Laser radiation color and active medium material. What are X, K, Ka, Laser bands, what is POP

Lasers are becoming increasingly important research tools in medicine, physics, chemistry, geology, biology and engineering. If used incorrectly, they can cause blinding and injury (including burns and electrical shock) to operators and other personnel, including bystanders in the laboratory, as well as significant property damage. Users of these devices must fully 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 B 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 various types lasers operating in different environments. Gases are used as the working medium (for example, argon or a mixture of helium and neon), hard crystals(eg 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 low frequencies 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, mirror surface, leads to significantly more injuries powerful radiation. 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 significant power: laser radiation, the sources of which are powerful with electronic pumping, 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 can't reach dangerous level, 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 normal reaction too much rejection bright light (blink reflex). When exposed to the beam, the human eye blinks within 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 devices special purpose 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 workday. 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.

Class 3b laser beam may cause injury from direct exposure or mirror image. 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. The impact of the beam, its mirror or diffuse reflections may 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 in the danger zone in non-working hours using barriers 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.

Protective glasses

When working with class 4 lasers with open danger zone or where there is a risk of reflection, safety glasses should be used. 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 devices 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;
• line of sight;
• 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 are susceptible to damage and wear, the laboratory safety program should include periodic inspection of these safety features.

A laser is a generator of optical waves that uses the energy of induced emitting atoms or molecules in media with an inverse population of energy levels, which have the property of amplifying light of specific wavelengths. To amplify the light many times over, an optical resonator is used, which consists of 2 mirrors. Due to various pumping methods, an active medium is created in the active element.

Figure 1 - Laser device diagram

Due to the above conditions, a spectrum is generated in the laser, which is shown in Figure 2 (the number of laser modes is controlled by the length of the resonator):

Figure 2 - Spectrum of longitudinal laser modes

Lasers have high degree monochromaticity, a high degree of directionality and polarization of radiation with a significant intensity and brightness, a high degree of temporal and spatial coherence, can be rearranged in wavelengths, and can emit light pulses of record short duration, unlike thermal light sources.

Throughout the development of laser technologies, a large list of lasers and laser systems has been created that satisfy the needs with their characteristics. laser technology, including biotechnology. Due to the fact that the complexity of the design of biological systems and the significant diversity in the nature of their interaction with light determine the need to use many types of laser systems in photobiology, and also stimulate the development of new laser devices, including delivery systems laser radiation to the object of research or influence.

Like ordinary light, laser radiation is reflected, absorbed, re-emitted and scattered by the biological environment. All of the listed processes carry information about the micro and macro structure of an object, the movement and shape of its individual parts.

Monochromaticity is a high spectral density power of laser radiation, or significant temporal coherence of radiation, provides: spectral analysis with a resolution several orders of magnitude higher than that of traditional spectrometers; a high degree of selectivity for excitation of a certain type of molecules in their mixture, which is essential for biotechnology; implementation of interferometric and holographic methods for diagnosing biological objects.

Due to the fact that the laser beams are practically parallel, with increasing distance the light beam slightly increases in diameter. Listed properties A laser beam allows you to selectively influence different areas of biological tissue, creating a large energy or power density in a small spot.

Laser installations are divided into the following groups:

1) High power lasers on neodymium, carbon monoxide, carbon dioxide, argon, ruby, metal vapors, etc.;

2) Lasers with low-energy radiation (helium-cadmium, helium-neon, nitrogen, dyes, etc.), which do not have a pronounced thermal effect on body tissues.

Currently, there are laser systems that generate radiation in the ultraviolet, visible and infrared regions of the spectrum. The biological effects caused by laser radiation depend on the wavelength and dose of light radiation.

In ophthalmology they often use: excimer laser (with a wavelength of 193 nm); argon (488 nm and 514 nm); krypton (568 nm and 647 nm); helium-neon laser (630 nm); diode (810 nm); ND:YAG laser with frequency doubling (532 nm), also generating at a wavelength of 1.06 μm; 10-carbon dioxide laser (10.6 µm). The scope of laser radiation in ophthalmology is determined by the wavelength.

Laser installations receive their names in accordance with the active medium, and a more detailed classification includes solid-state, gas, semiconductor, liquid lasers and others. The list of solid-state lasers includes: neodymium, ruby, alexandrite, erbium, holmium; gases include: argon, excimer, copper vapor; to liquid ones: lasers that operate on dye solutions and others.

The revolution was made by the emerging semiconductor lasers due to their cost-effectiveness due to high efficiency (up to 60 - 80% as opposed to 10-30% for traditional ones), small size and reliability. At the same time, other types of lasers continue to be widely used.

One of the most important properties for the use of lasers is their ability to form a speckle pattern when coherent radiation is reflected from the surface of an object. Light scattered by the surface consists of chaotically located light and dark spots - speckles. The speckle pattern is formed based on the complex interference of secondary waves from small scattering centers that are located on the surface of the object under study. Due to the fact that the vast majority of biological objects under study have a rough surface and optical heterogeneity, they always form a speckle pattern and thereby introduce distortions into the final results of the study. In turn, the speckle field contains information about the properties of the surface under study and the near-surface layer, which can be used for diagnostic purposes.

In ophthalmic surgery, lasers are used in the following areas:

In cataract surgery: to destroy cataract accumulation on the lens and discision of the posterior capsule of the lens when it becomes clouded in the postoperative period;

In glaucoma surgery: when performing laser goniopuncture, trabeculoplasty, excimer laser removal of deep layers of the scleral flap, when performing a non-penetrating deep sclerectomy procedure;

In ophthalmic oncosurgery: to remove certain types of tumors located inside the eye.

The most important properties inherent in laser radiation are: monochromaticity, coherence, directionality, polarization.

Coherence (from the Latin cohaerens, connected, connected) is the coordinated occurrence in time of several oscillatory wave processes of the same frequency and polarization; a property of two or more oscillatory wave processes that determines their ability, when added, to mutually enhance or weaken each other. Oscillations will be called coherent if the difference in their phases remains constant throughout the time interval and when summing the oscillations, an oscillation of the same frequency is obtained. The simplest example two coherent oscillations - two sinusoidal oscillations of the same frequency.

Wave coherence implies that at different points of the wave oscillations occur synchronously; in other words, the phase difference between two points is not related to time. Lack of coherence means that the phase difference between two points is not constant and therefore changes over time. This situation occurs if the wave is generated not by a single radiation source, but by a group of identical, but independent from each other, emitters.

Often simple sources emit incoherent oscillations, while lasers emit coherent oscillations. By virtue of of this property laser radiation is focused as much as possible, it has the ability to interfere, is less susceptible to divergence, and has the ability to obtain a higher spot energy density.

Monochromaticity (Greek monos - one, only + chroma - color, paint) - radiation of one specific frequency or wavelength. The radiation can be conditionally accepted as monochromatic if it belongs to the spectral range of 3-5 nm. If there is only one allowed electron transition from the excited to the ground state, then it is created monochromatic radiation.

Polarization is symmetry in the distribution of the direction of the electric and magnetic field strength vector in an electromagnetic wave regarding the direction of its propagation. A wave will be called polarized if two mutually perpendicular components of the intensity vector electric field oscillate with a constant phase difference over time. Non-polarized - if changes occur chaotically. In a longitudinal wave, the occurrence of polarization is not possible, since disturbances in this type waves always coincide with the direction of propagation. Laser radiation is highly polarized light (from 75 to 100%).

Directionality (one of the most important properties laser radiation) - the ability of radiation to exit a laser in the form of a light beam with a very low divergence. This feature is the simplest consequence of the fact that the active medium is located in a resonator (for example, a plane-parallel resonator). In such a resonator, only electromagnetic waves propagating along the axis of the resonator or in close proximity to it are supported.

The main characteristics of laser radiation are: wavelength, frequency, energy parameters. These characteristics are biotropic, that is, they determine the effect of radiation on biological objects.

Wavelength ( l) represents the smallest distance between two adjacent oscillating points of the same wave. Often in medicine, wavelength is specified in micrometers (µm) or nanometers (nm). Depending on the wavelength, the reflection coefficient, the depth of penetration into body tissue, the absorption and biological effect of laser radiation change.

Frequency characterizes the number of oscillations performed per unit time and is the reciprocal of the wavelength. Typically expressed in hertz (Hz). As the frequency increases, the energy of the light quantum increases. They are distinguished: the natural frequency of radiation (for a single laser oscillation generator is unchanged); modulation frequency (in medical laser systems can vary from 1 to 1000 Hz). The energy parameters of laser irradiation are also of great importance.

It is customary to distinguish three main physical characteristics dosing: radiation power, energy (dose) and dose density.

Radiation power (radiation flux, radiant energy flux, R) -represents full energy, which is transferred by light per unit time through a given surface; average power electromagnetic radiation, which is transferred through any surface. Typically measured in watts or multiples.

Energy exposure (radiation dose, H) is the energy irradiation by the laser over a certain period of time; the power of an electromagnetic wave that is emitted per unit time. Measured in [J] or [W*s]. The ability to do work is the physical meaning of energy. This is typical when the work makes changes in tissue with photons. The biological effect of light irradiation is characterized by energy. In this case, the same biological effect occurs (for example, tanning) as in the case of sunlight, can be achieved with low power and exposure time or high power and short exposure. The effects obtained will be identical, with the same dose.

Dose density “D” is the energy received per unit area of ​​exposure. The SI unit is [J/m2]. A representation in units of J/cm 2 is also used, due to the fact that the areas affected are usually measured in square centimeters.

The oscillatory system of a laser contains an active medium, so the spectrum of laser radiation should be determined as spectral properties environment and the frequency properties of the resonator. Let us consider the formation of the emission spectrum in cases of inhomogeneous and uniform broadening of the spectral line of the medium.

Emission spectrum with non-uniform spectral broadening; lines. Let us consider the case when the shape of the spectral line of the medium is mainly determined by the Doppler effect, and the interaction of particles of the medium can be neglected. The Doppler broadening of the spectral line is inhomogeneous (see.§ 12.2).

In Fig. 15.10, a shows the frequency response of the resonator, and in Fig. Figure 15.10b shows the contour of the spectral line of the medium. Typically, the width of the spectral line with Doppler broadening ∆ ν = ∆ νD is much larger than the interval ∆ νq between the frequencies of neighboring resonator modes. The value ∆ νq, determined by formula (15.2), for example, with a resonator length L = 0.5 m will be 300 MHz, while the spectral line width due to the Doppler effect ∆ νD in accordance with formula (12.31) can be about 1 GHz. In this example, within the spectral linewidth of the medium∆ ν≈∆ νД; three longitudinal modes are placed. With a larger resonator length, the number of modes within the line width increases, since the frequency interval ∆ νq of neighboring modes decreases.

Doppler broadening is inhomogeneous, i.e. spontaneous emission in a selected frequency range less than ∆ νD is created by a certain group of particles, and not by all

particles of the environment. Let us assume that the natural spectral linewidth of a particle is significantly less than the difference in frequencies of neighboring modes (for example, the natural linewidth

neon is close to 16 MHz). Then particles that excite a certain mode with their spontaneous emission will not cause excitation of other modes.

To determine the laser radiation spectrum, we will use the frequency dependence of the absorption coefficient æ in Bouguer’s law (12.50). This indicator is proportional to the difference in populations of the upper and lower transition levels. In a medium without population inversion æ >0 and characterizes energy absorption electromagnetic field. In the presence of inversionæ<0 и определяет усиление поля. В этом случае модуль показателя называют показателем усиления active mediumæ a (æ a =|æ |).

The frequency dependence of the gain æ a (ν) in accordance with formula (12.44) coincides with the shape of the spectral line of the medium when the level populations are constant or change slightly as a result of forced transitions. Such a coincidence will be observed if a population inversion is created, and the conditions for self-excitation of the laser have not yet been met (for example, there are no cavity mirrors). In Fig. 15.10, the dotted line shows such an initial frequency dependence. With Doppler broadening of the spectral line, the dependence is expressed by a Gaussian function and has a width ∆ νD as shown in Fig. 15.10, b.

Let us assume that the self-excitation conditions are met. Then the spontaneous emission of one particle will cause forced transitions of other particles if the frequency of the spontaneous emission of the latter lies approximately within the natural width of the spectral line of the exciting particle. Due to population inversion, forced transitions from top to bottom will prevail, i.e., population top level should decrease, the lower one should increase, and the gain index æ a should decrease.

The field in the resonator is maximum at the resonant frequencies of the modes. At these frequencies the greatest change in the populations of the transition levels will be observed. Therefore, dips will appear on the æ a (ν) curve in the vicinity of the resonant frequencies (see Fig. 15.10, c).

After the self-excitation condition is met, the depth of the dip at resonant frequencies increases until the regime occurs; stationary oscillations, at which the gain index will become equal to the loss index α in accordance with condition (15.13). The width of each dip is approximately equal to the natural width of the particle line if the power generated at the frequency in question is small. The greater the power, and therefore the volumetric field energy density, which affects the number of forced transitions, the wider the gap. At low power, the gain within one notch is independent of the gain within another notch, since the notches do not overlap due to the initial assumption that the natural linewidth is less than the distance between the resonant frequencies. Oscillations at these frequencies can be considered independent. In Fig. Figure 15.10d shows that the laser radiation spectrum contains three emission lines corresponding to three longitudinal modes of the resonator. The radiation power of each mode depends on the difference between the initial and stationary values ​​of the gain index,

as in formula (15.21), i.e., it is determined by the depth of the corresponding dips in Fig. 15.10, at. We will determine the width of each emission line δν at the end of the section, and now we will discuss the effect of pump power on the number of generated modes for given losses.

If the pump power is so low that the maximum value of the medium gain (curve 1 in Fig. 15.11, b) does not reach the threshold value equal to α, then none of the modes determined by the frequency response of the resonator is excited (Fig. 15.11, a). Curve 2 corresponds to a higher pump power, which ensures that the central frequency of the spectral line of the medium ν0 exceeds the threshold value. This case corresponds to one dip in Fig. 15.11,c and generation of one longitudinal mode (Fig. 15.11,d). A further increase in the pump power will ensure that the self-excitation conditions are met for other modes (curve 3). Accordingly, dips in the indicator curve and the emission spectrum will be depicted as in Fig. 15.10, in Ig.

Emission spectrum with uniform broadening of the spectral line. A uniform broadening of the spectral line is observed in the case when the main cause of broadening is the collision | (or interaction) of particles of the medium(§ 12.2) .

Let us assume, as in the case of inhomogeneous broadening, that several natural frequencies of the resonator fall within the spectral line of the medium. In Fig. 15.12a shows the frequency response of the resonator, indicating the frequency and width of the resonance curves of each mode ∆ νp. Curve 1 in Fig. 15.12b depicts the frequency dependence of the gain index of a medium with population inversion before self-excitation of the laser.

The spectral line of each particle and the entire medium coincide with uniform broadening, therefore spontaneous emission of any particle can cause stimulated

transitions of other particles. Consequently, during forced transitions in the specified environment with population inversion, the frequency dependence of æ a during generation (curve 2) will remain the same in shape as before generation (curve 1), but will be located below it. The dips observed with inhomogeneous line broadening (see Fig. 15.11c) are absent here, since now all particles of the medium participate in creating the laser radiation power.

In Fig. 15.12, b, the self-excitation conditions æ a > α are satisfied for three modes with frequencies νq-1, νq = ν0 and νq+1. However, at the central frequency of the spectral line ν0, the gain per single passage of radiation through the active medium is maximum. As a result of a larger number of passages, the main contribution to the radiation power will come from the mode with the central frequency.

Thus, in lasers with uniform broadening of the spectral line of the medium, it is possible to obtain a single-frequency regime with high power (Fig. 15.12c), since, unlike the case of inhomogeneous broadening, a reduction in the pump power is not required to obtain this regime.

Monochromaticity of laser radiation. The generation of oscillations in any quantum devices begins with spontaneous emission, the frequency dependence of the intensity of which is characterized by the spectral line of the medium. However, in the optical range, the width of the spectral line of the medium is significantly greater than the width of the resonance curves ∆ νp of a passive (without active medium) resonator due to the high quality factor Q of the latter. Value ∆ νP =ν0 /Q, where ν0 is the resonant frequency. If there is an active medium in the resonator, losses are compensated (regenerative effect), which is equivalent to an increase in the quality factor and a decrease in the width of the resonance curve ∆ νp to the value δ ν.

In the case of generation of one mode with frequency ν0, the laser radiation linewidth can be estimated using the formula

where P is the radiation power. An increase in radiation power corresponds to greater

compensation of losses, increasing the quality factor and reducing the emission linewidth. If ∆ νp =l MHz, ν0 =5·1014 Hz, Р =1 mW, then δ νtheor ≈ 10-2 Hz, and the ratio δ νtheor /ν 0 ≈2·10-17. Thus, theoretical value the width of the emission line turns out to be extremely

small, many orders of magnitude smaller than the width of the resonance curves ∆ νp. However, in real conditions because of acoustic influences and temperature fluctuations, instability of the resonator dimensions is observed, leading to instability of the natural frequencies of the resonator and, consequently, the frequencies of the laser radiation lines. Therefore, the real (technical) radiation linewidth, taking into account this instability, can reach δ ν = 104 –105 Hz.

The degree of monochromaticity of laser radiation can be assessed by the width of the laser radiation line and the width of the envelope of the laser radiation spectrum containing several emission lines (see Fig. 15.10, d). Let ∆ ν=104 Hz, ν0 =5·1014 Hz, and the width of the spectrum envelope δ o.c .=300 MHz. Then the degree of monochromaticity along one line will be δ ν/ν0 ≈ 2·10-11, and along the envelope δ ν/ν0 ≈ 6·10-7. The advantage of lasers is the high monochromaticity of the radiation, especially along one radiation line, or in a single-frequency operating mode

§ 15.4. Coherence, monochromaticity and directionality of laser radiation

IN When applied to optical vibrations, coherence characterizes the connection (correlation) between the phases of light vibrations. There are temporal and spatial coherence, which in lasers are associated with monochromaticity and directionality of radiation.

IN In the general case, when the correlation of radiation fields is studied at two points in space, respectively, at times shifted by a certain amount τ, the concept of mutual coherence function is used

where r 1 and r 2 are the radius vector of the first and second points; E 1 (r 1,t+ τ) and E* 2 (r 2, t) are the complex and complex conjugate values ​​of the field strength at these points. The normalized mutual coherence function characterizes the degree of coherence:

where I (r 1) and I (r 2) are the radiation intensity at selected points. Module γ 12 (τ) varies from zero to one. When γ 12 τ =0 there is no coherence, in the case of |γ 12 (τ )|=l there is complete coherence

Temporal coherence and monochromaticity of radiation. Temporal coherence is the correlation between field values ​​at one point in space at moments of time that differ by a certain amountτ. In this case, the radius vectors r 1 and r 2 in determining the mutual coherence function Г 12 (r 1, r 2, τ) and functions γ 12 (τ ) turn out to be equal, the mutual coherence function turns into an autocorrelation function, and the normalized function turns into a functionγ 11 (τ ), characterizing the degree of temporal coherence.

It was previously noted that during spontaneous transitions, the atom emits trains of vibrations that are not related to each other (Fig. 15.13). The correlation of oscillations at one point in space will be observed only in a time interval shorter than the duration of the train. This interval is called coherence time, and it is taken equal to the lifetime of spontaneous transitions m. The distance traveled by light during the coherence time is called coherence length£. At τ ≈ 10-8 с £ =c τ =300 cm. The coherence length can also be expressed through the width of the spectral line ∆ ν. Since ∆ ν≈ 1/τ, then £ ≈ c /∆ ν.

Temporal coherence and monochromaticity are related. Monochromaticity is quantitatively determined by the degree of monochromaticity ∆ ν/ ν0 (see § 15.3). The higher the degree of temporal coherence, i.e. the longer the coherence time, the less frequency spectrum∆ ν occupied by radiation and better monochromaticity. In the limit, with complete time coherence (τ →∞), the radiation became completely monochromatic (∆ ν→0).

Let us consider the temporal coherence of laser radiation. Let us assume that a certain particle of the active medium has emitted a quantum, which we will represent in the form of a train of oscillations (see Fig. 15.13). When a train interacts with another particle, a new train will appear, the phase of oscillations of which, due to the nature of forced transitions, coincides with the phase of oscillations of the original train. This process is repeated many times, while the phase correlation is maintained. The resulting oscillation can be considered as a train with a duration significantly greater than the duration of the initial train. Thus, the coherence time increases, i.e., the temporal coherence and monochromaticity of the radiation improves.

In connection with this consideration, it becomes obvious that an optical resonator increases the temporal coherence of laser radiation, since it ensures repeated passage of trains through the active medium. The latter is equivalent to an increase in the lifetime of the emitters, an increase in temporal coherence and a decrease in the linewidth

Spatial coherence and directionality of radiation, Spatial coherence is the correlation between field values ​​at two points in space at the same point in time. In this case, the formulas for the mutual coherence function Г 12 (r 1 ,r 2 , τ ) and normalized coherence functionγ 12 (τ ) should be substitutedτ =0. Function γ 12 (0) characterizes the degree of spatial coherence.

Radiation from a point source is always spatially coherent. The degree of spatial coherence of an extended source depends on its size and the distance between it and the observation points. From optics it is known that what larger size source, the smaller the angle within which the radiation can be considered spatially coherent. A light wave with the best spatial coherence should have a flat front.

In lasers, the radiation has a high directivity (flat front), determined by the properties of the optical cavity. The self-excitation condition is satisfied only for a certain direction in the resonator for the optical axis or directions close to it. As a result, very large number reflections from mirrors, the radiation travels a long way, which is equivalent to an increase in the distance between the source and the observation point. This path corresponds to the coherence length and can be tens of kilometers for gas lasers. The high directivity of laser radiation also determines high spatial coherence. It is significant that the effect of increasing distance in a laser is accompanied by an increase in radiation power due to its amplification in the active medium, whereas in conventional sources an improvement in spatial coherence is associated with a loss of light intensity.

The high degree of temporal coherence of radiation determines the use of lasers in systems for transmitting information, measuring distances and angular velocities, in quantum frequency standards. A high degree of spatial coherence (directivity) makes it possible to efficiently transmit light energy and focus the light flux into a spot of a very small size, comparable to the wavelength. This makes it possible to obtain enormous values ​​of energy density, field strength and light pressure necessary for scientific research and various technical applications.

Attention! Precautionary measures:

Do not direct laser radiation into your eyes! Direct contact with laser radiation into the eyes is dangerous for vision!

With the permission of the work supervisor, turn on the laser and install the screen and grid so that diffraction pattern was the clearest.

Changing the distance L, see how this affects the position of the maxima. Describe and sketch what you observed.

Place the diffraction grating at a certain distance L from the slot and measure the distances l 1 and l 2 (see Fig. 9.3) for first-order maxima. Calculate the wavelength of the laser radiation. Evaluate the absolute and relative error measurement, record the result for the laser wavelength.

Task 2.Determination of wavelengths of some colors of the spectrum

In this task, the light source is an incandescent lamp, which produces a continuous spectrum.

Measurements in task 2 carried out in accordance with the instructions at the workplace. The measurement results are entered into the table. 9.1. Distances should be determined l 1 and l 2 for each color four times: with two values k and two different distances L.

Table 9.1

Analysis and processing of measurement results

1. Describe the observed spectrum in the report, give an explanation for the fact that the maxima have such a significant width.

2. Fill out the table completely. 9.1. Constant value d get it at your workplace . Describe the picture you observe in the report. Make processing tables for each color and record final result according to general rules.

3. Compare the wavelength values ​​you obtained for each color with those given in the table. P. ...

Control questions

1. Define: wave diffraction, the Geygens-Fresnel principle, wave coherence. A written response to this question must be included in the report.

2. Name the components of the laboratory setup and their purpose.

3. What quantities are measured directly in this work? Which ones are calculated?

4. What is the phenomenon of light diffraction? Under what conditions is it observed?

5. What is a diffraction grating and what are its main parameters?

6. Derive the formula diffraction grating (9.3).

7. Define wavelength. How is it related to the frequency of light?

8. In what wavelength range does visible light lie?

9. Output and write calculation formulas to determine wavelengths visible light using a diffraction grating.

10. How does the angle of deviation of the diffraction maximum depend on the wavelength and grating period?

11. In what order are the colors of the diffraction maxima located from the central maximum? Explain the observed color order.

12.What is the difference between laser radiation and natural light?

Work No. 10. STUDYING LIGHT POLARIZATION

Goal of the work: investigate the passage of light through polaroids, check Malus's law, evaluate the quality of polaroids, investigate the polarization of light passing through several glass plates.

Equipment: optical bench, light source, polarizer in a frame, analyzer combined with a photocell, set of glass plates, power source, microammeter.

Brief theory

From Maxwell's theory it follows that light wave is transverse. The cross-section of light waves (as well as any other electromagnetic waves) is expressed in the fact that the oscillations of the vectors and are perpendicular to the direction of wave propagation (Fig. 10.1). Plane monochromatic wave propagating in vacuum along the axis x, is described by the equations:

;	(10.1)
,	(10.2)

where and are the current values ​​of the electric and magnetic field strengths; and – amplitudes of oscillations, w – frequency of oscillations, – initial phase hesitation.

When light interacts with matter, an alternating electric field acts on the negatively charged electrons of atoms and molecules of this substance, while the effect of the magnetic field on charged particles is insignificant. Therefore, in the processes of light propagation, the vector plays the main role, and in the future we will only talk about it.

Polarization of light is observed when light passes through anisotropic substances. The main property of such substances is that they can only pass through those light waves, in which vectors oscillate only in a strictly defined plane, which is called plane of oscillation. The plane in which the magnetic field is localized is called plane of polarization. In Fig. 10.1 the plane of oscillation is vertical, and the plane of polarization is horizontal.

To obtain and study polarized light, they are most often used polaroids. They are made from very small crystals of tourmaline or geropatite (iodine-quinine sulfate), applied to transparent film or glass. However, there are other ways to obtain plane-polarized light from natural light, for example, by reflection from a dielectric at a certain angle, depending on the refractive index of the dielectric. This method will be discussed in more detail below.

Let us mentally carry out the following experiment. Let's take two polaroids and a light source (Fig. 10.3). The first Polaroid is called polarizer, because it polarizes light. Its plane of oscillation is the plane PPS. After passing through the polarizer, the vector will oscillate only in this plane. By rotating the polarizer around the direction of the light beam, we will not notice any changes in the intensity of the light passing through it. Think why? Analysis of light for polarization is done using a second polaroid through which the light being tested is passed. In this case, the second polaroid is called analyzer, its plane of polarization is the plane AAc. By rotating the analyzer, we will notice that the intensity of the light passing through it will be maximum if the plane PPS And AAc coincide, and minimal if these planes are perpendicular. If these planes make a certain angle a (see Fig. 10.3), then the light intensity behind the analyzer will take an intermediate value.

Let's find the relationship between angle a and intensity I light passing through both polaroids. Let us denote the amplitude electric vector beam passing through the polarizer, letter E 0 . Analyzer oscillation plane AAc rotated relative to the polarizer oscillation plane PPS by angle a (see Fig. 10.4). Let us decompose the vector into components: parallel to the plane of oscillation of the analyzer кк and perpendicular to it ^. The parallel component кк will pass through the analyzer, but the perpendicular component ^ will not.

From Fig. 10.4 it follows that the amplitude of the light wave behind the analyzer

Where S– area over which energy is distributed; t- time. Since light energy is the total energy of electric and magnetic fields, its value is proportional to the squares of the strengths of these fields:

The resulting equality is called Malus's law: the intensity of light passing through the analyzer is equal to the intensity of light passing through the polarizer multiplied by the square of the cosine of the angle between planes of polarization analyzer and polarizer.

Note that the light passing through the polarizer will not only become plane polarized, but will also reduce its intensity by half. If the intensity of natural light is considered the same in all directions perpendicular to the velocity vector, then the intensity of light behind the polarizer

Where I max and I min – the highest and lowest light intensities behind the analyzer, corresponding to the voltages E max and E min in Fig. 10.2, c.

The phenomenon of polarization can also be observed when light is reflected or refracted at the interface of two isotropic dielectrics. In this case, the reflected beam will be dominated by vibrations perpendicular to the plane of incidence (they are indicated by dots in Fig. 10.5). It has been experimentally shown that the degree of polarization in the reflected beam depends on the angle of incidence, and as the angle of incidence increases, the proportion of polarized light increases, and at a certain value, the reflected light turns out to be completely polarized. Brewster found that the magnitude of this angle of total polarization depends on the relative refractive index and is determined by the relation:

The relationship is called Brewster's law, and angle a B is called Brewster's angle. With a further increase in the angle of incidence, the degree of polarization of light decreases again. Thus, at an angle of incidence equal to the Brewster angle, the reflected light is linearly polarized in the plane, perpendicular to the plane falls. Using (10.9) and the law of refraction, it can be shown that when incident at the Brewster angle, the reflected and refracted rays are 90°. Check it!.

When light is incident at the Brewster angle, the refracted beam is also polarized. The refracted beam will be dominated by vibrations, parallel planes falls (in Fig. 10.5 they are indicated by arrows). The polarization of refracted rays at this angle of incidence will be maximum, but far from complete. If you subject the refracted rays to the second, third, etc. refraction, the degree of polarization will increase. Therefore, 8–10 plates can be used to polarize light (the so-called Stoletov’s foot). The light passing through them will be almost completely polarized. Thus, this foot can serve as a polarizer or analyzer. In our setup, sets of 2–12 plates are used as a polarizer.

Description of installation

To study polarization, a setup mounted on an optical bench is used, the diagram of which is shown in Fig. 10.6.

The numbers on the diagram indicate: 1– lamp, 2 – removable polarizer, 3 – rotary table, 4 – glass plate set, put on the pins of the turntable, 5 – analyzer, 6 – photocell, 7 – meter light intensity (IIS), which converts light energy into an electrical signal; its readings are proportional to the luminous flux incident on the photocell. The turntable 3 can rotate around a vertical axis, thereby changing the angle of incidence of light on the glass plate 4. There is a special scale for measuring this angle of incidence. The position of the table is fixed with a screw. 5 Analyzer can rotate around horizontal axis, the arrow on it indicates the position of the plane of polarization. The analyzer has scale 8, which determines the position of its plane of polarization ( AAc). The removable polarizer 2 also has a vertical arrow that shows the position of its plane of polarization PPS. The photocell combined with the analyzer can also rotate around a vertical axis. This makes it possible to measure the intensity of light reflected from the set of plates 4.

Completing of the work

Exercise 1 . Checking Malus's Law

1. Install a removable polarizer 2 (remove the set of plates 4).

2. Turn on the lamp. Rotate the photocell-analyzer 6 so that the light from the lamp falls on it. Achieve a symmetrical arrangement of installation elements relative to the light beam.

3. Set the plane position AAc on a scale of 8 at 0°. Record the readings of meter 7 in the table. 10.1. This will be the intensity of the light passing through the polarizer and analyzer in relative units. Repeat the measurements, changing the angle between the polarization planes of the polarizer and analyzer from 0° to 360° every 10°, and also write them down in the table. 10.1.

Table 10.1

Task 2. Study of polarization of refracted light

1. Install the removable plate with two glasses ( N = 2).

2. Set the angle of incidence of light on the plate to 56° (this is the Brewster angle for glass with a refractive index n = 1,5).

3. Install a photocell to record the intensity of light passing through the plates according to Fig. 10.7 (the maximum value of the IIS readings confirms good light penetration into the photocell).

4. Please note that refracted light is polarized in the plane of incidence, so the maximum intensity value will be at position AAc 90° on a scale of 8 (questions 12, 13, 14). Measure the intensity of light transmitted through the plates at two positions AAc: at 90° and at 0°. Record the measurement results in the table. 10.2.

5. Carry out similar measurements for N= 4, 7, 12 plates. Record the measurement results in the table. 10.2.

Table 10.2

Related information.

1.1. Types of spectra.

At first glance, the laser beam seems very simple in structure. This is practically single-frequency radiation that has a spectrally pure color: the He-Ne laser has red radiation (633 nm), the cadmium laser emits Blue colour(440 nm, an argon laser emits several lines in the blue-green region of the spectrum (488 nm, 514 nm, etc.), a semiconductor laser emits red radiation (650 nm), etc. In fact, the laser emission spectrum has quite complex structure and is determined by two parameters - the emission spectrum of the working substance (for a He-Ne laser, for example, this is the red spectral line of neon excited electrical discharge) and resonance phenomena in the optical cavity of the laser.

For comparison, the figures on the right show the emission spectra of the sun (A) and a conventional incandescent light bulb (B) (top picture), the spectrum of a mercury lamp (picture right) and a greatly enlarged emission spectrum of a He-Ne laser (bottom picture).

The spectrum of an incandescent lamp, like the solar spectrum, refers to continuous spectra that are completely filled by the visible spectral range of electromagnetic radiation (400-700 nm). The spectrum of a mercury lamp belongs to the line spectra, which also fills the entire visible range, but consists of individual spectral components of varying intensities. By the way, before the advent of lasers, monochromatic radiation was obtained by isolating individual spectral components of the radiation from a mercury lamp.

1.2. Emission spectrum in a He-Ne laser.

The laser radiation spectrum is monochromatic, that is, it has a very narrow spectral width, but, as can be seen from the figure, it also has a complex structure.

Formation process laser spectrum Let's consider based on the well-studied He-Ne laser. Historically, it was the first continuous laser operating in the visible range of the spectrum. It was created by A. Javan in 1960.

In Fig. on the right are the energy levels of an excited mixture of helium and neon. An excited helium or neon atom is an atom that has one or more outer shell electrons in collisions with electrons and ions gas discharge move to higher energy levels and may subsequently move to a lower energy level or return back to a neutral level by emitting light quantum- photon.

Atoms are excited by an electric current passing through a gas mixture. For a He-Ne laser, this is a low-current, glow discharge (typical discharge currents are 20-50 mA). Painting energy levels and the radiation mechanism are quite complex even for such a “classical” laser, which is the He-Ne laser, so we will limit ourselves to considering only the main details of this process. Helium atoms excited to the 2S level in collisions with neon atoms transfer the accumulated energy to them, exciting them to the 5S level (therefore, helium in gas mixture more than neon). From the 5S level, electrons can move to a number of lower energy levels. We are only interested in the 5S - 3P transition (both levels are actually split into a number of sublevels due to the quantum nature of the excitation and emission mechanisms). The wavelength of photon emission during this transition is 633 nm.

Let's note one more important fact, fundamentally important for obtaining coherent radiation. With the correct proportions of helium and neon, the pressure of the gas mixture in the tube and the value of the discharge current, electrons accumulate at the 5S level and their number exceeds the number of electrons located at the lower 3P level. This phenomenon is called level population inversion. However, this is not laser radiation yet. This is one of the spectral lines in the neon emission spectrum. The width of the spectral line depends on several reasons, the main of which are: - the finite width of the energy levels (5S and 3P) involved in the radiation and determined by the quantum uncertainty principle associated with the residence time of neon atoms in the excited state, - line broadening associated with constant movement excited particles in a discharge under the influence of an electric field (the so-called Doppler effect). Taking these factors into account, the width of the line (experts call it the contour of the working transition) is approximately two ten thousandths of an angstrom. For such narrow lines in calculations it is more convenient to use its width in frequency domain. Let's use the transition formula:

dn 1 =dl c/l 2 (1)

where dn 1 is the width of the spectral line in the frequency domain, Hz, dl is the width of the spectral line (0.000002 nm), l is the wavelength of the spectral line (633 nm), c is the speed of light. Substituting all values ​​(in one measurement system), we obtain a line width of 1.5 GHz. Of course, such a narrow line can be considered completely monochromatic in comparison with the entire spectrum of neon radiation, but this cannot yet be called coherent radiation. To obtain coherent radiation, the laser uses an optical cavity (interferometer).

1.3. Laser optical cavity.

An optical resonator consists of two mirrors located on the optical axis and facing each other with reflective surfaces, Fig. on right. Mirrors can be flat or spherical. Flat mirrors are very difficult to align and laser output can be unstable. Resonator with spherical mirrors(confocal resonator) is much more stable, but the laser beam may be inhomogeneous across the cross-section due to the complex, multimode composition of the radiation. In practice, a semi-confocal resonator with a rear spherical and front flat mirror is most often used. Such a resonator is relatively stable and produces a homogeneous (single-mode) beam.

The main property of any resonator is the formation of standing electromagnetic waves in it. In the case of a He-Ne laser, standing waves are generated to emit a neon spectral line with a wavelength of 633 nm. This is facilitated by the maximum reflection coefficient of the mirrors, selected just for this wavelength. Laser cavities use dielectric mirrors with multilayer coating, allowing a reflection coefficient of 99% or higher. As is known, the condition of formation standing waves is that the distance between the mirrors must be equal to an integer number of half-waves:

nl =2L (2)

where n is an integer or order of interference, l is the wavelength of radiation inside the interferometer, L is the distance between the mirrors.

From the resonance condition (2) we can obtain the distance between the resonant frequencies dn 2:

dn 2 =c/2L (3)

For a one and a half meter gas laser cavity (He-Ne laser LGN-220) this value is approximately 100 MHz. Only radiation with such a frequency period can be repeatedly reflected from the resonator mirrors and amplified as it passes through an inverse medium - a mixture of helium and neon excited by an electric discharge. Moreover, what is extremely important, when this radiation passes along the resonator, its phase structure does not change, which leads to coherent properties of the amplified radiation. This is facilitated by the inverse population of the 5S level, which was mentioned above. An electron moves from the upper level to the lower level synchronously with the photon initiating this transition, therefore the phase parameters of the waves corresponding to both photons are the same. This generation of coherent radiation occurs along the entire radiation path inside the resonator. In addition, resonant phenomena lead to a much greater narrowing of the emission line, resulting in the greatest gain being obtained at the center of the resonant peak.
After a certain number of passes, the intensity of coherent radiation becomes so high that it exceeds the natural losses in the resonator (scattering in the active medium, losses on mirrors, diffraction losses, etc.) and part of it goes beyond the resonator. A day off for this, flat mirror made with a slightly lower reflectance (99.6-99.7%). As a result, the laser emission spectrum has the form shown in the third Fig. above. The number of spectral components usually does not exceed ten.

Let us summarize once again all the factors that determine the frequency characteristics of laser radiation. First of all, the working transition is characterized by the natural width of the contour. In real conditions due to various factors the outline widens. Within the broadened line, the resonant lines of the interferometer are located, the number of which is determined by the width of the transition contour and the distance between adjacent peaks. Finally, at the center of the peaks are extremely narrow spectral lines of laser emission, which determine the spectrum of the laser output.

1.4. Coherence of laser radiation.

Let us clarify what coherence length is provided by the He-Ne laser radiation. Let's use the formula proposed in the work:

as it passes through an inverse medium - a mixture of helium and neon excited by an electric discharge. Moreover, what is extremely important, when this radiation passes along the resonator, its phase structure does not change, which leads to coherent properties of the amplified radiation. This is facilitated by the inverse population of the 5S level, which was mentioned above. An electron moves from the upper level to the lower level synchronously with the photon initiating this transition, therefore the phase parameters of the waves corresponding to both photons are the same. This generation of coherent radiation occurs along the entire radiation path inside the resonator. In addition, resonant phenomena lead to a much greater narrowing of the emission line, resulting in the greatest gain being obtained at the center of the resonant peak.

dt =dn -1 (4)

where dt is the coherence time, which represents the upper limit of the time interval over which the amplitude and phase of the monochromatic wave are constant. Let's move on to the coherence length l that is familiar to us, with the help of which it is easy to estimate the depth of the scene recorded on the hologram:

l=c/dn (5)

Substituting the data into formula (5), including the full spectrum width dn 1 = 1.5 GHz, we obtain a coherence length of 20 cm. This is quite close to the real coherence length of a He-Ne laser, which has inevitable radiation losses in the cavity. Measurements of the coherence length using a Michelson interferometer give a value of 15-17 cm (at the level of a 50% decrease in the amplitude of the interference pattern). It is interesting to estimate the coherence length of an individual spectral component isolated by the laser cavity. The width of the resonant peak of the interferometer dn 3 (see the third figure from the top) is determined by its quality factor and is approximately 0.5 MHz. But, as mentioned above, resonance phenomena lead to an even greater narrowing of the laser spectral line dn 4, which is formed near the center of the resonant peak of the interferometer (third from the top in the figure). Theoretical calculation gives a line width of eight thousandths of a hertz! However, this value does not have much practical meaning, since the long-term existence of such a narrow spectral component requires values ​​of the mechanical stability of the resonator, thermal drift and other parameters that are absolutely impossible under real operating conditions of the laser. Therefore, we will limit ourselves to the width of the resonant peak of the interferometer. For a spectrum width of 0.5 MHz, the coherence length calculated using formula (5) is 600 m. This is also very good. All that remains is to isolate one spectral component, evaluate its power and keep it in one place. If, during the exposure of the hologram, it “passes” along the entire working circuit (due, for example, to the temperature instability of the resonator), we will again obtain the same 20 cm of coherence.

1.5. Spectrum of ion laser generation.

Let's talk briefly about the generation spectrum of another gas laser - argon. This laser, like the krypton laser, belongs to ion lasers, i.e. in the process of generating coherent radiation, it is no longer argon atoms that participate, but their ions, i.e. atoms, one or more electrons of the outer shell of which are torn off under the influence of a powerful arc discharge that passes through the active substance. The discharge current reaches several tens of amperes, the electrical power of the power supply is several tens of kilowatts. Intensive water cooling of the active element is necessary, otherwise its thermal destruction will occur. Naturally, under such harsh conditions, the picture of excitation of argon atoms is even more complex. Several laser lasers are generated at once. spectral lines, the width of the working contour of each of them is significantly greater than the width of the He-Ne laser line contour and amounts to several gigahertz. Accordingly, the laser coherence length is reduced to several centimeters. To record large format holograms, frequency selection of the generation spectrum is required, which will be discussed in the second part of this article.

1.6. Generation spectrum of a semiconductor laser.

Let us move on to consider the emission spectrum of a semiconductor laser, which is of great interest for the process of teaching holography and for beginning holographers. Historically, injection semiconductor lasers based on gallium arsenide were the first to be developed, Fig. on right.

Since its design is quite simple, let us consider the principle of operation of a semiconductor laser using its example. The active substance in which radiation is generated is a single crystal of gallium arsenide, which has the shape of a parallelepiped with sides several hundred microns long. The two side faces are made parallel and polished with a high degree of precision. Due to large indicator refraction (n = 3.6), at the crystal-air interface it turns out quite large coefficient reflection (about 35%), which is sufficient to generate coherent radiation without additional deposition of reflecting mirrors. The other two faces of the crystal are beveled at a certain angle; induced radiation does not escape through them. The generation of coherent radiation occurs in the p-n junction, which is created by the diffusion of acceptor impurities (Zn, Cd, etc.) into the region of the crystal doped with donor impurities (Te, Se, etc.). Thickness of the active region perpendicular to p-n junction direction is about 1 µm. Unfortunately, in this design of a semiconductor laser, the threshold pump current density turns out to be quite high (about 100 thousand amperes per 1 sq. cm.). Therefore, this laser is instantly destroyed when operated in continuous mode at room temperature and requires strong cooling. The laser operates stably at temperatures liquid nitrogen(77 K) or helium (4.2 K).

Modern semiconductor lasers are made on the basis of double heterojunctions, Fig. on right. In such a structure, the threshold current density was reduced by two orders of magnitude, to 1000 A/cm. sq. At this current density it is possible stable work semiconductor laser and at room temperature. The first laser samples operated in the infrared range (850 nm). With further improvement of the technology for forming semiconductor layers, lasers with an increased wavelength (1.3 - 1.6 µm) appeared for fiber optic lines connection, and with the generation of radiation in the visible region (650 nm). There are already lasers that emit in the blue region of the spectrum. The big advantage of semiconductor lasers is their high efficiency (ratio of radiation energy to electrical energy pumping), which reaches 70%. For gas lasers, both atomic and ion, the efficiency does not exceed 0.1%.

Due to the specific nature of the radiation generation process in a semiconductor laser, the width of the radiation spectrum is much greater than the width of the He-Ne laser spectrum, Fig. on right.

The width of the working contour is about 4 nm. Number spectral harmonics can reach several dozen. This imposes a serious limitation on the laser coherence length. If we use formulas (1), (5), the theoretical coherence length will be only 0.1 mm. However, as shown by direct measurements of the coherence length on a Michelson interferometer and recording of reflective holograms, the real coherence length of semiconductor lasers reaches 4-5 cm. This suggests that the real emission spectrum of a semiconductor laser is not so rich in harmonics and does not have such a large contour width worker transition, as theory predicts. However, in fairness, it is worth noting that the degree of coherence of semiconductor laser radiation varies greatly both from sample to sample and from its operating mode (pump current value, cooling conditions, etc.