RU2638110C1 - Laser doppler velocimeter - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: laser doppler velocimeter contains radiation source of two spatially superposed laser beams, the first lens, Bragg acousto-optic modulator of traveling wave, the second lens, first Wollaston prism, optical shaper of probing field, first light-sensitive detector, achromatic half-wave retarder, first and second half-wave dispersion retarders, first and second half-wave retarders, collimator, axicon, second and third Wollaston prisms, confocal lens system, chromatic filter, dichroic mirror, second light-sensitive detector.

EFFECT: increase in the accuracy of measurements is achieved through the use of Bessel beams, which ensure the uniformity of the spatial-frequency structure of probing field and the reduction of measuring volume.

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Description

The invention relates to measuring technique and can find application in experimental hydro- and aerodynamics, thermophysics, biophysics, medicine and industrial technologies related to the need to study the flow rates of gas and condensed matter.

[Ю.Н. Дубнищев, В.А. Арбузов, П.П. Белоусов, П.Я. Белоусов. Оптические методы исследования потоков. Новосибирск, Сибирское университетское из-во, 2003, 418 с.], где K₀ - пространственная частота в области пересечения перетяжек, R₀ - конфокальный параметр гауссова пучка, x - расстояние от плоскости пересечения перетяжек, K₀=2π/Λ₀, Λ₀ - пространственный период интерференционного поля в плоскости пересечения перетяжек.Known laser Doppler speed meters, the effect of which is based on the Doppler effect in laser radiation scattered by the medium under study. Laser Doppler velocity meters are used to measure the speed, in which the probe field is configured of three laser beams [US Pat. No. 4,838,687], or four laser beams [US Pat. No. 3,897,152 A], the devices comprising a laser source, a beam splitter, and an optical probe for the probing field , and the laser beams have a Gaussian intensity distribution in the cross section. A sounding field in such devices is formed as an interference field, which is formed when Gaussian beams intersect in the medium under study. In the region of intersection of Gaussian beams, the interference field is spatially limited by the surface of the ellipsoid. One of the axes of the ellipsoid is oriented along the bisector of the angle between the directions of the interfering laser beams. In sections of the probe field that do not coincide with the intersection of the constrictions of the Gaussian beams, the spatial frequency of the interference field K _x decreases according to the law

[Yu.N. Dubnischev, V.A. Arbuzov, P.P. Belousov, P.Ya. Belousov. Optical methods for studying flows. Novosibirsk, Siberian University University, 2003, 418 pp.], Where K ₀ is the spatial frequency in the region of the intersection of the constrictions, R ₀ is the confocal parameter of the Gaussian beam, x is the distance from the plane of intersection of the constrictions, K ₀ = 2π / Λ ₀ , Λ ₀ is the spatial period of the interference field in the plane of intersection of the constrictions.

However, in these devices, the disadvantage is the reduced measurement accuracy due to the indicated change in spatial frequency, determined by the curvature of the wavefront of Gaussian beams (this curvature is zero in the waist plane). It leads to an error in the measurement of speed. For example, in the case of a laminar flow (movement at a constant speed), this error is interpreted as the appearance of “virtual turbulence” with a relative intensity of the order of 10 ^-2 ÷ 10 ^-3 .

Also known is a laser Doppler speed meter described in [A. Voight, S. Heitrfm, L. Buttner, and J. Czarske, "A Bessel beam laser Doppler velocimeter," Opt. Commun., Vol. 282, No. 9, pp. 1874-1878, (2009)], containing a laser source, axicon, diffraction grating, optical probing field shaper, photodetector connected to the signal processing system, while the probing field for measuring one component of the velocity vector is proposed to be formed by the intersection of two Bessel beams. As is known, the wavefront of a Bessel beam is flat. Therefore, in the structure of the probing field formed by the interference of Bessel beams, the spatial frequency is constant. This allows us to exclude the measurement error associated with the inconstancy of the spatial frequency of the probe field formed by the interference of Gaussian beams. In this device, a Bessel beam is formed as a result of diffraction of a Gaussian laser beam on an axicon. The Bessel beam is directed to the diffraction grating, where, as a result of Raman-Nath diffraction, it is converted into two beams diffracted by +1 and -1 orders of magnitude. Zero diffraction order is blocked. The diffracted beams are sent by the optical system to the medium under study, where a probe field with a homogeneous spatial-frequency structure is formed in the intersection region of the Bessel beams.

However, in the indicated device, the main disadvantage is the impossibility of measuring two orthogonal components of the velocity vector and low energy efficiency due to the non-use (blocking) of a Bessel beam diffracted to zero order during Raman-Nath diffraction on a diffraction grating.

In addition, the known laser Doppler speed meter described in the book [Yu.N. Dubnischev, V.A. Arbuzov, P.P. Belousov, P.Ya. Belousov. Optical flow studies. Novosibirsk, Siberian University Publishing House, 2003, 418 pp., Pp. 230-231, Fig. 4.20], which is the prototype of the present invention and containing a source of two spatially aligned polarized laser beams, in series with which are located: an acousto-optic traveling wave modulator oriented at a Bragg angle to the direction of spatially aligned polarized laser beams, and a Wollaston prism, the orientation of which is consistent with the orientation of the acousto-optical modulator . A lens is placed between the source of two spatially aligned polarized laser beams and an acousto-optic modulator. A second lens is placed between the acousto-optical modulator and the Wollaston prism, which performs their optical conjugation. In series with the Wollaston prism, an optical probe of the probe field in the medium under study and its image formed in the light scattered by the medium under study are located on a photodetector connected to a signal processing system. The operation of the device consists in the fact that two polarized laser beams are spatially aligned and directed to an acousto-optic Bragg modulator of a traveling wave. The light beams diffracted in the first and minus first orders are directed to the Wollaston prism, the orientation of which is consistent with the polarization of the incident beams. The light beams split in pairs by the Wollaston prism coordinate according to polarization and direct to the medium under study. A probe field in the medium under study is formed by the intersection of light beams whose frequencies differ by the frequency of the ultrasonic wave in the modulator. An image of the probing field is formed in the light scattered by the test medium and its photoelectric conversion is performed. The frequency of the photoelectric field is measured.

However, this meter has the disadvantage of reduced measurement accuracy due to the formation of the probing field by Gaussian beams and the resulting spatial-frequency inhomogeneity.

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

The problem is achieved in that in the known device containing a sequentially located radiation source of two spatially aligned laser beams, consisting of two lasers and an optical element, spatially combining the directions of laser radiation, the first lens, a Bragg acousto-optical traveling wave modulator, the second lens, the first Wollaston prism the orientation of which is consistent with the orientation of the laser beams of the specified radiation source, the optical probe of the probe along I, in the path of the light beam scattered by the medium under study, a rotary mirror and the first photodetector connected to the signal processing system are arranged in series, an achromatic half-wave phase plate, the first and second dispersive half-wave phase plates, the first and second half-wave phase plates, a collimator, an axicon are introduced, second and third Wollaston prisms, confocal lens system, chromatic filter, dichroic mirror, second photodetector. In this case, a bichromatic source of polarized laser radiation was used as a radiation source of two spatially aligned polarized laser beams; an achromatic half-wave phase plate was placed in the path of one of the diffracted beams between the Bragg acousto-optic traveling wave modulator and A collimator, axicon, second and third Wollaston prisms are sequentially installed between the first Wollaston prism and the optical imager of the probing field, between the second and third Wollaston prisms a confocal lens system is installed, optically interfacing the planes of these prisms, between the confocal lens system and the third Wollaston prism on the way of each of the light beams placed the first and second phase half-wave plates, the optical probe of the probing field is made in Ideal confocal lens system. A chromatic filter is placed between the third Wollaston prism and the optical imaging probe field on the paths of one pair of orthogonally polarized split third light beams of the Wollaston prism, and the first and second half-wave phase plates are placed sequentially on the paths of another pair of identically polarized beams. A dichroic mirror is installed between the optical imaging probe of the probe field and the first photodetector in the path of the light beam scattered by the medium under study, the transmittance and reflection coefficients of which are consistent with the bichromatic laser radiation source, and a second photodetector is installed in the image plane of the probe field formed in the reflected dichroic mirror connected to a signal processing system.

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

In FIG. 2 shows a perspective view of the structural diagram of the proposed meter.

In FIG. 3 is a vector diagram of light beams forming a measuring volume.

In FIG. Figure 4 shows the polarization structure of laser beams in the plane after the second Wollaston prism.

In FIG. 5 shows the polarization structure of Bessel beams in a plane in front of the third Wollaston prism.

In FIG. Figure 6 shows the polarization structure of Bessel beams in the plane between the third Wollaston prism and phase half-wave plates.

In FIG. 7 shows the polarization structure of Bessel beams in the plane between the half-wave phase plates and the optical system for the formation of the probe field.

The proposed meter (Fig. 1) contains lasers 1, 2 and a mirror prism 3, forming a source of a bichromatic light beam. Consistently with the source are: lens 4, Bragg acousto-optic traveling wave modulator 5, lens 6, achromatic half-wave phase plate 7, first Wollaston prism 8, collimator consisting of lenses 9 and 10, axicon 11, second Wollaston prism 12, confocal lenses 13 and 14, half-wave dispersion phase plates 15 and 16, the third prism of Wollaston 17. Sequentially with the prism 17 are located: a chromatic filter 18, half-wave phase plates 19 and 20, an optical probe of the probe field, with consisting of confocal lenses 21 and 22. A rotary mirror 23 is placed between the lenses 21 and 22. In the path of the light beam reflected by the mirror 23, there are: a dichroic mirror 24, a lens 25 and a photodetector 26. In the path of the light beam reflected by a dichroic mirror 24 a lens 27 and a photodetector 28. The outputs of the photodetectors 26 and 28 are connected to a signal processing system.

The proposed laser Doppler speed meter works as follows. Linearly polarized light beams, one of which is emitted by laser 1 at a wavelength of λ ₁ , and the other by laser 2 at a wavelength of λ ₂ , are directed to a mirror prism 3, reflected in parallel directions and lens 4 are directed at Bragg angles, φ ₁ = λ ₁ / 2Λ _a and φ ₂ = λ ₂ / 2Λ _a , on the acousto-optic traveling wave modulator 5. Laser beams incident on mirror prism 3 have the same polarization, TE or TM. After reflection from prism 3, they are converted by lens 4 into light beams intersecting in the waist plane at an angle φ ₁₂ :

,

,

where Λ _a is the spatial period of the ultrasonic wave in the acousto-optical modulator 5.

The laser beams diffracted in the first and minus first orders have a relative frequency shift Ω equal to the frequency of the ultrasonic wave traveling in the modulator 5. These beams are directed by the lens 6 to the Wollaston prism 8. The lens 6 converts the diffracted bichromatic beams with an angular increase of γ equal to the ratio of the splitting angle α characterizing the Wollaston prism 8 to the doubled Bragg angle, which is calculated as:

,

,

.Where

.

On the path of laser beams with wavelengths λ ₁ and λ ₂ diffracted by first or minus first orders, an achromatic half-wave phase plate 7 is installed. It matches the polarization of the beams incident on the polarizing prism 8. The Wollaston prism 8 spatially combines the incident ones at an angle α orthogonally polarized beams. A collimator, consisting of lenses 9 and 10, expands the light beam and directs it to axicon 11. Axicon 11 forms a Bessel beam, which is directed to the Wollaston prism 12, where it splits into two bichromatic orthogonally polarized Bessel beams with a relative frequency shift Ω. Bessel beams by an optical system consisting of lenses 13 and 14 are directed to the Wollaston prism 17 at an angle equal to the angle of splitting of the polarizing prism.

In FIG. 4 shows the polarization structure of Bessel beams in a plane 29 immediately behind the lens 14. These bichromatic Bessel beams are orthogonally polarized. The dispersed half-wave phase plates 15 and 16 rotate the plane of polarization of one of the chromatic components λ ₁ by 90 °. The polarization structure of the Bessel beams in the plane 30 immediately after the half-wave phase plates 15 and 16 is shown in FIG. 5. These beams by the Wollaston prism 17 are split into four chromatically selected Bessel beams, the polarizations of which in plane 31 are shown in FIG. 6. The centers of these beams, as can be seen from FIG. 6 are located at the vertices of a rectangle close to a square. One diagonal of the rectangle (xx) passes through the section of beams whose color corresponds to the wavelength λ ₁ . Another diagonal (y'y ') passes through sections of beams whose color is determined by the wavelength λ ₂ . The intersecting diagonals xx and y'y 'define the directions of the axes of the coordinate measuring base, which is close to the orthogonal. The degree of approximation to the orthogonal basis (with the xx and yy axes) depends on the difference in the Bragg angles Δφ upon diffraction by ultrasonic wave of laser beams with wavelengths λ ₁ and λ ₂ :

where Δλ = λ ₁ -λ ₂ ;

Δ _a is the spatial period of the ultrasonic wave in the acousto-optical modulator.

From (1) follows the estimation of the nonorthogonality of the axes of the coordinate measuring base:

.

.

. Центры сечений бесселевых пучков, селектированных на длинах волн λ₁ и λ₂, соответственно локализованы на взаимно ортогональных диагоналях квадрата. В случае недостаточно качественной хроматической селекции бесселевых пучков, сформированных призмой Волластона 17, на пути соответствующих диагональных пар пучков устанавливаются хроматический фильтр 18 с пропусканием на длине волны λ₁. Согласование поляризаций монохроматических диагональных пар пучков достигается использованием полуволновых пластинок 19 и 20 на длинах волн λ₁ и λ₂. Согласованная поляризационная структура монохроматических диагональных пар бесселевых пучков в плоскости 32 после фазовых пластинок 19 и 20 показана на Фиг. 7. Конфокальная оптическая система, состоящая из объективов 21 и 22, формирует в исследуемой среде зондирующее оптическое поле, локализованное в области пересечения монохроматических пар бесселевых пучков. Масштаб изображения зондирующего поля определяется отношением фокусных расстояний объективов 21 и 22. Пространственно-частотная структура монохроматических зондирующих полей определяется интерференцией монохроматических пар бесселевых пучков. Взаимная ориентация интерференционных полос в пространственно совмещенных монохроматических зондирующих полях ортогональна. Пространственно-частотная структура монохроматических зондирующих полей однородна, поскольку волновой фронт бесселевых пучков плоский. Максимальный диаметр зондирующего поля определяется пространственным интервалом, соответствующим главному максимуму бесселевой функции нулевого порядка, описывающей поперечное сечение бесселевого пучка. Поперечный размер зондирующего поля оценивается как диаметр главного максимума бесселевой функции нулевого порядка, деленный на тангенс половины угла между волновыми векторами интерферирующих бесселевых пучков. Зондирующее поле ограничивается поверхностью эллипсоида вращения, ось которого ориентирована по биссектрисе угла между волновыми векторами бесселевых пучков.When λ ₁ = 0.532 μm (green) and λ ₂ = 0.65 μm (red) Δψ = 0.05. Correction of the orthogonality of the coordinate-measuring basis, if necessary, is achieved by rotating the polarization prism 8 relative to the optical axis by an angle Δψ and compensating rotation of the beam polarization at the output of the polarizing prism 3 using, for example, a half-wave achromatic phase plate or structurally. In practice, λ ₁ = 0.55 μm and λ ₂ = 0.65 μm, the orthogonality of the coordinate-measuring basis without any adjustments is performed with an angular deviation Δψ≈2 °, which contributes to the measurement error of the projections of the velocity vector, not exceeding

. The centers of the cross sections of Bessel beams selected at wavelengths λ ₁ and λ ₂ , respectively, are localized on mutually orthogonal diagonals of the square. In the case of insufficiently qualitative chromatic selection of Bessel beams formed by the Wollaston prism 17, a chromatic filter 18 with transmission at a wavelength of λ _{1 is} installed in the path of the corresponding diagonal pairs of beams. Coordination of polarizations of monochromatic diagonal beam pairs is achieved using half-wave plates 19 and 20 at wavelengths λ ₁ and λ ₂ . The coordinated polarization structure of monochromatic diagonal pairs of Bessel beams in plane 32 after phase plates 19 and 20 is shown in FIG. 7. The confocal optical system, consisting of lenses 21 and 22, forms a probing optical field in the medium under study, localized in the region of intersection of monochromatic pairs of Bessel beams. The image scale of the probe field is determined by the ratio of the focal lengths of the lenses 21 and 22. The spatial-frequency structure of the monochromatic probe fields is determined by the interference of the monochromatic pairs of Bessel beams. The mutual orientation of interference fringes in spatially aligned monochromatic probing fields is orthogonal. The spatial-frequency structure of monochromatic probe fields is homogeneous, since the wavefront of Bessel beams is flat. The maximum diameter of the probe field is determined by the spatial interval corresponding to the main maximum of the zero-order Bessel function, which describes the cross section of the Bessel beam. The transverse size of the probe field is estimated as the diameter of the main maximum of the zero-order Bessel function divided by the tangent of half the angle between the wave vectors of the interfering Bessel beams. The probe field is limited by the surface of the ellipsoid of revolution, whose axis is oriented along the bisector of the angle between the wave vectors of the Bessel beams.

Images of monochromatic probe fields in light scattered by optical inhomogeneities (particles) moving in the medium under study, a rotary mirror 23, a dichroic mirror 24, and lenses 25 and 27 are chromatically selected and formed respectively on photodetectors 26 and 28. The particle modulation frequency of the image is determined as the ratio of the velocity movement to the monochromatic spatial period of the interference structure:

where Λ _x and Λ _y are the sizes of interference fringes formed respectively by monochromatic Bessel beams with λ ₁ and λ ₂ .

Moreover, the indicated dimensions of the interference fringes are determined as:

where 2θ ₁ is the angle between the wave vectors of Bessel beams with a wavelength of λ ₁ ;

2θ ₂ is the angle between the wave vectors of Bessel beams with a wavelength of λ ₂ .

The frequencies ƒ _Dx and ƒ _{Dy are} determined through Doppler frequency shifts in light fields scattered by moving particles (optical inhomogeneities) when crossing Bessel monochromatic beams forming the corresponding probing field:

.Where

.

Similarly

.Where

.

The technical result consists in that it improves the measurement accuracy of the proposed device, which, in contrast to the known laser Doppler velocity meter based on the use of Gaussian beams, uses Bessel beams having a plane wave front in cross section, the size of which is determined by the main maximum of the Bessel function of zero order . The homogeneity of the spatial-frequency structure of the probe field in a Bessel LDIS (BLDIS) is an order of magnitude higher than that of a Gaussian LDIS, and the measurement volume is an order of magnitude smaller at the same angles between the directions of the laser beams forming the probe field.

Claims (1)

Laser Doppler speed meter containing a sequentially located radiation source of two spatially aligned laser beams, consisting of two lasers and an optical element that spatially combines the directions of laser radiation, the first lens, a Bragg acousto-optic traveling wave modulator, a second lens, the first Wollaston prism, the orientation of which is consistent with the orientation of the laser beams of the specified radiation source, the optical probe of the probing field, in the path of the light a sample dispersed by the medium under study, a rotary mirror and a first photodetector connected to a signal processing system are arranged in series, characterized in that an achromatic half-wave phase plate, the first and second dispersive half-wave phase plates, the first and second half-wave phase plates, a collimator, an axicon are inserted , the second and third prisms of Wollaston, a confocal lens system, a chromatic filter, a dichroic mirror, a second photodetector, while two x spatially aligned polarized laser beams, a bichromatic source of polarized laser radiation was used, between the Bragg acousto-optic traveling wave modulator and the first Wollaston prism, an achromatic half-wave phase plate was placed on the path of one of the diffracted beams, between the first Wollaston prism and the probing optical field shaper, the axial field probe was installed in series second and third prisms of Wollaston, between the second and third prisms of Wollaston last a confocal lens system was installed that optically matches the location planes of these prisms, between the confocal lens system and the third Wollaston prism, the first and second phase half-wave plates are placed in the path of each of the light beams, the optical probe shaper is made in the form of a confocal lens system, between the third Wollaston prism and the optical driver of the probing field on the paths of one pair orthogonally polarized by the light of the third Wollaston prism of the beams, a chromatic filter is placed, and the first and second half-wave phase plates are placed sequentially with it on the paths of another pair of equally polarized beams, a dichroic mirror is installed on the path of the light beam scattered by the medium under study, the transmission and reflection coefficients which are consistent with the bichromatic source of laser radiation in the image plane of the probe field formed in the reflected dichroic mirror th beam has a second photodetector connected to the signal processing system.

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