System and associated method for measuring central wavelength of a spectral line with high accuracy

Abstract

The present invention provides a system (10) for measuring a central wavelength (λc) 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); -Y-shaped optical fiber (FOY) The optical fiber comprises: A first inlet collects the optical signal (SLech) generated by the sample, called the sample signal; A second inlet collects an optical signal (SLref) generated by a reference source (Sref), called the reference signal; The outlet is coupled to the input of a spectrometer It is configured as follows: the measurement system is configured such that a detector detects the sample signal and the reference signal simultaneously or sequentially in time; The measurement system is processing the measured subject profile and the measured reference profile; determining a value of the center wavelength of the target based on the difference between the position of the target and a reference position, the known value of the reference wavelength, and the linear dispersion (DL) of the detection system; The present invention relates to a system (10) further comprising a processing unit (UT) configured to:

Description

[Technical Field]

[0001] The present invention relates to the field of spectroscopy, and more particularly to determining the central wavelength of a spectroscopic line with very high accuracy. [Background technology]

[0002] For certain applications in spectroscopy, e.g., atomic or molecular spectroscopy, or to determine the isotopic abundance of elements in a sample via an optical method (called LIBRIS, Laser Induced Breakdown self-Reversal Isotopic Spectrometry, see below), it is necessary to determine with high precision the value of the central wavelength of a spectroscopic line. The spectroscopic line to be characterized is produced by a light source and may be an atomic or molecular line, an absorption line or an emission line. Typically, an uncertainty of less than 5 pm, or even less than 1 pm, in the value of the central wavelength is required.

[0003] This problem has not yet arisen in fields such as laser-induced breakdown spectroscopy (LIBS) or laser ablation molecular isotope spectroscopy (LAMIS), where the widths of the observed lines are typically a few tens of pm. Thus, the wavelengths of the lines are usually measured with an uncertainty of about 10 pm to a few tens of pm, depending on the linear dispersion of the spectrometer used. This uncertainty does not affect these techniques, since the analyses performed are generally based on the intensity of the lines integrated over the same order, i.e., a width of about 10 to a few tens of pm.

[0004] Traditionally, detection systems are calibrated in terms of wavelength by a reference source, typically a mercury vapor lamp or hollow cathode lamp, that emits a known line. The positions of the analyte line and the reference line are specified in pixels on the detector, and the analyte line is determined from its relative position to the reference line position. The detector comprises at least N pixels Pi arranged in rows, where i runs from 1 to N. In the 2D case, an integration is performed over all pixels in a given column. For example, the detector is a CCD matrix array with 2048 x 512 pixels.

[0005] If the central wavelength of the reference line is λref and the wavelength to be determined is λ0, then let Pref be the position of λref within a detector pixel and P0 be the position of λ0 on the same detector. Of course, the wavelength λref is chosen so that it appears at the detector at the same time as λ0 for a given spectrometer configuration. Then the following is valid: λ0=λ ref +(P0-P ref ).DL(1) DL is the linear dispersion of the detection system, typically pm / pixel.

[0006] For various reasons (thermal fluctuations, vibrations), spectrometers and detectors drift very slightly even in the controlled environment of a laboratory, leading to wavelength drift. This drift is, of course, even more pronounced in the case of analysis outside the laboratory (in the field, online, with portable systems, etc.). As an example, for a grating spectrometer with a focal length of 1 m and a grating with 2400 lines / mm, a mere 10th of the grating angle can cause a -3 A variation in the wavelength of the ion beam causes a wavelength shift of 10 pm, which means that the uncertainty is less than 1 pm, for example. 6 This is not acceptable for LIBRIS analysis of lithium, which is targeted to obtain acceptable uncertainties in the isotopic abundance of Li.

[0007] The reference and analyte lines are detected sequentially in time. In the most common case, the signal generated by the sample is carried to the detection system by an optical fiber. To perform the two measurements, the optical fiber connected to the spectrometer must be positioned to first collect the light flux generated by the reference source and then the light flux generated by the emission to be characterized, or vice versa, which takes time. Typically, these two measurements are separated by a duration of about one minute, sufficient time for such drift to occur.

[0008] Therefore, it is not possible to perform accurate LIBRIS measurements without correcting the detection system for wavelength drift. This problem also arises in atomic spectroscopy, which attempts to accurately measure λ by calibration with a reference source.

[0009] Since the present invention is particularly advantageous in the context of the LIBRIS method, its principles as well as those of the LIBS and LAMIS methods are recalled below.

[0010] The principle of LIBS technology is to focus a laser pulse on the surface of a material sample (or the material itself), generating a transient plasma whose emission is analyzed by a spectrometer, as shown in Figure 1. By collecting the plasma's emission and analyzing its spectrum by spectroscopy, it is possible to use a database of emission lines to identify the elements present in the plasma and therefore determine the composition of the material. In LIBS, the intensity is integrated over the width of the line.

[0011] The LAMIS technique is described, for example, in the publication by R. Russo et al., Spectrochim. ACTA B 66 (2011) 99, and is an alternative to LIBS that allows isotopic analysis to be performed based on molecular beams formed by reactions between the ablated material and components of the surrounding medium or by reactions between two atoms of the ablated material.

[0012] A laser generator L0 generates a laser light FL0 which is focused on the sample 1 by a first optical system 2. This generates a plasma P10. The plasma emits an optical emission 3 which is collected by an optical system OS0. The focused optical emission is transmitted via an optical fiber FO to a spectrometer Spec0. The spectrometer Spec0 comprises (or is associated with) a detector Det0 synchronized with the laser generator L0. The spectrometer Spec0 makes it possible to record a line spectrum. Finally, processing means UT0 make it possible to process the recorded spectrum.

[0013] LIBS makes it possible to generate a spectrum 20 in the form of a set of spectral lines corresponding to the emission lines of the constituent elements of a material, and using the available data correlating the emission lines with the elements, it is possible to determine the elemental composition of a sample of material. The wavelength λ of a line provides information about one element present in the material, and its intensity I is related to the concentration of this element.

[0014] LIBS emission spectroscopy is also applicable to isotopic analysis, since the atomic lines of various isotopes of the same element have slightly different wavelengths. This spectral shift, called the isotope shift, is due to the mass effect (dominant for light elements) and modifications of the charge distribution within the nucleus (dominant for heavy elements). If it is desired to perform isotopic analysis by LIBS, it is essential to separate the lines of the two isotopes. However, this spectral shift is generally on the order of a few nm or even pm, as shown in Table I below.

[0015] [Table 1]

[0016] Such a shift is difficult to observe in plasmas produced by laser ablation under normal conditions because the confinement of the plasma by ambient air at atmospheric pressure leads to high density and therefore broadening of the emission line due to the Stark effect. This broadening typically reaches tens or even hundreds of pm and therefore masks the isotope shift, even if the spectrometer used has sufficient spectral resolution to resolve this shift. The limitation here is physical and not inherently significant.

[0017] The first solution consists in performing the analysis at low pressure, or even under vacuum. By limiting the confinement of the plasma by the surrounding medium in this way, its density is reduced, and sufficient spectral selectivity for specific isotopes can be achieved. A doublet is observed, and the isotope ratio is determined from the intensity ratio between the two lines associated with the two isotopes. This technique is not applicable to all isotopes and requires high-resolution, therefore bulky, spectrometers. The second solution consists in sending a second laser beam into the plasma to measure the resonance or fluorescence absorption signal, which constrains and complicates the measurement system.

[0018] In the field of isotope analysis at atmospheric pressure, it is possible to use the LAMIS technique, but this requires the fulfillment of several conditions: 1. The molecules must be formed in the plasma. 2. The formed molecules must be sufficiently stable under the temperature and density conditions of the plasma. 3. The formed molecules must have detectable lines, i.e., lines with sufficient lifetime, strong enough, and in the spectral band of the detection system. For example, in the case of lithium, no LAMIS signal is detected, possibly because the second condition is not met.

[0019] The LIBRIS technique is an optical technique that can determine the isotope abundance of elements in a sample (solid, liquid, or gas) from the emission spectrum of laser ablation plasma. This technique is described, for example, in the publication Spectrochim. ACTA B 168 (2020) 105868 by K. Touchet et al. and in the document U.S. Patent Application Publication No. 2019 / 0041336. It is a variation of the LIBS technique, using the same optical system. The LIBRIS technique overcomes various drawbacks of the LIBS method by enabling isotope ratio measurements to be performed at atmospheric pressure without a second laser.

[0020] It will be recalled that energy must be supplied in order for an atom's electron to transition to a higher energy level. This energy can take the form of a photon, in which case the atom absorbs the photon. One particular case is that of a laser-ablated plasma. To simplify the problem, the plasma can be considered to consist of two distinct parts: a core and a periphery. Photons emitted by the warmer plasma core can be absorbed by the cooler periphery. This effect therefore prevents a certain number of emitted photons from leaving the plasma: this is called self-absorption.

[0021] For an observer and measuring device outside the plasma, the line profile results from the emission and self-absorption at the same wavelengths corresponding to the transition between two levels of electrons in the observer's own or all atoms of interest placed in its line of sight. As a result, the measured intensity is not simply the sum of all the emission from the plasma, since such self-absorption must be taken into account.

[0022] The self-absorption effect, known in spectroscopy of plasmas for elemental analysis, is considered a rather undesirable effect, as it leads to distortion of the line profile and therefore to a nonlinearity of the signal relative to the concentration of the element of interest. LIBRIS exploits this self-absorption effect to deduce information about the isotopes of a given element in a material.

[0023] 2 and 3 show the lines RS0 of the elements of interest selected from the spectrum 20, obtained in two particular cases depending on the concentration of the element in the material.

[0024] Figure 2 shows the case where the concentration of the element in the plasma is lower, where the effect of self-absorption is barely noticeable or even nonexistent. A spectrally broad line profile without a central trough is obtained. The dotted and dashed curves ISO1 and ISO2 represent the emission of the two isotopes. Each line has a wide width relative to the separation between the two lines, mainly due to the Stark effect in the plasma, and therefore they are indistinguishable individually; a 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 isotope abundance, i.e., the ratio of the amplitudes of the two dotted and dashed lines. In this case, the value of the central wavelength λ0 is measured, which corresponds to the emission peak, i.e., the maximum point or apex 20 of the observed curve with a bell-shaped profile. This correlates with the ratio between the two isotopes ISO1 and ISO2 of the element in question and shifts depending on the isotope ratio.

[0025] Figure 3 shows the case where the concentration of an element in the plasma is high, where the effect of self-absorption is pronounced. A line profile with a trough at its center (double bell profile) is observed. This profile, called an inverted line, results from the superposition of a spectrally broad emission profile and a spectrally narrow 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 in the part of the profile corresponding to absorption, i.e., at the minimum point 30 of the observed trough. This is correlated with the ratio between the two isotopes Iso1 and Iso2 of the element in question and shifts depending on said isotope ratio. It is this measurement of the trough wavelength that defines the LIBRIS technique.

[0026] Therefore, in the LIBRIS technique, the isotope ratio is determined based on a very precise measurement of the wavelength λ of the Bell line maximum or the so-called inverted double Bell line minimum. R 1 and λ R 2 λ shifts linearly with isotopic abundance, and the indices 1 and 2 refer to the two isotopes of the element. R 1 and λ R 2is the physical data available in spectroscopic databases and / or scientific publications. Therefore, the analytical uncertainty of the isotope abundance is directly related to the uncertainty in the wavelength λ.

[0027] In the LIBRIS technique, the measurement of λ directly yields the isotope ratio. 6 Li and 7 Lithium isotopes containing only Li 6 The measured variation of λ as a function of the Li fraction is shown. The curve was constructed using inverted lines. The isotope shift is R 1 -λ R 2 and corresponds to the measurement range of the technique for a given line. For lithium, and for the 670.778 nm line used in LIBRIS, this shift is 15.8 ± 0.3 pm, and therefore the range from 0% to 100%. 6 corresponds to the total variation in the isotopic abundance of Li, and the complement is 7 The amount of Li present is therefore R An uncertainty of 1 pm in λ0 results in an uncertainty in the isotope abundance of 1 / 15.8 = 6.3%. Therefore, the precision of the measurement of the isotope ratio is directly correlated with the precision of the measurement of λ0. [Prior art documents] [Patent documents]

[0028] [Patent Document 1] US Patent Application Publication No. 2019 / 0041336 [Non-patent literature]

[0029] [Non-Patent Document 1] R.Russo et al.,Spectrochim.ACTA B 66(2011)99 [Non-patent document 2] K.Touchet et al.,Spectrochim.ACTA B 168(2020)105868 Summary of the Invention [Problem to be solved by the invention]

[0030] One object of the present invention is to remedy the aforementioned drawbacks by proposing a method and a system for determining the central wavelength of an atomic or molecular absorption or emission line produced by a light source with sub-picometer precision. [Means for solving the problem]

[0031] One subject of the present invention is a system for measuring a central wavelength of a spectral line of interest measured by a spectrometer, the spectral line of interest corresponding to an emission or absorption by a sample to be characterized, the optical signal produced by the sample being called a sample signal, the spectral line of interest having either a bell profile, said central wavelength of interest then corresponding to the apex of said bell 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 including a spectrometer associated with a detector including a plurality of pixels aligned in a -X direction, wherein spectral lines of interest are detected on the detector pixels; a reference source emitting an optical signal, referred to as the reference signal, having a reference spectral line of known value with a central wavelength, referred to as the reference wavelength, the reference wavelength being selected to be detected on at least one pixel of the detector; a Y-shaped optical fiber having a first inlet, a second inlet, and an outlet; Equipped with Optical fiber is a first inlet collecting the sample signal; a second inlet collecting the reference signal; The outlet is coupled to the input of a spectrometer It is configured as follows: the measurement system is configured such that a detector detects the sample signal and the reference signal simultaneously or sequentially in time to generate a measured object profile and a measured reference profile; The measurement system is processing the measured object profile and the measured reference profile to determine measured object positions and reference positions within detector pixels for the object central wavelength and the reference wavelength, respectively; determining a value of a central wavelength of the object based on a difference between a position of the object and a reference position, the known value of the reference wavelength, and the linear dispersion of the detection system; a processing unit further configured to: It is a system.

[0032] According to one embodiment, the measurement system is configured such that the detector simultaneously detects the sample signal and the reference signal, and the reference source has a reference wavelength that is located outside the spectral line of interest.

[0033] According to another embodiment, the measurement system is configured such that the detector detects the sample signal sequentially in time over a signal duration and detects the reference signal sequentially in time over a reference duration, the signal duration and the reference duration being separated by a duration referred to as the intermediate duration.

[0034] According to one embodiment, the sample signal is pulsed.

[0035] According to one embodiment, the measurement system according to the invention further comprises a pulsed laser configured to irradiate the sample to generate said sample signal.

[0036] According to one embodiment, the sample signal is emitted by the 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 entrance of the optical fiber.

[0037] According to one embodiment, the system according to the invention is configured to measure the isotopic abundance of an element present in a sample, the central wavelength of interest corresponding to a line resulting from the contribution of two isotopes of said element, the value of which makes it possible to determine said abundance.

[0038] According to another aspect, the present invention provides a first method for determining a central wavelength of interest of a spectral line of interest measured by a spectrometer, the method comprising: the spectral line of interest corresponds to emission or absorption by the sample to be characterized, the optical signal produced by the sample is called the sample signal, the spectral line of interest has either a bell profile, the central wavelength of said target then corresponding to the apex of said bell profile, or a double bell profile, the central wavelength of said target then corresponding to the trough between the two bells, the spectrometer is associated with a detector comprising a plurality of pixels aligned in the X direction, and the spectral line of interest is detected on the detector pixels; Regarding the first method.

[0039] The method comprises: - providing a Y-shaped optical fiber having a first inlet, a second inlet, and an outlet; - providing a reference source emitting an optical signal, referred to as the reference signal, the reference source having a reference spectral line with a central wavelength, referred to as the reference wavelength, of known value, the reference wavelength being selected to be detected on at least one pixel of the detector; - positioning an optical fiber such that a first inlet collects a sample signal, a second inlet collects a reference signal, and an outlet is coupled to an input of a spectrometer; - detecting the sample signal and the reference signal simultaneously in time to generate a measured object profile and a measured reference profile; - processing the measured target profile and the measured reference profile to determine target and reference positions for the target central wavelength and the reference wavelength, respectively; - determining a value of the central wavelength of the object based on the difference between the position of said object and a reference position, said known value of the reference wavelength, and the linear dispersion of the spectrometer and associated detector; Includes.

[0040] The present invention also provides a second method for determining a central wavelength of interest of a spectral line of interest measured by a spectrometer, comprising: the spectral line of interest corresponds to emission or absorption by the sample to be characterized, the optical signal produced by the sample is called the sample signal, the spectral line of interest has either a bell profile, the central wavelength of said target then corresponding to the apex of said bell profile, or a double bell profile, the central wavelength of said target then corresponding to the trough between the two bells, the spectrometer is associated with a detector comprising a plurality of pixels aligned in the X direction, and the spectral line of interest is detected on the detector pixels; Regarding the second method.

[0041] The method comprises: - providing a Y-shaped optical fiber having a first inlet, a second inlet, and an outlet; - providing a reference source emitting an optical signal, referred to as the reference signal, the reference source having a reference spectral line with a central wavelength, referred to as the reference wavelength, of known value, the reference wavelength being selected to be detected on at least one pixel of the detector; - positioning an optical fiber such that a first inlet collects a sample signal, a second inlet collects a reference signal, and an outlet is coupled to an input of a spectrometer; - detecting the sample signal over a signal duration and the reference signal over a reference duration sequentially in time to generate a measured object profile and a measured reference profile, the signal duration and the reference duration being separated by a duration referred to as the intermediate duration; - processing the measured target profile and the measured reference profile to determine target and reference positions for the target central wavelength and the reference wavelength, respectively; - determining a value of the central wavelength of the object based on the difference between the position of said object and a reference position, said known value of the reference wavelength, and the linear dispersion of the spectrometer and associated detector; Includes.

[0042] According to one embodiment, the intermediate duration is less than 5 seconds.

[0043] According to one embodiment, in a step of sequential detection, an additional detection of a reference signal, referred to as an additional reference signal, is then performed, such that the detection of the sample signal at time t0 is adjacent in time to the detection of the reference signal and such that an additional measured reference profile is generated; in the step of processing the measured profile, further reference positions are determined, and a reference position, called intermediate reference position, is determined at time t0 by interpolation on the basis of the reference position and the further reference position and on the basis of a predetermined law of variation of the reference positions as a function of time, A step of determining the central wavelength of the object is then carried out based on the difference between the position of said object and the intermediate reference position.

[0044] According to one embodiment, the processing step includes a sub-step of adjusting the values ​​of the measured object profile and the measured reference profile using known mathematical functions to determine by interpolation the position of the object and the reference position with an accuracy of better than one pixel.

[0045] According to one embodiment, the optical signal generated by the sample is pulsed.

[0046] According to one embodiment, the optical signal generated by the sample originates from the emission of plasma generated by the sample irradiated by a pulsed laser.

[0047] According to one embodiment, the method according to the invention is configured to determine the isotopic abundance of an element present in said sample, said central wavelength of interest corresponding to a line resulting from the contribution of two isotopes of said element, said value of the central wavelength of interest making it possible to determine said abundance.

[0048] The following description presents some examples of embodiments of the device of the present invention; the scope of the invention is not limited to these examples. These examples of embodiments have both essential features of the invention and additional features related to the embodiment in question.

[0049] The invention will be better understood and other features, objects and advantages will become apparent on reading the following detailed description given by way of non-limiting example and the accompanying drawings, in which: [Brief explanation of the drawings]

[0050] [Figure 1] It has already been explained and shows the principles of measurement using LIBS, LAMIS and LIBRIS techniques. [Figure 2] It has already been explained and shows spectroscopic lines measured at low concentrations of elements in the plasma, where the effect of self-absorption is hardly noticeable or even negligible. [Figure 3] As already explained, the spectral lines measured at high concentrations of elements in the plasma show that the effect of self-absorption is significant. [Figure 4] 10. As already explained, the variation of the measured central wavelength λ0 as a function of the isotopic abundance of the lithium isotope 6Li in the sample is shown. [Figure 5] 1 illustrates a system for measuring the central wavelength of an object according to the present invention. [Figure 6] The measured subject profile and the measured reference profile are shown. [Figure 7] 1 shows a system according to the invention in the context of LIBRIS, i.e. configured to measure the isotope ratios of elements present in a sample. [Figure 8] 1 illustrates a method for determining the central wavelength of an object according to the present invention. [Figure 9] The theoretical reference profile and theoretical target profile that best fit the experimental points of the measured reference profile and measured target profile, respectively, are shown. [Figure 10] The data obtained by repeating the measurement 18 times are shown (measurements n°i numbered from 1 to 18): for each measurement i, on the one hand, the raw value (cross) is determined and, on the other hand, the corrected value (dot) determined according to the method according to the invention. [Figure 11]The mean and standard deviation of these 18 measurements in both cases (raw and corrected) are shown. DETAILED DESCRIPTION OF THE INVENTION

[0051] The present invention relates to a system 10 for measuring a central wavelength λc of interest of a spectral line RSe of interest measured by a spectrometer, as shown in Figure 5. The present invention also relates to a method 100 for measuring the central wavelength.

[0052] The present invention is applicable to LIBS or any other spectroscopic technique, regardless of the pressure used, provided that the wavelength needs to be measured accurately. In LIBS / LAMIS, this is the case in physical applications where it is advantageous to determine the wavelength accurately, 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 present invention is also applicable to LIBRIS, for which it is particularly suitable.

[0053] The spectral line of interest corresponds to emission or absorption by the sample Ech being characterized, and the optical signal produced by the sample is called the sample signal SLech. The spectral line has either a bell profile λc corresponding to the wavelength at the apex of the bell profile, or a double bell profile λc corresponding to the wavelength at the trough between the two bells.

[0054] There can be different physical effects behind the excitation of the sample used to generate the sample signal. For example, a plasma is generated that 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 induced by an excitation source other than a pulsed laser, for example an electric discharge or a glow discharge as in spark spectroscopy. The plasma can also be an induced plasma.

[0055] According to one embodiment, the sample signal is pulsed. According to one embodiment, the system 10 according to the invention also comprises a pulsed laser L configured to irradiate the sample. In response to this irradiation, the irradiated sample emits a pulsed light signal SLech that can be characterized, in particular via physical effects such as plasma emission.

[0056] The system 10 according to the invention comprises a detection system comprising a spectrometer Spectro associated with (or comprising) a detector Det, which detector comprises a number of pixels Pi aligned in the X direction, on which a spectral line RSe of interest is detected.

[0057] The system 10 also includes a reference source Sref that emits an optical signal referred to as the reference signal SLref. The reference source has a reference spectral line RSref having a center wavelength referred to as the reference wavelength of a known value λref. The reference wavelength is selected to be detected by at least one pixel of the detector. The reference source Sref is independent of the sample excitation source that generates the sample signal SLech.

[0058] The system 10 also includes a Y-shaped optical fiber FOY having a first inlet E1 and a second inlet E2 and an outlet S.

[0059] The optical fiber FOY is positioned such that the system 10 is configured such that the first inlet E1 of the fiber collects the optical signal SLech generated by the sample, and the second inlet E2 of the fiber collects the optical signal SLref generated by the reference source. Thus, the rays RSe and RSref are detected on a detector with the same spectrometer configuration (same adjustment). The reference source is selected depending on the spectroscopic characteristics of the sample to be analyzed.

[0060] Furthermore, the outlet S of the fiber is coupled to the input of a spectrometer.

[0061] Thanks to the Y-fiber, the spectrometer and the detector can receive both signals SLech and SLref simultaneously.

[0062] 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).

[0063] This detection produces a measured object profile PSech and a measured reference profile PSref, as shown in Figure 6. The x-axis of the profile is the index i of the detector pixel Pi, and the y-axis is the intensity Ii detected by each pixel.

[0064] Finally, the system comprises a processing unit UT configured to process the measured object profile and the measured reference profile to determine the object position Pech of the object central wavelength λc and the reference position Pref of the reference wavelength λref, which are measured in the detector pixels.

[0065] Based on the difference between the positions Pech-Pref, the value of λref known with precision, and DL, the linear dispersion of the detection system [spectrometer + detector], the value of λ0 is determined, typically by equation (1). Of course, it is recommended to use the value of DL corresponding to the spectral region in which λref and λ0 are located.

[0066] Due to the instantaneous or quasi-instantaneous nature of the detection of the two spectra PSech and PSref, the wavelength drift mentioned above also becomes negligible, resulting in a very good sub-picometric precision of the value of λ0.

[0067] According to the first variant, two measurement profiles (target spectrum and reference spectrum) are acquired simultaneously. For this purpose, it is recommended that the reference source has a reference wavelength that is located outside the target spectral line. Otherwise, the reference signal may interfere with the sample signal.

[0068] In the particular case of LIBRIS, the plasma signal is strong and short in duration. Preferably, the reference source should be strong enough so that the optimized acquisition parameters of the detector for the detection of each spectrum are identical.

[0069] According to a second variant, the two profiles are acquired sequentially in time: a detector Det detects the sample signal over a signal duration Ds and the reference signal over a reference duration Dref, the signal duration and the reference duration being separated by a duration called the intermediate duration Dint that we wish to minimize.

[0070] As shown in Figure 5, the time series defined by [signal SLech in Ds / Dint / signal SLref in Dref] is denoted by ST. Of course, it is also possible to invert the signals SLech and SLref within the sequence.

[0071] The exposure times Ds and Dref are adjusted according to SLech and SLref, respectively, so that the signal-to-noise ratio is sufficient for good detection of each signal.

[0072] In this second variant, the reference wavelength may be identical to the central wavelength of interest, which is for example the case for a lithium hollow cathode lamp in the context of LIBRIS analysis of lithium.

[0073] The signals SLref and SLech 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, the signal SLech is a pulsed signal, typically strong and of short duration (large amount of photons for a very short time, e.g., a plasma signal).

[0074] Due to this difference in intensity profiles, in practice, a single acquisition for simultaneous detection of the two spectroscopic profiles by Det is not possible, since the detection of each type of profile requires a different detection configuration. Thus, in the typical case where the reference source emits weakly and continuously, while the plasma emits strongly for a short period, the optimized acquisition parameters for the detection of each spectrum cannot be identical, and sequential detection is required.

[0075] Preferably, Ds <Drefである。

[0076] Therefore, the system according to the invention is configured such that the detector detects two spectra successively with different acquisition parameters.

[0077] These parameters are (non-exhaustive list): delay of measurement relative to laser shot (sample signal only), width of acquisition time gate, number and rate of accumulations, detector gain, signal averaging.

[0078] These parameters are, for example:

[0079] [Table 2]

[0080] Preferably, the detector is an intensified CCD detector.

[0081] The separation between the two measurements corresponding to Dint can be made negligible with respect to the risk of drift in the wavelength of the detection system (preferably Dint<5 seconds, or even Dint<1 second).

[0082] According to an embodiment shown in Fig. 7, the optical signal SLech results from the emission of a plasma Pl emitted by a sample Ech irradiated by a pulsed laser L. In addition to the laser L, the system 10 according to the invention comprises an optical system 2 for focusing the laser beam on the sample and an optical system SO configured to inject a portion of the optical signal generated by the sample into a first inlet E1 of the optical fiber.

[0083] According to an embodiment, the processing unit UT is further configured to synchronize the detector Det with the laser L for the detection of the sample signal.

[0084] According to one embodiment, the system 10 according to the invention is associated with the implementation of the LIBRIS technique, i.e. is configured to measure the isotope ratio of an element present in a sample E. The central wavelength of interest corresponds to a line resulting from the contribution of two isotopes of the element, the value of which allows the isotope abundance to be determined as explained above.

[0085] The duration of detection of SLech depends on 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 the central wavelength λc of a spectroscopic line RSe of interest measured by a spectrometer comprises the following steps:

[0086] A Y-shaped optical fiber OFY is provided having a first inlet E1, a second inlet E2, and an outlet S, and a reference source Sref is provided emitting a reference signal and having a reference spectral line RSref having a center wavelength called the reference wavelength of a known value λref, the reference wavelength being selected to be detected on at least one pixel of the detector.

[0087] The optical fiber OFY is then positioned so that its first inlet collects the sample signal, its second inlet collects the reference signal, and its outlet is coupled to the input Espec of the spectrometer.

[0088] The sample signal SLech and the reference signal SLref are then simultaneously detected to generate a measured object profile PSech and a measured reference profile PSref.

[0089] The measured object profile and the measured reference profile are then processed to determine object positions P and reference positions P for the measured object central wavelength and reference wavelength within the detector pixels, respectively;

[0090] Finally, the value of the central wavelength of the object is determined based on the difference between the position of the object and the reference position, the known value of the reference wavelength, and the linear dispersion DL of the detection system (spectrometer + detector).

[0091] 8, in a method 200 for determining the central wavelength λ of interest, 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, rather than simultaneously. The signal duration and the reference duration are separated by a predetermined duration called the intermediate duration Dint. The other steps are identical.

[0092] According to one embodiment of the method 200, the time series is obtained using shutters O1 and O2 placed before the inlets E1 and E2 and programmed to adjust the durations Ds, Dref and Dint. If SLech is a pulsed signal with frequency f, shutter O1 is synchronized with the frequency of signal SLref to pass the desired number of pulses, and shutter O2 is configured to transmit signal SLech for a sufficient duration Dint before or after shutter O1 is activated, making the time Dint between the activation of two shutters as small as possible, limited by the speed of the electronics of the detection system.

[0093] According to one embodiment, the sample signal originates from the emission of plasma generated by the sample irradiated by the pulsed laser. According to one embodiment, the method is configured to measure the isotopic abundance of an element present in the sample. Thus, the central wavelength of interest corresponds to a line resulting from the contribution of two isotopes of the element, the value of which allows the abundance to be determined as described above.

[0094] According to one embodiment, in order to obtain a time series, in a pre-calibration step the brightness of Sref is adjusted so that the intensity of SLref detected during time Ds is negligible, i.e., undetectable above noise.

[0095] To implement the time series ST, the signal SLech is prepared to correspond to the last shot of the laser L, and the detector is synchronized with the last shot. The duration Ds is on the order of 1 μs or 10 μs and is limited by the frequency f of the laser. Then, after a duration Dint, which may be very short, the detector is configured to detect SLref during a time window Dref, typically on the order of 1 or a few seconds. In this embodiment, no shutter is required, the signal SLref is always present on the detector, even during the detection of SLech, and SLref is negligible in the time window Ds; in this case, it is the detector Det that is programmed according to the time series. The value of Dint is also limited by the speed of the detection system's electronics.

[0096] In order to be able to measure λc with very high precision, it is required to obtain its position Pech with better accuracy than one detector pixel. To do this, according to one embodiment, the processing step includes a sub-step of adjusting the values of the measured reference profile PSref and the measured target profile PSech using a known mathematical function, and determining the target position and the reference position with better accuracy than one pixel by interpolation. Thus, as shown in FIG. 9, the theoretical reference profile PSTref and the theoretical profile PSTech of the target that best fit the experimental points are determined. Typically, the mathematical functions used are selected from Gaussian, Lorentz distribution, and Voigt profile.

[0097] From these profiles, the positions Pref and Pech are determined in fractions of a pixel (typically with an accuracy up to the second decimal place).

[0098] According to an embodiment that enables even more accurate measurement, the sequential detection step further includes an additional detection of a reference signal called Sref / add, such that the detection of the sample signal at time t0 is temporally adjacent to the detection of the reference signal Sref at additional reference signals Sref / add at, for example, t1 < t0 and t2 > t0.

[0099] The sequence ST is, for example, the signal SLref / Ds / Dint in Dref / Dint, the signal SLech / Dref, and the signal SLref / add.

[0100] In this detection step, an additional measured reference profile PSref / add corresponding to the signal SLref / add is also generated.

[0101] In the step of processing the measurement profiles, an additional reference position Pref / add is further determined. Of course, in order to determine Pref / add, fitting to a theoretical curve may be applied. Due to the drift of the spectrometer, the positions Pref and Pref / add are slightly different.

[0102] In the processing step, a reference position, called intermediate reference position P0ref, is further determined at time t0 by interpolation based on the reference position Pref and the additional reference position Pref / add and based on a predetermined law of change of the reference position as a function of time, which is typically considered linear. This interpolation corrects the drift of the spectrometer from time t1 to t0 or from time t0 to t2. Therefore, the position P0ref is more accurate than Pref, which can be obtained through 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 sample.

[0103] In this particular embodiment, the step of determining the central wavelength of interest is then performed based on the difference Pech-P0ref.

[0104] In the linear case, we obtain: P ref (t0)=P0 ref =a.t0+b(2)

[0105] The parameters a and b are determined from measurements at times t1 and t2.

number

[0106] Below we briefly present some results demonstrating the benefits of the proposed correction method. A Jobin Yvon THR1000 spectrometer was used, equipped with a 2400 line / mm grating centered at 670 nm. The detector was an Andor iStar intensified camera with 2048 x 512 pixels and a linear dispersion DL of 2.774 pm / pixel at 670 nm.

[0107] The lines of the mercury vapor lamp were measured in the presence of spectrometer drift, and the lines of the mercury vapor source formed the spectral lines of interest.

[0108] Next (or beforehand) the line of a reference source, a lithium hollow cathode lamp (HCL), was measured, this line being known with an accuracy equal to λref=670.776 nm.

[0109] The spectrometer drift was then corrected using the method 100 according to the invention (coincidence detection). Specifically, in this case the lines of the two sources are sufficiently separated in wavelength and of comparable intensity, thus allowing for simultaneous measurement.

[0110] The acquisition parameters are shown in Table III below.

[0111] [Table 3]

[0112] The graph in Figure 10 shows data obtained by repeating the measurement 18 times (measurements i numbered from 1 to 18). For each measurement i, on the one hand, the raw value λc B (i) (crosses), and on the other hand the corrected values ​​λc(i) (dots) determined according to the method 100 according to the invention. The raw values ​​were obtained by direct measurement with the detection system. The variance in the raw data can be seen and arises from the drift of the spectrometer. The corrected values ​​of λc are almost uniform across the 18 measurements.

[0113] Figure 11 shows the mean and standard deviation σ of these 18 measurements in both cases, corrected for λref for raw and HCL lamps, respectively (λ B m, σ B ) and (λ c m, σ c ) The "true" value of the wavelength of a mercury vapor lamp, which is also known with great precision, is λ lvm = 671.643 nm. This value is also shown in Figure 11 and makes it possible to test the relevance of the method according to the invention. c m is the value λ B λ is higher than m lvm It can be seen that the measurement method according to the invention significantly improves the accuracy and fidelity of the measured wavelength.

Claims

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

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

3. 2. The measurement system of claim 1, wherein the detector is configured to detect the sample signal sequentially in time over a signal duration (Ds) and the reference signal sequentially in time over a reference duration (Dref), the signal duration and the reference duration being separated by a duration referred to as an intermediate duration (Dint).

4. The measurement system of any one of claims 1 to 3, wherein the sample signal is pulsed.

5. The measurement system according to any one of claims 1 to 4, further comprising a pulsed laser (L) configured to irradiate the sample to generate the sample signal.

6. 6. The measurement system according to claim 1, further comprising an optical system (SO) configured to inject a portion of the optical signal generated by the sample into the first entrance of the optical fiber, the sample signal being emitted by a plasma (Pl).

7. 7. The measurement system of claim 6, configured to measure the isotopic abundance of an element present in the sample, wherein the central wavelength of interest corresponds to a line resulting from contributions of two isotopes of the element, and wherein the value of the central wavelength of interest allows the abundance to be determined.

8. 1. A method (100) for determining a central wavelength (λ) of interest of a spectral line (RSe) of interest measured by a spectrometer, comprising: the spectral lines of interest correspond to emission or absorption by a sample (Ech) to be characterized, the optical signal generated by the sample is called a sample signal (SLech), the spectral lines of interest have either a bell profile, the central wavelength of the object then corresponding to the apex of the bell profile, or a double bell profile, the central wavelength of the object then corresponding to the trough between two of the bells, the spectrometer is associated with a detector (Det) comprising a plurality of pixels (Pi) aligned in the X direction, the spectral lines of interest being detected on the detector pixels, The method comprises: - providing an optical fiber Y (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) with a central wavelength called reference wavelength of known value (λref), said reference wavelength being selected 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 the sample signal and the reference signal simultaneously in time to generate a measured object profile (PSech) and a measured reference profile (PSref); - processing the measured object profile and the measured reference profile to determine the object position (Pech) and the reference position (Pref) of the central wavelength of the object and the reference wavelength, respectively; - determining a value of the central wavelength of the object based on the difference between the position of the object and a reference position, the known value of the reference wavelength, and the linear dispersion (DL) of the spectrometer and the associated detector; A method (100) comprising:

9. 1. A method (200) for determining a central wavelength (λ) of a spectral line (RSe) of interest measured by a spectrometer, comprising: the spectral lines of interest correspond to emission or absorption by a sample (Ech) to be characterized, the optical signal generated by the sample is called a sample signal (SLech), the spectral lines of interest have either a bell profile, the central wavelength of the object then corresponding to the apex of the bell profile, or a double bell profile, the central wavelength of the object then corresponding to the trough between two of the bells, the spectrometer is associated with a detector (Det) comprising a plurality of pixels (Pi) aligned in the X direction, the spectral lines of interest being detected on the detector pixels, The method comprises: - providing an optical fiber Y (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) with a central wavelength called reference wavelength of known value (λref), said reference wavelength being selected 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 the sample signal over a signal duration (Ds) and the reference signal over a reference duration (Dref) sequentially in time to generate a measured object profile (PSec) and a measured reference profile (PSref), the signal duration and the reference duration being separated by a duration called intermediate duration (Dint); - processing the measured object profile and the measured reference profile to determine the object position (Pech) and the reference position (Pref) of the central wavelength of the object and the reference wavelength, respectively; - determining a value of the central wavelength of the object based on the difference between the position of the object and a reference position, the known value of the reference wavelength, and the linear dispersion (DL) of the spectrometer and the associated detector; A method (200) comprising:

10. The method of claim 9 , wherein the intermediate duration is less than 5 seconds.

11. - in said step of sequential detection, an additional detection of said reference signal, called additional reference signal (Sref / add), is then carried out so that the detection of said sample signal at time t0 is adjacent in time to the detection of the reference signal and so that an additional measured reference profile (PSref / add) is generated, - in said step of processing the measurement profile, additional reference positions (Pref / add) are further determined, and a reference position, called intermediate reference position (P0ref), is determined at said time t0 by interpolation on the basis of said reference position (Pref) and said additional reference positions (Pref / add) and on the basis of a predetermined law of variation of said reference positions as a function of time, the step of determining the central wavelength of the object is then carried out based on the difference between the position of the object (Pech) and an intermediate reference position (P0ref), The method according to claim 9 or claim 10.

12. 12. The method according to claim 8, wherein the processing step comprises the sub-step of adjusting the values ​​of the measured object profile and the measured reference profile using known mathematical functions to determine the object position and the reference position by interpolation to an accuracy of better than one pixel.

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

14. 14. The method of claim 13, wherein the optical signal generated by the sample is derived from an optical emission of plasma emitted by the sample irradiated by a pulsed laser.

15. 15. The method of claim 14 configured to determine the isotopic abundance of an element present in the sample, wherein the central wavelength of interest corresponds to a line resulting from the contribution of two isotopes of the element, and wherein the value of the central wavelength of interest allows the abundance to be determined.

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