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    • 专利摘要:
    Device (1) of the LIDAR type for remote spectroscopy of a material, comprising an optical emission channel (2) comprising a laser source (3) and a frequency generator (4) of optical waves capable of generating a first comb (P1), a second comb (P2) and a local comb (POL), each comb comprising at least one line, an emission telescope (5) capable of emitting an emission signal (6), a reception (7) comprising a reception telescope (8) capable of receiving a signal reflected (9) by the material traversed by the emission signal (6), and a detection system (10) configured to detect at least a first beat signal (11), of the at least one line of the local comb with the first corresponding line of the first reflected comb; and at least one second beat signal (12), of the at least one line of the local comb with the second corresponding line of the second reflected comb; and at least a third beat signal (13) of the at least one first beat signal (11) with the at least one second beat signal (12).
    公开号:FR3082623A1
    申请号:FR1855413
    申请日:2018-06-19
    公开日:2019-12-20
    发明作者:Philippe Hebert；Francois Lemaitre；Nicolas CEZARD
    申请人:Office National dEtudes et de Recherches Aerospatiales ONERA；Centre National dEtudes Spatiales CNES；
    IPC主号:
    专利说明:

    Lidar with heterodyne detection by local oscillator and double sounding beam, at one or more simultaneous frequencies, and Lidar detection method by detection with double heterodyning.
    The invention relates to the field of detection and measurement lasers, or LIDAR, applied to probing the atmosphere by spectroscopy.
    It is known to use a laser source configured to, from a first light emission with a narrow spectral band, centered on a first frequency, generate a first light wave comprising a first plurality of spectral components, each component covering a band , or spectral line, narrow and centered on a frequency different from the other components, so that, in the frequency space, the spectral composition of the light wave has the shape of a first spectral comb, the height of each line of the comb corresponding to the light energy of the spectral line of the first light emission distributed over each line of the comb.
    It is also known to generate, from the first light emission, not only a first light wave, but also and simultaneously a second light wave, coherent with the first, and comprising like the first light wave, a second plurality of components spectral lines, forming a second spectral comb, each spectral line of the second comb being associated with a spectral line of the first comb slightly shifted in frequency with respect thereto.
    It is finally known to emit the first and the second light comb towards a target, for example a portion of atmosphere, then to pick up the signal returned by this target so as to make beat, ie to modulate the received signal corresponding to a line of the first comb with the received signal and associated with the line of the second comb, then to filter the radiofrequency (RF) component of the modulated signal. This modulation occurs for each pair of lines (first comb - second comb).
    The drawback of this device is that, for each line emitted, the signal received from the target is attenuated by the return path of the wave between the transmitter and the receiver, therefore of low level if this distance is important. The power of the filtered RF component is therefore itself at least twice as attenuated as the signal received from each line taken individually; the signal-to-noise ratio of the signal is therefore very degraded if the experimental conditions mean that the photon noise is not dominant.
    A known improvement in the case of a single or double emission line and of a single light wave emitted towards the target, consists in mixing its return with the line of a local oscillator to benefit from the power of the latter, which has not undergone attenuation over a long distance. This device called “heterodyne detection” or “coherent detection” can be limited in the optical frequencies by the coherence length of the laser source, which results in a random phase shift between the return light wave and the local oscillator. This phase shift modulates the radiofrequency signal randomly, which means that an average of several random intensity measurements must be made, affected by an unfavorable statistical standard deviation, and therefore by a degraded signal to final noise ratio.
    The invention therefore aims to solve all or part of these drawbacks.
    To this end, the present invention relates to a LIDAR type device for remote spectroscopy of a material, comprising:
    an optical transmission channel comprising:
    o a laser source capable of emitting a laser line at a generating frequency;
    o a frequency generator of optical waves capable of generating, from the emitted laser line:
    a first comb comprising at least a first line, the at least a first line having a stable frequency and derived from the generating frequency; and a second comb comprising at least a second line, the at least one second line having a stable frequency and derived from the generating frequency; and a local comb comprising at least one local line, the at least one local line having a stable frequency and derived from the generating frequency;
    o an emission telescope capable of transmitting an emission signal to the material through which the emission signal passes, the emission signal comprising the first and the second comb;
    a reception channel comprising:
    a reception telescope capable of receiving a signal reflected by the material through which the emission signal passes, the reflected signal comprising:
    a first reflected comb, the first reflected comb comprising at least a first line reflected by the material traversed by the at least one first line of the first comb of the emission signal, a second reflected comb, the second reflected comb comprising at least one second line reflected by the material traversed by the at least one second line of the second comb of the emission signal;
    each line of the local comb being associated with a first corresponding line of the first reflected comb, and a second corresponding line of the second reflected comb;
    o a detection system configured to detect:
    at least a first beat signal, of the at least one line of the local comb with the first corresponding line of the first reflected comb; and at least one second beat signal, of the at least one line of the local comb with the second corresponding line of the second reflected comb; and at least a third beat signal of the at least one first beat signal with the at least one second beat signal.
    Thanks to these arrangements, each line of the local comb, the signal of which is not attenuated by a round trip from the transmitter to the receiver, is mixed with the first corresponding line of the first reflected comb and the second corresponding line of the second reflected comb;
    The possible phase shift between each line of the local comb and, on the one hand the first corresponding line, on the other hand the corresponding second line, are the same at all times because the lines of the first and second combs are subjected, with respect to the lines of the local comb, the same disturbances; thus a third beat signal eliminates the common phase shift and cancels the effect of random modulation specific to heterodyne detection of the state of the art. It allows you to take full advantage of the power of the local oscillator; it thus allows a significant improvement in the signal-to-noise ratio, despite the attenuation linked to the halving of the energy of each comb, in comparison with the energy which would have been that of a single comb, and despite the losses related to the heterodyne mixture. This gain is the ratio between the power of the local reed and the power of the reflected combs and reducing bandwidth permitted by the low value of the frequency of the 3rd beat.
    According to one embodiment, the invention comprises one or more of the following characteristics, alone or in combination.
    According to one embodiment, the frequency generator is configured to generate the first comb separately from the second comb.
    According to one embodiment, the frequency generator comprises a first acousto-optical modulator and a second acousto-optical modulator, the frequency generator being configured to generate the first comb formed at the output of the first acousto-optical modulator separately from the second comb formed at the output of the second acousto-optical modulator.
    According to one embodiment, the first comb, the second comb and the local comb each comprise a number of lines greater than or equal to 1, preferably greater than or equal to 2, preferably included in 2 and 13.
    According to one embodiment, the first comb, the second comb and the local comb each comprise a number of lines greater than or equal to 2, preferably included in 2 and 13.
    Thanks to these arrangements, it is possible to probe the material simultaneously at several points of the spectrum in the absorption spectral band of the material considered. The different lines of the emission signal (first and second comb) will simultaneously cross the same area of the probed material and will simultaneously produce the different reflected lines which will compose the reflected signal. Thus a small uncertainty on the overall frequency positioning of the transmission signal can be compensated by wide frequency sampling, whose frequency step is well controlled by the technology described above, obtained by the emission of several simultaneous lines having all different frequencies within each comb. A refocusing on the absorption frequency will be possible by treatment after reception. This advantage, specific to the frequency comb lidar, is made all the more interesting since, combined with the other characteristics of the present invention, the signal to noise ratio is considerably improved. Thus, these provisions make it possible not only to benefit from the improvement in the signal to noise ratio brought by the local comb, but also to more effectively eliminate the biases induced by the non-simultaneity of the measurements carried out with a conventional LIDAR DIAL.
    According to one embodiment, the frequency of the lines of the local comb are regularly spaced, around the generating frequency, with a local pitch, the value of the local pitch being between 500 MHz and 2000 MHz, preferably equal to 1000 MHz.
    According to one embodiment of the invention, the generating frequency is an optical frequency, preferably located around 200 THz, or 200,000 GHz.
    According to one embodiment, the frequency of the lines of the first comb are spaced regularly in a first step, around a first central frequency equal to the sum of the generating frequency and a first frequency shift.
    According to one embodiment of the invention, the first frequency shift is obtained by means of a first acousto-optical modulator around a value equal to 100 MHz.
    According to one embodiment, the frequency of the lines of the second comb are regularly spaced in a second step, around a second central frequency equal to the sum of the generating frequency and a second frequency shift.
    According to one embodiment of the invention, the first and second frequency shifts are obtained respectively by means of a first and second acousto-optical modulators, preferably around a value equal to approximately 100 MHz.
    According to one embodiment, the value of the first step and the value of the second step are equal to the sum of the value of the local step with a difference.
    According to one embodiment of the invention, the value of the difference between the value of the steps of the first and second comb on the one hand, and the value of the step of the local comb on the other hand, is approximately 10 MHz.
    According to one embodiment, the detection system comprises a digital processing unit configured to process the at least one first beat signal and the at least one second beat signal so as to generate at least a third beat signal.
    This second level of beat can take place, for example, digitally and consist of spectral filtering of the heterodyne time signal and its elevation squared. A new filtering isolates the frequency of the new beat, which carries the spectroscopic information sought in the target material.
    According to one embodiment, the digital processing unit comprises: an analog-to-digital converter configured to digitize a time signal comprising the at least one first beat signal and the at least one second beat signal, so as to generate a digital signal;
    a first calculation unit configured to calculate a first digital spectrum of the digital signal;
    a first filtering unit configured to separate different spectral components from the previously calculated digital spectrum; a second calculation unit configured to, in parallel, convert from the spectral domain to the time domain, each spectral component separated by the first filtering unit, so as to generate, at least one temporal component comprising at least a first temporal component of the at least a first beat signal and at least a second time component of the at least one second beat signal, each first time component of the at least one time component and each second time component of the at least one second component temporal corresponding to a line of the local comb;
    a mixer configured to beat, in parallel for each line of the local comb, the first corresponding time component of the at least one first beat signal and the second corresponding time component of the at least one second beat signal, so generating, corresponding to each line of the local comb, a third beat signal of the at least one third beat signal;
    a third calculating unit, configured to calculate a digital spectrum of each third beat signal of the at least one third beat signal, in parallel for each third beat signal;
    a second filtering unit configured to isolate, within each third beat signal from the at least one third beat signal, a low frequency spectral component.
    According to these provisions, the detection of the third beat signal by the detection system can be carried out digitally.
    According to one embodiment, each final pair of signals corresponds to a line of the local comb and to a line of the first comb as well as to a line of the second comb. Each pair of signals comprises a first beat signal of a line of the local comb with a first corresponding line of the first reflected comb, and a second beat signal of the line of the local comb with the second corresponding line of the second reflected comb.
    According to one embodiment, the third beat signal is filtered around a frequency equal to an absolute value of a difference between the first frequency shift and the second frequency shift.
    According to these provisions, the filtering of the third beat signal makes it possible to isolate the signal which carries the spectroscopic information sought, while benefiting from the improved signal to noise ratio.
    According to one embodiment of the invention, the value of the third intermediate frequency is approximately 1 MHz.
    According to one embodiment, the invention also relates to a LIDAR detection method for remote spectroscopy of a material, comprising the following steps:
    emission by a laser source of a laser line at a generating frequency;
    from the laser line, generation by a frequency generator: o of a first comb comprising at least one first line, the at least one first line having a stable frequency and derived from the generating frequency; and o a second comb comprising at least a second line, the at least one second line having a stable frequency and derived from the generating frequency; and o a local comb comprising at least one local line, the at least one local line having a stable frequency and derived from the generating frequency;
    emission by an emission telescope of an emission signal towards the material traversed by the emission signal, the emission signal comprising the first and the second comb;
    reception by a reception telescope:
    a signal reflected by the material traversed by the emission signal, the reflected signal comprising:
    a first reflected comb, the first reflected comb comprising at least a first line reflected by the material traversed by the at least one first line of the first comb of the emission signal, a second reflected comb, the second reflected comb comprising at least one second line reflected by the material traversed by the at least one second line of the second comb of the emission signal;
    each line of the local comb being associated with a first corresponding line of the first reflected comb, and a second corresponding line of the second reflected comb;
    detection by a detection system:
    at least a first beat signal, at least one line of the local comb with the first corresponding line of the first reflected comb; and at least one second beat signal, at least one line of the local comb with the second corresponding line of the second reflected comb; and a third beat signal of the at least one first beat signal with the at least one second beat signal.
    For a good understanding, the invention is described with reference to the attached drawings representing, by way of nonlimiting example, an embodiment of a device according to the invention. Elements which are similar or whose function is similar are designated in the different figures by the same references.
    Figure 1 is a schematic view of a remote spectroscopy device according to an embodiment of the invention
    Figure 2 is a schematic view of the frequency spectrum of the different combs.
    Figure 3 is a schematic view of a digital embodiment of part of the detection system.
    Figure 4 is a simplified representation of the different steps of the method according to the invention.
    In FIG. 1, the device 1 for remote spectroscopy comprises an optical transmission channel 2 and an optical reception channel 7.
    The optical emission channel 2 comprises a laser source 3 configured to emit a laser line at a generator frequency FG; the laser source is connected to a frequency generator 4 by an optical fiber.
    In general, the transmission of the optical signals between the different optical components of the device 1 is carried out by optical fiber, which, preferably, ensures that the polarization of the transmitted optical signals is maintained.
    At the input of the frequency generator 4, the laser beam is divided into a first beam which is led to a first electro-optical modulator 41 configured to generate a first local comb POL. This first local comb comprises at least one local line, each line having a stable frequency and derived from the generating frequency FG.
    According to one embodiment, the generating frequency is an optical frequency, preferably close to 200 THz, and the local comb comprises a number of lines greater than 2, preferably between 2 and 13; the frequencies of the lines are distributed around the generating frequency FG, and are offset from each other by a predetermined FMEOL value, between 500 MHz and 2000 MHz, preferably equal to 1000 MHz, as illustrated in FIG. 2 , on which 13 frequency lines regularly distributed on either side of the generating frequency are represented along a frequency axis f.
    At the input of the frequency generator 4, the laser line is simultaneously divided into a second beam which is led to a second electro-optical modulator 42 which, in combination with a first acousto-optical modulator 43 and a second acousto-optical modulator 44, is configured to generate a first comb PI formed at the output of the first acousto-optical modulator 43, separated from a second comb P2 formed at the output of the second acousto-optical modulator 44. These first and second combs P1, P2 include, at least one line , each line having a stable frequency and derived from the generating frequency FG.
    Preferably, according to an embodiment illustrated in FIG. 2, the first and second comb P1, P2, each comprise a number of lines identical to the number of lines of the local comb, i.e. greater than 2, preferably between 2 and 13; the frequencies of the lines of each of the combs P1, P2 are distributed around a central frequency FC1, FC2, derived from the generating frequency FG, and are offset from each other respectively by a value FMEO1 for the first comb PI, FMEO1 being, for example, between 500 MHz and 2000 MHz, preferably equal to 1000 MHz, and with a value FMEO2 for the second comb P2, FMEO2 being for example between 500 MHz and 2000 MHz, preferably equal to 1000 MHz . According to one embodiment, the central frequency FC1 of the first comb and the central frequency FC2 of the second comb are offset from the generating frequency
    FG of a value respectively FMAO1, FMAO2, determined respectively by each of the two acousto-optical modulators 43, 44.
    According to one embodiment, the value of the FMAO1 offset between the generating frequency, ie the central frequency of the local comb, and the central frequency of the first PI comb, and the value of the FMAO2 offset between the generating frequency, and the central frequency of the second comb P2, are preferably fixed around 100 MHz, for example 110 MHz for FMAO1 and 111 MHz for FMAO2.
    Each line of the local comb is associated with a first corresponding line of the first comb PI, typically that of the lines of the first comb whose frequency is closest to the frequency of the line considered on the local comb; similarly, each line of the local comb is associated with a second corresponding line of the second comb P2, typically that of the lines of the second comb whose frequency is closest to the frequency of the line considered on the local comb;
    According to one embodiment, the value of the first step FMEO1 of the first comb PI and the value of the second step FMEO2 of the second comb P2, are equal to each other, and differ from the value of the local step FMEOL of the local comb by a value d 'deviation D. According to one embodiment of the invention, the value of the deviation is approximately 10 MHz.
    According to a particular embodiment, the values of the following parameters are thus fixed to define the local comb POL and the first and second combs P1, P2: Deviation D = 10.6 MHz
    No local comb FMEOL: FMEOL = 889.4 MHz
    Step FMEO1 of the first comb Pl: FMEO1 = FMEOL + D = 900 MHz
    Step FMEO2 of the second comb P2: FMEO2 = FMEOL + D = 900 MHz
    FMAO1 offset between the generating frequency, i.e. the central frequency of the local comb, and the central frequency of the first comb Pl: FMAO1 = 110 MHz
    FMAO2 offset between the generating frequency, i.e. the center frequency of the local comb, and the center frequency of the second comb P2: FMAO2 = 111 MHz
    According to this particular embodiment, given by way of example, the differences, expressed in MHz, between the generating frequency and the frequencies of each of the 13 lines which make up the local comb POL, as distributed around the order line. 0 (at the generating frequency), are summarized in the second column POL of table 1 below.
    D'' HzFMEOL 889.4 MHzFMEO1, FMEO ^ Ha"FMAO1MHZFMAO2MH: Bat may B (POL, PI) B (POL, PI ORDER ° OL □ 1 -'2 B (POL. PI) BiPOL, Pz) B (POL.Pz) B (POL Pz -6 -5356.4 -5290 -5289 45.4Θ 47.48 i 1 46.90 -5 -4447 -4390 -4389 57.00 58.00 1 57.50-3557.6 ”Z M - RiRR 67.50 68.60 1 1 68.10 - *> -2CG8.2 -2590 -2589 70.20 7S .28 1 78.70 -2 -1778.8 -1690 -1689 88.80 89.88 1 i 89.30 -1 -889.4 -790 -789 99.40 100.40 1 99.90 β at 110 111 110.00 111.00 1 lia. w 1: 869.4 1010 1011 120.60 121.50 1: 121.10 2 1778.8 1910 1911 131.20 132.20 1: 151.70 3 2668.2 2810 2811 141.88 X4Z î83 1 · 142.30 4 3557.6 3710 3711 152.40 153,40 1 152.90 5 "47 4618 4611 163.00 164.00 1 163.50 Ê 5336.4 5510 5511 173.60 174.60 1 174.10
    Table 1
    Similarly, the differences, expressed in MHz, between the generating frequency FG and the frequencies of each of the 13 lines which make up the first comb PI, as distributed around the line of order 0 (at the generating frequency), are summarized in 10 the third column PI of table 1.
    Similarly, the differences, expressed in MHz, between the generating frequency and the frequencies of each of the 13 lines that make up the second comb P2, as distributed around the line of order 0 (at the generating frequency), are summarized in the fourth column P2 of Table 1.
    The frequencies appearing on the same line of table 1 are on the one hand, on the column POL, the frequency of a line of the local comb, and on the other hand, on the columns PI and P2, the frequencies of the first line and second corresponding line belonging respectively to the first and second combs P1, P2, which are associated with said line of the local comb.
    The first comb PI and the second comb P2 simultaneously formed separately, respectively at the output of the first acousto-optical modulator 43 and of the second acousto-optical modulator 44, compose, after their spatial superposition by means of a coupler preferably holding of polarization, an optical signal sent, via an optical fiber, to an optical amplifier 45 configured to amplify the power of the signal which it receives and thus generate an optical emission signal 6 which will be emitted by a transmitting telescope 5 to the material to be probed remotely by spectroscopy.
    According to one embodiment, the optical amplifier 45 will be of the EDFA type (from the English “Erbium Doped Fiber Amplifier”) or of the YDFA type (Ytterbium), known per se in the state of the art.
    The emission telescope 5, also known in the state of the art, is configured to receive the amplified signal 6 and transmit it to the material to be probed, so that the emission signal 6 passes through the material to be probed and then is reflected on a hard target, thus generating a reflected signal 9 which is returned to a receiving telescope 8.
    The reception optical channel 7 comprises a reception telescope 8, an optical coupling module 14, and a detection system 10, these components being connected by a fiber optic link, as indicated above.
    According to one embodiment, the transmitting telescope 5 and the receiving telescope 7 can be in the form of a single component, in the case of a so-called monostatic configuration.
    The reception telescope 7 is configured to receive the reflected signal 9, after the emission signal 6 has passed through the material to be probed, the reflected signal 9 comprising:
    a first reflected comb, the first reflected comb comprising at least a first line reflected after the material has passed through the at least one first line of the first comb PI of the emission signal 6, a second reflected comb, the second reflected comb comprising at least one second line reflected after the material has passed through the at least one second line of the second comb P2 of the transmission signal 6.
    In the same way that each line of the local comb is associated with a first corresponding line of the first comb PI, and with a second corresponding line of the second comb P2, each line of the local comb is thus associated with a first corresponding line of the first reflected comb , ie to the corresponding line of the first comb PI as reflected after the emission signal 6 has passed through the material, and to a second corresponding line of the second reflected comb, ie to the corresponding line of the second comb P2 as reflected after the material has passed through the emission signal 6.
    The coupling module 14 is configured to couple the signal from the local comb POL and the reflected signal 9 and to send the coupled signal to the detection system 10.
    According to one embodiment, the detection system 10 comprises a photodetector configured to produce:
    at least a first beat signal 11, of the at least one line of the local comb with the first corresponding line of the first reflected comb; and at least one second beat signal 12, of the at least one line of the local comb with the second corresponding line of the second reflected comb;
    According to one embodiment, the photodetector detects and thus produces a first optical beat level, in which is generated at least a first heterodyne beat signal 11 between each line of the local comb POL and the first corresponding line of the first reflected component comb the reflected signal 9, as well as at least one second heterodyne beat signal 12 between each line of the local comb POL and the second corresponding line of the second reflected comb also making up the reflected signal 9.
    The at least one first beat signal 11 comprises, for each line of the local comb, a first beat signal between the line of the local comb and the first corresponding line of the first reflected PI comb. Likewise the at least one second beat signal 12 comprises, for each line of the local comb, a second beat signal between the line of the local comb and the first corresponding line of the second reflected comb P2.
    It is known to pay particular attention, within each beat signal, to the signal of intermediate frequency equal to the difference of the frequencies of the coupled signals.
    Thus, according to an embodiment corresponding to the values of the parameters set above, by way of example, to define the local comb POL and the first and second combs P1, P2 according to table 1 above, the same table 1 supplied to column B (POL, PI) the intermediate frequencies of the signals which make up each first beat signal 11 of the different lines of the local comb with the corresponding lines of the first reflected comb which make up in part the reflected signal 9; the same table 1 supplied in column B (POL, P2) the intermediate frequencies of the signals which make up each second beat signal 12 of the different lines of the local comb with the corresponding lines of the second reflected comb, which also make up the reflected signal 9.
    As illustrated in FIG. 3, the detection system 10 is also configured to beat, in a second beat level, electric, and for each stripe of the local comb, the first beat signal, corresponding to this stripe of the comb local, of the at least one first beat signal 11 with the second beat signal, corresponding to this same line of the comb local, of the at least one second beat signal 12, to thus generate, for each line of the comb local, at least a third beat signal 13, the at least a third beat signal 13 comprising as many beat signals CSBF1, CSBF2, CSBF3, ... CSBFi, ... as there are different lines in the local comb.
    To do this, for each line of the local comb:
    the beat signal of the at least one first beat signal 11 is filtered around a frequency equal to the average of the respective frequencies of the line of the local comb and of the corresponding line of the first reflected PI comb, which generates a first filtered beat signal;
    similarly, each beat signal of the at least one second beat signal 12 is filtered around a frequency equal to the average of the respective frequencies of the line of the local comb and the corresponding line of the second reflected comb P2, which generates a second filtered beat signal.
    Thus, for each line of the local comb is generated a first filtered beat signal and a second filtered beat signal.
    Then in a second beat level, these first and second filtered beat signals are mixed to beat and generate a third beat signal CSBF1.
    This process is repeated so on as many times as there are lines in the local combs, PI and P2, thus generating the sequence of beat signals CSBF1, CSBF2, CSBF3, ... CSBFi, ... which compose the at least one third beat signal 13.
    According to one embodiment, to isolate the signal which carries the desired spectroscopic information, it is necessary to filter, for each line of the local comb, the third beat signal 13 around a frequency equal to an absolute value of a difference between the first frequency shift FMAO1 and the second frequency shift FMAO2.
    Thus, according to an embodiment corresponding to the values of the parameters set above by way of example, to define the local comb POL and the first and second comb Pl, P2 according to table 1 above, it appears in the penultimate column of table 1 that by filtering the at least one third beat signal 13 around an intermediate frequency of 1 MHz, the signal which carries the desired spectroscopic information is isolated, with a maximum signal to noise ratio. This improvement in the signal to noise ratio is brought about by the processing described above, and a digital embodiment of which has now been described.
    According to an embodiment illustrated in FIG. 3, the detection system 10 comprises a digital processing unit UTN configured to process in parallel, for each line of the local comb, the at least one first beat signal 11 and the at at least one second beat signal 12, so as to generate the at least one third beat signal 13.
    According to this embodiment, the digital processing unit UTN comprises:
    an analog-to-digital converter ADC configured to digitize a time signal comprising the at least one first beat signal 11 and the at least one second beat signal 12, so as to generate a digital signal UN, 12N;
    a first FFT calculation unit configured to calculate a first digital spectrum SN11, SN12 of the digital signal UN, 12N;
    a first filtering unit UF1 configured to separate different spectral components of the digital spectrum SN11, SN12 previously calculated;
    a second FFT 1 calculation unit configured to, in parallel, convert from the spectral domain to the time domain, each spectral component separated by the first filtering unit UF1, so as to generate, at least one time component comprising at least one first time component of the at least one first beat signal 11 and at least one second time component of the at least one second beat signal 12, each first time component of the at least one time component and each second time component of the at least a second time component corresponding to a line of the local comb;
    a MEL mixer configured to beat, in parallel for each line of the local comb, the first corresponding time component of the at least one first beat signal 11 and the second corresponding time component of the at least one second beat signal 12 , so as to generate, corresponding to each line of the local comb, a third beat signal of the at least one third beat signal;
    a third FFT2 calculation unit, configured to calculate a digital spectrum of each third beat signal of the at least one third beat signal, in parallel for each third beat signal;
    a second filtering unit UF2 configured to isolate, within each third beat signal from the at least one third beat signal, a low frequency spectral component CSBF1, CSBF2, CSBF3, ..., CSBFi, ....
    According to these provisions, the detection of the third beat signal by the detection system is carried out for example digitally. The second beat level eliminates, at all times, the phase shifts between the PI comb and the local oscillator on the one hand, and between the P2 comb and the local oscillator on the other hand, which are the same in the 1st order , cancel each other out. This cancels the decorrelation which would otherwise degrade the final heterodyne signal.
    The invention also relates to a method 100 of detection by LIDAR for a remote spectroscopy of a material, the method comprising the following steps:
    emission 101 by a laser source 3 of a laser line at a generating frequency FG;
    from the laser line, generation 102 by a frequency generator 4:
    a first comb PI comprising at least a first line, the at least a first line having a stable frequency and derived from the generating frequency FG; and of a second comb P2 comprising at least a second line, the at least one second line having a stable frequency and derived from the generating frequency; and a local comb POL comprising at least one local line, the at least one local line having a stable frequency and derived from the generating frequency;
    o emission 103 by an emission telescope 5 of an emission signal 6 towards the material traversed by the emission signal 6, the emission signal 6 comprising the first and the second comb;
    reception 104 by a reception telescope 8:
    a signal reflected 9 by the material through which the emission signal 6 passes, the reflected signal 9 comprising:
    a first reflected comb, the first reflected comb comprising at least a first line reflected by the material traversed by the at least one first line of the first comb of the emission signal, a second reflected comb, the second reflected comb comprising at least one second line reflected by the material traversed by the at least one second line of the second comb of the emission signal;
    each line of the local comb being associated with a first corresponding line of the first reflected comb, and a second corresponding line of the second reflected comb;
    o detection 105 by a detection system 10:
    at least a first beat signal 11, at least one line of the local comb with the first corresponding line of the first reflected comb; and at least one second beat signal 12, at least one line of the local comb with the second corresponding line of the second reflected comb; and a third beat signal 13 of the at least one first beat signal with the at least one second beat signal.
    权利要求:
    Claims (11)
    [1" id="c-fr-0001]
    1. Device (1) of the LIDAR type for remote spectroscopy of a material, comprising:
    an optical transmission channel (2) comprising:
    o a laser source (3) capable of emitting a laser line at a generating frequency (FG);
    o a frequency generator (4) of optical waves capable of generating, from the emitted laser line:
    a first comb (PI) comprising at least a first line, the at least one first line having a stable frequency and derived from the generating frequency (FG); and a second comb (P2) comprising at least a second line, the at least one second line having a stable frequency and derived from the generating frequency (FG); and a local comb (POL) comprising at least one local line, the at least one local line having a stable frequency and derived from the generating frequency (FG);
    an emission telescope (5) capable of transmitting an emission signal (6) to the material through which the emission signal (6) passes, the emission signal (6) comprising the first and the second comb ( P1, P2);
    a reception channel (7) comprising:
    a reception telescope (8) capable of receiving a signal reflected (9) by the material traversed by the emission signal (6), the reflected signal (9) comprising:
    a first reflected comb, the first reflected comb comprising at least a first line reflected by the material traversed by the at least one first line of the first comb of the emission signal, a second reflected comb, the second reflected comb comprising at least one second line reflected by the material traversed by the at least one second line of the second comb of the emission signal;
    each line of the local comb being associated with a first corresponding line of the first reflected comb, and a second corresponding line of the second reflected comb;
    o a detection system (10) configured to detect:
    at least a first beat signal (11), of the at least one line of the local comb with the first corresponding line of the first reflected comb; and at least a second beat signal (12), of the at least one line of the local comb with the second corresponding line of the second reflected comb; and at least a third beat signal (13) of the at least one first beat signal (11) with the at least one second beat signal (12).
    [2" id="c-fr-0002]
    2. Device according to claim 1, in which the first comb, the second comb and the local comb each comprise a number of lines greater than or equal to 1, preferably greater than or equal to 2, preferably included in 2 and 13.
    [3" id="c-fr-0003]
    3. Device according to claim 2, in which the first comb, the second comb and the local comb each comprise a number of lines greater than or equal to 2, preferably included in 2 and 13.
    [4" id="c-fr-0004]
    4. Device according to claim 3, in which the frequency of the lines of the local comb are regularly spaced, around the generating frequency, with a local step (FMEOL), the value of the local step (FMEOL) being between 500 MHz and 2000 MHZ, preferably equal to 1000 MHz.
    [5" id="c-fr-0005]
    5. Device according to one of claims 3 or 4, wherein the frequency of the lines of the first comb are regularly spaced according to a first step (FMEO1), around a first central frequency (FC1) equal to the sum of the frequency generator and a first frequency offset (FMAO1).
    [6" id="c-fr-0006]
    6. Device according to one of claims 3 to 5, in which the frequency of the lines of the second comb are regularly spaced in a second step (FMEO2), around a second central frequency (FC2) equal to the sum of the frequency generator and a second frequency offset (FMAO2).
    [7" id="c-fr-0007]
    7. Device according to claim 6, in which the value of the first step (FMEOl) and the value of the second step (FMEO2) are equal to the sum of the value of the local step (FMEOL) with a difference (D).
    [8" id="c-fr-0008]
    8. Device according to one of the preceding claims, the detection system comprises a digital processing unit (UTN) configured to process the at least one first beat signal (11) and the at least one second beat signal ( 12) so as to generate at least a third beat signal (13).
    [9" id="c-fr-0009]
    9. Device according to claim 8, in which the digital processing unit (UTN) comprises:
    an analog to digital converter (ADC) configured to digitize a time signal comprising the at least one first beat signal (11) and the at least one second beat signal (12), so as to generate a digital signal (UN, 12N);
    a first calculation unit (FFT) configured to calculate a first digital spectrum (SN11, SN12) of the digital signal (UN, 12N);
    a first filtering unit (UF1) configured to separate different spectral components of the digital spectrum (SN11, SN12) previously calculated;
    a second calculation unit (FFT 1 ) configured to, in parallel, convert from the spectral domain to the time domain, each spectral component separated by the first filtering unit (UF1), so as to generate, at least one temporal component comprising at least a first time component of the at least one first beat signal (11) and at least a second time component of the at least one second beat signal (12), each first time component of the at least one time component and each second time component of the at least one second time component corresponding to a line of the local comb;
    a mixer (MEL) configured to beat, in parallel for each line of the local comb, the first corresponding time component of the at least one first beat signal (11) and the corresponding second time component of the at least one second beat signal (12), so as to generate, corresponding to each line of the local comb, a third beat signal of the at least one third beat signal;
    a third calculating unit (FFT2), configured to calculate a digital spectrum of each third beat signal of the at least one third beat signal, in parallel for each third beat signal;
    a second filtering unit (UF2) configured to isolate, within each third beat signal from the at least one third beat signal, a low frequency spectral component (CSBFI, CSBF2, CSBF3, ..., CSBFi ,. ..).
    [10" id="c-fr-0010]
    10. Device according to claim 1, in which the third beat signal is filtered around a frequency equal to an absolute value of a difference between the first frequency shift (FMAO1) and the second frequency shift (FMAO2).
    [11" id="c-fr-0011]
    11. Method (100) of detection by LIDAR for a remote spectroscopy of a material, comprising the following steps:
    emission (101) by a laser source (3) of a laser line at a generating frequency (FG);
    from the laser line, generation (102) by a frequency generator (4):
    i. a first comb (PI) comprising at least a first line, the at least one first line having a stable frequency and derived from the generating frequency (FG); and ii. a second comb (P2) comprising at least a second line, the at least one second line having a stable frequency and derived from the generating frequency; and iii. a local comb (POL) comprising at least one local line, the at least one local line having a stable frequency and derived from the generating frequency;
    emission (103) by an emission telescope (5) of an emission signal (6) towards the material through which the emission signal (6) passes, the emission signal (6) comprising the first and the second comb (P1, P2); reception (104) by a reception telescope (8):
    b. of a signal reflected (9) by the material traversed by the emission signal (6), the reflected signal (9) comprising:
    i. a first reflected comb, the first reflected comb comprising at least a first line reflected by the material traversed by the at least one first line of the first comb of the emission signal, ii. a second reflected comb, the second reflected comb comprising at least a second line reflected by the material traversed by the at least one second line of the second comb of the emission signal;
    each line of the local comb being associated with a first corresponding line of the first reflected comb, and a second corresponding line of the second reflected comb;
    detection (105) by a detection system (10):
    iii. at least a first beat signal (11), at least one line of the local comb with the first corresponding line of the first reflected comb; and iv. at least one second beat signal (12), at least one line of the local comb with the second corresponding line of the second reflected comb; and
    v. a third beat signal (13) of the at least one first beat signal with the at least one second beat signal.
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    EP3584604A1|2019-12-25|
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    引用文献:
    公开号 | 申请日 | 公开日 | 申请人 | 专利标题
    FR3027116A1|2014-10-08|2016-04-15|Centre Nat D'etudes Spatiales|REMOTE SPECTROSCOPY DEVICE OF LIDAR TYPE|
    FR3039331A1|2015-07-22|2017-01-27|Centre Nat D'etudes Spatiales |REMOTE SPECTROSCOPY METHOD, COMPUTER PROGRAM PRODUCT, AND ASSOCIATED SPECTROSCOPY DEVICE|
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    FR3105439B1|2019-12-20|2022-01-14|Thales Sa|LIDAR SYSTEM INCLUDING TWO DIFFRACTIVE COMPONENTS|
    CN111190160B|2020-01-08|2022-02-15|南京航空航天大学|Microwave photon multiband radar detection method and microwave photon multiband radar|
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    2019-12-20| PLSC| Search report ready|Effective date: 20191220 |
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