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This article will take an in-depth look at carbon dioxide lasers.
The article will bring more detail on topics such as:
Principle of Carbon Dioxide Lasers
Types of Carbon Dioxide Lasers
Applications and Benefits of Carbon Dioxide Lasers
And Much More…
Chapter 1: Principle of Carbon Dioxide Lasers
This chapter will discuss what carbon dioxide lasers are, their construction, and how they function.
What is a Carbon Dioxide Laser?
A carbon dioxide laser is a device that utilizes carbon dioxide as the gain medium and Nitrogen (N2), Helium (He). To some extent, it also uses hydrogen (H2), water vapor, Oxygen and/or Xenon (Xe) to improve the laser's effectiveness for light application, by emission of radiation that is stimulated. The laser is pumped electrically via an electrical gas discharge.
The electrical gas discharge can be operated using AC current, DC current or in the radio frequency domain. It operates at a wavelength of 10,6 micrometers. It is used in dermatology for surgeries like scar removal, removal of wrinkles, and to repair solar skin damage. Carbon dioxide lasers can also be used as a scalpel in surgeries like gynecological and neurosurgery.
The Construction of Carbon Dioxide Lasers
The construction of carbon dioxide lasers involves:
Materials Used in Constructing Carbon Dioxide Lasers
Modified materials are needed in the construction of Carbon dioxide Lasers because they operate in the infrared region of the spectra. The mirrors are coated with silver and the windows and lenses are made of germanium or zinc selenide. For high power applications gold mirrors and zinc selenide windows and lenses are used. Others use diamond windows and lenses.
Design of Carbon Dioxide Lasers
A carbon dioxide laser consists of a quartz discharge tube with a diameter of 2,5cm and a length of about 5metres. The discharge tube contains a mixture of CO2, N2 and He in the ratio 1:2:3 with water vapor. Individuals are maintained at P (for He) = 7 Torr, P (for N2) = 1.2 Torr and P (CO2) = 0.33 Torr. Transitions between vibrational states of Carbon dioxide molecules and rotational levels of carbon dioxide molecules produce a laser action. The construction of a carbon dioxide laser is simple and the output of this laser is continuous.
Active Medium and Active Centers
Carbon dioxide, nitrogen, and helium constitute the active medium in the ratio 1:2:3 respectively. Carbon dioxide molecules are the active centers as they play the main role in producing the laser.
An electrical discharge is used as a means of pumping and achieving population inversion. Electrons collide with carbon dioxide molecules and pump them to excited states.
During an electric discharge inside the machine, electrons and nitrogen molecules collide and the electrons are raised to their excited state. The following equation represents this process.
N2 + e* = N2* + e
N2 = Nitrogen molecule in ground state
e* = electron with kinetic energy
N2* = nitrogen molecule in excited state
e= same electron with lesser energy
N2 molecules in the excited state then collide with CO2 atoms in the ground state and are excited to higher electronic, vibrational, and rotational levels.
The following equation represents this process.
N2* + CO2 = CO2* + N2
N2* = Nitrogen molecule in excited state.
CO2 = ground state carbon dioxide atoms
CO2* = excited state carbon dioxide atoms
N2 = ground state nitrogen atoms
Because of the closeness of the excited level of nitrogen and the E5 level of the carbon dioxide atom, a population increase in the E5 level occurs. The laser action is triggered by any of the spontaneously emitted photons in the tube. This occurs when the population inversion is reached.
The laser transitions possible are of two types mentioned 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
Optical Resonator System
All the gas mixtures will be enclosed between a set of mirrors forming the optical resonator system. One of the mirrors is totally reflecting and the other mirror reflects partially. Due to the solely operation of CO2 lasers within the infrared region of the spectra and attaining of high power outputs, typically, their optical components are made from specialized materials like, zinc selenide, germanium, silver, diamond and gold.
How Carbon Dioxide Lasers Work
The working of a carbon dioxide laser involves:
Production of the Laser Beam
When stimulated by electric current, nitrogen molecules in the gas mixture become excited, gaining energy and acquiring a high energy state. The reason why nitrogen is used is because it is capable of holding its excited state for long periods of time, without any discharge of energy in the form of light or photons.
The carbon dioxide molecules are excited by the high-energy vibrations of the nitrogen. At this stage, a state called population inversion is achieved by the laser - the point at which there are more excited particles than non-excited particles in a system. For the production of a beam of light by the laser, the atoms of nitrogen must lose their excited state by the release of energy as photons. This occurs when the excited nitrogen atoms come into contact with the very cold helium atoms, which causes a release of light by nitrogen.
Direct excitation of carbon dioxide molecules can be made in the upper laser level though, but it has been proved that it is most efficient to make use of a resonant transfer of energy from the molecules of nitrogen. Here, the electric discharge excites the molecules of nitrogen into a metastable vibrational level and transfers their excitation energy to the molecules of carbon dioxide colliding with them.
The molecules of carbon dioxide that have been excited then greatly participate in the laser transition. The function of helium is to both depopulate the lower level of the laser and to remove the heat. Water vapor and hydrogen can help in the reoxidation of carbon monoxide that is formed in the discharge to carbon dioxide.
Carbon dioxide lasers emit a typical wavelength of 10.6 micrometers, but there are many other laser lines in the region of 9-11 micrometers, particularly at 9.6 micrometers. This is due to the two different vibrational states of the molecules of carbon dioxide that can be used as the lower level, and there is a substantial number of rotational states for each vibrational state, leading into multiple sub-levels. There is a possibility of dipole transitions with Δj = plus or minus 1, where Δj = 1 (R branch) leads to higher photon energies and Δj = -1 (P branch) to lower energies.
The stronger band transitions that involve one of the two possible final vibrational levels, have their P branch around 10.6 micrometers, with the dominant transition being P20, and the R branch is around 10.2 micrometers.
The other band‘s transitions have the P branch around 9.6 micrometers and the R branch around 9.3 micrometers.
It is possible to make a carbon dioxide laser to lase on one of more than a dozen transitions with wavelengths that are relatively closely spaced in each branch, but it is impossible to make continuous wavelength tuning because of the discrete states of rotation of the molecules. If there is no wavelength selective element in the resonator, only simultaneous lasing on a few transitions is obtained, or there will be occasional jumps to other transitions during operation. Emission wavelengths that are not standard make carbon dioxide lasers suitable for additional areas of application.
The carbon dioxide lasers that are commercially available emit at the standard wavelength of 10.6 micrometers. However, other devices are optimized specially for other wavelengths like 10.25 micrometers or 9.3 micrometers. These are more suitable for certain applications, for instance, in the processing of laser material, because the absorption of that radiation is much more in certain materials, such as polymers. For making lasers like that and for making use of their radiation, infrared optics may be required, as there may be the exhibition of too strong reflections by standard transmissive 10.6 micrometers optics. All emissions of carbon dioxide lasers can be considered to fall in the long-wavelength infrared region, which is a portion of the mid infrared region.
Output Powers and Efficiency
Average output powers are between tens of watts and much more kilowatts in most cases. The efficiency of the power conversion can be around 10% to 20%. This means it is higher than most gas lasers (because of a particular pathway that is favorable), also higher than solid state lamp pumped lasers, but lower than for multiple lasers that are diode pumped. Because of their long emission wavelengths and high output powers, carbon dioxide lasers require infrared optics of high quality which are often made of materials such as zinc sulfide or zinc selenide. Because of their high drive voltages and high powers, carbon dioxide lasers raise serious issues of laser safety. However, they are made relatively eye-safe at low intensities by their long operation wavelength.
When the stimulation of the molecules of nitrogen in the gas mixture occurs, they enter into their excited state. This means that they gain energy. The reason why nitrogen is used is that it is able to hold its excited state for a long time without any discharge of energy in the form of light or photons. The carbon dioxide molecules will in turn get excited by the high-energy vibrations of the nitrogen. This is the point when the laser achieves the population inversion state. For the production of a beam of light by the laser, the atoms of nitrogen must lose their excited state by releasing energy as photons. This occurs during the contact of the excited nitrogen atoms with the very cold helium atoms and light is released.
Maintaining Sustainable Carbon Dioxide Levels
CO2 can get degraded into carbon monoxide (CO) during laser operation. This is an undesirable side-effect, and having small amounts of water vapor or hydrogen can help trigger the conversion of CO back into CO2.
Characteristics of Carbon Dioxide Lasers
It is a molecular gas laser type.
Active medium: A mixture of CO2 , N2 and helium or water vapor is used as an active medium.
The pumping method used is the Electrical discharge method.
It consists of two concave mirrors forming a resonant cavity.
The carbon dioxide laser has a power output of about 10kW.
A continuous or pulse wave may be the output.
The output of the wavelength is 0.6μm and 10.6μm.
Principle of Carbon Dioxide Lasers
The gas mixture of N2, He, and CO2 is the active medium. The transition of the laser occurs between the vibrational states of the molecules of carbon dioxide.
Components of Carbon Dioxide Lasers
There are three components of carbon dioxide lasers: a gain (or laser) medium, an energy source (also known as a pump), and an optical resonator. The function of the pump is to provide energy which is then amplified by the gain medium. A conversion of this energy into light will occur eventually and the optical resonator will reflect the light; the reflected light will then emit the final output beam.
The electrical current serves as the laser pump, which excites the gas medium.
Mixture of Gasses
The mixture of gasses serves as the gain medium. The mixture of gasses is composed of carbon dioxide, nitrogen, hydrogen, and helium. The nitrogen, carbon dioxide, and helium constitute the vast majority of the mixture, even though the specific concentrations vary according to the intended use of the laser. The typical ratio is 1:1:8 for N2:CO2:He gas mixtures.
Specialized Optical Resonator
CO2 lasers operate entirely within the infrared spectrum and can attain high power outputs; their optical components are typically made of specialized (and often expensive) materials for example germanium, zinc selenide, silver, gold, and diamond
Chapter 2: Types of Carbon Dioxide Lasers
The longitudinal-flow and transverse-flow lasers, the sealed-off laser, the waveguide laser and the TEA laser are the most common types of lasers.
Longitudinal Flow and Transverse Flow 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.
A portion of the carbon dioxide contained in the gas mixture is split into carbon monoxide and oxygen by means of a direct current discharge. The gas mixture is continuously circulated, allowing more efficient removal of heat loss through several pumps in the system.
Sealed Off Laser
These are lasers that have a glass tube filled with the CO2-N2-He mixture. Hydrogen, water vapor and oxygen are added to the gas mixture instead of the gas mixture being replaced by the pump. This is because an electrical discharge quickly breaks down CO2, generally in a few minutes, hydrogen or water vapor is added to react with the carbon monoxide and oxygen to reform the CO2.CO2 is therefore catalytically regenerated.
There are mirrors at each end which form a resonant cavity. A nickel cathode heated to 300°C can also catalyze the recombination process. These two procedures increase operating life to several thousand hours.
This type of laser is formed when the sealed tube is replaced by a waveguide, with an inner diameter of a few millimeters. It is also known as the slab laser. The lasing volume of the waveguide laser is less and thus it produces a smaller power output. It has a resonator that has a relatively large surface area compared to the volume. This allows efficient removal of heat loss. The resonator is cuboidal in shape.
Transversely Excited Atmospheric Pressure Laser
This design uses a discharge voltage which is applied in short pulses of under one microsecond across the gas flow. This method prevents arcing. This design is used when high pressures are required. Since the required voltage for a longitudinal discharge is too high, transverse excitation occurs with electrodes that are in series along the tube.
The operation of TEA lasers is done in pulsed mode only since the discharge of the gas won’t be stable at pressures that are high. They produce average output power that is below 100 W more often, but can also be constructed for tens of kilowatts of power (combined with high pulse repetition rates).
High Power Diffusion-Cooled Slab Lasers
These types of carbon dioxide lasers have their gas in a gap that is between a pair of planar RF electrodes that are water cooled. Diffusion is used for the transfer of excess heat to the electrodes, if there is a small spacing of the electrodes compared to the width of the electrodes.
For energy extraction that is efficient, an unstable resonator is used with output coupling at the side of a mirror that is highly reflecting. It is possible to have several kilowatts of output in conjunction with reasonable beam quality.
Gas Dynamic Carbon Dioxide Lasers
These types are a kind of chemical lasers for multi-megawatt powers for example anti-missile weapons, where there is no provision of the energy by a discharge of gas but by a chemical reaction.
Carbon Dioxide Lasers vs. Solid State Lasers
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 10 micro meter laser beams, there are no optical fibers. Besides, it is possible to more tightly focus 1 micro meter beam, when there is a high beam quality. Concerning absorption, carbon dioxide laser beams are more favored for certain materials such as ceramics and polymers. Even when there is less favorability of absorption than for solid-state lasers, a carbon dioxide laser may be preferred as a relatively robust and cheap solution.
However, there is a substantial disadvantage that there are no high-power fiber cables for carbon dioxide radiation. Carbon dioxide lasers still have a wide application in the welding and cutting business, particularly for parts with a thickness greater than a few millimeters, and their sales contribute to a substantial part of all global sales of lasers.
Chapter 3: Applications and Benefits of Carbon Dioxide Lasers
This chapter will discuss the applications and benefits of carbon dioxide lasers.
Applications of Carbon Dioxide Lasers
The applications of carbon dioxide lasers include:
Carbon Dioxide Lasers in the Medical Field
A carbon dioxide laser is used to treat many skin problems. It is used in head and neck surgery and gynecological surgeries.
Carbon Dioxide Laser Surgery
Carbon dioxide lasers are used in medical surgeries because water is capable of absorbing the wavelength of a Carbon dioxide laser. It is the best laser for soft tissue because of its output wavelength of 10.6micrometers. They produce less bleeding and shorter surgery time. They also pose less risk of infection and less swelling after the operation.
Carbon dioxide lasers can be used in operations like oral, gynecological, dentistry and maxillofacial surgery. Soft tissues consist of 90% water and carbon dioxide has unique absorption properties, making carbon dioxide lasers the best in intraoral soft tissue surgery. The CO2 laser has been used for many otolaryngology. The carbon dioxide laser has been utilized for head and neck surgical processes and for the treatment of condylomata acuminata, intraepithelial neoplasms, and other lesions in gynecologic surgery. For airway surgery, it is the most commonly used type of laser and is particularly suitable for head and neck, and laryngeal surgical procedures.
Carbon Dioxide Laser in Dermatology
Carbon dioxide lasers are used in skin resurfacing. Carbon dioxide lasers very precisely remove thin layers of skin with minimal heat damage to the surrounding structures. They use the concept of vaporizing the skin using a high energy beam of laser light. This produces an injury to the skin but the injury is controlled. Collagen is produced as part of the healing process and this helps in restoring the skin's elasticity. They are used to treat wrinkles, photo damage, and removal of birthmarks, warts, scars and rhinophyma.
Industrial Application of Carbon Dioxide Lasers
A laser beam has the power to concentrate a huge power on a small area. As a result, it is used in the industry to weld, cut, and make holes.
Carbon Dioxide Laser Welding
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.
Carbon Dioxide Laser Cutting
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 makes use of radio waves in the detection of objects and for the arranging of objects in space. Carbon dioxide lasers are also used this way for monitoring the environment. This is called light detection and ranging. This technique identifies, observes, and measures inaccessible objects.
Benefits of Carbon Dioxide Lasers
It has simple construction.
It has continuous output of the laser.
It has high efficiency.
It has very high output power.
The gas tube length can be extended to increase the output.
Drawbacks of Carbon Dioxide Lasers
The action of the laser can get affected by the contamination of oxygen by carbon monoxide.
The operating temperature is a major contributor in determination of the output power of the laser.
The reflecting plates may corrode.
Damage of eyes because of the invisible (infra-red region) can occur to our eyes.
Consideration When Selecting Carbon Dioxide Lasers
The considerations when selecting a carbon dioxide laser include:
Compared to other lasers, CO2 lasers are confined to a relatively small range of wavelengths occurring entirely within the infrared (IR) spectrum. Most Carbon dioxide lasers emit light within the 9.4 μm to 10.6 μm band. As mentioned above, the ability to vary gas concentrations within the gain medium enables CO2 lasers to be manufactured to emit precise discrete frequencies within its general range.
The power rating of the carbon dioxide lasers is typically provided by the manufacturer. They are classified as high power devices often having some continuous emitting of 60 kW beams. A laser's power typically determines its application with a high power laser most suitable for cutting and welding, while a lower power device is most suitable for marking barcodes and labels.
Laser safety is considered in the use of carbon dioxide lasers due to their high power capabilities. For example, being exposed to a 200 mW laser emitting 100 yards away can cause permanent eye damage, considering that a CO2 laser may emit thousands of watts of power at a distance that is close, the operator’s eyes or skin can get burnt by direct contact.
Maintenance of Carbon Dioxide Lasers
The daily maintenance of carbon dioxide lasers include:
Changing Water and the Water Tank
It must be made sure that the laser tube is fully circulating water. The quality of the circulating water as well as the temperature of the water directly affects the service life of the laser tube. It is recommended that pure water must be used and the temperature of the water must be controlled below 35 degrees Celsius. To clean the water tank, turn off the power and remove the water inlet pipe, and let the laser tube of water automatically into the tank. After that open the water tank and pump and clear all dirt that is present on the water pump. Replace the circulating water and restore the water pump back into the water tank. Turn on the power of the pump alone and run for 2 to 3 minutes to see if the laser tube is fully circulating water.
Cleaning the Fan
After a long time of use, the fan can accumulate a lot of dust on the inside. This can make the fan make a lot of unusual sounds. When the fan makes these signs, remove the fan and clean the inside of the fan. The blades of the fan must be pulled out and wiped. This will ensure the efficient performance of the carbon dioxide laser.
The carbon dioxide laser has a lens that can easily catch dirt or other contaminants. These contaminants can cause damage to the lens. The lens must be removed and cleaned to ensure the efficient performance of the laser machine. When cleaning the lens, do not dip them in the cleaning fluid, but carefully wipe along the edges of the lens. Great care must be taken when wiping the lens since the surface coating can be damaged. Make sure that the lens does not fall because they are 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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