LIDAR system for differential absorption and background distance measurements

Abstract

To provide a new LIDAR system attainable by an optical fiber that can execute IPDA (Integrated Path Differential Absorption) and can determine separation distance from an obstacle existing in the background of a measurement region.SOLUTION: A LIDAR system is configured so as to execute differential absorption measurement of a chemical compound between two separate optical frequencies (ν1, ν2) and measure separation distance from an obstacle existing in the background of a measurement region in which absorption occurs. Since the LIDAR system can implement an optical fiber technique while having sufficient light emitting power, a light emitting light power value is changed at a different time interval between radiation light emitting sequences. The LIDAR system can evaluate the amount of chemical compounds included in the measurement region, and separation distance from an obstacle positioned in the background of the measurement region.SELECTED DRAWING: Figure 1a

Description

Detailed Description of the Invention

[0001] [Technical field] The description provided herein relates to a LIDAR system suitable for performing differential absorption and background distance measurements. The invention also relates to a method for measuring the amount of a chemical compound using such a system.

[0002] [Prior art] It is known to use differential absorption measurements to evaluate the amount of a chemical compound contained in a measurement area. For this purpose, the absorption of radiation emitted toward the measurement area is measured for a first frequency of radiation that does not correspond to the absorption band of the chemical compound, and for a second frequency of radiation that corresponds to the absorption band of the chemical compound. Therefore, it is necessary to emit radiation of both frequencies toward the measurement area and compare the absorption levels measured for each frequency. This type of measurement method is commonly called IPDA (Integrated Path Differential Absorption). It is also necessary to know the depth of the measurement area, for example, the distance to the location of obstacles present in the background of the measurement area.

[0003] Performing such IPDA measurements using LIDAR systems is particularly useful due to the detection sensitivity and ability to analyze the detection signals they provide. Therefore, the ability to use LIDAR systems made from optical fibers is an important challenge, as such systems offer reduced size, greater energy efficiency, and greater resistance to loss of coordination between the optical components they are made of.

[0004] However, IPDA measurements must meet the following requirements: It must be possible to emit at least two radiation pulses at different optical frequency values ​​in rapid succession, compared to the variations that may affect the chemical compounds contained in the measurement region. More precisely, both radiation pulses must be emitted at a repetition rate that is sufficiently high, compared to the variations in the chemical compounds in the measurement region, to allow the analysis of the detection signal at repetition rates higher than, for example, 1 kHz (kilohertz); Both radiations, i.e., radiation outside the absorption bands of the chemical compound to be measured and radiation corresponding to one of the absorption bands of said chemical compound, must each be emitted with sufficient energy; The radiation corresponding to one of the absorption bands of the chemical compound must have a sufficiently narrow spectral linewidth to provide sufficient accuracy in determining absorption by the chemical compound. In particular, if the central wavelength of the radiation is about 1.6 μm (micrometers), this spectral linewidth of the radiation corresponding to one of the absorption bands of the chemical compound may need to be less than 100 MHz (megahertz); Measuring the separation distance of background obstacles by characterizing the elapsed time requires the use of emitted light pulses of individual duration that are very short, typically less than about 100 ns (nanoseconds).

[0005] However, the well-known phenomenon of stimulated Broughton scattering in optical fibers limits the peak power of the radiation pulses that can be emitted by optical fiber-based LIDAR systems, thereby limiting the maximum distance of these fiber-optic LIDAR systems.

[0006] [Technical issues] Taking these constraints into account, the object of the present invention is to provide a novel LIDAR system, achievable using optical fibers, that both performs IPDA measurements and determines the separation distance from obstacles present in the background of the measurement area.

[0007] More specifically, the present invention aims to provide a LIDAR system implemented with at least one optical fiber to transmit emitted radiation, wherein the peak power limitations caused by stimulated Broughton scattering are extended or eliminated.

[0008] [Summary of the Invention] To achieve this and other objects, a first aspect of the present invention provides a LIDAR system configured to perform differential absorption measurements between two distinct optical frequencies and to measure the distance from obstacles in the background of a measurement region where absorption occurs, the LIDAR system comprising a laser source assembly suitable for generating radiation at either of two optical frequencies, intensity modulation means configured to apply a pulse envelope shape, including a pulse duration and a pulsed optical power value, to each of the radiation, and an emission controller configured to control the intensity modulation means.

[0009] In the context of the present invention, pulsed light power is understood to mean a value characterizing the intensity of each pulse, this value possibly corresponding to the peak power of the pulse or to the average power evaluated over the duration of the pulse, it being understood that the peak and average power values ​​increase one as a function of the other for a given duration and pulse shape.

[0010] According to the invention, the LIDAR system is configured to emit a sequence of radiation emissions in a target direction in which differential absorption measurements and separation distance measurements from background obstacles are made during operation of the LIDAR system, the radiation emission sequence comprising: the radiated emission is spectrally located at a first optical frequency of the two optical frequencies; and The first spectral width, the first pulse duration, and the first pulse optical power value are have a first time interval the radiated emission is spectrally located at a second optical frequency of the two optical frequencies; and The second spectral width, the second pulse duration, and the second pulse optical power value are have a second time interval, have The radiation emission sequence is

[0011] The first and second time intervals may be consecutive in any manner in the radiation emission sequence, and the radiation emission sequence may have any number of first time intervals between two second time intervals, or vice versa. Additionally or alternatively, the radiation emission sequence may have any number of pulses spaced around the first optical frequency between two pulses spaced around the second optical frequency, or vice versa.

[0012] Furthermore, the radiation emission sequence is i) The first and second spectral widths are determined by the following equations: Do not overlap each other Spectrum region and the first spectral width is greater than the second spectral width, ii) the first pulse light power value is greater than the second pulse light power value, and iii) the first pulse period is shorter than the second pulse period.

[0013] Such LIDAR systems may be optical fiber based, and in particular their laser source assemblies may be of the MOPFA type (Master Oscillator Power Fiber Optic). In this MOPFA type LIDAR system, emitted light pulses are first generated with a desired spectral width, modulated according to a desired envelope shape, separated between successive pulses, and then amplified and emitted externally.

[0014] By virtue of the first spectral width being greater than the second spectral width, the first pulse optical power value can be selected to be high, or very high, without the optical fiber used to create such a LIDAR system experiencing stimulated Brookhaven scattering effects that would interfere with the operation or use of the system.

[0015] For use in IPDA measurements, the radiation emission during a first time interval is selected to be outside the absorption band of a chemical compound relevant to the measurement. The radiation emission during the first time interval is further used to estimate the distance from obstacles present in the background of the measurement area. This remote measurement is performed by determining the elapsed time of radiation pulses emitted during the first time interval between the LIDAR system and the background obstacles and back. During these first time intervals, the high pulse light power achieved by the widened spectral width and the short pulse duration improve accuracy in estimating the distance from the background obstacles. The radiation emission during a second time interval is selected to be within one of the absorption bands of the chemical compound. During the second time interval, the low spectral width of the pulses improves accuracy in estimating the amount of the chemical compound. The amount of the chemical compound is estimated based on an absorption ratio determined by the LIDAR system between radiation emitted during a second time interval around the second optical frequency and radiation emitted during the first time interval around the first optical frequency, taking into account the estimated distance from the background obstacles.

[0016] Preferably, i) the repetition frequency of the radiation emission sequence may be between 1 kHz and 50 kHz; ii) the first time intervals may have individual durations of 10 ns to 200 ns, preferably 50 ns to 100 ns; iii) the second time intervals may have individual durations of 0.1 μs (microseconds) to 10 μs, preferably 0.5 μs to 5 μs; iv) a first spectral width of the radiation emitted by the LIDAR system in the measurement area during the first time intervals may be between 100 MHz and 2000 MHz, preferably between 500 MHz and 1000 MHz; v) a first spectral width of the radiation emitted by the LIDAR system in the measurement area during the second time intervals may be between 10 MHz and 200 MHz, preferably between 50 MHz and 100 MHz; and one or more of the above features i) to v) may be selected by the light emission control device.

[0017] Finally, the laser source assembly may be configured such that the radiation emitted by the LIDAR system within the measurement region during the first and second time intervals, respectively, has first and second optical frequency values ​​corresponding to wavelengths between 1.3 μm and 1.8 μm, particularly between 1.5 μm and 1.6 μm, or at approximately 2 μm, which wavelength range is particularly suitable for measuring the amount of carbon dioxide contained within the measurement region.

[0018] Additionally, the LIDAR system of the present invention includes detection paths corresponding to the first and second optical frequencies and configured to independently detect, process, and analyze backscattered radiation corresponding to the emissions during the first and second time intervals, respectively.

[0019] In some cases, the LIDAR system may further comprise a calculation unit connected from the input of the detection path to at least one output, the calculation unit being configured to provide an estimate of the separation distance from background obstacles and the amount of chemical compound contained in the measurement region based on the analytical signal generated by the detection path.

[0020] Depending on the different possible embodiments of the present invention, and in particular depending on the different types of laser oscillators used in the laser source assembly to generate radiation at each of the two optical frequencies, the first and / or second spectral widths may be inherent or may be generated by dedicated spectral broadening means. Inherent spectral width is understood to mean the spectral width of the radiation generated by the corresponding laser oscillator. In other words, in the former case, the spectral width of the pulses emitted during the first and / or second time intervals corresponds to the spectral width of the corresponding laser oscillator. In other cases, the LIDAR system further comprises spectral broadening means arranged to modify the spectral width of at least one of the radiation generated by the laser source assembly.

[0021] In a first embodiment of the present invention, the light emission controller may be configured to control the laser source assembly, the intensity modulation means and, where appropriate, the spectral broadening means, so that in a radiated light emission sequence, a first optical frequency of the two optical frequencies is exclusively associated in a first radiated light pulse with a first spectral width, a first pulse duration and a first pulse optical power value excluding the second spectral width, the second pulse duration and the second pulse optical power value, and a second optical frequency of the two optical frequencies is exclusively associated in a second radiated light pulse separated from the first pulse with a second spectral width, a second pulse duration and a second pulse optical power value excluding the first spectral width, the first pulse duration and the first pulse optical power value.

[0022] For such a first embodiment, the LIDAR system may be configured such that the laser source assembly comprises a first laser oscillator configured to generate radiation at a first optical frequency and a second laser oscillator configured to generate radiation at a second optical frequency having a second spectral width. The spectral broadening means then comprises a phase modulator disposed in a path of the radiation generated by the first laser oscillator and controlled by the emission controller to provide the radiation generated by the first laser oscillator with the first spectral width. Furthermore, the LIDAR system further comprises an optical switch controlled by the emission controller to transmit either the radiation from the phase modulator or the radiation generated by the second laser oscillator to a downstream portion of the emission optical path shared by the radiation from the phase modulator and the radiation generated by the second laser oscillator and comprising the intensity modulation means.

[0023] According to another configuration that is also practicable with respect to the first embodiment of the present invention, the laser source assembly comprises a first laser oscillator configured to generate radiation at a first optical frequency and a second laser oscillator configured to generate radiation at a second optical frequency having a second spectral width. The spectral broadening means comprises a phase modulator arranged in the path of the radiation generated by the first laser oscillator and controlled by the emission controller to provide the radiation generated by the first laser oscillator with the first spectral width. However, in this alternative configuration, the intensity modulation means comprises a first intensity modulator arranged in the path of the radiation emerging from the phase modulator and controlled by the emission controller to exert an effect on the radiation emerging from the phase modulator, and a second intensity modulator arranged in the path of the radiation generated by the second laser oscillator and controlled by the emission controller to exert an effect on the radiation generated by the second laser oscillator. The LIDAR system then further comprises an optical coupler configured to transmit the radiation emerging from the first intensity modulator and the second intensity modulator into a downstream portion of the radiation path shared by the radiation emerging from the first intensity modulator and the second intensity modulator.

[0024] In these two configurations, the downstream portion of the emission light path may comprise an optical emission amplifier, or an optical emission amplification chain, controlled by an emission control device and generating the first and second optical power values ​​in accordance with the inventive features of the emission emission sequence.

[0025] In a second embodiment of the present invention, which is an alternative to the first embodiment described above, the light emission control device is configured to control the laser source assembly, the spectral broadening means and the intensity modulation means so that the radiated light emission sequence comprises a series of radiated light pulses that are spectrally located at either a first optical frequency or a second optical frequency of two optical frequencies, and all of the radiated light pulses have the same envelope shape for both the first optical frequency and the second optical frequency, with a first period during which the radiated light emission has a first spectral width and a first optical power value, and a second period during which the radiated light emission has a second spectral width and a second optical power value, and the first period is shorter than the second period and occurs before or after this second period for each radiated light pulse.

[0026] Generally, with respect to the present invention, LIDAR systems may implement fiber optic technology.

[0027] Also generally with regard to the present invention, the LIDAR system may comprise polarization means configured such that the radiation emitted by the LIDAR system towards the measurement area has orthogonal polarizations, in particular opposite circular polarizations, when emitted during the first or second time interval, in which case the detection path comprises a first spectrum comprising a first optical frequency combined with a first spectral width. region a first detector sensitive to a second spectrum including a second optical frequency combined into a second spectral width; region The second detector may include a polarizing beam splitter configured to transmit the backscattered radiation depending on the polarization of the backscattered radiation to either the first detector or the second detector that is sensitive in the common sensitivity spectrum of the two detectors. region The two detectors may be identical if the optical frequency comprises both a first optical frequency combined with a first spectral width and a second optical frequency combined with a second spectral width.

[0028] A second aspect of the present invention provides a method for measuring the amount of a chemical compound present in a target direction, comprising: selecting a LIDAR system according to the first aspect of the present invention such that the chemical compound has a lower absorption capacity value at a first optical frequency than at a second optical frequency; pointing the selected LIDAR system toward a measurement area likely to contain the chemical compound in a target direction to emit radiation according to a radiation emission sequence, thereby triggering operation of the LIDAR system; estimating a separation distance from an obstacle present in the background of the measurement area based on backscattered radiation associated with the first optical frequency and corresponding to the emission during a first time interval; and estimating the amount of the chemical compound contained in the measurement area, integrated over a path of pulses between the LIDAR system and the background obstacle, based on brightness values ​​separately associated with backscattered radiation at the first and second optical frequencies corresponding to the first and second time intervals in the radiation emission sequence, respectively, which backscattered radiation is detected by a detection path of the LIDAR system.

[0029] Preferably, the separation distance from obstacles present in the background of the measurement region may be estimated based on the elapsed time measured for the backscattered radiation associated with the first optical frequency, in which case the separation distance estimated based on the backscattered radiation associated with the first optical frequency can be used to estimate the amount of chemical compound integrated over the path of a pulse contained within the measurement region, in combination with intensity values ​​separately associated with the backscattered radiation detected at the first optical frequency and the second optical frequency and corresponding to the emission during the first and second time intervals, respectively.

[0030] The chemical compounds relevant to the assay of the present invention may be any of carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and water (H2O).

[0031] Finally, different conditions for carrying out the present invention are enumerated below. Regarding a first implementation condition, a LIDAR system may be installed on the Earth's surface and directed to measure the amount of chemical compounds present between the LIDAR system and an obstacle. Regarding a second implementation condition, a LIDAR system may be mounted on an aircraft in flight and directed toward a geographical area to measure the separation distance between the LIDAR system and the surface of the geographical area and to measure the amount of chemical compounds present between the LIDAR system and the surface of the geographical area. And regarding a third implementation condition, a LIDAR system may be mounted on a satellite in orbit around the Earth and directed toward a geographical area of ​​the Earth's surface to measure the separation distance between the LIDAR system and the surface of the geographical area and to measure the amount of chemical compounds present between the LIDAR system and the surface of the geographical area.

[0032] For the second and third implementation conditions, the ground surface within the geographical area towards which the LIDAR system is directed acts as an obstacle present in the background of the measurement area.

[0033] [Brief description of the drawing] The features and advantages of the present invention will become more clearly apparent in the following detailed description of certain non-limiting embodiments, by reference to the accompanying drawings.

[0034] FIG. 1a shows the spectral variation of a first radiation emission sequence implemented in a first possible embodiment of the present invention.

[0035] FIG. 1b shows the radiation power fluctuations for the first radiation emission sequence of FIG. 1a.

[0036] FIG. 2a shows the spectral variation of a second radiation emission sequence implemented in a second possible embodiment of the present invention.

[0037] [Figure 2b] shows the radiation power fluctuations for the second radiation emission sequence of [Figure 2a].

[0038] [Figure 3a] is a block diagram of an emission path of a LIDAR system according to the present invention configured to generate the first radiation emission sequence of [Figures 1a] and [Figure 1b].

[0039] [Figure 3b] is a block diagram of an arrangement for generating the first radiation emission sequence of [Figures 1a] and [Figure 1b], a first variation of the emission path compared to the system of [Figure 3a] for another LIDAR system according to the present invention.

[0040] [Figure 3c] is a block diagram of a second variation of the emission path for yet another LIDAR system according to the present invention, compared to the system of [Figure 3a], for similarly generating the first radiation emission sequence of [Figures 1a] and [Figure 1b].

[0041] [Figure 3d] is a block diagram of a third variation of the emission path for yet another LIDAR system according to the present invention, compared to the system of [Figure 3a], for similarly generating the first radiation emission sequence of [Figures 1a] and [Figure 1b].

[0042] [Figure 3e] is a block diagram of a fourth variation of the emission path for yet another LIDAR system according to the present invention, compared to the system of [Figure 3a], for similarly generating the first radiation emission sequence of [Figures 1a] and [Figure 1b].

[0043] FIG. 4a is a block diagram of the emission path of yet another LIDAR system according to the present invention, similarly configured to generate the first radiation emission sequence of FIGS. 1a and 1b.

[0044] [Figure 4b] is a block diagram of a variation of the emission path for yet another LIDAR system according to the present invention, similarly configured to generate the first radiation emission sequence of [Figures 1a] and [Figure 1b], compared to the system of [Figure 4a].

[0045] [Figure 5a] is a block diagram of the emission path of yet another LIDAR system according to the present invention, similarly configured to generate the second radiation emission sequence of [Figures 2a] and [Figure 2b].

[0046] [Figure 5b] is a block diagram of a variation of the emission path compared to the system of [Figure 5a] for yet another LIDAR system according to the present invention, which is also configured to generate the second radiation emission sequence of [Figures 2a] and [Figure 2b].

[0047] [Fig. 6a] is a block diagram of the detection path of the LIDAR system according to the present invention.

[0048] [Figure 6b] is a block diagram of a variation of the detection path compared to the system of [Figure 6a].

[0049] [Figure 6c] is a block diagram of another variation of the detection path compared to the system of [Figure 6a].

[0050] [Figure 6d] is a block diagram of yet another variation of the detection path compared to the system of [Figure 6a].

[0051] [Mode for Carrying Out the Invention] For clarity, the dimensions of the elements shown in [Figure 1a], [Figure 1b], [Figure 2a], and [Figure 2b] do not correspond to actual dimensions or actual dimensional ratios. In addition, all elements are only symbolically represented in the drawings, and the same reference symbols shown in different drawings indicate the same elements or elements having the same function.

[0052] In Figures 1a, 1b, 2a, and 2b, the x-axis identifies times, denoted t, during radiation emission sequences consisting of radiation pulses spectrally positioned either around a first optical frequency, denoted v1, or around a second optical frequency, denoted v2. The durations of the first and second time intervals between these radiation emission sequences are denoted Δt1 and Δt2, respectively. In Figures 1a and 2a, the y-axis indicates the value of the optical frequency, denoted v, for each time in the radiation emission sequence, and in Figures 2a and 2b, the y-axis indicates the instantaneous radiation intensity, denoted P. Generally, the radiation pulses, denoted 1, are spectrally positioned around the optical frequency v1 and are assumed to be identical to each other. Similarly, the radiation pulses, denoted 2, are spectrally positioned around the optical frequency v2 and are assumed to be identical to each other. To apply the present invention to the measurement of the differential absorption of a chemical compound between two different optical frequencies, all emitted optical pulses 1 and 2 are emitted in the direction of a measurement region likely to contain the amount of the chemical compound to be measured. The optical frequency v1 is intended to be selected outside the absorption band of the chemical compound, and the optical frequency v2 is intended to be selected within one of the absorption bands of this chemical compound. In the case where the chemical compound is carbon dioxide, the optical frequency v1 may be selected to be equal to 190.81 THz, which corresponds to a wavelength value λ1 equal to 1572.2 nm, and the optical frequency v2 may be selected to be equal to 190.84 THz, which corresponds to a wavelength value λ2 equal to 1572.02 nm.

[0053] 1a and 1b show a first radiation emission sequence that can be implemented according to the present invention. In such a first radiation emission sequence, all radiation pulses 1 have a first common spectral width Δν1, a common peak emission power P1, and a pulse duration equal to Δt1. Similarly, all radiation pulses 2 of the same radiation emission sequence have a second common spectral width Δν2, a common peak emission power P2, and a pulse duration equal to Δt2. The peak emission power values ​​P1 and P2 correspond to radiation emitted by the LIDAR system according to the present invention, and the radiation exits the LIDAR system in the direction of the measurement region, particularly after final optical amplification within the LIDAR system. In the first radiation emission sequence, pulses 1 of individual duration Δt1 correspond to the first time interval introduced in the general description of this specification, and pulses 2 of individual duration Δt2 correspond to the second time interval. Due to the larger spectral broadening for pulse 1 compared to pulse 2, i.e., Δν>Δν, the emission peak power value P occurs in the optical fiber of the LIDAR system, P SBS and the emission peak power value P2 may be greater than the power limit due to stimulated Brillouin scattering, which is expressed as SBS It may be smaller than

[0054] 2a and 2b show the same second radiation emission sequence, which can also be implemented in accordance with the present invention. In such a second emission sequence, all radiation pulses 1 and 2 emitted toward a measurement region likely to contain a measured amount of a chemical compound have the same envelope shape displaced by the optical frequency of each pulse, with v1 representing pulse 1 and v2 representing pulse 2 appearing alternately. The envelope shape is such that within each of pulses 1 and 2, the radiation of pulse 1 or pulse 2 has a spectral width value Δv1 and an emission peak power value P1 at a first time during a period Δt1. Intervaland a second time interval of duration Δt2 during which the emitted light of Pulse 1 or Pulse 2 has a spectral width value Δν2 and an emission peak power value P2. The chronological order between the first and second time intervals, with their own associated values ​​for spectral width and emission peak power, may be reversed within each pulse, and the first and second time intervals may be separated by an intermediate envelope pattern shape within each pulse. As noted above, the power limit P associated with the stimulated Broughton scattering effect is SBS may be smaller than the peak emission power value P1 and larger than the peak emission power value P2.

[0055] For the first emission sequence (FIGS. 1a and 1b) and the second emission sequence (FIGS. 2a and 2b), the radiation emission at optical frequency v1 during time period Δt1 is used to determine the separation distance from the background obstacle. In the second emission emission sequence, the portion of Pulse 1 corresponding to time period Δt2 having a spectral width value Δv2 and a peak emission power value P2 may not be usable to estimate the separation distance from the background obstacle. However, the portion of Pulse 2 corresponding to time period Δt1 having a spectral width value Δv1 and a peak emission power value P1 may optionally be used in addition to the portion of Pulse 1 corresponding to time period Δt1 to determine the separation distance from the background obstacle.

[0056] The second pulse sequence can be generalized in that pulse 1 or pulse 2, around optical frequency ν or optical frequency ν, respectively, may exhibit a spectrally broad, rapid rise in optical power that allows remote measurements to be performed, followed by a slower, spectrally narrow, decrease in optical power that is suitable for differential absorption measurements.

[0057] Regarding these two radiation emission sequences, the first sequence follows [Fig. 1a] and [Fig. 1b], and the second sequence follows [Fig. 2a] and [Fig. 2b], respectively, and the following values ​​are given as non-limiting examples: The pulse repetition frequency may be between 1 kHz (kilohertz) and 50 kHz; Δt1 may be between 50 ns and 100 ns; Δt2 may be between 0.5 μs and 5 μs; Δν1 may be between 500 MHz and 1000 MHz; Δν2 may be between 50 MHz and 100 MHz; P1 can be greater than 200W, P2 can be greater than 50W.

[0058] Therefore, the duration Δt1 of the first time interval may be shorter than the duration Δt2 of the second time interval. Furthermore, the first spectral width value Δν1 may be greater than the second spectral width value Δν2, and the first emission peak power value P1 may be greater than the second emission peak power value P2. Then, by virtue of the amplified first spectral width value Δν1, the value P1 corresponds to a wider emission spectrum than the value P2. region Therefore, the value P1 is the stimulated Brookhaven scattering threshold P corresponding to the optical fiber used to create the LIDAR system. SBS Preferably, the value P2 may be selected to be less than or equal to the stimulated Bourjois scattering threshold in order to limit the loss of energy efficiency in producing the radiation emitted during the second time interval of each period Δt2.

[0059] Several LIDAR system structures according to the present invention are described herein, each designed to emit a radiation light sequence as described above. It should be understood that the description of these structures is limited to the organization of their main components, and that those skilled in the art will recognize commercially available components and know how to combine them into the described structures without difficulty or inventive step. It should also be understood that additional components used in these structures, but not directly related to the principles of the present invention, are not described for clarity. It is advantageous to implement all of these LIDAR system structures described below using fiber optic technology or integrated optical circuit technology for the fabrication of the optical, electro-optical, and interconnect components used. In the diagrams illustrating the light path structures, reference numeral 50 denotes a light emission control device, designated CTRL, connected to the components of the light path to generate a radiation light emission sequence having desired characteristics. The control mode implemented by the light emission control device 50 is within the capabilities of those skilled in the art once the radiation light emission sequence to be generated is provided to them.

[0060] FIG. 3a shows a first possible light emission path configuration for a LIDAR system according to the present invention, designed to generate a radiation emission sequence according to FIGS. 1a and 1b. Reference numeral 10 denotes a laser source, commonly called a laser oscillator, capable of generating a continuous laser beam with an optical frequency ν1. It may be, for example, a laser diode or a fiber laser. The laser beam generated by the laser source 10 passes through a phase modulator 11, labeled MOD.PHASE, which gives the laser beam a spectral width Δν1. In this case, this is a phase modulation external to the laser source. The phase modulator 11 may be, for example, an electro-optic modulator. A random generator of binary signals, commonly known by the acronym PRBS for Pseudo-Random Binary Sequence, or a radio-frequency noise generator of arbitrary waveforms, commonly known by the acronym AWG for Arbitrary Waveform Generator, may be connected to the electrical control input of the phase modulator 11. Among these alternative control modes of the phase modulator 11, the mode used is designated by reference 11c and denoted GENERATOR. If a PRBS generator is used, it generates phase jumps equal to -π or π in a random or pseudorandom sequence. The output of the phase modulator 11 is then advantageously attached to an optical apodization filter (not shown) to remove the secondary lobe that such spectral broadening methods can produce in the spectrum of the radiation directly emitted from the phase modulator 11. If an AWG generator is used, the generator may be programmed to generate various waveforms, such as a series of ramps with randomly variable slopes between successive ramps. Alternatively, it may be programmed to generate an electrical control signal that is sinusoidal or a linear combination of several sinusoidal components. Alternatively, other forms of electrical control signal for the phase modulator 11 may be used, and those skilled in the art will understand how to select the characteristics of such electrical control signal to provide the radiation exiting the phase modulator 11 with the desired spectral envelope waveform having a spectral width Δν.The generator 11c may be selectively activated by the light emission control device 50 to generate pulse 1, or may be activated continuously. Reference numeral 20 denotes another laser source, i.e., another laser oscillator, capable of generating another continuous laser beam with an optical frequency ν2 having a direct spectral width Δν2. For example, the laser source 20 may be a fiber laser. In fact, the spectral width Δν2 of the present invention is low and can be provided directly or inherently by the laser source 20, i.e., without the use of additional components specifically dedicated to generating said spectral width value. The two laser sources 10 and 20 constitute a laser source assembly, as generally indicated herein. The two radiation beams generated by the phase modulator 11 and the laser source 20, respectively, are then injected into two inputs of an optical switch 30, designated COMMUTATOR. This may be a 2x1 optical switch controlled by an emission controller 50 to output radiation received at one or the other of its two inputs in a desired sequence alternating between radiation pulses of optical frequency v1 and radiation pulses of optical frequency v2 during the first and / or second time intervals. Alternatively, the optical switch 30 may be replaced by a fiber optic Y-coupler, e.g., with a 50 / 50 intensity ratio and selectively with maintaining polarization, or by a polarization coupler, e.g., of the polarizing beam splitter cube type. The radiation output from the optical switch 30 is then introduced into an intensity modulator 31, denoted MOD.INT., and controlled by the emission controller 50. This introduction and control is performed so that the radiation ultimately emitted towards the measurement region has an instantaneous power value P1 during the period Δt1 of the first time interval, when the optical frequency is closer to the value v1, and an instantaneous power value P2 during the period Δt2 of the second time interval, when the frequency is closer to the value v2. The intensity modulator 31 may be of the electro-optical, electro-acoustic, or semiconductor optical amplifier type. As is known, such intensity modulators may incorporate an internal controller or may be associated with an external controller that is interposed between the intensity modulator and the light emission controller 50.The emitted light from the intensity modulator 31 is then transmitted to an optical amplifier assembly 32, or optical amplifier chain 32, denoted AMPL., to actually generate the emitted light power values ​​P1 and P2. Finally, the emitted light from the optical amplifier assembly 32 is transmitted to the measurement region by the output optics 33 of the emission path of the LIDAR system, denoted OPT.

[0061] Several alternative architectures for LIDAR systems can be derived from the alternative architecture of [Figure 3a] using at least one of the following equivalence principles, each applied to the emission path architecture of [Figure 3a]: If the laser source 10 is of a type capable of directly generating a laser beam at an optical frequency ν1 having a spectral width value Δν1, such as the laser source 20 of [Fig. 3a], which corresponds to a spectral width value Δν2, the phase modulator 11 may be omitted so that the laser beam generated from the laser source 10 can be sent directly to the optical switch 30, as is done for the laser source 20. In this way, the configuration of [Fig. 3b] is obtained; If the laser source 10 is tunable, the electrical control signal used to impart a spectral width Δν1 to the emitted radiation around the optical frequency ν1 may be applied directly to the control input of the tunable laser source 10. This mode of obtaining the desired spectral width is sometimes called internal phase modulation, as opposed to the use of a phase modulator external to the laser source shown in [Fig. 3a]. Internally modulated laser sources are, for example, laser diodes, which can modulate the current injected into the gain region with a low modulation amplitude, or distributed Bragg reflector diodes (DBR diodes), which can modulate the injection into a phase, grating, or semiconductor optical amplifier region. Additionally or alternatively, this method of obtaining the desired spectral width may be internal to the laser source and applied to the laser source 20, if the latter is tunable, to obtain the spectral width Δν2. In this way, the configuration of [Fig. 3c] is obtained, where references 11c and 21c indicate modulation signal generators connected to the control inputs of the tunable laser source 10 and the tunable laser source 20, respectively; Two separate external phase modulators may be used simultaneously, one between the laser source 10 and the optical switch 30 to impart a spectral width Δν1 to the radiation emitted at the optical frequency ν1, as in [Fig. 3a], and the other between the laser source 20 and the optical switch 30 to impart a spectral width Δν2 to the radiation emitted at the optical frequency ν2. In this way, the configuration of [Fig. 3d] is obtained, in which the references 11 and 21 denote the two external phase modulators associated with the laser sources 10 and 20, respectively, and the references 11c and 21c denote the phase modulation signal generators connected to the respective control inputs of these phase modulators 11 and 21; A single phase modulator can be effectively used for two radiation beams separately generated by laser sources 10 and 20 at optical frequencies v1 and v2. In this case, the laser beams from the two laser sources 10 and 20 are transmitted directly to the inputs of optical switch 30, and a single phase modulator is positioned between the output of optical switch 30 and the input of intensity modulator 31. This single phase modulator can then be controlled in one of the ways described above to generate a spectral width Δν1 during a first time interval when optical switch 30 transmits radiation having optical frequency v1, and a spectral width Δν2 during a second time interval when optical switch 30 transmits radiation having optical frequency v2. In this way, the configuration of FIG. 3e is obtained, where reference 34 denotes the external phase modulator common to the two radiation beams at optical frequencies v1 and v2, and reference 34c denotes a phase modulation signal generator connected to the control input of this phase modulator 34.

[0062] The embodiment of FIG. 4a can be obtained from the embodiment of FIG. 3a by modulating the intensity of radiation around optical frequencies v1 and v2 upstream of the combination of the separate paths for generating the radiation. The radiation generation path around optical frequency v1 is identical to that of FIG. 3a, except for the addition of intensity modulator 12. Similarly, the radiation generation path around optical frequency v2 is identical to that of FIG. 3a, except for the addition of intensity modulator 22. The two intensity modulators 12 and 22 can be controlled by the light emission control device 50 in a manner that is temporally correlated with the modulation signal generated by generator 11c. In particular, they generate transmission time windows that limit the first time interval of discrete duration Δt1 and the second time interval of discrete duration Δt2 to a desired repetition frequency. In such an embodiment, the separate radiation generation paths around the two optical frequencies v1 and v2 can be combined using coupler 35 toward the downstream portion of the radiation path shared by the two optical frequencies and constituting the optical amplifier assembly 32. Coupler 35 may be a conventional Y-coupler, or a polarization coupler capable of imparting a particular polarization to radiation transmitted during a first time interval of discrete duration Δt1 and an orthogonal polarization to radiation transmitted during a second time interval of discrete duration Δt2. For example, radiation transmitted during a first time interval of discrete duration Δt1 may be linearly polarized parallel to a fixed direction by polarization coupler 35, and radiation transmitted during a second time interval of discrete duration Δt2 may be linearly polarized perpendicular to the fixed direction by polarization coupler 35.

[0063] The embodiment of [Figure 4b] can be obtained in the same way as the embodiment of [Figure 4a], but is based on the embodiment of [Figure 3d] instead of the embodiment of [Figure 3a]. By applying the same approach, a person skilled in the art will be able to derive further conceivable embodiments, for example by replacing the single intensity modulator of [Figures 3b], [Figure 3c], or [Figure 3e] with two intensity modulators, each dedicated to radiation around the two optical frequencies v1 and v2.

[0064] All of the embodiments of [Figures 3a] to [Figures 3e] and [Figures 4a] to [Figures 4b] are suitable for generating radiation emission sequences according to [Figures 1a] and [Figure 1b].

[0065] Unlike the embodiments of FIGS. 3a-3e and 4a-4b, in the embodiments of FIGS. 5a and 5b, the phase modulation used to separately obtain the desired spectral widths Δν and Δν during the intervals Δt and Δt is shared between the two optical frequencies ν and ν. In this case, a single phase modulator can be used, which is located downstream of an optical switch or coupler that groups together the separately arriving beams from the two laser sources 10 and 20 in the shared downstream portion of the emitted light path. Therefore, cost reductions for the LIDAR system can be achieved.

[0066] These embodiments of [Fig. 5a] and [Fig. 5b] are suitable for generating radiation emission sequences according to [Fig. 2a] and [Fig. 2b].

[0067] In the embodiment of FIG. 5a, both phase modulation and intensity modulation are performed downstream of the coupling of the optical paths of radiation generated separately from the laser source 10 and the laser source 11. The coupling of the optical paths is performed by a switch 30, with the phase modulation generated by a phase modulator 34 and the intensity modulation generated by an intensity modulator 31. The phase modulation signal generator 34c may also be of the type of PRBS generator, RF noise generator, or AWG generator. The switch 30, the phase modulation signal generator 34c, and the intensity modulator 31 can all be synchronously controlled by an emission controller 50.

[0068] In the embodiment of FIG. 5b, the radiation emitted separately from laser sources 10 and 20 is intensity modulated using two separate modulators, designated by references 12 and 22. The intensity-modulated radiation emitted therefrom may be introduced by coupler 35 in a shared downstream portion of the emission path. Coupler 35 may again be a Y-coupler or polarization coupler, as shown above. The downstream portion of the emission path then includes phase modulator 34, optical amplifier assembly 32, and output optics 33.

[0069] FIG. 6a illustrates a first detection path architecture that can be used in a LIDAR system according to the present invention. Reference 40 denotes the input optics of the detection path, denoted OPT. Their function is to collect a portion of the backscattered radiation corresponding to the radiation emission sequence generated by the LIDAR system. This collected radiation portion is directed onto an optical sensor 43, denoted DETECT.OPT. The optical sensor 43 generates an electrical detection signal, the intensity of which is a function of the power of the detected radiation portion. The optical sensor 43 may operate in direct or coherent detection mode. An optical sensor performing direct detection may, for example, consist of a photodiode associated with a transimpedance amplifier. An optical sensor performing coherent detection, also known as heterodyne detection, requires mixing the backscattered radiation collected by the input optics 40 with a portion of the radiation generated by the laser source assembly. The electrical output of the optical sensor 43 is connected to the input of the analysis chain 512, denoted ANALYS. The analysis chain 512 processes the electrical signals provided by the sensors 43, whether each electrical signal corresponds to pulse 1 or 2.

[0070] FIG. 6b shows a second detection path architecture using two separate analysis chains, each dedicated to radiation pulse 1 and radiation pulse 2. An electrical switch 44, designated COMM.ELEC, then transmits the detection electrical signal to two separate analysis chains 51 and 52, designated ANALYS.1 and ANALYS.2, based on the detection electrical signal corresponding to a first or second time interval during the radiation emission sequence. For this purpose, synchronization of the operation of the electrical switch 44 can be controlled by an emission control device 50. Thus, analysis chain 51 may be designed to be dedicated to a first time interval of discrete duration Δt1 and to determine the residual absorption of the measurement region outside the spectral absorption band of the chemical compound whose amount is to be determined and to determine the distance from obstacles located in the background of the measurement region. Separately, analysis chain 52 may be designed to be dedicated to a second time interval of discrete duration Δt2 and to determine the absorption of the measurement region within the spectral absorption band of the chemical compound. A calculation module (not shown) generates an estimate of the amount of chemical compound as a function of the absorption level determined by the two analysis chains 51 and 52 and the separation distance from background obstacles.

[0071] FIG. 6c shows a third possible detection path architecture. In this architecture, two optical sensors 41 and 42, denoted DETECT.OPT.1 and DETECT.OPT.2, generate separate electrical detection signals that are sent to analysis chains 51 and 52, respectively. The advantage of this architecture is the possibility of using optical sensors 41 and 42 with different sensitivity levels, adapted to the respective instantaneous power values ​​of the relevant portions of the backscattered radiation, i.e., P1 in a first time interval of a respective duration Δt1 for sensor 51 and P2 in a second time interval of a respective duration Δt2 for sensor 52. In this case, the backscattered radiation collected by input optics 40 is directed to sensor 41 or sensor 42 by optical switch 45, the operation of which is controlled by light emission controller 50. For this third detection path architecture, the coupler 35 of the light emission path may be a Y-coupler with a 50 / 50 ratio.

[0072] The three architectures shown in Figures 6a-6c, which represent the detection pathway, are compatible with the modified architectures shown in Figures 3a-3e, 4a-4b, and 5a-5b, which represent the emission pathway.

[0073] Finally, [Fig. 6d] shows a fourth possible detection path architecture, in which the optical switch 45 of [Fig. 6c] is replaced by a polarizing beam splitter 46 based on the linear polarization of the emitted light, denoted SEP.POLAR. This fourth architecture representing the detection path is compatible with the embodiment for the emission path in which the coupler 35 is of the polarizing coupler type, as described above.

[0074] Typically, the optical sensor or at least one optical sensor used in the detection path has a short enough reaction time to allow estimation of the separation distance from background obstacles based on the emitted light during the period Δt1.

[0075] Additionally, the output of each analysis chain of the detection path may be connected to a calculation unit (not shown) integrated along the path of the pulse, which is configured to provide an estimate of the separation distance from background obstacles and an estimate of the amount of chemical compound present in the measurement area based on the signals generated by one or two analysis chains. Such a calculation unit may optionally be integrated into the LIDAR system.

[0076] It will be understood that the invention can be reproduced by modifying the second aspect of the embodiment described in detail above, but still retaining at least some of the cited advantages. In particular, optical components having equivalent functions may be used instead of those described above. Furthermore, the following modifications are mentioned as examples of alternatives that do not involve any inventive step available to a person skilled in the art: One and the same optical frequency switching laser source may be used to generate pulses at optical frequencies ν1 and ν2; Separate optical amplifiers may be used for the pulses at optical frequency ν1 and the pulses at optical frequency ν2; The interlacing of pulses at optical frequency ν1 and pulses at optical frequency ν2 to generate the radiative emission sequence may be performed before or after applying phase and / or intensity modulation to the respective pulses; The interlacing of pulses at optical frequency v1 and pulses at optical frequency v2 to produce the radiative emission sequence may be performed before or after optical amplification of the pulses.

[0077] Finally, all numerical values ​​quoted are for illustrative purposes only and may vary depending on the chemical compound for which the amount is being determined. [Brief explanation of the drawings]

[0078] [Figure 1a] FIG. 2 shows the spectral variation of a first radiation emission sequence implemented in a first possible embodiment of the present invention. [Figure 1b] FIG. 1B is a diagram showing fluctuations in radiation power for the first radiation emission sequence in FIG. [Figure 2a] 4 shows the spectral variation of a second radiation emission sequence implemented in a second possible embodiment of the present invention; [Figure 2b] FIG. 2B is a diagram showing fluctuations in radiation power for the second radiation emission sequence in FIG. 2A. [Figure 3a] FIG. 1B is a block diagram of an emission path of a LIDAR system according to the present invention configured to generate the first radiation emission sequence of FIGS. 1a and 1b. [Figure 3b] FIG. 3B is a block diagram of a first variation of the emission path compared to the system of FIG. 3A for another LIDAR system according to the present invention, which is also configured to generate the first radiation emission sequence of FIG. 1A and FIG. 1B. [Figure 3c] FIG. 3B is a block diagram of a second variation of the emission path for yet another LIDAR system according to the present invention, compared to the system of FIG. 3A, for similarly generating the first radiation emission sequence of FIG. 1A and FIG. 1B. [Figure 3d] FIG. 3B is a block diagram of a third variation of the emission path for yet another LIDAR system according to the present invention, as compared to the system of FIG. 3A, for similarly generating the first radiation emission sequence of FIG. 1A and FIG. 1B. [Figure 3e] FIG. 3B is a block diagram of a fourth variation of the emission path for yet another LIDAR system according to the present invention, compared to the system of FIG. 3A, for similarly generating the first radiation emission sequence of FIG. 1A and FIG. 1B. [Figure 4a] FIG. 1B is a block diagram of the emission path of yet another LIDAR system according to the present invention, similarly configured to generate the first radiation emission sequence of FIGS. 1a and 1b. [Figure 4b] FIG. 4B is a block diagram of a variation of the emission path for yet another LIDAR system according to the present invention, similarly configured to generate the first radiation emission sequence of FIGS. 1A and 1B, compared to the system of FIG. 4A. [Figure 5a]FIG. 2B is a block diagram of the emission path of yet another LIDAR system according to the present invention, similarly configured to generate the second radiation emission sequence of FIGS. 2a and 2b. [Figure 5b] FIG. 5B is a block diagram of a variation of the emission path for yet another LIDAR system according to the present invention, similarly configured to generate the second radiation emission sequence of FIGS. 2a and 2b, compared to the system of FIG. 5a. [Figure 6a] FIG. 2 is a block diagram of the detection path of the LIDAR system according to the present invention. [Figure 6b] FIG. 6B is a block diagram of a variation of the detection path compared to the system of FIG. 6a. [Figure 6c] FIG. 6B is a block diagram of another variation of the detection path compared to the system of FIG. 6a. [Figure 6d] FIG. 6B is a block diagram of yet another variation of the detection path compared to the system of FIG. 6a.

Claims

1. Two distinct optical frequencies (ν １ , ν ２ 1. A LIDAR system configured to perform differential absorption measurements between a target and a target object, and a distance measurement from an obstacle present in the background of a measurement area where the absorption occurs, The two optical frequencies (ν １ , ν ２ a laser source assembly suitable for generating radiation at either Pulse duration and pulse light power value (P １ , P ２ an intensity modulation means configured to apply a pulse envelope shape to each of the radiation beams, the pulse envelope shape including a light emission control device (50) configured to control the intensity modulation means, the LIDAR system is configured to emit a sequence of radiation emissions in a target direction for which differential absorption and separation distance from background obstacles are measured during operation of the LIDAR system; The radiation emission sequence comprises: The radiation is at a first optical frequency (ν １ ) and has a first spectral width (Δν １ ), first pulse period (Δt １ ), and the first pulse light power value (P １ a first time interval having a first time period The radiated light is emitted at a second optical frequency (ν ２ ) and has a second spectral width (Δν ２ ), second pulse period (Δt ２ ), and the second pulse light power value (P ２ a second time interval having The first spectral width (Δν １ ) and the second spectral width (Δν ２ ) are widths such that the radiation emissions during the first time interval and the second time interval correspond to non-overlapping spectral regions, the first spectral width being greater than the second spectral width; The first pulsed light power value (P １ ) is the second pulsed light power value (P ２ ) is larger than The first pulse period (Δt １ ) is the second pulse period (Δt ２ ) shorter, The first optical frequency (ν １ ) and the second optical frequency (ν ２ ) and configured to independently detect, process, and analyze backscattered radiation corresponding to the emission of light during the first time interval and the second time interval, respectively.

2. 10. The LIDAR system of claim 1, further comprising spectral broadening means arranged to modify a spectral width of at least one of the emitted light beams generated by the laser source assembly.

3. The light emission control device (50) controls the first light frequency (ν ) of the two light frequencies in the radiation light emission sequence. １ ) within the first radiation pulse (1), the second spectral width (Δν ２ ), the second pulse period (Δt ２ ), and the second pulsed light power value (P ２ ) except for the first spectral width (Δν １ ), the first pulse period (Δt １ ), and the first pulsed light power value (P １ ) and the second optical frequency (ν ２ 3. The LIDAR system of claim 2, configured to control the laser source assembly and the intensity modulation means, and selectively control the spectral broadening means, such that, in a second emitted light pulse (2) separated from the first pulse, a first spectral width, a second pulse duration, and a second pulsed optical power value are exclusively associated with the second spectral width, the second pulse duration, and the second pulsed optical power value, excluding the first spectral width, the first pulse duration, and the first pulsed optical power value.

4. The laser source assembly emits light at the first optical frequency (ν １ a first laser oscillator (10) configured to generate radiation of the second spectral width (Δν ２ ) having the second optical frequency (ν ２ a second laser oscillator (20) configured to generate radiation of The spectral broadening means is disposed in a path of the radiation generated by the first laser oscillator (10) and provides the laser radiation generated by the first laser oscillator with the first spectral width (Δν １ a phase modulator (11) controlled by the light emission control device (50) to provide 4. The LIDAR system of claim 2, further comprising an optical switch (30) controlled by the light emission control device (50) to transmit either the radiation light originating from the phase modulator (11) or the radiation light generated by the second laser oscillator (20) to a downstream portion of an emission light path shared by the radiation light originating from the phase modulator and the radiation light generated by the second laser oscillator and including the intensity modulation means (31).

5. The laser source assembly emits light at the first optical frequency (ν １ a first laser oscillator (10) configured to generate the radiation having a second spectral width (Δν ２ ) having the second optical frequency (ν ２ a second laser oscillator (20) configured to generate the radiation of The spectral broadening means is disposed in a path of the radiation generated by the first laser oscillator (10) and provides the radiation generated by the first laser oscillator with the first spectral width (Δν １ a phase modulator (11) controlled by the light emission control device (50) to provide the intensity modulation means comprises a first intensity modulator (12) arranged in a path of the radiation light generated from the phase modulator (11) and controlled by the light emission control device (50) to exert an effect on the radiation light generated from the phase modulator, and a second intensity modulator (22) arranged in a path of the radiation light generated by the second laser oscillator (20) and controlled by the light emission control device to exert an effect on the radiation light generated by the second laser oscillator, 4. The LIDAR system of claim 2, further comprising an optical coupler configured to transmit the radiation emerging from the first intensity modulator and the second intensity modulator to a downstream portion of an radiation path shared by the radiation emerging from the first intensity modulator and the radiation emerging from the second intensity modulator.

6. The light emission control device (50) is configured to control the radiation light emission sequence to be at the first optical frequency (ν １ ) or the second optical frequency (ν ２ ), wherein all of the radiation pulses are such that the radiation emission falls within the first spectral width (Δν １ ) and the first optical power value (P １ ) having a first period (Δt １ ) and the radiated emission has the second spectral width (Δν ２ ) and a second optical power value (P ２ ) having a second period (Δt ２ 3. The LIDAR system of claim 2, configured to control the laser source assembly, the spectral broadening means, and the intensity modulation means so that each emitted light pulse has the same envelope shape comprising:

7. 7. The LIDAR system of any one of claims 1 to 6, which implements optical fiber technology.

8. polarizing means (35) configured so that radiation emitted by the LIDAR system towards the measurement area has orthogonal polarizations when emitted during the first time interval or the second time interval; The detection path is the first spectral width (Δν １ ) in combination with the first optical frequency (ν １ a first detector (41) sensitive in a first spectral region including the second spectral width (Δν ２ ) in combination with the second optical frequency (ν ２ 8. The LIDAR system of claim 1, further comprising a polarizing beam splitter (46) configured to transmit the backscattered radiation to one of a second detector (42) sensitive in a second spectral region including a first polarized light source (100) and a second detector (42) sensitive in a second spectral region including a second polarized light source (100).

9. 1. A method for measuring the amount of a chemical compound present in a target direction, comprising: The chemical compound reacts with the second optical frequency (ν ２ ) than the first optical frequency (ν １ The LIDAR system of any one of claims 1 to 8 is selected to have a low absorption capacity value in the LIDAR system is aimed in a target direction to emit radiation in accordance with the radiation emission sequence toward a measurement area that may contain the chemical compound, and an operation of the LIDAR system is triggered; The distance from the obstacle in the background of the measurement area is determined by the first optical frequency (ν １ ) and estimated based on backscattered radiation corresponding to the emission during the first time interval; The amount of chemical compound contained in a measurement area integrated over a path of the pulse between the LIDAR system and a background obstacle is determined by the first optical frequency (ν １ ) and the second optical frequency (ν ２ ) based on intensity values ​​separately associated with backscattered radiation at the target point, the backscattered radiation being detected by a sensing path of the LIDAR system.

10. The distance from the obstacle in the background of the measurement area is determined by the first optical frequency (ν １ 10. The method of claim 9, wherein the backscattered radiation is estimated based on a measured transit time for the backscattered radiation associated with the backscattered radiation.

11. The first optical frequency (ν １ The separation distance from the obstacle present in the background of the measurement area, estimated based on the backscattered radiation associated with the first optical frequency and the second optical frequency (ν ２ 11. The method of claim 10, wherein intensity values ​​separately associated with the backscattered radiation detected in the first time interval and the second time interval are used to estimate the amount of the chemical compound contained within the measurement area.

12. The method according to any one of claims 9 to 11, wherein the chemical compound is any one of carbon dioxide, methane, nitrous oxide, and water.

13. the LIDAR system is mounted on the surface of the earth and oriented to measure the amount of the chemical compound present between the LIDAR system and the obstacle; or the LIDAR system is mounted on an aircraft in flight and pointed at a geographical area of ​​the Earth's surface to measure the separation distance from the LIDAR system to the surface of the geographical area and to measure the amount of the chemical compound present between the LIDAR system and the surface of the Earth in the geographical area; Alternatively, the method of any one of claims 9 to 12, wherein the LIDAR system is mounted on a satellite in orbit around the Earth and pointed at a geographical region of the Earth's surface to measure the separation distance from the LIDAR system to the surface of the geographical region and to measure the amount of the chemical compound present between the LIDAR system and the surface of the geographical region.

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