Lidar type device for remote spectroscopy of matter and detection method thereof

Abstract

A laser radar type device (1) for remote spectroscopy of matter, comprising an optical transmission channel (2), which includes a laser source (3) and a light wave frequency generator (4), which is suitable for generating a first comb (P1), a second comb (P2) and a local comb (POL), each comb including at least one strip, a transmission telescope (5) suitable for transmitting a transmission signal (6), a receiving channel (7) including a receiving telescope (8), which is suitable for receiving a signal (9) reflected by a matter through which the transmission signal (6) passes, and a detection system (10), which is configured to detect at least one first beat signal (11) between at least one strip of the local comb and a corresponding first strip of the first reflection comb; and at least one second beat signal (12) between at least one strip of the local comb and a corresponding second strip of the second reflection comb; and at least one third beat signal (13) between the at least one first beat signal (11) and the at least one second beat signal (12).

Description

Technical Field

[0001] The invention relates to the field of detection and measurement lasers, or lidar, for probing the atmosphere by spectroscopy. Background Art

[0002] It is known to use a laser source configured to generate a first light wave comprising a first plurality of spectral components by light emission from a first narrowband frequency band centered on a first frequency, each component covering a segment or bar which is a narrowband spectral band or bar centered on a frequency different from that of the other components, such that in frequency space the spectral composition of the light wave has the shape of a first spectral comb, the height of each bar of the comb corresponding to the energy of the spectral bar of the first light emission distributed over each bar of the comb.

[0003] It is also known that from this first light emission not only a first light wave is generated, but also a second light wave is simultaneously generated, which is coherent with the first light wave and includes a second plurality of spectral components like the first light wave, which forms a second spectral comb, each spectral strip of the second comb being related to a spectral strip of the first comb slightly shifted relative to its frequency.

[0004] Finally, it is known to emit the first and second optical combs in the direction of a target, such as a portion of the atmosphere, and then to collect the signal sent back by this target, to generate a beat, that is to simulate the received signal corresponding to the strip with the received signal of the first comb and associated with the strip of the second comb, and then to filter the radio frequency (RF) component of the simulated signal. This simulation is carried out for each pair of strips (first comb-second comb).

[0005] The disadvantage of this arrangement is that for each emitted strip, the signal received from the object is attenuated due to the round trip of the wave between emitter and receiver, so it is at a low level if this distance is large. The power of the filtered RF component is in turn attenuated by a factor of two compared to the signal received from each strip considered individually; if the experimental conditions are such that the photon noise is not dominant, the signal-to-noise ratio of this signal is therefore greatly reduced.

[0006] In the case of a single or double emission bar and a single light wave emitted to a target, a known improvement consists in mixing its feedback with a bar of a local oscillator in order to exploit the power of the latter, which has not yet experienced long-distance attenuation. This arrangement, called "heterodyne detection" or "coherent detection", can be limited to the optical frequency through the correlation length of the laser source, which causes a random phase drift between the feedback light wave and the local oscillator. This phase drift randomly simulates a radio frequency signal, which requires averaging of several random intensity measurements and has an unfavorable statistical standard deviation and, therefore, a reduced final signal-to-noise ratio. Summary of the invention

[0007] The present invention therefore aims to overcome all or part of these disadvantages.

[0008] To this end, the invention relates to a device of the LiDAR type for remote spectroscopy of matter, comprising:

[0009] - An optical transmission channel comprising:

[0010] ○ A laser source, suitable for emitting laser strips at a generation frequency;

[0011] ○ A light wave frequency generator, suitable for generating from the emitted laser bar:

[0012] ■ a first comb comprising at least one first strip having a stable frequency derived from the occurrence frequency; and

[0013] ■ a second comb comprising at least one second strip having a stable frequency derived from the occurrence frequency; and

[0014] ■ a local comb comprising at least one local strip having a stable frequency derived from the occurrence frequency;

[0015] o a transmit telescope adapted to transmit a transmit signal towards matter through which the transmit signal passes, the transmit signal comprising the first and second combs;

[0016] - Receiving channels, including:

[0017] A receiving telescope adapted to receive signals reflected from the material through which the transmitted signal passes, the reflected signals comprising:

[0018] ■ a first reflection comb, the first reflection comb comprising at least one first stripe reflected by a substance through which at least one first stripe of the first comb of the transmitted signal passed,

[0019] ■ a second reflection comb, the second reflection comb comprising at least one second strip reflected by the substance through which at least one second strip of the second comb of the transmitted signal passed,

[0020] Each strip of the partial comb is associated with a corresponding first strip of the first reflective comb and a corresponding second strip of the second reflective comb;

[0021] ○ A detection system, configured to detect:

[0022] ■ at least one first beat signal of at least one stripe of the local comb and a corresponding first stripe of the first reflection comb; and

[0023] ■ at least one second beat signal of at least one stripe of the local comb and a corresponding second stripe of the second reflected comb; and

[0024] ■At least one third beat signal of the at least one first beat signal and the at least one second beat signal.

[0025] Due to these arrangements, each stripe of this local comb (whose signal has not been attenuated in the round trip between the transmitter and the receiver) is mixed with a corresponding first stripe of the first reflected comb and a corresponding second stripe of the second reflected comb;

[0026] The possible phase drifts between each strip of the local comb and the corresponding first strip on the one hand and the second strip on the other hand are the same at all times, since the strips of the first and second combs experience the same disturbances as the strips of the local comb; therefore, the third beat signal eliminates the usual phase drifts and cancels the random analog effects characteristic of the heterodyne detection of the prior art. It allows to fully exploit the power of the local oscillator; it therefore allows to significantly improve the signal-to-noise ratio, although this attenuation involves the energy of each comb being divided in two, in contrast to the energy of a single comb, and although the losses involve heterodyne mixing. This gain is a function of the ratio between the local comb power and the reflected comb power and the bandwidth reduction allowed by the low frequency value of the 3rd beat.

[0027] According to one embodiment, the present invention includes one or more of the following features, considered alone or in combination.

[0028] According to one embodiment, the frequency generator is configured to generate the first comb separately from the second comb.

[0029] According to one embodiment, the frequency generator includes a first AOM and a second AOM, the frequency generator being configured to generate a first comb formed at an output of the first AOM, and a second comb formed at an output of the second AOM, respectively.

[0030] According to one embodiment, the first comb, the second comb and the partial comb each comprise a number of bars greater than or equal to 1, preferably greater than or equal to 2, preferably comprised between 2-13.

[0031] According to one embodiment, the first comb, the second comb and the partial comb each comprise a number of bars greater than or equal to 2, preferably comprised between 2-13.

[0032] Due to these arrangements, the material under consideration can be detected simultaneously at several points of the spectrum in the absorption spectral band of the material. Different strips of the transmitted signal (first and second combs) will simultaneously pass through the same sampling of the detected material, and will simultaneously produce different reflection strips, which include the reflection signal. Therefore, small uncertainties in the overall frequency arrangement of the transmitted signal can be compensated by large frequency sampling, and its frequency step is well controlled by the above-mentioned technology, which is obtained by the emission of several synchronized strips of all different frequencies in each comb. Through post-receive processing, refocusing on the absorption frequency will be possible. The unique advantage of this laser radar with a frequency comb is even more noteworthy, that is, when combed with other features of the present invention, the signal-to-noise ratio is greatly improved. Therefore, these arrangements allow not only to take advantage of the improvement in the signal-to-noise ratio provided by the local comb, but also to more effectively eliminate the bias caused by the non-synchronization of measurements made with conventional laser radar DIALs.

[0033] According to one embodiment, the frequency of the bars of the local comb is regularly spaced around the frequency of occurrence in local steps, the value of which is comprised between 500 MHz and 2000 MHz, preferably equal to 1000 MHz.

[0034] According to one embodiment of the invention, the frequency of occurrence is an optical frequency, preferably located at about 200 THz, which represents 200000 GHz.

[0035] According to one embodiment, the bar frequencies of the first comb are regularly spaced according to a first step around a first center frequency equal to the sum of the occurrence frequency and a first frequency drift.

[0036] According to one embodiment of the invention, the first frequency shift is obtained via a first acousto-optic modulator at a value approximately equal to 100 MHz.

[0037] According to one embodiment, the frequencies of the bars of the second comb are regularly spaced according to a second step around a second central frequency equal to the sum of the occurrence frequency and the second frequency shift.

[0038] According to one embodiment of the invention, the first and second frequency shifts are obtained via first and second acousto-optic modulators, respectively, preferably around a value equal to about 100 MHz.

[0039] According to one embodiment, the value of the first step and the value of the second step are equal to the sum of the local step value and the deviation.

[0040] According to one embodiment of the invention, the deviation between the value of the first step amplitude and the values ​​of the second comb step amplitude on the one hand and the local comb step amplitude on the other hand is approximately 10 MHz.

[0041] According to one embodiment, the detection system includes a digital processing unit configured to process the at least one first beat signal and the at least one second beat signal to generate at least one third beat signal.

[0042] This second beat level can be run digitally, for example, and can consist of a spectral filtering of the heterodyne time signal and its square. The new filtering separates the frequencies of the new beats that carry the spectroscopic information for the target substance search.

[0043] According to one embodiment, the digital processing unit comprises:

[0044] - an analog-to-digital converter configured to digitize a time signal including the at least one first beat signal and the at least one second beat signal to generate a digital signal;

[0045] - a first calculation unit configured to calculate a first digital spectrum of the digital signal;

[0046] - a first filtering unit configured to separate different spectral components of the previously calculated digital spectrum;

[0047] - a second calculation unit configured to convert each spectral component separated by the first filtering unit from the spectral domain to the time domain in parallel to generate at least one time component, the at least one time component comprising at least one time component of the at least one first beat signal and at least one 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 time component corresponding to a strip of the local comb;

[0048] a mixer configured to generate the beats of the corresponding first time component of the at least one first beat signal and the corresponding second time component of the at least one second beat signal in parallel for each strip of the local comb to generate a third beat signal of the at least one third beat signal corresponding to each strip of the local comb;

[0049] a third calculation unit configured to calculate, for each third beat signal, a digital spectrum of each third beat signal of the at least one third beat signal in parallel;

[0050] - a second filtering unit configured to separate low frequency spectral components within each third beat signal of the at least one third beat signal.

[0051] According to these arrangements, detection of the third beat signal by the detection system can be performed digitally.

[0052] According to one embodiment, each final paired signal corresponds to the bar of the local comb and the bar of the first comb and the bar of the second comb. Each pair of signals includes a first beat signal of the bar of the local comb and the first bar corresponding to the first reflection comb, and a second beat signal of the bar of the local comb and the second bar corresponding to the second reflection comb.

[0053] According to one embodiment, the third beat signal is filtered around a frequency equal to the absolute value of the difference between the first frequency shift and the second frequency shift.

[0054] According to these arrangements, filtering of the third beat signal allows separation of the signal carrying the sought spectroscopic information while taking advantage of an improved signal-to-noise ratio.

[0055] According to one embodiment of the present invention, the third intermediate frequency value is approximately 1 MHz.

[0056] According to one embodiment, the present invention also relates to a lidar detection method for remote spectroscopy of a substance, comprising the following steps:

[0057] - emitting a laser strip at a generation frequency by means of a laser source;

[0058] -Generated by the laser bar via a frequency generator:

[0059] o a first comb comprising at least one first strip having a stable frequency derived from the occurrence frequency; and

[0060] o a second comb comprising at least one second strip having a stable frequency derived from the occurrence frequency; and

[0061] o a local comb comprising at least one local strip having a stable frequency derived from the occurrence frequency;

[0062] - transmitting a transmit signal through a transmit telescope towards the matter through which the transmit signal passes, the transmit signal comprising the first and second combs;

[0063] - Reception via receiving telescope:

[0064] ○The signal reflected by the material through which the transmitted signal passes, the reflected signal includes:

[0065] ■ a first reflection comb, the first reflection comb comprising at least one first stripe reflected by a substance through which at least one first stripe of the first comb of the transmitted signal passed,

[0066] ■ a second reflection comb, the second reflection comb comprising at least one second strip reflected by the substance through which at least one second strip of the second comb of the transmitted signal passed,

[0067] Each strip of the partial comb is associated with a corresponding first strip of the first reflective comb and a corresponding second strip of the second reflective comb;

[0068] -Tested by the testing system:

[0069] ■ at least one first beat signal of at least one stripe of the local comb and a corresponding first stripe of the first reflection comb; and

[0070] ■ at least one second beat signal of at least one stripe of the local comb and a corresponding second stripe of the second reflected comb; and

[0071] ■At least one third beat signal of the at least one first beat signal and the at least one second beat signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] For a better understanding thereof, the invention is described with reference to the accompanying drawings, which represent, as a non-limiting example, a device according to one embodiment of the invention. Similar elements (whose functions are similar) are indicated by the same reference numerals in the different figures.

[0073] Figure 1 is a schematic diagram of a remote spectroscopy apparatus according to one embodiment of the present invention.

[0074] Figure 2 is a schematic diagram of the frequency spectrum of different combs.

[0075] Figure 3 is a schematic diagram of a detection system that is part of a digital implementation scheme.

[0076] Figure 4 is a simplified diagram of the different steps of the method according to the invention. DETAILED DESCRIPTION

[0077] exist Figure 1 In the embodiment, the remote spectroscopy device 1 comprises an optical transmitting channel 2 and an optical receiving channel 7 .

[0078] The optical transmission channel 2 comprises a laser source 3 configured to emit a laser bar at a generation frequency FG; the laser source is connected to a frequency generator 4 via an optical fiber.

[0079] In general manner, the transmission of optical signals between the different optical components of the device 1 is carried out by means of optical fibers, which preferably ensure the maintenance of the polarization of the transmitted optical signals.

[0080] At the input of the frequency generator 4, the laser beam is divided into a first beam directed to a first electro-optical modulator 41 configured to generate a first local comb POL. This first local comb comprises at least one local stripe, each stripe having a stable frequency derived from the generation frequency FG.

[0081] According to one embodiment, the frequency of occurrence is an optical frequency, preferably close to 200 THz, and the local comb comprises a number of bars greater than 2, preferably comprised between 2 and 13; the frequencies of the bars are distributed around the frequency of occurrence FG and are shifted from one another by a predetermined value FMEOL comprised between 500 MHz and 2000 MHz, preferably equal to 1000 MHz, such as Figure 2 As shown, 13 bars with frequencies regularly distributed on either side of the frequency of occurrence are displayed along the axis of frequency f.

[0082] At the input of the frequency generator 4, the laser stripe is simultaneously divided into a second beam directed to a second electro-optic modulator 42, which, in combination with a first acousto-optic modulator 43 and a second acousto-optic modulator 44, is configured to generate a first comb P1 formed at the output of the first acousto-optic modulator 43, separated from a second comb P2 formed at the output of the second acousto-optic modulator 44. These first and second combs P1, P2 comprise at least one stripe, each having a stable frequency derived from the generation frequency FG.

[0083] Preferred according to Figure 2 In one embodiment shown, the first and second combs P1, P2 each include a number of bars equal to the number of bars of the local comb, that is, greater than 2, preferably included between 2-13; the bar frequencies of each comb P1, P2 are distributed around a center frequency FC1, FC2 originating from the generation frequency FG, and are shifted from each other by the value FMEO1 of the first comb P1 (FMEO1 is for example included between 500MHz-2000MHz, preferably equal to 1000MHz), and by the value FMEO2 of the second comb P2 (FMEO2 is for example included between 500MHz-2000MHz, preferably equal to 1000MHz).

[0084] According to one embodiment, the center frequency FC1 of the first comb and the center frequency FC2 of the second comb are shifted from the generation frequency FG by the values ​​FMAO1, FMAO2, respectively (which are determined by each of the two AOMs 43, 44, respectively).

[0085] According to one embodiment, the drift value FMAO1 between the generation frequency (i.e., the center frequency of the local comb) and the center frequency of the first comb P1, and the drift value FMAO2 between the generation frequency and the center frequency of the second comb P2 are preferably set at approximately 100 MHz, for example 110 MHz for FMAO1 and 111 MHz for FMAO2.

[0086] Each bar of the local comb is associated with a corresponding first bar of the first comb P1, typically associated with such a bar of the first comb, its frequency is closest to the frequency of the bars on the local comb under consideration; similarly, each bar of the local comb is associated with a corresponding second bar of the second comb P2, typically associated with such a bar of the second comb, its frequency is closest to the frequency of the bars on the local comb under consideration.

[0087] According to one embodiment, the first step amplitude FMEO1 of the first comb P1 and the second step amplitude FMEO2 of the second comb P2 are equal to each other and differ from the local step amplitude FMEOL of the local comb by a deviation value D. According to one embodiment of the invention, the deviation value is about 10 MHz.

[0088] According to a specific embodiment, the following parameter values ​​are therefore set to define the local comb POL and the first and second combs P1, P2:

[0089] Deviation D = 10.6 MHz

[0090] The stride of the local comb FMEOL: FMEOL = 889.4 MHz

[0091] The stride FMEO1 of the first comb P1: FMEO1 = FMEOL + D = 900 MHz

[0092] The stride FMEO2 of the second comb P2: FMEO2 = FMEOL + D = 900 MHz

[0093] The drift FMAO1 between the generation frequency (i.e. the center frequency of the local comb) and the center frequency of the first comb P1 is: FMAO1 = 110 MHz

[0094] The drift between the generation frequency (ie the center frequency of the local comb) and the center frequency of the second comb P2 is FMAO2: FMAO2 = 111 MHz.

[0095] According to this specific embodiment, given as an example, the occurrence frequency and the deviation (expressed in MHz) of the frequency of each of the 13 bars (which constitute the local comb POL), as distributed around the 0th order bar (at this occurrence frequency), are summarized in the second column POL of Table 1 below.

[0096]

[0097]

[0098] Table 1

[0099] Similarly, the deviations (expressed in MHz) between the frequency of occurrence FG and the frequency of each of the 13 bars constituting the first comb P1 are summarized in the third column P1 of Table 1 as a distribution around the 0th order bar (at this frequency of occurrence).

[0100] Similarly, the deviations (expressed in MHz) between the occurrence frequency and the frequency of each of the 13 bars constituting the second comb P2 are summarized in the fourth column P2 of Table 1 as a distribution around the 0th order bar (at this occurrence frequency).

[0101] The frequencies appearing in the same row of Table 1 are on the one hand in column POL, which are the bar frequencies of the local comb, and on the other hand in columns P1 and P2, which are the frequencies of the respective first and second bars, belonging respectively to the first and second combs P1, P2, associated with the bars of the local comb.

[0102] The first comb P1 and the second comb P2 are simultaneously separately formed at the output of the first acousto-optic modulator 43 and the second acousto-optic modulator 44, respectively, which constitute optical signals after their spatial superposition by means of the coupler's preferred polarization-maintaining coupler, which are sent via an optical fiber to an optical amplifier 45, which is configured to amplify the power of the signal it receives and thus generate an optical transmission signal 6, which will be emitted through a transmitting telescope 5 towards the matter that must be remotely detected by spectroscopy.

[0103] According to one embodiment, the optical amplifier 45 will be of the EDFA (“Erbium Doped Fiber Amplifier”) type or of the YDFA (Ytterbium) type, which are known per se in the prior art.

[0104] The transmitting telescope 5 (which is also known in the prior art) is configured to receive the amplified signal 6 and transmit it towards the material to be detected so that the transmitted signal 6 passes through the material to be detected and then reflects on the hard target, thereby generating a reflected signal 9 which is sent back to the receiving telescope 8.

[0105] The optical receiving channel 7 comprises a receiving telescope 8, an optical coupling module 14 and a detection system 10, which are connected by an optical fiber connection, as shown above.

[0106] According to one embodiment, the transmitting telescope 5 and the receiving telescope 7 may be in the form of a single component, in the case of a configuration known as a single-state configuration.

[0107] The receiving telescope 7 is configured to receive the reflected signal 9 after the transmitted signal 6 passes through the material to be detected, and the reflected signal 9 includes:

[0108] a first reflection comb, comprising at least one first strip of the first comb P1 of the transmitted signal 6 reflected after passing through the substance,

[0109] a second reflection comb comprising at least one second strip of the second comb P2 of the transmitted signal 6 reflected after passing through said substance.

[0110] In the same way that each bar of the local comb is associated with the corresponding first bar of the first comb P1, and the corresponding second bar of the second comb P2, each bar of the local comb is therefore associated with the corresponding first bar of the first reflection comb, i.e., the bar corresponding to the first comb P1 reflected after the emission signal 6 passes through the substance, and with the second bar corresponding to the second reflection comb, i.e., the bar corresponding to the second comb P2 reflected after the emission signal 6 passes through the substance.

[0111] The coupling module 14 is configured to couple the signal of the local comb POL and the reflected signal 9 , and send the coupled signal to the detection system 10 .

[0112] According to one embodiment, the detection system 10 includes a light detector configured to generate:

[0113] - at least one first beat signal 11 of at least one strip of the local comb and a corresponding first strip of the first reflected comb; and

[0114] - at least one second beat signal 12 of at least one strip of the local comb and a corresponding second strip of the second reflected comb;

[0115] According to one embodiment, the optical detector thus detects and generates a first optical beat level, in which at least one first heterodyne beat signal 11 is generated between each strip of the local comb POL and a corresponding first strip of a first reflection comb constituting the reflection signal 9, and at least one second heterodyne beat signal 12 between each strip of the local comb POL and a corresponding second strip of a second reflection comb also constituting the reflection signal 9.

[0116] For each strip of the local comb, the at least one first beat signal 11 includes a first beat signal between the strip of the local comb and the corresponding first strip of the first reflection comb P1. Similarly, for each strip of the local comb, the at least one second beat signal 12 includes a second beat signal between the strip of the local comb and the corresponding first strip of the second reflection comb P2.

[0117] It is known that, more specifically focusing on, within each beat signal, the intermediate frequency signal is equal to the frequency difference of the coupled signals.

[0118] Therefore, according to one embodiment, corresponding to the above-mentioned parameter setting values, as an example, in order to define the local comb POL and the first and second combs P1, P2 according to the above Table 1, the same Table 1 provides in column B (POL, P1) the intermediate signal frequency of each first beat signal 11 constituting different strips of the local comb and the corresponding strip of the first reflection comb (which partially constitutes the reflection signal 9); the same Table 1 provides in column B (POL, P2) the intermediate signal frequency of each second beat signal 12 constituting different strips of the local comb and the corresponding strip of the second reflection comb (which also constitutes the reflection signal 9).

[0119] like Figure 3 As shown, the detection system 10 is also configured to generate a beat in the second electrical beat level, and for each strip of the local comb, the at least one first beat signal 11 corresponding to the first beat signal of this strip of the local comb, and the at least one second beat signal 12 corresponding to the second beat signal of the same strip of the local comb, so as to thereby generate at least one third beat signal 13 for each strip of the local comb, and the at least one third beat signal 13 includes many beat signals CSBF1, CSBF2, CSBF3, ...CSBFi, ..., because there are different strips in the local comb.

[0120] For this purpose, for each strip of the partial comb:

[0121] - 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 local comb strips and the corresponding strips of the first reflection comb P1, which produces a first filtered beat signal;

[0122] - Similarly, each of the at least one second beat signal 12 is filtered around a frequency equal to the average of the respective frequencies of the local comb strips and the corresponding strips of the second reflection comb P2, which produces a second filtered beat signal.

[0123] Thus, for each strip of the partial comb, a first filtered beat signal and a second filtered beat signal are generated.

[0124] Therefore, in the second beat level, these first and second filtered beat signals are mixed to beat and generate the third beat signal CSBF1.

[0125] This method is repeated the same number of times as the number of bars in the partial combs, P1 and P2, thereby generating a beat signal sequence CSBF1, CSBF2, CSBF3, ...CSBFi, ..., which constitutes at least one third beat signal 13.

[0126] According to one embodiment, in order to separate the signal carrying the sought spectroscopic information, for each strip of the local comb, it is necessary to filter the third beat signal 13 around a frequency equal to the absolute value of the difference between the first frequency shift FMAO1 and the second frequency shift FMAO2.

[0127] Thus, according to one embodiment, corresponding to the parameter settings used as an example above, in order to define the local comb POL and the first and second combs P1, P2 according to the above Table 1, what appears on the second to last row of Table 1 is filtering the at least one third beat signal 13 around an intermediate frequency of 1 MHz, separating the signal carrying the sought spectroscopic information, which has a maximum signal-to-noise ratio. The improvement in the signal-to-noise ratio is provided by the processing described above, and its digital implementation is now described.

[0128] according to Figure 3 In one embodiment shown, the detection system 10 includes a digital processing unit UTN configured to process in parallel the at least one first beat signal 11 and the at least one second beat signal 12 for each strip of the local comb to generate at least one third beat signal 13.

[0129] According to this embodiment, the digital processing unit UTN comprises:

[0130] - an analog-to-digital converter ADC, configured to digitize a time signal including the at least one first beat signal 11 and the at least one second beat signal 12 to generate digital signals 11N, 12N;

[0131] - a first calculation unit FFT configured to calculate a first digital spectrum SN11, SN12 of the digital signal 11N, 12N;

[0132] - a first filtering unit UF1 configured to separate the different spectral components of the previously calculated digital spectra SN11 , SN12 ;

[0133] The second calculation unit FFT -1 , which is configured to convert each spectral component separated by the first filtering unit UF1 from the spectrum domain to the time domain in parallel to generate at least one time component, which includes 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 one second time component corresponding to a strip of the local comb;

[0134] - a mixer MEL configured to generate the beats of the corresponding first 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 in parallel for each strip of the local comb, to generate a third beat signal of the at least one third beat signal corresponding to each strip of the local comb;

[0135] a third calculation unit FFT2 configured to calculate, for each third beat signal, a digital spectrum of each third beat signal of the at least one third beat signal in parallel;

[0136] - a second filtering unit UF2 configured to separate low frequency spectrum components CSBF1, CSBF2, CSBF3, ..., CSBFi, ... within each third beat signal of the at least one third beat signal.

[0137] According to these arrangements, the detection of the third beat signal by the detection system is performed, for example digitally. This second beat level eliminates at all times the phase drift between the comb P1 and the local oscillator on the one hand, and the phase drift between the comb P2 and the local oscillator on the other hand, which are identical to the first level, canceling each other. This eliminates the de-correlation which would otherwise impair the final heterodyne signal.

[0138] The present invention also relates to a method 100 for remote spectroscopy of a substance using a laser radar, comprising the following steps:

[0139] - emitting 101 laser strips at a generation frequency FG by means of a laser source 3;

[0140] -Generated 102 by the laser strip via the frequency generator 4:

[0141] ■ a first comb P1 comprising at least one first strip having a stable frequency derived from the frequency of generation FG; and

[0142] ■ a second comb P2 comprising at least one second strip having a stable frequency derived from the occurrence frequency; and

[0143] ■ a local comb POL comprising at least one local strip having a stable frequency derived from the occurrence frequency;

[0144] o transmitting 103 a transmission signal 6 through the transmission telescope 5 towards the material through which the transmission signal 6 passes, the transmission signal 6 comprising the first and second combs;

[0145] - Receiving 104 via receiving telescope 8:

[0146] ○ The signal 9 reflected by the material through which the transmitted signal 6 passes, the reflected signal 9 includes:

[0147] ■ a first reflection comb, the first reflection comb comprising at least one first stripe reflected by a substance through which at least one first stripe of the first comb of the transmitted signal passed,

[0148] ■ a second reflection comb, the second reflection comb comprising at least one second stripe reflected by a substance through which at least one second stripe of the second comb of the transmitted signal passed;

[0149] Each strip of the partial comb is associated with a corresponding first strip of the first reflective comb and a corresponding second strip of the second reflective comb;

[0150] ○ Through the detection system 10 detection 105:

[0151] ■ at least one first beat signal 11 of at least one stripe of the local comb and a corresponding first stripe of the first reflection comb; and

[0152] ■ at least one second beat signal 12 of at least one stripe of the local comb and a corresponding second stripe of the second reflected comb; and

[0153] ■At least one third beat signal 13 of the at least one first beat signal and the at least one second beat signal.

Claims

1. A laser radar type device (1) for remote spectroscopy of matter, comprising: An optical transmission channel (2), comprising: A laser source (3) adapted to emit a laser strip at a generation frequency (FG); A light wave frequency generator (4) adapted to generate from the emitted laser strip: a first comb (P1) comprising at least one first strip having a stable frequency derived from said frequency of generation (FG); and a second comb (P2) comprising at least one second strip having a stable frequency derived from said frequency of occurrence (FG); and a local comb (POL) comprising at least one local strip having a stable frequency derived from said frequency of generation (FG); a transmitting telescope (5) adapted to transmit a transmission signal (6) towards a substance through which the transmission signal (6) passes, the transmission signal (6) comprising the first and second combs (P1, P2); The receiving channel (7) includes: A receiving telescope (8) is adapted to receive a signal (9) reflected by a substance through which the transmitting signal (6) passes, wherein the reflected signal (9) comprises: a first reflection comb, the first reflection comb comprising at least one first stripe reflected by a substance through which at least one first stripe of the first comb of the transmitted signal passed, a second reflection comb, the second reflection comb comprising at least one second stripe reflected by the substance through which at least one second stripe of the second comb of the transmitted signal passed, each strip of the partial comb is associated with a respective first strip of the first reflective comb and a respective second strip of the second reflective comb; A detection system (10) is configured to detect: at least one first beat signal (11) of at least one stripe of the local comb and a corresponding first stripe of the first reflection comb; and at least one second beat signal (12) of at least one stripe of the local comb and a corresponding second stripe of the second reflected comb; and at least one third beat signal (13) generated by the at least one first beat signal (11) and the at least one second beat signal (12), The third beat signal is filtered at a frequency equal to the absolute value of the difference between the first frequency drift (FMAO1) and the second frequency drift (FMAO2). 2 . The device according to claim 1 , wherein the first comb, the second comb and the partial comb each include a number of bars greater than or equal to 1. 3 . 3 . The device according to claim 2 , wherein the first comb, the second comb and the partial comb each include a number of bars greater than or equal to 2. 4 . 4 . The device according to claim 3 , wherein the frequency of the bars of the local comb is regularly spaced at the frequency of occurrence with a local step (FMEOL), the value of the local step (FMEOL) being comprised between 500 MHz and 2000 MHz.

5. A device according to any one of claims 3 or 4, wherein the frequency of the bars of the first comb are regularly spaced with a first step (FMEO1) at a first center frequency (FC1) equal to the sum of the occurrence frequency and the first frequency drift (FMAO1).

6. Device according to claim 3, wherein the frequency of the bars of the second comb is regularly spaced with a second step size (FMEO2) at a second center frequency (FC2) equal to the sum of the occurrence frequency and the second frequency shift (FMAO2).

7. Device according to claim 6, wherein the value of the first step (FMEO1) and the value of the second step (FMEO2) are equal to the sum of the value of the local step (FMEOL) and the deviation (D).

8. According to the device of claim 1, 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) to generate at least one third beat signal (13).

9. The apparatus according to claim 8, wherein the digital processing unit (UTN) comprises: an analog-to-digital converter (ADC) configured to digitize a time signal including the at least one first beat signal (11) and the at least one second beat signal (12) to generate a digital signal (11N, 12N); A first calculation unit (FFT) configured to calculate a first digital spectrum (SN11, SN12) of the digital signal (11N, 12N); a first filtering unit (UF1) configured to separate different spectral components of the previously calculated digital spectrum (SN11, SN12); a second calculation unit (FFT-1) configured to convert each spectral component separated by the first filtering unit (UF1) from the spectral domain to the time domain in parallel to generate at least one time component, the 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 one second time component corresponding to a strip of the local comb; a mixer (MEL) configured to generate the beats of the corresponding first 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) in parallel for each strip of the local comb to generate a third beat signal of the at least one third beat signal corresponding to each strip of the local comb; a third calculation 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) is configured to separate low frequency spectrum components (CSBF1, CSBF2, CSBF3, . . . , CSBFi, . . . ) in each of the at least one third beat signal.

10. A laser radar detection method (100) for remote spectroscopy of matter, comprising the following steps: emitting (101) a laser bar at a generation frequency (FG) by a laser source (3); The frequency generator (4) generates (102) from the laser strip: i. a first comb (P1) comprising at least one first strip having a stable frequency derived from said frequency of occurrence (FG); and ii. a second comb (P2) comprising at least one second strip having a stable frequency derived from said occurrence frequency; and iii. a local comb (POL) comprising at least one local strip having a stable frequency derived from said occurrence frequency; Transmitting (103) a transmission signal (6) through a transmission telescope (5) toward a substance through which the transmission signal (6) passes, wherein the transmission signal (6) includes the first comb and the second comb (P1, P2); Received (104) via receiving telescope (8): A signal (9) reflected by the material through which the transmitted signal (6) passes, wherein the reflected signal (9) comprises: i. a first reflection comb, the first reflection comb comprising at least one first line reflected by a substance through which at least one first line of the first comb of the transmitted signal passes, ii. a second reflection comb, the second reflection comb comprising at least one second strip reflected by a substance through which at least one second strip of the second comb of the transmitted signal passes, each strip of the partial comb is associated with a respective first strip of the first reflective comb and a respective second strip of the second reflective comb; Detection (105) by detection system (10): iii. at least one first beat signal (11) of at least one strip of the local comb and a corresponding first strip of the first reflection comb; and iv. at least one second beat signal (12) of at least one strip of the local comb and a corresponding second strip of the second reflected comb; and v. at least one third beat signal (13) generated by the at least one first beat signal and the at least one second beat signal, The third beat signal is filtered at a frequency equal to the absolute value of the difference between the first frequency drift (FMAO1) and the second frequency drift (FMAO2).

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