Raman spectroscopy device

Abstract

The present invention relates to a Raman spectroscopy device (1) comprising a radiation emission source (2), a probe (4), and a spectrometer (5), at least one first optical fiber (3), referred to as the emission fiber, adapted to carry the emitted radiation from the emission source (2) to the probe (4), and at least one second optical fiber (3'), referred to as the collection fiber, adapted to carry the return radiation from the probe (4) to the spectrometer (5). The first optical fiber (3) and / or the second optical fiber (3') is a solid fiber having a metallic coating (33) on at least part of its outer surface. Figure for the abstract: Fig. 1

Description

Title of the invention: Raman spectroscopy device technical field

[0001] The invention relates to a Raman spectroscopy device. This analytical method is effective for identifying and / or quantifying chemical species in a given medium, particularly in a liquid or gaseous fluid, and thus has numerous applications, notably in monitoring and quality control of industrial manufacturing processes. This type of equipment generally uses a laser light source, the radiation from which strikes, for example, a sample to be analyzed, the return signal being collected and then processed to obtain the desired information.To convey the signal emitted from the radiation source to the sample, and then to collect the return signal from the sample to the analysis methods, optical fiber guidance methods are commonly used, which have the advantage of being very flexible in their implementation: their shape and length can easily be adapted to suit various environments, which is valuable whether the device is intended to be fixed or portable. Previous technique

[0002] Raman spectroscopy devices are very useful analytical tools, particularly in industrial settings. Patents FR2951384 and FR2951385 thus propose, by way of illustration, the use of such analytical tools to measure the H2S concentration of an absorbing solution used to deacidify a gas containing H2S.

[0003] The Raman spectroscopic technique uses a laser (generally in the visible range) to extract information located in the infrared range, therefore at a much lower energy level. This approach is made possible by exploiting the inelastic behavior of the radiation / matter interaction.

[0004] We will be interested here in Raman spectroscopy devices using optical fibers to carry the emission signals and / or the return signals (as opposed to Raman devices operating in free optics).

[0005] However, the use of this technique raises the problem of the phenomenon of spurious fluorescence, which can be generated by the container of the sample to be analyzed or by the various optics necessary for irradiation and collection of the light (Raman) produced by the sample. This phenomenon has led to the proposal of various solutions, some more restrictive than others, aimed at filtering the fluorescence accumulated during the optical path of the emission signal to the sample and then of the collection / return signal from the sample.

[0006] Two origins are generally identified for this fluorescence, depending on whether it comes from the sample to be analyzed or from the optical chain used to perform this spectroscopy: When the origin is intrinsically linked to the sample to be analyzed, such as the nature of the solvent or the presence of impurities, it can be remedied by changing the excitation wavelength, which is done in the majority of cases.

[0007] This can also be addressed in a more complex way by separating the Raman spectral response from the fluorescence response through a time-domain analysis of the phenomena. Indeed, it is known to specialists (see, in particular, the publication Wei, Dong; Chen, Shuo; Liu, Quan (2015) Review of Fluorescence Suppression Techniques in Raman Spectroscopy. In: Applied Spectroscopy Reviews, vol. 50, no. 5, pp. 387-406. DOI: 10.1080 / 05704928.2014.999936) that the lifetime of the electronic excitation levels of fluorescence is longer than that associated with the inelastic Raman re-emission phenomenon. However, to implement this solution, the analytical equipment is more complex and very expensive because it must differentiate the signals with windows of comparable duration to a few hundred picoseconds.

[0008] When the fluorescence originates in the optical components of the excitation beam's optical chain, filtering the excitation line can be simply achieved using a dichroic bandpass filter (also known as a "notch filter"). A dichroic filter, and often also a high-pass filter (also known as an "edge filter"), is placed at the end of the fiber to eliminate any unwanted reflection or elastic scattering (Rayleigh line) of the laser excitation line. However, adding these filters increases the overall size of the device, particularly the dimensions of the probe where the signal-emitting optical fiber terminates and from which the return signal-collecting optical fiber originates.

[0009] It is also known, notably from the publication by Yerolatsitis, Stephanos; Kufcsâk, Andrâs; Ehrlich, Katjana; Wood, Harry AC; Fernandes, Susan; Quinn, Tom et al. (2021) (Sub millimeter flexible fiber probe for background and fluorescence-free Raman spectroscopy. In: Journal of Biophotonics, vol. 14, no. 10, e202000488. DOI: 10.1002 / jbio.202000488), to use hollow optical fibers: laser excitation is carried by means of hollow fibers (called "NCF" or "Negative Curvature Fiber"), in order to eliminate fluorescence from the core of the silica fiber. This publication reports that the result obtained is good, compared to a conventional fiber, with an attenuation factor of 99.9%. However, because the fiber is hollow, a small window must be fitted to its end for working with liquids. This solution also raises questions about its resistance to high pressures and its behavior under temperature conditions.

[0010] The invention aims to develop an improved Raman spectroscopy device that can, in particular, counteract or at least limit some of the fluorescence phenomena that disrupt analyses, while remaining reliable and robust, and without significantly complicating its design or implementation. Summary of the invention

[0011] The invention relates firstly to a Raman spectroscopy device comprising a radiation emission source, a probe, and a spectrometer, at least one first optical fiber, referred to as the emission fiber, capable of carrying the emitted radiation from the emission source to the probe, and at least one second optical fiber, referred to as the collection fiber, capable of carrying return radiation from the probe to the spectrometer. The first optical fiber and / or the second optical fiber is a solid fiber having a metallic coating on at least part of its outer surface.

[0012] The optical fibers according to the invention preferably comprise an optical core and cladding, the metallic coating then being disposed on at least a part of the outer surface of the optical cladding, in particular its entire outer surface.

[0013] The invention discovered that the protective layer, usually polymer-based, covering the outer surface of the optical fiber generated a specific fluorescence phenomenon, which disrupted the signal guided by the optical fiber and, consequently, disrupted the sharpness of the signals (emission and / or collection), thus affecting the results of Raman analysis. The invention then proposed replacing this polymer protective layer with a fluorescence-neutral coating, and it was found that a metallic material met this criterion while still adequately fulfilling its protective role.Furthermore, this choice of a metallic protective coating allows the use of these optical fibers, and the spectroscopy device that integrates them, in high-temperature environments (for example, at least 100° or at least 200° or 300°, or even at least 500 or even at least 700°C), which was not possible with polymer-clad fibers that generally degrade at such high temperatures, but also in cold, or even very cold, environments, for example down to -296°C.

[0014] According to the invention, preferably, the optical fiber in question does not include a polymer-based coating, either above or below the metallic coating in question. Even more preferably, the metallic coating is in direct contact with the outer surface of the optical fiber, in particular with its optical cladding.

[0015] According to one variant, the fiber is covered with several superimposed metallic coatings, of different nature or composition, of the same thickness or of different thicknesses.

[0016] Preferably, the first and / or second optical fibers are entirely covered by said metallic coating. The coating thus ensures complete protection of the fiber in question.

[0017] Preferably, the metallic coating material is chosen from at least one of the metals belonging to groups 8, 9, 10, 11 and 12 according to the IUPAC classification.

[0018] The metallic coating material may in particular include at least one of the following metals: gold Au, aluminium, copper Cu, nickel Ni.

[0019] The metallic coating material may consist of a single metal, or an alloy of at least two metals.

[0020] Advantageously, the first and / or second optical fiber can be cylindrical in cross-section with different core diameters, therefore single-mode or multi-mode.

[0021] Preferably, the metallic coating has a thickness of at least 10 nm, and preferably of at most 100 nm. Its thickness may, for example, be between 30 and 50 nm.

[0022] Preferably, the first and / or second optical fiber is silica-based for the core and doped silica for the optical cladding.

[0023] According to one embodiment, the first and / or second optical fiber is cylindrical and has a core diameter of at least 9 microns, preferably of at most 600 microns, and preferably between 100 and 300 microns.

[0024] The radiation emission source used is preferably capable of emitting monochromatic radiation, in particular a laser, the wavelength of which is preferably chosen in the infrared, ultraviolet or visible range.

[0025] The device according to the invention may also include radiation filtering means, which may be: - arranged in the probe, - and / or positioned at one end of the first and / or second optical fiber, - and / or interposed between two portions of the first and / or second optical fiber.

[0026] The various optical means such as these filtering means can indeed have different locations, in particular depending on whether or not there is a constrained dimensioning for the probe (for example), or depending on the environment of the device and what one seeks to analyze.

[0027] The invention also relates to the use of the device according to the invention to determine the nature and / or content of a compound in a medium, in particular in a fluid.

[0028] The invention also relates to the manufacturing process of the device according to the invention, integrating the step of depositing the metallic coating on the optical fiber(s), in particular by a thin film deposition technique, for example by physical deposition (in particular by sputtering, vacuum evaporation, pulsed laser deposition, electrohydrodynamic deposition...) or chemical deposition (in particular by a sol-gel process, centrifugal coating, vapor phase deposition possibly assisted by plasma...).

[0029] The invention is described below in more detail with the aid of non-limiting figures and examples of embodiments. List of figures

[0030] [Fig.1] Figure [1] represents a first configuration of a Raman spectroscopy device according to the invention. [Fig.2] Figure [Fig. 2] represents a second configuration of a Raman spectroscopy device according to the invention. [Fig.3] Figure 3 represents one end of an optical fiber used to transmit the emission signal into either of the devices in the previous figures, illustrating the phenomenon known as injection fluorescence. [Fig.4] Figure 4 represents one end of an optical fiber used to transmit the emission signal into either of the devices in Figures 1 or 2, illustrating the phenomenon of so-called injection reflections. [Fig. 5] Figure 5 represents one end of an optical fiber used to transmit the collected (return) signal into either of the devices in Figures 1 or 2, illustrating the phenomenon known as collection fluorescence. [Fig.6] Figure 6 represents one end of an optical fiber used to transmit the collected (return) signal into either of the devices in Figures 1 or 2, illustrating the phenomenon of reflections / scattering known as collection. [Fig.7] Figure 7 represents a graph where the x-axis corresponds to the emission wavelength of the radiation source of a Raman spectroscopy device in nm. and whose ordinate quantifies in arbitrary units ua the fluorescence due to the use of comparative optical fibers according to the invention.

[0031] It is emphasized that the figures, and in particular figures 1 to 6, are intentionally very schematic, that the different components represented are not necessarily to scale or in the spatial position of operation of the device.

[0032] The same references refer to the same components / luminous flux from one figure to another. Description of the implementation methods

[0033] The invention relates to Raman spectroscopy devices, which are described with reference to Figures 1 and 2, which are non-limiting examples: these devices 1 comprise, in a known manner, a radiation emission source 2, such as a laser, which emits a signal called the emission signal SI. This signal is transmitted by signal transmission means, in particular one or more optical fibers 3, to a probe 4, which directs this emission signal against, for example, a sample to be analyzed (not shown). The return signal S2 is captured by the probe 4 and then transmitted by signal transmission means, in particular one or more optical fibers 3' – called collection or return fibers – to a spectrometer 5.These are the components necessary for the operation of the device, but of course the device may include other components not detailed here because they are not essential to the implementation of the present invention and / or are otherwise known to specialists in this technique (optical components such as filters, mirrors, lenses, electronic / computer / electrical components and display devices that can be integrated into the spectrometer, etc.). Figures 1 and 2 show, as examples of these other possible components of the optical component type: - at least one filter intended to filter the SI emission signal, called the laser filter 6, which is either integrated into the probe ([Fig.1]) or placed on the path of the emission optical fiber 3 ([Fig.2]), depending on the configuration of the device in its operating environment, and in particular any size constraints concerning the probe 4 - a mirror or a mirror system(s) 7 integrated into the probe 4 in the example of [Fig.1] - at least one filter 8 intended to filter the return signal S2, this filter 8 being able to be integrated into the probe 4 ([Fig.1], this filter can be designated by the English term "filter edge") or be placed on the path of the optical collection fiber 3' ([Fig.2], this filter can be of the high-pass type).

[0034] The sample to be analyzed may be in liquid, solid or gaseous form. The aim may be to identify one or more compounds in a given medium (solid, liquid or gaseous) and / or to quantify their content(s) using this spectroscopic technique.

[0035] The device can, in operation, be permanently installed, for example in an industrial site or an analytical laboratory, or be portable and intended for field analysis (in the fields of geology, water analysis, etc.).

[0036] According to current technique, when using an optical fiber (or a bundle of optical fibers: in this text we will use the term "fiber" to include a single optical fiber or a set / bundle of optical fibers, for the sake of brevity), various phenomena appear which impair the quality of the analysis by Raman spectroscopy.

[0037] Thus, [Fig.3] illustrates a first disturbing phenomenon, which is that of the generation of fluorescence at the input of the emission signal SI in the optical fiber 3: here the fiber 3 is solid, essentially made of silica, with a known core 31, an optical cladding 32, and a protective layer 33 made of polymer, for example based on polyimide.

[0038] The signal SI passes through a lens 9, so as to concentrate the signal SI and guide it towards the "upstream" end of the fiber 3 ("upstream" and "downstream" being understood according to the general direction of propagation of the signal considered in the optical fiber considered). The arrows fl (solid lines) represent the propagation of the signal SI within the fiber 3.

[0039] But it tends to create so-called fluorescence ZI zones near the upstream end of the fiber 3, in the vicinity of / within the protective layer 33, with parasitic fluorescence signals which will also propagate (dashed arrows f2) in the fiber with a delay relative to the propagation of the SLA signal at the downstream end of the fiber 3, so we have two signals, the SI emission signal and an S3 fluorescence signal.

[0040] Fig. 4 illustrates a second disturbing phenomenon, which is that of the generation of multiple reflections (dashed arrows f3) generated from so-called reflection zones Z2, close to the upstream end of the fiber and in the vicinity / within the protective layer 33 also, hence, at the output, other parasitic signals S4.

[0041] Figure 5 illustrates a third disturbing phenomenon, namely the generation of fluorescence called collection fluorescence at the output of the collection signal S2 in the optical fiber 3', at its downstream end: there tends to be created areas Z3 called fluorescence zones near the downstream end of the fiber 3, in the vicinity of / within the protective layer 33, with parasitic fluorescence signals which will also propagate in the fiber with a delay relative to the propagation of the collection signal S2 and relative to the parasitic signals, in particular created during the guidance of the emission signal SI in the fiber 3. We thus end up, at the output of the fiber 3' (of the same design here as the fiber 3), at the output of the fiber 3', a multitude of signals including the collection signal S2, but also parasitic signals S5 coming from different diffusions / reflections.

[0042] Fig. 6 illustrates, symmetrically to Fig. 4, a fourth disturbing phenomenon, which is that of the generation of reflections / scattering at the output of the collection fiber 3', with reflection zones Z4 in the vicinity of the downstream end of the fiber 3' and located in the vicinity of / within the protective layer 33: here again, at the output other parasitic rays S6 are added to the signal S2 by reflection and / or scattering: part of the intensity of the laser excitation will, by reflection, noted (2), or by scattering, noted (1), excite the fluorescence of the polymer material of the protective layer 33 of the collection fiber 3'.

[0043] The inventors of the present invention have thus observed that the perturbation phenomenon observed for the emission signal SI (Figures 3, 4) and for the collection signal S2 (Figures 5, 6) is at least partly a fluorescence phenomenon. This fluorescence can be excited during the injection of the signal from the laser source into the fiber, but can also appear throughout the signal propagation, due to scattering produced, for example, by core defects in the fiber. Additional fluorescence can appear when the optical fiber is bent, which limits the advantages (flexibility, adaptability) of its use. This fluorescence can also appear during the collection of the return signal, particularly when using a probe in a confined geometry (Figures 5, 6).

[0044] The Raman spectroscopy device was then modified according to the invention by removing the protective polymer layer 33 from the optical fibers 3, 3'. This protective layer was replaced by a coating that is essentially, and in particular, entirely metallic. Preferably, this coating constitutes a protective layer that completely covers the outer wall of the fiber. Advantageously, it has a relatively constant thickness sufficient to protect the outer wall of the fiber from contact with its environment. Furthermore, it is preferable not to choose an excessively thick coating, particularly to best preserve the flexibility of the optical fiber.

[0045] Surprisingly, it turned out that this simple modification considerably improved the quality of the analysis by Raman spectroscopy, by significantly reducing parasitic phenomena, which arose at least in part from the choice of a polymer material to protect the fiber. Comparative Example 1

[0046] This is a 3.3' optical fiber as described above, the core of which is made of silica and the cladding of which is made of fluorine-doped silica, with a core diameter of 200 micrometers and coated with a protective polyimide coating, for example available from THORLABS under the commercial reference FG200UEP - 0.22 NA. Example 2 according to the invention

[0047] An example of an embodiment according to the invention consisted of replacing the protective polyimide layer 33 of the optical fiber of comparative example 1 with a metallic protective layer, made of gold, with a thickness of, for example, at least 30 nm. This metallic coating was deposited on the surface of the fiber by a thin-film deposition technique, for example by sputtering. Alternatively, a protective layer made of another metal or metal alloy, particularly aluminum, can be chosen, with the same advantages.

[0048] The radiation emission source is a laser emitting radiation centered on a wavelength of 532 nm.

[0049] The graph in [Fig.7] represents the fluorescence spectra 1, 2, 3 of three materials (with wavelength in nm on the x-axis and an arbitrary unit ua on the y-axis): - that of an optical fiber 3 with the gold coating defined above: example 2 according to the invention, it is the curve Cl - that of the same optical fiber 3 equipped with a protective polyimide layer: comparative example 1, this is curve C2 - that of a polyimide polymer sample: this is the C3 curve

[0050] On the graph, they have been circled to better highlight them: - a zone A around the 532 nm wavelength line corresponding to the emitted laser radiation, where a fluorescence phenomenon is seen generating a secondary peak in the Cl and C2 curves, which has been analyzed as corresponding to the fluorescence of the silica constituting the optical fiber 3, - a zone B extending between approximately 580 and 680 nm, with, for curves C2 and C3, a rounded, spreading portion with peaks of significant intensity, which has been analyzed as corresponding to the fluorescence of the polyimide material, while curve Cl is flat in this zone, - a "flat" zone C of the Cl curve corresponding to an absence of fluorescence in the case of the optical fiber according to the invention with metallic coating.

[0051] Thus, in the case of polyimide-clad optical fiber probes, a fluorescence amplitude varies from probe to probe, indicating an uncontrolled parasitic phenomenon. These differences can be attributed to different radii of curvature of the installed fibers, the presence of scattering defects, or be related to a degraded laser injection process in the transmitting fiber. Conversely, in the case of the fiber coated with a protective metallic sheath, the absence of the fluorescence phenomenon is observed.

[0052] In conclusion, the solution according to the invention does not eliminate all fluorescence phenomena, particularly that due to the very nature of the material. (Silica here) constitutes the optical fiber. However, it eliminates the fluorescence phenomenon caused by the protective polymer layer of the optical fiber, which significantly distorted the obtained spectrum. By opting for a metallic protective layer, the invention combats fluorescence whose cause was previously unknown, while retaining the protective role of a protective layer, but this time made of a material that does not generate fluorescence, and this applies to both the signal transmitted from the source and the signal collected from the probe. Therefore, it is advantageous to apply the invention to both the transmission and collection fibers when optical fibers are used to guide the transmission and collection signals.

[0053] Any metal can be considered for the coating of the invention, including any metallic alloy. It is also possible to choose a coating that is not entirely metallic, or even one that is at least partially polymeric, provided that the chosen material does not generate, or does not generate significantly, fluorescence while still being capable of providing mechanical and chemical protection to the optical fiber.

[0054] Note that the invention can also be applied to devices using optical fibers only for transmission or only for collection.

[0055] The invention is particularly advantageous for limiting fluorescence created in optical fibers in Raman spectroscopy devices using high-power laser sources.

[0056] It can also be noted that the invention is also advantageous if the species sought by the Raman spectroscopy device are located at Raman shifts greater than the last characteristic peaks of the silica composing the optical fibers, i.e., beyond 800 cm⁻¹. It is then possible to eliminate the need for a dichroic bandpass filter in the device in these cases, particularly in the 532-1085 nm emission range of the laser source, which allows: - to have the maximum laser power available in the analysis area, - to simplify the design of the spectroscopy device as a whole, - to obtain better spectral resolutions at constant laser source power, or to obtain identical spectral resolutions with a lower laser source power (and therefore potentially to switch to a less powerful laser source category) - and possibly to have shorter data acquisition times.

Claims

Demands

1. Raman spectroscopy device (1) comprising a radiation emission source (2), a probe (4) and a spectrometer (5), at least one first optical fiber (3), referred to as the emission fiber, capable of conveying the emitted radiation from the emission source (2) to the probe (4), and at least one second optical fiber (3'), referred to as the collection fiber, capable of conveying return radiation from the probe (4) to the spectrometer (5), characterized in that the first optical fiber (3) and / or the second optical fiber (3') is a solid fiber having on at least a part of its outer surface a metallic coating (33).

2. Raman spectroscopy device (1) according to the preceding claim, characterized in that the first fiber (3) and / or the second fiber (3') are entirely covered by said metallic coating (33).

3. Raman spectroscopy device (1) according to any one of the preceding claims, characterized in that the metallic coating material (33) is made of at least one metal selected from metals belonging to groups 8, 9, 10, 11 and 12 according to the IUP AC classification.

4. Raman spectroscopy device (1) according to any one of the preceding claims, characterized in that the metallic coating material (33) comprises at least one of the following metals: gold Au, aluminium Al, copper Cu, nickel Ni.

5. A device according to any one of the preceding claims, characterized in that the metallic coating (33) has a thickness of at least 10 nm, in particular of at most 100 nm, and is preferably between 30 and 50 nm

6. Raman spectroscopy device (1) according to any one of the preceding claims, characterized in that the first (3) and / or the second (3') optical fiber is silica-based.

7. Raman spectroscopy device (1) according to any one of the preceding claims, characterized in that the radiation emission source (2) is capable of emitting monochromatic radiation, in particular a laser, the wavelength of which is preferably chosen in the infrared, ultraviolet or visible range.

8. Raman spectroscopy device (1) according to any one of the preceding claims, characterized in that it comprises radiation filtering means, which are: - disposed in the probe (4), - and / or disposed at one end of the first (3) and / or the second (3') optical fiber, - and / or intercalated between two portions of the first (3) and / or the second (3') optical fiber.

9. Use of the Raman spectroscopy device (1) according to any one of the preceding claims to determine the nature and / or content of a compound in a medium, in particular in a fluid.

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