Structure and principle of operation of the laser. Optically pumped quantum devices operating according to a “three-level scheme”

Quantum generators emitting in the range of visible and infrared radiation are called lasers. The word “laser” is an abbreviation for the expression: Light Amplification by Stimulated Emission of Radiation, which means the amplification of light as a result of induced or, as sometimes called, stimulated emission of quanta.

Laser device

A generalized laser consists of a laser active medium, a “pumping” system - a voltage source and an optical cavity.

The pumping system transfers energy to atoms or molecules of the laser medium, giving them the opportunity to go into an excited “metastable state” creating a population inversion.

· Optical pumping uses photons provided by a source, such as a xenon gas-filled flash lamp or other laser, to transfer energy to the laser substance. The optical source must provide photons that match the acceptable transition levels in the laser material.

· Collision pumping is based on the transfer of energy to a laser substance as a result of collisions with atoms (or molecules) of the laser substance. At the same time, energy corresponding to permissible transitions must also be provided. This is usually accomplished by using an electrical discharge in a pure gas or a mixture of gases in a tube.

· Chemical pumping systems use the binding energy released as a result of chemical reactions to transform the laser substance into a metastable state.

An optical cavity is required to provide the desired force in the laser and to select photons that move in the desired direction. When the first atom or molecule in a metastable state of population inversion is discharged, due to stimulated emission, it initiates the discharge of other atoms or molecules in a metastable state. If photons travel toward the walls of the laser substance, usually a rod or tube, they are lost and the amplification process is interrupted. Although they may be reflected from the walls of the rod or pipe, sooner or later they will be lost from the system and will not contribute to the creation of the beam.

On the other hand, if one of the destroyed atoms or molecules releases a photon parallel to the axis of the laser substance, it can initiate the release of another photon, and they will both be reflected by a mirror at the end of the generating rod or tube. The reflected photons then pass back through the substance, initiating further radiation along exactly the same path, which is again reflected by the mirrors at the ends of the laser substance. As long as this amplification process continues, some of the amplification will always exit through the partially reflective mirror. As the gain or gain of this process exceeds the losses from the cavity, lasing begins. Thus, a narrow, concentrated beam of coherent light is formed. The mirrors in the laser optical cavity must be precisely adjusted to ensure that the light rays are parallel to the axis. The optical resonator itself, i.e. the substance of the medium should not strongly absorb light energy.

Laser Medium (Lasing Material) – Lasers are usually designated by the type of laser substance used. There are four such types:

solid,

Dye,

Semiconductor.

Solid-state lasers use laser material distributed in a solid matrix. Solid-state lasers occupy a unique place in laser development. The first working laser medium was a pink ruby ​​crystal (sapphire crystal doped with chromium); since then, the term "solid-state laser" has generally been used to describe a laser whose active medium is a crystal doped with ion impurities. Solid-state lasers are large, easy-to-maintain devices capable of generating high power energy. The most remarkable thing about solid-state lasers is that the output power is usually not constant, but consists of a large number of individual power peaks.

One example is the Neodymium-YAG laser. The term YAG is short for the crystal: yttrium aluminum garnet, which serves as a carrier for neodymium ions. This laser emits an infrared beam with a wavelength of 1,064 micrometers. In addition, other doping elements can be used, such as erbium (Er:YAG lasers).

Gas lasers use gas or a mixture of gases in a tube. Most gas lasers use a mixture of helium and neon (HeNe), with a primary output signal of 6,328 nm (nm = 10-9 meters), visible red. This laser was first developed in 1961 and became the forerunner of a whole family of gas lasers.

All gas lasers are quite similar in design and properties. For example, a CO2 gas laser emits a wavelength of 10.6 micrometers in the far infrared region of the spectrum. Argon and krypton gas lasers operate at multiple frequencies, emitting predominantly in the visible part of the spectrum. The main wavelengths of argon laser radiation are 488 and 514 nm.

Dye lasers use a laser medium that is a complex organic dye in a liquid solution or suspension.

The most significant feature of these lasers is their “adaptability”. The correct choice of dye and its concentration allows laser light to be generated over a wide range of wavelengths in or near the visible spectrum. Dye lasers typically use an optical excitation system, although some types of dye lasers use chemical excitation.

Semiconductor (diode) lasers - consist of two layers of semiconductor material stacked together. A laser diode is a light-emitting diode with an optical capacitance to amplify the emitted light from a backlash in a semiconductor rod, as shown in the figure. They can be tuned by changing the applied current, temperature or magnetic field.

The different time modes of laser operation are determined by the frequency at which the energy is supplied.

Continuous wave (CW) lasers operate with a constant average beam power.

Single-pulse lasers typically have pulse durations ranging from several hundred microseconds to several milliseconds. This mode of operation is usually called long pulse or normal mode.

Single-pulse Q-switched lasers are the result of an intracavity delay (Q-switched cell), which allows the laser medium to retain maximum potential energy. Then, under the most favorable conditions, single pulses are emitted, usually with a time interval of 10-8 seconds. These pulses have high peak power, often in the range of 106 to 109 watts.

Pulsed pulsed lasers, or scanning lasers, operate in principle the same as pulsed lasers, but at a fixed (or variable) pulse rate that can vary from a few pulses per second to as high as 20,000 pulses per second.

Laser operating principle

The physical basis of laser operation is the phenomenon of forced (induced) radiation. The essence of the phenomenon is that an excited atom is capable of emitting a photon under the influence of another photon without its absorption, if the energy of the latter is equal to the difference in the energies of the levels of the atom before and after the radiation. In this case, the emitted photon is coherent with the photon that caused the radiation (it is its “exact copy”). This way the light is amplified. This phenomenon differs from spontaneous emission, in which the emitted photons have random propagation directions, polarization and phase.

The probability that a random photon will cause stimulated emission from an excited atom is exactly equal to the probability of absorption of this photon by an atom in an unexcited state. Therefore, to amplify light, it is necessary that there be more excited atoms in the medium than unexcited ones (the so-called population inversion). In a state of thermodynamic equilibrium, this condition is not met, therefore various systems for pumping the laser active medium (optical, electrical, chemical, etc.) are used.

The primary source of generation is the process of spontaneous emission, therefore, to ensure the continuity of generations of photons, the existence of a positive feedback is necessary, due to which the emitted photons cause subsequent acts of induced emission. To do this, the laser active medium is placed in an optical cavity. In the simplest case, it consists of two mirrors, one of which is translucent - through it the laser beam partially exits the resonator. Reflecting from the mirrors, the radiation beam passes repeatedly through the resonator, causing induced transitions in it. The radiation can be either continuous or pulsed. At the same time, using various devices (rotating prisms, Kerr cells, etc.) to quickly turn the feedback off and on and thereby reduce the period of the pulses, it is possible to create conditions for generating radiation of very high power (the so-called giant pulses). This mode of laser operation is called Q-switched mode.

The radiation generated by a laser is monochromatic (one or a discrete set of wavelengths), since the probability of emission of a photon of a certain wavelength is greater than that of a closely located one, associated with the broadening of the spectral line, and, accordingly, the probability of induced transitions at this frequency also has a maximum. Therefore, gradually during the generation process, photons of a given wavelength will dominate over all other photons. In addition, due to the special arrangement of the mirrors, only those photons that propagate in a direction parallel to the optical axis of the resonator at a short distance from it are retained in the laser beam; the remaining photons quickly leave the resonator volume. Thus, the laser beam has a very small divergence angle. Finally, the laser beam has a strictly defined polarization. To do this, various polaroids are introduced into the resonator; for example, they can be flat glass plates installed at a Brewster angle to the direction of propagation of the laser beam.

Applications of lasers

laser quantum generator radiation

Since their invention, lasers have established themselves as “ready solutions to yet unknown problems.” Due to the unique properties of laser radiation, they are widely used in many branches of science and technology, as well as in everyday life (CD players, laser printers, barcode readers, laser pointers, etc.). In industry, lasers are used for cutting, welding and soldering parts made of various materials. The high temperature of the radiation allows you to weld materials that cannot be welded using conventional methods (for example, ceramics and metal). The laser beam can be focused into a point with a diameter of the order of a micron, which makes it possible to use it in microelectronics (so-called laser scribing). Lasers are used to obtain surface coatings of materials (laser alloying, laser surfacing, vacuum laser deposition) in order to increase their wear resistance. Laser marking of industrial designs and engraving of products made from various materials are also widely used. During laser processing of materials, there is no mechanical impact on them, so only minor deformations occur. In addition, the entire technological process can be fully automated. Laser processing is therefore characterized by high precision and productivity.

A semiconductor laser used in the image generation unit of a Hewlett-Packard printer.

Lasers are used in holography to create holograms themselves and obtain a holographic three-dimensional image. Some lasers, such as dye lasers, are capable of generating monochromatic light of almost any wavelength, and the radiation pulses can reach 10−16 s, and therefore enormous powers (the so-called giant pulses). These properties are used in spectroscopy, as well as in the study of nonlinear optical effects. Using a laser, it was possible to measure the distance to the Moon with an accuracy of several centimeters. Laser ranging of space objects clarified the meaning of the astronomical constant and contributed to the refinement of space navigation systems, expanded the understanding of the structure of the atmosphere and surface of the planets of the Solar System. In astronomical telescopes equipped with an adaptive optical system for correcting atmospheric distortions, lasers are used to create artificial guide stars in the upper layers of the atmosphere.

Ultrashort laser pulses are used in laser chemistry to trigger and analyze chemical reactions. Here, laser radiation allows for precise localization, dosage, absolute sterility and high speed of energy input into the system. Currently, various laser cooling systems are being developed, and the possibilities of implementing controlled thermonuclear fusion using lasers are being considered (the most suitable laser for research in the field of thermonuclear reactions would be a laser using wavelengths in the blue part of the visible spectrum). Lasers are also used for military purposes, for example, as guidance and aiming aids. Options for creating air, sea and ground-based combat defense systems based on high-power lasers are being considered.

In medicine, lasers are used as bloodless scalpels and are used in the treatment of ophthalmic diseases (cataracts, retinal detachment, laser vision correction, etc.). They are also widely used in cosmetology (laser hair removal, treatment of vascular and pigmented skin defects, laser peeling, removal of tattoos and age spots). Currently, the so-called laser communication is rapidly developing. It is known that the higher the carrier frequency of a communication channel, the greater its throughput. Therefore, radio communications tend to move to ever shorter wavelengths. The wavelength of light is on average six orders of magnitude shorter than the wavelength of the radio range, so laser radiation can transmit a much larger amount of information. Laser communication is carried out through both open and closed light-guide structures, for example, optical fiber. Due to the phenomenon of total internal reflection, light can propagate through it over long distances, practically without weakening.

Everyday production and scientific activities. Over the years, this “tool” will become more and more improved, and at the same time the scope of lasers will continuously expand. The increasing pace of research in the field of laser technology is opening up the possibility of creating new types of lasers with significantly improved characteristics, allowing them to expand their areas of application in...

Not only for particularly hard materials, but also for materials characterized by increased fragility. The laser drill turned out to be not only a powerful, but also a very delicate “tool.” Example: the use of a laser when drilling holes in chip substrates made of alumina ceramics. Ceramics are unusually fragile. For this reason, mechanical drilling of holes in the chip substrate...

The laser necessarily consists of three main components:

1) active medium, in which states with population inversion are created;

2) systemspumping− devices for creating inversion in the active medium;

3) opticalabout the resonator− a device that shapes the direction of the photon beam.

In addition, the optical resonator is designed for multiple amplification of laser radiation.

Currently as active (working) environments lasers use different aggregate states of matter: solid, liquid, gaseous, plasma.

To create an inverse population of the laser environment, various pumping methods . The laser can be pumped either continuously or pulsed. In long-term (continuous) mode, the pump power introduced into the active medium is limited by overheating of the active medium and related phenomena. In the single pulse mode, it is possible to introduce significantly more energy into the active medium than during the same time in the continuous mode. This results in greater power of a single pulse.

Laser- this is a light source with properties that differ sharply from all other sources (incandescent lamps, fluorescent lamps, flames, natural luminaries, and so on). The laser beam has a number of remarkable properties. It spreads over long distances and has a strictly linear direction. The beam moves in a very narrow beam with a low degree of divergence (it reaches the moon with a focus of hundreds of meters). The laser beam has great heat and can punch a hole in any material. The light intensity of the beam is greater than the intensity of the strongest light sources.
Name laser is an abbreviation of the English phrase: Light Amplification by Stimulated Emission of Radiation (LASER). light amplification using stimulated emission.
All laser systems can be divided into groups depending on the type of active medium used. The most important types of lasers are:

solid state

semiconductor

liquid

gas
An active medium is a collection of atoms, molecules, ions or a crystal (semiconductor laser), which under the influence of light can acquire amplifying properties.

So, each atom has a discrete set of energy levels. The electrons of an atom located in the ground state (state with minimal energy), when absorbing light quanta, move to a higher energy level - the atom is excited; When a light quantum is emitted, the opposite happens. Moreover, the emission of light, that is, the transition to a lower energy level (Fig. 1b) can occur spontaneously (spontaneously) or under the influence of external radiation (forced) (Fig. 1c). Moreover, if quanta of spontaneous radiation are emitted in random directions, then a quantum of stimulated radiation is emitted in the same direction as the quantum that caused this radiation, that is, both quanta are completely identical.

Fig.1 Types of laser radiation

In order for transitions in which energy emission occurs (transitions from an upper energy level to a lower one) to prevail, it is necessary to create an increased concentration of excited atoms or molecules (to create a population inversion). This will lead to an increase in the light incident on the substance. The state of a substance in which an inverse population of energy levels is created is called active, and a medium consisting of such a substance is called an active medium.

The process of creating an inverse population of levels is called pumping. And another classification of lasers is made according to the pumping method (optical, thermal, chemical, electrical, etc.). Pumping methods depend on the type of laser (solid-state, liquid, gas, semiconductor, etc.).
The main task of the pumping process can be considered using the example of a three-level laser (Fig. 2)

Fig. 2 diagram of a three-level laser

The lower laser level I with energy E1 is the main energy level of the system, at which all active atoms are initially located. Pumping excites atoms and, accordingly, transfers them from ground level I to level III, with energy E3. Atoms that find themselves at level III emit light quanta and move to level I, or quickly move to the upper laser level II. In order for the accumulation of excited atoms to occur at the upper laser level II, with energy E2, it is necessary to have a rapid relaxation of atoms from level III to II, which must exceed the decay rate of the upper laser level II. The inverted population created in this way will provide conditions for amplification of radiation.

However, for generation to occur, it is still necessary to provide feedback, that is, so that stimulated emission, once arising, causes new acts of stimulated emission. To create such a process, the active medium is placed in an optical resonator.

An optical resonator is a system of two mirrors, between which the active medium is located (Fig. 3). It provides multiple origins of light waves propagating along its axis through the amplifying medium, as a result of which high radiation power is achieved.

Fig.3 Laser diagram

When a certain power is reached, the radiation exits through a translucent mirror. Due to the participation in the development of generation of only that part of the quanta that are parallel to the axis of the resonator, the efficiency. lasers usually does not exceed 1%. In some cases, sacrificing certain characteristics, efficiency. can be increased to 30%.

The diagram shows: 1 - active medium; 2 - laser pump energy; 3 - opaque mirror; 4 - translucent mirror; 5 - laser beam.

All lasers consist of three main parts:

active (working) environment;

pumping systems (energy source);

optical resonator (may be absent if the laser operates in amplifier mode).

Each of them ensures that the laser performs its specific functions.

Active environment

Currently, various aggregate states of matter are used as the working medium of a laser: solid, liquid, gaseous, plasma. In the normal state, the number of atoms located at excited energy levels is determined by the Boltzmann distribution:

Here N- the number of atoms in an excited state with energy E, N 0 - number of atoms in the ground state, k- Boltzmann constant, T- environment temperature. In other words, there are fewer such atoms in the excited state than in the ground state, therefore the probability that a photon propagating through the medium will cause stimulated emission is also small compared to the probability of its absorption. Therefore, an electromagnetic wave, passing through a substance, expends its energy to excite atoms. The radiation intensity decreases according to Bouguer’s law:

Here I 0 - initial intensity, I l is the intensity of radiation traveling the distance l in matter a 1 is the absorption rate of the substance. Since the dependence is exponential, the radiation is absorbed very quickly.

In the case when the number of excited atoms is greater than non-excited ones (that is, in a state of population inversion), the situation is exactly the opposite. Acts of stimulated emission prevail over absorption, and the radiation increases according to the law:

Where a 2 - quantum gain factor. In real lasers, amplification occurs until the amount of energy received due to stimulated emission becomes equal to the amount of energy lost in the resonator. These losses are associated with the saturation of the metastable level of the working substance, after which the pumping energy is used only to heat it, as well as with the presence of many other factors (scattering by inhomogeneities of the medium, absorption by impurities, imperfection of reflecting mirrors, useful and unwanted radiation into the environment, etc.).

Pumping system

Various mechanisms are used to create an inverse population of the laser medium. In solid-state lasers, it is carried out by irradiation with powerful gas-discharge flash lamps, focused solar radiation (the so-called optical pumping) and radiation from other lasers (in particular, semiconductor ones). In this case, it is possible to operate only in a pulsed mode, since very high pump energy densities are required, which cause a strong heating and destruction of the working substance rod during prolonged exposure. Gas and liquid lasers use electric discharge pumping. Such lasers operate in continuous mode. Pumping chemical lasers occurs through the occurrence of chemical reactions in their active medium. In this case, population inversion occurs either directly in the reaction products or in specially introduced impurities with a suitable structure of energy levels. Pumping of semiconductor lasers occurs under the influence of a strong forward current through the p-n junction, as well as a beam of electrons. There are other pumping methods (gas-dynamic, which involve sharp cooling of preheated gases; photodissociation, a special case of chemical pumping, etc.).

In the figure: a - three-level and b - four-level pumping circuits for the laser active medium.

The classic three-level system for pumping the working medium is used, for example, in a ruby ​​laser. Ruby is a corundum crystal Al 2 O 3 doped with a small amount of chromium ions Cr 3+, which are the source of laser radiation. Due to the influence of the electric field of the corundum crystal lattice, the external energy level of chromium E 2 is split (see Stark effect). This is what makes it possible to use non-monochromatic radiation as pumping. In this case, the atom passes from the ground state with energy E 0 in excited with energy about E 2. An atom can remain in this state for a relatively short time (about 10−8 s); a nonradiative transition to the level occurs almost immediately E 1, where an atom can remain for much longer (up to 10 −3 s), this is the so-called metastable level. The possibility arises of induced radiation under the influence of other random photons. As soon as there are more atoms in a metastable state than in the main state, the generation process begins.

It should be noted that to create a population inversion of chromium atoms Cr using pumping directly from the level E 0 per level E 1 is not possible. This is due to the fact that if absorption and stimulated emission occur between two levels, then both of these processes proceed at the same rate. Therefore, in this case, pumping can only equalize the populations of the two levels, which is insufficient for generation to occur.

In some lasers, for example, in neodymium lasers, in which radiation is generated by Nd 3+ neodymium ions, a four-level pumping scheme is used. Here between metastable E 2 and main level E 0 there is an intermediate - working level E 1 . Stimulated emission occurs when an atom transitions between levels E 2 and E 1 . The advantage of this scheme is that in this case it is easy to fulfill the population inversion condition, since the lifetime of the upper working level ( E 2) several orders of magnitude longer than the lifetime of the lower level ( E 1). This significantly reduces the requirements for the pump source. In addition, such a scheme makes it possible to create high-power continuous-wave lasers, which is very important for some applications. However, such lasers have a significant drawback in the form of low quantum efficiency, which is defined as the ratio of the energy of the emitted photon to the energy of the absorbed pump photon (η quantum = hν radiation / hν pump)

It is difficult nowadays to find a person who has never heard the word "laser", however, very few clearly understand what it is.

In the half century since their invention, lasers of various types have found application in a wide range of areas, from medicine to digital technology. So what is a laser, what is its principle of operation, and what is it for?

What is a laser?

The possibility of the existence of lasers was predicted by Albert Einstein, who back in 1917 published a paper talking about the possibility of electrons emitting light quanta of a certain length. This phenomenon was called stimulated emission, but for a long time it was considered unrealizable from a technical point of view.

However, with the development of technical and technological capabilities, the creation of a laser became a matter of time. In 1954, Soviet scientists N. Basov and A. Prokhorov received the Nobel Prize for creating a maser - the first microwave generator operating on ammonia. And in 1960, the American T. Maiman produced the first quantum generator of optical beams, which he called a laser (Light Amplification by Stimulated Emission of Radiation). The device converts energy into narrow-directional optical radiation, i.e. light beam, a stream of light quanta (photons) of high concentration.

Laser operating principle

The phenomenon on which the operation of a laser is based is called forced, or induced, radiation of the medium. Atoms of a certain substance can emit photons under the influence of other photons, and the energy of the acting photon must be equal to the difference between the energy levels of the atom before and after radiation.

The emitted photon is coherent with the one that caused the radiation, i.e. exactly like the first photon. As a result, the weak flow of light in the medium is amplified, and not chaotically, but in one given direction. A beam of stimulated radiation is formed, which is called a laser.

Laser classification

As the nature and properties of lasers were studied, various types of these rays were discovered. Depending on the state of the initial substance, lasers can be:

• gas;
• liquid;
• solid state;
• on free electrons.

Currently, several methods have been developed for producing a laser beam:

• using an electric glow or arc discharge in a gaseous environment - gas discharge;
• using the expansion of hot gas and the creation of population inversions - gas-dynamic;
• by passing current through a semiconductor with excitation of the medium - diode or injection;
• by optical pumping of the medium with a flash lamp, LED, other laser, etc.;
• by electron beam pumping of the medium;
• nuclear pumping when radiation comes from a nuclear reactor;
• using special chemical reactions - chemical lasers.

All of them have their own characteristics and differences, thanks to which they are used in various fields of industry.

Practical use of lasers

Today, lasers of various types are used in dozens of industries, medicine, IT technologies and other fields of activity. With their help, the following is carried out:

• cutting and welding of metals, plastics, and other materials;
• applying images, inscriptions and marking the surface of products;
• drilling ultra-thin holes, precision machining of semiconductor crystal parts;
• formation of product coatings by spraying, surfacing, surface alloying, etc.;
• transmission of information packets using fiberglass;
• performing surgical operations and other therapeutic interventions;
• cosmetic procedures for skin rejuvenation, removal of defective formations, etc.;
• targeting various types of weapons, from small arms to missiles;
• creation and use of holographic methods;
• application in various research works;
• measurement of distances, coordinates, density of working media, flow speed and many other parameters;
• launching chemical reactions to carry out various technological processes.

There are many more areas in which lasers are already used or will find application in the very near future.