CO2 Laser

CO2 Laser

A little info on all the pieces that make this work.

Laser Tube
The laser tube, the piece that looks like a florescent light above is the heart of a CO2 laser. It works by exciting the atoms in the gas using an electrical discharge. The gas is a mix of Co2, Nitrogen, and Helium. The exited nitrogen atoms bump into the CO2 atoms and cause them to transfer their energy. The excited CO2 atoms will fall to their lower energy state by releasing energy in the form of light radiation. This light will bounce off the mirrors placed at either end of the tube and cause other CO2 atoms to release energy. Some of this bouncing light will escape from the partially reflective mirror and viola, you have a laser.

Vacuum Pump
To get the atoms of gas to excite you need to send a current through them. If you were to try to get electricity to flow at atmospheric pressure you would need and incredibly high voltage to get at arc started. You would need this really high voltage because the electrodes are almost a meter apart from each other. If you take out some of the pressure in the tube and then try to start an arc it will initiate much easier. This is why you need a vacuum pump. Even at this low pressure you need apx. 15,000V to get a stable arc.

Above: Cenco vacuum pump attached to a 1/2hp 115V motor.


Power Supply
To get the 15,000V necessary to get an arc across the laser tube you need a power supply. The easiest supply is a neon sign transformer or NST. The nst I have is a 15/30 which means it is 15Kv at 30mA, this is a little undersized for the tube I have built but should work for initial test purposes. I am using AC to excite the tube for simplicity although DC is really the recommended method. More on that later.






Water Cooler


The water cooler is simply to keep the laser tube cool. A CO2 laser will not function very well or at all if the gas becomes to hot. This is why helium is introduced into the laser gas. The helium will help transfer heat to the laser tube walls and then the water will carry it out of the tube. I am using a Miller water cooler for a Tig torch, as I had one lying around.






Mirrors
The mirrors in a laser are responsible to increasing the energy output to achieve a useful beam. For CO2 you have to use special materials because of the wavelength of light it emits. A CO2 laser emits light at 10,600nM which is in the far infrared band. This light is totally invisible to the human eye. Because of this long wavelength materials like glass will not allow light to pass and are essentially opaque. Materials which will transmit CO2 laser light are NaCl (Table Salt) ZnSe (Zinc Selenide) and some other semiconductor materials.

A picture at the output mirror, the partially transparent one. You can see the premix laser gas bottle in the background.