Lidar system for measuring fluid speed

Abstract

The invention relates to a pulsed lidar system capable of varying the radiation modulation frequency within each pulse such that a component in a detection signal for measuring the velocity of a target is spectrally shifted depending on its distance from the target. Such a lidar system is able to spectrally separate useful components of a detection signal from a Nhakos signal caused by an output optic (18) shared by a transmission channel (10) and a detection channel (20) of the lidar system. The lidar system according to the present invention can advantageously be used to measure and indicate airspeed.

Description

Technical Field

[0001] The present invention relates to a laser radar (LIDAR) system for performing speed measurement and a corresponding measurement method. Background Art

[0002] LIDAR systems stand for "Light Detection and Ranging" and are used to measure the distance to a target as well as the target's velocity. They use a laser beam directed at the target and analyze the portion of this beam that is reflected or backscattered by the target. The distance measurement comes from a direct or indirect estimate of the propagation time of the distance traveled by the radiation from the LIDAR system to the target and back, while the target's velocity is derived from the measurement of the Doppler frequency shift that affects the portion of the radiation that is reflected or backscattered by the target.

[0003] In certain cases and for certain applications, it is particularly advantageous that the radiation beam emitted towards the target consists of a series of continuous pulses. In fact, each pulse can thus have a much higher instantaneous power value than a continuous radiation, so that the power of the portion of radiation collected after being backscattered by the target can be increased accordingly. 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 a pulsed LiDAR system is that the power of the continuous laser source integrated in such a system will not be harmful to the eyes of the operator.

[0004] This pulsed lidar system can be used to measure the distance and speed of solid objects forming a target.

[0005] But some pulsed lidar systems are dedicated to airspeed measurement. In this case, the target consists of particles located in a part of the atmosphere, the laser radar system's transmission channel transmits pulses toward this atmospheric part, and the laser radar system's detection channel is suitable for collecting the part of the pulse that is backscattered by particles located in the atmospheric part. The particularity of these laser radar systems dedicated to airspeed measurement is that they are suitable for detecting the backscattered part of the transmitted pulse with very low power values. According to a feasible design of a laser radar system for airspeed measurement, as described in WO 2021 / 053290 A1, the part of the atmosphere involved in the measurement is determined by the convergence of the radiation beam of the transmitted pulse. This part of the atmosphere is superimposed on a region of space (called the Rayleigh zone), where the radiation emitted by the laser radar system is most concentrated.

[0006] However, for the most easily implemented LiDAR systems, in which the transmission channel and the detection channel use the same output optics, this output optics produces partial reflections of the transmitted radiation on some of its optical components. This internal reflection produces one or more target-independent components in the detection signal. These components originate from the optical interface between the LiDAR system and the space in which the radiation propagates freely, and are usually called Narcisse signals (Narcissus signals), which can have high or even extremely high intensity. This Narcisse signal can then be superimposed on the useful component of the detection signal generated by the target at a shorter distance from the LiDAR system, causing this useful component to not be effectively detected. In particular, in the case of wind speed measurement applications, the Narcisse signal is much stronger than the useful component of the detection signal.

[0007] Technical issues

[0008] Based on this situation, an object of the present invention is to provide a LiDAR system which can distinguish useful components of the detection signal relating to a target located at a close distance from the Narcissus signal and which can be used to provide an estimate of the range and / or speed of this target.

[0009] A secondary object of the present invention is to provide a laser radar system which is effective over short measurement distances and which is capable of airspeed measurements. Summary of the invention

[0010] In order to achieve at least one of these or other purposes, a first aspect of the present invention provides a laser radar system suitable for measuring the speed of at least one target, and comprising:

[0011] - a transmit channel adapted to transmit radiation pulses toward a target through the output optics of the lidar system;

[0012] a detection channel comprising a photodetector and which is also adapted to collect, through the output optics, the portion of the pulse that is reversely reflected or backscattered by the target and direct it onto the photodetector to generate a detection signal; and

[0013] A signal processing unit configured to output a speed measurement result related to the target based on the detection signal.

[0014] The laser radar system of the present invention also has the following features:

[0015] - the transmission channel is also suitable for changing the modulation frequency of the radiation in each pulse so that the component of the detection signal useful for measuring the velocity, called the useful component, undergoes a spectral shift according to the distance of the target from the lidar system; and

[0016] -The signal processing unit is also configured to extract a useful component from the spectrum of the detection signal so as to isolate the useful component from at least one other component in the detection signal and to obtain a velocity measurement result based on the extracted useful component, wherein at least one other component of the detection signal is obtained by reverse reflection or partial backscattering of the emitted pulse in the output optical device of the lidar system.

[0017] 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 selectively digitally filtering the useful component of the detection signal relative to the Narcissus signal. This allows the LiDAR system of the invention to provide velocity measurements and possibly even telemetry measurements, even for targets located at a short distance in front of the output optics. Such telemetry measurements can then be made by determining the frequency shift that affects the back-reflected or back-scattered portion of the pulse at the moment of reception.

[0018] In such a laser radar system, the modulation frequency that changes within each pulse can be directly the frequency of light radiated by the pulse. In other words, the laser radar system can implement optical frequency modulation.

[0019] Alternatively, the modulation applied to the pulses emitted to the target may be an amplitude modulation, in particular a sinusoidal amplitude modulation, the sinusoidal frequency of which varies over time within each pulse. This amplitude modulated sinusoidal frequency is then the modulation frequency that varies according to the invention.

[0020] The detection channel may be arranged to perform heterodyne detection on the reverse reflected or backscattered portion of the pulse. In this case, the transmit channel may be adapted to apply a constant additional frequency offset to the transmit pulses in addition to the modulation frequency variation within each pulse, and to advantageously perform heterodyne detection by mixing a reference signal without the constant additional frequency offset with the reverse reflected or backscattered portion of the pulse.

[0021] However, the laser radar system according to the present invention can optionally implement a non-heterodyne detection mode. For example, the detection channel can be adjusted to directly detect the portion of the pulse that is back-reflected or back-scattered by the target. In particular, this direct detection can be used when the modulation applied within each pulse is of the amplitude modulation type.

[0022] In particular, the laser radar system according to the invention may be adapted to measure airspeed. As described above, for this application, the target consists of particles located in a portion of the atmosphere, pulses are emitted by 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 are backscattered by the particles located in the portion of the atmosphere. In particular, also for this airspeed measurement application, the output optics may be adapted to send the pulses to the outside of the laser radar system in the form of a converging radiation beam, the convergence of which determines the portion of the atmosphere involved in the airspeed measurement. The measurement distance, i.e. the distance from the portion of the atmosphere involved in the airspeed measurement, may then be determined based on the convergence applied to the emitted pulse beam.

[0023] In a preferred embodiment of the present invention, the transmission channel can be adapted to change the modulation frequency within each pulse according to a frequency modulation ramp of a constant slope. In addition, the transmission channel can also be adapted to change the sign of the slope of the modulation frequency variation ramp between two consecutively transmitted pulses. In particular, the absolute value of the slope of the modulation frequency variation ramp between two consecutively transmitted pulses can be equal, but the sign is opposite. In this case, the signal processing unit can be configured to obtain the speed measurement result according to the sum of two corresponding frequency offsets associated with the first extracted useful component and the second extracted useful component, the first extracted useful component corresponding to those transmitted pulses with a positive slope of the modulation frequency variation ramp, and the second extracted useful component corresponding to those transmitted pulses with a negative slope of the modulation frequency variation ramp. At the same time, the signal processing unit can also be configured to obtain an evaluation of the distance of the target from the laser radar system according to the difference between the corresponding frequency offsets associated with the first extracted useful component and the second extracted useful component, the first extracted useful component corresponding to those transmitted pulses with a positive slope of the modulation frequency variation ramp, and the second extracted useful component corresponding to those transmitted pulses with a negative slope of the modulation frequency variation ramp. This evaluation of the target distance is independent of the determination of the same distance obtained from the convergence of the pulse beam.

[0024] However, when the distance of the target from the LiDAR system is known by other means, a velocity measurement can be obtained by varying the modulation frequency, which is the same for all pulses. This is particularly applicable to airspeed measurements when the distance to the part of the atmosphere involved in the measurement is determined by the convergence of the radiation beam of the emitted pulses.

[0025] In such an embodiment of the invention, since the distance to the target is already known by other means, the transmit channel may be adapted to transmit all radiation pulses with the same frequency modulation ramp. The signal processing unit may then be configured to obtain a velocity measurement based on a second contribution of the frequency offset relative to the extracted useful component, the second contribution being calculated by subtracting from the frequency offset a first contribution resulting from the frequency modulation ramp in combination with the distance to the target.

[0026] A second aspect of the present invention provides a method for measuring the speed of at least one target using a laser radar system, the method comprising the following steps:

[0027] / 1 / Send a radiation pulse to the target through the output optical device;

[0028] / 2 / using an output optical device to detect a portion of the pulse that is reversely reflected or backscattered by the target;

[0029] / 3 / detecting the reversely reflected or backscattered portion of the pulse using a photodetector that generates a detection signal; and

[0030] / 4 / Output the speed measurement result relative to the target based on the detection signal.

[0031] According to the invention, the method has the following additional features:

[0032] - during step / 1 / , the modulation frequency of the radiation varies within each pulse so that in step / 4 / the component of the detection signal used for speed measurement, called the useful component, undergoes a spectral shift depending on the distance of the target from the lidar system; and

[0033] -In step / 4 / , a useful component is extracted from the spectrum of the detection signal so as to isolate the useful component from at least one other component of the detection signal resulting from a reverse reflection or partial backscattering of the emitted pulse in the output optical device, and a velocity measurement result is then obtained based on the extracted useful component.

[0034] This method may be particularly, but not exclusively, used for airspeed measurement.In addition, the lidar system may be mounted on an aircraft and the method performed while the aircraft is in flight.

[0035] Generally speaking, the method according to the second aspect of the present invention can be implemented using the lidar system according to the first aspect of the present invention.

[0036] Different embodiments of the method of the present invention may employ at least one of the following additional features:

[0037] - the modulation frequency that varies within each pulse can be directly the optical frequency of the radiation of the pulse, or it can be a sinusoidal frequency of the amplitude modulation of the radiation of each pulse;

[0038] - detecting the portion of the pulse that is back-reflected or back-scattered may be performed using heterodyne detection or direct detection;

[0039] - the modulation frequency within each pulse may be varied according to a frequency modulation ramp having a constant slope. It is possible that the sign of the slope of such a modulation frequency variation ramp between two consecutively transmitted pulses may be changed. In particular, the absolute value of the slope of the modulation frequency variation ramp between two consecutively transmitted pulses may be equal but of opposite sign; and

[0040] - For wind speed measurement applications, the output optics can send pulses outside the LiDAR system in the form of a converging radiation beam, the convergence of which determines the atmospheric portion involved in the airspeed measurement. In this case, all radiation pulses can be emitted with the same frequency modulation ramp, and the velocity measurement can be obtained from a second contribution of frequency offset relative to the extracted useful component. This second contribution is calculated by subtracting the first contribution resulting from the frequency modulation ramp combined with the distance to the atmospheric portion from the frequency offset.

[0041] In particular, when two consecutively transmitted pulses have a frequency modulation ramp with a constant slope and opposite signs, the target speed measurement result can be obtained based on the sum of two corresponding frequency offsets associated with a first extracted useful component and a second extracted useful component, the first extracted useful component corresponding to those transmitted pulses with a positive slope of the radiation frequency modulation ramp, and the second extracted useful component corresponding to those transmitted pulses with a negative slope of the radiation frequency modulation ramp. At the same time, an estimate of the distance of the target from the lidar system can be obtained based on the difference of the corresponding frequency offsets associated with the first extracted useful component and the second extracted useful component. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] 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 drawings, in which:

[0043] [ Figure 1 ] is a block diagram of a laser radar system for airspeed measurement applications using optical frequency modulation and heterodyne detection according to the present invention;

[0044] [ Figure 2a ]and[ Figure 2b ] is to use [ Figure 1 ]’s lidar system obtains two spectrograms for two measured distance values;

[0045] [ Figure 3 ] shows the use of [ Figure 1 ] two spectra of heterodyne detection signals acquired by the laser radar system;

[0046] [ Figure 4 ] shows [ Figure 1 ]’s possible applications for lidar systems; and

[0047] [ Figure 5 ] corresponds to [ Figure 1 ] for target speed measurement applications using amplitude modulation and direct detection. DETAILED DESCRIPTION

[0048] For the sake of clarity, the sizes of the elements shown in these drawings do not correspond to actual sizes or size ratios. In addition, some of these elements are only shown symbolically, and the same reference numerals shown in different drawings represent the same or elements with the same function.

[0049] The invention will first be described for an airspeed measurement application using optical frequency modulation.Secondly, it will be described for an application involving the use of amplitude modulation to measure the velocity of a solid object constituting a target.

[0050] according to[ Figure 1 ], the laser radar system for airspeed measurement includes a transmitting channel 10, a detection channel 20 and a signal processing unit 30.

[0051] The emission channel 10 comprises a laser source 11, marked as SOURCE, an optical modulator 14, marked as MAO, an optical amplifier 15, marked as AO, and an output optical device 18. The laser source 11 can be of the continuous emission type, for example, generating radiation with a wavelength λ equal to about 1.55 μm (micrometers). The modulator 14 can be of the acousto-optic type and is controlled to chop the radiation from the laser source 11 into continuous pulses. Typically, but not necessarily, the modulator 14 can also apply a constant frequency offset to the optical frequency of the radiation, for example an offset equal to about 40 MHz (megahertz). The optical amplifier 15 increases the power of the radiation pulses thus formed, which are transmitted outwardly by the output optical device 18 in the form of a beam F, the central propagation direction of the beam F being AA. As is well known, chopping the radiation into continuous pulses makes it possible to obtain high instantaneous radiation power values ​​in each pulse, which is particularly advantageous for airspeed measurements. This type of pulse emission operation can be used to achieve 100 W (watts) to 5.10 in each pulse with a roughly rectangular profile. 5 Instantaneous radiant power values ​​between 100 W, for example 500 W. Such values ​​cannot be achieved with continuous laser radiation using optical amplification components that are currently available or compatible with airspeed measurement applications. The output optical device 18 can be composed of one or more converging lenses and can determine the size of the output pupil of the transmission channel 10. For example, the output pupil may have a radius of approximately 0.07 m (meter). In addition to the output optical device 18, the optical components of the transmission channel 10 can be advantageously made using fiber optic technology, thereby reducing the size of the lidar system and facilitating the optical alignment of these components.

[0052] The output optics 18 is designed to generate a beam F of radiation pulses having a converging beam structure in a spatial region located downstream of the output optics with respect to the propagation direction of the radiation away from the emission channel 10. The beam F thus has a cross section perpendicular to its central propagation direction AA, which gradually decreases between the output optics 18 and the focal zone ZF and then increases in the form of a divergent beam outside the focal zone ZF. In a known manner, in particular using a Gaussian beam model, the focal zone ZF can be compared to a cylinder, the axis of which is superimposed on the central propagation direction AA, with a radius w0=λ / (π·θ), usually called the "waist", and a length of 2·|R, where I R is the Rayleigh length, equal to λ / (π·θ 2 ), θ is the divergence half angle of the beam F beyond the focal region ZF (expressed in radians). Typically, the distance between the output optical device 18 and the focal region ZF (labeled D, which can be selected by adjusting the focal length or longitudinal position of the output optical device 18) can vary between a few meters and a few hundred meters, and the Rayleigh length I R The divergence half angle θ of the beam F can be estimated at a distance of 1 km (kilometer) from the focal zone ZF, downstream of the focal zone ZF, in the direction of radiation propagation.

[0053] To perform heterodyne detection, the detection channel 20 may include a photodetector 22, labeled PD, and optical couplers 13, 16 and 21, labeled CO, arranged to couple the backscattered portion of the beam F with the radiation F from the laser source 11. REF Partially merged. This part radiates F REF Serves as reference radiation for heterodyne detection. In a known manner, the backscattered portion of the light beam F thus detected comes essentially from the focal zone ZF and is produced by the 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 (marked λ / 4) is inserted between the optical coupler 16 and the output optics 18. The optical coupler 16 and the quarter-wave plate 17 thus form an optical circulator that couples the emission channel 10 and the detection channel 20 to the shared output optics 18.

[0054] Finally, the signal processing unit 30 may be a PC-based computer module carrying a suitable program for processing the detection signal transmitted by the photodetector 22. The signal processing unit 30 outputs an estimate of the velocity component of the backscattered particles in the focal zone ZF, which is parallel to the central propagation direction AA and is denoted by V A-A .

[0055] The operation of such a LiDAR system for airspeed measurement is well known to those skilled in the art and need not be repeated here. The system is oriented and the divergence half-angle θ is adjusted so that the focal zone ZF is in the portion of the atmosphere where the wind speed is to be measured. The particles of the backscattered light beam F are dust, microcrystals or aerosol droplets suspended in the atmosphere within the focal zone ZF. The structure of the output optics 18, and its control when it allows variable adjustment of the divergence half-angle θ, can provide an estimate of the distance between the center of the focal zone ZF and the LiDAR system, which distance corresponds to the distance D introduced above.

[0056] For the present invention, the transmission channel 10 also comprises additional modulation means, which are suitable for changing the optical frequency of the radiation within each pulse formed by the acousto-optic modulator 14. For example, these additional frequency modulation means can be constituted by an electro-optic modulator 12, referenced MEO, with its appropriate control unit, indicated by reference numeral 40, referenced CTRL and called modulation controller. They can be inserted between the laser source 11 and the acousto-optic modulator 14. In various embodiments of the present invention, these frequency modulation means can be arranged to vary the optical frequency of the radiation within each pulse formed by the acousto-optic modulator 14. REF Upstream or downstream of the optical coupler 13 where sampling is performed. In the following, it will be assumed that these frequency modulation devices are arranged upstream of the optical coupler 13 relative to the propagation direction of the radiation in the transmission channel 10, and a person skilled in the art will know how to apply the following description to the case where the additional modulation device is arranged downstream of the optical coupler 13.

[0057] For example, the modulation controller 40 is configured so that the acousto-optic modulator 14 chops the radiation initially generated by the laser source 11 into a single continuous pulse of duration equal to 1 μs (microseconds), and so that the electro-optic modulator 12 applies a linear increase in the optical frequency of the radiation within each pulse, from about 0 MHz (megahertz) at the beginning of the pulse to about 500 MHz at the end of the pulse. The slope p of the optical frequency variation is then equal to +0.500 MHz / ns (megahertz per nanosecond). Subsequently, the portion of the pulse generated by the emission channel 10 and backscattered in the focal zone ZF is increased relative to the reference radiation F REF The propagation delay of the radiation F t reaches the photodetector 22, which corresponds to the round trip propagation time between the emission optics 18 and the focal zone ZF. As a result of the frequency modulation introduced according to the invention, it is relative to the reference radiation F REF has a first frequency offset contribution equal to -p·2·D / C, where C is the radiation propagation speed of the laser radar system outside between the output optics 18 and the focal zone ZF. This first frequency offset contribution is referred to below as Δf1. In a known manner, when the airspeed V A-A like Figure 1When the orientation is shown, the velocity V of the backscattered particles contained in the focal region ZF is A-A The component (which is parallel to the central propagation direction AA) produces a second frequency shift contribution, denoted Δf2 and equal to 2·V A-A / λ. Figure 1 In the embodiment of the present invention, the two contributions Δf1 and Δf2 are added to generate the frequency of the heterodyne detection signal output by the photodetector 22. The two contributions Δf1 and Δf2 are also added to a constant frequency offset that may be generated by the AOM 14.

[0058] Figure 2a and Figure 2b The frequency of the spectral components of the heterodyne detection signal is shown as a function of time within each pulse. For each spectrum graph, the horizontal axis represents time, labeled t and expressed in microseconds (μs), the vertical axis represents frequency, labeled f and expressed in megahertz (MHz), and the color scale on the right side of each spectrum graph represents the instantaneous spectral power, labeled P i It is expressed in decibels (dB) relative to a noise-related reference level. Figure 2a The output optical device 18 is adjusted so that the distance D from the focal area ZF is equal to 15 m (meters). Then, the 2·| R The length is 90cm. Figure 2b Spectrum diagram in, D equals 30m and 2·| R is equal to 3.5 m. The values ​​of the other lidar system parameters are those already quoted, and the two spectrograms are identical except for the value of the constant frequency offset produced by the acousto-optic modulator 14, which is zero. The two components of the heterodyne detection signal appearing in each spectrogram correspond to the partial back reflection of the pulse on the output optics 18, marked as "Narcisse" in the spectrograms, called the narcissus signal, and the part of the pulse backscattered in the focal zone ZF, marked as "Mes", called the measurement signal. The Narcisse signal corresponds to D = 0 and f = 0, because it is identical to the reference radiation F REF In contrast, the Narcissus signal has no significant propagation delay when arriving at the photodetector 22, nor any Doppler effect. The measurement signal arrives at the photodetector 22 with a delay equal to 2·D / C compared to the Narcissus signal and has a frequency f that deviates from the first contribution Δf1 in the absence of wind in the focal zone ZF. For the adopted working conditions, Figure 2a The delay Δt is equal to 0.10μs, Figure 2b The delay Δt is equal to 0.20μs, Figure 2a The delay Δf1 is equal to 50MHz, Figure 2bThe delay Δf1 is equal to 100 MHz. The initial time (t=0) in these two spectra corresponds to the start of the backscattered part of the detection radiation pulse. In addition, due to Figure 2b The focal region ZF in is much longer, so Figure 2b The spectrum width of the measured signal is greater than Figure 2a The spectral width of the measured signal.

[0059] The signal processing unit 30 calculates the Fourier transform of the heterodyne detection signal output by the photodetector 22 with respect to time. The signal processing unit 30 then extracts the measurement signal from the frequency spectrum of the heterodyne detection signal, for example by applying adaptive digital filtering, and determines the frequency f of the measurement signal. In general, this frequency f is equal to Δf0+Δf1+Δf2, where Δf0 (if any) is a constant frequency offset generated by the acousto-optic modulator 14 in all pulses. When the distance D is known by other means, in particular by adjusting the focal length of the output optical device 18, the frequency offset contribution Δf1 is calculated by Δf1=-p·2·D / C, and the two contributions Δf0 and Δf1 are subtracted from the determined value of the frequency f of the measurement signal. The result of this subtraction is the frequency offset contribution Δf2, which corresponds to the Doppler effect, and the signal processing unit 30 calculates the frequency offset contribution Δf2 according to the formula V A-A =λ·Δf2 / 2 calculates the wind speed component parallel to the central propagation direction AA in the focusing area ZF.

[0060] The measurement signal "Mes" just described is the spectral component extracted from the heterodyne detection signal and is referred to in the general part of this description as the useful component of the detection signal.

[0061] However, for airspeed measurements, the distance D from the ZF focal area may depend on factors external to the lidar system, such as atmospheric turbulence and / or air thermal variations present in the pulse path between the output optics 18 and the focal area ZF, and / or concentration variations of backscattered particles along the central propagation direction AA. The improvement now described makes it possible to derive the actual value of the distance D from the heterodyne detection signal. To achieve this, the modulation controller 40 is configured so that some of the pulses are emitted with a specific (e.g. positive) linear optical frequency variation ramp slope and other pulses are emitted with a negative linear optical frequency variation ramp slope. Preferably, every other pulse is emitted with a positive linear ramp slope p, while the other pulses are emitted using -p as the linear ramp slope. Then, for these first pulses, as previously described, the frequency of the measurement signal obtained by heterodyne detection is Δf0+Δf1+Δf2 and is labeled f + For a pulse with a linear ramp slope of -p, the frequency of the measured signal obtained by heterodyne detection is Δf0-Δf1+Δf2, denoted by f -The signal processing unit 30 determines two frequencies f in the same manner as described above. + and f - Then, the signal processing unit 30 determines the contribution amounts Δf1 and Δf2 as follows: Δf1 = (f + -f - ) / 2 and Δf2=(f + +f - ) / 2-Δf0. Then, as described above, the airspeed V is calculated based on the value of the contribution Δf2 A-A , and the distance D is evaluated by applying the following formula: D = -C·Δf1 / (2·p).

[0062] Figure 3 1 shows a heterodyne detection signal obtained in this embodiment of the invention, in which the slopes of the linear optical frequency variation ramps between two consecutive pulses are opposite. The figure separates the spectrum of the pulses with positive slope values, shown in solid lines, from the spectrum of the pulses with negative slope values, shown in dashed lines. The horizontal axis also indicates the frequency value f in megahertz, and the vertical axis indicates the frequency value in watts per hertz (W-Hz). -1 ) and are marked P. The composite heterodyne detection spectrum is established for a value of constant frequency offset Δf0 equal to 40 MHz generated by the AOM 14. The two spectra each show several Narcissus signal components corresponding to partial reflections of the pulse on several components of the output optics 18 and the quarter wave plate 17. These Narcissus signal components differ by less than 15 MHz from the Δf0 corresponding to the 40 MHz value of f. The measurement signals corresponding to the two opposite slopes are represented by the respective frequency values ​​f + and f - The other signals correspond to peaks in the spectrum with frequencies f above 90 MHz, which come from reflections of pulses of opposite slope by obstacles in the background of the focal zone ZF. The peaks at 115 MHz and 205 MHz indicate the presence of an obstacle with high backscatter power, located at a distance D' = -C · Δf1 / (2·p), where Δf1 = (205 MHz - 115 MHz) / 2 = 45 MHz, i.e. D' = 13.5 m. For these same spectra, the values ​​of f are read separately according to the values ​​of f. + 15MHz, f - The value is 65 MHz, the distance D from the focal area ZF is approximately equal to 7.5 m, and the wind speed in the focal area ZF is less than 1 m.s -1 (m / s).

[0063] The above examples illustrate the advantages of the frequency modulation introduced by the invention for spectrally separating the measurement signal from the Narcissus signal or all components of the Narcissus signal, as the case may be. Thanks to the frequency modulation, even at very low wind speeds, the part of the transmitted pulse that is backscattered by particles in the air can be distinguished from the Narcissus signal. This distinction is also possible at small values ​​of the distance D.

[0064] A laser radar system according to the present invention and adapted for making airspeed measurements may be used in many applications, including but not limited to:

[0065] - Aircraft applications, where the small size and light weight of the LiDAR system are the main advantages. Figure 4 A helicopter 100 is shown equipped with such a Lidar system for airspeed measurement according to the invention. Preferably, the system is mounted on the helicopter so that the output optics 18 are located towards the nose of the helicopter 100 and turned towards the half space in front of the helicopter. Figure 4 shows the setting of the central propagation direction AA and the resulting focal region ZF;

[0066] - low or very low airspeed measurement applications, such as measurements at ground level or at low altitude, for example to optimize the operation of a wind turbine, or measurements from an aircraft that may be in hover. In this case, the AOM 14 advantageously generates a constant frequency offset Δf0 which is applied to the transmit pulses but not to the reference radiation F REF In this way, low space velocity values ​​correspond to heterodyne beat frequencies close to non-zero values ​​of Δf0, thereby improving the measurement accuracy without requiring a large number of pulses in each measurement sequence, that is, without making the measurement time too long; and

[0067] - Applications where the measured distance between the output optics 18 and the focal zone ZF is variable. To this end, the output optics 18 can be adjusted so that the convergence of the beam F of continuous pulses is variable when it exits through the optics. For example, when the beam originates from a fiber end, the output optics 18 can be a converging lens mounted on a support that can be translated parallel to the AA direction in order to move the object focus of the lens relative to the fiber end. In this way, the center O of the focal zone ZF can be located at a controllable distance from the output optics 18, for example between 1 m and 1000 m.

[0068] Figure 5 The embodiment of the invention shown is suitable for measuring the velocity of a retroreflective target T. If the target T has sufficient retroreflectivity, it is no longer necessary to converge the light beam F for transmitting pulses. In this case, the light beam can be collimated, thereby enabling the velocity of a target located at a distance D, which varies within a very wide range.

[0069] Figure 5 The embodiment in FIG. 1 uses direct detection of the portion of the pulse that is back-reflected by the target T, rather than Figure 1 heterodyne detection. To this end, the optical frequency modulation previously used can be replaced by amplitude modulation, which is performed using a variable modulation frequency. When the modulation controller 40 is appropriately configured, the acousto-optic modulator 14 can generate such an amplitude modulation with a variable modulation frequency. For example, the acousto-optic modulator 14 generates a sinusoidal amplitude modulation within each pulse, and the frequency of the sinusoidal amplitude modulation varies between the start and the end of the pulse. Preferably, the frequency varies linearly with time, i.e., has a rate of change. For example, the modulation frequency of the radiation amplitude within each pulse can vary between an initial value of 10 MHz at the start of the pulse and a final value of 100 MHz at the end of the pulse.

[0070] The instantaneous power of the pulse portion reflected back by the target T changes slowly enough to be detected in real time by the photodetector 22. Subsequently, the detection signal output by the photodetector 22 can be analyzed by the signal processing unit 30 in the same manner as before. For this reason, the description of this analysis will not be repeated. It is only necessary to explain that the refinement of the continuous transmission pulse with opposite slope of linear variation of the modulation frequency still applies by applying it to the variable amplitude modulation frequency.

[0071] It will be appreciated that the invention can be reproduced by modifying minor aspects of the embodiments already described in detail above, while retaining at least some of the advantages mentioned. In particular, the components mentioned may be replaced by other components or combinations of components that produce equivalent functions to the components described. Finally, the numerical values ​​cited are for illustrative purposes only and may vary depending on the application in question.

Claims

1. A laser radar system, adapted to measure the velocity of at least one target, the laser radar system comprising: - a transmission channel (10) adapted to transmit radiation pulses towards a target via output optics (18) of the laser radar system; - a detection channel (20) comprising a photodetector (22), which is also suitable for collecting the portion of the pulse that is reflected or backscattered by the target through the output optical device (18) and directing the portion of the pulse to the photodetector to generate a detection signal; as well as - a signal processing unit (30) configured to output a speed measurement result related to the target according to the detection signal, The laser radar system is characterized by: - the transmitting channel (10) is also suitable for changing the modulation frequency of the radiation in each pulse so that the component of the detection signal useful for measuring the speed, called the useful component, undergoes a spectral shift according to the distance of the target from the laser radar system; and -The signal processing unit (30) is also configured to extract the useful component from the spectrum of the detection signal so as to isolate the useful component from at least one other component of the detection signal and obtain a velocity measurement result based on the extracted useful component, wherein at least one other component of the detection signal is obtained by partial back reflection or back scattering of the transmitted pulse in the output optical device (18) of the lidar system.

2. The laser radar system according to claim 1 is suitable for measuring airspeed, the target is a particle located in an atmospheric part, the pulse is emitted by the transmitting channel (10) toward the atmospheric part, and the detection channel (20) is suitable for collecting and detecting part of the pulse that is backscattered by the particles located in the atmospheric part.

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

4. A laser radar system according to any one of the preceding claims, wherein: The transmit channel (10) is adapted to vary the modulation frequency within each pulse according to a frequency modulation ramp with a constant slope.

5. The laser radar system according to claim 4, wherein: The transmission channel (10) is adapted to transmit all radiation pulses with the same frequency modulation ramp, and wherein the signal processing unit (30) is configured to obtain the velocity measurement result based on a second contribution of the frequency offset relative to the extracted useful component, the second contribution being calculated by subtracting a first contribution resulting from the frequency modulation ramp in combination with the distance to the target from the frequency offset.

6. The laser radar system according to claim 4, wherein: The transmission channel (10) is suitable for changing the sign of the slope of the modulation frequency variation ramp between two consecutively transmitted pulses.

7. The laser radar system according to claim 6, wherein: The transmitting channel (10) is adapted to ensure that the slopes of the modulation frequency variation ramps between two consecutively transmitted pulses are equal in absolute value but opposite in sign.

8. The laser radar system according to claim 7, wherein: The signal processing unit (30) is configured to obtain the speed measurement result based on the sum of two corresponding frequency offsets associated with a first extracted useful component and a second extracted useful component, wherein the first extracted useful component corresponds to those transmitted pulses with a positive slope of the modulation frequency change ramp, and the second extracted useful component corresponds to those transmitted pulses with a negative slope of the modulation frequency change ramp.

9. The laser radar system according to claim 8, wherein: The signal processing unit (30) is also configured to obtain an estimate of the distance of the target from the lidar system based on the difference between the corresponding frequency offsets associated with the first extracted useful component and the second extracted useful component, wherein the first extracted useful component corresponds to those transmitted pulses with a positive slope of the modulation frequency change ramp, and the second extracted useful component corresponds to those transmitted pulses with a negative slope of the modulation frequency change ramp.

10. A laser radar system according to any one of the preceding claims, wherein: The detection channel (20) is arranged to perform heterodyne detection on the reverse reflected or backscattered portion of the pulse.

11. A method for measuring the velocity of at least one target using a laser radar system, the method comprising the steps of: / 1 / emitting a radiation pulse in the direction of a target through an output optical device (18); / 2 / using the output optical device (18) to collect the portion of the pulse that is reversely reflected or backscattered by the target; / 3 / detecting the reversely reflected or backscattered portion of the pulse using a photodetector (22) that generates a detection signal; as well as / 4 / outputting a speed measurement result relative to the target according to the detection signal, The method is characterized in that: - during step / 1 / , the modulation frequency of the radiation varies within each pulse so that in step / 4 / the component of the detection signal used for speed measurement, called the useful component, undergoes a spectral shift depending on the distance of the target from the lidar system; and - In step / 4 / , the useful component is extracted from the spectrum of the detection signal so as to isolate the useful component from at least one other component of the detection signal, and a velocity measurement result is then obtained based on the extracted useful component, wherein at least one other component of the detection signal is obtained by partial back reflection or back scattering of the emitted pulse in the output optical device (18).

12. A method according to claim 11 for measuring airspeed, whereby the target consists of particles located in a portion of the atmosphere, the pulses are emitted in the direction of the portion of the atmosphere, and the portion of the pulses backscattered by the particles located in the portion of the atmosphere are collected and detected.

13. The method according to claim 11 or 12, wherein: The laser radar system is carried on an aircraft (100), and the method is performed while the aircraft is in flight.

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