Properties of laser radiation. Basic principles and biological mechanisms of the effect of laser radiation on the skin. Technology and engineering
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The design of the laser and the properties of stimulated emission determine the difference between laser radiation and the radiation of conventional light sources. Laser radiation (LR) is characterized by the following important properties.
1. Highly coherent. Radiation is highly coherent, which is due to the properties of stimulated stimulated emission. In this case, not only temporal, but also spatial coherence takes place: the phase difference at two points of the plane perpendicular to the direction of propagation remains constant (Fig. a) (as a result of spatial coherence, radiation can be focused in a very small volume).
2. Monochromatic. Laser radiation is highly monochromatic, that is, it contains waves of almost the same frequency (photons have the same energy). This is due to the fact that stimulated emission is associated with duplication of photons (each stimulated photon is completely similar to the original one). In this case, an electromagnetic wave of constant frequency is formed. The spectral line width is 0.01 nm. In Fig. c shows a schematic comparison of the linewidth of a laser beam and a beam of ordinary light.
Before the advent of lasers, radiation with a certain degree of monochromaticity could be obtained using devices - monochromators, which distinguish narrow spectral intervals (narrow wavelength bands) from a continuous spectrum, but the light power in such bands is low.
3. High power. Using a laser, you can provide very high monochromatic radiation power - up to 10 5 W in continuous mode. The power of pulsed lasers is several orders of magnitude higher. This is how a neodymium laser generates a pulse with energy E= 75 J, the duration of which t= 3·10 –12 s. The pulse power is equal to R= E/t= 2.5 10 13 W (for comparison: hydroelectric power R~ 10 9 W).
4. High intensity. In pulsed lasers, the laser radiation intensity is very high and can reach I= 10 14 -10 16 W/cm 2 (cf. intensity of sunlight near the earth’s surface I= 0.1 W/cm2).
5. High brightness. For lasers operating in the visible range, brightness laser radiation (light intensity per unit surface) is very high. Even the weakest lasers have a brightness of 10 15 cd/m 2 (for comparison: the brightness of the Sun L~ 10 9 cd/m2).
6. Pressure. When a laser beam hits a surface, it has pressure (p). With complete absorption of laser radiation incident perpendicular to the surface, a pressure is created R= I/s, where I– radiation intensity, With– speed of light in vacuum. With total reflection, the pressure is twice as high. At intensity I= 10 14 W/cm 2 = 10 18 W/m 2, R= 3.3·10 9 Pa = 33000 atm.
7. Small divergence angle in the beam. Collimation. Radiation is collimated, that is, all the rays in the beam are almost parallel to each other (Fig. 6). Over a long distance, the laser beam increases only slightly in diameter (for most lasers the divergence angle is 1 arc minute or less). Since the divergence angle is small, the intensity of the laser beam decreases slightly with distance. High directionality allows signals to be transmitted over vast distances with little attenuation of their intensity.
8. Polarization. Laser radiation is completely polarized.
When scientists learned what the properties of laser radiation are, the public gained great opportunities for interferometry. Currently, the scientific community has fairly accurate methods for determining quantitative estimates of displacements and lengths. At first, interferometers were used quite limitedly, since the light wave sources were not sufficiently coherent and bright, so the picture accessible to humans was correct only in the case when the measuring arm was 50 cm or less. Much has changed when the possibility of using more high-precision laser radiation became possible.
Hemostatics
This term is usually used to briefly denote the property of laser radiation expressed through soldering and welding. The process is caused by necrosis associated with temperature treatment. Coagulation controlled necrosis, provoked by a change in the heating level, is accompanied by the formation of a marginal film from elements of cells and tissues. This connects several layers of the organ to a single level.
Working with a laser always involves interaction with very high temperatures. Due to this feature, the liquid that is normally found inside cells and between tissues evaporates almost instantly, and the dry components burn. Dystrophy is determined by what type of laser radiation (the properties are slightly different) is used in a particular installation. Much also depends on the type of organic tissue being processed and the duration of contact. If the laser is moved, it provokes evaporation, which results in a linear cut.
Important qualities
When considering what properties laser radiation has, it is important to mention a monochromatic spectrum, a high level of coherence, low divergence, and increased spectrum density. In total, this makes it possible to design high-precision laser-based devices that are reliable and applicable in a wide variety of climate conditions, geological, and hydrological factors.
In recent years, high-precision instruments with lasers have been designed for surveyors. They are based on the properties of laser radiation already known to mankind. The use of lasers in such installations is widespread not only in our country, but also abroad. As can be seen from practice, for pipe layers and earth-moving machines, laser systems are indispensable as a method for determining the direction of movement. They are also important when creating roads (railroads, roads) and many other works.
It is important
The laser found its application in the formation of trenches. Using a special installation, a laser beam is created that defines the route. By focusing on it, a person operating an excavator can work steadily. The operation of such modern devices is a guarantee of high-quality execution of all stages of work and the creation of trenches exactly as specified in the design documentation.
The laser is irreplaceable!
If in a school or university course, in a test paper, a student is given the task “Name the characteristic properties of laser radiation,” the first things that come to mind are coherence and brightness. If we compare laser and plasma, the first exceeds the brightness parameters by several times and is applicable for creating serial flashes, and the frequency can reach 1010 Hz. One pulse can last (in picoseconds) several tens. At the same time, the divergence is low, the frequency can be adjusted. These qualities turned out to be applicable in installations that make it possible to study processes occurring at very high speeds.
Due to the described features, lasers have become indispensable in analytics using thermo-optical spectroscopy technology.
Fine structures
The basic properties of laser radiation identified by scientists (listed above) made it possible to use this technology in the development of modern weapons and the design of machines for cutting various materials. But the range of possibilities is not limited to just this. Using particularly accurate and technological methods for constructing a working structure, based on laser radiation, it is possible to create a system for studying molecules, their structure, and properties. By obtaining the latest information in this way, scientists are forming the foundation for creating new types of lasers. As can be seen from the most optimistic forecasts, in the near future it will be through laser radiation that the nature of photosynthesis will be revealed, which means that scientists will receive all the keys to understanding the essence of life on the planet and the mechanisms of its formation.
Understanding the world: secrets and discoveries
It is believed that all the basic properties of laser radiation have now been studied. Scientists know the basic principles of stimulated emission of radiation and have been able to apply them in practice. The monochromatic spectrum of radiation, its intensity, pulse length, and clear direction are considered especially important. Due to such features, the laser beam enters into an atypical interaction with matter.
As physicists additionally point out, the indicated properties of laser radiation cannot be called independent characteristics that describe all varieties of the mentioned phenomenon without exception. There are certain connections between them. In particular, coherence is determined by the directionality of the radiation, and the pulse length is directly related to the monochromatic spectrum of the beam. The duration and direction determine the intensity of the radiation.
Raman effect
This phenomenon is one of the most important for assessing, understanding, and applying the properties of laser radiation. The term is usually used to denote a condition, the initiation of which requires the installation of high power. Under its influence, scattering occurs when a frequency shift of the radiation is observed. When identifying the specifics of the spectral composition and assessing the power, you can notice that the frequency is adjusted in accordance with a rather complex pattern. If the Raman effect is stimulated artificially, it is possible to create a correction method for the optics of coherent signals.
This is interesting
As studies of the properties of laser radiation and the processes that it initiates in matter have shown, the picture is in many ways similar to that observed in the structure of ferromagnets and superconductors. If a higher pump level is achieved using a low power cavity, the beams emitted by the laser become chaotic. Moreover, chaos itself is a state of light that is completely different from the chaos created by heat-emitting objects.
The scope of use is expanding
Since laser radiation has the following properties: monochromatic spectrum, strictly defined directionality, therefore, it can be used as a light source. Currently, active developments are underway in the field of exploitation of this technology for signal transmission. It is known that light and matter can interact in such a way that the process is practical in a variety of settings, but correct approaches have yet to be developed. There are other, high-tech, complex, knowledge-intensive urgent problems, for the solution of which sooner or later it will be possible to use high-power laser radiation.
The properties of the described phenomenon make it possible to design spectral devices. This is to some extent explained by the low beam divergence, accompanied by an increased spectral density.
There are many possibilities
As scientists found out, to create the most efficient and widely used installations, it is reasonable to use lasers for which the frequency can be adjusted during operation. They are relevant primarily for spectral devices with increased resolution. In such installations, it is possible to achieve correct research results without resorting to a dispersing element.
Systems based on a laser, the frequency of which is adjusted during operation, have currently found application in various fields and fields of scientific activity, medicine, and industry. In many ways, the purpose of a particular device is determined by the specifics of the laser radiation implemented in it. The generation line determines the spectral resolution, the half-width of the functionality of the device. The shape depends on the given intensity spectral distribution.
Technical features
Typically, a laser is designed as a resonator where a specific environment is created. Its key feature is the negative absorption of electromagnetic energy. Such a resonator makes it possible to reduce radiation losses in a specialized environment. This is due to the creation of a cycle for electromagnetic energy. In this case, only a narrow band of frequencies is taken. This approach makes it possible to replenish energy losses caused by the fact that the emission is stimulated.
To generate electromagnetic energy with the characteristics of a laser, you do not need to use a resonator. The result will still be coherent, characterized by high collimation and a narrow spectrum.
About holography
To implement such processes, you must have at your disposal a source that generates radiation with a high level of coherence. Currently these are lasers. As soon as such radiation was discovered for the first time, physicists almost immediately realized that its properties could be used to implement holography. This became the impetus for the widespread practical application of promising technology.
About application
As soon as lasers were invented, the scientific community, and then the whole world, valued them as a unique solution to any problem. This is due to the properties of radiation. Currently, lasers are used in technology, science, and in solving numerous everyday problems: from playing music to reading codes when selling goods. The industry uses such systems for soldering, cutting, and welding. Thanks to the ability to reach very high temperatures, it is possible to weld materials that are not amenable to classical joining methods. This made it possible, for example, to create solid objects from ceramic and metal parts.
Using modern technology, a laser beam can be focused so that the diameter of the resulting point is estimated in microns. This allows the technology to be used in microscopic electronic devices. Currently, this possibility is known under the term “scribing”.
Where else?
Due to their unique qualities, lasers are quite actively used in industry to create coatings. This helps increase the wear resistance of various products and materials. Laser marking and engraving are no less relevant - with the help of a modern installation, almost any surface can be processed in this way. This is largely due to the absence of direct mechanical influence, that is, the working process provokes less deformation than with any other common method. The current level of development of technology and science is such that it is possible to fully automate all stages of working with a laser, while maintaining a high level of productivity and increased accuracy of task execution.
Technology and engineering
Recently, dye laser systems have been widely used. They produce monochromatic radiation with different wavelengths, pulses are estimated at 10-16 s. The power of such installations is very large, and the generated pulses are estimated as gigantic. This possibility is especially significant for spectroscopy and studies in optics of relatively non-linear effects.
The use of laser has become the basic technology for accurately estimating the distance between our planet and the nearest celestial body, the Moon. Measurement accuracy is up to centimeters. Laser location allows one to increase astronomical knowledge, clarify navigation in space, and increase the database about the characteristics of the atmosphere and what the planets of our system are made of.
Chemistry is not left behind
Modern laser technologies are used to initiate chemical reactions and study how they occur. When using such capabilities, it is possible to identify the location, dose, sterility with extreme precision, and provide the necessary energy indicators at the moment the system starts.
Scientists are actively working on the development of laser cooling systems and developing the possibility of using such radiation to control thermonuclear reactions.
Features of laser radiation and types of lasers.
Lasers have given rise to new technologies with unique capabilities. What is extraordinary? properties of laser radiation, laser beam?
Firstly, the laser beam spreads without expanding at all. The word “almost” means that the beam of laser light is not completely parallel: there is a divergence angle, but it is relatively small - about 10 ^ (-5) rad and nevertheless, at large distances it is noticeable: on the Moon there is such a beam directed from the Earth , produces a spot with a diameter of approximately 3 km.
Secondly, laser light is extremely monochromatic, i.e. it has only one wavelength, one color. Unlike conventional light sources, whose atoms emit light independently of each other, in lasers the atoms emit light in concert. Thanks to this property of the laser beam, high-density optical recording of information has become possible - tiny optical disks can hold a huge amount of information - hundreds of megabytes.
Thirdly, laser is the most powerful light source. In a narrow range of the spectrum, for a short time (10 ^ (-11) s), a radiation power of 10 ^ 12-10 ^ 13 W per square centimeter is achieved, while the radiation power of the Sun from the same area is only 7 10 ^ 3 W, and in total across the entire spectrum.
Types of lasers
In 1960, T. Maiman (USA) created the first laser - ruby , working in pulse mode. But still this is a short light pulse. They can punch a hole, weld two metal wires and do many other useful things.
gas lasers . The gas laser was created almost simultaneously with the ruby laser, in the same 1960. It worked on a mixture of helium and neon. Modern gas lasers operate on many gases and vapors. They all give continuous radiation in a very wide range of wavelengths: from ultraviolet to infrared light.
gas dynamic laser , similar to a jet engine. In its combustion chamber, carbon monoxide (carbon monoxide) is burned with the addition of fuel (kerosene, gasoline, alcohol). The resulting mixture of gases consists of carbon dioxide, nitrogen and water vapor. Rushing between the mirrors, gas molecules emit energy in the form of light quanta, giving birth to a laser beam with a power of 150 - 200 kW. And this is not the power of a single flash, but of a constant, steady beam, shining until the laser runs out of fuel.
semiconductor lasers also provide continuous radiation. The semiconductor laser was created in 1962 by the American scientist R. Hall. It is based on optical recording, which is known to many users of personal computers who have held a laser disk in their hands, attractive not only for its appearance, but also for its information capacity: hundreds of thousands of pages of text can be recorded on a disk with a diameter of 12 cm.
dye lasers (liquid lasers). They are called so because their working fluid is solutions of aniline dyes in water, alcohol, acid and other solvents. Liquid lasers can emit pulses of light of various wavelengths (from ultraviolet to infrared light) and power from hundreds of kilowatts to several megawatts, depending on the type of dye.
Chemical lasers are being developed in which atoms go into an excited state when exposed to pump energy from chemical reactions. Much attention is paid to the development of high-power chemical lasers that convert the energy of a chemical reaction into coherent radiation, and an atomic laser that emits not light, but a beam of atoms.
Lecture 8
“Laser” is an abbreviation formed from the initial letters of the English phrase Light amplification by stimulated emission of radiation - amplification of light by creating stimulated radiation.
Laser (optical quantum generator) is a generator of electromagnetic radiation in the optical range, based on the use of stimulated radiation.
Laser radiation is electromagnetic radiation that is formed in ( lasers ) with a wavelength of 0.2-1000 µm: 0.2...0.4 µm - ultraviolet, 0.4...0.75 µm - visible light, near infrared 0.75...1.4 µm, infrared 1.4...10 2 microns.
Distinctive peculiarity laser radiation is: monochrome radiation ( strictly one wavelength); radiation coherence (all radiation sources emit electromagnetic waves in the same phase); sharp beam focus (small discrepancy).
Laser radiation is distinguished by type of radiation on
- direct(enclosed in a limited solid angle)
- scattered(scattered from a substance that is part of the medium through which the laser beam passes)
- mirror-reflected ( reflected from the surface at an angle equal to the angle of incidence of the radiation)
- diffuse-reflected(reflected from the surface in all possible directions)
As a technical device, a laser consists of three main elements:
- active medium
- resonator
- pumping systems.
Depending on the character active medium lasers are divided into the following types: solid-state (on crystals or glass); gas (He-Ne, Ar, Kr, Xe, Ne, He-Cd, CO 2, etc.); liquid; semiconductor, etc.
As resonator Usually parallel mirrors with a high reflectivity are used, between which the active medium is placed.
Pumping, i.e. the transfer of atoms of the active medium to the upper level is ensured either by a powerful light source or by an electric discharge.
There are continuous and pulsed lasers.
The classification of lasers can be presented as follows (Fig):
According to the degree of danger of the generated radiation, lasers are classified according to GOST 12.1.041-83 (1996):
Class 1 ( safe)- output radiation does not pose a danger to eyes and skin;
Class II ( low-hazard) - the output radiation is dangerous when the eyes are irradiated with direct or specularly reflected radiation;
Class III ( moderately dangerous) – direct, specular, and diffuse-reflected radiation is dangerous for the eyes;
Class IV ( highly dangerous) – diffusely reflected radiation at a distance of 10 cm from the reflected surface is dangerous for the skin.
Lasers are classified according to the degree of danger based on the temporal, energy and geometric (point or extended source) characteristics of the radiation source and the maximum permissible levels of laser radiation.
Laser Specifications : wavelength, µm; emission line width; radiation intensity (determined by the energy or power of the output beam and expressed in J or W); pulse duration, s; pulse repetition frequency, Hz.
Lasers are widely used for scientific purposes, in practical medicine, as well as in various fields of technology. The areas of laser application are determined by the energy of the laser radiation used:
Biological effect of laser radiation depends on radiation energy E, pulse energy E and, power (energy) density W p( W e), irradiation time t, wavelength l, pulse duration t, pulse repetition frequency f, radiation flux F, surface radiation density E e, radiation intensity I.
Characterized object | Index | Designation | Unit |
Laser beam | Laser energy | E | J |
Laser pulse energy | E and | J | |
Laser power | R | W | |
Energy (power) density of laser radiation | W e , W p | J/cm 2 (W/cm 2) | |
Radiation field | Radiation flux | F, F, R | W |
Surface radiation flux density | E uh | W/m2 | |
Radiation intensity | I, S | W/m2 | |
Radiation source | Emissivity | R e | W/m2 |
Energy radiation force | I e | Tue/Wed | |
Energy brightness | L e | W/m 2 sr | |
Radiation receiver | Irradiance (irradiance) | E e | W/m2 |
Energy quantity of lighting | H e | J/m 2 |
Under the influence of laser radiation, the vital functions of both individual organs and the body as a whole are disrupted. Currently, the specific effect of laser radiation on biological objects has been established, which differs from the effect of other hazardous industrial physical and chemical factors. When exposed to laser radiation on a continuous biological structure (for example, the human body), three stages are distinguished: physical, physicochemical and chemical.
At the first stage ( physical) interactions of radiation with matter occur, the nature of which depends on the anatomical, optical-physical and functional characteristics of the tissues, as well as on the energy and spatial characteristics of the radiation and, above all, on the wavelength and intensity of the radiation. At this stage, the substance is heated, the energy of electromagnetic radiation is transferred into mechanical vibrations, the ionization of atoms and molecules, the excitation and transition of electrons from valence levels to the conduction band, the recombination of excited atoms, etc. When exposed to continuous laser radiation, the thermal mechanism of action predominates mainly, in as a result of which protein coagulation occurs, and at high powers - evaporation of biological tissue. In pulse mode (with pulse duration<10 -2 с) механизм взаимодействия становится более сплошным и приводит к переходу энергии излучения в энергию механических колебаний среды, в частности ударной волны. При мощности излучения свыше 10 7 Вт и высокой степени фокусировки лазерного луча возможно возникновение ионизирующих излучений.
At the second stage ( physico-chemical ) free radicals are formed from ions and excited molecules, which have a high ability for chemical reactions.
At the third stage ( chemical ) free radicals react with the molecules of substances that make up living tissue, and in this case molecular damage occurs, which further determines the overall picture of the effect of laser radiation on the irradiated tissue and the body as a whole. Schematically, the main factors determining the biological effect of laser radiation can be represented as follows:
Laser radiation poses a danger mainly to tissues that directly absorb radiation, therefore, from the standpoint of the potential danger of exposure and the possibility of protection from laser radiation, we mainly consider the eyes and skin.
The cornea and lens of the eye are highly sensitive to electromagnetic radiation, and the optical system of the eye is capable of increasing the energy density of the visible and near-infrared range in the fundus relative to the cornea by several orders of magnitude.
Long-term exposure to laser radiation in the visible range (not much less than the burn threshold) on the retina of the eye can cause irreversible changes in it, and in the near-infrared range it can lead to clouding of the lens. Retinal cells do not recover after damage.
The effect of laser radiation on the skin, depending on the initial absorbed energy, leads to various lesions: from mild erythema (redness) to superficial charring and, ultimately, the formation of deep skin defects.
Distinguish 6 types of exposure to radiation on a living organism :
1) thermal (heat) effect. When laser radiation is focused, a significant amount of heat is released in a small volume in a short period of time;
2) energetic effect. Determined by a large electric field gradient due to high power density. This action can cause polarization of molecules, resonance and other effects.;
3) photochemical action. Manifests itself in the fading of a number of dyes;
4) mechanical action. It manifests itself in the occurrence of ultrasonic-type vibrations in the irradiated body.
5) electrostriction – deformation of molecules in the electric field of laser radiation;
6) formation of a microwave electromagnetic field within the cell.
Energy exposures are accepted as maximum permissible levels (MALs) of radiation exposure. For continuous laser radiation remote control, the energy exposure of the lowest value is selected that does not cause primary and secondary biological effects (taking into account the wavelength and duration of exposure). For pulsed-periodic radiation, the exposure rate is calculated taking into account the repetition rate and exposure to a series of pulses.
When operating lasers, there are other types of hazards in addition to laser radiation. These are the release of harmful chemicals, noise, vibration, electromagnetic fields, ionizing radiation, etc.
INTRODUCTION
1.2 SEMICONDUCTOR LASER
1.3 LIQUID LASER
1.3.1 DYE LASERS
1.4 CHEMICAL LASER AND OTHERS
1.5 POWERFUL LASERS
2. APPLICATION OF LASERS
2.3 HOLOGRAPHY
2.3.3 APPLICATION OF HOLOGRAPHY
CONCLUSION
PRINCIPLE OF OPERATION OF LASERS
Laser radiation is the glow of objects at normal temperatures. But under normal conditions, most atoms are in the lowest energy state. Therefore, at low temperatures substances do not glow. When an electromagnetic wave passes through matter, its energy is absorbed. Due to the absorbed energy of the wave, some of the atoms are excited, that is, they move to a higher energy state. In this case, some energy is taken away from the light beam:
where hν is the value corresponding to the amount of energy spent,
E2 - energy of the highest energy level,
E1 is the energy of the lowest energy level.
An excited atom can give up its energy to neighboring atoms in a collision or emit a photon in any direction. Now let's imagine that in some way we have excited most of the atoms of the medium. Then, when an electromagnetic wave with a frequency passes through a substance
Where v- wave frequency,
E2 - E1 - the difference between the energies of the higher and lower levels,
h- wavelength,
this wave will not be weakened, but, on the contrary, will be amplified due to induced radiation. Under its influence, atoms consistently transform into lower energy states, emitting waves that coincide in frequency and phase with the incident wave.
SEMICONDUCTOR LASER
In the 60s, it was discovered that semiconductors were excellent materials for lasers.
If you connect two wafers of different types of semiconductors together, a transition zone is formed in the middle. The atoms of the substance located in it are capable of being excited when an electric current passes across the zone and generating light. The mirrors required to produce laser radiation can be the polished and silver-plated faces of the semiconductor crystal itself.
Among these lasers, the best is considered to be a laser based on gallium arsenide, a compound of the rare element gallium with arsenic. Its infrared radiation has a power of up to ten watts. If this laser is cooled to the temperature of liquid nitrogen (-200°), its radiation power can be increased tenfold. This means that with an area of the emitting layer of 1 cm2, the radiation power would reach a million watts. But it is not yet possible to produce a semiconductor with a transition layer of this size for technical reasons.
It is possible to excite semiconductor atoms with a beam of electrons (as in solid-state lasers - with a flash lamp). Electrons penetrate deep into the substance, exciting more atoms; the width of the emitting zone turns out to be hundreds of times wider than when excited by electric current. Therefore, the radiation power of such electron-pumped lasers reaches two kilowatts.
The small size of semiconductor lasers makes them very convenient for use where a miniature high-power light source is needed.
LIQUID LASER
In solids it is possible to create a large concentration of radiating atoms and, therefore, obtain greater energy from one cubic centimeter of a rod. But they are difficult to make, expensive, and can also burst due to overheating during operation.
Gases are very homogeneous optically, light scattering in them is small, so the size of a gas laser can be quite impressive: a length of 10 meters with a diameter of 10-20 centimeters is not the limit for it. But such an increase in size does not please anyone. This is a necessary measure necessary in order to compensate for the insignificant amount of active gas atoms located in the laser tube under pressure of hundredths of an atmosphere. Pumping the gas somewhat saves the day by allowing the size of the emitter to be reduced.
Liquids combine the advantages of both solid and gaseous laser materials: their density is only two to three times lower than the density of solids (and not hundreds of thousands of times like the density of gases). Therefore, the number of their atoms per unit volume is approximately the same. This means that a liquid laser can easily be made as powerful as a solid-state laser. The optical homogeneity of liquids is not inferior to the homogeneity of gases, which means it allows the use of large volumes. In addition, the liquid can also be pumped through the working volume, continuously maintaining its low temperature and high activity of its atoms.
DYE LASERS
They are called so because their working fluid is a solution of aniline dyes in water, alcohol, acid and other solvents. The liquid is poured into a flat cuvette. The cuvette is installed between the mirrors. The energy of the dye molecule is pumped optically, only instead of a flash lamp, pulsed ruby lasers were first used, and later gas lasers. The pump laser is not built into the liquid laser, but is placed outside the laser, introducing its beam into the cuvette through a window in the housing. Now we have succeeded in generating light with a flash lamp, but not with all dyes. The solutions can emit pulses of light of varying wavelengths - from ultraviolet to infrared light - and with powers ranging from hundreds of kilowatts to several megawatts (millions of watts), depending on what dye is poured into the cuvette. Dye lasers have one special feature. All lasers emit exactly the same wavelength. This property lies in the very nature of the stimulated emission of atoms, on which the entire laser effect is based. In large and heavy molecules of organic dyes, stimulated emission occurs immediately in a wide band of wavelengths. To achieve monochromaticity from a dye laser, a light filter is placed in the path of the beam. This is not just painted glass. It is a set of glass plates that transmit only light of one wavelength. By changing the distance between the plates, you can slightly change the wavelength of the laser radiation. Such a laser is called tunable. And in order for the laser to generate light in different parts of the spectrum - to move, say, from blue to red light or from ultraviolet to green - it is enough to change the cuvette with the working fluid. They turned out to be most promising for studying the structure of matter. By adjusting the frequency of the radiation, you can find out what wavelength of light is absorbed or scattered along the path of the beam. In this way, you can determine the composition of the atmosphere and clouds at a distance of up to two hundred kilometers, measure the pollution of water or air, immediately indicating the size of the particles polluting it. That is, it is possible to build a device that automatically and continuously monitors the purity of water and air.
But along with broadband liquid lasers, there are also those whose monochromaticity, on the contrary, is much higher than that of solid-state or gas lasers.
The wavelength of laser light can change, shortening and lengthening by about one hundredth (for good lasers). The smaller the distance between the mirrors, the wider this band. For semiconductor lasers, for example, it is already several wavelengths, and for a laser based on neodymium salts this band is one ten-thousandth. Such constancy of the wavelength can only be achieved with large gas lasers, and only if all necessary measures are taken for this: ensure the stability of the temperature of the tube, the strength of the current feeding it, and include in the laser circuit a system for automatically adjusting the wavelength of the radiation. In this case, the radiation power should be minimal: as it increases, the band expands. But in a liquid neodymium laser, a narrow radiation band is obtained by itself and is preserved even with a noticeable increase in the radiation power, and this is extremely important for all kinds of precise measurements.
Therefore, the accuracy of the measurements depends on how accurately the wavelength of the light emitted by the laser is maintained. Reducing the laser radiation bandwidth by a hundred times promises a hundredfold increase in the accuracy of length measurements.
CHEMICAL LASER AND OTHERS
The search for new lasers, new ways to increase the power of laser radiation, is being carried out in different directions. Among them, for example, is a quantum generator with chemical pumping, the first version of which was created at the Institute of Chemical Physics of the USSR Academy of Sciences in the laboratory of Corresponding Member of the Academy of Sciences V. L. Talrose. In such a laser, during the reaction of fluorine F with hydrogen H2 or deuterium D2, the resulting HF or DF molecules move to a high energy level. Descending from this level, they create laser radiation - HF molecules at a wavelength of 2700 nm, DF molecules at a wavelength of 3600 nm. Lasers of this type achieve powers of up to 10 kW.
One of the relatively powerful pulse-periodic gas lasers uses copper vapors at a temperature of 1500°C or, in a simpler version, pairs of copper salts at a temperature of 400°C as a working substance. Pumping is carried out by the energy of electrons moving in a gas discharge. Laser radiation occurs when copper atoms transition from an excited state to one of two metastable states, and radiation is possible at two wavelengths 510.6 nm and 578.2 nm, corresponding to two shades of green. In the resonator, which is an intensively pumped pipe with a diameter of 5 cm and a length of 1 m, a pulse power of 40 kW was achieved with a pulse duration of 15-20 ns, a repetition frequency of 10-100 kHz, an average power of several tens of watts and an efficiency of more than 1% - Work is underway to increase the average power of a “copper” laser to 1 kW.
A special class is formed by powerful dye lasers, the main advantage of which is the ability to smoothly change the frequency. The liquid media used in them have “blurred” energy levels and allow generation at many frequencies. The choice of one of them can be done by changing the parameters of the resonator, for example, by rotating the prism inside it. If powerful radiation sources, in particular pulsed lasers, are used for pumping and intensive circulation of liquid dye is carried out, then it becomes possible to create tunable frequency lasers with an average power of the order of 100 W and a pulse repetition frequency of 10-50 kHz.
When it comes to prospects, the one most often mentioned is the iodine laser, in the resonator of which the compound of iodine, fluorine and carbon CF3J or more complex molecules dissociate and fall apart under the influence of ultraviolet pumping. The separated iodine atoms find themselves in an excited state and subsequently produce infrared laser radiation with a wavelength of 1315 nm. Lasers based on so-called excimer molecules, which generally can only be in an excited state, are also often mentioned. During the pumping process, energy is expended to combine scattered atoms into a molecule, and at the same time it immediately becomes excited, ready to emit. And, having given up its radiation quantum, making a contribution to the formation of the laser beam, the excimer molecule simply disintegrates, its atoms scatter almost instantly. The first excimer laser was created ten years ago in the laboratory of Academician N. G. Basov; ultraviolet laser radiation at a wavelength of 176 nm was obtained here by exciting liquid xenon Xe2 with a powerful electron beam. Five years later, several American laboratories produced laser radiation on other excimer molecules, mainly compounds of inert gases with halogens, for example, XeF, XeCl, XeBr, KrF and others. Excimer lasers operate in both the visible and ultraviolet ranges, and they allow some frequency variation. Lasers have been created with an efficiency of 10% and an energy of 200 J per pulse.
POWERFUL LASERS
One of the main trends in the development of modern applied physics is obtaining ever higher energy densities and finding ways to release it in an ever shorter time. The rapid progress of quantum electronics has led to the creation of a large family of high-power lasers. Powerful lasers have opened up fundamentally new possibilities both for obtaining record high concentrations of energy in space and time, and for a very convenient supply of light energy to matter. Before getting acquainted with the specific results on the creation of high-power lasers, it is useful to remember that they can be divided into three groups - pulsed, pulse-periodic and continuous. The first ones emit light in single pulses, the second ones - in continuous series of pulses, and, finally, the third ones give continuous radiation.
Power is a relative characteristic; it tells us what work is done, what energy is expended or received per unit of time. The unit of power, as is known, is the watt (W) - it corresponds to the energy of 1 J released in 1 second (s). If the release of this energy lasts for 10 seconds, then for each second there will be only 0.1 J and, therefore, the power will be 0.1 W. Well, if 1 J of energy is released in a hundredth of a second, then the power will already be 100 W. Because with such an intensity of the process, 100 J would be released per second. There is no need to pay attention to this “would” - when determining the power, it does not matter that the process lasted only one hundredth of a second and little energy was released during this time. Power does not speak about the complete, final action, but about its intensity, about its concentration in time. If the work went on long enough, at least more than a second, then the power indicates what was actually done in one second.
In a pulsed laser, the radiation lasts very briefly, some insignificant fractions of a second, and even with a small emitted energy, the process turns out to be highly compressed, concentrated in time, and the power is enormous. Here, for example, is what was in the first laser, in the first ruby laser, created in 1960: it emitted a pulse of light with an energy of about 1 J and a duration of 1 ms (millisecond, thousandth of a second), that is, the pulse power was 1 kW. After some time, lasers appeared that emitted the same joule of energy in a much shorter pulse - up to 10 ns (nanosecond, billionth of a second). At the same time, the pulse power with energy of the same joule already reached 100 thousand kW. This is not yet the Kuibyshev hydroelectric station, which has a capacity of 2 million kW, but already a power plant for a small city. With the difference, of course, that the laser develops this enormous power only in billionths of a second, while the power plant develops it continuously around the clock. Current lasers produce pulses with a duration of up to 0.01 ns, and with the same energy of 1 J their power reaches 100 million kW.
A laser beam is a stream of extremely ordered coherent radiation, highly directed, concentrated within a small solid angle. It is for all these qualities that we pay such a high price - the efficiency of lasers is a fraction of a percent, and at best several percent, that is, for each joule of laser radiation we need to spend tens, or even hundreds of joules of pump energy. But often even such a high fee is completely justified - by losing quantity, we gain quality. In particular, coherence, the directionality of the laser beam, combined with subsequent focusing in a very small volume, for example, to a sphere with a diameter of 0.1 mm, and compression of the process in time, that is, radiation in very short pulses, makes it possible to obtain enormous energy densities. Table 1 reminds us of this. The table shows that the energy concentration in a focused powerful laser beam is only a thousand times less than the unique record value for the complete annihilation of matter of normal density, the complete conversion of mass into energy. An increase in laser power is associated with some general problems, primarily with the properties of the working fluid, that is, the substance itself where the radiation is generated. But there are also problems specific to pulsed, pulse-periodic and continuous wave lasers. For example, for pulsed lasers one of the important problems is the durability of optical elements in a strong light field of very short pulses. For continuous and pulse-periodic lasers, the problem of heat removal is very important, since these lasers develop high average power. For a laser operating in long-burst mode, pulse power indicates how concentrated the energy of one pulse is in time, and average power indicates the work performed by a series of pulses lasting a second. So, for example, if a laser produces 20 pulses per second with a duration of 1 ms and an energy of 1 J each, then the pulse power will be 1 kW, and the average power will be 20 W.
All types of lasers began with rather modest energy indicators, and were often improved in different ways. In particular, the first pulsed laser operated in the free generation mode - an avalanche of laser radiation spontaneously arose in it and again stopped by itself after the excitation ended. The pulse lasted a long time by today's standards, and this determined the relatively low pulse power.
A few years later, they learned how to control generation using the Q-switching method, introducing a Kerr cell or another similar element into the resonator, which changes its optical properties under the influence of electrical voltage. In the normal state, the cell is closed, opaque, and a laser avalanche does not occur in the resonator. Only under the influence of a short electrical pulse does the cell open, and a short laser pulse appears in the working fluid. Its duration can be only several times longer than the time it takes for light to travel between the laser mirrors, that is, it can be 10-20 ns.
This method gave a noticeable increase in pulse power by reducing the pulse duration. Very short pulses, up to picosecond pulses, are obtained in the synchronization mode, or, in other words, in the mode capture mode. Here a special nonlinear element is introduced into the resonator; it behaves differently, brightens differently for bursts of radiation of different intensity, and, as it were, cuts out very short picosecond bursts of intensity from a nanosecond light pulse.
APPLICATION OF LASERS
APPLICATION OF LASERS IN MEDICINE
In medicine, laser systems have found their application in the form of a laser scalpel. Its use for surgical operations is determined by the following properties:
1. It makes a relatively bloodless cut, since simultaneously with tissue dissection, it coagulates the edges of the wound by “sealing” not too large blood vessels;
2. The laser scalpel has constant cutting properties. Contact with a hard object (for example, bone) does not disable the scalpel. For a mechanical scalpel, such a situation would be fatal;
3. The laser beam, due to its transparency, allows the surgeon to see the operated area. The blade of an ordinary scalpel, as well as the blade of an electric knife, always to some extent blocks the working field from the surgeon;
4. The laser beam cuts the tissue at a distance without exerting any mechanical effect on the tissue;
5. The laser scalpel ensures absolute sterility, because only radiation interacts with the tissue;
6. The laser beam acts strictly locally, tissue evaporation occurs only at the focal point. Adjacent areas of tissue are damaged significantly less than when using a mechanical scalpel;
7. As clinical practice has shown, a wound caused by a laser scalpel hardly hurts and heals faster.
The practical use of lasers in surgery began in the USSR in 1966 at the A.V. Vishnevsky Institute. The laser scalpel was used in operations on the internal organs of the thoracic and abdominal cavities. Currently, laser beams are used to perform skin plastic surgery, operations of the esophagus, stomach, intestines, kidneys, liver, spleen and other organs. It is very tempting to perform operations using a laser on organs containing a large number of blood vessels, for example, on the heart and liver.
Currently, a new direction in medicine is intensively developing - laser eye microsurgery. Research in this area is being conducted at the Odessa Institute of Eye Diseases named after V.P. Filatov, at the Moscow Research Institute of Eye Microsurgery and in many other “eye centers” of the Commonwealth countries. The first use of lasers in ophthalmology was associated with the treatment of retinal detachment. Light pulses from a ruby laser are sent into the eye through the pupil (pulse energy 0.01 - 0.1 J, duration about 0.1 s). They freely penetrate the transparent vitreous body and are absorbed by the retina. By focusing the radiation on the exfoliated area, the latter is “welded” to the fundus due to coagulation. The operation is quick and completely painless.
In general, there are five of the most serious eye diseases that lead to blindness. These are glaucoma, cataracts, retinal detachment, diabetic retinopathy and malignant tumor. Today, all these diseases are successfully treated with lasers, and three methods have been developed and used only for the treatment of tumors:
1. Laser irradiation - irradiation of a tumor with a defocused laser beam, leading to the death of cancer cells and their loss of ability to reproduce
2. Laser coagulation - destruction of the tumor with moderately focused radiation.
3. Laser surgery is the most radical method. It consists of excision of the tumor along with adjacent tissues using focused radiation.
HOLOGRAPHY
THE EMERGENCE OF HOLOGRAPHY
The photographing method used to save images of objects has been known for quite a long time and now it is the most accessible way to obtain an image of an object on any medium (photographic paper, film). However, the information contained in the photograph is very limited. In particular, there is no information about the distances of various parts of the object from the photographic plate and other important characteristics. In other words, an ordinary photograph does not allow us to completely restore the wavefront that was recorded on it. The photograph contains more or less accurate information about the amplitudes of the recorded waves, but completely lacks information about the phases of the waves. Holography allows you to eliminate this drawback of conventional photography and record on a photographic plate information not only about the amplitudes of the waves incident on it, but also about the phases, that is, complete information. The wave restored using such a recording is completely identical to the original one and contains all the information that the original wave contained. Therefore, the method was called holography, that is, the method of complete wave recording. In order to implement this method in the light range, it is necessary to have radiation with a sufficiently high degree of coherence. Such radiation can be obtained using a laser. Therefore, only after the creation of lasers producing radiation with a high degree of coherence, it was possible to practically implement holography. The idea of holography was put forward back in 1920 by the Polish physicist M. Wolfke (1883-1947), but was forgotten. In 1947, independently of Wolfke, the idea of holography was proposed and substantiated by the English physicist D. Gabor, who was awarded the Nobel Prize for this in 1971.
HOLOGRAPHING METHODS
Speaking about the process of creating a holographic image, it is necessary to highlight the stages of holography:
1. Registration of both amplitude and phase characteristics of the wave field reflected by the object of observation. This registration occurs on photographic plates called holograms.
2. Extracting from the hologram information about the object that is registered on it. To do this, the hologram is illuminated with a light beam.
There are several ways to implement these steps in practice.
The most common of them are the plane wave method and the colliding beam method.
The standard interference pattern is produced by the interference of coherent light waves. Thus, in order to register phase relationships in the wave field, which is obtained as a result of wave reflection by the object of observation, it is necessary that the object be illuminated with monochromatic and spatially coherent radiation. Then the field scattered by the object in space will have these properties.
If you add to the field under study created by the object an auxiliary field of the same frequency, for example, a plane wave (usually called reference wave), then throughout the entire space where both waves intersect, a complex but stationary distribution of regions of mutual amplification and weakening of waves is formed, that is, a stationary interference pattern, which can already be recorded on a photographic plate.
In order to restore a holographic image that has already been recorded on a hologram, the latter must be illuminated with the same laser beam that was used during recording. The image of an object is formed as a result of light diffraction on inhomogeneous blackening of the hologram.
In 1962, the Soviet scientist Yu. N. Denisyuk proposed a method for obtaining holographic images, which is a development of the color holography method, which was practically not used at that time Lippman. The object of observation is illuminated through a photographic plate (it is completely transparent to light even in its undeveloped state). The glass substrate of the photographic plate is coated with a photoemulsion with a layer thickness of about 15 - 20 microns. The wave field reflected from the object propagates back towards the emulsion layer. The initial light beam from the laser traveling towards this wave acts as a reference wave. That is why this method is called the colliding beam method. The interference of waves that occurs in the thickness of the photographic emulsion causes its layered blackening, which registers the distribution of both amplitudes and phases of the wave field scattered by the object of observation. Color holography is based on holography using the method of colliding light beams. To understand the principle of operation of color holography, it is necessary to recall in what cases the human eye perceives an image in color rather than black and white.
Experiments on the physiology of vision have shown that a person sees an image in color or at least close to the natural color of an object if it is reproduced in at least three colors, for example, blue, red and green. The combination of these colors is carried out using the most primitive color reproduction, performed by the method lithographs(for highly artistic reproductions, 10 - 15 color printing is used)
Taking into account the peculiarities of human perception, in order to restore a color image of an object, it is necessary to illuminate the object itself when recording a hologram simultaneously or sequentially with laser radiation of three spectral lines spaced far enough apart in wavelengths. Then, three systems of standing waves and, accordingly, three systems of spatial lattices with different distributions of blackening are formed in the thickness of the photographic emulsion. Each of these systems will form an image of the object in its own spectral region of white color, used in image restoration. Thanks to this, a diverging beam of white light reflected from the processed hologram will produce a color image of the object as a result of the superposition of three sections of the spectrum, which corresponds to the minimum physiological requirements for human vision. Holography using the Denisyuk method is widely used to obtain high-quality three-dimensional copies of various objects, for example, unique works of art.
APPLICATION OF HOLOGRAPHY
As already indicated, the original task of holography was to obtain a three-dimensional image. With the development of holography on thick-layer plates, it became possible to create three-dimensional color photographs. On this basis, ways to implement holographic cinema, television, etc. are being explored. One of the methods of applied holography, called holographic interferometry, has found very widespread use. The essence of the method is as follows. Two interference patterns are sequentially recorded on one photographic plate, corresponding to two different but slightly different states of the object, for example, during deformation. When such a “double” hologram is illuminated, two images of the object are obviously formed, changed relative to each other to the same extent as the object in its two states.
The reconstructed waves forming these two images are coherent, interfere, and interference fringes are observed in the new image, which characterize a change in the state of the object. In another embodiment, a hologram is made for a specific state of an object. When transilluminating it, the object is not removed and it is re-illuminated, as in the first stage of holography. Then again two waves are obtained, one forms a holographic image, and the other propagates from the object itself. If now some changes occur in the state of the object (in two successive waves a difference appears in comparison with what was during the exposure of the hologram), then between the indicated moves, the image is covered with interference fringes.
The described method is used to study the deformations of objects, their vibrations, translational motion and rotation, inhomogeneity of transparent objects, etc. The interference pattern clearly indicates the difference in deformations, stresses in the body, torques, temperature distribution, etc. Holography can be used for ensuring precision processing of parts.
CONCLUSION
The laser is one of the most powerful tools of today's science. It is not possible to list all the areas of its application, since new tasks are found for the laser every day.
This paper examined the main types of lasers and their operating principles. The main areas of application were also covered, namely: industry, medicine, information technology, science.
Such diverse tasks can be performed using laser due to its properties. Coherence, monochromaticity, high energy density make it possible to solve complex technological operations.
The laser is a tool of the future that has already firmly entered our lives.
INTRODUCTION
1. PRINCIPLE OF OPERATION AND TYPES OF LASERS
1.1 BASIC PROPERTIES OF LASER BEAM
1.2 SEMICONDUCTOR LASER
1.3 LIQUID LASER
1.3.1 DYE LASERS
1.4 CHEMICAL LASER AND OTHERS
1.5 POWERFUL LASERS
1.5.1 MULTI-STAGE AND MULTI-CHANNEL SYSTEMS
2. APPLICATION OF LASERS
2.1 APPLICATION OF LASER BEAM IN INDUSTRY AND ENGINEERING
2.2 APPLICATION OF LASERS IN MEDICINE
2.3 HOLOGRAPHY
2.3.1 THE EMERGENCE OF HOLOGRAPHY
2.3.2 HOLOGRAPHING METHODS
2.3.3 APPLICATION OF HOLOGRAPHY
2.4 LASER TECHNOLOGY – A MEANS OF INFORMATION RECORDING AND PROCESSING
CONCLUSION
BIBLIOGRAPHY
PRINCIPLE OF OPERATION OF LASERS
Lasers are based on the phenomenon of stimulated emission, the existence of which was predicted by Einstein in 1917. According to Einstein, along with the processes of conventional radiation and resonant absorption, there is a third process - forced (induced) radiation. Light of a resonant frequency, that is, the frequency that atoms are capable of absorbing when moving to higher energy levels, should cause the glow of atoms already at these levels, if any in the medium. A characteristic feature of this radiation is that the emitted light is indistinguishable from the driving light, that is, it coincides with the latter in frequency, phase, polarization and direction of propagation. This means that stimulated emission adds to the light beam exactly the same quanta of light that resonant absorption removes from it.
Atoms of the medium can absorb light while being at a lower energy level, but they emit light at higher levels. It follows that if there are a large number of atoms at the lower levels (at least more than the number of atoms at the upper levels), light passing through the medium will be attenuated. On the contrary, if the number of atoms at the upper levels is greater than the number of unexcited ones, then the light passing through this medium will intensify. This means that stimulated radiation predominates in this environment. The space between the mirrors is filled with an active medium, that is, a medium containing a greater number of excited atoms (atoms located in the upper energy levels) than non-excited ones. The medium amplifies the light passing through it due to induced radiation, which begins with the spontaneous emission of one of the atoms.
Laser radiation is the glow of objects at normal temperatures. But under normal conditions, most atoms are in the lowest energy state. Therefore, at low temperatures