Sinusitis

Wavelengths of laser radiation. Extending the spectral range of the laser

1. The passage of monochromatic light through a transparent medium.

2. Creation of population inversion. Pumping methods.

3. The principle of laser operation. Types of lasers.

4. Features of laser radiation.

5. Characteristics of laser radiation used in medicine.

6. Changes in the properties of tissue and its temperature under the influence of continuous powerful laser radiation.

7. Use of laser radiation in medicine.

8. Basic concepts and formulas.

9. Tasks.

We know that light is emitted in separate units - photons, each of which arises as a result of the radiative transition of an atom, molecule or ion. Natural light is a collection huge number such photons, differing in frequency and phase, emitted at random times in random directions. Obtaining powerful beams of monochromatic light using natural sources is an almost impossible task. At the same time, the need for such beams was felt by both physicists and specialists in many applied sciences. The creation of a laser made it possible to solve this problem.

Laser- a device that generates coherent electromagnetic waves due to the stimulated emission of microparticles of the medium in which a high degree of excitation of one of the energy levels is created.

Laser (LASER Light Amplification by Stimulated of Emission Radiation) - amplification of light using stimulated radiation.

The intensity of laser radiation (LR) is many times greater than the intensity of natural light sources, and the divergence of the laser beam is less than one arc minute (10 -4 rad).

31.1. Passage of monochromatic light through a transparent medium

In Lecture 27, we found out that the passage of light through matter is accompanied by: photon excitation its particles and acts stimulated emission. Let us consider the dynamics of these processes. Let it spread in the environment monochromatic light, the frequency of which (ν) corresponds to the transition of particles of this medium from the ground level (E 1) to the excited level (E 2):

Photons hitting particles in the ground state will be absorbed and the particles themselves will go into excited state E 2 (see Fig. 27.4). Photons that strike excited particles initiate stimulated emission (see Fig. 27.5). In this case, photons are doubled.

In a state of thermal equilibrium, the ratio between the number of excited (N 2) and unexcited (N 1) particles obeys the Boltzmann distribution:

where k is Boltzmann's constant, T is the absolute temperature.

In this case, N 1 >N 2 and absorption dominates over doubling. Consequently, the intensity of the emerging light I will be less than the intensity of the incident light I 0 (Fig. 31.1).

Rice. 31.1. Attenuation of light passing through a medium in which the degree of excitation is less than 50% (N 1 > N 2)

As light is absorbed, the degree of excitation will increase. When it reaches 50% (N 1 = N 2), between absorption And doubling equilibrium will be established, since the probabilities of photons hitting the excited and unexcited particles will become the same. If the illumination of the medium stops, then after some time the medium will return to the initial state corresponding to the Boltzmann distribution (N 1 > N 2). Let's make a preliminary conclusion:

When illuminating the environment with monochromatic light (31.1) impossible to achieve such a state of the environment in which the degree of excitation exceeds 50%. Still, let's consider the question of the passage of light through a medium in which the state N 2 > N 1 has been achieved in some way. This state is called a state with inverse population(from lat. inversio- turning).

Population inversion- a state of the environment in which the number of particles at one of the upper levels is greater than at the lower level.

In a medium with an inverted population, the probability of a photon hitting an excited particle is greater than that of an unexcited one. Therefore, the doubling process dominates over the absorption process and gain light (Fig. 31.2).

As light passes through a population inverted medium, the degree of excitation will decrease. When it reaches 50%

Rice. 31.2. Amplification of light passing through a medium with inverted population (N 2 > N 1)

(N 1 = N 2), between absorption And doubling equilibrium will be established and the light amplification effect will disappear. If the illumination of the medium stops, then after some time the medium will return to a state corresponding to the Boltzmann distribution (N 1 > N 2).

If all this energy is released in radiative transitions, then we will receive a light pulse of enormous power. True, it will not yet have the required coherence and directionality, but will be highly monochromatic (hv = E 2 - E 1). This is not a laser yet, but it is already something close.

31.2. Creation of population inversion. Pumping methods

So is it possible to achieve population inversion? It turns out that you can if you use three energy levels with the following configuration (Fig. 31.3).

Let the environment be illuminated with a powerful flash of light. Part of the emission spectrum will be absorbed in the transition from the main level E 1 to the broad level E 3 . Let us remind you that wide is an energy level with a short relaxation time. Therefore, the majority of particles that enter the excitation level E 3 non-radiatively transfer to the narrow metastable level E 2, where they accumulate. Due to the narrowness of this level, only a small fraction of flash photons

Rice. 31.3. Creation of population inversion at a metastable level

capable of causing a forced transition E 2 → E 1. This provides the conditions for creating an inverse population.

The process of creating a population inversion is called pumped up. Modern lasers use various types of pumping.

Optical pumping of transparent active media uses light pulses from an external source.

Electric discharge pumping of gaseous active media uses an electric discharge.

Injection pumping of semiconductor active media uses electric current.

Chemical pumping of an active medium from a mixture of gases uses energy chemical reaction between the components of the mixture.

31.3. The principle of laser operation. Types of lasers

The functional diagram of the laser is shown in Fig. 31.4. The working fluid (active medium) is a long narrow cylinder, the ends of which are covered by two mirrors. One of the mirrors (1) is translucent. Such a system is called an optical resonator.

The pumping system transfers particles from the ground level E 1 to the absorption level E 3 , from where they transfer non-radiatively to the metastable level E 2 , creating its population inversion. After this, spontaneous radiative transitions E 2 → E 1 begin with the emission of monochromatic photons:

Rice. 31.4. Schematic laser device

Spontaneous emission photons emitted at an angle to the cavity axis exit through lateral surface and do not participate in the generation process. Their flow is quickly drying up.

Photons, which, after spontaneous emission, move along the axis of the resonator, repeatedly pass through the working fluid, reflecting from the mirrors. At the same time, they interact with excited particles, initiating stimulated emission. Due to this, an “avalanche-like” increase in induced photons moving in the same direction occurs. A multiply amplified stream of photons exits through a translucent mirror, creating a powerful beam of almost parallel coherent rays. In fact, laser radiation is generated first a spontaneous photon that moves along the axis of the resonator. This ensures coherence of radiation.

Thus, the laser converts the energy of the pump source into the energy of monochromatic coherent light. The efficiency of such a transformation, i.e. Efficiency depends on the type of laser and ranges from fractions of a percent to several tens of percent. Most lasers have an efficiency of 0.1-1%.

Types of lasers

The first laser created (1960) used ruby as a working fluid and an optical pumping system. Ruby is a crystalline aluminum oxide A1 2 O 3 containing about 0.05% chromium atoms (it is chromium that gives ruby its pink color). Chromium atoms embedded in the crystal lattice are the active medium

with the configuration of energy levels shown in Fig. 31.3. The wavelength of the ruby laser radiation is λ = 694.3 nm. Then lasers using other active media appeared.

Depending on the type of working fluid, lasers are divided into gas, solid-state, liquid, and semiconductor. In solid-state lasers, the active element is usually made in the form of a cylinder, the length of which is much greater than its diameter. Gas and liquid active media are placed in a cylindrical cuvette.

Depending on the pumping method, continuous and pulsed generation of laser radiation can be obtained. With a continuous pumping system, population inversion is maintained long time due to an external energy source. For example, continuous excitation by an electric discharge in a gaseous environment. At pulse system pumping, a population inversion is created in pulsed mode. Pulse repetition frequency from 10 -3

Hz up to 10 3 Hz.

31.4. Features of laser radiation

Laser radiation in its properties differs significantly from the radiation of conventional light sources. Let us note its characteristic features.

1. Coherence. Radiation is highly coherent, which is due to the properties of stimulated emission. In this case, not only temporal, but also spatial coherence takes place: the phase difference at two points of the plane perpendicular to the direction of propagation remains constant (Fig. 31.5, a).

2. Collimation. Laser radiation is collimated, those. all rays in the beam are almost parallel to each other (Fig. 31.5, b). At greater distances, the laser beam only slightly increases in diameter. Since the divergence angle φ is small, then the intensity of the laser beam decreases slightly with distance. This allows signals to be transmitted over vast distances with little attenuation of their intensity.

3. Monochromatic. Laser radiation is highly monochromatic, those. contains waves of almost the same frequency (the width of the spectral line is Δλ ≈0.01 nm). On

Figure 31.5c shows a schematic comparison of the linewidth of a laser beam and a beam of ordinary light.

Rice. 31.5. Coherence (a), collimation (b), monochromaticity (c) of laser radiation

Before the advent of lasers, radiation with a certain degree of monochromaticity could be obtained using devices - monochromators, which isolated narrow spectral intervals (narrow bands) from the continuous spectrum wavelengths), however, the light power in such bands is low.

4. High power. Using a laser, it is possible to provide very high monochromatic radiation power - up to 10 5 W in continuous mode. The power of pulsed lasers is several orders of magnitude higher. Thus, a neodymium laser generates a pulse with energy E = 75 J, the duration of which is t = 3x10 -12 s. The power in the pulse is equal to P = E/t = 2.5x10 13 W (for comparison: the power of a hydroelectric power station is P ~ 10 9 W).

5. High intensity. In pulsed lasers, the intensity of laser radiation is very high and can reach I = 10 14 -10 16 W/cm 2 (cf. the intensity of sunlight near earth's surface I = 0.1 W/cm2).

6. High brightness. For lasers operating in the visible range, brightness laser radiation (light intensity per unit surface) is very high. Even the weakest lasers have a brightness of 10 15 cd/m 2 (for comparison: the brightness of the Sun is L ~ 10 9 cd/m 2).

7. Pressure. When a laser beam falls on the surface of a body, it creates pressure(D). With complete absorption of laser radiation incident perpendicular to the surface, pressure D = I/c is created, where I is the radiation intensity, c is the speed of light in vacuum. With total reflection, the pressure is twice as high. For intensity I = 10 14 W/cm 2 = 10 18 W/m 2 ; D = 3.3x10 9 Pa = 33,000 atm.

8. Polarization. Laser radiation is completely polarized.

31.5. Characteristics of laser radiation used in medicine

Radiation wavelength

The radiation wavelengths (λ) of medical lasers lie in the range of 0.2 -10 µm, i.e. from ultraviolet to far infrared region.

Radiation power

The radiation power (P) of medical lasers varies within wide limits, determined by the purposes of application. For lasers with continuous pumping, P = 0.01-100 W. Pulsed lasers are characterized by pulse power P and pulse duration τ and

For surgical lasers P and = 10 3 -10 8 W, and the pulse duration t and = 10 -9 -10 -3 s.

Energy in a radiation pulse

The energy of one pulse of laser radiation (E and) is determined by the relation E and = P and -t and, where t and is the duration of the radiation pulse (usually t and = 10 -9 -10 -3 s). For surgical lasers E and = 0.1-10 J.

Pulse repetition rate

This characteristic (f) of pulsed lasers shows the number of radiation pulses generated by the laser in 1 s. For therapeutic lasers f = 10-3,000 Hz, for surgical lasers f = 1-100 Hz.

Average radiation power

This characteristic (P avg) of pulse-periodic lasers shows how much energy the laser emits in 1 s, and is determined by the following relationship:

Intensity (power density)

This characteristic (I) is defined as the ratio of the laser radiation power to the cross-sectional area of the beam. For continuous lasers I = P/S. In the case of pulsed lasers there are pulse intensity I and = P and /S and average intensity I av = P av /S.

The intensity of surgical lasers and the pressure created by their radiation have the following values:

for continuous lasers I ~ 10 3 W/cm 2, D = 0.033 Pa;

for pulsed lasers I and ~ 10 5 -10 11 W/cm 2, D = 3.3 - 3.3x10 6 Pa.

Pulse energy density

This value (W) characterizes the energy per unit area of the irradiated surface per pulse and is determined by the relation W = E and /S, where S (cm 2) is the area of the light spot (i.e., the cross section of the laser beam) on the surface biological tissues. For lasers used in surgery, W ≈ 100 J/cm 2.

The parameter W can be considered as the radiation dose D per 1 pulse.

31.6. Changes in the properties of tissue and its temperature under the influence of continuous powerful laser radiation

Changes in temperature and fabric properties

under the influence of continuous laser radiation

Absorption of high-power laser radiation by biological tissue is accompanied by the release of heat. To calculate the heat released, a special value is used - volumetric heat density(q).

The release of heat is accompanied by an increase in temperature and the following processes occur in the tissues:

at 40-60°C, enzyme activation, edema formation, changes and, depending on the time of action, cell death, protein denaturation, the onset of coagulation and necrosis occur;

at 60-80°C - denaturation of collagen, membrane defects; at 100°C - dehydration, evaporation of tissue water; over 150°C - charring;

over 300°C - evaporation of fabric, gas formation. The dynamics of these processes are shown in Fig. 31.6.

Rice. 31.6. Dynamics of changes in tissue temperature under the influence of continuous laser radiation

1 phase. First, the tissue temperature rises from 37 to 100 °C. In this temperature range, the thermodynamic properties of the fabric remain practically unchanged, and the temperature increases linearly with time (α = const and I = const).

2 phase. At a temperature of 100 °C, the evaporation of tissue water begins, and until the end of this process the temperature remains constant.

3 phase. After the water evaporates, the temperature begins to rise again, but more slowly than in section 1, since the dehydrated tissue absorbs energy less than normal.

4 phase. Upon reaching a temperature T ≈ 150 °C, the process of charring and, consequently, “blackening” of the biological tissue begins. In this case, the absorption coefficient α increases. Therefore, a nonlinear increase in temperature, accelerating with time, is observed.

5 phase. When the temperature T ≈ 300 °C is reached, the process of evaporation of the dehydrated charred biological tissue begins and the temperature increase stops again. It is at this moment that the laser beam cuts (removes) the tissue, i.e. becomes a scalpel.

The degree of temperature increase depends on the depth of the tissue (Fig. 31.7).

Rice. 31.7. Processes occurring in irradiated tissues at different depths: A- in the surface layer the fabric heats up to several hundred degrees and evaporates; b- the radiation power weakened by the top layer is insufficient to evaporate the tissue. Tissue coagulation occurs (sometimes together with charring - a thick black line); V- tissue heating occurs due to heat transfer from the zone (b)

The extent of individual zones is determined both by the characteristics of the laser radiation and the properties of the tissue itself (primarily the absorption and thermal conductivity coefficients).

Exposure to a powerful focused beam of laser radiation is accompanied by the appearance of shock waves, which can cause mechanical damage to adjacent tissues.

Ablation of tissue under the influence of powerful pulsed laser radiation

When tissue is exposed to short pulses of laser radiation with a high energy density, another mechanism of dissection and removal of biological tissue is realized. In this case, very rapid heating of the tissue fluid occurs to a temperature T > T boil. In this case, the tissue fluid finds itself in a metastable overheated state. Then “explosive” boiling occurs tissue fluid, which is accompanied by tissue removal without charring. This phenomenon is called ablation. Ablation is accompanied by the generation of mechanical shock waves that can cause mechanical damage to tissue in the vicinity of the laser irradiation zone. This fact must be taken into account when choosing the parameters of pulsed laser radiation, for example when grinding skin, drilling teeth or when laser correction visual acuity.

31.7. Use of laser radiation in medicine

The processes characterizing the interaction of laser radiation (LR) with biological objects can be divided into 3 groups:

non-disturbing influence(not having a noticeable effect on the biological object);

photochemical action(a particle excited by a laser either itself takes part in the corresponding chemical reactions, or transfers its excitation to another particle participating in a chemical reaction);

photodestruction(due to the release of heat or shock waves).

Laser diagnostics

Laser diagnostics is a non-perturbing effect on a biological object using coherence laser radiation. Let us list the main diagnostic methods.

Interferometry. When laser radiation is reflected from a rough surface, secondary waves arise that interfere with each other. As a result, a picture of dark and light spots (speckles) is formed, the location of which provides information about the surface of the biological object (speckle interferometry method).

Holography. Using laser radiation, a 3-dimensional image of an object is obtained. In medicine, this method allows one to obtain three-dimensional images of the internal cavities of the stomach, eyes, etc.

Scattering of light. When a highly directed laser beam passes through a transparent object, light is scattered. Registration of the angular dependence of the intensity of scattered light (nephelometry method) allows one to determine the size of particles of the medium (from 0.02 to 300 μm) and the degree of their deformation.

When scattered, the polarization of light can change, which is also used in diagnostics (polarization nephelometry method).

Doppler effect. This method is based on measuring the Doppler frequency shift of LR, which occurs when light is reflected even from slowly moving particles (anenometry method). In this way, the speed of blood flow in the vessels, the mobility of bacteria, etc. are measured.

Quasielastic scattering. With such scattering, a slight change in the wavelength of the probing LR occurs. The reason for this is a change in the scattering properties (configuration, conformation of particles) during the measurement process. Temporary changes in the parameters of the scattering surface manifest themselves in a change in the scattering spectrum compared to the spectrum of the supply radiation (the scattering spectrum either broadens or additional maxima appear in it). This method allows you to obtain information about the changing characteristics of scatterers: diffusion coefficient, speed of directed transport, dimensions. This is how protein macromolecules are diagnosed.

Laser mass spectroscopy. This method is used to study chemical composition object. Powerful beams of laser radiation evaporate matter from the surface of a biological object. The vapors are subjected to mass spectral analysis, the results of which determine the composition of the substance.

Laser blood test. A laser beam passed through a narrow quartz capillary through which specially treated blood is pumped causes its cells to fluoresce. The fluorescent light is then detected by a sensitive sensor. This glow is specific to each type of cell passing individually through the cross section of the laser beam. The total number of cells in a given volume of blood is calculated. Precise quantitative indicators for each cell type are determined.

Photodestruction method. It is used to study surface composition object. Powerful LR beams make it possible to take microsamples from the surface of biological objects by evaporating the substance and subsequent mass spectral analysis of this vapor.

Use of laser radiation in therapy

Low-intensity lasers are used in therapy (intensity 0.1-10 W/cm2). Low-intensity radiation does not cause a noticeable destructive effect on tissue directly during irradiation. In the visible and ultraviolet regions of the spectrum, irradiation effects are caused by photochemical reactions and do not differ from the effects caused by monochromatic light received from conventional incoherent sources. In these cases, lasers are simply convenient monochromatic light sources that provide

Rice. 31.8. Scheme of using a laser source for intravascular irradiation blood

providing precise localization and dosage of exposure. As an example in Fig. Figure 31.8 shows a diagram of the use of a laser radiation source for intravascular irradiation of blood in patients with heart failure.

The most common laser therapy methods are listed below.

Red light therapy. He-Ne laser radiation with a wavelength of 632.8 nm is used for anti-inflammatory purposes to treat wounds, ulcers, and coronary heart disease. Therapeutic effect associated with the influence of light of this wavelength on the proliferative activity of the cell. Light acts as a regulator of cellular metabolism.

Blue light therapy. Laser radiation with a wavelength in the blue region of visible light is used, for example, to treat jaundice in newborns. This disease is a consequence of a sharp increase in the concentration of bilirubin in the body, which has a maximum absorption in the blue region. If children are irradiated with laser radiation of this range, bilirubin breaks down, forming water-soluble products.

Laser physiotherapy - the use of laser radiation in combination with various methods of electrophysiotherapy. Some lasers have magnetic attachments for the combined action of laser radiation and a magnetic field - magnetic laser therapy. These include the Milta magnetic-infrared laser therapeutic device.

The effectiveness of laser therapy increases when combined with medicinal substances previously applied to the irradiated area (laser phoresis).

Photodynamic therapy of tumors. Photodynamic therapy (PDT) is used to remove tumors that are accessible to light. PDT is based on the use of photosensitizers localized in tumors, which increase the sensitivity of tissues during their

subsequent irradiation visible light. The destruction of tumors during PDT is based on three effects: 1) direct photochemical destruction of tumor cells; 2) damage to the blood vessels of the tumor, leading to ischemia and tumor death; 3) the occurrence of an inflammatory reaction that mobilizes antitumor immune protection body tissues.

To irradiate tumors containing photosensitizers, laser radiation with a wavelength of 600-850 nm is used. In this region of the spectrum, the depth of light penetration into biological tissues is maximum.

Photodynamic therapy is used in the treatment of tumors of the skin, internal organs: lungs, esophagus (at the same time internal organs laser radiation is delivered using light guides).

Use of laser radiation in surgery

In surgery, high-intensity lasers are used to cut tissue, remove pathological areas, stop bleeding, and weld biological tissues. By properly choosing the radiation wavelength, its intensity and duration of exposure, various surgical effects can be obtained. Thus, to cut biological tissues, a focused beam of a continuous CO 2 laser is used, having a wavelength λ = 10.6 μm and a power of 2x10 3 W/cm 2.

The use of a laser beam in surgery provides selective and controlled exposure. Laser surgery has a number of advantages:

Non-contact, providing absolute sterility;

Selectivity, which allows the choice of radiation wavelength to destroy pathological tissues in doses without affecting the surrounding healthy tissues;

Bloodlessness (due to protein coagulation);

Possibility of microsurgical interventions due to the high degree of beam focusing.

Let us indicate some areas of surgical application of lasers.

Laser welding of fabrics. The connection of dissected tissues is a necessary step in many operations. Figure 31.9 shows how welding of one of the trunks of a large nerve is carried out in contact mode using solder, which

Rice. 31.9. Nerve welding using a laser beam

drops from a pipette are applied to the lasing site.

Destruction of pigmented areas. Pulsed lasers are used to destroy pigmented areas. This method (photothermolysis) used to treat angiomas, tattoos, sclerotic plaques in blood vessels etc.

Laser endoscopy. The introduction of endoscopy revolutionized surgical medicine. To avoid big open operations, laser radiation is delivered to the site of treatment using fiber-optic light guides, which allow laser radiation to be delivered to the biological tissues of internal hollow organs. This significantly reduces the risk of infection and postoperative complications.

Laser breakdown. Short-pulse lasers in combination with light guides are used to remove plaques in blood vessels, stones in gallbladder and kidneys.

Lasers in ophthalmology. The use of lasers in ophthalmology makes it possible to perform bloodless surgical interventions without compromising the integrity of the eyeball. These are operations on vitreous body; welding of the detached retina; treatment of glaucoma by “needing” laser beam holes (diameter 50÷100 µm) for outflow intraocular fluid. Layer-by-layer ablation of corneal tissue is used for vision correction.

31.8. Basic concepts and formulas

End of the table

31.9. Tasks

1. In a phenylalanine molecule, the energy difference in the ground and excited states is ΔE = 0.1 eV. Find the relationship between the populations of these levels at T = 300 K.

Answer: n = 3.5*10 18.

OPTICAL FREQUENCY STANDARDS - lasers with a frequency stable over time (10 -14 - 10 -15), its reproducibility (10 -13 - 10 -14). O. s. hours are used in physical sciences. research and find practical application in metrology, location, geophysics, communications, navigation and mechanical engineering. Frequency division O.s. hours before the radio range made it possible to create a time scale based on the use of the optical period. .
O. s. h. have advantages compared to quantum frequency standards Microwave range: experiments related to measuring frequency when using lasers require less time, because abs. the frequency is 10 4 - 10 5 times higher than non-laser frequency standards. Abs. intensity and width, which are frequency references, in optical. range 10 5 - 10 6 times more than in the microwave range, at the same relative. width. This allows you to create O. s. hours with a higher short-term duration. frequency stability. When dividing the frequency of O. s. h. refers to the radio range. the width of the emission line practically does not change (if a microwave standard is used, the fluctuation spectrum of its signal expands significantly when the frequency is multiplied by 10 5 - 10 6 times). The role of quadratic Doppler effect, limiting longevity. frequency stability and reproducibility are the same.

The principle of stabilization. Stabilization of the laser frequency, as well as radio standards, is based on the use of spectral lines of atomic or molecular gas (optical reference points), to the center of which the frequency is “linked” v using an electronic automatic system. frequency adjustments. Because laser gain lines usually significantly exceed the bandwidth optical resonator, then instability ( v) frequencies v Generation in most cases is determined by a change in optical. resonator length Main. sources of instability l are thermal drift, mechanical. and acoustic disturbances of structural elements, fluctuations of the refractive index of gas-discharge plasma. Using optical reference point, the auto-tuning system produces a signal proportional. the magnitude and sign of the detuning between the frequency v and frequency v 0 center of the spectral line, with the help of which the laser frequency is tuned to the center of the line ( = v - v 0= 0). Relates. setting accuracy inversely proportional the product of the spectral line ( - line width) and the signal-to-noise ratio during its display.
To obtain a narrow emission line and high short duration. frequency stability (stability over time s), it is necessary to use benchmarks of sufficiently high intensity with a width significantly exceeding the characteristic range of frequency disturbances. gas lasers characteristic width of the acoustic spectrum. disturbances ~ 10 3 - 10 4 Hz, therefore the required resonance width is Hz (relative width 10 -9 - 10 -10). This allows the use of automatic systems. frequency adjustments with a wide band (10 4 Hz) for eff. suppression of fast fluctuations in the resonator length.
To achieve high durability. stability and frequency reproducibility are required optical. lines of high quality factor, since this reduces the influence of decomposition. factors on frequency shifts of the line center.

Optical benchmarks. The methods used in the microwave range for obtaining narrow spectral lines turned out to be inapplicable in optical applications. spectral region (Doppler broadening is small in the microwave range). For O. s. Particularly important are the methods that make it possible to obtain resonances in the center of the spectral line. This makes it possible to directly relate the radiation frequency to the frequency of the quantum transition. Three methods are promising: the method of saturated absorption, two-photon resonance and the method of spaced optical beams. fields. Basic Results on laser frequency stabilization were obtained using the saturated absorption method, which is based on the nonlinear interaction of counterpropagating light waves with a gas. A nonlinear absorbing cell with low-pressure gas can be located inside the laser cavity (active reference) and outside it (passive reference). Due to the saturation effect (equalization of the population levels of gas particles in a strong field), a dip with a uniform width appears in the center of the Doppler-broadened absorption line, the edges can be 10 5 - 10 6 times less than the Doppler width. In the case of an internal absorbing cell, a decrease in absorption in the center of the line leads to the appearance of a narrow peak in the contour of the dependence of power on generation frequency. The width of a nonlinear resonance in a low-pressure molecular gas is determined primarily by collisions and effects caused by the finite time of flight of a particle through a light beam. A decrease in the resonance width is accompanied by a sharp drop in its intensity (proportional to the cube of pressure).
Naib. narrow saturated absorption resonances with a width of 10 -11 were obtained in CH 4 on components E oscillatory-rotate. lines R(7) stripes v 3 (see Molecular spectra), which are close to the center of the gain line of the helium-neon laser at = 3.39 microns. To accurately align the amplification and absorption lines, use 22 Ne and increase the He pressure in the active medium of the laser or place the active medium in a magnetic field. field (for E-components).
Scheme O. s. h., using ultra-narrow resonance (with a relative width of 10 -11 - 10 - 12 ) as a reference, consists of an auxiliary frequency-stable laser 2 with a narrow radiation line, a tunable laser 2 and a system for obtaining a narrow resonance (Fig. 1). The narrow emission line of a tunable laser, which is used to obtain an ultra-narrow resonance, is ensured by phase synchronization of this laser with a stable one.

Rice. 1. Scheme of the optical frequency standard: FFA - frequency-phase auto-tuning; SUR - system for obtaining ultra-narrow resonance; AFC - automatic frequency control system; ZG - sound generator; RG - radio generator; D - photo detector.

We'll have a long time. The stability of the tunable laser is achieved by smoothly adjusting its frequency to the maximum ultra-narrow resonance using an extreme auto-tuning system. In this case, it is possible to simultaneously obtain high values for a short time. and long lasting. stability and frequency reproducibility.
Frequency stability. Naib. high frequency stability was obtained in the IR range with a He - Ne laser ( = 3.39 μm) with internal. absorption cell. Because abs. its frequency is known with high accuracy (10 -11), then this laser can be used independently. secondary frequency standard for measuring abs. frequencies in optical and IR ranges. The emission linewidth of such a laser is 0.07 Hz (Fig. 2). Frequency stability for averaging times = 1 - 100 s is equal to 4 x 10 -15 (Fig. 3).
We'll have a long time. stability and frequency reproducibility of He - Ne lasers with telescopic. beam expansion, stabilized by resonances in CH 4 on absorption lines F 2 2 and E(see above) with a quality factor of ~10 11, reach ~10 -14. The principal factor limiting frequency reproducibility and accuracy is quadratic.

Lit.: Basov N. G., Letokhov V. S., Optical frequency standards, "UFN", 1968, v. 96, p. 585; Jennings D. A., Petersen F. R., Evenson K. M., Direct frequency measurement of the 260 THz (1.15mm) 20 Ne Laser and beyond, in: Laser spectroscopy. IV. Proc. 4th-Intern. Conf., Rottach-Egern, Fed. Rep. of Germany, June 11 - 15 1979, ed. by H. Walther, K. W. Kothe, V. -, 1979, p. 39; Proceedings of Third Symposium on Freq. Standards and Metrology, Aussois, France, 12 - 15 Oct. 1981, "J. Phys.", 1981, v. 42, Colloq. S 8, No. 12; Bagaev S.N., Chebotaev V.P., Laser frequency standards, "UFN", 1986, v. 148, p. 143; Knight D. J. E., A tabulation of absolute laser - frequency measurements, "Metrologia", 1986, v 22, p. 251.

V. P. Chebotaev.

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 at 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 the police radar in which it emits 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, the police 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!

Lasers are becoming increasingly important research tools in medicine, physics, chemistry, geology, biology and engineering. At misuse they can cause blinding and injury (including burns and electrical shock) to operators and other personnel, including bystanders, as well as significant property damage. Users of these devices must to the fullest understand and apply the necessary safety precautions when handling them.

What is a laser?

The word “laser” (LASER, Light Amplification by Stimulated Emission of Radiation) is an abbreviation that stands for “light amplification by stimulated emission of radiation.” The frequency of the radiation generated by the laser is within or near the visible part electromagnetic spectrum. The energy is amplified to extremely high intensity through a process called laser-induced emission.

The term radiation is often misunderstood because it is also used to describe In this context, it means the transfer of energy. Energy is transferred from one place to another through conduction, convection and radiation.

There are many different types of lasers operating in different environments. The working medium used is gases (for example, argon or a mixture of helium and neon), solid crystals (for example, ruby) or liquid dyes. When energy is supplied to the working medium, it becomes excited and releases energy in the form of particles of light (photons).

A pair of mirrors at either end of a sealed tube either reflects or transmits light in a concentrated stream called a laser beam. Each operating environment produces a beam of unique wavelength and color.

The color of laser light is typically expressed by wavelength. It is non-ionizing and includes ultraviolet (100-400 nm), visible (400-700 nm) and infrared (700 nm - 1 mm) parts of the spectrum.

Electromagnetic spectrum

Each electromagnetic wave has a unique frequency and length associated with this parameter. Just as red light has its own frequency and wavelength, all other colors - orange, yellow, green and blue - have unique frequencies and wavelengths. Humans are able to perceive these electromagnetic waves, but are unable to see the rest of the spectrum.

Ultraviolet radiation also has the highest frequency. Infrared, microwave radiation and radio waves occupy the lower frequencies of the spectrum. Visible light lies in a very narrow range between the two.

impact on humans

The laser produces an intense, directed beam of light. If directed, reflected, or focused onto an object, the beam will be partially absorbed, raising the temperature of the surface and interior of the object, which can cause the material to change or deform. These qualities, which are used in laser surgery and materials processing, can be dangerous to human tissue.

In addition to radiation that has a thermal effect on tissue, laser radiation that produces a photochemical effect is dangerous. Its condition is a sufficiently short, i.e., ultraviolet or blue part of the spectrum. Modern devices produce laser radiation, the impact of which on humans is minimized. Low-power lasers do not have enough energy to cause harm, and they do not pose a danger.

Human tissue is sensitive to energy, and under certain circumstances, electromagnetic radiation, including laser radiation, can cause damage to the eyes and skin. Studies have been conducted on threshold levels of traumatic radiation.

Eye hazard

The human eye is more susceptible to injury than the skin. The cornea (the clear outer front surface of the eye), unlike the dermis, does not have an outer layer of dead cells to protect it from environmental influences. The laser is absorbed by the cornea of the eye, which can cause harm to it. The injury is accompanied by swelling of the epithelium and erosion, and in case of severe injuries - clouding of the anterior chamber.

The lens of the eye can also be susceptible to injury when it is exposed to various laser radiation - infrared and ultraviolet.

The greatest danger, however, is the impact of the laser on the retina in the visible part of the optical spectrum - from 400 nm (violet) to 1400 nm (near infrared). Within this region of the spectrum, collimated beams are focused onto very small areas of the retina. The most unfavorable impact occurs when the eye looks into the distance and is hit by a direct or reflected beam. In this case, its concentration on the retina reaches 100,000 times.

Thus, a visible beam with a power of 10 mW/cm 2 affects the retina with a power of 1000 W/cm 2. This is more than enough to cause damage. If the eye does not look into the distance, or if the beam is reflected from a diffuse, non-mirror surface, significantly more powerful radiation leads to injury. Laser exposure There is no focusing effect on the skin, so it is much less susceptible to injury at these wavelengths.

X-rays

Some high-voltage systems with voltages greater than 15 kV can generate X-rays of significant power: laser radiation, the sources of which are powerful electronically pumped ones, as well as plasma systems and ion sources. These devices must be tested to ensure proper shielding, among other things.

Classification

Depending on the power or energy of the beam and the wavelength of the radiation, lasers are divided into several classes. The classification is based on the device's potential to cause immediate injury to the eyes, skin, or fire when directly exposed to the beam or when reflected from diffuse reflective surfaces. All commercial lasers must be identified by markings applied to them. If the device was home-made or otherwise not marked, advice should be obtained regarding its appropriate classification and labeling. Lasers are distinguished by power, wavelength and exposure duration.

Secure Devices

First class devices generate low-intensity laser radiation. It cannot reach dangerous levels, so sources are exempt from most controls or other forms of surveillance. Example: laser printers and CD players.

Conditionally safe devices

Second class lasers emit in the visible part of the spectrum. This is laser radiation, the sources of which cause in humans a normal reaction of aversion to too bright light ( blink reflex). When exposed to beam human eye blinks after 0.25 s, which provides sufficient protection. However, laser radiation in the visible range can damage the eye with constant exposure. Examples: laser pointers, geodetic lasers.

Class 2a lasers are special-purpose devices with an output power of less than 1 mW. These devices only cause damage when directly exposed for more than 1000 seconds in an 8-hour work day. Example: barcode readers.

Dangerous lasers

Class 3a includes devices that do not cause injury during short-term exposure to an unprotected eye. May pose a hazard when using focusing optics such as telescopes, microscopes or binoculars. Examples: 1-5 mW helium-neon laser, some laser pointers and building levels.

A Class 3b laser beam can cause injury through direct exposure or specular reflection. Example: Helium-neon laser 5-500 mW, many research and therapeutic lasers.

Class 4 includes devices with power levels greater than 500 mW. They are dangerous to the eyes, skin, and are also a fire hazard. Exposure to the beam, its specular or diffuse reflections can cause eye and skin injuries. All safety measures must be taken. Example: Nd:YAG lasers, displays, surgery, metal cutting.

Laser radiation: protection

Each laboratory must provide adequate protection for persons working with lasers. Room windows through which radiation from a Class 2, 3, or 4 device may pass through causing harm in uncontrolled areas must be covered or otherwise protected while such device is operating. To ensure maximum eye protection, the following is recommended.

The bundle must be enclosed in a non-reflective, non-flammable protective enclosure to minimize the risk of accidental exposure or fire. To align the beam, use fluorescent screens or secondary sights; Avoid direct contact with eyes.
Use the lowest power for the beam alignment procedure. If possible, use low-class devices for preliminary alignment procedures. Avoid the presence of unnecessary reflective objects in the laser operating area.
Limit the passage of the beam into the danger zone during non-working hours using shutters and other barriers. Do not use room walls to align the beam of Class 3b and 4 lasers.
Use non-reflective tools. Some equipment that does not reflect visible light becomes mirrored in the invisible region of the spectrum.
Do not wear reflective jewelry. Metal jewelry also increases the risk of electric shock.

Safety glasses

Safety glasses should be worn when working with Class 4 lasers with an open hazardous area or where there is a risk of reflection. Their type depends on the type of radiation. Glasses should be selected to protect against reflections, especially diffuse reflections, and to provide protection to a level where the natural protective reflex can prevent eye injury. Such optical instruments will maintain some visibility of the beam, prevent skin burns, and reduce the possibility of other accidents.

Factors to consider when choosing safety glasses:

wavelength or region of the radiation spectrum;
optical density at a certain wavelength;
maximum illumination (W/cm2) or beam power (W);
type of laser system;
power mode - pulsed laser radiation or continuous mode;
reflection possibilities - specular and diffuse;
field of view;
the presence of corrective lenses or sufficient size to allow the wearing of glasses for vision correction;
comfort;
the presence of ventilation holes to prevent fogging;
influence on color vision;
impact resistance;
ability to perform necessary tasks.

Because safety glasses subject to damage and wear, the laboratory safety program should include periodic inspection of these protective features.

Expanding the spectral range of the laser. One of the main tasks of specialists developing laser devices is to create sources of coherent radiation, the wavelength of which can be tuned over the entire spectral range from the far infrared region to ultraviolet and even shorter wavelength radiation.

The creation of a dye laser turned out to be an extremely important event from this point of view, since their radiation can be tuned in a wavelength range beyond the visible region of the spectrum. However, there are significant gaps in the spectrum of laser radiation, i.e., regions in which known laser transitions are rare, and tuning their frequency is possible only in narrow spectral ranges.

The broad fluorescence bands on which the operation of a tunable dye laser is based are not detected in the far infrared region of the spectrum, and the dyes used in lasers are quickly destroyed by intense pump radiation when the dye is excited, when it is necessary to generate lasing in the ultraviolet region of the spectrum.

Nonlinear optics.

In search of ways to fill these gaps, many laser scientists have exploited nonlinear effects in some optical materials. In 1961, researchers from the University of Michigan focused the light of a ruby laser with a wavelength of 694.3 nm into a quartz crystal and detected in the radiation passed through the crystal not only the ruby laser light itself, but also radiation with a double frequency, i.e., at a wavelength of 347. 2 nm. Although this radiation was much weaker than at a wavelength of 694.3 nm, nevertheless, this short-wave radiation had the monochromaticity and spatial coherence characteristic of laser light.

The process of generating such short-wave radiation is known as frequency doubling, or second harmonic generation. SHG is one example of many nonlinear optical effects that have been used to expand the tunable spectral range of laser radiation. SHG is often used to convert 1.06 μm infrared radiation and other lines of a neodymium laser into radiation falling in the yellow-green region of the spectrum, such as 530 nm, in which only a small number of intense laser lines can be obtained.

Harmonic generation can also be used to produce radiation with a frequency three times higher than that of the original laser radiation. The nonlinear characteristics of rubidium and other alkali metals are used, for example, to triple the frequency of a neodymium laser to a value corresponding to a wavelength of 353 nm, i.e., falling in the ultraviolet region of the spectrum.

Theoretically, processes of generating harmonics higher than the third are possible, but the efficiency of such conversion is extremely low, so from a practical point of view they are of no interest. The possibility of generating coherent radiation at new frequencies is not limited to the process of harmonic generation. One such process is the process of parametric amplification, which is as follows.

Let a nonlinear medium be affected by three waves: a powerful light wave with a frequency of 1 pump wave and two weak ones light waves with lower frequencies 2 and 3. When condition 1 23 and the wave synchronism condition are met, the energy of a powerful wave with frequency 1 is transferred into the energy of waves with frequencies 2 and 3. If a nonlinear crystal is placed in an optical cavity, we obtain a device very reminiscent of a laser and called a parametric generator.

Such a process would be useful even if its use were limited to obtaining the differences between the frequencies of two existing ones. laser sources. In fact, a parametric oscillator is a device capable of generating coherent optical radiation, the frequency of which can be tuned in almost the entire visible range. This reason is that there is no need to use additional sources of coherent radiation at frequencies 2 and 3. These oscillations can themselves arise in the crystal from noise photons of thermal noise, which are always present in it.

These noise photons have wide range frequencies located predominantly in the infrared region of the spectrum. At a certain temperature of the crystal and its orientation relative to the direction of the pump wave and to the axis of the resonator, the above-mentioned condition of wave matching is satisfied for a certain pair of frequencies 2 and 3. To adjust the radiation frequency, it is necessary to change the temperature of the crystal or its orientation.

The operating frequency can be any of the two frequencies 2 and 3, depending on what frequency range of the device’s radiation is needed. Rapid frequency tuning in a limited spectral range can be obtained using electro-optical changes in the refractive indices of the crystal. As with a laser, there is a threshold pump power level that must be exceeded to obtain steady-state oscillations. Most parametric oscillators use visible lasers, such as an argon laser or the second harmonic of a neodymium laser, as a pump source.

The output of the device produces tunable infrared radiation. 2.

End of work -

This topic belongs to the section:

Dye laser

The emission parameters of a solid-state laser largely depend on the optical qualities of the crystal used. Inhomogeneities in the crystal structure can seriously limit.. At the same time, liquid lasers are not as bulky as gas systems and are easier to operate. Of the calculated types..

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