The Legend of the Holy Rose, part 2
MacGyver sets up three rods with crystal heads, a golden globe (with ruby center), and a shiny mirror artifact on a stone (created for this purpose) to create a ruby laser that ignites what he believes is gunpowder in a stone wall. (http://wikibin.org/articles/list-of-problems-solved-by-macgyver.html)
At that age, I didn’t know about gain saturation. In my CO2 laser, the gain medium consists of three gases: nitrogen, carbon dioxide, and helium. My laser uses an electrical discharge to excite the nitrogen. The metastable diatomic nitrogen molecule vibrates with just the right amount of energy that when it knocks into a CO2 molecule, it sets the CO2 molecule vibrating similarly. (The nitrogen loses the remainder of its potential energy through collisions with relatively plentiful helium molecules.) However, the CO2 molecule can lose some of its energy by emitting a photon somewhere between 10.4 μm and 10.7 μm. For a little more detail, see here. As for carving my name in the moon, once all the excitable parts of the gain medium are... uh... excited (less losses), that’s all the power you’re likely to get.
First, I have not yet built my optical cavity. The tubes were assembled September 2017. The following is my current design for the optical cavity:
- Two 1.2m discharges. (Ld = 2300 mm)
- 1.75" OD, .157" Wall Pyrex jacket with hose barbs at either end
- 17mm OD, 1.2mm Wall Pyrex bore
2-3/4 in Conflat-style flanges at either end with graded glass-to-metal sealsCustom aluminum flanges were designed, as shown below, to reduce weight and optical resonator length. A double-sided and dual-ported (1/8 NPT) 2-3/4 in CF flange on HR and OC ends.Too much overhead here. DST-531 thermocouple vacuum gauge on one of the above flange ports. The other ports will serve as gas input ports.Probably a Kurt Lesker Perani gauge nearer the gas supply. Vacuum port at beam folderA single tube was assembled first; maybe this later. Two cathodes, one at each end, with a shared anode as part of the beam folderJust one channel for now.
- Overall resonator length
(Lo ~ 2500 mm)(Lo ~1200mm)
- Optical Coupler Transmissivity:
50%60% (Plano-Plano, Zinc Selenide, Laser Research Optics)
- Radius of Curvature:
~5 m10m, Silver Enhanced on Silicon
w1 = 2.9 mm (radius of spot size on flat optical coupler)
w2 = 4.1 mm (radius of spot size on spherical high reflector)
Nf = 2.044
And, by the CORD LEOT Class I CO2 laser empirical power formula*:
Power Output ≈ 147.0 W
* A Class I CO2 laser is defined as a laser incorporating the following design characteristics: (1) slow flowing gas system (1-20L/min), (2) water-cooled plasma tube, (3) long-radius hemispherical optical cavity, and (4) beam-limiting aperture located at or near high-reflector.
Optics and Mounts
Later... I purchased a couple of 2-3/4" Conflat flange blanks to get an idea of weight and dimensionality. I determined that their weight was just too much to accept just for the sake of modularity and standardization. Each flange weighs between a half-pound and a full pound. To get the gas input port was going to cost even more weight. Instead I am now considering getting custom flanges made out of 304SS. These flanges should have the same glass-to-metal size, but allow for (1) gas input hose barb, (2) electrode (C10200 Copper tube) mountable, and (3) Viton O-ring groove.
The above tube has been manufactured to these specs and I will be picking it up in mid-October 2014. Pretty cool to get such a major part of the laser head soon.
Here's the design I've come up with for the flanges on the laser tube:
FreeCAD is an excellent program. I got a bit nervous when the mouse stopped working with a new version of Mac OS, but when that got ironed out this year, I jumped on the opportunity to get some stuff done.
The files are here, here, and here.
Here is a photograph of the flange (Rev 1), printed from Shapeways. I just happened to have some 1/4-28 12-pointed bolts hanging around from a previous vision of using Conflat flanges for the whole project. The O-ring is AS568-214 Viton. The hole is a few thousandths of an inch wider than the high-purity copper electrode tubes.
This particular part was printed in polished metallic plastic, where nylon and alumina particles are fused together in the printing process. It actually had a weird sparkling/rough finish that I didn't expect.
The Rev 2 flange has a drill hole in the side to permit the tapping of a 1/8 NPT port. I bought 4 304SS 1/8 NPT to 1/4 ID hose barb adapters to be permanently epoxied into these holes.
The Rev 3 design included a notch similar to the one in the above design that would allow a set screw in the copper electrode to secure the electrode in place.
Revision 3 Optical Assembly
On 6 March 2015, I received my full optical assembly model (Rev3, .stl) from Shapeways [The 1/4-28 holes in this model are incorrect for machining]. For those who care, it was done in Polished White Flexible and Strong. It was a bit less expensive than the Metallic Plastic above (Rev1). The first time around I was a little put off by the name "Flexible and Strong," not understanding how flexible it was going to turn out to be.
Below is the disassembled version:
Referring to the Paschens curve info (left): Ideal pd for laser gas mix (I'm guessing here based on only Helium and Nitrogen) is about 2 Torr-cm. Since the discharge is roughly 120 cm, then the lowest voltage (between 200V and 1000V) needed to strike a discharge is 2/120, or .016 Torr. That's a pretty low easiest-case scenario. Let's see what we can do for maximum pressure allowable, given pump strength and leaky hoses, etc.
Well, I have 20kV to play with. Assume worst case, all nitrogen (I think worst case is actually CO2, but I don't have good data). Pd for nitrogen and 20kV is about 400 Torr-cm, thus 400/120, or 3.3 Torr. So, I should be able to strike a plasma in pure nitrogen at 3.3 Torr at 120cm with 20kV potential.
CNC Fabrication of Optical Assembly
Then, I discovered that I had the exact washers that would fit (you can see them just inside the photo on the right.
I also received the Loctite Hysol 1C with which to epoxy these couplers to the borosilicate glass tube (and act as thread sealant for the 1/8 NPT gas input port.)