定义：
光是通过二氧化碳分子实现放大的激光器。

CO2激光器是采用混合气体作为增益介质，其中包括二氧化碳，氦气，氮气以及可能包含一些氢气，水蒸气和氙气。这种激光器是由气体放电进行电学泵浦，可以采用直流电流，交流电流或者射频范围。
氮分子由放电电流激发到亚稳态，然后与CO2分子发生碰撞时将能量转移给它。氦气可以使低能态粒子数减少，并且能够消除产生的热。其它的组分，例如氢气和水蒸气可以使一氧化碳重新氧化成二氧化碳。 

图1：二氧化碳激光器的示意图。气体管具有布儒斯特窗口，并且进行了水冷处理。 
CO2激光器通常辐射光的波长为10.6μm，但是在9-11μm期间（尤其是9.6μm）还有其它的辐射线。大多数情况下，辐射光的功率在几十瓦到几千瓦之间。功率转化效率能大于10%，即比大多数固态激光器要高，但是比二极管泵浦激光器低。 

激光器类型 
CO2激光器中包含多个类型：

1. 激光器功率在几瓦特到几百瓦特，常采用封闭管或者无流量激光器，其中气体在封闭管内。这种激光器体积很小并且结构稳定，寿命可以达到几千小时。 
2. 高功率扩散冷却平板激光器中，气体在一对平板水冷射频电极的缝隙之间。如果电极间隙与电极宽度相比很小，产生的热会有效的通过扩散转到到电极。输出功率可以达到几千瓦。 
3. 快速轴向流动激光器和快速横向流动激光器也可以产生几千瓦的输出功率。快速流动的气体混合物会带走额外的热能，在被气体放电重新利用之前会经过外置的冷却装置。 
4. 横向激励气体(TEA)激光器具有很高的气压。由于纵向放电所需的电压很高，可以在管子中采用一系列电极进行横向激励。TEA激光只能工作于脉冲模式，因为气体放电在高电压时不稳定。它们通常产生的平均输出功率小于100W，但是有时也可以得到几十千瓦的光功率。 
5. 还有气体动力学CO2激光器可以产生即兆瓦的功率，其中能量不由气体放电提供，而是由一种火箭引擎中的化学反应提供。 

以上的这些概念的差别主要在除热技术，气压和采用的电极结构上。在低功率封闭管激光器中，多余的热通过扩散或者慢的气流转移到管壁。光束质量很高。高功率的CO2激光器采用快速的强制气体对流，可以在轴向或者横向。 

应用 
CO2激光器广泛应用于材料加工行业，尤其是： 

1. 切割塑料材料，木制品，刀模等，在10.6μm处具有很强的吸收，需要功率在20-200W 
2. 切割和焊接金属，例如不锈钢，铝或铜，需要几千瓦功率 
3. 在各种材料上进行激光打标 

还有其它的应用包括激光手术和测距。
CO2激光器在材料加工方面与工作在1μm波长的固态激光器形成竞争。波长更短的优势在于会被金属台更好的吸收，并且可以通过光纤光缆传输光束。（没有光纤可以传输高功率10μm激光光束。）1-μm激光器具有更小的光束参数乘积也是有利的。
但是，后者通常很难在高功率灯泵浦激光器得到，而二极管泵浦激光器则非常昂贵。基于以上原因，CO2激光器仍然广泛应用于切割和焊接领域中，尤其是部件厚度大于几毫米时，它们的销量占整个激光器销量的10%还多（2013年）。
未来，由于高功率薄片激光器的发展以及光纤光缆的技术提高，这种格局可能会有变化。 
由于激光器具有很高的功率和需要很高的驱动电压，因此存在很严重的激光安全问题。但是，它们工作波长很长，因此在低光强时是相对人眼安全的。

Definition: infrared lasers based on a gas mixture in which light is amplified by carbon dioxide molecules

Alternative term: carbon dioxide lasers

The CO₂ laser (carbon dioxide laser) is a molecular gas laser with emission in the long-wavelength infrared spectral region. It is based on a gas mixture as the gain medium, which contains carbon dioxide (CO₂), helium (He), nitrogen (N₂), and possibly some hydrogen (H₂), oxygen (O₂), water vapor and/or xenon (Xe). Such a laser is electrically pumped via an electrical gas discharge, which can be operated with DC current, with AC current (e.g. 20–50 kHz) or in the radio frequency (RF) domain.

Although direct excitation of CO₂ molecules into the upper laser level is possible, it has proven to be most efficient to use a resonant energy transfer from nitrogen molecules. Here, nitrogen molecules are excited by the electric discharge into a metastable vibrational level and transfer their excitation energy to the CO₂ molecules when colliding with them. The exited CO₂ molecules then largely participate in the laser transition. Helium serves both to depopulate the lower laser level and to remove the heat. Other constituents such as hydrogen or water vapor can help (particularly in sealed-tube lasers) to reoxidize carbon monoxide (CO, formed in the discharge) to carbon dioxide.

Figure 1: Schematic setup of a sealed-tube carbon dioxide laser. The gas tube has Brewster windows and is water-cooled.

Spectral Lines

CO₂ lasers typically emit at a wavelength of 10.6 μm, but there are dozens of other laser lines in the region of 9–11 μm (particularly at 9.6 μm). This is because two different vibrational states of the CO₂ molecules can be used as the lower level, and for each vibrational state, there is a substantial number of rotational states, leading to many sub-levels. Dipole transitions (the only ones with a relatively high strength) are possible with ΔJ = ±1, where ΔJ = 1 (R branch) leads to higher photon energies (shorter wavelengths) and ΔJ = −1 (P branch) to lower energies:

• Transitions of the stronger band, involving one of the two possible final vibrational levels, have their P branch around 10.6 μm, with P20 being the dominant transition, and the R branch around 10.2 μm.
• Transitions of the other band have the P branch around 9.6 μm and the R branch around 9.3 μm.

With a suitable wavelength tuning element in the laser resonator, a CO₂ laser can be made to lase on one of more than a dozen transitions with relatively closely spaced wavelengths in each branch, but continuous wavelength tuning is not possible due to the discrete rotational states of the molecules. Without a wavelength-selective element in the resonator, one may obtain simultaneous lasing on a few transitions, or occasional jumps to other transitions during operation.

Non-standard emission wavelengths make CO₂ lasers suitable for additional application areas.

While most commercially available CO₂ lasers emit at the standard wavelength of 10.6 μm, there are devices which are specially optimized for other wavelengths such as 10.25 μm or 9.3 μm, which are far better suited for certain applications for example in laser material processing, because that radiation is much more absorbed in certain materials (e.g. polymers). For making such lasers and for using their radiation, one may require special infrared optics, as standard transmissive 10.6-μm optics may e.g. exhibit too strong reflections.

All CO₂ laser emissions lines can be considered to be in the long-wavelength infrared region, which is part of the mid-infrared region according to ISO 20473:2007.

Output Powers and Efficiency

In most cases, average output powers are between some tens of watts and many kilowatts. The power conversion efficiency can be around 10% to 20%, i.e., it is higher than for most gas lasers (due to a particularly favorable excitation pathway), also higher than for lamp-pumped solid-state lasers, but lower than for many diode-pumped lasers.

Due to their high output powers and long emission wavelengths, CO₂ lasers require high-quality infrared optics, often made of materials like zinc selenide (ZnSe) or zinc sulfide (ZnS).

Due to their high powers and high drive voltages, CO₂ lasers raise serious issues of laser safety. However, their long operation wavelength makes them relatively eye-safe at low intensities.

CO₂ Laser Types

The family of CO₂ lasers is very diverse:

• For laser powers between a few watts and a several hundred watts, it is common to use sealed-tube or no-flow lasers, where the laser bore and gas supply are contained in a sealed tube. Waste heat is transported to the tube walls by diffusion (with a quite helpful effect of the helium) or a slow gas flow. Such lasers are compact and rugged, and easily reach operation lifetimes of several thousands of hours or more. Here, one needs to employ methods for continuously regenerating the gas – in particular, for counteracting the dissociation of CO₂ by catalytic re-oxidization of CO. The beam quality can be very high.
• High-power diffusion-cooled slab lasers (not to be confused with solid-state slab lasers) have the gas in a gap between a pair of planar water-cooled RF electrodes. The excess heat is efficiently transferred to the electrodes by diffusion, if the electrode spacing is made small compared with the electrode width. For efficient energy extraction, one often uses an unstable resonator with output coupling at the side of a highly reflecting mirror. Several kilowatts of output are possible in conjunction with reasonable beam quality.
• Fast axial flow lasers and fast transverse flow lasers are also suitable for multi-kilowatt continuous-wave output powers and high beam quality. The excess heat is removed by the fast-flowing gas mixture, which passes an external cooler (heat exchanger) before being used again in the discharge. The gas can be continuously regenerated and occasionally replaced. Transverse flow lasers reach highest output powers, but typically with lower beam quality.
• Transverse excited atmosphere (TEA) lasers have a very high (about atmospheric) gas pressure. As the voltage required for a longitudinal discharge would be too high, transverse excitation is done with a series of electrodes along the tube. TEA lasers are operated in pulsed mode only, as the gas discharge would not be stable at high pressures. They often produce average output powers below 100 W, but can also be made for powers of tens of kilowatts (combined with high pulse repetition rates).
• There are gas dynamic CO₂ lasers (a kind of chemical lasers) for multi-megawatt powers (e.g. for anti-missile weapons), where the energy is not provided by a gas discharge but by a chemical reaction in a kind of rocket engine.

These concepts lead to quite different laser architectures, with specific strengths and weaknesses concerning output power potential, beam quality, gas consumption and device lifetime.

Applications of CO₂ Lasers

CO₂ lasers are widely used as industrial lasers for laser material processing, in particular for

• cutting and structuring plastic materials, wood, die boards, glass pieces, etc., which exhibit high absorption at 10.6 μm, mostly applying moderate power levels of 20–200 W
• cutting, welding and cladding metals such as stainless steel, aluminum or copper, applying multi-kilowatt powers
• laser marking of various materials
• laser hardening e.g. of machine parts made of steel
• laser soldering in electronics
• laser 3D printing of polymer materials

Other applications include laser surgery (including ophthalmology), range finding (LIDAR) and spectroscopy.

Competition with Solid-state Lasers

CO₂ lasers used for laser material processing (e.g. welding and cutting of metals, or laser marking) are in competition with solid-state lasers (particularly YAG lasers and fiber lasers) operating in the 1-μm wavelength regime. These shorter wavelengths have the advantages of more efficient absorption in a metallic workpiece and the potential for beam delivery via fiber cables. (There are no optical fibers for high-power 10-μm laser beams.) Besides, 1-μm beams can be more tightly focused, provided that the beam quality is high. However, the latter potential normally cannot be realized with high-power lamp-pumped lasers, and diode-pumped lasers tend to be more expensive. Concerning absorption, CO₂ laser beams are actually quite favorable for certain materials like polymers and ceramics. Even when absorption is less favorable than for a solid-state laser, a CO₂ laser may be preferred as a relatively cheap and robust solution. A substantial disadvantage, however, is that there are no high-power fiber cables for CO₂ laser radiation.

CO₂ lasers are still widely used in the cutting and welding business, particularly for parts with a thickness greater than a few millimeters, and their sales still make a substantial part of all global laser sales. This may to some extent change in the future due to the development of high-power thin-disk lasers and advanced fiber cables in combination with techniques which exploit the high beam quality of such lasers.

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