Pulse Lidar System

Abstract

The pulsed LIDAR system (100) has a transmission path (10) configured such that two successively emitted pulses (I) are spectrally separated and associated with different respective central wavelength values. In this way, the signal-to-noise ratio of the heterodyne detection signal is improved. This type of LIDAR system can be implemented using optical fibers and is particularly suitable for air velocity measurements.

Description

[Technical Field]

[0001] The present invention relates to a pulsed LiDAR system, and more particularly to a LiDAR system adapted for performing atmospheric velocity measurements. LiDAR is an acronym for Light Detection And Ranging, and LiDAR systems are very well suited for performing velocity measurements from a distance. [Background technology]

[0002] Determining wind speed from a distance is useful in many fields, particularly in aircraft safety, for example, to detect the presence of turbulence near airport runways or to compensate for the effects of premature wear caused by gusts of wind on the aircraft's structure, or to detect wind gusts while on board an aircraft in flight. Other fields where such knowledge is useful include the surveying and management of wind farm sites or measuring atmospheric flow from space for weather forecasting.

[0003] Known methods allow pulsed LiDAR systems to measure the velocity component of a target parallel to the LiDAR system's emission direction, as well as the distance separating the target from the LiDAR system. In particular, pulsed LiDAR systems configured for atmospheric velocity measurement allow obtaining an estimate of the wind velocity component parallel to the LiDAR system's emission direction as a function of the separation distance measured along this emission direction. However, in such atmospheric velocity measurements, the signal detected by the LiDAR system, from which the wind velocity measurement result is obtained, is generated by the backscattering of the emission pulse caused by particles suspended in the air. Since these detected signals have very low intensity, it is important to improve the signal-to-noise ratio associated with them.

[0004] Furthermore, in known cases, when a pulsed LiDAR system uses heterodyne detection, i.e., when the system is coherent between emission and detection, its signal-to-noise ratio is E·PRF. 1 / 2is proportional to, where E is the energy of each pulse that is backscattered and then detected, and PRF is the pulse repetition frequency. Efforts are therefore made to increase the values of energy E and frequency PRF.

[0005] An increase in energy E can be achieved by increasing the energy of each pulse emitted by the LIDAR system. In fact, radiation is initially generated by a laser radiation source, which itself does not impose a limit on the power of radiation emitted to the outside. However, implementing a LIDAR system by using optical fiber connection technology provides considerable advantages, in particular increased robustness of the system and elimination of a mechanism for aligning the optical components of the system relative to each other. However, a known phenomenon called stimulated Brillouin scattering (SBS) that occurs in optical fibers limits the peak power value that each emitted pulse can have.

[0006] Furthermore, frequency PRF is limited by the range of the LIDAR system. In fact, a pulse of radiation emitted towards a target needs to return and be detected before the next pulse is emitted, in order to correlate each detected radiation portion with the correct moment of pulse emission and thereby estimate the value of the distance from the target. In other words, frequency PRF is limited by the range L defined for the LIDAR system according to the formula: PRF < C / (2·L), where C is the speed of light.

[0007] Therefore, these limitations on the energy of emitted pulses and the pulse repetition frequency, due to the resulting effect on the signal-to-noise ratio of the heterodyne detection signal, prevent improving the accuracy of measurement results, particularly atmospheric velocity measurement results. Summary of the Invention Problem to be Solved by the Invention

[0008] Based on this situation, one objective of the present invention is to propose a novel pulsed LiDAR system in which the signal-to-noise ratio of the detection signal is improved.

[0009] A supplementary object of the present invention is that such a LiDAR system is compatible with the use of optical fibers for interconnecting optical components within the LiDAR system.

[0010] Another supplementary objective of the present invention is to adapt such a LIDAR system to atmospheric velocity measurements. [Means for solving the problem]

[0011] To achieve at least one of these or other objectives, a pulsed LiDAR system provided in one aspect of the present invention is adapted to determine the value of the Doppler effect frequency shift received by a series of radiation pulses continuously emitted toward a target by the system between the portion of the pulse received after retroreflection or backscattering on the target and the same pulse emitted by the system. The system then provides an estimate of the velocity component of the target parallel to the light emission direction of the system, based on the value determined for the frequency shift. For this purpose, the system - A transmission path configured to generate the series of pulses, -A detection path configured to detect the pulse portion received after retroreflection or backscattering on the target and to generate a heterodyne detection signal corresponding to the series of pulses, -The frequency shift (ν) from the heterodyne detection contribution corresponding to the series of pulses. Doppler The system comprises a spectral analysis module adapted to perform spectral analysis of the heterodyne detection signal such that a value of ) is produced. The use of multiple pulses to perform the spectral analysis provides an initial improvement in the signal-to-noise ratio, and the accuracy of the measurement results provided by the LIDAR system is improved accordingly.

[0012] According to the present invention, the LIDAR system has the following additional features.

[0013] - the transmission path is further configured to form each of the pulses as a superposition of a plurality of pulse spectral components that are emitted simultaneously, are spectrally discrete, and are associated one-to-one with different central wavelength values, - the system is adapted such that the value of said frequency shift determined by said spectral analysis module arises from the contribution of a plurality of heterodyne detection contributions respectively corresponding to spectrally separated pulses having different central wavelength values.

[0014] In the context of the present invention, pulses spectrally separated from each other are understood to mean pulses whose respective spectra do not overlap, which means that there is no wavelength interval where the respective spectral intensity of several pulses is greater than 1% of the respective maximum spectral intensity value of the pulses.

[0015] Accordingly, two pulses consecutively emitted by the LIDAR system of the present invention are distinguished by different respective spectral intervals. In this case, the same distinction exists between the pulse portions received after retroreflection or backscattering on a target, as a result, the system can assign each pulse portion received after retroreflection or backscattering to its corresponding emitted pulse independently of the fact that another pulse was emitted therebetween. In this way, by means of the spectral difference introduced between consecutive pulses, the pulse repetition frequency PRF can be increased without reducing the measurement range L of the LIDAR system.

[0016] Furthermore, each pulse may still have a peak power value that is just below the threshold suitable for stimulated Brillouin scattering. Then, regarding the determination of frequency shift values, combining heterodyne detection contributions respectively corresponding to spectrally separated pulses and having different center wavelength values is equivalent to increasing the pulse repetition frequency PRF. A further improvement in the signal-to-noise ratio of the heterodyne detection signal is obtained, which is proportional to the square root of the increase in the repetition frequency PRF provided by the operation of the LIDAR system of the present invention. Accordingly, the accuracy of the value obtained for the Doppler frequency shift is improved. According to another aspect, for a constant value of the temperature range L, while maintaining the same accuracy in the measurement result, the LIDAR system of the present invention can enable the integration time for the heterodyne detection signal to be reduced by a factor equal to the number of different center wavelength values for the pulses.

[0017] The fact that the peak power value of each pulse remains below the stimulated Brillouin scattering threshold allows the use of optical fiber technology to implement the transmission path of the system.

[0018] Furthermore, all heterodyne detection contributions respectively corresponding to spectrally independent pulses and having different center wavelength values can contribute to obtaining the value of the frequency shift resulting from the Doppler effect generated by the movement of the target. Therefore, the system of the present invention can effectively operate to multiply the repetition frequency PRF by the number of different center wavelength values of the pulses while maintaining an unchanged value for the temperature range L of the LIDAR system.

[0019] Accordingly, the present invention provides a LiDAR system that determines the value of the Doppler effect frequency shift based on multiple spectral contributions present in the heterodyne detection signal. These spectral contributions, which constitute exactly the same number of components as spectrally distinct components in the heterodyne detection signal, correspond one-to-one to the center wavelength values ​​of pulses emitted toward the target, which differ between two consecutive pulses. For example, a baseline value for the Doppler effect frequency shift can be determined based on each heterodyne detection spectral contribution, independently of other heterodyne detection spectral contributions, and then a final value for the Doppler effect frequency shift can be calculated by averaging the baseline values.

[0020] Generally speaking, with respect to the present invention, the transmission path of the LIDAR system of the present invention is -A laser radiation source adapted to generate initial laser radiation, wherein the initial laser radiation is preferably monochromatic or quasi-monochromatic, - At least one modulator configured to modify the initial laser radiation according to a modulation signal applied to at least one control input of this modulator, -Includes a controller connected to apply a modulated signal to at least one control input of a modulator.

[0021] Next, the modulated signal is generated such that the initial laser radiation is converted by the modulator into a series of pulses where two consecutive pulses are spectrally separated from each other and have different center wavelength values. Furthermore, the reference input of the detection path used for heterodyne detection can be connected to the secondary output of the transmission path located between the laser radiation source and the modulator. The optical reference signal used for heterodyne detection can then be monochromatic. In the heterodyne detection signal generated by the detection path, heterodyne detection contributions, which arise from spectrally separated pulses and have different center wavelength values, are then spectrally shifted relative to each other. In other words, these heterodyne detection contributions also have different center frequency values. The spectral analysis module then estimates the value of the Doppler effect frequency shift from all of these different center frequency values ​​for the heterodyne detection contributions.

[0022] Alternatively, but less favorably, to obtain heterodyne detection, the secondary output of the transmission path to which the reference input of the detection path is connected can be located downstream of the modulator with respect to the direction of radiation propagation in the transmission path.

[0023] In a first embodiment of the present invention, the transmission paths may be configured to generate continuous pulses with spectrally separated and different center wavelength values ​​by celodyne modulation. To achieve this, the modulator may be a phase modulator, and the modulation signal may be a phase-modulated signal consisting of a time-independent array of linear phase-shift ramps, where the linear phase-shift ramps are identical and continuous within each array and have different gradients between different arrays. The array of linear phase-shift ramps then corresponds one-to-one with the pulses emitted by the LIDAR system. In such a first embodiment with celodyne modulation, the phase modulator used may be an electro-optic type modulator.

[0024] In a second embodiment of the present invention, the system can be configured to generate consecutive pulses by I / Q modulation, where the transmission paths are spectrally separated from each other and have different center wavelength values. To achieve this, the modulator may comprise a recombined Mach-Zehnder interferometer and two secondary Mach-Zehnder interferometers, one each positioned on the two separate optical propagation paths of the recombined Mach-Zehnder interferometer. The system then further comprises means for applying the following phase shift.

[0025] -Applied between the two separate optical propagation paths of the first of two second-order Mach-Zehnder interferometers, the first phase shift is equal to the sum of π, which changes sinusoidally as a function of time, and the first phase shift component, -Applied between the two separate optical propagation paths of the second of two second-order Mach-Zehnder interferometers, the first and second phase-shift components vary sinusoidally as a function of time and have a common frequency, and are in phase perpendicular to each other, with the sum of π, which varies sinusoidally as a function of time, and the second phase-shift component, - Applied between the two optical propagation paths of a recombined Mach-Zehnder interferometer, and comprising a third phase shift equal to ± half of π.

[0026] Next, the common frequency of the first and second phase-shift components, which change sinusoidally as a function of time, determines the difference between the center wavelength of the emitted pulse and the wavelength of the initial laser radiation generated by the laser source. In such embodiments with I / Q modulation, a recombined Mach-Zehnder interferometer and two secondary Mach-Zehnder interferometers may constitute an integrated optical circuit.

[0027] In preferred embodiments of the present invention, at least one of the following additional features can be reproduced, either individually or in combination.

[0028] - The LIDAR system may be adapted to provide an estimate of the air velocity component when the system is directed to emit a radiated pulse toward a portion of the atmosphere containing suspended particles that form a target, with the particles acting as backscatterers for the radiation.

[0029] Each pulse may be monochromatic or semi-monochromatic.

[0030] -The transmission path can be further configured such that any two consecutively emitted pulses are spectrally separated from each other by at least 10 MHz, preferably at least 20 MHz, and at most 2000 MHz.

[0031] - The transmission path may be further configured such that a series of pulses repeats a constant sequence of pulse center wavelength values. Furthermore, within the repeating sequence, the difference between the center wavelength values ​​related to consecutively emitted pairs of pulses can be constant.

[0032] -The transmission path may be further configured such that a number of different center wavelength values ​​for the series pulses are between 2 and 16.

[0033] -The transmission path can be further configured such that the duration between consecutively emitted pulses changes throughout the course of the series of pulses. In this way, the measurement area that is interfered with by the reflection of radiated pulses on the optical components of the transmission path can be eliminated.

[0034] - The transmission path and / or detection path may be implemented using optical fiber technology to interconnect the components of this transmission path and / or detection path.

[0035] The features and advantages of the present invention will become more apparent with reference to the accompanying drawings in the following detailed description of some non-limiting exemplary embodiments. [Brief explanation of the drawing]

[0036] [Figure 1a] This is a block diagram of a pulsed LiDAR device with heterodyne detection, as known from conventional technology. [Figure 1b] The two spectral diagrams related to the operation of the LIDAR system in Figure 1a are grouped together. [Figure 2] This is a timing diagram showing possible spectral distributions for the operation of the LIDAR system according to the present invention. [Figure 3a] This corresponds to Figure 1a, which shows a possible embodiment of the LIDAR system according to the present invention. [Figure 3b] Figure 3a corresponds to Figure 1b of the LIDAR system. [Figure 4] This is a group of two figures, each showing a possible temporal variation of the modulated signal used in the first embodiment of the present invention, and a corresponding spectral diagram. [Figure 5] This is a block diagram of an I / Q modulator usable in a second embodiment of the present invention. [Modes for carrying out the invention]

[0037] In these figures, all components are represented symbolically, and the same reference numerals shown in different figures indicate elements that are identical or have the same function. For clarity, components whose use in LIDAR systems is known to those skilled in the art and which are not directly related to the present invention are not described below. In such cases, their possible adaptation to the present invention is within the scope of those skilled in the art. In Figures 1a and 3a, the following references used have the meanings set forth herein. 100 General name for pulsed LIDAR using heterodyne detection 10 Transmission Path 11 Laser source labeled "LASER" 12 MAO frequency shift and pulse separation modulator 13 AMPL (Optical amplifier) 14. Light Circulator 15 OPT (Optical Emission Optical System) 16. Secondary output of the transmission path 20 Detection Path 21 Heterodyne detector labeled DETECT 30. Spectral Analysis Module (ANALYS) Figure 1a shows a system 100 known prior to the present invention.

[0038] The transmission path 10 comprises a laser radiation source 11, a modulator 12, an optical amplifier 13, an optical circulator 14, and an emission optical system 15. The laser radiation source 11 may be a continuous emission source with an emission wavelength of approximately 1550 nm (nanometers) and a power of 600 μJ (microjoules). Thus, it generates initial laser radiation R0, which is monochromatic or quasi-monochromatic. The initial laser radiation R0 is transmitted to the modulator 12. The modulator 12 may be an acousto-optic type modulator. It is controlled to form identical pulses I from the received radiation, with individual durations of 200 ns (nanoseconds) to 800 ns, using a pulse repetition frequency PRF, which may be, for example, 10 kHz (kilohertz). At the same time, the modulator 12 can be controlled to shift the optical frequency of the radiation by applying a frequency shift Δν0, which can be equal to, for example, 100 MHz (megahertz). The pulse I generated by the modulator 12 is amplified by the amplifier 13 and transmitted to the emission optical system 15 via the optical circulator 14. The emission optical system 15 may have, for example, a telescope structure. The amplified pulse I is therefore outside the LIDAR system 100 and measured along the emission direction of the system 100, and then transmitted toward a target T located at a distance D. In principle, the separation distance D is less than the range L of the system 100, which, as an example, is perhaps equal to about 15 km (kilometers).

[0039] Thus, all pulses I emitted by the system 100 in [Figure 1a] are identical and monochromatic or quasi-monochromatic.

[0040] The secondary output 16 is located in the transmission path 10 between the laser radiation source 11 and the modulator 12, which is dedicated to shifting and separating pulse I.

[0041] The detection path 20 shares the emission optical system 15 and optical circulator 14 with the transmission path 10 and further includes a heterodyne detector 21. Within the detection path 20, one function of the optical system 15 is to collect a portion of the RI of a pulse I retroreflected or backscattered by the target T. The heterodyne detector 21 receives the retroreflected or backscattered portion of the RI of the pulse collected by the optical system 15 via the optical circulator 14 and is optically coupled to simultaneously receive an optical reference signal RR collected from the transmission path 10 via a secondary output 16 of this transmission path. In other words, the secondary output 16 is optically coupled to the heterodyne detector 21 in addition to the output of the optical circulator 14 dedicated to the detection path 20. The heterodyne detector 21 may be a photodiode, particularly an ultrafast photodiode, on which the optical reference signal RR coming from the secondary output 16 and the pulse portion of the RI coming from the target T are focused.

[0042] The spectral analysis module 30 is configured to spectrally analyze the heterodyne detection signal generated by the detector 21 during the operation of system 100. It is configured to estimate the value of the frequency shift present between the optical reference signal RR and the pulsed portion RI from this spectral analysis. The frequency shift value thus obtained is then used to determine the velocity component value V for the target T, parallel to the emission direction of system 100. T It is further configured to convert to V. T =-λ0·(ν m -Δν(0) / 2, where, λ0 represents the wavelength of the laser radiation source 11, which in the example above is equal to approximately 1550 nm. Δν0 further represents the frequency shift applied by modulator 12, which is equal to 100 MHz in the example above. ν mThis is a frequency in the radio frequency domain or RF domain that relates to the position of maximum intensity or central peak in the spectral decomposition of the heterodyne detection signal.

[0043] System 100 is preferably implemented using optical fiber technology. In such a case, the optical amplifier 13 can be of the type specified by EDFA for "erbium-doped fiber amplifier". The initial laser radiation R0 is transmitted from the laser radiation source 11 to the modulator 12 by the first optical fiber segment S1, and then to the amplifier 13 via the second optical fiber segment S2. Furthermore, the retroreflected or backscattered pulsed partial RI collected by the optical system 15 is injected into the third optical fiber segment S3 at the output of the optical circulator 14 to transmit them to the heterodyne detector 21. In parallel, the secondary output 16 of the transmission path 10 is implemented by an optical fiber coupler and connected to the heterodyne detector 21 by the fourth optical fiber segment S4.

[0044] For the operation of the system 100 described above, which has a retroreflective point target, the heterodyne detection signal has a frequency ν mhas sinusoidal fluctuations. The upper diagram in [Fig. 1b] shows the spectral composition of the radiation received by the heterodyne detector 21. The horizontal axis in this upper diagram of [Fig. 1b], represented by λ, identifies wavelength values in the optical region, expressed in nanometers (nm). The vertical axis identifies spectral intensity values in arbitrary units. The radiation received by the heterodyne detector 21 comprises a first contribution consisting of the optical reference signal RR transmitted from the secondary output 16 and a second contribution corresponding to the pulsed portion RI retroreflected by the target T. For the system 100 in [Fig. 1a], the optical reference signal RR is a portion of the initial laser radiation R0, and as a result the corresponding contribution in the upper diagram of [Fig. 1b] is a very narrow peak indicated as RR. When the target T is located at a single position along the emission direction of the system 100, the second contribution also has the shape of a narrow peak indicated as RI. The lower diagram in [Fig. 1b] shows the spectral composition of the heterodyne detection signal corresponding to the spectral composition of the radiation received by the detector 21 as shown in the upper diagram. The heterodyne detection signal consists of a single peak, the frequency of which is ν m =Δν0+ν Doppler , where ν Doppler ≒-2·V T / λ1, λ1 is the wavelength of the radiation emitted by the LIDAR system 100. The horizontal axis in the lower diagram of [Fig. 1b], represented by f, identifies frequency values in the RF region, expressed in megahertz (MHz). The vertical axis is also arbitrary units for identifying the spectral intensity value of the heterodyne detection signal.

[0045] Regarding the operation of system 100, which is dedicated to atmospheric velocity measurement, pulse I starts from the emission optical system 15 and is backscattered by a number of targets distributed along the path of the pulse beam outside system 100. These targets, consisting of particles or aerosols suspended in the air, are pulled along as a function of the local velocity of air movement present at each position in the beam path. Those skilled in the art generally refer to such a distribution of targets as “extended targets,” “distributed targets,” or “volume targets.” Thus, the pulsed partial RI collected by the optical system 15 and transmitted to the detector 21 spreads over time in correspondence with different separation distances along the emission direction of system 100, where partial backscattering of pulse I occurs. Furthermore, they are frequency-shifted to vary according to the local wind velocity parallel to the emission direction at each location where partial backscattering occurs. The heterodyne detection signal then has more complex temporal variations. Spectral analysis performed by module 30 is assumed to be known, and as a result, a set of velocity values ​​V assigned one-to-one to different values ​​of separation distance D T This provides the following: In known methods, the resolution at separation distance D is determined by the individual duration of the emitted pulses I, which is equal to the individual duration divided by twice the pulse propagation velocity outside the LIDAR system 100. Compared to the figure in [Figure 1b], the peaks corresponding to the pulsed partial RI in the spectral composition of the radiation received by the detector 21 are expanded. The peaks in the spectral composition of the heterodyne detection signal in the RF region are broadened correlated.

[0046] In Figure 2, the horizontal axis of the diagram specifies time, denoted by t, and the vertical axis specifies the instantaneous emission wavelength λ1 of the LIDAR system 100 according to the present invention. The wavelength λ1 is expressed in nanometers (nm). According to this diagram, a series of pulses I emitted by the LIDAR system 100 may consist of repetitions of an array S of several pulses I, for example, 100 repetitions. For example, array S may have a duration of 100 μS (microseconds) and may consist of 10 pulses I, each having an individual duration that may be 0.5 μS. Within array S, pulses I are distributed with a separation duration that is advantageously variable between two consecutive pulses. In fact, due to the reflection of each pulse I on a specific optical component at the terminal portion of the transmission path 10 shared with the detection path 20, the emission of each pulse I generates a detection signal of very high intensity that causes saturation of the detector 21. This detection signal is due to internal reflection within the system 100 and is generally called a narcissistic signal. During its duration, it simultaneously prevents the detection of pulsed partial RI received by detector 21, corresponding to a previously emitted pulse I, which is then retroreflected or backscattered by the target. Therefore, if the separation times between consecutively emitted pulses are all the same, the narcissus signal prevents the measurement of velocity relative to a target located within a constant interval along the direction of emission, known as a blind interval. By varying the separation time of consecutive pulses within the array S, it becomes possible to obtain velocity measurements of a target located at any position within the range of system 100, with some pulses filling the blind intervals caused by other pulses. Each pulse I is monochromatic or quasi-monochromatic. Thus, the described array S corresponds to 10 different values ​​for emission wavelength λ. The order in which these 10 wavelength values ​​are generated by system 100 does not matter, as long as two consecutively emitted pulses have different wavelength values. Furthermore, the difference between these wavelength values ​​can be any value, as long as any two pulses of array S are spectrally sufficiently separated, and as a result, the frequency shift of the retroreflected or backscattered pulse portion RI is contained within all the separation intervals between different pulses of array S.For illustrative purposes, in [Figure 2], each successive pulse I has a wavelength value that increases over time within the array S, with a constant increment relative to the wavelength value indicated by Δλ1. The wavelength increment Δλ1 is -CΔλ1 / λ0. 2 This corresponds to a frequency increment Δν1 equal to . This latter increment can be equal to 200 MHz in the RF domain, for example. However, generally, the difference between pulse wavelength values ​​may be invariant from one pair of adjacent values ​​to another.

[0047] In the embodiment described herein, the repetition frequency of array S is equal to 10 kHz, while the effective pulse frequency for measuring the target velocity, i.e., frequency PRF, is equal to the product of this repetition frequency of array S and the number of pulses in the array, i.e., 100 kHz.

[0048] Such operation according to the present invention can be performed by the LIDAR system 100, as shown in [Figure 3a]. This system has a hardware architecture similar to that of [Figure 1a], except that the transmission path 10 further comprises an additional modulator 17 indicated by MOD and a controller 18 indicated by CTRL. The modulator 17 is inserted into a first optical fiber segment S1 between the laser radiation source 11 and the electroacoustic modulator 12. Two possible configurations of the modulator 17 are described below. The modulator 17, in relation to the controller 18, converts the initial laser radiation R0 into a series of monochromatic pulses having a variable wavelength, as described above in relation to [Figure 2]. The controller 18 simultaneously controls the modulator 12 to generate a variable separation duration between consecutive pulses. Furthermore, the modulator 12 applies a frequency shift Δν0 to each of the pulses generated by the modulator 17.

[0049] Upon retroreflection, each pulse I is spectrally shifted by the Doppler effect. Assuming that the frequency increment Δν1 is much lower than the optical frequency corresponding to wavelength λ0, all pulses will have the same Doppler effect frequency shift ν Doppler In addition, the frequency increment Δν1 is the Doppler effect applied to the frequency shift Δν0, which is then applied to the frequency shift ν. DopplerThe values ​​are selected to be greater than all other values ​​that are expected to be achievable.

[0050] The secondary output 16 of the transmission path 10 is positioned between the laser radiation source 11 and the modulator 17. In this way, the optical reference signal RR transmitted to the heterodyne detector 21 still consists of the portion of the initial laser radiation R0. In particular, it remains monochromatic.

[0051] As shown in the upper diagram of [Figure 3b], the spectral composition of the radiation received by the heterodyne detector 21 still includes the peak RR corresponding to the emission from the laser radiation source 11, but also includes several additional peaks RI corresponding to the pulse portions retroreflected or backscattered by the optical system 15 and subsequently collected. These peaks RI arise from all wavelength values ​​of the emitted pulse I and contain measurement information. They are ν with respect to optical frequency. Doppler The Doppler effect frequency shift ν is spectrally shifted with respect to pulse I. During heterodyne detection, each peak RI forms interference with peak RR. As shown in the lower figure of [Figure 3b], the heterodyne detected signal consists of many peaks as long as the wavelength values ​​of pulse I are different. The two figures in [Figure 3b] correspond to the separation of the wavelength values ​​of pulse I according to a constant frequency increment Δν1. The spectral analysis unit 30 then calculates the Doppler effect frequency shift ν based on the high-frequency values ​​measured for all the peaks of the heterodyne detected signal. Doppler Determine the value of ν based on the center frequency value of each peak in the heterodyne detection signal. Doppler By finding the baseline values ​​and averaging these baseline values, ν Doppler The final value of is calculated. Assuming that all peaks in the heterodyne detection signal correspond to mutually incoherent contributions, the heterodyne detection signal is n 1 / 2 The signal-to-noise ratio value is increased by a factor of n, where n is the number of different wavelength values ​​for pulse I.

[0052] In a first embodiment of the present invention, monochromatic pulses I having a variable wavelength value in array S can be generated by celodyne modulation. In this case, the modulator 17 can be an electro-optic type modulator, and the controller 18 is adapted to apply the celodyne-modulated signal to the control input of modulator 17. It is assumed that the principle of such modulation is known to those skilled in the art. If necessary, refer to the papers entitled "New coherent Doppler Lidar engine integrating optical transceiver with FPGA signal processor" by Toshiyuki Ando and Eisuke Haraguchi (a) and Hitomi Ono (a), 18th Coherent Laser Radar Conference (2016). According to the first two figures in [Figure 4], this modulated signal consists of a series of identical, time-coupled linear phase ramps for each pulse I of emitted radiation. Each phase ramp varies individually from 0 to 2π. The sequence of phase ramps occupies the entire duration of the pulse. These phase ramps cause an increase in the rate of variation of the phase of the radiation, and thus produce the desired optical frequency shift for the pulse in question. This optical frequency shift is directly equal to the phase ramp gradient divided by 2π. This phase ramp gradient, which is constant with respect to the duration of each pulse I, changes between two consecutive pulses. This can be positive or negative depending on whether the wavelength of the pulse at the output from modulator 17 is smaller or larger than the wavelength λ0 of the initial laser radiation R0. The upper figure in [Figure 4] shows such a celodyne-modulated signal. The horizontal axis represents time t, and the vertical axis represents the phase shift produced by the modulation, denoted as ph. and expressed in radians. The first pulse, indicated by I1, can correspond to an optical frequency shift equal to 40 MHz with respect to the optical frequency of the initial laser radiation R0. For this, the gradient of its phase ramp is equal to 2π40 MHz. The phase ramp of the second pulse, indicated by I2, is twice as steep as the phase ramp of pulse I1, and the corresponding optical frequency shift of pulse I2 is equal to 80 MHz. Similarly, the phase ramp of the third pulse, indicated by I3, is three times steeper than the phase ramp of pulse I1, and the optical frequency shift of pulse I3 is equal to 120 MHz, etc.For clarity in the diagram in [Figure 4], only three of the ten pulses in array S are shown. In the operation of the LIDAR system 100 in [Figure 3a], the shift Δν0 generated by modulator 12 is added to the previous shift generated by modulator 17. The intermediate diagram in [Figure 4] shows that the serodyne modulation does not change the amplitude of the radiation transmitted by modulator 17. The horizontal axis of this intermediate diagram again specifies time t, and the vertical axis specifies the attenuation coefficient of the attenuation generated by modulator 17 with respect to the radiation intensity in arbitrary units (au), which is denoted as A. This coefficient is substantially constant and as close to a single unit as possible. Finally, the lower diagram in [Figure 4] shows the frequency distribution of the obtained heterodyne detection signal. The horizontal axis of this lower diagram specifies the value of frequency f in the RF region, and the vertical axis specifies the power spectral density of the heterodyne detection signal. Thus, the peak corresponding to pulse I1 has a value of 40 MHz + Δν0 + ν in all iterations of the array S of emitted pulses. Doppler Centered at this point, the peak corresponding to pulse I2 has a value of 80MHz + Δν0 + ν Doppler The peak corresponding to pulse I3 is centered around a value of 120MHz + Δν, etc. This cellodyne modulation corresponds to a frequency increment of Δν1 = 40MHz.

[0053] In a second embodiment of the present invention, a monochromatic pulse I with a variable wavelength can be generated by I / Q modulation. In this case, the modulator 17 may be of the type described in the paper entitled "Tunable Frequency Shift Based on LiNbO3 I / Q Modulators" by Alexandre Motte, Nicolas Bourriot and Jerome Hauden, Photline Technologies, ZI Les Tilleroyes-Trepillot, 16 rue Auguste Jouchoux, 25000 Besancon, France, or in the paper entitled "Integrated optical SSB modulator / Frequency Shifter" by Izutsu Masayuki, Shinsuke Shikama and Tadasi Sueta, IEEE Journal of Quantum Electronics. It consists of a Mach-Zehnder type main interferometer, also called a recombination interferometer, which is connected to receive initial laser radiation R0 from a laser radiation source 11 as an input and is connected at the output to the optical input of an acousto-optic modulator 12. As shown in [Figure 5], this recombination interferometer has two optical propagation paths arranged in parallel between the light source 11 and the modulator 12, namely path A1A2A3A4 and path A1A5A6A4, as indicated by reference numeral 170. Path A1A2A3A4 comprises an electro-optic modulator M5 between points A1 and A2, and another Mach-Zehnder interferometer between points A2 and A3, and is called a secondary interferometer, designated by reference numeral 171. The secondary interferometer 171 itself comprises two optical propagation paths arranged in parallel between points A2 and A3. Each of these two paths of the secondary interferometer 171 comprises an electro-optic modulator, M1 and M2, respectively. Path A1A5A6A4 has the same configuration as path A1A2A3A4. It comprises another electro-optic modulator M6 between points A1 and A5, and another secondary Mach-Zehnder interferometer between points A5 and A6, which is indicated by reference numeral 172. The secondary interferometer 172 itself comprises two optical propagation paths arranged in parallel between points A5 and A6. Each of these last two paths includes an electro-optic modulator, M3 and M4, respectively.Such modulators 17 can be implemented in the form of an integrated optical circuit having electro-optic modulators M1 to M6 that are implemented based on lithium niobate (LiNbO3) portions associated with each electrode. Several techniques for integrated optical circuits are known to those skilled in the art and can be used for such embodiments of the LIDAR system 100 according to the present invention.

[0054] The controller 18 applies a voltage to each electrode of the electro-optic modulators M1 to M6, and as a result, each of these generates an optical phase shift with respect to the portion of the initial laser radiation R0 through which it passes. Thus, modulator Mi generates an optical phase shift φ i This generates a function where i is a natural integer exponent that varies from 1 to 6. Under these conditions, the secondary Mach-Zehnder interferometer 171 applies a first phase shift between the two optical propagation paths connecting points A2 and A3. This first phase shift is Φ1 = φ1 - φ2. Similarly, the secondary Mach-Zehnder interferometer 172 applies a second phase shift Φ2 between the two optical propagation paths connecting points A5 and A6: Φ2 = φ3 - φ4. Finally, the recombining Mach-Zehnder interferometer 170 applies a third phase shift Φ3 between the two optical propagation paths A1A2A3A4 and A1A5A6A4: Φ3 = φ5 - φ6. Thus, the modulator 17 has an optical frequency shift function for the initial laser radiation R0 when the controller 18 applies voltage to the electro-optic modulators M1~M6.

[0055] Φ Φ1 =φ1-φ2=π+α·sin(Δν1·t), Φ Φ2 =φ3-φ4=π+α·sin(Δν1·t+±π / 2), and Φ Φ3 =φ5-φ6=±π / 2.

[0056] Therefore, phase shift Φ Φ1 and Φ Φ2The waveform has sinusoidal fluctuations with respect to time t, according to the frequency which is intended to be equal to the optical frequency shift Δν1 belonging to the high-frequency region introduced above. To generate control voltages for the electro-optic modulators M1 to M6, controller 18 may incorporate an AWG type generator for the "arbitrary waveform generator".

[0057] The implementation of distance resolution is not described in relation to the implementation of the present invention presented above in order to obtain velocity measurement results with respect to sampled separation distance values, but the principle for obtaining such distance resolution can be used as described for system 100 in [Figure 1a].

[0058] In all embodiments of the present invention, each pulse can have an individual peak power value that is just below the threshold of stimulated Brillouin scattering occurring in the optical fiber segments S1 and S2, as well as in the optical amplifier 13, the optical circulator 14, and the optical fiber segments between them leading to the emission optics 15. For the same value over the temperature range L of the system 100, the total number of pulses is multiplied by the number of different wavelength values ​​n for the pulses, but the individual energy of each pulse can be the same as that used before the present invention. This allows for an n-factor operation of the LIDAR system using heterodyne detection. 1 / 2 Improvements can be obtained. Therefore, the pulsed LiDAR system with heterodyne detection according to the present invention is particularly suitable for measurement conditions in which the retroreflection or backscattering pulse portion has low power or very low power. Therefore, they are particularly suitable for performing atmospheric velocity measurements.

[0059] It is understood that the present invention can be reproduced by modifying secondary aspects of the embodiments described in detail above, while retaining at least some of the cited advantages. In particular, the following modifications are possible:

[0060] Some of the components used in the embodiments described may be replaced by other components or by combinations of components that produce equivalent functionality. For example, each acousto-optic modulator may be replaced by a semiconductor optical amplifier, or SOA, used as a modulator.

[0061] In the LIDAR system architecture of [Figure 3a], the secondary output 16 of the transmission path 10 can move between the electro-optic modulator 17 and the electro-acoustic modulator 12. Heterodyne detection operation is still obtained by connecting the reference input of the detection path 20 to the secondary output 16 at this new position. The heterodyne detection signal then consists of one or more primary peaks corresponding to the detection of one or more targets present within the temperature range L of the LIDAR system, limited by the pulse repetition frequency PRF, and pulse portions that are shifted mainly according to the spectral difference between a series of pulses and correspond to one or more additional targets present beyond the temperature range L, and are backscattered after the emission of at least one subsequent pulse following one, which gives rise to each pulse portion backscattered by one of the additional targets.

[0062] All figures cited are for illustrative purposes only and may be modified depending on the application being considered for pulsed LiDAR systems with heterodyne detection.

Claims

1. A pulsed LiDAR system (100) is provided, and the Doppler effect frequency shift (ν) is caused by a series of radiation pulses (I) continuously emitted toward a target (T) by the system. Ｄｏｐｐｌｅｒ A system adapted to determine the value of between the portion of the pulse (RI) received after retroreflection or backscattering on the target and the pulse emitted by the system, and to provide an estimate of the velocity component (VT) of the target parallel to the light emission direction of the system based on the value determined for the Doppler effect frequency shift, The aforementioned system (100) A transmission path (10) configured to generate the series of pulses (I), A detection path (20) is configured to detect the pulse portion (RI) received after retroreflection or backscattering on the target (T) and to generate a heterodyne detection signal corresponding to the series of pulses (I), From the heterodyne detection contribution corresponding to the series of pulses (I), the Doppler effect frequency shift (ν Ｄｏｐｐｌｅｒ The system comprises a spectral analysis module (30) adapted to perform spectral analysis of the heterodyne detection signal such that a value of ) is produced, The system (100) is adapted to provide an estimate of the air velocity component when directed to emit the radiation pulse (I) toward a portion of the atmosphere containing suspended particles forming the target (T), wherein the particles backscatter the radiation. The transmission path (10) is further configured such that two pulses (I) continuously emitted toward the target (T) are spectrally separated and associated with different central wavelength values. The Doppler effect frequency shift (ν) determined by the spectral analysis module (30) is calculated by determining a baseline value for the Doppler effect frequency shift based on each heterodyne detection spectral contribution, independently of other heterodyne detection spectral contributions, and then averaging the baseline values ​​to calculate the final value for the Doppler effect frequency shift. Ｄｏｐｐｌｅｒ The system (100) is adapted such that the aforementioned value of ) results from a combination of several heterodyne detection contributions, each corresponding to a spectrally separated pulse and having a different center wavelength value.

2. The pulsed LiDAR system (100) according to claim 1, wherein the transmission path (10) is further configured such that any two continuously emitted radiation pulses (I) are spectrally separated by at least 10 MHz.

3. The pulse LiDAR system (100) according to claim 1, wherein the transmission path (10) is further configured such that the series of pulses (I) repeat a constant sequence of the center wavelength values ​​of the pulses.

4. The pulsed LiDAR system (100) according to claim 3, wherein the transmission path (10) is further configured such that the difference between the center wavelength values ​​related to the pairs of pulses (I) continuously emitted in the repeating array is constant.

5. The pulse LiDAR system (100) according to claim 1, wherein the transmission path (10) is further configured such that a plurality of different center wavelength values ​​for the series of pulses (I) are between 2 and 16.

6. The pulse LiDAR system (100) according to claim 1, wherein the transmission path (10) is further configured such that the duration between consecutively emitted pulses (I) changes over the course of the series of pulses.

7. The aforementioned transmission path (10) Initial laser radiation (R ０ A laser radiation source (11) adapted to generate ) According to a modulation signal applied to at least one control input, the initial laser radiation (R ０ At least one modulator configured to correct ) The controller (18) is connected to apply the modulation signal to at least one of the control inputs, The modulated signal is the initial laser radiation (R ０ ) is converted by the modulator (17) into the series of pulses (I), and two consecutive pulses are spectrally separated, with different center wavelength values. The LIDAR system (100) according to claim 1, wherein the reference input of the detection path (20) used for heterodyne detection is connected to the secondary output (16) of the transmission path (10) located between the laser radiation source (11) and the modulator (17).

8. The modulator (17) is a phase modulator, The modulated signal is a phase-modulated signal composed of a time-discrete array (S) of linear phase-shift ramps, The linear phase shift ramp is identical and continuous within each array, and has different gradients between different arrays. The LIDAR system (100) according to claim 7, wherein the arrangement of the linear phase shift lamps corresponds one-to-one with the pulses (I) emitted by the LIDAR system (100).

9. The modulator (17) The system includes a recombined Mach-Zehnder interferometer (170) and two secondary Mach-Zehnder interferometers (171, 172), each positioned on two separate optical propagation paths of the recombined Mach-Zehnder interferometer and equipped with means for applying the following phase shift: Applied between two separate optical propagation paths of the first of the two secondary Mach-Zehnder interferometers (171, 172), the first phase shift is equal to the sum of π, which changes sinusoidally as a function of time, and the first phase shift component, Applied between the two separate optical propagation paths of the second of the two secondary Mach-Zehnder interferometers (171, 172), the second phase shift is equal to the sum of π, which changes sinusoidally as a function of time, and the second phase shift component, which has a common frequency and changes sinusoidally as a function of time, and is in phase perpendicular to each other, Applied between the two optical propagation paths of the recombined Mach-Zehnder interferometer (170), and including a third phase shift equal to ± half of π, The common frequency of the first and second phase shift components is the center wavelength value of the radiation pulse (I) and the initial laser radiation (R) generated by the laser radiation source (11). ０ The LIDAR system (100) according to claim 7, which changes sinusoidally as a function of time to determine the difference between the wavelength value and the other wavelength value.

10. The LIDAR system (100) according to claim 1, wherein at least one of the transmission paths (10) and the detection path (20) are implemented by optical fiber technology to interconnect the components of the transmission path and the detection path, respectively.

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