Apparatus for optical or raman micro-spectrometry

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) arranged 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) on the input slit of the optical spectrometer (20, 21), the astigmatic image being elongated by astigmatism in the height direction of the input slit, the optical spectrometer being able to spectrally disperse the astigmatic image and form a spectrally dispersed image of the field of view 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.

Description

Optical microspectrometry or Raman instrument TECHNICAL FIELD OF THE INVENTION

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

[0002] In optical microspectrometry, 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. STATE OF THE ART

[0004] In the field described above, optical or Raman microscopy instruments exist, 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, forming a beam through 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 10 6A 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 which a confocal hole is positioned 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 input of the spectrometer.

[0006] An optical spectrometer based on a diffraction grating is generally used to spectrally separate the components of a collected light beam, for example, Raman scattering. There are different types of optical spectrometers, including the Czerny-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 detector plane. These aberrations cause the image point on the detector to elongate in one direction. A spectrometer 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 implemented on 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 offer an optical or Raman microspectrometry instrument that provides excellent 3D spatial resolution and very good spectral resolution.

[0010] Furthermore, it is desirable to offer an optical or Raman micro-spectrometry device with high spatial and spectral resolution, high brightness, simple settings and a relatively low cost. PRESENTATION OF THE INVENTION

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

[0012] More particularly, the invention proposes an optical or Raman microspectrometry apparatus comprising an optical microscope objective and an optical spectrometer, the optical spectrometer comprising a diffraction grating and a detector, the microscope objective having a front focal plane, the micro-spectrometry 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 disposed 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 disposed 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 disposed 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 beam collected on the detector, the astigmatic optical system being configured to reduce an astigmatism optical aberration of the optical spectrometer in the spectrally dispersed image.

[0014] Thus, the astigmatic optical system allows for the pre-compensation or reduction of the astigmatism aberration of the optical spectrometer in the spectrally dispersed image on the detector. Furthermore, this enables the creation of a high-brightness, high-spectral-resolution optical or Raman microspectrometry instrument while using a low-cost, uncorrected optical spectrometer. astigmatism, without changing its settings, the astigmatic optical system being 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 arranged in a part of the device where the collected light beam is collimated.

[0017] Alternatively, the astigmatic optical system is arranged in a part of the device 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 includes 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 example of implementation, the astigmatic optical system includes 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-Turner 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 towards the planar diffraction grating, the planar diffraction grating forming a spectrally dispersed light beam towards 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] In a particularly advantageous manner, 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 Raman backscatter beam onto 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]

[0023] 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 as long as they are not incompatible or mutually exclusive. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Furthermore, 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] Figure 1 is a schematic view of an optical or Raman micro-spectrometry device according to an example embodiment based on a Czerny-Turner spectrometer;

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

[0032] Figure 3 is a schematic view of an example of a spectrometer entrance slit;

[0033] Figure 4 is a schematic view of an astigmatic optical system according to a first example of its realization;

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

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

[0036] Figure 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 example.

[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 examples of implementation.

[0039] We will first describe the elements common to the different modes of implementation.

[0040] The optical microspectrometry or Raman apparatus 100, 200, 300 comprises an optical microscope objective 10 and an optical spectrometer 20, 21. The optical microscope objective 10 defines a principal optical axis 14 of the microspectrometry apparatus. For example, a 10x, 40x, 50x, or 100x microscope objective 10 is used. As described in detail below, the optical spectrometer is a Czerny-Turner 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 mirrors 26, 27 for reflection.

[0041] In optical microspectrometry, the optical microspectrometry apparatus generally includes an illumination light source 4, 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 generally includes a light excitation source 4, 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 objective 10 of microscope collects the light beam 42 formed by back scattering of the excitation laser beam 41 by the spot 1 on or in the sample.

[0043] In one example of an 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, a lens, to focus 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 µm. However, the confocal hole is not used directly as the input of the spectrometer. 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 located on the principal optical axis of the microscope objective 10. The confocal hole 12 is a circular or polygonal hole with 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 Figure 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 arranged and configured to receive the collected light beam 42, possibly 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 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 example of an embodiment illustrated in Figure 1, the optical or Raman microspectrometry apparatus 100 is based on a Czerny-Turner spectrometer 20. The Czerny-Turner spectrometer 20 has an entrance slit 28, a collimating spherical mirror 23, and a focusing spherical mirror 24. The diffraction grating 22 is 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 collimated beam towards the planar diffraction grating, which operates, for example, in reflection. The planar diffraction grating diffracts the light beam in first order and forms a beam of light that is angularly dispersed as a function of wavelength towards the focusing spherical mirror 24.The spherical focusing mirror is positioned to form an image of the spectrally dispersed beam 420 onto detector 25. Detector 25 is a linear detector or an imaging detector. For example, a CCD-type 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 spherical mirror. The collimating spherical mirror 23 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 θ on the collimating spherical mirror 23. The collimated beam forms an angle of incidence θ, denoted i, on the diffraction grating 22. The beam diffracted by the diffraction grating forms a deviation angle θ, denoted 0, with the collimated beam. The diffracted beam forms an angle of incidence θ2 on the focusing spherical mirror 24. Detector 25 is inclined at an angle noted [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 lines of the grating (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 at θ and θ2 are not numb 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, 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, positioned between the microscope objective 10 and the entrance slit 28, is arranged and configured to form an astigmatic image of the backscattered light beam on the entrance slit. This pre-compensates for or reduces the astigmatism aberration of the Czerny-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 Figure 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 grating The concave diffraction grating 22 is preferably a spherical grating arranged along 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 order 1 or -1, for example. The concave diffraction grating 22 forms a light beam that is angularly dispersed according to the wavelength and forms an image of the spectrally dispersed beam 420 on the detector 25. The entrance slit 28 is located at a distance Ls from the diffraction grating 22. The diffraction grating is located at a distance Le from the detector 25. The optical axis of propagation of the light beam from the entrance slit forms an angle of incidence αs on the diffraction grating 22. The beam diffracted by the diffraction grating forms a deviation angle, denoted θ, with the incident beam.Detector 25 is inclined at an angle [3] with respect to the median optical axis of the diffracted light beam. 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 arranged in 256 rows and 1024 columns.

[0054] In one example of an embodiment, a planar field spectrometer is used having a spherical 22 diffraction grating with a radius of curvature of 200mm, the grating having 1000 lines / mm and being used in the 1st order. The spherical 22 diffraction grating 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 includes 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 entrance slit 28, while also allowing the backscattered light beam to be focused onto 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 also allowing the backscattered light beam to be focused onto the entrance slit 28 of the spectrometer.

[0059] According to a first embodiment, illustrated in Figure 4, the apparatus includes 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 include 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 instrument with a... better spatial resolution and a spectrum collected over fewer pixels in height, therefore less noise and a better signal-to-noise ratio.

[0060] In one variant, the cylindrical lens is placed in a part of the microspectrometry apparatus where the collected light beam is uncollimated. A particularly advantageous feature is that 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 positioned between the microscope objective 10 and the entrance slit 28, and the cylindrical lens 34 is positioned 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 µm 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 µm or 200 µm. 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 ai of 10°, / of 7°, and 02 of 16°. The optical spectrometer (without astigmatism correction) thus has an astigmatism height on the CCD of the order of 1 to 2 mm (image height). (of the confocal hole on the detector). With the correction of the astigmatic optical system 30, the height of astigmatism is approximately 100pm.

[0063] In a second embodiment, illustrated in Figure 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. If the device includes a confocal hole, the astigmatic optical system 30 further comprises a first spherical lens 36 positioned 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 collimated collected light beam 42. 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 two-axis cylindrical lens 37 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 Figure 6, the astigmatic optical system 32 includes at least one concave mirror 38. In the case where the device includes a confocal hole, the astigmatic optical system 32 further includes 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 Czerny-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 parameter values ​​of the astigmatic optical system 30, 31, 32, optical design simulation software is advantageously used and optimization, for example Zemax Optics Studio, 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 the astigmatism of the optical spectrometer.

[0066] For example, a simulation is performed of a spectrometry device comprising a Czerny-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 image height of the confocal hole in 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 us to evaluate the resulting astigmatism in the detector plane at different wavelengths with this astigmatic optical system. At a wavelength of 640 nanometers (nm), the astigmatism in the detector plane is estimated at 280 pm. At a wavelength of 598 nm, the astigmatism in the detector plane is estimated at 116 pm. At a wavelength of 552 nm, the astigmatism in the detector plane is estimated at 500 pm.A drastic reduction in astigmatism is observed in the detector plane thanks to the astigmatic optical system, compared to the optical system without the cylindrical lens. In this example, the reduction in astigmatism is approximately one order of magnitude at wavelengths of 640 nm and 598 nm, and approximately twofold at a wavelength of 552 nm.

[0068] The astigmatic optical system 30, 31, 32 described in this disclosure has the advantage of being located outside the spectrometer 20, 21. In the case of a Czerny-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 single-axis or dual-axis cylindrical lens requires no adjustments other than positioning and orienting the astigmatic optical system between the entrance slit of the optical spectrometer and the microscope objective, possibly including the confocal hole. Overall, the 100, 200, and 300 apparatus is simpler to manufacture, adjust, and less expensive than an apparatus equipped with an optical spectrometer that includes 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 spectrometer's spectral range, as in the numerical example shown above. The astigmatic optical system 30, 31, 32 increases the brightness of the optical microspectrometry or Raman instrument, thereby increasing 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 spectrometer's spectral range, 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 Figure 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 the subbeam 421 at the wavelength of 552 nm, subbeam 422 at wavelength of 598 nm and subbeam 423 at wavelength of 640 nm. These subbeams are imaged at different points on the detector.

[0072] For example, a chromatic cylindrical lens 34 is chosen with 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 and 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 detector plane.

[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 lens Additional cylindrical is chosen to be chromatic or hyperchromatic so as to further reduce the residual astigmatism of the spectral image in the detector plane.

[0075] Alternatively, two cylindrical lenses are used, positioned side by side and having a radius of curvature lying in a plane parallel to the height of the slit. These two cylindrical lenses have different radii of curvature. Made from different materials to increase the chromatic aberration of the cylindrical optics, these lenses allow for better astigmatism correction over a wider spectral range.

[0076] In another example, the cylindrical lens has a variable radius of curvature, for example, increasing in a direction perpendicular to the slit height. 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 arranged 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 aberration of astigmatism 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 in which 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-Turner 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 7 in which the optical spectrometer (21) is a planar field spectrometer comprising a concave diffraction grating (22), 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 (25).

10. Raman microspectrometry apparatus (100, 200, 300) according to any one of claims 1 to 9 comprising a light source (4) capable of emitting a beam excitation light (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.

Images