System for measuring central wavelength of spectral line with high precision and related method

Abstract

Disclosed is a system (10) for measuring a central wavelength of interest ([lambda] c) of a spectral line of interest (RSe) measured by a spectrometer, the system comprising:-a detection system comprising a spectrometer (Spectro) associated with a detector (Det),-a Y-shaped optical fiber (FOY) configured such that: a first input collects an optical signal (SLeck) generated by a sample, and a second input collects an optical signal (SLeck) generated by the sample; the invention relates to a measurement system comprising a first input, referred to as a sample signal, a second input collecting an optical signal (SLref) generated by a reference source (Sref), referred to as a reference signal, an output coupled to an input of a spectrometer, the measurement system being configured such that the detector simultaneously or sequentially detects said sample signal and said reference signal in time, the measurement system further comprising a processing unit (UT), the processing unit is configured to: process the measured profile of interest and a measured reference profile, determine a value of a central wavelength of interest based on a difference between the position of interest and a reference position, the known value of a reference wavelength and a linear dispersion (DL) of the detection system.

Description

Technical Field

[0001] The present invention relates to the field of spectroscopy and more particularly to determining the central wavelength of spectral lines with very high accuracy. Background Art

[0002] In certain applications in spectroscopy, such as atomic or molecular spectroscopy, or for determining the isotopic abundance of elements in a sample by an optical method (called LIBRIS, Laser Induced Breakdown Self-Reversal Isotope Spectroscopy, see below), it is necessary to determine with high accuracy the value of the central wavelength of a spectral line. The spectral lines to be characterized are generated by a light source and can be atomic or molecular, absorption or emission lines. Typically, an uncertainty in the value of the central wavelength of less than 5 pm or even less than 1 pm is sought.

[0003] This problem does not arise in fields such as laser induced breakdown spectroscopy (LIBS) or laser ablation molecular isotope spectroscopy (LAMIS), because the width of the observed lines is typically tens of pm. The wavelength of the line is therefore usually measured with an uncertainty of about ten pm to tens of pm, depending on the linear dispersion of the spectrometer used. This uncertainty has no impact on these techniques, because the analysis performed is based on the intensity of the line, which is usually integrated over a width of the same order of magnitude (i.e. about ten pm to tens of pm).

[0004] Typically, the detection system is calibrated with respect to wavelength by a reference source emitting a known line, 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 the pixels on the detector, and the line to be analyzed is determined according to its relative position with respect to the position of the reference line. The detector comprises at least N pixels Pi arranged in a row, where i varies from 1 to N. In 2D, the integration over all pixels of a given column is performed. For example, the detector is a CCD matrix array with 2048x512 pixels.

[0005] Let λref be the central wavelength of the reference line and λ0 be the wavelength to be determined, Pref be the position of λref in the detector pixel and P0 be the position of λ0 on the same detector. For a given spectrometer configuration, the wavelength λref is of course chosen so that it appears on the detector simultaneously with λ0. Then the following holds:

[0006] λ0=λ ref +(P0-P ref ).DL (1)

[0007] where DL is the linear dispersion of the detection system, usually expressed 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, resulting in wavelength drift. In the case of analysis outside the laboratory (on-site, at-line, portable systems, etc.), this drift is of course even more pronounced. As an example, in the case of a grating spectrometer with a focal length of 1 m and a grating of 2400 lines / mm, only 10 of the grating angle will be -3 The change in the wavelength results in a wavelength shift of 10 pm, which is unacceptable for example for LIBRIS analysis of lithium, where in order to obtain 6 The acceptable uncertainty in the isotopic abundance of Li is targeted to be less than 1 pm.

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

[0010] Therefore, it is not possible to make accurate LIBRIS measurements without correcting the detection system for wavelength drift. This problem arises in the same way as for atomic spectroscopy, where λ0 should be measured accurately by calibration with a reference source.

[0011] Since the invention is particularly advantageous in the context of the LIBRIS method, its principles are reviewed below together with those of the LIBS and LAMIS methods.

[0012] like Figure 1 As shown, the principle of LIBS technology is to focus a laser pulse on the surface of a sample of a material (or the material itself) to generate a transient plasma, and analyze the light emission of the transient plasma by a spectrometer. By collecting the light emission of the plasma and analyzing its spectrum by spectroscopy, it is possible to identify the elements present in the plasma, and thus determine the composition of the material using a database of emission lines. In LIBS, the intensity is integrated over the entire width of the line.

[0013] The LAMIS technique, for example described in the publication by R. Russo et al. (Spectrochim. ACTA B 66 (2011) 99), is an alternative derived from LIBS that allows isotopic analysis based on lines of molecules formed by reactions between the ablated material and components of the surrounding medium or by reactions between two atoms of the ablated material.

[0014] A laser generator L0 generates a laser beam FL0 focused on a sample 1 by a first optical system 2. This generates a plasma PlO. The plasma emits a light emission 3 which is collected by an optical system OS0. The focused light emission is sent via an optical fiber FO to a spectrometer Spec0. The spectrometer Spec0 comprises a detector Det0 synchronized with the laser generator L0 (or associated therewith). The spectrometer Spec0 allows recording a line spectrum. Finally, a processing device UT0 allows processing the recorded spectrum.

[0015] LIBS allows the generation of a spectrum 20 in the form of a set of spectral lines corresponding to the emission lines of the constituent elements of the material and the determination of the elemental composition of the material sample using the available data relating the emission lines to the elements. The wavelength λ of the line provides information about one element present in the material and its intensity I is related to the concentration of this element.

[0016] LIBS emission spectroscopy is also suitable for isotope analysis because the atomic lines of various isotopes of the same element have slightly different wavelengths. This spectral shift, called isotope shift, is due to mass effects (mainly in the case of light elements) and changes in the charge distribution inside the nucleus (mainly in the case of heavy elements). If isotope analysis is desired by LIBS, the lines of the two isotopes need to be separated. However, this spectral shift is usually on the order of fractions of nm or even a few pm, as shown in Table 1 below:

[0017] isotope Emission line Isotope shift 7Li→6Li 670.775nm +15.8pm 10B→11B 208.891nm -2.5pm 238U→235U 424.437nm +25pm 239Pu→240Pu 594.522nm +13pm

[0018] Table I

[0019] This shift is difficult to observe under normal conditions in plasmas produced by laser ablation, because of the high density caused by the confinement of the plasma by the ambient air at atmospheric pressure, and thus the broadening of the emission lines due to the Stark effect. This broadening typically reaches tens or even hundreds of pm, thus masking the isotopic shift, even if the spectrometer used has a spectral resolution sufficient to resolve this shift. The limitation is here physical and not instrumental in nature.

[0020] A first solution consists in carrying out the analysis at low pressure or even in vacuum. Thus, by limiting the confinement of the plasma by the ambient medium, its density is reduced and a spectral selectivity sufficient for certain isotopes can be achieved. A double line is observed and the isotope ratio is determined from the intensity ratio between the two lines associated with the two isotopes. This method is not applicable to all isotopes and requires a spectrometer with high resolving power and therefore large size. A second solution consists in sending a second laser beam through the plasma in order to measure the resonance or fluorescence absorption signal, which limits and complicates the measurement system.

[0021] In the technique of isotope analysis at atmospheric pressure, the LAMIS technique can also be used, but this requires that many conditions be met: 1. It is necessary to form molecules in the plasma; 2. The formed molecules must be sufficiently stable under the temperature / density conditions of the plasma; and 3. The formed molecules must have detectable lines, i.e. lines with sufficient lifetime, sufficient intensity and located in the spectral band of the detection system. In the case of lithium, for example, no LAMIS signal was detected, probably because the second condition was not met.

[0022] The LIBIS technique is an optical technique that allows the isotopic abundance of elements in a (solid, liquid or gas) sample to be determined based on the emission spectrum of a laser ablation plasma. The technique is described, for example, in the publications of K. Touchet et al. (Spectrochim. ACTA B 168 (2020) 105868) and document US2019 / 0041336. It is a variation of the LIBS technique and uses the same optical system. The LIBIS technique overcomes the various shortcomings of the LIBS method by allowing the measurement of isotope ratios at atmospheric pressure and without a second laser.

[0023] It should be remembered that the electrons of the atoms must be supplied with energy to transition to a higher energy level. This energy can take the form of photons, in which case the atoms absorb the photons. A specific case is that of laser ablation of plasmas. To simplify matters, the plasma can be considered to consist of two separate parts, the core and the periphery. Photons emitted by the hotter plasma core can be absorbed by the cooler periphery. This effect therefore prevents a certain number of emitted photons from leaving the plasma: it is called self-absorption.

[0024] For an observer outside the plasma, and for the measuring device, the line profile results from emission and self-absorption of the same wavelength, corresponding to the transitions between the two electronic levels of all the atoms in question placed on her, his or its line of sight. The measured intensity is therefore not simply the sum of all the emissions from the plasma, since this self-absorption must be taken into account.

[0025] The well-known self-absorption effect in plasma spectroscopy for elemental analysis is considered an undesirable effect because it leads to a distortion of the line profile and thus to a non-linearity of the signal with respect to the concentration of the element of interest. LIBIS exploits this self-absorption effect to infer from it information about the isotopes of a given element in a material.

[0026] Figure 2 and Figure 3 Shown are lines RS0 of an element of interest selected from the spectrum 20 obtained in two specific cases, depending on the concentration of the element in the material.

[0027] Figure 2 The case of a low concentration of the element in the plasma is shown, where the effects of self-absorption are rarely marked or even absent. A spectral broad line profile is obtained without a trough at its center. The dotted curve ISO1 and the dashed curve ISO2 represent the emission of 2 isotopes. The width of each line is wide relative to the interval between the two lines, mainly because of the Stark effect in the plasma, so they cannot be distinguished individually: the solid line RS0 corresponding to the sum of the two lines is detected. The principle of LIBRIS is that the central wavelength of the solid line varies with the isotopic abundance (i.e. the amplitude ratio of the 2 dotted lines and the dashed line). In this case, the value of the central wavelength λ0 is measured, which corresponds to the emission peak, i.e. to the maximum point or vertex 20 of the observed curve (which has a bell-shaped profile). It is related to the ratio between the two isotopes Ioso1 and Iso2 of the element in question, and it is shifted according to the isotopic ratio.

[0028] Figure 3 The case where the concentration of the element in the plasma is high is shown, where the effect of self-absorption is marked. A line profile with a trough in its center (double bell-shaped profile) is observed, which is called an inverse line and results from the superposition of a spectrally broad emission profile and a spectrally narrower absorption profile. In this case, the value of the central wavelength λ0 corresponding to the absorption trough is measured. In this case, the central wavelength λ0 is measured on a portion of the profile corresponding to the absorption, i.e. at the lowest point 30 of the observed trough. It is related to the ratio between the two isotopes Ioso1 and Iso2 of the element in question, and it is shifted according to the isotope ratio. It is this measurement of the trough wavelength that defines the LIBRIS technique.

[0029] Therefore, in the LIBRIS technique, the isotope ratio is measured based on a very precise measurement of the wavelength λ0 of the maximum of the bell-shaped line or the minimum of the so-called inverse double bell-shaped line. This wavelength λ0 increases with the isotope abundance in λ R 1 and λ R 2 , where indices 1 and 2 refer to the two isotopes of an element. R 1 and λ R 2 is the physical data available in spectral databases and / or scientific publications. Therefore, the analytical uncertainty of the isotopic abundance is directly related to the uncertainty of the wavelength λ0.

[0030] In the LIBRIS technique, the measurement of λ0 directly yields the isotope ratio. Figure 4 is shown as having only two isotopes 6 Li and 7 Lithium isotopes 6The measured variation of λ0 as a function of the proportion of Li. The curve was generated using the reverse curve. The isotope shift is determined by λ R 1 -λ R 2 is given by , and corresponds to the technical measurement range 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, thus corresponding to 6 The total change in isotopic abundance of Li from 0% to 100% is complemented by 7 The abundance of Li. Therefore, the wavelength λ R An uncertainty of 1 pm in λ results in an uncertainty in the isotopic abundance of 1 / 15.8 = 6.3%. Therefore, the measurement accuracy of the isotope ratio is directly related to the measurement accuracy of λ0.

[0031] It is an object of the present invention to remedy the above-mentioned drawbacks by proposing a method and a system for determining with sub-picometer accuracy the central wavelength of an atomic or molecular, absorption or emission line generated by a light source. Summary of the invention

[0032] A subject of the invention is 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 the emission or absorption of a sample to be characterized, the optical signal generated by the sample being called sample signal, the spectral line of interest having: a bell-shaped profile, the central wavelength of interest corresponding to the apex of the bell-shaped profile; or a double bell-shaped profile, the central wavelength of interest corresponding to the trough between the two bells, the system comprising:

[0033] a detection system comprising a spectrometer associated with a detector, the detector comprising a plurality of pixels aligned in the X direction, the spectral line of interest being detected at the detector pixels,

[0034] a reference source emitting an optical signal called reference signal, said reference source having a reference spectral line with a central wavelength of known value called reference wavelength, said reference wavelength being selected so as to be detected on at least one pixel of said detector,

[0035] - a Y-shaped optical fiber having a first inlet, a second inlet and an outlet, the optical fiber being configured such that:

[0036] ● The first inlet collects the sample signal,

[0037] ● The second entry collects the reference signal,

[0038] ● The outlet is coupled to an input of the spectrometer, the measurement system being configured such that the detector detects the sample signal and the reference signal simultaneously or sequentially in time to generate a measured profile of interest and a measured reference profile,

[0039] The measurement system further comprises a processing unit, wherein the processing unit is configured to:

[0040] ● processing the measured profile of interest and the measured reference profile to determine positions of interest and reference positions measured in units of detector pixels for the central wavelength of interest and the reference wavelength, respectively,

[0041] ● The value of the central wavelength of interest is determined based on the difference between the position of interest and a reference position, the known value of the reference wavelength, and the linear dispersion of the detection system.

[0042] According to one embodiment, the measurement system is configured such that the detector detects the sample signal and the reference signal simultaneously, the reference source having a reference wavelength located outside the spectral line of interest.

[0043] According to another embodiment, the measuring system is configured such that the detector detects the sample signal for a signal duration and the reference signal for a reference duration sequentially in time, the signal duration and the reference duration being separated by a duration called the intermediate duration.

[0044] According to one embodiment, the sample signal is pulsed.

[0045] According to one embodiment, the measurement system according to the present invention further comprises a pulsed laser, which is configured to illuminate the sample in order to generate the sample signal.

[0046] According to one embodiment, the sample signal is emitted by a plasma and the system according to the invention further comprises an optical system configured to inject a portion of the optical signal generated by the sample into the first inlet of the optical fiber.

[0047] According to one embodiment, the system according to the invention is configured to measure the isotopic abundance of an element present in the sample. The central wavelength of interest corresponds to the line resulting from the contribution of two isotopes of the element, the value of the central wavelength of interest allowing the determination of the abundance.

[0048] According to another aspect, the invention relates to a first method for determining a central wavelength of interest of a spectral line of interest measured by a spectrometer,

[0049] The spectral line of interest corresponds to the emission or absorption of the sample to be characterized, and the light signal generated by the sample is called the sample signal. The spectral line of interest has: a bell-shaped profile, and the central wavelength of interest corresponds to the apex of the bell-shaped profile; or a double bell-shaped profile, and the central wavelength of interest corresponds to the trough between the two bells. The spectrometer is associated with a detector, and the detector includes a plurality of pixels aligned in the X direction, and the spectral line of interest is detected on the detector pixels.

[0050] The method comprises the following steps:

[0051] - providing a Y-shaped optical fiber having a first inlet, a second inlet and an outlet,

[0052] - providing a reference source emitting an optical signal called reference signal, said reference source having a reference spectral line having a central wavelength of known value called reference wavelength, said reference wavelength being selected so as to be detected on at least one pixel of said detector,

[0053] - positioning the optical fiber such that the first inlet collects the sample signal, the second inlet collects the reference signal, and the outlet is coupled to the input of the spectrometer,

[0054] - detecting the sample signal and the reference signal simultaneously in time so as to generate a measured profile of interest and a measured reference profile, - processing the measured profile of interest and the measured reference profile so as to determine a position of interest and a reference position of the central wavelength of interest and the reference wavelength, respectively,

[0055] - determining a value of the central wavelength of interest based on a difference between the position of interest and a reference position, the known value of the reference wavelength and the linear dispersion of the spectrometer and the associated detector.

[0056] The present invention also relates to a second 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 the emission or absorption of a sample to be characterized, the light signal generated by the sample being referred to as the sample signal, the spectral line of interest having: a bell-shaped profile, the central wavelength of interest corresponding to the apex of the bell-shaped profile; or a double bell-shaped profile, the central wavelength of interest corresponding to the trough between the two bells, the spectrometer being associated with a detector comprising a plurality of pixels aligned in the X direction, the spectral line of interest being detected on the detector pixels.

[0057] The method comprises the following steps:

[0058] - providing a Y-shaped optical fiber having a first inlet, a second inlet and an outlet,

[0059] - providing a reference source emitting an optical signal called reference signal, said reference source having a reference spectral line having a central wavelength of known value called reference wavelength, said reference wavelength being selected so as to be detected on at least one pixel of said detector,

[0060] - positioning the optical fiber such that the first inlet collects the sample signal, the second inlet collects the reference signal, and the outlet is coupled to the input of the spectrometer,

[0061] - sequentially in time detecting said sample signal for a signal duration and detecting said reference signal for a reference duration, said signal duration and said reference duration being separated by a duration called the intermediate duration, so as to generate a measured profile of interest and a measured reference profile,

[0062] - processing said measured profile of interest and said measured reference profile in order to determine a position of interest and a reference position of said central wavelength of interest and said reference wavelength, respectively,

[0063] - determining a value of the central wavelength of interest based on a difference between the position of interest and a reference position, the known value of the reference wavelength and the linear dispersion of the spectrometer and the associated detector.

[0064] According to one embodiment, the intermediate duration is less than 5 s.

[0065] According to one embodiment,

[0066] - in the step of sequential detection, an additional detection of the reference signal is performed, then called the additional reference signal, so that the detection of the sample signal at time t0 is temporally flanked by the detection of the reference signal and so that an additional measurement reference profile is generated, and

[0067] - in the step of processing the measured profile, an additional reference position is also determined and a reference position, called the intermediate reference position, is determined at time t0 by interpolation, based on said reference position and said additional reference position and on a predetermined law of variation of said reference position as a function of time,

[0068] - A step of determining said central wavelength of interest is then performed based on the difference between said position of interest and an intermediate reference position.

[0069] According to one embodiment, the processing step comprises the sub-step of adjusting the values ​​of the measured profile of interest and the measured reference profile using a known mathematical function so as to determine the position of interest and the reference position with an accuracy better than one pixel by interpolation.

[0070] According to one embodiment, the optical signal generated by said sample is pulsed.

[0071] According to one embodiment, the optical signal generated by the sample comes from the emission of a plasma emitted by the sample illuminated by a pulsed laser.

[0072] According to one embodiment, the method according to the invention is configured to determine the isotopic abundance of an element present in the sample, the central wavelength of interest corresponding to the line resulting from the contribution of two isotopes of the element, the value of the central wavelength of interest allowing determination of the abundance.

[0073] The following description presents several examples of embodiments of the device of the invention: the scope of the invention is not limited to these examples. These examples of embodiments have the essential features of the invention and additional features related to the embodiment in question. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] The invention will be better understood, and other characteristics, objects and advantages thereof will become apparent, from the following detailed description, made with reference to the accompanying drawings, which are given by way of non-limiting examples, and in which:

[0075] Already described Figure 1 The measurement principle using LIBS, LAMIS and LIBRIS techniques is shown;

[0076] Already described Figure 2 shows the spectral lines measured in the case of low concentrations of the element in the plasma, then the effects of self-absorption are rarely marked or can even be neglected;

[0077] Already described Figure 3 The spectra measured in the case of high concentrations of the elements in the plasma are shown, and the effects of self-absorption are then marked;

[0078] Already described Figure 4 shows the variation of the measured central wavelength λ0 as a function of the isotopic abundance of the lithium isotope 6Li in the sample;

[0079] Figure 5 A system for measuring a central wavelength of interest according to the present invention is shown;

[0080] Figure 6 A measured profile of interest and a measured reference profile are shown;

[0081] Figure 7 A system according to the invention is shown in the context of LIBRIS, i.e. it is configured to measure isotope ratios of elements present in a sample;

[0082] Figure 8 A method for determining a central wavelength of interest according to the present invention is shown;

[0083] Fig. 9 showing a theoretical reference profile and a theoretical profile of interest that best fit the experimental points to the measured reference profile and the measured profile of interest, respectively;

[0084] Fig.10 shows data obtained by repeating the measurement 18 times (measurements n°i numbered 1 to 18): for each measurement i, on the one hand, the original value (cross) and, on the other hand, the corrected value determined according to the method of the invention (dot);

[0085] Fig.11 The mean and standard deviation of these 18 measurements in both cases (original values ​​and corrected values) are shown. DETAILED DESCRIPTION

[0086] The invention relates to a system 10 for measuring a central wavelength λc of interest of a spectral line of interest RSe measured by a spectrometer, such as Figure 5 The present invention also relates to a method 100 for measuring a central wavelength.

[0087] The invention is applicable to LIBS or any other spectroscopic technique, regardless of the pressure employed, as long as it is necessary to measure the wavelength accurately. In LIBS / LAMIS, this is the case for physical applications where it is advantageous to determine the wavelength accurately, such as when measuring problems with spectral shifts of lines due to the Stark effect or the Doppler effect. The invention is also applicable to LIBRIS, which is particularly suitable.

[0088] The spectral line of interest corresponds to the emission or absorption of the sample Ech to be characterized, and the light signal generated by the sample is called the sample signal SLech. The spectral line has a bell-shaped profile, λc corresponding to the wavelength of the apex of the bell-shaped profile, or a double bell-shaped profile, λc corresponding to the wavelength of the trough between the two bells.

[0089] There may be different physical effects behind the excitation of the sample for generating the sample signal. For example, a plasma is generated which emits the sample signal. According to one embodiment, the sample is irradiated by a pulsed laser. According to other embodiments, the emission of the sample is caused by an excitation source other than a pulsed laser (e.g., an electric discharge or glow discharge as in spark spectroscopy). The plasma may also be an induced plasma.

[0090] According to one embodiment, the sample signal is pulsed. According to one embodiment, the system 10 according to the invention further comprises a pulsed laser L configured to illuminate the sample. In response to this illumination, the illuminated sample emits a pulsed light signal SLech to be characterized via a physical effect, such as in particular the emission of a plasma.

[0091] The system 10 according to the invention comprises a detection system comprising a spectrometer Spectro associated with (or comprising) a detector Det comprising a plurality of pixels Pi aligned in the X direction. A spectral line of interest RSe is detected on the detector pixels.

[0092] The system 10 further comprises a reference source Sref, which emits a light signal SLref called a reference signal. The reference source has a reference spectral line RSref having a central wavelength λref called a reference wavelength of known value. The reference wavelength is selected so as to be detected on at least one pixel of the detector. The reference source Sref is independent of a sample excitation source, which causes the generation of a sample signal SLech.

[0093] The system 10 also includes a Y-shaped optical fiber FOY having a first entrance E1 and a second entrance E2 and an exit S.

[0094] The optical fiber FOY is positioned and the system 10 is configured so that the first inlet E1 of the optical fiber collects the light signal SLech generated by the sample and the second inlet E2 of the optical fiber collects the light signal SLref generated by the reference source. Lines RSe and RSref are thus detected on the detector in the same spectrometer configuration (same adjustments). The reference source is selected according to the spectral characteristics of the sample to be analyzed.

[0095] Additionally, the outlet S of the optical fiber is coupled to the input of a spectrometer.

[0096] By means of the Y-shaped optical fiber, the spectrometer and the detector can receive both signals SLech and SLref simultaneously.

[0097] The system 10 according to the invention is configured such that the detector Det detects the sample signal SLech and the reference signal SLref simultaneously (first variant) or sequentially in time (second variant).

[0098] like Figure 6 As shown, this detection generates a measured profile of interest PSech and a measured reference profile PSref. The x-axis of the profile is the index i of the pixel Pi of the detector and the y-axis is the intensity Ii detected by each pixel.

[0099] Finally, the system comprises a processing unit UT configured to process the measured profile of interest and the measured reference profile in order to determine positions of interest Pech of a central wavelength of interest λc and reference positions Pref of a reference wavelength λref, which are measured in the detector pixels.

[0100] Based on the difference between the positions Pech-Pref, the value of λref known with precision, and the linear dispersion DL of the detection system [spectrometer + detector], the value of λ0 is determined, usually using formula (1). Of course, it is recommended to use the value of DL corresponding to the spectral region where λref and λ0 are located.

[0101] Due to the instantaneous or quasi-instantaneous detection of the two spectra PSech and PSref, the aforementioned wavelength drift is also made negligible and a very good sub-picosecond accuracy of the value of λ0 is achieved.

[0102] According to a first variant, the two measured profiles (spectrum of interest and reference spectrum) are acquired simultaneously. For this purpose, it is recommended that the reference source has a reference wavelength located outside the spectral line of interest. If this is not the case, the reference signal may interfere with the sample signal.

[0103] In the specific case of LIBRIS, the plasma signal is intense and short-lived. Preferably, the reference source should be strong enough so that the optimized acquisition parameters of the detector used to detect each spectrum are identical.

[0104] According to a second variant, two profiles are acquired sequentially in time. The detector Det detects the sample signal for a signal duration Ds and the reference signal for a reference duration Dref. The signal duration and the reference duration are separated by a duration called the intermediate duration Dint, which is sought to be minimized.

[0105] The time series defined by [the signal SLech / Dint during Ds / the signal SLref during Dref] (e.g. Figure 5 Of course, the signals SLch and SLref in the sequence may also be reversed.

[0106] The exposure times Ds and Dref are adjusted according to SLech and SLref, respectively, so that the signal-to-noise ratio is sufficient to detect each signal well.

[0107] In this second variation, the reference wavelength may be identical to the central wavelength of interest, such as in the case of a lithium hollow cathode lamp in the context of LIBRIS analysis of lithium.

[0108] The signals SLref and SLech typically have very different intensity profiles as a function of time. Sref (typically a hollow cathode lamp) emits very few photons continuously. According to a preferred embodiment, the signal SLech is a pulsed signal, which is typically intense and short-lived (a large number of photons in a very short time, such as a plasma signal).

[0109] Due to this difference in intensity profiles, simultaneous detection by Det while acquiring two spectral profiles at once is not actually possible, since detection of each type of profile requires a different detection configuration. Thus, for the typical case where the reference source emits weakly but continuously while the plasma emits intensely for a short time, the optimized acquisition parameters for each of the detected spectra cannot be the same, and sequential detection is necessary.

[0110] Preferably, Ds <Dref。

[0111] Therefore, the system according to the invention is configured such that the detector detects two spectra consecutively using different acquisition parameters.

[0112] These parameters are (non-exhaustive list): delay of measurement relative to laser emission (for sample signal only), width of acquisition time gate, number and rate of accumulations, detector gain, signal averaging.

[0113] These parameters are, for example, as follows:

[0114]

[0115]

[0116] Table II

[0117] Preferably, the detector is an intensified CCD detector.

[0118] The interval between two measured values ​​(corresponding to Dint) can be made negligible with respect to the risk of wavelength drift of the detection system (preferably Dint<5 s, or even Dint<1 s).

[0119] according to Figure 7 In one embodiment shown, the optical signal SLech is due to the emission of a plasma Pl emitted by a sample Ech irradiated by a pulsed laser L. The system 10 according to the invention then comprises, in addition to the laser L, an optical system 2 which focuses the laser beam onto the sample and an optical system SO configured to inject part of the optical signal generated by the sample into a first inlet E1 of the optical fiber.

[0120] According to one embodiment, the processing unit UT is further configured to synchronize the detector Det with the laser L to detect the sample signal.

[0121] According to one embodiment, the system 10 according to the invention is associated with an implementation of the LIBRIS technique, i.e. it is configured to measure the isotopic ratio of an element present in a sample Ech. The central wavelength of interest corresponds to the line resulting from the contribution of the two isotopes of the element, and the value of the central wavelength of interest makes it possible to determine the isotopic abundance as described above.

[0122] The duration of the detection of SLech is determined according to the duration of the laser pulse and the laser frequency f. Typically, Ds is of the order of 1 μs. According to a first variant, a method 100 for determining a central wavelength λc of interest of a spectral line of interest RSe measured by a spectrometer comprises the following steps.

[0123] A Y-shaped optical fiber OFY having a first entrance E1, a second entrance E2 and an exit S is provided, and a reference source Sref is provided, which emits a reference signal and has a reference spectral line RSref, the reference spectral line RSref having a central wavelength λref of a known value, called the reference wavelength, which is selected so as to be detected on at least one pixel of the detector.

[0124] The optical fiber OFY is then positioned so that the first inlet collects the sample signal, the second inlet collects the reference signal, and the outlet is coupled to the input Espec of the spectrometer.

[0125] Next, the sample signal SLech and the reference signal SLref are detected simultaneously to generate a measured profile of interest PSech and a measured reference profile PSref.

[0126] Next, the measured profile of interest and the measured reference profile are processed in order to determine the position of interest Pech and the reference position Pref measured in detector pixels for the central wavelength of interest and the reference wavelength, respectively.

[0127] Finally, the value of the central wavelength of interest is determined based on the difference between the position of interest and the reference position, the known value of the reference wavelength and the linear dispersion DL of the detection system (spectrometer+detector).

[0128] according to Figure 8 In the second variant shown, in the method 200 for determining the central wavelength of interest λc, the sample signal SLech and the reference signal SLref are detected sequentially in time during the signal duration Ds and the reference duration Dref, respectively, instead of detecting both signals simultaneously. The signal duration and the reference duration are separated by a predetermined duration called the intermediate duration Dint. The other steps are the same.

[0129] According to one embodiment of the method 200, the time sequence is obtained using shutters O1 and O2 placed in front of the entrances E1 and E2, and programmed to adjust the durations Ds, Dref and Dint. When SLech is a pulse signal with a frequency of f, the shutter O1 is synchronized with the frequency of the signal SLref to let the desired number of pulses pass, and the shutter O2 is configured to send the signal SLech within a sufficient duration Dech before or after the shutter O1 is activated, wherein the time Dint between the activation of the two shutters is as small as possible and is limited by the speed of the electronic devices of the detection system.

[0130] According to one embodiment, the sample signal is derived from the emission of a plasma emitted by the sample irradiated by a pulsed laser. According to one embodiment, the method is configured to measure the isotopic abundance of an element present in the sample. The central wavelength of interest then corresponds to the line produced by the contribution of the two isotopes of the element, and its value can determine the abundance, as described above.

[0131] According to one embodiment, in order to obtain the time series, in a pre-calibration step, the brightness of Sref is adjusted so that the intensity of SLref detected during the time Ds is negligible, ie cannot be detected in the presence of noise.

[0132] To realize the time sequence ST, it is provided that the signal SLech corresponds to the last emission of the laser L and the detector is synchronized with the last emission. The duration Ds is of the order of 1 μs or 10 μs and is limited by the frequency f of the laser. Next, after a possibly very short duration Dint, the detector is configured to detect SLref during a time window Dref, typically of the order of 1 s or a few s. In this embodiment, no shutter is required, the signal SLref is always present on the detector, even during the detection of SLech, SLref is negligible in the time window Ds, and the detector Det 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.

[0133] In order to be able to measure λc with very high accuracy, its position Pech should be obtained with an accuracy better than one detector pixel. To this end, according to one embodiment, the processing step comprises the following sub-steps: Adjust the values ​​of the measured reference profile PSref and the measured profile of interest PSech using known mathematical functions so as to determine the position of interest and the reference position by interpolation with an accuracy better than one pixel. Thus, a theoretical reference profile PSTref and a theoretical profile of interest PSTech of the best fitting experimental points are determined, such as Fig. 9As shown. Generally, the mathematical functions used are selected from: Gaussian profile, Lorentz profile, and Voigt profile.

[0134] According to these profiles, the positions Pref and Pech are determined as fractions of a pixel (usually accurate to the second decimal place).

[0135] According to an embodiment that allows for more precise measurements, the sequential detection step further includes an additional detection of a reference signal, which is referred to as an additional reference signal Sref / add, such that the detection of the sample signal at time t0 is flanked in time by the detection of the reference signal Sref (e.g., at t1 < t0) and the additional reference signal Sref / add (at t2 > t0).

[0136] Then, the sequence ST is, for example: the signal SLref / Dint during Dref / the signal SLecf / Dint during Ds / the signal SLref / add during Dref.

[0137] In this detection step, an additional measurement reference profile PSref / add corresponding to the signal SLref / add is also generated.

[0138] In the step of processing the measurement profiles, an additional reference position Pref / add is also determined. Of course, a fit to a theoretical profile can be applied to determine Pref / add. Due to the drift of the spectrometer, the positions of Pref and Pref / add are slightly different.

[0139] In the processing step, based on the reference position Pref and the additional reference position Pref / add, and based on a predetermined law of variation of the reference position as a function of time (usually considered to be linear), an additional reference position P0ref, referred to as the intermediate reference position, is determined by interpolation at time t0. By virtue of this interpolation, the drift of the spectrometer between times t1 and t0 or between times t0 and t2 is corrected. Thus, the position P0ref is more precise than Pref obtained via a single detection of the spectrum of the reference source at a time before or after t0. This corresponds to the case of quasi-instantaneous detection of the reference and the sample.

[0140] In this particular embodiment, the step of determining the central wavelength of interest is then performed based on the difference Pech - P0ref.

[0141] In the linear case, the following is obtained:

[0142] P ref (t0) = P0 ref = a.t0 + b (2)

[0143] The parameters a and b are determined based on the measurements at times t1 and t2:

[0144]

[0145] b=P ref (t2)-a.t2 (4)

[0146] Below, the results of the advantages of the proposed correction method are briefly presented. A Jobin Yvon THR1000 spectrometer equipped with a grating of 2400 lines / mm centered at 670nm was used. The detector was a 2048x512 pixel Andor iStar intensified camera with a linear dispersion DL of 2.774pm / pixel at 670nm.

[0147] The lines of the mercury vapor lamp are measured in the presence of spectrometer drift, and the lines of the mercury vapor source form the spectral lines of interest.

[0148] Next (or beforehand), a line is measured as a reference source of a hollow cathode lamp (HCL), the accuracy of which is known and equal to λref=670.776 nm.

[0149] The spectrometer drift is then corrected (simultaneous detection) using the method 100 according to the invention. Specifically, in this case the lines of the two sources are sufficiently separated in wavelength and comparable in intensity and can therefore be measured simultaneously.

[0150] The acquisition parameters are given in Table III below.

[0151] Width of the acquisition time gate 50ms The amount and rate of accumulation 100 to 18 Hz Detector gain 4000

[0152] Table III

[0153] Fig.10 The graph of shows the data obtained by repeating the measurement 18 times (measurements i are numbered 1 to 18). For each measurement i, the following is determined: On the one hand, the original value λc B (i) (crosses), on the other hand, the correction values ​​λc(i) (dots) determined according to the method 100 according to the invention. The original values ​​were obtained by direct measurement with the detection system. The dispersion in the original data can be seen and is caused by the drift of the spectrometer. The correction values ​​λc are very slightly dispersed in the 18 measurements.

[0154] Fig.11 The mean and standard deviation σ of these 18 measurements in two cases (original and corrected with λref of HCL lamp) are shown, respectively (λ B m,σ B ) and (λ c m,σ c ). In addition, the "true" value of the wavelength of a mercury vapor lamp, which is known very accurately, is λ lvm=671.643nm. This value is also Fig.11 and makes it possible to test the relevance of the method according to the invention. c m ratio λ B m is closer to λ lvm It can be seen that the measurement method according to the present invention significantly improves the accuracy and fidelity of the measured wavelength.

Claims

1. A system (10) for measuring a central wavelength of interest (λc) of a spectral line of interest (RSe) measured by a spectrometer, the spectral line of interest corresponding to the emission or absorption of a sample (Ech) to be characterized, the optical signal generated by the sample being referred to as a sample signal (SLech), the spectral line of interest having: a bell-shaped profile, the central wavelength of interest corresponding to the apex of the bell-shaped profile; or a double bell-shaped profile, the central wavelength of interest corresponding to the trough between the two bells, the system comprising: a detection system comprising a spectrometer (Spectro) associated with a detector (Det), the detector comprising a plurality of pixels (Pi) aligned in the X direction, the spectral line of interest being detected at the detector pixels, a reference source (Sref) emitting an optical signal called reference signal (SLref), said reference source having a reference spectral line (RSref) having a central wavelength called reference wavelength of known value (λref), said reference wavelength being selected so as to be detected on at least one pixel of said detector, - a Y-shaped optical fiber (FOY) having a first inlet (E1), a second inlet (E2) and an outlet (S), the optical fiber being configured such that: ● The first inlet collects the sample signal (SLech), ● The second entry collects the reference signal (SLref), ● The outlet is coupled to an input of the spectrometer, The measuring system is configured such that the detector detects the sample signal and the reference signal simultaneously or sequentially in time in order to generate a measured profile of interest (PSech) and a measured reference profile (PSref), The measurement system further comprises a processing unit (UT) configured to: ● processing the measured profile of interest and the measured reference profile in order to determine a position of interest (Pech) and a reference position (Pref) measured in detector pixels for the central wavelength of interest and the reference wavelength, respectively, ● The value of the central wavelength of interest is determined based on the difference between the position of interest and a reference position, the known value of the reference wavelength, and the linear dispersion (DL) of the detection system.

2. The measurement system according to claim 1, wherein: The measurement system is configured such that the detector detects the sample signal and the reference signal simultaneously, and the reference source has a reference wavelength that is outside the spectral line of interest.

3. The measurement system according to claim 1, wherein: The measuring system is configured such that the detector detects the sample signal for a signal duration (Ds) and the reference signal for a reference duration (Dref) sequentially in time, the signal duration and the reference duration being separated by a duration (Dint) referred to as an intermediate duration.

4. The measuring system according to claim 1, wherein: The sample signal is pulsed.

5. The measurement system according to one of the preceding claims, further comprising a pulsed laser (L) configured to illuminate the sample in order to generate the sample signal.

6. The measuring system according to claim 1, wherein: The sample signal is emitted by a plasma (P1), and the measurement system further comprises an optical system (SO) configured to inject a portion of the optical signal generated by the sample into the first inlet of the optical fiber.

7. The measurement system according to the preceding claim, configured to measure the isotopic abundance of elements present in the sample, wherein: The central wavelength of interest corresponds to the line resulting from the contribution of two isotopes of the element, the value of the central wavelength of interest allowing the abundance to be determined.

8. A method (100) for determining a central wavelength of interest (λc) of a spectral line of interest (RSe) measured by a spectrometer, The spectral line of interest corresponds to the emission or absorption of the sample (Ech) to be characterized, the light signal generated by the sample is called the sample signal (SLech), the spectral line of interest has: a bell-shaped profile, the central wavelength of interest corresponds to the apex of the bell-shaped profile; or a double bell-shaped profile, the central wavelength of interest corresponds to the trough between the two bells, the spectrometer is associated with a detector (Det), the detector comprises a plurality of pixels (Pi) aligned in the X direction, the spectral line of interest is detected on the detector pixels, The method comprises the following steps: - providing a Y-shaped optical fiber (OFY) having a first inlet (E1), a second inlet (E2) and an outlet (S), - providing a reference source (Sref) emitting an optical signal called reference signal (SLref), said reference source having a reference spectral line (RSref) having a central wavelength called reference wavelength of known value (λref), said reference wavelength being selected so as to be detected on at least one pixel of said detector, positioning said optical fiber so that said first inlet collects said sample signal, said second inlet collects said reference signal and said outlet is coupled to an input (Espec) of said spectrometer, - detecting said sample signal and said reference signal simultaneously in time in order to generate a measured profile of interest (PSech) and a measured reference profile (PSref), - processing said measured profile of interest and said measured reference profile in order to determine a position of interest (Pech) and a reference position (Pref) of said central wavelength of interest and said reference wavelength, respectively, - determining a value of the central wavelength of interest based on a difference between the position of interest and a reference position, the known value of the reference wavelength and the linear dispersion (DL) of the spectrometer and the associated detector.

9. A method (200) for determining a central wavelength of interest (λc) of a spectral line of interest (RSe) measured by a spectrometer, The spectral line of interest corresponds to the emission or absorption of the sample (Ech) to be characterized, the light signal generated by the sample is called the sample signal (SLech), the spectral line of interest has: a bell-shaped profile, the central wavelength of interest corresponds to the apex of the bell-shaped profile; or a double bell-shaped profile, the central wavelength of interest corresponds to the trough between the two bells, the spectrometer is associated with a detector (Det), the detector comprises a plurality of pixels (Pi) aligned in the X direction, the spectral line of interest is detected on the detector pixels, The method comprises the following steps: - providing a Y-shaped optical fiber (OFY) having a first inlet (E1), a second inlet (E2) and an outlet (S), - providing a reference source (Sref) emitting an optical signal called reference signal (SLref), said reference source having a reference spectral line (RSref) having a central wavelength called reference wavelength of known value (λref), said reference wavelength being selected so as to be detected on at least one pixel of said detector, positioning said optical fiber so that said first inlet collects said sample signal, said second inlet collects said reference signal and said outlet is coupled to an input (Espec) of said spectrometer, - sequentially in time detecting said sample signal for a signal duration (Ds) and detecting said reference signal for a reference duration (Dref) in order to generate a measured profile of interest (PSech) and a measured reference profile (PSref), said signal duration and said reference duration being separated by a duration called an intermediate duration (Dint), - processing said measured profile of interest and said measured reference profile in order to determine a position of interest (Pech) and a reference position (Pref) of said central wavelength of interest and said reference wavelength, respectively, - determining a value of the central wavelength of interest based on a difference between the position of interest and a reference position, the known value of the reference wavelength and the linear dispersion (DL) of the spectrometer and the associated detector.

10. The method according to the preceding claim, wherein: The intermediate duration is less than 5 seconds.

11. A method according to claim 9 or claim 10, wherein: - in the step of sequential detection, an additional detection of the reference signal is performed, then called additional reference signal (Sref / add), so that the detection of the sample signal at time t0 is temporally flanked by the detection of the reference signal and so that an additional measurement reference profile (PSref / add) is generated, and - in the step of processing the measured profile, an additional reference position (Pref / add) is also determined and, at time t0, a reference position called intermediate reference position (P0ref) is determined by interpolation, based on said reference position (Pref) and said additional reference position (Pref / add) and on a predetermined law of variation of said reference position as a function of time, - A step of determining said central wavelength of interest is then performed based on the difference between said position of interest (Pech) and an intermediate reference position (P0ref).

12. The method according to any one of claims 8 to 11, wherein: The processing step comprises the sub-step of adjusting the values ​​of the measured profile of interest and the measured reference profile using a known mathematical function so as to determine the position of interest and the reference position with an accuracy better than one pixel by interpolation.

13. The method according to any one of claims 7 to 11, wherein: The optical signal generated by the sample is pulsed.

14. A method according to the preceding claim, wherein: The optical signal generated by the sample results from emission of a plasma emitted by the sample illuminated by a pulsed laser.

15. The method according to the preceding claim, configured to determine the isotopic abundance of an element present in the sample, the central wavelength of interest corresponding to the line resulting from the contribution of two isotopes of the element, the value of the central wavelength of interest allowing determination of the abundance.

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