Generation method for frequency combs, corresponding generation module, telespectroscopic method and corresponding spectroscopic device

Description

[0001] The present invention relates to a method for generating frequency combs and an associated generation module.

[0002] The present invention also relates to a remote spectroscopy method and an associated spectroscopy device.

[0003] Various remote spectroscopy methods are already known in the state of the art. An example of such a spectroscopy method is described in particular in application FR 3039331.

[0004] This spectroscopy process is called "LIDAR" type, which means that it is implemented by a remote sensing and optical measurement tool known as "LIDAR". The term "LIDAR" comes from the English expression "light detection and ranging" (or "detection and measurement by light" in French).

[0005] In particular, this method makes it possible to study the composition of a targeted material located at a distance from the LIDAR by emitting a light wave of a determined frequency towards this material and then receiving a light wave reflected by this material.

[0006] Thus, by comparing this reflected light wave with the initial wave or with a reference signal, it is for example possible to determine the absorption coefficient by the material, which generally makes it possible to determine the precise composition of this material.

[0007] The light wave is emitted by the LIDAR in the form of a frequency comb generally composed of a plurality of light lines. These lines are generated from a laser signal by one or more modulators with a predetermined frequency, using generation methods known per se. This frequency is chosen in particular according to the desired sampling fineness.

[0008] All the comb lines are sent simultaneously towards the targeted material, subsequently reflected by this material and received by the LIDAR. By mixing with the reference signal, the received light wave makes it possible to obtain an interferogram of the targeted material. Its Fourier transform then provides a spectrum of the targeted material, that is to say a precise composition of this material.

[0009] The quality of the sampling therefore depends considerably on the quality of the combs emitted towards the targeted material.

[0010] In particular, to ensure good sampling quality, the lines of these combs must be perfectly controlled and stable.

[0011] Furthermore, in order to obtain a more complete decomposition of the targeted material, these combs must contain a large number of lines, which therefore makes it possible to cover a large frequency spectrum of the targeted material.

[0012] However, when applying existing phase modulation comb generation methods, it is often difficult to increase the number of lines in a single comb beyond nine while ensuring the stability of this comb.

[0013] The aim of the present invention is to propose a generation method making it possible to obtain a particularly stable comb whose number of lines is greater than nine and preferably equal to thirteen.

[0014] To this end, the invention relates to a method for generating frequency combs composed of a predetermined number of lines, the method comprising steps of: A) generation of a main line by a laser source; B) generation of a first plurality of lines by modulating the main line by a first radiofrequency signal; C) generation of a second plurality of lines by modulating each line of the first plurality of lines by a second radiofrequency signal other than the first radiofrequency signal; D) generation of a third plurality of lines by modulating each line of the second plurality of lines by a third radiofrequency signal other than the first and second radiofrequency signals;and E) forming a comb from the lines of the third plurality of lines, each of the first, second and third radio frequency signals being at a respective frequency value, and the respective frequency values ​​of the first, second and third radio frequency signals comprising a first frequency value, a second frequency value being twice the first frequency value and a third frequency value equal to three times the first frequency value.;

[0015] According to other advantageous aspects of the invention, the generation method comprises one or more of the following characteristics, taken in isolation or in all technically possible combinations: the modulation by the first radiofrequency signal is carried out with a modulation amplitude substantially equal to 1.6418 radians and a relative phase shift substantially equal to 21.8°; the modulation by the second radiofrequency signal is carried out with a modulation amplitude substantially equal to 1.3549 radians and a relative phase shift substantially equal to 43.6°; the modulation by the third radiofrequency signal is carried out with a modulation amplitude substantially equal to 2.2829 radians and a relative phase shift substantially equal to 245.4°;

[0016] The invention also relates to a method for generating frequency combs composed of a predetermined number of lines, the method comprising the following steps: A') generation of a main line by a laser source; B') generation of a complex signal corresponding to the sum of three distinct radiofrequency signals; C') generation of a main plurality of lines by modulating the main line by the complex signal; D') formation of a comb from the lines of the main plurality of lines, each of the radiofrequency signals being at a respective frequency value, and the respective frequency values ​​of the radiofrequency signals comprising a first frequency value, a second frequency value equal to twice the first frequency value and a third frequency value equal to three times the first frequency value;

[0017] The method may also include one or more of the following features: the comb is composed of thirteen lines regularly spaced apart from each other by the first frequency value and substantially equal to each other in power; the method further comprises a step of modulating the lines of the comb to an additional frequency value greater than the third frequency value; and the additional frequency value is between 20 and 40 GHz.

[0018] The invention also relates to a module for generating frequency combs composed of a predetermined number of lines, comprising means configured to implement the generation method as defined above.

[0019] The invention also relates to a method of remote spectroscopy of the LIDAR type of a targeted material comprising the following steps: generating a transmission signal comprising at least one comb generated as previously described; transmitting the transmission signal towards the targeted material; receiving a response signal corresponding to the transmission signal reflected by the targeted material; and analyzing the response signal.

[0020] According to other advantageous aspects of the invention, the remote spectroscopy method comprises the following characteristic: the first frequency value is chosen based on the physical characteristics of the target material.

[0021] The invention also relates to a remote spectroscopy device of the LIDAR type implementing the spectroscopy method as defined above.

[0022] These characteristics and advantages of the invention will appear on reading the description which follows, given solely by way of non-limiting example, and made with reference to the appended drawings in which: there figure 1 is a schematic view of a remote spectroscopy device according to the invention, the device comprising in particular a frequency comb generation module; the figure 2 is a schematic view of the generation module of the figure 1 , implemented according to a first embodiment; the figure 3 is a flowchart of a remote spectroscopy method according to the invention, the remote spectroscopy method being implemented by the remote spectroscopy device of the figure 1 and including in particular a method for generating frequency combs; figure 4 is a flowchart of the frequency comb generation process of the figure 3 , implemented according to a first embodiment; the figure 5 is a schematic view illustrating a comb obtained following the implementation of the frequency comb generation method of the figure 3 ; there figure 6 is a schematic view of the generation module of the figure 1 , implemented according to a second embodiment; and the figure 7 is a flowchart of the frequency comb generation process of the figure 3 , implemented according to a second embodiment.

[0023] In the remainder of the description, the expression “substantially equal to” is understood as a relationship of equality within plus or minus 10%.

[0024] The spectroscopy device 10 of the figure 1 is for example embarked on a satellite located in an Earth orbit and carrying out observations of the Earth, or of another planet, and in particular of a layer of atmosphere of thickness equal to for example 15 km.

[0025] The spectroscopy device 10 makes it possible to study a targeted material in this layer of atmosphere. The targeted material is, for example, a gas composed of several elementary gases such as CO 2 , H 2 O or CH 4 .

[0026] Each elementary gas is capable of absorbing a light wave of a given frequency with an absorption coefficient known a priori.

[0027] 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.

[0028] According to an alternative embodiment, the spectroscopy device 10 is embarked in another space or terrestrial vehicle, or even an aircraft.

[0029] According to yet another embodiment variant, the spectroscopy device 10 is arranged in a fixed manner, for example on the Earth's surface.

[0030] In at least some of the aforementioned embodiments, the spectroscopy device 10 also makes it possible to study a targeted material placed in any medium other than the atmosphere, such as, for example, the underwater or subterranean environment.

[0031] In reference to the figure 1 , the spectroscopy device 10 comprises a frequency comb generation module 12, a transmission module 14, a reception module 15, a post-processing module 16 and a control module 18.

[0032] The generation module 12 is capable of forming a transmission signal and a reference signal intended respectively for the transmission module 14 and the post-processing module 16. Each of these signals comprises at least one frequency comb generated as will be explained later.

[0033] The emission module 14 is a known emission telescope which is capable of receiving the emission signals from the generation module 12 and emitting them towards the targeted material.

[0034] The receiving module 15 is a receiving telescope also known per se which is capable of receiving response signals corresponding to the transmission signals emitted by the transmission module 14 and reflected by the targeted matter or other matter.

[0035] According to an alternative embodiment, the telescopes of the transmission 14 and reception 15 modules are in the form of a single component.

[0036] The post-processing module 16 is capable of receiving the reference signals generated by the generation module 12 and the response signals received by the reception module 15.

[0037] The post-processing module 16 makes it possible in particular to convert the reference and response signals into digital signals, the analysis of which gives a precise composition of the targeted material. This analysis is carried out according to analysis methods known per se.

[0038] The control module 18 makes it possible to control the operation of all the modules of the spectroscopy device 10.

[0039] The control module 18 is for example connected to a central computer (not shown) of the satellite from which it is controlled.

[0040] The generation module 12 according to a first embodiment will now be explained with reference to the figure 2 .

[0041] So, as illustrated in this figure 2 , the generation module 12 comprises a laser source 20, two optical channels 21, 22 connected to this laser source 20 by optical guidance means and an electronic device 24 for managing these channels 21, 22.

[0042] The laser source 20 is capable of emitting a laser signal at a generating frequency which is substantially equal, for example, to 200 THz. This laser signal notably comprises a laser line, hereinafter referred to as the main line.

[0043] The optical channels 21, 22 are capable of receiving the laser signal emitted by the laser source 20 and, from this signal, of forming respectively an emission signal and a reference signal.

[0044] The optical paths 21, 22 are similar to each other and in particular comprise substantially the same components.

[0045] Unlike the optical channel 21, the optical channel 22 further comprises means for differentiating the reference signal from the transmission signal.

[0046] These differentiation means are configured according to the construction method chosen to determine the composition of the targeted material. In the exemplary embodiment described, these means include in particular delay means making it possible to delay the emission of the reference signal relative to the emission signal in order to synchronize this reference signal with the corresponding response signal. These means are known per se and will not be detailed.

[0047] Subsequently, only optical path 21 will be described in detail.

[0048] So, as illustrated in the figure 2 , the optical path comprises three modulators, namely a first modulator 31, a second modulator 32 and a third modulator 33, connected in series to the laser source 20.

[0049] Each modulator 31 to 33 has, for example, an electro-optical modulator known per se.

[0050] Thus, each modulator 31 to 33 is capable of generating by phase modulation a plurality of lines from each line passing through the optical path 21 as a function of a radiofrequency signal controlling the operation of this modulator 31 to 33.

[0051] For this purpose, the electronic management device 24 comprises three management units, namely a first management unit 41 associated with the first modulator 31, a second management unit 42 associated with the second modulator 32 and a third management unit 43 associated with the third modulator 33.

[0052] Each management unit 41 to 43 makes it possible to control the modulator 31 to 33 associated with it by generating a sinusoidal radiofrequency signal with a given modulation amplitude, a given relative phase shift and a given frequency value.

[0053] The remote spectroscopy method implemented by the spectroscopy device 10 will now be explained with reference to the figure 3 illustrating a flowchart of its steps.

[0054] Initially, the target material is determined and the transmitting 14 and receiving 15 modules are configured to respectively transmit a transmit signal towards this material and receive a response signal reflected by this material.

[0055] During step 110, the control module 18 controls the generation of a transmission signal by the generation module 12.

[0056] During the following step 120, the generation module 12 then generates a transmission signal intended for the transmission module 14 and a reference signal intended for the post-processing module 16.

[0057] The transmission signal comprises a frequency comb generated by the optical channel 21 and the reference signal comprises a frequency comb generated by the optical channel 22 and possibly delayed relative to the comb included in the transmission signal.

[0058] Each of these combs is generated according to a frequency comb generation method which is implemented by each of the optical channels 21, 22. The implementation of this method by the optical channel 21 will be explained in more detail later.

[0059] In the following step 130, the emission module 14 emits the emission signal generated by the generation module 12 towards the targeted material.

[0060] In the following step 140, the reception module 15 receives a response signal corresponding to the emission signal reflected by the targeted material.

[0061] During the following step 150, the post-processing module 16 analyzes the received response signal and determines the composition of the targeted material by comparing in particular this response signal with the corresponding reference signal.

[0062] The implementation of the method for generating frequency combs by the optical channel 21 will now be explained with reference to the figure 4 illustrating a flowchart of the steps in this process and figure 5 illustrating the implementation of these steps.

[0063] During the initial step A), the laser source 20 generates a laser signal then comprising a main line. This main line is referenced by the reference 50 on the figure 5 .

[0064] During the following step B), the first modulator 31 generates a first plurality of lines 51 by modulating the main line with a first radiofrequency signal at a first frequency value F.

[0065] The first radio frequency signal is generated by the first management unit 41 and is defined with a modulation amplitude substantially equal to 1.6418 radians and a relative phase shift substantially equal to 21.8°.

[0066] The first frequency value F is chosen according to the physical characteristics of the targeted material, for example to have an analysis result which best reproduces these characteristics.

[0067] The first frequency value F is equal to, for example, 1 GHz.

[0068] The modulation implemented during this step results in the generation of three lines of equal power and a multitude of lines of much lower power which will be neglected subsequently.

[0069] Thus, the first plurality 51 is composed of three lines, one of which corresponds to the main line and two others are placed on either side of the main line and are spaced from this main line by the first frequency value F.

[0070] During the following step C), the second modulator 31 generates a second plurality of lines 52 by modulating each line of the first plurality of lines 51 by a second radiofrequency signal at a second frequency value 2F.

[0071] The second radio frequency signal is generated by the second management unit 42 and is defined with a modulation amplitude substantially equal to 1.3549 radians and a relative phase shift substantially equal to 43.6°.

[0072] The second frequency value 2F is equal to twice the first frequency value F.

[0073] The modulation implemented during this step results in the generation of three lines of equal power from each line of the first plurality of lines 51, and a multitude of lines of much lower power which will be neglected subsequently.

[0074] Thus, given the overlap of at least some of the lines as illustrated in the figure 5 , the second plurality of lines 52 is composed of seven lines of equal power which are spaced from each other by the first frequency value F.

[0075] During the following step D), the third modulator 33 generates a third plurality of lines 53 by modulating each line of the second plurality of lines 52 by a third radiofrequency signal at a third frequency value 3F.

[0076] The third radio frequency signal is generated by the third management unit 43 and is defined with a modulation amplitude substantially equal to 2.2829 radians and a relative phase shift substantially equal to 245.4°.

[0077] The third frequency value 3F is equal to three times the first frequency value F.

[0078] The modulation implemented during this step results in the generation of three lines of equal power from each line of the second plurality of lines 52, and a multitude of lines of much lower power which will be neglected subsequently.

[0079] Thus, given the overlap of at least some of the lines as illustrated in the figure 5 , the third plurality of lines 53 is composed of thirteen lines of equal powers which are spaced from each other by the first frequency value F.

[0080] Finally, in the following step E), a comb is formed from the third plurality of lines 53.

[0081] This comb thus comprises thirteen lines spaced apart by the first frequency value F.

[0082] The method for generating frequency combs by the optical channel 22 is implemented in a similar manner by choosing the first frequency value F slightly different from that chosen for the optical channel 21. The difference between the first two frequency values ​​F corresponding to the different optical channels 21, 22 is for example equal to 100 kHz.

[0083] According to an alternative embodiment of the generation device 12, the modulators 31 to 33 are arranged in series in an order different from that of the figure 3 .

[0084] In fact, the order of arrangement of these modulators 31 to 33 has no influence on the shape of the final comb which always has thirteen lines spaced from the first frequency value F.

[0085] Thus, for example, when the third modulator 33 is put in place of the first modulator 31 and the first modulator 31 is put in place of the third modulator, during step B), it is the third modulator 33 which generates the first plurality of lines from the main line. These lines of the first plurality are then spaced by the third frequency value 3F.

[0086] Then, as in the previous case, during step C), the second modulator 32 generates the second plurality of lines in which the lines are spaced apart from the first frequency value F or from the second frequency value 2F.

[0087] Finally, during step D), the first modulator 31 generates the third plurality of lines in which the lines are spaced by the first frequency value F.

[0088] According to another embodiment variant compatible with one of the variants previously described, the optical path 21 further comprises an additional modulator placed downstream of the modulators 31 to 33 described previously.

[0089] Such a modulator is driven by an additional drive unit to modulate each line passing through this modulator to an additional frequency value FA greater than the third frequency value 3F.

[0090] The additional frequency value FA is for example between 20 and 40 GHz.

[0091] In this case, the generation method described above further comprises a step F) implemented after step E) during which all the lines of the comb formed during step E) are modulated by the additional modulator.

[0092] This has the effect of tripling the initial comb, that is, generating two combs on either side of the initial comb.

[0093] It is therefore understood that the present invention presents a certain number of advantages.

[0094] Indeed, the frequency combs obtained by the generation method according to the invention each comprise exactly thirteen lines which are spaced from each other by a given frequency value. These lines are stable and perfectly controlled.

[0095] This then makes it possible to improve the quality of the analysis result of the corresponding target material.

[0096] This quality can be further improved by adding a modulation at a high frequency to triple the initial comb.

[0097] Thus, the emission signal formed with the three combs makes it possible to cover a larger wavelength range.

[0098] A frequency comb generation module 112 according to a second embodiment of the invention is illustrated in the figure 6 .

[0099] So, with reference to this figure 6 , the generation module 112, as in the previous case, comprises a laser source 120, two optical channels 121, 122 connected to this laser source 120 by optical guidance means and an electronic device 124 for managing these channels 121, 122.

[0100] The laser source 120 is analogous to the laser source 20 described previously.

[0101] As in the previous case, the electronic management device 124 comprises three management units, namely a first management unit 141 similar to the first management unit 41 and generating the first radiofrequency signal, a second management unit 142 similar to the second management unit 42 and generating the second radiofrequency signal, and a third management unit 143 similar to the third management unit 43 and generating the third radiofrequency signal.

[0102] Unlike the electronic management device 24, the management device 124 further comprises a unit 145 for generating a complex signal corresponding to the sum of the first radiofrequency signal, the second radiofrequency signal and the third radiofrequency signal.

[0103] Also as in the previous case, the optical paths 121, 122 respectively deliver a transmission signal and a reference signal, and differ from each other only by the presence in the optical path 122 of means for differentiating the reference signal from the transmission signal, similar to those described previously.

[0104] Thus, subsequently, only optical path 121 will be described in detail.

[0105] Unlike the optical channel 21, the optical channel 121 comprises a single modulator 150, called the main modulator, similar to one of the modulators 31 to 33 described previously.

[0106] The main modulator 150 is controlled by the complex signal delivered by the generation unit 145. Thus, with such a complex signal, the modulator 150 is able to modulate the main line passing through the optical path 121 by each of the first radiofrequency signal, the second radiofrequency signal and the third radiofrequency signal.

[0107] A method for generating frequency combs implemented by the frequency comb generation module 112 according to the second embodiment and for the optical channel 121, will now be described with reference to the figure 7 illustrating a flowchart of its steps.

[0108] During step A') similar to step A) described previously, the laser source 120 generates a laser signal then comprising a main line.

[0109] In the following step B'), the generation unit 145 generates a complex signal corresponding to the sum of said radiofrequency signals. These radiofrequency signals are defined by the same frequency values, the same modulation amplitudes and the same relative phase shifts as in the previous case.

[0110] In the following step C'), the modulator 150 generates a main plurality of lines by modulating the main line with the complex signal.

[0111] The plurality of lines thus generated comprises, as in the previous case, thirteen lines spaced from each other by the first frequency value F.

[0112] Finally, in the following step D'), a comb is formed from the lines of the main plurality of lines.

[0113] As in the previous case, according to an alternative embodiment, an additional modulator similar to the additional modulator described previously, is placed downstream of the main modulator 150.

[0114] In this case, the generation method according to the second embodiment of the invention further comprises a step E') during which the additional modulator triples the comb generated during step D').

[0115] The frequency comb generation device according to the second embodiment of the invention is advantageous in that it allows the number of optical components used to be reduced. This allows in particular to reduce the costs relating to the installation.

Claims

1. A method of generating frequency combs consisting of a predetermined number of rays, the method comprising steps of: A) Generation of a main ray by a laser source (20); B) generation of a first plurality of rays (51) by modulating the main ray with a first radio frequency signal; C) generation of a second plurality of rays (52) by modulating each ray of the first plurality of rays (51) with a second radio-frequency signal other than the first radio-frequency signal; D) generation of a third plurality of rays (53) by modulating each ray of the second plurality of rays (52) with a third radio-frequency signal other than the first and second radio-frequency signals; and E) formation of a comb from the rays of the third plurality (53) of rays, each of the first, second and third radio-frequency signals being at a respective frequency value, and the respective frequency values of the first, second and third RF signals comprising a first frequency value, a second frequency value (2F) equal to twice the first frequency value (F) and a third frequency value (3F) equal to three times the first frequency value (F).

2. A method according to claim 1, wherein modulation by the first radio-frequency signal is performed with a modulation amplitude substantially equal to 1.6418 radians and a relative phase shift substantially equal to 21.8°.

3. A method according to any of the preceding claims, wherein modulation by the second radio frequency signal is carried out with a modulation amplitude substantially equal to 1.3549 radians and a relative phase shift substantially equal to 43.6°.

4. Method according to any of the preceding claims, wherein modulation by the third radio frequency signal is carried out with a modulation amplitude substantially equal to 2.2829 radians and a relative phase shift substantially equal to 245.4°.

5. Method for generating frequency combs composed of a predetermined number of rays, the method comprising steps of: A') generation of a main ray by a laser source (120); B') generation of a complex signal corresponding to the sum of three distinct radio-frequency signals; C') generation of a main plurality of rays by modulating the main ray with the complex signal; D') forming a comb from the rays of the main plurality of rays, each of the radio frequency signals being at a respective frequency value, and the respective frequency values of the radio frequency signals comprising a first frequency value, a second frequency value (2F) equal to twice the first frequency value (F) and a third frequency value (3F) equal to three times the first frequency value (F).

6. A method according to claim 1 or 2, wherein each of the radio frequency signals is at a respective frequency value, and the respective frequency values of the radio frequency signals comprise a first frequency value, a second frequency value (2F) equal to twice the first frequency value (F) and a third frequency value (3F) equal to three times the first frequency value (F).

7. Method according to any of the preceding claims, further comprising a step of modulating (F; E') the comb rays at an additional frequency value (FA) higher than the third frequency value (3F).

8. Method according to claim 8, wherein the additional frequency value (FA) is between 20 and 40 GHz.

9. Module for generating frequency combs (12; 112) composed of a predetermined number of rays, comprising means configured to implement the process according to any of the preceding claims.

10. Method for LIDAR-type remote spectroscopy of a target material, comprising the following steps: - generation (120) of a transmission signal comprising at least one comb generated according to the method of any one of claims 1 to 8; - emission (130) of the emission signal to the target material; - reception (140) of a response signal corresponding to the transmission signal reflected by the target material; and - analysis (150) of the response signal.

11. Method according to claim 10, wherein the first frequency value (F) is selected based on the physical characteristics of the targeted material.

12. LIDAR remote spectroscopy device (10) configured to implement the method according to claim 10 or 11.