FR2761162A1 - Optical Doppler effect speed detection system - Google Patents

Abstract

The device for measuring speed by the Doppler effect comprises a laser (10) emitting a light beam through a reference material. The speed is determined by the difference in frequency between the emitted beam and the beam which has passed through the reference material. In the channel which lies between the output (12) of the laser source (10) and the objective (28) of the device transmitting the light beam through the reference material, the beam is conveyed by an optical fibres (11,14,26). An optical separator with an optical fibre (18), such as a circulator, directs the beam towards the objective lens (28). The light signal returning from the circulator is directed towards a mixer of a coupler (32).

Description

DOPPLER EFFECT SPEED MEASURING DEVICE, NORMALLY FOR
FLYING MACHINES
The invention relates to a Doppler effect speed measuring device of the optical type.

 It relates more particularly to such a device in which a light beam, in particular a laser beam, is emitted from a mobile to a reference medium - that is to say a medium relative to which one wishes to measure the speed of the mobile - and the beam returned by backscattering by this medium is used to measure the speed of the mobile. It is known, in fact, that the frequency of the returned beam differs from the frequency of the emitted beam by a value which is proportional to the speed of the mobile with respect to the reference medium.

 Devices of this type are used in particular in aircraft to measure the speed of the machine relative to the surrounding air. In this case, the device is called an "anemometer".

 In this application, a laser beam emitted from the aircraft, through a window, is focused at a distance of approximately 100 m or more from the craft. Aerosols in the atmosphere backscatter the beam. The Doppler frequency - that is to say the difference between the frequency of the return beam and the frequency of the incident beam - is detected by an interferometer which performs the beat between the incident beam and the return beam. Compared to a conventional Pitot tube measurement device, such an anemometer has the advantage of allowing an exact measurement - which does not need to be corrected - of the speed of the aircraft, since the reference, c that is to say the aerosols on which the laser beam is directed, is practically not subjected to the agitation caused by the movement of the machine.

 In general, provision is made for scanning the laser beam or at least three separate beams so as to make it possible to know not only the amplitude of the speed, but also its direction.

 Such an anemometer is described, for example, in French patent 90 02946 in the name of Sextant Avionique.

 A common anemometer includes a laser emitting a beam with rectilinear polarization. The beam passes through a half-wave plate which is oriented in such a way that it delivers a beam having a principal component in parallel polarization and an auxiliary component in perpendicular polarization, the intensity of the auxiliary component being weak compared to the intensity of the main component. This beam is supplied to a first polarization splitter cube which transmits the parallel polarization and reflects the perpendicular polarization. The perpendicular polarization, which is reflected, is transmitted to a reference channel. The parallel polarization is transmitted to another polarization splitter cube and, from there, through a quarter-wave plate, to a focusing objective. The second separator cube transmits the parallel polarization while the quarter-wave plate transforms this parallel polarization into a circular polarization.

The objective is used to focus the beam on aerosols at a distance. The beam backscattered by the aerosols returns by the same objective, but the circular polarization has changed direction.

This back-scattered beam crosses, in the opposite direction, the quarter-wave plate. After the latter, on the return path, the beam polarization is perpendicular. This beam is therefore reflected by the second polarization splitter cube and this beam reflected in perpendicular polarization is mixed with the reference signal formed by said auxiliary component in perpendicular polarization. At the output of the homodyne or heterodyne mixer, a signal is obtained at the Doppler frequency.

 The half-wave plate, the quarter-wave plate and the separator cubes are expensive elements. Alignment is not easy to carry out, and the assembly has a space requirement which is not always optimum, especially for aeronautical applications.

 The invention overcomes these drawbacks. It significantly reduces the mechanical alignment constraints of the device.

 The device according to the invention is more compact and less expensive.

 It is characterized in that between the optical generator, in particular a laser, and the output of the device, the beam is led in a path with optical fiber (s) and, in that an optical separation means to optical fiber is arranged in this channel to direct the signal emitted by the generator towards the output and to direct the light signal back to a mixer or coupler.

 The separation means is for example an optical circulator or a polarization splitter coupler.

 Such an optical fiber assembly (s) is easier to manufacture and implement than a free propagation assembly. It is therefore less expensive and can be accommodated in a reduced volume. In addition, the weight can be minimized.

 In a particularly advantageous embodiment, the laser emits a beam at approximately 1.5 m. In this way, it is possible to use optical fibers and associated components of the type which are commonly used in the field of telecommunications; thus the costs are further reduced. In addition, this wavelength does not penetrate the human eye, the precautions for use are much less restrictive in comparison with the precautions to be taken when using a laser emitting in the visible.

 It will be noted here that, unlike a usual use of a circulator in telecommunications, the level of the signal 1 at the input of the circulator is high, while the signal at the output is at a level substantially lower than at the input.

In the telecommunications field, it is generally the output signal which is at a higher level than the input signal, a circulator being usually associated with an optical amplifier.

 In other words, in the invention, the optical fibers and the separation means are used in a strong signal path, whereas usually these components are used in weak signal paths.

 If the means of separation between the outward and return beams is an optical circulator, making it possible to separate the beams without it being necessary to impose a polarization constraint on the latter, it is possible to use single-mode optical fibers without maintaining polarization. In this case, the assembly is particularly simplified because it is not necessary to impart a particular angular orientation to the optical fibers.

Other characteristics and advantages of the invention will appear with the description of some of its embodiments, this being carried out with reference to the attached drawings in which
FIG. 1 is a diagram of an embodiment of the invention,
FIG. 2 is a diagram similar to that of FIG. 1, but for a variant,
FIG. 3 is a sectional view of an optical fiber without maintaining polarization, and
Figure 4 is a sectional view of an example of polarization maintaining optical fiber.

 The examples which will be described in relation to the figures relate to laser beam anemometry, that is to say to the measurement of the speed of an aircraft, or of winds, by emission of a laser beam towards the aerosols suspended in the atmosphere, the beam being focused at a distance from the aircraft and the speed being measured by the Doppler frequency of the beam backscattered by the aerosols.

We know that the Doppler frequency fd has the value fd = 2v /
in this formula, v is the projection onto the line of sight of the laser of the speed of the aircraft relative to the ambient atmosphere, that is to say the reference with respect to which we want to measure the speed of displacement of the aircraft, and x is the wavelength of the beam emitted.

 In all these examples, use is made, in accordance with one aspect of the invention, of optical fibers or optical conductors optimized for approximately 1500 nanometers.

 We first refer to Figure 1. In this embodiment, there is provided a laser 10, for example fiber, in particular fiber doped with erbium, emitting at a wavelength of about 1,550 nanometers. Its power output 12 is connected to a coupler 13 by an optical fiber 11. This coupler 13 has a power output 131 which is connected to the input port 16 of a circulator 18 via an optical fiber 14 .

 The coupler 13 has a reference output 132 emitting a beam at the same wavelength but of a much lower power, for example 1000 times less than that of the beam on its power output 131. As a variant (not shown) , the reference beam is supplied by a reference output of the laser 10.

 The optical circulator 18 has two other ports 22 and 24. Port 22 constitutes an input-output, while port 24 constitutes a single output. In a manner known per se, the circulator has the property that a light signal entering through port 22 can only go towards port 24 and not towards input port 16.

 The input-output port 22 is also connected to an optical fiber 26, the output signal of which leads to a lens 28. The end 27 of the fiber 26 is placed in a location such that the conjugate point is between a few tens of meters and about 100 meters or more. In this way, the light beam is focused on aerosols suspended in the atmosphere at a location where the atmosphere is practically not disturbed by the movement of the aircraft.

These aerosols (not shown) scatter the beam which returns to the objective 28 and returns to the optical fiber 26. The back-scattered beam directs from port 22 towards port 24.

 Port 24 is connected to the first input 30 of an optical coupler 32 via another optical fiber 34.

 The second input 36 of the coupler 32 is connected to the second output 132, of reference, of the coupler 13 by an optical fiber 37.

 The output 38 of the coupler 32 is connected to an optical fiber 40 which is itself connected to a detector 42 via another fiber 41.

The coupler 32 therefore receives on an input 36 a signal at the frequency of the laser (the local oscillator), while on its input 30, it receives the backscattered signal whose frequency is equal to the frequency of the laser increased by the frequency
Doppler.

 The coupler 32 constitutes an interferometer at the output of which a beat signal at the Doppler frequency is obtained on the detector 42.

 The optical fibers 11, 14, 26, 34, 37 and 40 are of the single-mode type without polarization maintenance if the laser source is depolarized. The fiber 41 is of any type, that is to say multimode or monomode with or without polarization maintenance.

 The fibers 11, 14, 26, 34, 37 and 40 are of the polarization maintaining type in the case where the laser source emits a polarized signal.

 A fiber without polarization maintenance (Figure 3) has an axis 70 of symmetry of revolution, the core 72 of this fiber being of circular section. It is therefore not necessary to angularly position such a fiber, which simplifies assembly.

 In the example of FIG. 1, the output 132 of the coupler 13 is connected directly (by the fiber 37) to the input 36 of the coupler 32. This example corresponds to a homodyne detection.

In the case of heterodyne detection, an acousto-optical modulator (not shown) is provided between the output 132 and the port 36.

 At the output of this modulator, a signal is obtained at the frequency fL i Af, fL being the frequency of the laser and df having a value of approximately 70 MHz. This last value is chosen so as to allow the speed sign to be determined.

 The device described can be used not only to measure the speed of a machine, but also to measure the wind speed in the area of the machine. In this case, the beam is directed in sequence at several distances from the machine. For this purpose, the objective is adjustable so as to move the focal point. As a variant, the objective is an afocal system delivering a parallel or focused beam at long distance; the return signals are then analyzed at different times, each measurement instant corresponding to a distance from the machine.

 Reference is now made to FIG. 2 which is a variant of that of FIG. 1. For this reason, the analogous elements bear the same reference numbers.

 The laser 10 emits, at its output 12, a main beam with parallel polarization which is transmitted, along the slow axis of the optical fiber 11 'of the monomode type with polarization maintenance, to the coupler 13' having the same role as the coupler 13 of FIG. 1. The power output 13'1 of this coupler is connected by a fiber 14 ', also monomode with polarization maintenance, to the first input port 16' of a coupler 50 polarization splitter. This coupler 16 ′ has one or two input-output ports 52, 54. The port 52 delivers radiation which is transmitted, along the slow axis of a single-mode polarization maintaining optical fiber 26 ′, to the input of a bidirectional type quarter-wave element 56 which transforms a rectilinear polarization into a circular polarization.

 The output 27 ′ of the quarter-wave element 56, for example with fiber, is associated with the objective 28 so that the conjugate point is between a few tens of meters and a hundred meters or more.

 The beam scattered by the aerosols therefore returns to port 52 through the objective 28, the element 56 and the fiber 26 '.

 The output port 24 'of the coupler 50 is connected, by an optical fiber 34' with polarization maintenance, to the input 30 'of a coupler 32' whose second input 36 'is connected to the reference output 13' 2 of the laser 10 via an optical fiber 37 ′ also with polarization maintenance. The fiber 37 'is in two parts, one 37'1 leaving the coupler 13' and the other 37'2 entering the coupler 32 '. These parts are interconnected by splicing, so that a signal emitted by the part 37'1, along the slow axis, is received by the part 37'2 along the fast axis.

 The coupler 32 'is connected to a detector 42 via a single-mode polarization-maintaining optical fiber 40 and 41 of any type.

 The beam backscattered by the particles has a circular polarization in the opposite direction to that of the incident beam. Consequently, the return beam propagates along the fast axis of the fiber 26 'while the outgoing beam propagates along the slow axis. Similarly, the signal on the output 24 'of the coupler 50 propagates along the fast axis of the fiber 34'. As indicated above, the signal on the input 36 'of the coupler 32' is along the fast axis.

 A polarization-maintaining fiber is shown in FIG. 4. It can be seen that such a fiber does not have a symmetry of revolution about its axis. In fact, in the example of FIG. 4, the internal part, or heart, 72 ′ has an oval or elliptical shape. The optical fiber must therefore be oriented correctly around its axis 70 'relative to the polarized beam. The slow axis 80 corresponds to the major axis of the ellipse 72 'and the fast axis 82 corresponds to the minor axis of this ellipse.

 The connection of an optical fiber to an element such as a circulator or a coupler is carried out via a connector which, in general, causes losses of the order of 1 decibel if single-mode fibers are used with polarization maintenance and of the order of 0.3 decibel, if single-mode optical fibers are used without polarization maintenance.

 To avoid these losses, in one embodiment, some of the elements are spliced or fabricated together. For example, in the case of FIG. 1, the coupler 13, the circulator 18 and the coupler 32 can be manufactured together. Indeed, the coupler and the circulator being composed of optical fibers extended by input and output fibers , a set of elements can be manufactured in such a way that the output fiber of one element also constitutes the input fiber of the next element. A component is thus produced having an input intended to receive the output signal 12 from the laser 10, an input-output constituted by the end 27 of the fiber 26, and an output 38 (that of the coupler 32). Alternatively, the laser 10 is part of the component.

 Similarly, in the case of FIG. 2, a single component can be provided comprising the coupler 13 ', the coupler 50 polarization splitter, the coupler 32' and the optical fibers 11 ', 14', 37 ', 40 and 26 ', as well as the quarter wave element 56. Likewise, the laser 10 can be incorporated into this single component.

 We have so far described only the application of the invention to an anemometer, that is to say a device for measuring the speed of an aircraft relative to the ambient atmosphere or for measuring the speed of winds. The invention is not limited to this application. It can also be used to measure a speed relative to the ground, in particular for a low-flying craft, such as a missile or for landing assistance. The invention also applies to the measurement of the speed of movement of a land vehicle relative to the ground.

 The beam emitted by the laser 10 is either continuous or in the form of pulses (pulsed).

 In a variant which applies, both to the embodiment of FIG. 1 and to the embodiment of FIG. 2, provision is made between the output 132 of the coupler 13 and the input 36 of the coupler 32 (FIG. 1) or between the output 13'2 of the coupler 13 'and the input 36' of the coupler 32 '(Figure 2), a length of fiber chosen so that the signal journey time in channel 37 or 37' is substantially equal at the travel time of the beam from the output 131 or 13'1 of the coupler 13 or 13 'towards the outside then from the outside towards the input 30 or 30' of the coupler 32 or 32 '.

 In this way, a continuous laser source with a low coherence length can be used, since the beams appearing at the inputs 30 and 36 (or 30 'and 36') correspond to beams emitted practically at the same times by the laser source. The use of a continuous laser source with a short coherence length has the advantage of allowing good spatial discrimination of the area measured. In fact, the zones which do not correspond to substantially equal optical paths will not be taken into account by the anemometer.

 The simultaneous appearance of signals on inputs 30 and 36 (or 30 'and 36') is also useful when using a pulsed source. Indeed, in this case, if we do not take precautions in the choice of the length of the fiber 37 or 37 'the signals may not arrive simultaneously on the inputs 30 and 36 (or 30' and 36 ') and , in such a situation, the anenometer will not be able to function, that is to say that no signal will appear at the output of the detector 42.

 The speed measurement accuracy along the line of sight is of the order of 0.3 m / s to 1 m / s and varies depending on the spectral quality of the laser source.

 Although a speed measuring device installed on board a moving vehicle has been described, it is easily understood that this device can also be used to measure, from the ground or from another vehicle, the relative speed d 'a device in relation to the reference medium constituted by the ground or said other device.

Claims (16)

 1. Doppler effect speed measuring device comprising a light radiation source1 preferably a laser (10), emitting a light beam towards a reference medium, the speed being determined by the frequency difference (fd) between the beam emitted and the beam returned by the reference medium, characterized in that, in the path between the output (12) of the source (10) and the output (28) of the device emitting the light beam towards the reference medium , the beam is led by a fiber optic means (11, 14, 26; 11 ', 14', 26 ', 56) and in that an optical separation means (18; 50) fiber optic directs, in said channel, the signal from the source (10) to the output (28) and directs the light signal back to a mixer or coupler (32; 32 ').  2. Device according to claim 1, characterized in that the separation means comprises an optical circulator (18).  3. Device according to claim 2, characterized in that the optical fiber means comprises single-mode optical fibers (11, 14, 26) without maintaining polarization.  4. Device according to claim 2 characterized in that the optical fiber means comprises single-mode optical fibers with polarization maintenance.  5. Device according to claim 1, characterized in that the separation means comprises a blow their polarization separator (50) and a quarter wave element (56) disposed between this separator coupler (50) and the output (28) of the device.  6. Device according to claim 5, characterized in that the optical fiber means comprises optical fibers (11 ', 14', 26 ') single-mode polarization maintaining.  7. Device according to any one of the preceding claims, characterized in that the outlet (24; 24 ') of the separation means (18; 50) is connected to an inlet (30; 30') of a coupler (32 ; 32 ') whose second input (36; 36') receives a reference signal at the frequency of the source (10), the output of the coupler (32; 32 ') delivering a signal representing the frequency difference between the source and the signal returned by the reference medium.  8. Device according to claim 7 characterized in that it comprises means (37; 37 ') so that the signals arriving at each instant on the inputs of the coupler (36; 36') correspond to signals which have been emitted substantially in marl time by the source (10).  9. Device according to claims 3 and 7, characterized in that the output (24) of the optical circulator (18) is connected to the input (30) of the coupler (32) by an optical fiber (34) without maintaining polarization .  10. Device according to any one of the preceding claims, characterized in that the source (10) comprises an auxiliary output emitting a reference signal serving as a local oscillator for extracting the Doppler frequency.  11. Device according to any one of claims 1 to 9 characterized in that it comprises a coupler (13; 13 ') transmitting the signal from the laser source and providing a reference signal serving as a local oscillator for extracting the Doppler frequency .  12. Device according to any one of the preceding claims, characterized in that the optical output of said device intended for transmitting to the reference medium comprises a lens (28) or an afocal system.  13. Device according to any one of the preceding claims, characterized in that the optical fiber means, the optical fiber optical separation means and the mixer or coupler (32; 32 ') are spliced.  14. Application of the device according to any one of the preceding claims to the measurement of the speed of an aircraft relative to the ambient atmosphere.  15. Application of the device according to any one of claims 1 to 13 for measuring the wind speed at distance (s) from a machine.  16. Application of the device according to any one of claims 1 to 13 to the measurement of the speed of a machine relative to the ground.