LIDAR SYSTEM FOR VELOCIMETRIC MEASUREMENTS

Abstract

A pulsed LIDAR system is adapted to vary the radiation modulation frequency within each pulse, such that a component of a detection signal useful for measuring the velocity of a target is spectrally shifted as a function of the distance from the target. Such a LIDAR system allows the useful component of the detection signal to be spectrally separated from a Narcissus signal generated by an output optic (18) common to a transmission channel (10) and a detection channel (20) of the LIDAR system. A LIDAR system according to the invention can advantageously be used to measure airspeed. (Shorthand figure: Figure 1)

Description

Title of the invention: LIDAR SYSTEM FOR VELO-CEMETRIOUES MEASUREMENTS technical field

[0001] The present description relates to a LIDAR system for performing velocimetric measurements, as well as a corresponding measurement method. Previous technique

[0002] LIDAR systems, for "Light Detection and Ranging," are useful for measuring the distance and velocity of a target. They employ a laser beam directed at the target and analyze the portion of this beam that is back-reflected or backscattered by the target. The distance measurement results from a direct or indirect estimation of the time of flight of the radiation to travel the distance from the target to the LIDAR system and back, and the target's velocity is deduced from a measurement of the Doppler frequency shift affecting the portion of the radiation that has been back-reflected or backscattered by the target.

[0003] In certain circumstances and for certain applications, it is particularly advantageous for the radiation beam emitted towards the target to consist of a series of successive pulses. Indeed, each pulse can thus have instantaneous power values ​​that are much higher than those of continuous radiation, thereby increasing the power of the portion of the radiation that is collected after being backscattered by the target. This first advantage is particularly important for a LIDAR system designed to measure airspeed, since the power of the collected portion of the radiation is then very low. A second advantage of using a pulsed LIDAR system stems from the fact that a continuous laser source incorporated into such a system can then have a power level that does not pose an eye hazard to operators.

[0004] Such pulsed LIDAR systems can be used to measure the repulsion distance and repulsion speed of a solid object that constitutes the target.

[0005] However, some pulsed LIDAR systems are specifically dedicated to anemometric measurements. The target then consists of particles located in a portion of the atmosphere; the pulses are emitted by an emission channel of the LIDAR system towards this portion of the atmosphere, and a detection channel of this LIDAR system is adapted to collect the portion of the pulses that has been backscattered by the particles located in the portion of the atmosphere. One by A key feature of these LIDAR systems dedicated to anemometric measurements is their ability to detect backscattered portions of emitted pulses with very low power values. According to one possible design of LIDAR systems for anemometric measurements, as described in WO 2021 / 053290 A1, the portion of the atmosphere targeted by a measurement is determined by a convergence of the radiation beam from the emitted pulses. This portion of the atmosphere overlaps with the region of space, known as the Rayleigh zone, where the radiation emitted by the LIDAR system is most concentrated.

[0006] However, for the easiest-to-implement LIDAR systems, in which the same output optics are common to both the emission and detection channels, these output optics generate a partial reflection of the emitted radiation on some of its optical components. This internal reflection produces one or more components in the detection signal that are not related to the target. These components, which originate from the optical interface between the LIDAR system and the free propagation space of the radiation, and which are commonly called the Narcissus signal, can have a significant, even very significant, intensity. Therefore, such a Narcissus signal can be superimposed on a useful component of the detection signal generated by a target located at a short distance from the LIDAR system, preventing the valid detection of this useful component.In particular, the Narcissus signal is much more intense than the useful component of the detection signal in the case of anemometric applications. Technical problem

[0007] From this situation, an object of the present invention is to provide a LIDAR system for which a useful component of the detection signal relating to a target which is located at a short distance can be distinguished from the Narcissus signal, and can be exploited to provide an estimate of the distance and / or speed of this target.

[0008] An ancillary object of the invention is to provide such a LIDAR system which is effective for a short measurement distance, and which allows anemometric velocity measurements to be made. Summary of the invention

[0009] To achieve at least one of these goals or another, a first aspect of the invention proposes a LIDAR system that is adapted to measure the speed of at least one target, and which comprises: - an emission channel, adapted to emit radiation pulses towards the target through an output optic of the LIDAR system; - a detection channel, comprising a photodetector, and adapted to collect, also through the output optics, and direct onto the photodetector, a portion of the im impulses that have been back-reflected or back-scattered by the target, so as to produce a detection signal; and - a signal processing unit, configured to deliver a velocity measurement result that is relative to the target, from the detection signal.

[0010] Such a LIDAR system of the invention further possesses the following characteristics: - the emission path is further adapted to vary a modulation frequency of the radiation within each pulse, so that a component of the detection signal that is useful for measuring velocity, called the useful component, is spectrally shifted as a function of the distance of the target from the LIDAR system; and - the signal processing unit is further configured to extract the useful component from a spectrum of the detection signal, so as to isolate this useful component from at least one other component of the detection signal which results from a back-reflection or partial back-scattering of the emitted pulses occurring in the output optics of the LIDAR system, and to obtain the result of the velocity measurement from the extracted useful component.

[0011] In the LIDAR system of the invention, the useful component of the detection signal is distinguished from the Narcissus signal by an apparent frequency shift that depends on the distance of the target from the LIDAR system. This frequency shift makes it possible to separate the useful component from the Narcissus signal, for example, by performing digital filtering of the useful component of the detection signal, selectively with respect to the Narcissus signal. It is thus possible for the LIDAR system of the invention to provide a velocimetric measurement result, and possibly also a rangefinding measurement result, even for a target located at a short distance in front of the output optics. Such a rangefinding measurement can then be performed by determining the frequency shift that affects, at the time of its reception, the backscattered portion of the pulses.

[0012] In such a LIDAR system, the modulation frequency that is varied within each pulse can be directly the optical frequency of the radiation from that pulse. In other words, the LIDAR system can implement optical frequency modulation.

[0013] Alternatively, the modulation applied to the pulses emitted towards the target can be amplitude modulation, in particular sinusoidal amplitude modulation, with a sinusoid frequency that varies with time within each pulse. This sinusoidal frequency of amplitude modulation is then the modulation frequency that is varied according to the invention.

[0014] The detection pathway can be arranged to produce heterodyne detection of the back-reflected or back-scattered portion of the pulses. In this case, the transmission channel can further be adapted to apply to the emitted pulses an additional frequency shift which is constant, in addition to the variation of the modulation frequency within each pulse, and heterodyne detection is advantageously carried out by mixing a reference signal which does not have this additional frequency shift which is constant, with the back-reflected or back-scattered portion of the pulses.

[0015] However, a LIDAR system according to the invention may alternatively implement a detection mode that is not heterodyne. For example, the detection channel may be adapted to perform direct detection of the portion of the pulses that has been backreflected or backscattered by the target. Such direct detection may be used, in particular, when the modulation applied within each pulse is of the amplitude modulation type.

[0016] A LIDAR system according to the invention can, in particular, be adapted to measure airspeed. As described above for this application, the target consists of particles located in a portion of the atmosphere, the pulses are emitted through the emission channel towards this portion of the atmosphere, and the detection channel is adapted to collect and detect the portion of the pulses that has been backscattered by the particles located in the portion of the atmosphere. In particular, again for this application of measuring airspeed, the output optics can be adapted to transmit the pulses out of the LIDAR system in the form of a convergent beam of radiation, with the convergence of this beam determining the portion of the atmosphere relevant to the airspeed measurement.The measurement distance, that is to say the distance from the portion of the atmosphere concerned by the anemometric velocity measurement, can then be determined from the convergence that is applied to the beam of emitted pulses.

[0017] In preferred embodiments of the invention, the transmission channel can be adapted to vary the modulation frequency within each pulse according to a constant-slope frequency modulation ramp. Furthermore, the transmission channel can be further adapted to change the sign of the slope of this frequency modulation ramp between two successively transmitted pulses. In particular, the slope of the frequency modulation ramp can be equal in absolute value but opposite in sign between two successively transmitted pulses. In this case, the signal processing unit can be configured to obtain the velocity measurement result from a sum of two respective frequency shifts relative to a first extracted useful component corresponding to those of the transmitted pulses for which The first useful component is extracted from the first, corresponding to the emitted pulses for which the slope of the modulation frequency variation ramp is positive, and a second useful component is extracted, corresponding to those emitted pulses for which the slope of the modulation frequency variation ramp is negative. Simultaneously, the signal processing unit can be further configured to obtain an evaluation of the target's distance from the LIDAR system based on the difference between the respective frequency shifts of the first useful component extracted, corresponding to those emitted pulses for which the slope of the modulation frequency variation ramp is positive, and the second useful component extracted, corresponding to those emitted pulses for which the slope of the modulation frequency variation ramp is negative.Such an assessment of the distance to the target is then independent of a determination of that same distance obtained from the convergence of the pulse beam.

[0018] However, when the distance of the target from the LIDAR system is known from other sources, the velocity measurement can be obtained from a variation in the modulation frequency, which is the same for all pulses. This applies in particular to an anemometric velocity measurement where the distance of the portion of the atmosphere being measured is determined by the convergence of the radiation beam of the emitted pulses.

[0019] In such an embodiment of the invention, for which the distance from the target is known, the transmission path can be adapted to emit the radiation pulses all with the same frequency modulation ramp. The signal processing unit can then be configured to obtain the velocity measurement result from a second contribution to a frequency shift that is relative to the extracted useful component, this second contribution being calculated by subtracting from the frequency shift a first contribution that is produced by the frequency modulation ramp in combination with the distance from the target.

[0020] A second aspect of the invention proposes a method for measuring the speed of at least one target using a LIDAR system, comprising the following steps: / 1 / emit radiation pulses towards the target through an output optic; / 2 / using the output optics, collect a portion of the pulses that have been back-reflected or backscattered by the target; / 3 / detect the back-reflected or backscattered portion of the pulses, using a photodetector that produces a detection signal; and / 4 / From the detection signal, deliver a velocity measurement result that is relative to the target.

[0021] According to the invention, the method has the following additional characteristics: - a modulation frequency of the radiation is varied within each pulse during step / 1 / , so that a component of the detection signal which is used in step / 4 / for velocity measurement and called the useful component, is spectrally shifted as a function of the distance of the target from the LIDAR system; and - the useful component is extracted from a spectrum of the detection signal during step / 4 / , so as to isolate this useful component from at least one other component of the detection signal which results from a back-reflection or partial back-scattering of the emitted pulses occurring in the output optics, then the result of the velocity measurement is obtained from the extracted useful component.

[0022] Such a method can be implemented, in particular, for performing an anemometric measurement, but not exclusively. Furthermore, the LIDAR system can be mounted on board an aircraft, and the method performed during an aircraft flight.

[0023] In general, a method conforming to the second aspect of the invention can be implemented using a LIDAR system that conforms to the first aspect of the invention.

[0024] Different implementations of the method of the invention may use at least one of the following additional features: - the modulation frequency which is varied within each pulse can be directly the optical frequency of the radiation of that pulse, or can be a sinusoidal frequency of amplitude modulation of the radiation of each pulse; - the detection of the part of the pulses that has been back-reflected or back-scattered can be of the heterodyne type or be a direct detection; - The modulation frequency within each pulse can be varied according to a constant-slope frequency modulation ramp. Possibly, the sign of the slope of this frequency modulation ramp can be reversed between two successively emitted pulses. In particular, the slope of the frequency modulation ramp can be equal in absolute value but opposite in sign between two successively emitted pulses; and - For anemometric applications, the output optics can transmit pulses outside the LIDAR system as a converging beam of radiation, with the convergence of this beam determining the portion of the atmosphere relevant to the anemometric velocity measurement. In this case, all radiation pulses can be emitted with the same frequency modulation ramp, and the velocity measurement result can be obtained from a second contribution to a frequency shift that is relative to the component useful extracted, this second contribution being calculated by subtracting from the frequency shift a first contribution which is produced by the frequency modulation ramp in combination with the distance away from the portion of atmosphere.

[0025] In particular, when two successively emitted pulses have frequency modulation ramps with constant slopes and opposite signs, the target velocity measurement can be obtained from the sum of two respective frequency shifts relative to a first extracted useful component corresponding to those emitted pulses for which the slope of the radiation frequency modulation ramp is positive, and to a second extracted useful component corresponding to those emitted pulses for which the slope of the radiation frequency modulation ramp is negative. In parallel, an evaluation of the target's distance from the LIDAR system can be obtained from the difference between the respective frequency shifts relative to these first and second extracted useful components. Brief description of the figures

[0026] The features and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments, with reference to the accompanying figures, among which:

[0027] [Fig.l] is a synoptic diagram of a LIDAR system according to the invention, for an anemometric speed measurement application, with optical frequency modulation and heterodyne detection;

[0028] [Fig.2a] and [Fig.2b] are two spectrograms obtained with the LIDAR system of [Fig.1], for two measurement distance values;

[0029] [Fig.3] reproduces two spectra of a heterodyne detection signal obtained with the LIDAR system of [Fig.1];

[0030] [Fig.4] illustrates a possible application of the LIDAR system of [Fig.1]; and

[0031] [Fig.5] corresponds to [Fig.1] for a target velocity measurement application with Amplitude modulation and direct detection. Detailed description of the invention

[0032] For clarity, the dimensions of the elements shown in these figures do not correspond to actual dimensions or ratios of actual dimensions. Furthermore, some of these elements are represented only symbolically, and identical reference numerals shown in different figures designate identical elements or elements with identical functions.

[0033] The invention is first described for an anemometric velocity measurement application using optical frequency modulation. Secondly, it will be described for an application measuring the velocity of a solid object which constitutes the target, using amplitude modulation.

[0034] According to [Fig.1], a LIDAR system for anemometric velocity measurement comprises an emission channel 10, a detection channel 20 and a signal processing unit 30.

[0035] The emission channel 10 comprises a laser source 11, denoted SOURCE, an optical modulator 14, denoted MAO, an optical amplifier 15, denoted AO, and an output optic 18. The laser source 11 may be of the continuous emission type, for example, producing radiation with a wavelength X of approximately 1.55 pm (micrometers). The modulator 14 may be of the acousto-optic type and controlled to slice the radiation from the laser source 11 into successive pulses. In a manner that is common but not mandatory, the modulator 14 may also apply a constant frequency shift to the optical frequency of the radiation, for example, a shift of approximately 40 MHz (megahertz). The optical amplifier 15 increases the power of the radiation pulses thus formed, and these pulses are transmitted outwards by the output optics 18 in the form of a beam F whose central direction of propagation is AA.As is known, dividing the radiation into successive pulses allows for high instantaneous power values ​​within each pulse, which is particularly advantageous for anemometric measurements. Such pulsed emission can indeed achieve instantaneous radiation power values ​​within each pulse, with a substantially rectangular profile, ranging from 100 W to 5-10⁵ W, for example, 500 W. Such values ​​are not attainable for continuous laser radiation with the optical amplification components currently available or compatible with the anemometric application. The output optics 18 can consist of one or more converging lenses and can determine the size of the output pupil of the emission channel 10.For example, this exit pupil can have a radius of approximately 0.07 m (meter). The optical components of the emission channel 10, apart from the output optic 18, can advantageously be implemented using fiber optic technology, reducing the size of the LIDAR system and facilitating the optical alignment of these components.

[0036] The output optic 18 is designed to produce the beam F of radiation pulses with a convergent beam structure in a spatial region located downstream of this output optic, relative to the direction of propagation of the radiation exiting the emission channel 10. Thus, the beam F has cross-sections, perpendicular to its central propagation direction AA, which decrease between the output optic 18 and a focusing zone denoted ZF, and then which increase in the form of a diverging beam beyond this focal zone ZF. In a known way, notably using a Gaussian beam model, the focal zone ZF can be approximated as a cylinder with axis superimposed on the central propagation direction AA, of radius w0 = X / (jt-0), commonly called "waist", and of length 2-lR, where 1R is the Rayleigh length equal to X / (ir-02), 0 being the half divergence angle of the beam F beyond the focal zone ZF, expressed in radians. Typically, the distance between the output optic 18 and the focusing zone ZF, which is denoted D and which can be selected by adjusting a focal length or longitudinal position of the output optic 18, can vary from a few meters to several hundred meters, the Rayleigh length 1R then varying correlatively from a few tens of centimeters to a few meters, and the radius w0 being on the order of a few centimeters.In general, the half-angle of divergence 0 of the beam F can be evaluated downstream of the focal zone ZF in the direction of radiation propagation, at a distance from the focal zone ZF which can be equal to 1 km (kilometer).

[0037] To perform heterodyne detection, the detection channel 20 may include a photodetector 22, denoted PD, and optical couplers 13, 16, and 21, denoted CO, which are arranged to combine a backscattered portion of the beam F with a portion FRu of the radiation from the laser source 11. This portion of FRu radiation acts as the reference radiation for heterodyne detection. As is known, the backscattered portion of the beam F that is thus detected originates essentially from the focusing zone ZF and is produced by backscattering particles located in this zone. The optical coupler 16 may advantageously be of the polarization-separating type, in which case the radiation from the optical amplifier 15 is linearly polarized, and a quarter-wave plate 17, denoted X / 4, is inserted between the optical coupler 16 and the output optics 18.The optical coupler 16 and the quarter-wave plate 17 thus constitute an optical circulator which couples the emission channel 10 and the detection channel 20 to the output optics 18 thus shared.

[0038] Finally, the signal processing unit 30 can be constituted by a computer module, denoted PC, which hosts a suitable program for processing the detection signal delivered by the photodetector 22. The signal processing unit 30 provides as output an evaluation of the velocity component of the backscattering particles that are in the focusing zone ZF, this component being parallel to the central propagation direction AA and denoted VA-a-

[0039] The operation of such a LIDAR system for performing anemometric velocity measurements is well known to those skilled in the art, so it is not necessary to repeat it here. The system is oriented and the half-angle of divergence 0 adjusted so that the focal zone ZF is in a portion of the atmosphere where the wind speed is to be measured. The particles that backscatter the beam F are then dust, microcrystals, or aerosol droplets suspended in the atmosphere within the focusing zone ZF. The structure of the output optics 18, and its control when it allows variable adjustment of the half-angle of divergence 0, can provide an evaluation of the distance from a center of the focusing zone ZF to the LIDAR system, corresponding to the distance D introduced previously.

[0040] For the invention, the emission channel 10 further comprises additional modulation means, which are adapted to vary the optical frequency of the radiation within each of the pulses as formed by the acousto-optic modulator 14. For example, these additional frequency modulation means may consist of an electro-optic modulator 12, denoted MEO, with a suitable control unit thereof, designated by reference numeral 40, denoted CTRL and called the modulation controller. They may be inserted between the laser source 11 and the acousto-optic modulator 14. In various embodiments of the invention, these frequency modulation means may be arranged upstream or downstream of the optical coupler 13 which performs the sampling of the reference radiation Fref.In the following, we will assume that they are arranged upstream of the optical coupler 13 with respect to the direction of propagation of the radiation in the emission channel 10, and a person skilled in the art will be able to adapt the following description to the case of an arrangement of the additional modulation means downstream of the optical coupler 13.

[0041] For example, the modulation controller 40 is configured so that the acousto-optic modulator 14 slices the radiation initially produced by the laser source 11 into successive pulses of individual durations equal to 1 ps (microsecond), and so that the electro-optic modulator 12 applies a linear increase in the optical frequency of the radiation within each pulse, from approximately 0 MHz (megahertz) at the beginning of the pulse to approximately 500 MHz at the end of the pulse. The slope p of the variation of the optical frequency is then equal to +0.500 MHz / ns (megahertz per nanosecond). The portion of the pulses produced by the emission channel 10 which is backscattered in the focusing zone ZF then reaches the photodetector 22 with a propagation delay relative to the reference radiation Fref, this delay corresponding to the round-trip propagation time between the emission optics 18 and the focusing zone ZF.Due to the frequency modulation introduced according to the invention, it then exhibits a first frequency shift contribution relative to the reference radiation Fref, which is equal to -p-2D / C, where C is the propagation speed of the radiation outside the LIDAR system between the output optic 18 and the focusing zone ZF. This first frequency shift contribution is denoted Afi hereafter. As is known, the . The VA-a component of the velocity of the backscattering particles contained within the focusing zone ZF, this component being parallel to the central propagation direction AA, produces a second frequency shift contribution, denoted Af2 and equal to 2-VA a / ^ when the anemometric velocity VA a is oriented as shown in [Fig. 1]. For the embodiment of the invention in [Fig. 1], the two contributions Af1 and Af2 are added together to constitute the frequency of the heterodyne detection signal delivered by the photodetector 22. They are also added to the constant frequency shift that may be produced by the acousto-optic modulator 14.

[0042] [Fig.2a] and [Fig.2b] indicate the frequencies of the spectral components of the signal of Heterodyne detection is performed as a function of time within each pulse. For each spectrogram, the horizontal axis represents time, denoted t and expressed in microseconds (ps), the vertical axis represents frequency, denoted f and expressed in megahertz (MHz), and the tone scale to the right of each spectrogram represents instantaneous spectral power, denoted P, and expressed in decibels (dB) relative to a baseline noise level. For the spectrogram in [Fig. 2a], the output optics 18 are adjusted so that the distance D from the focal zone ZF is equal to 15 m (meter). The length 2-lR of this focal zone ZF is then equal to 90 cm (centimeter). For the spectrogram in [Fig. 2b]: D is equal to 30 m and 2-lR is equal to 3.5 m.The numerical values ​​of the other LIDAR system parameters are those already mentioned, identical for both spectrograms, except for the value of the constant frequency shift produced by the acousto-optic modulator 14, which is zero. The two components of the heterodyne detection signal appearing in each spectrogram correspond respectively to a partial back-reflection of the pulses on the output optic 18, designated as "Narcissus" in the spectrograms and called the Narcissus signal, and to the portion of the pulses that was backscattered in the focusing zone ZF, designated as "Mes" and called the measurement signal. The Narcissus signal corresponds to D=0 and f=0, since it does not exhibit a significant propagation delay upon arrival at the photodetector 22 relative to the reference radiation Fref, nor does it exhibit a Doppler effect.The measurement signal reaches the photodetector 22 with a delay At equal to 2-D / C relative to the Narcissus signal, and has a frequency f that is shifted by the first contribution Afi in the absence of wind in the focusing zone ZF. For the adopted operating conditions, the delay At is equal to 0.10 ps for [Fig. 2a] and 0.20 ps for [Fig. 2b], and Afi is equal to 50 MHz for [Fig. 2a] and 100 MHz for [Fig. 2b]. The initial time (t=0) in these two spectrograms corresponds to the beginning of the detection of the backscattered portion of the radiation pulse. Furthermore, the spectral width of the measurement signal is greater for [Fig. 2b] compared to [Fig. 2a], due to the focusing zone ZF which... is much longer for [Fig.2b].

[0043] The signal processing unit 30 calculates a time-dependent Fourier transform of the heterodyne detection signal delivered by the photodetector 22. It then extracts, for example by applying digital adaptive filtering, the measurement signal from the spectrum of the heterodyne detection signal and determines the frequency f of this measurement signal. Generally, this frequency f is equal to Af0 + Afi + Af2, where Af0 is the constant frequency shift within all the pulses produced by the acousto-optic modulator 14, if applicable. When the separation distance D is known from other sources, in particular by adjusting the focus of the output optics 18, the frequency shift contribution Afi is calculated by Afi = -p - 2 - D / C, and the two contributions Af0 and Afi are subtracted from the value determined for the frequency f of the measurement signal.The result of this subtraction is the frequency shift contribution Af2 which corresponds to the Doppler effect, and the wind speed component in the focus zone ZF, parallel to the central propagation direction AA, is calculated by the signal processing unit 30 according to the formula: VA A=^-Af2 / 2. .

[0044] The measurement signal “Mes” as just described as a spectral component extracted from the heterodyne detection signal, has been called the useful component of the detection signal in the general part of this description.

[0045] However, for anemometric measurements, the separation distance D from the focusing zone ZF can depend on factors external to the LIDAR system, such as atmospheric turbulence and / or temperature variations of the air present along the pulse path between the output optics 18 and the focusing zone ZF, and / or variations in the concentration of backscattering particles along the central propagation direction AA. The improvement described herein makes it possible to deduce the effective value of the separation distance D from the heterodyne detection signal. For this purpose, the modulation controller 40 can be configured so that some of the pulses are emitted with a predetermined linear ramp slope of optical frequency variation, for example, positive, and other pulses with a negative linear ramp slope of optical frequency variation.Preferably, every other pulse is emitted with a positive linear ramp slope p, and the other pulses are emitted using -p as the linear ramp slope. Then, for the first pulses, the frequency of the measurement signal resulting from heterodyne detection is Af0 + Afi + Af2, as before, and is denoted f+. For the pulses with a linear ramp slope of -p, the frequency of the measurement signal resulting from heterodyne detection is Af0 - Afi + Af2, denoted f'. The two frequencies f+ and f' are determined by the signal processing unit 30 in the same way as before. Then the unit 30 determines the... contributions Afi and Af2 as follows: Afi=(f+-f_ ) / 2, and Af2=(f++f_ ) / 2 - Af0. It then calculates the anemometric velocity VA-a as before from the value of the contribution Af2, and provides an evaluation of the distance D by applying the following formula: D=-C-Afi / (2-p).

[0046] [Fig. 3] shows the heterodyne detection signal obtained for such an implementation of the invention with opposing linear ramp slopes of the optical frequency variation between two successive pulses. This diagram separates the spectrum of pulses with a positive slope, represented by a solid line, from that of pulses with a negative slope, represented by dashed lines. The horizontal axis also marks the frequency values ​​f expressed in megahertz, and the vertical axis marks the spectral intensity values ​​expressed in watts per hertz (WHz') and denoted P. This composite heterodyne detection spectrum was established for a constant frequency shift of 40 MHz, Af0, which is produced by the acousto-optic modulator 14.Both spectra exhibit several Narcissus signal components corresponding to partial reflections of the pulses off several components of the output optics 18, as well as on the quarter-wave plate 17. These Narcissus signal components are less than 15 MHz away from the 40 MHz value for f, corresponding to Af0. The measurement signals corresponding to the two opposing slopes are designated by their respective frequency values ​​f+ and f. The other signals, corresponding to spectral peaks with values ​​for the frequency f greater than 90 MHz, originate from reflections of the opposing-slope pulses off obstacles present in the background of the focusing zone ZF. The peaks which are located at the values ​​115 MHz and 205 MHz reveal an obstacle with high backscattering power which is located at the distance D'=-C-Afi / (2-p) with Afi = (205 MHz - 115 MHz) / 2 = 45 MHz, i.e. D'=13.5 m.For these same spectra, according to the 15 MHz and 65 MHz values ​​read for f+ and f, respectively, the distance D from the focal zone ZF is approximately 7.5 m and the wind speed in this focal zone ZF is less than 1 ms (meter per second).

[0047] The examples just provided demonstrate the advantage of the frequency modulation introduced by the invention for spectrally separating the measurement signal from the Narcissus signal or from all the components of the Narcissus signal, as appropriate. Indeed, thanks to frequency modulation, the portion of the emitted pulses that is backscattered by particles suspended in the air can be distinguished from the Narcissus signal even for very low wind speeds. This distinction can also be made for small values ​​of the separation distance D.

[0048] A LIDAR system that conforms to the invention and is adapted to perform anemometric velocity measurements can be used in numerous applications, including, but not limited to: - applications on board an aircraft, for which reduced size and weight of the LIDAR system are significant advantages. [Fig. 4] shows a helicopter 100 equipped with such a LIDAR system to perform airspeed measurements according to the invention. The system is preferably installed on board the helicopter so that the output optic 18 is located towards the nose of the helicopter 100, and pointed towards the half-space that is forward of the helicopter. [Fig. 4] shows the arrangement of the central propagation direction AA and the resulting focusing zone ZF; - applications where the airspeed to be measured may be low or very low, such as ground-level or low-altitude measurements, for example, to optimize wind turbine operation, or measurements from aircraft that may be hovering. In this case, the acousto-optic modulator 14 advantageously generates the constant frequency shift Af0, which is applied to the emitted pulses without being applied to the reference radiation Fref. In this way, a low airspeed value corresponds to a heterodyne beat frequency close to the non-zero value of Af0, thus improving measurement accuracy without requiring a large number of pulses per measurement sequence, i.e., without the measurement time being excessively long; and - Applications where the measurement distance between the output optic 18 and the focusing zone ZF is variable. For this purpose, the output optic 18 can be adapted to vary the convergence of the beam F of successive pulses as it exits through this optic. For example, when this beam originates from the end of an optical fiber, the output optic 18 can be a converging lens mounted on a support that is movable in translation parallel to the direction AA, so as to move the object focal point of the lens relative to the end of the optical fiber. Thus, the center O of the focusing zone ZF can be located at a controllable distance from the output optic 18, for example, between 1 m and 1000 m.

[0049] The embodiment of the invention illustrated in [Fig. 5] is adapted for measuring the velocity of a retroreflective target T. If the target T has sufficient retroreflectivity, the beam F of the emitted pulses no longer needs to be convergent. This beam can then be collimated, allowing the measurement of velocities of targets located at very wide distances D.

[0050] The embodiment of [Fig. 5] uses direct detection of the portion of the pulses that is back-reflected by the target T, instead of the heterodyne detection of [Fig. 1]. For this, the optical frequency modulation that was implemented previously can be replaced by amplitude modulation, which is carried out with a variable modulation frequency. Such amplitude modulation with variable modulation frequency can be produced by the acousto-optic modulator 14, when the modulation controller 40 is configured appropriately. For example, the acousto-optic modulator 14 produces sinusoidal amplitude modulation within each pulse, and the frequency of this sinusoidal amplitude modulation varies between the beginning and end of the pulse. Preferably, it varies linearly with time, that is, with a constant slope. As an example, the modulation frequency of the radiation amplitude in each pulse can vary between an initial value of 10 MHz at the beginning of the pulse and a final value of 100 MHz at the end of the pulse.

[0051] The resulting instantaneous power variations for the portion of the pulses back-reflected by the target T are sufficiently slow to be detected in real time by the photodetector 22. The detection signal delivered by the latter can then be analyzed by the signal processing unit 30 in the same way as before. For this reason, the description of this analysis is not repeated. It is only noted that the refinement of successively emitted pulses with opposite slopes of linear variation of the modulation frequency is still applicable, by applying it to the variable amplitude modulation frequency.

[0052] It is understood that the invention can be reproduced by modifying secondary aspects of the embodiments described in detail above, while retaining at least some of the advantages mentioned. In particular, the components mentioned can be replaced by other components or combinations of components that produce a function equivalent to that mentioned. Finally, the numerical values ​​cited are for illustrative purposes only and can be changed depending on the application considered.

Claims

Claims

1. LIDAR system adapted to measure a speed of at least one target, and comprising: - an emission path (10), adapted to emit radiation pulses towards the target through an output optic (18) of the LIDAR system; - a detection path (20), comprising a photodetector (22), and adapted to collect, also through the output optics (18), and direct onto the photodetector, a part of the pulses which has been retroreflected or retroscattered by the target, so as to produce a detection signal; and - a signal processing unit (30), configured to deliver a result of the speed measurement which is relative to the target, from the detection signal, the LIDAR system being characterized in that: - the emission path (10) is further adapted to vary a modulation frequency of the radiation within each pulse, said modulation frequency which is varied within each pulse being an optical frequency of the radiation of said pulse or a sinusoid frequency of amplitude modulation of the radiation of said pulse, so that a component of the detection signal which is useful for measuring the speed and called useful component, is spectrally shifted as a function of a distance of the target from the LIDAR system; and - the signal processing unit (30) is further configured to extract the useful component from a spectrum of the detection signal, so as to isolate said useful component from at least one other component of the detection signal which results from a partial back-reflection or back-scattering of the emitted pulses occurring in the output optics (18) of the LIDAR system, and to obtain the result of the speed measurement from the extracted useful component.

2. LIDAR system according to claim 1, adapted to measure an anemometric speed, the target being constituted by particles which are located in a portion of atmosphere, the pulses being emitted by the emission path (10) in the direction of the portion of atmosphere, and the detection path (20) being adapted to collect and detect the part of the pulses which has been backscattered by the particles located in said portion of atmosphere.

3. A LIDAR system according to claim 1 or 2, adapted so that the modulation frequency which is varied within each pulse is an optical frequency of the radiation of said pulse.

4. A LIDAR system according to any preceding claim, wherein the transmission path (10) is adapted to vary the modulation frequency within each pulse according to a constant slope frequency modulation ramp.

5. LIDAR system according to claim 4, wherein the emission path (10) is adapted to emit the radiation pulses all with the same frequency modulation ramp, and wherein the signal processing unit (30) is configured to obtain the result of the velocity measurement from a second contribution to a frequency shift which is relative to the extracted useful component, said second contribution being calculated by subtracting from said frequency shift a first contribution which is produced by the frequency modulation ramp in combination with the distance away from the target.

6. LIDAR system according to claim 4, in which the transmission channel (10) is adapted to change a sign of the slope of the modulation frequency variation ramp between two pulses which are transmitted successively.

7. LIDAR system according to claim 6, in which the transmission channel (10) is adapted so that the slope of the modulation frequency variation ramp is equal in absolute value but opposite in sign between two pulses which are transmitted successively.

8. LIDAR system according to claim 7, wherein the signal processing unit (30) is configured to obtain the result of the speed measurement from a sum of two respective frequency shifts relating to a first extracted useful component which corresponds to those of the emitted pulses for which the slope of the modulation frequency variation ramp is positive, and to a second extracted useful component which corresponds to those of the emitted pulses for which the slope of the modulation frequency variation ramp is negative.

9. The LIDAR system of claim 8, wherein the signal processing unit (30) is further configured to obtain an estimate of the distance away from the target to the LIDAR system from a difference between the fre- respective quantities relating to the first extracted useful component which corresponds to those of the emitted pulses for which the slope of the modulation frequency variation ramp is positive, and to the second extracted useful component which corresponds to those of the emitted pulses for which the slope of the modulation frequency variation ramp is negative.

10. A LIDAR system according to any preceding claim, wherein the detection path (20) is arranged to perform heterodyne detection of the retroreflected or backscattered portion of the pulses.

11. A method for measuring a velocity of at least one target using a LIDAR system, comprising the following steps: / 1 / emitting radiation pulses toward the target through an output optic (18); / 2 / using the output optic (18), collecting a portion of the pulses that has been retroreflected or backscattered by the target; / 3 / detecting the retroreflected or backscattered portion of the pulses, using a photodetector (22) that produces a detection signal;and / 4 / from the detection signal, delivering a result of the speed measurement which is relative to the target, the method being characterized in that: - a modulation frequency of the radiation is varied within each pulse during step / 1 / , said modulation frequency which is varied within each pulse being an optical frequency of the radiation of said pulse or a sinusoid frequency of amplitude modulation of the radiation of said pulse, so that a component of the detection signal which is used in step / 4 / for the speed measurement and called useful component, is spectrally shifted as a function of a distance of the target from the LIDAR system;and - the useful component is extracted from a spectrum of the detection signal during step / 4 / , so as to isolate said useful component with respect to at least one other component of the detection signal which results from a partial retro-reflection or retro-scattering of the emitted pulses occurring in the output optics (18), then the result of the speed measurement is obtained from the extracted useful component.;

12. A method according to claim 11, used for measuring an anemometric speed, wherein the target consists of particles which are located in a portion of atmosphere, the pulses are emitted towards the portion of atmosphere, and the part of the pulses which has been backscattered by the particles located in said portion of atmosphere is collected and detected.

13. The method of claim 11 or 12, wherein the LIDAR system is carried on board an aircraft (100), and the method is performed during a flight of the aircraft.

Images