38 – How Do Lasers Work: The Destructive Power of Light (25:43)

The Lightblade Learning Lab with Russ Sadler

The Lightblade Learning Lab is a series of videos that Russ did for Thinklaser Limited based on using the Lightblade 4060 Laser Cutting and Engraving Machine. Thinklasers Lightblade 4060 has a 400 x 600mm bed size and was supplied with a 60W EFR laser tube. In this session, Russ explains the interaction of the laser beam and materials and answers the following question: So, how do lasers work?

How Do Lasers Work? The Destructive Power of Light!
How Do Lasers Work? The Destructive Power of Light!

Contents

  • Different damage thresholds for materials.
  • The ways that a laser beam cuts and engraves different materials.
  • The beam in cross-section.
  • Test firing the laser at a piece of wood and looking at the effect.
  • Heating organic materials leaving carbon.
  • Result of firing a beam at an organic block.
  • Colour temperatures and absolute zero.
  • Effect of a beam on a surface – reflections and ‘absorption’, i.e. stimulation of the surface atoms.
  • Effect of focusing the beam.
  • Focused beam tests and effect on the time.
  • Effect of longer pulses.
  • Ideally cut with highest power as quickly as possible.

My thanks go out to Tom at Thinklaser for giving permission to embed these videos on this site. If you are looking for a new laser machine from a quality supplier, then I would suggest you check out their website: www.thinklaser.com.

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Transcript for How Do Lasers Work?

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0:10The Destructive Power of Light

0:15Welcome to another Lightblade Learning Lab.

0:19Today I think we’re starting off on a short series of videos about materials,

0:26different materials that we can use and cut on this machine. When I say cut,

0:34engrave, damage; I think damage is a good word because

0:41that’s effectively what we’re doing with this machine we’re damaging material. Now

0:46we’ve talked in the past about various materials and the way in which they have

0:51different… something I call damage thresholds, in other words there is a

0:56certain amount of energy which if you exceed would damage the surface of the

1:01material. Now whether that damage is mechanical

1:04damage or some other form of damage like heat, there will be a threshold for each

1:12material and that’s one of the strange things about this particular technology.

1:19The Laser Technology, because we’ve taken it for granted that it cuts, that

1:28it engraves, that it does strange things to our materials, but have we really got

1:35an understanding of what’s going on? That’s what we’re going to try and

1:40uncover in this session and then we’re going to go on to look at how the

1:46mechanism by which the laser beam works, can damage different materials in

1:51different ways. Now you might imagine that this laser

1:56beam is like a hot knife cutting through butter, wrong!

2:02Neither is it like a hacksaw or any sort of saw, where you cut through your

Transcript for How Do Lasers Work? (Cont…)

2:09material evenly all the way through. These are I suppose, basically what I

2:15would call linear damage mechanisms, they damage the whole of the material at the

2:20same time. The laser beam is not like that at all,

2:27you can watch it cutting through a piece of clear acrylic and you can see a

2:32straight line going through there and you think that it’s a magical hacksaw.

2:36I’m afraid it’s not, the mechanism for cutting with a laser beam is completely

2:43different. Now when we were doing rotary engraving and we use glass, we touched on

2:48the subject of just how it is that glass gets engraved. How you can damage the

2:53surface of the glass and that’s basically where we’re going to pick up

2:58and carry on today, the discussion about the damage mechanism. Glass is just one

3:05damage mechanism, along with stone slate and other mineral materials where you

3:15damage the surface by effectively “stone chipping” but it’s a thermal stone

3:22chipping as opposed to a mechanical stone chipping mechanism. Then we’ve got

3:27wood that looks as though it’s a burning process, I chose my words carefully there

3:35and then we’ve got other things like this, which is a synthetic rubber or a

3:41rubbery type material which to be honest I haven’t got a clue how this operates. I

3:46suspect this is evaporation as well, but it’s one of the products that we’re

3:49going to test in the next few sessions. Now we’ve already spoken of the nature

3:55of our beam of light that we’re producing, our laser beam and the fact

4:00that it has got a brighter intensity in the centre of the beam and it disappears

Transcript for How Do Lasers Work? (Cont…)

4:07away to nothing towards the outside of the beam, but of course we can’t see it

4:10because it’s an invisible beam. So when we look at the beam from the side

4:15here’s what we shall see. We shall see a very dense central part to the beam and the

4:20light will be getting fainter as it gets towards the outside.

4:22Well we’ve also spoken in the past about the energy profile across this beam and

4:27the fact that it is this sort of shape and you can clearly see that from the

4:34way in which this colour density changes. Right, so I’ve got a piece of 3

4:40millimeter plywood which I’ve got sitting in the laser beam well away from

4:45the mirrors and everything else. I’ve got the power turned up towards maximum and

4:49I’m going to hold the pulse button on and we’re going to burn a mark on that

4:55face. Now I’m gonna have to turn the extraction on because we are going to

4:59produce a little bit of smoke.

5:05Now, I’ve stopped on and off, but what I’d like you to see is to watch what happens

5:12right in the centre of that burn, especially when I stop and the flame

5:17disappears. I’ll try and blow the flame away so that you can see what’s going on.

5:25Can you see the intense bright light behind that flame?

6:01The intensity of light is so high there that I’ve actually got those sunspots

Transcript for How Do Lasers Work? (Cont…)

6:07that you see on your retina. Now, the flame has now stopped,

6:17and we’ve just got the glow.

6:21Why is that do you think? Let’s move in and take a closer look, and

6:26there’s the answer to the question, look I’ve burnt a hole in it! Let’s understand what

6:35this wood is, it basically is a cellulose material and it’s an organic material

6:44which burns and it burns with a flame at about 300 ~ 350 degrees C and

6:51once the organic material has burnt away what we’re left with is this black stuff

6:57here. Which basically is pure carbon, this centre part has not burned away it’s

7:04actually evaporated away. You can’t evaporate wood! Well true,

7:11because wood does not have a liquid phase it doesn’t go from solid to liquid

7:17to vapor.

7:20But carbon is a rather special material, carbon exists as a solid material and

7:27at about 3,000 degrees C it changes directly from a solid material to a gas

7:37and just disappears.

7:41Hence the reason we have a hole in there. We’ve turned the wood to carbon and then

7:48we’ve supplied so much heat to the carbon at the center of our beam here,

7:52we’ve evaporated the carbon away and the other thing I want you to note about

7:56this, although the flame was blowing across this way and we got a little bit

8:01of a distortion or a halo over here. What we have got is from the centre, we’ve got

Transcript for How Do Lasers Work? (Cont…)

8:06a gradual burning process which is from a very high temperature to a much lower

8:11temperature out here, where it was scorching and as you can see, look it’s gone away to a

8:15very faint Brown right at the edge. Where we’re just having enough heat to damage

8:21the surface of the material. We can almost measure the diameter of our laser

8:25beam by looking at the size of this. Although the power is in the middle,

8:32there is energy that goes out to maybe 10 or 12 millimetres diameter.

8:37Now we’ve also witnessed this energy profile in the beam by firing the beam

8:42at a block of acrylic and here we can see the energy density at the centre of

8:48the beam is much higher, because it’s penetrated deeper into the acrylic. We

8:53just witnessed exactly the same thing happening with the wood, this very high

8:58intensity part of the centre of the beam has been able to generate huge

9:03temperatures and that’s before it passes through the lens. Anybody that’s

9:08messed around with lighting or photography will understand this color

9:12temperature chart and it was a scientist called Kelvin who basically realized

9:17that the filament in a light bulb, glowed at different colours and those

9:24colours were representative of different temperatures. Now, here we’re talking

9:29about temperatures that are going up into the thousands of degrees Kelvin or

9:34thousands of degrees C. They’re more or less the same thing with an offset, let

9:41me explain. This same guy, was also responsible for determining something

9:50called absolute zero. Now all molecules sit

9:58there, at room temperature doing this. They’ve got vibration and basically the

Transcript for How Do Lasers Work? (Cont…)

10:09amount of vibration is actually an indication of their temperature. The

10:17faster they vibrate the higher the temperature. Now that sounds like a

10:22strange concept, but that is actually what temperature is and here it’s

10:28defined; “how fast its atoms and molecules oscillate” that’s what temperature of an

10:33object is. He took that concept to the opposite extreme and said; well, look if

10:40they’re doing this at room temperature there must be a point at which, when I

10:45lower the temperature all motion will stop and that is what absolute zero was

10:55defined as. Now in modern day terms, with quantum physics and all the rest of it,

11:00they have established that at absolute zero there is still a small amount of motion

11:05that goes on, but to all intents and purposes absolute zero means the atoms

11:12are not vibrating and as you raise the temperature, the atoms vibrate faster and

11:18faster and faster. Now that concept of temperature and

11:22atoms moving fast, was also discussed when we talked about the nature of the

11:29laser tube and the mechanism that takes place within the tube itself and how the

11:34nitrogen gets very very excited by being threatened with 25,000 volts, and if you

11:43allow more current to pass through the nitrogen, it gets more and more and more

11:48excited and it can do damage to the co2 molecules because it’s so excited. It’s

11:56all related to this same concept of atomic motion, so if you can carry

12:02forward this idea of vibrating molecules, vibrating faster and faster, getting

Transcript for How Do Lasers Work? (Cont…)

12:08hotter and hotter, that is basically the mechanism by which

12:11the laser beam works. Back to this little picture here. Now Mr. Kelvin, working the

12:18other way, away from absolute zero determined that the colour of a filament

12:24in a bulb was able to determine its temperature.

12:28Now absolute zero was determined as minus 273 degrees C and so that is where

12:36the kelvin scale starts, at minus 273 degrees C. So these values here are

12:44degrees K Kelvin and they start at minus 273, I mean they are degrees C but

12:55they’ve got this offset on here so to get them into numbers that we understand

13:00we had to have to add about another 300 say another 300 degrees C onto these.

13:05Well look at these numbers these are thousands of degrees C so another 300

13:09degrees C added onto these numbers it’s not actually going to give us a great

13:13deal of change, but the point that I want you to understand is that we’ve been

13:19looking at colors for our glowing wood up here in this bright yellow to white

13:27region and that is somewhere in the region of anything between four and five

13:33thousand degrees C. At 3,000 degrees C carbon will sublimate, it will go

13:41directly from a solid to a gas and so here we’ve got confirming information

13:47that what we’re looking at in the center of that beam. I’ve got to be very careful

13:52what I say here, because the temperature is not in the beam itself, but the beam

13:58can, depending on the material that we’re firing at and we fired it at wood. In

Transcript for How Do Lasers Work? (Cont…)

14:02that particular instance it was able to generate temperatures up in the four to

14:07five thousand degrees C, because it was hot enough to sublimate carbon. Now that

14:14does not mean to say that the beam itself is hot,

14:19that is a misconception. The beam is a beam of light and it has no temperature,

14:27so how do we get the damage? How do we burn things if there’s no

14:32temperature in this light? I’ll bring you back to this little statement here, where

14:37it says temperature is actually defined by how fast the atoms and molecules of a

14:43material vibrate or oscillate. When this light hits the surface of that material,

14:47it could do one of two things. If this is a metallic surface, it will certainly

14:55reflect, but of course like all light the surface has to be flat to act like a

15:02mirror, if it’s in any way distorted the light will disappear off at different

15:08angles. Things like gold, silver, copper and aluminium, particularly those four

15:14metals will reflect 99% or better. So they are literally mirrors, lower grade

15:22materials like mild steel for example or iron will be somewhere in the region

15:27about 60 or 65 percent reflective. The other thing that could happen is the

15:33light can be absorbed into the surface. Oooh, I’ve got to be careful about this

15:39word absorption, because it gives you the impression the material is like a sponge,

15:43wrong! What actually happens is, just at the surface of the material, there are

15:50atoms which will be stimulated just like when you put things in your microwave

15:58and it heats up, that’s what would happen here. These light particles are

Transcript for How Do Lasers Work? (Cont…)

16:05stimulating the atoms on the surface of the material. It’s light, it can’t

16:11penetrate the material itself, it can only interact with a surface and that’s

16:17the important thing to remember throughout this. The solid starts to

16:22vibrate because it’s stimulated by the light and as we’ve just discussed,

16:29vibration, extra stimulation, heat. Because these materials that we’re

16:34going to be firing this at are non metallic materials, they have got very

16:40poor conduction properties, they do not transfer heat easily and this heat

16:47builds up on the surface and because the heat can’t disappear, it stays on the

16:53surface and it gets hotter and hotter and hotter. As we’ve seen in the Wood

16:59experiment, it starts to glow white-hot and it glows white-hot in the centre here.

17:06Basically the wood has burnt away, but then after that we saw this white glow

17:11in the center here and the white glow was the carbon, it wasn’t burning it was

17:17heating up and it heated up to a white-hot temperature where it couldn’t

17:21resist anymore and it turned into a gas and so gradually what was happening is

17:26here, we get a little bit of erosion in the surface as the carbon

17:32evaporates and it leaves clean carbon behind and so the light beam stimulates

17:37this new carbon that’s behind here and gradually what we’ve got is a process of

17:45erosion and it eventually worked its way through the three millimeter thick

17:54material. The important thing I need to stress here is this is not like the

18:01hacksaw, this is not like the knife, this this is not a continuous process this is

Transcript for How Do Lasers Work? (Cont…)

18:06almost like a wood pecking action, it’s a gradual erosion process, even with the

18:11very high energy levels that we had here it took time to burn through the wood.

18:16But what we can categorically say is, that when we fire this beam at wood it’s

18:21capable, because of the carbon content of generating at least 3,000 degrees C. Now

18:29the beam itself is not 3,000 degrees C, it’s just a beam of light now if this

18:35was just a piece of acrylic,

18:40this damage here took place at a much lower temperature than 3000 degrees C,

18:47because the threshold, the damage threshold on this material is different

18:52to the damage threshold of wood. So every material will react to this light

18:57stimulation in a different way, but it is the light stimulation that heats up the

19:04material, it’s not the light itself that is hot. That’s an important concept to

19:13remember, we are now going to take that beam and we’re going to magnify it.

19:18We’re going to concentrate it down and we’re going to concentrate it down from

19:22six millimeters diameter to 0.1 millimeters diameter. Basically we’re

19:28going to amplify the light at this point, the energy density, to some phenomenal

19:34value and in fact if we do a very simple bit of maths you’ll find that that, is

19:40about three thousand five hundred times smaller than that. So we’ve amplified

19:50potentially, the ability to do three thousand degrees C’s worth of damage,

19:57there is a huge amplification of the energy density.

Transcript for How Do Lasers Work? (Cont…)

20:03Watch very carefully here, I’m just going to do a very simple, very very quick

20:10pulse. You may or may not have seen what happened there, but let’s first of all

20:17check. How long was that pulse? Maybe a tenth of a second? The pulse has burnt

20:24right the way through and here’s what the underneath looks like, it’s burnt

20:29right through it in less than say a tenth of a second or maybe even less

20:34than that. We’ll do it one more time, now if you watch very carefully you’ll see

20:39two things happening, you’ll see a little bit of a flame coming up where the wood

20:44is burning away, but then you’ll see this white flash as the carbon evaporates.

20:56We’ve still got the same energy profile, it’s just been magnified up to some

21:06phenomenal intensity as it’s been decreased to 0.1 of a millimeter

21:13diameter. The same damage principle is existing after the lens as the one that

21:22I demonstrated to you before the lens and whereas with the unfocused beam it

21:29was taking it’s possibly a minute to burn through this piece of 3 millimeter

21:33material. When we amplified the beam it was taking less than 0.1 of a second!

21:42This piece of wood is the same piece of wood that took over a minute to pierce

21:47through with low energy density, once we amplify the energy density up, we

21:55can do the same amount of work in a tenth of a second. Time is something

22:00that’s going to come into our cutting and our engraving discussions as we push

Transcript for How Do Lasers Work? (Cont…)

22:06on with this subject. Now, we saw that in a tenth of a second we could burn a very

22:12compact little hole through. Let’s see what happens if I leave the beam on for

22:18about a second. One second. We’ve got our scorch Corona around the

22:27outside, the same as we did here. We’ve started to generate additional damage

22:35around the outside of the hole, whereas here,

22:40we had it on for such a short period of time that all we did was to pierce a

22:44hole, instantly through there, right with the centre of the energy beam and we

22:50didn’t get a chance for the lower energy levels around the outside to have any

22:55burning effect on the wood. So that’s another important lesson I’m trying to

23:00get over to you about time, you need just enough time to do the damage that you

23:06want to do and not so much time that you actually cause collateral damage as well.

23:14Now, these are very important concepts when it comes to understanding the

23:18cutting process, because this is effectively the process that causes

23:24charring on the edge of your cut. The idea is to cut as quickly as possible

23:32with as high an energy level as you can. The way in which the laser beam

23:37damages material is a difficult concept to try and describe to you, but I

23:44hope that I’ve broken it down into small enough chunks, that like a jigsaw puzzle

23:49you’ll be able to put the pieces together for yourself. The fact that

23:53we’ve got energy density which causes gradual erosion and not an

23:58instantaneous cut, even that instantaneous tenth of a second cut that

Transcript for How Do Lasers Work? (Cont…)

24:04took place as we burn that hole through, was not an instantaneous pierce. It was

24:10still done by exactly the same pecking mechanism, it’s just that it happened in

24:15such a short period of time that it looked as though it was an instant cut,

24:21like a hacksaw or a knife, but it’s not, it’s a woodpecker.

24:27A very fast-acting woodpecker and that process applies to every material that

24:35we’re going to fire this beam at, whether we’re doing engraving or whether we’re

24:39doing cutting, we’re basically either going, we’re going to be damaging the

24:44material surface with this high-energy beam of light and it’s the interaction

24:50of the light with the surface that causes the damage. It causes heating and

24:56that heating causes different types of damage in different types of material.

25:01Well, I think that’s enough mental gymnastics for today we’ve basically

25:06talked about the damage process for wood and organic materials in this session

25:12and we’ve used that as a demonstration of how the laser beam damages material,

25:18because it’s one of the easier concepts to understand. Now there are other damage

25:24concepts which we’ll move on to in future sessions, but they’re all based on

25:28this same principle. So the hard work is done, if you can understand

25:34what’s been going on today. So thank you very much for your time and I’ll catch

25:39up with you in the next session

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