SU1099284A1 - Laser doppler speed meter - Google Patents

Abstract

. A LASER DOPPLER SPEED METER containing a laser and optically focusing and collecting lenses optically matched with it, an aperture diaphragm, a polarizing accelerator, two photodetectors and a Doppler frequency processing unit, characterized in that, in order to increase the sensitivity, a duplicator has been inserted into it; with different frequencies and mutually orthogonal polarizations, installed between the laser and the focusing lens and made mainly in the form of optically matched expansions Divisor, the pair of rotators polarization plane and a pair of quarter-wave plates arranged in series in the path of the split beams, and mixer inputs connected to the outputs of the photodetectors, and output - to the Doppler processing unit. Have

Description

The invention relates to a measurement technique and can be used to measure the local flow rate of a liquid or gas by optical means. A laser Doppler velocity measurer is known, which contains a laser and a lens that focuses and collects optically compatible with it, an aperture, a polarization splitter, two photodetectors and a block of Doppler frequency, and a splitter between the laser and the focus lens and the differential amplifier, through which the outputs of photodetectors are connected to the processing unit of the Doppler frequency l) However, the known device has insufficient sensitivity, which limits the lower azone measurable velocities. The aim of the invention is to increase the sensitivity of a laser Doppler velocity meter. The goal is achieved by the fact that a laser aperture velocity meter, which contains a laser and a focusing aperture diaphragm and a lens aperture diaphragm, a polarization splitter, two photodetectors and a Doppler frequency processing unit, optically aligned, has a shaper of two dual beams with different frequencies and mutually orthogonal frequencies polarization, installed between the laser and the focusing lens and made mainly in the form of optically matched splitters; pairs of rotators polarization-plane and a pair of quarter-wave plates arranged in series in the path of the split beams, and mixer inputs connected to the outputs of the photodetectors, and output - to block Obra botki.doplorovskoy frequency. Ya FIG. Figure 1 shows a block diagram of a laser Doppler velocity meter, operating on scattered forward light; in fig. 2-6 are options for constructing a device for forming two beams, each of which has different frequencies and mutually orthogonal linear polarizations; in fig. 7 is a block diagram of a laser Doppler velocity meter operating on back scattered light; in fig. 8 through 11 are variants for constructing a device for forming two beams, each of which has different frequencies and mutually orthogonal circular polarizations; in fig. 12 - block diagram of a laser doplerob speed meter operating in backscattered light. The laser Doppler meter (Fig. 1) consists of laser 1 emitting beam 2, device 3 forming two beams 4 and 5, each of which represents a superposition of two spatially .7 aligned beams having different frequencies f and fn to mutually orthogonal linear polarization, focusing of the lens 6, the measurement area 7 moving with the velocity V of the stream collecting the scattered light 8. of the objective 9, the aperture stop 10 with a cross-shaped hole, polarization 11 .11, two photodetectors 12 and 13, the mixer 14 and the meter 15 oplorovskoy frequency. The two beamforming devices 3 consist of a splitter 16, rotators 17 and 18 of the polarization plane, quarter-wave plates 19 and 20 (Fig. 2); from a frequency shift device 21, consisting of quarter-wave plates 22 and 23 and a rotating quarter-wave plate 24, and a half-wave plate 25 ("Hz"); a quarter-wave plate 26 and a half-wave plate 27, with a 28 Zeeman laser (Fig. 4); from the Jamin interferometer, consisting of two translucent mirrors 29 and 30 and two mirrors 31 and 32, the frequency shift of the device 33, the half-wave plates 34 and 35, the mirror 36 (figure 5); from two quarter-wave plates 37 and 38, with 39 being a ring laser emitting two spatially separated beams 2 (Fig. 6). In the laser speed meter shown in FIG. 7, unlike the circuit in FIG. 1, a polarization splitter 11 is installed, consisting of a quarter-wave plate 40 and a polarization prism 41. In this scheme, a two-beam forming device 3 consists of a splitter 42 and rotators 43 and 44 of the polarization plane (Fig. 8); an orthogonal polarization converter 45 consisting of quarter-wave plates 46 and 47 and a half-wave plate 48, with 49 being a Zeeman laser (Fig. 9); from a frequency shift device 50, consisting of a rotating half-wave plate 51 to a quarter-wave plate 52, two quarter-wave plates 53 and 54 (Fig. 10); of the beam splitter 55 and two pivoting mirrors 56 and. 57, with 58 being a Zeeman laser (Fig. P) In the laser speed meter shown in Fig. 12, in contrast to the diagrams in Figs. 1 and 7, a mirror 59 is installed and a diaphragm 60 with a slit-like opening lying flat in the beams 4 and 5. The laser Doppler velocity meter (Fig. 1) works as follows: Laser 1 emits a linearly polarized beam 2, which enters the input device 3 of the formation of two beams, whose output produces two parallel beams 4 and 5, and beam 4 represents the superposition of two spatially aligned beams, one of which, vertically polarized., has a frequency ff, and the other has horizontal polarization, a frequency f2. Beam 5 also has one of two beams, one of which polarized in a vertical plane has a frequency fe, and the other, horizontally polarized - frequency f. Next, rays 4 and 5 are focused by lens 6 into the stream area under study 7. Radiation I scattered by moving particles is collected by lens 9 within the spatial region limited by a diaphragm 10, and directed by lens 9 to the input of polarization prisms 11, which divides the scattered radiation into two beams, one of which, horizontal polarization, enters the photodetector T2, and the other, having vertical polarization, photodetector 13. As a result of optical heterodyne, the output of the photodetector 12 is formed the signal component is variable at the frequency .. -An + (0tf, and at the photo detector device TK - is equal to it in amplitude the signal component is variable at the frequency BL-oz, where b. D 2i (,). The signals from the outputs of the photodetectors T2 and 13 are fed to the inputs of the mixer 4 and 14, the output of which is different. The difference difference frequency is 2 V | which is measured by a Doppler frequency meter 15. Let us consider in more detail the work of various options for constructing the diagrams of the device 3 forming two beams, which can be used in the considered scheme. In device 3 (FIG. 2), beam 2 is split by splitter 16 into two beams. One of the beams 4 is fed to a rotator 17 of the plane of polarization, which, for example, represents an electro-optical plate placed in a rotating with angular velocity D P half-wave electric field. In this case, at the output of the quarter-wave plate 19, there are vertically and horizontally polarized wave components with frequencies 00, Wo + LL and 4; .i (iq; -ftnj fMJ (j-frequency of laser radiation). Beam 5 passes through a similar, rotating The polarization plane 18 and the quarter-wave plate 20, the azimuth of the axis of which is b, 0, + 90 ° where is the azimuth of the axis of the quarter-wave plate 19. In this case, the output of the plate 19 also forms vertically and horizontally polarized component waves, respectively Oj Sd) o- bn, and W, COo + u «,. In device 3 (FIG. 3), a linearly polarized beam 2 is fed to the input of a frequency shifting device 21, which consists of a series-installed quarter-wave plate 22 rotating with an angular velocity LL, a quarter-wave plate 24 (for example, an electro-optical crystal placed in a rotating quarter-wave electric field) and a quarter-wave plate 23. At the output of such a device 21, there are two spatially combined horizontally and vertically polarized waves with frequencies of 0 ° IO-2La. This beam is divided by splitter 16 into two beams 4 and 5, and beam 5 passes through a half-wave plate 25, which is used to rotate the plane of polarization of the wave a 90, In device 3 (FIG. 4), a Zeemaw two-frequency laser 26 emits an AC 2, representing the superposition of two mutually orthogonally polarized waves with Si and -Yo frequencies. This beam is converted into two waves with mutually orthogonal linear polarizations, which are split by splitter 16 into two beams 4 and 5, using a quarter-wave plate 26, on the path of the beam 5 y The half-wave plate 27 is set to rotate the plane of polarization of the beam 5 by 90. In device 3 (Fig. 5), the linearly polarized beam 2 with the frequency SO is divided by the beam splitter 29 into two beams, one of which is then divided by the beam splitter 30 into two beams 4 and 5. Beam 5, reflected from the beam splitter 30, passes through a half-wave plate 35, designed to rotate the plane of polarization of the beam through 90 °. The second beam, reflected from the beam splitter 29 and the mirror 31, proho. Dit through the frequency-shifting device 33 (for example, the Bregt acousto-optic cell), where it is shifted in frequency. Further, this beam, after reflection from the mirror 32 and passing the half-wave plate 34, intended to rotate the polarization plane by 90, is divided by the beam splitter also into two beams 4 and 5. Thus, beam 4 represents the superposition of two spatially aligned beams having horizontal and vertical polarization and, respectively, the frequencies O ioo, G3o + uQ ,. Beam 5 also consists of horizontally and vertically polarized components of the waves at respectively CO frequencies (Op + yH, IMO. In device 3 (Fig. 6), a ring laser 39 with an anisotropic resonator reHepHpyet two spatially separated beams 2, each of which consists of two orthogonally polarized components at frequencies of Co, HW. Using quarter-wave plates 37 and 38, the azimuth of the axis of one of which is rotated by 90 relative to the azimuth-axis of the second plate, two beams 4 and 5 are formed, orthogonal linear Hbie p Consider the feature of the laser Doppler meter scheme (Fig. 7), which uses a 3 dp device to form two beams 4 and 5, each of which consists of two mutually orthogonal circularly polarized beams with frequencies and f 2. and 5. is focused by the lens 6 into the measurement area. The scattered radiation 8 is collected by the lens 9 and guided to the input of a polarization splitter 11 consisting of a quarter-wave plate 40 and a polarization prism 41. The scattered radiation 8 has a circular polarization It is converted by a quarter wave plate 40 into linearly polarized radiation. A polarization prism 41 divides the scattered radiation 8 into two beams, one of which is directed to the photodetector 12, and the other to the photodetector 13. As a result of optical heterodyning, a signal is generated at the frequency of the O. Urfi and at the output of the photodetector 13 - at the frequency DY-COg-. These signals are received by n, and the mixer, at the output of which the differential frequency is 2D-, which is measured by the Doppler frequency meter 15. . Consider the operation of the various device 3 forming two beams, which may be used in the circuit of FIG. 7. In device 3 (FIG. 8), beam 2 is divided by splitter 42 into two beams 4 and 5, which further pass through rotators 43 and 44 of the polarization plane. If device 43 rotates the plane of polarization of the beam, for example, clockwise, and device 44 rotates counterclockwise, then beam 4 consists of right and left circularly polarized waves, respectively, at frequencies CJ, iy, and beam 5 consists of left and right polarized waves, respectively, at frequencies from to to GJ, In device 3 (FIG. 9), a Zeeman laser 49 emits beam 2, which is divided by splitter 42 into two beams 4 and 5, with beam 5 passing through an orthogonal polarization converter (consisting of located even ertvolnovoy, half-wave and quarter-wave plates), which converts the polarization of the input beam field on mutually orthogonal polarization state. In device 3 (Fig. 10), beam 2 enters the input of frequency shifter 50, for example, consisting of a rotating half-wave plate 51 and a quarter-wave plate 52, at the output of which two orthogonal linearly polarized component waves are formed. with Further, beam 2 is divided by splitter 42 into two beams 4 and 5, on the way of which a quarter wave plates 53 and 54 are installed, and the azimuth 0 of the axis of the plate 53 is equal, where S is the azimuth of the axis of the plate 54. In device 3 (Fig. 11) Zeeman laser 58 emits beam 2, which is divided by a beam splitter 55 into two beams. One beam passes through the beam splitter 55, and the second, reflected from the beam splitter 55, is directed to mirrors 56 and 57. Since the reflection causes a change in the direction of the electric vector, beam 5 consists of two components at frequency f and f that have mutually orthogonal circular polarization with respect to the corresponding components of beam 4 at frequencies and 2. Laser Doppler velocity meter (Fig. 12) works as follows. The laser 1 emits a beam 2, which enters the input of a device for forming two beams, which can be performed either according to the schemes in FIG. 2-6 or according to the schemes in FIG. 8-11. Parallel beams 4 and 5 are focused by a tube 6 into area 7 of measurement. The scattered back radiation 8 is collected by 1 84. lens 6 within the range of aperture 60 and, after reflection from mirror 59, is directed by lens 9 to a polarization splitter 11. If beams 4 and 5 consist of two linearly polarized components, one of which are polarized in the plane of the rays 4 and 5, the scattered radiation retains its polarization state unchanged when receiving radiation within a slit-like diaphragm. In this case, the polarization splitter 11 divides the beam 8 into two beams, one of which, polarized in the horizontal plane, is directed to the photodetector 12, and the other, polarized in the vertical plane, to the receiver 13. As a result of optical heterodyning of the photodetectors, Hai output 12 and 13, respectively, the signals + Og and U p -% are formed, which at the dj frequency G4 mixer. Sigrye come on cash differential frequency output in the meter 15 mixer comes Doppler frequency. The proposed meter has a higher sensitivity than the known. A twofold increase in sensitivity is achieved due to the use of two beams and a two-channel receive circuit in the meter device. Increasing the sensitivity allows the lower range of measured speeds to be expanded and the accuracy of measuring very low speeds to be improved.

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Claims (1)

LASER DOPLER A SPEED MEASURER containing a laser and a focusing and collecting lenses optically matched with it, an aperture diaphragm, a polarizing splitter, two photodetectors and a Doppler frequency processing unit, characterized in that, in order to increase the sensitivity, a shaper * of two twin beams with different beams is introduced into it frequencies and mutually orthogonal polarizations, mounted between the laser and the focusing lens and made mainly in the form of optically matched splitter, a pair of rotators polarization-plane and a pair of quarter-wave plates arranged in series in the path of the split beams, and mixer inputs connected to the outputs of the photodetectors, and output - to the Doppler processing unit. Φοι 7