The gas discharge tube is placed in an optical resonator, which is formed by mirrors with an interference coating. The mirrors are fixed in flanges, the design of which allows the mirrors to be rotated in two mutually perpendicular planes during adjustment by rotating the adjusting screws. The gas mixture is excited by supplying high-frequency voltage from the power supply to the electrodes. The power supply is a high-frequency generator that generates electromagnetic oscillations with a frequency of 30 MHz using several tens of watts.

Power supply of gas lasers with direct current at a voltage of 1000 ... 2000 V, obtained with the help of stabilized rectifiers, is widespread. In this case, the gas discharge tube is heated and cold cathode and anode. To ignite the discharge in the tube, an electrode is used to which a pulse voltage of about 12 kV is applied. this voltage is obtained by discharging a capacitor with a capacity of 1 ... 2 μF through the primary winding of a pulse transformer.

The advantage of helium-neon lasers is the coherence of their radiation, low power consumption (8 ... 10 W) and small size. The main disadvantages are low efficiency (0.01 ... 0.1%) and low output power, not exceeding 60 mW. These lasers can operate in a pulsed mode if a high-amplitude pulsed voltage with a duration of a few microseconds is used for excitation. The main areas of practical application of helium-neon lasers are research and measurement technology.

Of the ion lasers, the most widespread is the argon continuous-wave laser at a wavelength of 0.48 μm. Argon ions are formed in the cell as a result of the ionization of neutral Ag II atoms by a high-density current (~ 10 3 A / cm 3).

Population inversion in such a laser between the upper (4 p) and lower (4s) operating levels are created in this way. Level 4 p, which has a longer lifetime compared to the 4s level, are populated by argon ions due to collisions with fast electrons in a gas discharge due to transitions of excited ions from the group of higher levels 5 p... At the same time level 5 p, which has a very short lifetime, is rapidly emptied due to the return of ions to the ground state. Since levels 5 p, 5s, 4 p consist of groups of sublevels, generation can occur simultaneously at several wavelengths: from 0.45 to 0.515.

Currently, argon ion lasers are the most powerful sources of continuous coherent radiation in the ultraviolet and visible spectral ranges. The widespread use of high-power argon lasers is hindered by their high cost, complexity, low efficiency (~ 0.1%) and high power consumption (3 ... 5 kW).

BRIEF HISTORICAL OVERVIEW

The first calculations concerning the possibility of creating lasers, and the first patents related mainly to gas lasers, since the energy level schemes and excitation conditions in this case are more understandable than for substances in the solid state. However, the ruby ​​laser was discovered first, although a gas laser was soon created. At the end of 1960, Javan, Bennett, and Herriott created a helium-neon laser operating in the infrared region on a number of lines in the 1 μm region. In the next two years, the helium-neon laser was improved, and other gas lasers, operating in the infrared region, were also discovered, including lasers using other noble gases and atomic oxygen. However, the greatest interest in gas lasers was caused by the discovery of the generation of a helium-neon laser on the 6328 A red line under conditions that only slightly differed from the conditions under which lasing was obtained in the first gas laser. Obtaining lasing in the visible region of the spectrum stimulated interest not only in the search for additional transitions of this type, but also in laser applications, since many new and unexpected phenomena were discovered, and the laser beam received new applications as a laboratory instrument. The two years following the discovery of lasing on the 6328 A line were full of many technical improvements, mainly aimed at achieving higher power and more compactness of this type of laser. In the meantime, the search for new wavelengths continued and many infrared and several new transitions in the visible spectrum were discovered. The most important of these is the discovery by Mathias of pulsed laser transitions in molecular nitrogen and carbon monoxide.

The next most important stage in the development of lasers was apparently the discovery by Bell at the end of 1963. a laser operating on mercury ions. Although the mercury ion laser by itself did not meet the initial hopes of obtaining high CW powers in the red and green regions of the spectrum, this discovery indicated new discharge modes in which laser transitions in the visible region of the spectrum can be detected. Searches for such transitions were also carried out among other ions. It was soon discovered that argon ions are the best source of high-power laser transitions in the visible region and that cw lasing can be obtained from them. Further improvements to the CW argon laser have yielded the highest power available in the visible region. As a result of searches, lasing was discovered on 200 ion transitions, concentrated mainly in the visible and also in the ultraviolet parts of the spectrum. Such a search, apparently, is not yet over; in the journals on applied physics and in technical journals there are frequent reports of generation at new wavelengths,

In the meantime, technological advances in lasers rapidly expanded, with the result that many of the "witchcraft" tricks of the first designs of helium-neon and other gas lasers disappeared. Bennett's research on such lasers continued until a helium-neon laser was developed that could be placed on a conventional bench with complete confidence that the laser would perform as expected. The argon ion laser is not as well researched; however, a large number of original works by Gordon Bridges make it possible to predict, within reasonable limits, the possible parameters of such a laser.

Over the past year, a number of interesting papers have appeared on gas lasers, but it is too early to determine their relative value. To everyone's surprise, the most important achievement was the discovery by Peitel of stimulated emission in CO2 in the 1.6 micron band with high efficiency. the output power of these lasers can be increased to hundreds of watts, promising to open up a whole new area of ​​laser applications.
List of used literature:

Encyclopedic Dictionary of a Young Physicist (editor-in-chief Migdal A.B.)

Moscow "Pedagogy" 1991

N.M. Shakhmaev, S.N. Shakhmaev, D.Sh. Shodiev "Physics 11"

Moscow "Education" 1993

OFKabardin "Physics" Moscow "Education" 1988

“Gas Lasers” (ed. By NN Sobolev) Moscow “Mir” 1968.

"Fundamentals of laser technology" Baiborodin Yu. V. 2nd ed., K.: 1988, 383s.

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

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

The 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 pump source (for example, a powerful lamp - a flash), 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 influence of spontaneous emission, the process of stimulated emission of light begins. Due to population inversion, this process is avalanche in nature and leads to an exponential amplification of light. The streams of light going 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 emitted outside, forming a laser beam.

Fig. 6. Laser concept. 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.

1). The laser consists of a gas-discharge tube T with a length of several tens of cm to 1.5-2 m and an inner diameter of 7-10 mm. The tube is filled with a mixture of helium (pressure ~ 1 mm Hg) and neon (pressure ~ 0.1 mm Hg). The ends of the tube are closed by plane-parallel glass or quartz plates P 1 and P 2, set at a Brewster angle to its axis. This creates linear polarization of the 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 the mirror, through which the laser radiation predominantly exits, is usually 2%, the other 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 milliamperes. The laser generates red light with a wavelength  = 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 reflectivity in the infrared region.

2). In lasers, stimulated radiation is used to generate coherent light waves. This idea was first expressed in 1957 by A.M. Prokhorov, N.G. Basov and independently of them C. Towns. In order to transform the active substance of the laser into a generator of light oscillations, it is necessary to carry out a feedback. This means that part of the emitted light must constantly return to the zone of the active substance and cause a forced emission of more and more atoms. For this, the active substance is placed between two mirrors S 1 and S 2 (see Fig. 7), which are feedback elements. A ray of light, undergoing multiple reflections from mirrors S 1 and S 2, will pass many times through the active substance, while amplifying as a result of forced transitions from a higher energy level  "3 to a lower level " 1. The result is an open resonator, in which the mirrors provide multiple transmission (and thereby amplification) of the luminous flux in the active medium. In a real laser, some of the light must be released from the active medium to the outside to be usable. 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 suggest that 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, the oblique rays will eventually hit the side wall of the cylinder, where they will scatter or come out. It is therefore clear that the beams propagating parallel to the axis of the cylinder will be maximally amplified. This explains the collimation of the rays. Of course, strictly parallel rays cannot be obtained. This is prevented by light diffraction. The divergence angle of the rays, in principle, cannot 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 be Z is the optical path length between the mirrors. If 2 Z= m, that is, along the length Z fits an integer number of half-waves m, then the light wave, leaving from 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. Closest wavelength  , for which the same amplification should occur, can be found from the condition 2 Z=(m1)( ). Hence,  = / m, that is  , as expected, coincides with the spectral range of Fabry-Perot interferometers. Let us now take into account that the energy levels "3 and " 1 and the spectral lines arising 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 the atoms is less than the dispersed region of the device. Then, of all wavelengths emitted by atoms, the condition 2 Z= m only one wavelength can satisfy ... 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 monochromatization of light.

Basic properties of a laser light beam:

    monochromaticity;

    spatial and temporal coherence;

    high intensity;

    small divergence of the beam.

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.

Ion lasers

Ion lasers are a type of gas lasers in which the upper level is populated by two successive collisions with electrons in an electrical discharge (ionization + excitation). Ion energies are superior to atomic ones, so ion lasers generate in the visible and UV spectral regions.

Due to the high current density in the discharge tube, ions can be pumped to the cathode; therefore, an additional bypass drip is required. To prevent destruction of the tube during bombardment with fast ions, it is made of ceramic and placed in a longitudinal magnetic field created by a solenoid. Radially moving charged particles experience the deflecting action of the Lorentz force, as a result, their trajectories are curved, reducing the rate of diffusion of charges to the walls. An example is an argon laser that generates in the visible region on the lines l 1 = 488 im (blue) and l 2 = 514.5 im (green).

When designing transmitting devices for optical communication systems, an engineer is inevitably faced with the need to choose a radiation source - an optical quantum generator. The choice of the generator depends on the specific conditions of the communication system application: its location (ground or space, mobile or stationary versions), spectral range of operation, pulsed or continuous mode, required output power, required beam divergence and frequency stability, transmitter efficiency, generator resource and period service of the system, types of modulation and reception, the need to take into account the atmosphere, etc. Each of these factors must be considered. From the most general considerations, the following recommendations can be made .

Gas laser systems have high monochromaticity and frequency stability, as well as a small beam divergence angle; they can operate in both continuous and pulsed modes at high repetition rates. The disadvantages of gas laser are low efficiency (excluding carbon dioxide laser) and relatively large dimensions. Solid state lasers are characterized by high pulse power and the ability to receive very short pulses. However, their inherent disadvantages - low efficiency and difficulties in implementing a continuous mode of operation - limit to a certain extent their use in communication systems. Semiconductor lasers have high efficiency, small dimensions, and the possibility of direct modulation by the pump current. However, a very wide spectrum of the output signal and a large beam divergence angle restrain widespread introduction into optical communication systems.

The most suitable for broadband optical communication systems are helium-neon lasers, argon ion lasers, YAG: Nd 3+ (in main mode or with frequency doubling) and carbon dioxide laser. In communication systems with power less than 100 mW, for which the size of the laser and low efficiency are not limiting factors, helium-neon laser with good spectral properties, low beam divergence and long service life are acceptable. In communication systems with an output power of more than 100 mW, argon ion lasers, YAG: Nd 3+ and CO 2 are considered the most suitable. The first two OGCs, although they have low efficiency, can be effectively used in multichannel communication systems with increased bandwidth operating in the pulse-code modulation mode. For this, the laser must emit in the mode locking mode. The main obstacle to the widespread use of carbon dioxide lasers, which have a high efficiency and provide the required level of output power, is the need to develop broadband photodetectors with cooling for receiving radiation with a wavelength of 10.6 μm. This obstacle is currently being successfully overcome.

  • by type of active medium:

o solid-state;

o gas;

o liquid;

o semiconductor;

o plasma.

  • by pumping type:

types of pumping:

o optical;

o electrical discharge in gases;

o electroionization;

o thermal (gas-dynamic);

o chemical.

2. Solid state lasers.

Solid state lasers are lasers that use a crystalline or amorphous dielectric.

Main features of solid state lasers:

  • high concentration of particles: up to 10 19 and even up to 10 21 cm -3;
  • high specific energy output;
  • generation at small lengths;
  • optical uniformity (inferior to gas lasers);
  • luminescence line width (units of A ° - tens of A °),
  • the main type of pumping is optical pumping.

Active medium for solid-state lasers:

Matrix (base) + activator (impurity).

The activator is usually from fractions to several percent in relation to the matrix.

The principle of operation of solid-state lasers.

In a 2-level optical pumping system, inversion cannot be created.

In practice, they use 3 or 4-level systems.

Multiple levels can be used as level 3 in a 3-level circuit, and level 4 in a 4-level circuit.

A 4-level circuit has a lower generation threshold.

As matrices a wide class of substances is used, in particular, salts of tungsten, molybdenic and hydrofluoric acids (H 2 WO 4, H 2 MoO 4, HF), corundum Al 2 O 3, yttrium garnets Y 3 Me 5 O 12(where Me – Al, Cu, Fe), for example Y 3 Al 5 O 12 - YAG, glasses of various compositions.

As activator - chromium, cobalt, nickel, titanium, as well as many rare earth elements.

Examples of effective laser media:

Al 2 O 3: Cr 3+; Y 3 Al 5 O 12: Nd 3+; CaF: Nd 3+; glass: Nd 3+ etc. (see reference).

The active elements of solid-state lasers have various forms:

The most commonly used form is a).

Optical pumping systems for solid-state lasers.

Optical pumping system designed to create inversion in active environments.

Both coherent (laser) pumping and incoherent (lamp) pumping are used.

In the case of incoherent (lamp) pumping, the optical pumping system consists of source of optical radiation(special lamp), illuminator(reflector) and electrical power supply feeding the source of optical radiation.

For example, an optical pumping system may include the following elements:

  1. step-up transistor;
  2. rectifier;
  3. capacity (capacitive storage);
  4. pump lamp;
  5. illuminator;
  6. flash lamp ignition system;
  7. active element.

Special flash lamps are used, as well as continuous lamps.

The pump energy should not exceed the lamp limit.

U c

Ignition system ( 6 ) controls the start of pumping (discharge in the lamp).

Pump lamps are most often in the form of a cylinder with electrodes ( rice. 4). Since the lamp emits in all directions, a very small fraction of its radiation falls on the active element ( rice. 5). Therefore, a reflector (illuminator) is needed, which would send as large a fraction of the radiation as possible to the active element. Examples of such illuminators are an elliptical cylinder ( rice. 6) and circular cylinder ( rice. 7), the inner surfaces of which have high reflection coefficients.

In the case of high-power lasers, multi-tube pumping and a large-diameter element are required. On rice. eight A schematic representation of such a system is shown, along the central axis of which an active element (AE) is located, and along the focal lines of semi-ellipses, a pump lamp (BP):

The pumping system must provide:

o high efficiency of radiation transmission from the pump lamp to the active element;

o high uniformity (uniformity) of pumping in the volume of the active element (both along the length and in the cross section).

The non-uniformity of the optical pumping of the active element (especially in the cross section) leads to thermo-optical distortions due to the non-uniformity of its heating, and strongly affects the characteristics of laser radiation (lasing threshold, angular divergence, radiation energy) and can even lead to lasing disruption. Thermo-optical distortions arise due to the dependence of the refractive index on heat transfer and its unevenness in the active element.

The appearance of thermo-optical distortions is equivalent to a change in the configuration of the resonator, since the optical length of the resonator is equal to.

In solid-state lasers, thermo-optical effects are strongly manifested, since the refractive index n highly temperature dependent T. On rice. nine the case is shown where the central region of the active element has a higher temperature (shaded) than the peripheral region.

On rice. ten a possible case of nonuniform pumping (and, consequently, temperature) of an active element under isotropic illumination of its cylindrical lateral surface is shown. The cylindrical active element behaves like a cylindrical lens.

The appearance of thermo-optical distortions of solid-state lasers is caused, in addition to pump irregularities, by the cooling of the lateral surface, since the thermal conductivity is limited, and the central part of the active element will have a higher temperature than the lateral surface.

To increase the uniformity of pumping, in particular, the so-called immersion cladding is used.

It also increases the density of the pumping energy in the active element, since the size of the cross-section that "captures" the pump radiation increases.

This harmful phenomenon "eats up" the inversion and reduces the generation energy in the direction of the fundamental radiation, that is, degrades the characteristics of the radiation.

To combat it, immersion shells are used, and the lateral surface (in whole or in part - a strip and rings) of the active element is roughened.

The disadvantage of lamp pumping is that its spectrum is much wider than absorption bands ( rice. 13).

With coherent (laser) pumping, the pump radiation can be ideally matched to the absorption bands.

Coherent pumping is the most efficient in terms of spectrum matching. For coherent pumping of solid-state lasers, semiconductor lasers are most widely used. An example of such pumping is shown in fig. 14.

  1. semiconductor laser power supply;
  2. semiconductor laser;
  3. matching optics;
  4. pumped up by tt. laser.

Let us consider as an example the working circuits of some solid-state lasers.

Ruby laser.

Al 2 O 3: Cr 3+- ruby, where chromium ions are used as active centers Cr 3+ introduced as an activator into the matrix Al 2 O 3... The laser works according to the three-level scheme shown in fig. 15.

The generation energy per pulse is up to 100 J.

Neodymium glass laser.

The active medium of the laser is glasses of various compositions, where neodymium ions are used as active centers. Nd 3+ introduced as an activator into a glass matrix, the laser of which operates according to the four-level scheme shown in rice. 16.

YAG laser.

The active medium of the laser is Y 3 A l 5 O 12: Nd- yttrium - aluminum garnet, where neodymium ions are used as an activator ( Nd 3+) introduced into the YAG as an activator. The operation of the laser is similar to that of a neodymium glass laser. The laser works according to a four-level scheme.

Generation in continuous mode is possible (up to 500 W-1 kW).

Solid state microlasers.

Miniature solid-state lasers can be realized with a high concentration of particles - up to 10 21 cm -3 (tens - hundreds of times more than in YAG and glass). Pumping is carried out by LEDs or semiconductor lasers (coherent pumping).

Materials allowing the introduction of a high concentration of the activator:

  • neodymium petnophosphate NdP 5 O 14;
  • potassium neodymium tetrophosphate KNdP 4 O 12;
  • neodymium-aluminum borate NdAl 3 (BO 3) 4;
  • neodymium lithium tetrophosphate LiNdP 4 O 12;

Pulse power - several W, .

They can provide single-mode lasing and compete with semiconductor lasers. They can operate in a stable single-frequency mode, provide high coherence and monochromaticity of radiation, low temperature dependence.

  • gadolinium-scandium-gallium garnets (GHA), etc.

In the field of glasses, KNFS glasses (lithium-neodymium-lanthanum-phosphate glasses) are considered the most promising. Concentration Nd up to 10 21 cm -3.

Tunable solid state lasers.

Tunable solid-state lasers are divided into 3 groups:

1. Crystals activated by ions of transition elements.

Examples:

Alexandrite BeAl 2 O 4: Cr 3+(0.70-0.82 μm);

· Al 2 O 3: Ti 3+(0.68-0.93μm);

· KZn 3: Cr 3+(0.78-0.86μm);

· ZnWO 4: Cr 3+(0.9-1.1μm).

2. Lasers on color centers (LCO).

Color centers (CCs) are crystal lattice defects that absorb light in the spectral region where there is no intrinsic absorption of the crystal ( rice. 17).

Crystal lattice defects:

· Vacancies (ions removed from the crystal lattice sites);

· Interstitial ions;

· Impurity atoms;

Color centers are labeled differently according to the type of defect. So, for example, centers caused by anionic vacancies capturing electrons are called f centers.

They work according to a 4-level scheme, have a low excitation threshold, broadband absorption and luminescence spectrum.

On rice. 17 a possible structure of the energy levels of a solid-state laser based on color centers is shown.

CW lasers use laser pumping. CO lasers can generate subnanosecond pulses.

Tuning 0.7-3.3 microns.

LiF (0.62-1.25μm);

NaF (0.99-1.4μm);

RbCl: Li (2.55-3.28μm)

Currently, lasers are being improved on precious and semi-precious stones (diamond, sapphire, alexandrite)

3. Solid state liquid lasers .

Solid-state lasers are widely used in many fields of science and technology, including medicine.

Pulsed YAG lasers are widely used (in particular) in medicine:

With holmium Ho (λ = 2.1μm);

With erbium Er (λ = 2.79-2.9 μm) - the best absorption in water;

With thulium Tm (λ = 1.96-2.01μm).

In surgery, in addition:

YAG: ( λ = 1.06μm);

YAG: ( λ = 1.32μm);

KDR-532 ( λ = 0.532μm).

Crystals of chromium-containing scandium garnets can serve as the basis for creating a wide range of medical lasers:

· ISGG: Cr-Nd (yttrium-scandium-gallium garnet).

Erbium glass miniature lasers (chromium-ytterbium-erbium glass)

LGS-X λ = 1.54μm.

Gas lasers.

Gas lasers lasers are called, the active medium of which is in a gaseous state. These can be gases proper, or vapors of liquid or solid substances.

Key Features:

· High homogeneity of the active medium;

· A high degree of monochromaticity and coherence of radiation as a result of less mutual influence of particles.

Because of the line spectra (narrow bands) of absorption, optical pumping is rarely used.

The most widespread are pumping by means of an electric discharge (both independent and non-self-sustained), as well as chemical pumping and thermal (gas-dynamic) pumping.

The design of the active medium is a cuvette (for example, a tube) in which there is a gaseous medium, and the cuvette windows are often inclined at the Brewster angle to the cuvette axis to reduce the Fresnel losses at the windows (see. fig. 18)

1. a cuvette filled with gas.

2. Brewster windows (installed at Brewster angle i Br). i Br = arctan n,
where n is the relative refractive index of the window material.

In this case, radiation polarized in the plane of incidence will not experience Fresnel reflection at the windows and for it there will be the smallest losses in the cavity. It is on this polarization that radiation will be generated, that is, the radiation will be linearly polarized.

Gas lasers are classified into:

· Atomic (neutral atoms are used);

· Molecular (neutral molecules are used);

· Ionic (ions are used).

Depending on the type of pumping, gas lasers are subdivided into:

Gas-electric discharge (independent electric discharge)

Electroionization (non-self-sustaining electric discharge)

Gas-dynamic (heat pumping)

Chemical (chemical pumping)

Mechanisms for creating inversion in gas-discharge lasers.

Gas discharge is called a set of processes associated with the passage of an electric current through a gaseous medium.

When a discharge occurs, a gas-discharge plasma is formed (a special medium), which is characterized by a significant concentration of charged and excited particles.

Gas lasers use a glow discharge and an arc. Pumping with direct current, both continuous and pulsed, as well as high-frequency excitation are used.

The following processes lead to the excitation of particles and the formation of inversion:

Direct electronic excitation (inelastic collisions of electrons with particles)

e+ A → e + A *

Step electronic excitation

e+ A * → e + A **

In addition to these processes, in the case of using auxiliary (impurity) gases, these processes can be supplemented by the excitation of the main gas due to collisions and resonant energy exchange between the particles of the auxiliary and main gases:

e+ B = e + B *

B * + A = B = A *,

where A- particles of the main gas.

V- particles of auxiliary gas (impurity gas).

This mechanism significantly increases the efficiency of creating inversion in gas-discharge lasers, since it makes it possible to selectively populate the upper working (laser) levels.

In addition, impurity gases are used for more efficient cooling, unloading lower laser levels (for example, He in a laser on CO 2).

Gas lasers use both longitudinal and transverse electrical discharge.

Lasers with high blood pressure(to atmospheric and more) use , a low pressure(units, tens of torus), as a rule, longitudinal discharge.

To cool the working mixture, gas lasers are used as longitudinal and transverse blowing gas, moreover cross blowing is more efficient, since the change of the mixture occurs faster than when blowing along the cuvette (see Fig.), since the cuvette width is much less than its length: h<.

Gas lasers high blood pressure, use transverse electric discharge and cross blow and denoted as TEM lasers.

To ensure a uniform electric discharge in the entire volume of the working mixture of TEA lasers, it is used preionization system, which creates in the working volume of the gas a sufficient number of charged particles (electrons and ions) before the moment the main voltage is applied between the electrodes.

Electron guns, UV radiation, and creeping discharge are used to preionize TEA lasers.

The greater the gas pressure, the greater the concentration of active particles per unit volume and, accordingly, more specific energy consumption.

In lasers low pressure The broadening of the emission line is mainly determined by the Doppler effect and is inhomogeneous, and at significant pressures, collisional processes prevail, which determine the homogeneous broadening.

Thus, the nature of the broadening of the emission line depends on the gas pressure.

V atomic lasers are used electronic transitions(transitions between electronic levels), and in molecular, mainly transitions between vibrational and rotational levels.

Molecular lasers give the longest-wavelength radiation, since they use transitions between vibrational and rotational levels: the energy of transitions between them is much less than between electronic levels: ∆E el<<∆E к << ∆E вр .

The emission characteristics of gas lasers depend both on the total gas pressure and on the partial pressures of the mixture components (their ratio) - the main and auxiliary gases.

V ion lasers nessesary to use high current densities since in addition to the excitation of ions, it is necessary to create their high concentration from neutral atoms.

A feature of electron-beam-controlled lasers is the ability to provide optimal values ​​of electron energies for exciting the required energy levels, which cannot be realized in lasers with a self-sustained electric discharge. Let us explain this.

In gas-discharge lasers, the energy of electrons is spent both on the creation of a conducting plasma and on the excitation of active particles. Moreover, the energy optima for these two functions are different. The separation of these functions is carried out in electron-beam-controlled lasers using a non-self-sustaining charge.

Let us consider as an example some types of gas-discharge lasers.

Ionic lasers.

Cuvette- capillary (to obtain high current densities at not very large values).

As an active medium in gas lasers are widely used CO 2, N 2, CO, H 2, HF, HCl, NO 2 and many other molecules.

Excimer lasers

(scattering molecule lasers).

A feature of excimer lasers is generation in the UV and visible region of the spectrum.

As active environment they use quasimolecules or excimer atomic complexes that appear and exist only in an excited state.

Laser radiation arises during the transition of the excimer complex from an excited state (2) to an unexcited state (1), after which they decay into atoms.

Excimer lasers operate on electronic-vibrational transitions in such a way that when a molecule enters level (1), where there is no potential well, it decays into atoms.

An active medium on scattered molecules is an environment with a constantly empty lower working level.

Excimer molecules include molecules such as:

Ar 2 *, Xe 2 *, Kr 2 *, ArO *, KrO *, XeO *, XeF * and etc.

Excimer lasers operate at elevated pressures (up to 10 atm.) To increase the likelihood of molecular formation.

Excitation is produced by a beam of high-energy electrons e(hundreds of keV - 1 MeV), electric discharge, fast transverse discharge and optical excitation.

An example of a reaction leading to the formation of molecules:

Xe + + Xe → Xe 2 + + e → Xe 2 *

Xe * + Xe → Xe 2 *

The duration of the excitation pulse is several tens of ns.

Gas-dynamic lasers

Such lasers are called lasers, in which population inversion is created by the rapid expansion of a preheated gas mixture.

The energy source is vibrationally excited molecules in a highly heated gas, and amplification arises due to the difference in the rates of relaxation of the lower and upper laser levels during gas outflow through a supersonic nozzle. This unique type of laser is a direct conversion of thermal energy into coherent radiation energy.

Thus, population inversion in a gas-discharge laser is provided by heating and rapid expansion of the working gas.

N 2: CO 2: H 2 O

91,3 % 7,5 % 1,2 %

Active centers - molecules CO 2; t up to 1500 ° C.

Behind the nozzle, due to a sharp expansion of gases and a drop in temperature, the distribution of atoms over levels should relax to a new equilibrium state corresponding to a lower temperature (about 300 ° C).

At new temperature (behind the nozzle):

The power of such a laser is determined by the gas flow rate.

Pre-excitation (heating) can be provided by both chemical reactions and electric discharge.

t u- the moment the inversion appears.

Z and = t and V gas is the distance from the nozzle where the inversion area starts.

V gas a is the gas flow rate.

Chemical lasers.

Chemical lasers- these are lasers in which the excitation and inversion of populations is achieved through the implementation of chemical reactions. The connections are rearranged in such a way that the components are in an excited state.

There are 2 types of chemical lasers:

· with the initiation of a chemical reaction when to ensure the conditions,

necessary for a chemical reaction to proceed, preliminary excitation of the reactants entering into the reaction (dissociation, photodissociation, heating) is required. This leads to the need for special initiating devices;

A chemical reaction occurs spontaneously when mixing components

(no initiation). The generation of chemical lasers is due to the appearance of an inversion between the vibrational-rotational or rotational levels of diatomic molecules, which is formed as a result of chemical interaction.

An example of a chemical laser without initiating a chemical reaction:

H 2 + F = HF * + H F- atomic fluorine.

(D 2) (DF *)

F 2 + NO → ONF + F- this is how atomic fluorine is obtained as a result of a chemical reaction.

HF *- vibrational excited molecule.

V = 1…..6

λ = (3.5 ÷ 5.0) μm

There are a large number of chemical lasers (see literature).

Liquid lasers

Liquid lasers- these are lasers, where liquid media are used as an active medium.

In this regard, they have a number of features:

· The volume of the active medium is not limited;

· Higher optical uniformity in comparison with solids;

· The possibility of a higher concentration of active centers in comparison with gases, which makes it possible to generate high power;

· The problem of heat removal is easily solved, since the liquid can be pumped through the working volume;

· The shape of the active element is determined by the shape of the cuvette, which is filled with liquid.

For example:

Depending on the type of active medium, liquid lasers are divided into 3 types:

1. Rare-earth chelate solution lasers(complex organic

complexes in which ions of rare earth elements are surrounded by oxygen atoms belonging to an organic molecule);

2. Lasers based on solutions of inorganic compounds of rare-earth elements

(typical ionic systems). They are characterized by high efficiency and photochemical stability (for example, a solution of neodymium oxide in selenium oxychloride Nd (SeOCl 2). Operation is similar to a solid-state neodymium glass laser.

3. Organic dye solutions. These lasers are the most widely

are widespread and provide the ability to tune the wavelength in a wide range of wavelengths (from UV to IR).

Active environment liquid lasers consist of a solvent and an active substance dissolved in it.

Various substances are used as a solvent, for example, such as:

· distilled water;

· Alcohols;

Acids;

Glycerin;

· Acetone.

In lasers based on solutions of organic dyes, organic dyes are used, which constitute a wide class of complex organic compounds, which, unlike other laser materials, is characterized by a wide luminescence band (up to 0.2 μm) and has an unstable upper laser level (excited state duration 10 -8 h 10 -9 s).

Laser radiation was obtained on dyes belonging to 3 groups:

1. Xanthene dyes;

2. Polymethine dyes;

3. Derivatives of coumarin.

Currently, the following dyes, in particular, are widely used:

Rhodamine 6G (λ - 0.55 μm) I

Rhodamine G (λ - 0.585 μm) I Solvent -

Rhodamine B (λ - 0.608 μm) I ethyl alcohol.

Acridone (λ - 0.437 μm) I

And others (see reference materials).

Basic physical concepts of the mechanism of generation of dye solutions.

At the beginning, when creating liquid lasers, they tried to obtain lasing in the same way as in solids. They introduced impurity ions, looked for narrow energy levels (metastable), introduced elements of rare earths, iron, etc. The generation was very inefficient.

Then they realized that if the levels are wide enough, then you can get generation in a two-level system, which is impossible if the levels are narrow, since it is impossible to carry out the inversion.

So, the main feature of dye lasers is the use of two levels of significant width.

The dye molecules are very complex and have broad energy levels (bands). A stripe is a wide layer, consisting of a huge number of sublevels. The diagram below shows the lower electronic vibrational levels of the dye molecule.

τ v.y., τ n..y- time of internal relaxation;

S- singlet levels (have compensated spins);

T- triplet levels (have uncompensated spin).

Singlet-singlet transitions are more probable than singlet-triplet transitions, since the latter are associated with spin reorientation. The spin reorientation is associated with particle collisions.

S 0: ↓↓↓ compensated

S 1: ↓↓ ↓ spin

T 0: ↓↓ uncompensated spin

Pumping is done from the bottom of the strip S 0 to the top of the strip S 1... In this case, thermal equilibrium (Boltzmann distribution) is violated both between the levels S 1 - S 0, and between sublevels inside each of the stripes S 1 and S 0... Relaxation time between levels S 1 and S 0 is ~ 10 -8 ÷ 10 -9 s(interlevel relaxation time) and is much longer than the relaxation time between sublevels of the band S 0 and stripes S 1 which is ~ 10 -12 s(time of intra-level relaxation).

Thus, the interlevel relaxation time S 1 → S 0 much longer intralevel relaxation time in bands S 1 and S 0.

This circumstance makes it possible to obtain the population inversion between the lower part of the strip S 1 and the top of the strip S 0 when exposed to the pumping described above. In this case, generation is possible in a wide range of wavelengths corresponding to transitions between different sublevels of the lower part of the S 1 band and the upper part of the band. S 0 and it is possible to tune the generated wavelengths in a wide range!

Note that the pump pulse duration should be short and not exceed the relaxation time S 1 → T 1, since otherwise the molecules will begin to move to the level T 0 then go up to the level T 1 and the generation will stop, since the molecules will not return to their original state S 0.

Thus, although in this case 2 levels (but wide) are used, generation occurs as in a four-level scheme with all its advantages.

Additional clarification:

On rice. 35 the dotted line shows the distribution of particles before the start of pumping (equilibrium Boltzmann distribution), and the solid lines show the distribution that is established inside the bands S 1 and S 0 after pumping during the intralevel relaxation time and indicating the occurrence of inversion between a part of the sublevel band S 1 and S 0.

Methods of excitation (pumping) of liquid lasers .

Dye solution lasers work with optically pumped.

An important feature is that the pulse should not exceed the interlevel relaxation time S 1 → T 0, that is, to be no more 10 -6 s... With a short pulse, the transitions S 1 → T 0 do not have time to manifest. For pumping, they are used as lasers (laser pumping), usually operating in the Q-switched mode ( generation τ ~ 10 -8 ÷ 10 -9 s), and special pump lamps (in particular, of a coaxial design, having a low inductance), emitting short pulses.

With laser pumping (for example, using a ruby ​​laser) Q-switched (especially for phthalocyanine dyes), a Q-switched neodymium laser (for polymethine dyes), nitrogen laser ( λ ~ 3000Å) there are 2 options:

  1. Longitudinal pumping:

  1. Transverse pumping:

For lamp pumping, coaxial lamps are used, in particular.