Gas Laser

a laser with a gaseous active medium. A tube containing the active gas is placed in an optical resonator, which in the simplest case consists of two parallel mirrors, one of which is semitransparent. A light wave emitted at any point in the tube is amplified upon propagation through the gas by events of induced emission, which give rise to a photon avalanche. Upon reaching the semitransparent mirror, the wave partially passes through it. This part of the light energy is radiated outward by the gas laser. The other part is reflected from the mirror and gives rise to a new photon avalanche. All the photons are identical in frequency, phase, and direction of propagation. Because of this, laser radiation may have extraordinarily high monochromaticity and power and sharp directivity.

The first gas laser was developed in the USA in 1960 by A. Javan. Existing gas lasers operate in a very broad range of wavelengths—from ultraviolet to far infrared—in both the pulse and continuous modes. Certain data on the most widely used continuous-wave lasers are presented in Table 1.

Of the gas lasers operating only in the pulse mode, lasers in the ultraviolet band operating on neon ions (λ = 0.2358 microns [μm]) and on N₂ molecules (λ = 0.3371 μm) are of the greatest interest. The nitrogen laser has a high pulse power.

The characteristic properties of laser radiation—high directivity and monochromaticity—are manifested most distinctly in the radiation of a gas laser. A significant advantage of gas lasers is their ability to operate in the continuous-wave mode. The use of new methods of excitation and the transition to higher gas pressures can sharply increase the power of gas lasers. By using gas lasers it is possible to make use of the far infrared band, as well as the ultraviolet and X-ray bands. New fields for the use of gas lasers—for example, in space studies—are opening up.

Particular features of gases as laser materials. In comparison with solids and liquids, gases have a much lower density and higher homogeneity. Therefore a light ray in a gas is virtually not distorted, does not disperse, and experiences no energy losses. In such lasers it is comparatively simple to excite just one type of electromagnetic wave (one mode). As a result, the directivity of the laser radiation is sharply increased, reaching a limit determined by light diffraction. The divergence of the light ray of a gas laser is 10^-5 to 10^-4 radians in the visible light band and 10^-4 to 10^-3 radians in the infrared region.

In contrast to solids and liquids, the particles comprising a gas (atoms, molecules, or ions) interact with each other only upon collisions in the process of thermal motion. This interaction has little effect on the grouping of particle energy levels. Therefore, the energy spectrum of the gas corresponds to the energy levels of the individual particles. The spectral lines corresponding to transitions of the particles from one energy level to another are broadened only slightly in a gas. The narrowness of the spectral lines in a gas leads to a situation in which few resonator modes are incident upon the line.

Since a gas has virtually no effect on the propagation of radiation in a cavity, the stability of the radiation frequency of a gas laser depends primarily on the immobility of the mirrors and the entire structure of the resonator. This leads to the extremely high stability of the radiation frequency of a gas laser. The frequency ω of gas laser radiation is reproduced to an accuracy of 10^-11, and the relative stability of the frequency Δω/ω = 10^-14.

The low density of gases impedes the production of a high concentration of excited particles. Therefore the density of the energy generated in gas lasers is much lower than in solid-state lasers.

Production of an active gaseous medium. The aggregate of the excited particles of a gas (atoms, molecules, or ions) that have population inversion is the active medium of a gas laser. This means that the number of particles “populating” higher energy levels is greater than the number of particles at lower energy levels. Under ordinary conditions of thermal equilibrium, the reverse is true—the population of lower levels is greater than that of higher levels. In case of a population inversion, events of induced photon emission at an energy hv = ε_h — ε_l which accompany the forced transition of particles from a higher level ε_h to a lower level ε_l, predominate over instances of absorption of these photons. As a result of this, the active gas may generate electromagnetic radiation with frequency v = (ε^h — ε_l)/h (or with wavelength λ. = c/v).

One of the specific features of a gas (or of a mixture of gases) is the diversity of the physical processes that lead to its excitation and the creation within it of a population inversion. The excitation of an active medium by the radiation of gas-discharge tubes, which has found extensive application in solid-state and liquid lasers, is not very effective in producing a population inversion in a gas laser, since gases have narrow absorption lines, but the tubes radiate light in a broad range of wavelengths. Hence only a negligible part of the power of the pumping source can be used (the efficiency is low). In the overwhelming majority of gas lasers a population inversion is created in an electrical discharge (gas-discharge lasers). The electrons formed in a discharge excite the gas particles upon collision with them (electron impact), causing them to pass into higher energy levels. If the lifetime of particles at a higher energy level is greater than at a lower level, a stable population inversion is formed in the gas. The excitation of atoms and molecules by electron impact is the best-developed method of producing a population inversion in gases. The method may be used for exciting a gas laser in both the continuous-wave and pulse modes.

Excitation by electron impact may be successfully combined with another excitation mechanism—the transfer of the energy needed to excite the particles of one type from particles of another type upon inelastic collision (resonant excitation transfer). Such transfer is extremely efficient when the energy levels of particles of different types coincide (see Figure 1). In these cases the creation of the active medium takes place in two stages: first, the electrons excite the particles of the auxiliary gas; then, in the process of inelastic collisions with the particles of the working gas, these particles transfer their excitation energy to them. As a result of this, the upper laser level is populated. To ensure good energy accumulation, the upper energy level of the gas should have a long intrinsic lifetime. It is according to precisely this scheme that population inversion is accomplished in the helium-neon laser.

Figure 1. Diagram of energy levels of auxiliary and working particles of a gas-discharge laser

The helium-neon laser (A. Javan, USA, 1960). In a helium-neon laser the neutral atoms of neon (Ne) are the working medium. Atoms of helium (He) serve to transfer the excitation energy. In an electrical discharge part of the Ne atoms pass from the ground level ε₁ to the excited higher energy level ε₃. But in pure Ne the lifetime at level ε₃ is brief, and the atoms quickly “jump” from there to levels ε₁ and ε₂, which hinders the formation of a sufficiently high population inversion for the pair of levels ε₂ and ε₃. An admixture of He substantially alters the situation. The first excited level of He coincides with the upper level ε₃ of Ne. Therefore, upon the collision of He atoms (excited by electron impact) and unexcited Ne atoms (with energy ε₁), excitation transfer takes place, as a result of which the Ne atoms will be excited and the He atoms will return to the ground state. When there is a sufficiently large number of He atoms, preferential population of the ε₃ level of neon can be attained. The emptying of the ε₂ level of neon, which takes place during collisions between atoms and the walls of the gas-discharge tube, also contributes to this. For efficient emptying of the ε₂ level, the tube diameter must be sufficiently small; however, a small tube diameter limits the amount of Ne and hence the generated power. From the standpoint of maximum generated power, a diameter of about 7 mm is optimal. Thus, as a result of the special selection of quantities (partial pressures) of Ne and He and upon proper selection of the diameter of the gas-discharge tube, a stationary population inversion of energy levels ε₂ and ε₃ of neon is established.

The ε₂ and ε₃ levels of neon are of complex structure, that is, they consist of many sublevels. As a result, a helium-neon laser may operate at 30 wavelengths in the visible light band and the infrared region. The mirrors of the optical resonator have multilayer dielectric coverings. This makes it possible to create the necessary reflection factor for a given wavelength and thus to excite generation at the required frequency in the gas laser.

The main structural component of a helium-neon laser is the gas-discharge tube, which is usually made of quartz. The gas pressure in the discharge is 1 mm of mercury (mm Hg), and there is usually ten times more He than Number Error. The design of a helium-neon laser developed for use in outer space is shown in Figure 2. A discharge tube with an internal diameter of 1.5 mm, made of corundum ceramic, is placed between the semitransparent mirror and a reflecting prism that are mounted on a rigid beryllium tube (cylinder). Discharge is accomplished with direct current (8 milliamperes, 1,000 volts) in two sections (each 127 mm long) with a common central cathode. A cold oxide-tantalum cathode (44 mm in diameter and 51 mm long) is divided in half by a dielectric separator, which ensures more uniform distribution of the current over the cathode’s surface. Stainless-steel vacuum bellows, which are the anodes, form a movable connection between each pipe and the retainers of the mirror and prism. The housing has an outlet at the left side. The laser is designed to operate in space for 10,000 hours.

The radiation power of helium-neon lasers may reach tenths of a watt, and the efficiency does not exceed 0.01 percent, but the high monochromaticity and directivity of the radiation, the simplicity of handling, and the reliability of design have caused them to be widely used. A red helium-neon laser (λ. = 0.6328 μm) is used in alignment and leveling (mining operations, shipbuilding, and the construction of large structures). The helium-neon laser is widely used in optical communications and location, in holography, and in quantum gyroscopes.

The carbon-dioxide laser (C. Patel, USA; F. Legay and N. Legay-Sommaire, France, 1964). Molecules, unlike atoms, have not only electron but also vibrational energy levels, which result from oscillations of the atoms constituting the molecule with respect to the equilibrium positions. The transitions between vibrational energy levels correspond to infrared radiation. Lasers in which these transitions are utilized are called molecular lasers. Among the molecular lasers, the CO₂ laser, which utilizes the vibrational levels of CO₂ molecules between which a population inversion is formed, is of particular interest.

In gas-discharge CO₂ lasers the population inversion is also accomplished by exciting the molecules by electron impact and resonant excitation transfer. Molecules of nitrogen (N₂), which are in turn excited by electron impact, serve to transfer the excitation energy. Under conditions of a glow discharge, about 90 percent of the nitrogen molecules usually pass into the excited state, whose lifetime is very great. Molecular nitrogen accumulates excitation energy well and transfers it easily to the CO₂ molecules in the course of inelastic collisions. High population inversion is accomplished by adding He to the discharge mixture, which first facilitates conditions for the discharge and second, because of its high thermal conductivity, cools the discharge and contributes to the emptying of the lower laser levels of the CO₂ molecule. Efficient excitation of CO₂ lasers may be accomplished by chemical or gas-dynamics methods.

The fine structure of the vibrational levels of the CO₂ molecule makes it possible to change the wavelength (to re-tune the laser) in increments of 30-50 hertz (Hz) in the wavelength interval between 9.4 and 10.6 μm.

Carbon-dioxide lasers have great power (the maximum power of laser radiation occurs in the continuous-wave mode) and high efficiency. Upon excitation of CO₂ molecules by electron impact and with a gas-discharge tube 200 m long, a CO₂ laser radiates a power of 9 kilowatts (kW). Compact designs with a 1-kW output also exist. In addition to high output, CO₂ lasers have high efficiency, reaching 15-20 percent (efficiency of 40 percent may be attained). In principle CO₂ lasers can also operate efficiently in the pulse mode. These features of CO₂ lasers account for the diversity of their application—in industrial processes (cutting and welding), location and communications (the atmosphere is transparent to waves with λ = 10 μm), physics research connected with the production and study of high-temperature plasma (high radiation power), and the study of materials.

The gas-discharge tubes of CO₂ lasers have a diameter of 2 to 10 cm, and their length may be very great. Modular (sectional) designs with a discharge current of up to several amperes at voltages of up to 10 kV per section are usually used. Since the power of continuous-wave CO₂ lasers attains very high values, the manufacture of sufficiently long-lived mirrors of good optical quality is a serious problem. Gold-plated sapphire or metal mirrors are used. The radiation is often tapped through apertures in the mirrors. Plastics made of high-resistance germanium, gallium arsenide, or other materials are used as the semitransparent discharge mirrors.

Undesirable effects that destroy the population inversion —heating of the gas and dissociation of its molecules— take place in the electrical discharge of CO₂ lasers. To eliminate them the gas mixture is continuously “driven” through the discharge tubes of the lasers, thus renewing the active medium. In order to produce high power (several kW) in the continuous mode, the gas is driven through the pipe at high speed, and the discharge takes place in a supersonic flow. To avoid losses of costly He, the gas mixture is circulated through a closed loop. Electron impact excitation is accomplished either in the resonator or immediately prior to the mixture’s entry into the resonator. In the best devices virtually all CO₂ molecules that enter the resonator are already excited and during their passage through the resonator yield their excitation energy in the form of a quantum of radiation.

Ion lasers (W. Bridges, USA, 1964). In ion lasers the population inversion is created between the electron energy levels of the ionized atoms of the inert gases and metal vapor. Population inversion is accomplished by selecting a pair of levels for which the lower level has a shorter lifetime and the upper level a longer lifetime. The need to produce a large number of ions leads to gas-discharge current densities in ion lasers of as high as tens of thousands of amperes per square centimeter. The electrical discharge is effected in fine capillaries up to 5 mm in diameter. At higher current densities the gas is diverted by the current from the anode to the cathode. To compensate for this effect the anode and cathode regions of the discharge tube are joined by an additional long tube of small diameter, which ensures the reverse flow of the gas.

Because of the high current density, cermet designs or pipes made of beryllium ceramics, which have high thermal conductivity, are used to manufacture the gas-discharge tubes for ion lasers. The efficiency of ion lasers does not exceed 0.01 percent. In the visible-light region argon lasers have comparatively high power in the continuous mode. An argon-ion laser generates a radiation with λ = 0.5145 μm (a green beam) with a power of up to several dozen watts. It is used in the processing of solid materials, in physics research, in optical communications lines, and in optical tracking of artificial earth satellites.

An ion laser using a mixture of argon and krypton ions may, be tuned (by replacing the mirrors) to wavelengths throughout the entire visible spectrum. It radiates a power of up to 0.1 W at wavelengths of 0.4880 μm (blue), 0.5145 μm (green), 0.5682 μm (yellow), and 0.6471 μm (red).

The cadmium-vapor laser, which operates in continuous mode in the blue (0.4416 μm) and ultraviolet (0.3250 μm) regions of the spectrum and has high monochromaticity, is extremely promising. Cadmium vapors are formed in the evaporator placed along the anode (see Figure 3). They are strongly diluted with He. Uniform distribution of cadmium in the gas discharge tube and the selection of cadmium concentrations are attained by deflecting cadmium vapors from the anode to the cathode by means of helium ions. The density of the cadmium vapors is determined by the temperature of the heater. The cadmium condenses in the coolant around the cathode. At a helium pressure of 4.5 mm Hg, a heater temperature of 250° C, a discharge current of 0.12 amps, and a voltage of 4 kV, a pipe 2.5 mm in diameter and 140 cm long makes it possible to produce 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, oceanography, photobiology, and photochemistry.

Figure 3. Schematic representation of a cadmium laser: (1) mirrors, (2) radiation outlets, (3) cathode (left) and anode (right), (4) cadmium vaporizer, (5) condenser of cadmium vapors, (6) gas-discharge tube

Gas-dynamic lasers (V. K. Koniukhov and A. M. Prokhorov, USSR, 1966). A characteristic feature of gases is the possibility of creating high-speed flows of gaseous masses. If a gas that has been strongly preheated is abruptly expanded—for example, when it passes through a nozzle at supersonic velocity—its temperature drops sharply. When the temperature of a molecular gas drops suddenly, the vibrational energy levels of the molecules may be excited (gas-dynamic excitation). A CO₂ laser using gas-dynamic excitation exists. During gas-dynamic excitation the thermal energy is directly transformed into the energy of electromagnetic radiation. The radiation power of gas-dynamic lasers operating in the continuous mode is as high as 100 kW.

Chemical lasers. In some gases population inversion may result from chemical reactions in which excited atoms, radicals, or molecules are formed. A gaseous medium is convenient for chemical excitation, since the reacting substances intersperse easily and quickly and are easily transported. Chemical lasers are interesting in that direct conversion of chemical energy into the energy of electromagnetic radiation takes place within them. The excitation that occurs during the chain reaction of the combination of fluorine with deuterium, which results in the production of excited fluorine deuteride (DF), which subsequently transfers its excitation energy to the CO₂ molecules, may serve as an example of chemical excitation. The removal of reaction products ensures the continuous operation of these lasers.

Gas lasers in which population inversion is accomplished with the aid of photodissociation reactions (in which the molecules decay when exposed to light) border on the chemical lasers. These are high-speed reactions during which excited radicals or atoms appear. A laser using the photodissociation of the CF₃ I molecule has been developed by S. G. Rautian and I. I. Sobel’man of the USSR. Dissociation takes place under the effect of the radiation of a xenon flashtube. The excited atomic ion I⁺ is the reaction fragment.