Ion lasers, ionic argon lasers, ionic krypton lasers. Laser Vario Racurs. Some Recommendations for Choosing OCG Lasers for the Shortwave Region

In the case of ion lasers, we are talking about gas-discharge systems close to lasers with atomic gases. An ion is an atom in which one or more electrons are usually freed from their outer orbits. Therefore, the ion is positively charged, and this charge corresponds to one or more elementary charges. The remaining electrons can be excited in the same way as in an atom and generate radiation upon transition to the ground state or other excited states. Here exactly the same laser transitions are possible as in atoms. Since several ions belong to each atom, many additional laser lines arise by virtue of their existence.

Ions are formed in every gas discharge as electrons, excited atoms, or other ions collide with atoms, causing the atoms to "ionize." In addition, in a gas discharge, ions are excited due to various collision processes with electrons or other particles, so that gas discharges, along with atomic transitions, also generate radiation through electronic transitions in ions.

Ions can be formed not only as a result of electrical discharges, but also in plasmas initiated by laser radiation. For this purpose, the beam of a pulsed high-power laser is directed to a stationary target, which begins to evaporate. On the basis of the high density of the supplied energy, electrons and ions are formed, and a very high degree of ionization is achieved, that is, many electrons are separated from the atoms. These ions emit short wavelength light and are suitable for making X-ray lasers.

Ions can also be present in solids in the form of crystal lattice modules or so-called impurity centers. They even have a stable shape, while ions in gas discharges and other plasmas are characterized by recombination with electrons, which again leads to the formation of atoms. Such impurity ions are the basis for the development of the most important types of solid-state lasers.

1.1 Lasers for shortwave region

The energy states En of the most extreme ("emitting") electron of an ion can be approximately described by the example of a model close to hydrogen, according to which the charge of an atomic nucleus minus the charge of internal electrons gives the effective (charge) atomic number Z. According to this simplified model, an optical electron moves in the field of a point charge, so that the energy states - in accordance with Bohr's theory of the hydrogen atom - are expressed through:

En = - (13.6 eV) Z2 / n2, (1)

where n is the principal quantum number or, respectively, the orbit number.

Equation (1) and Fig. 1 show that the electron energy of ions (Z> 2) exceeds the electron energy of atoms (Z = 1). This is because, due to the higher effective charge of the atomic nucleus, the optical electron in the ion is bound more strongly than in the atom. Equation (1) can be considered accurate only for H (hydrogen atom, Z = 1), He + (positive singly charged helium ion, Z = 2), L ++ (doubly charged lithium ion, Z = 3) and other fully ionized atoms ... But in the case of other ions, one can observe that the energy of electrons, on average, increases with the degree of ionization.

Rice. 1. Energy states of an electron in a hydrogen atom (H) and a helium atom (He +)

The generation of laser radiation usually occurs between the excited states of atoms and ions, since inversion with respect to the ground state is created only with great difficulty. Transitions between excited states in the hydrogen atom give wavelengths predominantly in the visible and infrared regions of the spectrum. The same goes for more complex neutral atoms. In comparison with this, the generation of short-wave, ultraviolet radiation is possible through transitions in excited states of ions, as can be seen from Fig. 1 for helium. With atoms of a higher degree of ionization, it is possible, according to equation (1), to obtain even shorter waves. In addition to the above, it should be noted that the helium atom and the He ++ ion are still considered not very suitable for generating laser radiation and are presented here exclusively as the simplest examples of considering the spectral properties of the radiation of atoms and ions.

The more successful generation of short-wavelength regions by ion lasers (in comparison with atomic lasers) is also clearly manifested in the parallel consideration of the ionic argon and helium-neon lasers. This atomic laser produces red and green lines, while the argon ion laser generates green, blue and ultraviolet lines.

In conclusion, it can be stated that an ion, on average, emits shorter wavelengths than an atom. Another way of generating waves with a length shorter than that of an atom is through the use of molecules. The fact is that molecules have the same electron energy as atoms, but as a result of vibrations, the ground state splits or even becomes unstable, as in excimers. And then laser transitions with high energies are possible, corresponding approximately to the transition energy in the hydrogen atom of the Lyman series.

1.2 Ion noble gas lasers

With the help of ionized inert gases Ne, Ar, Kr and Xe, laser radiation is generated in gas discharges on more than 250 lines in the spectral range from 175 to 1100 nm. In this case, as a rule, the higher the ionization state, the shorter the wavelengths and the higher the photon energy, since an ever stronger coupling of optical electrons is noted (see Sec. 1.1). Some of the laser lines arise from transitions in inert gases, sometimes multiply ionized. Such a high state of ionization with the required ion density is possible only in the pulsed mode.

Of particular importance are cw lasers (cw) in inert gases, singly and doubly ionized. The main representative of this type is the argon ion laser, which in special versions is capable of generating powers above 100 W in the blue-green region of the spectrum and up to 60 W in the near ultraviolet region. It is one of the most popular commercial lasers. A krypton laser with continuous (cw) powers of several watts extends the spectrum to almost infrared. The most intense lines of cw ion lasers are shown in Fig. 2

Argon ion lasers

A schematic diagram of the generation process for upper laser levels is shown in Fig. 3 using argon as an example. As a result of the collision of electrons, an argon atom is ionized. Further, after a collision of the second kind, an argon ion is excited to the upper laser level. Other excitation mechanisms consist in the fact that the population is created due to the decays of the radiation of the higher levels, or the electron-collisional excitation arises from deeper metastable states of the argon ion. It is assumed that all three processes make a significant contribution to the population of the upper laser level, with, for example, cascade transitions from higher levels accounting for from 25 to 50%.

Rice. 3. Energy levels and pumping process for an argon laser. (ArII is the spectroscopic designation for the Ar + ion)

As can be seen from Fig. 3, the upper laser 4p level of 35.7 eV is located above the ground state of the argon atom, and 20> B - above the argon ion. Thus, excitation can only be promoted by high-energy electrons with a low quantum efficiency of the order of 10%. These data refer to the ground state of the argon ion, since it can be re-excited in the discharge. The lower 4y laser level is rapidly depleted as a result of a radiative transition (72 nm) with a lifetime of 1 ns. In comparison, the lifetime in the upper 4p state is 10 ns longer. The short lifetime at the lower laser level provides a very small population, as a result of which the inversion can occur despite the relatively weak excitation of the upper laser level.

Since the 4p and 4S states are split, a large number of laser transitions with different intensities are formed. In fig. 4 shows 10 laser lines, the most intense of which are in the wavelength range of 488.0 nm (blue) and 514.5 nm (green). In commercial lasers, these lines have powers of more than 10 watts (see table 1).

Rice. 4.4p-> 4s - transitions of an argon laser

Due to the two-stage electron-collisional excitation, the power of the argon laser increases almost squarely to the current. For a high-power argon laser, due to the necessary ionization and excitation, large currents are required at small cross sections. Of course, this will require much more serious - in comparison with helium-neon lasers - technological costs.

With a further increase in the current density, argon can be ionized twice. This requires an energy of 43 eV. Approximately 25-30 eV above the ground state of Ar2 +, there are further laser levels that generate ultraviolet radiation at 334, 351 and 364 nm. For lasers in a special design, the power can be several watts. Such ultraviolet argon lasers are quite expensive because they require special optics, as well as higher current densities and strong magnetic fields.

Table 1. Power of an ionic argon laser 20 W at different emission lines

Constructive execution

Effective excitation of the Ar + lines in the discharge requires an electron density of 10 14 cm -3. This value is achieved at a current density of up to 10 3 A · cm 2 in low pressure arc discharges. The field strength along the discharge is about 4 V cm -1. The temperature of the neutral gas can reach 5 · 10 3 K. High power densities require significant technical costs in the design of laser tubes. In most cases, we are talking about a ceramic tube with water cooling, for example, made of BeO - a substance with a high thermal conductivity, almost like aluminum. In other versions, the discharge is conducted through tungsten disks provided with holes, which conduct heat to the tube by means of copper holders (Fig. 5). Today, in most cases, it is BeO tubes that are used. Since BeO powder is highly toxic, the disposal of such tubes requires the utmost care with the involvement of specialized organizations.

Rice. 5. Gas discharge tube for argon ion laser

Due to their high density, the electrons are squeezed radially outward, which leads to a decrease in the current density. This effect is compensated for by the external magnetic field generated by a long coil around the laser tube. The electrons are affected by the Lorentz force directed perpendicular to the axis and the radial component of the motion. As a result, the movement from the radial direction is deflected to a circular or spiral trajectory, and the discharge is concentrated just on the axis. This softens the effect of the plasma on the materials of the laser tube, which greatly extends the tube's life. Additionally, the pumping speed and the efficiency of the laser are increased. High currents are taken from directly heated spare cathodes, and cooled structural elements from the sand can serve as anodes.

In the presence of high currents in the discharge, impulses are transferred from electrons to the gas, and the gas drifts towards the cathode. To equalize the resulting pressure gradients along the tube, special holes are made in the copper discs. The gas in the ion laser is depleted because the discharge drives the ions directly into the wall. With commercial lasers, this gas loss is compensated automatically from a connected reservoir. The gas pressure is at the level of 1 - 100 Pa, and, unlike many other types of lasers, it does not use a mixture of gases, but pure argon.

Unlike a helium-neon laser, shielding effects do not play a role in generating population inversion, and a large-diameter laser tube is possible here. The gain for the 488 nm line at a length of 50 cm is approximately: G = 1.35. Taking into account the quality of the beam, its diameter is limited to 1.5 to 2 mm. The efficiency here is usually less than 10 -3

Due to the use of laser mirrors with reflection in a wide range, radiation is simultaneously achieved on different lines. A Brewster prism in a laser cavity is used to select individual wavelengths. In order to avoid loss of reflection with such a prism, the rays fall on the prismatic surfaces at the Brewster angle (this is the angle of full polarization). The other side of the prism is oriented perpendicular to the laser beam refracted on the first surface. The vertical back surface has a high mirror finish. Depending on the wavelength, there is a different deflection of the beam in the first surface. Different lines can be set by rotating the prism. Typical powers for a commercial 20 W laser with and without wavelength selection are shown in Table 1. Due to the high temperature in the discharge region, the linewidth is - due to Doppler broadening - up to 6 GHz. Without frequency selective elements, the coherence length is in the centimeter range. For applications in the field of holography, this value is increased taking into account the intracavity standard. In this case, the bandwidth can be reduced to 5 MHz. Almost all commercial noble gas lasers generate in the fundamental TEM 00 mode. Despite the significant load on the tube from the side of high currents, its service life reaches many thousands of operating hours.

Ionic krypton lasers

A discharge tube for krypton lasers is very similar in design to an argon laser tube. But since here, during operation, the gas is depleted faster than with an argon laser, you will have to take care of a larger gas reservoir. The strongest krypton laser line is at 647 nm (red) with powers of several watts. The intensity of the other lines is heavily dependent on pressure. Therefore, lasers capable of line selection have active pressure control. The Kr + laser lines lie between 337 and 799 nm (Fig. 2). The moderate efficiency of a krypton ion laser requires a higher current density than an argon laser of identical power. In addition, enhanced scattering should be mentioned, since the Kr ions are heavier and more energetic. Therefore, most lasers of this type are water-cooled, while lower-power argon ion lasers are air-cooled.

Areas of use

Noble gas ion lasers are standard devices for visible continuous radiation in the range of a few milliwatts to 50 watts. Argon and krypton lasers are of particular importance in the red and blue-green regions of the spectrum. These lasers are used wherever He-Ne and He-Cd lasers are not powerful enough. Their use is especially important in the treatment of eyes for the treatment of the retina and for other medical purposes. Other applications include the printing industry (for display, video and audio disc production), high-speed laser printers, and holography. As a result of mixing argon and krypton, lines are obtained between red and blue, which can be mixed to achieve white: such lasers are very popular in creating a variety of laser shows. In addition, the argon ion laser is used to pump dye lasers and titanium-sapphire laser systems. In this case, by appropriate mode locking, pulses in the pico- and femtosecond ranges can be generated. Continuously tunable (cw) lasers are also subject to pumping using argon lasers.

A natural step forward in the development of gas lasers was the inclusion in the number of active media, along with gases consisting of neutral atoms, also ionized and molecular gases. Molecular lasers are discussed in the next chapter.

Bell was the first to report the achievement of pulsed lasing in mercury II vapor at a wavelength in 1964. In spectroscopy, the number indicated by the Roman numeral, if it is reduced by one, means the multiplicity of ionization. Thus, the symbol denotes mercury vapor in an atomic non-ionized state, II - in a state of single ionization. In April of the same year, Bridges is the bulk power density at all emitting modes, J- discharge current density.

Along with pumping the upper level, care must be taken to empty the lower one. For argon ions, the ratio of the lifetime of the working levels is 3p 4 4p and 3p 4 4s unfavorably (without external factors, the lower level is more long-lived). The presence of UV radiation from the lower level with a wavelength of about 72 nm helps. This radiative decay of the lower level provides the necessary conditions for inversion.

Radiation in argon was obtained on 10 lines of transition between states 4p and 4s, the most intense of which are the 514.5 nm (green) and 488.0 nm (blue) lines. The efficiency of an argon laser, bounded from above by a quantum limit of ~ 7% (which follows from the level diagram), is of the same order of magnitude as for a He-Ne laser (0.1 - 0.05)%. It can be estimated that for each W of output power there is no less than 1 kW of consumed power (for domestic lasers - no less than 5 kW).

To obtain a high current density, small diameter tubes are used. The discharge in this case is not purely glowing, where the degree of ionization is very small, it is closer to the arc one. A high concentration of active particles makes it possible to obtain about 1000 times higher levels of output power than in a He-Ne mixture at the same pressures and lengths of the active medium.

Ionic gas lasers were created almost simultaneously in many laboratories, both in the USSR and in the USA, in 1963-64, so it is difficult to indicate a priority development. It is important to note that their appearance was predetermined by the objective requirements for obtaining high-power coherent radiation in the visible range, moreover, in a continuous mode.

Due to the high current densities and low efficiency, the thermal load on the active element of the Ar + laser turns out to be very high. Therefore, the developers of Ar + lasers have to face very serious technical problems. The ion temperature in the discharge is ~ 3000 K (it can be estimated with a sufficient degree of accuracy from the Doppler linewidth of the laser, which is ~ 3500 MHz). This means that the electrodes and walls are subjected to intense bombardment with heavy ions and undergo impressive erosion during operation.

But heating and erosion are not all the troubles of ion lasers. Due to the high current density, Ar + ions intensively diffuse from the anode to the cathode, which leads to the appearance of longitudinal pressure gradients, gas stratification in the discharge column, and disruption of the discharge in general. So the developers of argon lasers were at first just right to grab their heads from the abundance of technological problems.

Nevertheless, to the credit of engineers and designers, very ingenious and elegant technical solutions were found that made it possible, if not to solve, then to a significant extent to mitigate these problems.

Thus, the need for the most efficient heat removal made it necessary to be very exacting in the choice of material for gas discharge tubes. The traditional heat-resistant material - fused quartz - withstands no more than 500 hours of operation. Ceramic materials such as beryllium ceramics (BeO) provide significantly better results. Active elements with discharge channels made of BeO have a service life of up to 5 thousand hours, which is comparable to neon-helium lasers.

But the service life is lengthened not only by the choice of material. To reduce the number of collisions of ions with the walls of the tube, it is placed in a longitudinal magnetic field - in a solenoid coaxial with the optical axis. The strong magnetic field not only protects the tube walls, but also increases the pumping efficiency, forcing the ions to collide more often and to be better excited.

Catophoresis (diffusion of ions to the cathode) is compensated for in an equally ingenious way: the gas-discharge tube is equipped with a bypass channel that circulates the gas and thereby “deceives” diffusion: the ions are, as it were, “dragged out” from under the cathode and flow into the anode region. True, here I immediately had to run into trouble: during ignition, it is easier to ignite the discharge through the bypass channel, and not along the working gap (the diameter of the bypass channel is much larger), which, of course, is unacceptable. Therefore, it is necessary to make a bypass channel with a length that significantly exceeds the length of the main channel. This is usually implemented in the form of a spiral quartz tube surrounding the discharge gap. So, the emitter of an argon laser, schematically shown in Figure 8.8, has a rather complex design.

The most widely used continuous-wave Ar + lasers with an output power from 1 to 20 W (on all lines from 451.5 to 514.5 nm). Such a "picket fence" of lines (10) is not always convenient; therefore, Ar + lasers are often equipped with dispersive elements (prisms, diffraction gratings). If we talk about record power levels of Ar + lasers, then in a continuous mode they can reach hundreds of watts, but such monsters are not used in medicine.

The thermal load on the active element can be significantly reduced in a pulsed mode, up to the rejection of water cooling. Such lasers are of undoubted interest for medicine (more on this in Section 4). However, in the pulsed mode for argon lasers, solid-state lasers operating at the second harmonic and fiber lasers, which significantly outperform both in most operational characteristics, are in strong competition.

LITERATURE for lecture 8.

1. N.V. Karlov. "Lectures on Quantum Electronics".

2. W. Bennett. Gas lasers (review).

3. O. Zvelto. "Principles of Lasers".

4. V.S. Letokhov, V.P. Chebotaev. Ultrahigh-resolution nonlinear laser spectroscopy.― Moscow, 1990, 512 p.

5. V.E. Privalov // Lazer-Inform, 2006, No. 19-20, p.5.

6.P.S. Krylov, V.E. Privalov // Letters to ZhTF, 2005, 31 , no. 5, page 7.

7. Raizer Yu.P. // Soros educational journal, 1997, N o 8, p. 99-104.

Ionic Lasers (W. Bridges, USA, 1964). In ion lasers, population inversion is created between the electronic energy levels of ionized rare-gas atoms and metal vapors. Population inversion is achieved by choosing a pair of levels for which the lower laser level has a shorter, and the upper one has a longer life time. The need to create a large number of ions leads to the fact that the current density of the gas discharge in ion lasers reaches tens of thousands of A / cm2. The electric discharge is carried out in thin capillaries up to 5 mm in diameter. At high current densities, the gas is carried away by the current from the anode to the cathode. To compensate for this effect, the anode and cathode regions of the discharge tube are connected by an additional long tube of small diameter, which provides reverse gas movement.

Due to the high current density, metal-ceramic structures or beryllium-ceramic tubes with high thermal conductivity are used for the manufacture of gas-discharge tubes of ion lasers. The efficiency of ion lasers does not exceed 0.01%. In the visible light region, argon lasers have a comparatively high power in continuous mode. An argon ion laser generates radiation with l = 0.5145 microns (green beam) with a power of up to several tens of watts. It is used in the technology of processing solid materials, in physical research, in optical communication lines, in the optical location of artificial earth satellites.

An ion laser based on a mixture of argon and krypton ions has the ability to tune in wavelength (by changing mirrors) in the entire visible range. It emits power up to 0.1 W at 0.4880 microns (blue), 0.5145 microns (green), 0.5682 microns (yellow) and 0.6471 microns (red).

A very promising cadmium vapor laser operating in a continuous mode in the blue (0.4416 μm) and ultraviolet (0.3250 μm) spectral regions and exhibiting high monochromaticity. Cd vapors are formed in an evaporator located near the anode (fig. 6)... They are highly diluted with He. The uniform distribution of Cd in the gas discharge tube and the selection of its concentration are achieved by the entrainment of Cd vapors by He ions from the anode to the cathode. The density of the Cd vapor is determined by the temperature of the heater. In the cooler, Cd condenses near the cathode. A tube with a diameter of 2.5 mm and a length of 140 cm at an He pressure of 4.5 mm Hg. Art., a heater temperature of 250 ° C, a discharge current of 0.12 A and a voltage of 4 kV allows you to obtain a power of 0.1 W in the blue and 0.004 W in the ultraviolet regions of the spectrum. The cadmium laser is used in optical research (see. Nonlinear optics), oceanography, as well as photobiology and photochemistry.

Gas-dynamic lasers (V.K.Konyukhov and A.M. Prokhorov, USSR, 1966). A characteristic feature of gases is the ability to create fast flows of gas masses. If a preliminarily highly heated gas suddenly expands, for example when flowing at supersonic speed through a nozzle, then its temperature drops sharply. With a sudden decrease in the temperature of a molecular gas, the vibrational energy levels of the molecules can become excited (gas-dynamic excitation). There is a CO2 laser with gas dynamic excitation. With gas-dynamic excitation, thermal energy is directly converted into energy of electromagnetic radiation. The radiation power of continuous gas-dynamic lasers reaches 100 kW.