The principle of the laser: features of laser radiation

The first principle of the laser action, the physics of which was based on the Planck radiation law, was theoretically substantiated by Einstein in 1917. He described the absorption, spontaneous and stimulated electromagnetic radiation with the help of probability coefficients (Einstein's coefficients).

Pioneers

Theodore Meiman was the first to demonstrate the principle of the action of a ruby laser based on optical pumping with a flash lamp of a synthetic ruby producing pulsed coherent radiation with a wavelength of 694 nm.

In 1960, Iranian scientists Javan and Bennett created the first gas quantum generator using a mixture of He and Ne gases in a ratio of 1:10.

In 1962, RN Hall demonstrated the first diode laser from gallium arsenide (GaAs), emitted at 850 nm. Later that year Nick Golonyak developed the first semiconductor quantum generator of visible light.

The device and principle of operation of lasers

Each laser system consists of an active medium placed between a pair of optically parallel and highly reflective mirrors, one of which is semitransparent, and an energy source for its pumping. The amplification medium can be a solid, liquid or gas that has the property of amplifying the amplitude of a light wave passing through it by stimulated emission with electrical or optical pumping. The substance is placed between a pair of mirrors in such a way that the light reflected in them passes through it each time and, reaching a considerable gain, penetrates through the translucent mirror.

Two-level mediums

Consider the principle of the action of a laser with an active medium, whose atoms have only two energy levels: excited E ₂ and base E ₁ . If atoms are excited to any state with the aid of any pumping mechanism (optical, electric discharge, current transmission, or electron bombardment) to the state E ₂ , then they return to the ground position after a few nanoseconds, emitting photons of energy hν = E ₂ - E ₁ . According to Einstein's theory, emission is produced in two different ways: either it is induced by a photon, or it happens spontaneously. In the first case, stimulated emission takes place, and in the second case - spontaneous emission. At thermal equilibrium, the probability of stimulated emission is much lower than the spontaneous emission (1:10 ³³ ), therefore most conventional light sources are incoherent, and laser generation is possible under conditions other than thermal equilibrium.

Even with very strong pumping, the population of two-level systems can only be made equal. Therefore, in order to achieve an inverted population by optical or other pumping, three- or four-level systems are required.

Multilevel Systems

What is the principle of the action of a three-level laser? Irradiation with intense light of frequency ν ₀₂ pumps a large number of atoms from the lowest energy level E ₀ to the upper E ₂ . The nonradiative transition of atoms from E ₂ to E ₁ establishes a population inversion between E ₁ and E ₀ , which in practice is possible only when the atoms are in a metastable state for a long time _, and the transition from E ₂ to E ₁ occurs rapidly. The principle of operation of the three-level laser is to fulfill these conditions, so that an population inversion is achieved between E ₀ and E ₁ and the photons are intensified by the energy E ₁ -E _{0 of the} induced radiation. A wider E ₂ level could increase the wavelength absorption range for a more efficient pump, resulting in an increase in stimulated emission.

The three-level system requires very high pumping power, since the lower level involved in the generation is the base one. In this case, in order for an population inversion to take place, more than half of the total number of atoms must be pumped to the state E ₁ . Thus energy is wasted. The pump power can be significantly reduced if the lower level of generation is not basic, which requires at least a four-level system.

Depending on the nature of the active substance, lasers are divided into three main categories, namely, solid, liquid and gas. Since 1958, when generation was first observed in a ruby crystal, scientists and researchers have studied a wide range of materials in each category.

Solid State Laser

The principle of operation is based on the use of an active medium, which is formed by adding a transition group (Ti ⁺³ , Cr ⁺³ , V ⁺² , Co ⁺² , Ni ⁺² , Fe ⁺² , etc.) to the insulating crystal lattice of the metal , Rare-earth ions (Ce ⁺³ , Pr ⁺³ , Nd ⁺³ , Pm ⁺³ , Sm ⁺² , Eu ^{+ 2, +3} , Tb ⁺³ , Dy ⁺³ , Ho ⁺³ , Er ⁺³ , Yb ⁺³ , Etc.), and actinides like U ⁺³ . Energy levels of ions are responsible only for generation. The physical properties of the base material, such as thermal conductivity and thermal expansion, are important for the effective operation of the laser. The arrangement of the lattice atoms around the doped ion alters its energy levels. Different wavelengths of generation in the active medium are achieved by doping different materials with the same ion.

Holmium laser

An example of a solid-state laser is a quantum generator in which holmium replaces the atom of the base material of the crystal lattice. Ho: YAG is one of the best generation materials. The principle of the holmium laser is that the aluminum-yttrium garnet is doped with holmium ions, optically pumped by a flash lamp and emits at a wavelength of 2097 nm in the IR band, which is well absorbed by the tissues. This laser is used for operations on the joints, in the treatment of teeth, for the evaporation of cancer cells, kidney stones and gallstones.

Semiconductor quantum generator

Lasers in quantum wells are inexpensive, they allow mass production and are easily scaled. The principle of operation of a semiconductor laser is based on the use of a diode with a pn junction, which produces light of a certain wavelength by recombining the carrier with a positive bias, like LEDs. LED emit spontaneously, and laser diodes - forced. To fulfill the population inversion condition, the operating current must exceed the threshold value. The active medium in a semiconductor diode has the form of a connecting region of two two-dimensional layers.

The principle of operation of this type of laser is such that no external mirror is required to maintain vibrations. The reflective power created by the refractive index of the layers and the internal reflection of the active medium is sufficient for this purpose. The end surfaces of the diodes are cleaved, which ensures the parallelism of the reflecting surfaces.

A compound formed by semiconductor materials of the same type is called a homojunction, and created by a combination of two different - a heterojunction.

Semiconductors of p and n type with a high carrier density form a pn junction with a very thin (≈1 μm) depleted layer.

Gas laser

The principle of operation and use of this type of laser makes it possible to create devices of almost any power (from milliwatts to megawatts) and wavelengths (from UV to IR) and allows to operate in pulsed and continuous modes. Based on the nature of the active media, there are three types of gas quantum generators, namely, atomic, ionic, and molecular.

Most gas lasers are pumped with an electric discharge. Electrons in the discharge tube are accelerated by an electric field between the electrodes. They collide with atoms, ions or molecules of the active medium and induce a transition to higher energy levels to achieve the population state of inversion and stimulated emission.

Molecular laser

The principle of the laser is based on the fact that, unlike isolated atoms and ions, in atomic and ionic quantum generators, molecules have broad energy bands of discrete energy levels. In this case, each electronic energy level has a large number of vibrational levels, and those, in turn, are somewhat rotational.

The energy between the electron energy levels is in the UV and visible regions of the spectrum, while between the vibrational-rotational levels - in the far and near IR regions. Thus, most molecular quantum generators operate in far or near-IR regions.

Excimer lasers

Excimers are such molecules as ArF, KrF, XeCl, which have a divided ground state and are stable at the first level. The principle of the laser is as follows. As a rule, the number of molecules in the ground state is small, so direct pumping from the ground state is not possible. Molecules are formed in the first excited electronic state by combining high-energy halides with inert gases. The population of the inversion is easily achieved, since the number of molecules at the base level is too small, in comparison with the excited one. The principle of the laser action, briefly, consists in the transition from the bound excited electronic state to the dissociative ground state. The population in the ground state always remains at a low level, because the molecules at this point dissociate into atoms.

The device and principle of operation of lasers is that a discharge tube is filled with a mixture of a halide (F ₂ ) and a rare earth gas (Ar). The electrons in it dissociate and ionize the halide molecules and create negatively charged ions. Positive ions Ar ⁺ and negative F ^- react and produce ArF molecules in the first excited bound state with their subsequent transition to a repulsive base state and the generation of coherent radiation. The excimer laser, the principle of operation and application of which we are now considering, can be used to pump the active medium on dyes.

Liquid laser

Compared to solids, the liquids are more homogeneous, and have a higher density of active atoms, compared to gases. In addition to this, they are not difficult to produce, they allow simply to remove heat and can be easily replaced. The principle of the laser is to use organic dyes such as DCM (4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran), rhodamine, styryl, LDS, coumarin, stilbene, etc., as an active medium ., Dissolved in an appropriate solvent. A solution of dye molecules is excited by radiation whose wavelength has a good absorption coefficient. The principle of the laser, in short, is to generate a longer wavelength, called fluorescence. The difference between the absorbed energy and the emitted photons is used by nonradiative energy transitions and heats the system.

The broader fluorescence band of liquid quantum generators has a unique feature - wavelength tuning. The principle of operation and use of this type of laser as a tunable and coherent light source is becoming increasingly important in spectroscopy, holography, and in biomedical applications.

Recently, quantum dye-based generators have been used to separate isotopes. In this case, the laser selectively excites one of them, prompting to enter into a chemical reaction.