LIDAR System Calibration

Abstract

A method for calibrating a LIDAR system (10), which proposes to incorporate a reference optical path formed by an optical fiber (20) in the LIDAR system. The length of the optical fiber determines a measurement reference value, which can then be used to evaluate the distance of a target to be characterized using the LIDAR system. The calibration method is simple and economical to implement. It can be used in particular for LIDAR systems designed to perform airspeed measurements especially on an aircraft.

Description

Technical Field

[0001] This specification relates to a method for calibrating a LIDAR system and a LIDAR system adapted to implement this calibration method. Background Art

[0002] LIDAR systems are used in many applications, some of which include measuring the separation distance of an object. Other applications include measuring an external quantity of at least one value of the separation distance of the location involved in the measurement, even if the measurement of this separation distance does not constitute the main objective of the application under consideration. This is the case for wind measurement, which is based on detecting a part of a laser beam backscattered by particles suspended in the air. The intensity of this part of the laser beam that is backscattered and then detected by the LIDAR system is thus extremely low. The particle velocity value is inferred from the spectral analysis of the backscattered part of the laser beam detected by the LIDAR system and then from the Doppler effect calculation of the frequency difference identified between this backscattered part of the laser beam and the laser beam emitted by the LIDAR system.

[0003] However, in a monostatic LIDAR system, residual reflections of the emitted laser beam occur on the optical components common to the emission and detection channels of the LIDAR system, even if these components are equipped with anti-reflection coatings. The radiation thus internally reflected into the LIDAR system is then detected by the detection channel. This reflected signal inside the LIDAR system is often referred to as "narcissus". It is often used to form a distance reference of zero or a low fixed value in order to calibrate the distance to be measured for the system, or the distance presented when measuring other quantities using the LIDAR system. However, the intensity of the emitted laser beam is high, and the detection sensitivity of the LIDAR system can be extremely high, especially when it is designed to detect the radiation backscattered by particles in the atmosphere. The narcissus signal thus has the following characteristics:

[0004] - It is generated by the reflection of the laser beam on several optical components common to the emission and detection channels of the LIDAR system, so it does not correspond to a single distance value where the reflection occurs; and

[0005] - Its intensity is much higher than the common level of the intensity of the radiation backscattered by atmospheric particles, such that it causes the detector of the LIDAR system designed to detect the radiation due to backscattered atmospheric particles to saturate.

[0006] For these reasons, the Narcissus signal cannot provide an accurate distance reference. Additionally, for a monostatic LIDAR system, it hinders the satisfactory detection of radiation generated by retroreflection or backscattering on targets at short distances from the LIDAR system, typically at distances between 20 m (meters) and 150 m. The region extending from the output of the monostatic LIDAR system to the minimum distance for which the measurement is inhibited by the Narcissus signal is thus commonly referred to as the blind zone.

[0007] Another method for distance calibration of a LIDAR system 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 on this target. However, this method is tedious to implement and thus cannot be repeated frequently. Additionally, when the LIDAR system is of the pulsed type, this calibration must be performed using pulses similar to those used for an effective measurement sequence. For longer pulses, such as pulses with an individual duration greater than 0.5 μs (microseconds), this requires placing the target at a distance of more than 75 m from the LIDAR system, which is very complex in practice.

[0008] Finally, document US 5,825,464 proposes a calibration device for a LIDAR system, which is external to the LIDAR system to be calibrated and optically couples an integrating sphere to the LIDAR system.

[0009] Technical problem

[0010] Based on this situation, an object of the present invention is to make it possible to perform distance calibration of a LIDAR system more easily and precisely than using previous methods.

[0011] More precisely, for a monostatic LIDAR system, an object of the present invention is to achieve distance calibration that is not hindered by the Narcissus signal.

[0012] Another object of the present invention is to allow this calibration to be repeated as often as needed without having to place a target at a determined distance in front of the LIDAR system.

[0013] Yet another object of the present invention may be to allow distance calibration to be performed whenever an effective measurement sequence is carried out, even simultaneously with this effective measurement sequence. Summary of the Invention

[0014] To achieve at least one of these or other objectives, a first aspect of the present invention proposes a new method for calibrating a LIDAR system to provide measurement reference values corresponding to free space propagation distance reference values, which measurement reference values and free space propagation distance reference values are established for the calibration of the LIDAR system in order to evaluate the distance of at least one target to be characterized using the LIDAR system. The method of the present invention includes the following steps:

[0015] / 1 / Coupling the LIDAR system to a reference optical path formed by at least one optical fiber that effectively guides the radiation emitted by the LIDAR system during operation of the system, such that at least a portion of the radiation travels between the emission and detection channels of the LIDAR system along the reference optical path, and the reference optical path has a known length;

[0016] / 2 / Performing a distance measurement sequence using the LIDAR system coupled to the reference optical path generated in step / 1 / to obtain a measurement value representing the time of radiation propagation in the reference optical path, and the measurement value is established to constitute a measurement reference value; then

[0017] / 3 / Converting the known length of the reference optical path into a free space propagation distance value of the radiation according to the principle of equal propagation time to form a free space propagation distance reference value, and associating this free space propagation distance reference value with the measurement reference value to constitute the calibration of the LIDAR system.

[0018] Thus, the method of the present invention uses a reference optical path to be optically coupled to the LIDAR system. This reference optical path based on at least one optical fiber physically determines the free space propagation distance reference value. This free space propagation distance reference value is thus known to have a high level of accuracy. With this calibration method, the distance values determined from the operation of the LIDAR system for the target to be characterized can have improved accuracy and high precision.

[0019] Generally, the method of the present invention is compatible with LIDAR systems of the monostatic or bistatic type. In the context of the present invention, monostatic should be understood to mean a LIDAR system in which the optical output and detection aperture of this LIDAR system coincide, are adjacent to, or are close to each other compared to the separation distance of the target to be characterized. In contrast, bistatic means a LIDAR system in which its optical output and detection aperture are separated.

[0020] Also, typically, a LIDAR system may be adapted to characterize a single target or a set of scattered multiple targets distributed in a spatial zone during each measurement operation of the LIDAR system. Specifically, the present invention can be applied to a LIDAR system that is designed to perform a measurement of wind force by implementing the laser radiation emitted by the LIDAR system at each measurement cycle, and a part of the laser radiation is backscattered by particles suspended in the air. These particles can be aerosols, dust, ice particles, etc., which are suspended in the atmosphere and together form the target to be characterized. The separation distance of the target is thus the distance from the particle-containing zone involved in the measurement result, such as the measurement of wind force.

[0021] Depending on the configuration of the reference optical path relative to the optical fiber used to form it, the length of this reference optical path can be associated with the length of the optical fiber or twice the length of the optical fiber. In addition, in step / 3 / , the conversion of the length of the reference optical path to the free-space propagation distance of the radiation takes into account the difference in the propagation speed of the radiation emitted by the LIDAR system, which exists between the propagation of the radiation in the form of a guided wave in the optical fiber and the propagation of the radiation in free space.

[0022] Advantageously, the optical fiber can have a certain length such that when the system is of the monostatic type, the free-space propagation distance corresponding to the length of the reference optical path is greater than any blind zone of the LIDAR system. For this purpose, the method of the present invention can further include the following preparatory steps, which will be performed before step / 1 / when the LIDAR system is of the monostatic type, whereby the optical output and the detection aperture of the LIDAR system are coincident:

[0023] / 0 / Determine a distance limit value such that the portion of the radiation emitted by the LIDAR system reflected by the components inside the LIDAR system corresponds to the time between the emission and detection of a target equivalent to being located less than this distance limit value in front of the optical output aperture of the LIDAR system.

[0024] The components inside the LIDAR system that are likely to partially reflect the emitted radiation (i.e., to generate the Narcissus signal) can be the exit window, the polarization control component, one or more focusing lenses, etc. The minimum distance limit value that can be determined in this way is the length of the blind zone. The length of the reference optical path can thus preferably be selected such that the free-space propagation distance value is greater than the distance limit value determined in step / 0 / .

[0025] For example, the length of the optical fiber can be such that the free-space propagation distance reference value is greater than 150 m.

[0026] According to the present invention, a reference optical path formed by the at least one optical fiber is incorporated inside the LIDAR system. It is thus available within the LIDAR system for performing or repeating the calibration of the system as often as needed during the lifetime of the LIDAR system, each time using the method of the present invention.

[0027] The reference optical path formed by the at least one optical fiber may be adapted to be optically coupled to the LIDAR system in a reversible manner in step / 1 / so as to be able to be optically decoupled during a measurement sequence available for characterizing the target. Specifically, it may optionally be optically decoupled from the optical output aperture of the LIDAR system and may also be optically decoupled from the optical detection aperture of the LIDAR system.

[0028] Advantageously, the optical fiber for the reference optical path may be arranged in the form of a coil. The reference optical path is thus lightweight and compact, making it easy to integrate into the LIDAR system, including a LIDAR system located on a means of transport such as an aircraft, for example, to perform measurements for determining wind force.

[0029] Generally, the reference optical path may be reversibly and temporarily optically coupled to the LIDAR system by a controlled optical coupling component for the duration of the calibration process and then decoupled, possibly for an effective measurement sequence.

[0030] Alternatively, the reference optical path may remain continuously optically coupled to the LIDAR system during an effective measurement sequence in order to perform the calibration of the LIDAR system using a portion of the same radiation as that emitted by the LIDAR system to perform the effective measurement. Thus, this calibration is directly applied to the conditions for performing the effective measurement, especially to the radiation used for the measurement. Thus, the validity of the calibration is not called into question by any defect in the repeatability of the radiation emitted by the LIDAR system. This advantage is even more significant when the radiation is emitted in the form of pulses for the effective measurement, where some characteristics such as the envelope shape or the central optical frequency value of each pulse may vary randomly or drift progressively.

[0031] The method of the present invention may further include the following additional steps, which constitute an effective measurement sequence carried out based on the calibration method:

[0032] / 4 / Performing a measurement related to the at least one target to be characterized by directing the line of sight of the LIDAR system towards this at least one target independently of the reference optical path in order to collect another measured value representing the separation distance of the at least one target; and then

[0033] / 5 / Calculating the value of this separation distance by combining the so-called another measured value with a measurement reference value and a free-space propagation distance reference value corresponding to the length of the reference optical path.

[0034] Thus, in some possible embodiments of the present invention, the reference optical path formed by the at least one optical fiber can be optically coupled to the LIDAR system in a continuous manner during step / 4 / , such that a first portion of the radiation emitted by the LIDAR system thereby is used to perform measurements related to the at least one target, and such that a second portion of the same radiation is simultaneously used to obtain a measurement value representing the propagation time in the reference optical path. The measurement reference value constituted by the propagation time in the reference optical path measured during step / 4 / can thus be used in step / 5 / .

[0035] The LIDAR system to which the method of the present invention is applied can be adapted to perform measurements according to any principle, including time-of-flight measurements and measurements based on the level of radiation absorption, etc. However, the LIDAR system can preferably be adapted to perform measurements according to the principle of heterodyne detection, including providing a measurement reference value in step / 2 / , and also providing, if appropriate, another measurement value representing the separation distance of the at least one target to be characterized in step / 4 / .

[0036] According to a first possible configuration of the reference optical path, a first end of the optical fiber can be coupled to the emission channel of the LIDAR system in step / 1 / , and a second end of this optical fiber, opposite to its first end, can be coupled to the detection channel of the LIDAR system. Thus, at least a portion of the radiation emitted by the LIDAR system enters the optical fiber via its first end and then exits via its second end while being transmitted to the detection channel of the LIDAR system. This configuration of the reference optical path is compatible with both monostatic and bistatic types of LIDAR systems. For this first configuration of the reference optical path, the free-space propagation distance reference value is related to one times the length of the optical fiber.

[0037] According to another possible configuration of the reference optical path compatible with a monostatic LIDAR system, the optical fiber of the reference optical path can have a first end and a second end, the second end being opposite to the first end and provided with a reflector. The radiation propagating in the optical fiber from the first end is thus retroreflected inside the optical fiber and towards the first end by the reflector at the second end. Step / 1 / thus includes coupling the LIDAR system to the optical fiber such that at least a portion of the radiation emitted by the LIDAR system enters the optical fiber via its first end, and at least a portion of the retroreflected radiation exiting via the same first end is transmitted to the detection channel of the LIDAR system. For this other configuration of the reference optical path, the free-space propagation distance reference value is related to two times the length of the optical fiber.

[0038] When the LIDAR system is monostatic and the reference optical path is coupled to this LIDAR system only through the first end of the optical fiber and the second end has a reflector, the first end of the optical fiber can be optically coupled to the optical input-output aperture of the optical circulator of the LIDAR system, and this optical circulator also couples the transmission and detection channels of the LIDAR system to the optical output aperture of the LIDAR system. The optical circulator thus transmits radiation from the LIDAR system to the optical fiber via the optical input-output aperture of the optical circulator during step / 2 / , and also transmits the radiation received from the optical fiber and coming from the same optical input-output aperture of the optical circulator to the detection channel of the LIDAR system during the same step / 2 / . In this case, the reference optical path can also be adapted or provided with a radiation polarization control member so that the optical circulator is effective during step / 2 / . Alternatively, this polarization control member can be integrated into the optical circulator. The input-output aperture of the optical circulator for coupling the first end of the optical fiber in steps / 1 / and / 2 / can or can not equally be used in step / 4 / for the radiation used during this effective measurement sequence. When the reference optical path is coupled to an optical input-output aperture of an optical circulator different from the one used for transmitting radiation towards the target to be characterized during the effective measurement sequence, a new distance calibration of the LIDAR system can be performed simultaneously for each effective measurement sequence. Specifically, this embodiment of the present invention can be achieved by coupling the transmission and detection channels of the monostatic LIDAR system together by means of an optical circulator using a polarization cube. In this case, the two optical input-output apertures of the optical circulator formed by two different faces of the polarization cube can be dedicated, one dedicated to transmitting the main part of the radiation towards the target to be characterized, and the other dedicated to simultaneously transmitting the secondary part of the radiation to the reference optical path.

[0039] Preferably, the reference optical path can be coupled downstream to the emission channel in the propagation direction of the radiation in the emission channel of the LIDAR system, starting from an optical amplifier that is part of the emission channel. In this way, the distortion caused by the optical amplifier and affecting the radiation pulses used during each measurement sequence is the same between the radiation traveling along the reference optical path and the radiation retroreflected or backscattered by the target to be characterized. Therefore, these distortions are effectively taken into account in the method of the present invention in order to reduce the error that they may cause to the distance measurement results obtained for the target to be characterized.

[0040] According to an optional improvement of the present invention that can further improve the accuracy of distance calibration of a LIDAR system, a free-space propagation distance reference value can be set based on a differential evaluation of delays in transmission, detection, and / or signal processing that are effective for the radiation of the LIDAR system between the emission and detection paths of this radiation used during step / 2 / and the other radiation emission and detection paths used in step / 4 / for characterizing the at least one target.

[0041] The method according to the present invention can advantageously be used especially for LIDAR systems of the type with individual radiation pulses. For such a pulsed LIDAR system, the distance to this (these) target(s) is evaluated based on the time between the emission of a radiation pulse towards the at least one target to be characterized and the detection of the portion of this radiation pulse that has been reflected or backscattered by the target.

[0042] In this case of a pulsed LIDAR system, the method of the present invention can further include performing at least one of the following:

[0043] - Characterization of the pulse envelope associated with the radiation pulse emitted and then detected by the LIDAR system;

[0044] - Characterization of the frequency shift presented between the radiation pulse emitted by the LIDAR system and the same radiation pulse subsequently detected by the LIDAR system; and

[0045] - Characterization of the distortion of the radiation pulse envelope, which is caused by the saturation of the detection channel of the LIDAR system.

[0046] This or these additional characterizations can be carried out based on the detection signals generated by the LIDAR system during step / 2 / for one or more consecutive executions of this step / 2 / . The calibration method of the present invention can thus be supplemented to constitute a test for the proper operation of the LIDAR system and also calibrate the LIDAR system in terms of frequency shift. This frequency shift calibration makes it possible to more accurately measure the target velocity especially for measurements for determining wind force.

[0047] Generally, for the present invention, the reference optical path can have an intensity attenuator, which is arranged to attenuate the intensity of the radiation transmitted by the optical fiber to the detection channel of the LIDAR system. This intensity attenuator is preferably variable. It can especially include an iris diaphragm arranged at one end of the optical fiber.

[0048] And, generally, for the present invention, the LIDAR system can be adapted for at least one of the following applications:

[0049] - Meteorological measurements, such as measurements of atmospheric turbulence;

[0050] - Measurements of the dispersion of atmospheric pollutants;

[0051] - Measurement of the shear force of the atmosphere at an airport, for example;

[0052] - Measurement of the position and lifetime of at least one eddy current present in a fluid flow, in particular the measurement of the position of the eddy current core;

[0053] - Measurement of the wind force performed from an aircraft in flight, in particular from a spaceship or a drone;

[0054] - Measurement of the wind force performed to optimize the operation of a wind turbine; and

[0055] - Measurement of the wind force performed to adjust the flight of an aircraft in a formation or to adjust the flight of a drone.

[0056] Finally, when the LIDAR system is monostatic and adapted to focus the radiation emitted in each measurement sequence at the focal length in front of the optical output aperture of this LIDAR system, the sequence for measuring this focal length may include the following steps:

[0057] - Control the LIDAR system to emit at least one radiation pulse, and then collect the detection signal corresponding to the portion of the emitted radiation that has been backscattered at different distances and generated by the detection channel of the LIDAR system;

[0058] - Determine the detection instant corresponding to the maximum value of the signal-to-noise ratio of the collected detection signal; then

[0059] - Calculate the distance value associated with the determined detection instant using the free space propagation distance reference value corresponding to the length of the reference optical path, and assign the calculated distance value to the focal length.

[0060] To increase the accuracy of the value of the focal length thus determined, short pulses may preferably be used.

[0061] A second aspect of the invention relates to a LIDAR system having a reference optical path formed by at least one optical fiber that effectively guides the radiation emitted by this LIDAR system during operation. Thus, at least a portion of the radiation travels between the emission channel and the detection channel of the LIDAR system along the reference optical path. The LIDAR system is further adapted to implement the calibration method according to the first aspect of the invention, possibly with the enumerated improvements and optional additional features. Specifically, the reference optical path formed by the at least one optical fiber is incorporated inside the LIDAR system.

[0062] Preferably, the reference optical path can be optically coupled to the emission channel and the detection channel of the LIDAR system, parallel to a path external to the LIDAR system that the radiation established to be emitted by the system towards the at least one target follows and that can be used to evaluate the separation distance to this target. In this way, the radiation from the emission channel is transmitted simultaneously in the reference optical path and towards the at least one target, and the same detection sequence provides both a measurement reference value and another measurement value related to the external path to the target.

[0063] Specifically, the LIDAR system according to the second aspect of the present invention can have at least one of the following characteristics:

[0064] - It can be adapted to implement the heterodyne detection mode;

[0065] - It can be of the type with individual radiation pulses, for which the separation distance of the target to be characterized is evaluated based on the time between the emission of the radiation pulse towards this (these) target and the detection of the part of the radiation pulse that has been reflected or backscattered by the target;

[0066] - It can include a laser source for generating the radiation emitted towards the target to be characterized; and

[0067] - It can be adapted to measure at least one atmospheric quantity (especially the wind speed) based on a part of the radiation emitted by the LIDAR system, this part of the radiation being backscattered by particles suspended in the atmosphere and then detected by the LIDAR system. Description of the Drawings

[0068] With reference to the accompanying drawings, the features and advantages of the present invention will be more clearly understood from the following detailed description of some examples of non - restrictive embodiments, in which:

[0069]

Figure 1a

[0070]

Figure 1b

Figure 1a

[0071]

Figure 2

[0072]

Figure 3a

Figure 3d

[0073]

Figure 4

[0074] For clarity, the dimensions of the elements shown in these figures do not correspond either to actual dimensions or to actual dimensional ratios. Moreover, the elements shown are only represented symbolically, and it should be understood that components not shown or described which are not directly involved in the present invention or which can be adapted spontaneously by a person skilled in the art are not included. Finally, the same reference numerals indicated in different figures refer to the same or functionally equivalent elements.

[0075] The present invention is described by way of example with a monostatic LIDAR system of the pulsed type with heterodyne detection. However, based on the following description, the present invention can be easily adapted to other types of LIDAR systems, in particular continuous emission LIDAR systems, and detection modes other than heterodyne detection. Generally speaking, the present invention relates to converting the measurement values delivered by a LIDAR system into distance values. Each measurement value represents the time between the emission of the radiation carried out by this system as measured by the LIDAR system and its subsequent detection. However, this measurement value can be a time offset, a voltage, a frequency deviation, etc., depending on the detection mode and the type of signal processing used in each LIDAR system. Moreover, by way of example, the LIDAR system described below is suitable for carrying out measurements of the wind force by using the backscattering of laser radiation produced by particles suspended in the air. These backscattering particles constituting the target to be characterized are designated by the reference numeral 100 in

Figure 2

[0076] According to

【 Figure 1a, the LIDAR system 10 includes a transmitting channel 10E and a detection channel 10D. The transmitting channel 10E includes a laser source 1 that generates monochromatic radiation which may have a wavelength equal to 1545 nm (nanometers) when propagating in air; at least one acousto-optic modulator 2, denoted as MAO and controlled to form continuous pulses from the laser radiation generated by the laser source 1; and an optical amplifier 3, denoted as AMPL. The acousto-optic modulator 2 further produces a frequency shift of the laser radiation emitted by the LIDAR system 10 towards the target with respect to the laser radiation generated by the laser source 1. In this way, the heterodyne detection signal delivered by the detection channel 10D has a non-zero beat frequency when the radiation is retroreflected by a stationary target. The detection channel 10D includes a detector 7 denoted as DETECT, and an acquisition card 8 denoted as ACQ. The acquisition card 8 is connected to receive the electrical heterodyne detection signal generated by the detector 7, to apply processing to this signal so as to infer therefrom a measurement of the time between the emission of a pulse of the laser radiation via the transmitting channel 10E and the detection of the portion of this pulse received by the detection channel 10D, and then to convert this measurement into a distance value D to the particles that backscattered the laser radiation. It is possible that an electrical signal amplifier (not shown) may be used on the link for transmitting the electrical signal between the detector 7 and the acquisition card 8. According to the heterodyne detection mode, the detector 7 receives, as input, a mixture of the portion of each pulse that has been received by the detection channel 10D and the portion of the emitted laser radiation sampled between the laser source 1 and the acousto-optic modulator 2. In a known manner, the portion of the emitted laser radiation transmitted from the transmitting channel 10E to the detector 7 may undergo various intermediate transformations between its sampling in the transmitting channel 10E and the detector 7. The acousto-optic modulator 2 and the acquisition card 8 are controlled by a controller 9, denoted as CTRL and providing the LIDAR functions to the system.

[0077] When the LIDAR system 10 is of the monostatic type, the transmitting 10E and detection 10D channels may be coupled to an optical aperture 5 by an optical circulator 4, the aperture serving as an optical output aperture for the transmitting channel 10E for outputting the radiation outside the LIDAR system, and as an optical detection aperture for the detection channel 10D for collecting the backscattered radiation from outside the LIDAR system. The transmitting channel 10E is then optically coupled to the optical input aperture 41 of the optical circulator 4, the detection channel 10D is optically coupled to the optical output aperture 42 of the optical circulator 4, and the optical aperture 5 of the LIDAR system 10 is optically coupled to the combined optical input-output aperture 43 of the optical circulator 4.

[0078] In the case of Figure 1bIn an alternative configuration of the system 10 shown, the optical output aperture 5a of the emission channel 10E and the optical detection aperture 5b of the detection channel 10D can be separated. Thus, an optical circulator is not used between the emission 10E and detection 10D channels. The optical output aperture 5a is arranged to transmit radiation from the emission channel 10E outside the LIDAR system, and the optical detection aperture 5b is arranged to collect the backscattered radiation from outside the LIDAR system. When the optical apertures 5a and 5b are adjacent and close enough to each other, the LIDAR system 10 remains of the monostatic type.

[0079] The invention, which is the subject of the present specification, particularly relates to the conversion, implemented within the acquisition card 8, of a measured value of the time between the emission of a pulse and its detection after backscattering into a value D of the separation distance of the particles that produced the backscattering. This conversion requires a calibration step, which is implemented according to the invention by coupling a reference optical path to the LIDAR system 10. This reference optical path includes an optical fiber 20 of known length (denoted as L) and can have several configurations. It is incorporated into the LIDAR system 10, for example, by being housed within its housing.

[0080] According to Figure 1a a first possible configuration of the reference optical path shown in ref the optical fiber 20 is coupled to the optical aperture 5 of the LIDAR system 10 through a first end of this fiber (designated by the reference numeral 21). The opposite end of the optical fiber 20, designated by the reference numeral 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 the round trip of the radiation within the optical fiber 20. It is denoted as D eff and is equal to 2·L·n eff where n eff designates 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 within the optical fiber 20 is C / n ref where C is the free - space propagation speed of the radiation in air. Throughout the present specification, D

[0081] According to Figure 1bThe second possible configuration of the reference optical path shown in 【】, the optical fiber 20 is coupled to the optical output aperture 5a of the LIDAR system 10 through the end 21, and at the same time is coupled to the optical detection aperture 5b through its other end 22, without using the reflector 23. The end 21 implements the transmission of radiation from the emission channel 10E of the LIDAR system 10 to the optical fiber 20, and the end 22 implements the return transmission of radiation from the optical fiber 20 to the reception channel 10R of the LIDAR system 10. Using the above notation, for this second configuration, D ref = L·n eff .

[0082] When the reference optical path includes additional components, the additional propagation length caused by these additional components can be considered by adjusting the above expression of the free space propagation distance reference value D ref so as to obtain a more accurate calibration. Those skilled in the art will be able to spontaneously adjust the above expression of D ref for this purpose.

[0083] For the two configurations just described related to the monostatic LIDAR system, the length L of the optical fiber 20 is preferably selected such that the corresponding free space propagation distance value D ref is greater than 150 m. In this way, the part of the radiation retransmitted by the optical fiber 20 to the detection channel 10D of the LIDAR system 10 is separated in time from any possible Narcissus signal.

[0084] The optical fiber 20 is preferably arranged in the form of a compact coil and such that its ends 21 and 22 are easily accessible.

[0085] Still for the two configurations just described, the reference optical path can include an intensity attenuator 24 so as to prevent the radiation for calibration that propagates in the reference optical path and is not backscattered by the particles suspended in the air from causing saturation of the detection channel 10D. The attenuator 24 can be formed in various ways and has a variable attenuation level. For example, it can be an iris aperture with an adjustable aperture. For these couplings to the optical apertures of the LIDAR system 10, a removable or reversible coupling mode can be provided such that the LIDAR system 10 can operate to perform effective measurements on the particles present in the air after calibration has been carried out.

[0086] In addition, the reference optical path for distance calibration of the LIDAR system 10 does not have to be coupled to the same optical aperture as the optical aperture that can be used for one or more targets to be characterized. For example, in

Figure 1aIn the monostatic LIDAR system configuration shown in 【

[0087] Furthermore, the end 21 of the optical fiber 20 may preferably be optically coupled to the emission channel 10E downstream of the optical amplifier 3, such that the distortions that the optical amplifier 3 may generate will similarly affect the radiation pulses transmitted into the optical fiber 20 and the other pulses emitted in the direction of the target to be characterized.

[0088]

Figure 2

Figure 1aA possible embodiment of the optical circulator 4 used in the LIDAR system 10 of 【

[0089] In general, the coupling of at least one of the ends 21 and 22 of the optical fiber 20 used for the purpose of distance calibration of the LIDAR system to the emission channel 10E or the detection channel 10D of the LIDAR system 10, respectively, may be removable. In other words, the reference optical path may be designed to reversibly optically couple to / decouple from the emission 10E and detection 10D channels of the LIDAR system 10, according to commands suitable for switching between calibration sequences and active measurement sequences of the LIDAR system. Alternatively, when the optical fiber 20 is coupled to the emission 10E and detection 10R channels of the LIDAR system 10 in parallel with the optical path to and from the target 100, its coupling may be permanent, making it possible to repeat the calibration process for each active measurement sequence used to characterize the target. In the case of this permanent coupling, it may advantageously be designed so that at each measurement sequence the main part of the radiation from the emission channel 10E (for example, more than 90%, or even more than 99%) is emitted in the direction of the target 100, and a small complementary part of this radiation is transmitted to the reference optical path.

[0090] The calibration sequence of the LIDAR 10 system consists of obtaining a measurement of the time between the emission and the detection of a pulse propagating in a reference optical path. It may consist of an operation sequence of the LIDAR system 10 that is identical to the operation sequence as performed for a valid measurement of the target to be characterized. The result of the calibration sequence is a measurement time value ΔT ref , which is consistent with the free space propagation distance reference value D defined above according to the configuration of the reference optical path ref Therefore, when the optical fiber 20 is used for the calibration sequence (for the free space propagation distance reference value D ref , the fiber length L corresponds to the value 38m), at the measurement time ΔT ref The calibration method makes it possible to take ΔT into account for subsequent valid measurements. ref and D ref The difference between the result of the quotient of C. represents the time ΔT ref The measured value of is referred to as the measurement reference value in the entire part of this specification.

[0091] An effective measurement sequence carried out to characterize a target (eg, to measure the moving speed of a dispersed particle group suspended in air) provides another measurement time value ΔT mesThis value is referred to as the measured value throughout the present specification and is obtained in step / 4 / . An effective measurement sequence is implemented by decoupling the reference optical path from the LIDAR 10 system when necessary, such that each laser pulse emitted by the LIDAR 10 is backscattered by the particles and then detected by the LIDAR system 10 in turn. Subsequently, the value D of the separation distance between the particle group and the LIDAR system 10 can be obtained in step / 5 / by combining the measurement time values ΔT mes and ΔT ref with the free space propagation distance reference value D ref as follows: D = D ref +(ΔT mes -ΔT ref )·C / 2. When the distance calibration of the LIDAR system 10 is implemented and then used for the effective measurement sequence in the manner just described, a residual error determined to be equal to 0.02 m for the exemplary LIDAR system may affect this calibration.

[0092] The residual error of the calibration method just described may partly stem from the fact that the optical path followed by the radiation inside the LIDAR system 10 is different from the reference optical path outside during the calibration sequence and the effective measurement sequence, and / or the free space propagation distance value cannot be determined accurately enough by calculation based on the length of the optical fiber 20. For example, this is the case when the reference optical path with the configuration shown in

Figure 1a

Figure 3a

Figure 3d

[0093]

Figure 3a

[0094]

Figure 3b

[0095]

Figure 3c

[0096]

Figure 3d

[0097] To be associated with the measurement reference value ΔT ref to form the free-space propagation distance reference value D for the calibration of the LIDAR system 10 ref Thus, it is D ref= C·(Δt1 + Δt2 - Δt3 - Δt4). This calibration will be used to calculate the separation distance D during the effective measurement sequence, rather than calculating D based on the length L of the optical fiber 20 as mentioned above ref of the result. This applies when used in conjunction with the configuration of

Figure 1a

[0098] Furthermore, the heterodyne detection signal obtained during the calibration sequence performed by coupling the reference optical path to the LIDAR system 10 according to the present invention constitutes a temporal image of the shape of the radiation pulse, since this pulse is received after propagation in the reference optical path. Its pulse envelope shape can be characterized, for example, by the full width at half maximum value specified by FWHM and the width at -30 dB (decibels) denoted as l 30 . Thus, for a radiation pulse extending over approximately 22.5 m in free space propagation (when considering its full width at half maximum) and approximately 36 m at -30 dB (decibels), values of FWHM = 75 ns and l 30 = 120 ns are obtained. Assuming that the reference optical path does not introduce significant distortion of each pulse, this characterization of the pulse envelope shape allows verification of whether the LIDAR system 10 is operating correctly.

[0099] The heterodyne detection signal obtained during the calibration sequence performed by coupling the reference optical path to the LIDAR system 10 according to the present invention also makes it possible to characterize the frequency shift of the pulse as detected after propagation in the reference optical path, compared to the pulse generated by the emission channel of the LIDAR system 10. Thus, by way of example, a frequency shift of 3 MHz (megahertz) is measured for a radiation wavelength value equal to 1545 nm. This frequency shift may be caused by one or more components of the LIDAR system 10 having a variable behavior depending on the wavelength, and thus will change the shape of the pulse, including changing its central wavelength value. This frequency shift calibration is particularly applicable to measurements for determining wind speed, in order to separate the contribution from the Doppler effect from the contribution within the LIDAR system 10 in the measured frequency shift.

[0100] Finally, when the reference optical path includes a variable attenuator 24, the characterization of the envelope shape of the repetitive radiation pulses is carried out to achieve a continuous adjustment of the attenuator 24 corresponding to increasingly lower levels of intensity attenuation. When the attenuation level is high enough, saturation of the detection channel 10D of the LIDAR system 10 does not occur, such that these possible saturations do not modify the measurements related to the characteristics of the established target that may depend on the envelope shape. On the other hand, when the attenuation level is gradually decreased to a sufficient extent, saturation caused by the various components of the detection channel 10D of the LIDAR system 10 modifies the shape of the envelope of the detected pulses. This modification of the envelope shape may modify the measurements of certain characteristics of the target as generated by the LIDAR system 10. For example, when the velocity measurement of the target is inferred from the frequency analysis of the heterodyne detection signal in combination with the Doppler effect, the modification of the envelope shape may modify this result. When the intensity of the portion of the radiation retroreflected or backscattered by the target is too high, knowledge of the envelope deformation caused to the detection channel of the LIDAR system makes it possible to at least partially correct this error in the velocity measurement of the target. In addition, this characterization of the operation of the detection channel 10D can be advantageously used to provide the maximum value of the intensity of the optical signal received by the detector 7, below which saturation is avoided.

[0101] These characterizations of the envelope shape, frequency shift, and envelope deformation make it possible, as proposed by the present invention, to verify the operation of the LIDAR system, starting from the calibration step, in a particularly easy, fast, and economical manner compared to the LIDAR system verification periods practiced in the shops.

[0102]

Figure 4

Figure 3a

Figure 3d

[0103] It should be understood that the present invention can be reproduced by modifying minor aspects of the embodiments described above while maintaining at least some of the recited advantages. Specifically, although the present invention has been described in detail for a monostatic LIDAR system, it can also be applied to a bistatic LIDAR system. In addition, all of the recited numerical values are given for illustrative purposes only and can vary depending on the application considered.

Claims

1. A method for calibrating a LIDAR system (10) to provide a measurement reference value (ΔT ref ), the measurement reference value corresponding to a free space propagation distance reference value (D ref ), the measurement reference value and the free space propagation distance reference value being established for calibrating the LIDAR system to evaluate a separation distance (D) of at least one target (100) to be characterized using the LIDAR system, the method comprising the steps of: / 1 / Couple the LIDAR system (10) to a reference optical path formed by at least one optical fiber (20) that effectively guides the radiation emitted by the LIDAR system during operation of the LIDAR system, such that at least a portion of the radiation travels between the emission channel (10E) and the detection channel (10D) of the LIDAR system along the reference optical path, the reference optical path having a known length; / 2 / Perform a distance measurement sequence using the LIDAR system (10) coupled to the reference optical path generated by step / 1 / to obtain a measurement value representing the time of propagation of the radiation in the reference optical path, and The established measured values constitute the measurement reference value (ΔT ref ); then / 3 / Convert the known length of the reference optical path into a value of the free space propagation distance of the radiation according to the principle of equal propagation time, so as to form the free space propagation distance reference value (D ref ), and associate this free space propagation distance reference value with the measurement reference value (ΔT ref ) to constitute the calibration of the LIDAR system (10), The method is characterized in that the reference optical path formed by the at least one optical fiber (20) is incorporated within the LIDAR system such that the method for calibration is implemented simultaneously with an effective measurement sequence for characterizing the target to be characterized.

2. The method according to claim 1, wherein the LIDAR system (10) is of the monostatic type, whereby the optical output and the detection aperture (5) of the LIDAR system coincide, and the method further comprises the following preparatory steps to be carried out before step / 1 / : / 0 / Determine a distance limit value such that the portion of the radiation emitted by the LIDAR system (10) reflected by components within the LIDAR system corresponds to a time between emission and detection equivalent to that of a target located less than this distance limit value in front of the output optical aperture (5; 5a) of the LIDAR system. The length of the reference optical path is selected such that the free space propagation distance value (D ref ) is greater than the distance limit value determined in step / 0 / .

3. The method according to claim 1, further comprising the following steps: / 4 / Perform measurements related to the at least one target (100) to be characterized by directing the line of sight of the LIDAR system (10) towards the at least one target independently of the reference optical path in order to collect another measurement value (ΔT mes ), said another measurement value representing the separation distance (D) of the at least one target; then / 5 / by combining said another measurement value (ΔT mes ) with said measurement reference value (ΔT ref ) and with said free space propagation distance reference value (D ref ) corresponding to the length of the reference optical path to calculate the value of the separation distance (D) of the at least one target (100).

4. The method according to claim 3, wherein The reference optical path formed by the at least one optical fiber (20) is optically coupled to the LIDAR system (10) in a continuous manner during step / 4 / such that a first portion of the radiation emitted by the LIDAR system is used to perform measurements related to the at least one target (100), and such that a second portion of the same radiation is simultaneously used to obtain a measurement value representing the propagation time in the reference optical path, and wherein the measured reference value (ΔT ref ) consisting of the propagation time in the reference optical path measured during step / 4 / is used in step / 5 / .

5. The method according to claim 3, characterized in that, The LIDAR system (10) is adapted to perform measurements according to the principle of heterodyne detection, including providing the measurement reference value (ΔT ref ) in step / 2 / , and, where appropriate, providing the other measurement value (ΔT mes ) representing the separation distance (D) of the at least one target (100) to be characterized in step / 4 / .

6. The method according to claim 1, 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 the fiber opposite the first end is coupled to the detection channel (10D) of the LIDAR system such that at least a portion of the radiation emitted by the LIDAR system enters the fiber via the first end and then exits via the second end while being transmitted to the detection channel of the LIDAR system.

7. The method according to claim 1, characterized in that The LIDAR system (10) is of the monostatic type; and The optical fiber (20) has a first end (21) and a second end (22), the second end being opposite the first end and provided with a reflector (23) such that the radiation propagating in the fiber from the first end is retroreflected inside the fiber and towards the first end by the reflector at the second end; and Step / 1 / includes coupling the LIDAR system (10) to the optical fiber (20) such that at least a portion of the radiation emitted by the LIDAR system (10) enters the optical fiber via the first end (21), and at least a portion of the retro-reflected radiation exiting via the first end is transmitted to the detection channel (10D) of the LIDAR system.

8. The method according to claim 7, wherein The emission channel (10E) and the detection channel (10D) of the LIDAR system (10) are coupled to the optical output aperture (5) of the 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 the optical input-output aperture (43; 44) of the optical circulator (4), and the optical circulator transmits the radiation from the LIDAR system to the optical fiber via the optical input-output aperture of the optical circulator during step / 2 / , and also transmits the radiation received from the optical fiber by the optical input-output aperture of the optical circulator to the detection channel (10D) of the LIDAR system during step / 2 / .

9. The method according to claim 1, characterized in that, Based on a differential evaluation of delays in the transmission, detection, and / or signal processing of radiation effective for the LIDAR system (10) between the emission and detection path of the radiation used during step / 2 / and other radiation emission and detection paths for characterizing the at least one target (100), the free space propagation distance reference value (D ref ) is set.

10. The method according to claim 1, characterized in that, The LIDAR system (10) is of the type having individual radiation pulses, and for this type, the separation distance of the at least one target (100) to be characterized is evaluated based on the time between the emission of the radiation pulse towards the at least one target to be characterized and the detection of the portion of the radiation pulse that has been reflected or backscattered by the at least one target to be characterized.

11. The method according to claim 10, further comprising performing at least one of the following operations: - Pulse envelope characterization related to the radiation pulses emitted by the LIDAR system (10) and then detected; - Characterization of the frequency shift presented between the radiation pulses emitted by the LIDAR system (10) and the radiation pulses subsequently detected by the LIDAR system; and - Characterization of the deformation of the radiation pulse envelope caused by saturation of the detection channel (10D) of the LIDAR system (10), for one or more consecutive executions of step / 2 / , performing each of the characterizations based on the detection signals generated by the LIDAR system (10) during step / 2 / .

12. The method according to claim 1, wherein The reference optical path includes an intensity attenuator (24) arranged to attenuate the intensity of the radiation transmitted from the optical fiber (20) to the detection channel (10D) of the LIDAR system (10).

13. The method according to claim 1, characterized in that The reference optical path formed by the 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 the LIDAR system during a measurement sequence available for characterizing the target.

14. The method according to claim 1, wherein The LIDAR system (10) is suitable for at least one of the following applications: - Meteorological measurements; - Measurements of the dispersion of atmospheric pollutants; - Measurements of the shear force of the atmospheric flow; - Measurements of the position and lifetime of at least one eddy present in the fluid flow; - Wind measurement performed from an aircraft or drone in flight; - Wind measurement performed to optimize the operation of a wind turbine; and - Wind measurement performed to adjust the flight of an aircraft in a formation or to adjust the flight of a drone.

15. The method according to claim 12, wherein The intensity attenuator is a variable attenuator.

16. The method according to claim 14, wherein The meteorological measurement is a measurement of atmospheric turbulence.

17. The method according to claim 14, wherein The aircraft is an airship.

18. The method according to claim 14, wherein The measurement of the shear force of the atmospheric flow is a measurement of the shear force of the atmospheric flow at an airport.

19. A LIDAR system (10) comprising a reference optical path formed by at least one optical fiber (20) that effectively guides the radiation emitted by the LIDAR system during operation of the LIDAR system, such that at least a portion of the radiation travels between the emission channel (10E) and the detection channel (10D) of the LIDAR system along the reference optical path, the LIDAR system being adapted to implement the method according to any one of the preceding claims 1-18, characterized in that the reference optical path formed by the at least one optical fiber (20) is incorporated inside the LIDAR system such that the LIDAR system (10) is arranged and / or configured to implement the method while performing an effective measurement sequence for characterizing a target to be characterized.

20. The LIDAR system (10) according to claim 19, characterized in that, The reference optical path is optically coupled to the emission channel (10E) and the detection channel (10D) of the LIDAR system, parallel to a path external to the LIDAR system that the radiation established to be emitted by the LIDAR system towards the at least one target (100) follows and that can be used to evaluate the separation distance (D) of the target, such that the radiation from the emission channel is transmitted simultaneously in the reference optical path and towards the at least one target, and the same detection sequence provides both the measurement reference value (ΔT ref ) and another measurement value (ΔT mes ) related to the external path to the target.

21. The LIDAR system (10) according to claim 19, having at least one of the following characteristics: - The LIDAR system (10) is adapted to implement a heterodyne detection mode; - The LIDAR system (10) is of the type having separate radiation pulses, for which the separation distance (D) of at least one target (100) to be characterized is evaluated based on the time between the emission of a radiation pulse towards the at least one target to be characterized and the detection of the portion of the radiation pulse that has been reflected or backscattered by the at least one target to be characterized; - The LIDAR system (10) includes a laser source (1) for generating radiation emitted towards the at least one target (100) to be characterized; and - The LIDAR system (10) is adapted to measure at least one atmospheric quantity based on a portion of the radiation emitted by the LIDAR system, this portion of the radiation being backscattered by particles suspended in the atmosphere and then detected by the LIDAR system.

22. The LIDAR system (10) according to claim 21, characterized in that, The at least one atmospheric quantity is wind speed.

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