Optical microspectrometry or Raman instrument

Abstract

The present invention relates to an optical or Raman micro-spectrometry apparatus (100) comprising an optical microscope objective (10) and an optical spectrometer (20).According to the invention, the micro-spectrometry apparatus comprises an astigmatic optical system (30, 31, 32) disposed outside the optical spectrometer (20, 21) between the microscope objective (10) and the optical spectrometer, the astigmatic optical system (30, 31, 32) being arranged and configured to form an astigmatic image of a light beam transmitted through the microscope objective (10) onto the entrance slit of the optical spectrometer (20, 21), the astigmatic image being elongated by astigmatism in the direction of the height of the entrance slit, the optical spectrometer being capable of spectrally dispersing the astigmatic image and forming a spectrally dispersed image of the field of view on the detector, the astigmatic optical system being configured to reduce an astigmatism optical aberration of the optical spectrometer in the spectrally dispersed image. Figure for the abstract: Fig. 1.

Description

Title of the invention: Optical or Raman microspectrometry apparatus. Technical field of the invention.

[0001] The present invention relates to the technical field of optical micro-spectrometry, and is notably applicable to Raman micro-spectrometry, for the analysis of materials of all types or of biological samples.

[0002] In optical micro-spectrometry, a sample is illuminated with an illumination beam and the light emitted by the sample is observed in reflection or transmission in the wavelength range of the illumination beam or in a different range.

[0003] In Raman microspectrometry, a sample is illuminated with an excitation beam focused at a point, and the scattered light is observed in a range of wavelengths different from the wavelength of the excitation light beam. Prior art

[0004] In the above-mentioned field, there are optical or Raman microscopy instruments comprising, for example, an optical microscope, a light source (e.g., a laser), a microscope objective, and a spectrometer. The microscope objective focuses the light beam onto a focal point on the sample and forms a beam by reflection, transmission, and / or scattering. In Raman spectroscopy, the scattering beam comprises, on the one hand, a component called elastic or Rayleigh scattering, at the wavelength of the excitation laser, and on the other hand, a component called Raman scattering, at wavelengths different from the excitation laser, which depend on the nature and structure of the sample. The intensity of Rayleigh scattering is much greater than the intensity of Raman scattering, the intensity ratio generally being on the order of 10⁶.A wavelength-selective filter, for example a high-pass filter (or "Edge filter"), allows the separation of Raman scattering from Rayleigh scattering, to enable the detection and analysis of the spectrum of the Raman scattering beam.

[0005] To obtain good spatial resolution in 3D, a confocal microscope configuration is used in particular, in which a confocal hole is arranged in a position optically conjugate to a point located in the front focal plane on the optical axis of the microscope objective. The confocal hole can form the input of the optical spectrometer. An optical fiber can also be used as a confocal hole to guide the signal to be analyzed towards the spectrometer input.

[0006] An optical spectrometer based on a diffraction grating is generally used to spectrally separate the components of the collected light beam, for example, Raman scattering. There are different types of optical spectrometers, notably the Czemy-Turner spectrometer based on a planar diffraction grating and the planar-field spectrometer based on a concave diffraction grating. It is known that the non-zero angle of incidence of the light beam on the optical components, diffraction grating and / or mirrors, of the spectrometer induces astigmatism in the plane of the detector. These optical aberrations induce an elongation of the image point on the detector in one direction. A spectrometer equipped with an imaging detector receives lines at each wavelength instead of points. These optical aberrations limit the spatial resolution of Raman microspectrometry instruments.

[0007] The publication Qun Yuan et al. “Comparative assessment of astigmatism-corrected Czerny-Turner imaging spectrometer using off-the-shelf optics”, Optics Communication, vol. 388, pp. 53-61 (2017), describes different optical configurations of Czerny-Turner imaging spectrometer to correct astigmatism, these optical configurations being based on a modification of a distance between optical components or on the addition of an aspheric optical component inside the spectrometer.

[0008] These modifications to the spectrometer's optical design require fine adjustments during manufacturing and cannot be made to existing devices. Furthermore, adjusting such a spectrometer is complex. The cost of such an optical spectrometer corrected for astigmatism is generally high.

[0009] Optical microscopy and Raman microscopy have numerous applications in materials microanalysis and the analysis of biological samples, in which a multitude of biological cells are arranged on a microscope slide. It is desirable to provide an optical or Raman microspectrometry device that exhibits excellent 3D spatial resolution and very good spectral resolution.

[0010] Furthermore, it is desirable to propose an optical or Raman micro-spectrometry device having high spatial and spectral resolution, high brightness while having simple settings and a relatively low cost. Presentation of the invention

[0011] In this context, the present invention proposes a low-cost, high-spectrum-resolution optical or Raman micro-spectrometry device.

[0012] More particularly, the invention proposes an optical or Raman microspectrometry apparatus comprising an optical microscope objective and a optical spectrometer, the optical spectrometer comprising a diffraction grating and a detector, the microscope objective having a front focal plane, the microspectrometry apparatus having a field of view in the front focal plane of the microscope objective.

[0013] According to the invention, the spectrometer has an entrance slit having a height and a width, the height being greater than the width, the height of the entrance slit being arranged in a plane parallel to the lines of the diffraction grating and the micro-spectrometry apparatus comprises an astigmatic optical system disposed outside the optical spectrometer between the microscope objective and the entrance slit of the optical spectrometer, the astigmatic optical system being arranged and configured to receive a collected light beam from the field of view of the microscope objective and transmitted through the microscope objective, the astigmatic optical system being arranged and configured to form an astigmatic image of the collected light beam on the entrance slit, the astigmatic image being elongated by astigmatism in the direction of the height of the entrance slit,The optical spectrometer is capable of spectrally dispersing the astigmatic image and forming a spectrally dispersed image of the beam collected on the detector, the astigmatic optical system being configured to reduce an astigmatism aberration of the optical spectrometer in the spectrally dispersed image.

[0014] Thus, the astigmatic optical system makes it possible to pre-compensate for or reduce the astigmatism aberration of the optical spectrometer in the spectrally dispersed image on the detector. Furthermore, this allows for a high-brightness, high-spectral-resolution optical or Raman microspectrometry instrument to be obtained while using a low-cost, uncorrected optical spectrometer without modifying its settings, as the astigmatic optical system is located outside the optical spectrometer.

[0015] Other non-limiting and advantageous features of the system according to the invention, taken individually or according to all technically possible combinations, are as follows.

[0016] According to a particular and advantageous aspect, the astigmatic optical system is disposed in a part of the device where the collected light beam is collimated.

[0017] Alternatively, the astigmatic optical system is disposed in a part of the apparatus where the collected light beam is non-collimated, for example convergent, just upstream of the spectrometer's entrance slit.

[0018] In a particular embodiment, the micro-spectrometry apparatus includes a confocal hole, the confocal hole being disposed between the microscope objective and the astigmatic optical system, the confocal hole being in a plane optically conjugate with the front focal plane of the microscope objective, the confocal hole delimiting the field of view in the front focal plane of the microscope objective.

[0019] According to another particular and advantageous aspect, the astigmatic optical system exhibits chromatic aberrations capable of reducing the optical aberration of astigmatism of the optical spectrometer in the spectrally dispersed image on the detector.

[0020] According to one embodiment, the astigmatic optical system comprises at least one uniaxial cylindrical lens, the uniaxial cylindrical lens having a radius of curvature in a plane parallel to the height of the entrance slit of the spectrometer.

[0021] According to another embodiment, the astigmatic optical system comprises a two-axis cylindrical lens, the two-axis cylindrical lens having a first radius of curvature in a plane parallel to the height of the entrance slit of the spectrometer and a second radius of curvature in a plane perpendicular to the height of the entrance slit, the first radius of curvature being greater than the second radius of curvature.

[0022] According to yet another embodiment, the astigmatic optical system comprises at least one concave mirror, the concave mirror being an off-axis spherical mirror or an aspherical mirror or a toric mirror.

[0023] According to a particular and advantageous aspect, the optical spectrometer is a Czerny-Tumer spectrometer comprising a spherical collimating mirror and a spherical focusing mirror, the diffraction grating being a planar grating arranged on an optical path between the spherical collimating mirror and the spherical focusing mirror, the spherical collimating mirror being arranged to collimate a light beam from the entrance slit and to reflect the collimated light beam in the direction of the planar diffraction grating, the planar diffraction grating forming a spectrally dispersed light beam in the direction of the spherical focusing mirror, the spherical focusing mirror being arranged to form the image of the spectrally dispersed beam on the detector.

[0024] According to another particular and advantageous aspect, the optical spectrometer is a planar field spectrometer comprising a concave diffraction grating, the concave diffraction grating being arranged to receive a light beam from the entrance slit and to form the image of the spectrally dispersed light beam on the detector.

[0025] Particularly advantageously, the apparatus is a Raman microspectrometry apparatus comprising a light source capable of emitting an excitation light beam, the microspectrometry apparatus being configured to focus the excitation light beam via the microscope objective into a spot, the microscope objective being capable of collecting a Raman backscatter beam emitted by the spot, the apparatus comprising an optical system disposed between the microscope objective and the confocal hole, the optical system being capable of focusing the backscatter beam Raman on the confocal hole and the detector being capable of detecting a Raman scatter signal.

[0026] According to a particular and advantageous aspect, the diffraction grating is fixed and the detector is a linear detector or an imaging detector.

[0027] According to another particular and advantageous aspect, the diffraction grating is mounted to rotate about an axis parallel to the features of the diffraction grating.

[0028] Of course, the different features, variants and embodiments of the invention can be combined with each other in various ways insofar as they are not incompatible or mutually exclusive. Brief description of the drawings

[0029] In addition, various other features of the invention become apparent from the attached description made with reference to the drawings which illustrate non-limiting embodiments of the invention and where:

[0030] [Fig-1] is a schematic view of an optical micro-spectrometry apparatus or Raman according to an example of an implementation based on a Czerny-Tumer spectrometer;

[0031] [Fig.2] is a schematic view of an optical or Raman micro-spectrometry device according to an example embodiment based on a plane field spectrometer;

[0032] [Fig.3] is a schematic view of an example of the spectrometer's entrance slit;

[0033] [Fig.4] is a schematic view of an astigmatic optical system according to a first example of implementation;

[0034] [Fig.5] is a schematic view of an astigmatic optical system according to a second embodiment;

[0035] [Fig.6] is a schematic view of an optical or Raman micro-spectrometry apparatus with an astigmatic optical system according to a third embodiment;

[0036] [Fig.7] is a schematic view of an optical or Raman micro-spectrometry device illustrating the optical paths of a polychromatic light beam based on an astigmatic optical system according to a variant of the first embodiment.

[0037] It should be noted that in these figures the structural and / or functional elements common to the different variants may have the same references. Detailed description

[0038] Figures 1, 2 and 6 schematically represent an optical or Raman 100, 200, 300 microspectrometry device according to different embodiment examples.

[0039] We will first describe the elements common to the different embodiments.

[0040] The optical micro-spectrometry or Raman 100, 200, 300 apparatus comprises a An optical microscope objective 10 and an optical spectrometer 20, 21 are used. The optical microscope objective 10 defines a principal optical axis 14 of the microspectrometry apparatus. For example, an xl0, x40, x50, or x100 microscope objective 10 is used. As described in detail below, the optical spectrometer is a Czemy-Tumer spectrometer 20 or a flat-field spectrometer 21. The optical spectrometer 20, 21 includes a diffraction grating 22 and a detector 25. For example, the diffraction grating 22 operates in reflection. Alternatively, the diffraction grating 22 operates in transmission. The diffraction grating 22 is fixed or mounted to rotate about an axis of rotation parallel to the grating lines. Optionally, the optical spectrometer also includes one or more flat reflecting mirrors 26, 27.

[0041] In optical microspectrometry, the optical microspectrometry apparatus comprises In general, a light source 4 provides illumination, for example, a white light source that emits an illumination beam. In a reflection or transmission configuration, respectively, the illumination beam illuminates a field of view of a sample to be analyzed in the front focal plane 2 of the objective 10. The microscope objective 10 collects a light beam 42 formed by reflection or transmission of the illumination beam on or through the sample.

[0042] In Raman microspectrometry, the Raman microspectrometry apparatus comprises In general, a light source 4 is used for excitation, for example, a laser source that emits an excitation light beam 41, for example, a laser beam. In a backscattering configuration, the collimated excitation light beam is injected into the microscope objective to be focused into the front focal plane 2 of the objective 10 at a point or spot 1 on or in a sample to be analyzed. The microscope objective 10 collects the light beam 42 formed by backscattering the excitation laser beam 41 by the spot 1 on or in the sample.

[0043] In one embodiment, the light beam 42 propagates while being collimated (represented by dashes in figures 1, 2 and 6), for example inside the microscope tube.

[0044] In another embodiment, the optical or Raman microspectrometry apparatus 100, 200, 300 comprises a confocal hole 12 and a focusing optical system 11, for example with a lens, for focusing the light beam 42 collected by the microscope objective onto the confocal hole 12. The confocal hole 12 is arranged in a plane optically conjugate to the front focal plane 2 of the microscope objective 10. The confocal hole 12 is generally of microscopic dimensions. The confocal hole 12 defines a 3D point field of view in the front focal plane of the microscope objective. In one example, the confocal hole 12 has a diameter of 200 pm. However, the confocal hole is not used directly as the spectrometer input. In this case, the collected light beam 42 entering the confocal hole is a convergent beam and not a collimated beam. Naturally, in this case, the collected light beam 42 exiting the confocal hole is a divergent beam.

[0045] The confocal hole 12 is arranged on the principal optical axis of the microscope objective 10. The confocal hole 12 is a circular or polygonal hole having a fixed or adjustable diameter, typically from 10 pm to several hundred pm.

[0046] The optical spectrometer 20, 21 here includes an entrance slit 28. The entrance slit 28 is elongated along a direction also called its height. For example, the entrance slit 28 is rectangular, with a height H and a width W as illustrated in [Fig. 3]. The height H is greater than the width W. The height of the entrance slit 28 is oriented parallel to the lines of the diffraction grating 22. In other words, the height of the entrance slit 28 is oriented perpendicular to the diffraction plane of the diffraction grating 22. In the examples in Figures 1, 2, 6, and 7, an orthonormal XYZ coordinate system is shown. The lines of the diffraction grating 22 are parallel to the X-axis, and the height of the entrance slit is also parallel to the X-axis.

[0047] According to this disclosure, an astigmatic optical system 30, 31, 32 is disposed between the microscope objective 10 and the entrance slit 28 of the optical spectrometer 20, 21. The astigmatic optical system 30, 31, 32 is disposed and configured to receive the collected light beam 42, optionally transmitted through the confocal hole 12, and to form an image of the collected light beam on the entrance slit 28. The astigmatic optical system 30, 31, 32 is oriented to form an image on the entrance slit 28 that exhibits astigmatism oriented in the vertical direction of the entrance slit 28. In other words, the image on the entrance slit of the light beam collected by the microscope objective is an elongated image due to astigmatism.

[0048] The width of the entrance slit is related to the spectral resolution of the spectrometer on the detector 25. The height of the entrance slit 28 is chosen here to receive the astigmatic image of the light beam collected by the microscope objective. In one example, the astigmatic optical system 30, 31, 32 has a magnification of 0.5, the entrance slit 28 has a width of 100 µm and a height of 2 mm.

[0049] According to an exemplary embodiment illustrated in [Fig. 1], the optical or Raman microspectrometry apparatus 100 is based on a Czemy-Tumer spectrometer 20. The Czemy-Tumer spectrometer 20 has an entrance slit 28, a collimating spherical mirror 23, and a focusing spherical mirror 24, the diffraction grating 22 being a planar grating arranged along an optical path between the collimating spherical mirror and the focusing spherical mirror. The collimating spherical mirror 23 receives the diverging light beam 428 from the entrance slit and reflects a beam collimated towards the plane diffraction grating, which operates, for example, in reflection mode. The plane diffraction grating diffracts the light beam in first order and forms a beam of light that is angularly dispersed according to the wavelength, directed towards the spherical focusing mirror 24. The spherical focusing mirror is positioned to form an image of the spectrally dispersed beam 420 onto the detector 25. The detector 25 is a linear detector or an imaging detector. For example, a CCD detector 25 is used, with its pixels arranged in 256 rows and 1024 columns (parallel to the X-axis).

[0050] The entrance slit 28 is located at a distance Li from the collimating spherical mirror 23. The diffraction grating is located at a distance L2 from the collimating spherical mirror 23. The focusing spherical mirror 24 is located at a distance L3 from the diffraction grating 22. The detector 25 is located at a distance L4 from the focusing spherical mirror 24. The optical axis of propagation of the light beam 428 from the entrance slit forms an angle of incidence ai on the collimating spherical mirror 23. The collimated beam forms an angle of incidence denoted i on the diffraction grating 22. The beam diffracted by the diffraction grating forms a deviation angle denoted 0 with the collimated beam. The diffracted beam forms an angle of incidence α2 on the focusing spherical mirror 24.Detector 25 is inclined at an angle of [3] with respect to the median optical axis of the diffracted light beam, which corresponds to the orientation of the optimum focal plane (for which the slit images at each wavelength are as narrow as possible along an axis perpendicular to the grating lines (or to the slit height), here the Y axis. This plane is generally inclined by a few degrees depending on the geometry of the spectrometer.

[0051] The off-axis configuration of the collimating spherical mirror 23 and the focusing spherical mirror 24 induces astigmatism on the detector. In other words, the fact that the angles of incidence ai and a2 are not harmed induces astigmatism on the detector. In the absence of the astigmatic optical system 30, 31, 32, this astigmatism of the optical spectrometer induces an elongation of the spectral image of the confocal hole on the detector in the X direction, that is to say along the slit height or parallel to the grating lines, therefore transverse to the direction of spectral dispersion.

[0052] The astigmatic optical system 30, 31, 32, arranged between the microscope objective 10 and the entrance slit 28, is positioned and configured to form an astigmatic image of the backscattered light beam on the entrance slit, so as to pre-compensate for or reduce the astigmatism aberration of the Czemy-Turner optical spectrometer in the spectrally dispersed image on the detector. The astigmatic optical system 30, 31, 32 thus increases the intensity of the signal detected on the detector.

[0053] In another embodiment illustrated in [Fig. 2], the optical or Raman microspectrometry apparatus 100 is based on a plane-field spectrometer 21, also called a plane-field spectrometer. The plane-field spectrometer 21 has an entrance slit 28, a concave diffraction grating 22, and a detector 25. The concave diffraction grating 22 is preferably a spherical grating arranged in an optical path between the entrance slit 28 and the detector 25. The concave diffraction grating 22 receives the diverging light beam 428 from the entrance slit and diffracts the light beam in the order 1 or -1, for example. The concave diffraction grating 22 forms a light beam that is angularly dispersed as a function of wavelength and forms an image of the spectrally dispersed beam 420 on the detector 25. The entrance slit 28 is located at a distance L5 from the diffraction grating 22. The diffraction grating is located at a distance L6 from the detector 25.The optical axis of propagation of the light beam from the entrance slit forms an angle of incidence α5 on the diffraction grating 22. The beam diffracted by the diffraction grating forms a deviation angle, denoted θ, with the incident beam. The detector 25 is inclined at an angle [3] with respect to the median optical axis of the diffracted light beam. The detector 25 is a linear detector or an imaging detector. For example, a CCD-type detector 25 is used, having pixels of 26 µm on each side distributed over 256 rows and 1024 columns.

[0054] In one embodiment, a plane field spectrometer is used having a spherical diffraction grating 22 with a radius of curvature of 200mm, the grating having 1000 lines / mm and being used in order 1. The spherical diffraction grating 22 is placed at a distance of 200mm from the entrance slit and 200mm from the detector.

[0055] The off-axis configuration of the diffraction grating 22 induces astigmatism on the detector in the plane-field spectrometer. The astigmatic optical system 30, 31, 32, located between the microscope objective and the entrance slit 28, is arranged and configured to form an astigmatic image of the backscattered light beam on the entrance slit, so as to pre-compensate for or reduce the astigmatism aberration of the plane-field optical spectrometer in the spectrally dispersed image on the detector. Here too, the astigmatic optical system 30, 31, 32 thus increases the intensity of the signal detected on the detector.

[0056] In a first example, the astigmatic optical system 30 includes at least one uniaxial cylindrical lens 34 introducing the astigmatism necessary to correct the astigmatism of the spectrometer upstream of the entrance slit 28 of the optical spectrometer.

[0057] In another example, the astigmatic optical system 31 comprises at least one two-axis or biaxial cylindrical lens 37 introducing the astigmatism necessary to pre-compensate or reduce the astigmatism of the spectrometer upstream of the slit of the entrance 28, while allowing the backscattered light beam to be focused on the entrance slit 28 of the spectrometer.

[0058] In yet another example, the astigmatic optical system 32 includes at least one off-axis mirror introducing the astigmatism necessary to pre-compensate or reduce the astigmatism of the spectrometer upstream of the entrance slit 28, while allowing the backscattered light beam to be focused on the entrance slit 28 of the spectrometer.

[0059] According to a first embodiment, illustrated in [Fig. 4], the apparatus comprises a confocal hole and the astigmatic optical system 30 comprises a first spherical lens 33, a cylindrical lens 34, and a second spherical lens 35. The confocal hole 12 is located at the object focus of the first spherical lens. Alternatively, the apparatus does not comprise a confocal hole, and the astigmatic optical system 30 comprises only the cylindrical lens 34 and the second spherical lens 35. The entrance slit 28 is located at the image focus of the second spherical lens. The cylindrical lens 34 is positioned in the space where the collected light beam is collimated, either between the first spherical lens 33 and the second spherical lens 35, in the confocal case, or between the microscope objective 10 and the second spherical lens 35, in the non-confocal case.The position of the cylindrical lens 34 along the longitudinal axis of propagation of the light beam is not critical when it is within the collimated space of the collected beam. The cylindrical lens 34 is said to be single-axis or mono-axis. The cylindrical lens 34 has a radius of curvature in a plane parallel to the height of the entrance slit of the spectrometer. For example, the cylindrical lens 34 has a flat face oriented towards the first spherical lens 33 and a cylindrical face oriented towards the second spherical lens 35. The cylindrical lens 34 has the effect of spreading the image of the light beam focused on the entrance slit of the optical spectrometer along the height of the entrance slit. The optical spectrometer induces astigmatism which compensates on the detector for the spreading of the imaged light beam on the entrance slit.This results in a micro-spectrometry device with better spatial resolution and a spectrum collected over fewer pixels in height, therefore less noise and a better signal-to-noise ratio.

[0060] According to one embodiment, the cylindrical lens is placed in a part of the microspectrometry apparatus where the collected light beam is uncollimated. Particularly advantageously, the cylindrical lens is placed just upstream of the spectrometer's entrance slit, in a part of the microspectrometry apparatus where the collected beam is convergent. For example, the microspectrometry apparatus includes a focusing spherical lens 35 disposed between the microscope objective 10 and the entrance slit 28, and the cylindrical lens 34 is disposed between the focusing spherical lens 35 and the entrance slit 28.

[0061] According to one embodiment, the cylindrical lens has a spherical face and a cylindrical face. Such a lens advantageously combines the functions of the cylindrical lens and the spherical lens 33 and / or 35.

[0062] In one embodiment, the microscope objective is an Olympus X50 objective with a focal length F of 3.6 mm and a numerical aperture NA of 0.8. The microscope objective produces a spot less than 1.5 pm in diameter. The optical system 11 consists of an achromatic lens with a focal length of 120 mm. The confocal hole has a diameter of 100 pm or 200 pm. The astigmatic optical system 30 comprises a first achromatic spherical lens 33 with a focal length of 80 mm, a cylindrical lens 34 with a radius of curvature of 20 mm, and a second achromatic spherical lens 35 with a focal length of 30 mm. The first achromatic spherical lens 33 is placed at a distance of 80 mm from the confocal hole 12. The second spherical lens 35 is placed at a distance of 30 mm from the confocal hole 12.The spectrometer is a Czerny-Turner spectrometer 20 with a collimating spherical mirror 23 having a radius of curvature of 200 mm and a focusing spherical mirror 24 also having a radius of curvature of 200 mm. The diffraction grating is placed 100 mm from the collimating spherical mirror 23. The diffraction grating is placed 100 mm from the focusing spherical mirror 24. The angles of incidence are respectively α = 10°, α = 7°, and α₂ = 16°. The optical spectrometer (without astigmatism correction) thus has an astigmatism height on the CCD of approximately 1 to 2 mm (height of the image of the confocal hole on the detector). With the correction of the astigmatic optical system 30, the astigmatism height is approximately 100 µm.

[0063] In a second embodiment, illustrated in [Fig. 5], the astigmatic optical system 31 comprises at least one two-axis cylindrical lens 37 upstream of the entrance slit 28 of the optical spectrometer. In the case where the device includes a confocal hole, the astigmatic optical system 30 further comprises a first spherical lens 36 disposed between the confocal hole and the two-axis cylindrical lens 37, the confocal hole 12 being located at the object focus of the first spherical lens 36. In other words, preferably, the cylindrical lens 34 receives the collected light beam 42, which is collimated. The entrance slit 28 is located at the image focus of the two-axis cylindrical lens 37. The two-axis cylindrical lens has a first face oriented towards the microscope objective or, where applicable, towards the first spherical lens 36, and a second face oriented towards the entrance slit 28.The first face has a first radius of curvature in a plane parallel to the height of the entrance slit, and the second face has a second radius of curvature in the same plane, parallel to the height of the entrance slit. The lens. cylindrical 37 with two axes has a focal length in the direction of the height of the slit and a sagittal focal length in the direction of the width of the slit.

[0064] In a third embodiment, illustrated in [Fig. 6], the astigmatic optical system 32 comprises at least one concave mirror 38. In the case where the device comprises a confocal hole, the astigmatic optical system 32 further comprises a first spherical lens 36 disposed between the confocal hole 12 and the concave mirror 38, the confocal hole 12 being disposed at the object focus of the first spherical lens 36. The entrance slit 28 is disposed at the image focus of the concave mirror 38. The concave mirror 38 is chosen from an off-axis spherical mirror or an off-axis aspherical mirror, for example an off-axis parabolic mirror or a toric mirror. In one embodiment, the spectrometer is a Czemy-Turner spectrometer or a plane-field spectrometer and the astigmatic optical system 32 includes a spherical lens 36 and a concave mirror 38 placed upstream of the entrance slit 28.

[0065] To obtain the values ​​of the parameters of the astigmatic optical system 30, 31, 32, an optical design simulation and optimization software, for example Zemax Optics Studio, is advantageously used to estimate the astigmatism of the optical spectrometer 20, 21 in a band of wavelengths as a function of the parameters of the optical spectrometer 20, 21 and to calculate the values ​​of the geometric optical parameters of the astigmatic optical system 30, 31, 32 so as to pre-compensate for the astigmatism of the optical spectrometer.

[0066] For example, a simulation is performed of a spectrometry device comprising a Czemy-Turner optical spectrometer based on spherical mirrors 23, 24 combined with an optical system consisting solely of a first spherical lens 33 and a second spherical lens 35 to image a confocal hole on the entrance slit 28 of the optical spectrometer. A CCD detector is used. The simulation software allows the astigmatism in the detector plane to be evaluated at different wavelengths. At a wavelength of 640 nanometers (nm), the astigmatism in the detector plane is estimated at 2000 pm. At a wavelength of 598 nm, the astigmatism in the detector plane is estimated at 1626 pm. At a wavelength of 552 nm, the astigmatism in the detector plane is estimated at 1248 pm.

[0067] A new simulation is performed, using the same optical spectrometer and the same spherical lenses 33, 35, by inserting a cylindrical lens 34 at an axis between the first spherical lens 33 and the second spherical lens 35. The cylindrical lens 34 has a focal length of 40 mm. The height of the image of the confocal hole on the entrance slit 28 is 1728 pm at a wavelength of 598 nm, 1722 pm at a wavelength of 640 nm, and 1734 pm at a wavelength of 532 nm. The simulation software allows the resulting astigmatism to be evaluated in the plane of the The detector was tested at different wavelengths using this astigmatic optical system. At a wavelength of 640 nanometers (nm), the astigmatism in the detector plane was estimated at 280 pm. At a wavelength of 598 nm, the astigmatism in the detector plane was estimated at 116 pm. At a wavelength of 552 nm, the astigmatism in the detector plane was estimated at 500 pm. A drastic reduction in astigmatism in the detector plane was observed with the astigmatic optical system, compared to the optical system without the cylindrical lens. In this example, the reduction in astigmatism was approximately one order of magnitude at wavelengths of 640 nm and 598 nm, and approximately two times greater at a wavelength of 552 nm.

[0068] The astigmatic optical system 30, 31, 32 according to the present disclosure has the advantage of being disposed outside the spectrometer 20, 21.In the case of a Czemy-Turner spectrometer, it is thus possible to use a spectrometer with spherical mirrors 23 and 24 while compensating for the astigmatism of the optical spectrometer on the detector using the astigmatic optical system 30, 31, 32. A spectrometer with spherical mirrors 23 and 24 is less expensive than a spectrometer with a toroidal mirror designed to correct astigmatism. Advantageously, the two spherical mirrors 23 and 24 are identical. Furthermore, the optical spectrometer requires no adjustment; it can be adjusted conventionally, at the factory, independently of the astigmatic optical system 30, 31, 32. The astigmatic optical system with a cylindrical lens on one axis or two axes requires no adjustment, except for the positioning and orientation of the astigmatic optical system between the entrance slit of the optical spectrometer and the microscope objective, possibly the confocal hole.Overall, the 100, 200, 300 device is simpler to manufacture, adjust, and less expensive than a device equipped with an optical spectrometer including a toroidal mirror.

[0069] The astigmatic optical system 30, 31, 32 allows for first-order correction of the astigmatism of the optical spectrometer, for example, at the center of the spectral range of the optical spectrometer, as in the numerical example shown above. The astigmatic optical system 30, 31, 32 makes it possible to increase the brightness of the optical or Raman microspectrometry instrument, and thus to increase the intensity of the detected signal without increasing the power of the light source. However, residual astigmatism aberrations persist, particularly at the extremes of the spectral range of the optical spectrometer, for example, at the wavelength of 532 nm in the example above.

[0070] In a particular and advantageous embodiment, the astigmatic optical system 30, 31, 32 is chromatic.

[0071] According to an example illustrated in [Fig. 7], the astigmatic optical system, for example based on a cylindrical lens 34 between two spherical lenses 33, 35, also exhibits chromatic aberration. In particular, the cylindrical lens 34 is chosen to be chromatic or hyperchromatic. Figure 7 illustrates in particular the optical path of the collected light beam 42 and then spectrally dispersed within the spectrometer at different wavelengths, for example subbeam 421 at a wavelength of 552 nm, subbeam 422 at a wavelength of 598 nm and subbeam 423 at a wavelength of 640 nm. These subbeams are imaged at different points on the detector.

[0072] For example, a chromatic cylindrical lens 34 is chosen having a focal length of 43 mm at a wavelength of 598 nm, a focal length of 57 mm at a wavelength of 552 nm, and a focal length of 35 mm at a wavelength of 640 nm. A new simulation is performed, using the same optical spectrometer and the same spherical lenses 33, 35 as before, and with the chromatic cylindrical lens 34. The image height of the confocal hole on the entrance slit 28 via this astigmatic and chromatic system is 1620 pm at a wavelength of 598 nm, 2000 pm at a wavelength of 640 nm, and 1240 pm at a wavelength of 532 nm. The simulation software allows for the evaluation of the resulting astigmatism in the detector plane at different wavelengths with this astigmatic and chromatic optical system. At a wavelength of 640 nanometers (nm), the residual astigmatism in the detector plane is estimated at 40 pm.At a wavelength of 598 nm, the residual astigmatism in the detector plane is estimated at 26 pm. At a wavelength of 552 nm, the residual astigmatism in the detector plane is estimated at 35 pm. This results in an almost complete reduction of astigmatism in the detector plane thanks to the astigmatic and chromatic optical system. This reduction in astigmatism increases the brightness, spectral resolution, and spatial resolution of the Raman microspectrometry instrument. Furthermore, this improvement is achieved at a relatively low cost and without requiring any recalibration of the optical spectrometer.

[0073] According to a variant of the second embodiment, the astigmatic optical system 31 comprises a spherical lens and a two-axis cylindrical lens, the two-axis cylindrical lens being chromatic or hyperchromatic so as to further reduce the residual astigmatism of the spectral image in the plane of the detector.

[0074] According to a variant of the third embodiment, the astigmatic optical system 31 comprises a spherical lens and an off-axis mirror, and includes an additional cylindrical lens, disposed between the spherical lens and the off-axis mirror or between the off-axis mirror and the entrance slit 28. This additional cylindrical lens is chosen to be chromatic or hyperchromatic so as to further reduce the residual astigmatism of the spectral image in the plane of the detector.

[0075] Alternatively, two cylindrical lenses are used, arranged side by side and having a radius of curvature located in a plane parallel to the height of the slit, the two Cylindrical lenses with different radii of curvature. The two cylindrical lenses, made of different materials to increase the chromatic aberration of cylindrical optics, allow for better correction of astigmatism over a wide spectral range.

[0076] In another example, the cylindrical lens has a variable radius of curvature, for example increasing in a direction perpendicular to the height of the slit. Other examples, for example based on an acylindrical lens, are of course possible without departing from the scope of this disclosure.

Claims

Demands

1. An optical or Raman microspectrometry apparatus (100, 200, 300) comprising an optical microscope objective (10) and an optical spectrometer (20, 21), the optical spectrometer comprising a diffraction grating (22) and a detector (25), the microscope objective (10) having a front focal plane, the microspectrometry apparatus having a field of view (1) in the front focal plane (2) of the microscope objective, characterized in that: the spectrometer has an entrance slit (28) having a height and a width, the height being greater than the width, the height of the entrance slit being disposed in a plane parallel to the lines of the diffraction grating, and in that the microspectrometry apparatus comprises an astigmatic optical system (30, 31, 32) disposed outside the optical spectrometer (20, 21) between the microscope objective (10) and the slit input (28) of the optical spectrometer, the astigmatic optical system (30, 31,32) being arranged and configured to receive a collected light beam from the field of view of the microscope objective and transmitted through the microscope objective (10), the astigmatic optical system (30, 31, 32) being arranged and configured to form an astigmatic image of the collected light beam on the entrance slit, the astigmatic image being elongated by astigmatism in the direction of the height of the entrance slit, the optical spectrometer being capable of spectrally dispersing the astigmatic image and forming a spectrally dispersed image of the collected beam on the detector, the astigmatic optical system being configured to reduce an optical aberration of astigmatism of the optical spectrometer in the spectrally dispersed image.

2. Apparatus (100, 200) according to claim 1 in which the astigmatic optical system (30, 31, 32) is disposed in a part of the apparatus where the collected light beam is collimated.

3. Apparatus (100, 200) according to claim 1 comprising a confocal hole (12), the confocal hole (12) being disposed between the microscope objective and the astigmatic optical system (30, 31, 32), the confocal hole (12) being in a plane optically conjugate with the front focal plane (2) of the microscope objective, the confocal hole (12) delimiting the field of view (1) in the front focal plane of the microscope objective.

4. Apparatus (100, 200) according to any one of claims 1 to 3 wherein the astigmatic optical system (30, 31, 32) has chromatic aberrations capable of reducing the optical astigmatism aberration of the optical spectrometer in the spectrally dispersed image on the detector.

5. Apparatus (100, 200) according to any one of claims 1 to 4 in which the astigmatic optical system (30) comprises at least one uniaxial cylindrical lens (34), the uniaxial cylindrical lens (34) having a radius of curvature in a plane parallel to the height of the entrance slit of the spectrometer.

6. Apparatus (100, 200) according to any one of claims 1 to 4 wherein the astigmatic optical system (31) comprises a two-axis cylindrical lens (37), the two-axis cylindrical lens having a first radius of curvature in a plane parallel to the height of the entrance slit of the spectrometer and a second radius of curvature in a plane perpendicular to the height of the entrance slit, the first radius of curvature being greater than the second radius of curvature.

7. Apparatus (300) according to claim 1 to 4 in which the astigmatic optical system (32) comprises at least one concave mirror (38), the concave mirror (38) being an off-axis spherical mirror or an aspherical mirror or a toric mirror.

8. Apparatus (100) according to any one of claims 1 to 7 wherein the optical spectrometer (20) is a Czerny-Tumer spectrometer comprising a collimating spherical mirror (23) and a focusing spherical mirror (24), the diffraction grating (22) being a planar grating arranged on an optical path between the collimating spherical mirror and the focusing spherical mirror, the collimating spherical mirror (23) being arranged to collimate a light beam from the entrance slit and to reflect the collimated light beam towards the planar diffraction grating, the planar diffraction grating forming a spectrally dispersed light beam towards the focusing spherical mirror, the focusing spherical mirror being arranged to form the image of the spectrally dispersed beam on the detector.

9. Apparatus (200) according to any one of claims 1 to 8, wherein the optical spectrometer (21) is a plane-field spectrometer comprising a concave diffraction grating (22), the grating of concave diffraction being arranged to receive a light beam from the entrance slit and to form the image of the spectrally dispersed light beam on the detector (25).

10. Raman micro-spectrometry apparatus (100, 200, 300) according to any one of claims 1 to 9 comprising a light source (4) capable of emitting an excitation light beam (41), the micro-spectrometry apparatus being configured to focus the excitation light beam via the microscope objective into a spot, the microscope objective being capable of collecting a Raman backscatter beam emitted by the spot, the apparatus comprising an optical system (11) disposed between the microscope objective and the confocal hole, the optical system (11) being capable of focusing the Raman backscatter beam onto the confocal hole and the detector being capable of detecting a Raman scatter signal.

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