Lidar system for differential absorption and background range measurement

Abstract

A lidar system suitable for differential absorption measurements of a compound between two different optical frequencies (v1, v2) and for measuring the stand-off distance from an obstacle present in the background of the measurement region in which the absorption occurs. During a radiation emission sequence, the emitted optical power value is varied during a plurality of different time intervals so that the lidar system implements fiber optic technology while having sufficient emitted power. The lidar system is capable of evaluating the amount of compound contained in the measurement region and the stand-off distance from an obstacle located in the background of the measurement region.

Description

Technical Field

[0001] This invention relates to a lidar system suitable for performing differential absorption measurement and background distance measurement, and to a method for using the system to measure the amount of a compound. Background Technology

[0002] Using differential absorption measurement to assess the amount of a compound contained in a measurement region is a well-known technique. For this purpose, the absorption of radiation emitted into the measurement region is measured at a first frequency that does not correspond to the compound's absorption band, and a second frequency that corresponds to the compound's absorption band. Therefore, it is necessary to emit radiation into the measurement region at both frequencies and compare the absorption levels measured at each frequency. This measurement method is commonly referred to as IPDA, or "Integrated Path Differential Absorption." This method also requires prior knowledge of the depth of the measurement region, such as the distance to obstacles present in the background of the measurement region.

[0003] Using laser systems to perform such IPDA measurements is advantageous, especially because such systems offer superior detection sensitivity and the ability to analyze detection signals. Therefore, the feasibility of using fiber-optic laser systems is an important consideration, given their smaller size and weight, higher energy efficiency, and robustness that resists alignment losses between their constituent optical components.

[0004] However, IPDA measurement needs to meet the following requirements:

[0005] - It must be able to emit at least two types of radiation at different optical frequency values ​​in a sufficiently fast, continuous manner relative to changes that may affect the content of the measurement area. More precisely, it must emit two radiation pulses at a repetition frequency sufficiently high relative to the changes in the content of the measurement area, and allow analysis of the detection signal, for example, at a repetition frequency greater than 1 kHz (kilohertz);

[0006] - Two types of radiation: one outside the absorption band of the compound being tested, and one corresponding to one of its absorption bands, both of which must be emitted with sufficient energy of their respective.

[0007] The radiation corresponding to one of the compound's absorption bands must have a sufficiently narrow spectral linewidth to provide adequate accuracy in determining the compound's absorption. In particular, when the center wavelength of the radiation is approximately 1.6 μm (micrometers), the spectral width of the radiation corresponding to one of the compound's absorption bands may need to be less than 100 MHz (megahertz); and

[0008] - The flight time is characterized by extremely short radiation pulses to measure the separation distance from background obstacles, typically with a single duration of less than about 100 ns (nanoseconds).

[0009] However, it is well known that stimulated Brinell scattering in optical fibers limits the peak power of radiation pulses that can be emitted by fiber-based lidar systems, thus limiting the maximum range of these fiber lidar systems.

[0010] Purpose of the invention

[0011] In view of these limitations, one object of the present invention is to provide a novel lidar system that can perform IPDA measurements and determine the separation distance from obstacles present in the background of the measurement area, and can be implemented using optical fiber.

[0012] More specifically, the present invention aims to provide a lidar system that uses at least one optical fiber to transmit emitted radiation, but for this system, it extends or eliminates the peak power limitation caused by stimulated Brinell scattering. Summary of the Invention

[0013] To achieve this or other objectives, a first aspect of the present invention provides a lidar system suitable for performing differential absorption measurements between two different optical frequencies, and for measuring separation distances calculated from obstacles present in the background of the measurement region where absorption occurs. The lidar system of the present invention includes:

[0014] - Laser source assembly suitable for generating radiation at either of two optical frequencies;

[0015] - An intensity adjustment device suitable for applying the pulse envelope shape to each radiation, including pulse duration and pulse power value; and

[0016] - A launch controller suitable for controlling intensity adjustment devices.

[0017] In this specification, pulsed optical power can be understood as a value representing the intensity of each characteristic pulse, which may correspond to the peak power of the pulse or the average power evaluated over the entire pulse duration. It is understood that, under constant duration and pulse shape, an increase in either the peak power value or the average power value may be a function of the other.

[0018] According to the present invention, a lidar system is adapted such that, during operation of the lidar system, the lidar system is suitable for emitting radiation along the target direction for performing differential absorption measurement and separation distance measurement of background obstacles, the radiation emission sequence including:

[0019] - During a plurality of first time intervals, radiation is emitted at a first optical frequency located in one of two optical frequencies in the spectrum, having a first spectral width, a first pulse duration, and a first pulse optical power value; and

[0020] - During multiple second time intervals, radiation is emitted at a second optical frequency located in one of two optical frequencies in the spectrum, having a second spectral width, a second pulse duration, and a second pulse optical power value.

[0021] Multiple first time intervals and multiple second time intervals can form any continuum in a radiative emission sequence, with any number of first time intervals between two second time intervals, or vice versa. Alternatively, a radiative emission sequence can include any number of pulses located near a first optical frequency between two pulses located near a second optical frequency, or vice versa.

[0022] In addition, the radiative emission sequence has the following characteristics:

[0023] The plurality of first spectral widths and the plurality of second spectral widths are radiative emissions corresponding to a plurality of non-overlapping spectral intervals during a plurality of first time intervals and a plurality of second time intervals, wherein the first spectral width is greater than the second spectral width;

[0024] The power value of the first pulse optical light is greater than the power value of the second pulse optical light; and

[0025] The duration of the first pulse is shorter than the duration of the second pulse.

[0026] This type of lidar system can be made of optical fiber, and in particular, its laser source component can be of the MOPFA type, namely "Master Oscillator Power Fiber Amplifier". For this type of lidar system, radiation pulses with the required spectral width are first generated and adjusted according to the required envelope shape, and then these pulses are amplified before being emitted to the outside.

[0027] Since the first spectral width is greater than the second spectral width, the power value of the first pulse can be selected as high or very high, without stimulated Brinell scattering, an effect that would occur in the optical fiber used to build such a lidar system and interfere with the operation or use of the system.

[0028] For use in IPDA measurements, the radiative emission during multiple first time intervals is selected to be outside the absorption band of the compound involved in the measurement. This radiative emission during the multiple first time intervals can be further used to estimate the separation distance from obstacles present in the background of the measurement area. This telemetry measurement is performed by determining the flight time of the emitted radiative pulses during the multiple first time intervals, which can be used for round trips between the lidar system and background obstacles. The higher pulse power achieved by a larger spectral width and shorter pulse duration during these first time intervals allows for a more accurate estimation of the separation distance from background obstacles. The radiative emission during multiple second time intervals is selected to be within one of the compound's absorption bands. During the multiple second time intervals, the lower spectral width of the pulses provides higher accuracy in estimating the amount of compound. Between the radiation emitted during the multiple second time intervals near the second optical frequency and the radiation emitted during the multiple first time intervals near the first optical frequency, the amount of compound can be estimated based on the absorptivity determined by the lidar system and taking into account the estimated separation distance from background obstacles.

[0029] Preferably, the transmit controller may select one or more of the following features:

[0030] - The repetition frequency of the radiated emission sequence can be between 1 kHz and 50 kHz;

[0031] - Multiple first time intervals may have individual durations between 10 ns and 200 ns, preferably between 50 ns and 100 ns;

[0032] - Multiple second time intervals may have individual durations between 0.1 μs (microseconds) and 10 μs, preferably between 0.5 μs and 5 μs;

[0033] - During multiple first time intervals, the first spectral width of the radiation emitted by the lidar system in the measurement area can be between 100MHz and 2000MHz, preferably between 500MHz and 1000MHz; and

[0034] - During multiple second time intervals, the second spectral width of the radiation emitted by the lidar system in the measurement area can be between 10 MHz and 200 MHz, preferably between 50 MHz and 100 MHz.

[0035] Finally, the laser source assembly can be adapted such that the radiation emitted by the lidar system in the measurement area has multiple first optical frequency values ​​and multiple second optical frequency values ​​during multiple first time intervals and multiple second time intervals, corresponding to wavelengths between 1.3 μm and 1.8 μm, particularly between 1.5 μm and 1.6 μm, or near 2 μm. This wavelength range is particularly suitable for measuring the amount of carbon dioxide contained in the measurement area.

[0036] Furthermore, the lidar system of the present invention includes a detection path adapted to independently detect, process, and analyze backscattered radiation, which corresponds to a first optical frequency and a second optical frequency, and respectively corresponds to emission during a plurality of first time intervals and a plurality of second time intervals.

[0037] The lidar system may also include a computing unit connected from an input to at least one output of the detection path. Based on the analysis signal generated by the detection path, the computing unit is suitable for providing estimates of the separation distance from background obstacles and the amount of compounds contained in the measurement area.

[0038] Depending on the possible embodiments of the invention, and particularly on the use of multiple laser oscillators of different types in the laser source assembly to generate radiation at each of the two optical frequencies, the first spectral width and / or the second spectral width may be inherent or generated by a dedicated spectral broadening device. An inherent spectral width can be understood as representing the spectral width of the radiation generated by the respective laser oscillator. In other words, in the first case, the spectral width of multiple pulses emitted during multiple first time intervals and / or multiple second time intervals corresponds to the spectral width of the respective laser oscillator. Otherwise, the lidar system further includes:

[0039] - A spectral broadening device for modifying the spectral width of at least one type of radiation produced by a laser source assembly.

[0040] In a first embodiment of the invention, the emission controller is adapted to control the laser source assembly, the intensity adjustment device, and the appropriate spectral broadening device. Therefore, in the radiative emission sequence, within a plurality of first radiation pulses, the first optical frequency of the two optical frequencies is associated only with the first spectral width, the first pulse duration, and the first pulse optical power value, but does not include the second spectral width, the second pulse duration, and the second pulse optical power value. Within a plurality of second radiation pulses separate from the plurality of first radiation pulses, the second optical frequency of the two optical frequencies is associated only with the second spectral width, the second pulse duration, and the second pulse optical power value, but does not include the first spectral width, the first pulse duration, and the first pulse optical power value.

[0041] Regarding the first embodiment, the lidar system may have an arrangement in which the laser source assembly includes a first laser oscillator adapted to generate radiation of a first optical frequency, and a second laser oscillator adapted to generate radiation of a second optical frequency having a second spectral width. The spectral broadening device includes a phase modulator disposed in the path of the radiation generated by the first laser oscillator and controlled by a transmission controller to provide the first spectral width to the laser radiation generated by the first laser oscillator. Furthermore, the lidar system also includes an optical switch controlled by the transmission controller to transmit radiation originating from the phase modulator or radiation generated by the second laser oscillator to a downstream portion of the transmission optical path, wherein the radiation originating from the phase modulator and the radiation generated by the second laser oscillator share the downstream portion of the transmission optical path, and the downstream portion of the transmission optical path includes an intensity adjustment device.

[0042] According to another possible configuration of the first embodiment of the invention, the laser source assembly similarly includes a first laser oscillator adapted to generate radiation of a first optical frequency, and a second laser oscillator adapted to generate radiation of a second optical frequency (ν2) having a second spectral width. The spectral broadening device further includes a phase modulator disposed in the path of the radiation generated by the first laser oscillator and controlled by a transmission controller to provide the first spectral width to the radiation generated by the first laser oscillator. However, in this configuration, the intensity adjustment device includes a first intensity modulator disposed in the path of the radiation originating from the phase modulator and controlled by a transmission controller to be effective for the radiation originating from the phase modulator. The intensity adjustment device also includes a second intensity modulator disposed in the path of the radiation generated by the second laser oscillator and controlled by a transmission controller to be effective for the radiation generated by the second laser oscillator. Furthermore, the lidar system also includes an optical coupler adapted to transmit radiation originating from the first and second intensity modulators to a downstream portion of the transmission optical path, wherein the radiation originating from the first intensity modulator and the radiation originating from the second intensity modulator (22) share the downstream portion of the transmission optical path.

[0043] For the two configurations described above, the downstream portion of the transmitting optical path may include an optical radiation amplifier or an optical radiation amplification chain, which may be controlled by the transmitting controller to generate a first optical power value and a second optical power value for the radiation emission sequence according to the technical features of the present invention.

[0044] The second embodiment of the present invention is an alternative to the first embodiment described above. In the second embodiment, the emission controller is adapted to control the laser source assembly, the spectral broadening device, and the intensity adjustment device. Therefore, the radiative emission sequence includes a series of radiative pulses located at either a first optical frequency (ν1) or a second optical frequency (ν2) in the spectrum. For the first optical frequency and for the second optical frequency, all radiative pulses having the same envelope shape include a first pulse duration during which the radiative emission has a first spectral width and a first pulse power value, and a second pulse duration during which the radiative emission has a second spectral width and a second pulse power value (P2). The first pulse duration is shorter than the second pulse duration, and the first pulse duration occurs before or after the second pulse duration in each radiative pulse.

[0045] In general, the lidar system of this invention can implement fiber optic technology.

[0046] In general, the lidar system of this invention may include an adapted polarization device that, when emitting radiation during a plurality of first time intervals or a plurality of second time intervals, causes the radiation emitted by the lidar system toward the measurement area to have orthogonal polarization, particularly opposite circular polarization. In this case, the detection path includes a polarization beamsplitter, configured as a function of the polarization of the backscattered radiation, to transmit the backscattered radiation to a first detector, which may have sensitivity in a first spectral range including a first optical frequency combined with a first spectral width, or to a second detector, which may have sensitivity in a second spectral range including a second optical frequency combined with a second spectral width. Possibly, the two detectors may be identical if their common sensitivity spectral range includes a first optical frequency combined with a first spectral width and a second optical frequency combined with a second spectral width.

[0047] A second aspect of the present invention provides a method for determining the amount of a compound present in a target direction, characterized in that:

[0048] - Select a lidar system as described in the first aspect above, such that the absorption capacity value of the compound at the first optical frequency is lower than the absorption capacity value at the second optical frequency.

[0049] - The lidar system is positioned facing the target so that it emits radiation toward the measurement area that may contain compounds according to the radiation emission sequence, thus triggering the operation of the lidar system;

[0050] - The separation distance from obstacles present in the background of the measurement area is estimated based on backscattered radiation associated with a first optical frequency, the separation distance corresponding to the emission of multiple first time intervals; and

[0051] The amount of compounds contained in the measurement area, accumulated on the pulse path between the lidar system and the background obstacle, can be estimated separately based on the intensity values ​​associated with backscattered radiation at the first and second optical frequencies, corresponding to multiple first time intervals and multiple second time intervals in the radiation emission sequence, respectively, where the backscattered radiation can be detected by the detection path of the lidar system.

[0052] Preferably, the separation distance from obstacles in the background of the measurement area is estimated based on the time of flight, which is measured for backscattered radiation associated with the first optical frequency. In this case, the separation distance from obstacles in the background of the measurement area estimated based on the backscattered radiation associated with the first optical frequency, combined with the intensity values ​​associated with the backscattered radiation detected at the first and second optical frequencies, respectively, and corresponding to the emission of multiple first time intervals and multiple second time intervals, can be used to estimate the amount of compound contained in the measurement area accumulated along the pulse path.

[0053] The compound involved in the measurement method of the present invention can be any one of carbon dioxide or CO2, methane or CH4, nitrous oxide or N2O, and water or H2O.

[0054] Finally, the different conditions for implementing this invention are as follows:

[0055] - First implementation condition: The lidar system is installed on the Earth's surface to measure the amount of compounds present between the lidar system and the obstacle;

[0056] - Second implementation condition: The lidar system is set on the aircraft in flight and pointed to a geographical area on the Earth's surface to measure the separation distance between the lidar system and the Earth's surface within the geographical area, and to measure the amount of compounds present between the lidar system and the Earth's surface in the same geographical area.

[0057] - Third implementation condition: The lidar system is installed on a satellite orbiting the Earth and pointed at a geographic area on the Earth's surface to measure the separation distance between the lidar system and the Earth's surface within the geographic area, and to measure the amount of compounds present between the lidar system and the Earth's surface in the same geographic area.

[0058] For the second and third implementation conditions, the Earth's surface in the geographic area pointed to by the lidar system can be considered as an obstacle existing in the background of the measurement area. Attached Figure Description

[0059] Other features and advantages of the invention will become apparent when reading the detailed description below, which is by way of example only and is presented in a non-limiting manner. Please refer to the following figures:

[0060] Figure 1a The spectral variations of a first radiative emission sequence implemented as in a possible first embodiment of this application are shown;

[0061] Figure 1b It shows Figure 1a The change in emitted optical power of the first radiative emission sequence;

[0062] Figure 2a The spectral variations of the second radiation emission sequence implemented as in a possible second embodiment of this application are shown;

[0063] Figure 2b It shows Figure 2a The change in emitted optical power of the second radiative emission sequence;

[0064] Figure 3a This is a block diagram of the emission path of the lidar system according to this application, which is suitable for generating... Figure 1a and Figure 1b The first radiative emission sequence;

[0065] Figure 3b It shows the relationship with Figure 3a The block diagram of the first variant of the transmission path compared to the system is used for another lidar system according to this application, also for generating... Figure 1a and Figure 1b The first radiative emission sequence;

[0066] Figure 3c It shows the relationship with Figure 3a The block diagram of the second variant of the transmission path compared to the system is used for yet another lidar system according to this application, also for generating... Figure 1a and Figure 1b The first radiative emission sequence;

[0067] Figure 3d It shows the relationship with Figure 3a The block diagram of the third variant of the transmission path compared to the system is used for yet another lidar system according to this application, also for generating... Figure 1a and Figure 1b The first radiative emission sequence;

[0068] Figure 3e It shows the relationship with Figure 3a The block diagram of the fourth variant of the transmission path compared to the system is used for yet another lidar system according to this application, also for generating... Figure 1a and Figure 1b The first radiative emission sequence;

[0069] Figure 4a This is a block diagram of the transmission path of another lidar system according to the present invention, which is again applicable to generating... Figure 1a and Figure 1b The first radiative emission sequence;

[0070] Figure 4b It shows the relationship with Figure 4a The block diagram of the system compared to the emission path variant is used for yet another lidar system according to this application, and it is also suitable for generating... Figure 1a and Figure 1b The first radiative emission sequence;

[0071] Figure 5a This is a transmission path block diagram of another lidar system according to the present invention, which is suitable for generating... Figure 2a and Figure 2b The second radiative emission sequence;

[0072] Figure 5b It shows the relationship with Figure 5a The block diagram of the variant of the transmission path compared to the system is used for yet another lidar system according to this application, and is again applicable to generating... Figure 2a and Figure 2b The second radiative emission sequence;

[0073] Figure 6a This is a block diagram of the detection path of the lidar system according to the present invention;

[0074] Figure 6b It shows the relationship with Figure 6a A block diagram of a variant of the detection path compared to the system;

[0075] Figure 6c It shows the relationship with Figure 6a A block diagram of another variant of the detection path compared to the system; and

[0076] Figure 6d It shows the relationship with Figure 6a This is a block diagram of another variant of the detection path compared to the system. Detailed Implementation

[0077] For clarity, Figure 1a , Figure 1b , Figure 2a and Figure 2b The dimensions of the elements shown in the figures do not correspond to actual dimensions or actual size ratios. Furthermore, all elements are shown symbolically in the figures only, and the same reference numerals indicating the same element or element having the same function in different figures.

[0078] exist Figure 1a , Figure 1b , Figure 2a and Figure 2bIn the diagram, the x-axis represents the time interval during a radiative emission sequence, denoted by t, which consists of radiative pulses spectrally located near a first optical frequency (denoted as ν1) or a second optical frequency (denoted as ν2). Δt1 and Δt2 represent the durations of multiple first time intervals and multiple second time intervals during these radiative emission sequences, respectively. Figure 1a and Figure 2a In the diagram, for each time point of the relevant emission sequence, the y-axis represents the value of the light emission frequency, denoted as ν. Figure 2a and Figure 2b In the diagram, the y-axis represents the instantaneous emitted power of the radiation, denoted as P. Generally, the radiation pulse designated as element symbol 1 is located near the optical frequency ν1 in the spectrum and is assumed to be the same as each other. Similarly, the radiation pulse designated as element symbol 2 is located near the optical frequency ν2 in the spectrum and is also assumed to be the same as each other. To apply the invention to the measurement of differential absorption of compounds between two different optical frequencies, all radiation pulses 1 and 2 are emitted in the direction of the measurement region where the compound to be measured may be present. The optical emission frequency ν1 is intended to be selected outside the absorption band of the compound, while the optical emission frequency ν2 is intended to be selected within one of the compound's multiple absorption bands. In the case of carbon dioxide, the optical emission frequency ν1 can be selected to be equal to 190.81 THz, corresponding to a wavelength value λ1 of 1572.2 nm, while the optical transmission frequency can be selected to be equal to 190.84 THz, corresponding to a wavelength value λ2 of 1572.02 nm.

[0079] Figure 1a and Figure 1b This relates to the same first radiation emission sequence that can be used to implement the present invention. In this first emission sequence, all radiation pulses 1 have a first common value of spectral width Δν1 and emission peak power P1, and have a pulse duration equal to Δt1. Similarly, all radiation pulses 2 in the same emission sequence have a second common value of spectral width Δν2 and emission peak power P2, and have a pulse duration equal to Δt2. The emission peak power values ​​P1 and P2 correspond to radiation emitted by the lidar system according to the present invention, such that the emitted radiation is directed away from the lidar system in the direction of the measurement area, particularly after final optical amplification within the lidar system. For this first radiation emission sequence, multiple pulses 1 with a single duration Δt1 correspond to multiple first time intervals described in the general part of this description, and multiple pulses 2 with a single duration Δt2 correspond to multiple second time intervals. Since the spectral broadening of multiple pulses 1 is greater than that of multiple pulses 2, i.e., Δν1 > Δν2, due to stimulated Brinell scattering (denoted as P) occurring in the fiber of the lidar system... SBS The peak transmit power value P1 can be greater than the power limit, while the peak transmit power value P2 can be less than the same power limit P.SBS .

[0080] Figure 2a and Figure 2b The same second emission sequence can also be used to implement the present invention. In such a second emission sequence, all radiation pulses 1 and 2 emitted into a measurement region that may contain a quantity of the analyte have the same envelope shape, which can be transposed at the optical frequency of each pulse, the latter alternating between ν1 of multiple pulses 1 and ν2 of multiple pulses 2. Within each pulse 1, 2, the envelope shape includes a first time interval of duration Δt1 during which the radiation of pulse 1 or 2 has a spectral width value Δν1 and an emission peak power value P1, and a second time interval of duration Δt2 during which the radiation of pulse 1 or 2 has a spectral width value Δν2 and an emission peak power value P2. The temporal order between the multiple first time intervals and the multiple second time intervals can be reversed within each pulse, each time interval having its associated spectral width and emission peak power value, and the multiple first time intervals and the multiple second time intervals may also be separated by an intermediate envelope pattern shape within each pulse. As described above, the power limit P associated with the stimulated Brinell scattering effect SBS It can be less than the peak transmit power value P1 and greater than the peak transmit power value P2.

[0081] For the first transmission sequence ( Figure 1a and Figure 1b ) and second launch sequence ( Figure 2a and Figure 2b In the second radiative emission sequence, the radiative emission of optical frequency ν1 over multiple durations Δt1 is used to determine the separation distance from background obstacles. In the case of the second radiative emission sequence, multiple pulses 1 corresponding to multiple portions of multiple durations Δt2 with spectral width Δν2 and emission peak power P2 may not be usable for estimating the separation distance from background obstacles. However, in addition to multiple pulses 1 corresponding to portions of multiple durations Δt1, multiple pulses 2 corresponding to multiple portions of multiple durations Δt1 with spectral width Δν1 and emission peak power P1 can also be selectively used to determine the separation distance from background obstacles.

[0082] The second pulse sequence can be roughly such that the optical power of each pulse 1 or 2 near optical frequency ν1 or ν2 has a steep rising edge and a wide spectral range, which makes telemetry measurements possible. Subsequently, the optical power decreases slowly and the spectrum narrows, making it suitable for differential absorption measurements.

[0083] For these two emission sequences, the first emission sequence can be based on Figure 1a and Figure 1b As shown, the second transmission sequence can be based on Figure 2a and Figure 2b As shown, the following values ​​are non-limiting examples:

[0084] The pulse repetition frequency can be between 1 kHz (kilohertz) and 50 kHz.

[0085] Δt1 can be between 50 ns and 100 ns.

[0086] Δt2 can be between 0.5 μs and 5 μs.

[0087] Δν1 can be between 500MHz and 1000MHz.

[0088] Δν2 can be between 50MHz and 100MHz.

[0089] P1 can be greater than 200W, and

[0090] P2 can be greater than 50W.

[0091] Therefore, the duration Δt1 of the first time interval can be shorter than the duration Δt2 of the second time interval. Furthermore, the first spectral width Δν1 can be greater than the second spectral width Δν2, and the first emission peak power P1 can be greater than the second emission peak power P2. Then, because the first spectral width Δν1 increases, the value P1 is distributed over a wider range of emission spectral intervals than the value P2. Therefore, the value P1 can be greater than the stimulated Brillouin scattering threshold P corresponding to the fiber used to establish the lidar system. SBS Preferably, the value P2 can be selected to be less than or equal to the stimulated Brinell scattering threshold to limit energy efficiency loss during the emission of radiation over multiple second time intervals of multiple individual durations Δt2.

[0092] Several lidar system architectures according to this application are described below, designed to emit radiation sequences as described above. The description of these architectures is limited to the organization of their main components; it is understood that those skilled in the art will recognize that these components are commercially available and will know how to combine them into the described architecture without difficulty or any inventive step. Furthermore, it should be understood that, for clarity, additional components that will be used in these architectures but are not directly related to the principles of the invention and are commonly used will not be described herein. All of the lidar system architectures described below can be advantageously implemented using fiber optic technology or integrated optical path technology to produce the optical, optoelectronic, and interconnect components used. In the diagrams showing the emission path structure, component numeral 50 denotes the emission controller, denoted as CTRL, and is connected to the component of the emission path to generate a radiation emission sequence with desired characteristics. When the generated radiation emission sequence is provided to those skilled in the art, the control modes implemented by the emission controller 50 are within the capabilities of those skilled in the art.

[0093] Figure 3a This illustrates a first possible transmission path architecture for a lidar system according to the present invention, designed to generate according to Figure 1a and Figure 1bThe laser beam emitted by the laser source 10, commonly referred to as a laser oscillator, is a continuous laser beam with a frequency ν1. For example, it can be a laser diode or a fiber laser. The laser beam generated by the laser source 10 is transmitted through a phase modulator 11, denoted as MOD.PHASE, with a spectral width of Δν1. In this case, this is an external phase modulation of the laser source. For example, the phase modulator 11 can be a photoelectric modulator. A random generator for binary signals, commonly referred to as the acronym PRBS (Pseudo-Random Binary Sequence), a radio frequency noise generator, or an arbitrary waveform generator known as the acronym AWG (Arbitrary Waveform Generator), can be connected to the electrical control input of the phase modulator 11. The mode used in the alternative control mode of the phase modulator 11 is indicated by the component denoted as 11c and denoted as GENERATOR. When a PRBS generator is used, it produces phase jumps equal to -π or π in a random or pseudo-random sequence. The output of phase modulator 11 can advantageously be associated with an optical apodized filter (not shown) to eliminate secondary lobes generated by this spectral broadening method in the radiation spectrum directly from phase modulator 11. When using an AWG generator, the generator can be programmed to produce various waveforms, such as a series of ramps with slopes randomly varying between successive ramps. Alternatively, the generator can be programmed to produce an electrical control signal of sinusoidal or linear combinations of sinusoidal components. Other forms of electrical control signals for phase modulator 11 can also be used alternatively, and it should be understood that those skilled in the art will understand how to select the characteristics of such electrical control signals to provide a desired spectral envelope shape with a spectral width Δν1 for the radiation exiting phase modulator 11. Generator 11c can be selectively activated by emitter controller 50 to produce multiple pulses 1, or it can be activated continuously. Component designation 20 indicates another laser source, i.e., another laser oscillator, which generates another continuous laser beam with an optical frequency of ν2 and a direct spectral width Δν2. For example, laser source 20 can be of the type of fiber laser. In fact, since the spectral width Δν2 of the present invention is very low, it can be provided directly or inherently by laser source 20, i.e., without the use of any additional components specifically designed to generate this spectral width value. The two laser sources 10 and 20 constitute the laser source assembly specified in the general part of this description. Subsequently, two radiations, respectively, originating from phase adjuster 11 and laser source 20, are injected into the two inputs of optical switch 30, denoted as COMMUTATOR.During multiple first time intervals and / or multiple second time intervals, and according to the desired sequence for alternating between radiation pulses at optical frequencies ν1 and ν2, this can be a 2x1 optical switch, which can be controlled by the emission controller 50 to output the radiation received at one or the other of its two inputs. Alternatively, the optical switch 30 can be replaced by a fiber optic Y-coupler, for example, with a 50 / 50 intensity ratio and optional polarization holding, or by a polarization coupler, such as a cubic polarization beam splitter. The radiation output from the optical switch 30 is then introduced into an intensity modulator 31, denoted as MOD.I NT., and controlled by the emission controller 50, such that the radiation ultimately emitted into the measurement area has an instantaneous power value P1 during multiple first time intervals of duration Δt1, where the optical frequency is closer to the numerical value ν1, and an instantaneous power value P2 during multiple second time intervals of duration Δt2, where the optical frequency is closer to the numerical value ν2. The intensity modulator 31 can be of the type of photoelectric, acousto-electric, or semiconductor optical amplifier. As is well known, such an intensity modulator can be combined with an internal controller or associated with an external controller inserted between the intensity modulator and the emission controller 50. The radiation originating from the intensity modulator 31 is then transmitted to the optical amplification assembly 32, or optical amplification chain 32, denoted as AMPL, to actually generate the emitted optical power values ​​P1 and P2. Finally, the radiation originating from the optical amplification assembly 32 is transmitted to the measurement area via the output optics 33 of the lidar system's emission path, denoted as OPT.

[0094] Several alternative architectures for lidar systems can be derived from Figure 3a The architecture in the text applies at least one of the following equivalent principles each time. Figure 3a Launch path architecture:

[0095] -If the laser source 10 is a type of laser beam with a spectral width of Δν1 and capable of producing an optical frequency of ν1, for example... Figure 3a The laser source 20 is used for the spectral width value Δν2. The phase adjuster 11 is then omitted, allowing the laser beam from the laser source 10 to be directly transmitted to the optical switch 30, just as it would be for the laser source 20. This achieves the following: Figure 3b Configuration;

[0096] When the laser source 10 is tunable, the electrical control signal used to impart a spectral width Δν1 to the radiation emitted near the optical frequency ν1 can be directly applied to the control input of the tunable laser source 10. This method of obtaining the desired spectral width is sometimes called internal phase adjustment, which is the opposite of using an external phase adjuster for the laser source. Figure 3aAs shown. The internally adjustable laser source can be, for example, a laser diode whose injected current in the gain region can be adjusted with a low adjustment amplitude, or a DBR (Distributed Bragg Reflector) diode, to which the injected phase, grating, or semiconductor optical amplification region can be adjusted. Additionally or alternatively, this method of obtaining the desired spectral width from within the laser source can also be applied to laser source 20, where the latter itself is tunable, thus obtaining a spectral width Δν2. Therefore, a spectral width Δν2 is obtained. Figure 3c The configuration, wherein component designations 11c and 21c denote the modulated signal generators connected to the respective control inputs of the tunable laser sources 10 and 20;

[0097] - Two independent external phase modulators can be used simultaneously, one positioned between the laser source 10 and the optical switch 30, to provide a spectral width Δν1 for radiation emitted at optical frequency ν1, such as... Figure 3a As shown. Another element is located between the laser source 20 and the optical switch 30 to provide a spectral width Δν2 to the radiation emitted at optical frequency ν2. Therefore, the following is obtained: Figure 3d The configuration, wherein component designations 11 and 21 represent two external phase adjusters associated with laser sources 10 and 20, respectively, and component designations 11c and 21c represent phase adjustment signal generators connected to the respective control inputs of phase adjusters 11 and 21; and

[0098] - A single-phase modulator that can be used to effectively control two types of radiation generated by laser sources 10 and 20 at optical frequencies ν1 and ν2, respectively. In this case, laser beams from the two laser sources 10 and 20 can be directly transmitted to the input of optical switch 30, and the single-phase modulator is located between the output of optical switch 30 and the input of intensity modulator 31. The single-phase modulator can then be controlled in one of the above-described ways to generate a spectral width Δν1 during multiple first time intervals when optical switch 30 transmits radiation with optical frequency ν1, and a spectral width Δν2 during multiple second time intervals, wherein optical switch 30 can transmit radiation with optical frequency ν2. Therefore, a... Figure 3e The configuration includes an external phase adjuster, denoted by component 34, which is shared by the two radiations of optical frequencies ν1 and ν2, and a phase adjustment signal generator, denoted by component 34c, which is connected to the control input of the phase adjuster 34.

[0099] Figure 4a The embodiments can be derived from Figure 3a In the embodiment, this radiation is generated by adjusting the radiation intensity near optical frequencies ν1 and ν2 located upstream of the joint of individual paths. The radiation generation path located near optical frequency ν1 is... Figure 3aSame as above, but with the addition of an intensity modulator 12. Similarly, the radiation generation path located near the optical frequency ν2 is the same as... Figure 3a The same, but with the addition of intensity modulator 22. The two intensity modulators 12 and 22 can be controlled by the transmit controller 50 in a time-dependent manner with respect to the modulation signal generated by the generator 11c. In particular, the two intensity modulators 12 and 22 can generate transmission time windows that limit a plurality of first time intervals of each duration Δt1 and a plurality of second time intervals of each duration Δt2 to a desired repetition frequency. In such an embodiment, independent radiation generation paths located near the two optical frequencies ν1 and ν2 can be combined using coupler 35 in the direction of the downstream portion of the transmission path, which is shared by the two optical frequencies and includes an optical amplification assembly 32. Coupler 35 can be a conventional Y-coupler, or alternatively, a polarization coupler capable of assigning determined polarizations to the radiation transmitted during the plurality of first time intervals, each duration Δt1, and assigning determined orthogonal polarizations to the radiation transmitted during the plurality of second time intervals, each duration Δt2. For example, linear polarization parallel to a fixed direction can be imparted by polarization coupler 35 to radiation transmitted during multiple first time intervals of various durations Δt1, and linear polarization perpendicular to a fixed direction can be imparted by polarization coupler 35 to radiation transmitted during multiple second time intervals of various durations Δt2.

[0100] Figure 4b The embodiments in can be compared with Figure 4a The embodiments in the text are obtained in the same way, but are based on Figure 3d The embodiments in the example, not Figure 3a The embodiments described herein. By applying the same method, those skilled in the art will be able to derive other possible embodiments of the invention, for example, by... Figure 3b , Figure 3c or Figure 3e The single intensity modulator in the image is replaced with two intensity modulators, each dedicated to radiation located near two optical frequencies ν1 and ν2.

[0101] Figures 3a-3e and Figures 4a-4b All embodiments are applicable to generating a basis Figure 1a and Figure 1b The radiative emission sequence.

[0102] and Figures 3a-3e and Figures 4a-4b The implementation methods differ, in Figure 5a and Figure 5bIn the embodiments, the phase modulation used achieves the desired spectral widths Δν1 and Δν2 during multiple time intervals of durations Δt1 and Δt2, respectively, and the phase modulation is shared by the two optical frequencies ν1 and ν2. A single-phase modulator can then be used, located downstream of a combined optical switch or optical coupler, with the beam originating from two laser sources 10 and 20 respectively in the shared downstream portion of the emission path. This reduces the cost of the lidar system.

[0103] Figure 5a and Figure 5b The embodiments are suitable for generating according to Figure 2a and Figure 2b The radiative emission sequence.

[0104] exist Figure 5a In this embodiment, both phase adjustment and intensity adjustment are performed downstream of the optical path junction of the radiation from laser sources 10 and 20, respectively. The optical path junction is achieved by switch 30, phase adjustment is generated by phase adjuster 34, and intensity adjustment is generated by intensity adjuster 31. The phase adjustment signal generator 34c can also be one of the types described above: a PRBS generator, an RF noise generator, or an AWG generator. Switch 30, phase adjustment signal generator 34c, and intensity adjuster 31 can all be synchronously controlled by the transmit controller 50.

[0105] exist Figure 5b In this embodiment, the radiation emitted individually from laser sources 10 and 20 is intensity-modulated using two separate modulators, denoted by element reference numerals 12 and 22. The intensity-modulated radiation can be introduced by coupler 35 in a shared downstream portion of the emission path. Coupler 35 can also be a Y-coupler or polarization coupler as described above. The downstream portion of the emission path includes a phase modulator 34, an optical amplification assembly 32, and an output optics 33.

[0106] Figure 6aA first detection path architecture that can be used in a lidar system according to the invention is shown. Component reference numeral 40 denotes multiple input optics of the detection path, denoted as OPT., which function to collect a portion of the backscattered radiation corresponding to the emission sequence generated by the lidar system. The collected portion of the radiation is directed to an optical sensor 43, denoted as DETECT.OPT., and generates an electrical detection signal whose intensity is a function of the power of the detected radiation portion. The optical sensor 43 can be implemented in a direct detection mode or in a coherent detection mode. For example, an optical sensor with direct detection function may consist of a photodiode associated with a transimpedance amplifier, while an optical sensor with coherent detection, also known as heterodyne detection, requires mixing the backscattered radiation collected by the multiple 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 an analysis chain 512, denoted as ANALYS., which processes the electrical signals transmitted by the sensor 43, regardless of whether each electrical signal corresponds to pulse 1 or pulse 2.

[0107] Figure 6b A second detection path architecture is shown, employing two distinct analysis chains dedicated to multiple radiation pulses 1 and multiple radiation pulses 2, respectively. An electrical switch 44, denoted COMM.ELEC., directs the electrical detection signal to two separate analysis chains 51 and 52, denoted ANALYS.1 and ANALYS.2, based on portions of the detection electrical signal corresponding to multiple first or second time intervals in the radiation emission sequence, respectively. The operation of the electrical switch 44 can be synchronized by the emission controller 50 for this purpose. Thus, analysis chain 51 can be dedicated to multiple first time intervals of various durations Δt1 and can be designed to determine the residual absorption in the measurement region outside the spectral absorption band of the compound to be determined, and to determine the separation distance from obstacles in the background of that measurement region. Independently, analysis chain 52 can be dedicated to multiple second time intervals of various durations Δt2 and is designed to determine the absorption in the spectral absorption band of the compound in the measurement region. A calculation module (not shown) generates an assessment of the amount of compound based on the absorption levels determined by the two analysis chains 51 and 52 and the separation distance from background obstacles.

[0108] Figure 6cA third possible detection path architecture is shown, in which two optical sensors 41 and 42, denoted as DETECT.OPT.1 and DETECT.OPT.2, respectively generate electrical detection signals, which are transmitted to analysis chains 51 and 52. An advantage of this third architecture is the ability to use optical sensors 41 and 42 with different sensitivity levels, adapted to the corresponding instantaneous power values ​​of the relevant portions of the backscattered radiation: P1 in multiple first time intervals of multiple durations Δt1 for sensor 51, and P2 in multiple second time intervals of multiple durations Δt2 for sensor 52. In this case, the backscattered radiation collected by the input optics 40 is directed to sensor 41 or sensor 42 by an optical switch 45. The operation of this optical switch 45 is controlled by the emission controller 50. For this third detection path architecture, the coupler 35 of the emission path can be a Y-coupler type with a 50 / 50 ratio.

[0109] Figures 6b-6c All three detection path architectures are similar to Figures 3a-3e , Figures 4a-4b Compatible with architectural variants, while Figures 5a-5b This is used for the launch path.

[0110] at last, Figure 6d A fourth possible detection path architecture is shown, in which Figure 6c The optical switch 45 is replaced by a polarization beam splitter 46, denoted as SEP.POLAR., which is based on the linear polarization of the radiation. This fourth architecture for the detection path is compatible with the embodiment for the transmission path, wherein the coupler 35 is a polarization coupler, as described above.

[0111] Generally, the optical sensor used in the detection path, or at least one of multiple optical sensors, has a sufficiently short response time to allow the separation distance from background obstacles to be estimated based on the radiation emitted during multiple durations Δt1.

[0112] Furthermore, the output of each analysis chain in the detection path can be connected to a computing unit (not shown), which is adapted to provide an estimate of the separation distance from background obstacles and the amount of compound present in the measurement area and accumulated along the pulse path, based on the signals generated by one or two analysis chains. This computing unit can optionally be integrated into a lidar system.

[0113] It should be understood that the invention can be reproduced with modifications to minor aspects of the embodiments detailed above, while retaining at least some of the advantages cited. In particular, optical elements with equivalent functionality can be used instead of the mentioned elements. Furthermore, those skilled in the art can employ the following modifications as alternatives without inventive step:

[0114] - A single, frequency-switched laser source that can be used to generate multiple pulses at optical frequencies ν1 and ν2;

[0115] - A separate optical amplifier that can be used for multiple pulses at optical frequencies ν1 and ν2;

[0116] - To generate a radiative emission sequence, the interleaving of multiple pulses at optical frequency ν1 and multiple pulses at optical frequency ν2 can be performed before or after phase and / or intensity adjustment of each pulse; and

[0117] - To generate a radiative emission sequence, the interleaving of multiple pulses at optical frequency ν1 and multiple pulses at optical frequency ν2 can be performed before or after the optical amplification of the pulses.

[0118] Finally, all numerical values ​​cited are for illustrative purposes only and may vary depending on the amount of the compound being determined.

Claims

1. A lidar system adapted to perform differential absorption measurements between two different optical frequencies (vi, v2) and to measure the separation distance from an obstacle present in the background of the measurement region where the absorption occurs, the lidar system comprising: a laser source assembly adapted to generate radiation at either one of the two optical frequencies (vi, v2); intensity modulation means adapted to apply a pulse envelope shape to each radiation, including a pulse duration and a pulse optical power value (Pi, P2); and a transmission controller (50) adapted to control the intensity modulation means, so that, during operation of the lidar system, the lidar system is adapted to transmit along a target direction where differential absorption measurements and separation distance measurements from background obstacles are performed, a radiation transmission sequence including: during a plurality of first time intervals, the radiation transmission is spectrally located at a first one of the two optical frequencies (vi), having a first spectral width (Avi), a first pulse duration (Ati) and a first pulse optical power value (Pi); and during a plurality of second time intervals, the radiation transmission is spectrally located at a second one of the two optical frequencies (v2), having a second spectral width (Av2), a second pulse duration (At2) and a second pulse optical power value (P2); the first spectral width (Avi) and the second spectral width (Av2) are radiation transmissions corresponding to a plurality of disjoint spectral intervals during the plurality of first time intervals and the plurality of second time intervals, the first spectral width being greater than the second spectral width; the first pulse optical power value (Pi) is greater than the second pulse optical power value (P2); and the first pulse duration (Ati) is shorter than the second pulse duration (At2); the lidar system further comprises a detection path adapted to independently detect, process and analyze backscattered radiation corresponding to the first optical frequency (vi) and to the second optical frequency (v2), and respectively corresponding to the transmission during the plurality of first time intervals and the plurality of second time intervals.

2. The lidar system according to claim 1, further comprising: spectral broadening means for modifying the spectral width of at least one of the radiations generated by the laser source assembly (10, 20).

3. The lidar system of claim 2, wherein, The emission controller (50) is adapted to control the laser source assembly, the intensity adjustment device, and optionally the spectral broadening device, so that, in the sequence of radiation emissions, in a plurality of first radiation pulses (1), the first one (v1) of the two optical frequencies is associated only with the first spectral width (Av1), the first pulse duration (At1) and the first pulse optical power value (P1), but not with the second spectral width (Av2), the second pulse duration (At2) and the second pulse optical power value (P2), and in a plurality of second radiation pulses (2) separate from the first ones, the second one (v2) of the two optical frequencies is associated only with the second spectral width, the second pulse duration and the second pulse optical power value, but not with the first spectral width, the first pulse duration and the first pulse optical power value.

4. The lidar system according to claim 2 or 3, characterized in that the laser source assembly comprises a first laser oscillator (10) adapted to generate radiation of the first optical frequency (v1) and a second laser oscillator (20) adapted to generate radiation of the second optical frequency (v2) having the second spectral width (Av2); the spectral broadening device comprises a phase adjuster (11) arranged in the path of the radiation generated by the first laser oscillator (10) and controlled by the emission controller (50) to provide the first spectral width (Av1) to the laser radiation generated by the first laser oscillator (10); and the lidar system further comprises an optical switch (30) controlled by the emission controller (50) to transmit radiation originating from the phase adjuster (11) or radiation generated by the second laser oscillator (20) to a downstream portion of the emission optical path, the radiation originating from the phase adjuster and the radiation generated by the second laser oscillator sharing the downstream portion of the emission optical path, the downstream portion of the emission optical path comprising the intensity adjustment device (31).

5. The lidar system of claim 2 or 3, wherein, the laser source assembly comprises a first laser oscillator (10) adapted to generate radiation of the first optical frequency (v1) and a second laser oscillator (20) adapted to generate radiation of the second optical frequency (v2) having the second spectral width (Av2); the spectral broadening device comprises a phase adjuster (11) arranged in the path of the radiation generated by the first laser oscillator (10) and controlled by the emission controller (50) to provide the first spectral width (Av1) to the radiation generated by the first laser oscillator; the laser source assembly comprises a first laser oscillator (10) adapted to generate radiation of the first optical frequency (v1) and a second laser oscillator (20) adapted to generate radiation of the second optical frequency (v2) having the second spectral width (Av2); The intensity adjustment device comprises a first intensity adjuster (12) arranged in the path of the radiation originating from the phase adjuster (11) and controlled by the emission controller (50) to be active on the radiation originating from the phase adjuster, and a second intensity adjuster (22) arranged in the path of the radiation generated by the second laser oscillator (20) and controlled by the emission controller to be active on the radiation generated by the second laser oscillator, and the lidar system further comprises an optical coupler (35) suitable for transmitting the radiation originating from the first intensity adjuster (12) and from the second intensity adjuster (22) to a downstream portion of the emission optical path, the radiation originating from the first intensity adjuster and the radiation originating from the second intensity adjuster sharing the downstream portion of the emission optical path.

6. The lidar system of claim 2, wherein, The emission controller (50) is suitable for controlling the laser source assembly, the spectral widening device and the intensity adjustment device, so that the radiation emission sequence comprises a series of radiation pulses spectrally located in the first light frequency (v1) or in the second light frequency (v2) of the two light frequencies, all the radiation pulses having the same envelope shape for the first light frequency and for the second light frequency comprise a first pulse duration (At1) during which the radiation emission has the first spectral width (Av1) and the first pulse optical power value (P1), and a second pulse duration (At2) during which the radiation emission has the second spectral width (Av2) and the second pulse optical power value (P2), the first pulse duration being shorter than the second pulse duration and the first pulse duration being preceded or followed by the second pulse duration in each radiation pulse.

7. The lidar system according to any one of claims 1 to 3 and 6, which implements fiber-optics technology.

8. The lidar system according to any one of claims 1 to 3 and 6, which comprises adapted polarization means which, when radiation is emitted during the plurality of first time intervals or during the plurality of second time intervals, cause the radiation emitted by the lidar system towards the measurement region to have orthogonal polarizations; The detection path comprises a polarization beam splitter (46) arranged to transmit the backscattered radiation to a first detector (41) having sensitivity in a first spectral interval comprising the first light frequency (v1) combined with the first spectral width (Av1), or to a second detector (42) having sensitivity in a second spectral interval comprising the second light frequency (v2) combined with the second spectral width (Av2), as a function of the polarization of the backscattered radiation.

9. The lidar system according to claim 8, the radiation emitted by the lidar system towards the measurement region having opposite circular polarizations.

10. A method of determining the amount of a compound present in a target direction, characterized in that: The lidar system of any one of claims 1 to 9, such that the absorption capacity value of the compound at the first optical frequency (v1) is lower than the absorption capacity value at the second optical frequency (v2); The lidar system is arranged towards the target direction so as to emit radiation according to the radiation emission sequence towards a measurement region possibly containing the compound, and to trigger the operation of the lidar system; The separation distance of the obstacle present in the background of the measurement region is estimated from the backscattered radiation related to the first optical frequency (v1), the separation distance corresponding to the emission of the plurality of first time intervals; and The amount of the compound contained in the measurement region, accumulated on the path of the pulses between the lidar system and the background obstacle, is estimated from intensity values related to the backscattered radiation at the first optical frequency (v1) and at the second optical frequency (v2), respectively, corresponding to the plurality of first time intervals and to the plurality of second time intervals in the radiation emission sequence, respectively, the backscattered radiation having been detected by the detection path of the lidar system.

11. The method of claim 10, wherein, The separation distance of the obstacle present in the background of the measurement region is estimated on the basis of a time of flight measured for the backscattered radiation related to the first optical frequency (v1).

12. The method of claim 11, wherein, The separation distance of the obstacle present in the background of the measurement region, estimated from the backscattered radiation related to the first optical frequency (v1), in combination with intensity values related to the backscattered radiation detected at the first optical frequency and at the second optical frequency (v2), respectively, corresponding to the emission of the plurality of first time intervals and of the plurality of second time intervals, respectively, is used to estimate the amount of the compound contained in the measurement region.

13. The method according to any one of claims 10 to 12, characterized in that, The compound is any one of carbon dioxide, methane, nitrous oxide and water.

14. The method according to any one of claims 10 to 12, characterized in that, The lidar system is installed on the Earth's surface to measure the amount of the compound present between the lidar system and the obstacle, or The lidar system is arranged on board a flying vehicle in flight and directed towards a geographical region of the Earth's surface to measure the separation distance from the Earth's surface within the geographical region with respect to the lidar system and to measure the amount of the compound present between the lidar system and the Earth's surface of the geographical region, or The lidar system is installed on board a satellite orbiting the Earth and directed towards a geographical region of the Earth's surface to measure the separation distance from the Earth's surface within the geographical region with respect to the lidar system and to measure the amount of the compound present between the lidar system and the Earth's surface of the geographical region.

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