RU2106658C1 - Laser doppler meter - Google Patents

Abstract

FIELD: measurement technology, ecology, meteorology, physics of atmosphere and other fields of science and technology. SUBSTANCE: characteristic feature of invention lies in usage of additional laser, unit for convergence and change over of beams, acoustooptical modulator with control unit, second beam splitter, power measurement unit and computation unit. EFFECT: expanded functional capabilities of meter thanks to measurement of concentration of substances along route of propagation of laser radiation. 2 dwg

Description

 The invention relates to the field of laser Doppler measuring instruments and can be used in ecology, meteorology, atmospheric physics and other fields of science and technology, where it is necessary to measure the speed of gas flows and / or the concentration of gaseous substances by remote sensing in local areas of space or along long paths.

 Laser Doppler meters are based on the selection and subsequent measurement of the parameters of the Doppler signal obtained by scattering laser radiation by the object under study or the environment.

 Known laser Doppler anemometer [1], consisting of a frequency-stabilized laser that generates linearly polarized radiation, an antenna of a scanning system, a homodyne detector and a system for processing a Doppler signal. This anemometer is designed to work in the field and is used at aerodromes to measure wind speed along the glide path.

 Known onboard laser Doppler true airspeed meter of an aircraft [2], consisting of an optical head, including a laser with a stabilization system of its parameters, an optical homodyne converter, a monostatic transmitting and receiving antenna unit, in which the receiving and transmitting optics are combined, as well as the processing unit Doppler signal, including a device for measuring and recording the frequency of the Doppler signal and the corresponding speed value.

 These meters work by reflection from natural aerosols contained in the air, have a long range (up to 300 - 1000 m) due to the use of an optical homodyne converter (GHP) with polarization isolation of the transmitted and received radiation.

 The closest technical solution, selected as a prototype, is a laser Doppler speed meter [3], containing a laser that generates linearly polarized radiation, OGP, a monostatic transceiver antenna unit and a processing unit of the converted Doppler signal.

The OGP is made according to the scheme of the Mach-Zehnder interferometer and includes a divider of the initial plane-polarized radiation into two channels: a reference beam formation channel, in which a rotary mirror and a birefringent half-wave plate are rotationally rotated by 90 ^° , and the transmitter-receiver antenna channel is turned into which is installed polarization isolation, consisting of a plate located at an angle of Brewster and a quarter-wave plate. The OGP also includes an optical mixer, a focusing lens and a photodetector.

 The devices discussed above allow not only measuring the air flow velocity [1, 2], but also determining its sign [3]. However, they do not provide information on the concentration of substances that absorb laser radiation.

 The aim of the invention is to expand the functionality of the meter by measuring the concentration of gaseous substances along the propagation path of laser radiation.

 This goal is achieved by the fact that a second laser with linearly polarized radiation, oriented so that the polarization planes of the first and second lasers coincide, the beam mixing and switching unit, and an acousto-optic modulator (AOM), is additionally introduced into the laser Doppler meter [3] ) with a control unit located in the channel for forming the reference beam in front of the mixer, a second beam splitter installed between the first beam splitter and the plate at an angle Brewster, and oriented in such a way that part of the radiation of the probe beam is directed to the power measurement unit, and a calculation unit, one input of which is connected to the output of the Doppler signal processing unit, the second to the output of the power measurement unit, and the third to the output of the information and switching unit rays.

 A comparative analysis of the proposed solution with the prototype shows that the proposed device differs from the known one by the presence of new blocks and the connections between them and other blocks of the laser Doppler meter. Additionally introduced elements (laser, beam converting and switching unit, AOM with control unit, light separator, measuring power measuring unit, calculation unit) are not fundamentally new in themselves. However, their use in the inventive device in this connection with other blocks and optical elements allows you to determine the concentration of substances along the propagation path of laser radiation, thereby expanding the functionality of the meter.

 In FIG. 1 presents a block diagram of the inventive device. Figure 2 is one of the possible options for a specific implementation of the circuit of the claimed device.

 A laser Doppler meter (LDI) contains lasers 1, 2, at the output of which a beam converting and switching unit 3 is installed, followed by a first beam splitter 4, connected to the channels for forming the reference and probe beams. Behind the first beam splitter 4, along the propagation of the laser radiation, there is a second beam splitter 5, a plate 6 mounted at a Brewster angle, a quarter-wave plate 7 and a monostatic transceiver antenna unit 8, and a radiation power measuring unit 9 is arranged along the propagation of the radiation reflected from the second beam splitter 5, the output of which is connected to the second input of the computing unit 18.

 In addition, along the propagation of the radiation reflected from the first beam splitter 4, a reflector 10, a half-wave plate 11, AOM 12 with a control unit 13, a mixer of the reference and probe beams 14, a focusing lens 15, a photodetector (FPU) 16, the output of which is connected to the input of the block, are installed processing the converted Doppler signal 17. In this case, the output of the processing unit of the Doppler signal 17 is connected to the first input of the unit for calculating the concentration of substances along the propagation path of the laser radiation 18, and a third the th input is connected to the output of the unit information and switching rays 3.

 The principle of measuring the concentration of substances is to measure the difference in the absorption of radiation at two wavelengths, one of which coincides with the maximum, and the other with the nearest absorption minimum of the test gas [4]. Knowing the difference in cross sections and absorption and the length of the radiation propagation region, we can determine the average concentration of gas impurities in this region.

где

- средняя концентрация исследуемого газа;
α - коэффициент поглощения (рассеяния), обусловленный всеми компонентами атмосферы, кроме исследуемого газа;
ρ - коэффициент диффузного рассеяния объекта;
A - площадь апертуры приемной антенны;
L - длина трассы;
γ - коэффициент, учитывающий потери в приемо-передающем тракте.If we denote by P ₀₁ and P _{02 the} initial laser radiation powers at wavelengths λ ₁ and λ _2, respectively, and wavelengths λ ₁ and λ ₂ are chosen so close that in the calculations it is sufficient to take into account only the difference in the absorption cross sections of the gas under study, σ ₁ and σ ₂ , then the radiation powers P ₁ and P ₂ arriving at the receiver are found from the expressions

Where

- the average concentration of the test gas;
α is the absorption coefficient (scattering) due to all components of the atmosphere, except for the test gas;
ρ is the diffuse scattering coefficient of the object;
A is the aperture area of the receiving antenna;
L is the length of the track;
γ - coefficient taking into account losses in the transceiver path.

где
σ₁₂= σ₁-σ₂ - дифференциальное сечение поглощения исследуемого газа.From equations (1), (2) we find the gas concentration:

Where
σ ₁₂ = σ ₁ -σ ₂ is the differential absorption cross section for the gas under investigation.

 LDI works as follows.

The radiation beam of the first laser 1 with a frequency of ν ₁ after the information unit and switching beams 3 is divided by the first beam splitter into two - probing and reference.

The probe beam, having passed the first beam splitter 4, the second beam splitter 5, a plate 6 mounted at a Brewster angle, a quarter-wave plate 7, is guided by a monostatic transceiver antenna unit 8 into the investigated area of space. Reflecting from particles located in the measuring volume remote to the distance L = F _АС , where F _АС is the focus length of the antenna system, and having a nonzero velocity relative to the LDI, the signal acquires a Doppler frequency shift Δν ₀₁ , is received by the antenna system 8 and is again directed to a quarter-wave plate, passing through which and reflected from the Brewster plate 6 falls on the mixer 14.

 The reference beam, reflected from the first beam splitter 4 and the swivel mirror 10, passes a half-wave plate 11, AOM 12 and the mixer 14.

The probe and reference beams are spatially aligned on the mixer 14 and form intensity beats with a Doppler frequency Δν ₀₁ on the sensitive area of the photodetector 16. These beats are converted into FPU 16 and an electric signal with the same frequency Δν ₀₁ and amplitude V ₁ , which enters the processing unit of the Doppler signal 17.

In all known devices, including the described analog devices and the prototype device, only information is contained in the frequency of the Doppler signal Δν ₁ , which is the source for determining the relative velocity of the laser meter, more precisely its carrier and the object that is the source of the scattered radiation (signal). However, the processing unit of these devices, as a rule, includes a spectral analyzer device [1, 3], the output of which automatically receives information not only about the magnitude of the Doppler "shift" of the reflected or scattered signal, but also about the amplitude of the Doppler signal, which is proportional to the power scattered or intercepted by the antenna LDI signal.

The signal V ₁ from the output of the processing unit of the Doppler signal 17 is fed to the input of the calculation unit 18. At the same time, the signal V ₀₁ from the output of the power measurement unit 9 is received at the other input of the calculation unit 18, and the identifier signal U ₁ from the block is received at the third input switching and information of beams 3. The identified signals V ₁ , V _{01 are} converted in the input unit of the computer (analog-to-digital converter) and stored in the storage device of the computer. After some time, the beam converting and switching unit turns off the first laser and turns on the second laser. At the same time, the signal V _{2 is} supplied to the first input of the calculation unit 18 from the output of the Doppler signal processing unit 17, the signal V ₀₂ to the second input from the output of the power measuring unit 9, and the signal-identifier U ₂ to the third input of the calculation unit 18 from the information and switching unit rays [3].

 The computer performs operations to implement a program for calculating the concentration of substances along the propagation path of laser radiation in accordance with formulas (1-3). Further, the measurement process is cyclically repeated.

 LDI can be performed, for example, as shown in figure 2.

As lasers, continuous CO ₂ lasers of the type LGN-901, LGN-903 can be used.

 The beam converting and switching unit is an optical circuit with a beam switching device and a synchronization device. The actuator of the switching device is a two-window opaque screen for radiation. The synchronization device of the mixing and beam switching unit can be made in the form of an optoelectronic circuit consisting of a pair of LED 21 - a photoresistor 22, a transistor 23 and resistors 24, 25, and a photoresistor 22 connected between the base and the emitter of the transistor 23.

 The synchronization of the measurement process is as follows.

With the upper position of the screen 19, the radiation of the first laser passes into the lower window of the screen and goes into the optical path. In this case, the radiation of the second laser and the LED 21 is blocked by the screen and the signal V _{1 is} supplied to the first input of the calculation unit 18 from the processing unit of the Doppler signal 17, and the signal-identifier U ₁ ≈ to the second input of the calculation unit 18 from the synchronization device of the mixing and beam switching unit 3 0, since the screen material is opaque to the radiation of the LED 21, and the dark resistance of the photoresistor 22 is large and the transistor 23 is open.

In the lower position of the screen 19, the radiation of the second laser passes into the upper window, the radiation of the first laser is blocked by the screen, and the radiation of the LED 21 passes through the lower window to the photoresistor 22. In this case, the signal V ₂ is received at the first input of the calculation unit 18 from the processing unit of the Doppler signal 17, and from the synchronization device of the switching unit and converting the rays 3 to the second input of the calculation unit, the signal U ₂ equal to U, since the resistance of the photoresistor 22 is small and the transistor 23 is closed at the collector-base junction.

 The elemental base of the synchronization device for the mixing and beam switching unit 3 can be as follows: LED - AL-310A, photoresistor - SF2-5, transistor - KT-315A.

When using CO ₂ lasers in LDI, the radiation of which lies in the IR range, the beam splitters can be made of germanium or zinc selenide.

 As a power measurement unit, a pyroelectric receiver of the MG-30 type with an electromechanical modulator can be used.

 As the AOM, you can use the ML-206 acousto-optical modulator, and the G4-154 generator as the AOM control unit.

 A spectrum analyzer of type SK4-59, from the output of which it is possible to take information about both the frequency (Doppler shift) and the amplitude of the Doppler signal, is one of the possible options for implementing a block of processing the Doppler signal.

 The calculator may consist of analog-to-digital measuring devices such as serial meters of signal ratios PB8-7, logarithmic amplifiers, etc. equipment or may be fully digital, made, for example, as shown in figure 2. In this case, the calculator 18 consists of [5]: from an input block 26 (ADC, voltage-code converter, input register), an arithmetic logic device 27, a control device 28, a storage device (operational and constant) 29 and an output block 30 (converter code-voltage) output register, buffer memory.

 This calculator can be implemented on microcircuits of wide application, for example, on a microprocessor set of integrated circuits of the K580 series in a typical inclusion [6] or on another element base.

Literature
1.Barbor AE Scanning laser doppler anemometr system Proc. SPIE, 1980, N 227, p. 85-90.

 2. UK patent N 2075787, CL G 01 S 17/58, H 01 S 3/13.

 3. French patent N 2556841, CL G 01 P 5/00, G 01 S 17/58.

 4. S. M. Kopylov, B. G. Lysoy et al. Tunable dye lasers and their application. - M.: Radio and Communications, 1991.

 5. V.L. Grigoriev, G.A. Petrov. Micro- and mini-computers: L .: Energoatomizdat, 1984.

 6. Microprocessor-based integrated circuit kits. Composition and structure. Directory. under the editorship of Vasenkova V.P. - M.: Radio and Communications, 1982.

Claims (1)

 A laser Doppler meter comprising a linearly polarized laser, an optical heterodyne converter including a first beam splitter optically coupled to a probe beam forming channel containing an optical plate located at a Brewster angle and a quarter wave plate, and a reference beam channel containing a reflector and a half wave as well as a mixer, a lens and a photodetector installed in the spatial alignment channel of the reference and probing beams, and m the output of the photodetector is connected to the input of the converted Doppler signal processing unit, and a monostatic transceiver antenna unit located along the probe radiation after the quarter-wave plate, characterized in that a second laser with linearly polarized radiation is oriented into it so that the polarization planes of the first and second lasers coincided, located in front of the first beam splitter unit information and switching beams, an acousto-optical modulator with a unit a control beam located in the channel for forming the reference beam in front of the mixer, a second beam splitter installed between the first beam splitter and the plate located at a Brewster angle and oriented so that part of the radiation of the probe beam is directed to the radiation power measuring unit, and the unit for calculating the concentration of substances along laser radiation propagation paths, one input of which is connected to the output of the Doppler signal processing unit, the second to the output of the power measurement unit, and the third to the output of the information unit and switching beams.

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