FR3027116A1 - REMOTE SPECTROSCOPY DEVICE OF LIDAR TYPE - Google Patents

Abstract

The present invention relates to a spectroscopy device (10) comprising a laser source (20) capable of forming a main comb, a pulse generator (22), a transmission telescope (26) able to emit an emission signal to a target material, a receiving telescope (40) adapted to receive a response signal corresponding to the emission signal reflected by the material, a detector (44) adapted to detect a final signal dependent on the response signal, and a unit (16) for analyzing the final signal. The pulse generator (22) is adapted to generate a second plurality of reference lines to form a reference comb separate from the main comb. The final signal also depends on the reference comb and comprises a combination of lines from the main comb and the reference comb.

Description

The present invention relates to a remote spectroscopy device of the LIDAR type, comprising an optical transmission path comprising a laser source capable of emitting a laser line at a generating frequency; a pulse generator adapted to generate, from the emitted laser line, a first plurality of side lines around the generating frequency to form a main strip comb; a transmission telescope capable of transmitting a transmission signal to a targeted material by spectroscopy, the transmission signal comprising the main comb; an optical reception channel comprising a reception telescope able to receive a response signal corresponding to the transmission signal reflected by the targeted material; a detector adapted to detect a final signal dependent on the response signal and transform it into a digitized signal; and a computing unit connected to the detector and capable of digitally analyzing the digitized final signal.

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 a LIDAR application to send several light waves of different frequencies to target the different elemental gases or its concentration profile. 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 light waves, for example three or four, of different frequencies.

For this, one solution is to provide this LIDAR with a laser source for emitting light waves at different frequencies. However, this solution has a considerable cost which is mainly due to high costs of optical components of an ultrastable laser source at several frequencies. Moreover, this solution presents difficulties of setting up in an unstable environment.

Another solution, mentioned in particular in document WO 2013/165945, consists in emitting a comb of lines of different frequencies and generated with a modulator from a main laser source. A first laser source is used internally in the transmission system to generate a reference signal for stabilizing the line comb before it is transmitted.

The emission of a signal comprising this comb makes it possible to study several elementary gases associated with a spectral dispersion system for detection. However, this solution is relatively complex and expensive to achieve The object of the present is to provide a remote spectroscopy device overcomes this disadvantage.

To this end, the subject of the invention is a remote spectroscopy device of the aforementioned type, in which the pulse generator is able to generate, from the emitted laser line, a second plurality of side lines of reference around the generating frequency to form a reference comb separate from the main comb, each reference line of the reference comb being associated with a line of the main comb, and wherein the final signal further depends on the reference comb and comprises a combination of lateral stripes from the main comb and the reference comb. According to other advantageous aspects of the invention, the remote spectroscopy device comprises one or more of the following characteristics, taken alone or in any technically possible combination: the number of lines of each of the main comb and the reference comb is limited between five and fifteen, and advantageously equal to ten; the lines of the main comb are spaced regularly and stably with a first step corresponding to a frequency value in the frequency range 500 MHz to 2000 MHz and advantageously equal to 1000 MHz; - the lines of the reference comb are spaced regularly and stably with a second pitch corresponding to a frequency value in the frequency range 500 MHz to 2000 MHz and substantially equal to 1000 MHz; - the second step is superior to the first step; The difference between the second step and the first step corresponds to a predetermined current offset value, the current offset value being less than or equal to 1 MHz; the difference between the second step and the first step is stable in time; the lines of the main comb are ordered according to an increasing order of the frequency values corresponding to these lines; each line of the reference comb is shifted from the corresponding line of the main comb by a value of frequency equal to the product of the ordinal number of this line of the main comb decremented by one and the current offset value; the reception optical channel further comprises a coupling module adapted to integrate the reference comb in the response signal in order to obtain the final signal; - It further comprises an optical interconnection channel connecting the pulse generator and the coupling module, and for delivering the reference comb to the coupling module; the pulse generator is able to generate the two combs in a sequential manner; the interconnection channel comprises delaying means making it possible to delay the delivery of the reference comb to the coupling module; the pulse generator is able to generate the reference comb after the generation of the main comb with a time delay substantially equal to the difference in the time of reception of the response signal by the reception telescope and the time of arrival of the transmission of the transmission signal by the transmission telescope; the optical transmission channel further comprises a coupling module able to make the main comb interact with the reference comb in order to obtain the transmission signal; and the pulse generator is able to generate the two combs in parallel. These features and advantages of the invention will appear on reading the description which follows, given solely by way of nonlimiting example, and with reference to the appended drawings, in which: FIG. 1 is a diagrammatic view of a remote spectroscopy device 35 according to a first embodiment of the invention; FIG. 2 is a schematic view of the evolution of different light signals during spectroscopy implemented by the device of FIG. 1; FIG. 3 is a schematic view of a remote spectroscopy device according to a second embodiment of the invention; and FIG. 4 is a schematic view of the evolution of different light signals during a spectroscopy implemented by the device of FIG. 3. In the remainder of the description, the expression "substantially equal to" is means as a relationship of equality to plus or minus 10%. In FIG. 1, the remote spectroscopy device 10 comprises an optical transmission channel 12 making it possible to emit a light signal towards a targeted material, an optical reception channel 14 making it possible to receive a signal reflected by this material, and transform it into a digitized signal, and a computing unit 16 for digitally analyzing this digitized signal. The spectroscopy device 10 further comprises a unit 18 for controlling the various components of the device 10. The spectroscopy device 10 is embedded in a satellite or any other vehicle situated in a terrestrial orbit and making observations of the Earth, or another planet, and especially its atmosphere, or a slice of distant atmosphere. The targeted material is, for example, a gas composed of several elemental gases such as, for example, 002, H20 or CH4. 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. For this, the transmission path 12 comprises a laser source 20, a pulse generator 22, an amplifier 24, a transmission telescope 26 and an optical fiber transmission link 28 connecting these components of the channel 12.

The laser source 20 is able to emit a laser line at a generating frequency FG which is substantially equal to, for example, 200 THz. The pulse generator 22 is able to receive the laser line emitted by the source 20 via the link 28 and to simultaneously generate, from this line, a plurality of lateral lines around the generating frequency FG to form a comb P of lines as described below.

Thus, to form the comb P, the generator 22 is able to duplicate or modulate the laser line as many times as the number Nr of lines in the comb P. This number Nr of lines is for example between five and fifteen. and advantageously equal to ten.

More particularly, the generator 22 comprises a first modulator 31 able to generate an elementary PE comb of regularly spaced lines with a pitch corresponding to a frequency value FmEo + df, where df is a current offset variable equal to either 0 or at a current offset value df ,, the choice between these two values being made by the control unit 18 as will be explained later. The lines of the PE elementary comb are stably spaced apart, i.e. the spacing pitch is substantially unchangeable over time. By "substantially unchangeable" is meant any relative change in spacing pitch less than 1%, preferably less than 0.1%, and preferably less than 0.001%.

This value FmEo + df is for example in the frequency range 500 MHz to 2000 MHz and advantageously equal to 1000 MHz. The lines of the elementary comb PE are ordered in order of increasing frequency values corresponding to these lines. The generator 22 further comprises a second modulator 32 able to shift each line of the elementary comb PE generated by the first modulator 31 by a frequency value equal to FmA0 + Elf to obtain the comb P at the output of the generator 22, where FMAO is a frequency value advantageously equal to 110 MHz; and Elf is an initial offset variable equal to either 0 or an initial offset value Mi equal to df, * (Nr-1) / 2, the choice between these two values being made by the control unit 18 as will be explained later. The second modulator 32 is advantageously an acousto-optic modulator. The value of frequency F, of the line of the comb P corresponding to the ordinal number n, is thus determined by the following formula: F, = FG + FmA0 + 15f + (n-1- (Nr-1) / 2) (FmEo + df), where n is an ordinal number of the corresponding line greater than or equal to one. The ordinal number n thus corresponds to the spectral sample number of the optical signal sent to the targeted material. The current offset value df, is substantially equal to 1 MHz so that the following expression is respected: Nr * dfl <FmE0 / 2.

Thus, at each emission of a laser line by the laser source 20, the control unit 18 is able to determine the values of the variables Elf and df. The current offset variable df makes it possible to determine the type of the comb P at the output of the generator 22.

If the current offset variable df is equal to 0, the comb P at the output of the generator 22 is a comb of a first type which will be designated hereinafter by the term "main comb P1". If the current offset variable df is equal to the offset value df ,, the comb P at the output of the generator 22 is a comb of a second type which will be designated hereinafter by the term "reference comb P2". . The initial offset variable Elf makes it possible to move each line of the reference comb P2 by a constant value so that the first line (n = 1) of the reference comb P2 coincides with the first line (n = 1) of the main comb. Pl. Thus, the initial offset variable Elf is equal to 0 for the main comb P1 and is equal to the initial offset value Mi for the reference comb P2. This then makes it possible to avoid the bidirectional dilation effect of the reference comb P2 with respect to the main comb P1. Under the control of the unit 18, according to the first embodiment, the generator 22 is able to generate the main comb. P1 and the reference comb P2 in a sequential manner.

Each line of the reference comb P2 is thus associated with a line of the main comb P1. This line of the reference comb P2 is slightly offset from the line of the main comb P1 corresponding to its ordinal number n. The amplifier 24 is adapted to receive the main comb P1 from the generator 22 via the link 28 and to amplify its power in order to obtain an emission signal SE.

The bandwidth of the amplifier 24 is adapted to cover all the comb P1. The amplifier 24 is a fiber amplifier of EDFA (of the "Erbium Doped Fiber Amplifier") type or of the YDFA (Ytterbium) type, to double sheath, known in itself in the state of the art. The transmission telescope 26, also known in the state of the art, is able to receive the amplified emission signal SE via the link 28 and to transmit it to the targeted material. The reception channel 14 comprises a reception telescope 40, a coupling module 42, a detector 44 and an optical fiber reception link 46 connecting these components of the channel 14.

The receiving telescope 40 is adapted to receive a response signal SR corresponding to the transmission signal SE emitted by the transmission telescope 26 and reflected by the targeted material. Alternatively, the transmit and receive telescopes 26 are in the form of a single component. The coupling module 42 is connected to the reception telescope 40 via the link 46 and able to receive the response signal SR. The coupling module 42 is furthermore connected to the pulse generator 22 via an interconnection optical channel 50 to receive the reference comb P2 coming from the generator 22. The coupling module 42 is thus able to couple the response signal. SR with the reference comb P2 to obtain a final signal SF. This coupling has a superposition of the two signals, namely that of the reference comb P2 and the response signal SR constructed with the comb P1. The final signal SF thus comprises all the beat combinations of each line of one of the signals with each line of the other signal. By "beat", one understands the sum of the two signals in the Fourier space, advantageously resulting in a pulsation at the frequency equal to the difference of the frequencies of these signals. In a variant of the spectroscopy device 10, the interconnection channel 50 additionally comprises delay means 52 for delaying the delivery of the reference comb P2 to the coupling module. This makes it possible to synchronize the signals temporally and, in particular, their initial points. The detector 44 is connected to the coupling module 42 via the link 46 and able to detect the final signal SF in order to transform it into a digitized signal SN.

The detector 44 further comprises a low-pass filter known per se in the state of the art, making it possible to choose in the signal SF the combinations of the neighboring lines coming from the different combs. That is, the filter chooses only the lines spaced by the values dfl, 2dfl, 3dfl, ... Nr * df1. Finally, the computing unit 16 is able to receive the digitized signal SN to analyze it numerically according to Fourier transform methods known in the state of the art. This treatment comprises more particularly the determination of the amplitude of each line, in the final signal SF, from the main comb P1 with the line of the reference comb P2 corresponding thereto.

The operation of the spectroscopy device 10 according to the first embodiment of the invention will now be explained with reference to FIG. 2 in which the evolution of different signals during the spectroscopy is illustrated. Initially, the unit 18 controls the laser source 20 to emit a laser line at the generating frequency FG. At first, the unit 18 sets the current variables df and initial offset Elf equal to 0 and the generator 22 emits the main comb P1. This main comb P1 is then amplified by the amplifier 22 to obtain the signal of SE emission.

Then, the transmission telescope 26 transmits the transmission signal SE that travels from the transmitter to the targeted material for a time of up to 2.7 ms in the case of a satellite. The emission signal SE is then absorbed in part by the targeted material, depending on the frequency of each line. The response signal SR reflected by the targeted material travels to the receiver during the same time and is received by the receiving telescope 40. During the time corresponding to the round trip time of the transmission signal SE, the unit 18 sets the current offset variable df equal to the current offset value df, and the initial offset variable Elf to the initial shift value Mi, the generator 22 outputs the reference comb P2 with a slightly increased pitch as described previously. This transmission is made after a delay equal to the time of the round trip to coincide with the arrival of the signal SR. The reference comb P2 is sent to the coupling module 46 via the interconnection channel 50 to arrive at the coupling module 42 at the same time as the response signal SR. In a variant, the reference comb P2 is delayed by approximately the round-trip time of the main comb P1 by the optical delay means 52. After receiving this signal SR, it is superimposed with the reference comb P2 in the coupling unit 42 to obtain the final signal SF. Finally, the detector 44 transforms the final signal SF into a digitized signal SN so that the latter is analyzed by the calculation unit 16. The spectroscopy device 10 according to a second embodiment of the invention is illustrated in FIG. 3. Unlike the device 10 according to the first embodiment, the coupling module 42 of the device 10 of FIG. 3 is integrated in the optical transmission channel 12 between the amplifier 24 and the transmission telescope 26 ..

The coupling module 42 is thus able to couple the main comb P1 with the reference comb P2 generated in parallel with the main comb P1 by the generator 22, after their amplification separated by the amplifier 24. For this, as illustrated in FIG. 3, the generator 22 comprises two parallel comb generation chains 5 each comprising a first modulator 31A, 31B and a second modulator 32A, 32B. Each of the second modulators 32A, 32B is advantageously an acousto-optic modulator. In this case, the amplifier 24 is in the form of two elementary amplifiers 54A, 54B connected in parallel. Each of the elementary amplifiers 54A, 54B is analogous to the amplifier 24 described in the first embodiment. The elementary amplifier 54A is able to amplify the main comb P1 and the amplifier 54B is able to amplify the reference comb P2. According to this embodiment, the optical transmission channels 12 and reception 14 are no longer interconnected within the device 10.

The operation of the spectroscopy device 10 according to the second embodiment of the invention will now be explained with reference to FIG. 4 in which the evolution of different signals during the spectroscopy is illustrated. In this embodiment, the control unit 16 controls the generation of the two combs P1 and P2 simultaneously by the generator 22 by setting the current variables df and initial offset Elf equal to 0 for one of the two chains, and equal to the current value df and the initial offset Mi respectively for the other channel. The generator 22 then transmits the main comb P1 and the reference comb P2 simultaneously to the coupling module 42 to obtain the transmission signal SE. This signal SE is then transmitted to the amplifier 24 and the transmission telescope 26 to be transmitted as previously described. Unlike the first embodiment, the response signal SR received by the receiving telescope 40 is transmitted directly to the detector 44. The final signal SF detected by the detector 44 thus comprises the response signal SR. It is then conceivable that the device according to the invention makes it possible effectively to perform remote spectroscopy of an inhomogeneous target material such as a complex gas by generating a light signal comprising a main strip comb and a reference comb from a laser source. More particularly, this avoids the use of an expensive laser source with multiple transmit frequencies or additional source to generate a reference signal.

In addition, the device according to the invention makes it possible to perform spectroscopy from a particularly restrictive environment such as for example a satellite. Finally, the device according to the invention comprises few expensive components, such as for example certain optical components, which makes it less expensive and more simple to produce compared to devices of the state of the art.

Claims (12)

CLAIMS1.- LIDAR-type remote spectroscopy device (10), comprising: an optical transmission channel (12) comprising: a laser source (20) capable of emitting a laser line at a generating frequency (FG); + a pulse generator (22) able to generate, from the emitted laser line, a first plurality of side lines around the generating frequency (FG) to form a main line comb (P1); + a transmission telescope (26) capable of transmitting an emission signal (SE) to a material targeted by the spectroscopy, the emission signal (SE) comprising the main comb (P1); an optical reception channel (14) comprising: a reception telescope (40) adapted to receive a response signal (SR) corresponding to the transmission signal (SE) reflected by the targeted material; + a detector (44) capable of detecting a final signal (SF) dependent on the response signal (SR) and transforming it into a digitized signal (SN); - a computing unit (16) connected to the detector (44) and capable of digitally analyzing the digitized final signal (SN); characterized in that the pulse generator (22) is adapted to further generate, from the emitted laser line, a second plurality of reference side lines around the generating frequency (FG) to form a comb. reference (P2) distinct from the main comb (P1), each reference line of the reference comb (P2) being associated with a line of the main comb (P1); and in that the final signal (SF) also depends on the reference comb (P2) and comprises a combination of lateral lines coming from the main comb (P1) and the reference comb (P2). 2.- Device (10) according to claim 1, characterized in that the number of lines (Nr) of each of the main comb (P1) and the reference comb (P2) is limited between five and fifteen, and preferably equal to ten. 3.- Device (10) according to any one of the preceding claims, characterized in that the lines of the main comb (P1) are spaced regularly and stably with a first step corresponding to a frequency value (FMEO + 0) 3027116 12 in the frequency range 500 MHz to 2000 MHz and preferably equal to 1000 MHz. 4.- Device (10) according to any one of the preceding claims, characterized in that the lines of the reference comb (P2) are evenly and stably spaced with a second pitch corresponding to a frequency value (FmE0 + dfl) in the frequency range 500 MHz to 2000 MHz and substantially equal to 1000 MHz. 10 5.- Device (10) according to claims 3 and 4, characterized in: - that the second step is greater than the first step; in that the difference between the second step and the first step corresponds to a current value (dfl) of predetermined offset, the current offset value (dfl) being less than or equal to 1 MHz; and in that the difference between the second step and the first step is stable over time. 6. Device (10) according to claim 5, characterized in that: - the lines of the main comb (P1) are ordered in an increasing order of the frequency values corresponding to these lines; and in that each line of the reference comb (P2) is shifted from the corresponding line of the main comb (P1) by a value of frequency equal to the product of the ordinal number (n) of this line of the main comb (P1) decremented by one and the current offset value (dfl). 25 7.- Device (10) according to any one of the preceding claims, characterized in that: - the optical reception channel (14) further comprises a coupling module (42) adapted to integrate the reference comb (P2) in the response signal (SR) 30 to obtain the final signal (SF); and in that it further comprises an interconnecting optical channel (50) connecting the pulse generator (22) and the coupling module (42), and making it possible to deliver the reference comb (P2) to the module coupling (42). 35 8.- Device (10) according to claim 7, characterized in that the pulse generator (42) is adapted to generate the two combs (P1, P2) sequentially. 3027116 13 9.- Device (10) according to claim 7 or 8, characterized in that the interconnection channel (50) comprises delay means (52) for delaying the delivery of the reference comb (P2) to the coupling module (42). 5 10.- Device (10) according to claim 8 or 9, characterized in that the pulse generator (42) is adapted to generate the reference comb (P2) after the generation of the main comb (P1) with a time delay substantially equal to the difference in the time of reception of the response signal (SR) by the receiving telescope (40) and the transmission time of the transmission signal (SE) by the transmission telescope (26) . 11.- Device (10) according to any one of claims 1 to 6, characterized in the optical transmission path (12) further comprises a coupling module (46) adapted to interact with the main comb (P1). with the reference comb (P2) to obtain the transmission signal (SE). 12.- Device (10) according to claim 11, characterized in that the pulse generator (22) is adapted to generate the two combs (P1, P2) in parallel.