Real radiation contains not one specific oscillation frequency, but a certain set of different frequencies, called the spectrum or spectral composition of this radiation. Radiation is said to be monochromatic if it contains a very narrow range of frequencies (or wavelengths). In the visible region, monochromatic radiation produces a light sensation of a certain color; for example, radiation covering the wavelength range from 0.55 to 0.56 μm is perceived as green. The narrower the frequency range of a given radiation, the more monochromatic it is. Formula (1.2) refers to ideally monochromatic radiation containing one oscillation frequency.
Hot solids and liquid bodies emit a continuous (or continuous) spectrum of electromagnetic waves over a very wide frequency range. Luminous rarefied gases emit a line spectrum consisting of individual monochromatic radiations called spectral lines; Each spectral line is characterized by a specific oscillation frequency (or wavelength) located in the middle of the narrow frequency range it covers. If the sources of radiation are not individual (isolated, free) atoms, but gas molecules, then the spectrum consists of bands (banded spectrum), each band covers a wider continuous wavelength interval than the spectral line.
The line (atomic) spectrum of each substance is characteristic of it; Thanks to this, spectral analysis is possible, i.e., determining the chemical composition of a substance from the wavelengths of the spectral lines of the radiation it emits.
Let us assume that an electromagnetic wave propagates along a certain straight line, which we will call a ray. You can be interested in the change in vector at a certain point of the ray with flow
time; it is possible that c. At this point, not only the magnitude of the vector changes, as follows from formula (1.2), but also the orientation of the vector in space. Next, you can fix the magnitude and direction of the vector at different points of the beam, but at a certain point in time. If it turns out that at different points along the beam all vectors lie in the same plane, then the radiation is called plane-polarized or linearly polarized; Such radiation is produced by a source that maintains the plane of oscillations during the radiation process. If the plane of oscillation of the wave source changes over time, then the vector in the wave does not lie in a certain plane and the radiation will not be plane-polarized. In particular, it is possible to obtain a wave in which the vector rotates uniformly around the beam. If the vector changes its orientation around the beam completely randomly, then the radiation is called natural. Such radiation is obtained from luminous solid, liquid and gaseous bodies, which have planes, vibrations elementary sources healing - atoms and molecules - are oriented in space randomly.
Thus, the simplest radiation is a monochromatic plane-polarized wave. The plane in which the vector and vector of the direction of wave propagation lie is called the plane of oscillation; the plane perpendicular to the plane of oscillation (i.e., the plane in which the vector H lies) is called the plane of polarization.
The speed of propagation of electromagnetic waves in a vacuum is one of the most important constants of physics and is equal to
In other media it is less than k and is determined by the formula (see Part III, § 29)
where are the dielectric and magnetic permeabilities of the medium, respectively.
When radiation passes from one medium to another, the oscillation frequency in the wave is maintained, but the wavelength K changes; Usually, unless otherwise specified, K denotes the wavelength in vacuum.
It was stated above that visible radiation(which we call light) covers wavelengths from 400 to, with special eye training, can perceive light with a wavelength from 320 to 900 nm. A wider range of wavelengths from 1 cm to , also covering the ultraviolet and infrared regions, is called optical radiation.
The first principle of laser operation, the physics of which 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
Theodore Maiman was the first to demonstrate the principle of operation based on optical pumping using 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 gallium arsenide (GaAs) to emit at 850 nm. Later that year, Nick Golonyak developed the first semiconductor visible light quantum oscillator.
The design 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 to pump it. 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. 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 the translucent mirror.
Two-tier environments
Let us 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 ground E 1 . If atoms are excited to the E 2 state using any pumping mechanism (optical, electrical discharge, current flow or electron bombardment), then after a few nanoseconds they will return to the ground 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 occurs spontaneously. In the first case, stimulated emission occurs, and in the second, spontaneous emission occurs. At thermal equilibrium, the probability of stimulated emission is much lower than spontaneous emission (1:10 33), therefore most conventional light sources are incoherent, and laser lasing 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 population inversion optical or other pumping methods require three- or four-level systems.
Multi-level systems
What is the operating principle 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 highest 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 only possible when the atoms long time are in a metastable state E 1, 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 population inversion is achieved between E 0 and E 1 and photons are amplified with energy E 1 -E 0 of the induced radiation. A wider E 2 level could increase the wavelength absorption range for more efficient pumping, resulting in increased stimulated emission.
A three-level system requires very high pump power, since the lower level involved in generation is the base level. In this case, in order for a population inversion to occur, more than half of the total number of atoms must be pumped to the E 1 state. In this case, energy is wasted. The pump power can be significantly reduced if the lower lasing level is not the base level, 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 range of materials in every 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 (Ti +3, Cr +3, V +2, Co +2, Ni +2, Fe +2, etc.) to the insulating crystal lattice. , 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. ions are responsible only for generation. Physical properties base material, such as thermal conductivity and are important for efficient work laser The arrangement of lattice atoms around a doped ion changes its energy levels. Different lasing wavelengths in the active medium are achieved by doping various materials the same ion.
Holmium laser
An example is a quantum generator in which holmium replaces an atom of the base substance crystal lattice. Ho:YAG is one of the best lasing 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 joints, in dental treatment, to evaporate cancer cells, kidney and gallstones.
Semiconductor quantum generator
Quantum well lasers are inexpensive, mass-producible, and easily scalable. The operating principle of a semiconductor laser is based on the use of a pn junction diode, which produces light of a specific wavelength by recombining the carrier at a positive bias, similar to LEDs. LEDs emit spontaneously, while laser diodes emit forced radiation. To satisfy the population inversion condition, the operating current must exceed a threshold value. Active environment in 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 reflectivity created by 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 connection formed by one type is called a homojunction, and one created by connecting two different ones is called a heterojunction.
P- and n-type semiconductors with high carrier densities form a p-n junction with a very thin (≈1 μm) depletion layer.
Gas laser
The operating principle and use of this type of laser allows the creation of devices of almost any power (from milliwatt to megawatt) and wavelengths (from UV to IR) and allows operation 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 by 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 the 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 large number vibrational levels, and those, in turn, are somewhat rotational.
The energy between electronic energy levels is in the UV and visible regions of the spectrum, while between vibrational-rotational levels is 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 separated ground state and are stable at the first level. The operating 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 great energy halides with inert gases. Population inversion is easily achieved since the number of molecules per basic level too little compared to excited. The principle of operation 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 low because the molecules at this point dissociate into atoms.
The design and principle of operation of 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+ and negative F- ions 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. An excimer laser, the principle of operation and application of which we are now considering, can be used to pump an active medium based on dyes.
Liquid laser
Compared to solids, liquids are more homogeneous and have a higher density of active atoms compared to gases. In addition to this, they are not difficult to manufacture, allow for simple heat dissipation and can be easily replaced. The operating 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 the 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 operation of a 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 operating principle and use of this type of laser as a tunable and coherent light source is becoming increasingly important in spectroscopy, holography, and biomedical applications.
Recently quantum generators dyes began to be used to separate isotopes. In this case, the laser selectively excites one of them, causing it to enter into a chemical reaction.
1.1. Types of spectra.
At first glance, the laser beam seems very simple in structure. This is practically single-frequency radiation that has a spectrally pure color: the He-Ne laser has red radiation (633 nm), the cadmium laser emits blue(440 nm, an argon laser emits several lines in the blue-green region of the spectrum (488 nm, 514 nm, etc.), a semiconductor laser emits red radiation (650 nm), etc. In fact, the laser emission spectrum has a rather complex structure and is determined by two parameters - the emission spectrum of the working substance (for a He-Ne laser, for example, this is the red spectral line of neon emission excited by an electric discharge) and resonance phenomena in the optical resonator of the laser.
For comparison, the figures on the right show the emission spectra of the sun (A) and a conventional incandescent light bulb (B) (top picture), the spectrum of a mercury lamp (picture right) and a greatly enlarged emission spectrum of a He-Ne laser (bottom picture).
The spectrum of an incandescent lamp, like the solar spectrum, belongs to continuous spectra that completely fills the visible spectral range electromagnetic radiation(400-700 nm). The spectrum of a mercury lamp refers to line spectra, which also fills the entire visible range, but consists of individual spectral components of varying intensities. By the way, before the advent of lasers, monochromatic radiation was obtained by isolating individual spectral components of the radiation from a mercury lamp.
1.2. Emission spectrum in a He-Ne laser.
The laser radiation spectrum is monochromatic, that is, it has a very narrow spectral width, but, as can be seen from the figure, it also has a complex structure.
We will consider the process of forming a laser spectrum on the basis of a well-studied He-Ne laser. Historically, it was the first continuous laser operating in the visible range of the spectrum. It was created by A. Javan in 1960.
In Fig. on the right are the energy levels of an excited mixture of helium and neon. An excited helium or neon atom is an atom that has one or more outer shell electrons in collisions with electrons and ions gas discharge move to higher energy levels and may subsequently move to a lower energy level or return back to a neutral level with emission light quantum- photon.
Atoms are excited by an electric current passing through a gas mixture. For a He-Ne laser, this is a low-current, glow discharge (typical discharge currents are 20-50 mA). The picture of energy levels and the radiation mechanism are quite complex even for such a “classical” laser, which is the He-Ne laser, so we will limit ourselves to considering only the main details of this process. Helium atoms excited to the 2S level in collisions with neon atoms transfer the accumulated energy to them, exciting them to the 5S level (therefore, helium in gas mixture more than neon). From the 5S level, electrons can move to a number of lower energy levels. We are only interested in the transition 5S - 3P (both levels are actually split into a number of sublevels due to quantum nature mechanisms of excitation and radiation). The wavelength of photon emission during this transition is 633 nm.
Let's note one more important fact, fundamentally important for obtaining coherent radiation. With the correct proportions of helium and neon, the pressure of the gas mixture in the tube and the value of the discharge current, electrons accumulate at the 5S level and their number exceeds the number of electrons located at the lower 3P level. This phenomenon is called level population inversion. However, this is not laser radiation yet. This is one of the spectral lines in the neon emission spectrum. The width of the spectral line depends on several reasons, the main of which are: - the final width of the energy levels (5S and 3P) involved in the radiation and determined quantum principle uncertainty associated with the residence time of neon atoms in an excited state - line broadening associated with the constant movement of excited particles in the discharge under the influence electric field(the so-called Doppler effect). Taking these factors into account, the width of the line (experts call it the contour of the working transition) is approximately two ten thousandths of an angstrom. For such narrow lines in calculations it is more convenient to use its width in frequency domain. Let's use the transition formula:
dn 1 =dl c/l 2 (1)
where dn 1 is the width of the spectral line in the frequency domain, Hz, dl is the width of the spectral line (0.000002 nm), l is the wavelength of the spectral line (633 nm), c is the speed of light. Substituting all values (in one measurement system), we obtain a line width of 1.5 GHz. Of course, such a narrow line can be considered completely monochromatic in comparison with the entire spectrum of neon radiation, but this cannot yet be called coherent radiation. To obtain coherent radiation, the laser uses an optical cavity (interferometer).
1.3. Laser optical cavity.
An optical resonator consists of two mirrors located on the optical axis and facing each other with reflective surfaces, Fig. right. Mirrors can be flat or spherical. Flat mirrors are very difficult to align and laser output can be unstable. A resonator with spherical mirrors (confocal resonator) is much more stable, but the laser beam may be inhomogeneous across the cross-section due to the complex, multimode composition of the radiation. In practice, a semi-confocal resonator with a rear spherical and front flat mirror is most often used. Such a resonator is relatively stable and produces a homogeneous (single-mode) beam.
The main property of any resonator is the formation of standing electromagnetic waves in it. In the case of a He-Ne laser, standing waves are generated to emit a neon spectral line with a wavelength of 633 nm. This is facilitated by the maximum reflection coefficient of the mirrors, selected just for this wavelength. Laser cavities use dielectric mirrors with multilayer coating, allowing a reflection coefficient of 99% or higher. As is known, the condition for the formation of standing waves is that the distance between the mirrors must be equal to an integer number of half-waves:
nl =2L (2)
where n is an integer or order of interference, l is the wavelength of radiation inside the interferometer, L is the distance between the mirrors.
From the resonance condition (2) we can obtain the distance between the resonant frequencies dn 2:
dn 2 =c/2L (3)
For a one and a half meter gas laser cavity (He-Ne laser LGN-220) this value is approximately 100 MHz. Only radiation with such a frequency period can be repeatedly reflected from the resonator mirrors and amplified as it passes through an inverse medium - a mixture of helium and neon excited by an electric discharge. Moreover, what is extremely important, when this radiation passes along the resonator, its phase structure does not change, which leads to coherent properties of the amplified radiation. This is facilitated by the inverse population of the 5S level, which was mentioned above. Electron s top level moves to the lower one synchronously with the photon initiating this transition, therefore the phase parameters of the waves corresponding to both photons are the same. This generation of coherent radiation occurs along the entire radiation path inside the resonator. Besides, resonant phenomena lead to a much greater narrowing of the emission line, resulting in the greatest gain being obtained at the center of the resonant peak.
After a certain number of passes, the intensity of coherent radiation becomes so high that it exceeds the natural losses in the resonator (scattering in the active medium, losses on mirrors, diffraction losses, etc.) and part of it goes beyond the resonator. A day off for this, flat mirror made with a slightly lower reflectance (99.6-99.7%). As a result, the laser emission spectrum has the form shown in the third Fig. above. The number of spectral components usually does not exceed ten.
Let us summarize once again all the factors that determine the frequency characteristics of laser radiation. First of all, the working transition is characterized by the natural width of the contour. IN real conditions due to various factors the outline widens. Within the broadened line, the resonant lines of the interferometer are located, the number of which is determined by the width of the transition contour and the distance between adjacent peaks. Finally, at the center of the peaks are extremely narrow spectral lines of laser emission, which determine the spectrum of the laser output.
1.4. Coherence of laser radiation.
Let us clarify what coherence length is provided by the He-Ne laser radiation. Let's use the formula proposed in the work:
as it passes through an inverse medium - a mixture of helium and neon excited by an electric discharge. Moreover, what is extremely important, when this radiation passes along the resonator, its phase structure does not change, which leads to coherent properties of the amplified radiation. This is facilitated by the inverse population of the 5S level, which was mentioned above. An electron moves from the upper level to the lower level synchronously with the photon initiating this transition, therefore the phase parameters of the waves corresponding to both photons are the same. This generation of coherent radiation occurs along the entire radiation path inside the resonator. In addition, resonant phenomena lead to a much greater narrowing of the emission line, resulting in the greatest gain being obtained at the center of the resonant peak.
dt =dn -1 (4)
where dt is the coherence time, which represents the upper limit of the time interval over which the amplitude and phase of the monochromatic wave are constant. Let's move on to the coherence length l that is familiar to us, with the help of which it is easy to estimate the depth of the scene recorded on the hologram:
l=c/dn (5)
Substituting the data into formula (5), including the full spectrum width dn 1 = 1.5 GHz, we obtain a coherence length of 20 cm. This is quite close to the real coherence length of a He-Ne laser, which has inevitable radiation losses in the cavity. Measurements of the coherence length using a Michelson interferometer give a value of 15-17 cm (at the level of a 50% decrease in the amplitude of the interference pattern). It is interesting to estimate the coherence length of an individual spectral component isolated by the laser cavity. The width of the resonant peak of the interferometer dn 3 (see the third figure from the top) is determined by its quality factor and is approximately 0.5 MHz. But, as mentioned above, resonance phenomena lead to an even greater narrowing of the laser spectral line dn 4, which is formed near the center of the resonant peak of the interferometer (third from the top in the figure). Theoretical calculation gives a line width of eight thousandths of a hertz! However, this value does not have much practical meaning, since the long-term existence of such a narrow spectral component requires values of the mechanical stability of the resonator, thermal drift and other parameters that are absolutely impossible under real operating conditions of the laser. Therefore, we will limit ourselves to the width of the resonant peak of the interferometer. For a spectrum width of 0.5 MHz, the coherence length calculated using formula (5) is 600 m. This is also very good. All that remains is to isolate one spectral component, evaluate its power and keep it in one place. If, during the exposure of the hologram, it “passes” along the entire working circuit (due, for example, to the temperature instability of the resonator), we will again obtain the same 20 cm of coherence.
1.5. Spectrum of ion laser generation.
Let's talk briefly about the generation spectrum of another gas laser - argon. This laser, like the krypton laser, belongs to ion lasers, i.e. in the process of generating coherent radiation, it is no longer argon atoms that participate, but their ions, i.e. atoms, one or more electrons of which outer shell are torn off under the influence of a powerful arc discharge, which passes through the active substance. The discharge current reaches several tens of amperes, the electrical power of the power supply is several tens of kilowatts. Intensive water cooling of the active element is necessary, otherwise its thermal destruction will occur. Naturally, under such harsh conditions, the picture of excitation of argon atoms is even more complex. Several laser lasers are generated at once. spectral lines, the width of the working contour of each of them is significantly greater than the width of the He-Ne laser line contour and amounts to several gigahertz. Accordingly, the laser coherence length is reduced to several centimeters. To record large format holograms, frequency selection of the generation spectrum is required, which will be discussed in the second part of this article.
1.6. Generation spectrum of a semiconductor laser.
Let us move on to consider the emission spectrum of a semiconductor laser, which is of great interest for the process of teaching holography and for beginning holographers. Historically, injection semiconductor lasers based on gallium arsenide were the first to be developed, Fig. right.
Since its design is quite simple, let us consider the principle of operation of a semiconductor laser using its example. Active substance, in which the generation of radiation occurs, is a single crystal of gallium arsenide, having the shape of a parallelepiped with sides several hundred microns long. Two side faces are made parallel and polished with a high degree of precision. Due to the large refractive index (n = 3.6), a sufficiently large reflection coefficient (about 35%) is obtained at the crystal-air interface, which is sufficient to generate coherent radiation without additional deposition of reflecting mirrors. The other two faces of the crystal are beveled at a certain angle; induced radiation does not escape through them. The generation of coherent radiation occurs in the p-n junction, which is created by the diffusion of acceptor impurities (Zn, Cd, etc.) into the region of the crystal doped with donor impurities (Te, Se, etc.). Thickness of the active region perpendicular to p-n junction direction is about 1 µm. Unfortunately, in this design of a semiconductor laser, the threshold pump current density turns out to be quite high (about 100 thousand amperes per 1 sq. cm.). Therefore, this laser is instantly destroyed when operated in continuous mode at room temperature and requires strong cooling. The laser operates stably at temperatures liquid nitrogen(77 K) or helium (4.2 K).
Modern semiconductor lasers are made on the basis of double heterojunctions, Fig. right. In such a structure, the threshold current density was reduced by two orders of magnitude, to 1000 A/cm. sq. At this current density, stable operation of a semiconductor laser is possible even at room temperature. The first laser samples operated in the infrared range (850 nm). With further improvement of the technology for forming semiconductor layers, lasers appeared with an increased wavelength (1.3 - 1.6 μm) for fiber optic lines connection, and with the generation of radiation in the visible region (650 nm). There are already lasers that emit in the blue region of the spectrum. Big advantage semiconductor lasers is their high efficiency (ratio of radiation energy to electrical energy pumping), which reaches 70%. For gas lasers, both atomic and ion, the efficiency does not exceed 0.1%.
Due to the specific nature of the radiation generation process in a semiconductor laser, the width of the radiation spectrum is much greater than the width of the He-Ne laser spectrum, Fig. right.
The width of the working contour is about 4 nm. The number of spectral harmonics can reach several tens. This imposes a serious limitation on the laser coherence length. If we use formulas (1), (5), the theoretical coherence length will be only 0.1 mm. However, as shown by direct measurements of the coherence length on a Michelson interferometer and recording of reflective holograms, the real coherence length of semiconductor lasers reaches 4-5 cm. This suggests that the real emission spectrum of a semiconductor laser is not so rich in harmonics and does not have such a large contour width worker transition, as theory predicts. However, in fairness, it is worth noting that the degree of coherence of semiconductor laser radiation varies greatly both from sample to sample and from its operating mode (pump current value, cooling conditions, etc.
A laser is a generator of optical waves that uses the energy of induced emitting atoms or molecules in media with an inverse population of energy levels, which have the property of amplifying light of specific wavelengths. To amplify the light many times over, an optical resonator is used, which consists of 2 mirrors. Due to in various ways pumping in the active element is created active medium.
Figure 1 - Laser device diagram
Due to the above conditions, a spectrum is generated in the laser, which is shown in Figure 2 (the number of laser modes is controlled by the length of the resonator):
Figure 2 - Spectrum of longitudinal laser modes
Lasers have a high degree of monochromaticity, a high degree of directionality and polarization of radiation with significant intensity and brightness, a high degree of temporal and spatial coherence, can be tuned in wavelengths, and can emit light pulses of record short duration, unlike thermal light sources.
Throughout the development of laser technologies, a large list of lasers and laser systems has been created that satisfy the needs with their characteristics. laser technology, including biotechnology. Due to the fact that the complexity of the structure of biological systems and the significant diversity in the nature of their interaction with light determine the need for the use of many types of laser installations in photobiology, and also stimulate the development of new laser means, including means of delivering laser radiation to the object of research or influence.
Like ordinary light, laser radiation is reflected, absorbed, re-emitted and scattered by the biological environment. All of these processes carry information about the micro and macro structure of the object, the movement and shape of its individual parts.
Monochromaticity is a high spectral power density of laser radiation, or significant temporal coherence of radiation, provides: spectral analysis with a resolution several orders of magnitude higher than that of traditional spectrometers; high degree selectivity of excitation of a certain type of molecules in their mixture, which is essential for biotechnology; implementation of interferometric and holographic methods for diagnosing biological objects.
Due to the fact that the laser beams are practically parallel, with increasing distance the light beam slightly increases in diameter. The listed properties of the laser beam allow it to selectively influence different areas of biological tissue, creating a high energy or power density in a small spot.
Laser installations are divided into the following groups:
1) High-power lasers using neodymium, carbon monoxide, carbon dioxide, argon, ruby, metal vapor, etc.;
2) Lasers with low-energy radiation (helium-cadmium, helium-neon, nitrogen, dyes, etc.), which do not have a pronounced thermal effect on body tissues.
Currently, there are laser systems that generate radiation in the ultraviolet, visible and infrared regions of the spectrum. The biological effects caused by laser radiation depend on the wavelength and dose of light radiation.
In ophthalmology they often use: excimer laser (with a wavelength of 193 nm); argon (488 nm and 514 nm); krypton (568 nm and 647 nm); helium-neon laser (630 nm); diode (810 nm); ND:YAG laser with frequency doubling (532 nm), also generating at a wavelength of 1.06 μm; 10-carbon dioxide laser (10.6 µm). The scope of laser radiation in ophthalmology is determined by the wavelength.
Laser installations receive their names in accordance with the active medium, and a more detailed classification includes solid-state, gas, semiconductor, liquid lasers and others. The list of solid-state lasers includes: neodymium, ruby, alexandrite, erbium, holmium; gases include: argon, excimer, copper vapor; to liquid ones: lasers that operate on dye solutions and others.
The revolution was made by the emerging semiconductor lasers due to their efficiency due to high efficiency (up to 60 - 80% as opposed to 10-30% for traditional ones), small size and reliability. At the same time, other types of lasers continue to be widely used.
One of the most important properties for using lasers is their ability to form a speckle pattern when coherent radiation is reflected from the surface of an object. Light scattered by the surface consists of chaotically located light and dark spots - speckles. The speckle pattern is formed based on the complex interference of secondary waves from small scattering centers that are located on the surface of the object under study. Due to the fact that the vast majority of biological objects under study have a rough surface and optical heterogeneity, they always form a speckle pattern and thereby introduce distortions into the final results of the study. In turn, the speckle field contains information about the properties of the surface under study and the near-surface layer, which can be used for diagnostic purposes.
In ophthalmic surgery, lasers are used in the following areas:
In cataract surgery: to destroy cataract accumulation on the lens and discision of the posterior capsule of the lens when it becomes cloudy in the postoperative period;
In glaucoma surgery: when performing laser goniopuncture, trabeculoplasty, excimer laser removal of deep layers of the scleral flap, when performing a non-penetrating deep sclerectomy procedure;
In ophthalmic oncosurgery: to remove certain types of tumors located inside the eye.
The most important properties inherent in laser radiation are: monochromaticity, coherence, directionality, polarization.
Coherence (from the Latin cohaerens, connected, connected) is the coordinated occurrence in time of several oscillatory wave processes of the same frequency and polarization; a property of two or more oscillatory wave processes that determines their ability, when added, to mutually enhance or weaken each other. Oscillations will be called coherent if the difference in their phases remains constant throughout the time interval and when summing the oscillations, an oscillation of the same frequency is obtained. The simplest example two coherent oscillations - two sinusoidal oscillations of the same frequency.
Wave coherence implies that at different points in the wave the oscillations occur synchronously; in other words, the phase difference between two points is not related to time. Lack of coherence means that the phase difference between two points is not constant, therefore changing over time. This situation arises if the wave is generated not by a single radiation source, but by a group of identical, but independent from each other, emitters.
Often simple sources emit incoherent oscillations, while lasers emit coherent oscillations. In force of this property laser radiation is maximally focused, it has the ability to interfere, is less susceptible to divergence, and has the ability to obtain a higher spot energy density.
Monochromaticity (Greek monos - one, only + chroma - color, paint) - radiation of one specific frequency or wavelength. The radiation can be conditionally accepted as monochromatic if it belongs to the spectral range of 3-5 nm. If there is only one allowed electron transition from the excited to the ground state, monochromatic radiation is created.
Polarization is symmetry in the distribution of the direction of the electric and magnetic field strength vector in an electromagnetic wave regarding the direction of its propagation. A wave will be called polarized if two mutually perpendicular components of the electric field strength vector oscillate with a phase difference that is constant over time. Non-polarized - if changes occur chaotically. IN longitudinal wave the occurrence of polarization is not possible, since disturbances in this type waves always coincide with the direction of propagation. Laser radiation is highly polarized light (from 75 to 100%).
Directivity (one of the most important properties of laser radiation) is the ability of radiation to exit the laser in the form of a light beam with a very low divergence. This feature is the simplest consequence of the fact that the active medium is located in a resonator (for example, a plane-parallel resonator). In such a resonator, only electromagnetic waves propagating along the axis of the resonator or in close proximity to it are supported.
The main characteristics of laser radiation are: wavelength, frequency, energy parameters. These characteristics are biotropic, that is, they determine the effect of radiation on biological objects.
Wavelength ( l) represents the smallest distance between two adjacent oscillating points of the same wave. Often in medicine, wavelength is specified in micrometers (µm) or nanometers (nm). Depending on the wavelength, the reflection coefficient, the depth of penetration into body tissue, the absorption and biological effect of laser radiation change.
Frequency characterizes the number of oscillations performed per unit time and is the reciprocal of the wavelength. Typically expressed in hertz (Hz). As the frequency increases, the energy of the light quantum increases. They are distinguished: the natural frequency of radiation (for a single laser oscillation generator is unchanged); modulation frequency (in medical laser installations can vary from 1 to 1000 Hz). The energy parameters of laser irradiation are also of great importance.
It is customary to distinguish three main physical characteristics dosing: radiation power, energy (dose) and dose density.
Radiation power (radiation flux, radiant energy flux, R) - represents the total energy that is transferred by light per unit time through a given surface; the average power of electromagnetic radiation that is transferred through any surface. Typically measured in watts or multiples.
Energy exposure (radiation dose, H) is the energy irradiation by the laser over a certain period of time; power electromagnetic wave, which is emitted per unit time. Measured in [J] or [W*s]. The ability to do work is the physical meaning of energy. This is typical when the work makes changes in tissue with photons. The biological effect of light irradiation is characterized by energy. In this case, the same biological effect occurs (for example, tanning) as in the case of sunlight, can be achieved with low power and exposure time or high power and short exposure. The effects obtained will be identical, with the same dose.
Dose density “D” is the energy received per unit area of exposure. The SI unit is [J/m2]. A representation in units of J/cm 2 is also used, due to the fact that the areas affected are usually measured in square centimeters.
The word “laser” itself is an abbreviation for the English “Light Amplification by Stimulated Emission of Radiation,” which means “light amplification using stimulated radiation.”
The era of laser medicine began more than half a century ago, when in 1960, Theodore Mayman first used a ruby laser in the clinic.
The ruby laser was followed by other lasers: 1961 - a neodymium yttrium aluminum garnet (Nd:YAG) laser; 1962 – argon; 1964 – carbon dioxide (CO 2) laser.
In 1965, Leon Goldman reported the use of a ruby laser for tattoo removal. Subsequently, until 1983, various attempts were made to use neodymium and argon lasers to treat vascular pathologies of the skin. But their use was limited high risk scar formation.
In 1983, Rox Anderson and John Parrish published their concept of selective photothermolysis (SPT) in the journal Science, which led to revolutionary changes in laser medicine and dermatology. This concept allowed us to better understand the processes of interaction of laser radiation with tissue. This, in turn, has facilitated the development and production of lasers for medical applications.
Features of laser radiation
Three properties inherent in laser radiation make it unique:
- Coherence. The peaks and troughs of the waves are parallel and in phase in time and space.
- Monochrome. Light waves, emitted by the laser, have the same length, exactly the one provided by the medium used in the laser.
- Collimation. The waves in a beam of light remain parallel, do not diverge, and the beam transfers energy with virtually no loss.
Methods of interaction of laser radiation with skin
Laser surgery methods are used to manipulate the skin much more often than any other tissue. This is explained, firstly, by the exceptional diversity and prevalence of skin pathologies and various cosmetic defects, and secondly, by the relative ease of performing laser procedures, which is associated with the superficial location of the objects requiring treatment. The interaction of laser light with tissue is based on the optical properties of the tissue and the physical properties of laser radiation. The distribution of light entering the skin can be divided into four interrelated processes.
Reflection. About 5-7% of light is reflected at the level of the stratum corneum.
Absorption (absorption). Described by the Bouguer-Lambert-Beer law. The absorption of light passing through tissue depends on its initial intensity, the thickness of the layer of material through which the light passes, the wavelength of the light absorbed, and the absorption coefficient. If the light is not absorbed, there is no effect on the tissue. When a photon is absorbed by a target molecule (chromophore), all of its energy is transferred to that molecule. The most important endogenous chromophores are melanin, hemoglobin, water and collagen. Exogenous chromophores include tattoo dyes, as well as dirt particles impregnated during injury.
Diffusion. This process is mainly due to the collagen of the dermis. The importance of the scattering phenomenon is that it rapidly reduces the energy flux density available for absorption by the target chromophore and, consequently, the clinical effect on the tissue. Dissipation decreases with increasing wavelength, making longer wavelengths ideal for delivering energy to deep dermal structures.
Penetration. The depth of light penetration into subcutaneous structures, as well as the intensity of scattering, depends on the wavelength. Short waves (300-400 nm) are intensely scattered and do not penetrate deeper than 100 microns .
Longer waves penetrate deeper because they are scattered less .
The main physical parameters of the laser that determine the effect of quantum energy on a particular biological target are the length of the generated wave and the energy flux density and exposure time.
Length of the generated wave. The wavelength of the laser radiation is comparable to the absorption spectrum of the most important tissue chromophores (Fig. 2). When choosing this parameter, it is imperative to take into account the depth of the target structure (chromophore), since the scattering of light in the dermis significantly depends on the wavelength (Fig. 3). This means that long waves are less absorbed than short ones; Accordingly, their penetration into tissues is deeper. It is also necessary to take into account the heterogeneity of the spectral absorption of tissue chromophores:
- Melanin Normally found in the epidermis and hair follicles. Its absorption spectrum lies in the ultraviolet (up to 400 nm) and visible (400 - 760 nm) spectral ranges. The absorption of laser radiation by melanin gradually decreases as the wavelength of light increases. Absorption weakens in the near-infrared region of the spectrum from 900 nm.
- Hemoglobin found in red blood cells. It has many different absorption peaks. The maximums of the absorption spectrum of hemoglobin lie in the UV-A region (320-400 nm), violet (400 nm), green (541 nm) and yellow (577 nm) ranges.
- Collagen forms the basis of the dermis. The absorption spectrum of collagen is in the visible range from 400 nm to 760 nm and the near-infrared region of the spectrum from 760 to 2500 nm.
- Water makes up up to 70% of the dermis. The absorption spectrum of water lies in the middle (2500 - 5000 nm) and far (5000 - 10064 nm) infrared regions of the spectrum.
Energy flux density. If the wavelength of light affects the depth at which it is absorbed by one or another chromophore, then for direct damage to the target structure, the amount of laser radiation energy and the power that determines the rate of arrival of this energy are important. Energy is measured in joules (J), power - in watts (W, or J/s). In practice, these radiation parameters are usually used in terms of per unit area - energy flux density (J/cm2) and energy flux rate (W/cm2), or power density.
Types of laser interventions in dermatology
All types of laser interventions in dermatology can be divided into two types:
- Type I Surgeries that involve ablation of an area of affected skin, including the epidermis.
- II type. Operations aimed at selective removal of pathological structures without compromising the integrity of the epidermis.
Type I. Ablation.
This phenomenon is one of the fundamental, intensively studied, although not yet fully resolved problems of modern physics.
The term “ablation” is translated into Russian as removal or amputation. In non-medical vocabulary, this word means erosion or melting. In laser surgery, ablation means the elimination of a section of living tissue directly under the influence of laser photons. This refers to an effect that manifests itself precisely during the irradiation procedure itself, in contrast to the situation (for example, with photodynamic therapy), when the irradiated tissue area remains in place after the cessation of laser exposure, and its gradual elimination occurs later as a result of a series of local biological reactions developing in the irradiation zone.
The energy characteristics and ablation performance are determined by the properties of the irradiated object, the radiation characteristics and parameters that inextricably link the properties of the object and the laser beam - the reflection, absorption and scattering coefficients of a given type of radiation in a given type of tissue or its individual components. The properties of the irradiated object include: the ratio of liquid and dense components, their chemical and physical properties, the nature of intra- and intermolecular bonds, the thermal sensitivity of cells and macromolecules, blood supply to tissue, etc. The characteristics of radiation are wavelength, irradiation mode (continuous or pulse), power, energy per pulse, total absorbed energy, etc.
The ablation mechanism has been studied in most detail using a CO2 laser (l = 10.6 µm). Its radiation at a power density of ³ 50 kW/cm 2 is intensively absorbed by tissue water molecules. Under such conditions, rapid heating of water occurs, and from it, the non-aqueous components of the tissue. The consequence of this is the rapid (explosive) evaporation of tissue water (vaporization effect) and the eruption of water vapor along with fragments of cellular and tissue structures outside the tissue with the formation of an ablation crater. Along with the overheated material, most of the thermal energy is removed from the fabric. A narrow strip of heated melt remains along the walls of the crater, from which heat is transferred to the surrounding intact tissue (Fig. 4). At low energy density (Fig. 5, A), the release of ablation products is relatively small, so a significant part of the heat from the massive melt layer is transferred to the tissue. With more high density(Fig. 5, B) the opposite picture is observed. In this case, minor thermal damage is accompanied by mechanical trauma to the tissue due to the shock wave. Part of the heated material in the form of a melt remains along the walls of the ablation crater, and it is this layer that serves as a reservoir of heat transferred into the tissue outside the crater. The thickness of this layer is the same along the entire contour of the crater. As the power density increases, it decreases, and as it decreases, it increases, which is accompanied by a corresponding decrease or increase in the thermal damage zone. Thus, by increasing the radiation power, we achieve an increase in the rate of tissue removal, while reducing the depth of thermal damage.
The scope of application of the CO 2 laser is very wide. In focused mode, it is used to excise tissue while simultaneously coagulating blood vessels. In the defocus mode, by reducing the power density, pathological tissue is removed layer-by-layer (vaporization). It is in this way that superficial malignant and potentially malignant tumors (basal cell carcinoma, actinic cheilitis, Queyr's erythroplasia), a number of benign neoplasms of the skin (angiofibroma, trichlemmoma, syringoma, trichoepithelioma, etc.), large post-burn scabs, inflammatory skin diseases (granulomas, nodular chondrodermatitis of the auricle), cysts, infectious skin lesions (warts, recurrent condylomas, deep mycoses), vascular lesions (pyogenic granuloma, angiokeratoma, annular lymphangioma), formations causing cosmetic defects (rhinophyma, deep post-acne scars, epidermal birthmarks, lentigo, xanthelasma), etc.
The defocused beam of a CO 2 laser is also used in a purely cosmetic procedure - the so-called laser dermabrasion, that is, layer-by-layer removal of the surface layers of the skin in order to rejuvenate the patient’s appearance. In pulsed mode with a pulse duration of less than 1 ms, 25-50 microns of tissue are selectively vaporized in one pass; in this case, a thin zone of residual thermal necrosis is formed in the range of 40-120 microns. The size of this zone is sufficient to temporarily isolate the dermal blood and lymphatic vessels, which in turn reduces the risk of scar formation.
Skin renewal after laser dermabrasion is due to several reasons. Ablation reduces the appearance of wrinkles and textural abnormalities through superficial tissue evaporation, thermal coagulation of cells in the dermis, and denaturation of extracellular matrix proteins. During the procedure, an immediate visible contraction of the skin occurs within 20-25% as a result of tissue shrinkage due to dehydration and compression of collagen fibers. The onset of a delayed, but longer-lasting result of skin renewal is achieved through processes associated with the tissue response to injury. After laser exposure, aseptic inflammation develops in the area of the formed wound. This stimulates post-traumatic release of growth factors and fibroblast infiltration. The onset of the reaction is automatically accompanied by a surge of activity, which inevitably leads to fibroblasts beginning to produce more collagen and elastin. As a result of vaporization, the renewal processes and kinetics of proliferation of epidermal cells are activated. In the dermis, the processes of regeneration of collagen and elastin are launched, followed by their arrangement in a parallel configuration.
Similar events occur when using pulsed lasers emitting in the near and mid-infrared region of the spectrum (1.54-2.94 µm): diode-pumped erbium (l = 1.54 µm), thulium (l = 1.927 µm), Ho: YSSG (l = 2.09 µm), Er:YSSG (l = 2.79 µm), Er:YAG (l = 2.94 µm). The listed lasers are characterized by very high absorption coefficients by water. For example, Er:YAG laser radiation is absorbed by water-containing tissues 12-18 times more actively than CO 2 laser radiation. As in the case of a CO 2 laser, a melt layer forms along the walls of the ablation crater in tissue irradiated with an Er:YAG laser. It should be borne in mind that when working on biological tissue with this laser, the energy characteristics of the pulse, primarily its peak power, are of significant importance for the nature of tissue changes. This means that even with minimal radiation power, but a longer pulse, the depth of thermal necrosis increases sharply. Under such conditions, the mass of the removed superheated ablation products is relatively less than the mass of the remaining ones. This causes deep thermal damage around the ablation crater. At the same time, with a powerful pulse the situation is different - minimal thermal damage around the crater with highly effective ablation. True, in this case positive effect achieved at the cost of extensive mechanical damage to the tissue shock wave. In one pass, the erbium laser ablates tissue to a depth of 25-50 microns with minimal residual thermal damage. As a result, the process of skin re-epithelialization is much shorter than after exposure to a CO 2 laser.
II type. Selective influence.
Operations of this type include procedures during which laser damage is achieved to certain intradermal and subcutaneous formations without violating the integrity of the skin. This goal is achieved by selecting the laser characteristics: wavelength and irradiation mode. They must ensure the absorption of laser light by the chromophore (colored target structure), which will lead to its destruction or discoloration due to the conversion of radiation energy into thermal (photothermolysis), and in some cases into mechanical energy. The targets of laser exposure can be: hemoglobin of erythrocytes located in numerous dilated dermal vessels with port-wine stains (PWS); melanin pigment of various skin formations; coal, as well as other differently colored foreign particles introduced under the epidermis during a tattoo or getting there as a result of other influences.
An ideal selective effect can be considered such an effect in which laser beams are absorbed only by the target structures, and there is no absorption beyond its boundaries. To achieve such a result, a specialist who has selected a laser with the appropriate wavelength would only have to establish the radiation energy density and the duration of exposures (or pulses), as well as the intervals between them. These parameters are determined taking into account (TTR) for a given target - the period of time during which the target temperature, which increased at the moment the pulse was applied, drops by half its increase relative to the initial one. Exceeding the pulse duration above the BTP value will cause unwanted overheating of the tissue around the target. Reducing the interval between pulses will have the same effect. In principle, all these conditions can be modeled mathematically before surgery, but the composition of the skin itself does not allow full use of the calculated data. The fact is that in the basal layer of the epidermis there are melanocytes and individual cratinocytes, which contain melanin. Since this pigment intensively absorbs light in the visible, as well as the near ultraviolet and infrared regions of the spectrum (the “optical window” of melanin is in the range from 500 to 1100 nm), any laser radiation in this range will be absorbed by melanin. This can lead to thermal damage and death of the affected cells. Moreover, radiation in the visible part of the spectrum is also absorbed by cytochromes and flavin enzymes (flavoproteins) of both melanin-containing cells and all other types of cells of the epidermis and dermis. It follows that when laser irradiation of a target located under the surface of the skin, some damage to epidermal cells becomes inevitable. Therefore, the real clinical problem comes down to a compromise search for laser irradiation modes in which it would be possible to achieve maximum target damage with minimal damage to the epidermis (with the expectation of its subsequent regeneration, mainly due to neighboring non-irradiated areas of the skin).
Compliance with all these conditions in relation to a specific target will lead to its maximum damage (heating or disintegration) with minimal overheating or mechanical injury to neighboring structures.
Thus, for irradiating pathological vessels of a port-wine stain (PWS), the most rational is to use a laser with the most long length waves corresponding to the light absorption peaks of hemoglobin (l = 540, 577, 585 and 595 nm), with a pulse duration of the order of milliseconds, since in this case the absorption of radiation by melanin will be insignificant (position 1 of the theory of selective photothermolysis). A relatively long wavelength will effectively provide deep heating of the tissue (position 2), and a relatively long pulse will correspond to a very large sizes targets (vessels with red blood cells; position 3).
If the goal of the procedure is to eliminate tattoo particles, then in addition to selecting the radiation wavelength corresponding to the color of these particles, it will be necessary to set the pulse duration, which is significantly shorter than in the case of port-wine stains, in order to achieve mechanical destruction of the particles with minimal thermal damage to other structures (position 4 ).
Of course, compliance with all these conditions does not provide absolute protection of the epidermis, but it prevents too severe damage to it, which would subsequently lead to a permanent cosmetic defect due to excessive scarring.
Tissue reactions to laser irradiation
When laser light interacts with tissue, the following reactions occur.
Photostimulation. Low-intensity therapeutic lasers are used for photostimulation. In terms of energy parameters, a therapeutic laser has an effect that does not damage the biosystem, but at the same time, this energy is sufficient to activate the vital processes of the body, for example, accelerating wound healing.
Photodynamic response. The principle is based on the effect of light of a certain wavelength on a photosensitizer (natural or artificially introduced), providing a cytotoxic effect on pathological tissue. In dermatology, photodynamic exposure is used to treat acne vulgaris, psoriasis, lichen planus, vitiligo, urticaria pigmentosa, etc.
Photothermolysis and photomechanical reactions - When radiation is absorbed, the energy of the laser beam is converted into heat in the area of the skin that contains the chromophore. With sufficient laser beam power, this leads to thermal destruction of the target . Selective photothermolysis can be used to remove malformations of superficial vessels, some pigmented formations of the skin, hair, and tattoos.
Literature
- Laser and light therapy. Dover J.S.Moscow. Reed Elsiver 2010.P.5-7
- Nevorotin A.I. Introduction to laser surgery. Tutorial. - St. Petersburg: SpetsLit, 2000.
- Nevorotin A.I. Laser wound in theoretical and applied aspects. // Laser biology and laser medicine: practice. Mat. report rep. seminar school. Part 2. - Tartu-Pyhäjärve: Publishing House of Tartu University of the ESSR, 1991, p. 3-12.
- Anderson R. R., Parish J. A. The optics of human skin. J Invest Dermatol 1981; 77:13-19.
- Anderson R. R., Parrish J. A. Selective photothermolysis: precise microsurgery by selective absorption of pulsed radiation. Science 1983; 220:524-527.
- Goldman L., Blaney D. J., Kindel D. J. et al. Effect of the laser beam on the skin: preliminary report. J Invest Dermatol 1963; 40:121-122.
- Kaminer M. S., Arndt K. A., Dover J. S. et al. Atlas of cosmetic surgery. 2nd ed. - Saunders-Elsevier 2009.
- Margolis R. J., Dover J. S., Polla L. L. et al. Visible action spectrum for melanin-specific selective photothermolysis. Lasers Surg Med 1989; 9:389-397.
