Fibre coupling a 40/50W CO2 Laser tube - looking for advice

[freemoore]

Hi all,

Not specifically Lasersaur related but I hope you can help me.

I've moved back from my Lasersaur build project and integrated what I learned from that into building my large CNC table - a compromise design that should do ok jobs of wood routing and plasma and laser cutting. I've built the table to take 5' x 10' sheets and at the moment have built in a tall vertical cage to move the laser tube and psu around the work so that the beam path is as short as possible - 100mm or so - and to make alignment easier.

I'm a bit nervous about the table shaking the tube to pieces; I'm considering switching to the TinyG firmware running on an Arduino Due just for laser cutting as it does something like S-curve acceleration profiles which should be the most gentle treatment possible for the tube. This is not ideal, what I really want is a stationary tube and an optical waveguide. 

I understand that CO2 laser light is absorbed by the medium if you point it down a standard optical fibre, and that some folks have developed a hollow (air) core fibre that will redirect that light without absorbing it.  So, does anybody here have any experience with hollow optical waveguide?  Can it be DIYed out of cheap materials? Is there any material that could coat the inside of, say, a silicon tube that would make it reflect the light? 

Thanks in advance for any insight!

cheers
Andy

[Steve Baker]

I don't think this idea has even the slightest chance of working.

Metallic surfaces reflect CO2 laser light quite well - but the problem
with using things like this is the efficiency of the reflection.

Suppose you are sending a milliwatt of laser light down an optical fibre
or a waveguide (eg in a communications laser) and the material is a 99%
efficient reflector - then if the light has to bounce 20 times off of the
sides then you'll get 0.99 to the power 20 efficiency which means that
about 81% of the light makes it out the other end - and (of course) the
remaining 19% is absorbed by the tube. If there are 40 bounces, you're
down to 67% efficiency, for 100 bounces, you're getting 36% efficiency.
For a communications laser, this is great - you can detect a third of a
milliwatt of light quite easily - so the loss is no big deal. That's why
optical fibres are so useful.

But for a cutting laser, 19% of 100 watts is 19 watts of laser power that
has to be dissipated from the waveguide to prevent it from getting too hot
and melting or catching fire or something. I don't know how many times
the light reflects off of the side (and I'm sure the angle of the
reflection matters) - but if even a few percent of light is absorbed, that
air-tube is going to get hot!

The kinds of mirrors that we use for laser cutting are typically made of
exotic materials like molybdenum and coated with thin layers of stuff like
gold. This gets the reflectivity high enough that very little of the
laser energy is absorbed by the mirror - and the amount of heating stays
manageable.

We have to be very careful not to touch or scratch our mirrors - and to
keep them very clean because any contamination whatever would cause a loss
of efficiency - and when that happens, within a matter of seconds, the
mirror will melt or distort.

I have no idea what the reflectivity of the materials used in these
air-core devices is - but if it's not something fairly exotic (like our
mirrors) - then I very much doubt it'll work. Worse still, the reflective
coating you need has to be super-flexible - if it cracks, it'll let 100%
of the laser light out into the surrounding material - and that'll melt or
catch fire within a small fraction of a second.

Worse still, when it does fail, the laser light will be directed in some
fairly random direction - and that's very dangerous indeed. You'll set
light to some part of your enclosure - or maybe the room it's sitting in -
or maybe the poor guy sitting next to it.

You *REALLY* can't mess around with big-assed lasers like this - they are
unbelievably DANGEROUS and very unforgiving. The mere fact that you're
even contemplating doing this tells me that you have no clue about the
dangers involved...and that's the kind of behavior that'll either burn
down the building or blind you.

Incidentally, I recall a Kickstarter laser cutter project that moved the
laser tube around with the head - and the result was such a high amount of
inertia in the head motion that the machine was dog slow - and needless to
say, the Kickstarter failed. He was using a small tube...I'm thinking 30
or 40 watts. With a larger tube, it would get MUCH harder.

Remember that affordable laser tubes are water cooled - so there will be
several pounds of water being moved around (and sloshing about) inside the
tube as it's being moved...what this does to the dynamics of the head is
likely to be complicated and hard to predict.

Also, if you're mounting the laser tube vertically, the water pump has to
push the cooling water uphill by another couple of feet (depending on the
length of your laser tube. Are you sure the chiller can do that and
maintain an adequate flow rate?

There is a reason that laser cutter designs don't move the tube around!

-- Steve

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-- Steve

[freemoore]

Hi Steve,

Thanks for your response. I ask about hollow waveguides tech because I know that it it exists in industrial and surgical applications and has been investigated up to at least 1.5kW (http://irfibers.rutgers.edu/resources/publications/1996/hgw_high_power_co2.pdf).  There are transmission losses here as you say, with the waveguide needing a cooling jacket and also requiring that the laser not be pointed into the guide above a certain angle else it'll melt.

I am aware of the dangers here; note that I am contemplating this by asking for advice from those likely to be more knowledgeable than myself, rather than storming in feet first, but fair play for mentioning it.  I figure I get much more useable beam power with much less hassle by bypassing the mirror alignment issue. It'd be different on a smaller machine or with a higher power laser - I'm not going to try moving a 100W tube around - 1.5m long or so? - but a 40W, 900mm tube fully supported at both ends with rubber mounts seems ok if I am happy with low overall speed.

The current system is able to move a router head around which is heavier than the water-filled laser tube plus psu, at good speeds and under control.  Granted it will be rather slow, but it's for cutting large shapes rather than fine detail in this incarnation, and the cutting speed should be comparable with wood routing.

I'll be building a smaller-footprint machine with flying optics for doing the detail work that benefits from very high speed head movement, but there's no doubt that being able to laser cut a full-size sheet will come in very handy. 

The water pump used claims a head of 7m and certainly pushes more water through the tube than did the smaller one that came with the A4 engraver that I took the tube from. 

So, anyone here with actual experience of hollow waveguides?  No disrespect intended, Steve, I won't dispute your statement of the problems with waveguides in theory, but knowing that they do exist in at least some form does incite me to look for more information. I do generally do my best not to do outright idiotic things around the nasty sharp heavy burning electrocuting things, I promise.


Andy

[Chris Uhlik]

I think you might be able to fly a solid state laser like this one

http://www.ebay.com/itm/40w-OPTO-Power-Laser-Diode-w-Focus-Head-Aiming-laser-Fiber-Case-Welder-Cutter-/131144013387

40 watts, 980 nm, laser diode, 48 amps at 2 volts.

These things are routinely used for laser surgery where the energy is guided by fiber optics.  They aren't hideously expensive either.  I can imagine cooling them with expanding (cold) assist gas.

Chris

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