It is difficult in our time to find a person who would never hear the word "laser", however, very few clearly understand what it is.

Half a century since the invention of lasers different types found application in a wide range of areas, from medicine to digital technology. So what is a laser, what is the principle of its 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 has become a matter of time. In 1954, Soviet scientists N. Basov and A. Prokhorov received Nobel Prize for the development of the maser, the first microwave generator powered by ammonia. And in 1960, the American T. Maiman manufactured the first quantum generator of optical rays, which he called a laser (Light Amplification by Stimulated Emission of Radiation). The device converts energy into optical radiation of a narrow direction, i.e. light beam, a stream of light quanta (photons) of high concentration.

The principle of operation of the laser

The phenomenon on which the operation of the laser is based is called stimulated, or induced, radiation of the medium. Atoms of a certain substance can emit photons under the action of other photons, while 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 to the one that caused the emission, i.e. exactly like the first photon. As a result, a weak light flux in the medium is amplified, and not randomly, but in one given direction. A beam of stimulated radiation is formed, which is called a laser.

Classification of lasers

As the nature and properties of lasers were studied, various types of these beams were discovered. According to the state of the initial substance, lasers can be:

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

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

• with the help of an electric glow or arc discharge in a gaseous medium - gas discharge;
• by expanding hot gas and creating population inversions - gas dynamic;
• by passing current through a semiconductor with excitation of the medium - diode or injection;
• by optically pumping the medium with a flash lamp, LED, another laser, etc.;
• by electron-beam pumping of the medium;
• nuclear pumping upon receipt of radiation from a nuclear reactor;
• with the help of special chemical reactions– chemical lasers.

All of them have their own characteristics and differences, due to which they are used in various industries.

Practical use of lasers

To date, lasers different types are used in dozens of industries, medicine, IT technologies and other fields of activity. They are used to:

• cutting and welding of metals, plastics, other materials;
• drawing images, inscriptions and marking the surface of products;
• drilling of ultrathin holes, precision processing of semiconductor crystalline parts;
• formation of product coatings by spraying, surfacing, surface alloying, etc.;
• transmission of information packets using fiberglass;
• performance of surgical operations and other therapeutic effects;
• cosmetic procedures for skin rejuvenation, removal of defective formations, etc.;
• targeting various kinds weapons, from small arms to rocket weapons;
• creation and use of holographic methods;
• application in various research projects;
• measurement of distances, coordinates, density of working media, flow rates and many other parameters;
• launch of chemical reactions for carrying out various technological processes.

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

All materials for which population inversion can be provided can be used as a laser medium. This is possible with the following materials:

a) free atoms, ions, molecules, ions of molecules in gases or vapors;

b) dye molecules dissolved in liquids;

c) atoms, ions embedded in a solid body;

d) doped semiconductors;

e) free electrons.

The number of media that are capable of generating laser radiation and the number of laser transitions is very large. About 200 different laser transitions are observed in the neon element alone. According to the type of laser active medium, gas, liquid, semiconductor and solid-state lasers are distinguished. As a curiosity, it should be noted that human breath, consisting of carbon dioxide, nitrogen and water vapor, is a suitable active medium for a weak CO 2 laser, and some varieties of gin have already generated laser radiation, since they contain a sufficient amount of quinine with blue fluorescence.

Laser generation lines are known from the ultraviolet region of the spectrum (100 nm) to millimeter wavelengths in the far infrared range. Lasers smoothly transition into masers. Intensive research is being carried out in the field of lasers in the range of X-ray waves (Fig. 16). But only two or three dozen types of laser have acquired practical significance. CO 2 lasers, argon and krypton ion lasers, CW and pulsed Nd:YAG lasers, CW and pulsed dye lasers, He-Ne lasers and GaAs lasers have now found the widest medical application. Excimer lasers, frequency doubling Nd:YAG lasers, Er:YAG lasers and metal vapor lasers are also increasingly used in medicine.

Rice. 16. Types of lasers most commonly used in medicine.

In addition, laser active media can be distinguished by whether they form discrete laser lines, i.e. only in a very narrow specific range of wavelengths, or radiate continuously over a wide range of wavelengths. Free atoms and ions have, due to their well-defined energy levels, discrete laser lines. Many solid-state lasers also emit on discrete lines (ruby lasers, Nd:YAG lasers). However, solid-state lasers have also been developed (colour-center lasers, alexandrite, diamond lasers), whose radiation wavelengths can vary continuously over a large spectral region. This applies in particular to dye lasers, in which this technique has progressed to the greatest extent. Due to the band structure of the energy levels of semiconductors, semiconductor lasers also do not have discrete clear laser generation lines.

Population inversion in lasers is created in different ways. Most often, light irradiation (optical pumping), electric discharge, electric current, and chemical reactions are used for this.

In order to switch from the amplification mode to the light generation mode, the laser, as in any generator, uses feedback. Feedback in the laser is carried out using an optical resonator, which in the simplest case is a pair of parallel mirrors.

Schematic diagram of the laser is shown in fig. 6. It contains an active element, a resonator, and a pump source.

The laser works as follows. First, a pumping source (for example, a powerful flash lamp), acting on the working substance (active element) of the laser, creates a population inversion in it. Then the inverted medium begins to spontaneously emit light quanta. Under the action of spontaneous emission, the process of stimulated emission of light begins. Owing to the population inversion, this process has an avalanche-like character and leads to an exponential amplification of light. Streams of light traveling in lateral directions quickly leave the active element without having time to gain significant energy. At the same time, a light wave propagating along the axis of the resonator repeatedly passes through the active element, continuously gaining energy. Due to the partial transmission of light by one of the resonator mirrors, the radiation is output to the outside, forming a laser beam.

Fig.6. Schematic diagram of the laser. 1 - active element; 2- pumping system;

3- optical resonator; 4 - generated radiation.

§5. The device and operation of a helium-neon laser

Fig.7. Schematic diagram of a helium - neon laser.

one). The laser consists of a gas-discharge tube T with a length of several tens of cm to 1.5-2m and an inner diameter of 7-10mm. The tube is filled with a mixture of helium (pressure ~1mmHg) and neon (pressure ~0.1mmHg). The ends of the tube are closed with plane-parallel glass or quartz plates P 1 and P 2 installed at a Brewster angle to its axis. This creates a linear polarization of laser radiation with an electric vector parallel to the plane of incidence. Mirrors S 1 and S 2 , between which the tube is placed, are usually made spherical with multilayer dielectric coatings. They have high reflectivity and practically do not absorb light. The transmittance of a mirror, through which the laser radiation predominantly exits, is usually 2%, while that of another mirror is less than 1%. A constant voltage of 1-2 kV is applied between the electrodes of the tube. The cathode K of the tube can be cold, but to increase the discharge current, tubes with a hollow cylindrical anode are also used, the cathode of which is heated by a low-voltage current source. The discharge current in the tube is several tens of milliamps. The laser generates red light with a wavelength of =632.8 nm and can also generate infrared radiation with wavelengths of 1.15 and 3.39 µm (see Fig. 2). But then it is necessary to have end windows that are transparent to infrared light and mirrors with high reflection coefficients in the infrared region.

2). In lasers, stimulated emission is used to generate coherent light waves. The idea of ​​this was first expressed in 1957 by A.M. Prokhorov, N.G. Basov and, independently of them, Ch. Towns. In order to turn the active substance of the laser into a generator of light vibrations, it is necessary to implement feedback. This means that part of the emitted light must always return to the zone of the active substance and cause stimulated emission of more and more new atoms. To do this, the active substance is placed between two mirrors S 1 and S 2 (see Fig. 7), which are feedback elements. A beam of light, undergoing multiple reflections from mirrors S 1 and S 2, will pass many times through the active substance, while being amplified as a result of forced transitions from a higher energy level " 3 to a lower level  " 1 . This results in an open resonator, in which the mirrors provide multiple passage (and thus amplification) of the light flux in the active medium. In a real laser, some of the light must be emitted from the active medium to the outside in order to be used. For this purpose, one of the mirrors, for example S 2 , is made translucent.

Such a resonator will not only amplify the light, but also collimate and monochromatize it. For simplicity, we first assume that the mirrors S 1 and S 2 are ideal. Then the rays, parallel to the axis of the cylinder, will pass through the active substance back and forth an unlimited number of times. However, oblique beams will eventually strike the side wall of the cylinder, where they will dissipate or escape. It is therefore clear that rays propagating parallel to the axis of the cylinder will be maximally amplified. This explains the collimation of rays. Of course, strictly parallel rays cannot be obtained. This is prevented by the diffraction of light. The divergence angle of the beams cannot, in principle, be less than the diffraction limit  D, where D- beam width. However, in the best gas lasers this limit is practically reached.

Let us now explain how the monochromatization of light occurs. Let Z is the optical path length between the mirrors. If a 2 Z= m , that is, on the length Z fits an integer number of half-waves m, then the light wave, leaving S 1, after passing back and forth will return to S 1 in the same phase. Such a wave will intensify during the second and all subsequent passages through the active substance in the forward and reverse directions. nearest wavelength  , for which the same amplification should occur, can be found from the condition 2 Z=(m 1)( ). Consequently,  =  / m, that is  , as expected, coincides with the spectral region of the Fabry-Perot interferometers. Let us now take into account that the energy levels " 3 and  " 1 and the spectral lines that appear during transitions between them are not infinitely thin, but have a finite width. Let us assume that the width of the spectral line emitted by atoms is smaller than the dispersed region of the device. Then, of all wavelengths emitted by atoms, the condition 2 Z= m can only satisfy one wavelength  . Such a wave will intensify as much as possible. This leads to a narrowing of the spectral lines generated by the laser, that is, to the monochromatization of light.

The main properties of a laser light beam:

monochromaticity;

spatial and temporal coherence;

high intensity;

low beam divergence.

Due to its high coherence, the helium-neon laser serves as an excellent source of continuous monochromatic radiation for studying all kinds of interference and diffraction phenomena, the implementation of which with conventional light sources requires the use of special equipment.

Let us first consider a four-level laser with, for simplicity, only one pump absorption band (band 3 in Fig. 5.1). However, the subsequent analysis will remain unchanged even if we deal with more than one pump absorption band (or level), provided that the relaxation from these bands to the upper laser level 2 is very fast. Denote

the populations of the four levels 0, 1, 2, and 3, respectively, through We assume that the laser generates only in one resonator mode. Let be the total number of photons in the resonator. Assuming that transitions between levels 3 and 2 and levels 1 and 0 are fast, we can put . Thus, we have the following rate equations:

In equation (5.1a), the quantity is the total number of active atoms (or molecules). In equation (5.16), the term takes into account pumping [see equation (1.10)]. Explicit expressions for the pumping rate for both optical and electrical pumping have already been obtained in Chap. 3. In the same equation, the term corresponds to stimulated emission. The speed of stimulated emission as shown in Chap. 2 is indeed proportional to the square of the electric field of the electromagnetic wave and is therefore proportional. Therefore, the coefficient B can be considered as the speed of stimulated emission per photon in the mode. The quantity is the lifetime of the upper laser level and, in general case is determined by expression (2.123). In equation (5.1 c), the term corresponds to the rate of change in the number of photons due to stimulated emission. Indeed, as we have already seen, the term in equation (5.16) is the rate of population decrease due to stimulated emission. Since each act of stimulated emission leads to the appearance of a photon, the rate of increase in the number of photons should be equal to where is the volume occupied by the mode inside the active medium (the exact definition of the mode volume is given below). Finally, the term [where is the photon lifetime (see Section 4.3)] takes into account the decrease in the number of photons due to losses in the resonator.

Rice. 5.1. Scheme of energy levels of a four-level laser.

A rigorous definition of the mode volume requires a detailed discussion, which is given in Appendix B. As a result, we have the following definition

where is the distribution of the electric field inside the resonator, E is the maximum value of this field, and integration is performed over the volume occupied by the active medium. If a resonator with two spherical mirrors is considered, then the ratio is equal to the real part of expression (4.95). It is appropriate to cite as an example a symmetric resonator consisting of two mirrors whose radii of curvature are much greater than the length of the resonator. Then the size of the mode spot will be approximately constant along the entire length of the resonator and equal to the value at the center of the resonator. Similarly, the radius of curvature of the equiphase surfaces will be sufficiently large and the wave fronts can be considered flat. Then from expression (4.95) for the mode we obtain

here we set From expressions (5.2) and (5.3) we have

where is the length of the active medium. When deriving this expression, we took into account the fact that is a slowly varying function compared to so that we can put Thus, the appearance of a quadruple in the denominator of expression (5.4) is the result of the following two circumstances: 1) the presence of the factor 1/2 is due to the fact that the mode has the character of a standing wave, so in accordance with the above reasoning ; 2) another factor 1/2 appears due to the fact that is the spot size for the field amplitude E, while the spot size for the field intensity (i.e., obviously, is several times smaller.

Before continuing our consideration, it should be noted that expression (5.1c) neglects the term that takes spontaneous radiation into account. In fact, as noted in Chap. 1, generation occurs due to spontaneous emission; therefore, it should be expected that equations (5.1) do not give a correct description of the onset of generation. Indeed, if in equation (5.1 c) we put at the moment of time, then we get , therefore, generation cannot occur. To take into account spontaneous emission, one could try again, based on simple condition balance, start consideration with a term which in equation (5.16) is included in the term. In this case, it may seem that

that in equation (5.1c) the term, which takes into account spontaneous radiation, should have the following form: However, this is not true. In fact, as shown in Sec. 2.4.3 [see, in particular, expression (2.115)], spontaneous radiation is distributed in a certain frequency interval and the shape of its line is described by the function radiation that contributes to the considered mode. The correct expression for this term can only be derived from a quantum mechanical consideration of the electromagnetic field of the resonator mode. The result thus obtained is very simple and instructive. In the case when spontaneous radiation is taken into account, equation (5.1 c) is transformed to the form

All this looks as if we have added an "extra photon" to the term corresponding to stimulated emission. However, for the sake of simplicity, we will not introduce such an additional term related to spontaneous emission in what follows, but instead assume that at the initial time a certain small number of photons are already present in the resonator. As we shall see, the introduction of this small number of photons, which is only necessary for the occurrence of generation, in fact, in no way affects the subsequent consideration.

Let us now take up the derivation of explicit expressions for the quantity B, which enters equations (5.16) and (5.1 c). A rigorous expression for this quantity is again derived in Appendix B. For most practical purposes, an approximate expression is suitable, which can be obtained from simple considerations. For this, we consider a resonator with a length in which there is an active medium with a length with a refractive index. We can assume that the resonator mode is formed by a superposition of two waves propagating in opposite directions. Let I be the intensity of one of these waves. In accordance with expression (1.7), when a wave passes through a layer of an active medium, its intensity changes by the value where a is the transition cross section at the frequency of the considered resonator mode. Let us now determine the following quantities: and are the transmission coefficients of the two resonator mirrors in terms of power; - the corresponding relative loss factors on the mirrors; 3) Г, - relative coefficient of internal losses per pass. Then the change in intensity for a complete passage of the resonator

Here and are the logarithmic losses per pass due to the transmission of the mirrors, and are the internal logarithmic losses. For brevity, we will call y, and transmission losses, and - internal losses. As will become clear in what follows, due to the exponential nature of laser amplification, recording with logarithmic losses is much more convenient for representing losses in lasers. However, it should be noted that although for small transmission values, this is not true for large transmission values. Let's give an example: if we put then we get i.e., while for we have It should also be noted that using expressions (5.7) it is possible to determine the total loss per pass:

Having determined the logarithmic losses , we substitute expressions (5.7) and (5.8) into (5.6). Introducing an additional condition

the exponential function in (5.6) can be expanded into a power series, and we get

Let us divide both parts of this expression by the time interval during which the light wave makes a complete passage of the resonator,

i.e., by the value where is determined by the expression

Using the approximation, we get

Since the number of photons in the resonator is proportional to the intensity, equation (5.12) can be compared with (5.1c). In this case, we obtain the following expressions:

We will call the value V the effective volume of the resonator mode. Note that formula (5.136) generalizes what was obtained in Sec. 4.3 expression for the lifetime of a photon. In addition, expression (5.14) for the resonator volume is valid only approximately. In fact, Appendix B shows that in (5.13a) a more rigorous expression for V should be used, namely

here the first integral is taken over the volume of the active medium, and the second - over the remaining volume of the resonator. We note, however, that for a symmetric resonator with mirrors of a large radius of curvature, both expressions (5.14) and (5.15) give

So far, our consideration has been directed to the justification of equation (5.1c) and to the derivation of explicit expressions for B and in terms of the measured laser parameters. However, it should be noted that we also indicated the limits of applicability of equation (5.1c). Indeed, when deriving equation (5.12), we had to use approximation (5.9), according to which the difference between gain and loss is small. For a cw laser, this condition is always satisfied, since in a steady state process (see Section 5.3.1). But for a pulsed laser, condition (5.9) will be valid only when the laser operates at a small excess over the threshold. If condition (5.9) is not satisfied, then the equations

Test

LASERS BASED ON CONDENSED MATTER

Introduction

2.2. ruby laser

3.2. neodymium laser

3.7. Fiber lasers

5. Semiconductor lasers

5.1. Operating principle

5.2. DHS lasers

5.3. DFB and VRPI lasers

BIBLIOGRAPHY

Introduction

Lasers based on substances in a condensed state include lasers whose active medium is created:

1) in solids ah mainly in dielectric crystals and glasses, where the active particles are ionized atoms of actinides, rare-earth and other transition elements alloying the crystal, and also in crystals with semiconductor properties,

2) in liquids containing active particles molecules of organic dyes.

In these media, stimulated laser radiation arises due toinduced radiativetransitions (see Section 1) between the energy levels of activator ions or terms of molecules. In semiconductor structures, stimulated emission occurs as a result of the recombination of free electrons and holes. In contrast to gas lasers (see Section 4), population inversion in solid-state and liquid lasers is always created at transitions that are close to the ground energy state of the active particle.

Since dielectric crystals do not conduct electric current, for them, as well as for liquid media, the so-called.optical pumping– pumping of the laser transition by optical radiation (light) from an auxiliary source.

In semiconductor lasers, electric current pumping is more often used ( injection current) flowing through the semiconductor in the forward direction, less often other types of pumping: optical pumping, or pumping by electron bombardment.

1. Specific features of optical pumping of the laser active medium

An important feature of OH is its selectivity , namely: by selecting the wavelength of OH radiation, it is possible to selectively excite the desired quantum state of active particles. Let us find the conditions that ensure the maximum efficiency of the process of excitation of active particles due to optical pumping (OH), as a result of which the active particle experiences a quantum transition from the energy state i to the higher excited state on the energy scale k . To do this, we use the expression for the radiation power of the OH source absorbed by the active particles of the irradiated medium (see Section 1.9)

. (1)

Eq. (1) includes the frequency dependence of the spectral energy density of the radiation of the OH source and the function of the shape of the absorption line of the medium, i.e. its frequency dependence (form factor).

Obviously, the absorption rate and the amount of absorbed power will be maximum when:

1) concentration of particles in state i will be the largest, i.e. OH is effective at a high density of active particles, namely, from the whole variety of media for media that are in a condensed state (solids and liquids);

2) In the TDS state, the distribution of particles over states with different meanings internal (potential) energy is described by the Boltzmann formula, namely: the ground (lowest) energy state of the particle and the ensemble as a whole has the maximum population. It follows from this that the state i should be the main energy state of the particle;

3) for the most complete absorption of the energy of the OH source (the largest Δ Pik ) it is desirable to have an environment with highest value absorption coefficient at the quantum transition: (see f-lu (1.35)), and since it is proportional to the Einstein coefficient B k i , a B ki A ki (see f-lu (1.11, b)), it is desirable that the absorbing transition be “allowed” and “resonant”;

4) It is desirable that the width of the radiation spectrum of the pump source would not be greater than the width of the absorption contour of active particles. When pumped by spontaneous emission of lamps, this, as a rule, cannot be achieved. Ideal from this point of view is “ coherent ” pumping pumping by monochromatic laser radiation, in which the entire line (entire spectrum) of OH radiation “falls” into the absorption contour. Such an absorption regime was considered by us in Section 1.9;

5) it is obvious that the OH efficiency will be the higher, the greater the fraction of radiation will be absorbed by active particles through a quantum transition with pumping of the required level. So, if the active medium is a crystal (matrix) doped with active particles, then the matrix should be chosen such that it does not absorb OH radiation, i.e. so that the matrix would be “transparent” for the pump radiation, which excludes, among other things, the heating of the medium. At the same time, the overall efficiency of the “OH source laser active medium” system is usually determined to a large extent by the conversion efficiency electrical energy, embedded in the pump source, in its radiation;

6) In section 1.9 it was shown that in quantum system with two energy levels, it is fundamentally impossible to obtain a population inversion for any values ​​of the intensity of external radiation (i.e., optical pumping): at →∞, it is only possible to equalize the populations of the levels.

Therefore, to pump a quantum laser transition with optical radiation and create a population inversion on it, active media with one or two auxiliary energy levels are used, which, together with two levels of the laser transition, forms a three- or four-level scheme (structure) of the energy levels of the active medium.

2. Quantum devices with optical pumping, operating according to the “three-level scheme”

2.1. Theoretical analysis three-level scheme. In such a scheme (Fig. 1), the lower laser level "1" is the ground energy state of the ensemble of particles, the upper laser level "2" is a relatively long-lived level, and the level "3", associated with level "2" by a fast nonradiative transition, isauxiliary. Optical pumping operates on channel "1" → "3".

Let us find the condition for the existence of inversion between levels "2" and "1". Assuming the statistical weights of the levels are the same g 1 = g 2 = g 3 , we write the system of kinetic (balance) equations for levels "3" and "2" in the stationary approximation, as well as the relation for the number of particles at the levels:

(2)

where n 1 , n 2 , n 3 particle concentrations at levels 1,2 and 3, Wn 1 and Wn 3 the rate of absorption and induced emission at transitions between levels "1" and "3" under the action of pump radiation, the probability of which W; wik the probability of transitions between levels, N

From (2) we can find the level populations n 2 and n 1 as a function of W , and their difference Δ n in the form

, (3)

which defines the unsaturated gainα 0 of the ensemble of particles at the transition "2"→"1". Toα 0 >0, it is necessary that, i.e. the numerator in (3) must be positive:

, (4)

where W then threshold level of pumping. Since always W then >0, then it follows that w 32 > w 21 , i.e. the probability of pumping level "2" by relaxation transitions from level "3" should be greater than the probability of its relaxation to state "1".

If

w 32 >> w 21 and w 32 >> w 31 , (5)

then from (3) we get: . And finally, if W >> w 21 , then the inversion Δ n will be: Δ n ≈ n 2 ≈ N , i.e. at level "2" you can "collect" all the particles of the environment. Note that relations (5) for the relaxation rates of the levels correspond to the conditions for the generation of spikes (see Section 3.1).

Thus, in a three-level system with optical pumping:

1) inversion is possible if w 32 >> w 21 and maximum when w 32 >> w 31 ;

2) inversion occurs when W > W then , i.e. creation wears threshold character;

3) for low w 21 conditions are created for the “spike” regime of free generation of the laser.

2.2. ruby laser. This solid-state laser is the first laser to operate in the visible wavelength range (T. Meiman, 1960). Ruby is a synthetic crystal A l 2 O 3 in the modification of corundum (matrix) with an admixture of 0.05% activator ions Cr3+ (ion concentration ~1.6∙10 19 cm 3 ), and is denoted as A l 2 O 3 : Cr 3+ . The ruby ​​laser operates according to a three-level scheme with OH (Fig. 2a). Laser levels are electronic levels Cr3+ : lower laser level "1" is the ground energy state Cr 3+ in A l 2 O 3 , upper laser level "2" long-lived metastable level withτ 2 ~10 3 With. Levels "3a" and "3b" areauxiliary. Transitions "1" → "3a" and "1" → "3b" belong to the blue (λ0.41 μm) and "green" (λ0.56 μm) parts of the spectrum, and are wide (with Δλ ~50nm) absorption contour (stripes).

Rice. 2. Ruby laser. (a) Energy Level Diagram Cr 3+ in Al 2 O 3 (corundum); (b ) constructive diagram of a laser operating in a pulsed mode with Q-switching. 1 ruby ​​rod, 2 pump lamp, 3 elliptical reflector, 4a fixed resonator mirror, 4b rotating resonator mirror modulating the resonator Q factor, C n storage capacitor, R charging resistor, " Kn » start button for current pulse through the lamp; shows the inlet and outlet of the cooling water.

Optical pumping method provides selective population of auxiliary levels "3a" and "3b" Cr3+ through channel "1"→"3" by ions Cr3+ when absorbed by ions Cr3+ radiation from a pulsed xenon lamp. Then, in a relatively short time (~10 8 c) there is a nonradiative transition of these ions from "3a" and "3b" to levels "2". The energy released in this case is converted into vibrations of the crystal lattice. With a sufficient density ρ of the radiation energy of the pump source: when, and at the “2” → “1” transition, population inversion occurs and radiation is generated in the red region of the spectrum at λ694.3 nm and λ692.9 nm. The threshold value of pumping, taking into account the statistical weights of the levels, corresponds to the transfer to level "2" of about ⅓ of all active particles, which, when pumped from λ0.56 μm, requires specific radiation energy E pore > 2J / cm 3 (and power P pore > 2 kW / cm 3 at pump pulse durationτ ≈10 3 s ). Such a high value of power deposited in the lamp and the ruby ​​rod at stationary RS can lead to its destruction; therefore, the laser operates in a pulsed mode and requires intensive water cooling.

The laser scheme is shown in fig. 2b. The pump lamp (flash lamp) and a ruby ​​rod to increase the pumping efficiency are located inside a reflector with a cylindrical inner surface and a cross section in the form of an ellipse, and the lamp and rod are located at the focal points of the ellipse. As a result, all the radiation coming out of the lamp is focused in the rod. A lamp light pulse occurs when a current pulse is passed through it by discharging a storage capacitor at the moment the contacts are closed with the “ Kn ". Cooling water is pumped inside the reflector. The laser radiation energy per pulse reaches several joules.

The pulse mode of operation of this laser can be one of the following (see Section 3):

1) “free generation” mode at a low pulse repetition rate (usually 0.1 ... 10 Hz);

2) “Q-switched” mode, usually optical-mechanical. On fig. 2b, Q-switching of the OOP is carried out by rotating the mirror;

3) “mode-locking” mode: with the width of the emission line Δν not one ~10 11 Hz,

number of longitudinal modes M~10 2 , pulse duration ~10 ps.

Ruby laser applications include holographic image recording systems, material processing, optical rangefinders, etc.

Widely used in medicine and laser on BeAl 2 O 4 : Cr 3+ (chrysoberyl doped with chromium, or alexandrite), emitting in the range of 0.7 ... 0.82 microns.

2.3. Erbium Fiber Optic Quantum Amplifier. Such an amplifier, often referred to as “ EDFA ” (abbreviation for “ Erbium Dopped Fiber Amplifier ”), works according to a three-level scheme on quantum transitions between electronic states Er 3+ in erbium-doped silica fiber: SiO2 : Er3+ (Fig. 3a). The lower quantum state "1" is the ground electronic state Er 3+ 4 I 15/2 . The upper quantum states "2" are the group of lower sublevels of the split electronic state 4 I 13/2 . Splitting into a number of closely spaced sublevels occurs due to the interaction of ions Er 3+ with intracrystalline field SiO2 (Stark effect). Upper sublevels of the electronic state 4 I 13/2 and separate level 4 I 11/2 are auxiliary levels "3a" and "3b".

Under the action of pump radiation at wavelengths of 980 nm (or 1480 nm), ions Er 3+ go from state "1" to short-lived states "3a" or "3b", and then fast nonradiative transitions ( w 32 ~10 6 s 1 ) to state “2”, which is quasi-metastable ( w 21 ~10 2 c 1 , and τ 2 ~10ms). Thus the requirement w 32 >> w 21 is carried out, and at level "2" there is an accumulation of particles, the number of which, when the pump level exceeds its threshold value, W > W then , exceeds the population of level "1", i.e. there will be a population inversion and amplification at wavelengths in the range of 1.52…1.57 μm (Fig. 3b). It turns out that the inversion threshold is reached when one third of the particles are transferred to level "2". Threshold OH W then and the frequency dependence of the gain are determined by the structure of the fiber (Fig. 3b), concentration Er 3+ and wavelength of OH radiation. The pump efficiency, namely the ratio of the unsaturated gain to the unit power of the OH source, is for pumping from λ980nm to 11dB m 1 ∙mW 1 , and for λ1480nmabout 6dB m 1 ∙mW 1 .

Gain Frequency Compliance EDFA the third “transparency window” of quartz fiber determines the use of such amplifiers as linear loss compensators of modern fiber-optic communication lines (FOCL) with frequency multiplexing of channels (systems WDM : Wavelength Division Multiplexing , and DWDM : Dense Wavelength Division Multiplexing ). The length of the cable-amplifier, pumped by the radiation of a semiconductor laser, is quite simply included in the FOCL (Fig. 3c). The use of erbium fiber amplifiers in FOCL replaces the technically much more complicated method of signal “regeneration” isolating a weak signal and recovering it.

Rice. 3. Erbium fiber optic quantum amplifier ( EDFA ). (a) energy level diagram Er 3+ in SiO 2 (quartz), (b)signal amplification in quartz with various additives, ( in )simplified scheme for switching on an amplifier in an FOCL: 1input radiation (from the transmission path), 2 semiconductor pump laser, 3multiplexer ( coupler ), 4 EDFA (SiO 2 : Er 3+ fiber ), 5optical isolator, 6output radiation (to the transmission path).

3. Optically pumped lasers operating according to the “four-level scheme”.

3.1. Theoretical analysis of the four-level scheme. In such a scheme of levels (Fig. 4), level “0” is the ground energy state of an ensemble of particles, level “1”, associated with a quantum transition with level “0”, is the lower laser level, long-lived level “2” is the upper laser level, and level "3" is auxiliary. Pumping operates on channel "0" → "3".

Let us find the condition for the existence of inversion between levels "2" and "1". Assuming the statistical weights of the levels to be the same, and also assuming that

and, (6)

write down simplified system kinetic equations for levels "3", "2" and "1" in the stationary approximation, as well as the ratio for the number of particles at all levels:

(7)

where n 0 , n 1 , n 2 , n 3 , concentration of particles at levels 0,1,2,3; Wn 0 and Wn 3 the rate of absorption and induced emission at transitions between levels "0" and "3" under the action of pump radiation, the probability of which W; wik probabilities of transitions between levels, N total number of active particles per unit volume.

From (6 and 7) we can find the level populations n 1 and n 2 as a function of W , and their difference Δ n in the form

, (8)

which determines the unsaturated gain α 0 at the transition "2"→"1".

Obviously, the gain will be positive and maximum when:

. (9)

From this we can conclude that in the case of a four-level scheme with OH, when conditions (6) and (9) are satisfied:

1) inversion is not of a threshold nature and exists for any W;

2) the laser output power, determined by expression (2.14), depends on the optical pumping speed Wn 0 .

3) compared to the three-level, the four-level scheme is more versatile and allows you to create a population inversion, as well as to implement both pulsed and continuous and generation at any pump levels (when the gain exceeds the losses in the OER).

3.2. neodymium laser. The laser uses a quantum transition between electronic energy levels Nd 3+ , laser generation is carried out according to a four-level scheme with OH (Fig. 5). The most widely used crystal matrix for ions Nd 3+ is yttrium aluminum garnet: Y 3 Al 5 O 12 , and the doped crystal is denoted as Y 3 Al 5 O 12 : Nd 3+ or YAG: Nd 3+ . Nd3+ concentration , which does not deform the YAG crystal up to 1.5%. Other matrices for Nd 3+ are phosphate and silicate glasses (denoted as glass : Nd 3+ ), crystals of gadolinium-scandium-gallium garnet (GSHG: Nd 3+ ), yttrium-lithium fluoride YLiF 4 : Nd 3+ , yttrium orthovanadate, organometallic liquids. Due to the cubic structure of the matrix, the YAG luminescence spectrum has narrow lines, which determines the high gain of neodymium solid-state lasers, which can operate in both pulsed and cw generation modes.

Simplified electronic energy level diagram Nd 3+ in YAG is shown in Fig. 5 Lower laser level "1" 4 I 11/2 the most intense quantum transition Nd 3+ with a wavelength of λ1.06 μm is located approximately 0.25 eV above the ground energy state "0" 4 I 9/2 , and under normal conditions is practically unpopulated (0.01% of the population of the ground state), which determines the low generation threshold of this laser. Level 4 F 3/2 , whose lifetime is 0.2ms, is the upper laser level "2". Groups of levels (energy “zones”) "3a" ... "3 d ” play the role of an auxiliary electronic level “3”. Optical pumping is carried out through the channel "0" → "3", the absorption bands have wavelengths near 0.52; 0.58; 0.75; 0.81 and 0.89 µm. From the states "3a" ... "3 d » there is a fast relaxation by nonradiative transitions to the upper laser state «2».

Krypton and xenon gas-discharge lamps are used for pumping, halogen lamps with alkali metal additives in the filling gas, as well as semiconductor GaAs lasers (λ0.88 µm) and LEDs based on Ga 1 x Al x As (λ0.81 µm) (Fig. 6).

YAG laser radiation power: Nd 3+ with a wavelength of λ1.06 μm in the continuous mode reaches 1 kW, the record values ​​achieved in the pulsed mode: the pulse energy is about 200 kJ, and the power is 200 TW at a pulse duration of ~ 1 ns (a laser designed for experiments on controlled laser thermonuclear fusion - LTS).

In a YAG crystal, a laser line Nd 3+ with λ1.06 μm is uniformly broadened (up to 0.7 nm), while in glasses there is a significant inhomogeneous broadening due to the Stark effect (Δν not one ≈3∙10 12 Hz,), which makes it possible to successfully apply the longitudinal mode locking mode (see Section 3.3) with M ~10 4 and receive ultrashort pulses with a duration of the order of 1 ps.

An increased concentration of activator ions in media such as neodymium pentaphosphate ( NdP 5 O 14 ), lithium neodymium tetraphosphate ( LiNdP 4 O 12 ) and others, provides efficient absorption of semiconductor laser radiation at distances of the order of fractions of a millimeter, which allows you to create miniature modules called minilasers : semiconductor laserneodymium laser.

The high radiation power of a neodymium laser with λ1.06 μm makes it possible to convert the frequency of its radiation using nonlinear crystals. To generate the second and higher optical harmonics, crystals with quadratic and cubic nonlinear susceptibility are used (potassium dihydrogen phosphate KDP , potassium titanyl phosphate KTP ), with direct and (or) sequential (cascade) conversion. So, if a chain of crystals is used for radiation of a neodymium laser, then it is possible to obtain, in addition to IR radiation at the fundamental frequency with λ1.06 μm , the generation of the 2nd, 4th and 5th harmonics with wavelengths λ0.53 μm (green radiation); λ0.35µm, λ0.26µm and λ0.21µm (UV radiation) (Fig. 7).

The main areas of application of neodymium lasers: technological and medical installations, experiments on controlled laser thermonuclear fusion, studies of the resonant interaction of radiation with matter, in underwater vision and communication systems (λ0.53 μm), optical information processing; spectroscopy, remote diagnostics of impurities in the atmosphere (UV radiation), etc.

In lasers using glasses as a matrix (silicate, borate, etc.), other activator ions can also be successfully used: Yb 3+ , Er 3+ , Tm 3+ , Ho 3+ with radiation in the range of 0.9 ... 1.54 μm.

3.3. Frequency conversion of radiation in a nonlinear medium. The phenomenon of doubling and adding the frequencies of light waves is as follows. When light propagates in a medium under the action of an electric field of an electromagnetic wave E , there is a corresponding displacement of atomic electrons relative to the nuclei, i.e. the medium is polarized. The polarizability of the medium is characterized by the magnitude of the electric dipole moment per unit volume - R associated with the magnitude of the field E through the dielectric susceptibility of the mediumχ : . If this field is small, then the dielectric susceptibilityχ \u003d χ 0 \u003d Const, p is a linear function of E : , and the displacement of charges causes radiation with the same frequency as the initial radiation (“ linear” optics).

At high power, when electric field radiation begins to exceed the value of the intraatomic field, the polarizability becomes a nonlinear function E : That is, apart from linearly dependent on E term at small E , when we are dealing with linear optics, in the expression for R appears nonlinear with respect to E term (“nonlinear ” optics). As a result, when a “pump” wave propagating in a medium with a frequency ν 0 and wave vector (where is the refractive index of the medium), a new wave appears the second optical harmonic with frequency and wave vector, as well as a number of higher-order harmonics. Obviously, the energy of a pump wave with a frequency will be most efficiently transferred into a new wave with a frequency if the propagation velocities of these two waves are the same, i.e. if there is a so-called.: . This condition can be met using a crystal with birefringence, when two waves propagate at a certain angle to its main optical axis.

When two waves propagate in the crystal with frequencies and and wave vectors and, in addition to the harmonics of each of the waves, a wave with a total frequency is generated in the crystal: , and a wave with a difference frequency. The condition of wave synchronism in this case has the form: .

In a certain sense, the described phenomena can be considered as the generation of harmonics during coherent optical pumping of a nonlinear crystal.

3.4. Tunable dye lasers. Lasers based on solutions of complex organic compounds (including dyes: rhodamines, coumarins, oxazoles, etc.) in alcohols, acetone and other solvents belong to the group liquid lasers. Such solutions have intense absorption bands at OH and emission bands in the near UV, visible, or near IR spectral regions. Their main advantage is a wide luminescence line (up to 50…100 nm), which makes it possible to smoothly tune the operating frequency of the laser within this line.

The electronic states of most dyes used in such lasers are wide, up to 0.1 eV, continuous energy bands resulting from the addition of hundreds of “overlapping” vibrational and rotational sublevels, which also leads to broad, as a rule, structureless absorption and luminescence bands. , as a result of the addition of "overlapping" transitions between such sublevels (Fig. 8a). Between sublevels “inside” these bands, there are fast nonradiative transitions with probabilities w ~10 10 …10 12 s 1 , and the probabilities of relaxation transitions between electronic states are two to four orders of magnitude lower (~10 8 c 1 ).

Generation occurs according to a “four-level” scheme on transitions of the dye molecule from the lower vibrational sublevels of the first excited singlet electronic state S1 (Fig. 8, a), analogues of level "2" in the diagram in Fig. 4 to the upper sublevels of the ground electronic state S0 , analogues of level "1". The analogue of level "0" is the lower sublevels of the main electronic term, and the analogue of the auxiliary level "3" is the upper vibrational sublevels of the excited electronic term S1.

Since fast transitions take place inside electronic terms, the population distribution of states corresponds to Boltzmann's law: the upper sub-levels "3" and "1" are weakly populated, and the lower "0" and "2" are strongly populated. Such a ratio for levels "0" and "3" determines for them a high efficiency of the RS along the channel "0" → "3", and the ratio for levels "2" and "1" determines the population inversion, amplification and generation at this transition.

To obtain a narrow generation line, as well as to be able to tune it in frequency within a wide luminescence band of dye molecules, a dispersive resonator with spectral selective elements (prisms, diffraction gratings, interferometers, etc.) is used (Fig. 8b).

The possibility of tuning in wavelength within the luminescence line (Fig. 8, in ) without power loss is determined by fast nonradiative transitions inside the electronic terms "2" and "1", the probability of which exceeds the probability of induced transitions. So, when tuning the resonator to any wavelength within the luminescence line of the transition "2" → "1", laser radiation occurs at the transition between the corresponding sublevels "2ʹ" and "1ʹ ”, resulting in sublevel “2ʹ » by induced transitions is “cleared”, and «1ʹ » is additionally populated. However, due to OH and fast transitions from neighboring sublevels within the term, the population of the “generating” sublevel “2ʹ » is continuously restored. At the same time, sublevel "1ʹ ” is continuously cleared by fast transitions, eventually relaxing to the “0” state. Thus, the entire pumping of the upper electron term "2" becomes the pumping of the transition "2ʹ»→«1ʹ » and turns into narrow-band monochromatic laser radiation at the tuning frequency of the dispersive resonator, and this frequency can be varied.

In addition to radiative transitions S 1 → S 0 ("2" → "1") There are also a number of transitions that reduce the generation efficiency. These are the transitions: S 1 → T 1 , which reduce the population of levels “2ʹ ”, transitions T 1 →"1", increasing the population of levels "1ʹ", and transitions T 1 → T 2 absorbing laser radiation.

There are two types of dye lasers: incoherent (tube) optically pumped by radiation flash lamps and pulse mode of operation; and also with coherent pumping by laser radiation of other types (gas or solid-state) in continuous, quasi-continuous or pulsed operation. If a change of dyes is used in the laser, and there are more than a thousand of them, then in this way it is possible to “block” the entire visible and part of the IR region of the spectrum (0.33 ... 1.8 μm) with radiation. In lasers with coherent pumping, ion pumps are used as pump sources to obtain a continuous regime. Ar - or Kr -gas lasers. To pump dyes in a pulsed mode, gas lasers are used on N 2 , copper vapor, excimers, as well as ruby ​​and neodymium lasers with frequency multiplication. It is often necessary to use pumping of the dye solution, as a result of which molecules that have undergone dissociation under the action of pump radiation are removed from the active zone and fresh ones are introduced.

Dye lasers, having Δν not one ~10 13 Hz and M>10 4 , make it possible to generate ultrashort radiation pulses (τ~10 14 …10 13 s).

Dye lasers with distributed feedback (DFB) form a special group. In DFB lasers, the role of a resonator is played by a structure with a periodically changing refractive index and (or) gain. It is usually created in an active medium under the action of two interfering pump beams. A DFB laser is characterized by a narrow generation line (~10 2 cm 1 ), which can be tuned within the gain band by changing the angle between the pump beams.

Dye laser applications include photochemistry, selective pumping of quantum states in spectroscopy, isotope separation, etc.

3.5 Tunable titanium-doped sapphire laser. A smooth tuning of the generation wavelength is also ensured by a solid-state laser based on a titanium-activated corundum crystal ( Al 2 O 3 : Ti 3+ ), called sapphire.

Every electronic state Ti 3+ , consists of a large number of "overlapping" vibrational sublevels, which leads to structureless absorption and luminescence bands even wider than those of a dye as a result of the addition of "overlapping" transitions between such sublevels. Inside these states, there are fast nonradiative transitions with probabilities w ~10 9 s 1 , while the relaxation probabilities between electronic states are of the order of 10 5 …10 6 s 1 .

The sapphire laser belongs to the group of so-called. vibronic lasers, characterized in that their main electronic term is a band of vibrational sublevels ( crystal lattice), due to which the laser operates according to a four-level scheme, and, like a dye laser, it creates the possibility of smooth generation tuning in the range of λ660…1180 nm. The absorption band extends from λ0.49 µm to λ0.54 µm. Short lifetime of the excited state "2" Ti 3+ makes the lamp pumping of this laser ineffective, which, as a rule, is carried out by a cw argon laser (λ488 nm and λ514.5 nm), the second harmonic of a neodymium laser (λ530 nm) or copper vapor laser radiation pulses (λ510 nm).

The undoubted advantages of a sapphire laser with titanium are a much higher permissible pump power without degradation of the working substance and a wider inhomogeneously broadened luminescence line. As a result, a sequence of pulses with a duration of about tens of femtoseconds (1fs=10 15 c), and with subsequent compression (compression) of pulses in non-linear optical fibersup to 0.6 fs.

3.6. Tunable color center lasers. Such lasers, like the solid-state lasers discussed above, use ionic crystals as an active substance, but with color centers called F - centers , which allows the tuning of their radiation. Laser materials for such lasers: crystals of fluorides and chlorides of alkali metals ( Li, Na, K, Rb ), as well as fluorides Ca and Sr . The impact on them of ionizing radiation: gamma quanta, high-energy electrons, X-ray and hard UV radiation, as well as the calcination of crystals in alkali metal vapors, leads to the appearance of point defects in the crystal lattice, localizing electrons or holes on themselves. A vacancy that captures an electron forms a defect whose electronic structure is similar to that of a hydrogen atom. Such a color center has absorption bands in the visible and UV regions of the spectrum.

The scheme of laser generation on color centers is similar to schemes of liquid lasers on organic dyes. For the first time, generation of stimulated emission at color centers was obtained in crystals of K Cl - Li under pulsed optical pumping. At the moment, generation has been observed at a large number of different color centers with IR radiation in pulsed and continuous modes with coherent RS. The radiation frequency is tuned using dispersive elements (prisms, diffraction gratings, etc.) placed in the resonator. However, poor thermal and photostability prevent widespread use such lasers.

3.7. Fiber lasers. fiber called lasers, the resonator of which is built on the basis of optical fiber-waveguide, which is also the active medium of the laser in which radiation is generated (Fig. 9). Rare earth doped quartz fiber is used ( Nd, Ho, Er, Tm, Yb etc.), or passive fiber using the effect of stimulated Raman scattering. In the latter case, the optical resonator forms a light guide in combination with “Bragg” refractive index gratings “embedded” in the fiber. Such lasers are called fiber Raman ” lasers. The laser radiation propagates inside the optical fiber, and therefore the fiber laser cavity is simple and does not require alignment. In a fiber laser, it is possible to obtain both single-frequency generation and generation of ultrashort (femtosecond, picosecond) light pulses.

4. Parametric light generation

Parametric light generation(POS) is carried out under the action of laser optical pumping radiation in solid-crystals with nonlinear properties, and is characterized by a fairly high conversion coefficient (tens of percent). In this case, it is possible to smoothly tune the frequency of the output radiation. In a certain sense, the OPO, as well as the phenomenon of frequency multiplication and addition considered above, can be considered as the generation of tunable radiation during coherent optical pumping of a nonlinear crystal.

At the heart of the OPO phenomenon, as in the case of multiplication and addition of frequencies, are non-linear optical phenomena in media. Let us consider the case when a medium with nonlinear properties and located in an open optical cavity (OOR) interacts with laser radiation of a sufficiently high intensity, having a frequency ν 0 (pumping). Due to pumping the energy of this wave, two new light waves can appear in the medium:

1) a wave of “noise” nature with a certain frequency ν 1 ;

2) a wave with a difference frequency (ν 0 v 1 ), which is the result of a nonlinear interaction between pump radiation and a random (noise) wave with a frequency ν 1 .

Moreover, the frequencies ν 1 and (ν 0 ν 1 ) must be natural frequencies of the OOP and for all three waves,wave synchronism condition: . In other words, the pump light wave with frequency ν 0 using an auxiliary noise wave with frequency ν 1 , transforms into a wave with a frequency (ν 0 ν 1 ).

Frequency tuning of the OPO radiation is carried out by selecting the orientation of a birefringent nonlinear crystal by rotating it, i.e. changing the angle between its optical axis and the axis of the resonator in order to performwave synchronism condition. Each value of the angle corresponds to a strictly defined combination of frequencies ν 1 and (ν 0 ν 1 ), for which the condition of wave synchronism is currently satisfied.

Two schemes can be used to implement PGS:

1) “two-resonator” scheme, when the generated waves with frequencies ν 1 and (ν 0 ν 1 ) occur in one OER, while the loss of OER for them should be small;

2) “single resonator” scheme, when only one wave with frequency (ν 0 ν 1 ).

A crystal can be used as an active medium LiNbO 3 (lithium niobate), pumped by the radiation of the second harmonic of the YAG: Nd 3+ (λ0.53 μm) and smooth tuning can be carried out in the range up to λ3.5 μm within 10%. A set of optical crystals with different areas of nonlinearity and transparency allows tuning in the IR region up to 16 µm.

5. Semiconductor lasers

semiconductorcalled such solid-state lasers in which semiconductor crystals of various compositions with population inversion at a quantum transition are used as an active medium (working substance). A decisive contribution to the creation and improvement of such lasers was made by our compatriots N.G. Basov, Zh.I. Alferov and their collaborators.

5.1. Operating principle. In semiconductor lasers, unlike lasers of other types (including other solid-state ones), radiative transitions are used not between isolated energy levels of atoms, molecules and ions that do not interact or weakly interact with each other, but between allowedenergy zonescrystal. Radiation (luminescence) and generation of stimulated emission in semiconductors is due to quantum transitions of electrons both between the energy levels of the conduction band and the valence band, and between the levels of these bands and impurity levels: transitions donor level acceptor level, conduction band acceptor level, donor level valence band, including through exciton states. Each energy zone corresponds to a very large (~10 23 …10 24 ) the number of allowed states. Since electrons are fermions; then, for example, valence the band can be completely or partially filled with electrons: with a density decreasing from bottom to top along the energy scale similar to the Boltzmann distribution in atoms.

The radiation of semiconductors is based on the phenomenonelectroluminescence. A photon is emitted as a result of an act recombination charge carrierselectron and “hole” (an electron from the conduction band occupies a vacancy in the valence band), while the radiation wavelength is determined byband gap. If we create such conditions that an electron and a hole before recombination will be in the same region of space for a sufficiently long time, and at this moment a photon with a frequency that is in resonance with the frequency of the quantum transition passes through this region of space, then it can induce the recombination process with emission second photon, and its direction, vector polarization and phase will exactly match the same characteristics as the first photon. For example, in own (“pure”, “impurity-free”) semiconductors, there is a filled valence band and an almost free conduction band. During interband transitions, in order to cause inversion and obtain generation, it is necessary to create excess nonequilibrium concentrations of charge carriers: in the conduction band, electrons, and in the valence band, holes. In this case, the interval between the quasi-Fermi levels must exceed the band gap, i.e., one or both quasi-Fermi levels will be inside the allowed bands at distances of no more than kT from their borders. And this presupposes excitation of such intensity that degeneration in the conduction band and in the valence band.

The first semiconductor lasers used gallium arsenide (GaAs), operated in a pulsed mode, emitted in the IR range and required intensive cooling. Further research has made it possible to make many significant improvements in the physics and technology of lasers of this type, and at present they emit in both the visible and UV ranges.

The degeneracy of a semiconductor is achieved by heavily doping it at a high dopant concentration, such that the properties of the dopant, rather than those of the intrinsic semiconductor, are exhibited. Every atom donor impurity gives one of its electrons to the conduction band of the crystal. On the contrary, the atomacceptorimpurity captures one electron, which was shared by the crystal and was in the valence band. degeneratena semiconductor is obtained, for example, by introducing intoGaAstellurium impurities (concentration 3...5 1018 cm3 ), and the degeneratepsemiconductor zinc impurities (concentration 1019 cm3 ). Generation is carried out at IR wavelengths from 0.82 µm to 0.9 µm. Structures grown on substrates are also widespread.InP(IR region λ1…3 µm).

The semiconductor crystal of the simplest laser diode operating on a “homojunction” (Fig. 10) has the form of a very thin rectangular plate. Such a plate is essentially an opticalwaveguidewhere the radiation propagates. The top layer of the crystaldopedfor creatingparea, and in the bottom layer is creatednregion. The result is a flatpnlarge area crossing. The two sides (ends) of the crystal are cleaved and polished to form smooth, parallel reflective planes that form an open optical cavity.- Fabry-Perot interferometer. Random photon of spontaneous emission emitted in a planepntransition perpendicular to the reflectors, passing along the resonator, will cause stimulated recombination transitions, creating new and new photons with the same parameters, i.e. the radiation will be amplified, generation will begin. In this case, the laser beam will be formed due to repeated passage through the optical waveguide and reflection from the ends.

The most important type of pumping in semiconductor lasers isinjectionpumping. In this case, the active particles are free charge carriers excess nonequilibrium conduction electrons and holes, whichinjectedinpn-transition (active medium), when passing through it electric current in the “direct” direction with a “direct” displacement, which reduces the height of the potential barrier. This allows direct conversion of electrical energy (current) into coherent radiation.

Other methods of pumping are electrical breakdown (in the so-called.streamerlasers), electron beam pumping, and optical pumping.

5.2. DHS lasers. If you arrange a layer with a narrowerforbidden zone(active region) between two layers with a wider bandgap, a so-called.heterostructure. The laser that uses it is called a double laser.heterostructure(DHS laser, or “double heterostructure”, DHS- laser). This structure is formed by joininggallium arsenide(GaAs) andaluminum gallium arsenide(AlGaAs). The advantage of such lasers lies in the small thickness of the middle layer of the active region, where electrons and holes are localized: light is additionally reflected from heterojunctions, and the radiation will be contained in the region of maximum amplification.

If two more layers with a lower refractive index compared to the central ones are added on both sides of the DHS laser crystal, then a resemblinglight guidestructure that more effectively traps radiation (DHS laserwith separate hold, or "separate confinement heterostructure”, SCHS- laser). Most of the lasers produced in recent decades are made using this technology. The development of modern optoelectronics, solar energy is based on quantum heterostructures: incl. with quantum "wells", quantum "dots".

5.3. DFB and VRPI lasers. In lasers withdistributed feedback(ROS or “distributedfeedback”– DFBlaser) nearp- ntransition, a system of transverse relief “strokes” is applied, forminggrating. Thanks to this grating, radiation with only one wavelength returns back to the resonator, and generation occurs on it, i.e. stabilization of the radiation wavelength is carried out (lasers for multi-frequency fiber-optic communication).

A semiconductor "edge" laser that emits light in a direction perpendicular to the crystal surface and is called a "vertical cavity surface-emitting" laser (VRTS laser, or "verticalcavitysurface- emitting”: VCSElaser), has a symmetrical radiation pattern with a small divergence angle.

In the active medium of a semiconductor laser, a very high gain (up to 104 cm-1 ), due to which the dimensions of the active element P. l. lasers are extremely small (resonator length 50 µm…1 mm). In addition to compactness, the features of semiconductor lasers are: ease of intensity control by changing the current value, low inertia (~109 c), high efficiency (up to 50%), the possibility of spectral tuning and a large selection of substances for generation in a wide spectral range from UV, visible to mid-IR. At the same time, compared with gas lasers, semiconductor lasers are characterized by a relatively low degree of monochromaticity and coherence of radiation and cannot emit at different wavelengths simultaneously. Semiconductor lasers can be either single-mode or multi-mode (with a large active zone width). Multimode lasers are used in cases where a device requires high radiation power, and the condition of low beam divergence is not set. The areas of application of semiconductor lasers are: information processing devices - scanners, printers, optical storage devices, etc., measuring devices, pumping of other lasers, laser designators, fiber optics and technology.

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