CO2 Slab Laser

Simplifying the construction of a gas laser by switching to a "tube" that's easier to build.

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In the path towards a lasercutter with as little premade parts as possible I'm building a slab laser to substitute the usual tubes.
A slab laser is a laser whose gain media has a rectangular cross section rather than the circular ones we're used to.
This format is common in solid state lasers, guess it's easy to make a slab of Nd:Glass or YAG, when it comes to gas lasers it is less common, maybe due to the requirement of the rather long glass-metal junctions that can become hard to get right when an hermetic seal with good resistance to thermal cycling is needed.
Still, it offers some advantages, namely easier cooling, possibility to get along with using cheaper materials and (most importantly) ease of building: more on that in later project logs, but basically a slab laser with gas media is an hermetically sealed metal sandwich with glass filling.
If the build is successful this design can become a good alternative for building gas laser equipment with limited res

Ok, let's expand a bit on this project:

As said, my main interest in this is demonstrating a CO2 gas laser that can be made in a place where resources are a little limited. Now, laser tubes aren't unobtanium. I know that. Aliexpress is there for anyone who wants one for relatively cheap and glassblowing materials aren't that fancy either, there's bunch of build logs at Sam's laser FAQ to prove that (though where I live I'd need to use fluorescent bulbs as glass tubing if I were to blow tubes) . But what if you could build a laser using stuff any small city metalworking shop has? This is the level of simplicity I want.

That's about it on the motivation side besides, of course, the usual "because I can" and "Because it's interesting".

Now some of the constraints regarding the design:

  • I need to pick a metal for the electrodes that has a similar thermal expansion coefficient to that of my chosen glass formulation, otherwise the seal between the two materials will likely crack. Still regarding the seal, there are a feel promissing techniques to ensure the glass will bond to the metal including not only direct joints between the two but also the use coatings on the metal and the use of salts and oxides that have affinity for the glass and the metal as a "glue" between two that don't bond to well on their own.
  • The metal must resist the plasma it'll be exposed to. Otherwise the windows could be blocked by sputtered metal from the electrodes.
  • Speaking of windows, they must be made of a material that is transparent at the IR wavelenghts. Regular glass will not cut it as its transmitance drops at near IR.

Once the tube is assembled we'd have the following challenges:

  • How will we pump the air out of the tube.
  • What discharge mode will be used? For simplicity sake the modes used along with flowing gas are out but there are still a lot of options and the necessity to balance power output, efficiency and simplicity.
  • What kind of gas formulation will be used? Just throwing CO2 and expecting it to lase is not going to work. And even if it does, how long until CO formation eventually poisons the gain media? Will the CO diffuse out and poison me? Will Jon Snow come back in the new season of GoT?

These are the main points regarding this idea in a write-up that is certainlly in not stellar form (I'm not a good writer)... Still I hope this gives a decent overall Idea.

If anything is unclear or there're any doubts, please ask.

About Licences:

Think of this as kindof CC-BY.

I'm not really bothering about it... use everything as you wish... but I'd appreciate if you referenced this project back when you expand upon it.

  • What's happened so far.

    Mateus Silva04/09/2017 at 23:50 0 comments

    Things have been kinda slow. Spring break ended and the first few weeks of college are always a mess of trying to figure a schedule that fits all the classes with the least amount of them taking place at the same time...By now I Still have a class on the math department happening at the same time as one on the chemical engineering, but I'll have to roll with that this semester.

    Anyways, I have been reading papers and press releases and patents to try and narrow the amount of time I'll spend on blind experimentation and brain damage inducing calculations.

    I've also been checking the locally available materials for a good choice of electrode and separator. I think the most promising material combination I can think of for them would be SS410 and soda lime glass. While there isn't references for this specific stainless steel, the 316 alloy is said to bond nicely to glass, so I can have some hope of getting a good seal. And the 410 has a thermal expansion coefficient similar enough to the soda lime glass that I can assume cracking because of the disparity in expansion is not going to be a problem. That still leaves the problem of figuring windows that will go along these two materials in a nice hermetic seal. But soon enough I hope to get the torch running and start playing with soldering glass and metal together.

    The other problem I want to check before trying things in the shop is stress distribution in the materials, I want to chose metal that is thick and glass that is resistant enough to not break due to the atmosphere trying to squeeze the vacuum tube to oblivion. And I can't just go around trying and checking if shrapnel will fly when the laser cavity is evacuated. Calculations are the way to go... but it seems anything that is not a sphere has really messy formulas (if there are any at all) so pretty much everything I've read about designing vacuum chambers tells that FEA software is the way to go. Therefore I've been familiarizing myself with a certain engineering software that offers free student licenses so that I can let a computer solve the partial differential equations for me. (Don't let the idea of software doing it fool you into thinking it's easy, setting the multiple solvers to work together and converge to a solution is not a small feat.)

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helge wrote 04/14/2018 at 20:33 point

Imho large area metal-to-glass seals are a bit cursed (also from a thermal management point of view). There are however a few other materials and processes that might be interesting: DBC (direct copper bonding) for Al2O3-Cu and AMB (active metal brazing) for Si3N4-Cu, both being high temperature processes. The key there is to create claddings on both sides and letting tensile strain cause plastic deformation.

Due to the unchanged CTE mismatch, DBC still hates thermal cycles. Si3N4 is a lot tougher though hard to come by.

I see the issues with simplicity you mention :) The take-away message there would be however that there are reactive bonding processes (e.g. with Ti based braze paste) and thin metal layers sandwiched between thicker glass or ceramic layers might be more benign than thick-on-thick mistmatched structures.

Hope that helped in any way ^^

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