Sl
Depending on what wavelength the quantum generator emits, it can be called differently:
laser (optical range);
maser (microwave range);
razer (X-ray range);
gaser (gamma range).
Sl
In reality, the operation of these devices is based on the use of Bohr's postulates:
An atom and atomic systems can remain for a long time only in special stationary or quantum states, each of which has a specific energy. In a stationary state, an atom does not emit electromagnetic waves.
Light emission occurs when an electron transitions from a stationary state with higher energy to a stationary state with lower energy. The energy of the emitted photon is equal to the energy difference between the stationary states.
The most common today are lasers, that is, optical quantum generators. In addition to children's toys, they have become widespread in medicine, physics, chemistry, computer technology and other industries. Lasers acted as " ready-made solution» many problems.
Let's take a closer look at the operating principle of the laser.
DC4-14
Laser - an optical quantum generator that creates a powerful, narrowly directed coherent monochromatic beam of light. (slides 1, 2)
( 1. Spontaneous and stimulated emission.
If the electron is at the lower level, then the atom will absorb the incident photon, and the electron will move from level E 1 to level E 2 . This state is unstable, electronspontaneously will move to level E 1 with photon emission. Spontaneous emission occurs spontaneously, therefore, the atom will emit light inconsistently, chaotically, therefore the light waves are inconsistent with each other neither in phase, nor in polarization, nor in direction. This is natural light.
But induced (forced) emission is also possible. If the electron is in the upper level E 2 (an atom in an excited state), then when a photon falls, a forced transition of an electron to a lower level by emitting a second photon can occur.
Sl
Radiation during the transition of an electron in an atom from an upper energy level to a lower one with the emission of a photon under the influence of an external electromagnetic field (incident photon) is calledforced or induced .
Properties of stimulated emission:
identical frequency and phase of primary and secondary photons;
same direction of propagation;
same polarization.
Consequently, stimulated emission produces two identical twin photons.
Sl
2. Use of active media.
The state of matter in a medium in which less than half of the atoms are in an excited state is calledstate with normal population of energy levels . This is a normal state of the environment.
Sl
An environment in which more than half of the atoms are in an excited state is calledactive medium with inverse population of energy levels
. (slide 9)
In a medium with an inverse population of energy levels, the light wave is amplified. This is an active environment.
The intensification of light can be compared to the growth of an avalanche.
Sl
To obtain the active medium, a three-level system is used.
At the third level, the system lives very briefly, after which it spontaneously goes into state E 2 without emission of a photon. Transition from state2 in a state 1 accompanied by the emission of a photon, which is used in lasers.
The process of a medium transitioning to an inverse state is calledpumped . Most often, light irradiation (optical pumping), electric discharge, electricity, chemical reactions. For example, after an outbreak powerful lamp the system goes into state3 , after a short period of time in the state2 , in which he lives for a relatively long time. This creates overpopulation at the level2 .
Sl
3. Positive feedback.
In order to move from the light amplification mode to the generation mode in the laser, feedback is used.
Feedback is carried out using an optical resonator, which is usually a pair of parallel mirrors. (slide 11)
As a result of one of the spontaneous transitions from the upper level to the lower a photon appears. When moving towards one of the mirrors, a photon causes an avalanche of photons. After reflection from the mirror, an avalanche of photons moves in the opposite direction, simultaneously causing more and more atoms to emit photons. The process will continue as long as it existsinverse population level
Inverse population energy levels - a nonequilibrium state of the environment, in which the number of particles (atoms, molecules) located at the upper energy levels, i.e., in an excited state, is greater than the number of particles located at the lower energy levels. .
Active element
pumping
pumping
Optical resonator
Streams of light moving in lateral directions quickly leave the active element without having time to gain significant energy. The light wave propagating along the axis of the resonator is amplified many times over. The bottom of the mirrors is made translucent, and from it the laser wave goes out into the environment.
Sl
4. Ruby laser .
The main part of a ruby laser isruby rod. Ruby is made up of atomsAl And Owith an admixture of atomsCr. It is chromium atoms that give ruby its color and have a metastable state.
Sl
A tube of a gas-discharge lamp, called pump lamp . The lamp flashes briefly and pumping occurs.
The ruby laser operates in pulsed mode. There are other types of lasers: gas, semiconductor... They can operate in continuous mode.
Sl
5. Properties of laser radiation :
most powerful source Sveta;
P of the Sun = 10 4 W/cm 2 , P of the laser = 10 14 W/cm 2 .
exceptional monochromaticity(monochromatic waves – spatially unlimited waves of one specific and strictly constant frequency) ;
gives a very small degree of angle divergence;
coherence ( those. coordinated occurrence in time and space of several oscillatory or wave processes) .
DC3
For laser operation
a pumping system is required. That is, we will give an atom or an atomic system some energy, then, according to Bohr’s 2nd postulate, the atom will move to more high level with a lot of energy. The next task is to return the atom to its previous level, while it emits photons as energy.
With sufficient lamp power, most chromium ions are transferred to an excited state.
The process of imparting energy to the working body of a laser to transform atoms into an excited state is called pumping.
The photon emitted in this case can cause the stimulated emission of additional photons, which in turn will cause stimulated emission)
DC15
The physical basis of laser operation is the phenomenon. The essence of the phenomenon is that an excited photon is capable of emitting under the influence of another photon without its absorption, if the latter is equal to the energy difference
Maser emits microwave, size – x-ray , and gaser – gamma radiation.
DC16
Maser - quantum generator emitting
coherent electromagnetic waves centimeter range (microwaves).
Masers are used in technology (in particular, in space communications), in physical research, and also as quantum generators of standard frequency.
Sl
Rather (X-ray laser) - a source of coherent electromagnetic radiation in the X-ray range, based on the effect of stimulated emission. It is a short-wave analogue of a laser.
Sl
Application of coherent x-ray radiation include research in dense plasma, X-ray microscopy, phase-resolution medical imaging, material surface exploration, and weapons. The soft X-ray laser can serve as a propulsion laser.
Sl
Work in the gaser field is ongoing, since an effective pumping system has not been created.
Lasers are used in a whole list of industries :
6. Application of lasers : (slide 16)
in radio astronomy to determine distances to bodies solar system with maximum accuracy (light locator);
metal processing (cutting, welding, melting, drilling);
in surgery instead of a scalpel (for example, in ophthalmology);
for obtaining three-dimensional images (holography);
communications (especially in space);
recording and storing information;
in chemical reactions;
for carrying out thermonuclear reactions in a nuclear reactor;
nuclear weapon.
Sl
Thus, quantum generators have firmly entered the everyday life of mankind, making it possible to solve many problems that were pressing at that time.
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 oscillators (OQOs) or lasers are the only sources of powerful monochromatic light. The principle of light amplification using atomic systems was first proposed in 1940 by V.A. Manufacturer. However, the justification for the possibility of creating an optical quantum generator was given only in 1958 by C. Townes and A. Shavlov based on 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 with a ruby crystal as a working substance. The creation of population inversion in it was carried out by the three-level pumping method, usually used in paramagnetic quantum amplifiers.
Currently, many different optical quantum generators have been developed, differing in working substances (crystals, glasses, plastics, liquids, gases, semiconductors are used) 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 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 anotheran inversion of populations and a resonant system is created (Fig. 62). As the latter, open resonators of the Fabry-Perot interferometer type are used in lasers, formed by a system of two mirrors located at a distance from each other.
The working substance enhances optical radiation due to the induced emission of active particles. The resonant system, causing repeated passage of the resulting optically induced radiation through the active medium, determines the effective interaction of the field with it. If we consider a laser as a self-oscillating system, then the resonator provides positive feedback as a result of the return of part of the radiation propagating between the mirrors into the active medium. For oscillations to occur, the power in the laser received from the active medium must be equal to or exceed the power lost in the resonator. This is equivalent to the fact that the intensity of the generation wave after passing through the amplifying medium, reflection from mirrors -/ and 2, returning to the original cross section must remain unchanged or exceed the initial value.
When passing through the active medium, the wave intensity 1^ changes according to the exponential law (ignoring saturation) L, ° 1^ ezhr [ (oc,^ - b())-c ], and when reflected from the mirror it changes in G once ( T - coefficient. mirror reflection), therefore the condition for generation to occur 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 is the gain of the active medium; b 0 - attenuation constant, taking into account energy losses in the working substance as a result of scattering by inhomogeneities and defects.
I. Resonators of optical quantum generators
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 their own types of oscillations, characterized by a certain field distribution in them and own frequencies. The natural types of oscillations 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 their own types of vibrations. A rigorous and most complete theory of open resonators is given in the works of L.A. Vaivestein.* 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 reflection from mirrors. Initially, an arbitrary field distribution is set on the surface of one of the mirrors. Then, using Huygens' principle, the field distribution on the surface of another mirror is calculated. The learned distribution is taken as the original one and the calculation is repeated. After multiple reflections, the distribution of the amplitude and phase of the field on the surface of the mirror tends to a stationary value, i.e. the field on each mirror reproduces itself unchanged. The resulting field distribution represents 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 Huygens’ principle, which allows one to find the bottom at the observation point A by 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 A And IN; Q - angle between the line connecting the points A And IN, and normal to the surface Sb
As the number of passes increases, the flow rate on the mirrors tends to a stationary distribution, which can be represented as follows:
Where V(x ,у) - 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 obtain the integral equation
It has a solution only for certain values [Gamma] = [gamma min.] called eigenvalues, Vmn functions , satisfying the integral equation, characterize the structure of the field of various types of oscillations of the resonator, which are called transverse vibrations and are designated as vibrations of the type TEMmn Symbol TEM indicates that the waters inside the resonator are close to transverse electromagnetic, i.e. having no field components along the direction of wave propagation. Indexes m and n denote the number of changes in field direction 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 electric field for the simplest transverse types of vibrations of open resonators with round mirrors. The intrinsic types of oscillations of open resonators are characterized not only by the transverse distribution of the field, 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, a third index is introduced into the designation of vibration types A, characterizing the number of half-waves that fit along the axis of the resonator.
Solid State Optical Quantum Generators
Solid-state optical quantum oscillators, or solid-state lasers, use crystals or amorphous dielectrics as the active gain medium. The working particles, transitions between energy states of which determine generation, are, as a rule, ions of atoms of transition groups of the Periodic Table. The ions Na 3+, Cr 3+, Ho 3+, Pr 3+ are most often used. Active particles constitute fractions or units of percent of total number atoms of the working medium, so that they 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 substance. Crystals of corundum (Al2O3) and yttrium-aluminum garnet are most often used as the basis for the active gain medium. YAG(Y3Al5O12), different brands glass, etc.
Population inversion in the working substance of solid-state lasers is created by a method similar to that used in paramagnetic amplifiers. It is carried out using optical pumping, i.e. exposure of a substance to high intensity light radiation.
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. 71, a). In the normal state, only the lower main level is populated 1 (the energy distance between levels is significantly greater than kT), since 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 a working pair of levels. It actually occupies a wide range of permissible energy values, due to the interaction of working particles with intracrystalline fields.
Baltic State Technical University
"Voenmekh" named after. D. F. Ustinova
Department I4
"Radio-electronic control systems"
Devices for receiving and converting signals
Coursework on the topic
«
Quantum generators »
Completed:
Peredelsky Oleg
Group I471
Checked:
Tarasov A.I.
Saint Petersburg
2010
1. Introduction
This paper discusses the principles of operation of quantum generators, generator circuits, their design features, issues of frequency stability of generators and principles of modulation in quantum generators.
1.1 General information
The operating principle of quantum generators is based on the interaction of a high-frequency field with atoms or molecules of matter. They allow the generation of oscillations of significantly higher frequency and high stability.
Using 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 Nowadays, quantum oscillators are widely used as frequency standards in time service systems. Quantum amplifiers used in receiving devices of various radio 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 effectively at very high frequencies, including the optical range, i.e., almost up to frequencies of the order of 10 9 MHz
Optical range generators make it possible to obtain high radiation directivity and high energy density in the light beam (about 10 12 -10 13 W/M 2
)
and a huge frequency range, allowing for 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 significantly increasing 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 diameters in superhard alloys and minerals, including the hardest mineral, diamond.
Quantum generators are usually distinguished:
- by the nature of the active substance (solid or gaseous), quantum phenomena in which determine the operation of devices.
by 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 separation of molecules by energy levels.
1.2 History of creation
The history of the creation of the 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 population of levels. Later, after the end of the Great Patriotic War, V.A. Fabrikant returned to this idea and, based on his research, submitted 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 of 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 excited states compared to the equilibrium one.”
Initially, this method of amplifying radiation was implemented in the radio range, or more precisely in the ultrahigh frequency range (microwave 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 put forward at Columbia University in the USA by the American physicist Charles Townes. In 1954, a molecular oscillator, soon called a maser, became a reality. It was developed and created independently and simultaneously in two places on the globe - at the P.N. Physics Institute. Lebedev Academy of Sciences of the USSR (group led by N.G. Basov and A.M. Prokhorov) and at Columbia University in the USA (group led by C. Townes). 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. Manufacturer. 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 became known as quantum electronics.
2. Operating principles of quantum generators.
In quantum generators, under certain conditions, a direct conversion of the internal energy of atoms or molecules into the energy of electromagnetic radiation is observed. This energy transformation occurs as a result of quantum transitions - energy transitions accompanied by the release of quanta (portions) of energy.
In the absence of external influence, energy is exchanged between molecules (or atoms) of a substance. Some molecules emit electromagnetic vibrations, moving from a higher energy level to a lower one, while others 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 a continuous exchange of energy, the amount of energy emitted is equal to the amount absorbed.
The population of energy levels, i.e. the number of atoms or molecules located at different levels is determined by the temperature of the substance. The population of levels N 1 and N 2 with energies W 1 and W 2 is determined by the Boltzmann distribution:
(1)
Where k– Boltzmann constant;
T– absolute temperature of the substance.
In a state of thermal equilibrium, quantum systems have fewer molecules at higher energy levels, and therefore they do not emit, but only absorb energy when exposed to external irradiation. In this case, molecules (or atoms) move to higher energy levels.
In molecular oscillators 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 associated with the transition from a high to a low energy level can be observed. Such radiation caused by an external field is called induced radiation.
An external high-frequency field of the fundamental frequency corresponding to the quantum transition frequency (this frequency is called the resonant frequency) not only causes intense stimulated radiation, but also phases the radiation of individual molecules, which
provides the addition of vibrations 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 transition level is called inverted.
There are several ways to obtain a high population of the upper energy levels (population inversion).
In gaseous substances, such as ammonia, it is possible to separate (sort) molecules into different energy states using an external constant electric field.
In solids, such separation is difficult, so various methods of excitation of molecules are used, i.e. methods of redistributing molecules across energy levels by irradiation with an external high-frequency field.
A change in the population of levels (inversion of the population of levels) can be produced by pulsed irradiation with a high-frequency field of a resonant frequency of sufficient intensity. With the correct selection of the pulse duration (the pulse duration should be much less than the relaxation time, i.e., the time to restore dynamic equilibrium), after irradiation it is possible to amplify the external high-frequency signal for some time.
The most convenient excitation method, currently widely used in generators, is the method of irradiation with an external high-frequency field, which differs significantly in frequency from the generated vibrations, under the influence of which the necessary redistribution of molecules across energy levels occurs.
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 generation occurs due to an induced transition from the level
3
per level 2
(see Fig. 1).
In order for the active substance to enhance at the transition frequency
3
-> 2,
need to make population level 3
above population level
2.
This task is performed by an auxiliary high-frequency field with a frequency ?
vsp
which “throws” some of the molecules from the level 1
per level 3.
Population inversion is possible with certain parameters of the quantum system and sufficient auxiliary radiation power.
A generator that creates an auxiliary high-frequency field to increase the population of a higher energy level is called a pump or backlight generator. The last term is associated with oscillation generators of visible and
infrared
spectra in which light sources are used for pumping.
Thus, to carry out 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, and also to select the most appropriate method of excitation or separation of molecules into energy levels.
Figure 1. Diagram of energy transitions
in quantum generators
3. Circuits of quantum generators
Quantum generators and amplifiers are distinguished by the type of active substance used in them. Currently, mainly two types of quantum devices have been developed, which use gaseous and solid active substances
capable of intense induced radiation.
3.1 Molecular generators with separation of molecules by energy levels.
Let us first consider a quantum generator with a gaseous active substance, in which, using an electric
fields, separation (sorting) of molecules located at high and low energy levels is carried out. This type of quantum oscillator is usually called a molecular beam oscillator.
Figure 2. Diagram of a molecular generator using an ammonia beam
1 – source of ammonia; 2- mesh; 3 – diaphragm; 4 – resonator; 5 – sorting device
In practically implemented molecular generators, ammonia gas (chemical formula NH 3) is used, in which molecular radiation associated with the transition between different energy levels is very pronounced. In the ultrahigh 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 the gaseous state is shown in Figure 2.
The main elements of the device, outlined in dotted lines in Figure 2, 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.
Ammonia molecules leave the reservoir at 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 quite small compared to the average free path of the molecules. To reduce the speed of movement of molecules and, therefore, reduce the likelihood of collisions and spontaneous, i.e., uninduced, radiation leading to fluctuation noise, the diaphragm is cooled with liquid helium or nitrogen.
To reduce the probability of collisions of molecules, one could go not along the path of decreasing temperature, but along the path of decreasing pressure, however, this would reduce the number of molecules in the resonator that simultaneously interact with the high-frequency field of the latter, and the power given off by excited molecules to the high-frequency field of the resonator would decrease.
To use gas as an active substance in a molecular generator, it is necessary to increase the number of molecules located at a higher energy level against their number determined by dynamic equilibrium at a given temperature.
In a generator of this type, this is achieved by sorting out low energy level molecules from the molecular beam using a 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 to 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, due to the symmetry of the system, is equal to zero and reaches its maximum value in the space between adjacent rods (Figure 3b).
Figure 3. Quadrupole capacitor circuit
The process of sorting molecules proceeds as follows. It has been established that molecules located in an electric field change their internal energy with increasing electric field strength; the energy of the upper levels increases and the lower levels decrease (Figure 4).
Figure 4. Dependence of energy levels on 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., 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., they move away from the axis of the capacitor, approaching the plates of the latter. As a result of this, the molecular beam is not only largely freed from molecules of the lower energy level, but also quite 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 a cavity resonator causes stimulated emission of molecules associated with a transition from an upper energy level to a lower one. If the energy emitted by the molecules is equal to the energy consumed in the resonator and transferred to an external load, then a stationary oscillatory process is established in the system and the device under consideration can be used as a generator of frequency-stable oscillations.
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, emitting energy quanta of electromagnetic energy and exciting the resonator. Initially, this excitation of the resonator is very weak, since the energy transition of the molecules is random. The electromagnetic field of the resonator, acting on the molecules of the beam, causes induced transitions, which in turn increase the field of the resonator. Thus, gradually increasing, the resonator field will increasingly influence the molecular beam, and the energy released during induced transitions will strengthen the resonator field. The process of increasing the intensity of oscillations will continue until saturation occurs, at which point the resonator field will be so large 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 partially also reverse transitions associated with absorption of electromagnetic energy. In this case, the power released by ammonia molecules no longer increases and, therefore, a further increase in the amplitude of vibrations becomes impossible. A stationary generation mode is established.
Therefore, this is not a simple excitation of the resonator, but a self-oscillatory system, including 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 stimulated emission 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 passing through 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 resonator walls are carefully processed and silver-plated. The holes for the entry and exit of molecules, which have a very small diameter, simultaneously serve as high-frequency filters. They are short waveguides, the critical wavelength of which is less than the natural 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, the latter uses some kind of tuning element. In the simplest case, it is a screw, the immersion of which into the resonator slightly changes the frequency of the latter.
In the future, it will be shown that the frequency of the molecular oscillator is somewhat “delayed” when the resonator tuning frequency changes. True, the frequency delay 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 with an accuracy of fractions of a degree. Figure 5 schematically shows a device of this type of generator.
The power of molecular generators using ammonia usually does not exceed 10 -7 W,
Therefore, in practice they are used mainly 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 frequency stability of the order of 10 -13.
One of the significant disadvantages of the considered generator design is the need for continuous pumping and maintenance of the molecular flow.
Figure 5. Design of a molecular generator
with automatic stabilization of the resonator temperature:
1- source of ammonia; 2 – capillary system; 3- liquid nitrogen; 4 – resonator; 5 – water temperature control system; 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 for energy-induced transitions of atoms or molecules excited by an external high-frequency field 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 range of positive qualities, and have found wide application in various radio communication systems, navigation, etc.
As in centimeter- and millimeter-wave quantum generators, lasers usually use three-level systems, that is, active substances in which a transition between three energy levels occurs.
However, one feature should be noted that must be taken into account when choosing an active substance for generators and amplifiers of the optical range.
From the relation W 2
–W 1
=h? It follows that as the operating frequency increases? in oscillators and amplifiers it is necessary to use a higher difference in energy levels. For optical range generators approximately 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 ions (Cr). In addition to ruby, glasses activated with neodymium (Nd), crystals of calcium tungstate (CaWO 4) with an admixture of neodymium ions, crystals of calcium fluoride (CaF 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 Generators with solid active substance
The most widespread type of optical range generator are generators in which ruby with an admixture of chromium (0.05%) is used as the active substance. Figure 6 shows a simplified diagram of the arrangement of energy levels of chromium ions in ruby. The absorption bands at which it is necessary to pump (excite) correspond to the green and blue parts of the spectrum (wavelength 5600 and 4100A). Typically, 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, move 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 emitting energy to the metastable level P, increasing the population of the latter. Chromium ions that have passed to level II remain in this excited state for a long time. Therefore, on the second level
can be accumulated large quantity active particles 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 vibrations in the red part of the visible spectrum (?
= 6943
A ). The role of a resonator in the optical range is performed by reflective surfaces parallel to each other.
Figure 6. Energy levels of chromium ions in ruby
- absorption bands under optical pumping
non-radiative transitions
metastable level
In the pulsed mode, the envelope of the radiation pulse of the ruby generator has the character of short-term flashes lasting on the order of tenths of a microsecond and with a period of the order of several microseconds (Fig. 7, V).
The relaxation (intermittent) nature of the generator radiation is explained by 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) accumulation of excited ions occurs at level II. After some time the population N 2 will exceed the threshold value and self-excitation of the generator will become 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, due to pumping, level II is enriched to a value exceeding the threshold (Fig. 7, b), and excitation of oscillations again becomes possible. As a result of the process considered, a series of short-term flashes will be observed at the laser output (Fig. 7, c).
Figure 7. Oscillograms explaining the operation of a ruby laser:
a) power of the pumping source
b) level II population
c) generator output power
In addition to ruby, other substances are used in optical range generators, for example, calcium tungstate crystal and neodymium-activated glass.
A simplified structure of the energy levels of neodymium ions in a calcium tungstate crystal is shown in Figure 8.
Under the influence of light from a pumping lamp, ions from level I are transferred to excited states indicated in diagram III. Then they move to level P without radiation. Level II is metastable, and excited ions accumulate on it. Coherent radiation in the infrared range with wavelength ?=
1,06 mk occurs when ions move from level II to level IV. Ions make the transition from level IV to the ground state without radiation. The fact that radiation occurs
during the transition of ions to level IV, which lies above the ground level, significantly
facilitates the excitation of the generator. The population of level IV is significantly less than level P [this follows from formula 1] and thus, to achieve the excitation threshold to level II, fewer ions must be transferred, and therefore less pumping energy must be expended.
Figure 8. Simplified structure of neodymium ion levels in calcium tungstate (CaWO 4 )
Glass doped with neodymium 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 (less often rectangular) rods, the ends of which are carefully polished and reflective coatings are applied to them in the form of special dielectric multilayer films. The plane-parallel end walls form a resonator in which a regime of multiple reflection of emitted oscillations (close to the regime of standing waves) is established, which enhances the 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 are much superior in durability. Metal mirrors fail after several flashes, and therefore modern models They don't use lasers.
The first laser models used spiral-shaped pulsed xenon lamps as a pumping source. Inside the lamp there was a rod of the active substance.
A serious disadvantage of this generator design is the low utilization rate of the light energy of the pumping source. In order to eliminate this drawback, generators use focusing of the light energy of the pumping source using special lenses or reflectors. The second method is simpler. The reflector is usually made in the form of an elliptical cylinder.
Figure 9 shows the circuit of a ruby oscillator. The backlight lamp, operating in a pulsed mode, is located inside an elliptical reflector that focuses the lamp light on the 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 mkf.
At the moment of applying a 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 accumulated 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 onto the ruby rod using reflectors.
For the one shown in Fig. 23.10 generator threshold pumping energy, i.e. the energy at which generation begins, is about 150 J.
With the storage capacity indicated on the diagram WITH
= 400 mkf
such energy is provided at a source voltage of about 900 IN.
Figure 9. Ruby oscillator with elliptical reflector for focusing the light of the pumping lamp:
- reflector
ignition spiral
xenon lamp
ruby
Due to the fact that the spectrum of pumping sources is much wider than the useful absorption band of the crystal, the energy of the pumping source is used very poorly and therefore it is necessary to significantly increase the power of the source in order to provide sufficient pumping power for generation in a narrow absorption band. Naturally, this leads to a strong increase in the temperature of the crystal. To prevent overheating, you can use filters whose bandwidth approximately coincides with the absorption band of the active substance, or use a forced cooling system for the crystal, for example, using liquid nitrogen.
Inefficient use of pump energy is the main reason for the relatively low efficiency of lasers. Generators based on ruby in pulse mode make it possible to obtain an efficiency of the order of 1%, generators based on glass - up to 3-5%.
Ruby lasers operate primarily in pulsed mode. The transition to continuous mode is limited by the resulting overheating of the ruby crystal and pumping sources, as well as burnout of the mirrors.
Research into lasers using semiconductor materials is currently underway. They use a semiconductor diode made of gallium arsenide as an active element, the excitation (pumping) of which is carried out not by light energy, but by a high-density current passed through the diode.
The design of the laser active element is very simple (see Figure 10) It consists of two halves of semiconductor material R-
And n-type. The lower half of n-type material is separated from the upper half of p-type material by a plane р-n
transition. Each of the plates is equipped with a contact for connecting a diode to a pumping source, which is used as a source direct current. 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 р-n
transition reaches values of 10 4 -10 6 a/cm 2
In this case, coherent oscillations of the infrared range with a wavelength of ?
= 8400A.
Figure 10. Structure of the active element of a semiconductor diode laser.
- polished edges
contact
pn junction plane
contact
When an energy quantum is absorbed, an electron moves from the valence band to the conduction band and two current carriers are formed.
In order for amplification (as well as generation) of oscillations to be possible, it is necessary that the number of transitions with energy release 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 indicated in Figure 10. When the junction is biased in the forward direction, electrons from n- areas diffuse into p- region. Due to these electrons, the population of the conduction band sharply increases R-conductor, and it can exceed the concentration of electrons in the valence band.
The diffusion of holes from R- V n- region.
Since the diffusion of carriers occurs to a small depth (on the order of a few microns), not the entire surface of the end of the semiconductor diode participates in the radiation, but only the areas immediately adjacent to the interface plane R- And n- regions.
In a pulsed mode of this type, lasers operating in liquid helium have a power of about 300 W with a duration of about 50 ns and about 15 W with duration 1 mks. In continuous mode, the output power can reach 10-20 mW with a pump power of about 50 mW.
Emission of oscillations occurs only from the moment when the current density in the junction reaches a threshold value, which for arsenic gallium is about 10 4 a/cm 2 . Such a high density is achieved by choosing a small area р-n transitions usually corresponds to a current through the diode of the order of several amperes.
3.2.2 Generators with gaseous active substance
In optical quantum generators, the active substance is usually a mixture of two gases. The most common is a gas laser using 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 influence of electromagnetic oscillations of a high-frequency generator in gas mixture, enclosed in a quartz glass tube, an electrical discharge occurs, leading to the transition of helium atoms from the ground state I to states 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.................
Quantum generators use the internal energy of microsystems - atoms, molecules, ions - to create electromagnetic oscillations.
Quantum generators are also called lasers. The word laser is made up of the initial letters of the English name for quantum generators - a light amplifier 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=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 a state with the lowest energy. Consequently, in the normal state, the electron moves in the orbit closest to the nucleus. This state of the atom is called ground or stationary.
Under the influence external factors- heating, lighting, electromagnetic field - the energy state of the atom can change.
If an atom, for example, of hydrogen interacts with an electromagnetic field, then it absorbs energy E 2 -E 1 = hv and its electron moves to a higher energy level. This state of the atom is called excited. An atom can remain in it for some very short time, called the lifetime of the excited atom. After this, the electron returns to the lower level, i.e., to the ground stable state, giving up excess energy in the form of an emitted energy quantum - a photon.
The emission of electromagnetic energy during the transition of a quantum system from an excited state to a ground state without external influence is called spontaneous or spontaneous. In spontaneous emission, photons are emitted at random times, in an arbitrary direction, with arbitrary polarization. That's why it's called incoherent.
However, under the influence of an external electromagnetic field, the electron can be returned to the lower energy level even before the lifetime of the atom in the excited state expires. 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 when its energy level decreases under the influence of an external electromagnetic field is called forced, induced or stimulated.
Induced radiation coincides in frequency, phase and direction with external radiation. Hence such radiation is called coherent (coherence - from the Latin cogerentia - cohesion, 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 enhanced 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 in which direct conversion occurs electrical energy into radiation energy in the optical range.
For a laser to work, that is, 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 higher low levels. Therefore, in its normal state, a semiconductor absorbs electromagnetic energy.
The presence of electrons at a particular level 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: using the injection of charge carriers when directly switching on the p-n junction, by irradiating the semiconductor with light, etc.
The energy source, creating a population inversion, performs work by transferring energy to the substance and then to the electromagnetic field. In a semiconductor with an inverted population, stimulated emission can be obtained, since it contains 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 transition frequency between energy levels, then electrons from the upper level are forced to move to the lower level, emitting photons. In this case, stimulated coherent emission occurs. It is enhanced. By creating a positive feedback circuit in such a device, we obtain a laser - a self-oscillator of electromagnetic oscillations in the optical range.
For the manufacture of lasers, gallium arsenide is most often used, from which a cube with sides a few tenths of a millimeter long is made.
Chapter 4. STABILIZATION OF TRANSMITTER FREQUENCY