05 – Let’s Test Some Fiber Laser Pulses (39:13)

The Fiber Laser Learning Lab Series with Russ Sadler

In this Series, Lotus Laser have lent Russ a MOPA 20 watt fiber laser to “play with”. Although Russ has a moderate understanding of laser technology (his words) and how constant power glass tube systems work, pulsing fiber laser marking machines are shrouded in a deeper mystery than the glass tube machines. So let’s test some Fiber Laser Pulses.

They have been designed for high speed marking and the technology has been well tried and proven. There are limited “tricks” that the pulsing laser technology can perform. You enter predefined parameters for each marking “trick” you wish the machine to deliver , then stand back in amazement. Most correspondents tell Russ that they have bought their machine direct from China and received a machine and EZCAD software, preloaded with a few default parameters. No other instructions beyond the EZCAD manual are forthcoming.

Fiber Laser Pulses: Test Program
Fiber Laser Pulses: Test Program

Russ states “I am neither a teacher or expert in this field so you join me in my learning adventure with the warning that I have a simple but inquisitive mind and will probably make mistakes on my way to discovering the truth. I WILL oversimplify and maybe distort the scientific detail in my quest to build a simple picture of why and how this technology works. I am not trying to reverse engineer anything, just to break through the seemingly impenetrable ‘techno cotton wool’ that surrounds this amazing piece of science.”

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Transcript for Let’s Test Some Fiber Laser Pulses

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00:00

welcome to another Fiber Laser Learning

00:03

lab today we’re going to go off on a

00:07

tangent now we’re not going to lose

00:09

sight of the goal that we’re trying to

00:11

achieve which is to work out how we

00:13

thicken the chromium oxide layer on the

00:16

surface of stainless steel to produce

00:18

refractive colors but before we can do

00:23

that we’ve got to understand how we can

00:26

manipulate that layer with what this

00:29

machine has to offer now this machine

00:31

has only got light which as I explained

00:35

to you before the light energy is like a

00:38

tsunami wave it’s very powerful but it

00:41

has to be and can be controlled because

00:43

this is a motor laser which is a pulsing

00:46

laser which enables us deliver very

00:49

exact amounts of power per pulse so

00:52

we’ve got all sorts of parameters to

00:53

work with to try and find a way of

00:56

manipulating the thickness of this

00:58

chromium oxide layer now the first thing

01:02

we’re going to do is take a quick look

01:03

at a program that I’ve written to try

01:07

and test this machine and at the same

01:10

time find out what the power is in each

01:13

one of these pulses so I’m trying to

01:16

achieve several things with this program

01:19

and I would explain what I’m trying to

01:21

achieve as we go through but I think one

01:24

of the most important things to say is

01:26

that the program that I’m going to be

01:28

used is called Lotus mark now any of you

01:31

guys out there with a fiber laser will

01:33

already have a copy of this program

01:35

because it’s called easy catcher but

01:39

it’s written in China and the one thing

01:42

the Chinese are not good at is quality

01:46

control of their software a piece of

01:49

software that’s completely bug free will

01:51

be upgraded and there will be bugs where

01:54

they were never bugs before all credit

01:57

to Lotus laser they have actually frozen

02:00

a version of easy care for their own use

Transcript for Let’s Test Some Fiber Laser Pulses (Cont…)

02:04

and agreed with the suppliers of the

02:06

software that they will always receive

02:08

the same thing so it will not get bugs

02:11

coming into it as they upgrade the

02:13

software which is an amazing quality

02:16

control approach which is reflected in

02:19

the rest of the design of this machine

02:20

know you had a chance to look around it

02:22

but as an engineer I look at this I

02:24

think to myself everything around this

02:26

machine is well-made and well built its

02:28

won’t designed okay the actual laser

02:31

itself comes from China the to hate the

02:36

lens and the head up here probably comes

02:38

from China as well but these are well

02:40

made pieces whereas my experience with

02:44

Chinese pieces so far

02:46

Chinese machines I mean I’ve got a

02:47

Chinese laser machine out the other side

02:49

of the workshop there I’ve got the

02:50

Chinese mill here I’ve got a Chinese

02:52

light behind me yeah they all work but

02:56

the Chinese lathe I had to spend several

02:59

hours fixing problems before I could use

03:01

it properly

03:02

the mill was one of the least trouble

03:04

things was one of the least troublesome

03:06

things I had from China the Chinese

03:08

laser machine well over there you could

03:12

see my other series and see how many

03:13

troubles I had fixing all the problems

03:15

that we encountered with that machine

03:17

as well as trying to understand the

03:18

technology because I said this is not a

03:20

corporate video I’m not being paid to do

03:22

any of this stuff I’m just saying it as

03:25

it is so yeah if we come across problems

03:28

don’t worry I shall tell you as well no

03:31

I don’t know who dramatized what I’m

03:33

just about to do but I am prepared now

03:37

you remember what happened to paper when

03:40

I exposed it to ten point six micron

03:42

wavelength light on the other machine

03:45

yeah it caught fire now you’ll

03:48

understand my precautionary measures

03:56

well that’s very interesting isn’t it

03:58

look we’ve them we’ve got the front of

Transcript for Let’s Test Some Fiber Laser Pulses (Cont…)

04:02

the paper

04:02

[Applause]

04:05

nothing on the back

04:10

I think you earlier stop and absolutely

04:16

no marks on the anodized aluminium well

04:19

that’s another interesting conundrum

04:20

isn’t it we’ve got the ability to

04:26

vaporize the ink which is probably a

04:30

water-based ink but we saw water

04:34

yesterday only absorbing a small

04:35

proportion of this light so it was able

04:38

to force its way through the water and

04:40

the glass and still have enough energy

04:41

to damage the anodized the black

04:44

anodized surface basically was able to

04:45

evaporate the dye that’s in the surface

04:49

not completely not as complete as it did

04:51

there but there was enough energy coming

04:54

through so there is a certain amount of

04:56

energy that gets absorbed by water and

04:58

one has to assume that that’s what’s

05:00

happened here but hey we’ve got paper

05:03

behind delicates absolutely nothing and

05:06

yet it has no effect on paper doesn’t

05:09

attempt to scorch it burn it go through

05:12

it we’ve just discovered something that

05:13

goes along with the square wheel paper

05:17

glasses to protect you against one

05:20

micron wavelength well here I’ve

05:23

designed a matrix to check out all sorts

05:26

of power and frequency combinations so

05:31

it’s easier to see if i zoom in on the

05:32

pattern itself and there we go look each

05:38

one of those little marks there’s a

05:43

little checkerboard pattern

05:46

and here we can see we run about pulse

05:50

duration from 1 all the way up to 350

05:55

doing 16 pulses that are available on

05:59

this machine and then what I’ve done

Transcript for Let’s Test Some Fiber Laser Pulses (Cont…)

06:02

here down the side we’ve got a pulse

06:04

frequency I’ve just chosen pulses all

06:07

the way down from a thousand to five now

06:12

these are not necessarily that peak

06:14

power pulses this is just a range now

06:19

somewhere in there I shall be able to

06:21

find my peak power pattern but we’ll

06:25

talk about that after we carried out the

06:27

test but before we do the test let me

06:29

explain why

06:31

I’ve gone for this interesting little

06:33

pattern from a distance as you can see

06:36

that looks like a Meade gray it’s not

06:41

white it’s not black because it’s a

06:43

pattern of 50% black and 50% white which

06:47

your eye perceives as mid gray and

06:50

that’s part of the design of the pattern

06:53

so that when we look at the pattern from

06:56

a distance we can immediately see which

06:58

parts are burnt

07:01

well back to white remember we’re going

07:04

to be doing this test on black anodized

07:06

aluminium and with black anodized

07:08

aluminium what we’ve got to try and do

07:10

is evaporate away the top surface which

07:13

is the salient and then we’ve got to

07:15

evaporate away the inter crystalline

07:17

black dye that’s hiding underneath the

07:20

top sailing surface now if we remove all

07:22

of the black dye which will get a nice

07:25

white mark like this but if we don’t

07:30

evaporate away all of the dye we shall

07:33

get something like this so this has been

07:37

set up so that it’s got a fixed dpi

07:39

in x and y 254 the machine knows that

07:45

it’s got to step down every time point

07:48

one okay so here’s that pattern and

07:52

that dimension there in both directions

Transcript for Let’s Test Some Fiber Laser Pulses (Cont…)

08:02

not 0.1 millimeters because the problem

08:07

with this and any other laser machine is

08:11

it cannot produce sharp corners at this

08:15

size this along with any other laser is

08:18

going to have a round beam so it cannot

08:26

produce sharp corners and the claim for

08:30

this machine is that it has got a beam

08:33

diameter of naught point O six five

08:38

millimeters diameter and that’s what

08:42

I’ve drawn here

08:43

that’s a naught point O six five beam

08:47

diameter now you might ask the question

08:53

why have you drawn it like this well

08:56

what happen is as the software comes in

08:59

and analyzes what it’s got to do it will

09:02

come across a black pixel and it will

09:04

turn on a black pixel signal just there

09:07

and then it will run out of black pixel

09:10

just there and then it will start

09:12

another black pixel there and so it will

09:15

go across and produce lines it will

09:19

produce signals to the light so it says

09:20

turn on turn off turn on turn off and I

09:23

don’t think this machine knows anything

09:25

about this little bit here and this

09:28

little bit here so the net result is

09:32

we’re gonna finish up I think but that’s

09:35

what we could find out with a pattern

09:37

that looks like this now it’s only going

09:41

to drive across the center of the pixels

09:44

because as I just pointed out to you

09:46

I’ve defined that these lines here are

09:50

254 pixels per inch which makes this

09:54

dimension here not point 1 so it’s going

09:57

to produce naught point 1 lines even

10:00

though it comes across black pixels all

Transcript for Let’s Test Some Fiber Laser Pulses (Cont…)

10:03

it’s going to do is is

10:05

the age of the pixel and the age of the

10:08

pixel and turn on off on off like that

10:13

so effectively even though I’ve drawn

10:15

black pixels it’s actually going to draw

10:19

single black lines which hopefully are

10:22

not 0.065 millimeters wide because this

10:25

is a pulsing system I anticipate we

10:28

cannot change the line width it is what

10:31

it is unless we mess around with the

10:33

focal distance the whole principle of

10:36

this test pattern was so that we can

10:37

have a quick visual check to see whether

10:41

or not we got 50% gray and 50% white to

10:45

give us a meet grey if these marks are

10:48

not completely white it means we haven’t

10:51

put enough power in to completely

10:53

evaporate the black Danny if we get

10:56

these marks nice and clean and white

10:59

then we shall still have approximately a

11:03

mid gray look to the pattern because if

11:07

we take a quick look at these two pieces

11:10

that hanging out the end here they’re

11:12

roughly 32 33 point 2 units of area and

11:17

then when I look at this little red area

11:20

here it’s 17.5 but of course there are

11:24

two of those so – 17.5 s is 35 33 give

11:30

or take a little bit we’re not going to

11:32

get too fussy we’re is still expecting

11:34

to see with this pattern a mid gray look

11:38

or if we see a completely white look it

11:40

means that we’ve either over burnt these

11:43

patterns which I can’t really see

11:46

happening because we’ve got to remember

11:48

the possibility that we have got a beam

11:50

which has got a power distribution and

11:55

I’ve drawn it upside down this time but

11:57

effectively what we’ve got is got more

11:59

power in the centre of the beam I’m

Transcript for Let’s Test Some Fiber Laser Pulses (Cont…)

12:01

anticipating that regardless of the

12:04

speed at which we run we shall always

12:08

get the same position power level in the

12:13

beam which will be that width of line

12:16

whatever

12:16

width of line happens to be this is not

12:19

the same as a continuous power layer

12:23

where when you change the speed you will

12:25

change the width of the line because

12:28

you’re only using the very high peak

12:30

power as you get faster and as you get

12:33

slower you allow more and more of this

12:36

Gaussian curve to have it’s burning

12:38

effect its power to come in I don’t

12:41

think that’s going to happen here so I’m

12:43

expecting to see a fairly constant

12:45

thickness line we might see some

12:47

ghosting of some sort which would

12:50

indicate this Gaussian distribution

12:54

power coming into play we’re going to

12:57

find out what this machine is actually

12:59

capable of with this simple test we can

13:01

see that the program is very nicely

13:03

outlined for us so we don’t have to

13:06

guess where to put the material

14:06

so let me catch this in the light right

Transcript for Let’s Test Some Fiber Laser Pulses (Cont…)

14:08

you’ll see that there’s a whole band of

14:11

results across there where the surface

14:13

has really been badly damaged and the we

14:17

worked our way properly even through to

14:20

the aluminium underneath we’ll have to

14:21

check it out under the microscope and we

14:26

can see that same band when we look at

14:28

them from a distance including these

14:30

three just here so we can study this

14:34

stuff under the microscope and see what

14:36

we can find

14:36

now I’ve had a quick look at the first

14:39

set of results that we’ve done there

14:41

before revealing anything to you guys

14:43

and there are some fairly interesting

14:48

pretty serious inconsistencies in the

14:50

results not what I would expect and I’m

14:54

sure these are not results that Lotus

14:57

will expect when I reveal them to them

14:59

the first thing I must do is to make

15:00

sure that the machine is acting

15:02

consistently now here we are I’ve

15:04

switched the machine off I’ve switched

15:05

the machine on again I’ve really origen

15:09

the machine and I reset the program but

15:13

we’re running the same program again

15:15

immediately beside the first one so that

15:17

we can do a comparison now the only

15:20

difference between the two programs is

15:22

the wording at the bottom the wording at

15:25

the bottom says you need errection on

15:27

the first one and it should say

15:29

bi-directional I’ve set all the

15:32

parameters to bi-directional but I

15:34

failed to change the wording so this one

15:37

has got the correct wording on it same

15:40

pattern

16:37

okay so I’m now going to examine this

Transcript for Let’s Test Some Fiber Laser Pulses (Cont…)

16:39

under my little microscope my little USB

16:42

microscope there I’m going to take a

16:44

look on the screen at exactly what’s

16:46

going on the one nanosecond results are

16:50

supposed to be CW continuous wave so as

16:54

I look down here at the various

16:55

frequencies that run from a thousand

16:57

kilohertz down to 5 kilohertz we should

17:01

see exactly the same pattern lovely

17:05

that’s just what we expect you to see so

17:08

we carry on down 800 700 600 500 400

17:17

kilohertz what’s going on there 300 –

17:25

and we’re back to normal

17:30

150 25 10 and 5 so we’ve got lovely

17:41

consistent results all the way down CW

17:43

as you would expect because that’s

17:45

exactly what it says constant and

17:47

throughout the frequency range we’ve got

17:50

good results except at 300 and 400

17:53

kilohertz now let’s take a look at some

17:56

others we’re not going to take a look at

17:58

all the results but let me just jump in

18:01

somewhere at somewhere like look at 6 9

Transcript for Let’s Test Some Fiber Laser Pulses (Cont…)

18:04

a second now interesting that we’ve got

18:11

several things that we can observe on

18:13

there first of all this is

18:17

bi-directional scaling and we can see

18:20

that because look here is the start

18:22

point of the scan when we go in this

18:24

direction and here is the start point of

18:27

the scan when we go in districts can you

18:29

see the intense dots at the beginning of

18:32

the scan now that is not something that

18:34

I was expecting I’m expecting these to

18:36

be uniform all the way along and in in

18:39

addition to that although these are very

18:41

very thin lines these are a lot thinner

18:44

than 0.06 because look point oh six how

18:48

many of these lines do you think I could

18:50

get

18:50

these two the answer is at least two of

18:53

them so that means that probably this

18:55

line here is about 0.03 thick so that’s

19:00

the great advantage of knowing the scale

19:02

of this picture point one long and point

19:04

one pitch wide well you can look and say

19:07

that the lines are already longer than

19:09

0.1 because first of all look they’re

19:11

overlap with each other so something

19:14

strange going on here with the timing

19:16

and with the power and we could see also

19:19

that we’ve got a halo around the outside

19:21

of each of these so let’s just push on a

19:24

little bit well let’s just go back a

19:26

little bit because I have seen something

19:28

else than 800 kilohertz and that is CW

19:35

continuous wave mode and those to be

19:38

approximately what we expected about

19:40

2006 5 long with little bits on the end

19:44

that sort of just about overlap each

19:46

other so that’s pretty representative of

19:49

what I was expecting if you remember

19:50

when I looked at the picture beforehand

19:52

that there is 800 kilohertz at 2

19:57

nanoseconds so that’s more or less one

19:59

of the numbers I’m allowed to use for

Transcript for Let’s Test Some Fiber Laser Pulses (Cont…)

20:02

peak power right so that’s 2 nanoseconds

20:07

850 kilohertz and this is 800 kilohertz

20:10

at 2 nanoseconds look at the width of

20:13

the line difference between this and

20:15

this these are both set at exactly the

20:17

same focus point okay so this is CW what

20:21

this is 2 nanoseconds time shouldn’t

20:23

make any difference basically what’s

20:25

happening here is we’re only just making

20:28

it to the most powerful part of the

20:31

center of the beam and that’s the part

20:33

that’s doing the drawing of this line

20:34

whereas here we’ve got more time and so

20:38

consequently what’s happening is we’re

20:39

we’re allowing more of the beam width to

20:44

do the burning it may get a lower power

20:46

but it’s still being allowed to burn

20:49

further up the Gaussian distribution

20:51

shape of the beam let’s go back to the

20:54

8th known a second hmm

20:59

where’s my dash is gone where my dash is

21:03

gone ah there they are they’re very thin

21:07

but they’ve also got ghosting around

21:09

them you’ll see and they have got a

21:13

little bit of heavy spot at the

21:16

beginning of the burn the beam is

21:20

getting a bit thicker now look what’s

21:22

happened there first of all we’ve got a

21:24

very distinct white patch at the

21:25

beginning of every burn and then what

21:29

we’ve got we’ve got a well you can see

21:32

we’ve got a little deep black bit in the

21:34

middle here which i think is probably

21:37

burning right the way through to the

21:40

aluminium underneath and you’ll also

21:42

notice got a yellow tinge to it as well

21:44

which to me would indicate that we’re

21:46

melting the aluminium oxide so let’s

21:51

push on that was at a frequency of 600

21:56

kilohertz with eight nanosecond pulse oh

22:00

dear

Transcript for Let’s Test Some Fiber Laser Pulses (Cont…)

22:03

we’ve got a pretty significant burn all

22:05

the way across there no dashes where my

22:07

dash is gone and there it is again look

22:15

how yellow it is dropping down from 200

22:18

to 100 kilohertz

22:19

we’ve got her dashes back nothing else

22:22

has changed now I was convinced that I

22:25

had got a problem with my programming so

22:28

I’ve been in and I’ve checked my program

22:30

everything is exactly as I expect it to

22:33

be on every one of these having found

22:36

that these tests have not exactly gone

22:38

quite to expectations I’ve got a

22:42

question whether or not first of all

22:43

I’ve been programming them right and the

22:47

answer that question is I’m absolutely

22:48

sure that I must be programming the

22:51

right because some of the results are

22:53

working perfectly fine and when I look

22:56

at my program everything is consistent

22:59

throughout the program all I’m doing is

23:02

changing basically one number at a time

23:05

it’s either pulse width or pulse

23:08

frequency

23:10

the power is always a hundred percent

23:12

and the scan speed in this particular

23:15

instance is always 250 millimeters a

23:18

second there’s something strange about

23:22

pulse frequency because when we look at

23:24

300 and 400 kilohertz that’s where we

23:28

seem to be getting most of the problems

23:30

I’m absolutely sure it’s not the XY

23:32

mirror system that’s at fault because

23:34

the mirror system is rotating at a

23:37

steady speed whatever that speed is and

23:39

it doesn’t go click click click click

23:41

click to make my dashes it’s going at a

23:46

steady speed and the laser itself is

23:48

switching on and off at a frequency

23:50

which causes those dots to happen so

23:53

there’s some sort of problem between the

23:56

control software and the firing software

23:59

where this is a unique problem to this

Transcript for Let’s Test Some Fiber Laser Pulses (Cont…)

24:02

machine because it’s got a problem I

24:04

can’t say but what we are going to have

24:07

to do is to get locus laser involved and

24:10

make them aware that I’ve got an issue

24:13

so even though we’ve come across this

24:15

problem it’s not going to stop us

24:18

investigating one or two other things

24:19

that we’ve found today I am particularly

24:25

interested in trying to find out how

24:28

pulse width and pulse frequency can be

24:31

used and manipulated to generate

24:33

specific amounts of power and how that

24:36

power goes down into the job now when I

24:39

say power we’re talking about light

24:41

energy we’re talking about light

24:43

intensity stimulating atoms remember so

24:46

when I use the word power that’s what I

24:48

really mean but never forget the fact

24:51

that it’s stimulation of atoms that’s

24:53

causing the effects that we’re looking

24:54

at we can obviously regulate the power

24:58

to bring it back down to whatever but

25:01

I’ve used 100% today and I’ve used a

25:03

standard scan speed of 250 millimeters a

25:07

second which as I said is one eighth of

25:09

the maximum that this machine could

25:11

deliver but different speeds are going

25:15

to produce different results when we

25:16

combine it with pulse frequency and

25:18

pulse width and that’s the strange

25:21

relationship that I want to

25:23

try and take a quick look at because

25:25

there’s some very strange but simple

25:27

maths involved now if I write this

25:30

example we’re going to use my little

25:32

north point one millimeter long line as

25:34

our reference to do some calculations

25:38

with now as I said we were doing these

25:42

tests at 250 millimeters a second and

25:46

therefore it’s fairly easy to calculate

25:48

that one millimetre of travel at 250

25:52

millimetres a second takes place in

25:55

North Point

25:56

0:04 of a second or 4 milliseconds per

25:59

millimetre okay so therefore my line is

Transcript for Let’s Test Some Fiber Laser Pulses (Cont…)

26:05

actually 0.1 1/10 of that travel so it’s

26:09

going to take point four of a

26:11

millisecond to travel 0.1 of a

26:14

millimeter so if I’m going to produce

26:16

that lined with a pulse repetition rate

26:19

of 1000 kilohertz that basically is a

26:24

million cycles per second so if I take

26:29

one second to divide it by a million I

26:32

finish up with a millionth of a second

26:34

so it didn’t actually take Einstein to

26:37

help us with that bit of the equation

26:39

and now for the next bit this is just as

26:42

simple if we take the time to travel 0.1

26:46

of a millimeter which is 0.4

26:49

milliseconds and and and the time it

26:51

takes to complete one cycle or one pulse

26:55

point four milliseconds divided by one

26:57

microsecond which when you mess around

27:00

with the Matteson that and the numbers

27:02

it comes out to four hundred pulses per

27:04

naught point 1/2 a millimeter so if we

27:07

go to the other extreme of my test where

27:09

I was using 5 kilohertz there it means

27:13

if we take one second to divide it by

27:15

5000 we finish up with a cycle time for

27:19

just one cycle of Northpoint two

27:22

milliseconds so therefore when we’re

27:25

travelling naught point one of a

27:27

millimeter at 5 kilohertz

27:29

we’re only getting two pulses per length

27:33

of line we’re getting 400 pulses

27:36

in the line when we do it at this speed

27:38

and two pulses when we do it at this

27:41

speed okay let’s put those numbers into

27:43

a little bit more of a reality check the

27:49

very first test that we did was using a

27:52

two nanosecond pulse so we worked out

27:55

that at 5 kilohertz each pulse each

28:00

cycle was not 0.2 milliseconds right but

Transcript for Let’s Test Some Fiber Laser Pulses (Cont…)

28:05

at the speed that we’re traveling we’re

28:07

doing point 4 milliseconds over the

28:11

whole distance so there’s our whole

28:14

distance of not 0.1 millimeters and

28:19

there’s our time to do half of it

28:23

because we’re getting two of those

28:25

pulses per point one of a millimeter so

28:31

we’ve got a time for one pulse if not

28:35

point to really seconds but we’re

28:38

actually using a pulse at the beginning

28:41

of that there and another pulse there

28:44

and another pulse here which is actually

28:47

only 2 nanoseconds wide so how many of

28:53

these two nanosecond pulses could we get

28:56

in point two of a millisecond well the

29:00

answer is 100,000 just by this very

29:06

simple bit of maths 2 milliseconds

29:07

divided by 2 nanoseconds but really

29:11

we’re only going to put one pulse into

29:14

that period of time and it’s there and

29:15

there and there regardless of how many

29:18

other pulses we could get in between so

29:21

that means that we’ve got one part in a

29:25

hundred thousand when we’re heating the

29:28

material and then 999,000 when the beam

29:33

or the power is off so that’s a very

29:38

very large cooling rate in relation to a

29:41

heating rate and that rate that heating

29:45

ratio

29:45

is very important to us when we’re

29:47

dealing with pulses we’re no longer

29:49

working as we were with the constant

29:52

wave laser machine a continuous power

29:55

here we’ve got little teeny-weeny pulses

29:58

and we’ve got a heating effect and a

Transcript for Let’s Test Some Fiber Laser Pulses (Cont…)

30:01

cooling effect and that’s the huge ratio

30:04

between the two so when we work in a

30:06

thousand kilohertz we’ve worked out that

30:10

the pulse rate was one microsecond a

30:13

millionth of a second per cycle so here

30:16

we are we’ve got all these millionths of

30:18

a seconds and in one point one of a

30:21

millimeter we’ve got four hundred pulses

30:25

but it does mean to say that we’ve still

30:28

got two nanoseconds for this pulse and

30:31

then a thousand nanoseconds later we’ve

30:35

got another pulse and then a thousand

30:39

nanoseconds later we’ve got another two

30:41

nanosecond pulse so that’s a ratio of

30:44

two nanoseconds per thousand nanoseconds

30:48

which is a microsecond and when you do

30:51

that very simple calculation it’s five

30:53

hundred to one so that’s the staggering

30:54

difference between these two frequencies

30:58

for the same pulse duration okay that’s

31:04

that’s the one thing that you’ve got to

31:06

remember we’ve got a beam which is

31:08

roughly not 0.06 diameter 60 micro

31:12

meters diameter so if we divide those

31:16

400 pulses equally amongst that not 0.1

31:20

millimeter we shall find that actually

31:23

we are going to advance naught point 2 5

31:31

microns per pulse so what that means is

31:38

we’re going to get these pulses which

31:41

are overlapping by a very very large

31:45

amount in other words this dot is only

31:47

going to move forward a small amount and

31:49

it’s going to almost stand on the same

31:51

spot repeating itself so that’s how

31:54

we’re going to get our heating effect

31:55

because we’re not moving very much

31:58

for every step of the pulse advancement

Transcript for Let’s Test Some Fiber Laser Pulses (Cont…)

32:01

well it’s pretty obvious really when we

32:03

look at this situation because we’ve got

32:05

a nought point six not point O six but

32:10

the type by the time we get to here we

32:14

will have already covered roughly not

32:17

point one so we’ve only got two pulses

32:21

as we said in this not point one so not

32:25

only have we got a huge off time in

32:29

relation to the on time we’ve got a huge

32:32

step difference so we’re going to get

32:34

virtually no repeat heating on top of

32:38

the one that we put down before this is

32:41

all very clear in my mind how this is

32:43

beginning to work this machine and I

32:45

hope I’m not be fuddling you with that

32:47

with the numbers and go away for

32:49

yourself and maybe think about this in a

32:51

bit more detail but what I am slightly

32:54

worried about is the inconsistent

32:56

results that we’re getting and why bear

32:59

in mind these two results are both for

33:01

the same pulse width of two nanoseconds

33:04

and the only thing that we’re changing

33:06

here at the moment is the frequency of

33:09

which with which we’re dispensing these

33:12

pulses so in this instance not only have

33:15

we got a much better heating to call

33:17

heating to cooling ratio we’ve also got

33:20

a much closer over burning of the dots

33:24

so we’re going to get a much great heat

33:26

buildup because of it remember I keep

33:29

talking about vibration and molecules

33:32

and atoms if I vibrate a molecule or an

33:36

atom with a beam of energy it’s not

33:39

going to vibrate on its own it’s going

33:42

to nudge and vibrate its neighbors and

33:45

nudging and vibrating its neighbors is

33:47

basically conduction we’re going to

33:50

vibrate those molecules into a hotter

33:53

state because they’re going to get

33:55

excited and that’s how conduction

33:57

happens through the material so the

34:00

other thing that we’ve got to take into

Transcript for Let’s Test Some Fiber Laser Pulses (Cont…)

34:01

account when we talk about pulses is the

34:03

fact that some of these pulses are long

34:06

and shallow just because of post is 350

34:09

nanosec

34:10

long it isn’t necessarily the same power

34:13

throughout its pulse length whereas the

34:16

shorter pulses tend to be just a spike

34:18

of energy there are these fill in the

34:20

middle here which are a mixture of a

34:23

spike of energy and a certain amount of

34:26

let’s call it a body energy so it’s yet

34:30

a further complication that we’ve got to

34:31

take into account when we try and choose

34:33

a waveform which is why it’s so

34:36

important to try and see and understand

34:39

what the relationship between damage and

34:43

frequency and pulse length is one other

34:47

rather interesting thought crossed my

34:50

mind as I was doing those very simple

34:54

calculations and that is if we are

34:58

running a naught point one line at two

35:02

thousand millimeters a second with the

35:05

slowest pulse possible when we tell the

35:08

pulse to stop that’s the last pulse

35:10

remember and the last pulse is on its

35:13

way and it will carry on for another 350

35:16

nanoseconds how far is that 350

35:22

nanoseconds going to cause my line to

35:25

overrun I’m going to make the line

35:28

longer by 350 nano seconds it’s a small

35:31

point but it’s something that I really

35:34

want to just build into my equation is

35:36

it important or is it not important

35:38

my very basic maths and I’m going to ask

35:40

the question in this way so that

35:42

everybody can understand how many 350

35:44

nano seconds chunks are there in one

35:48

second

35:49

and the answer is 2.8 million if we

35:53

travelled at one meter a second the

35:57

distance covered in 350 nano seconds

36:00

will actually be just one of those

Transcript for Let’s Test Some Fiber Laser Pulses (Cont…)

36:03

chunks ie it’ll be one meter divided by

36:07

this big number that we found here and

36:10

the answer to that question is not 0.35

36:13

micro meters not 0.35 microns so

36:19

therefore at 2 meters a second we’re

36:22

going to get an over trap

36:23

nor point seven microliters it’s it’s

36:27

it’s negligible amount when we do our

36:30

line we’ve automatically got half of

36:33

them whipped hanging at the end so a

36:35

tenth of that beam width isn’t gonna

36:38

make a lot of difference it’s just a

36:41

simple piece of information that I can

36:43

now put to one side and say the beam

36:46

length and the over travel has no effect

36:48

on this at all what I’m a lot more

36:50

concerned about when I look at these

36:51

pictures is why from all the theory that

36:57

we dealt with up to now technically we

37:00

should have a fairly steady stream of

37:02

energy coming out of the laser why is it

37:05

that when the laser switches on it goes

37:08

and does one power and then as it gets

37:14

into its stride it either goes heavier

37:18

or in some cases lighter

37:21

I haven’t yet worked that out because I

37:24

haven’t studied the patterns enough the

37:27

one thing that we can always say is that

37:28

there is a different power at the start

37:31

of the bee and you might say well that’s

37:33

because remember the pulse-width looks

37:37

like this therefore you know it will

37:39

always have a starting point which is

37:42

very sharp yes but what you’re

37:43

forgetting is that we’ve got four

37:46

thousand we’ve got four hundred of these

37:48

in that like that so there is no reason

37:54

to have that spot there any heavier than

37:57

the rest of the line length so I’ve got

Transcript for Let’s Test Some Fiber Laser Pulses (Cont…)

38:03

quite a lot of questions technical

38:05

questions which I’m not sure whether

38:08

Lotus laser can answer or whether this

38:12

is stuff that’s got to go back right to

38:14

people like Jay PT who probably won’t

38:17

even want to tell me so I’m going to

38:20

gather together some data and send it

38:22

off to Lotus later and we shall have to

38:25

put things on hold for a little while

38:27

until I can come up with some reasonably

38:29

satisfactory answers to these questions

38:31

if I just want to go blip-blip blip-blip

38:33

blip-blip blip-blip blip-blip

38:35

with this machine it does a fine job and

38:39

that’s what it’s designed to do I’m not

38:42

trying to find out how well it does that

38:45

I couldn’t care less what I want to find

38:47

out is how it works and at the moment

38:50

I’ve found my little simple test has

38:52

shown me that there are conditions under

38:55

which this system does not work in the

38:57

way in which I expected and on that note

39:00

I think we should finish and I’ll go and

39:02

get myself another cup of coffee and

39:04

I’ll catch up with you in the next

39:06

session whenever that is

39:09

cheerio now and thanks for your time

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