Carbon-dioxide laser

Form of gas laser

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A test target bursts into flame upon irradiation by a continuous-wave kilowatt-level carbon-dioxide laser.

The carbon-dioxide laser (CO2 laser) is recognized as one of the pioneering gas lasers that emerged in the field of laser technology. Developed by Kumar Patel at Bell Labs, it remains highly relevant and useful today. These lasers are the most powerful continuous-wave lasers available, showcasing impressive efficiency with output power to pump power ratios that can reach as high as 20%. The CO2 laser emits an infrared beam centered around wavelengths of 9.6 and 10.6 micrometers (µm).

Amplification

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The operating medium of a CO2 laser is typically a gas discharge system, which can be cooled by air or water depending on input power levels. Inside the sealed discharge tube, the gas mixture generally consists of approximately 10% to 20% carbon dioxide (CO2), nitrogen (N2), a small percentage of hydrogen (H2) and/or xenon (Xe), while the remaining component is helium (He). Variations in gas mixtures can occur depending on specific laser designs.

Population inversion, essential for laser operation, occurs through a sequence where electron collisions excite nitrogen's vibrational modes. In de-exciting collisions, nitrogen transfers its energy to carbon dioxide molecules, resulting in CO2 emitting photons at either 10.6 µm or 9.6 µm as it transitions between vibrational states. Collisions with cold helium atoms help sustain the inversion by facilitating the return of CO2 to its ground state. The effective cooling of heated helium atoms is crucial to maintain the inversion, either via contact with the discharge tube walls in sealed lasers or through constant gas flow in flow-through systems.

Helium’s involvement also enhances the initiation of vibrational excitation in nitrogen through resonance reactions. It's worth noting that using other noble gases may not yield similar benefits for laser output.

CO2 laser wavelengths are advantageous since they fall within a significant atmospheric transmission window, which allows for efficient communication through the atmosphere and interacts strongly with various materials, providing an effective range for numerous applications.

Additionally, altering the isotopic composition of carbon and oxygen atoms in CO2 can fine-tune the laser wavelength.

Construction

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Given that CO2 lasers function in the infrared spectrum, they require specific construction materials. The mirrors in these lasers are typically silvered, while windows and lenses are made from germanium or zinc selenide, particularly in high-power applications where gold mirrors may also be utilized. Diamond optics are also an option due to their exceptional thermal conductivity and durability, despite their high cost. Historically, materials like sodium chloride or potassium chloride were used for lenses and windows, but their susceptibility to moisture has led to their decreased use.

At a basic level, the CO2 laser consists of a gas discharge configuration with a total reflector on one end and a partially reflecting mirror on the output side. These lasers can produce continuous wave (CW) outputs from milliwatts to hundreds of kilowatts and can achieve peak powers in the gigawatt range when actively Q-switched.

CO2 lasers exhibit vibrational transitions that allow for tuning via diffraction gratings, facilitating the selection of specific rotational lines. This tuning process combined with isotopic substituents enables a continuous spectrum of frequencies to be produced which is particularly valuable in research settings. The output wavelengths can also vary based on the isotopes present in the CO2 gas.

Applications

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A medical CO2 laser

Industrial (cutting and welding)

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Given their capacity for high power output at a reasonable cost, CO2 lasers are commonly employed in industrial sectors for cutting and welding processes, while lower power variants are utilized for engraving tasks. In selective laser sintering, CO2 lasers effectively fuse plastic powder into solid parts.

Medical (soft-tissue surgery)

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The ability of CO2 lasers to interact with water—predominantly found in biological tissues—makes them invaluable in medical contexts. Common procedures include surgical applications and skin resurfacing, with the CO2 laser promoting collagen formation through tissue vaporization. They are also applied to treat specific skin conditions by removing unwanted growths. Furthermore, ongoing research is investigating their potential for welding human tissues as an alternative to stitches.

The 10.6 µm CO2 laser has established itself as a premier choice in soft tissue surgical procedures due to its ability to achieve both cutting and hemostatic effects. Its application scope extends to gynecology, dentistry, and oral and maxillofacial surgery, owing to advantages like reduced bleeding, shorter recovery times, and minimized risk of infection. The frequency of CO2 lasers is particularly well-suited for delicate procedures where traditional scalpels may not suffice.

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Additionally, CO2 dental lasers may be used to ablate hard tissue at a wavelength of 9.25-9.6 µm, effectively reaching extremely high temperatures.

Other

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CO2 lasers have also found applications in fabricating microfluidic devices from materials like poly(methyl methacrylate) (PMMA), thanks to their capacity to absorb infrared light. Furthermore, their ability to transmit infrared light through atmospheric conditions makes them suitable for military applications, particularly LIDAR rangefinding systems.

In spectroscopy and uranium enrichment processes, CO2 lasers are indispensable. They also play a role in semiconductor manufacturing, where they assist in the generation of extreme ultraviolet light.