DE4240769C2 - Method and device for detecting multiple speed components, in particular particles in fluids - Google Patents

Description

The invention relates to a method for detecting a plurality of speed components, in particular of Particles in fluids. It also includes two devices for Implementation of the method, as well as an electro-optical module for these devices.

The brochure epa-Aachen "Two-component laser Doppler velocimeter" or the DE 34 21 341 A1 show such a method and such a device. A polarized one Laser beam is switched between two polarization states in a Pockels cell. Depending on the direction of polarization, this laser beam is in a polarization beam splitter deflected or let through. The two resulting arms of the measurement setup are in their elements identical. The respective beam of an Ann is above a beam splitter split a double beam that defines a plane as a cross beam into a measuring volume entry. The two levels resulting from the two arms of the measurement setup are standing perpendicular to each other. Each pair of rays allows the formation of one Interference pattern, wherein the two interference patterns are perpendicular to each other. Kick now a particle through the measuring volume, velocity components are perpendicular Detectable to the optical axis because a detector is arranged in back or forward scatter is. However, using a Pockels cell means high switching capacities necessary.

As further emerges from the prospectus and DE 34 21 341 A1, 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 traverses to measure speed in one dimension is necessary. The required electro-optical switchover therefore requires high switching powers in order change the optical planes during the flight time of the particle to be measured cause. 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 provide a device in which with low control power both speed Components on the same particle at any particle speed are detectable.  

The problem is solved with the subject matter of claims 1, 8 or 9.

It is important that a quick switch in one (claim 1, 9) or two (Claim 1, 8) Y-coupler (s) can take place in two (or four) output beams and the Orientation of the interference pattern after each measurement value (very quickly) with low power can be changed. The almost complete one supports this Guiding the light measuring beams in optical fibers, such as fiber optics; only shortly before Measurement volume, the beams are decoupled from the optical fibers. This can be done with (usual) collimator lenses or GRIN lenses happen, which are designed as cylinders and with a continuous change in the refractive index, a parallel alignment of the grant emerging rays (claim 2).

The use of optical fibers and the implementation of beam switching (Light output changeover) by means of a Y-coupler controlled by a control circuit enables a very fast and low-loss switching. According to one advantageous embodiment, alternately one measurement value for each of the clock steps two speed components can be determined, making an exact It is possible to determine the speed of a single particle which is in the Measuring steps flies through the individual fringes of the respective interference pattern. Should one The speed component cannot be detected at all or cannot be detected well, or bothers or blocks this detection delay does not affect the detection of the other components.

If the switchover takes place after recording a measured value, the frequency of the Clock signal of the beam switching (light output switching) in the order of magnitude Frequency of the electrical signal received by the detector (claim 3).

Advantageous embodiments of the method are highlighted in claims 4 to 7.

Unlike the present invention, which uses active Y-couplers, the review article "Linear and Non-Linear Optical Fiber Devices "by GOURE, J.P. et al. in Journal of Physics, Part D: Appl. Phys. Vol. 22,1989, pages 1791 to 1805 under a large number of optical Components passive Y-coupler. They should be used for LAN networks. In DE 34 41 088 C2 - comparable to the characterizing part of claim 2 - GRIN lenses or collimator lenses for the parallel alignment of those coupled into a measurement volume Rays. The background to this description is the improvement of the measuring accuracy in the Detection of a single velocity component of a particle in a fluid.  

The simultaneous acquisition of several speed components on a single one Particles cannot be found from this site.

In a device (claim 8, 9) with which the method (claim 1) is advantageous there are two suggestions. Either two beam pairs are exchanged via two Y-couplers or one beam pair is replaced through a steady beam of light, that with the other - alternating - beams of light of the remaining pair forms the two differently oriented interference patterns.

By using Y couplers, fast changeover switches are available the light power fed into the input fiber optic cables with low switching power Switch (light beams) completely between the output fiber pairs. This makes it possible in particular to measure the measured values from the detector to switch and so the measured values required for speed determination in To obtain time division multiplexing from a single particle, so that it is in the measured values actually around the speed components of a individual particles.

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

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 according to 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 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 structure of the output optical fibers 9 , 19 , 29 and 39 preferably corresponds to that of 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-balanced, bridge modula tors), are constructed from an elongated LiNbO₃ 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. In addition to branches in two optical fibers, it is also possible to obtain bridge switching to more than two optical fibers by cascading the transitions internally. 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 is a second interference pattern, which is perpendicular to the above-mentioned first interference pattern, created by the angle between the rays emerging from the optical waveguides 19 and 39 . 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, the latter is temporarily stored and both Y-couplers 8 and 28 are quickly switched over. 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 50MHz 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 embodiment described offers the greatest sensitivity.

It is advantageous that the Y-couplers 8 and 28 , which are usually switchable with low switching powers 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 800nm 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 with high beam intensity can be applied , which 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 as well as 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 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, especially the one that includes the fluid 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 (12)

1. A method for detecting multiple velocity components, in particular particles in fluids, in which

• - A light beam into a first pair of rays, which is introduced into a measuring volume at an angle to one another in a first plane ( 44 ), and is broken down into a second pair of rays, which are at an angle to one another in a second plane ( 49 ) the measuring volume is introduced,
• the light beam is essentially guided in only one of the two pairs of beams,
• - The radiation emerging from the measurement volume is detected by a detector element ( 46 ) and the detector signal ( 47 ) is evaluated by a control and evaluation circuit ( 40 ), characterized in that
• - That at least one - controlled by the control and evaluation circuit ( 40 ) - Y-coupler ( 8 , 28 ) divides the light beam into two quickly switchable output light beams ( 9 , 19 ; 29 , 39 ), the switchover after detection of a measured value and
• - That the light measuring beams are almost completely in optical waveguides ( 6 , 26; 7 , 27 ; 9 , 19 ; 29 , 39 ), such as fiber optics, to be coupled out just before the measurement volume from the optical fibers.

2. The method according to claim 1, characterized in that GRIN lenses or collimator lenses couple the light rays from the optical fibers ( 6 , 26 ; 7 , 27 ; 9 , 19 ; 29 , 39 ) in order to obtain a parallel alignment of the emerging rays. 3. The method according to any one of claims 1 or 2, characterized in that the frequency of the clock signal of the beam switching 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 received by the detector ( 46 ) and one of the beam pairs to be assigned signals are converted depending on the clock signal of the beam switching 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 beam pairs ( 9 , 29 ; 19 , 39 ) each consist of a light beam from a switchable light beam pair and a common light beam ( 59 ), the two planes ( 44, 49 ) being in the common light beam ( 59 ) cut.

6. The method according to any one of claims 1 to 4, characterized in that at least two Y-couplers ( 8 , 28 ) with at least two output optical fibers ( 9 , 19 ; 29 , 39 ) are provided, the at least two through the output -Light rays formed planes ( 44, 49 ) are arranged at any angle to each other at a spatial distance from each other (multipoint). 7. The method according to claim 6, characterized in that the backscattered light from the different locations is imaged via a respective receiving optical waveguide ( 65 , 75 ) and an element ( 67 ) bringing together the receiving optical waveguide on the single detector ( 46 ). 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 pair of beams ( 9 , 29 ) - which 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 time sequence in essentially 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 in that
• (f) that the beam switching (8, 28) of at least two Y-couplers (8, 28), wherein from a beam splitter (3) of the beam-splitting means (3, 8, 28) emergent light beams (4, 24) in Input optical fibers ( 7 , 27 ) are coupled in,
• (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 , 19 , 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 V-coupler ( 28 ) are arranged in one plane ( 44, 49 ),
• (h) that a control signal from the control electronics ( 40 ) switches the beam of the light beam ( 4 , 24 ) entering each of the Y-couplers ( 8 , 28 ) to only one of the associated output optical fibers ( 9 or 19 ; 19 or 39 ).

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 arrangement ( 3 , 8 , 28 ) which splits the coherent light beam ( 2 ) into a first beam pair ( 9 , 19 ) and a second beam ( 59 ), the first beam pair ( 9 , 19 ) in one Plane ( 44 ) lies and is cut with the second beam ( 59 ) in a measuring volume ( 45 );
• (c) with a detector element ( 46 ) with which the emerging radiation 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 the control electronics ( 40 ) with a control signal and one ( 9; 19 ) of the switched beams ( 9 , 19 ) with the second beam ( 59 ) cuts 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-optical 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 ) with a fast analog-digital converter - in particular with an upstream sample / hold circuit - is connected, with which the signal obtained is 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 which three or four output optical fibers ( 9 , 19 , 29 , 39 ; 9 , 19 , 59 ) can be led out and a receiving optical fiber ( 65 ) can be connected;
• (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 ) for processing the signal picked up by the detector ( 46 ) and controlling the Y-coupler ( 8 , 28 ).