RU2707957C1 - Laser doppler velocity meter - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: present invention relates to measurement equipment and can be used in experimental hydro- and aerodynamics, in industrial technologies associated with the need to study velocities of flows of gas and condensed media, as well as speed of surfaces. Laser Doppler velocity meter comprises series-arranged bi-chromatic source of two spatially aligned orthogonally polarized laser beams with wavelengths λ_r and λ_g, first lens, acousto-optic Bragg traveling wave modulator, a second lens, an achromatic half-wave phase plate placed on the path of one of the diffracted beams, a polarization prism of Wollaston, located in the image plane of the source of the beams diffracted in the acousto-optic modulator. Speed meter also includes third lens, front focal plane of which is combined with plane of splitting of beams in polarization prism of Wollaston, half-wave phase plates on paths of two Wollaston-split beams with wavelengths λ_r and λ_g, fourth lens – probing optical field shaper in analyzed medium, rotary mirror installed between third and fourth lenses, mounted on path of light beam from monochromatic pair with wavelength λ_g beam splitting plate oriented at Brewster angle, and in series with it quarter-wave plate, in the path reflected by the beam-splitting beam of beams, there are successively installed quarter-wave phase plate and mirror, on path of light beam scattered in direction of incident wave with wavelength λ_r and reflected from the beam splitting plate, sequentially installed optical filter at wavelength λ_r and a photodetector connected through an analogue-to-digital converter to a signal processing system. At that, Bragg acousto-optic modulator is made as two-wave modulator with frequency ratio Ω_r and Ω_g of ultrasonic waves equal to ratio of wave numbers k_r and k_g diffracting light beams, and photodetector installed on path reflected by mirror of light field scattered by investigated medium is connected to signal processing system through band-pass filters, central frequencies of which are equal to frequencies Ω_r and Ω_g ultrasound waves in acousto-optic modulator.

EFFECT: high accuracy of measuring speed.

1 cl, 8 dwg

Description

The present invention relates to measuring technique and can be used in experimental hydro- and aerodynamics, in industrial technologies associated with the need to study the flow rates of gas and condensed matter, as well as the speeds of surfaces.

Known laser Doppler speed meter, the effect of which is based on the use of laser radiation and the Doppler effect [patent US 4838687 A]. To measure the velocity vector in the studied medium, a probing field is formed, the spatial structure of which determines 3D (D - “dimension” - dimension) coordinate-measuring basis. The probe field is limited by the region of intersection of the laser beams. The frequency of the scattered light changes due to the Doppler frequency shift proportional to the velocity of the medium under study. The Doppler frequency shift measurement carries speed information. To determine the velocity vector, the five-beam system for the formation of the orthogonal coordinate basis in the probing field is most widely used; the implementation is described in US Pat. a 2D coordinate basis of three laser beams is formed for velocity, and for measuring the third orthogonal velocity component, a coordinate basis anny in the area of intersection of two laser beams. In fact, two laser Doppler measuring systems (LDIS) with a combined probe field are used to measure the 3D velocity vector: one of them measures two velocity projections in an orthogonal coordinate basis, the other measures the third projection. The direction of the components of the velocity vector is determined by introducing the carrier frequencies into the laser beams using acousto-optical modulators, the number of which in the measuring system is equal to the number of components of the velocity vector.

However, the disadvantages in this device are reduced measurement accuracy and reliability due to the problem of spatial alignment of the 2D and 1D components of the coordinate measuring base in the probing field, since they are formed by separate and independent three-beam and two-beam optical systems in a spatial angle of the order of π / 2. In such a structure of a laser Doppler velocity meter, three and two beams independently pass through various optical elements in 2D and 1D measuring systems, which leads to an additional decrease in the measurement accuracy due to the influence of mechanical instability and thermal fields. In addition, the use of three acousto-optical modulators to determine the direction of the velocity vector complicates the structure of the measuring system and reduces the reliability of its operation.

Also known laser Doppler speed meter, described in the book [Yu.N. Dubnischev, V.A. Arbuzov, P.P. Belousov, P.Ya. Belousov. Optical methods for studying flows. Novosibirsk, Siberian University Institute, 2003, 418 pp.] On pages 206-209 fig. 4.6, in which a probe field with a 3D coordinate-measuring basis is formed by four laser beams passing through the same optical elements, including three acousto-optical modulators. The structure of the system forms three measuring channels. This system operates in the adaptive frequency selection mode of the velocity vector components with temporal switching of the measuring channels.

However, the disadvantage in this device is the reduced accuracy of velocity measurements, since the switching frequency of the measuring channels, which determine successively the velocity projection time, and, accordingly, the Nyquist frequency, depends on the dynamics of the process under study and the concentration of scattering particles in the medium.

In addition, a known laser Doppler velocity vector meter [patent RU 2638580 C1 G01P 3/36], which is the prototype of the invention. It contains a bichromatic source of two spatially aligned orthogonally polarized monochromatic, differing in wavelengths λ ₁ and λ ₂ laser beams. Sequentially located with it are: the first lens, the acousto-optic traveling wave modulator, oriented at the Bragg angle to the direction of spatially aligned laser beams incident on the modulator, the second lens, the Wollaston prism, whose orientation is consistent with the polarization of the diffracted beams. On the path of one of a pair of dichromatic beams diffracted to zero or minus the first diffraction order, an achromatic phase plate is installed between the acousto-optical modulator and the second Wollaston prism. The relative position of the acousto-optical modulator, the second lens and the Wollaston prism provide optical coupling of the diffracted beam source with its image in the medium under study. The front focus of the third lens is aligned with the splitting point of the light beams of the second polarizing prism. Half-wave phase plates are placed in the path of two of the four beams split by a Wollaston prism. Sequentially with the third lens, the fourth lens is placed — the probing field shaper in the medium under study. On the path of two of the four laser beams split by the Wollaston prism, half-wave phase plates are placed that match the polarizations of the laser beams that form the structure of the probe field in the medium under study. Between the third and fourth lenses there is a swivel mirror. A dichroic mirror is mounted in the path of the light beam reflected by this rotary mirror. On the path of light beams reflected by a dichroic mirror, photodetectors are installed, the outputs of which are connected to analog-to-digital converters (ADCs). A beam-splitting plate is placed between the fourth and third lenses in the path of one of the beams forming the probing field. A quarter-wave phase plate and a mirror are installed in the path of the reflected light beam. A filter and a photodetector, the output of which is connected to an analog-to-digital converter (ADC), are sequentially placed in the path reflected by this mirror and transmitted through a beam splitting plate. The outputs of the analog-to-digital converters are connected to a signal processing system. The action of this device is that two spatially aligned orthogonally polarized, differing wavelengths λ ₁ and λ ₂ beams are sent to the acousto-optic Bragg modulator of the traveling wave. The light beams diffracted into the first and minus first diffraction orders have a relative frequency shift Ω equal to the frequency of the ultrasonic wave in the acousto-optical modulator. The second lens sends these beams to the Wollaston prism. The polarizations of these beams by an achromatic half-wave phase plate are consistent with the orientation of the Wollaston prism. The Wollaston polarizing prism splits incident beams. The relative position of the objective, the acousto-optical modulator and the Wollaston prism provides optical coupling of the source of diffracted beams with its image when the angle between the beams with wavelengths λ _{1 and} λ ₂ incident on the prism, splitting angle is equal. Beams split by a prism, polarizations of which are corrected by half-wave phase plates, are directed by the third and fourth lenses into the medium under study. A probe field is formed in the region of intersection of these beams in the medium under study, which is optically coupled to a source of light beams diffracted in an acousto-optical modulator. The coordinate-measuring basis is determined by the structure of the wave vectors of the laser beams that form the probe field in the medium under study. Chromatically selected images of the probe field in the light scattered by the medium under investigation, using mirrors, lenses, beam splitting and phase plates, are sent to photodetectors, which convert optical fields into photoelectric signals in the optical mixing mode and, through analog-to-digital converters (ADCs), enter the processing system. The carrier frequency of these signals is determined by the frequency of the acousto-optic modulator, and the frequency deviation corresponds to the Doppler frequency shift proportional to the corresponding component of the velocity of the medium under study in the formed coordinate measuring basis.

However, this meter has the disadvantage of reduced speed measurement accuracy due to the difference in the Bragg diffraction angles in the acousto-optical modulator for the chromatic components of the diffracting beam, which is Δϕ = Δλ / Λ _a , where Δλ = λ ₁ -λ ₂ is the wavelength difference of the diffracting beams , Λ _a is the spatial period of the acoustic wave. The result is a distortion of the coordinate-measuring basis, which affects the measurement error and requires compensation. Compensation can be performed constructively, which complicates the implementation of the measuring system, or requires fine control of the spatial, polarization and phase parameters of the light beams diffracted in the modulator by adequate orientation of the polarization prism. In addition, the inconsistency of the Bragg diffraction conditions with the bichromatic structure of the diffracting beam leads to additional energy losses that affect the contrast of the interference structure of the probe field, the signal-to-noise ratio, and, ultimately, the measurement accuracy.

Another disadvantage is the use for measuring each of the three components of the velocity vector of a separate system of chromatic selection of scattered beams and a photodetector, which leads to additional losses of the energy of the light field and, accordingly, a decrease in the signal-to-noise ratio during photoelectric conversion. The measurement system is complicated due to the need to form three independent optical channels in the system, which select the chromatic components of the scattered field and carry information about the orthogonal projections of the velocity vector.

The task (technical result) of the present invention is to increase the accuracy of measuring speed.

The solution of this problem is achieved by the fact that in a known meter containing sequentially arranged: a bichromatic radiation source of two spatially combined orthogonally polarized laser beams with wavelengths λ _r and λ _{g the} first lens; acousto-optic Bragg modulator of a traveling wave; second lens; achromatic half-wave phase plate in the path of one of the diffracted beams; Wollaston’s polarizing prism located in the image plane of the source of the beams diffracted in the acousto-optic modulator, which is consistent with the polarization of the bichromatic beams diffracted in the acousto-optical modulator; the third lens, the front focal plane of which is aligned with the plane of the splitting of the beams in the polarizing prism of Wollaston; half-wave phase plates on the paths of two of a monochromatic pair of beams split by a Wollaston prism with a wavelength of λ _r ; the fourth lens is a probe field shaper in the test medium, a rotary mirror mounted between the third and fourth lens, mounted on the path of a light beam from another monochromatic pair with a wavelength λ _{g a} beam splitter oriented at a Brewster angle and sequentially with it a quarter-wave plate, on the path reflected a quarter-wave phase plate and a mirror are sequentially mounted with a beam splitting plate of the beams; in the path of the light beam scattered in the direction of the incident wavelength λ _r and reflected from the beam splitter plate, a filter for the wavelength λ _r and a photodetector connected through the ADC to the signal processing system are installed in series. In this case, the Bragg acousto-optic modulator is made two-wave with a frequency ratio of Ω _r and Ω _{g of} ultrasonic waves equal to the ratio of wave numbers k _r and k _{g of} diffracting light beams, Ω _r / Ω _g = k _r / k _g . The photodetector is connected to the signal processing system through bandpass filters, the center frequencies of which are equal to the frequencies Ω _r and Ω _{g of} ultrasonic waves in an acousto-optic modulator, in the path of a light field reflected by the medium scattered by the medium under investigation, connected to the signal processing system.

In FIG. 1 shows a structural diagram of the proposed meter.

In FIG. 2 shows the structure of the bichromatic source of two spatially aligned orthogonally polarized laser beams with wavelengths λ _r and λ _g used in the prototype.

In FIG. Figure 3 shows the polarization structure of diffracted beams in a plane in front of a half-wave achromatic phase plate. The solid line indicates the polarization vector of the diffracted beam with a wavelength λ _r , the dashed line indicates the polarization vector of the diffracted beam with a wavelength λ _g .

In FIG. Figure 4 shows the polarization structure of light beams in the plane between the half-wave achromatic phase plate and the Wollaston polarizing prism.

In FIG. Figure 5 shows the polarization structure of laser beams in the plane behind the Wollaston prism in front of half-wave phase plates.

In FIG. Figure 6 shows the polarization structure of the laser beams in the plane between the half-wave phase plates and the lens forming the probe field.

In FIG. 7 shows the polarization structure of light beams split by a Wollaston prism after passing through half-wave phase plates.

In FIG. 8 shows the structure of the probe field in the space of wave vectors.

The proposed meter (Fig. 1) contains a bichromatic radiation source of spatially combined monochromatic orthogonally polarized laser beams 1. In series with the source are located: lens 2; Bragg acousto-optic traveling wave modulator 3; second lens 4; achromatic half-wave phase plate 5; Wollaston 6 polarizing prism; third lens 7; quarter-wave phase plates 8 and 9 in the path of one of each pair of 6 beams split by a polarizing prism; the lens 10, forming a probing field in the studied environment. A rotary mirror 11 is installed between the lenses 7 and 10. On the path of the light beam reflected by the mirror 11, a lens 12 and a photodetector 13 are mounted in series, the output of which is connected to the signal processing system through bandpass filters 14 and 15. A beam splitter plate 16 is installed between the lenses 7 and 10 on the path of one of the monochromatic beams. A quarter-wave phase plate 17 is placed on the path of the light beam passing through the beam splitter 16. A quarter-wave phase plate 18 is sequentially installed in the path of the incident light beam reflected by the beam splitter plate 16 and mirror 19. On the way reflected by the mirror 19 and passed through the beam splitter plate 16, a light filter 20, a lens 21 and a photocopier are sequentially placed receiver 22.

The proposed laser Doppler speed meter works as follows. The bichromatic beam generated by source 1, consisting of spatially aligned and orthogonally polarized monochromatic components with wavelengths λ _r , λ _g , and, accordingly, wave numbers k _r and k _g , is directed by lens 2 to an acousto-optic modulator 3 operating in the Bragg diffraction mode . An exemplary structure of such a bichromatic source used in the prototype of the invention is shown in FIG. 2. The bichromatic source consists of a Wollaston 23 prism and two laser diodes (24 and 25), mutually oriented at an angle of prism splitting. Laser diodes emit, orthogonally polarized monochromatic beams with wavelengths λ _r and λ _g , which the Wollaston polarizing prism spatially align and form a bichromatic light beam. The frequencies Ω _r and Ω _{g of} traveling ultrasonic waves in the acousto-optical modulator 3 satisfy the condition Ω _r / Ω _g = k _r / k _g . Therefore, the Bragg angles for the orthogonally polarized monochromatic components of the bichromatic beam are equal, λ _r Λ _r = λ _g / Λ _g , and, accordingly, the angles between the beams with wavelengths λ _r and λ _g diffracted to zero and minus first are equal. The frequency difference of the diffracted r-components is equal to Ω _r , and the frequency difference of the diffracted g-components is equal to Ω _g . Therefore, the spatial and polarization structures of the diffracted bichromatic beam in the zero and minus first order are the same, and the frequencies of the monochromatic components are different.

An exemplary structure of an acousto-optical modulator is shown in Fig. 3. Here we show the wave vectors K _r and K _{g of} ultrasonic waves excited in the modulator by the electric voltage Usin (Ω _r t) and Usin (Ω _g t), whose frequencies satisfy the condition: Ω _r = K _r V _a ; Ω _r = K _g V _a , where V _a is the speed of the ultrasonic wave; K _r and K _g - wave numbers of ultrasonic waves. The solid and dashed lines are the directions of propagation of bichromatic beams whose orthogonally polarized spatially combined components are: A _r sin (ω _r tk _r ) and A _g sin (ω _g tk _g ), where A _r and A _g are the amplitudes; ω _r and ω _g are frequencies; k _r and k _g are wave vectors.

The polarization structure of diffracted bichromatic laser beams is shown in FIG. 4 (the plane in which diffracted beams propagate is selected vertically for example). The solid vector shows the polarization of the monochromatic r-component of the diffracted beam, and the dashed one shows the polarization of the monochromatic g-component. Achromatic half-wave plate 5 (Fig. 1), mounted on the path of the upper pair of diffracted bichromatic beams passing through the lens 4, rotates the plane of polarization by 90 °.

The polarization structure of the laser beams immediately after passing through the phase plate 5 is shown in FIG. 5. The lens 4 directs the bichromatic beams with a polarizing structure (Fig. 4) to the polarizing prism of Wollaston 6 (Fig. 5) oriented so that the plane of splitting of the orthogonally polarized beams is orthogonal to the plane in which diffracted bichromatic beams propagate.

Polarization prism 6 splits the bichromatic beams with the polarization structure shown in FIG. 5 into monochromatic beams, the polarization structure of which is shown in FIG. 6. The relative position of the acousto-optical modulator 3, lens 4, and polarizing prism 6 provide optical coupling of the source of diffracted beams in the modulator with the source of split beams. In FIG. 7 shows the polarization structure of light beams split by a polarization prism 6 after passing half-wave phase plates 8 and 9. Objectives 7 and 10 form a probe field in the medium under study as an image of a source of laser beams split by a Wollaston prism. For Gaussian beams, it is combined with the plane of intersection of the constrictions.

FIG. 8 illustrates the formation of the structure of the probe field in the space of wave vectors. The directions of the axes of the coordinate measuring basis X, Y, Z are determined by the differences of the wave vectors of the laser beams forming the probe field:

Here k _sr2 = -k _r1 is the wave vector of the light field scattered by the medium under study in the direction opposite to the direction of the wave vector k _{r1 of the} laser beam forming the probe field.

As is known (Yu.N. Dubnischev, BS Rinkevicius. Methods of laser Doppler anemometry. Moscow. Science. Main edition of the physics and mathematics literature. 1982.), the frequency structure of light scattered in the probe field by the investigated medium moving with speed v is determined by the Doppler frequency shifts in the r and g chromatic components of the scattered light, which are proportional to the projections of the velocity vector on the difference of the wave vectors of the laser beams that form the probing field and specify the directions of the axes About basis:

The light scattered in the sounding field by the lens 10, the rotary mirror 11 and the lens 12 is sent to the photodetector 13, operating in the optical mixing mode. As a result of optical mixing in the structure of the photoelectric current at the output of the photodetector 13, components appear whose frequencies are determined by difference combinations of frequencies of the light fields incident on the photosensitive surface of the photodetector:

The components of the photoelectric current are frequency modulated signals. They are selected by bandpass filters 14 and 15 with central frequencies, respectively, Ω _r and Ω _g , which coincide with the frequencies of traveling ultrasonic waves in the acousto-optical modulator 3.

From the structure of wave vectors of light beams forming an orthogonal coordinate-measuring basis, and expressions (7) and (8) it follows:

и

вектора скорости исследуемой среды:From these equations are uniquely determined

and

velocity vectors of the medium under study:

вектора скорости, такая же, как и в прототипе. Референтный пучок формируется из падающего в измерительном канале, содержащем светоделительную пластинку 16, фазовые пластинки 17-18, зеркало 19, фильтр 20, объектив 21 и фотоприемник 22.The width of the filter bands is determined by the maximum range of measured speeds. The filtered signals through the ADC enter the processing system that measures the frequencies and, accordingly, the components of the velocity vector. The structure and operation of the optical measuring channel, which determines

velocity vector, the same as in the prototype. The reference beam is formed from the incident in the measuring channel containing the beam splitter plate 16, phase plates 17-18, mirror 19, filter 20, lens 21 and photodetector 22.

The frequency of the photoelectric current at the output of the photodetector 22:

Hence, in view of FIG. 8:

The technical result of the invention is to improve the accuracy of speed measurement. Improving accuracy is achieved by performing a two-wave Bragg acousto-optic modulator with a ratio of traveling ultrasonic waves frequencies equal to the ratio of wave numbers of diffracted light beams, which ensures the identity of the spatial and polarization structure of the bichromatic components diffracted to zero and minus first diffraction orders and the conditions of their spatial transformations of the polarization prism . This implies the implementation of another technical solution consisting in the use of a single photodetector for photoelectric conversion of a bichromatic light field scattered by the medium under study and frequency selection of the photoelectric current by bandpass filters with subsequent parallel measurement of Doppler frequency shifts that carry information about the magnitude and direction of two orthogonal components of the velocity vector. In the prototype, the possibility of a single-channel photoelectric conversion of a bichromatic scattered field is absent due to the equality of the carrier frequencies of its chromatic components.

Claims (1)

Laser Doppler speed meter containing a sequentially located bichromatic source of two spatially aligned orthogonally polarized laser beams with wavelengths λ _r and λ _g , the first lens, an acousto-optic Bragg traveling wave modulator, a second lens, an achromatic half-wave phase plate placed in the path of one of the diffracted beams , Wollaston's polarizing prism located in the image plane of a beam diffracted in an acousto-optic modulator s, the third lens, the front focal plane of which is aligned with the plane beam splitting in the polarization prism of Wollaston, half-wave phase plate in the ways the two split prism of Wollaston beams with wavelengths λ _r and λ _g, the fourth lens - shaper probing optical field in the test medium, installed between the third and fourth lenses a swivel mirror mounted in the path of a light beam from a monochromatic pair with a wavelength λ _{g a} beam splitter oriented at an angle Brewster’s scrap, and sequentially with it a quarter-wave plate, in the paths of the beams reflected by the beam splitter plate, a quarter-wave phase plate and a mirror are sequentially installed; in the path of the light beam scattered in the direction of the incident wavelength λ _r and reflected from the beam splitter, an optical filter with a length of λ _r waves and a photodetector connected through an ADC to a signal processing system, characterized in that the Bragg acousto-optic modulator is made of a two-wave with the ratio of the frequencies Ω _r and Ω _{g of the} ultrasonic waves, equal to the ratio of the wave numbers k _r and k _{g of the} diffracting light beams, and the photodetector installed in the path of the light field scattered by the medium under study is connected to the signal processing system through bandpass filters whose central frequencies are equal frequencies Ω _r and Ω _{g of} ultrasonic waves in an acousto-optical modulator.

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