DE4240769A1 - Measuring velocity of particles suspended in fluid - projecting two pairs of measuring beams in perpendicular planes for measuring coordinate velocity components - Google Patents

Abstract

The method employs a coherent light source (1) and a beam splitter (3, 8, 28) providing two pairs of beams (9, 29, 19, 39) directed into a measuring vol. (45) in two perpendicular planes (44, 49) via respective optical fibres supplied by a pair of Y couplers (8, 28). An electronic control (40) provides a control signal for switching the light beam received by at least one Y coupler to a single output optical fibre to allow successive measurement of coordinate velocity components. ADVANTAGE - Simple measurement of coordinate velocity components of particles suspended in fluid.

Description

The invention relates to a method for detecting speeds, in particular of particle velocities in fluids. It also relates to two devices for Implementation of the process and an electro-optical module for this device gene.

The brochure of epa-Aachen "Two-component laser Doppler velocimeter" shows one such a method and device. A polarized laser beam is in one Pockels cell switched between two polarization states. Depending on the Polarisati direction, this laser beam is deflected in a polarization beam splitter or let through. The two resulting arms of the measurement setup are in their Ele elements identical. The respective beam of an arm is converted into a beam splitter Double beam split, defining a plane as a cross beam in a measuring volume entry. The two levels resulting from the two arms of the measurement setup are perpendicular to each other. The cross-blasting method allows blasting couple the formation of an interference pattern system, the two interference mu are perpendicular to each other. Now a particle enters the measuring volume men, all the speed components are successively perpendicular to the opti rule axis detectable when a single detector in backward or forward scatter is arranged.  

In this method and this device are through the use of a Pockels cell high switching capacities necessary. As further shown in the prospectus, both are Speed components perpendicular to one another on one and the same Particles can only be detected with slow particles, since a variety of, e.g. B. eight, fringe crossings to record the speed in one dimension is necessary. The required electro-optical switchover therefore requires high switching powers in order the change of the optical planes during the flight time of the particle to be measured to effect. If the high switching capacity is not available, it may be that the Velocity components of different particles can be measured.

Proceeding from this, the object of the invention is a method and a To create device in which / with less equipment both Ge velocity components on one and the same particle for any particle Speeds are detectable.

To solve the problem, the technical teachings of claims 1, 8 and / or 9 suggested.

It is important that a quick switch in one (claim 1, 9) or two (Claim 1, 8) Y-coupler (s) can be done and the orientation of the interference pattern can be changed very quickly with low power. The na has a supporting effect complete guidance of the light measuring beams in fiber optics or optical fibers; the rays are only coupled out of the fiber shortly before the measurement volume. This usually happens with collimator lenses or GRIN lenses, which are designed as cylinders are and with a continuous change in the refractive index a parallel out Grant direction of the emerging rays.

The use of fiber optics and the implementation of the switchover using Y Couplers enable a very quick and low-loss switchover. This can According to an advantageous embodiment, one measurement alternately for each cycle step value for one of the two speed components, which is one exact determination of the speed of a single particle is possible, which in the individual steps of the respective interference pattern are flown through the measuring steps (Claims 2, 3).

Advantageous embodiments of the method are set out in claims 2 to 7 .mu.m wrote.  

In a device (claim 8, 9) with which the method (claim 1) is advantageous can be carried out, there are two closely related proposals. Either two beam pairs are exchanged via two Y-couplers or one becomes a beam pair sets with a steady beam of light that with the other - alternating - light the remaining pair radiate the two differently oriented interference patterns forms.

By using Y couplers, fast changeover switches are available low switching power, the light power fed into the input fiber optic cables is full constantly switch between the output fiber pairs. So it is particularly especially possible to switch over in time with the recording of the measured values by the detector and so the measured values required for speed determination in the time-division multiplex of egg to obtain a single particle, so that it is in the recorded measured values are the speed components of a single particle.

Electro-optical elements can be used in the beam paths to shift the frequency be seen (claim 10). The AD circuit technology is closer to claim 11 wrote.

The electro-optical module (claim 12) is particularly space-saving for the described devices. In addition to its compact design, this module eliminates also a large number of adjustment and adjustment work due to the modular design omitted. A laser Doppler anemometer can be easily assigned to a specific task be adjusted, precisely by the fact that certain modules are exchanged.

Exemplary embodiments of the invention are described below with reference to the attached Drawings exemplified. These show

Fig. 1 is a schematic block diagram of the optical elements of an apparatus accelerator as a first embodiment of the invention,

Fig. 2 is a schematic block diagram of the optical elements of an apparatus according to a second embodiment of the invention with only a single Y coupler,

Fig. 3 is a schematic block diagram of a third embodiment of the present invention with optical elements containing modules and probes for determining the Ge speed of an object in two dimensions,

Fig. 4 is a schematic block diagram of a fourth embodiment of the present invention with optical elements containing modules and probes for determining the Ge speed of an object in one dimension in different places,

Fig. 5 is a module of Fig. 3 with essentially optical and electrical elements of the third embodiment for determining the speed of an object in two dimensions, and

FIG. 6 shows a module from FIG. 4 with essential optical and electrical elements of the fourth exemplary embodiment for determining the speed of an object in one dimension at different locations.

Fig. 1 shows a schematic block diagram of the optical elements of an on device according to a first embodiment of the invention. A laser 1 emits a monochromatic light beam 2 . This is divided in a beam splitter 3 into two substantially equal beams 4 and 24 , which are indicated schematically as lines. These are usually essentially parallel light beams 4 and 24 . These are coupled into the respective arm 5 or 25 of the device in optical waveguide couplers 6 or 26 arranged there in optical fibers 7 or 27 . In so far, this is a common beam splitting. In the course of the partial beams 4 , 24 - if necessary - a frequency shift for a directional sensitivity (at the measuring point) can be installed.

The optical fibers 7 and 27 can be multimode optical fibers. However, single-mode optical fibers, e.g. B. step index fibers with egg nem a few microns core.

These optical fibers 7 and 27 are connected to a Y-coupler 8 and 28 , respectively. This is a component which is connected on the input side to an input optical waveguide 7 and 27 and which has two output optical waveguides 9 and 19 or 29 and 39 on the output side. The output optical fibers 9, 19 , 29 and 39 preferably correspond in structure to the input optical fibers 7 and 27 .

The light fed via the input optical waveguide 7 or 27 into the respective Y-coupler 8 or 28 is almost completely guided into a single one of the two output optical waveguides, for example 9 or 29 . The other two output light waveguides 19 and 39 are then subjected to almost no light output.

Such Y-couplers 8 and 28 , which are also called bridge modulators (Y-balance, bridge modulators), are made up of an elongated LiNbO ₃ substrate and are commercially available for input light powers (pulse) up to 200 milliwatts (continuous power 1- 2 mW) and wavelengths of 800, 1300 and 1500 nm are available. It is possible to obtain branching in two optical fibers by cascading the transitions between bridge switches to more than two optical fibers. This is particularly advantageous for multi-point laser Doppler anemometry (multi-point LDA) in order to have a speed component at z. B. to measure three or four locations.

The two in this exemplary embodiment of a device for two-dimensional LDA light-guiding output optical waveguides 9 and 29 (as an example) are arranged in a plane 44 spanned by the optical axis 42 and the predetermined Y direction 43 . The laser light emerging from them is then bundled via a focusing optics (not shown in the drawing) onto a measuring volume 45 . There arises from the angle between the rays emerging from the optical fibers 9 and 29 be a first interference pattern. A particle passing through the measuring volume scatters the light, which can be used to determine a measured value for the speed component of the particle in the Y direction in accordance with the intensity resulting in forward scattering on a detector 46 . For this purpose, the detector 46 is connected via a data line 47 to a control and evaluation circuit 40 .

The Y-couplers 8 and 28 are now connected to the evaluation and control circuit 40 via control lines 10 and 30 . By a corresponding control signal, both Y-couplers 8 and 28 are switched synchronously, so that the light fed via the input light waveguides 7 and 27 into the couplers 8 and 28 is almost completely switchable to the other output optical fibers 19 and 39 . These latter from output optical fibers 19 and 39 are arranged in a plane 49 spanned by the optical axis 42 and the given X-direction 48 . The laser light emerging from them is then focused onto the measuring volume 45 via the focusing optics (not shown in the drawing). There arises from the angle between the rays emerging from the optical waveguides 19 and 39 , a second interference pattern, which is perpendicular to the above-mentioned first interference pattern. It is now possible in an analogous manner to determine measured values for the X component of the particle velocity.

In one embodiment of the method and the device, the one speed component is first determined and then the light power entering through the input optical waveguides 7 and 27 is redirected to the other output optical waveguide in order to measure the other component.

The control and evaluation circuit 40 can contain a memory chip and a fast switch. Then, after a clock step, ie the recording of a measured value for the speed component in one direction, it is temporarily stored and both Y-couplers 8 and 28 are switched over quickly. Then, in the next clock step, a measured value of the speed component is recorded in the other direction and then switched over again. This measuring method ensures that a multiplicity of measured values can be obtained for one and the same particle for both speed components in the tipmul time multiplex. After acquiring several measured values, the speed of the observed particle can be clearly deduced.

The planes of the beam pairs which are orthogonal to one another in FIG. 1 can also be arranged at any other angle to one another in order to determine a two-dimensional speed from the speed directions which can be determined in the process. The non-orthogonality only has to be taken into account when calculating the speed.

It is favorable to provide a sample and hold circuit in the evaluation circuit, so that at very different speeds observed in the two Directions Measured values for a fast direction in time with the switching of the Light output are recorded, whereas in the other direction a measured value is integrated over several cycles. Clock ratios of z. B. 1 to 5 or larger possible.

The measured value is advantageously obtained using a fast analog-to-digital converter directly converted, switching rates of 50 MHz per channel are possible.

The frequency of the clock signal of the power switch is of the order of magnitude the frequency of the electrical signal received by the detector.

In addition to the 4-beam cross-beam method shown in FIG. 1, it is also possible to create a device with only a single Y-coupler 8 . Such is shown in Fig. 2 is.

Fig. 2 shows a schematic block diagram of the optical elements of an on device according to a second embodiment of the invention with only a single Y-coupler 8th Identical features are provided with the same reference symbols. The arm 5 is configured analogously to the arm 5 of FIG. 1. The second arm 55, on the other hand, is designed differently, since the light in the optical waveguide coupler 26 is guided through an optical waveguide 59 directly to the decoupling point.

The two mutually interconnectable with light output light waveguides 9 and 19 are preferably arranged together with the optical fiber 59 in a plane at an angle of 120 degrees, so that there is an equilateral triangle with respect to the axis 42 with the corner points of the fiber ends. As a result, there is an angle of 120 degrees between said first and second planes. If the Y-coupler 8 is now switched over the control line 10, the light output guided in one pair of optical fibers 9 and 59 changes to the second pair of optical fibers 19 and 59 . The light component guided in the optical waveguide 59 is involved in both pairs of rays. Preferably, the beam splitter 3 also divides the light output equally between the two light beams 4 and 24 , so that two different measured values of speed components at an angle of 120 degrees to one another can be measured.

Of course, other angles are also possible, but the version described offers the greatest sensitivity.

It is advantageous that the Y-couplers 8 and 28 , which are usually switchable with low switching power in fast steps and high clock, have a high efficiency and that the guidance of the light beams in optical fibers 7 , 9 , 19 , 27 , 29 and 39 a low-loss propagation in the wavelength range of interest of z. B. allow 800 nm and at the same time allow an extremely compact design. Because instead of a large-scale lens arrangement, as indicated in FIG. 1, it is also possible to arrange the fiber ends of the optical fibers 9 , 19 , 29 , 39 and 59 in the desired manner close together, so that a small measurement volume can be acted upon with high beam intensity is what at the same time simplifies the recording of the scatter signal by the detector 46 .

Fig. 3 shows a schematic block diagram of a third embodiment of the invention with such optical elements containing modules and probes for determining the speed of an object in two dimensions. This can be the speed of a rotating disk, a particle in a fluid or another object.

FIG. 3 is particularly advantageous modular construction, which can be exchanged for various applications in a simple manner. The optical fiber couplers 6 and 26 can usually be used and adjusted on a bench. The adjoining output optical fibers 7 and 27 are fed into a fiber probe control unit 60 , which is shown in FIG. 5 in one embodiment.

FIG. 5 shows the module 60 from FIG. 3 with essential optical and electrical elements of the third exemplary embodiment for determining the speed of an object in two dimensions. Identical features are provided with the same reference symbols. The input optical fibers 7 'and 27 ' have been designated differently, since connectors are usually used, which make an adjustment of the connection to the Y-couplers 8 and 28 superfluous.

On the output side, the control unit 60 comprises the output optical fibers 9 , 19 , 29 and 39 and also a receiving optical fiber 65 , which leads to the detector 46 , which can be an avalanche photodiode. The measurement values thus recorded are converted directly in the evaluation device 40 and forwarded via a data bus 66 to a computer for further processing and storage.

The five optical fibers 9, 19, 29, 39 and 65 are plug-in couplings and a fiber strand 62 in a LDA-glass fiber probe 61 conducted in which these optical fibers are arranged as shown to FIG. 1 and FIG. 2 example, for carrying out the cross-beam method. This module thus contains the optics necessary for the parallelization and bundling of the incident light and the optics necessary for the collection of the scattered light, which in particular can be so-called rod lenses which can be placed directly on the fiber ends. With the reference numeral 63 , the incident collimated and scattered rays are designated net, which hit an object or a particle 64 or scattered by the who.

FIG. 4 shows a schematic block diagram of a fourth exemplary embodiment of the invention with further modules and probes containing optical elements for determining the speed of an object in one dimension at different locations. This so-called multi-point laser Doppler anemometry comprises measuring two LDA glass fiber probes 61 and 71 , which are arranged at spatially different locations. The advantage of the modular design is clear, since the modules 61 and 71 are identical in construction and are therefore interchangeable and can be used for different purposes. The fiber strands 72 and 73 are connected to a control unit 70 , which is shown in detail in FIG. 6.

FIG. 6 shows the module 70 from FIG. 4 with essential optical and electrical elements of the fourth exemplary embodiment for determining the speed of an object in one dimension at different locations. The optical structure of the control unit 70 is essentially identical to that of the module 60 .

The output optical fibers 9 and 29 are crossed in the sense that the first two output optical fibers 9 and 29 together with the receive optical fiber 65 form a three-fiber connection point. Likewise, the output optical waveguides 19 and 39 together with the receiving optical fiber 75 form a second such connection point.

As mentioned in connection with FIG. 4, each of these connection points is connected to an LDA glass fiber probe 61 or 71 , with which a velocity component of particles 64 is detected at different locations. The Y-couplers 8 and 28 switch namely be in one switching state, the module 61 and in the other switching state, the module 71 with the light of the laser 1 , so that the receiving optical fibers 65 and 75 one after the other pick up scattered light from the other location and guide it into a prism 67 for bringing together the receiving optical fibers 65 and 75 . The light emerging from the prism 67 is directed to the single detector 46 , at the output of which the electrical signal of the light scattered from an object or particle at various locations is available in time division multiplex, in the evaluation circuit 40 or after conversion into a digital one Signal, e.g. B. to be processed by a 4-bit direct converter via a bus 66 in a computer unit. In addition to a prism 67, it is also possible to use a simple device that bundles the various receiving optical fibers 65 and 75 , which in the simplest case can consist of a lens that collimates on the detector.

The embodiment shown in the more compact embodiments is used the backscattered signal. Of course there is also the use of forward scatter possible with slightly modified modules, in particular that can also include the fluid transmit space can be integrated into a module. It is, as in the presented Embodiments important that the measuring room volume on the received light wel lenleiter is imaged to maintain a high sensitivity of the equipment.  

Of course, a device of the type described above will advantageously also acousto-optic elements such as Bragg cells comprise the with the help of a mixer To be able to determine the sign of the speed of the particle with certainty. Possibly. is the It makes sense to use a spatial filter to get a cleaner jet.

Claims (14)

1. A method for detecting speeds, in particular particle speeds in fluids, in which a light beam is broken down into a first pair of beams, which is introduced into a measuring volume at an angle in a first plane ( 44 ), and into a second pair of beams is, which is introduced in a second plane ( 49 ) lying to each other in the measuring volume, in which with beam switching means ( 8 , 28 ) the light beam is guided in a timed sequence essentially in only one of the two pairs of beams, in which a Detector element ( 46 ) detects the radiation emerging from the measurement volume, and in which a control and evaluation circuit ( 40 ) evaluates the detector signal and switches the beam switching means,
characterized,

• a) that the light beams emerging from the beam switching means are each coupled into an input optical waveguide ( 6 , 26 ; 7 , 27 ), which in at least one Y-coupler ( 8 , 28 ) each have at least two output optical fibers ( 9 , 19 ; 29 , 39 ) to be shared,
• b) that the control electronics ( 40 ) generate a control signal with which a power switching of the light entering the at least one Y-coupler ( 8 , 28 ) can emit light onto only one of the output optical fibers,
• c) the output optical waveguides ( 6 , 26 ; 7 , 27 ) assigned to the at least one Y-coupler ( 8 , 28 ) each guide part of the first and second beam pairs and in the first and second planes ( 44 , 49 ) are arranged.

2. The method according to claim 1, characterized, that the power switchover is carried out after taking a measured value becomes. 3. The method according to any one of claims 1 to 2, characterized in that the frequency of the clock signal of the power switch is in the order of the frequency of the electrical signal received by the detector ( 46 ). 4. The method according to any one of claims 1 to 3, characterized in that the signals obtained from the detector ( 46 ) and one of the beam pairs to be assigned Si signals are converted depending on the clock signal of the power switchover from an analog-digital converter into a digital signal. 5. The method according to any one of claims 1 to 4, characterized in

• a) that only one Y-coupler ( 8 ) is provided and
• b) that the two pairs of beams ( 9 , 29 ; 19 , 39 ) consist on the one hand of the light beams emerging from the output optical fibers ( 9 , 19 ) and on the other hand of a light beam emerging from a common output optical waveguide ( 59 ), the two planes ( 44, 49 ) intersecting in the common output optical waveguide ( 59 ).

6. The method according to any one of claims 1 to 4, characterized in that at least two Y-couplers ( 8 , 28 ) are provided, which are connected to at least two output optical fibers ( 9 , 19 ; 29 , 39 ), the at least two planes ( 44, 49 ) formed by the output optical waveguides are arranged at any angle to one another and at a spatial distance from one another (multipoint). 7. The method according to claim 6, characterized in that the backscattered from the various places where light via a receiving optical waveguide ( 65 , 75 ) and the receiving optical waveguide merging element ( 67 ) on the only gene detector ( 46 ) can be mapped is. 8. Device for detecting speeds, in particular particle speeds in fluids, for carrying out the method according to one of claims 1 to 7,

• a) with a coherent light source ( 1 ) with which a light beam ( 2 ) can be generated,
• b) with a beam-splitting arrangement ( 3 , 8 , 28 ) with which the light beam ( 2 ) into a first beam pair ( 9 , 29 ) - which is at an angle in a first plane ( 44 ) to each other in a measuring volume ( 45 ) can be introduced - and can be dismantled into a second pair of beams ( 19 , 39 ) - which can be introduced into the measuring volume ( 45 ) at an angle to one another in a second plane ( 49 ),
• c) the arrangement ( 3 , 8 , 28 ) having beam switching means ( 8 , 28 ) with which the light beam ( 2 ) can be guided in chronological order essentially into only one of the two pairs of beams ( 9 , 29 or 19 , 39 ),
• d) with a detector element ( 46 ) with which the radiation emerging from the measurement volume ( 45 ) can be detected, and
• e) with a control and evaluation circuit ( 40 ) which is connected to the output of the detector element ( 46 ) and whose control output is connected to the control inputs of the beam switching means ( 8 , 28 ),

characterized,

• f) that the beam switching (8, 28) of at least two Y-couplers (8, 28) best hen, said from a beam splitter (3) of the beam-splitting means (3, 8, 28) emergent light beams (4, 24) can be coupled into input optical waveguides ( 7 , 27 ),
• g) that each of the input optical fibers ( 7 , 27 ) is divided by one of the Y-couplers ( 8, 28 ) into two output optical fibers ( 9 , 29 and 19 , 39 ), the output optical fibers ( 9 , 19th , 29 , 39 ) are arranged such that an output optical fiber ( 9 or 19 ) assigned to the first Y coupler ( 8 ) and an output optical fiber ( 29 or 39 ) assigned to the second Y coupler ( 28 ) in one level ( 44, 49 ) are arranged,
• h) that a control signal from the control electronics ( 40 ) switches the power of the light beam ( 4 , 24 ) entering the Y coupler ( 8 , 28 ) to only one of the associated output optical fibers ( 9 or 19 ; 19 or 39 ) is feasible.

9. Device for detecting speeds - in particular particle speeds - in fluids for carrying out the method according to one of claims 1 to 7,

• a) with a coherent light source ( 1 ) with which a light beam ( 2 ) can be generated;
• b) with a beam splitter assembly (3, 8, 28) (9, 19) and divides the coherent light beam (2) in a He Stes pair of beams a second beam (59), said first pair of beams (9, 19) in a plane ( 44 ) lies and is brought to the cut with the second beam ( 59 );
• c) with a detector element ( 46 ) with which the emerging radiation from the measuring volume ( 45 ) - in which the beams intersect - can be detected;
• d) with a control and evaluation circuit ( 40 ) which is connected to the output of the detector ( 46 ) and whose control outputs are connected to the beam splitting arrangement ( 3 , 8 , 28 );

characterized,

• e) that the beam splitting arrangement ( 3 , 8 , 28 ) has an input beam splitter ( 3 ) and a Y-coupler ( 8 ) connected downstream thereof;
• f) that a beam switching ( 9 ; 19 ) in the Y-coupler ( 8 ) is triggered by control electronics ( 40 ) with a control signal and one ( 9; 19 ) of the switched beams ( 9 , 19 ) with the second beam ( 59 ) can be cut to size in the measuring volume ( 45 );
• g) that after the input beam splitter ( 3 ) up to the measuring volume ( 45 ) the beams are guided in optical fibers.

10. Apparatus according to claim 8 or 9, characterized in that frequency-shifting acousto-optic elements are arranged in one or in both beam paths ( 4 , 24 ). 11. The device according to claim 8 or 9, characterized in that the detector element ( 46 ) is connected to a fast analog-to-digital converter - possibly with an upstream sample / hold circuit - with which the signal obtained can be further processed in direct conversion. 12. Electro-optical module ( 60 , 70 ) for a device according to claims 8 to 11 for carrying out the method according to one of claims 1 to 7,

• a) with input-side connections for input optical fibers ( 7 , 27 ), which can be connected to Y-couplers ( 8 , 28 );
• b) with an output-side connection to the three or four output optical fibers ( 9 , 19 , 29 , 39 ; 9 , 19 , 59 ) and a receiving optical fiber ( 65 ) can be closed;
• c) with a detector element ( 46 ) onto which the light guided by the receiving optical waveguide ( 65 , 75 ) can be imaged and
• d) with an evaluation circuit ( 40 ) with which the Y-coupler (s) ( 8 , 28 ) can be controlled and the signal recorded by the detector ( 46 ) can be processed.