Session 04 – The CO2 Laser Tube

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The Concise RDWorks Learning Lab with Russ Sadler.

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Session 4: The Laser Tube. Now here we’ve got the subject of the next two sessions, the laser tube and the high voltage power supply.

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Now the laser tube in itself is quite a large subject to tackle.

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So I’m going to push the power supply aside and we’ll deal with that as a separate entity.

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Now, in the last session,

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we spoke about the laser beam itself and the purpose of this session is to show you where and how that laser beam is generated.

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Well, in the first session, we talked about atoms. It didn’t have all this little complicated in the middle.

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It was just one lump.

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You’ll see that there are equal number of electrons which are negative, balancing a set of protons which are in the core, which are positive.

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So everything is nicely in equilibrium. Not all these orbits are the same as shown in this diagram here.

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Some of them are further out than others,

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which means that these electrons that are further out from the core are vulnerable to being attracted or displaced out of their orbit,

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which is going to make the atom a little bit unstable. Gas does not conduct electricity, and that’s because everything is nicely stable.

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There are no free electrons. There’s a continuous stream of very high energy particles that are flying through space.

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And those very high energy particles may or may not pass cleanly through this ninety nine point nine percent space that is in an atom.

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There are so many particles flying around that one or more of those particles will collide and it will

Transcript for the CO₂ Laser Tube (Cont…)

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knock one of those nitrogen electrons out of its orbit and it will then become a free floating electron.

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We’ve got here a connection for the called the Anode in which we put 25000 volts or something like that,

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D.C. And here we got the return back to the power supply so that we’ve got a circuit here.

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But of course, this is gas in here and gas is non conductive.

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So how can we have a circuit when we don’t have something here that conducts the electricity?

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We’ve got our nitrogen, 10 percent of it, remember, mixed in with 10 percent carbon dioxide and 80 percent helium,

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which is just literally existing in our laser tube.

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We haven’t switched anything on yet, but there are still free electrons floating around in there.

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One, a hundred, a million? Who knows how many there are, but there only needs to be one.

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And remember, it is an electron is negatively charged here.

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Here we’re going to add 25000 volts across this electrode here and this electrode is sitting at zero.

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So we’ve got 25000 volts difference across this part here.

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Now,

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what’s going to happen is, this one negative electron is going to be violently attracted towards this positive Anode. So by the time it’s wizzed down this tube,

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at nearly the speed of light,

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we start to generate a cascade effect whereby it’s colliding with more nitrogen electrons and producing a bigger and bigger shower of electrons,

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that’s being attracted to this positive Anode. So by adding 25000 volts to that column and attracting and creating a flow of electrons,

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we’ve actually turned this piece of non conductive gas that’s in here into effectively a piece of wire to complete the circuit.

Transcript for the CO₂ Laser Tube (Cont…)

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Now, that gas reservoir, which is around the outside there, the big tube, is containing the gas mixture.

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And that gas mixture is 10 percent nitrogen, 10 percent carbon dioxide and 80 percent helium, is busy flowing in a circular motion around this tube,

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spinning out, getting hot and traveling back along the tube and recirculating through here.

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So we’ve got a continuous circulation of the gas mixture through this central core because remember,

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we’ve got electrons which are effectively driving the gas flow from negative to positive.

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What we’ve got here with this ionized nitrogen, it is lightning.

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That’s exactly what lightning is. It’s ionized nitrogen, which is conducting electricity between the clouds and the ground.

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The atmosphere is about 80 percent nitrogen.

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We’ve got a second tube that sits around that central core and it carries water to carry the heat away from the core to the water jacket.

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And to do that, they add 80 percent helium, because helium is a very, very good conductor of heat.

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And that’s all the helium is doing. The helium will never run out, the nitrogen will never run out.

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We’ve got a third gas in there, carbon dioxide, which could run out even though this is a completely sealed system.

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It can’t leak out and it can’t leak in. Nothing is going to go anywhere.

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But as I will explain to you later, we can destroy the carbon dioxide.

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None of what I’m going to explain is physics perfection.

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These are concepts that would enable you to understand how your tube works without ever being a physicist.

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What we really got there is lightning in a bottle. So let’s take a few moments out and look at a real laser tube.

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This thick wire tells you there’s something going on here. This red wire can carry anything up to about 40000 volts.

Transcript for the CO₂ Laser Tube (Cont…)

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But most of the time, twenty to twenty five thousand volts is enough of a voltage to stimulate the nitrogen to go into ionization mode.

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Once the ionization has taken place and stimulated and got triggered, as they call it,

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the actual voltage in here will drop down to some slightly lower value of something like maybe 15 to 17000 volts.

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If you want to move your tube or change your tube or do something with your tube.

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I suggest that you leave your machine switched off for about half an hour before you touch this wire.

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And that might be oversimplifying the situation, but better to be safe than sorry. That 25000 volts is connected into this electrode here.

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As you can see, it’s a tubular electrode that allows the gas to flow through and then carry on back through this tube here.

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And wizz around in a spiral motion. That will then cause the gas to spin as it travels back along the tube and lose some of its heat.

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Now, it won’t ever feel, this tube will never feel hot. It might feel warm.

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Now, I think we’ve caught the tube in the light right there. So you might be able to see the three tubes, this outer tube, the second tube in there,

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and then the third inner tube, which is the ionization tube in the middle.

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If ever you change your tube, there is no specific orientation for what happens at this end of the tube.

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But at this end of the tube, which is the negative end of the tube, this has basically got zero votes sitting here.

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So this is not dangerous this end. We’ve got a metal cathode sitting in there, which is tubular, which allows the gas to flow through it.

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And here we’ve got a water jacket, which is allowing the water to escape out of the tube and return to the reservoir through a water jacket.

Transcript for the CO₂ Laser Tube (Cont…)

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This end of the tube, when you put a new tubein, is important that you orientate that tube

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there vertically, because bubbles always sit on the top.

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They will always hang on the ceiling of this inner tube here, the water jacket.

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And so what you want, any air bubbles to come along here and escape upwards?

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Many machines I’ve seen have been assembled like this or like this,

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and that’s absolutely useless because the air bubbles get trapped in the water jacket.

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So that’s an essential point to note when you assemble a tube.

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Now, there’s one other interesting thing that you should note.

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They often put a marker on here that says bracket placement or bracket position, and then there’s one at the other end.

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You don’t have to be too fussy about those bracket positions.

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If I put a pivot point in the middle there to support that tube, I think you can fairly clearly see that that tube is going to want to sag like that,

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because half the way is this way and half the weight is this way.

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So by dividing the tube approximately into quarters like this, if I move that pivot point to there and that pivot point to there,

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then what I’ve done, I’ve made that very short seesaw now and that very short seesaw there.

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We’ve got 50 percent of this hanging this way, 50 percent of the hanging that way.

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So it’s nicely balanced and that’s nicely balanced. So the stress in the middle here is nil because both sides are balanced about the center point.

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So there is no tendency for that tube to want to bend. And that is a very important point.

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These ends here are very accurately manufactured parallel to each other.

Transcript for the CO₂ Laser Tube (Cont…)

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And so if the tube bends in any way, shape or form, it means these could go like that.

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And that’s going to destroy the lasing action within your tube.

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Don’t get too stressed about it because the glass is very, very stiff.

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You haven’t got to be exact with these quarters.

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OK, let’s come back to our simple model of the tube, because this is much easier to explain what’s going on without any additional complexity.

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The key thing that we’re after is this nitrogen, ionized nitrogen in the tube.

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Without that nitrogen, you cannot generate a laser beam.

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That mauve beam that you see in the tube is not the laser beam.

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If that was the laser beam, it would be pink all the way round the front and you would see it.

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The laser beam is invisible and coexists inside this ionization tube with the nitrogen.

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The nitrogen is basically the engine that we’re going to use to create the lasing action.

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The helium, as I said, it’s doing nothing. It’s only carrying the heat away.

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So it’s not part of the lasing action at all. We’ve just got these two key gases involved, ionized nitrogen and carbon dioxide.

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What I’m going to describe to you now is going to horrify any physicist that’s listening.

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Don’t worry about them. What you need is the nice, simple model that you can carry in your mind of how this system works.

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Now, let’s go back to one of the first principles that I described to you.

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Heat is what? Molecular motion, if I can stimulate the molecules in here to vibrate faster.

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So more current flow through here, means that these ionized nitrogen atoms are vibrating faster and as they vibrate fast,

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what happens is, mixed in with this gas mixture

Transcript for the CO₂ Laser Tube (Cont…)

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you’ve got the carbon dioxide molecules. One carbon bonded to two oxygen atoms.

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And what will happen is that some of the energy that’s in this nitrogen will be transferred to the carbon dioxide molecule.

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It’s a bit like billiard balls or snooker balls colliding.

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There will be an energy transfer. That’s the model that I’d like you to carry.

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But it isn’t exactly how it works in physics terms.

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So we’ve now transferred some of the energy that this very violent nitrogen vibration has got and we’ve added it to a carbon dioxide molecule.

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That carbon dioxide molecule does not really want to carry this extra energy.

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It will find any excuse to get rid of it.

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Physics is very strange and a very peculiar thing happens.

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So we’ve added some extra energy to the carbon dioxide molecule.

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And basically what we’ve done, we’ve made one or more of the electrons,

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it’s jumped from one orbit to another orbit, and it doesn’t really want to be at that different orbit.

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It wants to get back to where it was, get back to stability. If nothing happens within a millionth of a second, the movement of this electron,

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which has gone out to a slightly bigger orbit, drops back to its normal orbit.

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And as it drops back to its normal orbit, it gives up the energy that it acquired during the collision.

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And that energy appears in the form of this strange little thing here, a little pulse of light called a photon, which has got a certain frequency.

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And that frequency is ten point six microns wavelength. Or put it another way, twenty eight thousand billion hertz.

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That’s the frequency of that little pulse there.

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Now, this is where physics gets really strange because remember, we are not just going to have one electron raise to a high level.

Transcript for the CO₂ Laser Tube (Cont…)

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We’ve probably promoted several of them, because we’ve got all these nitrogen collisions taking place.

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And so we’ve got lots of atoms in our ionization tube which have got extra energy that they do not want.

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As this photon, just one photon travels along the ionization tube,

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it will pass by atoms which have got electrons which have got too much energy, and they want to drop back to their lower orbit.

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And what will happen is, this, like the Pied Piper of Hamelin will just collect photons

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as it travels along the tube and all those photons will be synchronized like this.

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Okay. They will all be fully synchronized in terms of their phases.

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Remember what I said? Coherent light. Well, this is how coherent light has been created within the tube.

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One photon will stimulate another photon to join it.

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And of course, now we’ve got two photons which are creating more stimulation for other photons to join it.

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And so, again, we’ve got this massive cascade effect whereby you’ve got millions and billions of photons

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attracting millions and billions of photons and they’re all traveling in one direction.

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And here they are traveling in this direction and they travel here at the speed of light.

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And photons have not got any charge. So they’re not positive or negative.

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They’re just packets of light. So they won’t stop at the anode.

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They will pass right through the anode, leaving the pink beam ionized nitrogen behind and they will fly out to the end here.

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And on the end here, what we’ve got is a mirror. And remember, Mirror Metal, that will reflect the light and it reflects the light back down here.

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In the meantime, we’ve got all this energized nitrogen which is busy creating more and more energized carbon dioxide atoms.

Transcript for the CO₂ Laser Tube (Cont…)

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And as those photons passed down here, they will collect more and more photons, more and more energy,

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and they’ll fly at the end of here as well and bounce off this mirror and then they will go back and collect more photons.

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So we’ve got this continuous process of backwards and forwards photon reflection.

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This mirror here is 100 percent reflective, but this mirror here is not 100 percent reflective.

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All these photons that are flying backwards and forwards, some of them are escaping out here.

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Because we’ve got 90 percent mirror, so we’ve got, say, 10 percent of these photons which are flying out the end here,

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and that is our laser beam, that’s the invisible collection of coherent photons which make up the laser beam.

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So this is where the word laser comes from. It’s an acronym.

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And what it stands for is light amplification.

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And here we’ve got photons of light which are bouncing backwards and forwards, remember?

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And as they bounce backwards and forwards, they collect more light, collect more light.

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They’re amplifying the light by bouncing backwards and forwards between the mirrors.

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So we have light amplification by stimulated emission of radiation.

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Now the radiation is the photon that’s created and the stimulated emission is what happens

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when a photon passes by an energized carbon dioxide atom and triggers it to drop down.

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That’s the stimulated emission. You only get the radiation of light because you have stimulated the emission of it.

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If it’s not stimulated, it’s random and random light is not going to be any use to us at all.

Transcript for the CO₂ Laser Tube (Cont…)

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So that’s the marvelous physics that takes place inside the laser tube itself.

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I did say to you that the carbon dioxide is not forever.

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It can be destroyed.

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And under normal circumstances, that destruction will take several years current passing through the tube makes the nitrogen atoms energetic,

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vibrating faster, getting hotter. It is that interaction, the billiard ball collisions, if you like,

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with the carbon dioxide transferring energy to them, which creates the lasing action.

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So if you put too much current through this circuit, the nitrogen will go bonkers.

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There will be so much energy in the nitrogen that when it, if you like your cueball, hit’s a carbon dioxide molecule.

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It won’t just collide and send it to a different pocket.

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It will collide with it and smash it to pieces.

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Now, in physics terms, what that really means is you hit it so hard that you’ve broken away one of the oxygen atoms from the carbon dioxide.

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And it’s no longer carbon dioxide, it’s carbon monoxide and a free oxygen atom.

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Now the free oxygen atom will float around in here and it will eventually try and join with something else.

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And the chances are what it’s going to do, it’s going to start oxidizing this cathode.

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If you overdrive your tube with too much current, you will destroy the one thing in that tube which you need, which is carbon dioxide.

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You will still have your pink glow because the nitrogen is still in there. You can’t destroy the nitrogen, you can’t destroy the helium.

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So everything will appear to be working, but you get nothing out the end here.

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So be very careful how much current you put through your tube.

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I encourage anybody and everybody to fit a very cheap analog ammeter into their circuit to monitor the current flow in this circuit.

Transcript for the CO₂ Laser Tube (Cont…)

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Now, you don’t need a very special ammeter because we’re not going to put the meter in this high voltage line.

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What we do, we put the ammeter in here. We just break that line and put the ammeter in there.

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And that monitors the current flow back to the high voltage power supply.

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And it’s the same current flow that’s coming out, but it’s just at a different voltage.

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Current flow remains the same. The voltage doesn’t. Now, there are some basic guidelines for current that I can give you. Up to a certain point,

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the power of the tube goes with the length of the tube. Now, the most common thing that you will find is that you are being sold a 50 watt tube.

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And if you’re being sold a 50 watt tube, it will be a thousand millimeters long.

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I can absolutely guarantee it’ll only be 800 millimeters long because it’s what fits into the machine.

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And that’s, in reality, a 40 watt tube.

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It may well be capable of running at 50 watts maximum test power, but you mustn’t use it at that, because you will be then over driving the tube.

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So you must stay to the current, that’s basically the size of your tube, not what the tube is rated to.

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So, for example, a 30 watt tube will be 600 millimeters long and it will typically be 50 millimeters diameter,

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fifteen milliamps. A 40 watt tube, 800mm long, 18 milliamps. A 50 watt tube, a thousand millimeters long, 20 milliamps.

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Now things start to get a little bit flaky.

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At 70 watts, you will find that maybe it’s twelve hundred and fifty long and 55 millimeters diameter. At 80 watts

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it may well be only 1450 long and 80mm diameter, and these numbers here will change.

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These are the sorts of values, the old school values that used to exist when all tubes were 50 millimeters diameter.

Transcript for the CO₂ Laser Tube (Cont…)

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They’re not that way anymore. So you might need to check up on your manufacturer’s specification to the actual current that they will allow.

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So under normal circumstances, this system is well controlled and you will get a constant current passing through here.

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A constant current means a constant level of vibration and a constant level of vibrational energy means a constant number of collisions,

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which means we get a constant flow of power out of the end here. Now provide you don’t overdrive the current through here.

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You will get five, six, seven years life out of this tube.

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But if you overdrive the tube, then I’m afraid that could be down to months.

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Every time the program switches this tube on, there is a very, very high frequency and surge of current pulses that pass through here.

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This is a particular zone that you can exploit and use.

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It’s called the Pre-ionisation Zone. And during that pre-ionization zone, which normally exists between about four and six milliamps,

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you’ve got very, very high frequency, very, very high power pulses of energy. Now,

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very high pulses of power means we must be generating that power with very high

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current flows and very high current flows means we are destroying the CO2.

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But it happens for such a short period of time.

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Every time we switch on that, it really doesn’t have a major over driving effect, but it will build up over time.

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And that is the main reason why your tube will eventually die.

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The manufacturers know this and they do two things.

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First of all, they introduce a small amount of maybe hydrogen or water,

Transcript for the CO₂ Laser Tube (Cont…)

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into the gas mix, and that acts a bit like a marriage counselor.

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These dissociated oxygen and carbon monoxide atoms are encouraged to rebond to create carbon dioxide.

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And so that helps to prolong the life of your tube. Now, there will be something else that you will notice in a tube that is not switched on.

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Some of your tubes, well you’ll find that it’s got a very slight pinky hue to the ionization tube.

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That’s a very good sign because it does mean to say that the tube has been coated on the inside with

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an atomically thin layer of gold and gold does exactly the same thing as the water in the gas.

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It encourages recombination to try and prolong the life of the gas inside the tube.

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Unless you are reckless with your tube an A-Grade tube, as I said, should last you between maybe five and seven years.

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So that’s about as much as I really want to tell you about the laser tube.

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But there are a couple of little teeny weeny things that I think would be of interest to you.

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First of all, this laser tube will never just drop down dead.

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It doesn’t have a heart attack and stop. What will happen is, it will gradually, over a few weeks, maybe a month, lose its power.

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You will know that because all of a sudden, something that you cut a 10 millimeters a second doesn’t cut anymore.

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Don’t wind the power up because you’ll be over driving the machine, slow the speed down.

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That’s the way to deal with it. And then note the warning signs and think to yourself, I’d better start saving up for a new tube.

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There’s a second myth around that, Tubes have got a shelf-life.

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Tubes do not have a shelf life. They’re hermetically sealed.

Transcript for the CO₂ Laser Tube (Cont…)

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There’s all sorts of old wives tales about this. In reality, this tube can sit on your shelf for ten years and then you can use it.

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The only thing that will go out of life is the warranty.

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And so really when you buy a brand new A-Grade tube, you really need the longest life you can possibly get.

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You want something that’s only a few months old because at the time they charged the tube with gas and test it,

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that’s when the warranty starts. Doesn’t start from the point in time when you buy it.

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And of course, different manufacturers of tubes have got different warranty periods.

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And that brings me round to a final point, which is the manufacturer.

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Now, this tube clearly says it’s from a known manufacturer.

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It says LASEA on there. But in fact, that is an EFR brand.

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That is a quality tube and any quality tube,

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first of all, it will have a manufacturer’s name on it. It’ll have a manufacturer’s address.

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It won’t just say model number 1200 millimeters, it’ll have a model number on it, not the length of the tube.

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It will generally have a serial number. It will have a date.

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And it will have some sort of reference to the person that checked the tube.

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Many tubes that you buy with your laser machine will be B-grade tubes that will be factory rejects which have been relabeled,

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and one cannot say what quality of beam you’ve got and how much power you’ve got.

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All we can say is the tube will work and you, in your naivete, will think that you’ve got a good tube.

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But in fact, it’s a virtual certainty that you have not. One of the major names that you’ll find on tubes which are carrying out this,

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let’s call it fraud, because it is. Is a company called KH laser.

Transcript for the CO₂ Laser Tube (Cont…)

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Now, they’re not a company because they don’t make laser tubes, they pick up reject tubes,

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retest them to make sure they work in some way, shape or form, stick their label on, put them in a machine, ship them into warehouses around the world,

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and then they get somebody in China to sell them to you for commission.

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That somebody in China is also being duped because they have not seen the machine,

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they don’t know anything about the machine, they just selling a product for commission.

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So both you and he are being scammed by the manufacturer of the machine.

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Why do they do that? These tubes are quite expensive, if you can buy a reject tube for nothing or just the price of scrap glass.

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That’s a good deal. Now, that’s about as much as you need to know about Laser tubes to give you a pretty good start in life.

Transcript for the CO₂ Laser Tube