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HISTORY OF TECHNOLOGY, TECHNOLOGY, OBJECTS AROUND US
Laser. History of invention and production
Directory / The history of technology, technology, objects around us A laser (English laser, an acronym for light amplification by stimulated emission of radiation), or an optical quantum generator, is a device that converts pump energy (light, electrical, thermal, chemical, etc.) into coherent energy. , monochromatic, polarized and narrowly directed radiation flux. The physical basis of laser operation is the quantum mechanical phenomenon of stimulated (induced) radiation. Laser radiation can be continuous, with a constant power, or pulsed, reaching extremely high peak powers. In some schemes, the working element of the laser is used as an optical amplifier for radiation from another source. There are a large number of types of lasers that use all aggregate states of matter as a working medium. Some types of lasers, such as dye solution lasers or polychromatic solid-state lasers, can generate a whole range of frequencies (optical cavity modes) over a wide spectral range. Lasers range in size from microscopic for some semiconductor lasers to the size of a football field for some neodymium glass lasers. The unique properties of laser radiation made it possible to use them in various branches of science and technology, as well as in everyday life, from reading and writing CDs to research in the field of controlled thermonuclear fusion.
Despite the relatively simple device of the laser, the processes underlying its operation are extremely complex and cannot be explained in terms of the classical laws of physics. Since the time of Maxwell and Hertz, science has established the idea that electromagnetic and, in particular, light radiation has a wave nature. This theory explained well most of the observed optical and physical phenomena. But already at the end of the XNUMXth century, some experimental data were obtained that did not fit this theory. For example, the phenomenon of the photoelectric effect turned out to be completely incomprehensible from the point of view of classical ideas about the wave nature of light. In 1900, the famous German physicist Max Planck, trying to explain the nature of these deviations, made the assumption that the emission of electromagnetic radiation and, in particular, light does not occur continuously, but in separate microscopic portions. In 1905, Einstein, developing the theory of the photoelectric effect, reinforced Planck's idea and convincingly showed that electromagnetic radiation is indeed emitted in portions (these portions began to be called quanta), and later, in the process of propagation, each portion retains its "individuality", is not crushed and does not stacks with others, so you can only absorb it entirely. From this description, it turned out that quanta in many cases behave not like waves, but like particles. But at the same time, they do not cease to be waves (for example, a quantum has no rest mass and exists only moving at a speed of 300000 km / s), that is, they have a certain dualism. Quantum theory made it possible to explain many previously incomprehensible phenomena and, in particular, the nature of the interaction of radiation with matter. Let's take a simple example: why does a body emit light when heated? Heating, say, a nail on a gas burner, we will notice that at first it acquires a crimson color, then it turns red. If you continue heating, then the red color turns into yellow and then into a dazzling white. Thus, the nail begins to emit not only infrared (thermal), but also visible rays. The reason for this phenomenon is the following. All bodies (including our nail) are made up of molecules, and molecules are made up of atoms. Each atom is a small, very dense nucleus around which more or less electrons revolve. These electrons do not move around the nucleus at random, but each of them is at its precisely set level; Accordingly, some levels are located closer to the core, while others are farther from it. These levels are called energy levels, since each of the electrons located on them has its own specific, inherent only to this level, energy. While the electron is at its stationary level, it moves without radiating energy. This state of the atom can continue indefinitely. But if a certain amount of energy is imparted to the atom from the outside (as happens when a nail is heated), the atom is "excited". The essence of this excitation is that the electrons absorb the quanta of radiation penetrating the substance (in our example, the infrared thermal radiation of a gas burner), acquire their energy and, due to this, move to higher energy levels. However, electrons can stay at these higher levels only for a very short time (thousandths and even millionths of a second). After this time, each electron again returns to its stationary level and at the same time emits a quantum of energy (or, what is the same, a wave of a certain length). Among these waves, some are in the visible range (these quanta of visible light are called photons; we observe the emission of photons by excited atoms like the glow of a heated nail). In our example with a nail, the process of absorption and emission of quanta proceeds chaotically. In a complex atom, a large number of transitions of electrons from upper levels to lower ones are observed, and each of them emits radiation with its own frequency. Therefore, the radiation goes simultaneously in several spectra and in different directions, with some atoms emitting photons, while others absorb them. In the same way, quanta are emitted by any heated body. Each of these bodies (whether it be the Sun, arc welding, or a filament of an incandescent lamp) simultaneously emits many waves of different lengths (or, what is the same, quanta of different energies). That is why, no matter how perfect a lens or other optical system we have, we will never be able to focus the radiation emitted by a heated body into a strictly parallel beam - it will always diverge at a certain angle. This is understandable - after all, each wave will be refracted in the lens at its own angle; therefore, under no circumstances will we be able to achieve their parallelism. However, the founders of quantum theory have already considered another possibility of radiation, which does not take place in natural conditions, but may well be simulated by man. Indeed, if it were possible to excite all the electrons of a substance belonging to one specific energy level, and then to force them to emit quanta at once in one direction, then it would be possible to obtain an extremely powerful and at the same time extremely homogeneous radiation pulse. By focusing such a beam (since all the waves that make it up are of the same length), it would be possible to achieve an almost perfect parallelism of the beam. For the first time, Einstein wrote about the possibility of such, as he called it, stimulated radiation in 1917 in his works "Emission and absorption of radiation according to quantum theory" and "On the quantum theory of radiation." Stimulated emission can in particular be achieved in the following way. Let us imagine a body whose electrons are already "overexcited" and are at the upper energy levels, and suppose that they are irradiated with a new portion of quanta. In this case, a process resembling an avalanche occurs. The electrons are already "oversaturated" with energy. As a result of additional irradiation, they break down from the upper levels and go like an avalanche to the lower ones, emitting quanta of electromagnetic energy. Moreover, the direction and phase of oscillations of these quanta coincides with the direction and phase of the incident wave. There will be, as it were, the effect of resonant amplification of the wave, when the energy of the output wave will many times exceed the energy of the one that was at the input. But how to achieve strict parallelism of emitted photons? It turns out that this can be done with a very simple device called an open mirror resonator. It consists of an active substance placed in a tube between two mirrors: a regular one and a translucent one.
The photons emitted by the substance, falling on a translucent mirror, partially pass through it. The rest are reflected and fly in the opposite direction, then reflected from the left mirror (now all) and again reach the translucent mirror. In this case, the photon flux after each passage through the excited substance is greatly enhanced. However, only the wave that moves perpendicular to the mirrors will be amplified; all the rest, which fall on the mirror with at least a slight deviation from the perpendicular, without receiving sufficient amplification, leave the active substance through its walls. As a result, the outgoing stream has a very narrow directivity. It is this principle of obtaining stimulated emission that underlies the operation of lasers (the word laser itself is composed of the first letters of the English definition of light amplification by stimulated emission and radiation, which means amplification of light by stimulated emission). The creation of this remarkable device was preceded by a long history. It is curious that technology owes the invention of the laser to specialists who, at first glance, are far from both optics and quantum electrodynamics, namely, radio physicists. However, this has its own deep pattern. It has already been said before that since the beginning of the 40s, radio physicists all over the world have been working on mastering the centimeter and millimeter wave ranges, since this made it possible to significantly simplify and reduce the equipment, especially antenna systems. But it soon became clear that the old tube generators could hardly be adapted to work in the new conditions. With their help, it was hardly possible to generate waves of 1 mm (the frequency of electromagnetic oscillations in these generators reached several billion per second), but the creation of generators for even shorter waves turned out to be impossible. A fundamentally new method for generating electromagnetic waves was needed. Just at that time, Soviet radio physicists Alexander Prokhorov and Nikolai Basov began to study a very interesting problem - the absorption of radio waves by gases. Even during the war, it was discovered that waves of a certain length emitted by a radar do not reflect, like others, from surrounding objects and do not give an "echo". For example, a 1 cm wave beam seemed to dissolve in space - it turned out that waves of this length are actively absorbed by water vapor molecules. Later it turned out that each gas absorbs waves of a certain length in such a way that its molecules are somehow "tuned" to it. From these experiments there was only a step to the next idea: if atoms and molecules are able to absorb waves of a certain length, then they can also emit them, that is, act as a generator. Thus, the idea was born to create a gas generator of radiation, in which, instead of electron tubes, billions of molecules of a specially excited gas would be used as radiation sources. The prospects for such work seemed very tempting, since it became possible to master for the needs of radio engineering not only the range of microwave waves, but also much shorter ones, for example, the range of visible waves (the wavelength of visible light is 0-4 microns, which corresponds to a frequency of the order of thousands billion vibrations per second). The most important problem along the way was how to create an active environment. Basov and Prokhorov chose ammonia as such. To ensure the operation of the generator, it was necessary to separate the active gas molecules, whose atoms were in an excited state, from the unexcited ones, whose atoms were oriented towards the absorption of quanta. The installation scheme developed for this purpose was a vessel in which a vacuum was created. A thin beam of ammonia molecules was let into this vessel. A high voltage capacitor was installed in their path. High-energy molecules freely flew through its field, while low-energy molecules were carried away by the field of the capacitor. This is how molecules are sorted by energy. The active molecules entered a resonator designed in the same way as the one described above. The first quantum generator was created in 1954. He had a power of only one billionth of a watt, so that only precise instruments could register his work. But in this case it was much more important that the fundamental correctness of the idea itself was confirmed. It was a remarkable victory that opened a new page in the history of technology. In the same days, at Columbia University, a group of American radiophysicist Charles Towns created a similar device, called the "maser". (In 1963, Basov, Prokhorov, and Townes received the Nobel Prize for their fundamental discovery.) The Basov-Prokhorov quantum generator and the Towns maser were not yet lasers - they generated radio waves 1 cm long, and lasers emit electromagnetic waves in the visible range, which are tens of thousands of times shorter. However, the principle of operation of both devices is the same, so the creator of the laser had to solve only particular problems. First, it was necessary to find a suitable active substance that could go into an excited state, because not every substance has this property. Secondly, to create a source of excitation, that is, a device that has the ability to transfer the active substance to an excited state by imparting additional energy to it. Thirdly, an open resonator was required in order to force all excited particles of the active substance to participate in the excitation, and also to amplify only those vibrations that propagate along the longitudinal axis of the active substance. Fourth, a power source was needed to energize the excitation source, otherwise the laser would not work. All these problems can be solved in different ways. The work was carried out by many scientists in several directions at once. However, the American physicist Theodor Meiman, who in 1960 created the first ruby-based laser, was lucky to achieve the cherished goal before others.
The essence of the operation of a ruby laser is as follows. The energy from the power source is converted by the excitation source into an electromagnetic field, which irradiates the active substance. As a result of this irradiation, the active substance passes from an equilibrium state to an excited state. The internal energy of the active substance increases significantly. This process is called "pumping" or "pumping" the active substance, and the source of excitation is called the source of "pumping" or "pumping". When the atoms of the active substance pass into an excited state, it is enough for one electron to escape from the upper level for some reason, so that it starts emitting a photon of light, which, in turn, will drop several electrons from the upper level, which will cause an avalanche-like release of energy by the rest of the excited electrons . An open resonator will direct and amplify the radiation of the active substance in only one direction. Meiman used artificial ruby as an active substance (ruby is a crystalline substance consisting of aluminum oxide, in which some of the aluminum atoms are replaced by chromium atoms, which is especially important, since not all of the material, but only chromium ions, participates in the absorption of light). The excitation generator consisted of three blocks: a radiating head, a power supply unit, and a launch unit. The emitting head created the conditions for the operation of the active substance. The power supply provided energy for the charge of two capacitors - the main and auxiliary. The main purpose of the trigger unit was to generate a high voltage pulse and apply it to the trigger electrode of the flash lamp. The emitting head consisted of a ruby rod and two U-shaped flash lamps. Lamps were standard, filled with xenon. From all sides, the lamps and the ruby rod were covered with aluminum foil, which played the role of a reflector. The capacitor accumulated and applied a pulsed voltage of about 40 thousand volts, which caused a powerful flash of lamps. The flash instantly transferred the atoms of the ruby into an excited state. For the next pulse, a new charge of the capacitor was necessary.
This, in general, a very simple device aroused great interest. If the essence of the discovery of Basov and Towns was clear only to specialists, then the Meiman laser made a huge impression even on the uninitiated. In the presence of journalists, Meiman repeatedly turned on his device and demonstrated its operation. At the same time, a beam was emitted from the hole in the end, no more than a pencil thick. Almost without expanding, it rested against the wall, ending in a dazzling round spot. However, Meiman was only marginally ahead of other inventors. Not much time passed, and reports of the creation of new types of lasers began to come from all sides. In addition to ruby, many other compounds can be used as an active substance in lasers, for example, strontium fluoride with impurities, barium fluoride with impurities, glass, etc. They may be gas. In the same 1960, a helium-neon based gas laser was created by Ali Javan. The excited state of the gas mixture was achieved by means of a strong electric field and gas discharges. However, both solid-state and gas lasers have very low efficiency. Their output energy does not exceed 1% of the consumed. Consequently, the remaining 99% is spent uselessly. Therefore, the invention in 1962 by Basov, Krokhin and Popov of the semiconductor laser became very important.
Soviet physicists discovered that if semiconductors are affected by an electric or light pulse, then some of the electrons will leave their atoms, and "holes" are formed here, which play the role of positive charges. The simultaneous return of electrons to the orbits of atoms can be considered as a transition from a higher energy level to a lower one, due to which photons are emitted. The efficiency of a semiconductor laser when excited by an electron beam can reach 40%. Gallium arsenide containing n-type impurities was used as the active substance. From this material blanks were made either in the form of a cube or in the form of a parallelepiped - the so-called semiconductor diode. The diode plate was soldered to a molybdenum sheet coated with gold to provide electrical contact with the n-region. An alloy of gold with silver was deposited on the surface of the p-region. The ends of the diode played the role of a resonator, so they were carefully polished. At the same time, during the polishing process, they were placed parallel to each other with high accuracy. The radiation came out precisely from these sides of the diode. The top and bottom sides served as contacts to which voltage was applied. Pulses were applied to the input of the device. Lasers very quickly entered human life and began to be used in many areas of technology and science. Their industrial production began in 1965, when more than 460 companies in America alone took up the development and creation of laser systems. Author: Ryzhov K.V.
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