Laser principle: features of laser radiation

The first laser principle, 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 using probability coefficients (Einstein coefficients).

Pioneers

Theodore Meiman was the first to demonstrate the principle of the ruby laser, based on optical pumping with a flash lamp of 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, R.N. Hall demonstrated the first diode laser made of gallium arsenide (GaAs), emitting at a wavelength of 850 nm. Later that year, Nick Golonyak developed the first semiconductor quantum visible-light generator.

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 translucent, and an energy source for pumping it. The amplification medium can be a solid, liquid, or gas, which have the property of amplifying the amplitude of the light wave passing through it, stimulated by radiation with electric 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, having achieved significant amplification, penetrates through a translucent mirror.

Duplex environments

Consider the principle of operation of a laser with an active medium, the atoms of which have only two energy levels: excited E ₂ and basic E ₁ . If atoms by any pump mechanism (optical, electrical discharge, current transmission or electron bombardment) are excited to the state E ₂ , then after a few nanoseconds they will return to their main position, 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, spontaneous emission. Under thermal equilibrium, the probability of stimulated emission is much lower than spontaneous (1:10 ³³ ), so 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, to achieve an inverse population by optical or other pumping methods, three- or four-level systems are required.

Tiered systems

What is the principle of operation 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 the population inversion between E ₁ and E ₀ , which in practice is only possible when atoms are in a metastable state of E ₁ for a long time and the transition from E ₂ to E ₁ occurs quickly. The principle of operation of a three-level laser is to fulfill these conditions, due to which a population inversion is achieved between E ₀ and E ₁ and photons are amplified by the energy E ₁ -E _{0 of the} induced radiation. A wider level of E ₂ could increase the absorption range of wavelengths for more efficient pumping, resulting in an increase in stimulated emission.

The three-level system requires a very high pump power, since the lower level involved in the generation is the base. In this case, in order for the population to be inverted, more than half of the total number of atoms must be pumped to the state E ₁ . In this case, energy is wasted. The pump power can be significantly reduced if the lower generation level 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 ruby ​​crystal generation was first observed, 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 metal to the insulating crystal lattice (Ti ⁺³ , Cr ⁺³ , V ⁺² , Co ⁺² , Ni ⁺² , Fe ⁺² , etc.) 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 . The 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 efficient operation of the laser. The arrangement of lattice atoms around a doped ion changes its energy levels. Different wavelengths of generation in the active medium are achieved by doping various 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 operation of a holmium laser is that yttrium aluminum garnet is doped with holmium ions, is optically pumped by a flash lamp and emits at a wavelength of 2097 nm in the infrared range, which is well absorbed by tissues. This laser is used for operations on joints, in the treatment of teeth, for the evaporation of cancer cells, kidney and gallstones.

Semiconductor quantum generator

Quantum pit lasers are inexpensive, allow mass production and are easily scalable. 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 recombination of the carrier at a positive bias, like LEDs. LEDs emit spontaneously, and laser diodes - forcedly. To fulfill the population inversion condition, the operating current must exceed a 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 oscillation. The reflectivity created due to 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 chipped, which ensures parallelism of the reflecting surfaces.

A compound formed by semiconductor materials of the same type is called a homojunction, and created by a connection of two different ones is 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 the use of this type of laser allows you to create devices of almost any power (from milliwatts to megawatts) and wavelengths (from UV to IR) and allows you to work in pulsed and continuous modes. Based on the nature of the active media, three types of gas quantum generators are distinguished, namely atomic, ionic, and molecular.

Most gas lasers are pumped by electrical discharge. The electrons in the discharge tube are accelerated by the 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 a state of population inversion and stimulated emission.

Molecular laser

The principle of laser operation is based on the fact that, in contrast to isolated atoms and ions, in atomic and ionic quantum generators, molecules have wide energy zones of discrete energy levels. Moreover, each electronic energy level has a large number of vibrational levels, and those, in turn, are somewhat rotational.

The energy between the electronic 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 the far or near-IR regions.

Excimer Lasers

Excimers are molecules such as ArF, KrF, XeCl, which have a divided ground state and are stable at the first level. The principle of laser operation is as follows. As a rule, in the ground state the number of molecules 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 inversion population is easily achieved, since the number of molecules at the basic level is too small compared to the excited one. The principle of the laser, in short, consists in the transition from a bound excited electronic state to a dissociative ground state. The population in the ground state always remains low, because the molecules at this point dissociate into atoms.

The device and the principle of operation of the lasers is that the discharge tube is filled with a mixture of halide (F ₂ ) and rare earth gas (Ar). The electrons in it dissociate and ionize the halide molecules and create negatively charged ions. Positive Ar ⁺ ions and negative F ^- ions react and produce ArF molecules in the first excited bound state, followed by their transition to the repulsive base state and generation of coherent radiation. An 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, liquids are more homogeneous, and have a higher density of active atoms, compared with gases. In addition to this, they are not difficult to manufacture, make it easy to remove heat and can be easily replaced. The principle of the laser is to use organic dyes as an active medium, such as DCM (4-dicyanomethylene-2-methyl-6-p-dimethylaminostyr-4H-pyran), rhodamine, styrene, LDS, coumarin, stilbene, etc. . 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 at a longer wavelength, called fluorescence. The difference between the absorbed energy and the emitted photons is used by non-radiative energy transitions and heats the system.

The wider fluorescence band of liquid quantum generators has a unique feature - wavelength tuning. The principle of operation and the 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, dye quantum generators have been used to separate isotopes. In this case, the laser selectively excites one of them, prompting them to enter into a chemical reaction.