The first principle of the laser, whose physics was based on Planck's law of radiation, was theoretically substantiated by Einstein in 1917. He described absorption, spontaneous and stimulated electromagnetic radiation using probability coefficients (Einstein coefficients).
Pioneers
Theodor Meiman was the first to demonstrate the principle of operation of a ruby laser based on optical pumping of synthetic ruby with a flash lamp, which produced pulsed coherent radiation with a wavelength of 694 nm.
In 1960, Iranian scientists Javan and Bennett created the first gas quantum generator using a 1:10 mixture of He and Ne gases.
In 1962, RN Hall demonstrated the first gallium arsenide (GaAs) diode laser emitting at a wavelength of 850 nm. Later that year, Nick Golonyak developed the first semiconductor visible light quantum generator.
Design and principle of operation of lasers
Each laser system consists of an active medium placedbetween a pair of optically parallel and highly reflective mirrors, one of which is translucent, and an energy source for its pumping. The amplification medium can be a solid, liquid or gas, which has the property of amplifying the amplitude of a light wave passing through it by stimulated emission with electrical or optical pumping. A 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 reached a significant amplification, penetrates a translucent mirror.
Two-tier environments
Let's consider the principle of operation of a laser with an active medium, the atoms of which have only two energy levels: excited E2 and basic E1. If atoms are excited to the state E2 by any pumping mechanism (optical, electric discharge, current transmission or electron bombardment), then after a few nanoseconds they will return to the ground position, emitting photons of energy hν=E 2 - E1. 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. At thermal equilibrium, the probability of stimulated emission is much lower than spontaneous emission (1:1033), so most conventional light sources are incoherent, and laser generation is possible under conditions other than thermal equilibrium.
Even with very strongpumping, the population of two-level systems can only be made equal. Therefore, three- or four-level systems are required to achieve population inversion by optical or other pumping methods.
Multilevel systems
What is the principle of the three-level laser? Irradiation with intense light of frequency ν02 pumps a large number of atoms from the lowest energy level E0 to the highest energy level E2. The nonradiative transition of atoms from E2 to E1 establishes a population inversion between E1 and E0 , which in practice is possible only when the atoms are in a metastable state for a long time E1, and the transition from E2 to E 1 is going fast. The principle of operation of a three-level laser is to fulfill these conditions, due to which between E0 and E1 a population inversion is achieved and photons are amplified by energy E 1-E0 induced emission. A wider level of E2 could increase the wavelength absorption range for more efficient pumping, resulting in an increase in stimulated emission.
A three-level system requires a very high pump power, since the lower level involved in generation is the base one. In this case, in order for the population inversion to occur, more than half of the total number of atoms must be pumped to the state E1. In doing so, energy is wasted. The pumping power can be significantlydecrease if the lower generation level is not the base one, 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 lasing was first observed in a ruby crystal, scientists and researchers have studied a wide variety 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, С+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 the ions are responsible only for generation. The physical properties of the base material, such as thermal conductivity and thermal expansion, are essential for efficient laser operation. 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 different materials with the same ion.
Holmium laser
An example of a solid-state laser is a quantum generator, in which holmium replaces an atom of the base substance 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, optically pumped by a flash lamp and emits at a wavelength of 2097 nm in the IR range, which is well absorbed by tissues. This laser is used for operations on the joints, in the treatment of teeth, for the evaporation of cancer cells, kidney and gallstones.
Semiconductor quantum generator
Quantum well lasers are inexpensive, mass-producible, and easily scalable. The principle of operation of a semiconductor laser is based on the use of a p-n junction diode, which produces light of a certain wavelength by carrier recombination at a positive bias, similar to 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 oscillations. The reflectivity 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 chipped, which ensures that the reflective surfaces are parallel.
A connection formed by semiconductor materials of the same type is called a homojunction, and a connection created by a connection of two different ones is calledheterojunction.
P- and n-type semiconductors with high carrier density form a p-n junction with a very thin (≈1 µm) depletion 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 active media, there are three types of gas quantum generators, namely atomic, ionic, and molecular.
Most gas lasers are pumped with an 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 population state of inversion and stimulated emission.
Molecular Laser
The operating principle of a laser is based on the fact that, unlike isolated atoms and ions, molecules in atomic and ion quantum generators have wide energy bands of discrete energy levels. Moreover, each electronic energy level has a large number of vibrational levels, and those, in turn, have several rotational ones.
The energy between electronic energy levels is in the UV and visible regions of the spectrum, while between the vibrational-rotational levels - in the far and near IRareas. Thus, most molecular quantum generators operate in the far or near infrared regions.
Excimer lasers
Excimers are molecules such as ArF, KrF, XeCl, which have a separated ground state and are stable at the first level. The principle of operation 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 compared to the excited one. The operating principle of a laser, in short, is the transition from a bound excited electronic state to a 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 the discharge tube is filled with a mixture of halide (F2) and rare earth gas (Ar). The electrons in it dissociate and ionize 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 the repulsive base state and generation of coherent radiation. The excimer laser, the principle of operation and application of which we are now considering, can be used to pumpactive medium on dyes.
Liquid Laser
Compared to solids, liquids are more homogeneous and have a higher density of active atoms than gases. In addition to this, they are easy to manufacture, allow for simple heat dissipation and can be easily replaced. The operating principle of the laser is to use organic dyes as an active medium, such as DCM (4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran), rhodamine, styryl, 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 operation of the laser, in short, is to generate at a longer wavelength, called fluorescence. The difference between absorbed energy and emitted photons is used by non-radiative energy transitions and heats up 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 biomedical applications.
Recently, dye quantum generators have been used for isotope separation. In this case, the laser selectively excites one of them, prompting them to enter into a chemical reaction.
