System for measuring the central wavelength of a spectral line with high precision and associated method.

Abstract

System (10) for measuring a central wavelength of interest (λc) of a spectral line of interest (RSe) measured by a spectrometer, the system comprising: a detection system including a spectrometer (Spectro) associated with a detector (Det), an optical fiber (FOY) in a Y configuration such that: the first input collects a light signal (SLech) from the sample, called the sample signal, the second input collects a light signal (SLref) from a reference source (Sref), called the reference signal, the output is coupled to an input of the spectrometer, the measurement system being configured so that the detector detects said sample signal and said reference signal simultaneously or sequentially in time, the measurement system further comprising a processing unit (TU) configured to: process said measured profiles of interest and reference,to determine a value for the central wavelength of interest from the difference between the positions of interest and reference, the known value of the reference wavelength, and a linear dispersion (LD) of the detection system. Figure 5,

Description

Title of the invention: System for measuring a central wavelength of a spectral line with high precision and associated method. FIELD OF THE INVENTION

[0001] The present invention relates to the field of spectroscopy, and more particularly to the determination of the central wavelength of a spectral line with very high precision. STATE OF THE ART

[0002] For certain applications in spectroscopy, for example in atomic or molecular spectroscopy, or to determine the isotopic abundance of an element in a sample by an optical method (called LIBRIS for Laser Induced Breakdown self-Reversal Isotopic Spectrometry, see below) a high precision on the determination of the value of the central wavelength of a spectral line is required. The spectral line to be characterized is produced by a light source and can be an absorption or emission line, atomic or molecular. An uncertainty of less than 5 pm, or even less than 1 pm, on the value of the central wavelength is typically sought.

[0003] This problem has not arisen to date in the field of laser ablation plasma spectroscopy (LIBS techniques for "Laser Induced Breakdown Spectroscopy" or laser-induced plasma optical emission spectrometry, LAMIS for Laser Ablation Molecular Isotopic Spectrometry, etc.), because the width of the lines observed is typically a few tens of pm. The wavelength of the lines is therefore usually measured with an uncertainty of about ten pm to a few tens of pm depending on the linear dispersion of the spectrometer used. This uncertainty has no impact on these techniques because the analysis is made from the intensity of the lines generally integrated over a width of the same order, from about ten to a few tens of pm.

[0004] Conventionally, the wavelength of the detection system is calibrated using a reference source emitting known lines, typically a mercury vapor lamp or a hollow cathode lamp. The position of the line to be analyzed and the position of the reference line are identified in pixels on the detector, and the line to be analyzed is determined from its relative position with respect to that of the reference line. The detector comprises at least N pixels Pi aligned along a line, with i varying from 1 to N. When it is 2D, an integration over all the pixels of the same column is carried out. For example, the detector is a CCD technology matrix, with 2048x512 pixels.

[0005] Let Xref be the central wavelength of the reference line and X0 the wavelength to be determined, Pref the position of Xref identified in pixels of the detector and PO the position of X0 on this same detector. The wavelength Xref is of course chosen so that it appears on the detector simultaneously with X0 for the same configuration of the spectrometer. We have:

[0006] 4 = (PQ-P,.ef).DL(.^

[0007] with DL linear dispersion of the detection system, typically in pm / pixel.

[0008] For various reasons (thermal fluctuations, vibrations), spectrometers and detectors drift very slightly even in the controlled environment of a research laboratory, which leads to a wavelength drift. This drift is of course even more pronounced in analysis situations outside the laboratory (in the field, online, by a portable system, etc.). For example, in the case of a 1 m focal length grating spectrometer with a 2400 lines / mm grating, a variation of only 103 degrees in the grating angle causes a wavelength shift of 10 pm, which is prohibitive for the LIBRIS analysis of lithium, for example, for which an uncertainty of less than 1 pm is aimed for to obtain an acceptable uncertainty on the isotopic abundance in 6Li.

[0009] The detection of the reference line and that of the line to be analyzed are carried out sequentially in time. In the most common case, the signal from the sample is routed to the detection system by an optical fiber. To carry out the two measurements, it is then necessary to position the optical fiber connected to the spectrometer first to collect the light flux from the reference source then that from the emission to be characterized or vice versa, which takes a certain amount of time. Typically, these two measurements are separated by a duration of the order of a minute, which is sufficient for such a drift to occur.

[0010] It is therefore impossible to carry out precise LIBRIS measurements without correcting the wavelength drift of the detection system. The problem arises in the same way in atomic spectroscopy where we seek to measure X0 precisely by calibrating on a reference source.

[0011] The invention being of particular interest for the LIBRIS method, its principle is recalled below as well as the principle of the LIBS and LAMIS methods.

[0012] The principle of LIBS technology, illustrated [Fig. 1], is to focus a laser pulse on the surface of a material sample (or the material) to generate a transient plasma whose light emission is analyzed using a spectrometer. By collecting the light emission from the plasma and analyzing the spectrum by spectrometry, it is possible to identify the elements present in the plasma, and therefore to determine the composition of the material, from emission line databases. In LIBS the intensity is integrated over the entire width of the line.

[0013] LAMIS technology, for example described in the publication by R. Russo et al., Spectrochim. Acta B 66 (2011) 99, is an alternative derived from LIBS which allows isotopic analysis to be carried out from the lines of molecules formed by reaction between the ablated material and a constituent of the ambient medium, or by reaction between two atoms of the ablated material.

[0014] A laser generator L0 generates a laser beam FL0 which is focused on the sample 1 using a first optical system 2. This generates a plasma P10. The plasma emits a light emission 3 which is collected by an optical system OS0. The focused light emission is sent to a spectrometer SpecO via an optical fiber FO. The spectrometer SpecO comprises (or is associated with) a detector DetO synchronized with the laser generator L0. The spectrometer SpecO makes it possible to record line spectra. Finally, processing means UT0 make it possible to process the recorded spectra.

[0015] LIBS makes it possible to generate a spectrum 20, which is presented in the form of a set of spectral lines which correspond to the emission lines of the elements composing the material, and make it possible - using the available correlation data between the emission lines and the elements - to determine the elemental composition of the material sample. The wavelength X of a line provides information on an element present in the material and the intensity I is linked to the concentration of this element.

[0016] LIBS emission spectrometry also applies to isotopic analysis because the atomic lines of different isotopes of the same element are at slightly different wavelengths. This spectral shift, called isotopic shift, is due to mass effects (majority for light elements) and modification of the charge distribution inside the nucleus (majority for heavy elements). If we want to do this isotopic analysis by LIBS it is imperative to separate the lines of the 2 isotopes. However, this spectral shift is generally of the order of a fraction of nm or even a few pm, as shown in Table I below: Isotopes Emission line Isotopic shift 7Li 6Li 670.775 nm + 15.8 pm 10B 11B 208.891 nm - 2.5 pm 238U 235U 424.437 nm + 25 pm 239Pu 240Pu 594.522 nm + 13 pm

[0017] Table I

[0018] Such a shift is difficult to observe in a plasma generated by laser ablation. under usual conditions, because the confinement of the plasma by ambient air at atmospheric pressure results in a high density, and therefore a broadening of the emission lines due to the Stark effect. This broadening commonly reaches several tens or even hundreds of pm and therefore masks the isotopic shift, even if the spectrometer used has sufficient spectral resolution to resolve this shift. The limitation here is physical and not instrumental.

[0019] A first solution consists of carrying out the analysis at reduced pressure, or even under vacuum. By thus limiting the confinement of the plasma by the ambient medium, its density is reduced and sufficient spectral selectivity can be found for certain isotopes. A double line is visualized, and the determination of the isotopic ratio is carried out from the intensity ratio between the two lines associated with the two isotopes. This approach is not applicable to all isotopes and requires a spectrometer with high resolution power, and therefore bulky. A second solution consists of sending a second laser beam through the plasma, in order to measure a resonant absorption or fluorescence signal, which is restrictive and complicates the measurement system.

[0020] In the state of the art of isotopic analysis at atmospheric pressure, the LAMIS technique can also be used, but this requires several conditions to be met: 1. Molecules must form in the plasma; 2. They must be sufficiently stable under the temperature / density conditions of the plasma; 3. They must have detectable lines, i.e. lines of sufficient lifetime, sufficiently intense, and in the spectral band of the detection system. In the case of lithium, for example, no LAMIS signal is detected, probably because the second condition is not met.

[0021] The LIBRIS technique is an optical technique for determining the isotopic abundance of an element in a sample (solid, liquid or gaseous) from the emission spectrum of a laser ablation plasma. This technique is for example described in the publication by K. Touchet et al., Spectrochim. Acta B 168 (2020) 105868 and in document US 2019 / 0041336. It is a variant of the LIBS technology and uses the same optical system. The LIBRIS technology overcomes the various drawbacks of the LIBS method by allowing measurement of an isotopic ratio at atmospheric pressure and without a second laser.

[0022] It is recalled that the electronic transitions of atoms to higher energy levels require an input of energy. This energy can be in the form of photons, in which case there is absorption of photons by the atom. A special case is that of laser ablation plasma. To simplify, we can consider that the plasma is made up of two distinct parts, the core and the periphery. Photons emitted by the core of the plasma, which is hotter, can be absorbed by the periphery, which is colder. This This phenomenon therefore prevents a certain number of emitted photons from leaving the plasma: this is the phenomenon of self-absorption.

[0023] For an observer outside the plasma, and for a measuring device, the line profile results from the emission and self-absorption at the same wavelength corresponding to the electronic transitions between two levels of all the atoms considered placed on its line of sight. Consequently, the measured intensity is not only the sum of all the emissions from the plasma, because this self-absorption must be taken into account.

[0024] The phenomenon of self-absorption, well known in plasma spectroscopy for elemental analysis, is rather considered as an undesirable phenomenon because it leads to a distortion of the line profile, and therefore to a non-linearity of the signal with respect to the concentration of the element of interest. LIBRIS exploits this self-absorption effect to deduce information on the isotopes of a given element in a material.

[0025] Figures 2 and 3 illustrate an RS0 line of an element of interest, selected from a spectrum 20, obtained in two cases, depending on the concentration of the element in the material.

[0026] [Fig.2] illustrates the case where the concentration of the element in the plasma is lower, the self-absorption phenomenon is not very marked or even absent. We obtain a spectrally broad line profile, not hollowed out in its center. The dotted curves ISOi and ISO2 represent the emission of the 2 isotopes. Each line has a significant width compared to the gap between the 2 lines, mainly due to the Stark effect in the plasma, and this is why we do not distinguish them individually: we detect the solid line RS0 which corresponds to the sum of the 2. The principle of LIBRIS is that the central wavelength of the solid line varies with the isotopic abundance, that is to say with the ratio of the amplitudes of the 2 dotted lines. In this case, we measure the value of the central wavelength XO corresponding to the emission peak, i.e. the maximum point or summit 20 of the observed curve which has a bell-shaped profile.It is correlated to the ratio between two isotopes Iso1 and Iso2 of the element considered, and it is shifted according to said isotopic ratio.

[0027] [Fig.3] illustrates the case where the element is in high concentration in the plasma, the self-absorption phenomenon is then marked. We observe a line profile hollowed out in its center (double bell profile), called inverted line, resulting from the superposition of a spectrally broad emission profile, with a spectrally narrower absorption profile. In this case, we measure the value of the central wavelength ko corresponding to the absorption trough. The central wavelength X0 is in this case measured on the part of the profile corresponding to the absorption, that is to say at the minimum point 30 of the observed trough. It is correlated to the ratio between two isotopes Isoi and Iso2 of the element considered, and it is shifted according to said isotopic ratio. It is this measurement of the wavelength of the trough that defines the LIBRIS technology.

[0028] Thus, in the LIBRIS technology the measurement of the isotopic ratio is carried out from the very precise measurement of the wavelength X0, maximum of the bell line or minimum of the line, called inverted, in double bell. This wavelength Xo shifts linearly with the isotopic abundance, between XR* and XR2, the indices 1 and 2 referring to two isotopes of the element. XR* and XR2 are physical data available in spectroscopic databases and / or in scientific publications. The analytical uncertainty on the isotopic abundance is therefore directly linked to the uncertainty on the determination of the wavelength XO.

[0029] In LIBRIS technology, the measurement of X0 directly gives the isotopic ratio. [Fig.4] illustrates this evolution of X0 measured as a function of the proportion of the 6Li isotope of Lithium, which has only two isotopes 6Li and 7Li. This curve was produced on an inverted line. The isotopic shift is given by XR* - XR2 and corresponds to the measurement range of the technique for a given line. In the case of lithium and for the line at 670.778 nm used in LIBRIS, this shift is 15.8 + / - 0.3 pm and therefore corresponds to the total variation of the isotopic abundance in 6Li from 0% to 100%, the complementary being the abundance in 7Li. Thus, an uncertainty of 1 pm in the determination of the XR wavelength leads to an uncertainty in the isotopic abundance of 1 / 15.8 = 6.3%. The measurement accuracy of the isotopic ratio is therefore directly correlated to the measurement accuracy on X0.

[0030] An aim of the present invention is to remedy the aforementioned drawbacks by proposing a method and a system for determining the central wavelength of an absorption or emission line, atomic or molecular, produced by a light source, with sub-picometric precision. DESCRIPTION OF THE INVENTION

[0031] The present invention relates to a system for measuring a central wavelength of interest of a spectral line of interest measured by a spectrometer, the spectral line of interest corresponding to an emission or an absorption of a sample to be characterized, the spectral line of interest having either a bell-shaped profile, said central wavelength of interest then corresponding to the peak of said bell-shaped profile, or a double-bell profile, said central wavelength of interest then corresponding to the trough between the two bells, the system comprising: - a detection system comprising a spectrometer associated with a detector comprising a plurality of pixels aligned in a direction X, the spectral line of interest being detected on pixels of the detector, - a Y-shaped optical fiber, having a first and a second input and a output, the optical fiber being configured so that: • the first input collects a light signal from the sample, called the sample signal, • the second input collects a light signal from a reference source, called the reference signal, the reference source having a reference spectral line having a central wavelength called the reference wavelength of known value, the reference wavelength being chosen so as to be detected on at least one pixel of the detector, • the output is coupled to an input of the spectrometer, • the measurement system being configured so that the detector detects said sample signal and said reference signal simultaneously or sequentially in time, so as to generate a measured profile of interest and a measured reference profile, - the measuring system further comprising a processing unit configured to: • process said measured profiles of interest and reference so as to determine a position of interest and a reference position, measured in pixels of the detector, respectively of the central wavelength of interest and the reference wavelength, • determine a value of the central wavelength of interest from a difference between said positions of interest and reference, from said known value of the reference wavelength and from a linear dispersion of the detection system.

[0032] According to a first variant, the measurement system is configured so that the detector detects said sample signal and said reference signal simultaneously, the reference source having a reference wavelength located outside the spectral line of interest.

[0033] According to a second variant, the measuring system is configured so that the detector sequentially detects in time said sample signal during a signal duration and said reference signal during a reference duration, the signal duration and the reference duration being separated by a so-called intermediate duration.

[0034] According to one embodiment, the measuring system according to the invention is suitable for measuring an isotopic abundance of an element present in the sample and further comprises: - a pulsed laser configured to illuminate the sample so as to generate a plasma capable of emitting said light signal from the sample, - an optical system configured to inject a portion of said light signal from the sample into the first input of the optical fiber, - the processing unit being configured to synchronize the detector with the laser, said central wavelength of interest corresponding to a line resulting from the contributions of two isotopes of said element, said value of the central wavelength of interest making it possible to determine said abundance.

[0035] According to another aspect, the invention relates, according to a first variant, to a method for determining a central wavelength of interest of a spectral line of interest measured by a spectrometer, the spectral line of interest corresponding to an emission or an absorption of a sample to be characterized, the spectral line of interest having either a bell-shaped profile, said central wavelength of interest then corresponding to the peak of said bell-shaped profile, or a double-bell profile, said central wavelength of interest then corresponding to the hollow between the two bells, the spectrometer being associated with a detector comprising a plurality of pixels aligned in a direction X, the spectral line of interest being detected on pixels of the detector, the method comprising the steps of: . - have a Y-shaped optical fiber, with a first and second input and an output, - have a reference source having a reference spectral line having a central wavelength called the reference wavelength of known value, the reference wavelength being chosen so as to be detected on at least one pixel of the detector, - position the optical fiber so that the first input collects a light signal from the sample, the second input collects a light signal from the reference source, the output is coupled to an input of the spectrometer, - simultaneously detecting in time said sample signal and said reference signal, so as to generate a measured profile of interest and a measured reference profile, - processing said measured profiles of interest and reference so as to determine a position of interest and a reference position, respectively of the central wavelength of interest and the reference wavelength, - determining a value of the central wavelength of interest from a difference between said positions of interest and reference, from said known value of the reference wavelength and from a linear dispersion of the spectrometer and the associated detector.

[0036] According to a second variant, the method for determining a central wavelength of interest of a spectral line of interest measured by a spectrometer comprises the steps of: - have a Y-shaped optical fiber, with a first and a second entrance and one exit, - have a reference source having a reference spectral line having a central wavelength called the reference wavelength of known value, the reference wavelength being chosen so as to be detected on at least one pixel of the detector, - position the optical fiber so that the first input collects a light signal from the sample, the second input collects a light signal from the reference source, the output is coupled to an input of the spectrometer, - sequentially detecting in time said sample signal for a signal duration and said reference signal for a reference duration, so as to generate a measured profile of interest and a measured reference profile, the signal duration and the reference duration being separated by a so-called intermediate duration, - processing said measured profiles of interest and reference so as to determine a position of interest and a reference position, respectively of the central wavelength of interest and the reference wavelength, - determining a value of the central wavelength of interest from a difference between said positions of interest and reference, from said known value of the reference wavelength and from a linear dispersion of the spectrometer and the associated detector.

[0037] According to one embodiment, the intermediate duration is less than 5s.

[0038] According to one embodiment: - in the sequential detection step, an additional detection of the so-called additional reference signal is carried out so that the detection of the sample signal at an instant t0 is temporally framed by the detection of the reference signals, and generates an additional reference measured profile, - in the step of processing the measured profiles, an additional reference position is also determined, and a so-called intermediate reference position is determined at time t0 by interpolation, from the reference and additional reference positions, and from a predetermined law of variation of the reference position as a function of time, - the step of determining the central wavelength of interest then being carried out from a difference between said positions of interest and intermediate reference.

[0039] According to one embodiment, the processing step comprises the sub-step of adjusting values ​​of the measured profiles of interest and reference with known mathematical functions so as to determine by interpolation said positions of interest and reference with a precision below the pixel.

[0040] According to one embodiment, the light signal from the sample is pulsed.

[0041] According to one embodiment, the light signal from the sample comes from an emission of a plasma emitted by the sample illuminated by a pulsed laser.

[0042] According to one embodiment, the method according to the invention is suitable for determining an isotopic abundance of an element present in said sample, said central wavelength of interest corresponding to a line resulting from the contributions of two isotopes of said element, said value of the central wavelength of interest making it possible to determine said abundance.

[0043] The following description presents several exemplary embodiments of the device of the invention: these examples are not limiting of the scope of the invention. These exemplary embodiments present both the essential characteristics of the invention as well as additional characteristics linked to the embodiments considered.

[0044] The invention will be better understood and other characteristics, aims and advantages thereof will appear during the detailed description which follows and with reference to the appended drawings given as non-limiting examples and in which:

[0045] The [Fig.l] already cited illustrates the principle of measurement by LIBS, LAMIS and LIBRIS technologies.

[0046] The [Fig.2] already cited illustrates a spectral line measured in a case where the element is in low concentration in the plasma, the self-absorption phenomenon is then not very marked or even negligible.

[0047] The [Fig.3] already cited illustrates a spectral line measured in a case where the element is in high concentration in the plasma, the self-absorption phenomenon is then marked.

[0048] The [Fig.4] already cited illustrates the evolution of the central wavelength X0 measured as a function of the isotopic abundance of the isotope 6Li of Lithium in the sample.

[0049] [Fig.5] illustrates a system for measuring a central wavelength of interest according to the invention.

[0050] [Fig.6] illustrates the measured profile of interest and the measured reference profile.

[0051] [Fig.7] illustrates a system according to the invention within the framework of LIBRIS, that is, it is suitable for measuring an isotopic ratio of an element present in the sample.

[0052] [Fig.8] illustrates the method of determining a central wavelength of interest according to the invention.

[0053] [Fig.9] illustrates the theoretical reference and interest profiles which best fit with experimental points respectively of the measured reference and interest profiles.

[0054] [Fig. 10] illustrates the data obtained by repeating the measurement 18 times (measurements no. i numbered from 1 to 18): for each measurement i, a raw value (cross) is determined on the one hand, and a corrected value (points) determined according to the method according to the invention on the other hand.

[0055] [Fig. 11] shows the mean and standard deviation of these 18 measurements in both cases, raw and corrected. DETAILED DESCRIPTION OF THE INVENTION

[0056] The invention relates to a system 10 for measuring a central wavelength of interest Xc of a spectral line of interest RSe measured by a spectrometer illustrated [Fig.5]. The invention also relates to a method 100 for measuring the central wavelength.

[0057] The invention can be applied to LIBS or to another spectroscopic technique, whatever the pressure, as long as it is necessary to measure a wavelength precisely. In LIBS / LAMIS this is the case for physical applications in which it is of interest to determine the wavelength precisely, for example when it is a question of measuring the spectral shift of a line due to the Stark effect or the Doppler effect. The invention also applies to LIBRIS for which it is particularly suitable.

[0058] The spectral line of interest corresponds to an emission or an absorption of a sample Ech to be characterized and presents either a bell-shaped profile, Xc then corresponding to the wavelength of the peak of the bell-shaped profile, or a double-bell profile, Xc then corresponding to the wavelength of the trough between the two bells.

[0059] The system 10 according to the invention comprises a detection system comprising a spectrometer Spectro being associated with (or comprising) a detector Det, this detector comprising a plurality of pixels Pi aligned in a direction X. The spectral line of interest RSe is detected on pixels of the detector.

[0060] The system 10 also comprises a Y-shaped FOY optical fiber, having a first input E1 and a second input E2 and an output S.

[0061] The optical fiber FOY is positioned and the system 10 is configured so that the first input E1 of the fiber collects the light signal SLech from the sample, called the sample signal, and the second input E2 of the fiber collects the light signal SLref from a reference source, called the reference signal. The reference source Sref has a reference spectral line RSref having a central wavelength called the reference wavelength of known value Xref, the reference wavelength being chosen so as to be detected on at least one pixel of the detector. The lines RSe and RSref are thus detected on the detector for the same setting (same configuration) of the spectrometer. The reference source is chosen according to the spectral characteristics of the sample to be analyzed.

[0062] In addition, the output S of the fiber is coupled to an input of the spectrometer.

[0063] Thanks to the Y fiber, the spectrometer and the detector potentially have both SLech and SLref signals simultaneously.

[0064] The system 10 according to the invention is configured so that the detector Det detects simultaneously (first variant) or sequentially (second variant) in time the sample signal SLech and the reference signal SLref.

[0065] This detection generates a measured profile of interest PSech and a measured reference profile PSref as illustrated [Fig.6]. The abscissa of the profiles is the index i of the pixels Pi of the detector and the ordinate is an intensity li detected for each pixel.

[0066] The system finally comprises a processing unit UT configured to process the measured profiles of interest and reference so as to determine a position of interest Pech of the central wavelength of interest Xc and a reference position Pref of the reference wavelength Xref, measured in pixels of the detector.

[0067] From the difference of the Pech-Pref positions, the value of Xref known precisely and DL, linear dispersion of the detection system [spectrometer + detector], the value of X0 is determined, typically with the formula (1). It is of course appropriate to take the value of DL corresponding to the spectral region in which Xref and X0 are located.

[0068] Due to the instantaneous or quasi-instantaneous detection of the two spectra PSech and PSref, the aforementioned wavelength drift is thus also rendered negligible and very good sub-picometric precision on the value of X0 is obtained.

[0069] According to the first variant, the acquisition of the two measured profiles (spectrum of interest and reference spectrum) is carried out simultaneously. For this, it is appropriate for the reference source to have a reference wavelength located outside the spectral line of interest. If this is not the case, the reference signal may disturb the sample signal.

[0070] In the particular case of LIBRIS, the plasma signal is intense and of short duration. Preferably, the reference source should be sufficiently intense so that the optimized acquisition parameters of the detector for the detection of each spectrum are identical.

[0071] According to the second variant, the acquisition of the two profiles is carried out sequentially in time. The detector Det detects said sample signal during a signal duration Ds and said reference signal during a reference duration Dref. The signal duration and the reference duration are separated by a so-called intermediate duration Dint which is sought to be minimized.

[0072] We call ST the time sequence defined by [SLech signal during Ds / Dint / SLref signal during Dref] illustrated [Fig.5]. Of course, we can also reverse the SLech and SLref signals in the sequence.

[0073] The exposure times Ds and Dref are adjusted as a function of SLech and Slref respectively so that the signal-to-noise ratio is sufficient for good detection of each signal.

[0074] In this second variant the reference wavelength may be identical to the central wavelength of interest, which is for example the case of a lithium hollow cathode lamp for the LIBRIS analysis of lithium.

[0075] The SLref and SLech signals generally have very different intensity profiles as a function of time. Sref, typically a hollow cathode lamp, emits few photons continuously. According to a preferred embodiment, SLech is a pulsed, intense and short-duration signal (large quantity of photons over a very short time, for example the plasma signal).

[0076] Due to this difference in intensity profile, in practice a single acquisition for the simultaneous detection by Det of the two spectral profiles is not possible, because the detection of each type of profile requires a different detection configuration. Thus for the typical case in which the reference source emits weakly but continuously, while the plasma emits intensely for a short time, the acquisition parameters optimized for the detection of each of the spectra cannot be identical, and sequential detection is required.

[0077] Preferably Ds <Dref.

[0078] Thus, the system according to the invention is configured so that the detector successively detects the two spectra with different acquisition parameters.

[0079] These parameters are (non-exhaustive list): measurement delay relative to the laser shot (for the sample signal only), width of the acquisition time gate, number and rate of accumulations, detector gain, signal averaging.

[0080] These parameters are for example the following: Sample Reference Measurement delay relative to laser shot 1 ps Not applicable Acquisition time gate width 500 ns 200 ms Number and rate of accumulations 20 at 20 Hz 10 at 3 Hz Detector gain 3000 3000 Signal averaged over... 10 acquisitions 1 acquisition

[0081] Table II

[0082] Preferably, the detector is of the intensified CCD type.

[0083] The difference between the two measurements, corresponding to Dint, can be made negligible compared to the risk of wavelength drift of the detection system, preferably Dint < 5s, or even Dint < 1s.

[0084] According to an illustrated embodiment [Fig.7], the system 10 according to the invention is associated with the implementation of the LIBRIS technology, that is to say that it is adapted for the measurement of an isotopic ratio of an element present in the sample Ech. The light signal SLech comes from an emission of a plasma PI emitted by the sample Ech, illuminated by a pulsed laser L. The system 10 according to the invention then comprises, in addition to the laser L, an optic 2 which focuses the laser beam on the sample and an optical system SO configured to inject a part of the light signal from the sample into the first input El of the optical fiber.

[0085] The processing unit UT is further configured to synchronize the detector Det with the laser L. The central wavelength of interest corresponds to a line resulting from the contributions of two isotopes of the element, and the value of the central wavelength of interest makes it possible to determine the isotopic abundance as explained above.

[0086] The detection time of SLech is determined as a function of the laser pulse duration and the laser frequency f. Typically Ds is the order of ps.

[0087] According to a first variant, the method 100 for determining the central wavelength of interest Xc of a spectral line of interest RSe measured by a spectrometer comprises the following steps.

[0088] A Y-shaped OFY optical fiber is provided, having a first input E1, a second input E2 and an output S, and a reference source Sref is provided having a reference spectral line RSref having a central wavelength called the reference wavelength of known value Xref, the reference wavelength being chosen so as to be detected on at least one pixel of the detector.

[0089] The OFY optical fiber is then positioned so that the first input collects a light signal from the sample, the second input collects a light signal from the reference source, the output is coupled to an Espec input of the spectrometer.

[0090] Then, the sample signal SLech and the reference signal SLref are simultaneously detected, so as to generate a measured profile of interest PSech and a measured reference profile PSref.

[0091] Then, the measured profiles of interest and reference are processed so as to determine a position of interest Pech and a reference position Pref, measured in pixels of the detector, respectively of the central wavelength of interest and the reference wavelength.

[0092] Finally, a value of the central wavelength of interest is determined from a difference between the positions of interest and reference, from the known value of the reference wavelength and from the linear dispersion DL of the spectrometer + detector detection system.

[0093] According to a second variant illustrated [Fig.8], in the method 200 for determining the central wavelength of interest Xc, the sample signal SLech is detected sequentially in time during the signal duration Ds and the reference signal SLref during the reference duration Dref, and no longer simultaneously the two signals. The signal duration and the reference duration are separated by a predetermined intermediate duration Dint. The other steps are identical.

[0094] According to one embodiment of the method 200, the time sequence is obtained with shutters 01 and 02 arranged in front of the inputs E1 and E2 and programmed to adjust the durations Ds, Dref and Dint. When SLech is a pulse signal of frequency f, the shutter 01 is synchronized with the frequency of the signal SLref to allow the desired number of pulses to pass and the shutter 02 is configured to transmit the signal SLech for a sufficient duration Dech, before or after the activation of the shutter 01, with a time Dint between the activation of the two shutters reduced to a minimum and limited by the speed of the electronics of the detection system.

[0095] According to one embodiment, the light signal from the SLech sample is pulsed. Preferably, the sample comes from an emission of a plasma emitted by the sample illuminated by a pulsed laser.

[0096] According to an embodiment suitable for a pulsed SLech signal originating from a plasma, to obtain the time sequence, in a calibration pre-step the brightness of Sref is adjusted so that the intensity of SLref detected during the time Ds is negligible, i.e. not detectable compared to the noise.

[0097] To implement the time sequence ST, it is arranged so that the signal SLech corresponds to the last shot of the laser L and the detector is synchronized with the last shot. The duration Ds is of the order of ps or tens of ps and limited by the frequency f of the laser. Then after a duration Dint which can be very short, the detector is configured to detect SLref during a time window Dref typically of the order of s or a few s. In this embodiment there is no need for shutters, the signal SLref is always present on the detector even during the detection of SLech, SLref being negligible over the time window Ds and it is the detector Det which is programmed in this case according to the time sequence. The value of Dint is also limited by the speed of the electronics of the detection system.

[0098] To be able to measure Xc with very high precision, we seek to obtain its position Pech with a precision better than the pixel of the detector. For this, according to one embodiment, the processing step comprises the sub-step consisting of adjusting the values ​​of the measured reference profiles PSref and of interest PSech with known mathematical functions so as to determine by interpolation the positions of interest and reference with a precision lower than the pixel. Thus, we determines theoretical profiles respectively of reference PSTref and of interest PSTech which fit best with the experimental points, as illustrated [Fig.9]. Typically the mathematical functions used are chosen from: Gaussian, Lorentzian, Voigt.

[0099] Thanks to these profiles the Pref and Pech positions are determined in fractions of pixels (typically with precision to the second decimal place).

[0100] According to an embodiment, allowing an even more precise measurement, the sequential detection step further comprises an additional detection of the reference signal called additional Sref / add so that the detection of the sample signal at an instant t0 is temporally framed by the detection of the reference signal Sref for example at ti<t0, et du signal de référence additionnel Sref / add à t2> t0.

[0101] The ST sequence is then for example: signal SLref during Dref / Dint / signal SLech during Ds / Dint / signal SLref / add during Dref.

[0102] In this detection step, an additional reference measured profile PSref / add corresponding to the signal SLref / add is also generated.

[0103] In the step of processing the measured profiles, an additional reference position Pref / add is also determined. Of course, adjustment with a theoretical curve can be applied for the determination of Pref / add. Due to the drift of the spectrometer, the positions Pref and Pref / add are slightly different.

[0104] In the processing step, the so-called intermediate reference position POref is also determined at time t0 by interpolation, from the reference positions Pref and additional reference Pref / add, and from a law of variation of the reference position as a function of predetermined time, typically considered to be linear. Thanks to this interpolation, the drift of the spectrometer is corrected between times ti and t0 or between times t0 and t2. The position POref is thus more precise than Pref obtained by a single detection of the spectrum of the reference source at a time before or after L. This amounts to returning to a case of quasi-instantaneity of the detection of the reference and the sample.

[0105] In this particular embodiment, the step of determining the central wavelength of interest is then carried out from the difference Pech - POref.

[0106] In the linear case we have:

[0107] Pref (^o) = POref = ^t0 + b (2)

[0108] The parameters a and b are determined from the measurements at times ti and t2:

[0109] _ PrAhfP^OO _ P^rPr.f (3) a “ hh “ hh

[0110] b = Pref(t2) -at2(4)

[0111] Below are briefly presented results illustrating the interest of the proposed correction method. A Jobin Yvon THR1000 spectrometer equipped with a 2400 lines / mm grating centered at 670 nm. The detector is a 2048x512 pixel Andor iStar intensified camera, with a linear dispersion DL of 2.774 pm / pixel at 670 nm.

[0112] A line from a mercury vapor lamp is measured in the presence of spectrometer drift; the line from the mercury vapor source constitutes the spectral line of interest.

[0113] Then (or before), we measure the line of the reference source constituted by a lithium hollow cathode lamp (HCL), which is known precisely, equal to Xref = 670.776 nm.

[0114] The drift of the spectrometer is then corrected according to method 100 according to the invention (simultaneous detection). Indeed, in this case the lines of the two sources are sufficiently separated in wavelength and of comparable intensity so that simultaneous measurement is possible.

[0115] The acquisition parameters are given in Table III below. Acquisition time gate width 50 ms Number and rate of accumulations 100 to 18 Hz Detector gain 4000

[0116] Table III

[0117] The graph in [Fig.10] illustrates the data obtained by repeating the measurement 18 times (measurements i numbered from 1 to 18). For each measurement i, a raw value XcB(i) (cross) is determined on the one hand and a corrected value Xc(i) (points) determined according to method 100 according to the invention on the other hand. The raw values ​​are obtained by direct measurement with the detection system. The dispersion of the raw data is obvious and results from the drift of the spectrometer. The corrected values ​​of Xc are very little dispersed over the 18 measurements.

[0118] [Fig. 11] shows the mean and standard deviation o of these 18 measurements in both cases, raw and corrected with Xref of the HCL lamp, respectively (XBm, oB) and (Xcm, oj. The "true" value, moreover known very precisely, of the wavelength of the mercury vapor lamp is Xivm = 671.643 nm. This value is also mentioned in [Fig. 11] and makes it possible to test the relevance of the method according to the invention. The value Xcm is much closer to Xivm than the value XBm. It emerges that the measurement method according to the invention significantly improves the accuracy and fidelity of the measured wavelength.

Claims

Claims

1. System (10) for measuring a central wavelength of interest (Xc) of a spectral line of interest (RSe) measured by a spectrometer, the spectral line of interest corresponding to an emission or an absorption of a sample (Ech) to be characterized, the spectral line of interest having either a bell-shaped profile, said central wavelength of interest then corresponding to the peak of said bell-shaped profile, or a double-bell profile, said central wavelength of interest then corresponding to the trough between the two bells, the system comprising: - a pulsed laser (L) configured to illuminate the sample so as to generate a plasma (PI), - a detection system comprising a spectrometer (Spectro) associated with a detector (Det) comprising a plurality of pixels (Pi) aligned in a direction X, the spectral line of interest being detected on pixels of the detector, - a Y-shaped optical fiber (FOY), having a first (El) and a second (E2) input and an output (S), the optical fiber being configured so that: • the first input collects a light signal (SLech) from the sample, called the sample signal, said sample signal being pulsed and coming from said plasma emitted by the sample, • the second input collects a light signal (SLref) from a reference source (Sref), called the reference signal, the reference source having a reference spectral line (RSref) having a central wavelength called the reference wavelength of known value (Xref), the reference wavelength being chosen so as to be detected on at least one pixel of the detector, • the output is coupled to an input of the spectrometer, the measurement system being configured so that the detector detects said sample signal and said reference signal simultaneously or sequentially in time, so as to generate a measured profile of interest (PSech) and a measured reference profile (PSref), the measuring system further comprising a processing unit (UT) configured to: • process said measured profiles of interest and reference so as to determine a position of interest (Pech) and a reference position (Pref), measured in pixels of the detector, respectively of the central wavelength of interest and the reference wavelength, • determine a value of the central wavelength of interest from a difference between said positions of interest and reference, from said known value of the reference wavelength and from a linear dispersion (DL) of the detection system.

2. A measurement system according to claim 1 wherein the measurement system is configured so that the detector detects said sample signal and said reference signal simultaneously, the reference source having a reference wavelength located outside the spectral line of interest.

3. Measuring system according to claim 1 wherein the measuring system is configured so that the detector sequentially detects in time said sample signal during a signal duration (Ds) and said reference signal during a reference duration (Dref), the signal duration and the reference duration being separated by a so-called intermediate duration (Dint).

4. Measuring system according to one of the preceding claims suitable for measuring an isotopic abundance of an element present in the sample further comprising: - an optical system (SO) configured to inject a portion of said light signal from the sample into the first input of the optical fiber, the processing unit being configured to synchronize the detector with the laser, said central wavelength of interest corresponding to a line resulting from the contributions of two isotopes of said element, said value of the central wavelength of interest making it possible to de-

5. end said abundance. Method (100) for determining a central wavelength of interest (Xc) of a spectral line of interest (RSe) measured by a spectrometer, the spectral line of interest corresponding to an emission or an absorption of a sample (Ech) to be characterized, the spectral line of interest having either a bell-shaped profile, said central wavelength of interest then corresponding to the peak of said bell-shaped profile, or a double-bell profile, said central wavelength of interest then corresponding to the trough between the two bells, the spectrometer being associated with a detector (Det) comprising a plurality of pixels (Pi) aligned in a direction X, the spectral line of interest being detected on pixels of the detector, the method comprising the steps of: - have an optical fiber (OFY) in Y, having a first (El) and a second (E2) input and an output (S), - have a reference source (Sref) having a reference spectral line (RSref) having a central wavelength called the reference wavelength of known value (Xref), the reference wavelength being chosen so as to be detected on at least one pixel of the detector, - positioning the optical fiber so that the first input collects a light signal from the sample, called the sample signal, said sample signal being pulsed and coming from an emission of a plasma emitted by the sample illuminated by a pulsed laser, so that the second input collects a light signal from the reference source, called the reference signal, and so that the output is coupled to an input (Espec) of the spectrometer, - simultaneously detecting in time said sample signal and said reference signal, so as to generate a measured profile of interest (PSech) and a measured reference profile (PSref), - process said measured profiles of interest and reference so as to determine a position of interest (Pech) and a reference position (Pref), respectively of the central wavelength of interest and the reference wavelength, - determine a value of the central wavelength of interest from a difference between said positions of interest and reference, from said known value of the reference wavelength and from a linear dispersion (DL) of the spectrometer and the associated detector.

6. Method (200) for determining a central wavelength of interest (Xc) of a spectral line of interest (RSe) measured by a spectrometer, the spectral line of interest corresponding to an emission or an absorption of a sample (Ech) to be characterized, the spectral line of interest having either a bell-shaped profile, said central wavelength of interest then corresponding to the top of said bell-shaped profile, or a double-bell profile, said central wavelength of interest then corresponding to the hollow between the two bells, the spectrometer being associated with a detector (Det) comprising a plurality of pixels (Pi) aligned in a direction X, the spectral line of interest being detected on pixels of the detector, the method comprising the steps of: - having an optical fiber (OFY) in Y, having a first (El) and a second (E2) input and an output (S),- have a reference source (Sref) having a reference spectral line (RSref) having a central wavelength called the reference wavelength of known value (Xref), the reference wavelength being chosen so as to be detected on at least one pixel of the detector, - position the optical fiber so that the first input collects a light signal from the sample, called the sample signal, said sample signal being pulsed and coming from an emission of a plasma emitted by the sample illuminated by a pulsed laser, so that the second input collects a light signal from the reference source, called the reference signal, and so that the output is coupled to an input (Espec) of the spectrometer, - sequentially detect in time said sample signal for a signal duration (Ds) and said reference signal for a reference duration (Dref),so as to generate a measured profile of interest (PSech) and a measured reference profile (PSref), the signal duration and the reference duration being separated by a so-called intermediate duration (Dint), processing said measured profiles of interest and reference so as to determine a position of interest (Pech) and a reference position (Pref), respectively of the central wavelength of interest and the reference wavelength, determining a value of the central wavelength of interest from a difference between said positions of interest and reference, from said known value of the reference wavelength and from a linear dispersion (DL) of the spectrometer and the associated detector.

7. Method according to the preceding claim in which the intermediate duration is less than 5s.

8. Method according to one of claims 6 or 7 in which: - in the sequential detection step, an additional detection of the so-called additional reference signal (Sref / add) is carried out so that the detection of the sample signal at an instant t0 is temporally framed by the detection of the reference signals, and generates an additional reference measured profile (PSref / add), - in the step of processing the measured profiles, an additional reference position (Pref / add) is further determined, and a so-called intermediate reference position (POref) is determined at the instant t0 by interpolation, from the reference positions (Pref) and additional reference positions (Pref / add), and from a predetermined law of variation of the reference position as a function of time,- the step of determining the central wavelength of interest then being carried out from a difference between said positions of interest (Pech) and intermediate reference (POref).,

9. Method according to one of claims 5 to 8 in which the processing step comprises the sub-step of adjusting values of the measured profiles of interest and reference with mathematical functions- known techniques so as to determine by interpolation said positions of interest and reference with sub-pixel precision.

10. Method according to one of claims 5 to 9 adapted to determine an isotopic abundance of an element present in said sample, said central wavelength of interest corresponding to a line resulting from the contributions of two isotopes of said element, said value of the central wavelength of interest making it possible to determine said abundance.

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