09 – Fiber Laser Marker: Let’s Zoom in on Focus (34:38)

0:00welcome to another fiber laser learning lab today we’ve got to stop chasing

0:05those colors that we were working on because well we’re waiting for some kit

0:10to arrive but in the meantime we can do something quite useful and that’s to

0:16examine the focus and the lens on this machine now I promised I wasn’t going to

0:22take this machine to pieces but you know promises are made to be broken the only thing is this lens in here it’s

0:33very expensive hence I’ve got a piece of soft material here I don’t intend to

0:40drop it but it’s just a precautionary measure now this is a very interesting line and it’s called an F theta menace

0:47I don’t know who supplies this lens because there isn’t as far as I could

0:53see a name on it I’ve looked at the price of these from a very reputable company called thought labs and I’m

1:02holding almost a thousand pounds here so yeah this is why I don’t want to drop it

1:07we’ll just have a quick look here at the lens and it’s got a flat surface on the

1:13front which implies that it’s got a protective cover on it and we’ve got I

1:18think maybe two elements in there there’s that one there and you can probably see another reflection off of

1:25another element inside and I think if we go and look on the front we shall see the same we shall see we should see

1:32we’ve got two or three elements in there now the thing that interests me about this lens is we’ve got the working

1:38elements here I’ve had a quick look at some diagrams for this lens and they

1:44have they haven’t specified a particular place for the focal length I mean it’s

1:52quite difficult to get to a focal length if you can imagine because we can’t physically get up there and check

1:57against the lens because we’ve got this piece of flat glass protection glass in between if we look here where they

Transcript for Fiber Laser Marker: Let’s Zoom in on Focus (Cont…)

2:04specify the focal length is 250 for 10 inches but where that’s measured from we don’t

2:11know this 261 millimeters here against 254 for the lens seemed a little bit

2:19strange and here in the software they

2:24put that same number 261 focus height it was a little puzzling so I got back in

2:31touch with Lotus later and asked the question and the answer is actually very simple because it’s much easier to and

2:38more accessible to measure something externally what they’re done they’ve set

2:43that to 61 dimension from this under face here down to the table so they set

2:50the focus up by a test and then they will specify what this dimension between

2:55the table and that is so it’s a practical dimension rather than an actual focal distance what I’m going to

3:02do is to check the focal distance for myself they have their own method of

3:08setting the focal distance which is to draw a series of lines they deduce from that somehow the focal distance although

3:17I’ve got the program it doesn’t work as expected so I suspect all the powers are

3:22maybe for a 30 watt laser as opposed to a 20-watt laser but we’re gonna go ahead and we’re going to check my method of

3:29doing it and I’ll explain why I’m doing it my way so at the end of this exercise we shall

3:34see how different this method is by which this machine has been set up I

3:40believe and my method is for those of you that are still awake you’ll notice that we’re not

3:45by the laser I want to demonstrate something to you using the constant

3:52power machine so we’re using a fixed power for all of these tests [Music]

Transcript for Fiber Laser Marker: Let’s Zoom in on Focus (Cont…)

4:10you’ll notice that the speed was going up maybe set that one

4:16something strange about that one so here we’ve got my fairly crude representation

4:21of a laser beam it’s parallel because that’s exactly what we used a parallel laser beam to produce these marks here

4:28so this parallel beam with a uniform light would do damage which looked like

4:34this it would produce a square cornered groove where we have eroded or burnt or

4:41evaporated the material away in reality that’s not what we’ve got here because

4:48the light from the laser beam is not of uniform intensity the light from the

4:54laser beam is more like this where you’ve got an intense beam in the center and it gets less and less towards the

5:00outside so if I draw a graph of the intensity within that beam what I’ll

5:06find is it looks like this okay where

5:13the most intense light is at the center and the least amount of light is at the edge so that’s zero and that’s max

5:21intensity if I use that beam to damage material now I will not produce a hole

5:29that looks like this with square corners the area of intensity at the center will

5:35do the most damage to the material now light intensity is a strange thing to think about now before I proceed with

5:42this diagram I’m going to show you another couple of experiments which I hope will try and explain to you what’s

5:50actually going on inside this laser beam okay so I’ve got a nice gentle flame here and look I can pass my hand through

6:01that flame quite slowly yes I’m creating a little bit of smoke on my hand there but it’s not hurting

Transcript for Fiber Laser Marker: Let’s Zoom in on Focus (Cont…)

6:08because I haven’t reached what would be called the damage threshold for that

6:14material I can feel that flame is warm on my hand but at the moment it hasn’t

6:20done any damage to my skin it’s just warmed it up I think you’ll appreciate that if I was to do that much slower

6:27than that then the amount of energy in that flame would be sufficient to burn my hand ie

6:34it would exceed the damage threshold of my flesh but that’s only dependent upon the speed

6:40at which I pass my hand through or over that flame although it’s a fairly low

6:46intensity of heat given enough time it will damage the material that you’re

6:52trying to damage but it’s time related energy against time not much energy lots

6:59of time we still get the same damage

7:10I think the point is made I’m certainly not gonna run my hand through that the

7:15flame at the same speed that I did this one because now we’ve got a more intense

7:20energy source a very high energy flame a high intensity beam of energy and here

7:29we are this is the high intensity blow lamp now I shall be able to do a lot

7:38more damage in the same period of time with that high intensity flame than I

7:45will be able to do with the low intensity flame and that’s the basis upon which this situation happens where

7:56I’ve got very high energy I can move fast and I will still produce a mark

Transcript for Fiber Laser Marker: Let’s Zoom in on Focus (Cont…)

8:04because the high energy is in the area there but when I move the beam slowly as

8:12in this case here I will allow enough time for that part of the beam to have

8:20an effect as well as this part of the beam so when we look at this here which

8:27you’ll find that not only is it wide but it’s also got a groove in it because the

8:32central part of the beam has taken the center of that out and the edge part of

8:38the beam this low-energy part of the beam has had enough time to make the beam burn wider so as we get faster and

8:45faster and faster we should be using less and less and less only the high

8:51intensity part of the beam will eventually exist here to do any damage here at this point we’re just about on a

8:59balance with the damage threshold for the material if we go much faster we won’t make any marks at all okay

9:05let’s put the constant power laser aside now and here we are we’ve got a change

9:12of beam we’re in blue beam now that’s the fiber later but the fiber laser has still got

9:18this Gaussian distribution energy in its and it starts off at roughly 7

9:26millimeters diameter so the spec sheet says now 7 millimeters diameter is

9:31approximately 38 square millimeters after that beam passes through our F theta lens doesn’t matter what the lens

9:38is the lens will actually cause the beam to compress down to a very small area

9:44we’ve got a beam which is now point zero seven millimeters diameter and it’s only

9:50four thousandths of a square millimeter as opposed to thirty eight square

9:55millimeters so if we do this quick calculation 38 divided by point zero zero three eight we find that we’ve

Transcript for Fiber Laser Marker: Let’s Zoom in on Focus (Cont…)

10:02actually got an amplification factor of 10,000 I’ve drawn that shape there like

10:09that which looks like a nice gentle little camel hump which is seven millimeters it’s probably not far off

10:15but this is a 20 watt laser so that area under there represents 20 watts doesn’t

10:25matter what the unit is that’s what the area represents 20 watts so when I squeeze that baseline down I wonder what

10:34happens now before I explain the next part of this I’m going to give you a couple of mechanical demonstrations to

10:41try and if you like put some pictorial sense into what I’m trying to explain

10:48here because some people might not understand or believe what I’m just about to tell you so what I’ve got here

10:54is basically a light beam yeah I know it might look like a plastic bowl full of water well I’ve had to waste some of my

11:01coffee to color it it’s a bit of a shame this is intense light and as it goes

11:07down the energy drops and it becomes less intense light I’ve got a hole in the bottom here and when I pull this

11:14piece of protective cover away you’ll see that because this is a very high intensity light here at the moment it

11:21can do a lot of damage in other words it’s going to send a sprout of water right out here somewhere I hope

11:29but as the light intensity gets less you’ll see that the amount of damage it

11:35can do will get less as well and as the

11:41level goes down you’ll very quickly see the water is getting closer and closer

11:52and closer

12:05got here is a piece of silicon tube which is stretchy and it’s got a small

Transcript for Fiber Laser Marker: Let’s Zoom in on Focus (Cont…)

12:13bore another round about three millimeter bore again any interest of

12:18science I gotta waste some of my coffee just get

12:25rid of all the air bubbles in there now I’m using this just here to mark where

12:31my fluid is as I stretch this tube I’m sure you can guess what’s going to

12:37happen the bore of the tube gets smaller and smaller and smaller and basically

12:45look we’ve increased the head of fluid we’ve basically increased the energy

12:50level in that water if it wants to escape at the bottom and the more I stretch it for greater the head so as I

12:59squeeze something down it will increase its energy level so in our bottle

13:07experiment what we tried to show was that when the light intensity ie the

13:12level of the fluid was very high it could do a lot of damage because the water was coming out and going a long

13:19way away from the bottle and as the water level went down ie the intensity

13:24dropped the amount of damage we could do got less and less and less now we took

13:30that one stage further with the second experiment where effectively what we did

13:36was to show that you can get a similar sort of effect a change of head ie

13:42we were increasing the energy level in the water in my lovely coffee cup column

13:47by stretching the tube by stretching the tube effectively what we were doing was

13:53this we were making the tube gets smaller and we were making the head we

13:59were making this shape here get taller and that’s the concept that I want you

Transcript for Fiber Laser Marker: Let’s Zoom in on Focus (Cont…)

14:04to imagine alright what we’ve got is 20 Watts worth of area under that curve now

14:12when I decrease it to a factor of 10 thousand times smaller

14:17not only is this going to get smaller but I’ve got the same amount of energy under that curve so this curve is going

14:25to get hideously large right which means I’m going to have a spike of energy a

14:31long way above and below this point here

14:36which is the focal point now it gets complicated by the fact that if it was a

14:42parallel beam then that would go a long way off the paper both ways but because

14:48the beam is actually expanding beyond the focal point you get a suppression of

14:54this effect and so effectively I’ve drawn it here look we’ve got a larger

15:00diameter here so we’ve got a larger base so it’s halfway it’s it’s partway between this effect and this effect and

15:08so consequently the net result is that we’re going to get a spike of energy very similar to what I’ve drawn here and

15:15there may well be sufficient energy in that beam to damage stuff 10 maybe 15 or

15:25maybe even 20 millimeters above and below the focal point that’s not what

15:31you’d normally expect because jpt have already told us that the depth of field is only for a 254 2.31

15:40i’ve only got about 1.1 millimetres either side of the focal point I’m

15:47absolutely convinced that is not the case we’ve got a lot of power either

15:52side at the focal point and that’s one of the things that we should go out to prove later on in this session but I’m

15:58trying to give you the logic and the reason why I’m convinced that we shall find something way up and way down on

Transcript for Fiber Laser Marker: Let’s Zoom in on Focus (Cont…)

16:05the focal point in fact there’s no point in going way up we might as well just start off with our material down here

16:12and lift the beam up and up and up and we’ll only look for one half of that because whatever is happening down here

16:17will also be happening up here as well because this is the beam coming in to

16:23power and then it’s going out of power so we come back to this concept

16:29of light energy hitting a surface when the light energy hits the surface it

16:35excites the molecules on the surface into a higher state of vibration and

16:40that higher state the vibration is the molecule getting hotter and hotter and

16:46hotter the higher the light intensity the more that effect is going to happen

16:52quickly you may think this wood has burnt yeah that’s what they tell you at

16:57school but in fact in reality what’s happened is the molecules are vibrated

17:03so fast that they’ve exceeded their damaged threshold and they have flown

17:08apart because they can’t stay in the relationship they had when it was ordinary cold wood and what they’ve done

17:16they’ve changed into a different sort of chemical and what we’ve got here is a sort of a mixture of carbon and various

17:22other chemicals that have combined with those chemicals that were previously in the wood and those chemicals that are in

17:28the air so this browning effect that we’ve got here is effectively self

17:34destruction of the molecules caused by the light but the light has only

17:39vibrated the molecules into a self-destruct mode I just hope I’m getting this message over to you it’s a

17:45very strange phenomenon light intensity is the key to the way in which this

17:50laser beam works and we’ve just shown you previously that there is a spike of

17:56intense light right down the center of this beam which looks something like this this intense light may affect the

Transcript for Fiber Laser Marker: Let’s Zoom in on Focus (Cont…)

18:05material what we’re firing at on the other hand if we’ve got something like

18:10this which is acrylic it may not affect it at all this is a different structure

18:17so therefore it doesn’t respond to this sort of frequency of light so there’s no

18:24guarantee that this light intensity will have an effect on the material but it

18:30may have an effect and it may have an effect which is different for different materials because not all materials will

18:36excite at the same rate the faster you can vibrate the molecules with this intensity of light

18:43the more it will damage now why am I telling you all of this well look here’s

18:49the focal point here’s the width of the beam at the focal point and what we

18:54should do when we fire a pulse at the material what we should do is wish your prettiest a mark like this

19:01in fact that’s wrong what we should do so we should produce a mark that’s like this where we shall have in the middle

19:13of the beam we shall have a shape which looks like this because that’s the

19:19damage profile that we’re going to produce with this beam the more intense the light the more damaged the material

19:25the less intense the light the less damage but what we’re going to do is we’re going to produce a conical spot

19:32with the depression in the center which represents the energy profile of the beam that is hitting it the more intense

19:38the light the more damage we should do so we shall not get a nice clean square

19:45spot out of our dot because of the shape of the profile because of the Gaussian

19:51shape even at the focal point okay it’ll be incredibly small but it’ll still be

19:56there what happens when we decrease the power in the spot we decrease the energy

Transcript for Fiber Laser Marker: Let’s Zoom in on Focus (Cont…)

20:03level from 20 watts to say 1 watt all that’s gonna happen is we’re going to

20:11produce an energy profile and I’ll draw it like that like that we should still

20:17have exactly the same spot size but it won’t be as deep the logic that I’m

20:25trying to force on you is that the spot will not change its size just because we

20:31change the power in the spot the spot will be remaining defined by the focal

20:38point and what that means is we shall not be able to see this effect if we

20:46travel at slow speeds with a given pulse rate we shall overlap the dots like this

20:54and so the amount of dot overlap depend upon the speed and if we speed up

21:00all that will happen is the dots will get further and further apart now I’ve

21:07drawn them like this in an exaggerated form but what that means is the lion will always be the line that we cut or

21:14draw with this fiber later will always be the same width it’ll be point O 7 until we reach this

21:28part of the beam where we will expect when we run out of power if we lift the

21:34focal point and this is what we shall be looking for later until we get to this point here there is sufficient power

21:41here to do damage but it’ll do damage with smaller and smaller dots like that

21:52until we run off the end of the energy profile if we put the beam in too far

21:58and gradually lift the beam out we would have exactly the same effect this end ok

Transcript for Fiber Laser Marker: Let’s Zoom in on Focus (Cont…)

22:04if we lowered and raised the focal point that’s what we expect to see on that

22:11basis I’m going to suggest a method by which we’re going to find out where the

22:17focal point is because this is a completely different approach to that which I think Lotus lakes are using I

22:24haven’t been able to use their method at the moment because I can’t get their program to run on this machine but

22:31that’s because I think it probably is a program designed for a slightly more powerful machine and and I just can’t

22:36get the it’s not correct for this machine we know that this machine has been set up by Lotus laser to an exact

22:45point on the table where they think the focal point is my plan is to use this ie

22:51a line through there to find out where the line starts and where the line

22:57finishes lines like this where we’ll change the focal point up and down to

23:04see if we can find the start and the finish of lines of different

23:10and if I get it right what I should do I shall find all these lines look like that and the center point of all of

23:16those lines represents this focal point here let’s see if we can put some of that theory into practice and see how

23:23much of it is right and how much of it is it on my face so here’s my program

23:29I’ve got five lines on here all done at different powers and different speeds

23:35and I’ve got the five top lines are an exact copy of the five bottom lines

23:42these are done from right to left and this copy over to make it run from left

23:48to right so I’ve got the same program running in each direction I’ve now integrated the pattern together

23:54so that it looks like that so what I’ve got here is a piece of ten millimeter thick material sitting on top of a piece

Transcript for Fiber Laser Marker: Let’s Zoom in on Focus (Cont…)

24:08top surface of this stainless steel relative to the base is half a mil plus

24:13half a mil so the top surface starts off one millimeter above the base and then it finishes up half a millimeter half a

24:21millimeter and ten it finishes up 11 millimeters above base er which whatever happens basically I’ve got a 10

24:27millimeter slope there but it’s at the center point of that slope regardless of

24:32the length of that piece of material at the centre of that piece of material it will be six millimeters above the table

24:40so if I can get my pattern completely symmetrical on there and the deepest cut

24:46happens to occur along the centre line of that material then I know that I’ve got the optimum focus point okay so now

24:53what I’m going to do is just move the material so that my pattern is

24:58symmetrically placed within the material just put my glasses on and turn the

25:04interlocks off [Music]

25:10okay now not all the lines are obvious in this artificial light there are

25:18further lines along the top there that go nearly to the end of my patent but those burn lines towards the bottom

25:25where I had a lot less power I’m measuring the width of that and it’s 80

25:30millimeters so the deepest cut is happening at the centre of my material

25:35so I have got the focus just right the midpoint of that plate is sitting at 6

25:43millimeters above the base plate so Lotus laser have set their machine up such that Z equals 0 so if I take my 6

25:51millimeters away from that it means that my setting is half a millimeter higher

25:56than their setting I feel very happy to leave their setting as it is but now you

Transcript for Fiber Laser Marker: Let’s Zoom in on Focus (Cont…)

26:02have another way in which you can check your focus if you need to so in real

26:09terms we don’t have to worry about exactly what this dimension is now this is a very interesting fact sheet and

26:14tutorial about how f theta lenses work

26:20there’s a calculation here for working out what the spot size is likely to be as well now I’m certainly no mathematician but

26:28to be honest it didn’t take a great deal of effort to do a quick calculation read through this and put the numbers in and

26:34in fact the calculation came out 2.07 for this lens now this is not

26:41necessarily a thorlabs lens so it might be slightly different and the claim for

26:46this lens is 0.065 so it’s about the

26:52same you know give or take half a hair i now understand quite a lot more about

26:57the lenses themselves and how to check and set the focal distance of this lens

27:02away from the work surface now grossly exaggerating this picture as I draw on my hand but you remember me

27:08saying there is an intensity distribution within this beam whoops which looks roughly something like this

27:16what that implies is that I shall be able to find power quite a long way below the focal point and I

27:24shall still have lots of cutting damage even though I am well below the focal

27:30point I don’t know what that level of damages I mean thinking that it could be ten maybe even as much as fifteen or

27:38twenty millimeters below the focal point but equally well what you must remember

27:43is whatever is below will also be above so if I’ve got something like this that

27:49I want to engrave for example so I should probably be able to draw off every wide logo within the focal range

27:56of this lens now it all depends on the pulse power because I’m very convinced

Transcript for Fiber Laser Marker: Let’s Zoom in on Focus (Cont…)

28:03that if you make the pulses very large then what we shall have we shall have

28:08sharp which I have much sharper shapes

28:14like this but when we start reducing the power then what we should do we shall

28:19finish up with much more softer shapes like this and we won’t have as much range so we’ll just go and investigate

28:27that a little bit and see what we can find so I’ve written a program here which

28:34starts off at position zero ie the focus point is on the table and that’s that

28:41zero there so I’ve got eight and three-quarter millimeters overall but I’m using a half millimeter test piece

28:47so therefore the focal point will be finishing up eight point two five millimeters above the work surface so

28:57this is where the two nano second pulse at 250 millimeters a second

29:23well here’s the stainless do with the two nanosecond pulse and that got about

29:30well that got to six millimeters above the surface with what I would class as

29:37damage and then there are a few speckles below this point so that got to six

29:44millimeters above the surface before it ran out of power but when we went to 200

29:49nanoseconds my goodness me okay so it ran out of lots of power just here

29:55but it carried on and it was damaging the surface in a strange sort of

Transcript for Fiber Laser Marker: Let’s Zoom in on Focus (Cont…)

30:02hammering way almost like a ball peen hammer with the focal point 12 millimeters above the surface there was

30:08still damage occurring to this surface there was even coloration occurring below that point right the way down to

30:14probably nearly 16 or 17 millimeters was still quite a lot of power in that beam

30:20at 17 millimeters above the surface as I might have mentioned to you before this

30:26material stainless steel is not what I call it binary material you can color it

30:32you can make different things happen to the surface so with aluminium you can only get two colors black and white and

30:38that’s why I call that a binary material these three here and I did them because

30:44I wanted to be to make sure that everything was repeating correctly they were the two nanosecond pedal pulses and

30:51they basically went out as far as about five millimeters before they ran out of

30:57puff now when we went to the 200 millisecond pulse here

31:05we ran all the way through to 8.75 millimeters and then we’ve gone above

31:11then we’ve gone above that and we’ve probably got as far as 12 millimeters

31:17before we run out of power but the last five lines on that pattern we’re getting

31:25thinner and thinner and thinner but they were showing the same sort of line width degradation that we were seeing on the

31:30wood with the continuous laser we were obviously reaching this is the wrong phrase to use but the pointy bit of the

31:38beam where the power is disappearing now the same was happening to this stainless

31:44steel I mean that went into this that went into this second section here where

31:49I repeated the pattern again but with a 9-mil offset that went up to about 12

31:54millimeters in total where the last five lines were gradually getting narrower and narrower so as I said to at the

32:00beginning of this session we’re now waiting for equipment to be able to look closely at these surfaces because trust

Transcript for Fiber Laser Marker: Let’s Zoom in on Focus (Cont…)

32:08me what you see through here is just mind-blowing you just cannot believe what’s happening

32:14to that surface this is only a low power system which is great because it gives me a lovely stereo view and I can see

32:21the depth of the cruise the depths of the cuts I can see the damage on the side of the cuts it’s absolutely amazing

32:26until we get a camera to stick on here you won’t be able to see what’s under there in about a week or so I hope to

32:33have another microscope where we can examine the surface very very closely but I’ve still got to wait until I get

32:40my camera from China China well they’ve got this problem at the moment that

32:45might delay things leaving China well I think that was a very useful session on focus and I think we can see that my

32:51prediction of what was likely to be happening around this focal point it appears that the long nanosecond pulses

32:58are the ones that are actually extending way way down beyond the focal point well

33:03that’s another little fragment of knowledge and information that we can put in the can and build into our analysis when we see things and it’s so

33:11sad at this moment in time that you can’t see what I can see I’ve now got the correct parameters

33:16for the Lotus laser focus test so we’re going to give that a try and see how we

33:22get on and how that compares to the results that I got

33:33they’re using a similar technique to me because obviously they’re running the power off at this end and they’re running the power off at the end but

33:40instead of just using a line they’ve drawn a scale and the most powerful part of the scale is supposed to be in the

33:47middle at zero and it’s not at the moment because we’ve got minus 3 and

33:54plus 5 hmm so it looks as though the

34:02Machine needs to go up by at least half or one millimeter this has now got a setting of 1.25 minus the thickness of

34:10material which is 0.5 so this is now point 7 5 above the material and my

34:19setting came out to 0.5 above the material so if I correct the Machine now

34:25to this calibration which I feel very very comfortable with then we will only

34:300.25 apart in our calibration techniques so for now thanks for your time and I’ll

34:36catch up with you in the next session