Carbon Dioxide Laser: Important Features and Applications.

Kumar Patel of Bell Labs invented the CO₂ laser in 1964. In this article, I discuss only some features of CO₂  laser. Carbon dioxide laser is another gas laser with a high power of about 60 kW.  Carbon dioxide laser is still one of the most used types of laser. Carbon dioxide lasers are the highest-power continuous-wave lasers that are currently available. They are also quite efficient: the ratio of output power to pump power can be as large as 20%. The CO2 laser produces a beam of infrared light with the principal wavelength bands centering on 9.6 and 10.6 micrometers (μm). [5] I will tell you about infrared light later in this article.

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

Lasing medium:

 It uses CO2 and N₂ in equal ratios, with traces of He and water vapor as the lasing medium.CO₂  is the laser-active material, and lasing transition happens between specific vibrational levels of CO₂ molecules. The specific proportions vary according to the particular laser.

Nitrogen in the lasing medium helps to achieve population inversion of CO2, while the additives help immediate depopulation to lower lasing levels.

Population inversion and lasing action in CO2 laser is in the following sequence:

1 Electron impact excites the quantum vibrational modes of nitrogen

2 N2 collisionally deexcites by transferring its vibrational mode energy to the CO2 molecule.

3 CO2 molecules excite to its vibrational mode quantum state.

4 The emission of radiation in two wavelengths

5 Output is infrared light with wavelengths 9.6 and 10.6 µm.

6 CO2 molecules deexcite to ground level by colliding with cold He atoms in the medium.

7 Because of this collision, Helium gets heated and should be cooled to sustain population inversion. The efficiency of this laser is 40% in continuous wave mode. Because of this high power, it has applications in tunneling and mining.

 The resulting hot helium atoms must be cool to sustain the ability to produce a population inversion in the carbon dioxide molecules. In sealed lasers, this occurs as the helium atoms strike the walls of the laser discharge tube.

The addition of helium also plays a role in the initial vibrational excitation of N2

 due to a near-resonant dissociation reaction with metastable He(23S1). Substituting helium with other noble gases, such as neon or argon, does not enhance the laser output. [2]

Because the excitation energy of molecular vibrational and rotational mode quantum states are low, the photons emitted due to the transition between these quantum states have comparatively lower power and longer wavelengths than visible and near-infrared light. 

The 9–12 μm wavelength of CO2 lasers falls into the significant window for atmospheric transmission (up to 80% atmospheric transmission at this wavelength).   Many natural and synthetic materials have strong characteristic absorption in this range. [3]

We can tune the laser wavelength by altering the isotopic ratio of the carbon and oxygen atoms comprising the CO2 molecules in the discharge tube.

 

Do you know what the atmospheric transmission window is?

The infrared atmospheric window refers to a region of the infrared spectrum where relatively little absorption of terrestrial thermal radiation by atmospheric gases. [20] The window plays a significant role in the atmospheric greenhouse effect, maintaining the balance between incoming solar radiation and outgoing IR to space. This window is roughly the region of 8 to 14 μm in the Earth's atmosphere. However, it can be narrowed or closed at times and places of high humidity because of the strong absorption in the water vapor continuum or the blocking by clouds. [21][22][23][24][25] It covers a substantial part of the spectrum from surface thermal emission, which starts at roughly five μm. It is a large gap in the absorption spectrum of water vapor. Carbon dioxide plays a significant role in setting the boundary at the long wavelength side. Ozone partly blocks transmission in the middle of the window.

The importance of the infrared atmospheric window in the atmospheric energy balance was discovered by George Simpson in 1928, based on G. Hettner's 1918[26] laboratory studies of the gap in the absorption spectrum of water vapor. In those days, computers were not available, and Simpson notes that he used approximations. He wrote about the need for this to calculate outgoing IR radiation: There was no hope of getting an exact solution; but by making suitable simplifying assumptions. [27] Nowadays, accurate line-by-line computations are possible, and careful studies of the spectroscopy of infrared atmospheric gases.

Construction

Because CO2 lasers operate in the infrared, selected materials are necessary for the construction. Typically, the mirrors are silvered, while for windows and lenses, either germanium or zinc selenide. For high-power applications, the preference is for gold mirrors and zinc selenide windows and lenses. There are also diamond windows and lenses in use. Diamond windows are expensive, but their high thermal conductivity and hardness make them useful in high-power applications and dirty environments. Historically, lenses and windows were made out of salt (either sodium chloride or potassium chloride). While the material was inexpensive, the lenses and windows degraded slowly with exposure to atmospheric moisture. [5]

The most basic form of a CO2 laser consists of a gas discharge (with a mix close to that specified above) with a total reflector at one end and an output coupler (a partially reflecting mirror) at the output end. [8]

The CO2 laser has continuous wave (CW) powers between milliwatts (mW) and hundreds of kilowatts (kW). [9] It is also easy to actively Q-switch a CO2 laser using a rotating mirror or an electro-optic switch, giving rise to Q-switched peak powers of up to gigawatts (GW). [10] To learn more about Q-switching using prisms, read my other article (https://retnacpn.blogspot.com/2023/10/solid-state-lasers-introduction-solid.html)

Because the laser transitions are actually on vibration-rotation bands of a linear triatomic molecule, we can select the rotational structure of the bands by a tuning element in the laser cavity. Prisms are not practical tuning elements because most media that transmit in the mid-infrared absorb or scatter some of the light. So, the frequency tuning element is almost always a diffraction grating. By rotating the diffraction grating, we can select a particular rotational line of the vibrational transition. The finest frequency selection is possible with an etalon. Such line-tunable carbon-dioxide lasers [11] are principally of interest in research applications. The laser's output wavelength is affected by the particular isotopes contained in the carbon dioxide molecule, with heavier isotopes and emission of longer wavelength.[11]

Applications:

 Industrial

Because of the high power levels available (combined with the reasonable cost of the laser), CO2 lasers have uses in industrial applications for cutting and welding. With low-power lasers, engraving is possible. To fuse particles of plastic powder in selective laser sintering, we use a CO2 laser.

•      Cutting and welding
•        Engraving
•     Selective laser sintering(SLS)

Medical

•     Laser surgery 
•     Skin resurfacing (laser facelifts, vaporizing the skin for collagen formation)

• ``` To remove vocal-fold cysts 

•      Weld human tissue

•     Tissue ablation in Dentistry

A medical CO₂ laser [5]

Soft-tissue surgery

Carbon dioxide lasers are applied in surgical procedures because water (which makes up most biological tissue) absorbs this frequency of light very well.

Some examples are:

Laser surgery and skin resurfacing.

Laser facelifts (which essentially consist of vaporizing the skin to promote collagen formation). [12] 

To treat certain skin conditions such as hirsuties papillaris genitalis by removing bumps.

To remove vocal-fold lesions, [13] 

Researchers in Israel are experimenting with using CO2 lasers to weld human tissues as an alternative to traditional sutures. [14]

The 10.6 μm CO2 laser remains the best surgical laser for soft tissue. [15][16][17][18] CO2 lasers have come into use with a scalpel for most procedures and in delicate areas where mechanical trauma could damage the surgical site. CO2 lasers are the best for soft-tissue procedures in humans and animals, compared to other lasers. Applications include gynecology, dentistry, oral and maxillofacial surgery, and many others.

A CO2 dental laser at the 9.25–9.6 μm wavelength has come to use in dentistry for hard-tissue ablation. The hard tissue is ablated at temperatures as high as 5,000 °C, producing bright thermal radiation. [19]

Advantages

•     Less bleeding 
•     Shorter surgery time 
•      Less risk of infection
•      Less post-op swelling   

Infrared radiation                                                                 

Infrared (IR) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with waves just longer than those of red light (the longest waves in the visible spectrum), so IR is invisible to the human eye. IR is generally understood to include wavelengths from around 750 nm to 1 mm. [28][29] IR between longer-wavelength is thermal IR, emitted from terrestrial sources, and shorter-wavelength IR or near-IR, part of the solar spectrum. [30] Terahertz radiation band includes longer IR wavelengths (30–100 μm). [31] Almost all black-body radiation from objects near room temperature is in the IR band.

Features:

• It is a form of electromagnetic radiation, IR.
• IR Carries energy and momentum.
• It exerts radiation pressure. 
• IR has properties corresponding to both a wave and a particle, the photon.

We know that fires emit invisible heat. In 1681, the pioneering experimenter Edme Mariotte showed that glass, though transparent to sunlight, obstructed radiant heat. [32][33] In 1800, the astronomer Sir William Herschel discovered that infrared radiation is a type of invisible radiation in the spectrum lower in energy than red light, using its effect on a thermometer. [34] Slightly more than half of the power from the Sun was eventually found, through Herschel's studies, to arrive on Earth in the form of infrared. The balance between absorbed and emitted infrared radiation has an important effect on Earth's climate.

Infrared radiation is emitted or absorbed by molecules when changing rotational-vibrational movements. It excites vibrational modes in a molecule through a change in the dipole moment, making it a frequency range for these energy states for molecules of the proper symmetry. Infrared spectroscopy examines the absorption and transmission of photons in the infrared range. [35]

Uses of IR radiation:

• IR has uses in industrial, scientific, military, commercial, and medical applications.
• We can observe people or animals in night-vision devices using active near-infrared illumination without detaching the observer. 
• Infrared astronomy uses sensor-equipped telescopes to penetrate dusty regions of space as molecular clouds, to detect objects such as planets, and to view highly red-shifted objects from the early days of the universe. [36] 
• Infrared thermal-imaging cameras detect heat loss in insulated systems, observe changing blood flow in the skin, assist firefighting, and detect the overheating of electrical components. [37]
• Military and civilian applications include target acquisition, surveillance, night vision, homing, and tracking.
• Non-military uses include thermal efficiency analysis, environmental monitoring, industrial facility inspections, detection of grow-ops, remote temperature sensing, short-range wireless communication, spectroscopy, and weather forecasting.

Do you know the wavelength of radiation emitted by your body in normal conditions?

It is around ten (10) μm. 

                                                   

   References:

1 Patel C K N, Physical Rev. 136, 5(A), 1964.

2 Andreeta M R B, et al., J. Micromechanics and Micro Engineering, 21(A), 2011.

3 Barten et al., Skin resurfacing in Charles Thorne (ed), 2014.

4 Israeli researchers pioneer test treatment for sealing

5 Wikipedia

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