A gas laser is a laser in which an electric current is discharged through a gas environment to produce optical radiationі. The gas laser was the first continuous-light laser. Gas lasers using many gases have been built and used for many purposes. The purpose of the article is the analysis of the possibility of application of gas laser technologies in the solar power systems of production and transportation of energy.

Helium-neon laser. The active environment of the laser is a gaseous mixture of helium and neon. The generation is achieved due to transitions between the power levels of neon. Helium executes the auxiliary role of buffer gas. Its energy is passed to the atoms of neon for creation of population inversion. Neon generates the laser radiation as a result of more than 130 different transitions. But the most intensive lines are of 632,8 nm, 1150 nm and 3390 nm (fig.1,a).

The generation of radiation by a laser is executed by a next way. At admission of electrical current through a helium-neon mixture, which is pumped into a quartz tube, the atoms of helium become excited to the levels of E₁' and E₂'.  These levels are metastable, because the transitions from the levels into the basic state are forbidden by the presence of the quantum-mechanical forbidden zones. Due to it the atoms are accumulated on these levels. When the excited atom of helium collides with the atom of neon (not excited), energy of excitation passes to the last. This transition occurs very effectively as a result of concordance of energies of the proper levels. At the levels of E₂ and E₁ of neon the population inversion is formed, that results in possibility of generation of laser radiations. A laser usually generates radiations in the continuous mode.

The ends of laser tube are closed by the proper transparent material so that axial modes fall on him under the Brewster’s angle. Due to this the radiation of laser is linearly polarized. Of course, pressure of helium in a chamber is equal of 332 Pa, and of neon - 66 Pa. On a tube a permanent voltage is given by a value near 4 kV. One of mirrors has coefficient of reflection near 0,999, and second (output) mirror - near 0,990. As mirror coverage the multilayer dielectrics are used.

Argon ionic laser. In this laser the life-span of some power levels of ions is much more than of other power levels. Power levels that are below have small life-span and are practically empty. Thus, the generation is possible in the unsettled levels that are above a basic level. An argon laser works as a system of four levels. Ions become excited and move to the strongly excited levels, which can be grouped into one (second) level. Then ions are accumulated on overhead power levels (third level) and generate the radiations at returning on lower power levels (fourth level). From a fourth level the ions go back quickly into the basic state and a cycle is repeated.

Argon laser generates the radiations on a few lines of blue-green part of spectrum with dominant lines of 488 nm and 514,5 nm.

Krypton laser. In accordance with the principle of work a krypton laser strongly reminds an argon device, but his output-input ratio is considerably smaller. The lines of radiations of krypton laser are located in the wide range of visible spectrum, and also in ultraviolet and near infrared regions of optical spectrum. Today more frequently a wave-length of 647 nm is used. In last time the yellow and green lines of radiations are also used.

CO₂ - laser. Gas CO₂ - lasers of the continuous and pulsed radiations find the wide application, thanking universality and power efficiency. As the active environment of laser is carbon dioxide. In a mixture the nitrogen and helium are also present.

CO₂ - laser radiates in the infrared range of wave-lengths within the limits of (9...11) μm. Energy of photon of CO₂ - laser is by 20 times weaker than energy of photon of argon laser.

Depending on the demands that are making to the power of radiations the CO₂ – lasers can have different constructions. In majority for excitation of molecules of carbon dioxide in a gas mixture an electric discharge is created. Molecules achieve the excited state, absorbing the energy of electrons of discharge. Thus nitrogen stimulates excitation of molecules of CO₂, so as the levels of vibrations of nitrogen are close to the power levels of molecules of CO₂. Accordingly, the molecules of nitrogen can absorb energy and then to pass the energy to the molecules of CO₂, exciting them to the top power level. On fig.1,b the simplified diagram of power levels of CO₂ - laser is represented. She characterizes the indicated power transformations.

Molecules of CO₂, situated on upper levels can lose part of energy with the radiations of photon and pass to lower power level. The bias to the symmetric mode causes the radiations on a wave-length of 10,6 µm. A bias to the rotatory mode causes the radiations on a wave-length of 9,6 µm. The laser radiations of gas mixture consist of two series that include almost 100 closely located lines.

Helium helps to decrease population density of two lower levels, supporting the population inversion and continuity of generation. Optimum proportion of gases within a tube is determined by construction of laser, but more often the following mixture is used: carbon dioxide (10 %), nitrogen (10 %), and helium (80 %). The power of laser is in the limits from a few Watts to a few hundreds of kilowatts.

Excimer laser. The term of "excimer" is abbreviation of expression of "excited dimer". Today this term is used in more wide sense and characterizes every diatomic molecule, the atoms of which are linked in the excited state and unconnected in the basic state. When two constituents of atom are excited, for example, by a discharge, they attract each other and form a stable molecule. But in the basic state two atoms are neutral. An overhead curve characterizes the power level of diatomic molecule.

In the process of relaxing of the excimer laser the excited molecule disintegrates onto component parts. This process stimulates the work of laser, so as in reality the basic state does not exist, and violation of population density comes at once after the formation of the excited molecules.

The molecules of salts of noble gases that do not meet in natural conditions are correspond most of all to the demands of use in this laser. But they can be easily got in an electric gas discharge. The greater part of gas mixture is presented by buffer gas, which is a bounding link for power transformations, but does not take part in the generation of pulses. Usually it is helium or neon. Small part is made by gases that become active excimer. In an excimer laser for the generation of radiations of different wave-lengths it is possible to use a few gases.

Structurally an excimer laser includes the tube filled by gas, through what perpendicular to the ray of laser the pulse of pumping is skipped. For the increase of power and of frequency of reiteration of pulses the gas can be given from the special reservoir. Impoverished gas is periodically renewed. Life-span of the excited molecules of excimer is equal about 10 ns. Pulses proceed from a few nanoseconds to a few tens of nanoseconds. Duration of pulses is limited by the discharge instability. The losses, determined by the fluorescence processes, can be diminished to the minimum with the use of rapid excitation. The value of output-input ratio of excimer molecules can be equal of 2 %. The typical width of lines is near of 0.3 nm. At lower output energy of radiations the width of line is about 3 nm. The laser with the use of fluorine radiates on a wave-length of 157 nm. The laser on a base of carbon dioxide radiates on a wave-length of 10.6 μm.

Greater part of excimer lasers is multicomponent. Their work is possible on the different wave-lengths, thanking choice of necessary composition of gas mixture. Range of typical values of wave-lengths of laser working in the UV - range of light-spectrum is represented in table 1. At replacement of excimer gases the additional adjusting is not needed.  But the change of output radiations from ultraviolet to infrared necessitates of replacement of resonator mirrors. The typical laser of Lambda Physik EMG 103 E generates about 40 W on a wave-length of 249 nm (krypton fluoride), about 20 W - on a wave-length of 193 nm (argon fluoride) and about 20 W - on a wave-length of 308 nm (xenon chloride).

Standard frequency of reiteration of pulses of EMG 103 E - laser is of 200 Hz. In EMG 103 E - laser (S- version) energy of 500 mJ in a pulse is attained on the wave-lengths of 249 nm and 308 nm. EMG 103 E - laser (F-version) is optimized for the generation of ultraviolet radiations of optical spectrum. On a wave-length of 157 nm laser provides middle power of 1,5 W. At the increase of reliability of lasers and decreasing of their cost they can become an alternative to Nd:YAG - lasers as sources of pumping of lasers on dyes. The important elements of excimer laser are a thick-walled gas reservoir with a connecting gas pipeline and low inductivity electric circuit. In some lasers a gas reservoir is made from aluminum by means of the extrusion method. He is equipped by the systems of circulation and cooling of gas and serves as optical table, on which the resonator mirrors are fastened.

There are three factors, providing high efficiency of excimer laser: correct choice of form of electrode; effective previous ionization of gas; presence of low inductivity electric circuit as an integrating element.

In a device an arc discharge must be absent. For this purpose the condensers are located closely to the electrodes, within a gas chamber.

As materials in excimer lasers only carbon fluorine dielectrics and aluminum or nickel are used, which are passive for fluorine or hydrogen-chloride environments. All components of electric circle, electrodes excluded, are placed outside the gas system. Pushing off from existent prices on xenon, the cost of gas for lasers on the xenon chloride is equal of 40 cents per hour and less, if a laser works with reiteration frequency of pulses close to 100 Hz.

Table 1

       Working wave-lengths of laser radiations of ultraviolet range  of  light-spectrum

Laser environment            Wave-length, nm

Fluoride (F₂)                                  157

Argon fluoride (ArF)                     193

Krypton chloride (KrCl)                222

Krypton fluoride (KrF)                  248

Xenon chloride (XeCl)                  308

Nitrogen (N₂)                                 337

Xenon fluoride (XeF)                    351

Lasers on the steam of copper and gold are the most known from the lasers on the steam of metals.

A laser on the steam of copper has two basic lines of radiations: 510,6 nm in green range of spectrum and 578,2 nm in the yellow range of spectrum. A laser on the steam of gold radiates on one line (628 nm) in a red part of light-spectrum.

Lasers on the steam of metals are pulsed emitters. For direct excitation of atoms of metals to the top level by means of electronic collisions an electric discharge is used. These collisions provide formation of population inversion on unsettled intermediate levels. Lower levels of laser transitions are metastable; a laser pulse arises up, when this level is filled. The repeated radiations are achieved after devastation of lower level to the basic state. Lasers on the steam of metals generate pulses with high-frequency of reiteration (a few kHz). Duration of pulses is of tens of nanoseconds, energy in a pulse is more than 10 mJ, and middle power is tens of Watts. A laser on the steam of copper is optimal for pumping of dye lasers. A laser on the steam of gold radiates on a wave-length of 628 nm.

Conclusion. The lasers presented in an article are based on gases as gain media. The laser-active components are either single atoms or molecules, or a mixture of single atoms or molecules with other substances having buffer functions. A population inversion for gain via stimulated emission is in most cases achieved by pumping the gas with an electric discharge, but there are also gas lasers using a chemical reaction, optically pumped, and that based on the Raman scattering effect. During operation, the gas is often in the state of plasma, containing a significant concentration of electrically charged particles. Most gas lasers provide high diffraction-limited quality of beam, since the gas introduces only weak optical distortions. Their operation usually requires a high-voltage supply, often with a high electrical power. Some high-power gas lasers use a system of quick circulation of gas.

Written by Vasil Sidorov on August 17, 2010 in queltanews.com

Technopark QUELTA,

Nizhyn Laboratories of Scanning Devices

sidorovvasil@gmail.com