ALTERNATIVE  STRUCTURES CO₂ LASER

      Alternative structures for a CO₂ laser include the waveguide structure, in which discharge is confined in a small-bore ceramic (usually, beryllium oxide) tube. The tube acts much as a fiber optic does, with total internal reflections confining radiation inside. The advantage of this arrangement is the high power extraction possible. Small waveguide lasers, the length of a typical HeNe tube, can be built which provide multi watt power outputs. Frequently, this configuration is used for a compact laser design, so most lasers of this type incorporate internal mirrors. Waveguide lasers can be sealed or flowing-gas types and usually incorporate a water-cooling jacket around the ceramic tube. These are usually small lasers with power outputs in the under-50-W range.

       Finally, no discussion of CO₂ laser structures would be complete without mentioning the largest, the gas dynamic structure. These lasers produce power outputs of well over 10 kW and are used primarily for military research purposes. Gas dynamic lasers do not use electrical excitation but rather, rely on the thermodynamic properties of gases as they are compressed and expanded. Resembling a rocket engine with an optical cavity perpendicular to the output, the laser works by burning fuel at high temperatures in a combustion chamber. One of the products of this combustion is CO₂ gas, and the hot CO₂-rich exhaust is mixed with nitrogen gas, which becomes thermally excited and transfers energy to the CO₂ molecules, raising them to a high vibrational energy state. The gas mixture is then allowed to expand rapidly, cooling the entire mixture. Excited CO₂ molecules remain at the high-energy state, but the low temperatures depopulate the lower energy states, so massive population inversions are achieved. Lasing occurs in this region of the laser around which an optical cavity is fabricated. Exhaust gas is then released.