FR3039331A1 - REMOTE SPECTROSCOPY METHOD, COMPUTER PROGRAM PRODUCT, AND ASSOCIATED SPECTROSCOPY DEVICE - Google Patents

Abstract

This method (100) of LIDAR type remote spectroscopy of a material comprises the following steps: - generating (115) from a laser signal lines of a first and a second type; - until an emitted comb counter reaches a factor, selecting (124) lines of the first and second types, forming (126) with lines a first comb and a second comb to be emitted from the selected lines detecting (140) a coincident line, identifying (142) a first initial line following and a second second line following, emitting (134) each formed comb and incrementing the transmitted comb counter by one; receiving (160) response signals to construct an interferogram of the targeted material.

Description

Remote spectroscopy method, computer program product and associated spectroscopy device

The present invention relates to a method of LIDAR type remote spectroscopy of a material having for example an atmospheric target.

The present invention also relates to a spectroscopy device implementing this method.

Such a spectroscopy device or more generally a remote sensing or optical measurement tool, is known in the prior art under the term "LIDAR" which comes from the English expression "light detection and ranging" meaning "detection and measurement by light ".

LIDAR is used in many fields, including archeology, geography, geology, altimetry and other areas related to earth sciences.

A particular area of application of LIDAR is remote spectroscopy to study a targeted material located at a distance from LIDAR. Such a material is for example a gas located in a specific location of the Earth's atmosphere and targeted from a LIDAR embedded in a satellite, an aircraft, or station on an industrial site.

More particularly, this radiometric application of the LIDAR consists in emitting a light wave of a determined frequency with a reference wave towards the targeted gas. Then, by knowing the absorption coefficient of the gas of the wave of this frequency, it is possible to determine for example the concentration of the gas by comparing a wave crossing the gas, reflected and received by the LIDAR, with a reference wave. which is not absorbed because, for example, of its different frequency.

For a complex gas having a mixture of several elemental gases or a concentration gradient, it is known an application of LIDAR for sending several light waves of different frequencies to target the different elemental gases or a concentration profile of this complex gas. This technique makes it possible in particular to determine the volume proportions of the elementary gases in such a complex gas.

Thus, for this application, the LIDAR must make it possible to emit several of the light signals of different frequencies.

One solution, mentioned in particular in document WO 2013/165945, consists in emitting a comb of lines generated with a modulator from a main laser source. Another laser source is used internally to the transmission system to generate a reference signal which is to be mixed with a measurement signal corresponding to the comb reflected by the material.

The comb lines are spaced uniformly by a predetermined time spacing value to ensure the desired sampling fineness.

Each line of the comb is sent consecutively to the targeted material in the form of an emission signal which is subsequently reflected by this material and received by the LIDAR. By mixing with the reference signal, the received signal makes it possible to obtain an interferogram of the targeted material. Its Fourier transform provides the spectrum of the targeted material.

However, this solution is not completely satisfactory.

In particular, when several lines are emitted successively, their response signals can be confused with other signals reflected by materials other than the targeted material. These signals reflected by materials other than the targeted material are also called "echoes" and thus present a considerable pollution of the desired signals. Indeed, a response signal corresponding to a signal transmitted according to the signal that generated the echoes can interfere with these echoes which will mix the information of the targeted materials to others.

The problem is illustrated in detail in Figure 1 schematically representing a spectroscopy of upper layers of the Earth's atmosphere from a satellite.

According to this figure, two lines ^ and R2 spaced temporally by a value 1 / en are sent to a target material M in a top layer of the atmosphere at altitude AXa equal for example to 15 km. The response signals corresponding to the signals reflected by the targeted material M are designated respectively by the references Si and S2. The echoes corresponding to the signals S-1 and S2 are designated respectively by the references Et and E2.

As illustrated in FIG. 1, when for example the value fr is substantially equal to 100 MHz, the echo Er corresponding to the signal reflected by the earth's surface is caught by the signal S2 so that these signals are superimposed on the reception. This superposition does not make it possible to study the pure signal and "pollutes" thus the corresponding sample.

It is thus conceived that the problem becomes more important when the need for the temporal sampling finesse increases to allow a consistent spectral coverage. Indeed, in this case, it is necessary to increase the transmission frequency of the lines which leads to overlapping signals corresponding to several different lines when received.

An object of the present invention is to provide a method of spectroscopy remedying this problem. To this end, the subject of the invention is a method of LIDAR-type remote spectroscopy of a material implemented by a spectroscopy device capable of generating a laser signal, of processing the laser signal to form an emission signal. transmitting the transmission signal to a spectroscopically targeted material, receiving a response signal corresponding to the transmission signal reflected by the targeted material, and processing the response signal.

The method comprises the following steps: - generating from a laser signal lines of a first type regularly spaced a first frequency spacing value and lines of a second type regularly spaced a second value d frequency spacing of the first frequency spacing value; until an emitted comb counter reaches a densification factor, performing the densification loop comprising the following sub-steps: + selecting lines of the first type and the second type according to a predetermined selection criterion; + form with selected lines of the first type, a first comb to be issued from a first initial current line; + form with the lines of the second type selected, a second comb to be issued from a second current initial line; + detecting a coincident line, the coincident line corresponding to a line of the first type emitted simultaneously with a line of the second type; + identifying a first following first line and a second subsequent initial line, as a function of the coincident line, and associating the first current initial line with the next first initial line, and the second initial current line with the next first initial line; + emit each comb formed and increment by one unit the comb counter issued; receiving response signals corresponding to all the lines emitted and processing these response signals to construct an interferogram of the targeted material.

According to other advantageous aspects of the invention, the remote spectroscopy method comprises one or more of the following characteristics, taken separately or in any technically possible combination: the lines of the first comb are spaced apart by a third a frequency spacing value whose inverse value is substantially equal to a time interval of reception of the echoes of the reflected signals corresponding to the same transmission signal; each initial first line is temporally offset from the corresponding coincident line by a first time offset value, the first time offset value being directly proportional to the transmitted comb counter and inversely proportional to a sampling frequency spacing value; and the sampling frequency spacing value is directly proportional to the densification factor and the third frequency spacing value; each second initial line is temporally offset from the first corresponding initial line of a second time offset value, the second time offset value being directly proportional to the transmitted comb counter and inversely proportional to the difference of the second spacing value. frequency and the first frequency spacing value; the construction of the interferogram of the targeted material comprises a processing of the response signals corresponding to the lines emitted, according to an order defined respectively by the time offset of each line with respect to the corresponding coincident line; the first frequency spacing value is substantially equal to 100 MHz; the selection criterion comprises a uniform selection from the first or second initial line of each nth line respectively of the first or second type in the set of generated lines, the parameter n being a natural integer; the selection criterion comprises a non-uniform selection of the lines from the first or the second initial line consisting of a uniform selection of each n-th line of the first type or the second type emitted respectively from the first line; initial and second line, during an observation interval, the parameter n being a natural number; and a uniform selection of each line of the first type or of the second type emitted outside the observation interval; the set of lines of the first type or of the second type selected according to the selection criterion during an observation interval respectively forms the first comb to be emitted or the second comb to be emitted; the observation interval is directly proportional to the first frequency spacing value and inversely proportional to the difference between the second frequency spacing value and the first frequency spacing value; the parameter n is directly proportional to the first frequency spacing value and inversely proportional to the third frequency spacing value; during the detection step, the coincidence of the lines of the first type and the second type is caused by a control module; and - during the line generation step, the lines of different types are generated by separate pulse generating units and are transmitted by separate optical paths. The invention also relates to a computer program product comprising software instructions which, when implemented by a computer equipment, implements the method as described. The subject of the invention is also a LIDAR-type remote spectroscopy device of a material, the device comprising a laser signal generation module, a laser signal pre-processing module capable of forming an emission signal, a module for transmitting the emission signal to a material targeted by the spectroscopy, a module for receiving a response signal corresponding to the emission signal reflected by the targeted material, and a module for post-processing the response signal. .

The device is able to: - generate from a laser signal lines of a first type regularly spaced a first frequency spacing value and lines of a second type regularly spaced a second value of frequency spacing different from the first frequency spacing value; until an emitted comb counter reaches a densification factor, perform the densification loop making it possible to: select lines of the first type and the second type according to a predetermined selection criterion; + form with selected lines of the first type, a first comb to be issued from a first initial current line; + form with the lines of the second type selected, a second comb to be issued from a second current initial line; + detecting a coincident line, the coincident line corresponding to a line of the first type emitted simultaneously with a line of the second type; + identifying a first following first line and a second subsequent initial line, as a function of the coincident line, and associating the first current initial line with the next first initial line, and the second initial current line with the next first initial line; + emit each comb formed and increment by one unit the comb counter issued; receiving response signals corresponding to all the lines emitted and processing these response signals to construct an interferogram of the targeted material.

These features and advantages of the invention will become apparent on reading the following description, given solely by way of nonlimiting example, and with reference to the appended drawings, in which: FIG. 1 is a schematic view illustrating a problem of spectroscopy of the prior art; FIG. 2 is a schematic view of a spectroscopy device according to the invention; FIG. 3 is a detailed view of the device of FIG. 2; FIG. 4 is a flowchart of a spectroscopy method according to the invention; and FIG. 5 is a schematic view illustrating certain steps of the flowchart of FIG. 4.

In the remainder of the description, the expression "substantially equal to" is understood as a relationship of equality at plus or minus 10%.

The spectroscopy device 10 of FIG. 2 is, for example, embedded in a satellite 12 situated in a terrestrial orbit and making observations of the Earth, or of another planet, and in particular of a layer of atmosphere of thickness AXa for example equal to 15 km.

The spectroscopy device 10 makes it possible to study a targeted material in this atmosphere layer. The targeted material is for example a gas composed of several elemental gases, for example CO 2, H 2 O or CH 4.

Each elemental gas is able to absorb a light wave of a determined frequency with a known absorption coefficient a priori.

Thus, the spectroscopy device 10 makes it possible, for example, to determine the densities of the elementary gases contained in the targeted gas by emitting a light signal towards the targeted gas and by analyzing a signal reflected by this gas, or absorbed by it and reflected by a surface in the background.

According to an alternative embodiment, the spectroscopy device 10 is embedded in another space or ground vehicle, or an aircraft.

According to yet another variant embodiment, the spectroscopy device 10 is fixedly arranged for example on the earth's surface.

In at least some of the aforementioned embodiments, the spectroscopy device 10 makes it possible to further study a targeted material disposed in any medium other than the atmosphere, for example the underwater or underground environment. The architecture of the spectroscopy device 10 is illustrated in more detail in FIG.

According to the exemplary embodiment illustrated in this figure, the spectroscopy device 10 comprises a module 20 for generating a laser signal, a module 22 for pretreatment of the laser signal capable of forming a transmission signal, a module 24 for transmitting the emission signal to a targeted material by the spectroscopy, a module 25 for receiving a response signal corresponding to the emission signal reflected by the targeted material, a module 26 for post-processing the response signal and a module 28 for driving the aforementioned modules.

The module 20 for generating a laser signal is a laser source capable of emitting a laser line at a generating frequency FG which is substantially equal to, for example, 200 THz.

According to the embodiment of FIG. 3, the generation module 20 comprises a light separator comprising two outputs 31, 32. This separator is able to divide the laser signal generated into two identical signals for each of the outputs 31, 32.

According to the same exemplary embodiment, the pretreatment module 22 of the laser signal is able to generate from the two signals from the generation module 20, two transmission signals intended for the transmission module 24 and at least three reference signals. for the post-processing module 26.

To do this, the pretreatment module 22 comprises a first optical channel Vi, a second optical channel V2, a detection unit 33 coincident lines, a reference optical unit 34 and a light mixer 36. These components will be described in more detail thereafter. Other variants of the generation module 20 and the preprocessing module 22 are also possible.

The transmission module 24 is a transmission telescope known per se and adapted to receive the transmission signals from the preprocessing module 22 and to transmit them to the targeted material.

The receiving module 25 is a reception telescope also known per se and adapted to receive a response signal corresponding to each transmission signal emitted by the transmission module 24 and reflected by the targeted material or other material.

According to an alternative embodiment, the telescopes of the transmission and reception modules 24 are in the form of a single component.

The post-processing module 26 is able to receive the reference signals generated by the preprocessing module 22 and the response signals received by the reception module 25.

The post-processing module 26 comprises a conversion unit 40 able to convert the received signals into digital signals and a computing unit 42 able to analyze the digital signals.

The control module 28 makes it possible to control the operation of all the modules of the spectroscopy device 10. In particular, the control module 28 is able to control the emission of a laser signal by the generation module 20 and the preprocessing of this signal by the preprocessing module 22.

In the embodiment of Figure 1, the control module 28 is connected to a central computer (not shown) of the satellite 12 and adapted to be controlled from this computer. The computing unit 42 is for example a computer capable of constructing an interferogram of the targeted material as will be explained later.

The first optical channel comprises a pulse generation unit 50, a line selection unit 52, a separator 53 and an amplification unit 54. The pulse generation unit 50 is capable of receiving the line laser emitted by the generation module 20 and to generate periodically, from this line, a plurality of side lines around the generating frequency FG. These lines will be designated later by the term "rays of a first type".

In an alternative embodiment, the generation module 20 and the pulse generation unit 50 are merged. In this case, the generation of the laser line at a generator frequency FG and the plurality of side lines are caused by a single optical method.

The lines of the first type generated by the unit 50 of the first optical channel V, are uniformly spaced from a first frequency spacing value fr.

Thus, in the time scale, these lines are spaced apart by a first time spacing value substantially equal to 1/1 / r.

The first frequency spacing value fr is substantially equal to 100 MHz. This value advantageously corresponds to a conventional frequency spacing value that can be produced by conventional generation units.

The first frequency spacing value fr is less than a second frequency spacing value fr + Afr whose meaning will be explained later. The selection unit 52 is able to select from the set of lines of the first type generated, lines satisfying a selection criterion explained later. The selection unit 52 is also able to identify a first initial line and to form, from this first initial line and selected lines, a first line comb Pu

By virtue of the selection, the lines of the first comb are spaced uniformly from a third frequency spacing value f ™ ax which is determined, for example, from the following relation:

where c is the speed of light.

The third frequency spacing value f ™ ax is less than or equal to the first frequency spacing value fr, i.e., fjnax <fr.

In the embodiment of FIG. 1, this value is substantially equal to 2.5 kHz.

Thus, in the time scale, the lines of the first comb P1 are temporally offset by a value substantially equal to 1 / fjnax.

Finally, the selection unit 52 is able to emit a signal comprising the lines of the first comb P1 towards the light separator 53.

The light separator 53 is able to divide the received signal into two signals with a predetermined power proportion, for example 50%: 50%. One of these signals is transmitted to the optical reference unit 34 and the other to the amplification unit 54. The amplification unit 54 is an EDFA type fiber amplifier (of the English "Erbium Doped Fiber Amplifier >>) or YDFA (of the "Ytterbium Doped Fiber Amplifier >>) type, potentially double-jacketed, known per se. The amplification unit 54 amplifies the signal from the separator 53 and sends a signal amplified to the mixer 36.

The second optical channel V2 is analogous to the first optical channel \ Λ.

Thus, with reference to FIG. 3, the second optical channel V2 comprises a pulse generation unit 60, a line selection unit 62, a separator 63 and an amplification unit 64. The generation unit of FIG. pulses 60 is adapted to receive the laser line emitted by the generation module 20 and to generate periodically, from this line, a plurality of side lines around the generating frequency FG. These lines will be designated later by the term "rays of a second type".

As in the previous case, according to an alternative embodiment, the pulse generation unit 60 is integrated in the generation module 20.

The lines of the second type generated by the unit 60 of the second optical channel V2 are uniformly spaced from a second frequency spacing value fr + Afr. Thus, in the time scale, these lines are spaced apart by a second time spacing value substantially equal to 1 / (fr + Afr).

The difference Afr between the second frequency spacing value fr + Afr and the first frequency spacing value fr, will be denoted thereafter by the term "frequency surplus".

The frequency surplus Afr is, for example, substantially equal to 3 Hz. The selection unit 62 is capable of selecting from among all the lines of the second type generated, lines satisfying the selection criterion. The selection unit 62 is also able to identify a second initial line and to form, from this second initial line and selected lines, a second line comb P2.

By virtue of the selection, the lines of the second comb P2 are spaced uniformly by a fourth frequency spacing value f ™ ax + Af ™ ax equal to the sum of the third frequency spacing value fjnax and a value determined by the following expression:

Thus, in the time scale, the lines of the second comb P2 are temporally offset by a value substantially equal to 1 / (j ™ ax + Af ™ ax).

Finally, the selection unit 62 is able to emit a signal comprising the lines of the first comb P2 to the light separator 63.

The light separator 63 and the amplification unit 64 are respectively identical to the light separator 53 and to the amplification unit 54 described above. The coincident line detection unit 33 is connected to the selection units 52 and 62, and is capable of detecting a line of the first type coinciding with a line of the second type.

More particularly, a line of the first type coincides with a line of the second type when these lines are emitted simultaneously by the corresponding pulse generation units 50 and 60. The reference optical unit 34, known per se, is able to generate two reference signals from the signals received from each optical channel V !, V2. These reference signals are intended for the post-processing module 26.

The mixer 36 is able to receive the amplified signals coming from each of the optical channels V 1, V 2 and to divide each signal into two signals with a predetermined power proportion, for example 99%: 1%. The higher power signal is the transmission signal that is transmitted to the transmission module 24. The lower power signal is the reference signal that is transmitted to the post-processing unit 26.

A method 100 of spectroscopy will now be explained with reference to Figure 4 illustrating a flow chart of its steps.

The spectroscopy method 100 is implemented by the spectroscopy device 10. The execution of the steps of this method 100 is controlled by the control module 28.

With reference to this FIG. 4, the method 100 comprises a line generation phase I, a phase II of preprocessing and emission of the generated lines and a phase III of reception and post-processing of the response signals.

These phases will be described consecutively one after the other. However, it should be understood that the spectroscopy device 10 is able to implement these phases in parallel when each phase has been executed at least once. Thus, for example, when phase I has been executed at least once, the spectroscopy device 10 is able to start the execution of phase II while continuing the execution of phase I.

During the initial step 110 of phase I, the control module 28 controls the generation of a laser signal by the generation module 20.

As indicated above, the laser signal comprises a generator frequency line FG which is then transmitted through the two outputs 31, 32 of the generation module 20 to each optical channel V 1, V 2 of the preprocessing module 22.

In the following step 115, the control unit 28 controls the periodic generation of the lines of the first type and the second type respectively by the pulse generation unit 50 and by the pulse generation unit. 60.

The first lines generated by the units 50 and 60 are transmitted simultaneously and are then coincident.

Then, the lines of the first type are emitted with the first frequency spacing value fr. The lines of the second type are emitted with the second frequency spacing value fr + Afr.

When the pulse generating units 50 and 60 are integrated with the generation module 20, steps 110 and 115 are executed simultaneously.

During the initial step 120 of phase II, the control module 28 identifies the targeted material. Thus, for example, during this step, the target material is identified according to the current position of the satellite 12 and the distance between the satellite 12 and the targeted material. In the embodiment of FIG. 2, the targeted material is determined as a function of the position of the satellite 12 and the thickness AXa of the atmosphere layer studied.

During the same step, the control module 28 initializes a counter k of combs emitted at zero.

Then, a densification loop 121 is started. The substeps of this loop 121 are described below.

In the sub-step 122, the control module 28 compares the transmitted comb counter k with a densification factor D determined by the following expression:

where Δν is the spectral range sought for the targeted material, determining the sampling fineness of the interferogram to be achieved. The spectral sampling range Δν is for example substantially equal to 1.3 THz.

With the numerical values cited by way of example above, the densification factor D is substantially equal to 32.

Thus, the densification factor D is directly proportional to the sampling spectral range Δν and the frequency surplus Afr, and inversely proportional to the first frequency spacing value fr and the third frequency spacing value / rma *.

When the transmitted comb counter k is less than or equal to the densification factor D, the control module proceeds to the substep 124, otherwise, the control module 28 proceeds to the phase III.

In the sub-step 124, the selection units 52 and 62 respectively select the lines of the first type and the lines of the second type from respectively the first and second initial lines, according to the selection criterion described herein. -Dessous.

Initially, the first initial line and the second initial line correspond to the first lines emitted by the pulse generation units 50 and 60.

According to a first embodiment, the selection criterion comprises a uniform selection of each nth line of the first type or of the second type respectively transmitted by the pulse generation units 50 and 60 respectively from the first initial line and the second initial line.

The parameter n is a natural number defined by the following expression:

where the symbol [... 1 designates the whole part by excess.

According to a second embodiment, the selection criterion comprises: a uniform selection of each n-th line of the first type or the second type respectively transmitted by the pulse generation units 50 and 60 respectively from the first line; initial and the second initial line, during an observation interval Tobs determined by the following expression:

where δν is a desired spectral resolution and is substantially equal to 3.3 GHz;

Tobs is a time necessarily greater than that required for kk to reach the densification factor D. - a uniform selection of each line of the first type or of the second type emitted respectively by the pulse generation units 50 and 60 outside the observation interval T0bs- These lines are not emitted. In particular, these lines are directed to the coincident line detection unit 33 to pass the described step 140, but not to the light separators 53 and 63.

In the following sub-step 126, the selection units 52 and 62 respectively form a first comb Pt to be emitted and a second comb P2 to be emitted.

Each comb? I and? 2 comprises all the lines selected respectively by the selection units 52 and 62 during the observation interval Tobs-

In the following sub-step 130, the separators 53 and 63 divide the signals comprising the lines of the combs. and 2 in two signals for the optical reference unit 34 and two signals for the amplification units 54 and 64, respectively.

In the following sub-step 132, the reference optical unit 34 generates two reference signals from the two signals from the corresponding separators 53 and 63.

During the same sub-step, the amplification units 54 and 64 amplify the corresponding signals and the mixer 36 divides the amplified signals into reference signals intended for the post-processing module 26 and for transmission signals intended for the module. issue 24.

In the next sub-step 134, the transmission module 24 transmits the transmission signals to the targeted material.

The sub-steps 140 and 142 described below are executed simultaneously with the steps 130 to 134.

In sub-step 140, the coincident line detection unit 33 detects each coincidence of the first and second type lines generated respectively by the pulse generating units 50 and 60 and respectively selected by the selection units 52. and 62.

In particular, the detection unit 33 detects a coincidence of the lines of the first and the second type when these lines were emitted simultaneously by the pulse generation units 50 and 60 and were selected by the selection units 52 and 62. .

Initially, the first lines emitted by the pulse generation units 50 and 60 are coincident.

A coincidence then occurs naturally periodically, at the end of each time interval substantially equal to 1 / ffax ax according to the first embodiment and 1 / Afr according to the second embodiment.

Sub-step 140 thus lasts the time necessary to detect a natural coincidence of the lines. During this time, all the lines generated by the pulse generation units 50 and 60 other than the nths are not selected and are therefore not used.

In the sub-step 142 following step 140, the selection units 52 and 62 respectively determine a first initial line and a second initial line for the next pass of the densification loop 121. These lines are determined from the set lines generated during step 115.

In the sub-step 144 following step 142, the control module 28 increments the transmitted comb counter k by one unit.

The next first initial line corresponds to a line of the first type generated during step 115 and offset temporally with respect to the coincident line of a first time offset value substantially equal to kAr, where Δτ is a sampling time value. determined by the following expression:

The value inverse to the sampling time value Δτ is designated by the term "fech sampling frequency spacing value".

The next second initial line corresponds to a line of the second type generated during step 115 and offset temporally with respect to the coincident line of a second time offset value substantially equal to / τ (Δτ-1 / Afr).

A passage of the densification loop is completed when substeps 142 and 134 are executed.

In the initial step 160 of phase III, the receiving module 25 receives each response signal reflected by the targeted material or other material.

In the same step, the receiving module 25 transmits each received signal to the post-processing module 26 and more particularly to the conversion unit 40 which converts this signal into a digital response signal.

On the other hand, the conversion unit 40 converts the three reference signals into digital response signals.

Then, in the next step 165, the computing unit 42 analyzes each digital response signal with the reference digital signals corresponding thereto to construct an interferogram of the targeted material.

More particularly, the construction of the interferogram of the targeted material comprises a processing of the response signals in the order defined by the values of time offsets of the transmitted lines with respect to the corresponding coincident line. Thus, the processing order is independent of the temporal times of emission of the corresponding lines.

This treatment principle is illustrated in greater detail in FIG.

For the sake of simplicity, only lines of the first type and response signals corresponding to these lines are illustrated in this figure. The lines of the second type and the response signals corresponding to these lines can be illustrated analogously.

FIG. 5 illustrates indeed in the zone E the phase II of pretreatment and emission of the lines generated to form two first combs Px of the first channel V ,. One of the first Pi combs is a previous comb, and the other is a next comb.

These combs Pi are constructed from the time lines emitted by the pulse generation unit 50 at the frequency fr and selected by the selection unit 52 as previously described.

In the figure, the continuous and interrupted lines correspond to the set of lines generated by the selection unit 52. On the time scale t, these lines are spaced temporally from the first time spacing value l / fr.

The continuous lines correspond to the lines selected by the selection unit 52. Thus, only the lines corresponding to the continuous lines form the combs Pi. Within a given comb, the continuous lines are spaced temporally by a spacing value. temporal equals 1 // rma *. As an example, the previous comb Pt is formed of three lines # io- # 2.o, # 3.o and the following comb P1 of three lines # 1.1, # 2.1, # 3.1 · The lines # i.0 , # 2.o, # 3.o are spaced here from 6 / en, compatible value of AXa thickness. The lines ff10 and ff1; L are the initial lines respectively for the combs P1. The selection criterion therefore comprises a selection of each sixth line.

According to the illustrated example, the densification factor D is equal to six and the final sampling frequency spacing value fech is equal to the first frequency spacing value fr. Thus, the lines # li0 and R [0 coincide with corresponding lines of the second type. The lines #ltl and # | .os are spaced temporally from each other by a value l / fech, equal in this example to l // r.

The lines are emitted one after the other according to the time scale t, towards the target material M of thickness AXa.

Zones R and T of FIG. 5 illustrate the phase III of reception and processing of the response signals.

In the zone R, the signals S10, S2 0, S30, S1: L and S21 are response signals corresponding to the set of signals reflected, in particular by the targeted material and the terrestrial surface, respectively corresponding to the lines emitted # 10, # 2.0, # 3.0, # 1.1 and # 2.1-

Given the time spacing of the transmission signals, their respective echoes do not overlap. Thus, these response signals S10, S2.0, S3 0, S1 ± and S21 are not "polluted" mutually.

Finally, according to the zone T illustrating the interferogram of the targeted material, the signals S10, # 2.o- # 3.0, # 1.1 and S21 are processed according to the order defined by the values of the offsets between the lines emitted by each comb. . This order is therefore the following: # 1.0, # 1.1, # 1.6, # 2.0- # 2.1 # 2.6 etc.

It will be appreciated that the present invention has a number of advantages.

Since the lines of each comb Pk are spaced by at least the third frequency spacing value f ™ ax, the echoes of each transmission signal do not disturb the response signals of the other transmission signals. In particular, this makes it possible to avoid overlapping of the response signals, which avoids the mixing of information carried by the different echoes.

In addition, the densification of the interferogram by several samplings makes it possible to obtain an extension of the desired spectroscopy range while avoiding the appearance of "pollutant" signals.

Finally, the selection criterion according to the second embodiment makes it possible to reduce the "hollow" moments between the emissions of the combs with respect to that of the first embodiment.

Indeed, contrary to the selection criterion according to the first embodiment, any line of the first type selected according to the criterion of the second embodiment by the selection module 52 at the given instant naturally coincides with a line of the second selected type. by the selection module 62 at a time offset from the given instant by the value 1 / Afr.

Thus, the total measurement time Tm of the spectroscopy of the targeted material according to the second embodiment is substantially equal to D / Afr, compared with the total measurement time Tm equal to D / Af ™ ax according to the first embodiment. Other embodiments of the method according to the invention are also possible.

Thus, according to an exemplary embodiment, during the sub-step 140 for detecting the coincident lines, the control module 28 controls the pulse generation units 50 and 60 to cause a coincidence of the lines.

When the pulse generating units 50 and 60 receive such a command, they transmit the corresponding lines simultaneously. Unlike the sub-step 140 previously described, the coincidence of the lines occurs in a controlled manner which eliminates the "hollow" moments during which the generated lines are not used.

According to this exemplary embodiment, the selection criteria according to the first and second embodiments are equivalent. The total time Tm of measurement is equal to the value DTobs.

Thus, according to this exemplary embodiment, the lines of the first type and of the second type are recalibrated as soon as the criterion k = D is fulfilled, without waiting for the natural coincidence occurring every 1 / Afr time interval.

According to another exemplary embodiment, the first and second initial lines are determined during the substep 142 by any other suitable method.

Thus, for example, according to one of such methods, the first initial line corresponds to a line generated during step 115 and offset temporally with respect to the coincident line of a value chosen by the dichotomy of the interval [0; DAt]

In other words, during the second pass of the densification loop 121, the first initial line corresponds to a line generated during step 115 and offset temporally with respect to the coincident line by a value equal to DAr / 2. During the third and fourth passages, the first initial line corresponds to a line generated during step 115 and offset temporally with respect to the coincident line by a value equal to DAt / 4 and 3DAt / 4, respectively. The loop is thus continued recursively.

The second initial line is chosen analogously but by the dichotomy of the interval [0; D (At - 1 / Afr) \.

This exemplary embodiment then makes it possible to interrupt the densification loop 121 when the desired sampling fineness is achieved while keeping the homogeneous distribution of the samples, or to obtain, by treatment of the interferogram, a spectrum of progressively increasing extent, without having to wait for completion of loop 121.

Of course, it is possible to combine the last two embodiments.

Claims (14)

1. - Method (100) of LIDAR type remote spectroscopy of a material implemented by a spectroscopy device (10) capable of generating a laser signal, of processing the laser signal to form an emission signal, to transmitting the emission signal to a spectroscopically targeted material, receiving a response signal corresponding to the emission signal reflected by the targeted material, and processing the response signal; the method comprising the following steps: - generating (115) from a laser signal lines of a first type regularly spaced a first frequency spacing value (/ r) and lines of a second type spaced apart regularly a second frequency spacing value (fr + Afr) different from the first frequency spacing value (/ r); until a counter (k) of transmitted combs reaches a densification factor (D), perform the densification loop (121) comprising the following sub-steps: + select (124) lines of the first type and of the second type according to a predetermined selection criterion; + forming (126) with selected first type lines, a first comb (Ρ ±) to be transmitted from a first initial current line; + forming (126) with selected lines of the second type, a second comb (P2) to be emitted from a second current initial line; + detecting (140) a coincident line, the coincident line corresponding to a line of the first type simultaneously transmitted with a line of the second type; + identifying (142) a first following initial line and a second subsequent initial line, as a function of the coincident line, and associating the first current initial line with the next first initial line, and the second initial initial line with the next first initial line ; + emitting (134) each comb (P1, P2) formed and incrementing by one unit the counter (k) of emitted combs; receiving (160) response signals corresponding to all the lines emitted and processing (165) these response signals to construct an interferogram of the targeted material. 2, - Method (100) according to claim 1, wherein the lines of the first comb (Pi) are spaced a third frequency spacing value (/ rma *) whose inverse value (1 // Γ ™ α * ) is substantially equal to a time interval for receiving echoes of the reflected signals corresponding to the same transmission signal. A method (100) according to claim 2, wherein: - each first initial line is temporally offset from the corresponding coincident line by a first time offset value, the first time offset value being directly proportional to the counter (k ) of emitted combs and inversely proportional to a sampling frequency spacing value (fech)> 6t - the sampling frequency spacing value (fech) is directly proportional to the densification factor (D) and the third value Frequency spacing (f ™ ax). A method (100) according to claim 3, wherein each second initial line is temporally offset from the first corresponding initial line of a second time offset value, the second time offset value being directly proportional to the counter (k). of emitted combs and inversely proportional to the difference (Afr) of the second frequency spacing value (fr + Afr) and the first frequency spacing value (/ r). 5. - Method (100) according to any one of the preceding claims, wherein the construction of the interferogram of the targeted material comprises a processing of the response signals corresponding to the lines emitted, in an order defined respectively by the time shift of each line with respect to the corresponding coincident line. 6. - Method (100) according to any one of the preceding claims, wherein the first frequency spacing value (fr) is substantially equal to 100 MHz. 7. - Method (100) according to any one of the preceding claims, wherein the selection criterion comprises a uniform selection from the first or the second initial line of each n-th line respectively of the first or the second type. in the set of generated lines, the parameter n being a natural number. The method (100) according to any one of claims 1 to 6, wherein the selection criterion comprises non-uniform selection of lines from the first or second initial line consisting of: - uniform selection each nth line of the first type or the second type emitted from the first and second initial lines, respectively, during an observation interval (Tobs), the parameter n being a natural integer; a uniform selection of each line of the first type or of the second type emitted outside the observation interval (Tobs). 9. - Method (100) according to any one of the preceding claims, wherein the set of lines of the first type or the second type selected according to the selection criterion during an observation interval (Tobs) form respectively the first comb (Pt) to emit or the second comb to emit (P2). 10. - Method (100) according to claim 8 or 9, wherein the observation interval (Tobs) is directly proportional to the first frequency spacing value (fr) and inversely proportional to the difference (Afr) of the second frequency spacing value (fr + Afr) and the first frequency spacing value (fr). 11. - Method (100) according to any one of claims 7 to 10 taken in combination with claim 2, wherein the parameter n is directly proportional to the first frequency spacing value (fr) and inversely proportional to the third frequency spacing value (/ rma *). 12. - Method (100) according to any one of the preceding claims, wherein during the detection step (140), the coincidence of the lines of the first type and the second type is caused by a control module (28). . A method (100) according to any one of the preceding claims, wherein in the line generation step (115), the lines of different types are generated by pulse generation units (50, 60). ) and are transmitted by separate optical paths (\ Λ, V2). 14. - Computer program product comprising software instructions which, when implemented by computer equipment, implements the method (100) according to any one of the preceding claims. 15. Device (10) for LIDAR-type remote spectroscopy of a material comprising a module (20) for generating a laser signal, a module (22) for pretreatment of the laser signal capable of forming an emission signal , a module (24) for transmitting the emission signal to a material targeted by the spectroscopy, a module (25) for receiving a response signal corresponding to the emission signal reflected by the targeted material and a module ( 26) post-processing the response signal; the device (10) being able to: - generate from a laser signal lines of a first type regularly spaced a first frequency spacing value (/ r) and lines of a second type spaced regularly a second frequency spacing value (fr + Afr) different from the first frequency spacing value (/ r); - until a counter (k) of emitted combs reaches a densification factor (D), perform the densification loop to: + select lines of the first type and the second type according to a predetermined selection criterion; + form with lines of the first type selected, a first comb (Pi) to be issued from a first initial current line; + form with the lines of the second type selected, a second comb (P2) to be issued from a second current initial line; + detecting a coincident line, the coincident line corresponding to a line of the first type emitted simultaneously with a line of the second type; + identifying a first following first line and a second subsequent initial line, as a function of the coincident line, and associating the first current initial line with the next first initial line, and the second initial current line with the next first initial line; + emit each comb (Pi, P2) formed and increment by one unit the counter (k) of emitted combs; receiving response signals corresponding to all the lines emitted and processing these response signals to construct an interferogram of the targeted material.