gas lasers. Helium neon laser

The helium-neon laser - along with the diode or semiconductor - is one of the most commonly used and most affordable lasers for the visible region of the spectrum. The power of laser systems of this kind, intended mainly for commercial purposes, is in the range from 1 mW to several tens of mW. Particularly popular are less powerful He-Ne lasers of the order of 1 mW, which are used mainly as quoting devices, as well as for solving other problems in the field of measuring technology. In the infrared and red ranges, the helium-neon laser is increasingly being replaced by the diode laser. He-Ne lasers are capable of emitting orange, yellow and green lines in addition to red lines, which is achieved thanks to appropriate selective mirrors.

Energy Level Diagram

The energy levels of helium and neon that are most important for the function of He-Ne lasers are shown in Figs. 1. Laser transitions occur in the neon atom, with the most intense lines resulting from transitions with wavelengths of 633, 1153, and 3391 (see Table 1).

The electronic configuration of neon in the ground state looks like this: 1s22s22p6 where the first shell (n = 1) and the second shell (n = 2) are filled with two and eight electrons, respectively. Higher states according to fig. 1 arise as a result of the fact that there is a 1s22s22p5 shell here, and a luminous (optical) electron is excited according to the scheme: 3s, 4s, 5s, ..., 3p, 4p, ... etc. We are talking, therefore, about the one-electron state, which carries out the connection with the shell. In the LS scheme (Russell-Saunders) for the energy levels of neon, a one-electron state (for example, 5s) is indicated, as well as the resulting total orbital momentum L (= S, P, D ...). In the notation S, P, D,..., the lower index shows the total orbital moment J, and the upper one shows the multiplicity 2S + 1, for example, 5s1P1. Often, a purely phenomenological designation according to Paschen is used (Fig. 1). In this case, the sublevels of excited electronic states are counted from 2 to 5 (for s-states) and from 1 to 10 (for p-states).

Rice. 1. Scheme of energy levels of a He-Ne laser. Neon levels are marked according to Pashen, that is: 3s2, 3s3, 3s4, 3s5, etc.

Table 1. Notations for the transitions of intense lines of a He-Ne laser

Excitation

The active medium of a helium-neon laser is a gas mixture, to which the necessary energy is supplied in an electric discharge. The upper laser levels (2s and 2p according to Paschen) are selectively populated on the basis of collisions with metastable helium atoms (23S1, 21S0). During these collisions, not only the exchange of kinetic energy occurs, but also the transfer of energy from excited helium atoms to neon atoms. This process is called a collision of the second kind:

He* + Ne -> He + Ne* + ΔE, (1)

where the asterisk (*) symbolizes the excited state. The energy difference in the case of excitation of the 2s level is: &DeltaE=0.05 eV. In a collision, the existing difference is converted into kinetic energy, which is then distributed in the form of heat. For the 3s level, identical relationships take place. Such a resonant energy transfer from helium to neon is the main pumping process in creating a population inversion. In this case, the long lifetime of the metastable state He has a favorable effect on the selectivity of the population of the upper laser level.

The excitation of He-atoms occurs on the basis of the collision of electrons, either directly or through additional cascade transitions from higher levels. Owing to long-lived metastable states, the density of helium atoms in these states is very high. The upper laser levels 2s and 3s can - subject to the selection rules for electrical Doppler transitions - pass only to the lower p-levels. For successful generation of laser radiation, it is extremely important that the lifetime of s-states (upper laser level) = approximately 100 ns exceeds the lifetime of p-states (lower laser level) = 10 ns.

Wavelengths

Next, we will consider the most important laser transitions in more detail, using Fig. 1 and data from Table 1. The most famous line in the red region of the spectrum (0.63 μm) appears due to the 3s2 → 2p4 transition. The lower level is split as a result of spontaneous emission during 10 ns into the 1s level (Fig. 1). The latter is resistant to splitting due to electric dipole radiation, so that it has a long natural life. Therefore, atoms are concentrated in this state, which turns out to be highly populated. In a gas discharge, atoms in this state collide with electrons, and then the 2p and 3s levels are again excited. In this case, the population inversion decreases, which limits the laser power. The depletion of the ls-state occurs in helium-neon lasers mainly due to collisions with the wall of the gas-discharge tube, and therefore, as the tube diameter increases, the gain decreases and the efficiency decreases. Therefore, in practice, the diameter is limited to about 1 mm, which, in turn, limits the output power of He-Ne lasers to several tens of mW.

The electronic configurations 2s, 3s, 2p, and 3p participating in the laser transition are split into numerous sublevels. This leads, for example, to further transitions in the visible region of the spectrum, as can be seen from Table 2. For all visible lines of the He-Ne laser, the quantum efficiency is on the order of 10%, which is not very high. The level diagram (Fig. 1) shows that the upper laser levels are approximately 20 eV above the ground state. The energy of red laser radiation is only 2 eV.

Table 2. Wavelengths λ, output powers, and line widths Δ ƒ of a He-Ne laser (Paschen transition notation)

Color	λ nm	Transition (according to Pashen)	Power mW	Δ ƒ MHz	Gain %/m
Infrared	3 391	3s2 → 3p4	> 10	280	10 000
Infrared	1 523	2s2 → 2p1	1	625
Infrared	1 153	2s2 → 2p4	1	825
Red	640	3s2 → 2p2
Red	635	3s2 → 2p3
Red	633	3s2 → 2p4	> 10	1500	10
Red	629	3s2 → 2p5
Orange	612	3s2 → 2p6	1	1 550	1.7
Orange	604	3s2 → 2p7
Yellow	594	3s2 → 2p8	1	1 600	0.5
Yellow	543	3s2 → 2p10	1	1 750	0.5

Radiation in the infrared range around 1.157 µm arises through transitions 2s → 2p. The same applies to a slightly weaker line at about 1.512 µm. Both of these infrared lines find use in commercial lasers.

A characteristic feature of the line in the IR range at 3.391 μm is a high gain. In the zone of weak signals, that is, with a single passage of weak light signals, it is about 20 dB / m. This corresponds to a factor of 100 for a 1 meter long laser. The upper laser level is the same as for the known red transition (0.63 µm). The high gain, on the one hand, is caused by the extremely short lifetime at the lower 3p level. On the other hand, this is due to the relatively long wavelength and, accordingly, the low frequency of radiation. Usually the ratio of stimulated and spontaneous emissions increases for low frequencies ƒ. The amplification of weak signals g, as a rule, is proportional to g ~ƒ2.

Without selective elements, the He-Ne laser would emit at the 3.39 µm line and not in the red region at 0.63 µm. The excitation of the infrared line is prevented either by the selective cavity mirror or by absorption in the Brewster windows of the gas-discharge tube. Due to this, the laser generation threshold can be raised to a level sufficient for 3.39 μm radiation, so that only a weaker red line appears here.

Design

The electrons necessary for excitation are formed in a gas discharge (Fig. 2), which can be used with a voltage of about 12 kV at currents from 5 to 10 mA. The typical length of the discharge is 10 cm or more, the diameter of the discharge capillaries is about 1 mm and corresponds to the diameter of the emitted laser beam. As the diameter of the gas discharge tube increases, the efficiency decreases, since collisions with the tube wall are required to empty the ls level. For optimum power output, the total filling pressure (p) is used: p·D = 500 Pa·mm, where D is the diameter of the tube. The ratio in the He/Ne mixture depends on the desired laser line. For the known red line, we have He: Ne = 5:l, and for the infrared line of about 1.15 µm - He:Ne=10:l. An important aspect is also the optimization of the current density. The efficiency for the 633 nm line is about 0.1%, since the excitation process in this case is not very efficient. The service life of a helium-neon laser is about 20,000 operating hours.

Rice. 2. Design of a He-Ne laser for polarized radiation in the mW range

The gain under these conditions is at g=0.1 m-1, so it is necessary to use highly reflective mirrors. To exit the laser beam only on one side, a partially transmitting (semitransparent) mirror (for example, with R = 98%) is installed there, and on the other side - a mirror with the highest possible reflectivity (~ 100%). The gain for other visible transitions is much less (see Table 2). For commercial purposes, these lines have been obtained only in recent years with the help of mirrors, which are distinguished by extremely low losses.

Previously, in a helium-neon laser, the output windows of the discharge tube were fixed with epoxy resin, and the mirrors were mounted outside. This caused helium to diffuse through the adhesive and water vapor entered the laser. Today, these windows are fastened by direct welding of metal to glass, which reduces helium leakage to about 1 Pa per year. In the case of small, mass-produced lasers, the mirror coating is applied directly to the output windows, which greatly simplifies the entire design.

Beam Properties

To select the direction of polarization, the gas-discharge lamp is equipped with two obliquely arranged windows or, as shown in Fig. 2, a Brewster plate is inserted into the resonator. The reflectivity on an optical surface vanishes if the light is incident at the so-called Brewster angle and polarized parallel to the plane of incidence. Thus, radiation with this direction of polarization passes without loss through the Brewster window. At the same time, the reflectivity of the component polarized perpendicular to the plane of incidence is quite high and is suppressed in the laser.

The ratio (degree) of polarization (the ratio of power in the direction of polarization to the power perpendicular to this direction) is 1000:1 for conventional commercial systems. When a laser operates without Brewster plates with internal mirrors, unpolarized radiation is generated.

The laser usually generates in the transverse TEM00 mode (the lowest order mode), and several longitudinal (axial) modes are formed at once. When the distance between the mirrors (length of the laser resonator) L = 30 cm, the intermode frequency interval is Δ ƒ` = c/2L = 500 MHz. The central frequency is at the level of 4.7 1014 Hz. Since amplification of light can occur within the range Δ ƒ = 1500 MHz (Doppler width), at L = 30CM three different frequencies are emitted: Δ ƒ/Δ ƒ`= 3. When using a smaller distance between the mirrors (<= 10см) может быть получена одночастотная генерация. При короткой длине мощность будет весьма незначительной. Если требуется одночастотная генерация и более высокая мощность, можно использовать лазер большей длины и с оснащением частотно-селективными элементами.

Helium-neon lasers around 10 mW often find use in interferometry or holography. The coherence length of such mass-produced lasers is from 20 to 30 cm, which is quite sufficient for holography of small objects. Larger coherence lengths are obtained by using serial frequency-selective elements.

When the optical distance between the mirrors changes as a result of thermal or other effects, the axial natural frequencies of the laser resonator are shifted. With single-frequency generation, a stable radiation frequency is not obtained here - it moves uncontrollably in the linewidth range of 1500 MHz. By additional electronic control, frequency stabilization can be achieved just in the center of the line (commercial systems can have a frequency stability of several MHz). In research laboratories, it is sometimes possible to stabilize a helium-neon laser to a range of less than 1 Hz.

By using suitable mirrors, the different lines from Table 4.2 can be excited to generate laser light. The most commonly used visible line is around 633 nm with typical powers of several milliwatts. After suppression of an intense laser line of about 633 nm, other lines in the visible range can appear in the resonator due to the use of selective mirrors or prisms (see Table 2). However, the output powers of these lines are only 10% of the output power of a heavy line or even less.

Commercial helium neon lasers are available in a variety of wavelengths. In addition to them, there are also lasers that generate on many lines and are capable of emitting waves of many wavelengths in various combinations. In the case of tunable He-Ne lasers, it is proposed to select the required wavelength by turning the prism.

Helium neon laser

In addition to Shavlov, two other Bell Labs researchers were working on the laser problem in 1958: Ali Javan and John Sanders. Javan was an Iranian by origin. He received his PhD in 1954 under Towns on the subject of radiospectroscopy. He remained with Towns' group for four years, working on radio spectroscopy and masers. After defending his dissertation, when Tau was not on sabbatical in Paris and Tokyo, Javan became more involved in masers and came up with the idea of a three-level maser before the Bell Labs group published experimental work on the topic. He found a method to obtain inversionless population gain, using in particular the Raman effect in a three-level system, but he published his results later than the Bell group.

In April 1958, when he was looking for a job at Bell Labs, he spoke with Shavlov, who told him about lasers. In August 1958 he was admitted to Bell Labs, and in October began systematic research on lasers. Initially, he had ethical difficulties there. RCA has previously examined his records of the three-level maser and determined that his dates predate those of the Bell group. RCA paid him $1,000 for the rights to the patent, and started a dispute with Bell, where Javan was already working. For about six months Javan dealt with lawyers from RCA and Bell Labs. Luckily, RCA did some market research and, convinced that this maser amplifier wasn't profitable, dropped the business, leaving the patent to Bell Labs.

So, Javan could devote himself entirely to the laser. He thought of building it using gases, and published his proposed design in Physical Review Letters in 1959. He decided to use a gas as the active medium, because he believed that this simple substance would facilitate research. However, he thought that it was impossible to use high-power lamps to pump atoms directly into an excited state, and considered excitation either by direct collisions with electrons in a pure neon medium, or by collisions of the second kind. In the latter case, the discharge tube is filled with two gases, which are chosen so that the atoms of the first gas, excited by collisions with electrons in an electric discharge, can transfer their energy to the atoms of the second gas, exciting them. Some mixtures of gases had an energy level structure that satisfied these conditions. In fact, it is necessary that the energy level of the second gas has an energy practically equal to the excitation energy of the first gas. From the possible combinations of gases, Javan chose a combination of helium and neon, the levels of which are shown in Fig. 54. He believed that any physical process tends to establish the Boltzmann distribution of energy over levels (ie, the population of the lower level is greater than the population of the upper one). Therefore, a medium with an inverse population can be obtained in a stationary process only as a result of the competition of various physical processes proceeding at different rates.

This can be better understood by looking at a tree with branches (two in Fig. 55) on which monkeys sit. Consider first the population according to the Boltzmann statistics, i.e., say, four monkeys sit on the top branch (1), five on the bottom (2), and six on the ground (3, main level). Of these three levels, the main one is the most populated, and the higher the level, the less populated it is. However, monkeys do not sit still, but jump on branches (for example, we can assume that this happens every minute). In this case, the populations at the levels remain the same in time (equilibrium situation). Suppose now that we continue to populate branches at the same rate (one monkey per minute), but at the same time we wet branch 2 and make it slippery. Now the monkeys cannot stay on it for more than, say, 10 seconds. Therefore, this branch quickly settles, and soon there are more monkeys on branch 1 than on branch 2. Thus, an inverse population is obtained due to the fact that the residence time of a monkey on different branches is different. Although these are very primitive considerations, they help to understand Javan's considerations.

The selection of the helium-neon mixture went through careful selection in order to obtain a system that promised an optimal environment, and only subsequent success brought a posteriori full confidence in Javan. Even after he was convinced that helium-neon was the best mixture, there were many skeptics who told him that the gas discharge was too chaotic. There were too many uncertainties, they said, and his attempts were like hunting wild goose.

Rice. 54. Energy levels of helium (He) and (Ne). Major laser transitions shown

Fig.55. The monkeys on the turf are distributed according to the Boltzmann statistics. There are more of them on the ground, and their number decreases with the height of the branches.

Javan spent a lot of money, but, fortunately, the system worked, otherwise the administration was ready to close the project and stop the experiments. By the end of the project, two million dollars had been spent on this study. Although this amount is apparently exaggerated, the project undoubtedly required significant costs.

Meanwhile, John Sanders, an experimental physicist at the University of Oxford, was invited to Bell Labs to try to implement an infrared laser. During the less than one year allotted for this study, Sanders did not waste time on theoretical study, but immediately decided to excite pure helium in a discharge tube with a Fabry-Perot resonator inside it. He tried to get the laser effect through trial and error, varying the parameters of the discharge. The maximum distance at which the mirrors could be mounted while still remaining parallel to each other was 15 cm. Sanders did not use any longer discharge tubes. Javan considered this a fundamental limitation. He assumed that the gain in the gas is very small and the Sanders resonator will not work. The tube that Javan used was much longer, and since it was extremely difficult to adjust the Fabry-Perot mirrors at such a distance, he decided to first determine the required parameters for the working device, and then try to adjust the mirrors by trial and error. That's how he worked. Without all the preliminary work in choosing the He-Ne mode to get the known gain, it was impossible to succeed.

Sanders sent a letter to Physical Review Letters saying that it was difficult to get enough excited atoms with a flash lamp and suggested using excitation produced by electron impacts. Such excitation can easily be carried out with an electric discharge in a gas or vapor. A population inversion could be obtained if the active material contains excited states with long lifetimes, as well as states with lower energies and short lifetimes (as we considered in the monkey example).

Immediately after this article, in the same issue of Physical Review Letters, A. Javan published his article in which he also considered these problems, and among other schemes he proposed one very original one. Consider a long-lived state in a gas. Under discharge conditions, this state can be appropriately populated due to its long lifetime. If the now excited state of the second gas has an energy very close to this long-lived state, then it is very likely that the collision will transfer energy from the first atom to the second, which will become excited. If this atom has other lower energy states, then they will remain unexcited and thus there may be an inverse population between the high energy state with respect to the lower energy state. In his work, Javan mentioned mixtures of krypton and mercury, as well as a mixture of helium and neon. This work was published in Physical Review Letters June 3, 1959.

Javan worked closely with William R. Bennett, Jr., a Yale University spectroscopist who was a friend of Javan at Columbia. They worked until late at night for a whole year. In the fall of 1959, Javan asked Donald R. Herriot, an optical technician at Bell Labs, to help with the project. One of the fundamental problems was to provide the discharge tube with two transparent windows of very high optical quality so as not to distort the output beam. It was also required to install resonator mirrors. A scheme was developed (Fig. 56) with mirrors inside the discharge tube, equipped with special devices with micrometer screws, which made it possible to fine-tune the mirrors at the corners. In September 1959, Bennett moved from Yale to Bell Labs and, together with Javan, began a program of intensive and thorough research, calculating and measuring the spectroscopic properties of helium-neon mixtures under various conditions, in order to determine the factors that determine the production of inversion. They found that under the best conditions, only a very small gain, on the order of 1.5%, can be obtained. This low gain made it absolutely necessary to minimize losses and use mirrors with the highest reflectivity possible. Such mirrors are obtained by depositing on a transparent surface (glass) many layers of suitable (transparent) dielectric materials with different refractive indices. A high reflection coefficient is obtained due to multipath interference with reflections at the boundaries between layers. Three researchers were able to use mirrors that had a reflectance of 98.9% at a wavelength of 1.15 µm.

Rice. 56. Diagram of a helium-neon laser built by Javan, Bennett and Heriott

In 1960 Javan, Bennett and Heriott finally tested their laser. First, they tried to carry out an electrical discharge in a quartz tube containing a gas mixture using a powerful magnetron, but the tube melted. I had to redo the equipment and make changes. On December 12, 1960, they began working on a new tube and discharge organization. They tried to adjust the mirrors to get lasing, but without success. Then, at noon, Heriott saw the signal: “I was turning the micrometer screws on one of the mirrors, as usual, when, suddenly, a signal appeared on the oscilloscope. We set up the monochromator and recorded the signal peak at a wavelength of 1.153 µm, i.e. at the expected wavelength. The first laser was born, using gas as an active medium, and operating in a continuous mode! Its radiation was in the near infrared range and therefore invisible to the eye. Registration required a suitable receiver connected to an oscilloscope.

And six months earlier, Ed Ballick, a technician who helped out, later earned a degree from Oxford University and taught in Canada, bought a hundred-year-old bottle of wine. It was intended for a solemn moment - on the occasion of the operation of the laser. When the laser experiments finally came to fruition, a few days later Javan called the head of Bell Labs and invited him to bathe the event in centenary wine. He was terribly delighted, but then exclaimed: “Damn, Ali. We have a problem!". This happened in the morning, Javan, and did not understand what the problem was. But at noon, a circular was circulated around the laboratory, clarifying the previous one, issued a few months earlier, and prohibiting the drinking of alcohol on the territory of the scientific center. The clarification forbade drinking any alcohol that was under 100 years old. After that, they raised their glasses for success without breaking the rules!

The first laser operated at a 1.15 µm transition, in the near-IR range. Javan used mirrors that had maximum reflection at this wavelength, which corresponds to one of the possible transitions of neon. He knew that there were other possible wavelengths. He chose this wavelength because his research showed that the greatest gain could be expected at it. To use transitions in the visible region, a tube with such a small diameter was required that it was impossible to adjust the flat mirrors that were used for the Fabry-Perot resonator at that time.

In the Javan laser, the discharge tube contained neon and helium at pressures of 0.1 and 1 Torr, respectively (1 Torr is almost a thousandth of the pressure of one atmosphere). The fused quartz tube was 80 cm long and 1.5 cm in diameter. At each end was a metal cavity containing high reflective flat mirrors. Flexible sleeves (bellows) were used, which made it possible to adjust (by precise tilting) Fabry-Perot mirrors with micrometer screws. This made it possible to ensure parallelism with an accuracy of 6 arc seconds. At the ends there were flat glass windows with surfaces polished with an accuracy better than 100 A. They made it possible to emit a radiation beam without distortion. The electrical discharge was excited with external electrodes using a 28 MHz oscillator with a power of 50 watts. Mirrors with high reflection were obtained by deposition of 13 layers of dielectric materials (MgF 2 , ZnS). Between 1.1 and 1.2 µm, the reflectance was 98.9%. The laser operated continuously and was the first laser of this type.

Following the example of Hughes, Bell Labs also gave a public demonstration of a helium-neon laser on December 14, 1960. To demonstrate the possible importance for communications, a telephone conversation was transmitted using a beam of laser radiation that was modulated by a telephone signal.

This laser became known as the He-Ne laser, using the chemical symbols of its components for the name. It was presented to the press on January 31, 1961. A paper describing it was published on December 30, 1960 in Physical Review Letters.

While Javan was conducting experiments in the spring of 1960, two Bell Labs researchers, A. Fox and T. Lee, began to study the question of what modes exist in the Fabry-Perot resonator. The fact is that the Fabry-Perot resonator is very different from microwave resonators in the form of closed cavities. They determined the shape of these modes, and their result prompted other Bell Labs researchers, Gary D. Bond, James Gordon, and Herwig Kogelnik, to find analytical solutions in the case of spherical mirrors. The importance of the study of optical cavities for the development of gas lasers cannot be underestimated. Before these results were obtained, the gas laser was, at best, a marginal device, the generation of which was highly dependent on the alignment of the end mirrors. Theoretical studies of resonators with spherical mirrors have shown that there can be configurations that are relatively weakly dependent on the alignment of the mirrors, and the internal losses in the resonator can be smaller than in a resonator with flat mirrors. This allows the use of active media with significantly lower gains than previously thought. The resonator with flat mirrors was practically abandoned, and all the discoveries of new gas lasers were made using resonators with spherical mirrors.

In 1961, a major laser research program began at Bell Labs. Researchers occupied with other problems were reoriented to new topics, new employees were hired. The decision to use two identical spherical mirrors in the resonator, located at their foci (this configuration is called a confocal resonator), showed what difficulties Javan could avoid if he used such a resonator. As a result, William W. Rygrod, Herwig Kogelnik, Donald R. Heriott, and D. J. Brangacio built in the spring of 1962 the first confocal resonator with spherical mirrors that concentrate light towards the axis of the discharge tube, these mirrors being placed outside the tube. This made it possible to obtain generation on the 6328 A red line. Part of the light is inevitably lost in reflections from window surfaces (Fresnel reflection). These losses, however, can be avoided by tilting the windows at a certain angle, called Brewster's angle. In this case, for light of a certain polarization, the losses are practically zero. This new laser configuration is shown in Fig. 57.

Rice. 57. Confocal optical resonator. The tube in which the gas is excited by an electric discharge is closed with windows inclined at the Brewster angle. Concave mirrors with equal radii of curvature are placed behind the tube so that the distance between them is equal to the radius of curvature

The red He-Ne laser has become widely used, and is still used, in particular, in medicine. In addition, it greatly contributes to understanding the fundamental differences between laser (highly coherent) and ordinary (incoherent) light. Using this laser, interference phenomena are easily observed, as well as the mode structure of the laser beam, which is easily and clearly changed by a slight tilt of the resonator mirror. The development of other, numerous types of lasers was also stimulated.

A modern He-Ne laser can generate on one of several transitions shown in Fig. 54. To do this, multilayer mirrors are made with maximum reflection at the desired wavelength. Generation is obtained at wavelengths of 3.39 μm, 1.153 μm, 6328 A°, and even when using special mirrors, at wavelengths of 5433 A (green line), 5941 A° (yellow line), 6120 A° (orange line).

From the author's book

The second solid-state laser In September 1959, Towns organized a conference on "Quantum Electronics - Resonance Phenomena" at which, although the laser had not yet been created, most of the informal discussions focused on lasers. This conference was attended by Peter

From the author's book

The cesium laser 1961 was the year of the implementation of two more lasers, on which specialists had been working since the very beginning of the laser concept. One of them was a cesium laser. After Townes and Shavlov wrote their paper, it was decided that Townes would try to build a laser.

From the author's book

Neodymium laser Another laser, launched in 1961 and still one of the main ones, is the neodymium glass laser. In 1959-1960. The American Optical Company also became interested in laser research, which was carried out by one of its scientists, Elias Snitzer. This

From the author's book

Does a laser exist in nature? The answer seems to be yes! Laser radiation with a wavelength of about 10 μm (a typical carbon dioxide emission line, which operate high-power CO2 lasers, which are widely used, in particular for the machining of materials) was

From the author's book

The Laser and the Moon Bell Labs used one of the first lasers to study the topography of the Moon's surface. During the Apollo 11 expedition, sent to the Moon on July 21, 1969, astronauts installed two corner reflectors on its surface capable of reflecting laser light,

The aim of this work is to study the main characteristics and parameters of a gas laser, in which a mixture of helium and neon gases is used as an active substance.

3.1. The principle of operation of a helium-neon laser

The helium neon laser is the typical and most common gas laser. It belongs to atomic gas lasers and its active medium is a mixture of neutral (non-ionized) atoms of inert gases - helium and neon. Neon is a working gas, and transitions occur between its energy levels with the emission of coherent electromagnetic radiation. Helium plays the role of an auxiliary gas and contributes to the excitation of neon and the creation of a population inversion in it.

To start generation in any laser, two important conditions must be met:

1. There must be a population inversion between the working laser levels.

2. The gain in the active medium must exceed all losses in the laser, including the "useful" losses for the output of radiation.

If the system has two levels E 1 and E 2 with the number of particles on each of them, respectively N 1 and N 2 and the degree of degeneracy g 1 and g 2 , then the population inversion will occur when the population N 2 /g 2 top levels E 2 there will be more population N 1 /g 1 lower level E 1 , that is, the degree of inversion Δ N will be positive:

If the levels E 1 and E 2 are non-degenerate, then for the inversion to occur it is necessary that the number of particles N 2 at the top level E 2 was more than the number of particles N 1 on the lower level E one . The levels between which the formation of a population inversion and the occurrence of forced transitions with the emission of coherent electromagnetic radiation are possible are called working laser levels.

The population inversion state is created using pumping– excitation of gas atoms by various methods. Due to the energy of an external source, called pump source, the Ne atom from the ground energy level E 0 , corresponding to the state of thermodynamic equilibrium, passes into the excited state Ne*. Transitions can occur to different energy levels depending on the pump intensity. Then there are spontaneous or forced transitions to lower energy levels.

In most cases, it is not necessary to consider all possible transitions between all states in the system. This makes it possible to speak of two-, three-, and four-level schemes of laser operation. The type of laser operation scheme is determined by the properties of the active medium, as well as by the pumping method used.

The helium-neon laser operates in a three-level scheme, as shown in Fig. 3.1. In this case, the channels for pumping and generating radiation are partially separated. Pumping the active substance causes transitions from the ground level E 0 to excited level E 2 , which leads to the appearance of a population inversion between the working levels E 2 and E one . The active medium, which is in a state with population inversion of the working levels, is capable of amplifying electromagnetic radiation with a frequency
due to stimulated emission processes.

Rice. 3.1. Diagram of the energy levels of the working and auxiliary gas, explaining the operation of a helium-neon laser

Since the broadening of the energy levels in gases is small and there are no broad absorption bands, it is difficult to obtain an inverse population using optical radiation. However, other methods of pumping are possible in gases: direct electronic excitation and resonant energy transfer upon collision of atoms. The excitation of atoms upon collision with electrons can be most easily carried out in an electric discharge, where the electrons accelerated by the electric field can acquire significant kinetic energy. In inelastic collisions of electrons with atoms, the latter pass into an excited state E 2:

It is important that process (3.4) has a resonant character: the probability of energy transfer will be maximum if the excited energy states of different atoms coincide, i.e., are in resonance.

The energy levels of He and Ne and the main working transitions are shown in detail in Fig. 1. 3.2. The transitions corresponding to inelastic interactions of gas atoms with fast electrons (3.2) and (3.3) are shown by dotted arrows upwards. As a result of electron impact, helium atoms are excited to the 2 1 S 0 and 2 3 S 1 levels, which are metastable. Radiative transitions in helium to the ground state 1 S 0 are forbidden by the selection rules. When excited He atoms collide with Ne atoms in the ground state 1 S 0 , excitation transfer (3.4) is possible, and neon passes to one of the 2S or 3S levels. In this case, the resonance condition is satisfied, since the energy gaps between the ground and excited states in the auxiliary and working gas are close to each other.

Radiative transitions can occur from the 2S and 3S levels of neon to the 2P and 3P levels. The P levels are less populated than the upper S levels, since there is no direct transfer of energy from He atoms to these levels. In addition, the levels Р have a short lifetime, and the nonradiative transition Р → 1S empties the levels Р. population inversion, which means that the transitions between them can be used for laser generation.

Since the number of S and P levels is large, a large set of different quantum transitions between them is possible. In particular, from four 2S levels to ten 2P levels, 30 different transitions are allowed by the selection rules, most of which generated generation. The strongest emission line during the 2S → 2P transitions is the 1.1523 μm line (infrared region of the spectrum). For 3S→2Р transitions, the most significant line is 0.6328 µm (red region), and for 3S→3Р – 3.3913 µm (IR region). Spontaneous emission occurs at all listed wavelengths.

Rice. 3.2. Energy levels of helium and neon atoms and the operation scheme of a He-Ne laser

As mentioned earlier, after radiative transitions to the P levels, nonradiative radiative decay occurs during the P → 1S transitions. Unfortunately, the levels of neon 1S are metastable, and if the gas mixture does not contain other impurities, then the only way for the transition of neon atoms to the ground state from the level 1S is by collision with the walls of the vessel. For this reason, the gain of the system increases as the diameter of the discharge tube decreases. Since the 1S states of neon are slowly depleted, the Ne atoms are retained in these states, which is highly undesirable and determines a number of features of this laser. In particular, as the pump current increases above the threshold value j then there is a rapid increase, and then saturation and even a decrease in the laser radiation power, which is precisely due to the accumulation of working particles at the 1S levels and then their transfer to the 2P or 3P states upon collision with electrons. This makes it impossible to obtain high output radiation powers.

The occurrence of an inverse population depends on the pressure of He and Ne in the mixture and on the electron temperature. The optimal values of gas pressures are 133 Pa for He and 13 Pa for Ne. The electron temperature is given by the voltage applied to the gas mixture. Usually this voltage is maintained at the level of 2…3 kV.

To obtain laser generation, it is necessary that positive feedback exist in the laser, otherwise the device will work only as an amplifier. To do this, the active gaseous medium is placed in an optical resonator. In addition to creating feedback, the resonator is used to select the types of oscillations and select the generation wavelength, for which special selective mirrors are used.

At pump levels close to the threshold, lasing on one type of oscillation is relatively easy. With an increase in the level of excitation, if no special measures are taken, a number of other modes arise. In this case, generation occurs at frequencies close to the resonant frequencies of the resonator, which are contained within the width of the atomic line. In the case of axial types of vibrations (TEM 00 -mode), the frequency distance between adjacent maxima
, where L is the resonator length. As a result of the simultaneous presence of several modes, beats and inhomogeneities arise in the emission spectrum. If only axial modes existed, then the spectrum would be separate lines, the distance between which would be equal to c / 2L. But it is also possible to excite non-axial types of oscillations in the resonator, for example, TEM 10 modes, the presence of which strongly depends on the tuning of the mirrors. Therefore, additional satellite lines appear in the emission spectrum, located symmetrically in frequency on both sides of the axial types of oscillations. The appearance of new types of oscillations with an increase in the pump level is easily determined by visual observation of the structure of the radiation field. It is also possible to visually observe the influence of the resonator alignment on the structure of coherent radiation modes.

Gases are more homogeneous than condensed media. Therefore, the light beam in the gas is less distorted and scattered, and the radiation of the helium-neon laser is characterized by good frequency stability and high directivity, which reaches its limit due to diffraction phenomena. Diffraction Limit of Divergence for a Confocal Resonator

where λ is the wavelength; d 0 is the diameter of the light beam in its narrowest part.

The radiation of a helium-neon laser is characterized by a high degree of monochromaticity and coherence. The width of the emission lines of such a laser is much narrower than the "natural" width of the spectral line and is many orders of magnitude smaller than the limiting degree of resolution of modern spectrometers. Therefore, to determine it, the spectrum of beats of various modes in the radiation is measured. In addition, the radiation of this laser is plane polarized due to the use of windows located at the Brewster angle to the optical axis of the resonator.

Evidence of the coherence of radiation can be the observation of a diffraction pattern in the superimposition of radiation received from different points of the source. For example, coherence can be estimated by observing interference from a system of multiple slots. It is known from Young's experience that in order to observe the interference of light from an ordinary "classical" source, radiation is first passed through one slit, and then through two slits, and then interference fringes are formed on the screen. In the case of using laser radiation, the first slit turns out to be unnecessary. This circumstance is fundamental. In addition, the distance between two slits and their width can be incommensurably greater than in classical experiments. At the exit window of the gas laser, there are two slits, the distance between which is 2 a. In the case when the incident radiation is coherent, on a screen located at a distance d from the slits, an interference pattern will be observed. In this case, the distance between the maxima (minimums) of the bands

The most common gas laser is the helium-neon ( He-Ne) laser (neutral atom laser), which operates on a mixture of helium and neon in a ratio of 10:1. This laser is also the first continuous laser.

Consider the energy scheme of helium and neon levels (Fig. 3.4). Generation takes place between neon levels, and helium is added to carry out the pumping process. As can be seen from the figure, the levels 2 3 S 1 and 2 1 S 0 helium are located, respectively, close to the levels 2s and 3s not she. Because helium levels 2 3 S 1 and 2 1 S 0 are metastable, then when metastable excited helium atoms collide with neon atoms, there will be a resonant energy transfer to neon atoms (collisions of the second kind).

So the levels 2s and 3s neon can be populated and, therefore, generation can proceed from these levels. Lifetime s-states ( t s» 100 ns) much longer lifetime R-states ( t p»10 ns), so the following condition is satisfied for the laser to operate according to the four-level scheme:

1 1 S z (3s, 2s) z(3p,2p) z 1s .

Laser generation is possible at one of the transitions a, b, c according to the wavelengths l a=3.39 µm, lb=0.633 µm, l s=1.15 μm, which can be obtained by selecting the reflection coefficient of the resonator mirrors or by introducing dispersive elements into the resonator.

Rice. 3.4. Scheme of energy levels of helium and neon.

Let us consider the generation characteristic of such a laser.

Fig.3.5. Generation characteristic of a helium-neon laser.

The initial increase in output power with increasing pump current is explained by population inversion. After the maximum power is reached, the curve begins to decrease with a further increase in the pump current. This is explained by the fact that the 2p and 1s levels do not have time to relax; electrons do not have time to go to a low energy level and the number of electrons in the neighboring 2p and 1s levels becomes the same. In this case, there is no inversion.

The efficiency of helium-neon lasers is on the order of 0.1%, which is explained by the low volume density of excited particles. Output power typical He-Ne–laser P~5-50 mW, divergence q~1 mrad.

Argon laser

These are the most powerful continuous-wave lasers in the visible and near ultraviolet spectral region related to ion gas lasers. The upper laser level in the working gas is populated due to two successive collisions of electrons during an electric discharge. In the first collision, ions are formed from neutral atoms, and in the second, these ions are excited. Therefore, pumping is a two-stage process, the efficiency of each of which is proportional to the current density. Sufficiently high current densities are required for efficient pumping.

Laser energy level diagram on Ar+ shown in fig. 3.3. Laser emission in lines between 454.5 nm and 528.7 nm occurs when a group of levels is populated 4p by excitation by electron impact of the ground or metastable states Ar + .

3.5 CO 2 laser

Molecular CO 2-Lasers are the most powerful cw lasers among gas lasers, due to the highest efficiency of converting electrical energy into radiation energy (15-20%). Laser generation occurs on vibrational-rotational transitions and the emission lines of these lasers are in the far infrared region, which are located at wavelengths of 9.4 μm and 10.4 μm.

AT CO 2 The laser uses a mixture of gases CO 2, N 2 and He. Pumping is carried out directly during collisions of molecules CO 2 with electrons and vibrationally excited molecules N 2. The high thermal conductivity of He in the mixture promotes cooling CO 2, which leads to depletion of the lower laser level populated as a result of thermal excitation. So the presence N 2 in the mixture contributes to the high population of the upper laser level, and the presence He– depletion of the lower level, and as a result, together they lead to an increase in the population inversion. Energy level diagram CO 2-laser is shown in Fig. 3.4. Laser generation is carried out during the transition between the vibrational states of the molecule CO 2 n 3 Jun 1 or n 3 Jun 2 with a change in rotational state.

Rice. 3.4. Energy level diagram N 2 and CO 2 in CO 2–laser.

CO 2 The laser can operate in both continuous and pulsed modes. In continuous mode, its output power can reach several kilowatts.