Carbon Dioxide (CO2) lasers make all the types discussed in previous chapters seem like LEDs compared to xenon arc search lights! Where common diode, HeNe, and even most Ar/Kr lasers put out mW of optical power, nearly all CO2 lasers are rated in WATTs or KILOWATTS. Even the smallest sealed CO2 laser looking much like a HeNe tube, will produce 10s of W continuously. Large CO2 lasers used in the metalworking industry for welding, cutting, annealing, and so forth may have continuous outputs of up to 30 KW or more. The distinguishing characteristic of the CO2 lasing process that makes these sustained power levels possible is its relatively high efficiency - at least compared to most other common gas lasers. The typical electrical power in to optical power out efficiency of a CO2 laser may be anywhere from 5 to 20 percent or more (compared to less than .1 percent for a HeNe or Ar/Kr ion laser).

    Unlike the other lasers producing visible or short near-IR light, the output of a CO2 laser is medium-IR radiation at 10.6 um. It is the classic heat ray of science fiction. I have no doubt that the Martians in H. G. Wells' "The War of the Worlds" used CO2 lasers powered by cold fusion generators (probably with superconducting electrical backup storage) for their directed energy weapons. (Chemical lasers would have required bulky reactant storage tanks to achieve the number and length of blasts and none were visible!)

    The basic construction of a CO2 laser is like that of any other gas laser: a gas filled tube between a pair of mirrors excited by an electrical discharge. Metal coated mirrors (e.g., solid molybdenum or gold or copper coated glass or another base metal) may be used for the high reflector (totally reflecting mirror). However, at the 10.6 um wavelength, a glass mirror cannot be used for the output coupler (the end at which the beam exits) as glass is opaque in that region of the E/M spectrum. Germanium is often used but must be cooled to minimize losses for high power lasers. Other materials that may be used for CO2 laser optics are common substances like NaCl (rock salt!), CaCl, and BaFl (but these are all hydroscopic - water absorbing - so moisture must be excluded from their immediate environment) and zinc selenide which has very low losses at 10.6 um.

    Many details differ between a 50 W sealed CO2 laser and a 10 KW flowing gas laser machining center but the basic principles are the same. While HeNe lasers are based on excited atoms and ion laser use ions, CO2 lasers exploit a population inversion in the vibrational energy states of CO2 molecules mixed with other gasses.

    Electron collisions raise CO2 molecules to higher vibrational energy level from which they decay to a metastable vibrational level. This has a lifetime of about 2 milliseconds at a gas pressure of a few Torr. The strongest and most common lasing wavelength is 10.6 um but depending on the specific set of energy levels, the lasing wavelength can also be at 9.6 um (which is also quite strong) and at a number of other lines between 9 and 11 um - but these are rarely exploited in commercial CO2 lasers.

    Additional gasses are normally added to the gas mixture (besides CO2) to improve efficiency and extend lifetime. The typical gas fill is: 9.5% CO2, 13.5% N2, and 77% He. Note how He is the largest constituent and CO2 isn't even second! (This also means that leakage/diffusion of He through the walls and seals of the laser tube may be a significant factor is degradation of performance and/or failure of a sealed CO2 laser to work at all due to age.)