This article will take an in-depth look at carbon dioxide lasers.
The article will bring more detail on topics such as:
This section will delve into the basic principles of carbon dioxide lasers, highlighting their design, components, and operational mechanisms.
A carbon dioxide laser is a kind of laser technology that uses carbon dioxide as its main gain medium. Nitrogen (N2), helium (He), and occasionally hydrogen (H2), water vapor, oxygen, or xenon (Xe) may be added to improve its performance. The laser functions through an electrical gas discharge, which stimulates the gas mixture, resulting in laser light production.
In carbon dioxide lasers, the electrical gas discharge can be powered by alternating current (AC), direct current (DC), or radio frequency (RF) energy. These lasers emit light at a wavelength of 10.6 micrometers and are widely used in dermatology for procedures such as scar removal, wrinkle reduction, and treating sun-damaged skin. They are also employed in surgical applications like gynecology and neurosurgery for accurate cutting and tissue removal.
Carbon dioxide lasers are constructed from several essential components:
Given their infrared operation, carbon dioxide lasers require specific materials. Their mirrors usually have a silver coating, while the windows and lenses are crafted from germanium or zinc selenide. For high-power applications, gold-coated mirrors and zinc selenide components are preferable. Sometimes, diamond can be used to enhance performance for windows and lenses.
The carbon dioxide laser is centered around a quartz discharge tube, typically 2.5 cm in diameter and about 5 meters long, containing a CO2, N2, and He gas mixture in a 1:2:3 ratio, with added water vapor. Pressures within the tube are maintained at around 7 Torr for helium, 1.2 Torr for nitrogen, and 0.33 Torr for carbon dioxide. Laser action occurs through transitions between the vibrational and rotational states of CO2 molecules. The design of the CO2 laser is simple, offering continuous laser output.
The active medium in a carbon dioxide laser comprises carbon dioxide, nitrogen, and helium in a 1:2:3 ratio. Carbon dioxide molecules primarily drive the laser's function, acting as the key centers for laser emission.
An electrical discharge facilitates population inversion by energizing the CO2 molecules. Here, electrons collide with CO2 molecules, boosting them to higher energy states.
The electric discharge inside the apparatus causes electrons to collide with nitrogen molecules, exciting the electrons. This process can be expressed by the following equation:
N2 + e* = N2* + e
Key:
N2 = Nitrogen molecule in ground state
e* = electron with kinetic energy
N2* = Nitrogen molecule in the excited state
e = same electron with lesser energy
The excited N2 molecules then interact with ground-state CO2 atoms, elevating them to higher electronic, vibrational, and rotational energy levels.
The process is represented by the following equation:
N2* + CO2 = CO2* + N2
Key:
N2* = Excited nitrogen molecule
CO2 = Ground state carbon dioxide atom
CO2* = Excited carbon dioxide atom
N2 = Ground state nitrogen atom
The close E5 energy level of carbon dioxide atoms sees an increase in population due to the excited state of nitrogen, achieving laser action as spontaneous photon emission occurs within the tube once population inversion is present.
There are two possible laser transitions, detailed below.
Transition E5 to E4
This transition results in a 10.6-micrometer wavelength laser beam.
Transition E5 to E3
This transition results in a 9.6-micrometer wavelength laser beam, though the 10.6-micrometer transition generally has greater intensity. CO2 lasers produce a power output of 10kW.
The gas mixtures are positioned between two mirrors forming the optical resonator system. One mirror is completely reflective, while the other is partially reflective. Operating solely in the infrared range and achieving significant power outputs, the optical parts of CO2 lasers are usually made from specialized materials such as zinc selenide, germanium, silver, diamond, and gold.
Understanding the operation of a carbon dioxide laser involves these key steps:
When exposed to electric current, nitrogen molecules in the gas mixture become excited. Nitrogen is used because it can remain in an energized state for extended durations without light emission or photon release. The carbon dioxide molecules then get excited from the high-energy vibrations transferred by nitrogen. At this stage, a condition called population inversion is achieved—where there are more energized particles than non-energized ones in the system. To produce a laser beam, nitrogen atoms must emit their excited state energy as photons, which occurs when excited nitrogen atoms interact with cooler helium atoms, provoking light emission.
Direct excitation of CO2 molecules in the upper laser level is possible but inefficient compared to the resonant energy transfer from nitrogen molecules. Through this process, electric discharge excites nitrogen molecules to a metastable vibrational level, which then transfers energy to carbon dioxide molecules during collisions.
Excited carbon dioxide molecules are crucial in the laser transition, with helium helping reduce the lower laser level's population and dissipating heat. Water vapor and hydrogen can assist in reconverting carbon monoxide, produced during discharge, back into carbon dioxide.
CO2 lasers primarily emit at a wavelength of 10.6 micrometers, although other wavelengths between 9-11 micrometers, such as 9.6 micrometers, are common. This variation is due to the carbon dioxide molecules' various vibrational states, which serve as the lower energy levels, and their numerous rotational states associated with each vibrational state. These conditions create multiple sub-levels and possible dipole transitions, where Δj = ±1. Transitions with Δj = +1 (R branch) generate higher photon energies, while Δj = -1 (P branch) results in lower photon energies.
While CO2 lasers can be tuned to various transitions closely situated within their respective branches, continuous wavelength tuning is not possible due to the discrete rotational states of the molecules. Without a wavelength-selective component, the laser might operate on multiple simultaneous transitions or occasionally switch between different ones during operation. Non-standard emission wavelengths increase the flexibility of CO2 lasers for specialized uses.
Carbon dioxide lasers on the market generally emit at 10.6 micrometers. However, some models are specifically designed to emit at alternative wavelengths like 10.25 micrometers or 9.3 micrometers, which are beneficial in applications such as laser material processing where certain materials absorb these wavelengths more efficiently. For these alternative wavelengths, infrared optics may be necessary since standard 10.6 micrometer optics might reflect excessive radiation. Typically, the emissions from CO2 lasers fall within the long-wavelength infrared spectrum, considered part of the mid-infrared range.
Carbon dioxide lasers typically have average powers ranging from tens of watts to several kilowatts. The power conversion efficiency generally lies between 10% and 20%, surpassing that of numerous gas lasers due to a favorable energy conversion pathway and higher than that of solid-state lamp-pumped lasers. Nevertheless, it is lower than diode-pumped lasers. The long emission wavelengths and high power output of carbon dioxide lasers necessitate high-quality infrared optics, often crafted from materials like zinc sulfide or zinc selenide. Despite their significant drive voltages and power levels, they are relatively safe for the eyes at low intensities due to their longer operational wavelength, though laser safety concerns persist.
Nitrogen molecules in the gas mixture are excited when stimulated, acquiring energy. Nitrogen is chosen for its ability to stay excited for long durations without energy emission as light or photons. The excited nitrogen molecules then pass their energy to carbon dioxide molecules, achieving population inversion within the laser. To produce light, nitrogen atoms must return to their ground state by emitting photons, a process that happens when excited nitrogen atoms interact with very cold helium atoms, resulting in light emission.
During operation, carbon dioxide (CO2) can degrade into carbon monoxide (CO), an undesirable byproduct. Small amounts of water vapor or hydrogen can help convert CO back to CO2 to counter this issue.
The active medium comprises a gas mixture of nitrogen (N2), helium (He), and carbon dioxide (CO2). Laser transitions occur between the carbon dioxide molecules' vibrational states.
Carbon dioxide lasers include three main parts: the gain medium, the energy source (or pump), and the optical resonator. The pump delivers energy to the gain medium, which is amplified. This energy is converted into light, which the optical resonator reflects, eventually emitting it as the final output beam.
The laser pump operates on electrical current, which energizes the gas medium.
The gain medium in a CO2 laser consists of a gas mixture including carbon dioxide, nitrogen, hydrogen, and helium. Proportions vary based on the laser’s use, but nitrogen, carbon dioxide, and helium are primary. Typically, the gas mixture ratio is 1 part nitrogen, 1 part carbon dioxide, and 8 parts helium.
Carbon dioxide lasers operate solely within the infrared spectrum and can deliver high power outputs. Therefore, their optical components are often made from specialized and expensive materials such as germanium, zinc selenide, silver, gold, and diamond.
The most common types of carbon dioxide lasers include the longitudinal-flow, transverse-flow, sealed-off, waveguide, and TEA (transversely excited atmospheric) lasers.
These are the simplest designs and are mostly used with high power output lasers. In these lasers laser gas is continuously vacuumed through a discharge tube by means of a vacuum pump.
In the laser system, a fraction of the carbon dioxide in the gas mixture is dissociated into carbon monoxide and oxygen using a direct current discharge. The gas mixture is continually circulated through the system with the help of multiple pumps, which enhances the efficiency of heat removal and minimizes heat loss.
These lasers utilize a glass tube filled with a CO2-N2-He gas mixture. Instead of replacing the gas mixture, hydrogen, water vapor, and oxygen are introduced into the system. This approach is necessary because an electrical discharge rapidly decomposes CO2, typically within minutes. To counteract this, hydrogen or water vapor is added to react with the resulting carbon monoxide and oxygen, facilitating the reformation of CO2. As a result, CO2 is effectively regenerated through a catalytic process.
Mirrors at both ends of the setup create a resonant cavity. Additionally, a nickel cathode heated to 300°C can facilitate the recombination process. These methods together enhance the operating lifespan of the laser to several thousand hours.
This type of laser is created by substituting the sealed tube with a waveguide, which has an inner diameter of just a few millimeters. Known as the slab laser, it has a smaller lasing volume and consequently generates lower power output. The waveguide's resonator features a comparatively large surface area relative to its volume, which aids in effective heat dissipation. The resonator typically has a cuboidal shape.
This design employs a discharge voltage applied in brief pulses of less than one microsecond across the gas flow, effectively preventing arcing. It is utilized in applications requiring high pressures. Due to the excessive voltage needed for a longitudinal discharge, transverse excitation is used instead, with electrodes arranged in series along the length of the tube.
TEA lasers operate exclusively in pulsed mode because the gas discharge cannot remain stable at high pressures. Typically, they produce average output powers under 100 W, although they can be designed to reach tens of kilowatts when paired with high pulse repetition rates.
In these carbon dioxide lasers, the gas is contained between two planar RF electrodes that are water-cooled. Heat is transferred to the electrodes through diffusion, especially when the spacing between the electrodes is minimal relative to their width.
To achieve efficient energy extraction, an unstable resonator is employed, featuring output coupling on the side of a highly reflective mirror. This setup can produce several kilowatts of output while maintaining good beam quality.
These lasers, which are a type of chemical laser, can achieve multi-megawatt power levels, such as those used in anti-missile systems. Instead of relying on gas discharge for energy, they generate power through chemical reactions.
Carbon dioxide lasers that are used for the processing of laser material (such as cutting and welding of metals or laser marking) are in competition with solid-state lasers (particularly fiber lasers and YAG lasers) that operate in the 1 micrometer wavelength. These shorter wavelengths offer advantages that include more efficient absorption in a workpiece that is metallic and the potential for delivery of beam via fiber cables.
For high-power laser applications at 10 micrometers, optical fibers are not used. However, a 1-micrometer beam can be focused more precisely when the beam quality is high. In terms of absorption, carbon dioxide laser beams are particularly effective for materials like ceramics and polymers. Although carbon dioxide lasers may offer less absorption efficiency compared to solid-state lasers, they are often chosen for their durability and cost-effectiveness.
Despite this, a major drawback is the lack of high-power fiber cables for carbon dioxide lasers. Nevertheless, carbon dioxide lasers remain widely used in the welding and cutting industries, especially for thick materials. They represent a significant portion of the global laser market.
This chapter will cover the uses and advantages of carbon dioxide lasers.
Carbon dioxide lasers are utilized in the following applications:
Carbon dioxide lasers are employed to address various skin conditions. They are also utilized in surgeries related to the head, neck, and gynecology.
Carbon dioxide lasers are utilized in medical surgeries due to their ability to be absorbed by water, making them highly effective for soft tissue procedures. Their wavelength of 10.6 micrometers is optimal for such applications. These lasers minimize bleeding, reduce surgery time, and lower the risk of infection and post-operative swelling.
Carbon dioxide lasers are highly effective in various medical procedures, including oral, gynecological, dental, and maxillofacial surgeries. These lasers excel in treating soft tissues due to their high water content, as carbon dioxide is specifically absorbed by water. This property makes CO2 lasers ideal for intraoral soft tissue operations. They have been extensively employed in otolaryngology and are valuable in head and neck surgeries, as well as for addressing condylomata acuminata, intraepithelial neoplasms, and other gynecological lesions. In airway surgery, CO2 lasers are preferred and are particularly beneficial for procedures involving the head, neck, and larynx.
Skin resurfacing is often performed using carbon dioxide lasers, which carefully eliminate thin layers of skin while minimizing heat damage to adjacent areas. These lasers operate by vaporizing skin tissue with a focused high-energy laser beam, resulting in a controlled injury. This process stimulates collagen production, contributing to the restoration of skin elasticity. Carbon dioxide lasers are effective for addressing wrinkles, sun damage, and for the removal of birthmarks, warts, scars, and rhinophyma.
A laser beam can focus intense energy on a very small area, making it highly effective for industrial applications such as welding, cutting, and drilling.
During welding two components are joined using a material. High temperatures are needed in the process of melting and then joining the material but however temperature must not be high enough to evaporate the material. Two dissimilar metals can be joined using a carbon dioxide laser. Carbon dioxide laser welding is used in the manufacture of aircrafts and automobiles. Laser welding is also used in electronics.
CO₂ laser cutting processing technique for sheets that utilizes a gas laser that is electrically driven. A carbon dioxide laser is used to remove part of a material from a substance. Materials cut can be metal or nonmetal such as titanium, stainless steel, ceramic glass, plastic and wood.
Radar technology employs radio waves to detect and map objects in space. Similarly, carbon dioxide lasers are utilized for environmental monitoring through a method known as light detection and ranging (LIDAR). This technique allows for the identification, observation, and measurement of objects that are otherwise difficult to access.
Marking lasers offer a rapid and precise method for marking various materials, surpassing the accuracy of traditional mechanical or chemical engraving techniques. The precision of carbon dioxide lasers minimizes the risk of burning or damaging the material being marked. They produce clear, clean lines, curves, and text over large surfaces with consistent quality and efficiency.
Carbon dioxide lasers are popular in industrial settings due to their quick setup, reduced labor costs, and fast production capabilities. They are commonly used for engraving logos, barcodes, part numbers, and 2D codes on components and products.
When choosing a carbon dioxide laser, the following factors should be considered:
Unlike other types of lasers, CO2 lasers operate within a narrow range of wavelengths, all of which fall within the infrared (IR) spectrum. Typically, these lasers emit light in the range of 9.4 μm to 10.6 μm. By adjusting the gas concentrations in the gain medium, CO2 lasers can be engineered to emit specific, discrete frequencies within this broader range.
Carbon dioxide lasers usually have their power ratings specified by the manufacturer. These lasers are often categorized as high-power devices, with some models capable of continuous output at up to 60 kW. The power level of a laser generally dictates its applications: high-power lasers are best suited for cutting and welding, whereas lower-power models are more appropriate for tasks like marking barcodes and labels.
Safety is a critical concern when using carbon dioxide lasers because of their high power output. For instance, even a 200 mW laser could cause permanent eye damage from 100 yards away. Given that CO2 lasers can emit thousands of watts, direct exposure at close range poses a significant risk of burns to the eyes or skin.
Daily maintenance tasks for carbon dioxide lasers include:
Ensure that the laser tube has proper water circulation, as both the quality and temperature of the circulating water directly impact the laser tube’s lifespan. Use pure water and maintain a temperature below 35 degrees Celsius. To clean the water tank, first turn off the power and disconnect the water inlet pipe to allow the water to drain from the laser tube into the tank. Next, open the water tank and pump, remove any debris from the water pump, and replace the circulating water. Reinstall the water pump, turn the power back on, and run the pump for 2 to 3 minutes to verify that the laser tube is receiving adequate water circulation.
Over time, dust can build up inside the fan, leading to unusual noises. If you notice such sounds, remove the fan and clean its interior. Make sure to detach and wipe the fan blades. This maintenance will help ensure the carbon dioxide laser operates efficiently.
The carbon dioxide laser's lens can attract dirt and contaminants, which may damage it. To maintain optimal performance, remove and clean the lens regularly. Avoid immersing the lens in cleaning fluid; instead, gently wipe along the edges. Exercise caution while cleaning to prevent damage to the lens’s surface coating and avoid dropping the lens, as it is fragile.
Carbon dioxide lasers are molecular gas lasers that have emissions in the long-wavelength infrared part of the spectra. They make use of carbon dioxide as well as Helium (He), Nitrogen (N2) and to some extent some hydrogen (H2), oxygen, water vapor, or Xenon (Xe) by emission of radiation that is stimulated, to improve their effectiveness in light application. There are different types of carbon dioxide lasers offering different properties and suitable for different specific applications for example gas dynamic carbon dioxide lasers which are kinds of chemical lasers for multi-megawatt powers for example anti-missile weapons. In short, carbon dioxide lasers are used for cutting, cladding, and welding metals, but their application is not limited to only these areas. They can also be used in surgery as well as dermatology. However, for the efficient performance of carbon dioxide lasers, proper maintenance must be conducted.
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