Carbon dioxide lasers are continuous-wave high-power lasers that operate at wavelengths of 10.6 μm and 9.6 μm, which fall in the infrared region of the electromagnetic spectrum. They were developed in 1964, by Kumar Patel at Bell Labs and were one of the first lasers to use gas as a gain medium. The output power of CO2 lasers can be as high as 15000 W. The CO2 lasers are typically continuous wave lasers, however, they can also produce pulsed output using techniques like q-switching where the power can peak up to several gigawatts.
The laser cavity in a CO2 laser consists of a mixture of carbon dioxide, nitrogen and helium gases. They are pumped using an electric discharge where energetically excited electrons collide with the atoms in the active medium. Here nitrogen gas is added to provide energy to the carbon dioxide molecules by collision. The nitrogen molecules are initially excited using the electric discharge because the upper metastable energy levels of the nitrogen atoms are long-lived when compared to other atoms. These nitrogen molecules collide with the CO2 molecules at the ground level and excite them to the upper vibrational levels. These excited CO2 molecules then deexcite to the ground level by transferring the energy to the helium atoms by collision and thereby emitting laser outputs at 10.6 μm and 9.6 μm.
In a CO2 laser, the nitrogen gas acts as a buffer gas similar to the He gas in the He-Ne laser. Each electronic state of the CO2 molecule consists of several vibrational and rotational states that result due to the vibrational and rotational motion of the molecule. The energy transition for lasing in a CO2 molecule occurs between their vibrational energy levels.
The energetic electrons produced through electric discharge collide with the nitrogen molecules and this collision results in the transfer of energy to the nitrogen molecules. This excites the nitrogen gas to the upper vibrational levels which is a metastable state. They then transfer this energy to the carbon dioxide molecule by collision which provides enough energy to excite the CO2 molecules in the lower vibrational levels to the upper vibrational levels. As this process continues, the upper vibrational levels of the CO2 molecule get densely populated and result in population inversion conditions.
The CO2 molecules then deexcite to the lower lasing level with the emission of wavelengths 10.6 μm and 9.6 μm. The deexcitation to the lower lasing level results in an increase in the population of that level. So it is important to remove the molecules from that level. This is done by cooling the CO2 gas. They can be cooled by colliding with the He atoms present in the laser cavity. The collision results in the energy transfer to the He atoms and thus depopulating the lower lasing level.
CO2 lasers are highly efficient because the transition occurs between vibration levels of the lowest electronic level. These levels are close to the lowest ground state level hence input energy is easily converted to output energy bearing the minimum loss.
High power is generated using carbon-dioxide lasers, which is why it finds application in welding, hole drilling, and cutting industries. Water that makes up the biological system absorbs the wavelength produced by carbon-dioxide lasers; hence it makes up a useful tool in surgical procedures (e.g. skin resurfacing: vaporizing the skin for collagen formation).
For soft tissue repair, a 10.6 μm carbon-dioxide laser is the best surgical laser where both cutting and repairing of soft tissues are achieved photo-thermally.
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