RU2589763C2 - Method for guiding pulsed electromagnetic radiation to remote object - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method for guidance onto a remote object of electromagnetic radiation based on formation in the material medium of radiation with a specified in the direction of the direction pattern with a wavelength of λ₀, pulse duration of τ₀ and simultaneous transmission, within the range of the formed direction patter in the direction of the object, of coherent radiation with a wavelength of λ₁ and duration of τ₁<τ₀. Coherent electromagnetic radiation with absorption factor α₁<α₀ is directed, relative to the axis of the direction pattern, at the angle of total internal reflection, and part of the coherent electromagnetic radiation reflected from the object with wave length λ₁<λ₀ is intercepted by the direction pattern, subject to amplification and complex conjugation.

EFFECT: increased accuracy of measurement and longer detection range with simultaneous decrease in power consumption.

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Description

The present invention relates to the field of radio and laser ranging, laser technology and can be used in optical communication systems to direct radiation and transport energy to remote objects.

A known method based on the illumination of a distant object, the reception and amplification of the radiation reflected by the object, the analysis and adaptive correction of its wavefront with the subsequent direction of radiation to the distant object.

The disadvantages of the method are: the use of electronic adaptive systems, the low accuracy of the formation of the phase conjugate wavefront, which is determined by the discreteness of the information count by the wavefront sensors and the low accuracy of the recovery of the phase function from the intensity measurements.

There is a method of pointing laser radiation at a distant object, which consists in illuminating the object with laser radiation and amplifying the radiation reflected from the object in a two-pass amplifier, reversing its wavefront and re-amplifying the radiation when it passes back through the amplifier.

The disadvantage of this method can be attributed to the short detection range of objects, the need to use optical elements to generate illumination radiation, the inability to track a moving object, considerable time for the initial detection of an object.

A device is known that implements a method of directing radiation to a spatial object using an unstable resonator with an output ring aperture and a beam-forming system in the form of a telescopic system. The device has the following disadvantages: the lack of feedback, providing tracking and control of the signal reflected from the object, the low accuracy of measuring the coordinates of the object and, as a result, the inoperability of decision-making. It is necessary to note the negative effect of atmospheric processes (dispersion, turbulence, etc.) on the shape of the probe signal, which leads to significant errors in tuning the optical system of the device.

A known method of focusing laser radiation, comprising converting a parallel laser beam into a beam of circular cross-section with a pre-cut central part and subsequent focusing of both beams at one point in space. The method has the following disadvantages: a negative effect on the dynamics of the formation of coherent laser radiation due to the introduction of an aperture diaphragm into the resonator, which, due to diffraction, distorts the wavefront formed by the resonator and the use of rotary elements necessary to reduce the two beams.

A device for automatically tracking objects and determining their coordinates by using radiation with different wavelengths is known. This device implements a method of pointing microwave radiation and optical range to pulsed electromagnetic radiation of a remote object - it is the closest in technical essence solution chosen by the author for the prototype - this method initially implies using electromagnetic radiation with a wavelength of λ ₀ and duration τ ₀ formed by a parabolic mirror, to search for an object, and further support of the object to implement coherent optical radiation of wavelength λ ₁ <λ ₀ and lastingly radiation pulse Stu ₁ τ <τ _0. For this purpose, two holes are used on the surface of a parabolic mirror.

A device that implements the method has the following disadvantages:

- the effects accompanying the joint passage of waves of different ranges that affect their passage in the atmosphere, which ultimately leads to errors in recording various parameters of the objects under study, have not been taken into account;

- lack of a compensation mechanism for spatial heterogeneities of the environment;

- the inability to work on surface paths due to the significant output aperture and the presence of side lobes of the radiation pattern with a length of λ ₀ .

Using the present invention, a technical result is achieved, which consists in increasing the detection range of distant objects while increasing the accuracy of measurements.

In accordance with the invention, the technical result is achieved by the fact that in the method of pointing pulsed electromagnetic radiation to a remote object, based on the formation of radiation in a material medium with a given directional pattern of the wavelength λ ₀ and pulse duration τ ₀ and simultaneous transmission within the formed radiation patterns in the direction of the object of coherent electromagnetic radiation with a wavelength of λ ₁ <λ ₀ and a pulse duration of τ ₁ <τ ₀ , diagram mu directivity is formed in the form of diverging wave fronts symmetrically spaced relative to the direction of the object with screening of a part of the cross section at a distance

D is the diameter of the cross section of the radiation pattern at a distance L from the antenna.

Coherent electromagnetic radiation is directed relative to the axis of the radiation pattern formed by diverging fronts, and part of the coherent radiation reflected from the object is intercepted by the radiation pattern and is amplified and complex conjugated.

As follows from the claims, the device uses wavelengths that belong to different ranges - one (λ ₀ ) has a higher absorption coefficient α ₀ compared to α _{1 of the} other wave (λ ₁ ). In accordance with the empirical formula [6, 8]

α is the absorption coefficient for the range λ ₀ = 0.5 ÷ 10 cm;

M = 7.5 g / m ³ - the density of water vapor at a temperature of 18 ÷ 20 ° C, we find the absorption coefficient for λ ₀ = 1 cm;

Under the same atmospheric conditions, the absorption coefficient (λ ₁ = 1.06 μm) is 0.5 dB / km (3).

The idea of the method is to use radiation λ ₀ to create a waveguide channel formed as a result of heating of the atmosphere by this radiation, and to transmit coherent radiation from the walls of the waveguide channel of the coherent radiation with a wavelength of λ ₁ and absorption coefficient α ₁ <α ₀ inside this channel.

In fig. 1 and 2 presents a device that implements the proposed method. The device operates as follows. After the excitation of two irradiators 2 with output apertures installed in the foci of asymmetrically truncated mirrors of a parabolic antenna 1 with shifted maxima, the radiation reflected from the surface of the mirrors and limited by mirror 3 is directed to the search zone of the proposed object 8. In a parabolic antenna, the optical path length of all the rays coming from the focus and reflected from the mirror in the aperture plane, have the same phase, therefore, the spherical fronts of the irradiators 2 are converted to flat and the antenna emits a pair lolly bunch. Moving the irradiators installed in the focus of the mirrors 1 along the optical axis allows the beam to be converted into a converging wave front 9 with the screened central part of the mirror 3. Using asymmetrically truncated antennas with a shifted radiation maximum allows reducing the influence of the shadow effect of irradiators 2 due to diffraction reflected from the mirror 1 radiation. Since the irradiators 2 have their radiation patterns, part of the radiation enters the space between the mirrors 1 simultaneously with the formation of channel 9 and the radiation of the irradiators 2 is recorded by the processing device 6, which allows a direct pulse to be received by the receiver 10 and the terminal device 11. In this case, the system operates on transmission of electromagnetic radiation. After the transmission is completed, using the antenna switch, made in the form of two spark gap switches that perform three functions - turning on the receiver 6 with the processing device, turning on the antenna and the pulse transmitter 12, a pulse of duration τ _{0 is} generated, which through the antenna switch falls into irradiators 2 and is radiated into an unbalanced antenna.

The receiver at this time is disconnected for a time τ ₀ from the antenna. The reflected pulse from the object 8, reflected from the mirror 1, enters the auxiliary mirror 7 and through the same antenna switch enters the receiver. According to the time stamps on the screen of the terminal device 11 (oscilloscope), the distance to the object is determined by the formula

D is the distance to the object;

C is the speed of light.

When the radar channel is in search and tracking mode, due to a fairly wide beam pattern of 1 antenna, significant errors are possible related to both the location of the group target and the location of low-flying targets. Several point targets within the allowed volume have an aggregate reflective surface; therefore, the backscatter diagram of this target contains many petals determined by the coherent addition of the beams reflected from the group target [8]. The influence of the earth leads to a decrease in the range of the radar for low-flying targets, since the radiation pattern of the antenna in free space is affected by the radiation pattern determined by the reflected radiation from the earth’s surface - F _land (ε), therefore

D _om - range in free space in the direction of the maximum without taking into account the influence of the earth;

F _E (ε) - radiation pattern in free space;

ε is the slip angle.

Interference interaction leads to the fact that the resulting diagram is distorted due to the influence of the earth. To detect an object and determine its coordinates, it is necessary to search for this object in space. Since the radar pattern is needle-shaped and has a considerable width, it usually takes a little time to search for an object in space, while when used in optical location by laser sources, quite a lot of time is required (the width of the radiation pattern for lasers is ~ 10 ^-3 ÷ 10 ^-4 glad). Therefore, it is convenient to use two locators - one operating in the radio range for the primary search, usually at a great height and at a considerable distance, and then use sources of coherent radiation.

For air, the relation for the refractive index

ρ is the density;

k is a constant.

Expression (6) can be reduced to

- начальные равновесные значения.

- initial equilibrium values.

Considering air as an ideal gas expanding at constant pressure, we obtain the expression [9]

γ is the adiabatic exponent;

P ₀ is the pressure;

α is the radiation absorption coefficient;

J (r) is the radiation intensity;

t ₀ - heating time at an arbitrary point r

For the selected wavelengths, we have:

Thus, we use λ ₀ to create a channel with a shaded central part of the beam, the shape of which is preserved in the near zone, i.e., at a distance

происходит формирование диаграммы направленности симметрично разнесенными относительно направления на объект расходящимися волновыми фронтами. Сохраняя свою форму в ближней зоне, излучение с λ₀ отдает энергию, формируя при этом тепловую линзу на всем пути распространения пучка. Для исключения возможности теплового расплывания пучка, формирующего канал, необходимо выполнить условиеfrom the aperture of an asymmetrically truncated antenna with a shifted radiation maximum. At a distance

radiation pattern is formed symmetrically spaced apart from the direction of the object by diverging wave fronts. Preserving its shape in the near field, radiation with λ ₀ gives off energy, forming a thermal lens along the entire path of the beam. To exclude the possibility of thermal spreading of the beam forming the channel, it is necessary to fulfill the condition

τ ₀ is the heating time;

a is the smallest value of the radius of the beam at the radiation propagation range λ ₀ ;

c _S is the speed of sound propagation. As is known [11], if an electromagnetic wave falls on the boundary of two homogeneous media, it is divided into two waves - transmitted to the second medium and reflected. If the second medium is less dense than the first, then the law of refraction implies

θ _i is the angle of incidence in a denser medium;

θ _t is the angle of refraction in a less dense medium;

n ₁ , n ₂ are the refractive indices of, respectively, dense and less dense media;

n ₁₂ is the relative refractive index of the second medium relative to the first.

Therefore, at incidence angles at which sin θ _i <n _1,2 , the effect of total internal reflection is realized, at which the incident wave returns entirely to the first medium. An optical location channel with λ is at the same time a channel forming an instrument for influencing a search object, and works as follows. Using rotary mirrors 3, 3a, coherent radiation generated by the backlight laser 4 is introduced into the inner region of the waveguide channel 5, the boundary of which 9, due to absorption of radiation with a wavelength of λ ₀ , represents a reflective surface for coherent radiation with a wavelength of λ ₁ . The characteristics of the laser radiation and the emission of the echo signal reflected in the form of a glare from the object 8 are measured by the registration system installed along the radiation according to the combined source and receiver scheme — this scheme, in which the same system of mirrors forms a highly directional antenna and receives an echo -signal from the object. The registration system contains a backlight 4, a telescopic system of mirrors 13, one of which is made with a communication hole, a beam splitter 14 with a measuring unit, with which measurements are taken to the distance to object 8. Before the power channel with radiation λ ₁ starts to work, a “cross” occurs atmospheric section in order to eliminate the influence of atmospheric factors (fog, snow, etc.) to ensure transparency of the channel in the area of beamforming and eliminate spurious reflection from particles located shaped channel region. After the operative detection of the object 8 by coherent optical radiation, a part of this radiation after reflection from the object returns to the inner region of the waveguide channel 5 and through the telescopic system 15 it enters the wavefront reversal device 16, in which its phase is complexly coupled, as a result of which the amplified signal returns to object 8, with full compensation of the path aberrations that occurred in a direct passage through the waveguide channel. The wavefront reversal phenomenon is based on stimulated Mandelstam-Brillouin scattering (SBS) [12, 13].

In order to provide a threshold power density (~ 10 ⁶ W / cm ² ) for the operation of phase conjugation, a combined amplifier containing a quantum amplifier 17, which acts as a noise filter, and a BP amplifier (stimulated scattering amplifier) 18, providing super gain (up to 10 ¹⁶ ) weak signals scattered by a distant object. Therefore, the combined amplifier is amplified as a BP amplifier with a noise level like that of a quantum amplifier. Considering that during phase conjugation, the light goes strictly backward, with compensation for inhomogeneities of the atmospheric channel, with a pump pulse duration of amplifier 17 of the order of 1 ms, repeated exposure to a glare object 8 in the amount of

τ is the duration of the pump pulse of the amplifier 17;

L ₁ is the distance to the object 8;

C is the speed of light.

Thus, the proposed device that implements the method can significantly increase the detection range of objects, especially those at low altitude, while using both a combined electromagnetic system consisting of radiation sources of various spectral ranges and a combined amplifier system in conjunction with a wavefront reversal system. The use of radiation with a wavelength of τ ₀ for the "lumbago" of the track allows to increase the measurement accuracy, since it eliminates the false signal determined by the reflection from various kinds of aberrations associated with various factors (turbulence, fog, snow, etc.). It should be noted that a combined search can dramatically reduce energy costs, since when searching for low-lying objects, the range is determined as the root of the eighth degree of the energy potential [8]

D _zem - the antenna pattern taking into account the earth;

P _p - transmitter power;

P _pmin - threshold power (minimally different signal).

Claims (1)

The method of pointing electromagnetic radiation to a remote object, based on the formation of radiation in a material medium with a radiation pattern specified in the direction of the object with a wavelength of λ ₀ pulse duration τ ₀ and simultaneous transmission within the generated radiation pattern in the direction of the coherent radiation object with a wavelength of λ ₁ and duration τ ₁ <τ ₀ , characterized in that the radiation pattern is formed by radiation with an absorption coefficient α ₀ > α ₁ in the form of symmetrically spaced, relates specifically the direction of the object, diverging wave fronts with shielding of the cross-section at a distance

, coherent electromagnetic radiation with an absorption coefficient α ₁ <α _{0 is} directed relative to the axis of the radiation pattern at an angle of total internal reflection, and part of the coherent electromagnetic radiation reflected from the object with a wavelength of λ ₁ <λ _{0 is} intercepted by the radiation pattern, subjected to amplification and complex conjugation, where D - the diameter of the output aperture of the electromagnetic radiation source with a wavelength of λ ₀ .

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