FR2556841A1 - Method and device for Doppler laser velocimetry with homodyne detection - Google Patents

Abstract

The laser velocimeter with homodyne detection is constructed from a laser 1 whose linearly polarised radiation is applied in 10 to an optical system 2 of the Mach-Zender interferometer type. The measurement beam strikes the object, whose velocity is to be measured, either directly or through a telescope 8. The signal beam S and the local oscillator beam OL are produced with a slight angular offset between them before striking a double detector 5. The angular offset is chosen so that the wavefronts of the two beams are offset by a half wavelength over the whole surface of the double detector 5. A comparison of the phases of the signals received by the two detectors gives the sign of the velocity.

Description

Method and device for Doppler laser velocimetry with homodyne detection.

 The invention relates to homodyne detection laser velocimetry, consisting in producing a beat signal between the beam of waves emitted by a laser towards a target and the back-scattered beam affected by a Doppler effect proportional to the speed of movement. relative of the target and the laser.

 The advantage of this particular detection technique, compared to heterodyne detection, is that it leads to the production of simpler speed measurement equipment, which is not very fragile and consumes little energy. This makes it very interesting for systems intended to be embarked on board an aircraft.

 it is for example advantageous to embark on board a helicopter an infrared laser velocimeter, in order to measure the speed of the helicopter with respect to the air, the sighting zone of the laser being located out of the aerodynamic turbulences generated by helicopter. This velocimeter. Must therefore provide the module and the sign of the speed of movement of the mobile relative to the target area. A difficulty then appears: difficult to embed, hetero-detection laser velocimeters are not suitable for this kind of application; For their part, laser velocimeters with homodyne detection, which can be shipped, have so far only provided module information or absolute speed value, without its sign.

The present invention overcomes this difficulty by proposing an improved homodyne detection laser velocimetry method and device.

In known manner, such a method comprises the following operations: a) a laser beam is sent in a direction where it is desired to measure the speed of the relative movement of one or more objects relative to the laser, b) the backscattered beam is selectively collected in the same direction, c) this backscattered beam is mixed with a reference beam taken from the initial laser beam, and d) the mixed beam is detected, the beat frequency or frequencies obtained being representative of the modulus of the relative speed of the said object or objects .

According to a general characteristic of the invention, in step c), the beams are mixed so as to obtain, at least at the detection level, a slight angular offset from their respective wave surfaces, and at stage d), are carried out separately at least two detections, in zones offset from one another in a predetermined manner transversely to the mixed beam and to the axis of the angular offset between the wave surfaces.

The comparison of the phases of the two detected signals (for the or each beat frequency) then gives the sign to be associated with the corresponding speed module information.

Preferably, to obtain a good signal / noise ratio on the phase measurement, the offset d between the two detection zones substantially verifies the relationship 8.d = A / 4 (to within kA / 2) where e is the angular offset in radian, At the wavelength of the laser radiation, measured in the same unit as d, and k an integer.

 Advantageously, the offset between the two detection zones checks the relation 0.d = A / 4 and the two detection zones are coplanar and located side by side.

The present invention applies in particular when the wavelength of the laser beam is located in the infrared.

There are also known laser velocimetry devices with homodyne detection, which include a laser source, optical means for sending part of the laser beam in a chosen direction, for selectively collecting the backscattered beam in the same direction, and for mixing the beam. backscattered to another part of the laser beam, and detector means capable of supplying a detected signal, containing one or more beat frequencies each representing speed information taken in absolute value.

In such a device, according to the invention, the optical means are arranged so that the beams are mixed so as to obtain, at least at the level of the detector means, a slight angular offset from their wave surfaces. respective, and the detector means comprise at least two detectors, offset from one another in a predetermined manner transversely to the mixed beam and to the axis of said angular offset between the wave surfaces, as well as means suitable for compare the phases of the two signals detected for the or each beat frequency.

In a preferred embodiment, the two detectors are located side by side, on the same substrate.

The distinction between the emitted beam, or measurement beam, and the backscattered beam can be made by applying linear polarization to the former, and using a polarization splitter.

Different embodiments of the invention can be constructed, in particular from structures of interferometers of the Mach-Zender type, or of the Michelson type. Furthermore, the beam can be applied to the steering members with or without prior focusing.

Finally, at least for certain applications, an optical device of the telescope type can be added to the device, placed on the path of the measurement laser beam, in order to allow it to illuminate a wider area.

Other characteristics and advantages of the invention will appear on examining the detailed description which follows, and the appended drawings, in which - FIG. 1 schematically illustrates a first embodiment of the invention - FIG. 2 illustrates in more detail the constitution of a double detector, and the situation of the wave surfaces incident on it; and - Figure 3 illustrates a second embodiment of the present invention.

In FIG. 1, the reference numeral 1 designates a coherent (or quasi-coherent) light source, such as an infrared carbon dioxide laser, operating for example at the wavelength of 10.6 microns.

The laser beam illustrated at 10 is linearly polarized in the direction materialized by the arrow P1. It is applied to optical means marked as a whole 2.

The laser beam 10 first strikes a partial reflection plate 20. The major part of the beam passes through this plate 1 while retaining the same polarization P1, to result in a polarization splitter 21. This can be produced in the form of '' a grid polarizer. I1 is arranged so as to allow the polarization P1 to pass, and to reflect on the other hand the perpendicular polarization (materialized, according to a known convention, by a point surrounded by a circle). The measurement laser beam thus transmitted by the polarization splitter 21 passes through a plate 22, which is a quarter wave plate for the frequency of the light radiation concerned. We know that such a suitably oriented quarter-wave plate transforms linear polarization into circular polarization.

The measurement beam thus modified is applied to the target, or to the targets.

In certain applications, such as those of infrared velocimeters on board aircraft, the measurement beam can be applied to a device 8 acting as a telescope, to reduce the measurement volume. It is then possible to obtain a measurement, in average value, of the speed of movement of the aircraft relative to the air by collecting the signals backscattered by particles in suspension, in the air, outside the aerodynamic turbulence zones generated by 3'aircraft. In other applications, such as those of anemometers, it is possible to access a measurement of the movement of the particles included in a fluid, hence the measurement of the speed of movement of this fluid, for example the speed of the wind.

The fluid or the object whose speed we want to measure, (or the particles, suspended in the air 'of the laser sighting zone and relative to which we want to measure the speed of movement of a body, such that a helicopter) produces a backscattering of the incident laser beam, which is thus returned ((if necessary, through the telescope 8) in the direction of the optical means 2.

After crossing the quarter-wave plate 22 again, the backscattered beam has now seen its polarization rotate by 900 relative to the measurement beam. At incidence on the polarization splitter 21, it is reflected along the optical path 15, with the transverse polarization illustrated in P2. This beam 15, also noted S (for Signal), strikes another partial reflection plate 26.

A generally weak part of the initial laser beam 10 is reflected by the blade 20, towards a mirror 23. At this moment, this beam has the polarization P1. As it operates like a local oscillator, this beam is noted O.L. It crosses a half-wave plate 24, which modifies its polarization by 900, so as to give it the same polarization P2 as the signal beam S. After that, the local beam O.L. strikes the partial reflection plate 26, downstream of which it is mixed with the signal beam S, for example on the optical path 19. This mixed beam is applied to a detector 5.

According to the invention, the detector 5 is a double detector, for example as illustrated in FIG. 2. It can be seen there that the detector 5 is here constructed of two elements 51 and 52, placed side by side, and with separate electrical outputs. One, teS dual detector can be realized in different ways. As a photosensitive material, structures of the HgCdTe type can be used in infrared. it is possible to place two detectors of this type side by side, either real; ment, or by using for example the two optical mixing paths 1 and 191 available at the outlet of the blade 26.

However, it is preferable that the two detectors are located on the same substrate, which makes it possible to pool their auxiliary elements, and in particular their cryostat.

For this purpose, we know in particular so-called four-quadrant detectors. By electrically connecting the quadrants two by two, two detectors which can be used according to the present invention are thus obtained. Such four-quadrant detectors operating in infrared are available in particular from the company Ar.onyme de Télécommunications. Of course, although this is a preferred application, the present invention is not limited to the case of infrared light.

By acting on the angle of the members 21 and 26 with respect to the optical path which concerns them, it is possible to ensure that, by remaining mixed with the local oscillator beam OL, the signal beam S arrives at the double detector 5 with a wave surface inclined at a small angle 9 relative to that of the local oscillating beam (or vice versa, depending on the way in which the detector 5 is arranged).

In practice, this angular offset e is adjusted so that the relative phase shift between the wave surfaces S and OL.

or about A / 4 when switching from one to the other of the detectors.

In FIG. 2, we denote the distance between the detectors S1 and 52. The relationship which must be substantially satisfied is then written 0.d = A / 4 (to k times A / 2 close; k integer 30 ) where X is the wavelength of the laser radiation.

This ensures a phase difference between the signals detected respectively by the photosensitive elements 51 and 52. In this respect, the relative average path difference of the waves arriving on each element of the detector is A / 4, that is to say ensures a phase shift of n / 2. To obtain a good homodyning efficiency it is necessary that the path difference between the wave surfaces is less than or equal to A / 4 over the entire surface of each of the detectors, the best efficiency being obtained for k = 0 (ed = A / 4).

As the sign of this phase shift is linked to the sign of the observed Doppler frequency shift, and consequently to the sign of the speed of the object or objects concerned, it suffices to measure, using a phase comparison device 6 , the phase difference between the two signals for the Doppler frequency (s) concerned. A prior calibration of the device makes it possible to determine the sign of the phase shift (+ w / 2) of the two electrical signals delivered by the elements 51 and 52, as a function of the direction of movement of the object, therefore of the sign of its speed. .

In the embodiment illustrated in FIG. 1, which has just been described, it is accepted that the mixed beam 19 remains in parallel light.

An interesting variant consists in focusing this beam, using a lens not shown in FIG. 1, the focal point of which will be the location of the detector 5.

The arrangement of the optical means 2 is then modified. Instead of acting jointly on some of these elements to modify the relative angle of the two beams when they are mixed, this action is done so as to offset the two beams laterally with respect to each other, whereas 'they remain parallel. Given the focal distance of the lens, this offset is adjusted so that, downstream of the lens, the two beams arrive at the detector 5 by making the above-mentioned angle O between them.

As a variant, it is possible to adjust the offset by translation of one of the blades 21 or 26 in a direction not situated in the plane of this blade, for example in a direction perpendicular to this plane.

reference is now made to FIG. 3, which illustrates a second embodiment of the invention, in which the optical means 3 are arranged in the manner d | a Michelson interferometer.

The initial laser beam 10 arrives at the input path 40 of the interferometer, the central element of which is again a polarization splitter 31, which can be produced as above. In known manner, the interferometer has an output path by transmission 45, along which the measurement laser beam propagates, which crosses a quarter-wave plate 32. It can subsequently meet a telescope 8 as before, or else go and strike directly at the object whose speed is to be measured. The backscattered beam again crosses the quarter-wave plate 32, after which its polarization has rotated 90 relative to the initial laser beam 10 (the polarizations are not illustrated in FIG. 3).

The polarization splitter 31 has reflected part of the incident laser beam 10 on its first transverse path 41. On this path is interposed a quarter-wave plate 34. The local beam thus obtained then strikes a right dihedral 35, which acts as a retroreflector with lateral offset to return the local beam to the polarization splitter 31.

At this time, the local beam has passed through the quarter-wave plate 34 twice, and therefore undergoes a rotation of 900 from its direction of polarization. It then crosses (partly) the polarization splitter 31, to arrive at the second transverse path 42. For its part, the backscattered beam at 45 is reflected on the polarization splitter 31, to join it cross path 42, on which it is offset from the local beam.

The offset is lateral, the two beams being parallel,
They then jointly strike a focusing lens 39, placed so that the two beams converge on the double detector 5. The position of the right dihedral 35 along the path 41 is adjusted so as to obtain an offset between beams such as the fronts of waves of these form an angle e on the detector 5, which corresponds to the angles of arrival of the two beams different from e on this same detector. The rest of the process takes place as before, the two outputs of the double detector 5 being applied to a phase comparison device 6.

Of course, in one or the other embodiment, the device 6 will generally carry out both the measurement of the speed in absolute value and the measurement of its sign.

Claims (12)

Claims. 1. Doppler laser velocimetry method with homodyne detection, according to which a) a laser beam (1) is sent in a direction where it is desired to measure the speed of the relative movement of one or more objects relative to the laser, b) the backscattered beam is selectively collected (21) in the same direction, c) the backscattered beam is mixed (26) with a reference beam drawn (20, 23) from the initial laser beam, and d) the beam is detected (5) mixed, the beat frequency (s) obtained being representative of the modulus of the relative speed of the said object (s), characterized in that - in step c) the beams are mixed so as to obtain, at least at the detection, a slight angular offset of their respective wave surfaces, and - in step d) are carried out separately at least two detections, in zones offset from one another in a predetermined manner transversely to the beam mixture and to the axis of the angular offset ent re the wave surfaces, the comparison of the phases of the two detected signals (5) at each beat frequency giving the sign of the corresponding speed. 2. Method according to claim 1, characterized in that the offset d between the two detection zones, substantially verifies the relation 0.d = A / 4 + k A / 2, where e is the angular offset in radians, À la wavelength of laser radiation and k a positive or zero integer. 3. Method according to one of claims 1 and 2, characterized in that the two detection zones are coplanar and located side by side and k is equal to zero. 4. Method according to one of claims 1 to 3, characterized in that the wavelength of the laser beam is located in the infrared. 5. Doppler laser velocimetry device with homodyne detection, of the type comprising: - a laser source (1), - optical means (2, 3) for sending part of the laser beam (10) in a chosen direction, for selectively collecting the backscattered beam in the same direction, and to mix the backscattered beam with another part of the laser beam, and - detector means (5) capable of providing a detected signal, containing one or more beat frequencies each representing a speed absolute value, characterized in that the optical means (2, 3) are arranged so that the beams are mixed so as to obtain, at least at the level of the detector means, a slight angular offset of their surfaces d respective waves, and in that the detector means comprise at least two detectors (51, 52), offset from one another in a predetermined manner transversely to the mixed beam and to the axis of said angular offset between e the wave surfaces, and means (6) suitable for comparing the phases of the two signals detected for each beat frequency. 6. Device according to claim 5, characterized in that the offset d between the two detectors (51,52) substantially verifies the relationship Qd = l / 4 + kX / 2 where O is the angular offset in radians, is the length d wave of laser radiation, and k a positive or zero integer. 7. Device according to one of claims 5 and 6, characterized in that k = O and in that the two detectors (51, = 2) are located side by side, on the same substrate. 8. Device according to one of claims 5 to 7, characterized in that the laser beam is linearly polarized and in that the optical means comprise the following constituents - a partial reflection plate (20), providing the measurement laser beam and the reference laser beam, - a polarization splitter (21), placed on the path of the measurement laser beam, and followed on this same path by a quarter wave plate (22), - a reflector assembly ( 23) and half-wave plate (24) placed on the path of the reference beam, - another partial reflection plate (26) for mixing the backscattered beam, selectively coming from the polarization splitter, with the reference beam, - the at least one of these constituents (20-26) being arranged so that the wave surfaces of the reference beam and of the backscattered beam are angularly offset with respect to each other on arrival at the detectors. 9. Device according to one of claims 5 to 8, characterized in that a focusing member is placed between the optical means and the detectors, said angular offset being obtained by lateral offset of the backscattered beam relative to the reference beam, upstream of the focusing member. 10. Device according to one of claims 5 to 7, characterized in that the laser beam is linearly polarized, and in that the optical means comprise - a polarization separator member (31) having an input path (40) which receives the laser beam (10), an output path (45) by direct transmission providing the measurement beam through a quarter wave plate (32), as well as two transverse optical paths. (41,42), - the first transverse path (41) comprising a quarter-wave plate (34) followed by a right reflective dihedral (35), which returns the reference beam to the separating member (3 -1) in slight axial offset and with a linear polarization having turned ae w / 2 for mixing with the backscattered beam on the second transverse optical path (42), which comprises a focusing member (39) of the beam mixed on the detectors ( 51.52). 11. Device according to one of claims 5 to 10, characterized in that the wavelength of the laser beam is located in the infrared. 12. Device according to one of claims 5 to 11, characterized in that it comprises an optical device of the telescope type (8) placed on the path of the measurement laser beam.