We are often asked the question - what do these letters mean in the description of radar detectors: X, K, Ka, L, POP, VG-2?

X, K And Ka These are the radio frequency ranges in which police radars operate.

L(laser) - means the ability to detect laser radars (lidars)

POP- this is not a range, this is the operating mode of a police radar (and for a radar detector - the detection mode).

VG-2 this is a detection system for radar detectors (and in radar detectors, accordingly, protection from such detection)

Let's take a closer look at this.

Range X(10.475 to 10.575 ghz) - The oldest radio frequency band used for speed control. Older drivers remember the large radars used by the police back in the USSR, which looked like a big gray pipe, which is why they got the name “pipe” or “headlight”. Now there are almost none of them left. Personally, the last time I saw such a thing on the roads of Ukraine was in 2007. Having any, even the cheapest, radar detector in service, you will easily have time to slow down, because... The operating speed of these radars is low.

K-band(24.0 to 24.25 ghz) - K-band is the most common range in which most police radars currently operate. This range was introduced in 1976 in the USA and is still widely used throughout the world for speed detection. Radars operating in the K-band are distinguished by their smaller size and weight compared to X-band radars, as well as higher operating speed. This range is used by the radars "Vizir", "Berkut", "Iskra", etc. All of which are presented in our store detect the K range.

Ka band(33.4 to 36.0 GHz) is a newer range. Radars operating in this range are more accurate. For radar detectors, detecting this range is more difficult. All modern radar detectors detect radar radiation in the Ka band, however, since such police radars operate very quickly, it is not a fact that you will be able to slow down enough to avoid being caught. Be careful!

Laser range. Radars (lidars) operating in the laser range are a nightmare for an intruder. It is used by speed cameras, such as the TruCam device. A laser speed meter emits a beam in the infrared spectrum. Reflecting from the headlights of a car or license plate, the laser beam returns back, and since all this happens at the speed of light, you simply have no chance to slow down. If your radar detector reported that a laser was detected, this means that you have already been caught: (It’s another matter if you were not caught at all and the radar detector “caught” the reflected signal, then you may still be lucky.
All radar detectors presented in our store have the laser radar detection function. But the most effective (the only reliable!) way to combat laser guns is the so-called “shifters” - devices that deceive the laser speed meter. Our store presents the Beltronics SHIFTER ZR4 complex, which allows you to detect and protect against laser detection. This is what truly allows you to protect yourself from TruCam! Beltronics Shifter ZR4 can work either independently or in conjunction with Beltronics radar detectors.

POP mode- this is the operating mode of a police radar in which it emits for a very short time (tens of milliseconds). This is often enough to determine the speed, but the speed is not recorded and the traffic cop, in principle, has nothing to show you. But he will present it, rest assured. Most radar detectors can detect signals in this mode, and many force this mode to be turned on. In this mode, your radar detector is more sensitive to interference, so use it outside the city.

VG-2-This is an anti-detection mode for your radar detector. In some European countries and in some states in the USA, the use of radar detectors is prohibited. Therefore, police officers are armed with so-called radar detectors (Radar Detector Detector-RDD). They detect specific radiation that the radar detector produces during operation. This way, a police officer can know from a distance that you have a radar detector installed in your car. All modern radar detectors are protected from detection by VG-2 devices. The funny thing is that VG-2 is a system invented in the early 90s and is currently practically not used. Now police officers use the new Specter (Stalcar) RDD systems. These RDDs are very difficult to defend against, almost no radar detector on the market can defend against the Specter system, except for the Beltronics STI Driver radar - this thing is 100% invisible.

After reading this article, you may get the impression that there is no point in radar detectors - it still won’t help. This is not true at all. Firstly, most radars operate in the K and Ka bands, so you will be warned in advance and have time to reduce speed.

Laser guns, stationary laser cameras are a problem. On the other hand, there are very few such devices, they are several times more expensive than a conventional radar and are less common than conventional K-band radars even in the USA, let alone Ukraine. Such radars cannot be used handheld, only from a tripod or permanently mounted. For one hundred percent protection against laser radars, you will need a shifter - expensive but reliable.

Even the simplest "radar detector" detects most K-band radars in advance, at a sufficient distance for you to stop. My favorite mid-priced radars are Stinger- better protected from interference and has greater sensitivity. Well, the premium class Beltronics radar detectors and especially the STI Driver are beyond competition!

Good luck on the roads!

Ghetto-style spectroscopy: Exploring the spectrum and (safe) dangers of lasers

• DIY or Do It Yourself

I think everyone who reads this article has played with laser pointers. Recently, the Chinese have been increasing the radiation power higher and higher - and we will have to take care of safety ourselves.

In addition to this, I also managed to look at the spectrum of the laser radiation on my knee - whether it generates at one frequency, or at several at once. This may be necessary if you want to try recording a hologram at home.

Let's remember the design of green DPSS lasers

The obvious problem here is that 808nm and 1064nm radiation can exit the laser (if there is no output filter, or it is of poor quality) at an unknown angle, and unbeknownst to us, artistic cutting on the retina can occur. The human eye does not see 1064 nm at all, and 808 nm radiation is very weak, but can be seen in the dark (this is not too dangerous only with scattered radiation at low power!).

However, what is the radiation in the focused part of the laser radiation? Let's try to find out.

First approach: a sheet of paper and a CD

Each wavelength forms several images at once - several positive and several negative orders.

As a result, with the eye and a regular camera we will see the following:

However, if we look at a sheet of paper with a camera without an IR filter, we notice a strange purple dot between the first and second dots from the center:

Second approach: dispersion prisms

The result is achieved: the 808nm, 1064nm and green 532nm points are clearly visible. The human eye, in place of the IR dots, sees nothing at all.

Using a 1W green laser, using a “finger high-precision power meter” (abbreviated PVIM), it was possible to find out that in my case the overwhelming majority of the radiation is 532 nm, and 808 nm and 1064 nm, although detectable by the camera, their power is 20 or more times less, below the limit detection of PVIM.

It's time to check the glasses

We put the glasses on the camera (if you put them on the laser, the hole will melt, they are plastic), and we get: 532nm and 808nm are weakened very much, a little remains from 1064nm, but I think it’s not critical:

Out of curiosity, I decided to test colored anaglyph glasses (with red and blue glass). The red half retains green well, but for infrared light they are transparent:

The blue half has virtually no effect at all:

Does the laser generate at one frequency or several?

The larger the size of the resonator, the greater the number of possible wavelengths at which the laser can generate. In the lowest power green lasers, the neodymium laser crystal is a thin plate, and often only 1 or 2 wavelengths are possible for lasing.

When the temperature (=resonator size) or power changes, the generation frequency can change smoothly or abruptly.

Why is this important? Lasers that generate light at a single wavelength can be used for holography at home, interferometry (ultra-precise distance measurements) and other fun things.

Well, let's check it out. We take the same CD, but this time we will observe the spot not from 10 cm, but from 5 meters (since we need to see a difference in wavelengths of the order of 0.1 nm, and not 300 nm).

1W green laser: Due to the large size of the resonator, the frequencies occur with a small interval:

10mW green laser: The resonator dimensions are small - only 2 frequencies fit in the same spectral range:

When the power is reduced, only one frequency remains. You can write a hologram!

Let's look at other lasers. Red 650nm 0.2W:

Ultraviolet 405nm 0.2W:

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 liquids 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, in which the planes, vibrations of the elementary sources of healing - atoms and molecules - are randomly oriented in space.

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, 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 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 by the high risk of scarring.

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:

1. Coherence. The peaks and troughs of the waves are parallel and in phase in time and space.
2. Monochrome. The light waves emitted by the laser have the same length, exactly the one provided by the medium used in the laser.
3. 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. At higher densities (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 the positive effect is achieved at the cost of extensive mechanical damage to the tissue by the 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 the irradiation of pathological vessels of a port-wine stain (PWS), the most rational is to use a laser with the longest wavelength 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 melanin will be insignificant (proposition 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 very large target sizes (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

1. Laser and light therapy. Dover J.S.Moscow. Reed Elsiver 2010.P.5-7
2. Nevorotin A.I. Introduction to laser surgery. Study guide. - St. Petersburg: SpetsLit, 2000.
3. 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.
4. Anderson R. R., Parish J. A. The optics of human skin. J Invest Dermatol 1981; 77:13-19.
5. Anderson R. R., Parrish J. A. Selective photothermolysis: precise microsurgery by selective absorption of pulsed radiation. Science 1983; 220:524-527.
6. 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.
7. Kaminer M. S., Arndt K. A., Dover J. S. et al. Atlas of cosmetic surgery. 2nd ed. - Saunders-Elsevier 2009.
8. 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.

Laser (from the English “light amplification by stimulated emission of radiation” " - "light amplification by stimulating radiation") or an optical quantum generator is a special type of radiation source with feedback, the emitting body in which is an inversely populated medium. The principles of laser operation are based on the propertieslaser radiation: monochromatic and high coherence (spatial and temporal). TAlso, the characteristics of radiation often include low angular divergence (sometimes you can come across the term “high directivity of radiation”), which, in turn, allows us to talk about high intensity of laser radiation. Thus, in order to understand the principles of laser operation, it is necessary to talk about the characteristic properties of laser radiation and the inversely populated medium - one of the three main components of the laser.

Spectrum of laser radiation. Monochromatic.

One of the characteristics of the radiation of any source is its spectrum. The sun and household lighting devices have a wide spectrum of radiation, which contains components with different wavelengths. Our eye perceives such radiation as white light, if the intensity of its different components is approximately the same, or as light with some shade (for example, green and yellow components dominate in the light of our Sun).

Laser radiation sources, on the contrary, have a very narrow spectrum. To some approximation, we can say that all photons of laser radiation have the same (or similar) wavelengths. Thus, the radiation of a ruby ​​laser, for example, has a wavelength of 694.3 nm, which corresponds to red light. The first gas laser, helium-neon, also has a relatively close wavelength (632.8 nm). An argon ion gas laser, on the other hand, has a wavelength of 488.0 nm, which is perceived by our eyes as a turquoise color (intermediate between green and blue). Lasers based on sapphire doped with titanium ions have a wavelength in the infrared region (usually around 800 nm), so their radiation is invisible to humans. Some lasers (for example, semiconductor lasers with a rotating diffraction grating as an output mirror) can tune the wavelength of their radiation. What all lasers have in common, however, is that the bulk of their radiation energy is concentrated in a narrow spectral region. This property of laser radiation is called monochromaticity (from the Greek “one color”). In Fig. To illustrate this property, Fig. 1 shows the radiation spectra of the Sun (at the level of the outer layers of the atmosphere and at sea level) and a semiconductor laser produced by the company Thorlabs.

Rice. 1. Radiation spectra of the Sun and a semiconductor laser.

The degree of monochromaticity of laser radiation can be characterized by the spectral width of the laser line (the width can be specified as the wavelength or frequency detuning from the maximum intensity). Typically the spectral width is set at a level of 1/2 ( FWHM ), 1/ e or 1/10 of the maximum intensity. Some modern laser systems have achieved emission peak widths of several kHz, which corresponds to a laser linewidth of less than one billionth of a nanometer. For specialists, we note that the width of the laser line can be orders of magnitude narrower than the width of the line of spontaneous emission, which is also one of the distinctive characteristics of the laser (compared, for example, with luminescent and superluminescent sources).

Laser coherence

Monochromaticity is an important, but not the only property of laser radiation. Another defining property of laser radiation is its coherence. Usually they talk about spatial and temporal coherence.

Let's imagine that the laser beam is divided in half by a translucent mirror: half of the beam energy passed through the mirror, the other half was reflected and went into the system of guide mirrors (Fig. 2). After this, the second beam is again brought together with the first, but with some time delay. The maximum delay time at which the beams can interfere (i.e., interact taking into account the phase of the radiation, and not just its intensity) is called the coherence time of laser radiation, and the length of the additional path that the second beam has passed due to its deviation is the length of the longitudinal coherence. The longitudinal coherence length of modern lasers can exceed a kilometer, although for most applications (eg, industrial materials processing lasers) such high spatial coherence of the laser beam is not required.

You can divide the laser beam in another way: instead of a translucent mirror, put a completely reflective surface, but block it not the entire beam, but only part of it (Fig. 2). Then the interaction of radiation that propagates in different parts of the beam will be observed. The maximum distance between points of the beam at which radiation will interfere is called the transverse coherence length of the laser beam. Of course, for many lasers, the transverse coherence length is simply equal to the diameter of the laser beam.

Rice. 2. Towards an explanation of the concepts of temporal and spatial coherence

Angular divergence of laser radiation. Parameter M 2 .

No matter how much we strive to make the laser beam parallel, it will always have a non-zero angular divergence. Minimum possible divergence angle of laser radiationα d (“diffraction limit”) in order of magnitude is determined by the expression:

α d~ λ /D, (1)

Where λ is the wavelength of laser radiation, and D is the width of the beam emerging from the laser. It is easy to calculate that with a wavelength of 0.5 microns (green radiation) and a laser beam width of 5 mm, the divergence angle will be ~10 -4 rad, or 1/200 degree. Despite this small value, the angular divergence can be critical for some applications (for example, for the use of lasers in combat satellite systems), since it sets an upper limit on the achievable laser power density.

In general, the quality of the laser beam can be set by the parameter M 2 . Let the minimum achievable spot area created by an ideal lens when focusing a Gaussian beam be equal to S . Then if the same lens focuses the beam from a given laser into a spot of area S 1 > S , parameter M 2 laser radiation is equal to:

M 2 = S 1 / S (2)

For the highest quality laser systems, the parameter M 2 is close to unity (in particular, lasers with the parameter M 2 , equal to 1.05). However, it must be kept in mind that not all classes of lasers today have a low value of this parameter, which must be taken into account when choosing a laser class for a specific task.

We have briefly summarized the main properties of laser radiation. Let us now describe the main components of the laser: the population inversion medium, the laser cavity, the laser pump, and the laser level circuit.

Environment with inverted population. Layout of laser levels. Quantum output.

The main element that converts the energy of an external source (electric, energy of non-laser radiation, energy of an additional pump laser) into light is the medium in which the inverse population of a pair of levels is created. The term “population inversion” means that a certain fraction of structural particles of the medium (molecules, atoms or ions) is transferred to an excited state, and for a certain pair of energy levels of these particles (upper and lower laser levels) there are more particles at the upper energy level than on the bottom.

When passing through a medium with an inverted population, radiation, the quanta of which have an energy equal to the difference in the energies of two laser levels, can be amplified, while removing the excitation of part of the active centers (atoms/molecules/ions). Amplification occurs due to the formation of new quanta of electromagnetic radiation having the same wavelength, direction of propagation, phase and state of polarization as the original quantum. Thus, the laser generates packets of identical (equal in energy, coherent and moving in the same direction) photons (Fig. 3), which determines the basic properties of laser radiation.

Rice. 3. Generation of coherent photons during stimulated emission.

It is, however, impossible to create an inversely populated environment in a system consisting of only two levels in the classical approximation. Modern lasers usually have a three- or four-level system of levels involved in lasing. In this case, excitation transfers the structural unit of the medium to the highest level, from which the particles relax in a short time to a lower energy value - the upper laser level. Laser generation also involves one of the underlying levels - the ground state of the atom in a three-level scheme or the intermediate state in a four-level scheme (Fig. 4). The four-level scheme turns out to be more preferable due to the fact that the intermediate level is usually populated by a much smaller number of particles than the ground state; accordingly, creating an inverse population (the number of excited particles exceeding the number of atoms at the lower laser level) turns out to be much simpler (to start laser generation, you need to inform environment with less energy).

Rice. 4. Three-tier and four-tier level systems.

Thus, during laser lasing, the minimum value of the energy imparted to the working medium is equal to the excitation energy of the highest level of the system, and lasing occurs between two underlying levels. This determines the fact that the laser efficiency is initially limited by the ratio of the excitation energy to the laser transition energy. This ratio is called the quantum efficiency of the laser. It is worth noting that usually the efficiency of a laser from the mains is several times (and in some cases even several tens of times) lower than its quantum output.

Semiconductor lasers have a special structure of energy levels. The process of generating radiation in semiconductor lasers involves electrons from two bands of the semiconductor, but due to impurities that form the light-emitting p-n transition, the boundaries of these zones in different parts of the diode turn out to be shifted relative to each other. Inverse population in the region p-n transition in such lasers is created due to the flow of electrons into the transition region from the conduction band n ‑site and holes from the valence band p - plot. More details about semiconductor lasers can be found in specialized literature.

Modern lasers use various methods to create population inversion, or laser pumping.

Laser pumping. Pumping methods.

In order for a laser to start generating radiation, it is necessary to supply energy to its active medium in order to create an inverse population in it. This process is called laser pumping. There are several basic pumping methods, the applicability of which in a particular laser depends on the type of active medium. Thus, for excimer and some gas lasers operating in a pulsed mode (for example, CO2 - laser) it is possible to excite the molecules of the laser medium by an electric discharge. In continuous gas lasers, a glow discharge can be used for pumping. Semiconductor lasers are pumped by applying a voltage to p‑n laser transition. For solid-state lasers, you can use an incoherent radiation source (flash lamp, line or array of light-emitting diodes) or another laser, the wavelength of which corresponds to the difference in energies between the ground and excited states of an impurity atom (in solid-state lasers, as a rule, lasing occurs on atoms or ions impurities dissolved in the matrix grid - for example, for a ruby ​​laser, chromium ions are the active impurity).

To summarize, we can say that the laser pumping method is determined by its type and the characteristics of the active center of the lasing medium. As a rule, for each specific type of laser there is the most effective pumping method, which determines the type and design of the system for supplying energy to the active medium.

Laser resonator. Laser generation condition. Stable and unstable resonators.

The active medium and the system for delivering energy to it are not yet sufficient for laser generation to occur, although it is already possible to build some devices on their basis (for example, an amplifier or a superluminescent radiation source). Laser generation, i.e. emission of monochromatic coherent light occurs only in the presence of feedback, or a laser cavity.

In the simplest case, the resonator is a pair of mirrors, one of which (the laser output mirror) is semitransparent. As a rule, a reflector with a reflection coefficient at the lasing wavelength close to 100% (“deaf mirror”) is installed as another mirror to avoid laser lasing “in both directions” and unnecessary energy loss.

The laser resonator ensures that part of the radiation returns back to the active medium. This condition is important for the emergence of coherent and monochromatic radiation, since photons returned to the medium will cause the emission of photons of the same frequency and phase. Accordingly, radiation quanta newly emerging in the active medium will be coherent with those that have already left the resonator. Thus, the characteristic properties of laser radiation are largely ensured by the design and quality of the laser cavity.

The reflectance of the output translucent mirror of the laser resonator is selected in such a way as to ensure maximum laser output power, or based on the technological simplicity of manufacturing. Thus, in some fiber lasers, an evenly chopped end of a fiber light guide can be used as an output mirror.

An obvious condition for stable laser lasing is the condition of equality of optical losses in the laser cavity (including losses in the radiation output through the cavity mirrors) and the radiation gain in the active medium:

exp( a× 2L) = R 1 × R 2 × exp( g× 2L) × X,(3)

where L = length of the active medium,a- gain in the active medium, R 1 and R 2 - reflection coefficients of the resonator mirrors andg- “gray” losses in the active medium (i.e., radiation losses associated with density fluctuations, defects in the laser medium, scattering of radiation and other types of optical losses that cause attenuation of radiation when passing through the medium, except for the direct absorption of radiation quanta by atoms of the medium). Last multiplier " X » denotes all other losses present in the laser (for example, a special absorbing element can be introduced into the laser so that the laser generates pulses of short duration), in their absence it is equal to 1. To obtain the condition for the development of laser generation from spontaneously emitted photons, it is obvious that must be replaced with the ">" sign.

From equality (3) the following rule follows for choosing an output laser mirror: if the radiation amplification factor of the active medium, taking into account gray losses (a- g) × L small, reflectance of the output mirror R 1 should be chosen large so that the laser generation does not decay due to radiation escaping from the cavity. If the gain is large enough, it usually makes sense to choose a lower value. R 1 , since a high reflectance will lead to an increase in the intensity of radiation inside the resonator, which may affect the lifetime of the laser.

However, the laser cavity needs to be adjusted. Let us assume that the resonator is composed of two parallel, but not adjusted mirrors (for example, located at an angle to each other). In such a resonator, radiation, having passed through the active medium several times, goes beyond the laser (Fig. 5). Resonators in which radiation goes beyond its limits in a finite time are called unstable. Such resonators are used in some systems (for example, in high-power pulsed lasers of special design), however, as a rule, they try to avoid cavity instability in practical applications.

Rice. 5. Unstable resonator with misaligned mirrors; stable resonator and

a stationary beam of radiation in it.

To increase the stability of the resonator, curved reflective surfaces are used as mirrors. At certain values ​​of the radii of the reflecting surfaces, this resonator turns out to be insensitive to small adjustment violations, which makes it possible to significantly simplify the work with the laser.

We have briefly described the minimum required set of elements to create a laser and the main features of laser radiation.