Pulsed lidar system

Abstract

A pulsed LIDAR system (100) has a transmission path (10) configured to form each pulse (I) as a superposition of a plurality of simultaneously emitted pulse spectral components. In this way the signal-to-noise ratio of the heterodyne detection signal is improved. This kind of LIDAR system can be implemented using optical fibers and is particularly suitable for performing airspeed measurements.

Description

Technical Field

[0001] This specification relates to pulsed LIDAR systems, and more specifically to LIDAR systems suitable for performing airspeed measurements. Although LIDAR is an acronym for Light Detection and Ranging, LIDAR systems are highly suitable for performing speed measurements over a distance. Background Technology

[0002] Determining wind speeds over a distance has applications in many fields, particularly aviation safety. For example, it can be used to detect the presence of turbulence near airport runways or to detect gusts blowing on aircraft in flight to compensate for premature wear caused by these gusts on the aircraft's structure. Other areas where this knowledge is useful include the surveying and management of wind farm sites and the measurement of atmospheric flow for weather forecasting.

[0003] In a known manner, pulsed LiDAR systems enable the measurement of the velocity components of a target parallel to the LiDAR system's launch direction, as well as the distance at which the target is separated from the LiDAR system. Specifically, pulsed LiDAR systems configured for airspeed measurement make it possible to obtain estimates of the wind speed components parallel to the LiDAR system, which vary with the separation distance measured along this launch direction. However, for such airspeed measurements, the signal from which the LiDAR system detects and from which the wind speed measurement is derived is generated by the backscattering of the emitted pulses caused by particles suspended in the air. These detected signals have extremely low intensity, therefore improving the associated signal-to-noise ratio is important.

[0004] In a known manner, when a pulsed LiDAR system uses heterodyne detection, it means that when the system is coherent between transmission and detection, its signal-to-noise ratio (SNR) is similar to that of the E·PRF. 1 / 2 Proportional, where E is the energy of each backscattered pulse that is subsequently detected, and PRF is the pulse repetition frequency. Therefore, efforts are made to increase the values ​​of energy E and frequency PRF.

[0005] Increasing the energy E can be achieved by increasing the energy of each pulse emitted by the LIDAR system. Indeed, the radiation is initially generated by a laser source, which itself does not impose a limit on the power of the radiation emitted outwards. However, implementations of LIDAR systems using fiber optic connections offer considerable advantages, particularly increased system robustness and the elimination of mechanisms for aligning the system's optical components relative to each other. However, the known phenomenon of stimulated Brillouin scattering (SBS) occurring in the fiber limits the peak power value that each emitted pulse can have.

[0006] Furthermore, the pulse repetition frequency (PRF) is limited by the range of the LIDAR system. In fact, it is necessary for the radiated pulse emitted towards the target to return for detection before the next pulse is emitted, so that each detected radiated portion can be associated with the correct pulse emission time, thereby deriving the value of the distance to the target. In other words, the PRF is limited by the maximum range L specified for the LIDAR system according to the following 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 hinder the improvement of the accuracy of measurement results, particularly airspeed measurement results, due to the resulting signal-to-noise ratio of heterodyne detection signals.

[0008] Technical Problem

[0009] Based on this situation, an object of the present invention is to provide a novel pulsed LIDAR system in which the signal-to-noise ratio of the detection signal is improved.

[0010] A complementary object of the present invention is that the LIDAR system is compatible with the use of optical fibers to interconnect optical components inside the LIDAR system.

[0011] Another complementary object of the present invention is that the LIDAR system is suitable for airspeed measurement. Summary of the Invention

[0012] To achieve at least one of these or other objects, an aspect of the present invention provides a pulsed LIDAR system adapted to determine the value of the Doppler frequency shift experienced by a series of radiation pulses continuously emitted by the system towards a target, said Doppler frequency shift being between the pulse received after retroreflection or backscattering on the target and a portion of the same pulse emitted by the system. The system then provides an estimate of the velocity component of the target parallel to the optical emission direction of the system based on the determined value of the frequency shift. For this purpose, the system comprises:

[0013] - a transmission path configured to generate said series of pulses,

[0014] - a detection path configured to detect the pulse portion received after retroreflection or backscattering on said target, and generate a heterodyne detection signal corresponding to the pulses of said series, and

[0015] - a spectral analysis module adapted to perform spectral analysis of said heterodyne detection signal, such that said value of said frequency shift is generated from heterodyne detection contributions corresponding to the pulses of said series.

[0016] Using multiple pulses to perform spectral analysis provides an initial improvement in signal-to-noise ratio, and thus improves the accuracy of the measurement results provided by the LIDAR system.

[0017] According to the present invention, the LIDAR system has the following additional features:

[0018] The transmission path is further configured to form each of the pulses as a superposition of multiple pulse spectral components, which are emitted simultaneously, do not intersect spectrally, and are associated one-to-one with different center wavelength values.

[0019] The system is adapted such that the value of the frequency shift determined by the spectral analysis module is generated by multiple heterodyne detection contributions from the pulse spectral components corresponding to the series of pulses, respectively.

[0020] In the context of this invention, the term "spectrally non-intersecting pulse spectral components" should be understood to mean that the spectral components of each pulse, between which the pulse spectral intensity becomes less than 5%, preferably less than 1%, of the maximum spectral intensity value of each pulse spectral component.

[0021] Therefore, each pulse can have a peak power value greater than the agreed stimulated Brillouin scattering threshold, while each pulse spectral component individually has a peak power value less than the stimulated Brillouin scattering threshold. In other words, the peak transmit power limitation imposed by the fiber-based implementation of the transmission path is satisfied, while allowing each pulse to have an increased energy value. For this reason, the present invention is particularly suitable when a LIDAR system uses fiber optic technology for its transmission path.

[0022] Furthermore, all heterodyne detection contributions corresponding to the pulse spectral components of each pulse contribute to obtaining the value of the frequency shift attributed to the Doppler effect caused by target movement. Therefore, the system of the present invention operates in which the effective frequency PRF is multiplied by the number of disjoint spectral components in each pulse, while maintaining the range L of the LIDAR system constant. The present invention thus provides an additional improvement in the signal-to-noise ratio associated with the heterodyne detection signal. This increases the accuracy of the value obtained for the Doppler effect frequency shift. According to another viewpoint, for a constant range L, and while maintaining the same accuracy in the measurement results, the LIDAR system of the present invention allows for a reduction in the accumulation time for the heterodyne detection signal by a factor equal to the number of spectrally disjoint pulse spectral components constituting each emitted pulse.

[0023] Generally speaking, for the present invention, the pulses consecutively emitted by the LIDAR system that each have a plurality of spectrally non-intersecting spectral components do not have to be identical. Thus, two pulses may differ from each other by the average wavelength values of at least some of their spectral components, in particular by the differences existing between the spectral components from one pulse to another. Therefore, the composition of pulses having a plurality of spectral components may vary during a series of pulses emitted to perform a measurement sequence, and may vary periodically or randomly during this series of pulses. By virtue of this distinction between consecutively emitted pulses, the range of the LIDAR system can be increased while maintaining a constant value for the pulse repetition frequency, or PRF. In fact, if two pulses are identical, it is necessary to receive the return detection of the radiation pulse emitted towards the target before emitting the next pulse, so as to associate each detected radiation portion with the correct time of pulse emission, thereby deriving the value of the distance to the target. In other words, when consecutive pulses are identical, the PRF frequency is limited by the range L specified for the LIDAR system according to the following formula: PRF < C / (2·L), where C is the speed of light. The use of different consecutive pulses therefore makes it possible to increase the range L of the LIDAR system for an equal value of the PRF frequency, or to increase the value of the PRF frequency for a constant range L of the LIDAR system. The individual duration of each measurement sequence can thus be reduced. When several different spectral compositions are used for pulses while being repeated periodically, the individual duration of each measurement sequence can thus be divided by the number of different spectral compositions at a constant range L of the LIDAR system.

[0024] The transmission path of the LIDAR system of the present invention may comprise:

[0025] - a laser emission source adapted to generate initial laser radiation, which is preferably monochromatic or quasi-monochromatic;

[0026] - a comb generation modulator arranged to modify said initial laser radiation in accordance with a modulation signal applied to a control input of said comb generation modulator; and

[0027] - a modulation signal generator connected to apply said modulation signal to said control input of said comb generation modulator.

[0028] Said modulation signal causes said initial laser radiation to be transformed by said comb generation modulator into a set of spectral components that are intended to form, one to one, said pulse spectral components. In other words, the optical radiation exiting the modulator has a comb spectral structure.

[0029] In a first embodiment of the invention, the reference input of the detection path can be connected to the secondary output of the transmission path to receive an optical reference signal comprising a reference spectral component corresponding one-to-one with a pulse spectral component, having a spectral shift between each pulse spectral component and its corresponding counterpart among the reference spectral components, the spectral shift being identical for all pulse spectral components. In this manner, all heterodyne detection contributions associated with the pulse spectral components in the heterodyne detection signal generated by the detection path are superimposed spectrally. In other words, the heterodyne detection signal is formed spectrally by a single peak that combines all contributions generated by the plurality of pulse spectral components. For this first embodiment, the secondary output of the transmission path connected to the reference input of the detection path can be located downstream of the comb-generating modulator in the transmission path relative to the propagation direction of radiation in the transmission path.

[0030] In a second embodiment of the invention, the reference input of the detection path can be connected to the secondary output of a differently located transmission path to receive a monochromatic or quasi-monochromatic optical reference signal. Subsequently, the heterodyne detection contribution associated with the pulse spectral components in the heterodyne detection signal generated by the detection path is spectrally shifted relative to each other according to the distribution of the pulse spectral components. In this case, the spectral analysis module is adapted to derive the value of the Doppler effect frequency shift based on the center frequency value relative to each of the heterodyne detection contributions. For this second embodiment, the secondary output of the transmission path connected to the reference input of the detection path can be located upstream of the comb-generating modulator in the transmission path relative to the propagation direction of the radiation in this transmission path.

[0031] In a preferred embodiment of the invention, at least one of the following additional properties may be optionally reproduced, individually or in combination:

[0032] - The LIDAR system can be adapted to provide an estimate of the airflow velocity component when the system is pointed to emit a radiation pulse toward a portion of the atmosphere containing suspended particles that form a target, the particles being backscatterers of the radiation;

[0033] - The transmission path can be further configured such that the spectral components of each pulse are spectrally separated by at least 10 MHz, preferably at least 20 MHz, and at most 2000 MHz;

[0034] - The transmission path can be further configured such that the spectral difference between any two that are spectral neighbors in the pulse spectral components is constant between different pairs of adjacent pulse spectral components.

[0035] - The transmission path can be further configured such that the number of spectrally non-intersecting pulse spectral components constituting each pulse is between 2 and 20, preferably between 4 and 12.

[0036] - Each pulse spectral component can be monochromatic or quasi-monochromatic;

[0037] -When using this modulator, the comb-generating modulator can be an electro-optic modulator;

[0038] - When using this generator, the modulation signal generator can be an electrical generator with arbitrary waveforms; and

[0039] - The transmission path and / or detection path may be implemented using fiber optic technology to interconnect the components of this transmission path and / or detection path. Attached Figure Description

[0040] The features and advantages of the invention will become clearer from the following detailed description of some non-limiting exemplary embodiments, with reference to the accompanying drawings, in which:

[0041] [ Figure 1a [This is a block diagram of a pulsed LIDAR device with heterodyne detection as known in the prior art;]

[0042] [ Figure 1b [will be with] Figure 1a The two spectral pattern groups related to the operation of the LIDAR system;

[0043] [ Figure 2 [This is a timing diagram showing the possible spectral composition of pulses used in the operation of the LIDAR system according to the present invention;]

[0044] [ Figure 3a The first embodiment of the present invention corresponds to [ Figure 1a ];

[0045] [ Figure 3b ]against[ Figure 3a The LIDAR system of the first embodiment of the present invention corresponds to [ Figure 1b ];

[0046] [ Figure 4a The second embodiment of the present invention corresponds to [ Figure 1a ];

[0047] [ Figure 4b ]against[ Figure 4a The LIDAR system of the second embodiment of the present invention corresponds to [ Figure 1b ];as well as

[0048] [ Figure 5 Another embodiment of the present invention corresponds to [ Figure 2 ]. Detailed Implementation

[0049] In these figures, all components are represented symbolically, and the same reference numerals indicated in different figures designate the same or functionally identical elements. For clarity, components known to those skilled in the art and not directly of concern in this invention are not described below in relation to their use in LIDAR systems. In this case, their possible adaptation to the invention is within the capabilities of those skilled in the art. [In [ Figure 1a ]、[ Figure 3a ]and[ Figure 4a In this context, the following reference numerals have the meanings indicated here:

[0050] 100 General markings of pulsed LIDAR with heterodyne detection

[0051] 10 Transmission Path

[0052] 11 laser emission sources, designated as LASER

[0053] A 12-frequency shift and pulse-cut modulator, denoted as MAO

[0054] 13. Optical amplifier, denoted as AMPL.

[0055] 14 Optical Circulator

[0056] 15. Emitting optical devices, denoted as OPT.

[0057] 16. Secondary output of the transmission path

[0058] 20 Detection Path

[0059] 21. Heterodyne detector, denoted as DETECT.

[0060] 30. Spectral analysis module, denoted as ANALYS.

[0061] [ Figure 1a The diagram shows a system 100 that was known prior to this invention.

[0062] The transmission path 10 includes a laser emission source 11, a modulator 12, an optical amplifier 13, an optical circulator 14, and an emitting optics 15. The laser emission source 11 can be a continuous emission source, for example, having an emission wavelength of approximately 1550 nm and a power of 600 μJ. It thus produces monochromatic or quasi-monochromatic initial laser radiation R0. The initial laser radiation R0 is transmitted to the modulator 12. The modulator 12 can be an acousto-optic type modulator. It is controlled to form identical pulses I from the radiation it has received, each with an individual duration between 200 ns and 800 ns, the pulses having a pulse repetition frequency PRF, for example, which can be 10 kHz. Simultaneously, 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. The pulses I generated by the modulator 12 are thus amplified by the optical amplifier 13 and subsequently transmitted to the emitting optics 15 via the optical circulator 14. The emitting optics 15 may have, for example, a stacked structure. The amplified pulse I is thus transmitted toward the target T, which is outside the LIDAR system 100 and located at a distance D from the system, the distance being measured along the emission direction of the system 100. In principle, the separation distance D is less than the range L of the system 100, which may be, for example, approximately 15 km.

[0063] Therefore, from [ Figure 1a All pulses I emitted by system 100 are identical and are monochromatic or quasi-monochromatic.

[0064] Secondary output 16 is located in transmission path 10, between laser emission source 11 and modulator 12 dedicated to shifting and separating pulse I.

[0065] Detection path 20 shares the transmitting optics 15 and optical circulator 14 with transmission path 10, and further includes a heterodyne detector 21. Within detection path 20, one function of optics 15 is to collect the portion RI of pulse I that has been reflected back or backscattered by target T. Heterodyne detector 21 is optically coupled to receive the portion RI of the reflected or backscattered pulse collected by optics 15 via optical circulator 14, and simultaneously receives the optical reference signal RR collected from transmission path 10 via secondary output 16 of this transmission path. In other words, in addition to the output of optical circulator 14 dedicated to detection path 20, secondary output 16 is also optically coupled to heterodyne detector 21. Heterodyne detector 21 may be a photodiode, specifically an ultrafast photodiode, onto which the optical reference signal RR from secondary output 16 and the pulse portion RI from target T are focused.

[0066] The spectral analysis module 30 is configured to perform spectral analysis on the heterodyne detection signal generated by the detector 21 during operation of the system 100. It is configured to derive the value of the frequency shift existing between the optical reference signal RR and the pulse portion RI from this spectral analysis. It is further configured to convert the frequency shift value thus obtained into a velocity component value V of the target T parallel to the emission direction of the system 100. T In a known manner: V T =-λ0·(ν m -Δν0) / 2, where:

[0067] λ0 indicates the wavelength of laser emission source 11, which is approximately 1550 nm in the example given above.

[0068] Δν0 further indicates the frequency shift applied by modulator 12, which in the example given above is equal to 100MHz, and

[0069] ν m It is a frequency in the radio frequency (RF) domain that is associated with the maximum intensity or central peak position in the spectral decomposition of the heterodyne detection signal.

[0070] System 100 is preferably implemented using fiber optic technology. In this case, the optical amplifier 13 may be of the type specified by an erbium-doped fiber amplifier (EDFA). The initial laser radiation R0 is transmitted from the laser emission source 11 to the modulator 12 via a first fiber segment S1, and then from the modulator to the amplifier 13 via a second fiber segment S2. Additionally, the backscattered or reflected pulse portion RI collected by the optics 15 is injected into a third fiber segment S3 at the output of the optical circulator 14 for delivery to the heterodyne detector 21. In parallel, the secondary output 16 of the transmission path 10 is implemented by a fiber coupler and connected to the heterodyne detector 21 via a fourth fiber segment S4.

[0071] For the operation of the system 100 just described with a target having a return reflection point, the heterodyne detection signal has a frequency ν m The sine change below. Figure 1b The upper part of the diagram shows the spectral composition of the radiation received by the heterodyne detector 21. Figure 1b The horizontal axis of this upper diagram identifies the wavelength values ​​in the optical domain, denoted as λ and expressed in nanometers (nm). The vertical axis identifies spectral intensity values ​​in arbitrary units. The radiation received by the heterodyne detector 21 includes a first contribution consisting of the optical reference signal RR transmitted from the secondary output 16, and a second contribution corresponding to the pulse portion RI that has been reflected back by the target T. For [ Figure 1aIn system 100, the optical reference signal RR is a portion of the initial laser radiation R0, such that [ Figure 1b The corresponding contribution in the upper diagram is a very narrow peak value, denoted as RR. When the target T is located at a single position along the emission direction of system 100, the second contribution also has a narrow peak value shape, denoted as RI. Figure 1b The lower part of the diagram shows the spectral composition of the heterodyne detection signal, which corresponds to the spectral composition of the radiation received by detector 21 as shown in the upper diagram. The heterodyne detection signal consists of a single peak with a frequency of ν. m =Δν0+ν 多普勒 , where ν 多普勒 ≈-2·V T / λ1, where λ1 is the wavelength value of the radiation emitted by the LIDAR system 100. Figure 1b The horizontal axis of the lower part of the diagram identifies the frequency values ​​in the RF domain, denoted as f and expressed in megahertz (MHz). The vertical axis is again in arbitrary units to identify the spectral intensity values ​​of the heterodyne detection signal.

[0072] For the operation of system 100 dedicated to airspeed measurement, pulse I is backscattered by a large number of targets distributed along the path of a pulse beam outside system 100, starting from the emitting optics 15. These targets, consisting of particles or aerosols suspended in the air, are pulled along by varying local velocities of air present at each location in the path of the beam. This distribution of targets is commonly referred to by those skilled in the art as “extended targets,” “distributed targets,” or “volume targets.” The portion of the pulse RI collected by optics 15 and subsequently transmitted to detector 21 thus expands over time, corresponding to different separation distances along the emission direction of system 100, at which partial backscattering of pulse I occurs. Furthermore, the pulse portion is frequency-shifted in a manner that varies depending on the local wind speed parallel to the emission direction at the location where partial backscattering occurs. The heterodyne detection signal then exhibits more complex time variations. The spectral analysis performed by module 30 assumes that, for the results, it provides a series of velocity values ​​V. T The velocity values ​​are assigned one-to-one to different values ​​of the separation distance D. In a known manner, the resolution of the separation distance D is determined by the individual duration of the transmitted pulse I, equal to this individual duration divided by twice the pulse propagation speed outside the LIDAR system 100. [With] Figure 1b Compared to the previous diagram, the peak value of the pulse portion RI in the spectral composition corresponding to the radiation received by detector 21 is broadened. The peak value of the spectral composition of the heterodyne detection signal in the RF domain is broadened in a correlated manner.

[0073] [ Figure 2The horizontal axis of the diagram in the figure represents time, denoted as t, and its vertical axis represents the instantaneous emission wavelength λ1 of the LIDAR system 100 according to the invention. Wavelength λ1 is expressed in nanometers (nm). According to this diagram, each pulse I emitted by the LIDAR system 100 consists of multiple simultaneous pulse spectral components, which are thus superimposed to form a pulse. Each pulse spectral component is monochromatic or quasi-monochromatic, and as an example, each pulse I may consist of ten such pulse spectral components. The difference between the wavelength values ​​of two spectrally adjacent pulse spectral components may be constant, but this is not necessarily the case. They can be any value, but sufficient to include the frequency shift of the backscattered or reflected pulse portion RI within all separation intervals between different pulses. For this purpose, these differences are preferably greater than 30 MHz or 50 MHz, a lower limit sufficient to obtain a reduction in stimulated Brillouin scattering. When they are constant, the wavelength difference between adjacent spectral components is called the wavelength increment and is denoted as Δλ1. The wavelength increment Δλ1 corresponds to the frequency increment Δν1, which is equal to -C·Δλ1 / λ0. 2 This frequency increment can be, for example, 200 MHz in the RF domain. Therefore, each pulse I is composed of a comb in the spectrum, and all pulses I continuously emitted by the LIDAR system 100 are identical, having this same individual composition. Such successfully emitted pulses I can have an individual duration of 2 μs (microseconds), emitted every 100 μs.

[0074] In the example just described, the pulse repetition frequency of pulse I is equal to 10 kHz, while the actual pulse frequency used to measure the target velocity, i.e., the frequency PRF, is equal to the product of this pulse repetition frequency and the number of pulse spectral components in each of pulses I, i.e., 100 kHz.

[0075] According to the present invention, this operation can be performed by, as [ Figure 3a The LIDAR system 100, as shown in [ ], generates this system. This system has similar characteristics to [ ] Figure 1aThe hardware architecture differs from that of the previous one, except that the transmission path 10 further includes an additional modulator 17 and a modulation signal generator 18. Modulator 17 may be of the electro-optic modulator type and is denoted as MEO. It is inserted in the first fiber segment S1, between the laser source 11 and the electroacoustic modulator 12. Modulation signal generator 18 may be of the arbitrary waveform generator type and is denoted as AWG. Generator 18 is programmed to transmit an electrically modulated signal, which may consist of the sum of n sinusoidal components, to the control input of modulator 17, where n is an integer. n may be between 1 and 9 (inclusive) and is preferably less than or equal to 5. In a known manner, the electro-optic modulator modulates the phase of the optical radiation supplied to it as input. The amplitude of each sinusoidal component of the electrically modulated signal is proportional to a dimensionless adjustment parameter commonly referred to as the modulation depth. The initial laser radiation R0 from laser source 11 is thus transformed by modulator 17 into radiation R1, which comprises the superposition of multiple spectral components in the form of disjoint lines. Each sinusoidal component of the electrical modulation signal generates multiple spectral components of radiation R1, which are spectrally symmetrical with respect to the wavelength value λ0 of the initial laser radiation R0. Therefore, the positions of all generated spectral components depend on the number of sinusoidal components in the electrical modulation signal and on the value used for their respective modulation depths. Radiation R1 may or may not still include a spectral component at the wavelength value λ0. Although [ Figure 2 The diagram shows that the spectral components are equidistant on the spectrum within each pulse I, but this is not always the case. For example, for an electrically modulated signal, the following composition is possible:

[0076] - A single sinusoidal modulation component associated with the value of 1.44 used for its modulation depth results in radiation R1 consisting mainly of three spectrally non-intersecting lines at the following wavelengths: λ0, λ0-Δλ1, and λ0+Δλ1, where Δλ1 is the wavelength difference between consecutive lines in radiation R1, and the three lines have substantially the same corresponding intensity.

[0077] - A single sinusoidal modulation component associated with the value 2.6 used for its modulation depth results in radiation R1 consisting primarily of four spectrally disjoint lines located at the following wavelengths: λ0-2·Δλ1, λ0-Δλ1, λ0+Δλ1, and λ0+2·Δλ1, which again have substantially the same corresponding intensity; and

[0078] - For example, two sinusoidal modulation components with corresponding frequencies of 100MHz and 30MHz and associated with a value of 1.44 for their corresponding modulation depths result in radiation R1 consisting mainly of nine spectrally disjoint lines of substantially the same corresponding intensity.

[0079] Modulator 12 then applies a frequency offset Δν0 to each of the spectral components of the radiation R1. It further applies time-separated pulses to all spectral components, such that each pulse I emitted outward from transmission path 10 consists of multiple spectral components that are superimposed over the duration of the pulse. In the terminology of those skilled in the art, the radiation R1 output by modulator 17 and each pulse I emitted by system 100 are now constituted by a comb of spectral components. Preferably, the frequency increment Δν1 is greater than the sum of the spectral widths of the electrical modulation signal plus the components of laser source 11, such that the spectral components of the comb do not intersect spectrally.

[0080] Instead of using an arbitrary waveform generator, the n sinusoidal components of an electrically modulated signal can be generated by a combination of n analog electrical oscillators. This alternative method for generating electrically modulated signals is practically simpler to implement.

[0081] Upon retroreflection, each pulse spectral component shifts in the spectrum due to the Doppler effect. Assuming the frequency increment Δν1 is much lower than the optical frequency corresponding to wavelength λ0, all pulse spectral components experience the same Doppler frequency shift ν. 多普勒 Additionally, the frequency increment Δν1 is chosen to be greater than the frequency shift ν for the Doppler effect added to the frequency shift Δν0. 多普勒 All possible expected values.

[0082] for[ Figure 3a In this embodiment, the secondary output 16 of transmission path 10 is located between modulators 17 and 12. In this way, the optical reference signal RR delivered to the heterodyne detector 21 is a portion of the radiation R1. It is therefore also constructed of a comb with a frequency increment Δν1.

[0083] As from [ Figure 3b As shown in the upper diagram, the spectrum of radiation received by the heterodyne detector 21 subsequently includes a comb with a frequency increment Δν1, denoted as RR, and an additional comb, denoted as RI, corresponding to the pulse portion that is reflected back or backscattered and subsequently collected by the optics 15. This additional comb contains measurement information. It is shifted by Δν0+ν relative to the comb RR in terms of optical frequency. 多普勒 It also has Δν1 as a frequency increment. During heterodyne detection, each spectral component of the comb RR interferes with each of the spectral components of the comb RI, but proper filtering eliminates all interference between the components of the comb RR and the components of the comb RI, which are not spectrally adjacent, meaning they are far apart from each other, for example, by more than 0.3·Δν1. Due to its characteristic response time, such filtering can potentially be performed by the detector 21 itself. All that remains in the heterodyne detection signal is the signal corresponding to the comb RR and its shift relative to it, Δν0+ν. 多普勒The contribution of interference between each component of the comb RI. These contributions are then superimposed on each other spectrally in the heterodyne detection signal, as shown in [ Figure 3b As can be seen in the lower part of the diagram. Although these pulse spectral components are incoherent with each other in the pulse portion RI reaching detector 21, they are related to [ Figure 1a Compared to system 100, the heterodyne detection signal has an increased signal-to-noise ratio calculated spectrally. For […] Figure 3a In this embodiment, the spectral analysis module 30 can be connected with [ Figure 1a The spectral analysis module in System 100 is the same.

[0084] [ Figure 4a The embodiment of ] is distinguished from [ ] by the position of the secondary output 16 in the transmission path 10. Figure 3a An embodiment of [ ]. It is now located between the laser emission source 11 and the comb-generating modulator 17. The modulator 17 and the modulation signal generator 18 can be [ ] Figure 3a The embodiments are the same as those, and the electrical modulation signal remains unchanged. The pulse I emitted by system 100 toward the outside therefore has the same spectral composition. On the other hand, the optical reference signal RR delivered from the new secondary output 16 is the same as that in [ Figure 1a The optical reference signal occurring in system 100 is the same as that in system 100. For example, [ Figure 4b As shown in the upper diagram, the spectrum of radiation received by the heterodyne detector 21 includes a peak RR corresponding to the emission from the laser emission source 11, and a comb formed by the collected backscattered or reflected pulse portions RI. This second comb is again labeled RI. Each spectral component of the comb RI again corresponds to a frequency shift ν. 多普勒 The affected pulse spectral components. These spectral components are again incoherent in the already collected backscattered or reflected pulse portion RI. When the heterodyne detector 21 has a sufficiently short response time, the heterodyne detection signal consists of the spectral components of the peak RR and the interference of each of the spectral components of the comb RI. The heterodyne detection signal is then composed of multiple spectral components with an RF center frequency value of Δν0 + ν 多普勒 +i·Δν1, where i is an integer index identifying the spectral component of the heterodyne detection signal. This spectral composition of the heterodyne detection signal consists of [ Figure 4b The lower part of the diagram is shown. The spectral analysis module 30 is based on the central value Δν0+ν. 多普勒 The Doppler effect frequency shift ν is determined by the RF frequency value measured by +i·Δν1. 多普勒 The value. For example, for the value based on the center value Δν0+ν 多普勒 ν for each of +i·Δν1 多普勒 The basic value is determined, and ν is calculated by averaging the basic value. 多普勒The final value. It is possible to use the RF frequency value corresponding to the maximum intensity value of the RI peak in the spectrum of the heterodyne detection signal instead of the center frequency value of the same peak in order to determine the ν inferred from individual positions of the RI peak. 多普勒 The basic value.

[0085] Although not yet combined Figure 3a ]and[ Figure 4a The embodiments described herein provide an implementation of a distance resolution scheme to obtain velocity measurement results involving sampling separation distance values, but the principle for obtaining this distance resolution can be as follows: Figure 1a It is used as described in System 100.

[0086] for[ Figure 3a ]and[ Figure 4a In an embodiment of [ ], each pulse spectral component may have an individual peak power value that is just below the threshold for stimulated Brillouin scattering occurring in fiber segments S1 and S2, and in the optical amplifier 13, optical circulator 14, and the fiber segment between them leading to the emitting optics 15. [ ] Figure 1a Compared to the system 100, the power of each pulse I emitted outward by system 100 to perform velocity measurement is multiplied by the number n of spectral components used in each pulse. comp For the operation of a LIDAR system with heterodyne detection, therefore, the factor n is obtained. comp 1 / 2 Improvements. The pulsed LIDAR system with heterodyne detection according to the invention is therefore particularly suitable for measurement conditions where the backscatter or reflective pulse portion has low or extremely low power. They are therefore particularly suitable for performing airspeed measurements.

[0087] In the embodiments of the invention described above, the corresponding spectral compositions of the pulses continuously emitted by the LIDAR system are identical. However, this characteristic is not essential for the present invention. Figure 5 The diagram illustrates a series of pulses I continuously emitted by a LiDAR system, each pulse I consisting of two disjoint spectral components separated by a wavelength difference of 2·Δλ1. This spectral difference 2·Δλ1 varies between two consecutive pulses I, for example, periodically during a complete series of pulses I. The use of this series of pulses with its variable spectral composition is based on […]. Figure 3a ]and[ Figure 4a The two embodiments of the LIDAR system invented by [ ] are compatible. Figure 5 The series of emission pulses shown are obtained in both cases by properly controlling the electro-optic modulator 17. Although [ Figure 5Only two spectral components per pulse are shown, but the number of spectral components per pulse can be greater. Specifically, but optionally, the control of the electro-optic modulator 17 can be configured such that the radiation R1 directly from the modulator 17 does not contain any significant spectral amplitude components at the wavelength value λ0 of the laser emission source 11 before being transmitted to the acousto-optic modulator 12.

[0088] It should be understood that the invention can be reproduced by modifying minor aspects of the embodiments described in detail above while retaining at least some of the enumerated advantages. Specifically, the following modifications are possible:

[0089] Some of the components used in the described embodiments may be replaced by other components or combinations of components that produce equivalent functionality. For example, each electro-optic modulator may be replaced by a semiconductor optical amplifier or SOA used as the modulator;

[0090] - Generally, for the purposes of this invention, the spectrally disjoint pulse spectral components used to constitute each pulse emitted by the LIDAR system do not necessarily have equal corresponding component intensities. In other words, the spectrally disjoint pulse spectral components used to constitute each pulse may have different maximum spectral intensities or corresponding total intensities within the same emitted pulse; and

[0091] - All values ​​cited are for illustrative purposes only and may be changed depending on the application being considered for a pulsed LIDAR system with heterodyne detection.

Claims

1. A pulsed LIDAR system (100) adapted to determine the Doppler effect frequency shift (v) experienced by a series of radiation pulses (I) continuously emitted by the system toward a target (T). 多普勒 The value of ), wherein the Doppler effect frequency shift is between a portion of the pulse received after retroreflection or backscattering on the target and the pulse emitted by the system, and the system is adapted to provide a velocity component (V) of the target parallel to the optical emission direction of the system based on the value determined for the frequency shift. T The estimate, The system (100) includes: - Transmission path (10), which is configured to generate the series of pulses (I). - Detection path (20), which is configured to detect the pulse portion (RI) received after retroreflection or backscattering on the target (T), and generate a heterodyne detection signal corresponding to the series of pulses (I), - Spectral analysis module (30), which is adapted to perform spectral analysis of the heterodyne detection signal, such that the frequency shift (v 多普勒 The value of ) is generated by the heterodyne detection contribution corresponding to the series of pulses (I), and The transmission path (10) is further configured to form each of the pulses (I) as a superposition of multiple pulse spectral components, which are emitted simultaneously, are spectrally non-intersecting, and are associated one-to-one with different center wavelength values. The system (100) is adapted such that the frequency determined by the spectral analysis module (30) is shifted (v 多普勒 The value of ) is generated by multiple heterodyne detection contributions from the pulse spectral components corresponding to the series of pulses (I), respectively. The system (100) is further adapted such that the non-intersecting pulse spectral components of two consecutively emitted pulses (I) are separated by a component-to-component spectral difference that varies between the two consecutive pulses, and such that the component-to-component spectral difference varies periodically over a complete series of pulses.

2. The pulsed LIDAR system (100) according to claim 1, which is adapted to provide an estimate of the airflow velocity component when the system is directed to emit the radiation pulse (I) toward a portion of the atmosphere containing suspended particles forming the target (T), the particles being backscatterers of the radiation.

3. The pulsed LIDAR system (100) according to claim 1 or 2, characterized in that, The transmission path (10) is further configured such that the spectral components of each pulse (I) are spectrally separated by at least 10 MHz and at most 2000 MHz.

4. The pulsed LIDAR system (100) according to claim 3, wherein, The spectral components are separated by at least 20 MHz in the spectrum.

5. The pulsed LIDAR system (100) according to claim 1 or 2, characterized in that, The transmission path (10) is further configured such that the spectral difference (Δv1) between any two of the pulse spectral components that are spectral neighbors is constant between different pairs of pulse spectral components that are neighbors.

6. The pulsed LIDAR system (100) according to claim 1 or 2, wherein the transmission path (10) is further configured such that the number of the spectrally disjoint pulse spectral components constituting each pulse (I) is between 2 and 20.

7. The pulsed LIDAR system (100) according to claim 6, wherein, The number of pulse spectral components is between 4 and 12.

8. The pulsed LIDAR system (100) according to claim 1 or 2, characterized in that, The transmission path (10) includes: - A laser emission source (11) is adapted to generate initial laser radiation (R0). - A comb-generating modulator (17) is arranged to modify the initial laser radiation (R0) according to a modulation signal applied to a control input to the comb-generating modulator; and - A modulation signal generator (18) is connected to apply the modulation signal to the control input of the comb-generating modulator (17). The modulation signal causes the initial laser radiation (R0) to be transformed by the comb-generating modulator (17) into a set of spectral components that form the pulse spectral components one-to-one.

9. The pulsed LIDAR system (100) according to claim 8, characterized in that, The comb-generating modulator (17) is electro-optical, and a pulse sequence (I) with a component spectral difference that varies between two consecutively emitted pulses is obtained by controlling the electro-optic comb-generating modulator.

10. The pulsed LIDAR system (100) according to claim 9, characterized in that, The control of the electro-optic comb generating modulator (17) is configured such that the radiation (R1) directly from the modulator does not contain spectral components at the emission wavelength value of the laser emission source (11) before being transmitted to the acousto-optic modulator (12) which is dedicated to frequency shifting and pulse separation.

11. The pulsed LIDAR system (100) according to claim 10, characterized in that, The reference input of the detection path (20) is connected to the secondary output (16) of the transmission path (10) to receive an optical reference signal (RR), which includes a reference spectral component corresponding one-to-one with the pulse spectral component of the pulse (I), and a spectral shift (Δv0) between each pulse spectral component and the reference spectral component corresponding to the pulse spectral component, the spectral shift being the same for all pulse spectral components. This causes all the heterodyne detection contributions associated with the pulse spectral components in the heterodyne detection signal generated by the detection path (20) to be superimposed on the spectrum.

12. The pulsed LIDAR system (100) according to claim 11, characterized in that, The secondary output (16) of the transmission path (10) is located downstream of the comb-generating modulator (17) in the transmission path relative to the propagation direction of the radiation in the transmission path.

13. The pulsed LIDAR system (100) according to claim 10, characterized in that, The reference input of the detection path (20) is connected to the secondary output (16) of the transmission path (10) to receive a monochromatic optical reference signal (RR). This causes the heterodyne detection contribution associated with the pulse spectral component in the heterodyne detection signal generated by the detection path (20) to shift spectrally relative to each other according to the distribution of the pulse spectral components. Furthermore, the spectral analysis module (30) is adapted to derive the Doppler effect frequency shift (v) based on the center frequency value relative to each of the heterodyne detection contributions. 多普勒 The value stated therein.

14. The pulsed LIDAR system (100) according to claim 13, characterized in that, The secondary output (16) of the transmission path (10) is located upstream of the comb-generating modulator (17) in the transmission path relative to the propagation direction of the radiation in the transmission path.

15. The LIDAR system (100) according to claim 1 or 2, characterized in that, The transmission path (10) and / or the detection path (20) are implemented using optical fiber technology to interconnect the components of the transmission path and / or the detection path.

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