CALIBRATION OF A LIDAR SYSTEM

Description

technical field

[0001] This description relates to a method for calibrating a LIDAR system, as well as a LIDAR system that is adapted to implement such a calibration method. Previous technique

[0002] LiDAR systems are used in numerous applications, some of which involve measuring the distance to a target. Other applications involve measuring an external quantity for at least a certain distance from a location to which the measurement relates, even if measuring that distance is not the primary objective of the application. This is the case with anemometric measurements, which are based on detecting a portion of a laser beam that is backscattered by airborne particles. The intensity of the backscattered portion of the laser beam detected by the LiDAR system is therefore very low.A particle velocity value is deduced from a spectral analysis of the backscattered portion of the laser beam that is detected by the LIDAR system, and then from a Doppler effect calculation that is applied to a frequency gap that is identified between this backscattered portion of the laser beam and the laser beam as emitted by the LIDAR system.

[0003] In a monostatic LiDAR system, residual reflection of the emitted laser beam occurs on optical components common to both the emission and detection channels, even if these components are equipped with anti-reflective coatings. The radiation reflected internally within the LiDAR system is then detected by the detection channel. This internal reflection signal is often called a "narcissus." It is commonly used to establish a zero or low-value distance reference for calibrating the system for specific distances, or for distances that arise when other quantities are measured using the LiDAR system. The intensity of the emitted laser beam is significant, and the LiDAR system's detection sensitivity can be very high, particularly when designed to detect radiation backscattered by atmospheric particles.The narcissus signal then presents the following characteristics: . It is generated by reflections of the laser beam on several optical components common to the emission and detection paths of the LIDAR system, so that it does not correspond to a single distance value where a reflection occurs; and its intensity is much greater than the usual intensity levels of radiation that is backscattered by atmospheric particles, so that it causes saturation of the detector of a LIDAR system that is designed to detect radiation such as is produced by backscattering atmospheric particles. For these reasons, the narcissus signal cannot provide an accurate distance reference. Furthermore, for a monostatic LIDAR system, it prevents the satisfactory detection of radiation resulting from backscattering or reflections from targets located at short distances from the LIDAR system, typically between 20 m and 150 m. The area extending from the output of a monostatic LIDAR system to the minimum distance at which a measurement can be inhibited by the narcissus signal is commonly referred to as the blind zone.

[0004] Another method for calibrating a LiDAR system's range involves placing a retroreflective target at a known distance in front of the LiDAR system, beyond the blind zone, and then performing a measurement sequence with this target. However, this method is time-consuming and therefore cannot be repeated frequently. Furthermore, when the LiDAR system is pulsed, such calibration must be performed with pulses of the same duration as those used in useful measurement sequences. For long pulses, for example, those with individual durations greater than 0.5 µs (microseconds), this requires placing the target more than 75 m (meters) from the LiDAR system, which is very difficult to achieve in practice.

[0005] Finally, US document 5,825,464 proposes a calibration device for a LIDAR system that is external to the LIDAR system to be calibrated, and to be optically coupled to it using an integrating sphere. Technical problem

[0006] From this situation, one aim of the present invention is to enable the calibration of a LIDAR system in distance, more easily and more accurately than using previous methods.

[0007] In particular, for a monostatic LIDAR system, the invention aims to enable such a distance calibration, which is not hindered by the narcissus signal.

[0008] Another objective of the invention is to allow such calibration to be repeated at will, without requiring a target to be placed at a predetermined distance in front of the LIDAR system.

[0009] Another objective of the invention may be to enable distance calibration to be performed each time a useful measurement sequence is carried out, or even at the same time as this useful measurement sequence. Summary of the invention

[0010] To achieve at least one of these goals, or another, a first aspect of the invention proposes a new method for calibrating a LiDAR system to provide a reference measurement value that corresponds to a reference free-scan distance value. These reference measurement and free-scan distance values ​​are intended to be used as calibration for the LiDAR system to evaluate the distance to at least one target to be characterized using the LiDAR system. The method of the invention comprises the following steps: / 1 / couple the LIDAR system to a reference optical path which is made up of at least one effective optical fiber to guide radiation emitted by the LIDAR system during its operation, such that at least part of the radiation travels along the reference optical path between an emission channel and a detection channel of the LIDAR system, the reference optical path having a known length; / 2 / carry out a distance measurement sequence using the LIDAR system coupled to the reference optical path as resulting from step / 1 / , to obtain a measurement value which is representative of a propagation time of the radiation in the reference optical path, and which is intended to constitute the measurement reference value;then / 3 / convert the known length of the optical reference path into a free propagation distance value for the radiation, according to a principle of equal propagation time, to form the reference value of free propagation distance, and associate this reference value of free propagation distance with the measurement reference value in order to establish the calibration of the LIDAR system. ;

[0011] Thus, the method of the invention uses a reference optical path to be optically coupled to the LIDAR system. This reference optical path, which is based on at least one optical fiber, materially determines the reference value of the free propagation distance. This reference value of the free propagation distance is therefore known with high accuracy. Thanks to such a calibration method, repulsion distance values ​​determined from LIDAR system operations for targets to be characterized can have improved accuracy and high precision.

[0012] In general, the method of the invention is compatible with a monostatic or bistatic LIDAR system. For the purposes of this invention, a monostatic LIDAR system is defined as one in which the optical output and detection ports of this LIDAR system are contiguous, adjacent, or close to each other relative to the distance from the target to be characterized. Conversely, a bistatic LIDAR system is defined as one in which the optical output and detection ports are spatially separated.

[0013] In general, the LIDAR system can also be adapted to characterize a single target or a diffuse set of multiple targets distributed over a spatial area during each measurement operation of the LIDAR system. In particular, the invention can be applied to a LIDAR system designed to perform anemometric measurements by implementing laser radiation emitted by the LIDAR system at each measurement cycle, a portion of which is backscattered by particles suspended in the air. Such particles can be aerosols, dust, ice grains, etc., suspended in the atmosphere, which together form the target to be characterized. The distance from the target is then that of an area containing the particles, to which the measurement result, for example, an anemometric measurement, relates.

[0014] Depending on the configuration of the reference optical path relative to the optical fiber used to construct it, the length of this reference optical path can be related to that of the optical fiber, or to twice the length of the optical fiber. Furthermore, in step / 3 / , the conversion of the reference optical path length into the free-space propagation distance for the radiation takes into account a difference in the propagation speed of the radiation emitted by the LIDAR system, which exists between its guided propagation within the optical fiber and its propagation in free space.

[0015] Advantageously, the optical fiber can have a length such that the free propagation distance, corresponding to the length of the reference optical path, is greater than any potential blind spot of the LIDAR system when it is of the monostatic type. To this end, the method of the invention may further include the following preliminary step, to be performed before step / 1 / when the LIDAR system is of the monostatic type in which the optical output and detection ports of the LIDAR system coincide: / 0 / determine a limiting distance value such that portions of the radiation emitted by the LIDAR system that are reflected by internal components of this LIDAR system correspond to emission-to-detection times that are equivalent to those of targets located within this limiting distance value in front of an optical output port of the LIDAR system.Internal components of the LIDAR system that are likely to partially reflect the emitted radiation, i.e., produce a narcissistic signal, can include an output window, polarization control components, one or more focusing lenses, etc. The smallest limiting distance value that can thus be determined is the length of the blind zone. The length of the reference optical path can then be selected, preferably such that the free-propagation distance is greater than the limiting distance value determined in step / 0 / . For example, the optical fiber length can be such that the reference free-propagation distance is greater than 150 m.

[0016] According to the invention, the optical reference path, which is formed from at least one optical fiber, is incorporated within the LIDAR system. It is thus available within the LIDAR system for performing or repeating calibrations at will during the system's lifetime, using the method of the invention each time.

[0017] The optical reference path, which consists of at least one optical fiber, can be adapted to be optically coupled reversibly to the LIDAR system in step / 1 / , so that it can be optically decoupled during measurement sequences useful for characterizing targets. Specifically, it can optionally be optically decoupled from the LIDAR system's optical output port, and possibly also optically decoupled from the LIDAR system's optical detection port.

[0018] Advantageously, the optical fiber used for the optical reference path can be arranged in the form of a reel. This allows the optical reference path to be lightweight and compact, making its integration into the LIDAR system easy, including for a LIDAR system intended to be mounted on a carrier such as an aircraft, for example, to perform anemometric measurements.

[0019] In general, the optical reference path can be optically coupled to the LIDAR system reversibly and temporarily for the duration of the calibration process, by a controlled optical coupling component, and then possibly decoupled for useful measurement sequences.

[0020] Alternatively, the optical reference path can remain continuously optically coupled to the LiDAR system during a useful measurement sequence. This allows for calibration of the LiDAR system using a portion of the same radiation emitted by the LiDAR system for the measurement. Such calibration is thus applied directly to the measurement conditions, specifically to the radiation used for the measurement. Consequently, the validity of the calibration is not compromised by potential repeatability issues in the radiation emitted by the LiDAR system. This advantage is particularly significant when the radiation is emitted as pulses for the measurement, as certain characteristics of these pulses, such as their envelope shape or center optical frequency, can vary randomly or drift gradually.

[0021] The method of the invention may further include the following additional steps, which constitute a useful measurement sequence carried out from the calibration method: / 4 / perform a measurement relative to at least one target to be characterized by directing a line of sight from the LIDAR system towards this at least one target independently of the optical reference path, so as to collect another measurement value which is representative of the distance away from the at least one target; then / 5 / calculate a value for this distance away by combining the so-called other measurement value with the measurement reference value and the free propagation distance reference value which corresponds to the length of the optical reference path. Therefore, in possible embodiments of the invention, the optical reference path, which is formed from at least one optical fiber, can be optically coupled to the LIDAR system continuously during step / 4 / , such that a first portion of the radiation emitted by this LIDAR system is used to perform the measurement relative to at least one target, and a second portion of the same radiation is used simultaneously to obtain the measurement value that is representative of the propagation time in the optical reference path. Thus, the measurement reference value, which is the propagation time in the optical reference path as measured during step / 4 / , can be used in step / 5 / .

[0022] The LIDAR system to which the method of the invention is applied can be adapted to perform measurements according to any principles, including time-of-flight measurements and measurements based on radiation absorption levels, etc. However, the LIDAR system can preferably be adapted to perform measurements according to a heterodyne detection principle, including providing the measurement reference value in step / 2 / , and where appropriate, also providing the other measurement value in step / 4 / which is representative of the distance from at least one target to be characterized.

[0023] According to a first possible configuration for the optical reference path, one end of the optical fiber can be coupled to the transmission channel of the LIDAR system at step / 1 / , and the other end of this optical fiber, opposite the first, can be coupled to the detection channel of the LIDAR system. Thus, at least some of the radiation emitted by the LIDAR system enters the optical fiber through its first end and then exits through its second end, being transmitted to the detection channel of the LIDAR system. This configuration of the optical reference path is compatible with both monostatic and bistatic LIDAR systems. For this first configuration of the optical reference path, the reference value of the free propagation distance is related to the length of the optical fiber.

[0024] According to another possible configuration of the reference optical path, which is compatible with monostatic LIDAR systems, the optical fiber of the reference optical path can have a first end and a second end, the latter being opposite the first end and equipped with a reflector. Thus, radiation propagating in the optical fiber from the first end is back-reflected by the reflector at the second end, inside the optical fiber, towards the first end. Step / 1 / then involves coupling the LIDAR system to the optical fiber such that at least some of the radiation emitted by the LIDAR system enters the optical fiber through its first end, and at least some of the back-reflected radiation exiting from this same first end is transmitted to the detection channel of the LIDAR system.For such an alternative configuration of the optical reference path, the reference value of free propagation distance is related to twice the length of the optical fiber.

[0025] When the LIDAR system is monostatic, and the optical reference path is coupled to this LIDAR system only via the first end of the optical fiber, the second end being equipped with a reflector, the first end of the optical fiber can be optically coupled to an input / output optical port of an optical circulator of the LIDAR system. This optical circulator also couples the emission and detection channels of the LIDAR system to an output optical port of this LIDAR system. Thus, in step / 2 / , the optical circulator transmits the radiation from the LIDAR system to the optical fiber via the optical input / output port of the optical circulator, and also transmits, in the same step / 2 / , to the detection channel of the LIDAR system the radiation received by this same input / output optical port of the optical circulator from the optical fiber.In this case, the reference optical path can be further adapted, or provided with means for controlling the polarization of the radiation, so that the optical circulator is effective during step / 2 / . Alternatively, such polarization control means can be integrated into the optical circulator. The input / output port of the optical circulator, which is used to couple the first end of the optical fiber in steps / 1 / and / 2 / , may or may not also be used in step / 4 / for the radiation implemented during that useful measurement sequence. When the reference optical path is coupled to an input / output port of the optical circulator that is different from the one used to emit the radiation toward the target to be characterized during a useful measurement sequence, a new range calibration of the LIDAR system can be performed concurrently with each useful measurement sequence.In particular, such an implementation of the invention can be achieved by coupling the emission and detection paths of the monostatic LIDAR system using an optical circulator based on a polarizing cube. In this case, two optical input-output ports of the optical circulator, formed by two different faces of the polarizing cube, can be dedicated, one to transmitting a primary portion of the radiation towards the target to be characterized, and the other to simultaneously transmitting a secondary portion of the radiation to the reference optical path.

[0026] Preferably, the optical reference path can be coupled to the emission path of the LIDAR system downstream of an optical amplifier that forms part of this emission path, according to the direction of radiation propagation in this emission path. In this way, distortions affecting the radiation pulses used during each measurement sequence, and caused by this optical amplifier, are identical between the radiation traveling along the optical reference path and the radiation back-reflected or backscattered by the target to be characterized. These distortions are thus effectively taken into account in the method of the invention, to reduce any error they might cause in a distance measurement result obtained for the target to be characterized.

[0027] According to an optional improvement of the invention, which can further improve the accuracy of the distance calibration of the LIDAR system, the reference value of free propagation distance can be established in accordance with a differential balance of transmission, detection and / or signal processing delays which are effective for the radiation of the LIDAR system, between emission and detection paths of this radiation which are used during step / 2 / , and other emission and detection paths of the radiation which are used to characterize at least one target, i.e. which are used in step / 4 / .

[0028] A method according to the invention can be advantageously used, in particular, for a pulsed-radiation LIDAR system. For such a pulsed LIDAR system, the distance to the at least one target to be characterized is evaluated according to a time interval between the emission of a radiation pulse towards that target(s) and the detection of a portion of that radiation pulse that has been reflected or backscattered by the target(s).

[0029] In such a case of a pulsed LIDAR system, the method of the invention may further comprise performing at least one of the following: a pulse envelope characterization, relative to a radiation pulse that was emitted and then detected by the LIDAR system; a frequency shift characterization, such as appearing between the radiation pulse as emitted by the LIDAR system, and the same radiation pulse as subsequently detected by the LIDAR system; and a radiation pulse envelope deformation characterization, this deformation being caused by saturation of the LIDAR system detection channel. This additional characterization (or these additional characterizations) can be performed using a detection signal produced by the LIDAR system during step / 2 / , for one or more successive executions of this step / 2 / . The calibration method of the invention can thus be completed to constitute a functional test of the LIDAR system, and to calibrate the LIDAR system also using frequency offset. Such frequency offset calibration allows for more accurate measurement of target velocities, particularly for anemometric measurements.

[0030] In general, for the invention, the optical reference path may be provided with an intensity attenuator, which is arranged to attenuate the intensity of the radiation transmitted through the optical fiber to the detection path of the LIDAR system. Such an intensity attenuator is preferably variable. In particular, it may include an iris arranged at one end of the optical fiber.

[0031] More generally, for the invention, the LIDAR system can be adapted for at least one of the following applications: meteorological measurements, for example measurements of atmospheric turbulence; measurements of the diffusion of atmospheric pollutants; measurements of shear of an atmospheric flow, for example at an airport; measurements of the position and lifetime of at least one vortex that is present in a fluid flow, in particular measurements of the positions of vortex cores; anemometric measurements that are carried out from an aircraft in flight, in particular from an airship or a drone; anemometric measurements that are carried out to optimize the operation of a wind turbine; and anemometric measurements that are carried out to adjust the flight of an aircraft in formation, or to adjust the flight of a drone.

[0032] Finally, when the LIDAR system is monostatic and adapted to focus the radiation emitted in each measurement sequence to a focusing distance in front of the optical output port of this LIDAR system, a measurement sequence of this focusing distance may include the following steps: control the LIDAR system to emit at least one radiation pulse, then collect a detection signal that is produced by the detection path of the LIDAR system, corresponding to portions of the emitted radiation that have been backscattered at varying distances; determine a detection instant that corresponds to a maximum of a signal-to-noise ratio for the collected detection signal; then calculate a recoil distance value that is associated with the determined detection instant, using the reference free propagation distance value that corresponds to the length of the reference optical path, and assign the calculated recoil distance value to the focusing distance. To increase the accuracy of the value thus determined for the focusing distance, a short pulse (or pulses) may preferably be used.

[0033] A second aspect of the invention relates to a LIDAR system equipped with a reference optical path made from at least one effective optical fiber to guide radiation emitted by the LIDAR system during its operation. Thus, at least a portion of the radiation travels along the reference optical path between an emission channel and a detection channel of the LIDAR system. The LIDAR system is further adapted to implement a calibration method that conforms to the first aspect of the invention, possibly with the aforementioned improvements and optional additional features. In particular, the reference optical path, which is made from at least one optical fiber, is incorporated within the LIDAR system.

[0034] Preferably, the optical reference path can be optically coupled to the emission and detection paths of the LIDAR system, in parallel with an external path to the LIDAR system. This external path is intended to be followed by radiation emitted by the LIDAR system towards at least one target, and is useful for evaluating the distance to that target. In this way, the radiation from the emission path is transmitted simultaneously to the optical reference path and towards at least one target, and a single detection sequence provides both the reference measurement value and another measurement value that is relative to the external path to the target.

[0035] In particular, a LIDAR system that conforms to the second aspect of the invention may have at least one of the following characteristics: It can be adapted to implement a heterodyne detection mode; it can be of a split-pulse type, in which the distance from the target(s) to be characterized is evaluated from a time between the emission of a radiation pulse towards that target(s) and the detection of a portion of the radiation pulse that has been reflected or backscattered by the target(s); it can include a laser source to produce the radiation emitted towards the target(s) to be characterized; and it can be adapted to measure at least one atmospheric quantity, in particular wind speed, from a portion of the radiation emitted by the LIDAR system, this portion of radiation being backscattered by particles suspended in the atmospheric air and then detected by the LIDAR system. Brief description of the figures

[0036] The features and advantages of the present invention will become clearer in the following detailed description of non-limiting examples of implementation, with reference to the accompanying figures, among which: [ Fig. 1a ] is a synoptic diagram of a first type of LIDAR system to which the invention can be applied; [ Fig. 1b ] corresponds to [ Fig. 1a ] for a second type of LIDAR system to which the invention can also be applied; [ Fig. 2 ] illustrates a possible use of a type of optical circulator within a LIDAR system to implement the invention; [ Fig. 3a] to [Fig. 3d ] are four optical assembly diagrams that can be used for an improvement of the invention; and [ Fig. 4 ] is a synoptic diagram of the process of the invention. Detailed description of the invention

[0037] For clarity, the dimensions of the elements shown in these figures do not correspond to actual dimensions or ratios of actual dimensions. Furthermore, the elements shown are only symbolic; it is understood that components not directly related to the invention, or that can be spontaneously adapted by a person skilled in the art, are neither shown nor described. Finally, identical reference numerals shown in different figures designate identical elements or elements with identical functions.

[0038] The invention is now described, by way of example, for a monostatic, pulsed, heterodyne-detecting LiDAR system. However, the invention can be readily applied, from the following description, to other types of LiDAR systems, particularly continuous-emission LiDAR systems, and to detection modes other than heterodyne detection. Generally, the invention relates to the conversion of measurement values ​​delivered by a LiDAR system into distance values. Each measurement value represents a time interval, measured by the LiDAR system, between the emission of radiation by that system and its subsequent detection. However, this measurement value can be a time shift, an electrical voltage, a frequency shift, etc., depending on the detection mode and the type of signal processing used in each LiDAR system.Furthermore, as examples, the LIDAR systems described below are suitable for performing anemometric measurements, using backscattering laser radiation produced by particles suspended in the air. These backscattering particles, which constitute the target to be characterized, are designated by reference 100 in [. Fig. 2 ].

[0039] In accordance with [ Fig. 1aThe LIDAR system 10 comprises a transmission channel 10E and a detection channel 10D. The transmission channel 10E includes a laser source 1, which produces monochromatic radiation with a wavelength of 1545 nm (nanometers) when propagating through air; at least one acousto-optic modulator 2, denoted AOM, controlled to generate successive pulses from the laser radiation produced by the source 1; and an optical amplifier 3, denoted AMPL. The acousto-optic modulator 2 further produces a frequency shift in the laser radiation emitted by the LIDAR system 10 toward a target relative to the laser radiation produced by the source 1. In this way, a heterodyne detection signal delivered by the detection channel 10D has a non-zero beat frequency when the radiation is back-reflected by a stationary target.The detection channel 10D comprises a detector 7, labeled DETECT, and a data acquisition card 8, labeled ACQ. The data acquisition card 8 is connected to receive a heterodyne detection signal produced by the detector 7, to process this signal to derive a measurement value representative of the time between the emission of a laser radiation pulse by the emission channel 10E and the detection of a portion of this pulse as received by the detection channel 10D, and then to convert this measurement value into a distance value D representing the separation distance of the particles that backscattered the laser radiation. Optionally, a signal amplifier, not shown, may be used on the transmission link between the detector 7 and the data acquisition card 8.In accordance with the heterodyne detection mode, detector 7 receives as input a mixture of the portion of each pulse received by the detection channel 10D, with a portion of the emitted laser radiation sampled between the source 1 and the acousto-optic modulator 2. As is known, the portion of the emitted laser radiation transmitted to detector 7 from the emission channel 10E can undergo various intermediate transformations between its sampling in the emission channel 10E and detector 7. The acousto-optic modulator 2 and the acquisition board 8 are controlled by a controller 9, which is denoted CTRL and provides the system with its LIDAR operation.

[0040] When the LIDAR system 10 is of the monostatic type, the emission channel 10E and the detection channel 10D can be coupled via an optical circulator 4 to an optical port 5. This latter port acts both as an optical output port for radiation to the outside of the LIDAR system for the emission channel 10E and as an optical detection port for the detection channel 10D, to collect backscattered radiation from outside the LIDAR system. Thus, the emission channel 10E is optically coupled to an optical input port 41 of the optical circulator 4, the detection channel 10D is optically coupled to an optical output port 42 of the optical circulator 4, and the optical port 5 of the LIDAR system 10 is optically coupled to a mixed optical input-output port 43 of the optical circulator 4.

[0041] In an alternative configuration of system 10, which is illustrated by [ Fig. 1bThe optical output port 5a of the transmit channel 10E and the optical detection port 5b of the detection channel 10D can be separated. No optical circulator is then used between the transmit channel 10E and the detection channel 10D. The optical output port 5a is configured to transmit radiation from the transmit channel 10E to the outside of the LIDAR system, and the optical detection port 5b is configured to collect backscattered radiation from outside the LIDAR system. When the optical ports 5a and 5b are contiguous or sufficiently close to each other, the LIDAR system 10 is still of the monostatic type.

[0042] The invention described herein relates in particular to the conversion, performed within the acquisition card 8, of a measurement value representing the time between the emission of a pulse and its detection after backscattering, into a value D representing the relocation distance of the particles that produced the backscattering. This conversion requires a calibration step, which is carried out according to the invention by coupling a reference optical path to the LIDAR system 10. This reference optical path comprises an optical fiber 20 of known length, denoted L, and can have several configurations. It is incorporated into the LIDAR system 10, for example, by being housed within a casing therein.

[0043] According to a first possible configuration of the reference optical path, which is represented in [ Fig. 1aThe optical fiber 20 is coupled to the optical port 5 of the LIDAR system 10 by a first end of this optical fiber, designated by reference 21. The opposite end of the optical fiber 20, which is designated by reference 22, can be provided with a reflector 23 effective for the wavelength of the radiation emitted by the LIDAR system 10. In this case, the length of the reference optical path corresponds to a round trip of the radiation in the optical fiber 20. It is denoted D ref and is equal to 2 · L · n eff , where n eff denotes the effective index of the optical fiber 20 for the wavelength of the radiation emitted by the LIDAR system 10. In other words, the propagation speed of the radiation emitted by the LIDAR system 10 in the optical fiber 20 is C / n eff , where C is the free propagation speed of the radiation in air.In the general part of this description, D ref has been called the free propagation distance reference value, "free propagation" denoting propagation of radiation in air, as opposed to guided propagation of radiation inside optical fiber 20. The wavelength of radiation denotes its spatial period when it propagates freely in air.

[0044] According to a second possible configuration of the reference optical path, represented in [ Fig. 1bThe optical fiber 20 is coupled to the optical output port 5a of the LIDAR system 10 by its end 21, and simultaneously coupled to the optical detection port 5b by its other end 22, without using a reflector 23. The end 21 performs the transmission of the radiation from the transmission channel 10E of the LIDAR system 10 to the optical fiber 20, and the end 22 performs the return transmission of the radiation from the optical fiber 20 to the reception channel 10R of the LIDAR system 10. For this second configuration, D ref = L · n eff , using the previous notations.

[0045] When the optical reference path includes additional components, the extra propagation lengths caused by these components can be accounted for by adjusting the previous expressions for the reference value of the free propagation distance Dref, to obtain an even more precise calibration. A person skilled in the art will be able to spontaneously adjust the previous expressions for Dref for this purpose.

[0046] For the two configurations just described, relating to monostatic LIDAR systems, the length L of the optical fiber 20 is preferably chosen so that the corresponding value of free propagation distance D ref is greater than 150 m. In this way, the part of the radiation that is retransmitted by the optical fiber 20 to the detection channel 10D of the LIDAR system 10 is temporally separated from a possible narcissus signal.

[0047] The optical fiber 20 is preferably arranged in the form of a compact reel, and such that its ends 21 and 22 are easily accessible.

[0048] For both configurations just described, the reference optical path can be equipped with an intensity attenuator 24 to prevent the calibration radiation, which propagates along the reference optical path instead of being backscattered by airborne particles, from saturating the detection channel 10D. The attenuator 24 can be constructed in various ways and have a variable attenuation level. For example, it could be an adjustable aperture iris. For such couplings to the optical port(s) of the LIDAR system 10, a removable or reversible coupling mode can be provided, so that the LIDAR system 10 is operational for useful measurements of airborne particles after calibration has been performed.

[0049] Furthermore, the optical reference path used to calibrate the LIDAR system 10 by distance does not need to be coupled to the same optical port(s) as those used for the target(s) to be characterized. For example, in a monostatic LIDAR system configuration such as that shown in [ Fig. 1aThe optical circulator 4 may have an additional optical port, designated by reference numeral 44. This additional optical port 44, which is a mixed input-output type, is not used for useful measurements performed on targets to be characterized. Therefore, the end 21 of the optical fiber 20 of the reference optical path, when this path consists of a round trip of radiation in the optical fiber 20, may be connected to the additional optical port 44 of the optical circulator 4. When such a configuration is used for the reference optical path and for its coupling to the LIDAR system 10, the reference optical path may further include means for controlling the polarization of the radiation.These polarization control means (not shown) ensure that radiation entering this optical circulator 4 through optical port 41 exits through the additional optical port 44, and that radiation entering through this additional optical port 44 exits through optical port 42, when optical port 43 is momentarily inhibited. As is known, such polarization control means can alternatively be integrated into the optical circulator 4, rather than into the reference optical path.

[0050] Furthermore, it may be preferable to optically couple the end 21 of the optical fiber 20 to the emission path 10E downstream of the optical amplifier 3, so that distortions which could be produced by the optical amplifier 3 affect identically radiation pulses which are transmitted to the optical fiber 20 and other pulses which are emitted in the direction of a target to be characterized.

[0051] [ Fig. 2 ] shows a possible implementation of the optical circulator 4 which is used in a LIDAR system 10 conforming to [ Fig. 1aIt also shows a possible implementation of this optical circulator 4 in which the optical fiber 20 is permanently coupled to the additional optical port 44. The optical circulator 4 comprises a four-sided polarizing cube, which constitutes the optical ports 41-44, respectively. The radiation to be emitted by the LIDAR system 10 comes from the optical amplifier 3. It has a primary component C1 with vertical linear polarization, intended to be transmitted towards the target to be characterized, designated by the reference 100. It also has a secondary component C2 with horizontal linear polarization, intended to be transmitted to the reference optical path.As is known, the intensity ratio between components C1 and C2 can be adjusted by rotating the polarizing cube with respect to a linear polarization direction of the laser radiation as originating from the optical amplifier 3, or with respect to a linear polarizer (not shown) which can be located between the optical amplifier 3 and the optical circulator 4. Both components C1 and C2 enter the polarizing cube through the face that constitutes the optical port 41. The principal component C1, with vertical linear polarization, is transmitted without deviation through the opposite face of the polarizing cube, which constitutes the optical port 43. A quarter-wave plate 45, which is located between the optical port 43 of the polarizing cube and the optical port 5 of the LIDAR system 10, transforms the vertical linear polarization of this principal component C1 into a left-handed circular polarization.After back-reflection or back-scattering on the target 100, part of the radiation from the main component C1 has a right-hand circular polarization, which is transformed into horizontal linear polarization by the quarter-wave plate 45, and which re-enters the polarizing cube through the optical port 43. It is then reflected within the polarizing cube to exit through the face that constitutes the optical port 42, in the direction of the detector 7. Simultaneously, the secondary component C2, with horizontal linear polarization, after also entering the polarizing cube through its face that constitutes the optical port 41 of the optical circulator 4, is reflected within the polarizing cube to exit through the face that constitutes the additional optical port 44.The end 21 of the optical fiber 20 of the reference optical path is coupled to this optical port 44, and its opposite end 22 is coupled to the optical input of the detection channel 10D, in the direction of the detector 7. The secondary component C2 thus propagates selectively in the reference optical path. Under these conditions, when the distance to the target 100 is different from the free propagation distance Dref of the reference optical path, the heterodyne detection signal delivered by the detector 7 has two separate contributions: a first contribution produced by the primary component C1, which allows the target 100 to be characterized during a useful measurement sequence, and a second contribution produced by the secondary component C2, which allows the calibration method of the invention to be applied simultaneously with the useful measurement sequence.

[0052] In general, the coupling of at least one of the ends 21 and 22 of the optical fiber 20 to the transmit channel 10E or the detect channel 10D, respectively, of the LIDAR system 10 for its range calibration, can be removable. In other words, the optical reference path can be designed to be optically coupled or decoupled from the transmit channel 10E and the detect channel 10D of the LIDAR system 10, reversibly according to a suitable control to switch between a calibration sequence and a measurement sequence useful for the LIDAR system. Alternatively, when the optical fiber 20 is coupled to the transmit channel 10E and the detect channel 10R of the LIDAR system 10 in parallel with the optical path to and from the target 100, its coupling can be permanent so that it is possible to repeat the calibration procedure at each measurement sequence useful for characterizing the target.In the case of such permanent coupling, it can be advantageously designed so that a main part, for example more than 90%, or even more than 99%, of the radiation which is emitted from the emission path 10E at each measurement sequence is emitted towards the target 100, and that a minor complementary part of this radiation is transmitted to the optical reference path.

[0053] The calibration sequence of the LIDAR 10 system consists of obtaining a measurement value for the time between emission and detection of a pulse propagating along the reference optical path. It can be comprised of an operating sequence of the LIDAR 10 system, identical to an operating sequence implemented for a useful measurement on a target to be characterized. The result of the calibration sequence is a measured time value ΔTref, which is associated with the reference value of the free propagation distance Dref, as defined above according to the configuration of the reference optical path. Thus, when an optical fiber 20 is used for the calibration sequence, with a fiber length L corresponding to a reference value of 38 m for the free propagation distance Dref, a value close to 254 ns (nanoseconds) is obtained for the measured time ΔTref.The calibration process allows us to take into account, for useful subsequent measurements, the difference that exists between ΔT ref and the result of the quotient of D ref by C. The measurement value that is representative of the duration ΔT ref has been called the measurement reference value in the general part of this description.

[0054] A useful measurement sequence performed to characterize a target, for example to measure the speed of movement of a diffuse collection of particles suspended in air, provides another measured time value ΔTmes. This latter has been called the measurement value in the general part of this description, and is obtained in step / 4 / . The useful measurement sequence is carried out by decoupling, if necessary, the optical reference path of the LIDAR 10 system, so that each laser pulse emitted by the LIDAR 10 is backscattered by the particles and then detected back by the LIDAR 10 system. Then, a value D can be obtained in step / 5 / for the distance of the set of particles from the LIDAR 10 system, by combining the measured time values ​​ΔT mes and ΔT ref with the reference value of free propagation distance D ref as follows: D = D ref + (ΔT mes - ΔT ref ) · C / 2.A residual error, which has been determined to be 0.02 m for a LIDAR system used as an example, can affect the distance calibration of the LIDAR system 10, when this calibration is performed and then used for useful measurement sequences in the way just described.

[0055] The residual error of the calibration procedure just described may partly result from the fact that the optical path followed by the radiation inside the LIDAR system 10 is not the same outside the reference optical path between the calibration sequence and the useful measurement sequence, and / or that the free propagation distance value cannot be determined with sufficient accuracy by calculation from the length of the optical fiber 20. This is the case, for example, when the reference optical path has the configuration shown in [ Fig. 1a] is coupled to the additional optical port 44 of the optical circulator 4, while the radiation is transmitted through the optical input-output port 43 of the optical circulator 4 and through the optical port 5 during a useful measurement sequence. To account for this source of error, the four [ Fig. 3a]-[Fig. 3d] can be performed and used to measure time differences Δt1 - Δt4. In these figures, reference numeral 11 designates a laser source which preferably has the same wavelength value as source 1 of the LIDAR system 10, and reference numeral 12 designates an optical coupler, for example of the 50%-50% evanescent wave type. Source 11 is connected to one input of the optical coupler 12, and the other input of the optical coupler 12 is not used. The optical reference path, which may consist in particular of components 20, 23 and 24, and any additional components, is the same as that used for step / 2 / with the LIDAR system 10 of [ Fig. 1aD1 and D2 are two photodetectors which, by comparing the detection signals they respectively produce, provide measurements of the time intervals separating a detection instant relative to photodetector D2 from a detection instant relative to photodetector D1, for portions of radiation arriving at these photodetectors respectively. For example, photodetectors D1 and D2 are fast or ultrafast photodiodes connected to the inputs of an oscilloscope. The optical port 42 of the optical circulator 4 is never used in these setups, and photodetector D1 is always optically coupled to the same first output of the optical coupler 12.

[0056] [ Fig. 3aFor this first setup, the optical input port 41 of the optical circulator 4 is coupled to a second output of the optical coupler 12, and the reference optical path is optically coupled to the optical port 43 of the optical circulator 4. The photodetector D2 is optically coupled to the additional optical port 44 of the optical circulator 4. The optical port 42 of the optical circulator 4 is inhibited. The time difference Δt1, which is thus measured, takes into account the propagation time of the radiation in the reference optical path.

[0057] [ Fig. 3b In this second setup, the two photodetectors D1 and D2 are directly optically coupled, one-to-one, to the two outputs of the optical coupler 12. The time difference Δt2 is thus measured. This second setup allows for consideration of any asymmetry that might exist between the two outputs of the optical coupler 12.

[0058] [ Fig. 3cThis third setup replicates the coupling of the optical circulator 4 to the optical coupler 12 as implemented in the first setup, but the reference optical path is removed and replaced by the photodetector D2 at the optical port 43 of the optical circulator 4. The optical ports 42 and 44 of the optical circulator 4 are inhibited. The time difference Δt 3 is thus measured.

[0059] [ Fig. 3d For this fourth configuration, the optical circulator 4 is reversed, as in the third configuration, and then coupled via its optical input / output port 43 to the second output of the optical coupler 12. The photodetector D2 is optically coupled to the additional optical port 44 of the optical circulator 4, and the latter's optical ports 41 and 42 are inhibited. The time difference Δt 4 is thus measured.

[0060] The reference value for free propagation distance Dref to be associated with the measurement reference value ΔTref to establish the calibration of the LIDAR system 10 is then Dref = C · (Δt1 + Δt2 - Δt3 - Δt4). This calibration is to be used to calculate the separation distance D during a useful measurement sequence, instead of the result of a calculation of Dref from the length L of the optical fiber 20 as mentioned above. It applies when the configuration of [ Fig. 1aThe optical path is used with the reference optical path, which is coupled to the additional optical input / output port 44 of the optical circulator 4 for the calibration sequence, and the optical output port 5 of the LIDAR system 10, which is coupled to the optical input / output port 43 of the optical circulator 4 for the useful measurement sequence. The correction thus applied to the reference free propagation distance value Dref is approximately 0.02 ns, corresponding to a free propagation distance deviation of approximately 4 mm (millimeters).

[0061] Furthermore, the heterodyne detection signal obtained during a calibration sequence performed according to the invention by coupling the reference optical path to the LIDAR system 10 constitutes a time-domain image of the shape of the radiation pulse, as received after propagation in the reference optical path. Its pulse envelope shape can be characterized, for example, by a full width at half maximum (FWHM) value and a -30 dB width, denoted I30. The values ​​FWHM = 75 ns and I30 = 120 ns were thus obtained for a radiation pulse that propagates freely over approximately 22.5 m when considering its full width at half maximum, and over approximately 36 m at -30 dB.Assuming that the optical reference path does not introduce significant distortion of each pulse, such characterization of the pulse envelope shape makes it possible to verify whether the operation of the LIDAR 10 system is correct.

[0062] The heterodyne detection signal obtained during a calibration sequence performed according to the invention by coupling the reference optical path to the LIDAR 10 system also allows for the characterization of a frequency shift of the pulse as detected after propagation in the reference optical path, relative to the pulse as produced by the emission channel of the LIDAR 10 system. A frequency shift of 3 MHz (megahertz) was thus measured as an example, for a radiation wavelength of 1545 nm. Such a frequency shift can be caused by one or more components of the LIDAR 10 system exhibiting variable behavior depending on the wavelength, thereby altering the shape of the pulses, including by modifying their central wavelength.Such frequency-shift calibration is particularly useful for anemometric measurements, to separate the Doppler effect contribution from the internal contribution of the LIDAR system 10 in the measured frequency shifts.

[0063] Finally, when the optical reference path is equipped with a variable attenuator 24, the characterization of the radiation pulse's envelope shape can be repeated for successive adjustments of the attenuator 24, corresponding to progressively lower intensity attenuation levels. When the attenuation level is sufficiently high, no saturation occurs in the 10D detection channel of the LIDAR system 10, so such potential saturations do not alter a measurement result concerning a characteristic of the target that might depend on the envelope shape. Conversely, when the attenuation level is decreased gradually and to a sufficient extent, saturations caused by various components of the 10D detection channel of the LIDAR system 10 modify the envelope shape of the detected pulse.This change in envelope shape can alter measurement results for certain target characteristics, such as those produced by the LIDAR system. For example, a change in envelope shape can alter a target velocity measurement result when this result is derived from a frequency analysis of the heterodyne detection signal combined with a Doppler effect calculation. Understanding the envelope deformation caused by the LIDAR system's detection path, when the intensity of the radiation backscattered by the target is too high, allows for at least partial correction of such an error in the target velocity measurement result.Furthermore, such a characterization of the operation of the detection channel 10D can be advantageously used to provide a maximum limit for the intensity of the optical signal which is received by the detector 7, below which saturation is avoided.

[0064] These characterizations of envelope shape, frequency shift and envelope deformation make it possible to verify the operation of the LIDAR system from the calibration stage, as proposed by the invention, in a way that is particularly easy, fast and economical compared to a LIDAR system control session that would be carried out in a workshop.

[0065] [ Fig. 4] summarizes the principle of the invention. Providing the reference optical path with the optical fiber 20 of length L yields the reference value of free propagation distance Dref, either by calculation or by using optical setups similar to those in the figures [ Fig. 3a]-[Fig. 3dObtaining the reference value Dref is step / 3 / of the calibration process of the invention. Furthermore, the reference measurement value ΔTref is obtained during the execution of an operating sequence of the LIDAR 10 system dedicated to its calibration, with the reference optical path optically coupled to the LIDAR 10 system. This obtaining of the reference value ΔTref, which is independent of obtaining Dref, constitutes steps / 1 / and / 2 / of the calibration process of the invention. Finally, executing an operating sequence of the LIDAR 10 system by pointing it at a target without using the reference optical path constitutes a useful measurement MES. This useful measurement sequence provides the measurement value ΔTmes and corresponds to step / 4 / .Finally, the combination of the ΔT mes value with the reference values ​​ΔT ref and D ref provides the result for the target distance value D, corresponding to step / 5 / introduced in the general part of this description.

[0066] It is understood that the invention can be reproduced by modifying minor aspects of the embodiments described above, while retaining at least some of the advantages mentioned. In particular, although the invention has been described in detail for monostatic LIDAR systems, it can also be applied to bistatic LIDAR systems. Furthermore, all the numerical values ​​cited are for illustrative purposes only and may be changed depending on the application.

Claims

1. A method for calibrating a LIDAR system (10), in order to provide a measurement reference value (ΔTref) which corresponds to a free-space propagation distance reference value (Dref), said measurement and free-space propagation distance reference values being intended to be used for calibration of the LIDAR system in order to evaluate a separating distance (D) of at least one target (100) to be characterized using said LIDAR system, the method comprising the following steps: / 1 / coupling the LIDAR system (10) to a reference optical path which is formed from at least one optical fiber (20) efficient for guiding radiation emitted by the LIDAR system during an operation of said LIDAR system, so that at least a portion of the radiation travels the reference optical path between an emission channel (10E) and a detection channel (10D) of the LIDAR system, the reference optical path having a known length; / 2 / carrying out a distance measurement sequence using the LIDAR system (10) coupled to the reference optical path as resulting from step / 1 / , in order to obtain a measurement value which is representative of a time of radiation propagation in the reference optical path, and which is intended to constitute the measurement reference value (ΔTref); then / 3 / converting the known length of the reference optical path into a free-space propagation distance value for the radiation, according to a principle of propagation time equality, to form the free-space propagation distance reference value (Dref), and associating said free-space propagation distance reference value with the measurement reference value (ΔTref) in order to constitute the calibration of the LIDAR system (10), the method being characterized in that the reference optical path formed from the at least one optical fiber (20) is incorporated within the LIDAR system.

2. The method according to claim 1, wherein the LIDAR system (10) is of a monostatic type, whereby optical output and detection apertures (5) of the LIDAR system are coincident, and the method further comprises the following preliminary step to be carried out before step / 1 / : / 0 / determining a distance limit value such that portions of the radiation emitted by the LIDAR system (10) which are reflected by components internal to said LIDAR system, correspond to times between emission and detection which are equivalent to those of targets located at less than this distance limit value in front of an output optical aperture (5; 5a) of the LIDAR system, the length of the reference optical path being selected such that the free-space propagation distance value (Dref) is greater than the distance limit value determined in step / 0 / .

3. The method according to claim 1 or 2, further comprising the following steps: / 4 / performing a measurement relating to said at least one target (100) to be characterized by directing a line of sight of the LIDAR system (10) towards said at least one target independently of the reference optical path, so as to collect another measurement value (ΔTmes), said another measurement value being representative of the separating distance (D) of said at least one target; then / 5 / calculating a value for the separating distance (D) of said at least one target (100) by combining said another measurement value (ΔTmes) with the measurement reference value (ΔTref) and with the free-space propagation distance reference value (Dref) which corresponds to the length of the reference optical path.

4. The method according to claim 3, wherein the reference optical path which is formed from the at least one optical fiber (20) is optically coupled to the LIDAR system (10) in a continuous manner during step / 4 / , so that a first portion of a radiation emitted by said LIDAR system is used to carry out the measurement relating to said at least one target (100), and so that a second portion of the same radiation is used simultaneously to obtain the measurement value representative of the propagation time in the reference optical path, and wherein the measurement reference value (ΔTref) which is composed of the time of propagation in the reference optical path as measured during step / 4 / is used in step / 5 / .

5. The method according to any one of the preceding claims, wherein the LIDAR system (10) is adapted to carry out measurements according to a principle of heterodyne detection, including to provide the measurement reference value (ΔTref) in step / 2 / , and where appropriate to provide in step / 4 / said another measurement value (ΔTmes) which is representative of the separating distance (D) of said at least one target (100) to be characterized.

6. The method according to any one of claims 1 to 5, wherein, in step / 1 / , a first end (21) of the optical fiber (20) of the optical reference path is coupled to the emission channel (10E) of the LIDAR system (10), and a second end (22) of said optical fiber, opposite to said first end, is coupled to the detection channel (10D) of the LIDAR system, so that at least a portion of the radiation emitted by the LIDAR system enters the optical fiber through said first end, then exits through said second end while being transmitted to the detection channel of the LIDAR system.

7. The method according to any one of claims 1 to 5, wherein the LIDAR system (10) is of a monostatic type; and the optical fiber (20) has a first end (21) and a second end (22), said second end being opposite to said first end and provided with a reflector (23) so that radiation which propagates in the optical fiber from the first end is retroreflected by the reflector at the second end, inside said optical fiber and toward the first end; and step / 1 / comprises coupling the LIDAR system (10) to the optical fiber (20) so that at least a portion of the radiation emitted by said LIDAR system (10) enters said optical fiber through the first end (21), and at least a portion of the retroreflected radiation that exits through said first end is transmitted to the detection channel (10D) of the LIDAR system.

8. The method according to claim 7, wherein the emission (10E) and detection (10D) channels of the LIDAR system (10) are coupled to an optical output aperture (5) of said LIDAR system by an optical circulator (4), and wherein the first end (21) of the optical fiber (20) of the reference optical path is optically coupled to an optical input-output aperture (43; 44) of the optical circulator (4), said optical circulator transmitting the radiation, during step / 2 / , from the LIDAR system to the optical fiber through the optical input-output aperture of the optical circulator, and also transmitting the radiation received by said optical input-output aperture of the optical circulator and coming from the optical fiber, to the detection channel (10D) of the LIDAR system during step / 2 / .

9. The method according to any one of the preceding claims, wherein the free-space propagation distance reference value (Dref) is set in accordance with a differential assessment of delays in transmission, detection, and / or signal processing which are effective for the radiation of the LIDAR system (10), between emission and detection paths of said radiation which are used during step / 2 / , and other radiation emission and detection paths which are used to characterize said at least one target (100).

10. The method according to any one of the preceding claims, wherein the LIDAR system (10) is of a type with separate radiation pulses, for which the separating distance of said at least one target (100) to be characterized is evaluated in accordance with a time between an emission of a radiation pulse toward said at least one target to be characterized, and a detection of a portion of the radiation pulse which has been reflected or backscattered by said at least one target to be characterized.

11. The method according to claim 10, further comprising performing at least one among: - a pulse envelope characterization, relating to a radiation pulse emitted and then detected by the LIDAR system (10); - a characterization of a frequency shift, as appearing between the radiation pulse as emitted by the LIDAR system (10) and said radiation pulse as subsequently detected by said LIDAR system; and - a characterization of a deformation of the radiation pulse envelope, said deformation being caused by saturation of the detection channel (10D) of the LIDAR system (10), each of said characterizations being carried out based on a detection signal produced by the LIDAR system (10) during step / 2 / , for one or more successive executions of said step / 2 / .

12. The method according to any one of the preceding claims, wherein the reference optical path is provided with an intensity attenuator (24), preferably a variable attenuator, arranged to attenuate an intensity of the radiation transmitted by the optical fiber (20) to the detection channel (10D) of the LIDAR system (10).

13. The method according to any one of the preceding claims, wherein the reference optical path which is formed from said at least one optical fiber (20) is adapted to be reversibly coupled to the LIDAR system (10) in step / 1 / , so as to be able to be optically decoupled from said LIDAR system during measurement sequences useful for characterizing targets.

14. The method according to any one of the preceding claims, wherein the LIDAR system (10) is adapted for at least one of the following applications: - meteorological measurements, for example measurements of atmospheric turbulence; - measurements of atmospheric pollutant diffusion; - measurements of shear of an atmospheric flow, for example at an airport; - measurements of position and lifetime of at least one vortex present in a flow of fluid; - anemometric measurements performed from an aircraft in flight, in particular from an airship or drone; - anemometric measurements performed to optimize an operation of a wind turbine; and - anemometric measurements performed to adjust a flight of aircrafts in formation, or to adjust a flight of a drone.

15. A LIDAR system (10), provided with a reference optical path which is formed from at least one optical fiber (20) efficient for guiding radiation emitted by the LIDAR system during operation of said LIDAR system, so that at least a portion of the radiation travels the reference optical path between an emission channel (10E) and a detection channel (10D) of the LIDAR system, the LIDAR system being adapted to implement a calibration method which is in accordance with any one of the preceding claims, the LIDAR system (10) being characterized in that the reference optical path formed from the at least one optical fiber (20) is incorporated inside said LIDAR system.

16. The LIDAR system (10) according to claim 15, wherein the reference optical path is optically coupled to the emission channel (10E) and to the detection channel (10D) of the LIDAR system, in parallel with a path external to the LIDAR system which is intended to be followed by radiation emitted by said LIDAR system toward the at least one target (100), and useful for evaluating the separating distance (D) of said target, so that the radiation coming from said emission channel is transmitted simultaneously in the reference optical path and toward of the at least one target, and that a same detection sequence provides both the measurement reference value (ΔTref) and another measurement value (ΔTmes) that relates to the external path to the target.

17. The LIDAR system (10) according to claim 15 or 16, having at least one of the following features: - said LIDAR system (10) is adapted to implement a heterodyne detection mode; - said LIDAR system (10) is of a type with separate radiation pulses, for which the separating distance (D) of said at least one target (100) to be characterized is evaluated based on a time between an emission of a radiation pulse toward said at least one target to be characterized, and a detection of a portion of the radiation pulse which has been reflected or backscattered by said at least one target to be characterized; - said LIDAR system (10) comprises a laser source (1), for producing the radiation emitted toward said at least one target (100) to be characterized; and - said LIDAR system (10) is adapted to measure at least one atmospheric quantity, in particular a wind speed, based on a portion of the radiation emitted by the LIDAR system, this portion of the radiation being backscattered by particles in suspension in air, then detected by the LIDAR system.