Lidar Systems for Measuring Fluid Velocity

Abstract

The present invention relates to a pulsed lidar system capable of varying the modulation frequency of radiation within each pulse so that the component of the detection signal useful for measuring velocity is spectrally shifted according to the distance from the target. This type of lidar system is capable of spectrally separating the useful component of the detection signal from a narcissus signal generated by an output lens (18) shared by the emission channel (10) and the detection channel (20) of the lidar system. A lidar system according to the present invention can be advantageously used to measure indicated airspeed.

Description

[Technical Field]

[0001] The present specification relates to a lidar system for performing velocity measurements and a corresponding measurement method. [Background technology]

[0002] Lidar systems, an English abbreviation for "light detection and ranging," are useful for measuring the distance and velocity of a target. They use a laser beam aimed at the target and analyze the portion of that light that is retroreflected or backscattered by the target. The distance measurement results from a direct or indirect estimation of the travel time of the radiation over the round-trip distance from the lidar system to the target, and the target's velocity is inferred from measuring the Doppler frequency shift that affects the portion of the radiation retroreflected or backscattered by the target.

[0003] In certain situations and applications, it is particularly advantageous for the radiation beam emitted toward the target to consist of a series of continuous pulses. Indeed, each pulse can have a much higher instantaneous power value than continuous radiation, which allows for a corresponding increase in the power of the portion of radiation collected after backscattering by the target. This first advantage is particularly important for lidar systems designed to measure airspeed, since the power of the portion of radiation collected is very low. A second advantage of using pulsed lidar systems is that the operator's eyes are not endangered by the power of the continuous laser source incorporated in this type of system.

[0004] This type of pulsed lidar system can be used to measure the distance and velocity of a solid target object.

[0005] However, some pulse lidar systems are specifically dedicated to airspeed measurement. In this case, the target consists of particles located in a portion of the atmosphere, toward which the lidar system's emission channel emits a pulse, and the lidar system's detection channel is adapted to collect the portion of the pulse backscattered by the particles located in that portion of the atmosphere. The particularity of these lidar systems dedicated to airspeed measurement is that they are adapted to detect the backscattered portion of the emitted pulse at very low power values. As described in Patent Document 1, according to a possible design of a lidar system for airspeed measurement, the portion of the atmosphere relevant to the measurement is determined by the convergence of the radiation beam of the emitted pulse. This portion of the atmosphere is superimposed on a zone of space known as the Rayleigh zone, where the radiation emitted by the lidar system is most concentrated.

[0006] Apart from the simplest lidar system implementations, in which the same output optic is used for both the emission and detection channels, this output lens generates partial reflection of the emitted radiation off some of the optical components. This internal reflection generates one or more components in the detection signal that are not related to the target. These components, which arise from the optical interface between the lidar system and the free-propagation space of the radiation, are commonly referred to as narcissus signals and can have high or even very high intensities. Such narcissus signals can then overlap with useful components of the detection signal generated by targets located close to the lidar system, preventing their effective detection. In particular, in anemometer applications, the intensity of the narcissus signals is much higher than the useful components of the detection signal. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2021 / 053290A1 Brochure Summary of the Invention [Problem to be solved by the invention]

[0008] In this context, the object of the present invention is to provide a lidar system that is able to distinguish the useful component of the detected signal relating to a target located at close range from the narcissus signal and that can be used to provide an estimate of the range and / or velocity of this target.

[0009] A secondary object of the present invention is to provide a lidar system that is effective over short measurement distances and allows for airspeed measurements. [Means for solving the problem]

[0010] To achieve at least one of these or other objects, a first aspect of the present invention proposes a lidar system suitable for measuring the velocity of at least one target, comprising: a launch channel adapted to launch pulses of radiation toward the target through an output lens of the lidar system; a detection channel including a photodetector and adapted to collect, through the output lens, and direct onto the photodetector, a portion of the pulse that is retroreflected or backscattered by the target; A signal processing unit configured to output a target relative velocity measurement from said detection signal.

[0011] This type of lidar system of the present invention also has the following features: the emission channel is further adapted to vary the modulation frequency of the radiation within each pulse such that a component of the detected signal useful for measuring velocity, referred to as a useful component, is spectrally shifted as a function of distance from the lidar system to the target; The signal processing unit is further configured to extract the useful component from the spectrum of the detection signal such that the useful component is separated from at least one other component of the detection signal resulting from retroreflection or partial backscattering of the emitted pulse within the output lens of the lidar system, and to obtain the velocity measurement result from the extracted useful component.

[0012] In the lidar system of the present 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 allows for separation of the useful component from the narcissus signal, for example, by selective digital filtering of the useful component of the detection signal relative to the narcissus signal. This allows the lidar system of the present invention to provide velocity measurements, and potentially remote measurements, even when the target is located at a short distance in front of the output lens. Such remote measurements can be performed by determining the frequency shift that affects the portion of the pulse that is retroreflected or backscattered upon reception.

[0013] In such a lidar system, the modulation frequency can be varied within each pulse directly to the optical frequency of the radiation in that pulse, i.e., the lidar system can implement optical frequency modulation.

[0014] Alternatively, the modulation applied to the pulses fired at the target can be amplitude modulation, specifically sinusoidal amplitude modulation, with a sinusoidal frequency that varies as a function of time within each pulse, the amplitude modulated sinusoidal frequency being the modulation frequency that is varied in accordance with the present invention.

[0015] The detection channel may be arranged to produce heterodyne detection of the retroreflected or backscattered portion of the pulse, in which case the emission channel may further be adapted to apply an additional frequency shift to the emitted pulses in addition to the variation in modulation frequency within each pulse, the heterodyne detection being advantageously performed by mixing the retroreflected or backscattered portion of the pulse with a reference signal that does not have this additional frequency shift.

[0016] However, a lidar system according to the invention can alternatively implement a non-heterodyne detection mode. For example, the detection channel can be adapted to perform direct detection of the portion of the pulse retroreflected or backscattered by the target. In particular, such direct detection can be used when the modulation applied within each pulse is of the amplitude modulation type.

[0017] In particular, a lidar system according to the present invention can be adapted to measure airspeed. For this application, as previously described, the target consists of particles located in a portion of the atmosphere, the emission channel emits pulses toward said portion of the atmosphere, and the detection channel is adapted to collect and detect portions of said pulses backscattered by particles located in said portion of the atmosphere. In particular, for this airspeed measurement application, the output lens can also be adapted to emit pulses external to the lidar system in the form of a converging radiation beam, the converging of which beam determines the portion of the atmosphere relevant to the airspeed measurement. The measurement distance, i.e., the distance from the portion of the atmosphere relevant to the airspeed measurement, can then be determined from the convergence applied to the emitted pulse beam.

[0018] In a preferred embodiment of the present invention, the emission channel can be adapted to vary the modulation frequency within each pulse according to a frequency modulation ramp with a constant slope. Additionally, the emission channel can be further adapted to change the sign of the slope of the modulation frequency variation ramp between two consecutively emitted pulses. In particular, the slope of the modulation frequency variation ramp can be equal in absolute value but opposite in sign between two consecutively emitted pulses. In this case, the signal processing unit can be configured to obtain the velocity measurement result from the sum of two respective frequency shifts associated with a first extracted useful component corresponding to an emitted pulse with a positive slope of the modulation frequency variation ramp and a second extracted useful component corresponding to an emitted pulse with a negative slope of the modulation frequency variation ramp. At the same time, the signal processing unit can be further configured to obtain the estimate of the distance from the lidar system to the target from the difference between the respective frequency shifts associated with the first extracted useful component corresponding to an emitted pulse with a positive slope of the modulation frequency variation ramp and the second extracted useful component corresponding to an emitted pulse with a negative slope of the modulation frequency variation ramp. This kind of assessment of the distance to the target is independent of the determination obtained from the convergence of the pulsed beam, in this case of the same distance.

[0019] However, when the distance from the lidar system to the target is somehow known, it is possible to obtain velocity measurements by varying the modulation frequency, which is the same for all pulses. This is especially true for airspeed measurements, when the distance to the part of the atmosphere relevant to the measurement is determined by the convergence of the radiation beam of the emitted pulse.

[0020] In such an embodiment of the invention, where the range to the target is somehow known, the emission channels can be adapted to emit radiation pulses all with the same frequency-modulated ramp, and the signal processing unit can then be configured to derive the velocity measurement result from a second contribution to the extracted useful component and a frequency offset relative to the extracted useful component, the second contribution being calculated by subtracting from the frequency offset the first contribution generated by the frequency-modulated ramp in combination with the range from the target.

[0021] A second aspect of the present invention proposes a method for measuring the velocity of at least one target using a lidar system, said method comprising the following steps: (1) projecting a pulse of radiation through an output lens toward the target; (2) using the output lens to collect a portion of the pulses retroreflected or backscattered by the target; (3) detecting the retroreflected or backscattered portion of the pulse using a photodetector to generate a detection signal; (4) outputting a velocity measurement for the target from the detection signal;

[0022] According to the invention, the method has the following additional features: In step (4), the modulation frequency of the radiation is varied within each pulse during step (1) so that a component of the detection signal used for velocity measurement, referred to as the useful component, is spectrally shifted as a function of the distance from the lidar system to the target; In step (4), the useful component is extracted from the spectrum of the detection signal such that the useful component is separated from at least one other component of the detection signal resulting from retroreflection or partial backscattering of the emitted pulse occurring within the output lens, and then the velocity measurement result is obtained from the extracted useful component.

[0023] This type of method can be used particularly, but not exclusively, to measure airspeed, and the lidar system can be mounted on an aircraft and the method can be performed while the aircraft is in flight.

[0024] Generally speaking, the method according to the second aspect of the invention can be implemented using a lidar system according to the first aspect of the invention.

[0025] Different implementations of the method of the present invention may employ at least one of the following additional features. The modulation frequency that varies within each pulse can be directly the optical frequency of the radiation of that pulse, or it can be a sinusoidal frequency amplitude modulation of the radiation of each pulse. Detection of the retroreflected or backscattered pulse portion can be heterodyne or direct detection. The modulation frequency within each pulse can be varied according to a frequency modulation ramp with a constant slope. Possibly, the sign of the slope of this modulation frequency variation ramp can be changed between two successively emitted pulses. In particular, the slope of the modulation frequency variation ramp can be equal in magnitude but opposite in sign between two successively emitted pulses. For airspeed measurement applications, the output lens can be adapted to transmit pulses in the form of a converging radiation beam outside the lidar system, the convergence of which determines the portion of the atmosphere relevant to the airspeed measurement. In this case, the same frequency-modulated lamp can be used to emit all radiation pulses, and the velocity measurement result can be obtained from the extracted useful component and a second contribution to the relative frequency shift, which is calculated by subtracting from the frequency shift the first contribution produced by the frequency-modulated lamp in combination with the distance from the portion of the atmosphere.

[0026] In particular, when two successively emitted pulses have frequency modulation ramps with a constant slope and opposite signs, it is possible to obtain a target velocity measurement result from the sum of two respective frequency shifts associated with a first extracted useful component corresponding to the emitted pulse with a positive slope of the frequency modulation ramp of the radiation and a second extracted useful component corresponding to the emitted pulse with a negative slope of the frequency modulation ramp of the radiation, and simultaneously, from the difference between the respective frequency shifts associated with the first and second extracted useful components, it is possible to obtain an estimate of the target's distance from the lidar system. [Brief explanation of the drawings]

[0027] The features and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments, which proceeds with reference to the accompanying drawings.

[0028] [Figure 1] FIG. 1 is a block diagram of a lidar system according to the present invention for airspeed measurement applications using optical frequency modulation and heterodyne detection. [Figure 2a] Figure 1 shows two spectrograms for two measured distance values ​​acquired with the lidar system. [Figure 2b] Figure 1 shows two spectrograms for two measured distance values ​​acquired with the lidar system. [Figure 3] Figure 1 shows two spectra of the heterodyne detection signal acquired using the lidar system. [Figure 4] Figure 1 is an illustration of a possible application of the lidar system. [Figure 5] FIG. 2 is a block diagram corresponding to FIG. 1 for a target velocity measurement application using amplitude modulation and direct detection. DETAILED DESCRIPTION OF THE INVENTION

[0029] For clarity, the dimensions of the elements shown in these figures are not to scale or to scale. In addition, some of these elements are shown symbolically only, and the same references shown in different figures indicate the same element or elements with the same function.

[0030] First, an airspeed measurement application using optical frequency modulation of the present invention will be described, followed by an application using amplitude modulation relating to measuring the velocity of solid objects constituting targets.

[0031] According to FIG. 1, a lidar system for airspeed measurement comprises a launch channel 10, a detection channel 20, and a signal processing unit 30.

[0032] The emission channel 10 comprises a laser source 11, designated "SOURCE", an optical modulator 14, designated "MAO", an optical amplifier 15, designated "AO", and an output lens 18. The laser source 11 may be of the continuous emission type, for example, generating radiation of wavelength λ equal to approximately 1.55 μm (micrometers). The modulator 14 is of the acousto-optical type and is controllable so as to chop the radiation from the laser source 11 into a series of pulses. Typically, but not necessarily, the modulator 14 can apply a constant frequency offset to the optical frequency of the radiation, for example equal to approximately 40 MHz (megahertz). The optical amplifier 15 increases the power of the radiation pulses thus formed, which are then emitted outward by the output lens 18 in the form of a beam F with a central propagation direction AA. As is known, chopping the radiation into a series of pulses makes it possible to obtain high values ​​of instantaneous radiation power within each pulse, which is particularly suitable for measuring airspeed. This type of pulse firing operation produces 5-10 watts (W) from 100 W within each pulse with a substantially rectangular profile. 5The output lens 18 can be used to achieve instantaneous radiation power values ​​of up to 100 W (e.g., 500 W). Such values ​​are not possible with continuous laser radiation using optical amplification components currently available or compatible with wind measurement applications. The output lens 18 can be composed of one or more converging lenses and can determine the size of the output pupil of the launch channel 10. For example, this output pupil can have a radius of approximately 0.07 m (meters). The optical components of the launch channel 10, except for the output lens 18, can be advantageously fabricated using fiber optic technology, reducing the size of the lidar system and facilitating optical alignment of these components.

[0033] The output lens 18 is designed to generate a beam F of radiation pulses in a converging beam configuration in a zone of space located downstream of this output lens with respect to the propagation direction of the radiation leaving the emission channel 10. The beam F therefore has a cross section perpendicular to its central propagation direction AA, which decreases between the output lens 18 and the focusing zone ZF, and then increases beyond this focusing zone ZF in the form of a diverging beam. In known methods, particularly those using the Gaussian beam model, the focusing zone ZF has an axis superimposed on the central direction of propagation AA, a radius w0=λ / (πθ), commonly called the "waist", and a length 2-I R It can be likened to a cylinder with R is the Rayleigh length, and λ / (πθ 2 ), and θ is the divergence half angle of the beam F beyond the focusing zone ZF, expressed in radians. Typically, the distance denoted D between the output lens 18 and the focusing zone ZF can be selected by adjusting the focal length or longitudinal position of the output lens 18 and can vary between a few meters and a few hundred meters, in which case the Rayleigh length I Rcan vary relative to the beam F from a few tens of centimeters to a few meters, and the radius w0 is on the order of a few centimeters. Generally speaking, the divergence half angle θ of the beam F can be evaluated downstream of the focusing zone ZF in the direction of propagation of the radiation and can be at a distance of 1 km (kilometer) from the focusing zone ZF.

[0034] To perform heterodyne detection, detection channel 20 includes a photodetector 22, designated "PD," and optical couplers 13, 16, and 21, designated "CO," which couple the backscattered portion of beam F with a portion of the radiation from laser source 11, F. REF The radiation can be arranged to combine with the part F REF serves as the reference radiation for the heterodyne detection. In a known manner, the backscattered portion of the beam F thus detected essentially comes from the focusing zone ZF and is generated by backscattering particles located in this zone. The optical coupler 16 can advantageously be of the polarization-splitting type, in which case the radiation coming from the optical amplifier 15 is linearly polarized and a quarter-wave plate 17, denoted λ / 4, is inserted between the optical coupler 16 and the output lens 18. The optical coupler 16 and the quarter-wave plate 17 thus form an optical circulator that combines the launch channel 10 and the detection channel 20 to a shared output lens 18.

[0035] Finally, the signal processing unit 30 can be a PC-based computer module hosting a suitable program for processing the detection signal coming from the photodetector 22. The signal processing unit 30 outputs an estimate of the velocity component of the backscattered particles in the focusing zone ZF, which is parallel to the central direction of propagation AA and is expressed as V A-A is shown as:

[0036] The operation of this type of lidar system for measuring airspeed is well known to those skilled in the art and need not be repeated here. The system is oriented so that the focal zone ZF is in the part of the atmosphere where wind speed is to be measured, and the divergence semi-angle O is adjusted. The particles that backscatter the beam F are dust, microcrystals, or aerosol droplets suspended in the atmosphere inside the focusing zone ZF. The structure of the output lens 18, and its control when allowing variable adjustment of the divergence semi-angle O, can provide an estimate of the distance from the lidar system to the center of the focal zone ZF, which corresponds to the distance D introduced earlier.

[0037] In the present case, the emission channel 10 further comprises additional modulation means 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, designated "MEO", and a suitable control unit called a modulation controller, designated "CTRL", with reference numeral 40. These may be inserted between the laser source 11 and the acousto-optic modulator 14. In various embodiments of the present invention, the reference radiation F REF These frequency modulation means can be arranged upstream or downstream of the optical coupler 13 that samples the . In the following, it is assumed that they are arranged upstream of the optical coupler 13 with respect to the direction of propagation of the radiation in the launch channel 10, but a person skilled in the art will understand how to adapt the following description if the additional modulation means are arranged downstream of the optical coupler 13.

[0038] For example, modulation controller 40 is configured so that acousto-optic modulator 14 chops the radiation initially generated by laser source 11 into successive pulses each having a duration equal to 1 μs (microsecond), and so that electro-optic modulator 12 linearly increases 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, with a slope p of the change in optical frequency equal to +0.500 MHz / ns (megahertz / nanosecond). The portion of the pulse generated by launch channel 10 that is backscattered within the focal zone ZF is then converted into reference radiation F REF , which corresponds to the round trip propagation time between the launch lens 18 and the focusing zone ZF. As a result of the frequency modulation introduced in accordance with the invention, the reference radiation F REF , the lidar system will have a first frequency shift contribution equal to -p-2-D / C, where C is the propagation velocity of the radiation outside the lidar system between the output lens 18 and the focusing zone ZF. In the following, this first frequency shift contribution will be called Δf1. In known manner, the velocity V of the backscattered particles contained in the focusing zone ZF A-A The component is parallel to the central direction of propagation AA and the airspeed V A-A When oriented as shown in Figure 1, this component is denoted as Δf2 and is 2-V A-A 1, the two contributions Δf1 and Δf2 sum to produce the frequency of the heterodyne detection signal output by photodetector 22. These are also in addition to any steady-state frequency shift that may be produced by acousto-optic modulator 14.

[0039] Figures 2a and 2b show the frequency of the spectral components of the heterodyne-detected signal as a function of time within each pulse. For each spectrogram, the horizontal axis is time, denoted t and expressed in microseconds (μs), the vertical axis is frequency, denoted f and expressed in megahertz (MHz), and the tone scale on the right side of each spectrogram is the instantaneous spectral power, Pi , expressed in decibels (dB) relative to a noise-related fundamental level. In the case of the spectrogram of FIG. 2a, the output lens 18 is adjusted so that its distance D from the focal zone ZF is 15 m (meters). The length of this focal zone ZF is 2-I R is equal to 90 cm (centimeters). In the case of the spectrogram in Figure 2b, D is equal to 30 m and 2-I R is equal to 3.5 m. The values ​​of the other lidar system parameters have already been mentioned and are identical for both spectrograms, except that the value of the steady-state frequency shift generated by the acousto-optic modulator 14 is zero. The two components of the heterodyne detection signal appearing in each spectrogram correspond respectively to the partial retroreflection of the pulse on the output lens 18, called the Narcissus signal, designated "Narcisse" in these spectrograms, and to the part of the pulse backscattered within the focal zone ZF, called the measurement signal, designated "Mes". The Narcissus signal is the reflection of the reference radiation F REF 2a and 2b, respectively. The first spectrogram (t=0) corresponds to D=0 and f=0, since there is no significant propagation delay in reaching the photodetector 22 compared to the Narcissus signal, and there is no Doppler effect. The measurement signal arrives at the photodetector 22 with a delay Δt equal to 2-D / C compared to the Narcissus signal, and has a frequency f offset from the first contribution Δf1 when there is no wind in the focal zone ZF. For the operating conditions employed, the delay Δt is equal to 0.10 μs in the case of FIG. 2a and 0.20 μs in the case of FIG. 2b, and Δf1 is equal to 50 MHz in the case of FIG. 2a and 100 MHz in the case of FIG. 2b. The first instant (t=0) of these two spectrograms corresponds to the start of detection of the backscattered portion of the radiation pulse. Additionally, due to the much longer focal zone ZF in FIG. 2b, the spectral width of the measurement signal is wider in FIG. 2b than in FIG. 2a.

[0040] The signal processing unit 30 calculates the Fourier transform with respect to time of the heterodyne detection signal output by the photodetector 22. Then, from the spectrum of the heterodyne detection signal, for example by applying adaptive digital filtering, a measurement signal is extracted and the frequency f of this measurement signal is determined. Generally speaking, this frequency f is equal to Δf0 + Δf1 + Δf2, where Δf0 is the constant frequency offset (if any) within all pulses generated by the acousto-optic modulator 14. When the distance D is somehow known, in particular by adjusting the focus of the output lens 18, the frequency shift contribution Δf1 is calculated by Δf1 = -p-2 - D / C, and the two contributions Δf0 and Δf1 are subtracted from the determined value for the frequency f of the measurement signal. The result of this subtraction is the frequency shift contribution Δf2, which corresponds to the Doppler effect, and is calculated by the signal processing unit 30 using the formula V: A-A = λΔf2 / 2, the wind speed component parallel to the central propagation direction AA within the focusing zone ZF is calculated.

[0041] The measurement signal "Mes", which was just described as the spectral component extracted from the heterodyne detection signal, is referred to in the general part of this description as the useful component of the detection signal.

[0042] However, in the case of airspeed measurement, the distance D from the focusing zone ZF may depend on factors external to the lidar system, such as turbulence and / or thermal fluctuations in the atmosphere present in the pulse path between the output lens 18 and the focusing zone ZF, and / or variations in the concentration of backscattered particles along the central propagation direction AA. The improvements described here make it possible to infer the actual value of the distance D from the heterodyne detection signal. To achieve this, the modulation controller 40 can be configured so that some pulses are fired with a particular, e.g., positive linear optical frequency variation ramp slope, and other pulses with a negative linear optical frequency variation ramp slope. Preferably, every other pulse is fired with a positive linear ramp slope p, and the other pulses are fired using a linear ramp slope −p. In that case, for the first pulse, the frequency of the resulting measurement signal from the heterodyne detection is Δf0+Δf1+Δf2, as already mentioned, and f + For a pulse with a linear ramp slope of -p, the frequency of the measurement signal obtained from heterodyne detection is Δf0-Δf1+Δf2, where f - The two frequencies f + and f - is determined by the signal processing unit 30 as above. Unit 30 then determines the contributions Δf1 and Δf2 as follows: Δf1=(f + -f - ) / 2, and Δf2=(f + +f - ) / 2-Δf0. Then, as already mentioned, the airspeed V is calculated from the value of the contribution Δf2. A-A Calculate and apply the following formula to provide an estimate of the distance D: D = -C·Δf1 / (2-p).

[0043] FIG. 3 shows the heterodyne detection signal obtained from such an implementation of the invention, where the linear optical frequency change ramp slope is reversed between two successive pulses. The figure separates the spectrum of a pulse with a positive slope value, shown as a solid line, from the spectrum of a pulse with a negative slope value, shown as a dashed line. The horizontal axis again shows the frequency value f in megahertz, and the vertical axis shows the power in watts per hertz (W / Hz), shown as P. -1 ) for spectral intensity values ​​expressed in Hz. This complex heterodyne detection spectrum was set for a steady-state frequency offset Δf0 generated by the acousto-optic modulator 14 equal to 40 MHz. These two spectra each show several narcissus signal components corresponding to some components of the output lens 18 and to the partial reflection of the pulse at the quarter-wave plate 17. These narcissus signal components are at values ​​of f between 40 MHz and less than 15 MHz, corresponding to Δf0. The measurement signals corresponding to the two opposite slopes are then scaled at their respective frequency values ​​f + and f - The other signals corresponding to the spectral peaks with values ​​of frequency f above 90 MHz result from the reflection of the oppositely inclined pulses at obstacles in the background of the focusing zone ZF. The peaks at 115 MHz and 205 MHz reveal that an obstacle with high backscattering power is located at a distance D' = -C - Δf1 / (2-p), i.e., D' = 13.5 m, with Δf1 = (205 MHz - 115 MHz) / 2 = 45 MHz. For these same spectra, f + and f - According to the values ​​read for 15MHz and 65MHz, respectively, the distance D from the convergence zone ZF is equal to about 7.5m, and the wind speed in this convergence zone ZF is 1m / s -1 (meters per second).

[0044] The above example demonstrates the value of the frequency modulation introduced by the present invention, which spectrally separates the measurement signal from the Narcissus signal, or in some cases from all components of the Narcissus signal. Frequency modulation makes it possible to distinguish the portion of the emitted pulse that is backscattered by airborne particles from the Narcissus signal, even at very low wind speeds. This distinction is also possible for small values ​​of the distance D.

[0045] Lidar systems suitable for performing airspeed measurements in accordance with the present invention can be used in many applications, including but not limited to: Airborne applications where reducing the size and weight of a lidar system is a major advantage. Figure 4 shows a helicopter 100 equipped with a lidar system for performing airspeed measurements in accordance with the present invention. The system is preferably mounted on the helicopter so that the output lens 18 is positioned toward the nose of the helicopter 100 and pointed into the half-space ahead of the helicopter. Figure 4 shows the orientation of the central direction of propagation AA and the resulting focal zone ZF. Applications where the measured airspeed may be low or very low, such as measurements at ground level or low altitude to optimize the operation of wind turbines, or measurements from aircraft that may be hovering. In this case, advantageously, an acousto-optic modulator 14 is applied to the emitted pulses but not to the reference radiation F REF This generates a constant frequency shift Δf0 that is not applied to Δf0. In this way, low airspeed values ​​correspond to heterodyne beat frequencies close to non-zero values ​​of Δf0, improving measurement accuracy without requiring the implementation of a large number of pulses per measurement sequence, i.e., without increasing measurement time too much. Applications in which the measuring distance between the output lens 18 and the focal zone ZF is variable. For this purpose, the output lens 18 can be adapted so that the convergence of the successive pulses of the beam F as it exits through it is variable. For example, if this beam originates from the end of an optical fiber, the output lens 18 can be a converging lens mounted on a support that is movable parallel to the direction AA so that the objective focus of the lens can be moved relative to the end of the optical fiber. In this way, the center O of the focusing zone ZF can be positioned at a controllable distance from the output lens 18, for example between 1 meter and 1000 meters.

[0046] The embodiment of the invention shown in Figure 5 is suitable for measuring the velocity of a retroreflective target T. If the target T is sufficiently retroreflective, it is no longer necessary to focus the emitted beam of pulses F. This beam can then be collimated, making it possible to measure the velocity of targets located at distances D that vary over a very wide range.

[0047] The embodiment of FIG. 5 uses direct detection of the portion of the pulse retroreflected by the target T instead of the heterodyne detection of FIG. 1. For this purpose, the optical frequency modulation used previously can be replaced by amplitude modulation performed with a variable modulation frequency. This type of amplitude modulation with a varying modulation frequency can be generated by the acousto-optic modulator 14 when the modulation controller 40 is appropriately configured. For example, the acousto-optic modulator 14 generates a sinusoidal amplitude modulation within each pulse, the frequency of which varies between the beginning and end of the pulse. Preferably, it varies linearly with time, i.e., with a rate of change. By way of example, the modulation frequency of the radiation amplitude within 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.

[0048] The resulting change in the instantaneous power of the portion of the pulse retroreflected by the target T is sufficiently slow to be detected in real time by the photodetector 22. The detected signal delivered by the latter can be analyzed in the same way as described above by the signal processing unit 30. For this reason, the description of this analysis will not be repeated. It is merely shown that the improvement of continuously emitted pulses with opposite slopes in the linear change of modulation frequency can still be applied by applying it to a variable amplitude modulation frequency.

[0049] It is to be understood that the present invention may be reproduced while retaining at least some of the cited advantages by modifying the secondary aspects of the embodiments described in detail above. In particular, elements referred to herein may be replaced by other elements or combinations of elements which produce equivalent functions to those referred to. Finally, the numerical values ​​cited are for illustrative purposes only and may vary depending on the application in question. [Explanation of symbols]

[0050] 10 Launch Channels 11 Laser light source 12 Electro-optic modulator 13 Optical Coupler 14 Acousto-optic modulators, optical modulators, modulators 15 Optical Amplifier 16 Optical Coupler 17 Quarter-wave plate 18 output lens 20 detection channels 21 Optical Coupler 22 Photodetector 30 Signal Processing Unit 40 Modulation Controller 100 helicopters AA center direction F beam F REF Reference Radiation O Center of the focusing zone T retroreflective target, target ZF Focusing Zone Δf1 first frequency shift contribution Δf2 second frequency shift contribution

Claims

1. 1. A lidar system adapted to measure the velocity of at least one target, comprising: an emission channel (10) adapted to emit a radiation pulse toward the target through an output lens (18) of the lidar system; a detection channel (20) containing a photodetector (22) and adapted to collect a portion of the pulse retroreflected or backscattered by the target through the output lens (18) and direct it onto the photodetector; a signal processing unit (30) configured to output a target-related velocity measurement from said detection signal; It encompasses the emission channel (10) is further adapted to vary the modulation frequency of the radiation within each pulse such that a component of the detected signal useful for measuring velocity, called the useful component, is spectrally shifted as a function of the distance from the lidar system to the target; the signal processing unit (30) is further configured to extract the useful component from the spectrum of the detection signal such that the useful component is separated from at least one other component of the detection signal resulting from partial retroreflection or backscattering of the emitted pulse occurring within the output lens (18) of the lidar system, and to obtain the velocity measurement from the extracted useful component. Lidar systems.

2. The lidar system is adapted to measure airspeed, the target is a particle located in a portion of the atmosphere, the pulse is emitted by the emission channel (10) toward the portion of the atmosphere, and the detection channel (20) is adapted to collect and detect a portion of the pulse backscattered by the particle located in the portion of the atmosphere.

10. The lidar system of claim 1.

3. the modulation frequency varied within each pulse is adapted to the optical frequency of the radiation of the pulse; A lidar system according to claim 1 or claim 2.

4. The emission channel (10) is adapted to vary the modulation frequency within each pulse according to a frequency modulation ramp with a constant slope. A lidar system according to any one of claims 1 to 3.

5. the emission channels (10) are adapted to emit all of the radiation pulses with the same frequency-modulated ramp, and the signal processing unit (30) is configured to obtain the result of the velocity measurement from a second contribution to a frequency offset relative to the extracted useful component, the second contribution being calculated by subtracting from the frequency offset a first contribution generated by the frequency-modulated ramp in combination with the distance from the target; 5. The lidar system of claim 4.

6. The emission channel (10) is adapted to change the sign of the slope of the modulation frequency variation ramp between two successively emitted pulses.

5. The lidar system of claim 4.

7. The emission 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 successively emitted pulses.

7. The lidar system of claim 6.

8. the signal processing unit (30) is configured to obtain the velocity measurement from the sum of two respective frequency shifts related to a first extracted useful component corresponding to an emitted pulse with a positive slope of the modulation frequency variation ramp and a second extracted useful component corresponding to an emitted pulse with a negative slope of the modulation frequency variation ramp, 8. The lidar system of claim 7.

9. The signal processing unit (30) is further configured to derive the estimate of the distance from the lidar system to the target from the difference between the respective frequency shifts associated with the first extracted useful component corresponding to the emitted pulses where the slope of the modulation frequency variation ramp is positive and the second extracted useful component corresponding to the emitted pulses where the slope of the modulation frequency variation ramp is negative.

9. The lidar system of claim 8.

10. the detection channel (20) is arranged to perform heterodyne detection on the retroreflected or backscattered portion of the pulse; A lidar system according to any one of claims 1 to 9.

11. 1. A method for measuring the velocity of at least one target using a lidar system, comprising: (1) emitting a pulse of radiation in the direction of the target through an output lens (18); (2) using the output lens (18) to collect a portion of the pulses that are retroreflected or backscattered by the target; (3) detecting the retroreflected or backscattered portion of the pulse using a photodetector (22) that generates a detection signal; (4) outputting a velocity measurement result relating to the target from the detection signal; It encompasses In step (4), the modulation frequency of the radiation is varied within each pulse during step (1) so that a component of the detection signal used for velocity measurement, called the useful component, is spectrally shifted as a function of the distance from the lidar system to the target; In step (4), the useful component is extracted from the spectrum of the detection signal such that the useful component is separated from at least one other component of the detection signal resulting from partial retroreflection or backscattering of the emitted pulse occurring within the output lens (18), and then the result of the velocity measurement is obtained from the extracted useful component; A method characterized by:

12. the method is used to measure airspeed, the target comprises particles located in a portion of the atmosphere, the pulse is emitted in the direction of the portion of the atmosphere, and a portion of the pulse backscattered by the particles located in the portion of the atmosphere is collected and detected; The method of claim 11.

13. The lidar system is mounted on an aircraft (100), and the method is performed while the aircraft is in flight. The method according to claim 11 or claim 12.

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