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 cover the fundamentals of carbon dioxide lasers, including their design, construction, and operational principles.
A carbon dioxide laser is a type of laser that uses carbon dioxide as the primary gain medium, often with the addition of nitrogen (N2), helium (He), and sometimes hydrogen (H2), water vapor, oxygen, or xenon (Xe) to enhance its performance. This laser operates by stimulating the emission of radiation through an electrical gas discharge, which energizes the gas mixture to produce laser light.
The electrical gas discharge in carbon dioxide lasers can be driven by alternating current (AC), direct current (DC), or radio frequency (RF) energy. These lasers emit light at a wavelength of 10.6 micrometers. They are commonly employed in dermatology for procedures such as scar removal, wrinkle reduction, and treatment of sun-damaged skin. Additionally, carbon dioxide lasers are used as surgical tools in fields like gynecology and neurosurgery for precise cutting and tissue removal.
The construction of carbon dioxide lasers involves several key components:
Due to their operation in the infrared spectrum, carbon dioxide lasers require specialized materials. The mirrors used in these lasers are typically coated with silver, while windows and lenses are made from germanium or zinc selenide. For high-power applications, gold-coated mirrors and zinc selenide components are preferred. In some cases, diamond is used for windows and lenses to enhance performance.
A carbon dioxide laser is built around a quartz discharge tube, typically 2.5 cm in diameter and about 5 meters long. This tube is filled with a gas mixture of CO2, N2, and He in a 1:2:3 ratio, along with water vapor. The pressures within the tube are maintained at approximately 7 Torr for helium, 1.2 Torr for nitrogen, and 0.33 Torr for carbon dioxide. Laser action is achieved through transitions between vibrational and rotational states of carbon dioxide molecules. The design of the carbon dioxide laser is straightforward, and it produces a continuous output.
The active medium of a carbon dioxide laser consists of carbon dioxide, nitrogen, and helium in the ratio of 1:2:3. Carbon dioxide molecules are the primary contributors to the laser's operation, serving as the main centers for laser emission.
An electrical discharge is employed to achieve population inversion by exciting the carbon dioxide molecules. In this process, electrons collide with the CO2 molecules, elevating them to higher energy states.
Inside the machine, an electric discharge causes collisions between electrons and nitrogen molecules, resulting in the excitation of the electrons. This process can be represented by the following equation:
N2 + e* = N2* + e
Key:
N2 = Nitrogen molecule in ground state
e* = electron with kinetic energy
N2* = nitrogen molecule in excited state
e= same electron with lesser energy
Excited N2 molecules then interact with CO2 atoms in their ground state, elevating the CO2 atoms to higher electronic, vibrational, and rotational energy levels.
The following equation illustrates this process:
N2* + CO2 = CO2* + N2
Key:
N2* = Nitrogen molecule in excited state.
CO2 = ground state carbon dioxide atoms
CO2* = excited state carbon dioxide atoms
N2 = ground state nitrogen atoms
Due to the proximity between the excited state of nitrogen and the E5 level of carbon dioxide atoms, there is an increase in the population of the E5 level. Laser action is initiated by photons emitted spontaneously within the tube, occurring once the population inversion is achieved.
There are two types of possible laser transitions, which are described below.
Transition E5 to E4
This transition produces a laser beam of 10.6 micrometers wavelength.
Transition E5 to E3
This transition produces a laser beam of 9.6 micrometers wavelength. Usually the 10.6micro meter transition has more intensity than the 9.6micro meter transition. The carbon dioxide laser produces a power output of 10kW
The gas mixtures are contained between a pair of mirrors that form the optical resonator system. One mirror is fully reflective, while the other mirror reflects partially. Because CO2 lasers operate exclusively in the infrared region and achieve high power outputs, their optical components are typically crafted from specialized materials such as zinc selenide, germanium, silver, diamond, and gold.
The operation of a carbon dioxide laser involves the following steps:
When exposed to electric current, nitrogen molecules in the gas mixture become energized, reaching a high-energy state. Nitrogen is used because it can maintain its excited state for extended periods without emitting light or photons.
Carbon dioxide molecules are then excited by the high-energy vibrations from the nitrogen. At this point, a state known as population inversion is achieved—where there are more excited particles than non-excited particles in the system. For the laser to produce a beam of light, the nitrogen atoms must release their excited state energy as photons. This release happens when the excited nitrogen atoms interact with cooler helium atoms, leading to the emission of light by nitrogen.
Although direct excitation of carbon dioxide molecules in the upper laser level is possible, it is more efficient to use resonant energy transfer from nitrogen molecules. In this process, the electric discharge excites nitrogen molecules to a metastable vibrational level, which then transfers their energy to colliding carbon dioxide molecules.
Excited carbon dioxide molecules play a crucial role in the laser transition. Helium helps by both reducing the population of the lower laser level and dissipating heat. Additionally, water vapor and hydrogen can assist in converting carbon monoxide, produced during the discharge, back into carbon dioxide.
Carbon dioxide lasers primarily emit at a wavelength of 10.6 micrometers, though other wavelengths in the 9-11 micrometer range, such as 9.6 micrometers, are also observed. This variation arises from the carbon dioxide molecules' different vibrational states, which serve as the lower energy levels, and the numerous rotational states associated with each vibrational state. These conditions allow for multiple sub-levels and potential dipole transitions, where Δj = ±1. Transitions with Δj = +1 (R branch) result in higher photon energies, while Δj = -1 (P branch) produces lower photon energies.
Carbon dioxide lasers can be tuned to lase on a variety of transitions, each with wavelengths that are relatively close together within their respective branches. However, continuous wavelength tuning is not feasible due to the discrete rotational states of the molecules. Without a wavelength-selective component in the resonator, the laser will either operate on a few simultaneous transitions or occasionally shift to different transitions during operation. Non-standard emission wavelengths enhance the versatility of carbon dioxide lasers for specialized applications.
Commercially available carbon dioxide lasers typically emit at the standard wavelength of 10.6 micrometers. However, some models are specifically engineered to emit at alternative wavelengths, such as 10.25 micrometers or 9.3 micrometers. These alternative wavelengths are advantageous for certain applications, such as laser material processing, where specific materials like polymers absorb these wavelengths more effectively. To accommodate these specialized wavelengths, infrared optics may be required, as standard 10.6 micrometer optics might reflect too much of the radiation. Generally, the emissions of carbon dioxide lasers are classified within the long-wavelength infrared spectrum, which is part of the mid-infrared range.
Carbon dioxide lasers typically have average output powers ranging from several tens of watts to several kilowatts. Their power conversion efficiency generally falls between 10% and 20%. This efficiency surpasses that of many gas lasers, thanks to a favorable energy conversion pathway, and is also higher than that of solid-state lamp-pumped lasers. However, it is lower compared to diode-pumped lasers. Due to their long emission wavelengths and high output power, carbon dioxide lasers require high-quality infrared optics, often made from materials like zinc sulfide or zinc selenide. Despite their significant drive voltages and power levels, carbon dioxide lasers are relatively safe for the eyes at low intensities due to their longer operational wavelength, although they still pose laser safety concerns.
When nitrogen molecules in the gas mixture are stimulated, they become excited, gaining energy in the process. Nitrogen is chosen for its ability to remain in an excited state for extended periods without emitting energy as light or photons. The excited nitrogen molecules then transfer their high-energy vibrations to carbon dioxide molecules, leading to the achievement of population inversion within the laser. To generate a beam of light, the nitrogen atoms must return to their ground state by emitting photons. This photon emission occurs when the excited nitrogen atoms interact with very cold helium atoms, resulting in the release of light.
During laser operation, carbon dioxide (CO2) can break down into carbon monoxide (CO), which is an unwanted byproduct. To mitigate this issue, the presence of small amounts of water vapor or hydrogen can facilitate the conversion of CO back into CO2.
The active medium in the laser consists of a gas mixture of nitrogen (N2), helium (He), and carbon dioxide (CO2). Laser transitions occur between the vibrational states of the carbon dioxide molecules.
Carbon dioxide lasers consist of three main components: the gain medium, the energy source (or pump), and the optical resonator. The pump delivers energy to the gain medium, which amplifies it. This energy is then converted into light. The optical resonator reflects this light, and the reflected light is emitted as the final output beam.
The laser pump is powered by electrical current, which energizes the gas medium.
The gain medium in a carbon dioxide laser is comprised of a gas mixture including carbon dioxide, nitrogen, hydrogen, and helium. Although the proportions can vary based on the laser’s application, nitrogen, carbon dioxide, and helium are the primary components. A common ratio for the gas mixture is 1 part nitrogen, 1 part carbon dioxide, and 8 parts helium.
Carbon dioxide lasers function exclusively within the infrared spectrum and are capable of delivering high power outputs. Due to this, their optical components are usually crafted from specialized and often costly 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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