Homegrown CO2 Laser


When Lasers Are Outlawed
Only Outlaws Will Have Lasers


Zen and the Art of Quantum Electromagnetics

Quantum Physics says beer only comes in discrete quanta, such as 12 oz, 16 oz, 32 oz, six packs and assorted kegs. The store dont sell 3 oz beers or 19 .747545322 oz beers. You can get an available quantum or be thirsty.

Actually it says that about electromagnetic energy, not beer. Lets assume water molecules can only exist in two states, corresponding to 20 degrees and 60 degrees Centigrade. A glass of water at 47 C has no molecules at 47 C, but that is the average value of all the molecules. A molecule can change temperature by making a QUANTUM JUMP between the two states by absorbing or emitting a QUANTUM OF ENERGY corresponding to the difference of energy between the two states. This quantum may be exchanged with another molecule of water, exchanged with the container wall or it may be a photon emitted or absorbed. The energy levels of molecules in an actual glass of water is far more complex, but the principle holds true.

Einstein was awarded the Nobel Prize not for E=mc2 or Relativity, but for his explanation of the photoelectric effect. When light is shined on a metal in a vacuum, it may knock loose electrons which are detected as a current flowing in the vacuum to another electrode. The light is not always strong enough to knock loose any electrons, depending on the metal and on the voltage between the electrodes.However, physicists saw a strange thing which they couldnt explain. Making the light BRIGHTER made no difference at all, photoelectric emission depended only on the COLOR of the light.Einstein explained that light was made of photons which each had a discrete quantum of energy proportional to their frequency (or wavelength, or color). An electron in the metal had to receive a photon with enough energy to promote it from the bound state to the free state. Brighter light has more photons, but the energy of each one remains the same. Using photons with a higher frequency ( shorter wavelength, or more blue ) gave enough energy to free the electrons.

Einsteins strange new paradigm explaining the photoelectric effect opened the way leading to solar cells, the laser, and to semiconductor physics.A gas laser is a simple device you can build which depends on quantum effects.

Low pressure gasses excited by energy normally emit photons in certain spectral lines according to the differences between their quantum energy states, as the molecules jump from state to state.CO2 (Carbon Dioxide) has many discrete energy levels, with most of the transitions producing or absorbing infrared photons. Normally a population of CO2 molecules will be distributed among these levels according to an EQUILIBRIUM DISTRIBUTION with most molecules in states near the average energy and fewer molecules in states with very high or very low energy. In the laser, we will mix CO2 with N2 (Nitrogen) and run electric current through the mixture. The N2 molecules have a simpler energy level structure, and a majority end up in a single state which is very near the energy of one of the upper CO2 states. This causes that particular state of CO2 molecules to be much more highly populated than normal because of RESONANT TRANSFER of energy between the CO2 and the N2. The CO2 moves towards equlibrium by decaying to lower energy states, emitting photons on each quantum jump. We add He (Helium) to the mix because the He absorbs energy from the long lived lower states of CO2, quickly returning the molecules in lower states to the ground state where they may again be pumped by N2 resonant absorption to the upper state.

When these conditions are in place, we are maintaining a POPULATON INVERSION where the an upper energy state contains an abnormally large population, and/or a lower state has an abnormally depleted population. We have disturbed the equilibrium distribution in this way for a purpose. Photons emitted when a molecule decays from an upper energy state to a lower energy state can pass near another molecule in the same upper state and cause it to decay to the same lower state by the emission of another photon of the same wavelength. This is called STIMULATED EMISSION, and can be viewed as a type of "chain reaction" producing more and more photons. In the CO2 laser, the N2 and the He cause the applied electrical energy to be distributed so as to maintain an inverted population where very many molecules are ready to decay from a particular upper state to a particular lower state. Photons emitted at 10.6 microns ( about 28 million megahertz ) will be amplified by stimulated emission as they propagate along the tube. When the tube is enclosed in an OPTICAL CAVITY the photons are fed back into the tube where the chain reaction continues. Any amplifier with more feedback than losses will oscillate. Thus as long as the population is inverted, oscillation will continue.

Heres how to make an oscillator producing about 40 W at 28302 GHz.

! ! DANGER ! !

Do Not Look Into Laser With Remaining Eye

Class IV Laser Device

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The output of this laser is an invisible beam that can blind, burn flesh, start fires, and can burn or shatter many common materials. Wear protective goggles. Ensure a beam stop is in place.

The beam is infrared, invisible to the naked eye but is sensed by the body as heat. Passing your hand quickly through the beam will feel like passing it through a jet of hot water. You will not want your body or anything flammable to remain in the beam.

Ingredients:

Procedure:

Fabricate Laser Tube according to diagram :

Dimensions no longer available. Hey ! I did this in 1980 or 1981, sorry !

Anyway, you get the idea. Your dimensions will vary according to what optics, tube, insulators, etc, you can scrounge up.

Laser Tube Assembly

Most lasers use one of these Optical Cavity Types Gas and Vacuum System Power Supply

My optics were slightly damaged optics salvaged from prototype military laser rangefinders. Edmund $$$cientific also has this type $tuff. The focal length of the rear optic will typically be 2 or more times the length of the optical cavity. My rear mirror was 20 m radius on a 1.2 m cavity, it was Copper coated Invar and had several deep but small pits. The front optic should be transparent to 10.6 micron radiation and have a partially reflective coating on the inside surface. My front mirror was an 80% reflective germanium flat which had been hit by quartz shrapnel, leaving a big jagged scratch and a few tiny pits in the coating. The optics dont have to be flawless, just perfectly aligned. However large flaws may cause local heating and fracture. But hey ! Try it and see ! If it cracks you needed a better one anyway.

My plasma tube was glass neon sign tubing, about 1/2 in dia and 40 in long.My insulators were surplus ribbed white porcelain with threaded studs and sleeves. They should be at least 1 inch long, mine were about 5 inches.It oughta be possible to use plastic endplates and eliminate the insulators and stabilizing rings, but I did not chose to go that way.

So now you have rustled up your optics, plasma tube and insulators. Time to figger up yer dimensions. Go on. Dont look at me, just do it.

Your mirror cells should of course be sized to hold your optics. And they should be vacuum tight.This REALLY NEAT mirror cell design uses some nice, fat O-rings to sandwich the optic between the two halves, because you can tweek the screws and squeeze the O-rings to align the mirrors. Trust me, that simple idea just saved you a world of hassle with glassblowing, metal bellows, glass-metal joints and a host of nightmares associated with traditional designs.Pay close attention to the picture, the optics are supported only by O-rings, they do not touch the metal parts of the cells. Thats the secret alignment shortcut.

Now your mirror cell dimensions should lead you to the diameter of the endplates and stabilizing rings. Your tubing length should lead you to your stabilizing rod length, and to your total optical cavity length. Tally up the miscellaneous O-ring grooves and fitting dimensions and youre ready.

If you, like me, are not a machinist you have several options. $pend $$$$. Be very nice to a machinist. Beg. Borrow. Steal. Grovel. Trade favors. I chose all the above.

Put It All Together :

Assemble with a bit of grease on the O-rings, but make sure the optics stay clean. The cooling jacket can be any big tube, preferably transparent, with rubber stoppers in the ends. For electrodes, slide a bit of metal tubing into each end of the plasma tube and wire it to the mirror cell. Hot spots here can lead to plasma tube rupture. I lost several plasma tubes, resulting in broken glass and 15,000 volts and water going everywhere. Very Nasty. Anyway, I recommend extending the electrode tips into the cooling jacket, but I still have to limit continuous laser operation to ten minutes.

PreAlignment :

Heres where those ingeniously simple mirror cells come into play. Be sure to clock the mirror cells. If theyre made exactly like the drawing, you can remove and replace the entire mirror cell with the three mounting screws without changing the mirror alignment, since it seats hard on the endplate. The mirror, however, is floating on its own O-rings, and can be fine aligned by tweeking the other three screws.

Coarse alignment is done with the nuts on the stabilizing rods. With one mirror cell removed, eyeball the plasma tube axis from the other end and tweek the stabilizing rod nuts until your eyeball is looking at your eyeball.

Now do the same at the other end.

Fine alignment is done with a small visible laser, like those pocket laser pointer thingys that cats go nuts for. I set up a crude optical bench on the living room floor using phone books. You can turn pages in the phone books to adjust the height. With one mirror cell removed, put a piece of clear tape over the opening and punch a hole in the exact center big enough to admit the beam from your alignment laser. Tweek the mirror cell alignment screws to get the returning beam centered.

Now do the same at the other end.

Operation :

CLASS IV LASER DEVICE
INFRARED LASER

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The output of this laser is dangerous ! Its invisible beam can blind, burn flesh, cause fires, and can burn or shatter many common materials. Reflections from ordinary objects can be as dangerous as the main beam. Wear protective goggles. Do not allow invisible beam to burn persons or objects. Ensure a beam stop is in place.

Ordinary glass completely blocks the output of this laser. Plastic safety goggles also work well, and typically burn slowly rather than shatter abruptly when exposed to a high power beam. A graphite block or refractory furnace tile makes a good beam stop, and the beam profile is easy to see as a hot spot on a graphite block.

EXPOSED HIGH VOLTAGE

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Exposed parts including the mirror cells and the alignment screws are at high voltage, possibly 15,000 volts or more. Do not work alone. Have a safety partner who knows how to shut off electrical power and give CPR. Use a WELL INSULATED alignment tool.

My favorite mirror tuning tool was an allen wrench epoxied into the empty barrel of a ballpoint pen.

The first oder of business is to get the laser tube airtight. Connect laser gas supply, vacuum pump, and cooling water as shown in the Gas and Vacuum System. Turn on cooling water supply. Close needle valve and turn on vacuum pump. Check for and correct all leaks. Flush the entire system with laser gas mixture and close needle valve again.

After plasma tube pressure falls (if no gauge is available, listen to the vacuum pump) connect High Voltage supply and energize at minimum current. Look for plasma to form in the plasma tube. Adjust high voltage drive and gas pressure to get a long, bright positive column in the plasma tube. Disregard any striations which may appear. If laser tube pressure goes so high that the discharge is interrupted, reduce the tube pressure to minimum, re-establish the discharge, then carefully increase laser tube pressure.

When your plasma tube has a nice, bright white glow along most of its length ( this is where a tranparent cooling jacket comes in handy ) maintain these conditions while fine tuning the mirror cell alignment screws.

The desired profile is Gaussian, brightest in the center, and fading symmetrically on all sides. Alternate ends, adjusting the mirrors to optimize both beam profile and power output. If best profile and maximum power do not coincide, theres probably a better maximum somewhere else. Tune for beam profile at the expense of power if necessary, until a stronger tuning is found.

This is a dangerous device, but also exciting and fascinating to build and operate. If you build one, drop me a line and let me know how it went. Have fun !



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