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Laser


Laser
Despite the relatively simple laser device, the processes that underlie his work, is extremely complex and can not be explained in terms of the classical laws of physics. Since the time of Hertz and Maxwell science confirmed the notion that the electromagnetic and in particular light radiation of a wave nature. This theory explains well the majority of the observed optical and physical phenomena. But at the end of the XIX century it has been obtained some experimental data that does not fit this theory. (For example, it is not clear in terms of classical ideas about the wave nature of light has a photoelectric effect.) 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, the light is not constantly and individual microscopic portions. In 1905, Einstein, developing the theory of the photoelectric effect, reinforced the idea of ​​Planck and convincingly shown that electromagnetic radiation is actually emitted portions (those portions became known as photons), and later, in the course of propagation, each piece retains its "personality", not crushed or It added to the other, so that it can only absorb the whole entirely. From this description, it turned out that the photons behave like waves, in many cases, as well as particles. But they do not cease to be waves (for example, the quantum has no rest mass, and there is only moving at a speed of 300,000 km / s), that is, they have a definite dualism.

Quantum theory allowed to explain many strange phenomena before and, in particular, the nature of the interaction of radiation with matter. Take a simple example: why the body when heated emits light? By heating, for example, on a gas burner nail, we note that he first becomes crimson, then turns red. If we continue heating, the red color becomes yellow and then to a dazzling white. Thus, the nail is not only begins to emit infrared (heat) and visible rays. The reason for this is as follows. All of the body (and including our nail) consist of molecules and molecules consist of atoms. Each atom is a small very dense core, around which revolves more or fewer electrons. These electrons move around the nucleus is not random, but each of them is at its exact prescribed level; accordingly some levels are closer to the nucleus, and the other farther away from him. These levels are called power, since each of the electrons located on them has its own specific, peculiar only to this level of energy. While the electron is in its steady-state level, it moves without radiating energy. This state of the atom can go on indefinitely. But if the atom is reported externally for a certain amount of energy (as is the case when heated nails), the atom is "excited." The essence of this field is that the electrons absorb photons of radiation that penetrates a substance (in this case the infrared heat radiation gas burners), gain their energy and thereby move to higher energy levels. However, at these higher levels electrons can only be in a very short time (a few thousandths and even millionths of a second). After this time, each electron again at a steady level and thus emits the quantum of energy (or, equivalently, a particular wave length). Among some of these waves occur in the visible range (the photons of visible light are called photons; photon emission by excited atoms, we see how the glow of the heated nails). In our example with a nail absorption and emission of photons process occurs randomly. In a complex atom there is a large number of electron transitions from the upper to lower levels, and each of them is emitted from its frequency. Therefore, the radiation is in several spectra in the different directions, the atoms emit photons alone, while others consume them.

Similarly, the emission of photons in any heated body. Each of these bodies (whether it be the sun, arc welding or filament incandescent lamp) emits at the same time a lot of waves of different lengths (or, equivalently, photons of different energies). That is why, no matter how perfect a lens or other optical system, we had no, we will never be able to focus the radiation emitted by the heated body in 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 under its own angle; therefore, under no circumstances we will not be able to achieve their parallelism. However, the founders of quantum theory have considered another possibility of radiation, which does not occur naturally, but it can be modeled by man. In fact, if it were possible to excite the electrons all substances belonging to one particular energy level, and then get them to emit photons at once in the same direction, it would be possible to obtain an extremely powerful and at the same time extremely homogeneous radiation pulse. While focusing such a beam (as all waves, its components are the same length) could be achieved almost perfect parallel beam. For the first time the possibility of such, as he calls it, stimulated emission wrote in 1917 in the works of Einstein, "Emission and absorption of radiation by the quantum theory" and "On the Quantum Theory of Radiation."

Stimulated emission can be in particular achieved by the following method. Imagine the body, the electrons which have "overexcited" and located on the upper levels of power, and assume that they are irradiated with a new portion of quanta. In this case, there is a process reminiscent of an avalanche. Electrons have "supersaturated" with energy. As a result of additional exposure, they break away from the upper levels and moving like an avalanche on the bottom, emitting photons of electromagnetic energy. Moreover, the direction and the phase of the oscillation of these quanta and coincides with the direction of the incident wave phase. There will be a resonant wave amplification, when the output of the wave energy will be many times greater than the energy of the one that was at the entrance.

But how to achieve the strict parallelism of the emitted photons? It turns out that this can be done using a very simple device which is called an open resonator mirror. It consists of an active substance in the tube is placed between two mirrors: normal and translucent. Substance emitted photons falling on semitransparent mirror, partially pass through it. Other recorded and fly in the opposite direction, and the left reflected from the mirror (now all) and again reach the semitransparent mirror. This photon flux after each pass through the excited substance magnified. Amplified, however, is only the wave that moves perpendicular to the mirror; all the others that fall on the mirror at least with a small deviation from the perpendicular, without receiving sufficient amplification, leaving the active substance through its walls. As a result, the effluent has a very narrow focus. This is the principle of obtaining stimulated emission is the basis of lasers (the word laser is composed of the first letters of the English definition of light amplification by stimulated emission and radiation, which means light amplification by stimulated emission).

The creation of this remarkable device was preceded by a long history. Curiously, the invention of the laser equipment is required to professionals at first glance distant from both the optics and quantum electrodynamics, namely - Radio. However, this has a deep pattern. First it mentioned that since the beginning of the 40s Radio Physics around the world worked on the development of the centimeter and millimeter wave band, as this can greatly simplify and reduce the equipment, especially the antenna system. But it was soon discovered that the old tube generators can hardly be adapted to work in new conditions. With their help, we struggled to generate waves in 1 mm (the frequency of electromagnetic waves in these generators has reached several billion per second), but the creation of generators for even shorter wavelengths proved impossible. Needed was a fundamentally new method for the generation of electromagnetic waves.

Just at this time, Soviet radio physics Alexander Prokhorov and Nikolai Basov took up the study of a very interesting problem - the absorption of radio waves gases. Even during the war, it was discovered that a certain wave length emitted by the radar, do not affect how others from surrounding objects and do not give "echo." For example, the beam wavelength of 1, 3, see if dissolved in the space - it turned out that this wavelength is actively absorbed by the molecules of water vapor. Later it turned out that each gas absorbs certain wavelengths in such a way as if its molecules are somehow "tuned" to it. From these experiments it was to the next step only the idea that if the atoms and molecules are able to absorb certain wavelengths, so they may emit them, that is to act as a generator. Thus was born the idea to create a gas-ray generator in which, instead of vacuum tubes as radiation sources, billions of molecules in a special way the excited gas. The prospects of this work seemed very attractive, because there was a possibility to learn the needs of not only the radio range of the microwave waves, but also much shorter, for example, the range of visible wavelengths (wavelength of visible light 0, 4-0, 76 microns, which corresponds to a frequency of the order of thousands billions of cycles per second).

The major problem in this way was how to create an active environment. As such, Basov and Prokhorov chose ammonia. To ensure the operation of the generator, it was necessary to separate the active molecules of the gas, whose atoms are in an excited state, from the non-excited, the atoms of which have been focused on the absorption of photons. Scheme installations designed for this purpose, was a vessel in which a vacuum has been created. To this vessel was admitted into a thin beam of ammonia molecules. On their way it was installed the high-voltage capacitor. High-energy molecules freely flying through his field, and the molecules of low-energy side of the field took a great interest in the condenser. This happens sorting molecules for energy. Active molecules fall into the cavity, arranged in the same manner as that described above.

The first maser was established in 1954. It had a capacity of only one-billionth of a watt, so that his work could only register precision instruments. But in this case is much more important was the fact that in principle confirmed the correctness of the idea. It was a great victory, which opened a new page in the history of technology. In those same days at Columbia University team of American radio physics Charles Townes created a similar device, called the "maser". (In 1963, Basov and Prokhorov, and Townes for his fundamental discovery received the Nobel Prize.)

Quantum Generator Basov - Prokhorov and Townes maser were not lasers - they generate radio wave lengths of 1, 27 cm, and the lasers emit electromagnetic waves in the visible range, which are ten thousand times shorter. However, both devices working principle is the same, so the creator of the laser was to allow only specific tasks. Firstly, it was necessary to find a suitable surfactant, which could move into an excited state, so that not all the substance has this property. Secondly, to create a source of excitation, i.e. a device which has the ability to convert the active substance into an excited state by a message to it additional energy. Thirdly, it required open cavity in order to force all involved in excitation of the excited particles of the active substance, as well as to enhance only the vibrations which propagate along the longitudinal axis of the active substance. Fourth, the needed power supply to fuel the energy of the excitation source, otherwise the laser would not work. Solve these problems a variety of ways. The work was done by many scientists in several directions. However, before the others fortunate enough to achieve the cherished goal of the American physicist Theodore Meiman, who created the first ruby ​​laser on the basis of in 1960.

The essence of the work of the ruby ​​laser is as follows. Energy from the power supply source of the excitation is converted into an electromagnetic field which is irradiated with the active substance. As a result of this irradiation the active substance passes from the equilibrium state to the excited state. The internal energy of the active agent increases significantly. This process is called "pumping" or "swap" of the active substance, and the excitation power source called "pumping" or "swap". When the atoms of the active substance pass into an excited state, rather one electron to break for whatever reason, from the top level, so that it began to emit a photon of light, which, in turn, lose a few electrons with higher-level than the cause avalanche release of energy the rest of the excited electrons . Open Resonator direct radiation and increase of the active substance only in one direction. The active substance used Meumann boule (ruby is a crystalline substance consisting of aluminum oxide, in which part of aluminum atoms replaced by atoms of chromium, which is particularly important because the absorption of light does not play all of the material, and only the chromium ions).

The generator excitation consisted of three parts: the accelerator head, power supply and start the unit. The radiating head is created conditions for the active substance. The power supply provides energy to charge two capacitors - the main and auxiliary. The main purpose of the unit was run to generate a high voltage pulse and feeding it to the trigger electrode of the flash lamp. Radiant head consisted of a ruby ​​rod and two U-shaped flash bulbs. The lamps were standard, filled with xenon. From all sides of the lamp and the ruby ​​rod covered aluminum foil, which played the role of the reflector. Condenser accumulated and filed a pulse voltage of about 40 thousand volts, causing a powerful flash lamp. Flash instant translation ruby ​​atoms in an excited state. For the next pulse needed was a new charging of the capacitor. It is in general a very simple device led to a huge interest. If you are opening Basov and Townes was understandable only to specialists, the laser Maiman produced a great impression on even the uninitiated. In the presence of journalists Maiman repeatedly included his device and demonstrated its operation. At this end of the openings in the emitted beam thickness not greater pencil. Almost without expanding it rested against the wall, ending dazzling round speck. However, Maiman is only slightly ahead of other inventors. It was not long, and messages to create new types of lasers began to arrive from all sides.

As the active substance in addition ruby ​​lasers may be used, and many other compounds, such as strontium doped with fluoride, barium fluoride, alloy, glass, etc. They can be and gas. In the same 1960 gas laser helium-neon based created Ali Javan. The excited state of the gas mixture is achieved due to a strong electric field and gas discharges. However, as a solid, and gas lasers have a very low efficiency. Their output power does not exceed 1% of the consumption. Consequently, the remaining 99% is spent useless. It is therefore very important was the invention in 1962, Basov, and Popov Krokhin semiconductor laser. Soviet physicists have discovered that if the impact on the semiconductor electric or light pulse, then some of the electrons leave their atoms, and here formed a "hole", which act as positive charges. Simultaneously the electrons return to the orbit of the atoms can be regarded as a transition from a higher energy level to a lower, due to which there photon radiation. Efficiency of a semiconductor laser excited by an electron beam can reach 40%. The active substance used gallium arsenide containing n-type impurity. From this material or preform made in the form of a cube or parallelepiped-shaped - the so-called semiconductor diode. The plate is soldered to the diode petal molybdenum, gold-plated to provide electrical contact with the n-region. On the surface of the p-region has suffered an alloy of gold and silver. The ends of the diode resonator played a role, so they are carefully polished. Simultaneously in the process of polishing with high accuracy exhibited parallel. Escape of radiation from these parties diode. The top and bottom sides serving terminals to which voltage is applied. On the input of the device applied pulse.

Lasers quickly entered the life of a person and have been used in many fields of technology and science. Their industrial production began in 1965, when only in the United States more than 460 companies took up the development and production of laser systems.

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