13 – The Anatomy of your CO2 Laser Beam (21:44)

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 investigates the anatomy of the CO2 laser beam and measures the actual diameter of the beam from his laser tube. Using a thermocouple, he confirms that the intensity is Gaussian.

Table of Contents

  1. The Lightblade Learning Lab with Russ Sadler
  2. Table of Contents
  3. Contents
  4. Video Resource Files
  5. External Resource Links
  6. What Next?
  7. Disclaimer
    1. WEBSITE DISCLAIMER
    2. EXTERNAL LINKS DISCLAIMER
    3. AFFILIATES DISCLAIMER
Picture of a laser beam glow in the tube
Picture of a CO2 Laser Beam Glow in the Tube

Contents

  • K-type Thermocouple (Note: this was not shown in the previous session)
  • Paper targets (as used previously) and acrylic targets
  • Looking at the characteristics of the beam, e.g. the diameter
  • The ‘Pulse’ control and configuring it to Manual (timed) or Continuous (controlled by how long the button is pressed) using the menus
  • Using a round target
  • Removing the lens assembly to prevent ash from the target falling onto the lens
  • Looking at the beam diameter
  • Total beam diameter coming out of the laser tube (12.7mm in this case)
  • Gaussian (‘bell-shaped’) energy density characteristic of the beam
  • Need to ensure beam is lined up correctly to ensure no energy is lost at the edges
  • Using the thermocouple to measure the relative energy of the beam in different places across a section of the beam
  • Using the acrylic target to visualise the energy across the beam diameter at the front of the machine, i.e. just before the final mirror
  • Repeating the test immediately after the laser tube
  • Looking at the difference in the shape of the hole made in the acrylic targets
  • Noticeable ‘step’ change in the power across the beam as it emerges from the laser tube
  • Measuring the power of the beam
  • Testing when replacing the molybdenum mirrors with gold-plated copper mirrors

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 Anatomy of your Laser Beam

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00:15

Welcome to another Lightblade learning

00:18

lab, today we are going to use something

00:22

that I exposed to you in the last

00:25

session which is our K-type thermocouple

00:30

and I’ve got some targets here, both

00:35

paper targets and some special Acrylic

00:39

targets made out eight millimeter

00:41

thick acrylic that I have designed and

00:43

made and they fit onto the same holder

00:46

that we made several weeks ago now and

00:51

today is going to be exciting, if I get

00:54

it wrong

00:55

we’re gonna have some fire if I get it

00:57

right

00:58

I won’t gain too much disapproval from

01:01

Thinklaser but what I’m going to show

01:03

you is not necessarily for you to do or

01:06

play with

01:07

I’m trying to demonstrate to you yet

01:11

more technical details about this laser

01:14

beam, because we’ve discovered over the past

01:17

few sessions that something that should

01:21

be maybe a four or five millimeter

01:25

diameter CO2 laser beam who knows? It isn’t

01:28

clearly specified but people believe

01:30

that we’ve got a four or five millimeter

01:33

CO2 laser beam on this 60 watt laser i’m

01:37

afraid my answer to that is hogwash and

01:41

I know that to be the case and I’ve

01:42

demonstrated it to you on one occasion

01:44

earlier you cannot burn an eight

01:48

millimeter hole in a piece of paper with

01:50

a four millimeter beam, you’ll burn a four

01:54

millimetre hole okay you could say it’s

01:57

carrying on burning outwards, but no…

Transcript For Anatomy of your CO2 Laser Beam (Cont…)

02:00

once it’s burnt out

02:02

it’s burnt out. The energy will pass

02:05

right through the middle of a hole and

02:06

it won’t carry on burning out this is

02:08

supposed to be a coherent beam of energy

02:11

which like this pencil stays the same

02:16

shape as it passes through the machine

02:18

from the laser it comes out that shape and

02:21

as it hits the last mirror it supposedly

02:23

that shape

02:23

let’s see what comes out of the tube and

02:26

what eventually hits this last mirror

02:27

because it makes a big difference to how

02:30

you set your mirrors up and how

02:31

accurately you set your mirrors up and

02:34

just how much energy you’re losing

02:36

because you haven’t set your mirrors up

02:38

properly. We’re gonna start off by taking

02:42

a quick look on this keyboard we’ve

02:45

already seen that the pulse control i

02:47

can just pulse it like that, just touch it

02:49

and it works if i move to this control

02:55

here and I select laser set, enter. I have

03:04

got two modes that i can choose from one

03:07

of them is called continuous and the

03:10

other one is called manual this is

03:13

rather ambiguous, manual basically means

03:18

that i press the button pulse button

03:20

once and I get this length of laser time

03:24

regardless of whether i’m holding the

03:26

button or not if I turn it over to

03:29

continuous like that it means I get what

03:34

I press on that button if I just go blip

03:36

that’s all i get, so today we’re going to

03:39

be using manual mode and these are

03:43

milliseconds so what I’ve got there at the

03:45

moment is half a second, i think half a

03:49

second is going to be long enough for me

03:51

to explore what I want to explore we

03:55

will be changing this to 5,000 which is

03:58

five seconds or maybe 4,000 shortly but

Transcript For Anatomy of your CO2 Laser Beam (Cont…)

04:01

we’ll start off with 500 milliseconds

04:03

enter we set our power to sixty-seven

04:07

percent which is maximum

04:10

Enter. We will just demonstrate or

04:13

test that because we’ll put this up in

04:14

front here and we’ll just blip the

04:17

button and hopefully you’ll see that

04:19

I’ve got no control

04:21

it’s running for half a second… right

04:26

we’ve got a round target this time

04:28

because we’re not interested in setting

04:31

the beam up

04:32

what we’re interested in doing is seeing

04:34

what the beam size is. Now i’m going to i’m

04:37

going to put smoke in here which could

04:39

possibly smoke the mirror now I’m not

04:42

worried about that because the mirror is

04:43

easily cleaned the thing that’s less

04:45

easily cleaned and more sensitive is the

04:47

lens in the bottom here, so i’m just gonna

04:49

remove the lens before i start this test

04:51

because I certainly don’t want the lens

04:53

to get smoked if there’s any debris comes in

04:55

there ash debris or anything it’s likely to

04:57

go all the way down and settle on top of

04:59

the lens. Those holes on there, those targets

05:02

they are 3, 6, 9, 12 and 15 millimeters

05:07

diameter so let’s just do a quick pulse

05:10

and see what we get. 9 12-15 we’re already

05:17

out at about six or seven millimeters and

05:20

I’ve only given one little pulse now I’m

05:23

purposely only doing it in pulses because

05:26

you’ll see otherwise it will catch fire

05:28

if I leave it running continuously so we

05:31

do it again

05:40

ok so we’ve got a nice ring there now

05:42

look now I’m already out to at least 12

05:47

millimetres and if the beam was going

05:52

right through the middle of that hole

05:53

it wouldn’t catch fire. Would it?

Transcript For Anatomy of your CO2 Laser Beam (Cont…)

06:05

still smoking how can i burn a hole

06:08

that’s 12 millimetres diameter which is

06:11

what it is basically if we look around here

06:13

and what you’ve got to see also is that there’s

06:16

scorch marks around the outside he said

06:19

ok well the scorch marks are at the top there

06:20

because heat travels upwards

06:22

yeah but that doesn’t account for the

06:24

scorch marks going downwards does it so

06:27

anything that scorching has also got heat

06:30

on it

06:30

so where does this beam finish it came

06:34

out of the laser tube at twelve point

06:36

seven millimeters diameter

06:38

how do I know that well that’s the size

06:41

of the aperture. Well I have to apologize

06:44

about me and my little sketches but

06:47

basically what happens is the laser tube

06:50

is two mirrors, this mirror here

06:54

allows some of the inside the laser tube

06:58

we’re getting energy that’s bouncing

06:59

backwards and forwards between two

07:01

mirrors and this is the output mirror

07:04

and it’s not hundred percent reflective

07:06

this one is a hundred percent reflective

07:08

and the one at this end is I don’t know

07:10

what the percentages but let’s just say

07:12

it’s eighty percent reflective that

07:14

means eighty percent of the energy gets

07:15

reflected backwards and forwards down

07:17

here but twenty percent of the energy

07:19

escapes out through the mirror

07:21

ok now we’ve got a mirror that is that

07:25

size there is a water jacket and the mirror

07:29

sits inside the water jacket and that is

07:32

approximately twelve point seven so one

07:36

has to assume that the beam coming out of

07:38

there although it’s not 12.7 diameter

07:41

it’s a beam and what they called TEM00

07:45

Now that M stands for mode and

07:50

this is something called a mode 0 shaped

07:55

beam and mode 0 shaped beam means its

07:58

energy density is what they call

Transcript For Anatomy of your CO2 Laser Beam (Cont…)

08:01

gaussian and basically that’s the shape

08:04

of a bell

08:05

so technically if I was to draw from

08:07

here the shape of a bell

08:12

like that, that is probably the energy

08:17

density that’s coming out here in other

08:20

words there’s more energy down the

08:21

center of the beam than there is

08:23

right at the extremity of the beam and

08:27

so yes we have got technically here

08:31

maybe a five-millimeter beam but what

08:37

about all this stuff around the outside here?

08:38

where does that go?

08:40

it’s still energy it might not be useful

08:44

energy as its traveling along but once

08:48

it gets to the lens every bit of energy

08:52

gets focussed down to something thinner

08:55

than that little line there it is about

08:59

two or three times the size of a human

09:03

hair now every ounce of energy is going

09:09

to be compressed into that little teeny

09:11

bit their so if we throw away even ten

09:14

percent of the energy at the extremes

09:16

there we’ve lost ten percent of our

09:18

energy down here it might not appear to

09:21

be useful energy but it really is useful

09:25

energy we want to capture the maximum we

09:27

can so that’s why it’s most important

09:31

that you line your beam up correctly

09:34

when you’re setting your mirrors we

09:37

can’t afford to lose any. In the last

09:40

session I assumed that we had a

09:43

10-millimeter beam and that we probably

09:46

had about a mil and a half or so

09:47

either side of the true centerline of

09:51

this mirror here to catch the beam

09:55

I’m getting a bit concerned now because

09:58

we’ve got something there that is close

Transcript For Anatomy of your CO2 Laser Beam (Cont…)

10:00

to well certainly 12 millimetres that

10:05

means I’ve got to be extremely accurate

10:07

with my beam alignment. I’ve got my little water

10:11

trough here and I much prefer to fire it

10:13

down into water

10:20

it’s just passing harmlessly through the

10:22

mirrors now

10:31

800

10:33

900

10:38

a 1000

10:44

1100

10:50

just hunting for the maximum point, it’sabout

10:52

1100, then I gradually drag this away

10:56

from the center you’ll see how quickly

10:59

the power disappears see the redness

11:02

disappearing out the probe. so i push it

11:04

back to Center, it gets red. as I pull it out

11:09

it goes down fairly quickly

11:16

600, 600, 500, 400

11:24

still at 200

11:27

185

11:33

push i in a little bit to see if we can make it

11:35

change

11:39

120

11:41

degrees C

11:45

so five millimetres in from each side is

11:49

10 and that’s 21 millimetres diameter so

11:54

that’s about an 11 millimetre beam and I’m

11:57

still at about a hundred and ten or 120

Transcript For Anatomy of your CO2 Laser Beam (Cont…)

12:01

degrees C now that’s above boiling point

12:03

that’s energy that you can’t afford to

12:06

lose

12:07

we’ve seen how difficult it is to

12:08

establish what size that beam is there

12:11

we’ve also seen how relatively difficult

12:15

it is to to measure what’s going on

12:18

although we’ve seen it’s very high

12:19

temperatures it would be nice to see the

12:22

profile of this beam as i described it

12:25

to you, the bell shape, well there is a way

12:28

that we can see that because acrylic

12:30

has got some fantastic properties it

12:33

will evaporate

12:34

proportional to the energy density or

12:37

the amount of temperature that hits its

12:40

surface so we should be able to burn a

12:43

hole in there a bell shape if we’re

12:47

lucky i’ve set the timer to five

12:51

thousand milliseconds now, which is five

12:53

seconds of burn time and what I’ve got

12:56

here I’ve got a gentle air supply because

13:01

what i want to do is make sure this

13:02

thing doesn’t catch fire because it will

13:05

do because the fumes that come off there

13:08

are flammable and so what I’ve got to

13:12

try and do is to blow the fumes away

13:13

before they get a chance to ignite

13:18

so let’s give it a go and see what we

13:21

see what we get

13:32

well there we are there’s five seconds worth of burn

13:34

we’ve got a funny-shaped beam to start

13:36

with the beam is not round

13:39

it’s about seven-and-a-half nearly eight

13:44

millimeters, along the width it’s not much

13:48

different so it is reasonably round so if we

13:55

take a look at the shape inside there

13:56

yes it is a sort of a a bit of a bullet

Transcript For Anatomy of your CO2 Laser Beam (Cont…)

14:00

shape rather than a bell shape but it

14:03

does show categorically how the energy

14:07

density varies across the beam from very

14:10

high in the middle where we saw that

14:12

very high glowing temperature dropping

14:14

off towards the side now it’s only eight

14:19

millimeters diameter here it takes quite

14:21

a lot of heat to evaporate and melt the

14:24

acrylic so what sort of temperature is

14:27

happening just outside here? another

14:29

millimetre out here which would take it

14:31

back to nearly 10 millimetres, well we

14:34

can’t say because we saw that at about 10 or

14:37

11 millimetres diameter we were seeing

14:39

over a hundred degrees C so there’s

14:41

still energy out here beyond this size

14:44

but we now do the same thing coming

14:49

straight out of the laser tube so just

14:53

down here I’ve got a little air supply

14:55

which is blowing air up into this area

14:57

so we’ll run the test and see what we

15:02

get

15:11

you can see it’s just trying to catch

15:13

fire

15:15

and now we can compare

15:18

what was at the front of the Machine and

15:20

what was at the back of the machine

15:22

well that’s a very interesting shape in

15:26

there

15:29

so you can see several things different

15:32

about those two shapes the first thing

15:35

is we’ve got a very very sharp-pointed

15:38

beam here where the energy density is

15:43

very high in the center , there’s a bit of a

15:47

strange step there where the laser beam

15:51

tends to change completely

15:54

let’s just take a look at that from the

15:55

top and you’ll see what i mean if i hold

15:58

it in the light you can probably see

Transcript For Anatomy of your CO2 Laser Beam (Cont…)

16:00

that step in there can you see that so

16:04

hey maybe that’s the three or four

16:06

millimetre beam that they talk about

16:08

and the rest of it they’re ignoring, you

16:11

can’t ignore all this energy out here

16:13

beyond that step you can see that step

16:15

very clearly look

16:19

that’s weird

16:21

well the good news is that it’s passed

16:24

through two mirrors and it hasn’t

16:27

changed its size very much at all that’s

16:29

still about 7.7 ~ 7.8 millimetres diameter

16:33

yes it may be grown about 0.2 across

16:37

two mirrors showing that the beam is

16:39

maybe slightly diverging but the bigger

16:42

tell-tale that we’ve got there is that

16:46

we’re losing energy. Well we’re supposed

16:50

to have about 60 watts here we decided

16:55

that we hadn’t got that before so let’s

16:57

just see what we got this time start

17:00

temperature 10.2

17:09

10.2

17:12

minus 37.0 times two equals

17:20

53.6

17:24

that’s what those numbers represent 56.4

17:28

watts here

17:31

and 53.6 watts here

17:36

so that’s ninety-five percent efficiency

17:39

but across two mirrors so I’m losing two

17:42

and a half percent per mirror

17:44

approximately which is in the realms of

17:48

acceptable

17:49

it’s actually a smaller loss than I

17:52

expected

17:53

so the energy is still here it’s just

17:56

not a spiky as is in this one which is

17:59

quite fascinating

Transcript For Anatomy of your CO2 Laser Beam (Cont…)

18:02

I suppose in a strange sort of way it

18:04

shows that we have got our mirrors set up

18:06

quite efficiently as well, using my Machen

18:09

meter which is what i call my Sunday instrument

18:13

the one I only get out occasionally just to

18:15

cross-check we got 55.2 as opposed to

18:19

53.6 sorry we got 55.2 as opposed to

18:25

56.4 and we got 53.6 as opposed to 53

18:29

so using these two numbers we

18:32

actually had ninety-six percent

18:33

efficiency using these two numbers we had

18:36

ninety-five percent efficiency so you

18:38

know with this sort of instrumentation

18:40

that’s an entirely acceptable variation

18:55

well then thats another interesting

18:57

observation that I’ve just done

19:02

this one here is slightly shallower than

19:06

that one there but this one is

19:09

supposedly a better quality mirror with

19:11

a higher transmission rate it’s a

19:13

gold-plated mirror and gold is supposed

19:16

to have a much higher transmission rate

19:19

than molybdenum well I say much higher

19:22

maybe one percent maybe one and a half

19:24

percent but you know it’s supposed to be

19:27

higher but two mirrors two gold mirrors

19:30

as opposed to molybdenum mirrors and i

19:34

have seen to have less power here and

19:36

the results tend to prove that look 54

19:38

and 50. Now I’ve started test work on my China

19:44

machine, my other China machine using

19:46

these gold mirrors and I’m struggling

19:49

because I’m finding similar sorts of

19:51

results and I was a bit puzzled because

19:56

I was convinced that I asked for copper

Transcript For Anatomy of your CO2 Laser Beam (Cont…)

20:00

mirrors but I got gold-plated copper

20:01

mirrors

20:02

I don’t really want gold-plated copper

20:05

mirrors, but I can’t actually buy copper mirrors

20:07

and that’s the biggest problem I’ve got

20:09

I may well have to polish the gold off of

20:11

them to get what I want because i’m not

20:14

so sure that it is gold, it might even be brass

20:16

plating for all I know umm it’s yellow and

20:20

shiny

20:20

it certainly doesn’t perform like gold

20:25

that’s an interesting observation that

20:26

we found at least molybdenum mirrors are

20:30

what they are

20:31

so this machine is fitted with basically

20:34

almost the best mirrors you can get.

20:36

it’s been a fascinating session today I

20:39

took the opportunity while I was playing

20:41

to put some gold mirrors in and yeah

20:45

very disappointing

20:48

in fact i’m gonna go to the jeweler’s

20:49

tomorrow and see if I can get the gold

20:53

assayed, get them tested so that we can

20:56

establish whether they are gold, they’re shiny,

21:00

they’re yellow but they don’t perform like gold

21:03

so i have to suspect that erm all that

21:06

glisters is not gold

21:08

William Shakespeare said I think it was

21:11

him wasn’t it? Or was it my wife? perhaps in

21:15

the next session we ought to start

21:17

making something again rather than just

21:19

looking at the Machine and I think

21:22

you’ve done enough learning about the

21:23

anatomy of the Machine the things to

21:26

watch for the dangers I think we’ve

21:29

covered most of the serious basics and

21:31

technicalities of the Machine now so

21:33

thanks to your time again next time it

21:36

won’t be quite so boring because I hope

21:37

we’re going to be able to do something a

21:38

little bit more interesting and start

21:40

thinking about craft session for you

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