Baltic State Technical University
"Voenmekh" them. D. F. Ustinova
Department I4
"Radioelectronic control systems"

Devices for receiving and converting signals
Coursework on the topic
« quantum generators »

Completed:
Peredelsky Oleg
Group I471
Checked:
Tarasov A.I.

St. Petersburg
2010

1. Introduction
This paper discusses the principles of operation of quantum generators, the circuits of generators, their design features, the stability of the frequency of generators, and the principles of modulation in quantum generators.
1.1 General information
The principle of operation of quantum generators is based on the interaction of a high-frequency field with atoms or molecules of a substance. They make it possible to generate oscillations of much higher frequency and high stability.
Based on quantum generators, it is possible to create frequency standards that exceed all existing standards in accuracy. Long-term frequency stability, i.e. stability over a long period is estimated at 10 -9 - 10 -10 , and short-term stability (minutes) can reach 10 -11 .

Currently in Time quantum generators are widely used as frequency standards in time service systems. Quantum amplifiers used in receivers of various radio engineering systems can significantly increase the sensitivity of the equipment and reduce the level of internal noise.
One of the features of quantum generators, which determines their rapid improvement, is their ability to operate efficiently at very high frequencies, including the optical range, i.e., practically up to frequencies of the order of 10 9 MHz
Optical range generators make it possible to obtain a high directivity of radiation, a high energy density in a light beam (of the order of 10 12 -10 13 w/m 2 ) and a huge frequency range, allowing the transmission of a large amount of information.
The use of optical range generators in communication, location and navigation systems opens up new prospects for a significant increase in the range and reliability of communications, the resolution of radar systems in range and angle, as well as the prospects for creating high-precision navigation systems.
Optical range generators are used in scientific research
research and industry. The extremely high concentration of energy in a narrow beam makes it possible, for example, to burn holes of very small diameter in superhard alloys and minerals, including the hardest mineral, diamond.
Quantum generators usually distinguish between:

by the nature of the active substance (solid or gaseous), the quantum phenomena in which determine the operation of devices.
by the operating frequency range (centimeter and millimeter range, optical range - infrared and visible parts of the spectrum)
by the method of excitation of the active substance or the separation of molecules according to energy levels.

According to the operating frequency range, quantum generators are divided into masers and lasers. Name maser is an abbreviation of the phrase "Microwave amplification by stimulated emission of radiation MASER". Name laser- abbreviation of the phrase "Light amplification by stimulated emission of radiation LASER"

1.2 History of creation
The history of the creation of a maser should begin in 1917, when Albert Einstein first introduced the concept of stimulated emission. This was the first step towards the laser. The next step was taken by the Soviet physicist V.A. Fabrikant, who in 1939 pointed out the possibility of using stimulated emission to amplify electromagnetic radiation as it passes through matter. The idea expressed by V.A. Fabrikant, assumed the use of microsystems with inverse level populations. Later, after the end of the Great Patriotic War, V.A. Fabrikant returned to this idea and, on the basis of his research, filed in 1951 (together with M.M. Vudynsky and F.A. Butaeva) an application for the invention of a method for amplifying radiation using stimulated emission. A certificate was issued for this application, in which, under the heading “Subject of the invention”, it is written: “A method for amplifying electromagnetic radiation (ultraviolet, visible, infrared and radio wavelengths), characterized in that the amplified radiation is passed through a medium in which, with the help of auxiliary radiation or in another way, they create an excess concentration of atoms, other particles or their systems at the upper energy levels corresponding to the excited states compared to the equilibrium one.
Initially, this method of radiation amplification turned out to be implemented in the radio range, and more precisely in the ultrahigh frequency range (UHF range). In May 1952, at the All-Union Conference on Radio Spectroscopy, Soviet physicists (now academicians) N.G. Basov and A.M. Prokhorov made a report on the fundamental possibility of creating a radiation amplifier in the microwave range. They called it a "molecular generator" (it was supposed to use a beam of ammonia molecules). Almost simultaneously, the proposal to use stimulated emission to amplify and generate millimeter waves was made at Columbia University in the USA by the American physicist C. Towns. In 1954, the molecular generator, soon called a maser, became a reality. It was developed and created independently and simultaneously at two points on the globe - at the P.N. Lebedev Academy of Sciences of the USSR (a group led by N.G. Basov and A.M. Prokhorov) and at Columbia University in the USA (a group led by C. Towns). Subsequently, the term “laser” came from the term “maser” as a result of replacing the letter “M” (the initial letter of the word Microwave - microwave) with the letter “L” (the initial letter of the word Light - light). The operation of both a maser and a laser is based on the same principle - the principle formulated in 1951 by V.A. Fabrikant. The appearance of the maser meant that a new direction in science and technology was born. At first it was called quantum radiophysics, and later it was called quantum electronics.

2. Principles of operation of quantum generators.

In quantum generators, under certain conditions, there is a direct conversion of the internal energy of atoms or molecules into the energy of electromagnetic radiation. This transformation of energy occurs as a result of quantum transitions - energy transitions, accompanied by the release of quanta (portions) of energy.
In the absence of external influence between the molecules (or atoms) of a substance, energy is exchanged. Some molecules emit electromagnetic vibrations, moving from a higher energy level to a lower one, and some absorb them, making the reverse transition. In general, under stationary conditions, a system consisting of a huge number of molecules is in dynamic equilibrium, i.e. as a result of the continuous exchange of energy, the amount of energy emitted is equal to the amount absorbed.
Population of energy levels, i.e. the number of atoms or molecules at different levels is determined by the temperature of the substance. The population of levels N 1 and N 2 with energy W 1 and W 2 is determined by the Boltzmann distribution:

(1)

where k is the Boltzmann constant;
T is the absolute temperature of the substance.

In a state of thermal equilibrium, quantum systems have a smaller number of molecules at higher energy levels, and therefore they do not radiate, but only absorb energy when externally irradiated. Molecules (or atoms) then move to higher energy levels.
In molecular generators and amplifiers that use transitions between energy levels, it is obviously necessary to create artificial conditions under which the population of a higher energy level will be higher. In this case, under the influence of an external high-frequency field of a certain frequency, close to the frequency of the quantum transition, intense radiation can be observed associated with the transition from a high to a low energy level. Such radiation caused by an external field is called induced.
The external high-frequency field of the fundamental frequency, corresponding to the frequency of the quantum transition (this frequency is called the resonant one), not only causes intense induced radiation, but also phases the radiation of individual molecules, which provides the addition of oscillations and the manifestation of the amplification effect.
The state of a quantum transition when the population of the upper level exceeds the population of the lower level of the transition is called inverted.
There are several ways to obtain a high population of the upper energy levels (population inversion).
In gaseous substances, for example, in ammonia, it is possible to carry out the separation (sorting) of molecules according to different energy states using an external constant electric field.
In solids, such a separation is difficult; therefore, various methods of excitation of molecules are used, i.e. methods of redistribution of molecules by energy levels by irradiation with an external high-frequency field.

A change in the population of the levels (inversion of the population of the levels) can be produced by pulsed irradiation with a high-frequency field of resonant frequency of sufficient intensity. With the correct selection of the pulse duration (the pulse duration should be much shorter than the relaxation time, i.e., the time of restoration of dynamic equilibrium), after irradiation, it is possible to amplify the external high-frequency signal for some time after irradiation.
The most convenient method of excitation, widely used at present in generators, is the method of irradiation with an external high-frequency field, which differs significantly in frequency from the generated oscillations, under the action of which the necessary redistribution of molecules occurs over energy levels.
The operation of most quantum generators is based on the use of three or four energy levels (although in principle a different number of levels can be used). Let us assume that the generation occurs due to the induced transition from the level 3 to the level 2 (see fig. 1).
In order for the active substance to amplify at the transition frequency 3 -> 2, need to make the population level 3 above population level 2. This task is performed by an auxiliary high-frequency field with a frequency ? vsp which "transfers" part of the molecules from the level 1 to the level 3. Population inversion is possible for certain parameters of the quantum system and a sufficient power of the auxiliary radiation.
An oscillator that creates an auxiliary high-frequency field to increase the population of a higher energy level is called a swap or backlight oscillator. The latter term is associated with generators of oscillations of the visible and infrared spectra in which light sources are used for pumping.
Thus, for the effective operation of a quantum generator, it is necessary to select an active substance that has a certain system of energy levels between which an energy transition could occur, as well as to choose the most appropriate method for excitation or separation of molecules according to energy levels.

Figure 1. Scheme of energy transitions
in quantum generators

3. Schemes of quantum generators
Quantum generators and amplifiers are distinguished by the type of active substance used in them. At present, two types of quantum devices have been developed, in which gaseous and solid active substances are used.
capable of intense induced radiation.

3.1 Molecular generators with separation of molecules according to energy levels.

Let us first consider a quantum generator with a gaseous active substance, in which, with the help of an electric fields, the separation (sorting) of molecules located at high and low energy levels is carried out. This type of quantum generator is commonly referred to as a molecular beam generator.

Figure 2. Diagram of a molecular generator based on an ammonia beam
1 – source of ammonia; 2- grid; 3 - diaphragm; 4 - resonator; 5 - sorting device

Practically implemented molecular generators use ammonia gas (chemical formula NH 3), in which molecular radiation associated with the transition between different energy levels is very pronounced. In the microwave frequency range, the most intense radiation is observed during the energy transition corresponding to the frequency f n= 23 870 MHz ( ? n=1.26 cm). A simplified diagram of a generator operating on ammonia in a gaseous state is shown in Figure 2.
The main elements of the device, outlined in Figure 2 by a dotted line, in some cases are placed in a special system cooled with liquid nitrogen, which ensures the low temperature of the active substance and all elements necessary to obtain a low noise level and high stability of the generator frequency.
The ammonia molecules leave the tank at a very low pressure, measured in units of millimeters of mercury.
To obtain a beam of molecules moving almost parallel in the longitudinal direction, ammonia is passed through a diaphragm with a large number of narrow axially directed channels. The diameter of these channels is chosen to be sufficiently small compared to the mean free path of the molecules. To reduce the speed of movement of molecules and, consequently, reduce the likelihood of collision and spontaneous, i.e., non-induced, radiation, leading to fluctuation noise, the diaphragm is cooled with liquid helium or nitrogen.
To reduce the probability of collision of molecules, it would be possible to go not along the path of temperature reduction, but along the path of pressure reduction, however, in this case, the number of molecules in the resonator that simultaneously interact with the high-frequency field of the latter would decrease, and the power given off by the excited molecules to the high-frequency field of the resonator would decrease.
To use gas as an active substance of a molecular generator, it is necessary to increase the number of molecules that are at a higher energy level, against their number determined by dynamic equilibrium at a given temperature.
In the generator of the type under consideration, this is achieved by sorting low-energy molecules from the molecular beam using the so-called quadrupole capacitor.
A quadrupole capacitor is formed by four metal longitudinal rods of a special profile (Figure 3a), connected in pairs through one with a high-voltage rectifier, which have the same potential, but alternating in sign. The resulting electric field of such a capacitor on the longitudinal axis of the generator is equal to zero due to the symmetry of the system and reaches its maximum value in the space between adjacent rods (Figure 3b).

Figure 3. Diagram of a quadrupole capacitor

The process of sorting molecules proceeds as follows. It has been established that molecules in an electric field change their internal energy with an increase in the electric field strength, the energy of the upper levels increases and the energy of the lower levels decreases (Figure 4).

Figure 4. Dependence of the level energy on the electric field strength:

upper energy level
lower energy level

This phenomenon is called the Stark effect. Due to the Stark effect, ammonia molecules, when moving in the field of a quadrupole capacitor, trying to reduce their energy, i.e., to acquire a more stable state, are separated: molecules of the upper energylevels tend to leave the region of a strong electric field, i.e., they move towards the axis of the capacitor, where the field is zero, and the molecules of the lower level, on the contrary, move into the region of a strong field, i.e., move away from the axis of the capacitor, approaching the plates of the latter. As a result, the molecular beam is not only largely freed from the molecules of the lower energy level, but also rather well focused.
After passing through the sorting device, the molecular beam enters a resonator tuned to the frequency of the energy transition used in the generator f n= 23 870 MHz .
The high-frequency field of the cavity resonator causes the induced emission of molecules associated with the transition from the upper energy level to the lower one. If the energy emitted by the molecules is equal to the energy consumed in the resonator and transferred to the external load, then a stationary oscillatory process is established in the system and the considered device can be used as a generator of oscillations stable in frequency.

The process of establishing oscillations in the generator proceeds as follows.
Molecules entering the resonator, which are predominantly at the upper energy level, spontaneously (spontaneously) make a transition to the lower level, while emitting energy quanta of electromagnetic energy and exciting the resonator. Initially, this excitation of the resonator is very weak, since the energy transition of molecules is random. The electromagnetic field of the resonator, acting on the beam molecules, causes induced transitions, which in turn increase the resonator field. Thus, gradually increasing, the resonator field will increasingly affect the molecular beam, and the energy released during induced transitions will enhance the resonator field. The process of increasing the intensity of oscillations will continue until saturation occurs, at which the resonator field will be so strong that during the passage of molecules through the resonator it will cause not only induced transitions from the upper level to the lower one, but also partially reverse transitions associated with with the absorption of electromagnetic energy. In this case, the power released by the ammonia molecules no longer increases and, consequently, a further increase in the oscillation amplitude becomes impossible. The stationary generation mode is set.
Therefore, this is not a simple excitation of the resonator, but a self-oscillating system that includes feedback, which is carried out through the high-frequency field of the resonator. The radiation of molecules flying through the resonator excites a high-frequency field, which in turn determines the induced radiation of molecules, the phasing and coherence of this radiation.
In cases where the self-excitation conditions are not met (for example, the density of the molecular flux penetrating the resonator is insufficient), this device can be used as an amplifier with a very low level of internal noise. The gain of such a device can be adjusted by changing the molecular flux density.
The cavity resonator of a molecular generator has a very high quality factor, measured in tens of thousands. To obtain such a high quality factor, the walls of the resonator are carefully processed and silvered. The holes for entry and exit of molecules, which have a very small diameter, simultaneously act as high-frequency filters. They are short waveguides, the critical wavelength of which is less than the intrinsic wavelength of the resonator, and therefore the high-frequency energy of the resonator practically does not escape through them.
To fine-tune the resonator to the transition frequency, some tuning element is used in the latter. In the simplest case, this is a screw, the immersion of which into the resonator somewhat changes the frequency of the latter.
Later it will be shown that the frequency of the molecular generator is somewhat "tightened" when the resonator tuning frequency is changed. True, the frequency pulling is small and is estimated at values ​​of the order of 10 -11 , but they cannot be neglected due to the high requirements placed on molecular generators. For this reason, in a number of molecular generators, only the diaphragm and the sorting system are cooled with liquid nitrogen (or liquid air), and the resonator is placed in a thermostat, the temperature in which is maintained constant by an automatic device to within fractions of a degree. Figure 5 schematically shows a device of this type of generator.
The power of molecular generators on ammonia usually does not exceed 10 -7 Tue,
therefore, in practice they are mainly used as highly stable frequency standards. The frequency stability of such a generator is estimated by the value
10 -8 - 10 -10. Within one second, the generator provides a frequency stability of the order of 10 -13 .
One of the significant drawbacks of the considered generator design is the need for continuous pumping and maintenance of the molecular flow.

Figure 5. The device of the molecular generator
with automatic stabilization of the resonator temperature:
1- source of ammonia; 2 - system of capillaries; 3- liquid nitrogen; 4 - resonator; 5 - system of water temperature control; 6 - quadrupole capacitor.

3.2 Quantum generators with external pumping

In the type of quantum generators under consideration, both solids and gases can be used as active substances, in which the ability of atoms or molecules excited by an external high-frequency field to induced energy transitions is clearly expressed. In the optical range, various sources of light radiation are used to excite (pump) the active substance.
Optical range generators have a number of positive qualities, and are widely used in various radio communication systems, navigation, etc.
As in quantum generators in the centimeter and millimeter ranges, lasers usually use three-level systems, i.e., active substances in which a transition occurs between three energy levels.
However, one feature should be noted that must be taken into account when choosing an active substance for oscillators and amplifiers in the optical range.
From the relation W 2 -W 1 =h? it follows that as the operating frequency increases? in oscillators and amplifiers, a higher energy level difference must be used. For generators of the optical range, roughly corresponding to the frequency range 2 10 7 -9 10 8 MHz(wavelength 15-0.33 mk), energy level difference W 2 -W 1 should be 2-4 orders of magnitude higher than for centimeter range generators.
Both solids and gases are used as active substances in optical range generators.
Artificial ruby ​​is widely used as a solid active substance - corundum crystals (A1 2 O 3) with an admixture of chromium (Cr) ions. In addition to ruby, glasses activated with neodymium (Nd), calcium tungstate crystals (СаWO 4) with an admixture of neodymium ions, crystals of calcium fluoride (СаF 2) with an admixture of dysprosium (Dy) or uranium ions and other materials are also widely used.
Gas lasers typically use mixtures of two or more gases.

3.2.1 Solid active generators

The most widely used type of optical range generator are generators in which ruby ​​with an admixture of chromium (0.05%) is used as an active substance. Figure 6 shows a simplified diagram of the arrangement of energy levels of chromium ions in ruby. The absorption bands on which it is necessary to pump (excite) correspond to the green and blue parts of the spectrum (wavelength 5600 and 4100A). Usually, pumping is carried out using a gas-discharge xenon lamp, the emission spectrum of which is close to that of the sun. Chromium ions, absorbing photons of green and blue light, go from level I to levels III and IV. Some of the excited ions from these levels return to the ground state (to level I), and most of them pass without energy emission to the metastable level II, increasing the population of the latter. Chromium ions that have passed to level II remain in this excited state for a long time. Therefore, at the second level
more active particles can be accumulated than at level I. When the population of level II exceeds the population of level I, the substance is able to amplify electromagnetic oscillations at the frequency of the II-I transition. If a substance is placed in a resonator, it becomes possible to generate coherent, monochromatic oscillations in the red part of the visible spectrum. (? = 6943 A ). The role of the resonator in the optical range is performed by reflecting surfaces parallel to each other.

Figure 6. Energy levels of chromium ions in ruby

absorption bands under optical pumping
nonradiative transitions
metastable level

The process of laser self-excitation proceeds qualitatively in the same way as in a molecular generator. Some of the excited chromium ions spontaneously (spontaneously) go to level I, while emitting photons. Photons that propagate perpendicular to reflective surfaces experience multiple reflections and repeatedly pass through the active medium and are amplified in it. There is an increase in the intensity of oscillations to a stationary value.
In the pulsed mode, the envelope of the radiation pulse of the ruby ​​generator has the character of short-term flashes with a duration of the order of tenths of a microsecond and with a period of the order of a few microseconds (Fig. 7, in).
The relaxational (discontinuous) nature of the oscillator radiation is explained by the different rates of ion arrival at level II due to pumping and a decrease in their number during induced transitions from level II to level I.
Figure 7 shows oscillograms that qualitatively explain the process
generation in a ruby ​​laser. Under the influence of pump radiation (Fig. 7, a) there is an accumulation of excited ions at level II. After a while the population N 2 exceeds the threshold value and self-excitation of the generator becomes possible. During the period of coherent emission, the replenishment of level II ions due to pumping lags behind their consumption as a result of induced transitions, and the population of level II decreases. In this case, the radiation either sharply weakens or even stops (as in this case) until level II is enriched to a value exceeding the threshold value (Fig. 7b) due to pumping, and oscillation excitation becomes possible again. As a result of the considered process, a series of short-term flashes will be observed at the output of the laser (Fig. 7c).

Figure 7. Oscillograms explaining the operation of a ruby ​​laser:
a) the power of the swap source
b) level II population
c) generator output power

In addition to ruby, other substances are also used in optical range generators, for example, a calcium tungstate crystal and neodymium-activated glasses.
A simplified structure of the energy levels of neodymium ions in a calcium tungstate crystal is shown in Figure 8.
Under the action of the pump lamp light, ions from level I are transferred to the excited states indicated in diagram III. Then they pass to level II without radiation. Level II is metastable, and the accumulation of excited ions takes place on it. Coherent radiation in the infrared range with a wavelength ?= 1,06 mk occurs during the transition of ions from level II to level IV. Ions make the transition from level IV to the ground state without radiation. The fact that radiation occurs
upon the transition of ions to level IV, which lies above the ground level,
facilitates excitation of the generator. The population of level IV is much less than that of level P [this follows from formula 1], and thus, in order to reach the excitation threshold, a smaller number of ions must be transferred to level II, and therefore less pumping energy must be expended.

Figure 8. Simplified structure of neodymium ion levels in calcium tungstate (CaWO 4 )

Neodymium-activated glass also has a similar energy level diagram. Lasers using activated glass emit at the same wavelength? = 1.06 microns.
Active solids are made in the form of long round (rarely rectangular) rods, the ends of which are carefully polished and reflective coatings are applied on them in the form of special dielectric multilayer films. Plane-parallel end walls form a resonator, in which the regime of multiple reflection of radiated oscillations (close to the regime of standing waves) is established, which contributes to the amplification of induced radiation and ensures its coherence. The resonator can also be formed by external mirrors.
Multilayer dielectric mirrors have low intrinsic absorption and make it possible to obtain the highest quality factor of the resonator. Compared to metal mirrors formed by a thin layer of silver or other metal, multilayer dielectric mirrors are much more difficult to manufacture, but they are far superior in durability. Metal mirrors fail after a few flashes, and therefore they are not used in modern laser models.
In the first models of lasers, pulsed helical xenon lamps were used as a pump source. Inside the lamp there was a rod of the active substance.
A serious disadvantage of this design of the generator is the low utilization of the light energy of the swap source. In order to eliminate this shortcoming, the generators focus the light energy of the pump source with the help of special lenses or reflectors. The second way is simpler. The reflector is usually made in the form of an elliptical cylinder.
Figure 9 shows a diagram of a ruby ​​generator. A lamp for illumination, operating in a pulsed mode, is located inside an elliptical reflector that focuses the light of the lamp on a ruby ​​rod. The lamp is powered by a high voltage rectifier. In the intervals between pulses, the energy of the high-voltage source is accumulated in a capacitor with a capacity of about 400 microf. At the moment of applying the starting ignition pulse with a voltage of 15 kV, removed from the secondary winding of the step-up transformer, the lamp lights up and continues to burn until the energy stored in the capacitor of the high-voltage rectifier is used up.
To increase the pumping power, several xenon lamps can be installed around the ruby ​​rod, the light of which is concentrated on the ruby ​​rod with the help of reflectors.
For the one shown in Fig. 23.10 of the generator, the threshold pump energy, i.e., the energy at which generation begins, is about 150 J. With the storage capacity indicated on the diagram FROM = 400 microf such energy is provided at a source voltage of about 900 AT.

Figure 9. Ruby oscillator with an elliptical reflector to focus the light of the pump lamp:

reflector
ignition coil
xenon lamp
ruby

Due to the fact that the spectrum of pump sources is much wider than the useful absorption band of the crystal, the energy of the pump source is used very weakly and, therefore, it is necessary to significantly increase the power of the source in order to provide pump power sufficient for generation in a narrow absorption band. Naturally, this leads to a strong increase in the crystal temperature. To prevent overheating, filters can be used whose transmission band approximately coincides with the absorption band of the active substance, or a system of forced cooling of the crystal, for example, using liquid nitrogen, can be used.
The inefficient use of pump energy is the main reason for the relatively low efficiency of lasers. Generators based on ruby ​​in a pulsed mode make it possible to obtain an efficiency of about 1%, generators on glass - up to 3-5%.
Ruby lasers operate predominantly in a pulsed mode. The transition to the continuous mode is limited by the resulting overheating of the ruby ​​crystal and pumping sources, as well as by the burnout of the mirrors.
Currently, research is underway on lasers using semiconductor materials. As an active element, they use a gallium arsenide semiconductor diode, the excitation (pumping) of which is carried out not by light energy, but by a high-density current passed through the diode.
The device of the active element of the laser is very simple (see Figure 10) It consists of two halves of a semiconductor material R- and n-type. The lower half of the n-type material is separated from the upper half of the p-type material by a plane district transition. Each of the plates is equipped with a contact for connecting the diode to a pumping source, which is a DC source. The end faces of the diode, strictly parallel and carefully polished, form a resonator tuned to the frequency of the generated oscillations corresponding to a wavelength of 8400 A. The dimensions of the diode are 0.1 x 0.1 x 1,25 mm. The diode is placed in a cryostat with liquid nitrogen or helium, and a pump current is passed through it, the density of which is district transition reaches values ​​10 4 -10 6 a/cm 2 In this case, radiation of coherent oscillations of the infrared range occurs with a wavelength ? = 8400A.

Figure 10. The device of the active element of the laser on a semiconductor diode.

polished edges
contact
p-n junction plane
contact

The emission of energy quanta in a semiconductor is possible when electrons pass from the conduction band to free levels in the valence band - from higher energy levels to lower ones. In this case, two current carriers "disappear" - an electron and a hole.
When an energy quantum is absorbed, an electron from the valence band passes into the conduction band and two current carriers are formed.
For amplification (as well as generation) of oscillations to be possible, the number of transitions with energy release should prevail over transitions with energy absorption. This is achieved in a semiconductor diode with heavily doped R- and n-regions when a forward voltage is applied to it, as shown in Figure 10. When the junction is biased in the forward direction, the electrons from n- areas diffuse into p- region. Due to these electrons, the population of the conduction band increases sharply R-conductor, and it can exceed the concentration of electrons in the valence band.
Diffusion of holes from R- in n- region.
Since the diffusion of carriers occurs at a shallow depth (on the order of a few microns), not the entire surface of the semiconductor diode end face participates in the radiation, but only the regions directly adjacent to the interface plane R- and n- areas.
In a pulsed mode of this type, lasers operating in liquid helium have a power of about 300 Tue with a duration of about 50 ns and about 15 Tue with duration 1 ms. In continuous mode, the output power can reach 10-20 mW at a pump power of about 50 mW.
Oscillations are emitted only from the moment when the current density in the junction reaches the threshold value, which for arsenic gallium is about 10 4 a/cm 2 . Such a high density is achieved by choosing a small area district transitions usually corresponds to a current through the diode of the order of several amperes.

3.2.2 Generators with gaseous active substance

In quantum generators of the optical range, the active substance is usually a mixture of two gases. The most common is a gas laser based on a mixture of helium (He) and neon (Ne).
The location of the energy levels of helium and neon is shown in Figure 11. The sequence of quantum transitions in a gas laser is as follows. Under the action of electromagnetic oscillations of a high-frequency generator in a gas mixture enclosed in a quartz glass tube, an electric discharge occurs, leading to the transition of helium atoms from the ground state I to the state II (2 3 S) and III (2 1 S). When excited helium atoms collide with neon atoms, an energy exchange occurs between them, as a result of which the excited helium atoms transfer energy to neon atoms and the population of the 2S and 3S levels of neon increases significantly.
etc.................

In quantum generators, the internal energy of microsystems - atoms, molecules, ions - is used to create electromagnetic oscillations.

Quantum generators are also called lasers. The word laser is made up of the initial letters of the English name of quantum generators - an amplifier of light by creating stimulated radiation.

The principle of operation of a quantum generator is as follows. When considering the energy structure of matter, it was shown that the change in the energy of microparticles (atoms, molecules, ions, electrons) does not occur continuously, but discretely - in portions called quanta (from the Latin quantim - quantity).

Microsystems in which elementary particles interact with each other are called quantum systems.

The transition of a quantum system from one energy state to another is accompanied by the emission or absorption of a quantum of electromagnetic energy hv: E 2 - Ei \u003d hv, where E 1 and E 2 - energy states: h - Planck's constant; v - frequency.

It is known that the most stable state of any system, including an atom and a molecule, is the state with the lowest energy. Therefore, each system tends to occupy and maintain the state with the lowest energy. Therefore, in the normal state, the electron moves in the closest orbit to the nucleus. This state of the atom is called the ground or stationary state.

Under the influence of external factors - heating, lighting, electromagnetic field - the energy state of the atom can change.

If an atom, for example, hydrogen interacts with an electromagnetic field, then it absorbs energy E 2 -E 1 = hv and its electron goes to a higher energy level. This state of the atom is called excited. An atom can stay in it for some very short time, called the lifetime of an excited atom. After that, the electron returns to the lower level, i.e., to the main stable state, giving off excess energy in the form of an emitted energy quantum - a photon.

The radiation of electromagnetic energy during the transition of a quantum system from an excited state to the ground state without external influence is called spontaneous or spontaneous. In spontaneous emission, photons are emitted at random times, in an arbitrary direction, with an arbitrary polarization. That is why it is called incoherent.

However, under the action of an external electromagnetic field, the electron can be returned to the lower energy level even before the expiration of the lifetime of the atom in the excited state. If, for example, two photons act on an excited atom, then under certain conditions the electron of the atom returns to the lower level, emitting a quantum in the form of a photon. In this case, all three photons have a common phase, direction and polarization of radiation. As a result, the energy of electromagnetic radiation is increased.

The emission of electromagnetic energy by a quantum system with a decrease in its energy level under the action of an external electromagnetic field is called forced, induced or stimulated.

The induced radiation coincides in frequency, phase and direction with external irradiation. Hence, such radiation is called coherent (coherence - from the Latin cogerentia - adhesion, connection).

Since the energy of the external field is not spent on stimulating the transition of the system to a lower energy level, the electromagnetic field is amplified and its energy increases by the value of the energy of the emitted quantum. This phenomenon is used to amplify and generate oscillations using quantum devices.

Currently, lasers are made from semiconductor materials.

A semiconductor laser is a semiconductor device that directly converts electrical energy into optical radiation energy.

For the operation of a laser, i.e., in order for the laser to create electromagnetic oscillations, it is necessary that there be more excited particles in its substance than unexcited ones.

But in the normal state of a semiconductor at higher energy levels at any temperature, the number of electrons is less than at lower levels. Therefore, in the normal state, the semiconductor absorbs electromagnetic energy.

The presence of electrons at one level or another is called the population of the level.

The state of a semiconductor in which there are more electrons at a higher energy level than at a lower level is called a population inversion state. An inverted population can be created in various ways: by injecting charge carriers with direct switching on of the p-n junction, by irradiating the semiconductor with light, etc.

The energy source, creating a population inversion, performs work by transferring energy to matter and then to the electromagnetic field. In a semiconductor with an inverted population, stimulated emission can be obtained, since it has a large number of excited electrons that can give up their energy.

If a semiconductor with an inverted population is irradiated with electromagnetic oscillations with a frequency equal to the frequency of the transition between energy levels, then the electrons from the upper level to the lower one are forced to emit photons. In this case, stimulated coherent emission occurs. It is reinforced. Having created a positive feedback circuit in such a device, we get a laser - an autogenerator of electromagnetic oscillations in the optical range.

For the manufacture of lasers, gallium arsenide is most often used, from which a cube is made with sides a few tenths of a millimeter long.

Chapter 4. STABILIZING THE FREQUENCY OF TRANSMITTERS

The successes achieved in the development and research of quantum amplifiers and oscillators in the radio range served as the basis for the implementation of the proposal to amplify and generate light based on stimulated emission and led to the creation of quantum oscillators in the optical range. Optical quantum generators (OQGs) or lasers are the only sources of powerful monochromatic light. The principle of amplifying light with the help of atomic systems was first proposed in 1940 by V.A. Fabrikant. However, the justification for the possibility of creating an optical quantum generator was given only in 1958 by Ch. Townes and A. Shavlov on the basis of the achievements in the development of quantum devices in the radio range. The first optical quantum generator was realized in 1960. It was a laser laser with a ruby ​​crystal as a working substance. The creation of population inversion in it was carried out by the method of three-level pumping, which is usually used in paramagnetic quantum amplifiers.

At present, a wide variety of optical quantum generators have been developed that differ in working substances (crystals, glasses, plastics, liquids, gases, semiconductors are used in this capacity) and methods for creating population inversion (optical pumping, discharge in gases, chemical reactions, etc.). ).

The radiation of existing optical quantum generators covers the wavelength range from the ultraviolet to the far infrared region of the spectrum adjacent to millimeter waves. Similar to a quantum generator in the radio range, an optical quantum generator consists of two main parts: a working (active) substance, in which in one way or another

an inversion of the populations is created, and the resonant system (Fig. 62). As the latter, open resonators of the Fabry-Perot interferometer type, formed by a system of two mirrors separated from each other, are used in the laser.

The working substance amplifies the optical radiation due to the induced emission of active particles. The resonant system, causing multiple passage of the emerging optical induced radiation through the active medium, determines the effective interaction of the field with it. If we consider the laser as a self-oscillatory system, then the resonator provides positive feedback as a result of the return of part of the radiation propagating between the mirrors to the active medium. In order for oscillations to occur, the power in the laser, obtained from the active medium, must be equal to the power of losses in the resonator or exceed it. This is equivalent to the fact that the intensity of the generation wave after passing through the amplifying medium, reflecting from the mirrors -/ and 2 , returning to the original cross section should remain unchanged or exceed the initial value.

When passing through the active medium, the intensity of the wave 1^ varies exponentially (neglecting saturation) L, ° 1^ ezhr [ (oc,^ - b())-c ] , and when reflected from the mirror, it changes into G once ( t - coefficient. reflection of the mirror), so the condition for the occurrence of generation can be written as

where L - length of the working active medium; r 1 and r 2 - reflection coefficients of mirrors 1 and 2 ; a u - gain of the active medium; b 0 - damping constant, taking into account energy losses in the working substance as a result of scattering on inhomogeneities and defects.

I. Resonators of optical quantum generators

The resonant laser systems, as noted, are open resonators. Currently, open resonators with flat and spherical mirrors are most widely used. A characteristic feature of open resonators is that their geometric dimensions are many times greater than the wavelength. Like volumetric open resonators, they have a set of natural modes of oscillation, characterized by a certain field distribution in them and own frequencies. The eigenmodes of an open resonator are solutions of the field equations that satisfy the boundary conditions on the mirrors.

There are several methods for calculating cavity resonators that allow one to find eigenmodes. A rigorous and most complete theory of open resonators is given in the works of L.A. Vaivshtein.* A visual method for calculating the types of oscillations in open resonators was developed in the work of A. Fox and T. Lee.

(113)

It is used in it. numerical calculation simulating the process of establishing the types of oscillations in the resonator as a result of multiple reflections from mirrors. Initially, an arbitrary distribution of the field on the surface of one of the mirrors is set. Then, applying the Huygens principle, the field distribution on the surface of another mirror is calculated. The resulting distribution is taken as the original one and the calculation is repeated. After multiple reflections, the distribution of the field amplitude and phase on the mirror surface tends to a stationary value, i.e. the field on each mirror is self-reproducing unchanged. The resulting field distribution is the normal type of oscillation of an open resonator.

The calculation of A. Fox and T. Lee is based on the following Kirchhoff formula, which is a mathematical expression of the Huygens principle, which allows you to find the hearth at the observation point BUT over a given field on some surface Sb

where Eb is the field at point B on the surface S b; k- wave number; R - distance between points BUT and AT; Q - angle between the line connecting the points BUT and AT, and normal to the surface Sb

With an increase in the number of passes, the hearth on the mirrors tends to a stationary distribution, which can be represented as follows:

where V(x ,y) - a distribution function that depends on the coordinates on the surface of the mirrors and does not change from reflection to reflection;

y is a complex constant independent of spatial coordinates.

Substituting formula (112) into expression (III). we get the integral equation

It has a solution only for certain values ​​[Gamma] = [gamma min.] called own values, Vmn functions , satisfying the integral equation, characterize the structure of the field of various types of oscillations of the resonator, which are called transverse oscillations and are designated as oscillations of the type TEMmn Symbol TEM indicates that the water inside the resonator are close to transverse electromagnetic, i.e. having no field components along the direction of wave propagation. Indices m and n denote the number of field direction changes along the sides of the mirror (for rectangular mirrors) or along the angle and along the radius (for round mirrors). Figure 64 shows the configuration of the electric field for the simplest transverse oscillation modes of open resonators with round mirrors. Eigenmodes of open resonators are characterized not only by the field distribution across, but also by its distribution along the axis of the resonators, which is a standing wave and differs in the number of half-waves that fit along the length of the resonator. To take this into account, the third index is introduced into the designations of vibration types a characterizing the number of half-waves that fit along the axis of the resonator.

Optical quantum generators on a solid state

Solid-state optical quantum generators, or solid-state lasers, use crystals or amorphous dielectrics as the active amplifying medium. The working particles, the transitions between the energy states of which determine the generation, as a rule, are ions of atoms of the transition groups of the Periodic Table. The most commonly used ions are Na 3+, Cr 3+, Ho 3+, Pr 3+. Active particles make up fractions or units of a percent of the total number of atoms of the working medium, so that they, as it were, form a "solution" of low concentration and therefore interact little with each other. The energy levels used are the levels of working particles split and broadened by strong inhomogeneous internal fields of the solid. As the basis of the active amplifying medium, crystals of corundum (Al2O3), yttrium-aluminum garnet are most often used. YAG(Y3Al5O12), different brands of glass, etc.

The population inversion in the working medium of solid-state lasers is created by a method similar to that used in paramagnetic amplifiers. It is carried out with the help of optical pumping, i.e. exposure to high intensity light.

As studies show, most of the currently existing active media used in solid-state lasers are satisfactorily described by two main idealized energy schemes: three- and four-level (Fig. 71).

Let us first consider the method of creating population inversion in media described by a three-level scheme (see Fig. 71a). In the normal state, only the lower main level is populated. 1 (the energy distance between the levels is much larger than kT), since the transitions 1->2, and 1->3) belong to the optical range. The transition between levels 2 and 1 is operational. Level 3 auxiliary and is used to create an inversion of the working pair of levels. It actually occupies a wide range of admissible energy values, due to the interaction of working particles with intracrystalline fields.

quantum generator

quantum generator- the general name of the sources of electromagnetic radiation, working on the basis of stimulated radiation of atoms and molecules. Depending on what wavelength the quantum generator emits, it can be called differently: laser, maser, raser, gaser.

History of creation

The quantum generator is based on the principle of stimulated emission proposed by A. Einstein: when a quantum system is excited and at the same time there is radiation corresponding to a quantum transition, the probability of the system jumping to a lower energy level increases in proportion to the density of already present radiation photons. The possibility of creating a quantum generator on this basis was indicated by the Soviet physicist V. A. Fabrikant in the late 1940s.

Literature

Landsberg G.S. Elementary textbook of physics. Volume 3. Oscillations and waves. Optics. Atomic and nuclear physics. - 1985.

Herman J., Wilhelmy B. "Lasers for generating ultrashort light pulses" - 1986.

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See what the "Quantum Generator" is in other dictionaries:

QUANTUM GENERATOR- electric generator magn. waves, in which the phenomenon of stimulated emission is used (see QUANTUM ELECTRONICS). K. g. of the radio range, as well as a quantum amplifier, called. maser. The first KG was created in the microwave range in 1955. The active medium in it ... Physical Encyclopedia

QUANTUM GENERATOR- a source of coherent electromagnetic radiation, the action of which is based on the stimulated emission of photons by atoms, ions and molecules. Quantum generators of the radio range are called masers, quantum generators of the optical range ... ... Big Encyclopedic Dictionary

quantum generator- A source of coherent radiation based on the use of stimulated emission and feedback. Note Quantum generators are divided according to the type of active substance, the method of excitation and other characteristics, for example, beam, gas ... Technical Translator's Handbook

QUANTUM GENERATOR- a source of monochromatic coherent electromagnetic radiation (optical or radio range), acting on the basis of stimulated radiation of excited atoms, molecules, ions. As a working substance, gases, crystalline ... Great Polytechnic Encyclopedia

quantum generator- a device for generating coherent electromagnetic radiation. Coherence is a coordinated flow in time and space of several oscillatory or wave processes, which manifests itself when they are added, for example. with interference... Encyclopedia of technology

quantum generator- a source of coherent electromagnetic radiation, the action of which is based on the stimulated emission of photons by atoms, ions and molecules. Quantum generators of the radio range are called masers, quantum generators of the optical range ... ... encyclopedic Dictionary

quantum generator- kvantinis generatorius statusas T sritis Standartizacija ir metrologija apibrėžtis Elektromagnetinių bangų generatorius, kurio veikimas pagrįstas sužadintųjų atomų, molekulių, jonų priverstinio spinduliavimo reiškiniu. atitikmenys: engl. quantum… … Penkiakalbis aiskinamasis metrologijos terminų žodynas

quantum generator- kvantinis generatorius statusas T sritis fizika atitikmenys: engl. quantum generator vok. Quantengenerator, m rus. quantum generator, m pranc. oscillateur quantique, m … Fizikos terminų žodynas

quantum generator- an electromagnetic wave generator that uses the phenomenon of stimulated emission (See Stimulated Emission) (see Quantum Electronics). K. g. of the radio range of superhigh frequencies (SHF), as well as the Quantum amplifier of this ... ... Great Soviet Encyclopedia

QUANTUM GENERATOR- a source of electromagnetic coherent radiation (optical or radio range), in which the phenomenon of induced radiation of excited atoms, molecules, ions, etc. is used. Gases, liquids, solids are used as a worker in VA in KG ... Big encyclopedic polytechnic dictionary

source of electromagnetic coherent radiation(optical or radio range), in which the phenomenon is used stimulated emission excited atoms, molecules, ions, and so on. Gases, liquids, solid dielectrics, and PP crystals are used as working substances in CG. The excitation of the working in-va, i.e., the supply of energy necessary for the work of K., is carried out by a strong electric. field, light from external source, electron beams, etc. The radiation of K. g., in addition to high monochromaticity and coherence has a narrow focus and means. power. see also Laser, Maser, Molecular generator.

• same as laser...

  The Beginnings of Modern Natural Science

• - quantum generator device for generating coherent electromagnetic radiation...

  Encyclopedia of technology

• - an optical quantum generator is the same as a laser ...

  Encyclopedia of technology

• - source of coherent el.-magnet. radiation, the action of which is based on the stimulated emission of photons by atoms, ions and molecules. K. g. radio band called. masers, K. g. optical. range -lasers ...
• same as laser...

  Natural science. encyclopedic Dictionary

• - a technical device for pulsed or continuous generation of monochromatic coherent radiation in the optical range of the spectrum ...

  Big Medical Dictionary

• - a source of electromagnetic coherent radiation, in which the phenomenon of induced radiation of excited atoms, molecules, ions, etc. is used. Gases, liquids, ...

  Big encyclopedic polytechnic dictionary

• - an electromagnetic wave generator that uses the phenomenon of stimulated radiation ...
• same as laser...

  Great Soviet Encyclopedia

• same as laser...

  Modern Encyclopedia

• - a source of coherent electromagnetic radiation, the action of which is based on the stimulated emission of photons by atoms, ions and molecules ...
• same as laser...

  Big encyclopedic dictionary

• - QUANTUM, -a, m. In physics: the smallest amount of energy given off or absorbed by a physical quantity in its non-stationary state. K. energy. K. light...

  Explanatory dictionary of Ozhegov

• - QUANTUM, quantum, quantum. adj. to quantum. quantum rays. Quantum mechanics...

  Explanatory Dictionary of Ushakov

• - quantum adj. 1. ratio with noun. quantum associated with it 2...

  Explanatory Dictionary of Efremova

• - sq "...

  Russian spelling dictionary

"QUANTUM GENERATOR" in books

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quantum generator

From the book Great Soviet Encyclopedia (KV) of the author TSB

Optical quantum generator

From the book Great Soviet Encyclopedia (OP) of the author TSB