What is the intensity of laser radiation. The use of laser radiation in surgery

We are often asked the question - what do these letters mean in the description of radar detectors: X, K, Ka, L, POP, VG-2?

X, K And Ka These are the radio frequency bands in which police radars operate.

L(laser) - means the ability to detect laser radars (lidars)

POP- this is not a range, this is the operating mode of the police radar (and for the radar detector, the detection mode).

VG-2 this is a radar detector detection system (and in radar detectors, respectively, protection against such detection)

Let's consider this in more detail.

Range X(10.475 to 10.575 ghz) - The oldest radio frequency band used for speed control. Drivers of the older generation remember the big radars used by the police back in the USSR, which looked like a big gray pipe, which is why they got the name "pipe" or "headlight". Now there are almost none left. Personally, I last saw such a thing on the roads of Ukraine in 2007. Having any, even the cheapest radar detector in service, you can easily slow down, because. the speed of these radars is low.

K-band(24.0 to 24.25 ghz) - The K-band is the most common band in which this moment Most police radars work. This range was introduced in 1976 in the USA and is still widely used worldwide for speed detection. K-band radars are smaller and lighter than X-band radars and operate faster. This range is used by the radars "Vizir", "Berkut", "Iskra", etc. All of which are presented in our store detect the K range.

Ka range(33.4 to 36.0 ghz) is a newer band. Radars operating in this range are more accurate. For radar detectors, detecting this range is more difficult. All modern radar detectors detect Ka-band radar radiation, however, since such police radars work very quickly, it is not certain that you will be able to slow down enough to avoid being caught. Be careful!

laser range. Radars (lidars) operating in the laser range are a nightmare for an intruder. It is used by speed cameras, such as the TruCam device. The laser speed meter emits a beam in the infrared spectrum. Reflecting from the headlights of a car or a license plate, the laser beam returns back, and since all this happens at the speed of light, you simply have no chance of slowing down. If your radar detector reported the detection of a laser, then this means that you have already been caught: (It’s another matter if you weren’t caught at all and the radar detector “caught” the reflected signal, then you might still be lucky.
The function of detecting laser radars has all the radar detectors presented in our store. But the most effective (the only reliable!) way to deal with laser guns is the so-called "shifters" - devices that deceive a laser speed meter. Our store presents the Beltronics SHIFTER ZR4-complex that allows you to detect and protect yourself from laser detection. That's what will really allow you to protect yourself from TruCam! Beltronics Shifter ZR4 can work both independently and in combination with Beltronics radar detectors.

POP mode- this is the mode of operation of the police radar in which it emits a very short time (tens of milliseconds). This is enough to determine the speed, but the speed is not fixed and the traffic cop, in principle, has nothing to show you. But he will, rest assured. Most radar detectors can detect signals in this mode, for many this mode is turned on forcibly. In this mode, your radar detector is more sensitive to interference, so use it outside the city.

VG-2- This is the protection mode against detection by your radar detector. In some European countries and in some US states, the use of radar detectors is prohibited. Therefore, the police are armed with so-called radar detectors (Radar Detector Detector-RDD). They capture the specific radiation that the radar detector produces during operation. Thus, a police officer from a distance can know that you have a radar detector installed in your car. All modern radar detectors are protected from detection by VG-2 devices. The laugh is that VG-2 is a system invented in the early 90s and is practically not used at the moment. Now the police are using the new Specter (Stalcar) RDD systems. These RDDs are very difficult to defend against, virtually no radar detector on the market is capable of defending against the Specter system, except for the Beltronics STI Driver radar - this thing is 100% invisible.

After reading this article, you may get the impression that there is no point in radar detectors - it still won't help. It's not like that at all. Firstly, most radars operate in the K and Ka bands, having you be warned in advance and have time to slow down.

Laser guns, stationary laser cameras are a problem. On the other hand, there are very few such devices, they are several times more expensive than a conventional radar and less common than conventional K-band radars even in the USA, let alone Ukraine. Such radars cannot be used handheld, only from a tripod or fixed permanently. For 100% protection against laser radars, you will need a shifter - expensive but reliable.

Even the simplest "anti-radar" detects most K-band radars in advance, at a sufficient distance for you to stop. My favorite mid-range radars are Stinger-better protected from interference and has greater sensitivity. Well, premium class - Beltronics radar detectors and especially STI Driver - out of competition!

Good luck on the roads!

FEDERAL RAILWAY TRANSPORT AGENCY

FEDERAL STATE BUDGET

EDUCATIONAL INSTITUTION OF HIGHER PROFESSIONAL EDUCATION

"MOSCOW STATE UNIVERSITY OF TRANSPORTATIONS"

Institute of Transport Engineering and Control Systems

Department "Technology of transport engineering and repair of rolling stock"

abstract

discipline: "Electrophysical and electrochemical methods of processing"

Topic: "Types and characteristics of lasers"

Introduction

The invention of the laser is on a par with the most outstanding achievements of science and technology of the 20th century. The first laser appeared in 1960, and the rapid development of laser technology immediately began. In a short time, various types of lasers and laser devices were created, designed to solve specific scientific and technical problems. Lasers have already won a strong position in many branches of the national economy. As academician A.P. Alexandrov, every boy now knows the word laser . And yet, what is a laser, why is it interesting and useful? One of the founders of the science of lasers - quantum electronics - Academician N.G. Basov answers this question in the following way: A laser is a device in which energy, such as thermal, chemical, electrical, is converted into energy electromagnetic field- laser ray. With such a conversion, part of the energy is inevitably lost, but the important thing is that the resulting laser energy has an incomparably higher quality. The quality of laser energy is determined by its high concentration and the possibility of transmission over a considerable distance. A laser beam can be focused into a tiny spot with a diameter of the order of the wavelength of light and obtain an energy density that exceeds that of a nuclear explosion today.

With the help of laser radiation, it has already been possible to achieve the highest values ​​of temperature, pressure, and magnetic field strength. Finally, the laser beam is the most capacious carrier of information and, in this role, a fundamentally new means of its transmission and processing. . Wide application lasers in modern science and technology is explained by the specific properties of laser radiation. A laser is a generator of coherent light. Unlike other light sources (such as incandescent lamps or lamps daylight) the laser produces optical radiation characterized by a high degree of ordering of the light field or, as they say, a high degree of coherence. Such radiation is characterized by high monochromaticity and directivity. Today, lasers successfully work in modern production, coping with a wide variety of tasks. Laser beam cuts fabrics and cuts steel sheets, weld car bodies and weld the smallest details in electronic equipment, punch holes in brittle and superhard materials. Moreover, laser processing of materials makes it possible to increase efficiency and competitiveness in comparison with other types of processing. The field of application of lasers in scientific research - physical, chemical, biological - is constantly expanding.

The remarkable properties of lasers - exceptionally high coherence and directivity of radiation, the possibility of generating coherent waves of high intensity in the visible, infrared and ultraviolet regions of the spectrum, obtaining high energy densities in both continuous and pulsed modes - already at the dawn of quantum electronics indicated the possibility of a wide range of applications for practical purposes. Since its inception, laser technology has developed at an exceptionally high rate. New types of lasers appear and at the same time old ones are improved: laser installations are created with a set of characteristics necessary for various specific purposes, as well as various types of beam control devices, and measuring equipment is being improved more and more. This was the reason for the deep penetration of lasers into many branches of the national economy, and in particular into machine and instrument making.

It should be especially noted that the development of laser methods or, in other words, laser technologies, significantly increases the efficiency of modern production. Laser technologies allow for the most complete automation of production processes.

Enormous and impressive are the achievements of laser technology today. Tomorrow promises even greater accomplishments. Many hopes are associated with lasers: from creating three-dimensional cinema to solving such problems. global problems, as the establishment of ultra-long terrestrial and underwater optical communications, unraveling the secrets of photosynthesis, the implementation of a controlled thermonuclear reaction, the emergence of systems with a large amount of memory and high-speed input-output devices.

1. Classification of lasers

It is customary to distinguish between two types of lasers: amplifiers and oscillators. At the output of the amplifier, laser radiation appears when an insignificant signal at the transition frequency arrives at its input (and it is already in an excited state). It is this signal that stimulates the excited particles to release energy. There is an avalanche of amplification. Thus - at the input is weak radiation, at the output - amplified. With a generator, the situation is different. At its input, radiation at the transition frequency is no longer supplied, but excites and, moreover, overexcites the active substance. Moreover, if the active substance is in an overexcited state, then the probability of a spontaneous transition of one or more particles from the upper level to the lower one increases significantly. This leads to stimulated emission.

The second approach to the classification of lasers is related to the physical state of the active substance. From this point of view, lasers are solid-state (for example, ruby, glass or sapphire), gas (for example, helium-neon, argon, etc.), liquid, if a semiconductor junction is used as an active substance, then the laser is called semiconductor.

The third approach to classification is related to the method of excitation of the active substance. There are the following lasers: excited by optical radiation, excited by an electron beam, excited by solar energy, excited by the energies of exploding wires, excited by chemical energy, excited by nuclear radiation. Lasers are also distinguished by the nature of the emitted energy and its spectral composition. If the energy is emitted in pulses, then they talk about pulsed lasers, if it is continuous, then the laser is called a continuous-wave laser. There are lasers with a mixed mode of operation, for example, semiconductor ones. If the laser radiation is concentrated in a narrow range of wavelengths, then the laser is called monochromatic, if in a wide range, then they speak of a broadband laser.

Another type of classification is based on the use of the concept of output power. Lasers that have a continuous (average) output power of more than 106 W are called high-power lasers. With an output power in the range of 105 ... 103 W, we have medium-power lasers. If the output power is less than 10-3 W, then one speaks of low-power lasers.

Depending on the design of an open mirror resonator, lasers with a constant Q factor and Q-switched lasers are distinguished - for such a laser, one of the mirrors can be placed, in particular, on the axis of an electric motor that rotates this mirror. In this case, the quality factor of the resonator periodically changes from zero to the maximum value. Such a laser is called a Q-modulated laser.

2. Characteristics of lasers

One of the characteristics of lasers is the wavelength of the emitted energy. The wavelength range of laser radiation extends from the X-ray region to the far infrared, i.e. from 10-3 to 102 microns. Beyond the region of 100 μm lies, figuratively speaking, virgin lands . But it extends only up to a millimeter section, which is mastered by radio operators. This undeveloped area is continuously narrowing, and it is hoped that its development will be completed in the near future. The share attributable to different types of generators is not the same. The widest range of gas quantum generators.

Another important characteristic of lasers is the pulse energy. It is measured in joules and reaches the highest value for solid-state generators - about 103 J. The third characteristic is power. Gas generators that emit continuously have a power of 10-3 to 102 watts. Milliwatt power has generators that use a helium-neon mixture as an active medium. CO2 generators have a power of about 100 W. With solid state generators, talking about power makes a lot of sense. For example, if we take the radiated energy of 1 J, concentrated in an interval of one second, then the power will be 1 W. But the duration of the radiation of the generator on the ruby ​​is 10-4 s, therefore, the power is 10,000 W, i.e. 10 kW. If the pulse duration is reduced by means of an optical shutter to 10-6 s, the power is 106 W, i.e. megawatt. This is not the limit! It is possible to increase the energy in the pulse to 103 J and reduce its duration to 10-9 s, and then the power will reach 1012 W. And that's a lot of power. It is known that when a beam intensity reaches 105 W/cm2 falls on a metal, then metal melting begins, at an intensity of 107 W/cm2 metal boils, and at 109 W/cm2 laser radiation begins to strongly ionize vapors of the substance, turning them into plasma.

Another important characteristic of a laser is the divergence of the laser beam. Gas lasers have the narrowest beam. It is a few arc minutes. Beam divergence of solid-state lasers is about 1…3 angular degrees. Semiconductor lasers have a petal opening of radiation: in one plane about one degree, in the other - about 10 ... 15 angular degrees.

The next important characteristic of a laser is the wavelength range in which the radiation is concentrated, i.e. monochromaticity. The monochromaticity of gas lasers is very high, it is 10-10, i.e. significantly higher than that of gas discharge lamps, which were previously used as frequency standards. Solid-state lasers, and especially semiconductor lasers, have a significant frequency range in their radiation, i.e., they are not highly monochromatic.

A very important characteristic of lasers is the efficiency. For solid state it ranges from 1 to 3.5%, for gas 1 ... 15%, for semiconductor 40 ... 60%. At the same time, all sorts of measures are being taken to increase the efficiency of lasers, because the low efficiency leads to the need to cool the lasers to a temperature of 4-77 K, and this immediately complicates the design of the equipment.

2.1 Solid state lasers

Solid-state lasers are divided into pulsed and continuous. Among pulsed lasers, devices based on ruby ​​and neodymium glass are more common. The neodymium laser wavelength is l = 1.06 μm. These devices are relatively large rods, the length of which reaches 100 cm, and the diameter is 4-5 cm. The pulse energy of generating such a rod is 1000 J for 10-3 sec.

The ruby ​​laser is also distinguished by a high pulse power; with a duration of 10-3 sec, its energy is hundreds of J. The pulse repetition frequency can reach several kHz.

The most famous continuous-wave lasers are made on calcium fluorite doped with dysprosium and lasers on yttrium-aluminum garnet, which contains rare-earth metal impurities. The wavelength of these lasers is in the region from 1 to 3 µm. The pulse power is approximately 1 watt or a fraction of it. Yttrium-aluminum garnet lasers are ways to provide a pulse power of up to several tens of watts.

As a rule, solid-state lasers use a multimode generation mode. Single-mode generation can be obtained by introducing selective elements into the resonator. Such a decision was caused by a decrease in the generated radiation power.

The complexity of the production of solid-state lasers lies in the need to grow large single crystals or melt large samples of transparent glass. These difficulties were overcome by the manufacture of liquid lasers, where the active medium is represented by a liquid into which rare-earth elements are introduced. Nevertheless, liquid lasers have a number of disadvantages that limit their application.

2.2 Liquid lasers

Liquid lasers are lasers with a liquid active medium. The main advantage of this type of device is the possibility of fluid circulation and, accordingly, its cooling. As a result, in both pulsed and continuous modes, more energy can be obtained.

The first liquid lasers were produced on the basis of rare earth chelates. The disadvantage of these lasers is the low level of achievable energy and the chemical instability of the chelates. As a result, these lasers have not found application. Soviet scientists suggested using inorganic active liquids in the laser medium. Lasers based on them are distinguished by high pulsed energies and provide average power indicators. Liquid lasers based on such an active medium are capable of generating radiation with a narrow frequency spectrum.

Another type of liquid lasers are devices operating on solutions of organic dyes, which are distinguished by broad spectral luminescence lines. Such a laser is capable of providing continuous tuning of the emitted wavelengths of light over a wide range. When replacing dyes, the entire visible spectrum and part of the infrared are covered. The source of pumping in such devices is, as a rule, solid-state lasers, but it is possible to use gas lamps that provide short bursts of white light (less than 50 μs).

2.3 Gas lasers

There are many varieties. One of them is a photodissociation laser. It uses a gas whose molecules, under the influence of optical pumping, dissociate (split) into two parts, one of which is in an excited state and is used for laser radiation.

A large group of gas lasers are gas-discharge lasers, in which the active medium is a rarefied gas (pressure 1-10 mm Hg), and pumping is carried out by an electric discharge, which can be glow or arc and is created by direct current or alternating current high frequency (10-50 MHz).

There are several types of gas discharge lasers. In ion lasers, radiation is obtained due to the transitions of electrons between the energy levels of the ions. An example is the argon laser, which uses a DC arc discharge.

Lasers based on atomic transitions generate due to the transitions of electrons between the energy levels of atoms. These lasers give radiation with a wavelength of 0.4-100 microns. An example is a helium-neon laser operating on a mixture of helium and neon at a pressure of about 1 mmHg. Art. For pumping, a glow discharge is used, created by a constant voltage of about 1000 V.

Molecular lasers also belong to gas-discharge lasers, in which radiation arises from electron transitions between the energy levels of molecules. These lasers have a wide frequency range, corresponding to wavelengths from 0.2 to 50 µm.

The most common molecular laser is carbon dioxide (CO2 laser). It can deliver power up to 10 kW and has a fairly high efficiency - about 40%. Nitrogen, helium and other gases are usually added to the main carbon dioxide. For pumping, a glow discharge of direct current or high-frequency is used. A carbon dioxide laser produces radiation with a wavelength of about 10 microns. It is shown schematically in Fig. one.

Rice. 1 - The principle of the CO2 laser device

A variety of CO2 lasers is gas-dynamic. In them, the inverse population required for laser radiation is achieved due to the fact that the gas, preheated to 1500 K at a pressure of 20–30 atm, enters the working chamber, where it expands, and its temperature and pressure decrease sharply. Such lasers can produce continuous radiation with a power of up to 100 kW.

Molecular lasers include the so-called excimer lasers, in which the working medium is an inert gas (argon, xenon, krypton, etc.), or its combination with chlorine or fluorine. In such lasers, pumping is carried out not by an electric discharge, but by a stream of so-called fast electrons (with an energy of hundreds of keV). The emitted wave is the shortest, for example, with an argon laser of 0.126 μm.

Higher radiation powers can be obtained by increasing the gas pressure and applying pumping using ionizing radiation in combination with an external electric field. ionizing radiation is a stream of fast electrons or ultraviolet radiation. Such lasers are called electroionization or compressed gas lasers. Schematically, lasers of this type are shown in Figs. 2.

Rice. 2 - Electroionization pumping

Excited gas molecules due to the energy of chemical reactions are obtained in chemical lasers. Here, mixtures of some reactive gases (fluorine, chlorine, hydrogen, hydrogen chloride, etc.) are used. chemical reactions in such lasers should proceed very quickly. For acceleration, special chemical agents are used, which are obtained by the dissociation of gas molecules under the action of optical radiation, or an electric discharge, or an electron beam. An example of a chemical laser is a laser using a mixture of fluorine, hydrogen, and carbon dioxide.

A special type of laser is the plasma laser. In it, the active medium is a highly ionized plasma of vapors of alkaline earth metals (magnesium, barium, strontium, calcium). For ionization, current pulses up to 300 A at voltages up to 20 kV are used. The duration of the pulses is 0.1-1.0 μs. The radiation of such a laser has a wavelength of 0.41-0.43 μm, but may also be in the ultraviolet region.

2.4 Semiconductor lasers

Although semiconductor lasers are solid state, they are usually classified as a separate group. In these lasers, coherent radiation is obtained due to the transition of electrons from the lower edge of the conduction band to the upper edge of the valence band. There are two types of semiconductor lasers. The first has a plate of a pure semiconductor, in which pumping is performed by a beam of fast electrons with an energy of 50-100 keV. Optical pumping is also possible. Gallium arsenide GaAs, cadmium sulfide CdS or cadmium selenide CdSe are used as semiconductors. Pumping with an electron beam causes a strong heating of the semiconductor, which degrades the laser radiation. Therefore, such lasers need good cooling. For example, a gallium arsenide laser is usually cooled to a temperature of 80 K.

Pumping by an electron beam can be transverse (Fig. 3) or longitudinal (Fig. 4). During transverse pumping, two opposite faces of the semiconductor crystal are polished and play the role of optical resonator mirrors. In the case of longitudinal pumping, external mirrors are used. With longitudinal pumping, the cooling of the semiconductor is significantly improved. An example of such a laser is a cadmium sulfide laser that generates radiation at a wavelength of 0.49 μm and has an efficiency of about 25%.

Rice. 3 - Transverse electron beam pumping

Rice. 4 - Longitudinal pumping by an electron beam

The second type of semiconductor laser is the so-called injection laser. It has a p-n junction (Fig. 5), formed by two degenerate impurity semiconductors, in which the concentration of donor and acceptor impurities is 1018-1019 cm-3. The faces perpendicular to the plane of the p-n junction are polished and serve as mirrors of the optical resonator. A forward voltage is applied to such a laser, under the action of which the potential barrier in the p-n junction is lowered and electrons and holes are injected. In the transition region, intense recombination of charge carriers begins, during which electrons pass from the conduction band to the valence band and laser radiation occurs. For injection lasers, mainly gallium arsenide is used. The radiation has a wavelength of 0.8-0.9 microns, the efficiency is quite high - 50-60%.

Rice. 5 - The principle of the device of the injection laser

amplifier generator beam wave

Miniature injection lasers with linear dimensions of semiconductors of about 1 mm give a radiation power of up to 10 mW in a continuous mode, and in a pulsed mode they can have a power of up to 100 W. Getting high power requires strong cooling.

It should be noted that there are many different features in the device of lasers. An optical resonator is only in the simplest case composed of two plane-parallel mirrors. More complex designs of resonators are also used, with a different shape of mirrors.

Many lasers include additional radiation control devices located either inside the resonator or outside it. With the help of these devices, the laser beam is deflected and focused, various radiation parameters are changed. The wavelength of different lasers can be 0.1-100 microns. With pulsed radiation, the duration of the pulses ranges from 10-3 to 10-12 s. The pulses can be single or follow with a repetition rate of up to several gigahertz. Achievable power is 109 W for nanosecond pulses and 1012 W for ultra-short picosecond pulses.

2.5 Dye lasers

Lasers that use organic dyes as the laser material, usually in the form of a liquid solution. They revolutionized laser spectroscopy and pioneered a new type of laser with a pulse duration of less than a picosecond (ultrashort pulse lasers).

Today, another laser is commonly used as pumping, such as a diode-pumped Nd:YAG, or an Argon laser. It is very rare to find a dye laser pumped with a flash lamp. The main feature of dye lasers is the very large width of the gain contour. Below is a table of parameters for some dye lasers.

There are two possibilities to use such a large working area of ​​the laser:

tuning the wavelength at which the generation occurs -> laser spectroscopy,

generation immediately in a wide range -> generation of ultra-short pulses.

In accordance with these two possibilities, laser designs also differ. If the wavelength is tuned conventional scheme, only additional blocks are added for thermal stabilization and emission of radiation with a strictly defined wavelength (usually a prism, a diffraction grating, or more complex circuits), then a much more complex installation is required to generate ultra-short pulses. The design of the cuvette with the active medium is changed. Due to the fact that the duration of the laser pulse is eventually 100 ÷30 10 ?15 (light in a vacuum has time to travel only 30 ÷ 10 μm during this time), the population inversion should be maximum, this can only be achieved by very fast pumping of the dye solution. In order to accomplish this, a special design of a cell with a free dye jet is used (the dye is pumped from a special nozzle at a speed of about 10 m/s). The shortest pulses are obtained using a ring resonator.

2.6 Free electron laser

A type of laser in which radiation is generated by a monoenergetic electron beam propagating in an undulator - a periodic system of deflecting (electric or magnetic) fields. Electrons, making periodic oscillations, emit photons, the energy of which depends on the energy of the electrons and the parameters of the undulator.

Unlike gas, liquid or solid-state lasers, where electrons are excited in bound atomic or molecular states, the FEL emits a beam of electrons in a vacuum passing through a series of specially arranged magnets - an undulator (wiggler), which makes the beam move along a sinusoidal path, losing energy, which is converted into a stream of photons. As a result, soft x-rays are produced, which are used, for example, to study crystals and other nanostructures.

By changing the energy of the electron beam, as well as the parameters of the undulator (the strength of the magnetic field and the distance between the magnets), it is possible to change the frequency of the laser radiation produced by the FEL over a wide range, which is the main difference between the FEL and lasers of other systems. The radiation produced by FEL is used to study nanometer structures - there is experience in imaging particles as small as 100 nanometers (this result was achieved using X-ray microscopy with a resolution of about 5 nm). The design for the first free electron laser was published in 1971 by John M. J. Maidy as part of his PhD project at Stanford University. In 1976, Maidy and colleagues demonstrated the first experiments with FEL using 24 MeV electrons and a 5-meter wiggler to amplify the radiation.

The laser power was 300 mW and the efficiency was only 0.01%, but the operability of this class of devices was shown, which led to huge interest and a sharp increase in the number of developments in the field of FEL.

Tutoring

Need help learning a topic?

Our experts will advise or provide tutoring services on topics of interest to you.
Submit an application indicating the topic right now to find out about the possibility of obtaining a consultation.

OPTICAL FREQUENCY STANDARDS - lasers with a frequency stable in time (10 -14 - 10 -15), its reproducibility (10 -13 - 10 -14). O. s. hours are used in physical. research and find practical. application in metrology, location, geophysics, communications, navigation and mechanical engineering. Frequency division O. s. h. to the radio range did possible creation time scale based on the use of the optical period. .
O. s. hours have advantages over quantum frequency standards microwave range: experiments related to the measurement of frequency using lasers require less time, because abs. the frequency is 10 4 - 10 5 times higher than non-laser frequency standards. Abs. intensity and width , which are frequency references, in optical. range 10 5 - 10 6 times greater than in the microwave range, with the same relates. width. It allows to create O. with. hours with a higher short time. frequency stability. When dividing the frequency of O. s. h. to the radio range refers. the width of the emission line remains practically unchanged (if a microwave standard is used, the fluctuation spectrum of its signal expands significantly when the frequency is multiplied by a factor of 105–106). The role of the quadratic Doppler effect limiting longevity. frequency stability and reproducibility are the same.

Principle of stabilization. Laser frequency stabilization, as well as radio band standards, is based on the use of spectral lines of an atomic or molecular gas (optical reference), to the center of which the frequency is "attached" v using an electronic automatic system. frequency adjustment. Since the amplification lines of lasers are usually much larger than the bandwidth optical resonator, then the instability ( v) frequency v generation in most cases is determined by a change in the optical. resonator length Main. sources of instability l are thermal drift, mechanical. and acoustic perturbations of structural elements, fluctuations in the refractive index of gas-discharge plasma. With the help of optical benchmark, the auto-tuning system generates a signal proportional to. magnitude and sign of the detuning between the frequency v and frequency v0 center of the spectral line, with the help of which the laser frequency is tuned to the center of the line ( = v-v0= 0). Relates setting accuracy inversely proportional. the product of the spectral line ( - line width) and the signal-to-noise ratio when it is displayed.
To obtain a narrow emission line and high short time. frequency stability (stability over time s), it is necessary to use reference points of sufficiently high intensity with a width significantly exceeding the characteristic range of frequency disturbances. gas lasers characteristic width of the acoustic spectrum. perturbations ~ 10 3 - 10 4 Hz, so the required resonance width is Hz (referred, width 10 -9 - 10 -10). This allows the use of automated systems. frequency adjustment with a wide band (10 4 Hz) for eff. suppression of fast fluctuations in the length of the resonator.
To achieve high durability stability and frequency reproducibility are required optical. line of high quality factor, since this reduces the influence of decomp. factors on frequency shifts of the center of the line.

Optical reference points. The methods used in the microwave range for obtaining narrow spectral lines turned out to be inapplicable in optical. spectral region (Doppler broadening is small in the microwave range). For O. with. Particularly important are methods that make it possible to obtain resonances at the center of a spectral line. This makes it possible to directly relate the radiation frequency to the frequency of the quantum transition. Three methods are promising: the method of saturated absorption, two-photon resonance, and the method of spaced optical. fields. Main The results on laser frequency stabilization were obtained using the saturated absorption method, which is based on the nonlinear interaction of counterpropagating light waves with a gas. A non-linearly absorbing cell with a low-pressure gas can be located inside the laser resonator (active reference) and outside it (passive reference). Due to the saturation effect (equalization of the population levels of gas particles in a strong field), a dip with a uniform width appears in the center of the Doppler-broadened absorption line, the edge can be 10 5 - 10 6 times less than the Doppler width. In the case of an internal absorbing cell, a decrease in absorption at the center of the line leads to the appearance of a narrow peak in the contour of the dependence of the power on the generation frequency. Width of a nonlinear resonance in a molecular gas low pressure is determined primarily by collisions and effects due to the finite time of flight of the particle through the light beam. A decrease in the width of the resonance is accompanied by a sharp drop in its intensity (proportional to the cube of the pressure).
Naib. narrow resonances of saturated absorption with relative, width 10 -11 were obtained in CH 4 on the components E oscillating-rotate. lines R(7) stripes v 3 (see Molecular spectra), which are close to the center of the gain line of a helium-neon laser at = 3.39 μm. To accurately match the gain and absorption lines, 22 Ne is used and the He pressure in the laser active medium is increased or the active medium is placed in a magnetic field. field (for E-Components).
O.'s scheme with. hours, using ultra-narrow resonance (with a relative width of 10 -11 - 10 - 12 ) as a reference, consists of an auxiliary frequency-stable laser 2 with a narrow emission line, a tunable laser 2, and a system for obtaining a narrow resonance (Fig. 1). The narrow emission line of a tunable laser, which is used to obtain a supernarrow resonance, is provided by phase locking this laser with a stable one.

Rice. 1. Scheme of the optical frequency standard: CHFAP - frequency-phase auto-tuning; SUR - system for obtaining ultra-narrow resonance; AFC - automatic frequency control system; ZG - sound generator; RG - radio generator; D - photo detector.

Long-term The stability of the tunable laser is achieved by smoothly tuning its frequency to the maximum ultranarrow resonance using an extreme auto-tuning system. At the same time, it is possible to simultaneously receive high values short-term and longevity. stability and frequency reproducibility.
frequency stability. Naib. high frequency stability was obtained in the IR range with an He - Ne laser ( = 3.39 μm) with an internal. absorption cell. Because abs. its frequency is known with high accuracy (10 -11), then this laser can be used as a standalone. secondary frequency standard for measuring abs. frequencies in optical. and IR bands. The emission linewidth of such a laser is 0.07 Hz (Fig. 2). Frequency stability for averaging times = 1 - 100 s is equal to 4 x 10 -15 (Fig. 3).
Long-term stability and frequency reproducibility of He - Ne-lasers with telescopic. beam expansion, stabilized by resonances in CH 4 on the absorption lines F 2 2 and E(see above) with a quality factor of ~10 11 reach ~10 -14 . The principal factor limiting frequency reproducibility and accuracy is quadratic.

Lit.: Basov N. G., Letokhov V. S., Optical frequency standards, "UFN", 1968, v. 96, p. 585; Jennings D. A., Petersen F. R., Evenson K. M., Direct frequency measurement of the 260 THz (1.15mm) 20 Ne Laser and beyond, in: Laser spectroscopy. IV. Proc. 4th Intern. Conf., Rottach-Egern, Fed. Rep. of Germany, June 11 - 15 1979, ed. by H. Walther, K. W. Kothe, B. - , 1979, p. 39; Proceedings of Third Symposium on Freq. Standards and Metrology, Aussois, France, 12 - 15 Oct. 1981, "J. Phys.", 1981, v. 42, Collog. S 8, No. 12; Bagaev S. N., Chebotaev V. P., Laser frequency standards, UFN, 1986, v. 148, p. 143; Knight D. J. E., A tabulation of absolute laser - frequence measurements, "Metrologia", 1986, v 22, p. 251.

V. P. Chebotaev.

The word "laser" itself is an abbreviation of the English "Light Amplification by Stimulated Emission of Radiation", which means "light amplification by stimulated emission."

The countdown of the era of laser medicine began more than half a century ago, when in 1960, Theodor Mayman first used the ruby ​​laser in the clinic.

Ruby was followed by other lasers: 1961 - neodymium yttrium-aluminum garnet laser (Nd:YAG); 1962 - argon; 1964 - carbon dioxide (CO 2) laser.

In 1965, Leon Goldman reported on the use of a ruby ​​laser for tattoo removal. Subsequently, until 1983, various attempts were made to use neodymium and argon lasers for the treatment of vascular pathologies of the skin. But their use has been limited by the high risk of scarring.

In 1983, Rox Anderson and John Parrish published their concept of selective photothermolysis (SPT) in the journal Science, which led to revolutionary changes in laser medicine and dermatology. This concept made it possible to better understand the processes of interaction of laser radiation with tissue. This, in turn, facilitated the development and production of lasers for medical applications.

Features of laser radiation

Three properties inherent in laser radiation make it unique:

1. Coherence. The peaks and falls of the waves are parallel and coincide in phase in time and space.
2. Monochrome. light waves, emitted by the laser, have the same length, exactly the one provided by the medium used in the laser.
3. Collimation. The waves in a beam of light remain parallel, do not diverge, and the beam transfers energy with virtually no loss.

Ways of interaction of laser radiation with the skin

Laser surgery methods are used for manipulations on the skin much more often than on any other tissues. This is explained, firstly, by the exceptional diversity and prevalence of skin pathology and various cosmetic defects, and secondly, by the relative ease of performing laser procedures, which is associated with the superficial location of objects requiring treatment. The interaction of laser light with tissues is based on the optical properties of tissues and the physical properties of laser radiation. The distribution of light on the skin can be divided into four interrelated processes.

Reflection. About 5-7% of the light is reflected at the level of the stratum corneum.

Absorption (absorption). Described by the Bouguer-Lambert-Beer law. The absorption of light passing through the tissue depends on its initial intensity, the thickness of the layer of substance through which the light passes, the wavelength of the absorbed light and the absorption coefficient. If the light is not absorbed, there is no effect on the tissues. When a photon is absorbed by a target molecule (chromophore), all of its energy is transferred to that molecule. The most important endogenous chromophores are melanin, hemoglobin, water and collagen. Exogenous chromophores include tattoo dyes, as well as dirt particles impregnated during trauma.

Diffusion. This process is mainly due to the collagen of the dermis. The importance of the scattering phenomenon lies in the fact that it quickly reduces the energy flux density available for absorption by the target chromophore, and, consequently, the clinical effect on tissues. Scattering decreases with increasing wavelength, making longer wavelengths ideal for delivering energy to deep skin structures.

Penetration. The depth of penetration of light into the subcutaneous structures, as well as the scattering intensity, depends on the wavelength. Short waves (300-400 nm) are intensely scattered and do not penetrate deeper than 100 µm . Longer wavelengths penetrate deeper because they scatter less. .

The main physical parameters of the laser, which determine the effect of quantum energy on a particular biological target, are the length of the generated wave and the energy flux density and exposure time.

The length of the generated wave. The laser radiation wavelength is comparable to the absorption spectrum of the most important tissue chromophores (Fig. 2). When choosing this parameter, one should definitely take into account the depth of the target structure (chromophore), since light scattering in the dermis depends significantly on the wavelength (Fig. 3). This means that long waves are absorbed more weakly than short ones; accordingly, their penetration into tissues is deeper. It is also necessary to take into account the inhomogeneity of the spectral absorption of tissue chromophores:

• Melanin normally found in the epidermis and hair follicles. Its absorption spectrum lies in the ultraviolet (up to 400 nm) and visible (400 - 760 nm) spectral ranges. The absorption of laser radiation by melanin gradually decreases as the wavelength of light increases. Weakening of absorption occurs in the near infrared region of the spectrum from 900 nm.
• Hemoglobin found in erythrocytes. It has many different absorption peaks. The absorption spectrum maxima of hemoglobin lie in the UV-A (320-400 nm), violet (400 nm), green (541 nm) and yellow (577 nm) ranges.
• Collagen forms the basis of the dermis. The absorption spectrum of collagen is in the visible range from 400 nm to 760 nm and in the near infrared region of the spectrum from 760 to 2500 nm.
• Water makes up to 70% of the dermis. The absorption spectrum of water lies in the middle (2500 - 5000 nm) and far (5000 - 10064 nm) infrared regions of the spectrum.

Energy flux density. If the wavelength of light affects the depth at which it is absorbed by one or another chromophore, then the magnitude of the laser radiation energy and the power that determines the rate of arrival of this energy are important for direct damage to the target structure. Energy is measured in joules (J), power is measured in watts (W, or J/s). In practice, these radiation parameters are usually used in terms of per unit area - energy flux density (J / cm 2) and energy flux rate (W / cm 2), or power density.

Types of laser interventions in dermatology

All types of laser interventions in dermatology can be divided into two types:

• I type. Operations during which ablation of the area of ​​the affected skin, including the epidermis, is performed.
• II type. Operations aimed at selective removal of pathological structures without violating the integrity of the epidermis.

Type I. Ablation.
This phenomenon is one of the fundamental, intensively studied, although not yet fully resolved, problems of modern physics.
The term "ablation" is translated into Russian as removal or amputation. In non-medical vocabulary, this word means blurring or melting. In laser surgery, ablation is understood as the elimination of a section of living tissue directly under the action of laser radiation photons on it. This refers to the effect that manifests itself precisely during the irradiation procedure, in contrast to the situation (for example, during photodynamic therapy), when the irradiated tissue area remains in place after the cessation of laser exposure, and its gradual elimination occurs later as a result of a series of local biological reactions developing in the irradiation zone.

The energy characteristics and performance of ablation are determined by the properties of the irradiated object, the characteristics of the radiation and the parameters that inextricably link the properties of the object and the laser beam - the coefficients of reflection, absorption and scattering of a given type of radiation in a given type of tissue or its individual components. The properties of the irradiated object include: the ratio of liquid and dense components, their chemical and physical properties, the nature of intra- and intermolecular bonds, the thermal sensitivity of cells and macromolecules, the blood supply to the tissue, etc. The radiation characteristics are the wavelength, the irradiation mode (continuous or pulse), power, pulse energy, total absorbed energy, etc.

The ablation mechanism has been studied in most detail using a CO2 laser (l = 10.6 μm). Its radiation at a power density of ³ 50 kW/cm 2 is intensely absorbed by tissue water molecules. Under such conditions, there is a rapid heating of the water, and from it the non-aqueous components of the tissue. The consequence of this is the rapid (explosive) evaporation of tissue water (vaporization effect) and the eruption of water vapor together with fragments of cellular and tissue structures outside the tissue with the formation of an ablation crater. Together with the superheated material, most of the thermal energy is removed from the fabric. A narrow strip of heated melt remains along the walls of the crater, from which heat is transferred to the surrounding intact tissues (Fig. 4). At a low energy density (Fig. 5a), the release of ablation products is relatively small, so a significant part of the heat from the massive melt layer is transferred to the tissue. At a higher density (Fig. 5, B), the opposite picture is observed. In this case, minor thermal damage is accompanied by mechanical trauma to the tissue due to the shock wave. Part of the heated material in the form of a melt remains along the walls of the ablation crater, and it is this layer that is the reservoir of heat transferred to the tissue outside the crater. The thickness of this layer is the same along the entire contour of the crater. With an increase in power density, it decreases, and with a decrease it increases, which is accompanied by a decrease or increase in the thermal damage zone, respectively. Thus, by increasing the radiation power, we achieve an increase in the rate of tissue removal, while reducing the depth of thermal damage.

The field of application of the CO 2 laser is very extensive. In focused mode, it is used to excise tissues with simultaneous coagulation of vessels. In the defocused mode, by reducing the power density, layer-by-layer removal (vaporization) of the pathological tissue is performed. It is in this way that superficial malignant and potentially malignant tumors (basal cell carcinoma, actinic cheilitis, erythroplasia of Queyrat), a number of benign neoplasms of the skin (angiofibroma, trichlemmoma, syringoma, trichoepithelioma, etc.), large post-burn scabs, inflammatory skin diseases (granulomas, nodular chondrodermatitis of the auricle), cysts, infectious skin lesions (warts, recurrent warts, deep mycoses), vascular lesions (pyogenic granuloma, angiokeratoma, annular lymphangioma), formations that cause cosmetic defects (rhinophyma, deep acne scars, epidermal birthmarks, lentigo, xanthelasma), etc.

The defocused CO 2 laser beam is also used in a purely cosmetic procedure - the so-called laser dermabrasion, that is, the layer-by-layer removal of the surface layers of the skin in order to rejuvenate the patient's appearance. In the pulsed mode with a pulse duration of less than 1 ms, 25-50 microns of tissue are selectively vaporized in one pass; this forms a thin zone of residual thermal necrosis within 40-120 microns. This zone is large enough to temporarily isolate the dermal blood and lymph vessels, which in turn reduces the risk of scar formation.

Skin renewal after laser dermabrasion is due to several reasons. Ablation reduces the appearance of wrinkles and textural abnormalities through surface evaporation of tissue, thermal coagulation of cells in the dermis, and denaturation of extracellular matrix proteins. During the procedure, there is an instantaneous visible contraction of the skin in the range of 20-25% as a result of tissue shrinkage (compression) due to dehydration and contraction of collagen fibers. The onset of a delayed, but longer lasting result of skin renewal is achieved due to the processes associated with the reaction of tissues to injury. After laser exposure, aseptic inflammation develops in the area of ​​the formed wound. This stimulates post-traumatic growth factor release and fibroblast infiltration. The oncoming reaction is automatically accompanied by a surge of activity, which inevitably leads to the fact that fibroblasts begin to produce more collagen and elastin. As a result of vaporization, the renewal processes and proliferation kinetics of epidermal cells are activated. In the dermis, the processes of regeneration of collagen and elastin are launched, followed by their arrangement in a parallel configuration.

Similar events occur when using pulsed lasers emitting in the near and mid-infrared region of the spectrum (1.54-2.94 μm): diode-pumped erbium (l = 1.54 μm), thulium (l = 1.927 μm), Ho: YSSG (l = 2.09 µm), Er:YSSG (l = 2.79 µm), Er:YAG (l = 2.94 µm). These lasers are characterized by very high water absorption coefficients. For example, Er:YAG laser radiation is absorbed by water-containing tissues 12-18 times more actively than CO 2 laser radiation. As in the case of the CO 2 laser, a melt layer forms along the walls of the ablation crater in tissue irradiated with the Er:YAG laser. It should be borne in mind that when working on a biological tissue with this laser, the energy characteristic of the pulse, primarily its peak power, is essential for the nature of tissue changes. This means that even at the minimum radiation power, but with a longer pulse, the depth of thermonecrosis sharply increases. Under such conditions, the mass of the removed superheated ablation products is relatively less than the mass of the remaining ones. This causes deep thermal damage around the ablation crater. At the same time, with a powerful pulse, the situation is different - minimal thermal damage around the crater with highly efficient ablation. True, in this case a positive effect is achieved at the cost of extensive mechanical damage to the tissue by the shock wave. In one pass with an erbium laser, tissue is ablated to a depth of 25-50 microns with minimal residual thermal damage. As a result, the process of re-epithelialization of the skin is much shorter than after exposure to CO 2 laser.

II type. Selective influence.
Operations of this type include procedures during which laser damage is achieved to certain intradermal and subcutaneous formations without violating the integrity of the skin. This goal is achieved by selecting the characteristics of the laser: wavelength and irradiation mode. They must ensure the absorption of laser light by the chromophore (colored target structure), which will lead to its destruction or discoloration due to the conversion of radiation energy into thermal energy (photothermolysis), and in some cases into mechanical energy. The target of laser exposure can be: hemoglobin of erythrocytes located in numerous dilated dermal vessels with wine stains (PWS); melanin pigment of various skin formations; coal, as well as other differently colored foreign particles injected under the epidermis during a tattoo or getting there as a result of other influences.

An ideal selective effect can be considered such an effect in which laser beams are absorbed only by the target structures, and there is no absorption outside of it. To achieve such a result, a specialist who chose a laser with an appropriate wavelength would only have to set the energy density of the radiation and the duration of exposures (or pulses), as well as the intervals between them. These parameters are determined taking into account (VTR) for a given target - the time interval during which the target temperature increased at the time of the pulse is lowered by half of its increase relative to the initial one. Exceeding the pulse duration over the TTR value will cause unwanted overheating of the tissue around the target. A decrease in the interval between pulses will also lead to the same effect. In principle, all these conditions can be modeled mathematically before surgery, but the composition of the skin itself does not allow full use of the calculated data. The fact is that in the basal layer of the epidermis there are melanocytes and individual cratinocytes that contain melanin. Since this pigment intensively absorbs light in the visible, as well as near ultraviolet and infrared regions of the spectrum (the "optical window" of melanin is in the range from 500 to 1100 nm), any laser radiation in this range will be absorbed by melanin. This can lead to thermal damage and death of the corresponding cells. Moreover, radiation in the visible part of the spectrum is also absorbed by cytochromes and flavin enzymes (flavoproteins) of both melanin-containing cells and all other types of cells of the epidermis and dermis. It follows from this that when laser irradiation of a target located under the surface of the skin, some damage to the epidermal cells becomes inevitable. Therefore, the real clinical problem is reduced to a compromise search for such regimes of laser irradiation, in which it would be possible to achieve maximum target damage with the least damage to the epidermis (with the expectation of its subsequent regeneration, mainly due to neighboring non-irradiated skin areas).

Compliance with all these conditions in relation to a specific target will lead to its maximum damage (heating or decay) with minimal overheating or mechanical injury to neighboring structures.

Thus, for the irradiation of pathological vessels of the wine stain (PWS), it is most rational to use a laser with the longest wavelength corresponding to the light absorption peaks of hemoglobin (l = 540, 577, 585 and 595 nm), with a pulse duration of the order of milliseconds, since in this case the absorption of radiation melanin will be negligible (position 1 of the theory of selective photothermolysis). A relatively large wavelength will effectively provide deep heating of the tissue (position 2), and a relatively long pulse will correspond to a very large target (vessels with red blood cells; position 3).

If the goal of the procedure is to eliminate tattoo particles, then in addition to selecting the radiation wavelength corresponding to the color of these particles, it will be necessary to set the pulse duration, which is much shorter than in the case of wine stains, in order to achieve mechanical destruction of the particles with minimal thermal damage to other structures (position 4 ).

Of course, compliance with all these conditions does not provide absolute protection of the epidermis, however, it excludes too gross damage to it, which would subsequently lead to a permanent cosmetic defect due to excessive scarring.

Tissue reactions to laser exposure

When laser light interacts with tissue, the following reactions occur.

Photostimulation. For photostimulation, low-intensity therapeutic lasers are used. The therapeutic laser in terms of energy parameters has an effect that does not damage the biosystem, but at the same time this energy is sufficient to activate the vital processes of the body, for example, accelerate wound healing.

photodynamic reaction. The principle is based on exposure to light of a certain wavelength on a photosensitizer (natural or artificially introduced), which provides a cytotoxic effect on pathological tissue. In dermatology, photodynamic exposure is used to treat acne vulgaris, psoriasis, lichen planus, vitiligo, urticaria pigmentosa, etc.

Photothermolysis and photomechanical reactions - when radiation is absorbed, the energy of the laser beam is converted into heat in the area of ​​the skin that contains the chromophore. With sufficient power of the laser beam, this leads to thermal destruction of the target . Selective photothermolysis can be used to remove malformations of superficially located vessels, some pigmented formations of the skin, hair, and tattoos.

Literature

1. Laser and light therapy. Dover J.S. Moscow. Reid Elsiver 2010.p.5-7
2. Nevorotin AI Introduction to laser surgery. Tutorial. - St. Petersburg: SpecLit, 2000.
3. Nevorotin AI Laser wound in theoretical and applied aspects. // Laser biology and laser medicine: practice. Mat. report rep. seminar schools. Part 2. - Tartu-Pyhäjärve: Publishing House of Tartu University of the Estonian SSR, 1991, p. 3-12.
4. Anderson R. R., Parish J. A. The optics of human skin. J Invest Dermatol 1981; 77:13-19.
5. Anderson R. R., Parrish J. A. Selective photothermolysis: precise microsurgery by selective absorption of pulsed radiation. Science 1983; 220:524-527.
6. Goldman L., Blaney D. J., Kindel D. J. et al. Effect of the laser beam on the skin: preliminary report. J Invest Dermatol 1963; 40:121-122.
7. Kaminer M. S., Arndt K. A., Dover J. S. et al. Atlas of cosmetic surgery. 2nd ed. - Saunders-Elsevier 2009.
8. Margolis R. J., Dover J. S., Polla L. L. et al. Visible action spectrum for melanin-specific selective photothermolysis. Lasers Surg Med 1989; 9:389-397.

The principle of laser operation, whose physics was based on Planck's law of radiation, was first theoretically substantiated by Einstein in 1917. He described absorption, spontaneous and forced electromagnetic radiation using probability coefficients (Einstein coefficients).

pioneers

Theodor Maiman was the first to demonstrate the principle of operation based on optical pumping of a synthetic ruby ​​with a flash lamp, which produced pulsed coherent radiation with a wavelength of 694 nm.

In 1960, Iranian scientists Javan and Bennett created the first gas quantum generator using a 1:10 mixture of He and Ne gases.

In 1962 R. N. Hall demonstrated the first of gallium arsenide (GaAs) emitting at 850 nm. Later that year, Nick Golonyak developed the first semiconductor visible light quantum generator.

The device and principle of operation of lasers

Each laser system consists of an active medium placed between a pair of optically parallel and highly reflective mirrors, one of which is semitransparent, and an energy source for its pumping. The amplification medium can be a solid, liquid or gas, which has the property of amplifying the amplitude of a light wave passing through it by stimulated emission with electrical or optical pumping. The substance is placed between a pair of mirrors in such a way that the light reflected in them passes through it each time and, having reached a significant amplification, penetrates the semitransparent mirror.

Two-tier environments

Let us consider the operating principle of a laser with an active medium whose atoms have only two energy levels: excited E 2 and base E 1 . If atoms are excited to the E 2 state by any pumping mechanism (optical, electric discharge, current transmission or electron bombardment), then after a few nanoseconds they will return to the ground position, emitting photons of energy hν = E 2 - E 1 . According to Einstein's theory, emission is produced by two different ways: either it is induced by a photon, or it happens spontaneously. In the first case, stimulated emission takes place, and in the second - spontaneous. At thermal equilibrium, the probability of stimulated emission is much lower than spontaneous emission (1:10 33), so most conventional light sources are incoherent, and laser generation possible under conditions other than thermal equilibrium.

Even with very strong pumping, the populations of two-level systems can only be made equal. Therefore, three- or four-level systems are required to achieve population inversion by optical or other pumping methods.

Layered systems

What is the principle of operation of a three-level laser? Irradiation with intense light of frequency ν 02 pumps a large number of atoms from the lowest energy level E 0 to the highest E 2 . The nonradiative transition of atoms from E 2 to E 1 establishes a population inversion between E 1 and E 0 , which in practice is possible only when the atoms long time are in the metastable state E 1, and the transition from E 2 to E 1 occurs quickly. The principle of operation of a three-level laser lies in the fulfillment of these conditions, due to which a population inversion is achieved between E 0 and E 1 and photons are amplified by the energy E 1 -E 0 of the induced radiation. A wider E 2 level could increase the wavelength absorption range for more efficient pumping, resulting in an increase in stimulated emission.

A three-level system requires a very high pump power, since the lower level involved in generation is the base one. In this case, in order for the population inversion to occur, more than half of the total number atoms. In doing so, energy is wasted. The pump power can be significantly reduced if the lower generation level is not the base level, which requires at least a four-level system.

Depending on the nature of the active substance, lasers are classified into three main categories, namely, solid, liquid and gas. Since 1958, when lasing was first observed in a ruby ​​crystal, scientists and researchers have studied a wide range of materials in each category.

solid state laser

The principle of operation is based on the use of an active medium, which is formed by adding a transition group metal (Ti +3, Cr +3, V +2, Co +2, Ni +2, Fe +2, etc.) to the insulating crystal lattice. , rare earth ions (Ce +3, Pr +3, Nd +3, Pm +3, Sm +2, Eu +2,+3, Tb +3, Dy +3, Ho +3, Er +3, Yb +3 , etc.), and actinides like U +3 . ions are responsible only for generation. Physical Properties base material such as thermal conductivity and are essential for effective work laser. The arrangement of lattice atoms around a doped ion changes its energy levels. Various generation wavelengths in the active medium are achieved by doping various materials the same ion.

Holmium laser

An example is a quantum generator in which holmium replaces an atom of the base substance of the crystal lattice. Ho:YAG is one of the best generation materials. The principle of operation of a holmium laser is that yttrium aluminum garnet is doped with holmium ions, optically pumped by a flash lamp and emits at a wavelength of 2097 nm in the IR range, which is well absorbed by tissues. This laser is used for operations on the joints, in the treatment of teeth, for the evaporation of cancer cells, kidney and gallstones.

Semiconductor quantum generator

Quantum well lasers are inexpensive, allow for mass production, and are easily scalable. The principle of operation of a semiconductor laser is based on the use of a p-n junction diode, which produces light of a certain wavelength by carrier recombination at a positive bias, similar to LEDs. LED emit spontaneously, and laser diodes - forced. To fulfill the population inversion condition, the operating current must exceed the threshold value. The active medium in a semiconductor diode has the form of a connecting region of two two-dimensional layers.

The principle of the laser of this type is such that no external mirror is required to maintain the oscillations. The reflectivity provided by the layers and the internal reflection of the active medium is sufficient for this purpose. The end surfaces of the diodes are chipped, which ensures the parallelism of the reflective surfaces.

A connection formed by one type is called a homojunction, and a connection created by a connection of two different types is called a heterojunction.

Semiconductors of p and n type with a high carrier density form a p-n junction with a very thin (≈1 μm) depletion layer.

gas laser

The principle of operation and the use of this type of laser allows you to create devices of almost any power (from milliwatts to megawatts) and wavelengths (from UV to IR) and allows you to work in pulsed and continuous modes. Based on the nature of active media, there are three types of gas quantum generators, namely atomic, ionic, and molecular.

Most gas lasers are pumped by an electrical discharge. The electrons in the discharge tube are accelerated by the electric field between the electrodes. They collide with atoms, ions or molecules of the active medium and induce a transition to higher energy levels to reach the state of the population of inversion and stimulated emission.

Molecular laser

The principle of laser operation is based on the fact that, unlike isolated atoms and ions, molecules in atomic and ion quantum generators have wide energy bands of discrete energy levels. Moreover, each electronic energy level has a large number of vibrational levels, and those, in turn, have several rotational ones.

The energy between electronic energy levels is in the UV and visible regions of the spectrum, while between the vibrational-rotational levels - in the far and near IR regions. Thus, most molecular quantum generators operate in the far or near infrared regions.

Excimer lasers

Excimers are molecules such as ArF, KrF, XeCl, which have a separated ground state and are stable at the first level. The principle of operation of the laser is as follows. As a rule, the number of molecules in the ground state is small, so direct pumping from the ground state is not possible. Molecules are formed in the first excited electronic state by combining high-energy halides with inert gases. The population of the inversion is easily achieved, since the number of molecules at the base level is too small compared to the excited one. The operating principle of a laser, in short, consists in the transition from a bound excited electronic state to a dissociative ground state. The population in the ground state always remains at a low level, because the molecules at this point dissociate into atoms.

The device and principle of operation of lasers is that the discharge tube is filled with a mixture of halide (F 2) and rare earth gas (Ar). The electrons in it dissociate and ionize halide molecules and create negatively charged ions. Positive ions Ar + and negative F - react and produce ArF molecules in the first excited bound state, followed by their transition to the repulsive base state and the generation of coherent radiation. The excimer laser, the principle of operation and application of which we are now considering, can be used to pump an active medium on dyes.

liquid laser

Compared to solids, liquids are more homogeneous and have a higher density of active atoms than gases. In addition to this, they are easy to manufacture, allow for easy heat dissipation and can be easily replaced. The operating principle of the laser is to use organic dyes as an active medium, such as DCM (4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran), rhodamine, styryl, LDS, coumarin, stilbene, etc. ., dissolved in an appropriate solvent. A solution of dye molecules is excited by radiation whose wavelength has a good absorption coefficient. The principle of operation of the laser, in short, is to generate at a longer wavelength, called fluorescence. The difference between the absorbed energy and the emitted photons is used by non-radiative energy transitions and heats up the system.

The wider fluorescence band of liquid quantum generators has a unique feature - wavelength tuning. The principle of operation and the use of this type of laser as a tunable and coherent light source is becoming increasingly important in spectroscopy, holography, and biomedical applications.

Recently, dye quantum generators have been used for isotope separation. In this case, the laser selectively excites one of them, prompting them to enter into a chemical reaction.