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

Fig. 3.2. Schemes of energy levels of a He-Ne laser.
The most powerful, possessing to the same high (~ 25%) efficiency are molecular lasers, operating at vibrational levels of molecules, for which small energy gaps (0.01-0.1 eV) are characteristic, high excitation efficiency, large quantum yield, and good energy selectivity for radiative transitions. One of the examples of molecular GL is a laser working on CO₂ molecules with an admixture of N₂, He and H₂O molecules, which contribute to a high population inversion, effectively populating the upper laser level (N₂) and emptying the lower laser (He and H₂O). The average power of these lasers is ~ 1 kW. To increase it, a fast-flow version of a molecular laser with a gas pressure of up to 50 atm is used. Such lasers, working in a continuous mode, allow to receive power in several tens of kilowatts, and in pulse-energy - the energy of radiation in a pulse of ~ 10⁴ Joules.

Perhaps the most exotic of all these GLs are excimer lasers, the generation of which is realized as a result of the transition of molecules from the upper bound, but very short-lived (~ 10^-8 s) excited state to the lower unstable lasers. The exotic feature of excimer molecules is that they can be formed from two atoms of inert gases (or from an atom of an inert gas and a halogen atom) only in an excited state, because free atoms repel each other and can not unite in the molecule in its lowest (basic) energy state. This feature of excimer molecules provides a rapid devastation of the lower (unstable) state, which contributes to the formation of an inverted population. Excimer lasers work on transitions between electronic levels of molecules. The emerging radiation is in the visible or ultraviolet region of the spectrum and is characterized by a large line width, which allows tuning the generation frequencies. The active medium of excimer lasers is an inert gas at a pressure of ~ 1 atm with a ~ 1% addition of halogen-containing molecules. In connection with the very short lifetime of the excited state of excimer molecules, a powerful electric-discharge pulse or an intense electron beam is used to pump excimer lasers. The most efficient generation is obtained for an ArF, KrF, and XeF based laser (the output energy is about 100 J, the efficiency is ~ 10% ~ 10^-8 s).

In addition to GLs pumped by an electric discharge and an electron beam, lasers with nuclear and chemical pumping are known in which the gas working medium is excited as a result of nuclear or chemical reactions. In the case of nuclear pumping, the excitation of gas atoms (for example, Ar or Xe) is produced by the products of nuclear reactions formed in the interaction of thermal neutrons (E = kT = 0.025 eV) with ¹⁰B, ³He or ²³⁵U, as a result of which 4He ions and ⁷Li, ¹H and ³H, or fission fragments. The construc- tion of the laser is a tube with a gas, on its inner surface a thin layer of ¹⁰B or ²³⁵U (3He is introduced as an admixture to the working gas) is applied. The source of thermal neutrons is usually a pulsed reactor with a moderator. The power on the infrared passages of inert gases is of the order of 10 kW, the efficiency is ~ 1%.

The described pump mechanism is close to the mechanism of pump plasma lasers, in which excitation of the working medium (weakly ionized plasma) is also produced by charged particles (electron beam). An important feature of plasma generators of coherent radiation is the possibility of smooth variation of the radiation frequency by changing the plasma density.

In the case of chemical pumping (chemical lasers), excitation of the working medium occurs due to the fact that the products of many exothermic reactions are formed in the excited state. In diatomic molecules, for example, it manifests itself in the form of excitation of vibrational-rotational levels. As a result, an inverse population of these levels appears, which (with a suitable variety of their energies) can be used to obtain a laser effect.

Typical reactions of this kind used in chemical lasers are substitution reactions that allow one to obtain an efficiency factor (the ratio of the laser radiation energy to the energy released in the reaction) of the order of 10%.

(3.1)

or
F+CH₄ (3.2)

The energy of the radiation of a chemical laser on HF in the pulse mode at a pulse duration of ~ 10 ns can reach 10 J. The maximum power in a continuous mode is obtained when the active substance is pumped through a resonator with supersonic velocity. In this case, the output power reaches several kilowatts, and the efficiency is 2-4%.

Below are the main parameters of various solid-state and gas lasers: