Light and general radiation laws. Coherence and incoherence. Emission, absorption and amplification of radiation. Units and physical constants

Gas Lasers

A gas laser (GL) in the broad sense of this naming is called a laser with an active medium in the form of gases, vapors or their mixtures, as well as a weakly ionized plasma. The most common feature of GL, which distinguishes them from solid-state, liquid and semiconductor, is the large homogeneity of the active medium, low density (even if the pressure is maintained in several tens of atmospheric spheres) and, as a consequence This is a high degree of transparency, narrowness of the emission and absorption lines, very low radiation divergence and high frequency stability. In addition, in connection with the possibility of rapid pumping of the working substance through the resonator, the GL makes it possible to obtain sufficiently large average radiation powers without overheating the active medium. Another feature of GL is the possibility of generating laser radiation in a very wide range of wavelengths (from ultraviolet to submillimeter wavelengths).

Specific types of GL are very diverse. Their constutions and modes of operation are determined by many factors, including the choice of working substance, the type of laser transitions (atomic, ion, molecular, excimer, etc.), the nature of the pump (optical, electrical discharge, nuclear or chemical reaction, electron beam). However, some factors are interrelated. Below we describe some specific types of GL and briefly characterize their features

According to the mode of operation GL, as well as solid and liquid, are divided into pulsed and continuous. The choice of the regime is determined by the nature of the pump. The severity of GL works continuously. A significant part of them is related to gas-discharge ones. In these lasers, population inversion is created by means of an electric discharge, in which electrons are generated that excite working gas particles (electron impact). At the same time neutral atoms (for example, He, Ne), ions (for example, Ar^{2 +}, Ar^{3 +}, Kr^{2 +}, Kr^{3 +}, Ne^{2 +}) can be working particles, atoms and ions of metal vapor (for example, Cu, Cd), molecules (vibrational and rotational levels of CO₂ and N₂), excimer (unstable) molecules (for example, Ar₂, ArF, Kr₂, KrCl, Xe₂, XeBr, etc.).

From the above list, a helium-neon laser was constructed earlier (in 1961) (Fig. 3.2), in which the main working substance is neon, and helium, resonantly transmitting neon energy to its metastable level, helps it to linger in the upper excited state State for a longer time than is possible for pure neon (in which it is not possible to obtain a sufficiently high inverted population). Due to the fact that the laser levels of neon have several sublevels, a helium-neon laser can operate at 30 wavelengths of visible (in the red region) and infrared radiation. The allocation of a resonant frequency is provided by a special tuning of the mirrors of the resonator. The power and efficiency of the He-Ne laser are not high (~ 0.1 Vt and ~ 0.1% respectively), but it is characterized by a continuous mode of operation, high monochromaticity and directionality of radiation, and simplicity of the device (gas-discharge tube with two electrodes and two mirrors).

In addition to the mixture of Ne and He, lasing was also obtained on several dozen neutral atoms, both in continuous and pulsed modes (including high peak power).

In 1964, the first laser was built using ionized gases, in which the population inversion was created between the energy levels of atomic ions. In total, there are several dozens of ion lasers that work on several hundred working sublevels. For ion lasers, a high current density and a large concentration of ions are characteristic, which makes it possible to obtain a higher output power for them than for lasers on neutral atoms. However, the efficiency of ion lasers is also not high (~ 0.1%). An order of magnitude greater efficiency (~ 1%) was obtained on atoms and metal vapor ions, which differ in the more efficient (collisional) type of depletion of the lower laser level than the usual spontaneous transition. Generation is obtained on pairs of several tens of metals with an average power of ~ 40 W (for Cu) and a peak power of 200 W.