Brillouin spectroscopic analysis device and method

Abstract

A sample observation device (1) comprising: a light source (10); an objective lens (13); a multimode optical fiber (15); an optical dispersion element (18) configured to allow angular dispersion of the light transmitted by the objective lens as a function of wavelength; an image sensor (19) configured to form an image of the light dispersed by the optical dispersion element; and a processing unit (30) configured to accumulate the intensity detected on pixels along each isofreference line, so as to obtain a frequency spectrum, taking into account the curved shape of each isofreference line. Figure 6A.

Description

Title of the invention: Device and method for analysis by Brillouin spectroscopy technical field

[0001] Brillouin spectroscopy is a laser spectroscopy technique used to probe the opto-mechanical properties of samples at frequencies on the order of GHz. It is based on the inelastic Brillouin scattering effect that occurs in a sample illuminated by a laser beam. Inelastic scattering results in a change in frequency related to the propagation speed of acoustic waves in the sample.

[0002] The frequency change is such that — ~V if ny,

[0003] where: - 2 is the wavelength of the laser in a vacuum; - n the refractive index of the material; - V is the speed of sound in the material; - 9 is the diffusion angle.

[0004] Brillouin scattering results in the appearance of a doublet of peaks at frequencies v = ± vB in the spectrum of the scattered light, where vl is the beam frequency laser. The position of each Brillouin scattering peak is representative of the elastic modulus of the sample, the latter being related to the speed of sound in the sample by the relation y - I p - where M is the longitudinal elastic modulus and P the mass volumetric. The width of these peaks indicates the attenuation of the sound in the sample according to the following relationship:

[0005] Afe = aV / Æ, where - is the width at half height of the Brillouin peaks; - a is the acoustic attenuation coefficient.

[0006] For example, when the sample is water, using a near-infrared laser (780 nm, vl = 3.8 x 10¹² Hz) yields a frequency variation vb = 5.7 GHz on either side of the laser frequency.

[0007] The frequency shift resulting from Brillouin scattering is small, typically less than 10 pm in wavelength. A typical Brillouin spectrometer must have a resolving power R = v / ΔV greater than 70,000. Such a resolving power is obtained using a Fabry-Perot standard such as a VIPA (Virtually Images Phased Array) standard. A VIPA is an optical component related to the Fabry-Perot standard that allows obtaining, from an angular distribution of beams, a spatial frequency distribution of these beams. The VIPA is usually associated with a lens and an image sensor. The image formed on the image sensor through the lens allows visualization and characterization of the spectral peaks separated by the VIPA.

[0008] Confocal spectroscopy allows for three-dimensional mapping of a Brillouin signature in biological media. A Brillouin signature is defined as the position and / or width of the peaks at frequencies V — ± vB. The signature Brillouin is correlated with the opto-mechanical properties of the system and can thus mark the potential effect of external stimuli or pathologies.

[0009] In most prior art devices, light propagates in free space, up to the VIPA, or is guided by a single-mode optical fiber. This choice maximizes spatial resolution but only allows for the collection of a low luminous flux.

[0010] The Meng publication "Optimizing signal collection efficiency of the VIPA-based Brillouin spectrometer" describes a Brillouin spectrometer in which a laser light source illuminates a sample. This publication describes a configuration in which a multimode optical fiber is placed upstream of the VIPA. However, it is noted that the use of a multimode optical fiber does not allow the observation of any Brillouin scattering peaks.

[0011] The inventors propose an alternative device, allowing a usable signal of higher intensity to be obtained than prior art devices. Description of the invention

[0012] A first object of the invention is a device for observing a sample, configured to maintain the sample in an observation position, the device comprising: - a light source emitting an incident beam with an emission wavelength; - a lens, configured to focus the incident beam towards the observation position, and to collect light scattered by the sample under the effect of illumination of the sample by the incident beam; - a multimode optical fiber, coupled to the lens, arranged to guide the light scattered by the sample; - an optical dispersion element, configured to allow angular dispersion of light, resulting from the multimode optical fiber, as a function of frequency; - an image sensor configured to form an image of the light dispersed by the optical dispersion element, the image sensor comprising pixels, the pixels being distributed in groups of pixels, each group of pixels being associated with the same frequency value; - a processing unit, configured to form a spectrum from the image formed by the image sensor, the spectrum being a histogram of the cumulative intensities of pixels belonging to the same pixel group;

[0013] the device being characterized in that: - the image sensor is configured so that each group of pixels extends along an isofreference line forming a curve, with a frequency value being assigned to each isofreference line; - the processing unit is configured to accumulate the intensity detected on pixels along each isofreference line, in order to obtain a frequency spectrum, taking into account the curved shape of each isofreference line.

[0014] The optical dispersion element may, in particular, be a Fabry-Perot type standard.

[0015] Advantageously, the diameter of the multimode optical fiber is greater than 10 pm. It may be between 10 pm and 500 pm, preferably between 20 pm and 200 pm.

[0016] According to one possibility, - the device includes a lens designed to focus the light, resulting from the optical fiber, along an optical axis; - the optical dispersion element is inclined with respect to the optical axis, at an angle of inclination between 0.5° and 10°.

[0017] The device may include an extinction chamber, comprising a gas configured to absorb light at the emission wavelength.

[0018] According to one embodiment, the extinction chamber is interposed between the optical dispersion element and the image sensor.

[0019] According to another embodiment, the optical dispersion element is interposed between the extinction chamber and the image sensor.

[0020] According to one possibility: - the curved shape of each isofreference line is parameterized by predetermined parameters; - the processing unit is connected to a memory containing said parameters.

[0021] The processing unit can be programmed to adjust the value of the parameters of the curved shape of an isofreference line or of each isofreference line by an analysis of the image formed by the image sensor.

[0022] A second object of the invention is a method for forming a frequency spectrum of light scattered by a sample, using a device according to the first object of the invention, the method comprising: a. disposition of the sample at the observation position defined by the device; b. illumination of the sample by the device's light source and ac- acquisition of an image of light by the image sensor of the device, the image comprising curved isofreference lines, extending transversely to an axis of angular dispersion, to each isofreference line being assigned a frequency value; c. using the device's processing unit, cumulatively the intensity detected on pixels along each curved line, so as to obtain the frequency spectrum.

[0023] The curved shape of the isofrequency lines can be predetermined. According to one possibility, - the curved shape is parameterized by parameters, stored in the processing unit; - the process includes a step of adjusting the value of the parameters from a processing of the image acquired by the image sensor.

[0024] The curved shape can be determined by implementing a calibration sample.

[0025] A third object of the invention is a device for observing a sample, configured to maintain the sample in an observation position, the device including: - a light source emitting an incident beam with an emission wavelength; - a lens, configured to focus the incident beam towards the observation position, and to collect light scattered by the sample under the effect of illumination of the sample by the incident beam; - an optical fiber, coupled to the lens, arranged to guide the light scattered by the sample; - an optical dispersion element, configured to allow angular dispersion of the light, resulting from the optical fiber, as a function of the wavelength; - an image sensor configured to form an image of the light dispersed by the optical dispersion element, the image sensor comprising pixels, the pixels being distributed in groups of pixels, each group of pixels being associated with the same frequency value; - a processing unit, configured to form a spectrum from the image formed by the image sensor, the spectrum being a histogram of the cumulative intensities of pixels belonging to the same pixel group;

[0026] the device being characterized in that:

[0027] The device may include an extinction chamber, comprising a gas configured to absorb light at the emission wavelength, the extinction chamber being interposed between the optical dispersion element and the image sensor.

[0028] Optical fiber can be multimode or single-mode.

[0029] The device according to the third object of the invention may include characteristics risks described in connection with the first object of the invention.

[0030] A fourth object of the invention is a method for forming a frequency spectrum of light scattered by a sample, using a device according to the third object of the invention, the method comprising: a. disposition of the sample at the observation position defined by the device; b. illumination of the sample by the device's light source and acquisition of an image of the light by the device's image sensor; c. using the device's processing unit, accumulate the intensity detected on pixels along each line, so as to obtain the frequency spectrum.

[0031] The device according to the fourth object of the invention may include features described in relation to the second object of the invention.

[0032] The invention will be better understood upon reading the description of the exemplary embodiments presented later in this description, in connection with the figures listed below. FIGURES

[0033] Fig. 1A represents a device according to the invention in a first embodiment.

[0034] [Fig.1B] is a detail of the device shown schematically in [Fig.1A].

[0035] Figure [Fig. 2A] represents a device according to the invention in a second embodiment.

[0036] [Fig.2B] is a detail of the device shown schematically in [Fig.2A].

[0037] Figure 3A is an example of an image formed by the image sensor of a device according to the invention.

[0038] The [Fig.3B] is a frequency spectrum resulting from the image shown in the [Fig.3A]

[0039] Fig. 4A represents two spectra respectively formed by considering two different angles of inclination of the VIPA of a device according to the invention.

[0040] Fig. 4B illustrates the inclination of the VIPA.

[0041] Figure 5A is an image resulting from the use of a device according to the invention, by implementing a multimode optical fiber with a diameter of 25 µm.

[0042] The [Fig.5B] is a spectrum obtained from the image shown in the [Fig.5A].

[0043] Figure 6A is an image resulting from the use of a device according to the invention, by implementing a multimode optical fiber with a diameter of 105 pm.

[0044] The [Fig.6B] is a spectrum obtained from the image shown in the [Fig.6A].

[0045] The [Fig.7A] is an image resulting from the use of a device according to the invention, implementing a single-mode optical fiber with a diameter of 5pm.

[0046] The [Fig.7B] is a spectrum obtained from the image shown in the [Fig.7A].

[0047] Figure 8 illustrates the main steps in implementing a method for forming a Brillouin scattering spectrum using the invention. Description of specific embodiments

[0048] Figure 1A describes a device 1 as described in the previously cited publication. A light source 10 produces an incident light beam that propagates through a half-wavelength plate 10', before being reflected by a reflector 11 towards a sample S, through a beam splitter 12 and an objective lens 13. The sample S is placed on a support 20, which may be part of the device. A quarter-wave plate 12' extends between the beam splitter and the objective lens 13. The support 20 defines an observation position. The sample S occupies the observation position. The objective lens 13 focuses the incident light beam onto the sample at the observation position.

[0049] The light scattered by the sample S is reflected, by the separator cube 12, towards a multimode optical fiber 15, then sent into an analysis system 16. Thus, the multimode optical fiber is coupled to the objective 13, so as to collect part of the light scattered by the sample, under the effect of illumination by the incident beam.

[0050] The analysis system includes an extinction chamber 17, filled with a gas designed to absorb some of the light resulting from the sample. The light from the extinction chamber is propagated to a VIPA standard 18. The VIPA is positioned upstream of an image sensor 19. The VIPA performs angular dispersion of the light from the optical fiber, depending on the wavelength. Thus, the VIPA enables spectral separation of the light along an angular dispersion axis. In [Fig. 1B], the angular dispersion axis is the Y-axis.

[0051] In this device, the optical fiber 15 is a multimode optical fiber. Using a multimode optical fiber increases the amount of light guided between the sample and the image sensor. Thus, the multimode optical fiber 15 increases the luminous flux reaching the image sensor 19. Furthermore, the coupling of the light scattered by the sample in a multimode optical fiber is simplified compared to a single-mode optical fiber. This facilitates the alignment of the analysis device 16 with the assembly formed by the objective 13 and the beam splitter cube 12 and increases the stability of the alignment over time. The diameter of the multimode optical fiber is preferably greater than 10 µm. It can be between 10 µm and 500 µm. Preferably, the diameter of the multimode fiber is between 20 µm and 200 µm.

[0052] The extinguishing chamber 17 comprises a gas selected to exhibit an ab High sorption occurs in the emission spectral band of the light source. In this example, the emission spectral band is centered around 780 nm (3.8 x 10¹² GHz). The extinction chamber contains Rubidium-85, which has an absorption peak at a wavelength of 780.24 nm. The dimensions of the extinction chamber are, for example, 75 mm (length) by 19 mm (diameter).

[0053] Fig. 1B is a detailed view of the analysis device 16 shown in Fig. 1A. The analysis device 16 is located downstream of a lens L1 configured to focus the beam, reflected by the beam splitter cube 12, onto a bandpass filter 15' centered on the emission wavelength. The bandpass filter 15' is located upstream of the multimode optical fiber 15.

[0054] The light from the multimode optical fiber 15 is relayed, by an optical system consisting of two lenses (L2, L3), to the extinction chamber 17. Lens L2 collimates the light, while lens L3 focuses the light onto an input window 18f of the VIPA, along an optical axis A. In this example, lens L3 is a cylindrical lens. The light exiting the extinction chamber is directed towards the input window 18f of the VIPA. Upon exiting the VIPA, the light is angularly split, depending on the wavelength, along the angular separation axis defined by the VIPA. Upon exiting the VIPA, the light propagates at an angle, relative to the optical axis A, that depends on the frequency. The light from the VIPA is then focused towards the image sensor 19 by a lens L4.

[0055] In the configuration shown in Figures IA and IB, the VIPA standard 18 is positioned downstream of the extinction chamber 17. The term downstream should be interpreted according to the direction of light propagation. A difficulty associated with this configuration is that the extinction cell 17 can become saturated, leading to glare on the sensor 19 from the light source 10. The risk of saturation of the extinction cell 17 is all the greater because the multimode fiber 15 can guide a potentially significant amount of light.

[0056] Figures 2A and 2B illustrate an embodiment in which the VIPA 18 is positioned upstream of the extinction cell 17. According to this embodiment, the light, spectrally separated by the VIPA, passes through the extinction chamber 17 before being focused by the lens L4 onto the image sensor 19. This arrangement allows the amount of light transmitted into the extinction chamber 17 to be adjusted by varying the tilt of the VIPA. Indeed, the tilt of the VIPA determines the size of the VIPA's input window 18f. By decreasing the tilt angle a, the size of the input window 18f decreases, which can prevent saturation of the extinction cell 17. According to the configuration shown in Figures 2A and 2B, adjusting the tilt angle a of the VIPA allows a compromise to be achieved between the intensity of the transmitted light and preventing saturation of the extinction cell. 17. This allows the image sensor to produce a signal with an optimized signal-to-noise ratio.

[0057] As previously mentioned, the entrance window 18f of the VIPA has a size defined by its inclination. The entrance window generally extends over a height of a few hundred micrometers. The VIPA is inclined, with respect to the optical axis A, at an angle of inclination α that can be between 0.5° and 10°, preferably between 0.5° and 5°, for example around 1°. The angle of inclination α is defined as the angle between the optical axis A and the normal N to the VIPA. It defines an angular acceptance range for the VIPA.

[0058] According to one possibility, lens L2 can be a spherical collimating lens, and lens L3 is a cylindrical focusing lens in the VIPA.

[0059] The pixels of the image sensor are arranged in rows and columns. In this example, the rows of pixels are aligned perpendicular to an angular dispersion axis Y, along which the light is spectrally distributed at the output of the VIPA. Each row of pixels is assigned a frequency value: these are called isofrequency lines. The columns are aligned parallel to the angular dispersion axis defined by the VIPA. More generally, the pixels of the image sensor are arranged in different pixel groups. The pixels in the same group are associated with a frequency value. Due to the angular dispersion of light in the VIPA, the pixels in the same pixel group are illuminated by light at the frequency associated with the pixel group. In this example, the pixel groups are straight segments, designated by the term "isofrequency lines."

[0060] Figure 3A shows an example of an image acquired by the image sensor 19, according to a configuration as described in Figures IA and IB. The x-axis corresponds to the angular dispersion axis. The y-axis corresponds to the pixel columns. The image in Figure 3A was obtained using a laser power of 10 mW and an acquisition time of 1 second. The sample was ethanol. The VIPA was silica, 3.37 mm thick, with a tilt angle of 1.5°. The image sensor used had 512 x 512 pixels, with each pixel measuring 16 pm on each side. The diameter of the multimode optical fiber was 25 pm, with a numerical aperture of 0.1.

[0061] By summing the intensities of the pixels in each isofrequency line (i.e., the pixels located along the same line, transverse to the angular dispersion axis), a spectrum can be formed, as shown in [Fig. 3B]. In such a spectrum, the x-axis is related to the frequency associated with each isofrequency line of pixels, and the y-axis corresponds to the sum of the intensities detected by each pixel in the same isofrequency line. The spectrum is a histogram of the intensities detected by the pixels in each isofrequency line.

[0062] The formation of the spectrum from the image is carried out by a processing unit 30. The processing unit can, for example, be a computer programmed to form the spectrum from each image, taking into account the shape of each isofreference line.

[0063] Figure 3B shows different orders on which the scattering peaks at frequencies V — Vl — can be distinguished. Each order is represented by a brace. It can be seen that the device makes it possible to obtain usable scattering peaks, provided that the laser light is sufficiently extinguished.

[0064] Observing different orders allows for calibration between pixel lines and frequency values. The frequency difference between the respective peaks of two adjacent modes is known. This frequency difference depends on the VIPA used, and more specifically on the VIPA's FSR. The FSR (Free Spectral Range) corresponds to the smallest frequency interval between two monochromatic beams interfering at the same location on the detector.

[0065] The FSR is such that:

[0066] FSR =

[0067] where n is the refractive index of the material forming the VIPA, d is the thickness of the VIPA etc is the speed of light in a vacuum.

[0068] In this example, the VIPA's FSR is 30 GHz. Calibration thus allows a frequency value to be assigned to each row of pixels.

[0069] In Figure 3B, it can be seen that the central peak, with frequency v1, is sufficiently attenuated to allow clear observation of the scattering peaks at -VB-. The frequency shift between two successive pixels was 150 MHz. A full width at half maximum (FWHM) of each Brillouin scattering peak is obtained, greater than 5 or 6 pixels, which allows for a correct evaluation of the FWHM of each peak. It should be noted that the FWHM of a scattering peak allows for the estimation of sound attenuation in the material.

[0070] Note the absence of a significant peak at the frequency the laser being sufficiently attenuated by the absorption cell.

[0071] Figures 4A and 4B show the influence of the VIPA tilt angle on the spectra. The smaller the angle, the fewer orders are visible in the spectrum, but the higher the spectral dispersion. In [Fig. 4A], curve a) corresponds to a small tilt angle and curve b) corresponds to a large tilt angle.

[0072] Another test was carried out using successively two multimode optical fibers of different diameters, respectively 25 pm and 105 pm. The device was implemented according to the configuration described in Figures IA and IB. The respective numerical apertures of the multimode fibers were 0.1 and 0.22. For For demonstration purposes, the sample was replaced by a neon lamp filtered around a wavelength of 815 nm, with an integration time of 60 seconds. Figures 5A and 6A show the images formed by the image sensor using, respectively, a 25 µm diameter multimode optical fiber and a 105 µm diameter multimode optical fiber. It can be observed that using a multimode optical fiber with a high numerical aperture results in a distortion of the isofrequency lines: while these lines are straight in [Fig. 5A] (low numerical aperture), they are curved in [Fig. 6A] (high numerical aperture). Figures 5B and 6B correspond to the spectra obtained from the images shown in Figures 5A and 6A, respectively. To form the spectrum shown in [Fig. 6B], the intensities of the pixels of the curved isofrequency lines were summed.The formation of the spectrum requires taking into account a curved shape of the isofrequency lines, then accumulating the intensities of the pixels along each curved isofrequency line.

[0073] Figures 7A and 7B represent the image and spectrum obtained using a single-mode optical fiber instead of the multi-mode optical fiber, on the same sample, and adopting the same acquisition time.

[0074] It is observed that using a multimode optical fiber increases the amount of detected signal, resulting in an increased signal-to-noise ratio of the diffusion spectra. On the most intense peak of the single-mode fiber, the peak height is 2500 counts (relative to the background below the peak), while the height of the most intense peak is 25,000 counts using the 25 pm diameter multimode fiber (see [Fig. 5B]) and 200,000 counts using the 105 pm diameter multimode fiber (see [Fig. 6B]). It is observed that the peaks obtained with both multimode fibers exhibit good spectral resolution. Thus, using a 105 pm diameter multimode optical fiber provides optimal performance in terms of spectral resolution and sensitivity. This, however, requires taking into account the curved shape of the isofrequency lines during spectrum formation.

[0075] At the sample level, the spatial extent of the useful signal is generally small: using a large-diameter fiber, typically greater than 500 pm, or even 200 pm, carries a risk of collecting stray light. Furthermore, increasing the fiber diameter generally requires a greater tilt of the VIPA, in order to increase the size of the VIPA's input window. However, the greater the tilt angle of the VIPA, the lower the angular dispersion. It is therefore necessary to reduce the pixel size of the image sensor in order to resolve the isofrequency lines.

[0076] Furthermore, when the VIPA is positioned upstream of the extinction chamber, the more it is inclined, the greater the amount of light entering the extinction chamber, which can lead to saturation of the extinction chamber.

[0077] It is understood from the above that the dimensioning of the diameter of the multimode fiber results from a compromise, allowing to optimize the signal to noise ratio of the isofreference lines formed on the image acquired by the image sensor.

[0078] Figure 8 schematically illustrates the main steps of a process for characterizing a sample using a device as previously described.

[0079] Step 100: The sample is positioned facing the objective lens of the device so that the incident beam emitted by the light source is focused on or into the sample. The device is configured to hold the sample in an observation position. The objective lens focuses the incident beam onto the observation position.

[0080] Step 110: Illumination of the sample and acquisition of an image, by the image sensor, of the light dispersed angularly by the VIPA.

[0081] Step 120: From the image acquired by the image sensor, a spectrum is formed representing a distribution of the cumulative intensities of the pixels respectively associated with the same frequency value. The spectrum is formed by taking into account, when summing the intensities of the pixels in the same isofreference line, the shape of the isofreference line, connecting the pixels associated with the same frequency. The line can be straight, particularly when the optical fiber has a small diameter, typically 25 µm. However, to obtain optimal performance, it is advantageous to use a multimode optical fiber with a larger diameter and / or a higher numerical aperture. With this type of multimode fiber, the isofreference lines formed on the image sensor are curved. This step is implemented using the processing unit 30.

[0082] The curved shape of each isofreference line may have been established beforehand and stored in a memory of the processing unit 30. Alternatively, the curved shape may be determined from the image acquired by the image sensor, for example by identifying pixels of the same intensity.

[0083] According to one possibility, a predetermined shape is available for each curved line, parameterized by specific parameters. For example, each curved line is considered to describe an arc of a circle, with the arcs of circles of the same mode being concentric. In this case, the parameter describing each curved line is a radius r and the position of the center of the circle. The parameters can be refined by comparing them with the results of processing the image acquired by the image sensor 19. This allows for obtaining more precise parameters from theoretical initial values. Although described in relation to circles, the method can be applied to other curved shapes.

[0084] The shape of each curved isofreference line can be predetermined by implementing the device on calibration samples, the composition of which is known.

[0085] The invention can be implemented for the analysis of samples, for example in the fields of biology, astronomy and materials science.

Claims

1. Demands Device (1) for observing a sample, configured to maintain the sample in an observation position, the device comprising: - a light source (10) emitting an incident beam along an emission wavelength; - an objective (13), configured to focus the incident beam towards the observation position and to collect light scattered by the sample under the effect of illumination of the sample by the incident beam; - a multimode optical fiber (15), coupled to the objective, arranged to guide the light scattered by the sample; - an optical dispersion element (18) configured to allow angular dispersion of the light, resulting from the multimode optical fiber, as a function of frequency; - an image sensor (19), configured to form an image of the light dispersed by the optical dispersion element, the image sensor comprising pixels, the pixels being distributed in groups of pixels, each group of pixels being associated with the same frequency value; - a processing unit (30), configured to form a spectrum from the image formed by the image sensor, the spectrum being a histogram of the cumulative intensities of the pixels belonging to the same pixel group; - an extinction chamber (17), comprising a gas configured to absorb light at the emission wavelength, the device being characterized in that: - the image sensor is configured so that each group of pixels extends along an isofreference line forming a curve, with a frequency value being assigned to each isofreference line; - the processing unit (30) is configured to accumulate the intensity detected on pixels along each isofrequency line, so as to obtain a frequency spectrum, taking into account the curved shape of each isofrequency line; - the extinguishing chamber is interposed between the element of optical dispersion and the image sensor or optical dispersion element being interposed between the extinction chamber and the image sensor.

2. Device according to claim 1, wherein the optical dispersion element is a Fabry-Perot type standard.

3. Device according to any one of the preceding claims, wherein the diameter of the multimode optical fiber is greater than 10 pm.

4. A device according to any one of the preceding claims, wherein: - the device comprises a lens for focusing the light resulting from the optical fiber along an optical axis (A); - the optical dispersion element is inclined with respect to the optical axis at an angle of inclination between 0.5° and 10°

5. Device according to any one of the preceding claims, wherein: - the curved shape of each isofreference line is parameterized by predetermined parameters; - the processing unit is connected to a memory, comprising said parameters.

6. Device according to claim 5, wherein the processing unit is programmed to adjust the value of the parameters of the curved shape of at least one isofreference line by an analysis of the image formed by the image sensor.

7. A method for forming a frequency spectrum of light scattered by a sample (S), using a device according to any one of the preceding claims, the method comprising: a. arranging the sample at the observation position defined by the device; b. illumination of the sample by the light source and acquisition of an image of the light by the image sensor of the device, the image comprising curved isofrequency lines extending transversely to a dispersion axis angular, with each isofreference line being assigned a frequency value; c. using the device's processing unit, accumulation of the intensity detected on pixels along each curved line, so as to obtain the frequency spectrum.

8. Method according to claim 7, wherein the curved shape of the isofreference lines is predetermined.

9. A method according to claim 8, wherein: - the curved shape is parameterized by parameters, stored in the processing unit; - the method includes a step of adjusting the value of the parameters from a processing of the image acquired by the image sensor.

10. A method according to any one of claims 8 or 9, wherein the curved shape is determined by implementing a calibration sample.

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