Spectral composition of lasers radiation. Spectrum of semiconductor laser generation

The first principle of the laser, whose physics was based on the Radiation Law of the Planck, theoretically substantiated Einstein in 1917. It described the absorption, spontaneous and forced electromagnetic radiation using probabilistic coefficients (Einstein coefficients).

Pioneer

Theodore Meiman was the first to demonstrate the principle of operation based on optical pumping using a synthetic rubbing lamp, which produced a pulsed coherent radiation with a wavelength of 694 nm.

In 1960, Iranian scientists Janesan and Bennett created the first gas quantum generator using a mixture of HE and NE gases in a ratio of 1:10.

In 1962, R. N. Hall demonstrated the first of Galliya Arsenide (GaAs), radiated at a wavelength of 850 nm. Later, in the same year, Nick Golonak developed the first semiconductor quantum generator visible light.

Device and principle of lasers

Each laser system consists of an active medium placed between a pair of optically parallel and high-reflecting mirrors, one of which is translucent, and an energy source for it pumping. As an amplification environment can perform solid, Liquid or gas that have a property to enhance the amplitude of the light wave passing through it, forced radiation with electrical or optical pumping. The substance is placed between a pair of mirrors in such a way that the light that reflects in them, every time passes through it and, having achieved significant amplification, penetrates through the translucent mirror.

Duplex environments

Consider the principle of the laser with the active medium, the atoms of which have only two levels of energy: an excited E 2 and the basic E 1. If atoms with any pumping mechanism (optical, electrical discharge, current transmission or bombardment of electrons) are excited to the state E 2, then through several nanoseconds they will return to the ground position, radiating the photons of the energy Hν \u003d E 2 - E 1. According to Einstein theory, emissions are produced by two different ways: either it is induced by a photon, or it happens spontaneously. In the first case, there is a forced radiation, and in the second - spontaneous. With thermal equilibrium, the probability of forced radiation is significantly lower than spontaneous (1:10 33 33), therefore, most of the usual light sources are incoherent, and laser generation is possible in conditions other than thermal equilibrium.

Even with a very strong pumping, the population of two-level systems can only be made to equal. Therefore, three- or four-level systems are required to achieve an inverse population to achieve an optical or other pumping.

Multi-level systems

What is the principle of action of a three-level laser? Irradiation intensive frequency light ν 02 pumped a large number of Atoms from the lowest energy level E 0 to the upper E 2. The non-radically transition of atoms with E 2 to E 1 establishes the inversion of the population between E 1 and E 0, which in practice it is possible only when atoms for a long time There are in the metastable state E 1, and the transition from E 2 to E 1 occurs quickly. The principle of operation of a three-level laser consists in performing these conditions, due to which the inversion of the population is achieved between E 0 and E 1 and photons are reinforced with the energy E 1 -e 0 induced radiation. The wider level E 2 could increase the wavelength absorption range for more efficient pumping, which is the growth of forced radiation.

The three-level system requires very high pump power, as the lower level involved in generation is basic. In this case, in order to occur in the inversion of the population, to the state of E 1, more than half of total Atoms. At the same time, the energy isted inust. Power power can be significantly reduced if the lower generation level is not basic, which requires at least a four-level system.

Depending on the nature of the active substance, the lasers are divided into three main categories, namely, solid, liquid and gas. Since 1958, when the generation of Rubin crystal was first observed, scientists and researchers studied a wide range of materials in each category.

Solid state laser

The principle of operation is based on the use of an active medium, which is formed by adding a transition group to an insulating crystal grid (Ti +3, Cr +3, V +2, CO +2, Ni +2, Fe +2, etc.) to an insulating crystal lattice , rare-earth ions (CE +3, PR +3, ND +3, PM +3, SM +2, EU + 2, + 3, TB +3, DY +3, HO +3, ER +3, YB +3 , etc.), and actinides like U +3. ions are responsible for generation only. Physical properties basic material, such as thermal conductivity and are important for efficient work laser. The location of the lattice atoms around the doped ion changes its energy levels. Different generation wavelengths in the active medium are achieved by doping various materials by the same ion.

Golmian laser

An example is a quantum generator in which the Golmia replaces the atom of the base substance. crystal lattice. Ho: YAG is one of the best generation materials. The principle of operation of the Holmium laser is that the aluminum grenade is doped by ions of the golmia, optically pumped by a flash lamp and radiates a wavelength of 2097 nm in an IR range, well absorbed tissues. This laser is used for joint operations, in the treatment of teeth, for evaporation cancer cells, renal and bile stones.

Semiconductor quantum generator

Lasers on quantum pits are inexpensive, allow mass production and easily scalable. The principle of action of the semiconductor laser is based on the use of a diode with a P-N-transition, which produces the light of a certain wavelength by recombining the carrier with a positive displacement, like LEDs. LED emit spontaneously, and laser diodes are forced. To perform the inversion condition of the population, the operating current must exceed the threshold value. The active medium in the semiconductor diode has the form of the connecting area of \u200b\u200bthe two two-dimensional layers.

Principle of laser action this type This is what no external mirror is required to maintain the oscillations. The reflectivity created by layers and the internal reflection of the active medium is sufficient for this purpose. The end surfaces of the diodes are cleaned, which ensures parallelism of reflective surfaces.

The compound formed by the same type is called a homochorter, and the two different - hetero-generated compound.

Semiconductors P and N type with high density The carriers form a p-n-transition with a very subtle (≈1 μm) layer.

Gas laser.

The principle of operation and using a laser of this type allows you to create devices of almost any power (from millivatt to megawatt) and wavelengths (from UV to IR) and allows you to work in pulse and continuous modes. Based on the nature of active media, three types of gas quantum generators differ, namely atomic, ionic, and molecular.

Most gas lasers are pumped up with an electric discharge. Electrons in the discharge tube are accelerated by an electric field between the electrodes. They face atoms, ions or active medium molecules and induce the transition to higher energy levels to achieve the state of the population of inversion and forced radiation.

Molecular laser.

The principle of the operation of the laser is based on the fact that, in contrast to isolated atoms and ions, the molecules in atomic and ion quantum generators have extensive energy zones of discrete energy levels. In addition, each electronic energy level has big number The oscillatory levels, and those, in turn, are somewhat rotational.

The energy between electronic energy levels is in UV and visible spectrum areas, while between the vibrational-rotational levels - in the far and near IR regions. Thus, most molecular quantum generators work in distant or neighboring IR regions.

Excimer lasers

Excimers are such molecules as ARF, KRF, XECL, which have a separated ground state and are stable at the first level. The principle of the laser is next. As a rule, in the main state the number of molecules is not enough, so the direct pumping from the ground state is not possible. Molecules are formed in the first excited electronic state by connecting halide halides with inert gases. Inversion population is easily achieved, since the number of molecules on basic level Too little, compared to excited. The principle of action of the laser, briefly, consists in the transition from the associated excited electronic state into a dissociative ground state. The population is in the ground state always remains low, because the molecules at this point are dissociated by atoms.

The device and the principle of the lasers are that the discharge tube is filled with a mixture of halide (F 2) and rare earth gas (AR). The electrons in it dissociate and ionize the halide molecules and create negatively charged ions. AR + positive ions and negative F - react and produce ArF molecules in the first excited bound state, followed by their transition to the repellent basic state and the generation of coherent radiation. Excimer laser, principle of operation and the use of which we are now considering, can be used to pump the active medium on dyes.

Liquid laser.

Compared with solid substances, liquids are more homogeneous, and have a greater density of active atoms, compared with gases. In addition to this, they are not complicated in production, allow you to simply allotten heat and can be easily replaced. The principle of the laser is used as an active organic dyes, such as DCM (4-ditianometall-2-methyl-6-p-dimethylaminarial-4N-Piran), Rhodamine, styril, LDS, Kumarina, Styben, and so on . dissolved in proper solvent. The solution of the dye molecules is excited by radiation, the wavelength of which has a good absorption coefficient. The principle of action of the laser, briefly, is to generate a larger wavelength called fluorescence. The difference between the absorbed energy and the radiated photons is used by non-radical energy transitions and heats the system.

The broader fluorescence strip of fluid quantum generators has unique feature - Perestroika wavelength. The principle of operation and the use of a laser of this type as a customizable and coherent light source acquires everything more important in spectroscopy, holography, and in biomedical applications.

Recently, quantum dye generators began to be used to separate isotopes. In this case, the laser selectively excites one of them, encouraging in a chemical reaction.

Attention! Precautions:

Do not send laser radiation into the eyes! Direct hit in the eyes of laser radiation is dangerous for sight!

With the permission of the paper executive, enable the laser and install the screen and grille so that the diffraction pattern is the most clear.

Changing distance L., Look, as it affects the position of the maxima. Describe and sketch what you watched.

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

Task 2.Determination of wavelengths of some spectrum colors

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

Measurements B. task 2. Conducted in accordance with the instructions in the workplace. Measurement results are recorded in Table. 9.1. You should define distances l. 1 I. l. 2 for each color four times: at two values k. and two different distances L..

Table 9.1.

No. P.P.	Color	k.	L.,	l. 1 ,	l. 2 ,	,	SIN A.	L,
			…
	Red green purple
	Red green purple		…
	Red green purple

Analysis and processing of measurement results

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

2. Fill in fully tab. 9.1. The value is constant d. Get in the workplace . Describe the picture you see. Make the processing tables for each color and write down the final result according to the general rules.

3. Compare the values \u200b\u200bof the wavelengths obtained by you with the wavelengths given in Table. P. ...

Control questions

1. Give the definition: wave diffraction, the principle of Geigense-Fresnel, the coherence of the waves. A written answer to this question must be included in the report.

2. Name composite parts of the laboratory installation and their purpose.

3. What values \u200b\u200bare measured in this work directly? What are the calculated?

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

5. What is the diffraction lattice and what is its main parameters?

6. Output the formula of the diffraction lattice (9.3).

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

8. In which wavelength intervals lies visible light?

9. Display and write down the calculated formulas to determine the wavelengths of the visible light using a diffraction lattice.

10. How depends an angle of deviation of the diffraction maximum from the wavelength and the lattice period?

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

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

Work number 10. Study of the polarization of light

purpose of work: Investigate the passage of light through polaroids, check the law of Malyus, to evaluate the quality of polaroids, to investigate the polarization of the light that has passed through several glass plates.

Equipment: Optical bench, light source, polarizer in frame, analyzer combined with photocell, set of glass plates, power supply, micro ammeter.

Brief theory

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

;	(10.1)
,	(10.2)

where and the current values \u200b\u200bof the tensions of electric and magnetic fields; And - the amplitudes of oscillations, W is the frequency of oscillations, the initial phase of oscillations.

In the interaction of light with a substance, the variable electrical field affects the negatively charged electrons of atoms and the molecules of this substance, while the action from the side magnetic field On charged particles slightly. Therefore, in the processes of light main role Plays vector, and we will only talk about it.

Most light sources consist of huge number emitting atoms, and therefore in the light beam there is a large amount of waves with different spatial orientation of vectors. In addition, this orientation randomly changes in extremely small intervals (Fig. 10.2, a). Similar radiation is called non-polarized, or natural light. The light in which the direction of oscillations of the vector is in any way ordered, called polarized, and the process of obtaining polarized light is called polarization. If the vibrations of the vector occur in the same plane, then the wave is called flat-polarized or linear polarized (Fig. 10.2, b). Partially polarized It is called the light in which there is a predominant direction of vibrations of vectors (Fig. 10.2, B).

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

To obtain and research polarized light, most often used polaroids. They are made of very small crystals of tourmaline or herropathy (sulfate iodine-quinine), deposited on transparent film or glass. However, there are other methods for producing flat-polarized light from natural, for example, when reflected from the dielectric at a certain angle, depending on the refractive index of the dielectric. For more details, this method will be discussed below.

We mentally the next experience. Take two polaroid and light source (Fig. 10.3). First polaroid called polarizerbecause It polarizes the light. Its plane of oscillations is the plane PPS. After passing through the polarizer, the vector will only fluctuate in this plane. Rotating the polarizer around the direction of the light beam, we do not note any changes in the intensity of the light passed through it. Think why? Analysis of light on polarization is made using a second polaride, through which the light studied is passed. In this case, the second polaroid is called analyzer, its polarization plane is the plane AAS. Rotating an analyzer, we note that the intensity of the light passed through it will be maximum if the plane PPS and AAS coincide, and minimal, if these planes are perpendicular. If these planes make up some angle A (see Fig. 10.3), the intensity of light for the analyzer will take an intermediate value.

Find the relationship between an angle A and intensity I. Light pasted through both polaroids. Denote the amplitude of the electric beam vector, which passed through the polarizer, letter E. 0. Plane of oscillations analyzer AASturning relative to the polarizer oscillation plane PPS At an angle A (see Fig. 10.4). We decompose the vector to the components: the parallel plane of the oscillations of the analyzer and perpendicular to it ^. The parallel component of the êê will take place through the analyzer, and perpendicular ^ - no.

From fig. 10.4 It follows that the amplitude of the light wave for the analyzer

where S. - area in which energy is distributed; t. - time. Since the energy of light is the total energy of electric and magnetic fields, then its value is proportional to the squares of the tension of these fields:

The resulting equality is called law of Malyus: The intensity of the light pasted through the analyzer is equal to the intensity of the light that passed through the polarizer multiplied by the kosinee kosinee between the corner between polarization planes Analyzer and polarizer.

Note that the light that passed through the polarizer will become not only flatly polarized, but also reduce its intensity twice. If the intensity of natural light is considered the same in all directions perpendicular to the velocity vector, the intensity of light beyond the polarizer

where I. Max I. I. MIN - the greatest and smallest intensity of light for the analyzer corresponding to voltages E. Max I. E. min in fig. 10.2, in.

Polarization phenomenon can also be observed when reflected or refracted light on the border of two isotropic dielectrics. At the same time, fluctuations perpendicular to the fall plane will prevail in the reflected ray (in Fig. 10.5, they are indicated by points). The experimental way was shown that the degree of polarization in the reflected ray depends on the value of the incidence angle, and with an increase in the angle of the fall, the proportion of polarized light is growing, and with its definite value, the reflected light is fully polarized. Brewster found that the magnitude of this angle of complete polarization depends on the relative refractive index and is determined by the ratio:

tG.a Br. \u003d N. 2 /n. 1 .

(10.9)

The ratio is called the Brewer's law, and the angle A B is called the corner of the Brewster. With a further increase in the incidence of the decline, the degree of polarization of light decreases again. Thus, at the corner of the fall, equal corner Brewster the reflected light is linearly polarized in the plane perpendicular to the fall plane. Using (10.9) and the refractive law, it can be shown that with a fall at the corner of the Brewster, the reflected and refracted rays are 90 °. Check this!.

When the light falls at the corner of the Brewster, the refracted beam also polarizes. In the refracted beam, fluctuations will prevail, parallel planes of the fall (in Fig. 10.5 they are marked with arrows). Polarization of refracted rays at the same inclination of the fall will be the maximum, but not complete. If you subjected the refracted rays to the second, third, etc. refraction, then the degree of polarization will increase. Therefore, 8-10 plates can be used to polarization of light (the so-called Stop Table). The light passed through them will be almost completely polarized. Thus, this foot can serve as a polarizer or analyzer. In our installation, sets of 2-12 plates are used as a polarizer.

Installation Description

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

The figures in the diagram are indicated: 1- lamp, 2 - removable polarizer, 3 - swivel table, 4 – set of glass platesdied on the pins of the turning table, 5 - analyzer, 6 – photocell, 7 – meter light intensity (IIS), transforming light energy into an electrical signal; His testimony is proportional to the light flow falling on the photocell. Rotary table 3 can rotate around the vertical axis, thereby you can change the angle of falling light on the glass plate 4. To measure this angle of fall there is a special scale. The position of the table is fixed with a screw. Analyzer 5 can rotate around the horizontal axis, the position of the polarization plane is indicated on it. The analyzer has scale 8, according to which the position of its polarization plane is determined ( AAS). On the removable polarizer 2, there is also a vertical arrow that shows the position of its PPP polarization plane. The photocell combined with the analyzer can also rotate around the vertical axis. Thus, it is possible to measure the light intensity, reflected from the set of plates 4.

Completing of the work

Exercise 1 . Checking the Law of Malyus.

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

2. Turn on the lamp. Turn the photocell analyzer 6 so that it falls on the light from the lamp. Get the symmetrical location of the installation elements relative to the beam of light.

3. Set the plane position AAS on a scale of 8 0 °. Write down the meter testimony 7 in Table. 10.1. It will be the intensity of the light that passed through the polarizer and the analyzer in relative units. Repeat the measurement, changing the angle between the polarization planes of the polarizer and the analyzer from 0 ° to 360 ° after 10 °, and also record them in Table. 10.1.

Table 10.1.

Task 2. Research of polarization of refracted light

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

2. Set the angle of the light drop on the plate 56 ° (this is an angle of the bustener for glass with refractive index n. = 1,5).

3. Install the photocell to register the intensity of the light in the plates according to fig. 10.7 (the maximum value of the IIS testimony confirms the good light on the photocell).

4. Note that the refracted light is polarized in the fall plane, so the maximum value of the intensity will be at position AAS 90 ° on scale 8 (Questions 12, 13, 14). Measure the intensity of the light passed through the plates at two positions. AAS: at 90 ° and at 0 °. Record the measurement results in Table. 10.2.

5. Similar measurements. Spend for N. \u003d 4, 7, 12 plates. Record the measurement results in Table. 10.2.

Table 10.2.

Similar information.

Optical frequency standards - lasers With a stable frequency in time (10 -14 - 10 -15), its reproducibility (10 -13 - 10 -14). O. s. h. Apply to Piz. Studies and find practical. The application in metrology, location, geophysics, communication, navigation and mechanical engineering. Frequency division O. p. h. before the radio band made possible creation Scale of time based on the use of the optch period. .
O. s. h possess benefits compared to quantum frequency standards Microwave range: experiments associated with frequency measurement when using lasers require less time, since ABS Frequency at 10 4 - 10 5 times the nonlazer frequency nonzero standards. Abs. The intensity and widths that are frequency repers, in Optic. The range of 10 5 - 10 6 times more than in the microwave range, with the same one relates. width. This allows you to create O. with. h. with a higher short. frequency stability. When dividing the frequency of O. with. h. to the radio parapasone believes. The width of the radiation line is practically not changed (if the microwave standard is used, the fluctuation. The spectrum of its signal is significantly expanding with a frequency multiply of 10 5 - 10 6 times). The role of quadratic Doppler effect Limiting the debt. Stability and reproducibility of frequency is the same.

Principle of stabilization. Stabilization of the frequency of the laser, as well as standard radio standards, is based on the use of atomic or molecular gas spectral lines (Optic. Reperas), to the center of K-rye "tied" frequency v. via electronic system Automatic. Frequency adjustments. T. K. Laser strengthening lines are usually significantly superior to the bandwidth optical resonator, then instability ( v.) Frequency v. Generation in most cases is determined by the change in optical. The length of the resonator is the OSN. Sources of instability l. are a thermal drift, mechanical. And acoustich. perturbations of structural elements, fluctuations of the refractive index of gas discharge plasma. With Optic. Repeper Auto-adjustment system generates a signal, proportion. the magnitude and sign of the disorder between the frequency v. and frequency v 0 The center of the spectral line, using a swarm frequency of the laser is configured to the center of the line (\u003d v - V 0 \u003d 0). Relates. Accuracy settings back proportion. The product of the spectral line (- line width) on the signal-to-noise ratio when it is indicated.
To obtain a narrow line of radiation and high short. Frequency Stability (Stability Over Time C) It is necessary to use the references sufficiently high intensity with a wide superior width. characteristic range Frequency perturbations for gas lasers Characteristic spectrum width acoustic. Perturbations ~ 10 3 - 10 Hz, so the required width of the Hz resonance (relates width 10 -9 - 10 -10). This allows you to use automatic systems. Frequency adjustment with a wide band (10 4 Hz) for Eff. Suppression of fast fluctuations of the length of the resonator.
To achieve high debt. Frequency and reproducibility of frequency are necessary optical. High Quality Lines, since this decreases the effect of Ring. Factors on the lines center frequency shifts.

Optical references. The methods of obtaining narrow spectral lines used in the microwave range were not applicable to Optic. Spectrum region (Doppler broadening is not enough in the microwave range). For O. p. h. Methods are important, to-rye allow you to receive resonances in the center of the spectral line. This makes it possible to directly connect the radiation frequency with the frequency of the quantum transition. Three methods are promising: the method of saturated absorption, two-photon resonance and the method of separated optical. Fields. OSN. The results for stabilizing the frequency of lasers were obtained using a saturated absorption method, which is based on a nonlinear interaction of the oncoming light waves with gas. The nonlinearly absorbing cell with low pressure gas may be inside the laser resonator (active reper) and outside it (passive reper). Due to the effect of saturation (leveling of population levels of gas particles in a strong field), a failure with a homogeneous width, K-paradium, may be at 10 5 - 10 6 times less than the Doppler width, occurs in the center of the Doppler-Eat absorption line. In the case of an internal absorbing cell, the absorption reduction in the center of the line leads to the appearance of a narrow peak on the circuit of the power dependence on the generation frequency. Width of nonlinear resonance in molecular gas low pressure It is determined primarily by collisions and effects caused by the end time of the particle span through the light beam. Reducing the width of the resonance is accompanied by a sharp drop in its intensity (proportion. Pressure cube).
Naib Narrow resonances of saturated absorption with relates, width of 10 -11 were obtained in CH 4 on a component E. oscillatory-rotate. lines R (7) stripes v. 3 (see Molecular spectra), C-center is close to the center of the amplification line of the helium-neon laser \u003d 3.39 μm. To accurately align the amplification and absorption lines, 22 NE are used and the pressure does not increase the active laser environment or put the active medium in Magn. Field (for E.-Components).
Scheme O. with. h., Using ultra-suspicious resonance (C relates. 10 -11 width - 10 - 12 ) As a reference, consists of auxiliary stable in the frequency of the laser 2 with a narrow radiation line, a rebuilt laser 2 and a system for producing a narrow resonance (Fig. 1). The narrow line of radiation of the tunable laser, which is used to obtain a supernuzzy resonance, is provided by phase synchronization of this laser with stable.

Fig. 1. Frequency optical standard scheme: FFAP - frequency-phase auto-tuning; SUR is a system of obtaining superspillable resonance; APCH - automatic frequency adjustment system; ZG - sound generator; RG is a radio generator; D - photo detector.

Long. The stability of the rebuilt laser is achieved by smooth adjustment of its frequency at the maximum of superspanitable resonance using an extreme auto-adjustment system. It is possible to simultaneously receive high values short-term and debt. Stability and frequency reproducibility.
Frequency stability. Naib The high frequency stability is obtained in the IR range with non-ne - laser (\u003d 3.39 μm) with ext. The absorption cell. T. K. abs. The frequency is known with high accuracy (10 -11), then this laser can be used as independent. Secondary frequency standard for ABS measurement. Frequencies in Optic. and IR Dpapazons. The width of the radiation line of such a laser is 0.07 Hz (Fig. 2). The frequency stability over the times of averaging \u003d 1 - 100 s is 4 x 10 -15 (Fig. 3).
Long. Stability and reproducibility of frequency non-ne-lasers with telescopic. Expansion of the beam stabilized by resonances in CH 4 on the absorption lines F. 2 2 I. E. (See above) with Quality ~ 10 11, reach ~ 10 -14. The principal factor limiting the reproducibility and frequency accuracy is quadratic.

LIT: Basov N. G., V. S., Optical frequency standards, "UFN", 1968, vol. 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 the book: Laser Spectroscopy. IV. Proc. 4 th-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. Standarts and Metrology, Aussois, France, 12 - 15 Oct. 1981, "J. Phys.", 1981, v. 42, COLLOQ. From 8, number 12; Bagaev S. N., Chebotaev V. P., Laser Frequency Standards, "UFN", 1986, vol. 148, p. 143; Knight D. J. E., A Tabulation of Absolute Laser - Frequence Measurements, Metrologia, 1986, V 22, p. 251.

V. P. Chebotaev.

Real radiation contains in itself not one specific frequency of oscillations, but some set of different frequencies, called the spectrum or spectral composition of this radiation. The radiation is called monochromatic if it contains a very narrow frequency range (or wavelengths). In the visible region, monochromatic radiation causes a light sensation of a certain color; For example, radiation covering the wavelength range from 0.55 to 0.56 microns is perceived as green color. The already the frequency range of this radiation, the more monochromatic it is. Formula (1.2) refers to ideally monochromatic radiation containing one frequency of oscillations.

Red solid and liquid bodies emit a continuous (or solid) spectrum of electromagnetic waves of a very wide frequency interval. Glow sprinkled gases emit a strollery spectrum consisting of separate monochromatic radiation, called spectral lines; Each spectral line is characterized by a certain frequency of oscillations (or wavelength), located in the middle of a narrow frequency interval covered by it. If the radiation sources are not separate (isolated, free) atoms, and the gas molecules, the spectrum consists of a strip (striped spectrum), each strip covers a wider continuous wavelength interval than the spectral line.

Line (atomic) spectrum of each substance is characteristic of PEGO; Thanks to this it is possible spectral analysis, i.e. definition chemical composition Substances on the wavelengths of the spectral lines of emissions emitted by them.

Suppose that the electromagnetic wave spreads along some straight, which we will call the beam. You can be interested in changing the vector at a certain point of the beam over the time

time; It is possible that in. This point changes not only the magnitude of the vector as follows from formula (1.2), but also the orientation of the vector in space. Next, you can fix the magnitude and direction of the vector at different points of the beam, but at a certain point in time. If it turns out that at various points along the ray, all vectors lie in the same plane, the radiation is called flat-polarized or linearly polarized; Such radiation gives a source that, in the process of radiation, retains the oscillation plane. If the plane of the oscillation of the wave source changes over time, the vector in the wave does not lie in a certain plane and the radiation will not be flat-polarized. In particular, you can get a wave in which the vector is uniformly rotates around the beam. If the vector changes its orientation around the beam completely randomly, then radiation is called natural. Such radiation is obtained from luminous solid, liquid and gaseous bodies, in which planes, oscillations of elementary sources of cure - atoms and molecules - are oriented in space randomly.

Thus, the simplest radiation is a monochromatic plasco-polarized wave. The plane in which the vector and vector of the direction of propagation of the wave is called the plane of the oscillation plane, perpendicular to the oscillation plane (i.e., the plane in which the vector H) is called polarization plane.

The speed of propagation of electromagnetic waves in vacuum is one of the most important constants of physics and is equal

In other environments, it is less as determined by the formula (see h. III, § 29)

where, respectively, dielectric and magnetic permeability of the medium.

When moving radiation from one medium to another frequency of oscillations in the wave is preserved, but the wavelength to changes; Usually, if this is not specified, it denotes the wavelength in vacuo.

It was above indicated that the visible radiation (which we call the light) covers wavelengths from 400 to with a special workout of the eye can perceive the light of the wavelength from 320 to 900 nm. A wider wavelength range from 1 cm to, covering also ultraviolet and infrared regions, is called optical radiation.

1.1. Types of spectra.

At first glance, the laser beam seems very simple in its structure. This is almost one-frequency radiation having a spectral clean color: He-ne Laser has red radiation (633 nm), the cadmium laser emits blue (440 nm, the argon laser emits several lines in a blue-green spectrum area (488 nm, 514 nm and Dr.), semiconductor laser - red radiation (650 nm), etc. In fact, the laser radiation spectrum has a rather complex structure and is determined by two parameters - the spectrum of the working substance radiation (for a laser HE-NE, for example, this is a red spectral line Emitting neon excited by electrical discharge) and resonant phenomena in the optical resonator of the laser.

For comparison, the figures of the sun (a) and the usual incandescent bulb (B) (A) and the usual incandescent bulb (B) are given (the upper rice), the spectrum of the mercury lamp (Fig. Right) and a highly enlarged spectrum of the he-ne laser generation (Fig. At thenime).

The spectrum of incandescent lamps, as well as the solar spectrum, refers to continuous spectra, which completely fills the visible spectral range electromagnetic radiation (400-700 nm). The spectrum of the mercury lamp refers to the bar spectra, which also fills the entire visible range, but consists of separate spectral components of various intensity. By the way, before the advent of lasers, monochromatic radiation was obtained, highlighting separate spectral components of the ruling lamp radiation.

1.2. Radiation spectrum in HE-NE laser.

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

The process of forming a laser spectrum Consider on the basis of a well-studied HE-NE laser. Historically, it was the first laser continuous actionoperating in the visible range of the spectrum. He was created by A. Javan in 1960

In fig. On the right shows the energy levels of the excited mixture of helium and neon. The excited atom of helium or neon is an atom who has one or more electrons of the outer shell in collisions with electrons and gas discharge ions go to higher energy levels and can continue to go to a lower energy level or return to the neutral level, with Emitting a light quantum - a photon.

The excitation of atoms is performed by an electric current passing through the gas mixture. For a laser HE-NE, this is a weak, smoldering discharge (typical discharge currents - 20-50 mA). The picture of energy levels and the radiation mechanism is quite complex even for such a "classic" laser, which is a HE-NE laser, so we will restrict ourselves to considering only the main parts of this process. Helium atoms excited to 2S levels in collisions with neon atoms transmit to them accumulated energy, exciting them to 5s levels (therefore, helium in the gas mixture is greater than neon). From the level of 5S electrons can go to a number of lower energy levels. We are only interested in the transition 5S - 3P (both levels are actually split into a number of sublevels due to the quantum nature of excitation and radiation mechanisms). The length of the photon radiation wave with this transition is 633 nm.

We note another important fact that is fundamentally important to obtain coherent radiation. With properly selected proportions of helium and neon, the pressure of the mixture of gases in the tube and the size of the discharge current electrons accumulate at the level of 5S and their number exceeds the number of electrons located at the lower level 3p. This phenomenon is called inverse level population. However, this is not yet laser radiation. This is one of the spectral lines in the emission spectrum of neon. The width of the spectral line depends on several reasons, the main of which are: - the final width of the energy levels (5s and 3p) participating in radiation and determined by the quantum principle of uncertainty associated with the time of stay of neon atoms in the excited state - the broadening of the line associated with the permanent movement of excited Particles in the discharge under the influence of the electric field (the so-called Doppler effect). Taking into account these factors, the width of the line (experts referred to its loop of the working transition) is equal to about two ten thousandth of angstrom. For such narrow lines in the calculations, it is more convenient to use its width in the frequency domain. We use the transition formula:

dN 1 \u003d DL C / L 2 (1)

where Dn 1 is the width of the spectral line in the frequency domain, Hz, DL - 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 \u200b\u200b(in one measurement system), we obtain the width of the line of 1.5 GHz. Of course, such a narrow line can be considered quite monochromatic compared to the entire spectrum of emitting neon, but it is still impossible to call it coherent radiation. To obtain coherent radiation, an optical resonator (interferometer) is used in the laser.

1.3. Optical laser resonator.

The optical resonator is two mirrors located on the optical axis and facing reflective surfaces to each other, fig. on right. Mirrors can be flat or spherical. Flat mirrors are very difficult to adjust and generate laser radiation can be unstable. The resonator with spherical mirrors (confocal resonator) is much more stable, but the laser beam can be inhomogeneous in cross section due to the complex, multimode composition of radiation. In practice, the half-focal resonator with a rear spherical and front flat mirror is most often used. Such a resonator is relatively stable and gives a homogeneous (single) bundle.

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 formed for emissions of the neon spectral line with a wavelength of 633 nm. This contributes to the maximum reflection coefficient of mirrors, shared just for this wavelength. In laser resonators, dielectric mirrors with multi-layered spraying, allowing to obtain a reflection coefficient of 99% and higher. As you know, the condition for the formation of standing waves is that the distance between the mirrors should be equal to an integer number of half-filled:

nL \u003d 2L (2)

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

From the condition of the resonance (2), you can get the distance between the resonant frequencies of DN 2:

dn 2 \u003d C / 2L (3)

For a 11-meter resonator of the gas laser (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 intensify as a mixture of helium and neon agitated by an electrical discharge through the inverse medium. Moreover, which is extremely important, when this radiation passes along the resonator, its phase structure does not change, which leads to coherent properties of reinforced radiation. This contributes to the inverse population of the level of 5S, which was mentioned above. The electron from the top level goes to the lower synchronously with the photon initiating this transition, so the phase parameters of the waves corresponding to both photons are the same. Such generation of coherent radiation occurs throughout the path of radiation inside the resonator. In addition, resonant phenomena lead to a much greater narrowing of the radiation line, as a result of which the greatest gain is obtained in the center of the resonant peak.
Through a certain number of passes, the intensity of coherent radiation becomes as high, which exceeds the natural losses in the resonator (scattering in an active medium, losses on mirrors, diffraction loss, etc.) and part of it goes beyond the resonator. For this, the outlet, the flat mirror is made with a slightly smaller reflection coefficient (99.6-99.7%). As a result, the laser generation spectrum has the appearance shown in the third rice. from above. The number of spectral components usually does not exceed ten.

All time, all the factors that determine the frequency characteristics of the laser radiation are summoning. First of all, the working transition is characterized by a natural width of the contour. In real conditions due to various factors The contour is broken. Within the agreed line, resonant lines of the interferometer are placed, the number of which is determined by the width of the transition circuit and the distance between adjacent peaks. Finally, in the center of the peaks there are extremely narrow spectral lines of the laser radiation, which determine the range of the output radiation of the laser.

1.4. Coherence of laser radiation.

We clarify what length of coherence provides the radiation of the HE-NE laser. We use the formula proposed in the work:

as you pass through the inverse environment, a mixture of helium and neon is excited by an electrical discharge. Moreover, which is extremely important, when this radiation passes along the resonator, its phase structure does not change, which leads to coherent properties of reinforced radiation. This contributes to the inverse population of the level of 5S, which was mentioned above. The electron from the top level goes to the lower synchronously with the photon initiating this transition, so the phase parameters of the waves corresponding to both photons are the same. Such generation of coherent radiation occurs throughout the path of radiation inside the resonator. In addition, resonant phenomena lead to a much greater narrowing of the radiation line, as a result of which the greatest gain is obtained in the center of the resonant peak.

dt \u003d dn -1 (4)

where DT is the coherence time, which is the upper limit of the time interval, on which the amplitude and phase of the monochromatic wave are constant. Let us turn to usually for us the length of coherence L, with which it is easy to estimate the depth of the scene recorded on the hologram:

l \u003d C / DN (5)

Substituting the data in formula (5), incl., Complete width of the spectrum Dn 1 \u003d 1.5 GHz, we obtain the coherence length of 20 cm. This is quite close to the real length of the coherence of the HE-NE laser having the inevitable loss of radiation in the resonator. Measurements of coherence lengths using the Michelson interferometer give a value of 15-17 cm (at the level of 50% reduction of the amplitude of the interference pattern). It is interesting to estimate the coherence length of a separate spectral component isolated by a laser resonator. The width of the resonant peak of the interferometer Dn 3 (see the third on top of Fig.) Is determined by its good quality and is equal to about 0.5 MHz. But, as mentioned above, resonant phenomena lead to an even greater narrowing of the laser spectral line Dn 4, which is forming near the center of the resonant peak of the interferometer (the third on top of Fig.). The theoretical calculation gives the width of the eight thousandth hertz! However, this value does not have a large practical meaning, since for a long existence of such a narrow spectral component, the values \u200b\u200bof the mechanical stability of the resonator, thermal drift and other parameters are needed, which are absolutely impossible in real conditions of operation of the laser. Therefore, we limit the width of the resonant peak of the interferometer. For the spectrum width of 0.5 MHz, the coherence length calculated by formula (5) is 600 m. It is also very good. It remains only to highlight one spectral component, to estimate its power and keep it in one place. If, during the exposure of the hologram "going through" throughout the work circuit (due to, for example, the temperature instability of the resonator), we again obtain the same 20 cm coherence.

1.5. Spectrum of the generation of ion laser.

We will tell you briefly about the generation spectrum of another gas laser - argon. This laser, like the crypton, refers to ion lasers, i.e. In the process of generating coherent radiation, there are no longer argon atoms, but their ions, that is, atoms, one or more electrons of the outer shell of which are tearned under the influence of a powerful arc discharge, which passes through the active substance. The discharge current reaches several dozen amps, the electrical power of the power supply is several tens of kilowatt. Mandatory intensive water cooling of the active element is necessary, otherwise its thermal destruction will occur. Naturally, in such harsh conditions, the picture of the excitation of argon atoms is even more complicated. There is a generation of several laser spectral lines at once, the width of the working circuit of each of them is significantly larger than the width of the HE-NE Laser line and is several gigahertz. Accordingly, the laser coherence length decreases to several centimeters. For the recording of the Big Format holograms, the frequency selection of the generation spectrum is necessary, which will be discussed in the second part of this article.

1.6. Spectrum of semiconductor laser generation.

Let us turn to the consideration of the spectrum of the generation of a semiconductor laser, which is of great interest to the process of learning holography and for beginner holders. Historically, injection semiconductor lasers based on Galia arsenide, Fig. on right.

Since its design is quite simple, consider the principle of operation of the semiconductor laser on its example. The active substance in which the generation of radiation occurs is a Galia arsenide monocrystal having a shape of parallepiped with the sides of a few hundred microns. Two side faces are made parallel and polished with a high degree of accuracy. Due to the large refractive index (n \u003d 3.6), the crystall-air border obtains a sufficiently large reflection coefficient (about 35%), which is sufficient to obtain generation of coherent radiation without additional sputtering of reflective mirrors. Two other faces of the crystal are beveled at some angle; Through them, the induced radiation does not come out. The generation of coherent radiation occurs in the P-N transition, which is created by diffusion of acceptor impurities (Zn, CD, etc.) into the crystal area doped with donor impurities (TE, SE, etc.). The thickness of the active region in perpendicular to the P-N direction of the direction is about 1 μm. Unfortunately, in such a design of the semiconductor laser, the threshold density of the pump current turns out to be quite large (about 100 thousand amps per 1 sq. Cm). Therefore, this laser instantly destroys when working in continuous mode at room temperature and requires strong cooling. The laser is steadily operating at a temperature of liquid nitrogen (77 K) or helium (4.2k).

Modern semiconductor lasers are made on the basis of double heterokers, rice. on right. In such a structure, the threshold density of the current was reduced by two orders of magnitude, up to 1000 A / cm. sq. With this density of the current, a stable operation of a semiconductor laser and at room temperature is possible. The first samples of lasers worked in the infrared range (850 nm). With further improvement of the technology of forming semiconductor layers, lasers appeared both with an increased wavelength (1.3 - 1.6 μm) for fiber-optic lines of communication and with radiation generation in the visible region (650 nm). Already there are lasers emitting in the blue field of the spectrum. The high advantage of semiconductor lasers is their high efficiency (the ratio of radiation energy to the electric pump energy), which comes to 70%. For gas lasers, both atomic and ionic, efficiency does not exceed 0.1%.

Due to the specifics of the radiation generation process in the semiconductor laser, the radiation spectrum width is much greater than the spectrum width of the HE-NE laser, fig. on right.

The width of the working circuit is about 4 nm. The number of spectral harmonics can reach several dozen. This imposes a serious limit on the length of the coherence of the laser. If you take advantage of formulas (1), (5), the theoretical coherence length will be only 0.1 mm. However, as shown direct measurements of coherence lengths at the Michelson interferometer and recording reflective holograms, the real coherence length of semiconductor lasers reaches 4-5 cm. This suggests that the real spectrum of the semiconductor laser generation is not so rich in harmonics and has not such a big outline width The working transition is predicted by the theory. However, for the sake of justice, it is worth noting that the degree of coherence of radiation of semiconductor lasers is strongly varied both from the sample to the sample, and on the mode of its operation (the magnitude of the pump current, cooling conditions, etc.