeter

19:56[Music]

Transcript for How Laser Lenses Cut (Cont…)

20:04intervals [Music]

20:10now you will see that there are a whole load of holes in that card now every one of those holes because the

20:18card is a piece of constant thickness material and the power is constant

20:26the holes will be created by the same intensity of light

20:31so that will tell me basically what the intensity is at various positions

20:36along the beam and that maybe helped me to determine uh some of the tracks of the rays now

20:44here we’ve got a little fixture that i’ve made for testing the focal lengths of my lenses

20:49and i’ve set up a target in here which i’m going to move backwards and forwards and i’m going to get you to look at that

20:55target through this 2-inch zinc selenide lens that i’ve put in here

21:00right so i’ve got the two inch lens roughly in focus now and as i move the target towards the

21:07lens only by a millimeter it’s already gone out of focus

21:12so let’s put it back into focus and take it a millimeter out of focus and this is very much in accordance with

21:19standard lens theory you have a very short focal range over

21:24which a lens works now the other thing that’s interesting about this image

21:30where’s all that red and orangey stuff coming from around the edge of the image

21:35so unlike this camera which has got a very good set of optics which focuses perfectly on the surface

21:42the lens that we’re using here has got something called spherical

21:48aberration and even as i hold it in front of this target you can see how it’s actually

21:54producing this strange orange glow around the outside which is the spherical aberration

21:59the problem is we’re looking at this in white uniform light

Transcript for How Laser Lenses Cut (Cont…)

22:06now standard lens theory works well for cameras as you can see telescopes

22:14microscopes projectors because they all rely on uniform

22:20light now i know there might be different brightnesses and colors in a projected image

22:28but that’s not quite the same as the intensity problem that we face with a laser beam

22:36we are not trying to focus images we are trying to focus light intensity

22:43and that’s a completely different subject which is not really covered by lens theory now in the in the

22:50description text below this video you’ll find some links to some other youtube videos where

22:57people are trying to link standard lens theory into laser cutting technology now one

23:06particular video i would love you to look at is this by a moderately famous company

23:13now you would think that these would be professional enough to explain correctly and technically what’s

23:20going on here what they’re trying to describe is that a four inch lens

23:25is to blame for this loss of text down here but hang about

23:33that implies that a lens has got some intelligence and it knows when to transmit light and

23:40when not to transmit light it’s been perfectly okay transmitting light here

23:45and burning these parts of the text so why is the four inch lens to blame for this missing text

23:52down here in reality it’s nothing to do with the lens at all it’s all to do with the light

23:59signals that are coming from their machine if there are no signals the lens cannot make light happen

Transcript for How Laser Lenses Cut (Cont…)

24:06it’s all to do with the parameters that they have their machine set to they are not transmitting every single

24:12pixel that’s in this bitmap image and these are actually

24:17missing pixels it’s not the fault of the lens that the pixel is not there it’s the

24:23fault of the encoding that the light is not there to be transmitted by the four inch lens

24:31now this four inch lens looks half reasonable it’s not doing a bad job around the edge it’s not blobby and lumpy around the

24:37edge even here look it’s not really blobby and lumpy but the verticals are missing because

24:44they’re not scanning the correct pixels what i’m saying is there’s a great deal

24:49of misinformation out there about lenses how they work people just

24:55don’t understand how lenses and laser systems work together so

25:00go and have a look at the video links that i’ve provided and you’ll see what the current

25:06understanding of lenses is you might want to do that before you push on with the rest of my video

25:13that shows you how lenses actually cut now as you can see i’ve already done

25:18some cad work and that cad work has allowed me to

25:24work out the essence of how a laser beam actually cuts

25:29in other words it’s the interaction between the laser beam and the lens that produces these strange effects that

25:35i’ve been looking for and after two years of searching i think i’ve now

25:40uncovered a model that actually describes how and why a laser beam cuts in the way

25:47that it does my basic principle of a spiky focus below the focal point has not proved to

25:55be wrong it’s just proved to be the wrong way of looking at the problem

Transcript for How Laser Lenses Cut (Cont…)

26:01the first thing we must do is quickly go back over the basic principle of a gaussian

26:06distribution curve now i know that my beam is approximately

26:1212 millimeters diameter you’ve seen me burning tape for longer and longer

26:18periods to try and work out exactly what the maximum diameter of my beam is because obviously

26:24when we get right down here to the bottom tail of this intensity it takes a long time

26:31for that very low intensity to damage the tape okay well this central middle third of

26:36the beam which in this case represents uh four millimeters

26:42so that central four millimeters contains roughly 70 percent of the power of the beam

26:49because the area under this curve represents 100 percent of the power of my beam 70 watts so therefore this

26:57central 70 percent power is roughly equivalent to 50 watts

27:03so in this four millimeters the yellow four millimeter section i’ve got 50

27:08watts of power being used that 50 watts is not being used as watts we are using those watts

27:17to develop intensity and the shape of this curve is very

27:23important and a proper gaussian curve means that at my 50 watt extremity here my 50 watts in the center

27:30here is developing 62

27:35intensity within the beam and as i get closer to the center of the beam my intensity

27:43starts going up and so this red zone here which is actually one-sixth of the diameter of

27:48the beam two millimeters contains roughly 89 of the intensity available

27:58in this gaussian distribution right at the center here we’ve got a hundred percent

Transcript for How Laser Lenses Cut (Cont…)

28:03okay so you mustn’t lose sight of the fact that it’s intensity that does damage

28:11the intensity of the naked beam is vital because if you’ve got a substandard

28:20gaussian distribution here and let’s just say that it’s a very flat distribution as they say in

28:27computing terms rubbish in rubbish out okay so don’t expect the performance

28:33that you you see me getting from your laser beam if it doesn’t look like a good gaussian

28:40distribution beam i’m very lucky that i’ve got a superb quality beam my rays if you like are colored

28:48to represent intensity boundaries within the beam itself so this represents zero percent

28:55intensity right at the extreme here because there is no power there there is a boundary sort of

29:01but then here 13 intensity boundary then we’ve got our 62 intensity boundary

29:07and here we’ve got our 89 or 90 intensity boundary so most of the

29:13intensity is passing right through this red zone at the center here this gaussian

29:20distribution exists here in the naked beam but it doesn’t last for very long

29:29i always thought that that was going to be compressed down to a very small version

29:35of it just here at the focal point that’s what i’ve believed all the way along

29:40in other words if you turn the telescope round you see a small version of the moon when you look at the moon

29:46you don’t see some distorted version of the moon or a white blob that’s not the case with this shape here

29:54when we start amplifying it down here it’s no longer gaussian at this point

29:59here and it’s totally destroyed when we get here but we’ll talk further about that in a

Transcript for How Laser Lenses Cut (Cont…)

30:06few moments when we’ve looked at more detail in this diagram so when we did this test if you remember

30:12we set the table at this height here okay and we did this first burn

30:19now this first burn the focal point was buried five millimeters into the material

30:26and then we dropped the table by a millimeter and now it’s four millimeters into the material

30:31we dropped it dropped it dropped it and eventually by the time we got to line number five here

30:37we were at supposedly the focal point which is what this blue line is and what

30:44i’ve then done is to measure the diameter the entry hole

30:49damage on each one of these test spikes and that’s what these

30:56red circles are the damage at the entrance to each one of these

31:01holes we can’t get damage unless there’s light intensity there to excite the molecules

31:07up above their vaporization point and make them disappear so these diameters here are very exact

31:15markers of light intensity that’s what i placed into my diagram first

31:20i then realized i was having quite a bit of difficulty trying to work out how my rays were actually passing

31:28through these various points couldn’t make a lot of sense of them

31:33and it was at that point that i decided i had to go back and use something that i’d used several

31:39years ago to plot what i call what i then called energy density within the beam

31:47and that’s what some people call it but i would much prefer to use the term light intensity not energy density

31:55because it is light intensity which is causing the damage to the

Transcript for How Laser Lenses Cut (Cont…)

32:01material it’s the thing that’s exciting the molecules making the molecules vibrate faster and

32:09faster vibrating molecules get hotter and the hotter they get we see it as

32:15burning but in fact what it’s doing it’s getting the molecules vibrating so fast that the molecular

32:22bonds start breaking down and we get chemical changes and that’s what you’re witnessing here

32:28you’re witnessing chemical changes to the material as it heats up

32:33as it vibrates faster and we’re getting a combination of oxygen in the air which is beginning

32:40to oxidize some of the material away and we’re leaving some

32:45carbon behind which is the black stuff we’ve got two materials involved here

32:51the red the red circles are these spikes which are disappearing

32:58and being created at 200 degrees c at 200 degrees c acrylic

33:05evaporates and what we’re left with is nothing we’re left with an edge so what’s inside

33:12that edge must have evaporated and gone above 200 degrees c to disappear so we’ve got basically

33:21a an intensity boundary that we’re setting with these red circles

33:26light intensity boundary now we’re doing exactly the same thing here and what in this instance we’ve got

33:33we’ve got two intensity boundaries that we can look at first of all we’ve got a hole in the

33:40center and i hope you might be able to see those see the light through those holes there but we’ve got holes

33:46through the center of these black dots now those holes can only exist because

33:53we have made the material evaporate we’ve burnt

33:59it we’ve made the molecules shake so hard that they’ve destroyed themselves and disappeared somewhere

Transcript for How Laser Lenses Cut (Cont…)

34:04this card is basically wood pulp and wood has got some very strange

34:10properties when it comes to let’s call it self-destruction you would like to see it as burning

34:16but i like to think of it as self-destruction because of stimulation by light this

34:21self-destruction happens in three basic stages number one

34:27you’ve got cellulose and the cellulose disappears at about 250

34:35to 300 degrees c what’s then left is the cell structure of the cellulose

34:42the cellular walls a material called lignin which shakes itself to pieces and disappears at about

34:50350 degrees c and then what we’re left with is this

34:56black stuff and everybody knows what that black stuff is it’s carbon

35:02but what is going to surprise some people is that that black stuff doesn’t shake itself to pieces

35:08until 5 000 degrees c it boils

35:15at about three and a half thousand degrees c but it disappears to gas at about 5000

35:22degrees c now on the way up to 5000 degrees c it may well combine with other elements

35:29mainly oxygen in the air and it will produce carbon dioxide gas and carbon monoxide gas but any carbon

35:36that’s left has to exceed about 5000 degrees c before it will

35:42disappear so there’s a very good marker of high intensity

35:49in this card but there’s also another marker in this

35:54card as well which is the outer brown mark what i want to do now is to

Transcript for How Laser Lenses Cut (Cont…)

36:01take a look at this particular focal point here because

36:06that is a vital point that helped me to construct these diagrams and i say these diagrams because i’ve

36:13done one of these diagrams for each of the four lenses that i have

36:18we’ll just take a look at the uh the first hole on here which you’ve got a a burn around the outside of the hole

36:25now you can clearly see the hole itself here but you would ask yourself the question

36:32why is this not black i mean look you can clearly see the

36:38clear card and the paper underneath and yet we’ve got lots of carbon around the outside

36:44and in the center here and we’ve got lots of scorching around here well this black carbonaceous material combined

36:50with the white gives the impression when you mix it in your eye that this is a brown

36:56material a scorched material and you get various shades of scorching as you move out from

37:02the center of this hole now this card is not quite what it seems it’s not

37:07simple wood pulp about 20 of this card is something called

37:14kaolin china clay and china clay is used widely in the paper industry in the card

37:20industry to a stiffen the card b to make it a slightly glossier

37:27harder finish so that you can write on it and it doesn’t soak the ink into the

37:32paper so kaolin is in itself

37:37a mineral material which does not burn when i say it doesn’t burn it melts at

37:44around about 1700 degrees c and probably by the time it gets to two and

37:51a half or three thousand degrees c it will vaporize

37:57but it’s nowhere near the vaporization temperature of this stuff which is the carbon so we’ve got a huge

Transcript for How Laser Lenses Cut (Cont…)

38:04range of temperatures going on as this material destroys itself this

38:10brown which is not evaporating and it’s not turning to carbon but it’s just scorching so we’ve got two

38:19light intensity boundaries when we look at these pictures so as the beam gets more condensed

38:26we’re getting a different pattern of burning and then we get to this rather strange one here which i said

38:33is very strange because you can see not only have we got our total evaporation of material in the center here

38:40we’ve also got a slightly brown halo around here and then we’ve got no damage at all and

38:47then we’ve got a rear currents of our brown halo around here we’ve got equal boundaries of intense

38:54light intensity at these points so again these are very important

38:59markers that tell us what is going on within the beam this one is getting close to the focal

39:06point we’ve got total destruction in the middle here where the material is shaking itself to pieces we’ve got partial destruction here

39:12around this boundary and then right around the outside we’ve got virtually no destruction

39:18you can just see a hint of brown around here where there’s obviously some very light energy around the outside but

39:23basically we’ve got a an intensity boundary here which is very obvious now the focal point is very interesting

39:31so we’ve got a very heavy layer of carbon just around here indicating we’ve had very high temperature there but not

39:37enough to make it disappear and then we’ve got our thin layer around the outside here

39:43of brown but you’ll notice that the brown and this destruction are very close

39:50together now intensity beams are all converging on this one point that people

39:56recognize as the focal point it’s not a focal point because if it was a focal point

Transcript for How Laser Lenses Cut (Cont…)

40:04we would not have any of this brown around the outside we would just have total destruction because all the rays

40:11would be passing through that hole most of the rains are passing through that hole

40:16but some of the rays are still not passing through the hole so this represents effectively the size

40:23of the focal point not this so the next part of this picture that i’d like you to understand

40:29is these green circles which represent the brown extremity of the scorch layer and the

40:35white circles which represent the total internal destruction of the material

40:42i.e a whole now this card pattern of burns was crucial

40:48to helping me define where the focal point for the lens really is because remember

40:55what i said to you a focal point is the point through which

41:01all the rays pass and so the smallest hole that i could find and

41:07let’s just zoom in you’ll see that what i’ve been able to do i’ve been able to concentrate my

41:12yellow rays my red rays my blue rays onto this hole here because this is

41:18where all the power is hitting to cause that amount of damage

41:23that must be the point of maximum intensity that white circle okay now there are some other things

41:29happening around the outside here which are going to cause scorching okay now remember this white beam

41:35although it’s got virtually no power in it at all because it’s the extremity of zero

41:40there is still some small amount of power there to do damage so combine the white with the blue

41:48and the yellow and even the red and we’re going to get this very very

41:53small scorch boundary around the focal point but this focal point basically defines

Transcript for How Laser Lenses Cut (Cont…)

42:01where i’ve got to make all my beams pass very close to it and

42:06buy it and it was this key feature the focal point on card which has allowed me to if you like

42:13project and guess where the various intensity of rays are passing through

42:22this focal point so now we can start looking at them in a slightly different way now what i’ve done here is to strip away

42:29all the useless data that was very useful for constructing this diagram so that we can

42:35see what’s actually happening and if we look carefully you’ll see that the outer rays

42:43are crossing over here the next row the next ones in which are

42:49the blue rays are crossing over here beyond the focal point

42:54and the yellow rays are crossing over even further beyond the focal point and eventually

43:01the red rays are crossing over beyond the focal point so there is

43:08no single focal point if there is any sort of focal point

43:14this has got the maximum amount of power the reds so that should technically be

43:19the focal point but it turns out not to be because there is a strange

43:27set of circumstances that exists just here and you think well hang on

43:33there must be power down here because this is the focal point that isn’t the

43:39way it works i’m afraid that’s why i said my focal point idea

43:44was both correct and incorrect there is a spike of focus way beyond the focal point but that’s

43:52not that spike of focus which is causing the damage let me try and explain that because this

43:57is a very difficult concept and i’m going to work back in a very simple way to start with

Transcript for How Laser Lenses Cut (Cont…)

44:03remember what i said to you the area under the curve equals 70 watts they’re all

44:09gaussian distribution curves but they have got different baselines as i shrink the baseline

44:16so the intensity within the beam goes up so you remember back to my

44:22picture of the two mode burns that i did one with parallel beam and one with a focusing beam well here

44:29we’ve got a focusing beam okay and if i take this shape here

44:37so let’s just assume that this is a section through the beam and it’s obviously above the focal point

44:43but what’s going to happen then we’re going to get a mode burn and the mode burn will be intensity

44:50times time so we should be able to burn quite a deep hole quite quickly so as you can see this one

44:57has got substantially more intensity than that one because i’ve got a slightly smaller

45:03baseline as you can see i’ve only moved a very small amount down the beam but now what i’ve got

45:10i’ve got a tremendously increased intensity

45:15and if we take a look you’ll see that that intensity is not spreading out here that intensity

45:22starts here at the focal point which is say 0.2 and very quickly bores its way in

45:30leaving a 0.2 diameter hole behind because that’s where it’s eventually going to grow to

45:35now you’ll remember when we do a normal mode burn we’ve got parallel rays of light hitting the surface here

45:41and it is the high intensity rays that are in the center of the beam that are actually working

45:47their way in quicker and quicker to make this point whereas the low in lower intensity rays at the outside are

45:54doing less damage but the high energy in the center doesn’t change because the rays

Transcript for How Laser Lenses Cut (Cont…)

46:00are parallel that isn’t the case here because as we start nibbling away

46:07into here with our erosion look what’s happening we’ve got yellow rays which are adding

46:13into the equation we’ve got the red rays which are actually still compressing down

46:19and they are actually increasing the intensity of the light even more

46:24and then we’ve got the blue rays which effect to a certain extent and then they stop working so we’ve got

46:31a very complex rate of erosion taking place within this

46:36orange spike of potential intensity that intensity is not

46:43constant that’s really what i’m saying so we will actually be not working with a proper

46:50gaussian spike of energy here as i’ve shown it this

46:57will be a really strange distorted spike of energy i do not have the mental capability to be

47:05able to analyze all these variables and tell you exactly what’s going to happen i can tell you in essence what’s going

47:12to happen and i’ve broken down the elements to show you the essence of what’s going to happen now i’ve done exactly the same

47:18drawing for all four of the lenses and what i’ve done i’ve put the focal points

47:24all in line so that you can clearly see that as the focal distance of a lens changes

47:33the red high intensity focal point moves further and further beyond the

47:39lenses natural focal point which is why we can cut thick material with the longer

47:47focal length so although this one a half inch has got a much smaller

47:54focal point it may well have a much higher intensity but

47:59its core its red core if you like of power does not project as far

Transcript for How Laser Lenses Cut (Cont…)

48:07as the four inch one the four inch one intensity beam is lower

48:14because it’s a bigger diameter but if you put more time into it in other words you cut

48:20slower you will be able to cut deeper you will never be able to cut deep

48:26with a one and a half inch lens although it has got a lot of intensity it does not have the potential to cut

48:34deep because its focal point disappears in a much shorter distance

48:40[Applause] so all this energy density at the focal

48:46point is meaningless first of all as i’ve indicated before there isn’t such a

48:53thing as an energy density at the focal point there can only be that can only be a

49:01nominal energy density and that doesn’t describe the model that we see here

49:06we don’t have an average density across the focal point

49:12it’s a gaussian distribution of some sort but a distorted gaussian distribution

49:19because we do not have all the beams perfectly lined up if we had the perfect lens would we be

49:27able to cut with it that’s an interesting question and i would think it would be very

49:32poor at cutting the only reason these lenses cut

49:38is because we’ve got this variation this aberration because it’s a property

49:44of a spherical shaped lens so my point really is no matter how

49:49good you make a spherical lens it can be as perfect as possible

49:54but it will still have this weakness of aberration which is the one thing

Transcript for How Laser Lenses Cut (Cont…)

50:00that we need for cutting i’m absolutely sure that there’ll be many people on the other side of this

50:07camera lens that will not accept the fact that hey we need this

50:12aberration to make a lens cut i’ve even doubted it myself all these

50:18diagrams are actually wrong technically because this parallel beam

50:23will refract at an angle inside this material and then have another refractive

50:29path when it comes out of the material so this is a very simplistic diagram but it

50:35describes what actually happens which is this confusion of focal points as i have

50:41described you in the previous parts of the video now we can get rid of this confusion

50:48and we can try and get a perfect focal point with something called an espheric lens

50:54which you can see has got a weird shape and again the paths through that lens are not technically correct

51:00but the end result is more or less correct in other words we shall be able to produce with some very

51:06special distortions a perfect focal point

51:11now i believe that that perfect focal point will not allow us to do anything other

51:17than engrave secondly we cannot get hold of any of these lenses

51:23to prove the point they’re very expensive and they’re not typically available for our little chinese machines so we have to

51:31suffer with either this type of lens which always has spherical aberration and is great for

51:37cutting or there is an attempt to try and reduce this problem

51:43with something called a meniscus lens a meniscus lens has got a spherical surface on the

51:49outside and it’s got a secondary spherical surface underneath the lens to try and correct

51:56for this problem here although you can’t see in great detail

Transcript for How Laser Lenses Cut (Cont…)

52:02this is not a perfect focal point this still has some aberration on it because we’ve got two spherical surfaces

52:10the second spherical surface attempts to remove the major part of the top surface spherical aberration but it

52:17doesn’t succeed completely so this lens will cut because it’s not

52:22perfect but it won’t cut as well as this lens

52:28you don’t believe me let me show you something now what we’ve got here is my dropping

52:35table test where the focal point is approximately the fourth or the fifth

52:40one in i can’t remember which but basically we’ve got very deep penetration

52:46because we’ve got very high intensity coming through the center of the lens where the spherical

52:52aberration is worst or in this case best because it produces the deepest

52:58cuts what i’m now going to show you and i’m going to slide it into the background is

53:05exactly the same shape size of lens except this is a gallium arsenide

53:11two and a half inch meniscus lens

53:21i think you can clearly see the cutting ability of that lens is compromised

53:29crap use whatever adjective you want but it’s not the sort of lens that you

53:34would use for deep cutting i think you can see that very clearly all i can do is show you the proof

53:41you can choose to believe it or not go ahead and do the tests yourselves

53:47let me just step back a second and say what about engraving well engraving is not any different

53:54to cutting we need to set the power of the machine lower and as we reduce the power of the

Transcript for How Laser Lenses Cut (Cont…)

54:00machine what we should be doing we should be changing the shape of the beam and making it blunter and if we make the

54:06beam blunt it means that we won’t be using the full extent of this red projection we should be pulling

54:13everything backwards to make it a much softer beam and so we

54:18won’t get as much cutting during our engraving process

54:25so i think i’ve reached a point where i don’t need to destroy any more lenses i’ve already proved that if you take the

54:31center away from a lens you can destroy its cutting ability and

54:37when you look at my diagrams you can see why because that’s where the power of cutting exists it isn’t necessarily a

54:43focus below the focal point that defines the cutting ability of a lens although it is a contributor

54:50it is the the strange non-gaussian mode burn that happens below the focal

54:55point which is actually responsible for the cut that we produce in materials

55:01so the longer the focal length the deeper you can cut but you either have to go slower

55:09or have a slightly more powerful laser tube to achieve it faster but again

55:16there is a physical limitation even on a four inch lens as to how deep you can go

55:21i should be limited by my 70 watts of power so in the next session i think what i

55:27should be doing is looking at that four inch lens to see whether or not i can make it cut

55:32deeper than the two and a half inch lens where i was able to achieve 26 millimeters at

55:38three millimeters a second can i achieve 50 millimeters at one millimeter a second with a four inch

55:45lens with my 70 watts interesting so thank you very much for your patience

55:51and I’ll see you in the next session