I. Introduction
In 1964, Patel achieved continuous laser output at wavelengths close to 10.4 microns and 9.4 microns in CO2 gas discharge, giving rise to the world's first CO2 molecular laser.
It has significant power and high energy conversion efficiency.
It utilizes the transition between the vibrational-rotational energy levels of CO2 molecules, which results in a rich spectrum. There are dozens of spectral lines for laser output close to 10 microns. Its wide application in industry, military, medicine and scientific research has brought many conveniences to our lives.
In 1966, the aerodynamic CO2 laser was born, drawing great attention to CO2 laser technology. The introduction of aerodynamics into laser technology has opened up broad prospects for the use of CO2 lasers.
With the advancement of science and technology, laser technology around the world has also developed accordingly. The carbon dioxide laser is currently one of the lasers with high continuous output power. Its early development and mature commercial products have been widely used in areas such as material processing, medical use, military weapons and environmental measurement.
In the development and application of lasers, the creation and application of CO2 lasers appeared earlier and more frequently. As early as the late 1970s, CO2 lasers were imported directly from abroad for industrial processing and medical applications.
Since the late 1980s, CO2 lasers have been widely introduced and applied in the area of materials processing.
This article mainly introduces the basic principles and structure of CO2 laser and focuses on the application of CO2 laser from three aspects. Finally, it presents the current state of research and future perspectives of the CO2 laser.
II. Lasers
2.1 Three conditions for laser production
The production of lasers requires three conditions:
(1) A gain medium that provides amplification as the laser working material, and its activated particles (atoms, molecules or ions) have an energy level structure suitable for stimulated emission;
(2) An external excitation source that pumps particles from lower energy levels to higher levels, causing an inversion of particle numbers between the upper and lower energy levels of the laser;
(3) An optical resonator that extends the working length of the activated medium, controls the direction of the light beam, and selects the frequency of the stimulated emission light to improve monochromaticity.
2.2 Characteristics of lasers
Compared to ordinary light sources, lasers have four main characteristics: excellent directionality, extremely high brightness, good monochromaticity and high coherence.
2.3 Laser Devices
A laser device is a mechanism capable of emitting lasers. The first quantum microwave amplifier was manufactured in 1954, producing a highly coherent microwave beam.
In 1958, AL Schawlow and CH Townes extended the principles of quantum microwave amplifiers to the optical frequency range and outlined methods for generating lasers.
In 1960, TH Maiman and his team built the first ruby laser. In 1961, a helium-neon laser was produced by A. Javan and others, and in 1962, RN Hall and his team created a gallium arsenide semiconductor laser. Since then, the types of lasers have continually expanded.
Other than free electron lasers, the fundamental working principles of many lasers are identical.
The essential conditions for producing a laser are particle number inversion and gain exceeding loss, so the indispensable components of a system include an excitation source (or pump) and a working medium with levels metastable energy sources.
Excitation is the state of excitement after the working medium absorbs external energy, creating conditions to achieve and maintain particle number inversion. Excitation methods include optical excitation, electrical excitation, chemical excitation and nuclear energy excitation.
The working medium with a metastable energy level allows the stimulated radiation to dominate, thus achieving light amplification. Common components in a laser device also include a resonant cavity.
However, the resonant cavity (see optical resonant cavity) is not an essential component. The resonant cavity can align the frequency, phase and direction of photons within the cavity, thus providing the laser with excellent directionality and coherence.
Furthermore, it can effectively shorten the length of the working material and adjust the mode of the laser produced by changing the length of the resonant cavity. Therefore, most laser devices have a resonant cavity.
There are many types of lasers. Below, we will categorize and introduce them based on laser working material, excitation method, and mode of operation.
(1) By work material
Lasers can be grouped into several categories based on the state of the work material:
① Solid lasers (crystal and glass);
② Gas lasers, divided into atomic gas lasers, ionic gas lasers, molecular gas lasers and quasi-molecular gas lasers;
③ Liquid lasers, the working materials of which mainly include two types: organic fluorescent dye solutions and inorganic compound solutions containing rare earth metal ions;
④ Semiconductor lasers;
⑤ Free electron lasers.
(2) By excitation method
① Optically pumped lasers;
② Electrically excited lasers;
③ Chemical lasers;
④ Nuclear pumped lasers.
(3) By operating mode
Due to the different working materials, excitation methods and application purposes of lasers, their operating modes and working states also vary. They can be divided into several main types:
① Continuous lasers;
② Single-pulse lasers;
③ Repetitive pulse lasers;
④ Modulated lasers;
⑤ Mode-locked lasers;
⑥ Single-mode and stable frequency lasers;
⑦ Adjustable lasers.
III. The Principle of CO2 Lasers
3.1 Basic Structure of a CO2 Laser
As shown in Figure 1, a typical CO2 laser structure is depicted. The two mirrors that form the resonant cavity of the CO2 laser are placed in an adjustable cavity holder. The simplest method is to directly attach the mirrors to both ends of the discharge pipe.
Basic structure:
① Laser Tube
This is the most critical part of the laser. It is generally composed of three parts (as shown in Figure 1): the discharge space (discharge pipe), the water cooling jacket (tube) and the gas reservoir.
The discharge pipe is typically made of hard glass and generally employs a cascading cylinder structure. Affects laser output and laser output power. The length of the discharge tube is proportional to the output power.
Within a certain length range, the power per meter of discharge pipe increases with the total length.
Generally speaking, the thickness of the discharge tube does not affect the output power. The water cooling jacket tube, like the discharge tube, is made of hard glass.
Its function is to cool the working gas, stabilizing the output power. The gas storage tube is connected to both ends of the discharge tube, which means that one end of the gas storage tube has a small hole connected to the discharge tube, and the other end is connected to the discharge tube through a spiral return gas tube.
Its function is to allow gas to circulate inside the discharge tube, facilitating constant gas exchange.
② Optical Resonator
The optical resonator is composed of a full reflection mirror and a partial reflection mirror, constituting a crucial part of the CO2 laser.
The optical resonator normally has three functions: controlling the propagation direction of the light beam, increasing monochromaticity; selecting a mode; extending the working length of the active medium.
The simplest and most commonly used laser optical resonator is composed of two flat mirrors (or spherical mirrors) placed opposite each other. The CO2 laser resonator generally uses a flat-concave cavity, with the reflective mirror made of K8 optical glass or optical quartz, processed into a concave mirror with a large radius of curvature.
A highly reflective metallic film – gold film – is deposited on the mirror surface, achieving a reflection rate of 98.8% for light with a wavelength of 10.6 μm and has stable chemical properties.
We know that the light emitted by carbon dioxide is infrared, so the mirror needs to be able to transmit infrared light. Because ordinary optical glass is opaque to infrared light, a small hole is needed in the center of the total reflection mirror, which is then sealed with a material that can transmit a 10.6 μm laser.
This seals the gas and allows part of the laser in the resonator to exit the cavity of this small hole, forming a laser beam.
③ Power supply and pump
The pump source provides energy to cause a population inversion between the upper and lower energy levels in the work material. The discharge current of a sealed CO2 laser is small, using a cold cathode, and the cathode is made in a cylindrical shape with molybdenum or nickel.
With a working current of 30-40mA and a cathode cylinder area of 500cm2, the mirror will not be contaminated. A light barrier is added between the cathode and the mirror.
3.2 Basic Operating Principle of CO2 Laser
As shown in Figure 2, the diagram illustrates the molecular energy levels responsible for laser generation in a CO2 laser.
The CO2 laser excitation process, as can be seen in Figure 2, mainly involves three gases: CO2, nitrogen and helium. CO2 is the gas that produces laser radiation, while nitrogen and helium serve as auxiliary gases.
Helium has two purposes: it accelerates the thermal relaxation process of the 010 level, which assists in the extraction of the 100 and 020 levels and facilitates effective heat transfer.
The introduction of nitrogen mainly facilitates the transfer of energy in the CO2 laser, contributing significantly to the accumulation of particles in the upper energy levels of the CO2 laser and to the production of high-power, high-efficiency lasers.
The pump employs continuous DC power excitation. Its DC power principle involves transforming the connected AC voltage using a transformer and then rectifying and filtering the high voltage to apply it to the laser tube.
The CO2 laser is a high-efficiency laser that minimizes damage to the working environment. It emits an invisible laser with a wavelength of 10.6 μm, making it an ideal laser.
According to the operating conditions of the gas, it can be divided into closed and circulating types. Based on the excitation method, it can be divided into electrical excitation, chemical excitation, thermal excitation, optical excitation and nuclear excitation. Almost all CO2 lasers used in medicine are electrically excited.
The basic operating principle of the CO2 laser is similar to other molecular lasers, with the stimulated emission process being quite complex.
The molecule has three different movements: the movement of electrons within the molecule, which determines the electronic energy state of the molecule; the vibrations of the atoms within the molecule, that is, atoms periodically oscillating around their equilibrium positions, determining the vibrational energy state of the molecule; and the rotation of the molecule, that is, the continuous rotation of the molecule in space as a whole, determining the rotational energy state of the molecule.
Molecular movements are extremely complex, hence the complexity of energy levels.
Laser generation in CO2 laser: A DC current of several tens to hundreds of milliamps is normally inserted into the discharge tube.
During discharge, nitrogen molecules in the mixed gas inside the discharge tube are excited due to electron collision. The excited nitrogen molecules then collide with the CO2 molecules.
The N2 molecule transfers its energy to the CO2 molecule, causing the CO2 molecule to transition from a lower energy level to a higher one, resulting in a population inversion and, consequently, the generation of laser.
3.3 Advantages and Disadvantages of CO2 Lasers
Compared to other lasers, CO2 lasers have the following advantages and disadvantages:
Benefits:
They exhibit superior directionality, monochromaticity, and frequency stability. Given the low density of the gas, it is difficult to achieve a high density of excited particles, so the energy density output of a CO2 gas laser is generally lower than that of a solid-state laser.
Disadvantages:
Although the energy conversion efficiency of CO2 lasers is quite high, it will not exceed 40%. This means that more than 60% of the energy is converted into thermal energy of the gas, resulting in an increase in temperature. Increasing gas temperature can cause depopulation of the upper laser level and thermal excitation of the lower level, both of which decrease the number of particle inversions.
Furthermore, an increase in gas temperature can cause the spectral line to broaden, leading to a decrease in the gain coefficient.
Especially, the increase in gas temperature can also cause the decomposition of CO2 molecules, reducing the concentration of CO2 molecules in the discharge pipe. These factors can decrease the laser output power and even lead to “thermal quenching”.
4. Applications of CO2 lasers
4.1 Military Applications
In recent years, the steady development of CO2 lasers has been notable in military applications. Laser weapons, as a new concept, have become preferred in the weaponry of the new century due to their advantages over traditional conventional weapons, such as high speed, good directionality, high energy density and high operational efficiency.
High-energy laser weapons play an increasingly important role in military applications, representing the direction of future weapons development. They are poised to profoundly change the current battlefield environment and ways of warfare, profoundly transforming the nature of future conflicts.
High-energy aerodynamic CO2 lasers with high output power have been designed by several countries for the development of high-energy laser weapons.
A basic feature of laser missile defense, or laser anti-missile tactics, is the use of high-energy lasers traveling at the speed of light to destroy missiles or other flying objects moving at the speed of sound.
We can safely say that this area is dominated by CO2 lasers due to their significant advantages.
Currently, the army is adopting small land-based laser anti-missile systems, while the air force is using airborne laser anti-missile systems, and the navy is using shipborne laser anti-missile systems, all of which use high-energy CO2 lasers. .
The main features of future CO2 laser weapons are ultra-high power and high portability. High-energy lasers will be a crucial component of future combat systems, contributing to counter-surveillance, active protection, air defense and mine clearance.
High portability will greatly increase each soldier's combat capabilities, maximizing each soldier's role, although this idea is currently theoretical. Laser weapons from several countries are being developed in this direction.
Future CO2 laser weapons are expected to evolve toward high functionality, portability, and lethal efficiency. As shown in Figure 3:
4.2 Medical Applications
Over the past 20 years, laser technology has rapidly advanced in the medical field, effectively curing many diseases and congenital disorders.
Free-beam CO2 lasers are used in surgeries, often without contact with the skin tissue, providing several advantages over conventional surgeries, such as reduced mechanical damage, increased protection of surrounding tissues and easier maintenance of aseptic conditions.
Compared to other laser surgeries, the CO2 laser scalpel has greater cutting power, a higher tissue absorption coefficient and a lower tissue penetration concentration (approximately 0.23 mm). This makes it less likely to damage arteries during surgery, leading to the widespread use of continuous CO2 lasers for clinical surgical treatment.
However, the damage of continuous CO2 lasers to tissues in clinical applications is not selective, often resulting in side effects such as skin scarring after surgery. Cutting or vaporizing lesions can also harm normal tissues to varying degrees, making it unsuitable for high-demand surgeries. This significantly limits the further application of CO2 lasers in medicine.
In 1983, Aderson and Parrish proposed the principle of “selective photothermolysis” for non-harmful laser treatment.
The essential idea is that when the laser passes through normal tissue to reach the target lesion, the absorption coefficient of the lesion for the laser must be greater than that of normal tissue – the greater the difference, the better – to avoid damaging the tissue. normal when destroying. the target lesion.
The thermal relaxation time of the target tissue must be greater than the pulse width or laser action time, preventing heat from spreading to the surrounding normal tissue during the laser heating process.
Based on the principle of “selective photothermolysis”, high-energy pulse medical devices represented by ultrapulse CO2 laser treatment machines appeared in the 1990s.
These devices have been successfully applied, enabling innovative progress in highly demanding applications, especially dominant in the field of laser cosmetics. The development prospects are very broad.
Ultrapulse CO2 lasers employ advanced pulse technology and PWM power control technology. They not only quickly increase the peak power of the laser, delivering sufficient energy to the target tissue, but also precisely control the width and repetition frequency of each pulse via PWM signals.
By calculating the thermal relaxation time of the target tissue, pulse width control can achieve optimal surgical results. For example, the thermal relaxation time of capillaries is about 10μs, requiring a pulse width of less than 10μs; The thermal relaxation time of skin tissue is approximately 1 ms, requiring a pulse width of less than 1 ms for a laser device used for skin resurfacing and wrinkle removal.
The most significant difference between modern laser devices and those of more than a decade ago lies in the precise control of pulse width, which fundamentally ensures the safety of modern laser treatment.
Ultrapulse CO2 laser treatment machines not only share the common characteristics of continuous CO2 laser scalpels, but also have their advantages. They can produce high-energy, high-repetition-frequency pulsed lasers, meeting the operational requirements of “selective laser photothermolysis.”
They can quickly and effectively remove target lesion tissues, minimizing laser damage to normal tissues and significantly increasing the accuracy and safety of medical clinics.
Clinical practice has shown that, when performing the same surgery, the laser power used by pulsed lasers is much lower than that of continuous lasers.
Therefore, the tissue reaction caused by laser surgery is milder, the damage to surrounding tissues is less, the time is shorter, and less smoke is produced during treatment, providing a clear visual field.
Ultrapulse CO2 lasers have been widely used in Otorhinolaryngology, Gynecology, Neurosurgery, General and Aesthetic Surgery.
Lumenis, the company that introduced Bridge Therapy, has researched and produced several CO2 laser treatment devices, such as the NovaPulse Series for use in Otolaryngology and Aesthetics.
Other examples include the MODEL CTL1401 surgical device produced by the Polish company CTL, and the GL-Ⅲ from Japan's NANO LASER, a CO2 laser treatment device for oral surgery.
4.3 Industrial Applications
(1) CO2 laser cutting technology
Laser cutting technology is widely used in processing metallic and non-metallic materials. Significantly reduces processing time, reduces costs and improves the quality of workpieces.
Laser cutting is achieved by high power density energy produced after focusing the laser.
Compared to traditional sheet metal processing methods, laser cutting offers superior cut quality, speed, flexibility (allowing arbitrary shapes), and broad material adaptability.
In terms of metal cutting, it constitutes the main domain of CO2 laser cutting. At present, considering economic factors, high-power laser cutting machines are generally employed for subcontracting in processing station format.
As medium-power CO2 lasers mature in the domestic market, several sheet metal factories will acquire their own laser cutting machines, leading to a substantial increase in demand.
Non-metal cutting is applied to mold cutting, wood and high-density fiberboard cutting, and plastic cutting.
(2) CO2 laser welding technology
Laser welding is a method of joining materials, predominantly used for joining metallic materials. Similar to traditional welding techniques, it connects two components or parts by melting the material in the connection area.
Given the high concentration of laser energy, the heating and cooling processes are incredibly fast.
Materials that are difficult to process with standard welding techniques, due to their brittleness, high hardness or strong flexibility, can be easily managed with lasers.
On the other hand, laser welding does not involve mechanical contact, making it easier to ensure that the welding area does not deform under stress.
By melting the smallest amount of material to obtain alloy connections, welding quality is greatly improved and productivity increases.
Laser welding offers a deep weld bead and minimal heat affected zone, resulting in superior quality.
For example, in welding of thin metal plates, medium power CO2 lasers are suitable for welding thin metal plates with a thickness of less than 1 mm, such as silicon steel laminated sheets often used in automobile parts, generators, wipers, starter motors, window lifters, etc.
Previously, they were fixed by punching and riveting, but now they can be laser welded.
Battery welding, particularly in the production of lithium batteries – such as tab welding, safety valve welding, negative electrode welding, casing seal welding – laser welding is the ideal process, requiring a large variety and number of laser welding machines.
Demand for laser welding in precision instrument parts is also increasing, such as welding stainless steel diaphragms and aviation instrument housings.
V. Current status of research and future perspectives of CO2 lasers
5.1 Current Status of CO2 Laser Research
For almost 50 years since its creation, the CO2 laser has been the focus of human attention. This type of gas laser operates using CO2 gas as the working medium. CO2 lasers are a significant category of gas lasers.
Current major research directions for CO2 lasers include:
1. High efficiency CO2 lasers.
Without a doubt, compared to solid-state lasers, its efficiency is extremely high. However, overall, relative to the CO2 laser itself, the efficiency is still comparatively low.
In 1964, with the use of N2, a conversion efficiency of 3% was achieved; in 1965, using a CO2-N2-He gas mixture, the conversion efficiency reached 6%. To date, the highest efficiency does not exceed 60%.
Many companies are researching efficiency improvements. For example, the American company Datong has achieved an efficiency of around 60% in its CO2 lasers.
2. Small, multifunctional CO2 lasers.
Most current CO2 lasers have a single function and can only perform a very specific task. We know that CO2 lasers used in large hospitals to remove freckles and hair are quite bulky, but their structures are fundamentally the same. The use of multifunctional CO2 lasers results in a smaller physical volume and, relatively, a much lower price.
3. High power CO2 lasers.
High power has always been a military pursuit. In this regard, the level of investigation of some national military companies is relatively backward. The US Air Force was the first to begin researching high-power CO2 lasers.
In 1975, the eleventh anniversary of the birth of the CO2 laser, the US Air Force developed a CO2 laser with a power level reaching 30KW. In 1988, the output power of the researched CO2 laser reached 380KW.
According to some data released by the US military, the output power of the developed CO2 lasers has now reached the level of tens of megawatts.
4. Research in industrial technology.
CO2 lasers dominate laser processing, being widely used for welding, cutting, heat treating and cleaning, among other things. Laser quality and power have very precise requirements.
Therefore, industrial CO2 lasers need to have high-quality laser beams and stable output power.
Laser applications have already permeated areas such as optics, medicine, nuclear energy, astronomy, geography and oceanography, marking the development of the new technological revolution.
If you compare the history of laser development with the history of electronics and aviation, you should realize that we are still in the early stages of laser development and that an even more exciting and promising future is on the horizon.
5.2 Future Perspectives of CO2 Lasers
The future of CO2 lasers will evolve in the following directions:
(1) High power cross flow CO2 laser.
This high-power cross-flow CO2 laser is used for laser processing and heat treatment, with an integrated box-shaped structure. The unit's top casing houses an integrated discharge chamber, heat exchanger, fan system, inlet/outlet guide, and an optical resonator.
The lower box contains the laser power source, gas charging and discharging system, vacuum pump, ballast resistor box and control box.
Compared to existing technology, it features compact structure, easy installation, maintenance, high work efficiency and can be miniaturized.
Its main applications are in the welding of diamond tools, automotive gears, automotive airbag gas generators, laser surface hardening and overlaying processes, and unique applications such as surface repair of petrochemical parts and surface hardening by melting steel rolls.
(2) Acousto-optic Q-Switched CO2 Laser.
To meet application requirements in areas such as laser ranging, environmental sensing, space communication and laser-material interaction mechanism research, an acoustic Q-switched CO2 laser has been developed.
Using the rate equations of Q-switched pulse lasers, the main technical parameters of the laser output were analyzed theoretically, calculated and then verified experimentally.
The laser pulse repetition frequency is 1 Hz to 50 kHz. At 1 kHz operation, the output laser pulse width is 180 ns and the peak power is 4062 W, which is basically consistent with theoretical calculations.
The results show that high repetition frequency, narrow pulse width and high peak power of a small-sized CO2 laser can be achieved by optimal selection of the acousto-optical (AO) crystal and reasonable design of the resonator.
Wavelength tuning and coded output of such lasers can be achieved through grid line selection design and TTL signal control.
(3) Compact CO2 laser with long-life RF-excited waveguide.
To broaden the application of CO2 lasers in industrial processing and military use, a compact long-life RF-excited waveguide laser was developed using extruded aluminum alloy profiles for the laser body, disk inductance instead of the traditional wirewound inductance and an all-metal sealing process.
It can output continuously or pulse at a modulation frequency of no more than 20kHz, with a maximum output power of 30W, a lifespan of more than 1,500 hours, and a storage life of more than 1.5 years.
The results show that this laser features compact structure, stable output power, long service life and can work in continuous and pulse modulation modes. It can not only process various materials but also be used in military applications.
(4) New portable TEA CO2 laser.
This is a new portable atmospheric pressure CO2 laser with transverse excitation. The laser is powered by four #5 rechargeable batteries and can operate continuously for 1 hour at a repetition rate of 1 Hz.
The size of the complete laser unit (including power supply and control system) is 200 nm×200 mm×360 mm and its weight is less than 8 kg. The laser uses ultraviolet corona pre-ionization for stable and uniform discharge.
Under free oscillation conditions, the output energy of the laser pulse reaches 35 mJ and the output pulse width is 70 ns.
(5) High power continuous CO2 laser.
In response to the problem of cracking and blade deformation in the continuous laser cladding of helicopter engine turbine blades, a new power control scheme was adopted in a 5 kW continuous cross-flow CO2 laser.
Through related software and controls, pulsed laser power output has been achieved, overcoming the cost and stability problems caused by high-power switching power supplies.
The pulse modulation frequency can reach 5 Hz and the modulation duty cycle can range from 5% to 100%.
In an experiment powder coating Stellite X-40 alloy on the K403 alloy surface of engine blades, a peak power of 4 kW, pulse repetition frequency of 4 Hz and duty cycle of 20% were used.
The results showed that the heat-affected zone was reduced by 50% after coating, the hardness increased by 5%, the interfacial bonding performance was comparable to that of the base material, and there were no coating cracks or blade deformation.
