19 – Fiber Laser Engraving Machine: Back Reflection (01:03:31)

0:00welcome to another fiber laser Learning Lab I might look as I’m relaxing this

0:09thing up here is gone bonkers I’m having all sorts of problems trying to

0:14understand something a major feature of this machine I spoke about it and I

0:20think it was the last session it’s a danger called back reflection which I’m

0:27told can cause very serious damage to this machine I’ve been warned that you

0:34first of all you need to be very very careful about where you set the focus on shiny materials shiny is a bit of a

0:42misnomer because it’s not a shiny surface that you’re worried about it’s a reflective surface and they’re

0:49not necessarily exactly the same thing a reflective surface will be something like gold or silver yeah looks as though

0:58I’ve got lots of that I’m gonna play with on the machine here doesn’t it or something that I have got which is almost the same level of reflection as

1:05cover now shiny stainless steel you would think is reflective it’s not quite

1:12as reflective as you think and it’s all down to the crystal structure or the

1:17atomic structure of the material itself but we’ll come on to that a little bit

1:23later the biggest problem I’ve got is trying to understand just how the actual

1:29rays I call them Ray’s because that’s what other people call them beam how the

1:36beam can reflect back on itself I don’t imagine the beam as being a beam

1:43I imagine the beam as being like a herd of sheep it’s an entity of photons each

1:50photon is a separate item but I’m not a physicist and I think that’s really the

1:57wrong concept and may well be a confusing concept I’m yet to try and find that out I suppose you could regard

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

2:05it almost like a tsunami as I’ve mentioned that word before the tsunami as a tsunami is a wave

2:10a huge wave of energy and I suppose photons when they all act together are

2:15just like that a big wave of energy but every one of those photons is a separate

2:23entity inside that wave so it is both at the same time a collection of bits

2:28pieces elements the photons and the entity which is the wave and it is the

2:35wave that maybe does the damage although the wave is a collection of photons very

2:41confusing I’m trying to get that picture in my mind when I should imagine one thing or when I should imagine something

2:48else the other thing that I’ve been told and I hope I don’t disbelieve this at

2:53all because this is from a very eminent physicist is that photons when they act

3:00together in total parallelism if they get out of synchronism they’ll cancel

3:05each other out and you’ll get nothing but when they collide with each other unlike two streams of water for example

3:12which when you hit them together just cups photons just don’t care about each

3:18other they can exist in the same space and can pass each other so from that point of view I can understand what back

3:25reflection is if you get a photon reflects off the surface it will not cancel out by an oncoming photon even

3:33though it collides with it well that’s what I’ve been led to believe so I’ve

3:39got all these confusing concepts in my brain at the moment which I can’t really put together and make sense of when

3:45people talk about the dangers and the damage of back reflection from this machine so today we’re going to do a

3:54little bit of an analysis from a practical point of view I’ve trying to find out what’s going on so we’re gonna

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

4:00do some rather strange things today which I very much doubt with a many people have done before but this is not

4:05a NASA laboratory I don’t have any sophisticated equipment there if I have to be very inventive with the very basic

4:12pieces of kit that I’ve got but quite often you can get very good not accurate

4:17but good answers basic equipment now what I’ve got on the work table here is a piece of wood which

4:24we know will not be affected by the beam in any way shape or form I’ve set the beam to go backwards and

4:30forwards this way so I know roughly where the beam is as it comes out of the lens it’s not going to change very much

4:36its position at all because remember down here it’s a long way away from the lens that means when it comes out of the

4:43lens there’s going to be virtually no movement on that beam at all when the beam hits this thermocouple

4:48there will be a heating effect so as the beam scans backwards and forwards if it

4:55manages to hit the thermocouple it should raise the temperature and that tells me where the edge of the beam is

5:01and then I can approximately work out what the diameter of the beam is when it

5:07comes out of the lens which is quite an important fact for me because I want to work out what the angle of the cone is

5:13that gets me down to the focal point just on the edge of the red beam there I’m going to pull it back just a shade

5:22there we go we’re working now wow look we’ve got some results so we know where

5:28the beam is so we’ve hit the edge of the beam there somewhere let’s just see

5:33let’s draw it back a little bit further run the test again

5:41no effect so let’s just edge it forward just a

5:47shade again and there we go so we found the edge of the beam I’ll put a black

5:54mark on there so this is not continuous beam passing over that thermocouple it’s just very quickly scanning over it

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

6:01leaving a little bit of energy behind getting towards the middle the beam now so let’s just see if we can find the

6:09highest power 180

6:21264 certainly up there somewhere it’s

6:28approximately the center so let me mark very carefully that as well okay so

6:37there’s my two marks on there and I would say there are about two millimeters apart so that represents the

6:44center to one half of the beam which would indicate that the beam is probably

6:49around about four millimeters diameter but of course that isn’t entirely true

6:55because remember it is a Gaussian distribution and probably I’m not

7:00picking up the tails of the beam I’m only picking up the most powerful part of the beam that’s enough to heat the

7:08thermocouple so it’s very likely that the beam might actually be as wide as

7:15they claim which is a seven millimeter beam and I’m only picking up maybe four

7:21millimeters or maybe four and a half millimeters that that beam these are two zinc selenide lenses on the co2 machine

7:29they’re fairly short focal lengths and I knew what I was doing when I tried to

7:35engrave stainless steel with them now as

7:41I said I knew what I was doing and I knew there was a risk of reflection and here’s a good example of exactly what

7:47happens these lenses only some 40 millimeters above the surface of the material the one on the right look as

7:53heated up the front face of the lens and it’s actually cracked right the way through the lens itself and the one on

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

8:00the left I’ve managed to burn the anti-reflective coating off the face of the lens the optics can be damaged by reflections I

8:08have no doubt how I can manage to achieve it with a lens system which is

8:15254 millimeters above the work surface I’m struggling with now often make it

8:23clear I’m not a teacher I’m on a learning journey I’ve got a lot

8:29of elemental facts about how lasers work and how materials up right but sometimes it’s a bit like

8:38having a jigsaw puzzle when you first open the box all the pieces are there but they don’t actually make any sense

8:44and that’s the problem I find quite often so one of the techniques that I

8:50use to try and help organize my thoughts is to write a document and put things down in a logical order and that’s what

8:57I’m attempting to do here I’ve got a series of titles here subject titles which I’m going to go through now I’m

9:02not going to apologize that some of these I’ve already discussed with you before but we’re going to discuss them

9:09again because they form part of this logical chain of working towards an

9:14answer now at the moment I’m giggling to myself here because I don’t have an

9:20answer I’ve still got the same confusion that I had when I made the first part of the video but I’m fairly confident that

9:28by the time we work our way through this list we will arrive at a sensible answer

9:34I shall probably have to do some research in the meantime but this is taking several days to create this video

9:40this is not quite the live video that I normally do so let’s push on and make a

9:48start now my early sessions of this series explained how an excited electron

9:56manages to be disturbed by a passing photon and it drops down to a lower

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

10:02energy level and then dropping down to a lower energy it emits something called a photon of light got no mass but it

10:08somehow has energy and as it goes past excited electrons it drags them all down

10:15and they just like this picture here they all remain phase synchronized doesn’t mean to say they’re all together

10:22but they remain basically locked together in the same sort of phase now

10:27this is a problem of having trying to decide when these little my new entities

10:34turn into something else but when they turn into something else they become more powerful you can pause the video

10:41and read what it says I come across this this concept that a beam of these

10:48Tong’s is a collective term and even though the photons remain individual

10:54little packets of energy just like sheep in a herd they can never be one big

11:00sheep they are just a herd it can be persuaded as an entity to move in

11:07different directions for example a sheepdog herding the Sheep will move the

11:12Sheep regard the sheepdog if you like is an external influence but it’s affecting

11:19the whole of the the wave it’s not just affecting one sheep when they’re all travelling in the same direction as a

11:25laser beam they technically should all be in-phase if for some reason they get

11:31out of phase then they will cancel and there will be no energy no light but

11:37we’ll come back to this little subject here in a minute this is this is what I have read so far my two gray cells are

11:44very easily confused when waves collide they will add or subtract but when

11:51photons collide they just passed through each other I’m glad I’m not a physicist now a photon is a little packet of

11:58energy which is a fixed packet of energy it has a certain if you want to imagine it as a wave if it has a certain

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

12:05amplitude or volume you can’t change that so if you want more light intensity

12:11or light density you need to pack more photons into the same physical area or

12:17volume so therefore I suppose if we imagine a single photon as being like a

12:22raindrop it will have very little damage potential but if you put millions and

12:29billions and gazillions of them together and move them in unison that now becomes

12:34a wave imagine a tsunami look at the potential damage you can do with the

12:40tsunami wave the beam on is 20 watt fiber laser machine is basically no

12:46different than the format of the beam on the co2 gas discharge later and what I’m

12:52saying here is the density of the photons is maximum right at this point

12:57here right down the center okay this is the highest point on the graph this is the greatest light intensity and the

13:04greatest light intensity means we’ve got maximum density of photons okay now this

13:10is a seven millimeter diameter beam so very crudely let’s just say that it’s

13:16six millimeters diameter because that makes it nice and easy to look at what we’ve got here then we’ve got one two

13:21three four five six sections so the centre two millimeters of that beam if

13:29it’s a 20 watt beam contains thirteen point six watts there’s no such thing as a section of

13:36the beam these are just convenient breakpoints to show you approximately what’s happening within

13:43the beam the next section out which is another millimetre on either side of the

13:48centre we’ve got only 2.7 watts and right on the outside we’ve got half of

13:55nothing so really the only moderately

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

14:01powerful part of the beam is the central two millimeters even though it’s a seven millimeter diameter beam and that’s a

14:09very important point to remember now a hundred percent power is not twenty

14:15watts twenty watts is down here if we chop the graph off across that 20 watt

14:20line and we should be able to take the top half of that graph and we’ll be able to fill in the gaps at the bottom here

14:27okay so that’s roughly what that 31 percent is that 31 percent of a hundred

14:33percent power as where our 20 watt average power is and if we like to consider then that 100 percent power is

14:40probably 65 Watts right at the peak so we’ve got 20 watts average power but

14:45probably 65 watts available to us at the peak now I’ll leave that part on the

14:51screen just in case you want to read it but I’ve just described what that says now in this section we’re not actually

14:59going to talk about how the lens focuses that’s a completely separate section

15:04that I’m going to come onto next what we want to do is look at how the light gets affected as it passes through the lens

15:11now this was another big problem that I was having trying to understand the concept of first of all what happens

15:18when the light hits the lens now I know enough about physics to know that when a

15:24ray of light hits a surface it gets refracted now refracted means change

15:31direction or does it it also means that it changes speed the light within the

15:40glass or that medium reduces its speed it slows down how can light slow down

15:57you now although this is not field glass

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

16:04it’s a glassy type material and the light will slow down inside the glass to

16:09about 200 million meters a second yeah

16:15okay that’s not really slowing down is it but it’s it has reduced by about a third this ray of light here enters at

16:23300 million and travels that distance this ray of light here enters at 300

16:28million and traveled that distance now as it goes in and slows down this ray

16:35here must exit before this right here okay so there no possibly no longer a

16:42laser beam because they must be out of phase reading between the lines I’ve

16:49been advised by very learned physics professor that I’m basically an idiot I

16:54didn’t need to be told because I know that already as I said I’m an engineer not a physicist and I don’t know how a

17:00physicists mind works certainly this stuff is almost like reading Chinglish but what I am and can

17:09understand is although this ray here exits before this ray here it has

17:16further to travel to get to the focal point so although it resumes its journey

17:22sooner it will get there later because it’s got further to travel and although

17:29this one exits later it will get there sooner because it’s got a shorter distance to travel and the

17:36idiot part of it was it wouldn’t be a lens if they didn’t all arrive at the same time that’s the whole point of a

17:43lens okay so I didn’t know that so I’ve just described what that text says but

17:49I’ll leave it on the screen so you can pause and read it if you wish okay now we come on to something rather interesting which gets us into a very

17:58controversial area because this is not an area where I can find any information

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

18:06the only information that I’ve got is information that I’ve developed for myself

18:12the is lens theory and everybody seems to rely on lens theory and not what happens

18:20in practice and I have had to investigate what happens in practice because I could not make lens Theory

18:28agree with what I was observing you cannot focus liked to appoint any

18:35smaller than its wavelength so the smallest possible spot we could get for

18:42this fiber laser is roughly 1 micron now

18:48the problem is it would be incredibly expensive to make a lens which

18:53physically focus down to the theoretical minimum 1 micron and in reality all

19:01lenses have got something called aberration which means that they do not

19:07pass through this theoretical focus point you will see the path of light

19:12described through a lens in the way that I’ve shown it on this diagram here that’s totally ludicrous because light

19:19cannot bend around corners so how does this diagram come about if we take a

19:27look at this Plano convex lens that I’ve drawn here you’ll see that the Rays from the outside of the lens are crossing

19:35over at a focal point which is less than where the arrow shows D and those rays

19:45they’re coming down the center of the lens are crossing over substantially

19:51beyond D and the profile that you see in

19:58the picture above has been sketched in there to show you that basically what

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

20:03you’ve got it’s that outline which encapsulates all the beams that are going towards the focal point focal

20:12point is a bit of a misnomer I like to call that D are a fuzzy focal point

20:20because the closer the beams are to the axis of the lens the further they

20:26crossover beyond – the actual focal point and very conveniently this diagram which I’ve

20:32picked up happens to be approximately three divisions on either side of center

20:38line now if you want to go back to the beginning and take a look at the distribution of light energy in that

20:45beam hitting the lens remember we’ve got roughly 70% of the light energy within

20:55those two central rays so therefore that maximum intensity of light than the

21:01center of the beam is actually focusing quite a long way beyond the nominal

21:07focal point so this is a very confusing practical situation we’ve got a very

21:13rapid or company here – six who make all sorts of optical systems very very high

21:19laboratory and industrial lenses two very important points we must note

21:26spherical aberration increases the spot size now let’s just turn that on its

21:31head for a moment and say if the spot size was theoretically perfect for this

21:38machine and with this lens we would have a 1 micron spot size we do not have a 1

21:47micron spot size on this machine we’ve got a 254 millimeter long focal length

21:52lens which has got a claimed spot size of about 65 microns that’s a

21:59lot more than the theoretical 1 micron that it could be and what we’re saying

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

22:05is 65 microns is pretty pretty damn good but what it does mean to say is we’re

22:13accepting a lot of spherical aberration now this will be important later on now

22:19the other thing that it’s clearly stated here is that this also causes the best

22:25focus to occur at a different location than the theoretical focal point notes

22:31on being cynical about the word theoretical I personally think that the phrase best focus is a little bit

22:38confusing in its own right focus employee a single point and as you can clearly

22:44see there is no single point what we have really is an integration of the

22:50intensities of all the Rays which come together at a single point where we get

22:56maximum light intensity so let’s come back to our seven millimeter diameter beam and we know that it’s roughly seven

23:04millimeters diameter because when we measured it at the beginning of this video we clearly saw that we had power

23:10roughly plus or minus two millimeters from centre and where’s plus or minus

23:17two millimeters from centre it’s that point where we’ve got maximum intensity

23:22we’ve got very little power beyond plus or minus two millimeters if you look at

23:28this graph here so instead of using the absolute spot size as defined by JP T

23:33for this particular machine I’m going to use a closed equivalent which is 0.07 70

23:40microns as opposed to 65 microns and the reason for that is because it’s a very nice factor of seven millimeters so what

23:48I’ve done I’ve taken the light intensity from a seven millimeter diameter beam and I’ve squashed it down to point zero

23:55seven as shown by the little picture on the right hand side there which means I

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

24:01have now increased the intensity by a factor of seven divided point point zero

24:08seven which is a hundred times what occurs at the focal point is still a

24:13Gaussian distribution when we amplify the power we take every single part of

24:19that graph that you can see here and we multiply its amplitude by a hundred if

24:27we look just here there’s the little Gaussian distribution that we had to

24:32start with but it isn’t like that now but the time we multiply every one of those amplitudes by a hundred so we’ve

24:39got a huge spike of energy at the center of this beam now now I don’t wait to run

24:46away with the idea that this is a drill or anything like that this is a graph of

24:53the intensity of light but obviously because we’ve got so much light intensity here at the center of the beam

25:02it’s going to have an influence way beyond the focal point itself because

25:07this beam is symmetrical on its way to the focal point and away from the focal

25:14point it clearly implies that we’re going to get exactly the same spike of

25:20energy above the focal point as well as below the focal points that are going to

25:25get this strange central core of energy around the focal point above and below

25:31it but extends both up and down from the focal point if we take a look here at

25:37what jpt tell us about the lens itself they tell us that there’s a focal length

25:44254 millimeters which is what I’ve got on this machine those who tell me that it’s roughly what sixty three point nine

25:5064 microns for the spot size across here

25:56okay they then go on to tell me that we’ve got something called a depth of

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

26:02field or depth of focus which extends in this particular instance to two

26:07millimeters ie one millimeter above and one millimeter below the nominal focal point we’ve got

26:16a useful working power from other work that I’ve done this depth of field stops

26:23at a point where approximately the area of the beam is twice the area of the

26:30waist so if we’ve got a certain energy density at this point here it will be

26:37half the energy density there and that approximately defines the working depth

26:42of field for a lens so at that point the beam starts to go out of focus this is a

26:47difficult concept for people to actually grab hold of I’ve tried to explain at many many times in my work and this is

26:55where theory and practice tend to diverge this is the practice that I’ve

27:01come across and I know that I can produce a lot of damage way beyond the

27:07focal point this is the method by which the laser focus the machined we fiddle with

27:12the lens and we adjust the height up and down above and below the focal point

27:18this is above the focal point and this is below the focal point until we get

27:23symmetry now if we were getting symmetry

27:29because the beam was going out of focus it would start going out of focus

27:35according to jpt at plus or minus one millimeter we should find that we’ve got

27:41a 65 micron line just there in the center but the time we get out to plus or minus one millimeter we should see

27:48that line growing to 90 microns does that look like a 90 micron line no it

27:56looks exactly the same as that 60 micron line what about at 2 millimetres no that

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

28:02hasn’t grown either and at 3 millimeters no so wherever we go along this graph

28:08we’ve got the same thickness of line so we cannot be going out of focus with

28:15this line what we’re running out of is damage power you’re here now the top

28:21picture was done when the power was set to 100% now let’s just backtrack for a

28:26second and how do we get to that picture there well we got to that picture by

28:32multiplying this graph by a hundred if we take this graph and reduce the power

28:39to 50% it’s going to get shorter and smoother it’s going to get blunter okay

28:47it’s amplitude is going to get less because we’re now going to multiply this

28:54graph by a hundred because we’ve still got the same lens which multiplies by a

29:00hundred but we’re multiplying a smaller graph by a hundred so we no longer have

29:07this very sharp-pointed 100% power we shall have something that’s roughly half

29:12the length and it will have less of a point on it it will still be sharpish but it’ll be blunter it’ll be softer

29:19gentler at its maximum intensity point so now we can begin to maybe understand

29:26why this picture at the top here is actually different to the picture at the

29:32bottom this is 50% power the influence beyond this so-called focal point here

29:39which we’ve defined has grown we’ve got a much longer length of influence of

29:46that intensity so it’s no longer able to just do damage at plus or minus three

29:52millimeters it’s now able to do damage at plus or minus six millimeters so we

29:58would just refer back again that we should theoretically I’m being cynical again be getting a 90 micron thickness

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

30:06at these plus and minus 1 millimeter points who knows what was your beginning at 3 or 4 5 & 6 millimeters you know

30:15there is no thickening of these lines we’re not picking up the out-of-focus

30:23part of the beam what we’re picking up here is the central core of energy in the beam this core of light intensity

30:33and that’s the difference between the theoretical focus point and what I’m

30:40describing to you is the practice that actually happens within a lens doesn’t

30:47just stop and go out of focus now this takes me back to the point where Lotus

30:52later asked me to make sure that if I tell you guys to use the focus as one of

31:01the parameters for limiting the power that you do it by moving the work

31:07towards the lens and not away from the lens I can accept what they say but I

31:13need to understand what they’re saying and why the first thing that I can see here is that when they set the Machine

31:19up they’re automatically going at least 3 millimeters below the focal point to set up zero now the zero that they’ve

31:26actually found is this point here point at which you get maximum energy density

31:32now remember this energy density graph is not only below the focal point it’s all so above the focal point so if I find

31:39the matching extremities above and below the focal point which is what I’ve done here and we could call that the focal

31:46point but I do know that this zero here is not that zero there and now we’re

31:56going to go on and try and explain why and to do that we need to look at this thing that I called earlier fuzzy focus

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

32:04we absolutely know that this lens has got aberration I’ve drawn this picture

32:12here on my CAD and yeah it’s designed to be a graphical representation of what’s

32:21happening in the B light rays travel in straight lines they cannot bend around corners and unless they’re affected by

32:27gravity or some other major effectors Einstein proved but what it does show me

32:33is that out of 254 millimeters long the red raise the outer rays which you’ve

32:40got insignificant amount of energy in them focus at roughly 2 3 4 5

32:49millimeters I don’t know exactly what the number is but I can be sure that it is some number of millimeters before and

32:56above the focal point this is the extremity of the blue section now which

33:02contains another 20% roughly of the energy that focuses maybe 3 4 5 again

33:12millimeters beyond the focal point and

33:18wherever that focuses the central core focuses another 4 5 6 millimeters beyond

33:28the blue zone so when we zoom in and look at actually what’s happening at the

33:35focal point this nominal focal point just here where my 70 microns spot is we

33:42can see that the blue zone was crossing here

33:48and the yellow zone as crossing substantially further forward but bear

33:54in mind that this yellow zone I’ve drawn is not at the center axis of the lens

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

34:02these are the extremities of the yellow zone as I get closer and closer to the

34:09center of axis of the lens this crossing point this focal point of that yellow

34:16zone is going to go out here to some stupid extreme I mean it’s just a fact

34:21of geometry if you make two lines parallel they will never cross but it is that intense part of the beam which is

34:28focused beyond the focal point I cannot tell you how much all I can demonstrate

34:35to you is the principle and not the theory the practice that they will

34:40absolutely focus beyond this nominal focal point that then asks the question

34:48where is this integration of power we’ve

34:54got the red beam there which is carry nothing but a hint of something we’ve

35:01got the blue beam which is moderately powerful and we’ve got the yellow beam which is carrying 70% of the energy of

35:10this beam by inference the yellow beam

35:16is going to win there’s going to be an addition and an integration of the blue

35:22beam as well but what we’re really saying is look just take a look at my

35:27little top picture there where it says three point six one and four point six okay that means the yellow beam is

35:34roughly a millimeter beyond the blue beam which is not really true because

35:40that yellow beam is lightened to be meters effectively beyond the yellow

35:46beyond the blue beam once it gets to its extremity let’s just guess and say that the integration point where we get

35:53maximum intensity of all these beams combining as five millimeters below the

35:59focal point so this is where the confusion comes in we’re happy to agree that at this point

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

36:06here we’ve got the integration of all the beams giving us the most intense point that’s not necessarily the focal

36:15point that’s just the point of maximum energy density but where is that point

36:22of maximum energy density in relation to the nominal focal point the answer is

36:28probably four five six millimeters below the nominal focal point I don’t know I

36:34can only show you in principle that it is not at the focal point okay now let’s

36:40just leave the uncertainty of the focal point itself for a moment and we’ll push on with something more important which

36:47is the material that we go to fire the laser beam at now we’ve got these three

36:55simple diagrams here the most important thing in the first diagram is that dotted line which is something called

37:01the normal that’s the point of perfect perpendicularity to the surface and when

37:07you fire a beam at the surface it will always reflect relative to that normal

37:13if the reflected ray is exactly the same as the incident ray then we have a

37:19lovely mirror reflection now the next two pictures show the types

37:25of surface that we’re going to be firing the laser beam at the first one is

37:30called a specular surface where the reflection bounces off exactly as a mirror reflection it applies that very

37:37simple rule all the Rays are bouncing off in a parallel direction and if you were looking at yourself in the mirror

37:44you would see the handsome person that you are now if you go to the next

37:50picture along which is a diffusive reflection where the surface now diffuses the light rays into random

37:58paths it obeys all the same rules as the first image mirror reflection but at the

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

38:04point where the ray hits we take the normal to the surface at that point and

38:10calculate the new angle of reflection and as you can see because the surfaces irregular the normals would

38:18not be parallel to each other therefore the reflected rays will not be parallel

38:23to each other and this is a bit like looking itself in a distorted mirror if

38:29you can see anything at all now the other thing that’s important about reflection it’s all to do with the

38:35wavelength of light the sort of materials that we’re worried about are metals and all metals have got a very

38:42dense crystalline structure where the atomic particles are very closely bound

38:49together if we have a short enough wavelength then that wavelength is able

38:55to penetrate into the gaps between the atoms and all of a sudden the light

39:01becomes absorbed but if the atomic distance is smaller than the wavelength

39:07of light that you fire at it then we will get a specular reflection we will

39:12get no penetration of the light into the surface of the material now we can begin

39:18to see that phenomenon in this little diagram here this is the wavelength of some typical materials that you will be

39:25using with this machine the blue one is aluminium then we’ve got gold which is red and silver which is the black one

39:32now if we take a look at the wavelength scale along the bottom we’re working

39:37with roughly one micron now as you can see above one micron we’ve got something

39:44like I don’t know 95% or more reflectivity at one micron

39:52things are starting to change very slightly if we drop down to 500 and

39:57maybe 400 nanometres you could see the serious drop off in the reflectivity of

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

40:03gold and silver that’s because the atomic spacing is such that at those

40:10points there the waves are actually penetrating into the material and being absorbed they’re no longer being

40:16reflected now our concern in this video is all about reflectance off of the surface of materials now the only

40:22materials that are really going to be at risk for us are these sorts of materials

40:27here in this table and you can see that at 10 microns where we may be used to working with the co2

40:33laser all of these materials are incredibly reflective it does change for

40:39some of these materials as we get down to one micron and that little table there shows you the risk associated with

40:46using these materials okay I think we’ve now collected all the pieces of the jigsaw puzzle that will enable us to

40:52probably go forward and try and decode the subject to this video which is back reflectance now we’re going to use this

41:01diagram quite a few times in our discussion um and it’s important that you understand what it actually is I

41:08know you can look at it and say yeah I’d say that’s a lens with some light rays going through it yes but this is a

41:14simulation of an F theta lens this is not an F theta Lin and F theta lens is a

41:20very special three-dimensional version of a lens because it sweeps through a

41:26spherical orbit and wherever it is in that orbit it has to focus that’s a very

41:31complex thing to even try to draw let alone to work with so what we’ve got here is a simple Plano convex lens which

41:39exhibits all the same properties as this 3-dimensional F theta lens but it’s

41:45condensed back into two dimensions where it’s much easier to comprehend what’s going on now this is not just some

41:52random drawing that I’ve created and out of my imagination the lens itself is a

41:58creation but it’s a creation with a circular arc on the top of the lens now

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

42:05the circular arc represents a part of a sphere and normally a lens would have a

42:10spherical top surface so this is a section a centerline section through a lens because it is a geometric surface a

42:17part of a spherical surface it possesses something called spherical aberration and as I’ve described before spherical

42:23aberration is when it rays from the outside focus at a different point to

42:30the rays passing closer to the axis of the lens what I’ve done here I’ve drawn

42:35the horizontal ray lines and I’ve used the same colors that I’ve used in the Gaussian distribution

42:42so that you can keep that in mind that the yellow section down the middle contains about 70% of the energy or

42:49power of the beam the next blue section there we’ve got roughly 10% at either

42:54side and then we’ve got about 4% out from the blue to the red zone so we’ve

43:00got this Gaussian distribution still implied in this beam once the beam hits

43:06the lens I then used the maths the well-known maths of refraction because

43:13I’ve assumed that the lens will be glass and that the beam is passing through air so I’ve used the respective refractive

43:19indices for each of those materials and I’ve constructed the correct geometric path of the Ray as it passes into the

43:27glass and out of the glass back into air so yeah this is not a figment of my

43:32imagination this is using lens theory which as I said I’m not deriding lens theory what

43:39I’m saying to you is that lens theory does not go as far as lens practice so

43:45here we can clearly see that lens Theory will show us the way in which the focal

43:51points do not coincide and this aberration is exactly what we get with

43:57the F theta lens if we take a look at that top diagram that’s an enlarged view of the focal point in the diagram below

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

44:05it look carefully and you’ll see that there’s a little gray wasted section

44:10there which I’ve mapped in to show you the if you like the way that the focus

44:15point is normally diagrammatically shown this is not a diagrammatic showing of

44:21the waist this is an actual showing of the various intersections of focal points from

44:28various parts of the lens and you can see it’s a bit of a mess but the other thing that I’d like you to note is that

44:35beyond these various focal points the Rays are no longer nice and evenly

44:42spaced out we’ve got a change of energy distribution below the focal point we’re

44:50now going to carry out some little tests to see what happens when we move the material above and below

44:55the nominal focal point now in this first test we’re going to move above the

45:01focal point which is supposedly the safe zone and here’s what happens we get the

45:08perfect mirror image of where the Rays would be if they were allowed to proceed

45:14normally all the happens as we get them reflected just in front of our mirror

45:20plane now this means that they’re not

45:26going to get anywhere near the lens and so you must consider that moving above

45:31the focal point is definitely safe let’s just see what happens when we project

45:37the Rays and see where they go when they reach the lens so the dotted lines here

45:42are showing the reflected rays and we can see that as we project those Rays

45:48out and hit the lens I’ve again mathematically calculated the correct path of the ray through the lens and

45:56then out back into air again and you can see that the path of those rays is

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

46:01always diverging that’s the important point they are diverging beyond the lens which

46:09means they are losing their power they have no capability of refocusing

46:14anywhere in the optical path so this is definitely safe to work above the focal

46:20point okay now let’s carry out exactly the same exercise but this time you can see

46:26the green line where we were reflecting above the focal point what I’ve done I’ve swapped it over and moved it to the other side of the focal

46:34point and again we’ve got dotted Ray’s there that reflect back off of a surface

46:42and if we look the red rays are just harmlessly disappearing out into nowhere

46:47it depends on how far we move beyond the focal point as to where those red rays

46:53will go but let’s face it those red rays are only got 4% of energy so even if

46:58they get refocused back through the lens they’re not exactly going to cause a great deal of problem the blue rays are

47:06slightly more interesting because at this coincidental distance which I chose they

47:13seem to be parallel with the previous red rays as they go back towards the

47:19optical system now that still means that they’re not particularly dangerous but I suspect that if I were to move that

47:25green plane in very slightly towards the focal point then my blue rays would

47:31start to converge in the same way that if you follow the yellow rays back

47:37they are definitely converging now okay

47:42it may well be a very slow convergence but but that means we’re going to go a

47:47long way back into the optical system before we get convergence and reach a focal point I don’t think we need to go

47:55any further to establish that the information that I’ve been given is correct I wouldn’t necessarily say the

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

48:03explanations that I’ve been given have been very clear so that’s why I’ve had

48:08to do this investigation myself to try and understand exactly what’s going on

48:14now I don’t want to exaggerate the situation out of all proportion because

48:20it doesn’t mean to say you can’t focus below the focal point only if you’ve got

48:26reflective materials should you be careful not all materials that you’re going to be using a reflective for

48:32example you might be using black anodized aluminium or coloured anodized aluminium that’s not a reflective

48:39surface that’s an absorptive surface so really it’s a matter of using your

48:44common sense the other important thing to remember is that if a material has a

48:50flat surface whether it’s polished or not and it’s one of these reflective

48:55materials it is a risk so if you like to recall what I mentioned about reflectivity it’s all to do with the

49:02wavelength of the light and the atomic spacing of the material that you’re firing the wavelength at if that atomic

49:10spacing is less than the wavelength of the light then there will be a reflection now I’m going to draw your

49:17attention to Section C basically the advice that I’ve been given is that high

49:23power as high risk when you fired it a reflective material okay that makes perfect sense but it leaves me with a

49:30little bit of a problem we can melt material with this later even though

49:38it’s only got 20 watts but that’s because it develops huge amount of power

49:44in a very very short period of time we’ve got a two nanosecond pulse but it

49:50probably takes less than a quarter of a nanosecond to rise from zero to twelve

49:56kilowatts if we can just sneak up on it quick enough we may well melt it before

Transcript for Fiber Laser Engraving Machine: Back Reflection (Cont…)

50:01it has a chance to realize that it’s been hit and if we melt the material and

50:06change it into a puddle all of a sudden it absorbs all the energy rather than

50:12reflect it but that then leaves one question which I’m afraid I cannot answer is there enough reflected energy

50:19in that very very short period of time to go back through the optical system

50:24and damage it before the energy starts being a hundred percent absorbed by the melt pool but if we stay above the focal

50:35point I don’t think we even have to worry about the problem because if there

50:41is focus as we’ve demonstrated it will not get back into the optical system

50:47however if for any reason you do want to work below the focal point with a

50:55reflective material and put your machine at risk then can I suggest the following

51:02precaution this diagram here with the top set of rays shows what happens when

51:09the Ray hits the surface of the table absolutely perpendicular we have the

51:15risk of reflection back into the system now if you move your work twelve and a

51:21half millimeters to one side what will happen is you will get the second ray but that means that any reflection off

51:29of your workpiece will now reflect to the third ray and that third lower ray

51:36as you could see misses the lens so provided you use a 25 millimeter

51:42diameter no-go zone in the center of your table you will stand no chance of

51:48reflecting any rays back up into your lens even if you work below the focus

51:54point now I do have one final thought maybe my exaggerated pictures we’re not

52:03necessarily telling me the truth so despite the fact that I’ve done all

52:09the refraction calculations and drawings correctly what I’ve done here now is to

52:15draw the complete system to scale so we’ve got a seven millimeter beam here

52:23which I’m only showing that blue in the yellow central power sections so there are the

52:28beams passing through the point zero seven spot size at the limits that I’ve

52:34chosen they’re roughly the same dimension about four millimeters past the focal point now I do keep mentioning

52:41this but that yellow beam there it hasn’t really got a focal point here

52:47coincident with the blue the blue is never going to get any closer to

52:53parallel because it is a limit but those yellow lines remember are only the 70%

52:59power limit within that power limit we’ve got all other power which is going

53:06closer to the axis of the lens and as we get closer to the axis of the lens the

53:12lines are going to become closer to parallel and their intersection point is going to go way out into space over

53:19there somewhere so you can see how impossible it would be to illustrate this on a piece of paper because I

53:24wouldn’t be able to scale in and out so in this second picture here I’ve scaled

53:30it properly with all the correct refractive indices directions and angles

53:36as I pass through the lens but this time I’ve dropped the beam by 10 millimeters

53:42below the focal point where we told if we come up through the center of the lens here we shall get a

53:48converging beam now what we’re seeing here off this 10 millimeter below the

53:54focal point plane as a reflection of the orange beam and the blue beam okay now

54:02you’re not seeing the original beam there you’re just seeing the reflected beams because I’ve I’ve pasted them out

54:08so that they’re not confusing so it certainly looks as though if we go below

54:14Center we will create a converging beam so let’s look what happens if we go

54:19above Center here’s what zero looks like 254 and here’s what 10 millimeter above

54:26Center looks like and if we put a reflection plane just here what we’re really going to do is to pick up this

54:33piece here and completely mirror it now it’s very confusing to put this little

54:39mirrored piece in here so I’ve left it out for the time being so you can see

54:44the reflected part of the beam but the whole point of it is look this part of the beam is going to focus where it

54:53would focus if it went beyond the 10 millimeter plane so this beam is

54:59immediately going to go into focus just above the reflection plane so that looks

55:05as though it’s completely safe but let’s follow these beams back and again I’ve

55:11removed the incoming beam and we’re only seeing the reflected beam as a dotted

55:17beam here and if we take a look at that dotted beam as it passes through the lens and then carries on for point three

55:24four four point six nine it’s going into a diverging condition it appears to be

55:30relatively safe well so where do I use my earlier exaggerated pictures or these

55:35real one to one scale pictures we still come up with the same confirming evidence lens theory says that yes the

55:43beam will hit the lens and it will then converge if you’re below the focal point and it will diverge if you are above the

55:50focal point regardless of what the theory says there’s only one way to really find out what’s going on when

55:56light reflects back through this lens and that is to reflect light back through the

56:01so here we’ve got the Machine Linds yeah we’ve got a mirror and here we’ve got a

56:08red LED pointer it’s a it’s a nicer die now this red laser diode has got a

56:14focusing element on the front of it which allows me to set it up so that the

56:19beam is virtually parallel it can be set to a focus at a certain distance but

56:24I’ve tried to get it as far as possible a parallel beam looks pretty small on

56:30the mirror I would estimate that spot as being probably half to three-quarters of

56:37a millimeter but as soon as I put something in front of it to look at it look at that dot goes to something like

56:46about two millimeter diameter or the light wave that’s going into the mirror

56:52is bouncing off the back face of the mirror and as it comes forward its cancelling some of the light that’s

56:57going in I don’t know which effect is happening there but all I can say is the

57:02mirror is got a smaller dot than the paper the spot size looked bigger than

57:10the 0.065 that’s claimed for it but hey we’re not gonna be worried about that because that’s not our argument in this

57:16case we’re not testing the spot size we’re testing what happens to that beam when it reflects back through the lens

57:21there’s the beam that’s going into the lens so if I had to put a circle around that dot I would say it’s probably about

57:28four millimeters diameter which in essence is equivalent to the blue and the yellow zones on my Gaussian curve so

57:36we’ve got basically a simulation of the high-powered path there so the way that

57:41I’ve got this set up at the moment is that returning spot is pretty close to the center line of the sending LED so

57:49you could see it just sitting there on the center of this window is just about

57:54to drop in the outgoing beam an incoming beam are now completely cancelling each

57:59other out and the interesting thing is to watch this Carozza

58:05just as I get in and the beam is on the edge of itself we get that interference

58:13I then carry on moving across the beam as I exit them on the other side so it’s

58:21not on the center so right on the center looks as though we’ve got cancellation

58:27by the look of it because the halo disappears fascinating but does it help

58:34us what I’m more interested in as what’s happening here to that God now that’s a

58:41pretty bright four millimeter dot at that point there that’s with the focus

58:47set at 254 now if I move the focus forward ie

58:53I’m going to push the focal point up towards the lens does it have an effect

59:03the answer is not really it’s not noticeable and I can’t see

59:11really any change at all therefore 10 millimeters the next thing is can we

59:17take it short by 10 millimeters and see an increase or a decrease in that dot

59:30now I’m looking at it well as I push the

59:37lens towards it it is actually

59:42decreasing so I can I think we can imply that as we move the focal point below

59:48the lens there is a focusing effect on the beam that returns to the optical

59:55system and that is the bad news that really we’re talking about okay so now

1:00:02we’re looking at the reflection in the mirror and what I’m going to do I’m gonna move that dot that previously was

1:00:09reflecting straight back into the optical device and I’m gonna move it off

1:00:14to the side and what you’ll be able to see by looking in there is how much the

1:00:20dot moves now my reflection is not

1:00:28hitting the sensor and now is if we have

1:00:36a dead zone in the center of the work there’s probably about six or seven millimetres diameter we won’t get any

1:00:43reflection back into the optical system well let’s just put this lens back where it belongs now we started off this

1:00:49session basically by asking the question why mustn’t I touch that big red button that you tell me I mustn’t touch no real

1:00:58answer not that was plausible anyway so the only way to find out is to take the

1:01:04plate off follow the wiring back and find out what that big red button controls yes I agree I mustn’t touch

1:01:12that big red button and that’s basically what we’ve achieved today having been told under certain circumstances the

1:01:18light will bounce back up through the lens and damage the optics and we’ve

1:01:26basically proved that in three different ways number one we’ve used a Plano

1:01:31convex lens analogy in a slightly exaggerated form and that worked and

1:01:37said we shouldn’t but below the focal point we’ve used the scale drawings of this system and that indicated that we

1:01:44shouldn’t work below the focal point but then we’ve used the real F theta lens to verify what’s actually happening when we

1:01:51bounce off a shiny surface and again proves the point that yes we should have

1:01:57to be careful when we work below the focal point but if you do work below the

1:02:03focal point all you’ve got to do is make sure you steer clear of something like a maybe a six or seven millimeter diameter

1:02:10no-go zone right in the middle of your table here and then you will have no chance of light going back up through

1:02:16the optical system now I’ve seen all sorts of fantastical images of where

1:02:22these light rays go when they bounce off the surface somewhere out here round Venus coming back and disappearing up my

1:02:29bottom okay so I’ve got a tendency to exaggerate but they’re just as wild and

1:02:34unbelievable as that plus the fact that I’m told that you could seriously damage the lens and the mirrors up here in the

1:02:41galvo any beam that goes back is on a diverging path so it’s going to be no

1:02:47more powerful coming through the lens this way that it is going back through the lens that way I don’t think the lens

1:02:54or the galvo mirrors are ever at risk that’s a personal opinion based on what

1:02:59I’ve experienced today but I can perceive that there is risk to the actual fiber-optic interface so the

1:03:07summary really is if you stay above the focal point for your defocusing you will never have a problem

1:03:13so really the nono is don’t focus below the focal point but in the next session

1:03:20I’m going to do something interesting where as you might imagine you’ve guessed it I’ll see you then bye for now

1:03:29and thanks for your time