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Principles of laser generation. Lasers based on condensed matter What substances belong to the active medium of a laser

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.

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) environment 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.

Test

CONDENSED MATTER LASERS

Introduction

2.2. Ruby laser

3.2. Neodymium laser

3.7. Fiber lasers

5. Semiconductor lasers

5.1. Operating principle

5.2. DGS lasers

5.3. DFB and VRPI lasers

BIBLIOGRAPHY

Introduction

Lasers based on condensed matter include lasers whose active medium is created by:

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

2) in liquids into which active particles and molecules of organic dyes are introduced.

In these environments, stimulated laser radiation occurs due toinduced radiativetransitions (see section 1) between 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. Unlike gas lasers (see Section 4), population inversion in solid-state and liquid lasers is always created at transitions close to the ground energy state of the active particle.

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

Semiconductor lasers often use electric current pumping ( injection current) flowing through the semiconductor in the forward direction, less often other types of pumping: optical pumping, or pumping by bombardment with electrons.

1. Specifics of optical pumping of the laser active medium

An important feature of HE is its selectivity , namely: by selecting the wavelength of OH radiation, one can selectively excite the desired quantum state of active particles. Let us find the conditions that ensure 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 an excited state higher on the energy scale k . To do this, we will 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)

(1) includes the frequency dependence of the spectral energy density of the OH source radiation and the function of the absorption line shape 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 greatest, i.e. OH is effective at a high density of active particles, namely, from the entire variety of media for media in a condensed state (solids and liquids);

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

3) for the most complete absorption of the energy of the OH source (maximum Δ Pik ) it is desirable to have a medium with the highest value of the 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)), then 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 should not be greater than the width of the absorption contour of active particles. When pumped by spontaneous radiation from lamps, this is usually not achieved. Ideal from this point of view is “ coherent ” pumping pumping with monochromatic laser radiation, in which the entire line (entire spectrum) of OH radiation “falls” into the absorption contour. This absorption mode was considered by us in section 1.9;

5) it is obvious that the efficiency of OH will be higher, the greater the fraction of radiation will be absorbed by active particles through a quantum transition with pumping of the required level. Thus, if the active medium is a crystal (matrix) doped with active particles, then the matrix must be chosen such that OH radiation is not absorbed by it, i.e. so that the matrix would be “transparent” for pump radiation, which also excludes heating of the medium. At the same time, the total efficiency of the “OH source laser active medium” system is usually determined to a large extent by the efficiency of converting the electrical energy deposited into the pump source into its radiation;

6) In section 1.9 it was shown that in a quantum system with two energy levels, at any intensity of external radiation (i.e., optical pumping), it is fundamentally impossible to obtain population inversion: 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 energy levels of the active medium.

2. Optically pumped quantum devices operating according to a “three-level scheme”

2.1. Theoretical analysis of the 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 level “3”, associated with level “2” by a fast non-radiative transition, isauxiliary. Optical pumping operates through 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 , let us write a system of kinetic (balance) equations for levels “3” and “2” in a stationary approximation, as well as the relationship for the number of particles at 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 stimulated emission at transitions between levels “1” and “3” under the influence of pump radiation, the probability of which W; w ik 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 determines the unsaturated gainα 0 ensemble of particles at the transition “2” → “1”. In order toα 0 >0, it is necessary that, i.e. the numerator in (3) must be positive:

, (4)

where W is threshold pumping level. As always W por >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 into state “1”.

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

then from (3) we get: . And finally, if W >> w 21, then the inversion of 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 rates of relaxation of levels correspond to the conditions for the generation of “spikes” (see Section 3.1).

Thus, in a three-level optically pumped system:

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

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

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

2.2. Ruby laser. This solid-state laser is the first laser to operate in the visible wavelength range (T. Meiman, 1960). A synthetic crystal is called ruby l 2 O 3 modified corundum (matrix) with an admixture of 0.05% activator ions Cr 3+ (ion concentration ~1.6∙10 19 cm 3 ), and is designated as A l 2 O 3 : Cr 3+ . The ruby laser operates according to a three-level scheme with OH (Fig. 2, a). Laser levels are electronic levels Cr 3+ : 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. The transitions “1” → “3a” and “1” → “3b” belong to the blue (λ0.41 µm) and “green” (λ0.56 µm) parts of the spectrum, and are broad (with Δλ ~50 nm) absorption contour (band).

Rice. 2. Ruby laser. (a) Energy level diagram Cr 3+ in Al 2 O 3 (corundum); (b ) design 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, C n storage capacitor, R charging resistor, " Kn » button to start a current pulse through the lamp; cooling water inlet and outlet are shown.

The optical pumping method ensures selective population of auxiliary levels “3a” and “3b” Cr 3+ via channel “1”→“3” by ions Cr 3+ when absorbed by ions Cr 3+ radiation from a pulsed xenon lamp. Then, in a relatively short time (~10 8 c) there is a non-radiative 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 radiation energy density ρ of the pump source: when, and at the “2” → “1” transition, population inversion occurs and generation of radiation in the red region of the spectrum at λ694.3 nm and λ692.9 nm. The threshold pumping value, taking into account the state weights of the levels, corresponds to the transfer to level “2” of about ⅓ of all active particles, which, when pumped with λ0.56 μm, requires specific radiation energy E pore >2 J/cm 3 (and power P pore > 2 kW/cm 3 at pump pulse durationτ ≈10 3 s ). Such a high value of power put into the lamp and ruby rod at a stationary ON can lead to its destruction, so the laser operates in a pulsed mode and requires intensive water cooling.

The laser circuit is shown in Fig. 2, b. To increase pumping efficiency, a pump lamp (flash lamp) and a ruby rod are located inside a reflector with a cylindrical inner surface and an ellipse-shaped cross-section, with the lamp and rod 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 “ button Kn " Cooling water is pumped inside the reflector. The laser radiation energy per pulse reaches several joules.

The pulsed operating mode 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. In Fig. 2b, the Q-switching of the OOR is carried out by rotating the mirror;

3) “mode locking” mode: with emission linewidth Δν several times ~10 11 Hz,

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

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

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

2.3. Erbium Fiber Optic Quantum Amplifier. This amplifier, often called “ 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 quartz fiber: SiO 2 : Er 3+ (Fig. 3, a). The bottom 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 a separate level 4 I 11/2 are auxiliary levels “3a” and “3b”.

Under the influence of pump radiation at wavelengths of 980 nm (or 1480 nm), ions Er 3+ transition from state “1” to short-lived states “3a” or “3b”, and then by fast non-radiative transitions ( w 32 ~10 6 s 1 ) into state “2”, which is quasi-metastable ( w 21 ~10 2 s 1, and τ 2 ~10ms). So 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 pumping level exceeds its threshold value W > W por , exceeds the population of level “1”, i.e. A population inversion and amplification will occur 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 level of OH W por and the frequency dependence of the gain are determined by the fiber structure (Fig. 3b), concentration Er 3+ and the wavelength of OH radiation. The pumping efficiency, namely the ratio of the unsaturated gain to unit power of the OH source, is for pumping from λ980 nm to 11 dB m 1 ∙mW 1 , and for λ1480nmabout 6dB m 1 ∙mW 1 .

Gain Frequency Range Matching EDFA the third “transparency window” of quartz fiber determines the use of such amplifiers as compensators for linear losses of modern fiber-optic communication lines (FOCL) with frequency division multiplexing (systems WDM: Wavelength Division Multiplexing, and DWDM: Dense Wavelength Division Multiplexing ). A section of an amplifier cable, pumped by semiconductor laser radiation, is quite simply connected to a fiber-optic link (Fig. 3c). The use of erbium fiber amplifiers in fiber-optic links replaces the technically much more complex method of “regeneration” of the signal - isolating a weak signal and restoring 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, ( V )simplified circuit for connecting an amplifier to a fiber-optic line: 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 a “four-level scheme”.

3.1. Theoretical analysis of the four-level scheme. In such a level scheme (Fig. 4), level “0” is the main energy state of the ensemble of particles, level “1”, connected by 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 through 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 are the same, and also assuming that

and, (6)

Let us write down a simplified system of kinetic equations for levels “3”, “2” and “1” in a stationary approximation, as well as the relation for the number of particles at all levels:

(7)

where n 0, n 1, n 2, n 3 , particle concentrations at levels 0,1,2,3; Wn 0 and Wn 3 the rate of absorption and stimulated emission at transitions between levels “0” and “3” under the influence of pump radiation, the probability of which W; w ik probabilities of transitions between levels, N the total number of active particles per unit volume.

From (6 and 7) we can find the populations of the levels 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 with a four-level scheme with OH, when conditions (6) and (9) are met:

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 universal and allows you to create population inversion, as well as carry out both pulsed and continuous lasing at any pumping levels (when the gain exceeds the losses in the OOR).

3.2. Neodymium laser. The laser uses a quantum transition between electronic energy levels Nd 3+ , laser lasing 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: Y3Al5O12 , and the doped crystal is denoted as Y 3 Al 5 O 12 : Nd 3+ or YAG: Nd 3+ . Nd 3+ 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+ ), gadolinium scandium gallium garnet crystals (GSGG: Nd 3+ ), yttrium-lithium fluoride YLiF4:Nd3+ , yttrium orthovanadate, organometallic liquids. Due to the cubic structure of the matrix, the luminescence spectrum of YAG has narrow lines, which determines the high gain of solid-state neodymium lasers, which can operate in both pulsed and continuous lasing modes.

Simplified electron 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 lasing threshold of this laser. Level 4 F 3/2 , whose lifetime is 0.2 ms, is the upper laser level “2”. Groups of levels (energy “zones”) “3a”…“3 d "play the role of auxiliary electronic level "3". Optical pumping is carried out through the “0” → “3” channel, the absorption bands have wavelengths near 0.52; 0.58; 0.75; 0.81 and 0.89 microns. From states “3a”… “3 d “Fast relaxation occurs due to non-radiative transitions to the upper laser state “2”.

For pumping, krypton and xenon gas-discharge lamps, halogen lamps with alkali metal additives in the filling gas, as well as semiconductor lamps are used. 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 continuous mode reaches 1 kW, record values achieved in pulsed mode: pulse energy about 200 kJ, and power 200 TW with a pulse duration of ~ 1 ns (laser created for experiments on controlled laser thermonuclear fusion - LTS).

There is a laser line in the YAG crystal Nd 3+ with λ1.06 μm is broadened uniformly (up to 0.7 nm), while in glasses there is a significant inhomogeneous broadening due to the Stark effect (Δν several ≈3∙10 12 Hz,), which allows you to successfully use the longitudinal mode synchronization mode (see section 3.3) with M ~10 4 and receive ultrashort pulses with a duration of about 1 ps.

Increased concentration of activator ions in media such as neodymium pentaphosphate ( NdP5O14 ), lithium neodymium tetraphosphate ( LiNdP 4 O 12 ) etc., ensures effective absorption of semiconductor laser radiation at distances of the order of fractions of a millimeter, which makes it possible 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 second and higher optical harmonics, crystals with quadratic and cubic nonlinear susceptibility (potassium dihydrogen phosphate KDP , potassium titanyl phosphate KTP ), with direct and (or) sequential (cascade) conversion. So, if you use a chain of crystals to emit a neodymium laser, you can obtain, in addition to IR radiation at the fundamental frequency with λ1.06 μm generation of the 2nd, 4th and 5th harmonics with wavelengths λ0.53 μm (green radiation); λ0.35 µm, λ0.26 µm and λ0.21 µm (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 glass (silicate, borate, etc.) as a matrix, other activator ions can be successfully used: Yb 3+ , Er 3+ , Tm 3+ , Ho 3+ with radiation in the range of 0.9...1.54 µm.

3.3. Conversion of radiation frequency in a nonlinear medium. The phenomenon of doubling and adding frequencies of light waves is as follows. When light propagates in a medium under the influence of the 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 a 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 = χ 0 = Const, р 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 the electric field of radiation begins to exceed the value of the intra-atomic field, polarizability becomes a nonlinear function E : That is, except linearly depending 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 with frequency ν propagates through the medium 0 and the 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. It is obvious that the energy of a pump wave with a frequency will be most efficiently pumped into a new wave with a frequency if the propagation speeds of these two waves are the same, i.e. if the 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 with frequencies and and wave vectors propagate in a crystal and, in addition to the harmonics of each wave, a wave with a total frequency is generated in the crystal: , and a wave with a difference frequency. The wave synchronism condition in this case has the form: .

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

3.4. Tunable dye lasers. Lasers using 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 regions of the spectrum. Their main advantage is a wide luminescence line (up to 50...100 nm), which makes it possible to smoothly adjust 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 leads to wide, usually structureless absorption and luminescence bands , as a result of the addition of “overlapping” transitions between such sublevels (Fig. 8, a). Between sublevels “inside” these zones, fast non-radiative transitions take place with the 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 s 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 S 1 (Fig. 8, a), analogues of level “2” in the diagram in Fig. 4 to the upper sublevels of the ground electronic state S 0 , 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 distribution of the population of states corresponds to Boltzmann’s law: the upper sub-levels “3” and “1” are weakly populated, and the lower sub-levels “0” and “2” are highly populated. This ratio for levels “0” and “3” determines for them the high efficiency of OH through the channel “0” → “3”, and the ratio for levels “2” and “1” determines population inversion, amplification and generation at this transition.

To obtain a narrow lasing 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-selecting elements (prisms, diffraction gratings, interferometers, etc.) is used (Fig. 8b).

Possibility of wavelength tuning within the luminescence line (Fig. 8, V ) without loss of power is determined by fast non-radiative transitions inside the electronic terms “2” and “1”, the probability of which exceeds the probability of induced transitions. Thus, when the resonator is tuned to any wavelength within the luminescence line of the “2” → “1” transition, laser radiation appears at the transition between the corresponding sublevels “2”ʹ " and "1 ʹ ", resulting in sublevel "2ʹ " by induced transitions is “cleared”, and “1ʹ » additional occupancy. However, due to OH and fast transitions from neighboring sublevels within the term, the population of the “generating” sublevel “2ʹ » is continuously being restored. At the same time, sublevel “1ʹ “With rapid transitions, it is continuously cleansed, eventually relaxing into the “0” state. Thus, the entire pumping of the upper electron term “2” becomes the pumping of the transition “2”ʹ »→«1 ʹ " 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 , reducing the population of levels “2ʹ ", transitions T 1 →"1", increasing the population of levels "1"ʹ ", and transitions T 1 → T 2 , absorbing laser radiation.

There are two types of dye lasers: incoherent (lamp) optical pumping by radiation from flash lamps and pulsed operating mode; and also with coherent pumping by radiation from other types of lasers (gas or solid-state) in continuous, quasi-continuous or pulsed operating modes. If you change dyes in a laser, and more than a thousand of them are known, then in this way you can “cover” with radiation the entire visible and part of the IR region of the spectrum (0.33...1.8 μm). In lasers with coherent pumping, ion sources are used as pump sources to obtain a continuous mode. Ar - or Kr -gas lasers. Gas lasers are used to pump dyes in a pulsed mode. N 2 , copper vapor, excimers, as well as ruby and neodymium lasers with frequency multiplication. It is often necessary to pump the dye solution, due to 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 , allow the generation of ultrashort radiation pulses (τ~10 14…10 13 s).

A special group consists of distributed feedback (DFB) dye lasers. 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 lasing line (~10 2 cm 1 ), which can be tuned within the gain band by changing the angle between the pump beams.

Among the areas of application of dye lasers are: photochemistry, selective pumping of quantum states in spectroscopy, isotope separation, etc.

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

Each electronic state Ti 3+ , consists of a large number of “overlapping” vibrational sublevels, which leads to structureless absorption and luminescence bands that are even wider than those of the dye as a result of the addition of “overlapping” transitions between such sublevels. Within these states, fast nonradiative transitions take place with the probabilities w ~10 9 s 1 , despite the fact that the probabilities of relaxation between electronic states are of the order of 10 5…10 6 s 1 .

The sapphire laser belongs to the so-called group. vibronic lasers, characterized in that their main electronic term is a strip of vibrational sublevels (crystal lattice), due to which the laser operates according to a four-level scheme, and, like a dye laser, creates the possibility of smooth tuning of generation in the range 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 lamp pumping of this laser ineffective, which, as a rule, is carried out by a continuous argon laser (λ488 nm and λ514.5 nm), the second harmonic of a neodymium laser (λ530 nm) or radiation pulses from a copper vapor laser (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 the order of tens of femtoseconds (1 fs = 10 15 c), and with subsequent compression (compression) of pulses in nonlinear optical fibers up to 0.6 fs.

3.6. Tunable lasers on color centers. Such lasers, like the solid-state lasers discussed above, use ionic crystals as the active substance, but with color centers called F - centers , which allows the restructuring 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 . Exposure of them to ionizing radiation: gamma quanta, high-energy electrons, X-rays and hard UV radiation, as well as calcination of crystals in alkali metal vapors leads to the appearance of point defects in the crystal lattice, localizing electrons or holes. A vacancy that has captured an electron forms a defect, the electronic structure of which is similar to the structure of the hydrogen atom. This color center has absorption bands in the visible and UV regions of the spectrum.

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

3.7. Fiber lasers. Fiber are called lasers whose resonator is built on the basis of an optical fiber-waveguide, which is also the active medium of the laser in which radiation is generated (Fig. 9). Uses quartz fiber doped with rare earth elements ( 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. Laser radiation propagates inside the optical fiber and therefore the fiber laser resonator is simple and does not require adjustment. 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(OGS) is carried out under the influence of laser optical pump 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 adjust the frequency of the output radiation. In a certain sense, OPO, like the phenomenon of frequency multiplication and addition discussed above, can be considered as the generation of tunable radiation under coherent optical pumping of a nonlinear crystal.

The OPO phenomenon, as well as the multiplication and addition of frequencies, is based on nonlinear optical phenomena in media. Let us consider the case when laser radiation of sufficiently high intensity, having a frequency ν, interacts with a medium having nonlinear properties and located in an open optical resonator (OOR). 0 (pumping). Due to pumping with the energy of this wave, two new light waves can arise in the medium:

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

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

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

The frequency tuning of the OPO radiation is carried out by selecting the orientation of the 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 angle value corresponds to a strictly defined combination of frequencies ν 1 and (ν 0 ν 1 ), for which the wave synchronism condition is currently satisfied.

To implement ASG, two schemes can be used:

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

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

A crystal can be used as an active medium LiNbO3 (lithium niobate), pumped by the second harmonic radiation 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 regions of nonlinearity and transparency allows for tuning in the IR region up to 16 microns.

5. Semiconductor lasers

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

5.1. Operating principle. In semiconductor lasers, unlike other types of lasers (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. Emission (luminescence) and generation of stimulated emission in semiconductors is caused by quantum transitions of electrons both between the energy levels of the conduction and valence bands, 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 excitonic states. Each energy band corresponds to a very large (~10 23 …10 24 ) number of allowed states. Since electrons are classified as fermions; then, for example, valence the zone can be completely or partially filled with electrons: with a density decreasing from bottom to top on 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 the act recombination charge carrierselectron and “hole” (an electron from the conduction band occupies a vacancy in the valence band), and the radiation wavelength is determinedband gap. If we create conditions such that the electron and hole before recombination will be in the same region of space for a sufficiently long time, and at that moment a photon with a frequency that is in resonance with the frequency of the quantum transition will pass through this region of space, then it can induce the process of recombination 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 for inversion to occur and generation to occur, it is necessary to create excess nonequilibrium concentrations of charge carriers: electrons in the conduction band, and holes in the valence band. In this case, the interval between quasi-Fermi levels must exceed the band gap, i.e. one or both quasi-Fermi levels will be located inside the allowed zones at distances no more than kT from their borders. And this presupposes excitation of such intensity that it creates degeneration in the conduction band and valence band.

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

The degeneracy of a semiconductor is achieved by heavily doping it at a high impurity concentration, such that the properties of the impurity are manifested mainly, and not the properties of the intrinsic semiconductor. Every atom donor The impurity gives up one of its electrons to the conduction band of the crystal. On the contrary, an atomacceptorThe impurity captures one electron, which was shared by the crystal and was located in the valence band. Degeneratena semiconductor is obtained, for example, by addingGaAstellurium impurities (concentration 3...5 1018 cm3 ), and 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 commonInP(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. Top layer of crystaldopedfor creatingparea, and in the lower layer it is creatednregion. The result is flatpnlarge area crossing. The two sides (ends) of the crystal are chipped 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 cavity, will cause forced recombination transitions, creating more and more photons with the same parameters, i.e. The radiation will intensify and generation will begin. In this case, the laser beam will be formed due to repeated passage along 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, whichare injectedVp-n-transition (active medium), when an electric current is passed through it in the “forward” direction with a “forward” bias, reducing the height of the potential barrier. This allows for the direct conversion of electrical energy (current) into coherent radiation.

Other pumping methods include electrical breakdown (so-calledstreamerslasers), electron beam pumping and optical pumping.

5.2. DGS lasers. If you place a layer with a narrowerprohibited area(active region) between two layers with a wider band gap, the so-called.heterostructure. The laser that uses it is called a double laser.heterostructure(DGS laser, or “double heterostructure”, DHS- laser). This structure is formed when connectinggallium arsenide(GaAs) andaluminum gallium arsenide(AlGaAs). The advantage of such lasers is the small thickness of the middle layer - the active region, where electrons and holes are localized: light is additionally reflected from heterojunctions, and the radiation will be confined to the region of maximum gain.

If two more layers with a lower refractive index are added to both sides of the GVD laser crystal compared to the central ones, then a reminiscentlight guidestructure that retains radiation more efficiently (GD laserwith separate retention, or "separate confinement heterostructure”, SCHS- laser). Most lasers produced in recent decades are made using this technology. The development of modern optoelectronics and solar energy is based on quantum heterostructures: incl. with quantum “holes”, 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, formingdiffraction grating. Thanks to this grating, radiation with only one wavelength returns back to the resonator, and generation occurs on it, i.e. The radiation wavelength is stabilized (lasers for multi-frequency fiber-optic communications).

A semiconductor “edge” laser that emits light in a direction perpendicular to the surface of the crystal and is called a “vertical cavity surface emitting laser” (VRC laser, or “verticalcavitysurface- emitting”: V.C.S.E.laser), has a symmetrical radiation pattern with a small divergence angle.

In the active medium of a semiconductor laser, very high gain can be achieved (up to 104 cm-1 ), due to which the dimensions of the active element of 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 tunability and a large selection of substances for generation in a wide spectral range from UV, visible to mid-IR. At the same time, compared to gas lasers, semiconductor lasers are distinguished 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 high radiation power is required from the device, and the condition of low beam divergence is not imposed. The areas of application of semiconductor lasers are: information processing devices - scanners, printers, optical storage devices, etc., measuring devices, pumping other lasers, laser target designators, fiber optics and technology.

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

Pioneers

Theodore Maiman was the first to demonstrate the principle of operation based on optical pumping using a flash lamp of synthetic ruby, producing pulsed coherent radiation with a wavelength of 694 nm.

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

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

The design and principle of operation of lasers

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

Two-tier environments

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

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

Multi-level systems

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

A three-level system requires very high pump power, since the lower level involved in lasing is the base level. In this case, in order for a population inversion to occur, more than half of the total number of atoms must be pumped to the E 1 state. In this case, energy is wasted. The pump power can be significantly reduced if the lower lasing level is not the base level, which requires at least a four-level system.

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

Solid State Laser

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

Holmium laser

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

Semiconductor quantum generator

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

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

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

P- and n-type semiconductors with high carrier densities form a p-n junction with a very thin (≈1 μm) depletion layer.

Gas laser

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

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

Molecular laser

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

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

Excimer lasers

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

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

Liquid laser

Compared to solids, liquids are more homogeneous and have a higher density of active atoms than gases. In addition to this, they are not difficult to manufacture, allow for simple heat dissipation and can be easily replaced. The operating principle of the laser is to use organic dyes such as DCM (4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran), rhodamine, styryl, LDS, coumarin, stilbene, etc. as the active medium ., dissolved in an appropriate solvent. A solution of dye molecules is excited by radiation whose wavelength has a good absorption coefficient. The principle of operation of a laser, in short, is to generate at a longer wavelength, called fluorescence. The difference between the absorbed energy and the emitted photons is used by non-radiative energy transitions and heats the system.

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

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

In such a scheme (Fig. 1), the lower laser level “1” is the main 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 non-radiative transition, is an auxiliary . Optical pumping operates through channel "1">"3".

Rice. 1. "Three-level" scheme with optical pumping

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 g1=g2=g3, we write a system of kinetic (balance) equations for levels “3” and “2” in a stationary approximation, as well as the relation for the number of particles at levels:

where n1, n2, n3 are the concentrations of particles at levels 1, 2 and 3, Wn1 and Wn3 are the rates of absorption and stimulated emission at transitions between levels “1” and “3” under the influence of pump radiation, the probability of which is W; wik - probabilities of transitions between levels, N - total number of active particles per unit volume.

From (2) we can find the populations of levels n2 and n1 as a function of W, and their difference Dn in the form

which determines the unsaturated gain b0 of the ensemble of particles at the transition "2">"1". In order for b0>0, it is necessary that, i.e. the numerator in (3) must be positive:

where Wthr is the threshold pump level. Since Wthr>0 is always, it follows that w32>w21, i.e. the probability of pumping level “2” by relaxation transitions from level “3” should be greater than the probability of its relaxation into state “1”.

w32 >>w21 and w32 >>w31, (5)

then from (3) we get: . And finally, if W>>w21, then the inversion of Dn will be: Dn?n2?N, i.e. at level "2" you can "collect" all the particles of the environment. Note that relations (5) for the rates of level relaxation correspond to the conditions for the generation of “spikes” (see Section 3.1).

Thus, in a three-level optically pumped system:

1) inversion is possible if w32>>w21 and maximum when w32>>w31;

2) inversion occurs at W>Wthr, i.e. creation is liminal;

3) at low w21, conditions are created for the “spike” mode of free laser generation.

This solid-state laser is the first laser to operate in the visible wavelength range (T. Meiman, 1960). Ruby is a synthetic crystal of Al2O3 in the modification of corundum (matrix) with an admixture of 0.05% activator ions Cr3+ (ion concentration ~1.6 1019 cm_3), and is designated as Al2O3:Cr3+. The ruby laser operates according to a three-level scheme with OH (Fig. 2, a). Laser levels are electronic levels of Cr3+: the lower laser level “1” is the ground energy state of Cr3+ in Al2O3, the upper laser level “2” is a long-lived metastable level with f2~10_3s. Levels "3a" and "3b" are auxiliary. The transitions “1”>”3a” and “1”>”3b” belong to the blue (λ0.41 μm) and “green” (λ0.56 μm) parts of the spectrum, and represent wide (with L ~ 50 nm) absorption contours (bands ).

Rice. 2. Ruby laser. (a) - Diagram of energy levels of Cr3+ in Al2O3 (corundum); (b) - design 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 that modulates the resonator quality factor, CH - storage capacitor, R - charging resistor, "Kn" - button for starting a current pulse through lamp; cooling water inlet and outlet are shown.

The optical pumping method ensures selective population of the auxiliary levels “3a” and “3b” of Cr3+ along the channel “1”>”3” with Cr3+ ions when Cr3+ ions absorb radiation from a pulsed xenon lamp. Then, in a relatively short time (~10_8 s), a nonradiative transition of these ions from “3a” and “3b” to levels “2” occurs. The energy released in this case is converted into vibrations of the crystal lattice. With sufficient radiation energy density from the pump source: when, and at the “2”>”1” transition, population inversion occurs and generation of radiation in the red region of the spectrum at l694.3 nm and l692.9 nm. The threshold pumping value, taking into account the state weights of the levels, corresponds to a transfer to level “2” of about? all active particles, which, when pumped with l0.56 μm, requires a specific radiation energy Epor>2 J/cm 3 (and power Rpore>2 kW/cm 3 with a pump pulse duration f? 10_3 s). Such a high value of power put into the lamp and ruby rod at a stationary ON can lead to its destruction, so the laser operates in a pulsed mode and requires intensive water cooling.

The laser circuit is shown in Fig. 2, b. To increase pumping efficiency, a pump lamp (flash lamp) and a ruby rod are located inside a reflector with a cylindrical inner surface and an ellipse-shaped cross-section, with the lamp and rod 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" button. Cooling water is pumped inside the reflector. The laser radiation energy per pulse reaches several joules.