High spectral resolution optical spectrometer

Abstract

The present invention relates to a high spectral resolution optical spectrometer (100). According to the invention, the optical spectrometer (100) comprises a polarization rectifier optical device (10) and a dispersive optical device (20) arranged and configured to receive a light beam (1) and to separate the light beam into N pairs of light beams dispersed in N spectral bands, where N is an integer, the N pairs of dispersed light beams having the same polarization state, a diffractive optical device (30) comprising a plurality of N diffraction gratings forming a plurality of N pairs of diffracted light beams, and a focusing optical system (5) arranged to form an image on a two-dimensional matrix detector (6), each diffracted light beam from the plurality of N pairs of diffracted light beams being imaged on a distinct area (61, 62, 63, 64, 65, 66) of the detector (6).Figure for the abridged version: Fig. 1.

Description

Title of the invention: High spectral resolution optical spectrometer. Technical field of the invention

[0001] The present invention relates to the technical field of high spectral resolution optical spectrometers.

[0002] It relates in particular to an optical spectrometer based on a diffraction grating. The invention finds particular applications in Raman microspectrometry, which requires very high spectral resolution. State of the art

[0003] In the above-mentioned field, it is known to use an optical spectrometer comprising a multichannel detector based on a pixel array. To obtain high spectral resolution and a significant spectral bandwidth, it is necessary to separate the spectral components of the light beam entering the spectrometer over a large number of pixels. Many spectrometers separate the spectral components along a single line of the detector. Other spectrometers are configured to separate the spectral components over several lines of a two-dimensional pixelated image sensor, for example, on a CCD or CMOS camera. In particular, optical spectrometers based on a ladder grating or on a dispersive device of the virtual imaged phased array (VIPA) type advantageously allow this separation of the spectral components over several lines of the image sensor.However, these ladder-type or VIPA grating spectrometers offer little versatility in terms of design, and do not provide the possibility of imaging the spectrum from several spatially separated points at the spectrometer's input.

[0004] French patent document FR2956204 describes a compact, broadband spectrograph comprising a plurality of concave diffraction gratings. However, this spectrograph suffers from low efficiency because all available light is divided among several diffraction gratings, each grating operating in a spectral range, so that for a given wavelength only one of these gratings directs the light to the sensor. A spectrometer incorporating a separation stage based on a dichroic element, designed to illuminate each grating with the appropriate spectral subband, partially solves the efficiency problem, but is not applicable to very high-resolution spectrometers requiring sub-banding into spectral subbands with a width of less than 100 nanometers (nm), because the cutoff wavelength of the filters is known to be Dichroic wavelengths under oblique incidence can differ by one or more tens of nanometers depending on the polarization, preventing separation into narrow sub-bands. Furthermore, the efficiency of the gratings used in spectrometers also limits the spectrometers' overall efficiency. The use of blazed diffraction gratings, whose efficiency can reach 70% over a spectral band, has been reported. It is known that so-called lamellar gratings can exhibit an efficiency greater than 90% over a spectral band and for a given incidence range, but this efficiency is only achieved for a single magnetic transverse linear polarization (TM). The efficiency for the other electric transverse linear polarization (TE) is generally less than 20%, making the average efficiency for unpolarized light less than 55%.

[0005] It is desirable to propose a high spectral resolution optical spectrometer, particularly on spectral bands with a width of less than 100 nm, for Raman applications, this spectrometer being compact, insensitive to the polarization of light and which also allows a spatial resolution of the beams entering the spectrometer. Presentation of the invention

[0006] In this context, the present invention proposes a high spectral resolution optical spectrometer.

[0007] More particularly, the invention proposes a high spectral resolution optical spectrometer comprising: an optical dispersion and polarization rectification device comprising a polarization rectifier optical device and a dispersive optical device arranged optically in series, the optical dispersion and polarization rectification device being arranged to receive a light beam and configured to separate the light beam into N pairs of light beams dispersed in N spectral bands, where N is an integer, the N pairs of dispersed light beams having the same polarization state, each pair of dispersed light beams comprising a first dispersed light beam and a second dispersed light beam in the same spectral band among the N spectral bands,the first dispersed light beam and the second dispersed light beam of each pair of dispersed light beams being spatially and / or angularly separated; a diffractive optical device comprising a plurality of N diffraction gratings, each diffraction grating of the plurality of N diffraction gratings being arranged and configured to selectively receive a pair of dispersed light beams in a spectral band and to form by diffraction a pair of diffracted beams comprising a first diffracted light beam and a second diffracted light beam in the spectral band, , The plurality of N diffraction gratings forming a plurality of N pairs of diffracted light beams, and an optical focusing system arranged to receive the plurality of N pairs of diffracted light beams and form an image on a two-dimensional array detector comprising at least 2N rows of photodetectors, each diffracted light beam from the plurality of N pairs of diffracted light beams being imaged on a distinct area of ​​the detector

[0008] Thus, the optical spectrometer provides excellent spectral resolution while remaining compact. Furthermore, it allows the spectrum of both polarization components to be obtained on each spectral band.

[0009] The spectrometer makes it possible to obtain a complete spectrum distributed across several superimposed spectral bands on a pixel array detector, with high spectral resolution and very high efficiency. This spectrometer has the additional advantage of being able to image, that is, to form the image of several light sources placed at the spectrometer's entrance slit on the detector, these images being resolved spatially and spectrally on the detector. The spectrum of each of these light sources is thus obtained. The spectrometer of the present disclosure also has the advantage of providing not only spectral information, but also polarization information for the source beam.

[0010] 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.

[0011] According to a first embodiment, the polarization rectifier optical device is arranged and configured to receive the light beam and separate the light beam into a first light beam having a first polarization state and a second light beam having a second polarization state, the polarization rectifier optical device comprising a wave plate disposed on the optical path of the second light beam and adapted to modify the polarization of the second light beam so as to form a second rectified light beam of the same polarization state as the first light beam, the dispersive optical device being disposed downstream of the polarization rectifier optical device to receive the first light beam and the second rectified light beam,the dispersive optical device being configured to spatially and / or angularly separate the first light beam into a plurality of N spectrally dispersed first light beams and the second rectified light beam into a plurality of N spectrally dispersed second light beams, forming said N pairs of light beams dispersed in N spectral bands.

[0012] According to a second embodiment, the dispersive optical device is arranged to receive the light beam, the dispersive optical device being configured to spatially and / or angularly separate the light beam into a plurality of N spectrally dispersed light beams in said N spectral bands, and the polarization rectifier optical device is disposed downstream of the dispersive optical device and configured to receive and polarize the plurality of N spectrally dispersed light beams into N first light beams having a first polarization state and N other light beams having a second polarization state, the polarization rectifier optical device comprising at least one waveplate disposed on the optical path of the N other light beams and adapted to modify the polarization of the N other light beams so as to form the N second light beams of the same polarization state as the first N light beams.

[0013] According to a particular and advantageous aspect of the first or second embodiment, the plurality of N diffraction gratings comprises N planar diffraction gratings, each diffraction grating having parallel lines and each diffraction grating having a direction normal to its plane, the plurality of N diffraction gratings being arranged so that the lines of the N diffraction gratings are parallel to the same alignment axis, the plurality of N diffraction gratings is arranged in one or more columns parallel to the alignment axis of the lines of the gratings and in which the normal directions of the gratings of each column are oriented in a plane perpendicular to the alignment axis so as to form a non-zero angle between them two by two.

[0014] According to another particular and advantageous aspect of the first or second embodiment, the plurality of N diffraction gratings comprises N planar diffraction gratings, each diffraction grating having parallel lines, the plurality of N diffraction gratings being arranged so that the lines of the N diffraction gratings are in the same plane defined by an alignment axis and the normals to the planes of the diffraction gratings, in which the plurality of N diffraction gratings is arranged in one or more columns parallel to the alignment axis of the lines of the gratings, and in which each diffraction grating in each column has a line density different from that of the other gratings in the same column.

[0015] According to yet another particular and advantageous aspect of the first or second embodiment, the plurality of N diffraction gratings comprises N planar diffraction gratings, each diffraction grating having features parallel to an alignment axis, each diffraction grating having a normal to the plane of the diffraction grating, and in which the normals of the plurality of N diffraction gratings are oriented so as to form a non-zero angle between them two by two.

[0016] Advantageously in this case, the alignment axes of the plurality of N diffraction gratings are in the same plane.

[0017] According to yet another particular and advantageous aspect of the first or second embodiment, the plurality of N diffraction gratings comprises N planar diffraction gratings, the plurality of N diffraction gratings being arranged in the same plane, each diffraction grating having features parallel to an alignment axis and in which the alignment axes of the plurality of N diffraction gratings are oriented so as to form a non-zero angle between them two by two.

[0018] Advantageously, the dispersive optical device comprises at least one dichroic blade, a diffraction grating and / or a dichroic optical fiber coupler.

[0019] According to another particular and advantageous aspect, the inlet opening is arranged upstream of the polarization rectifier optical device.

[0020] Alternatively, the optical spectrometer includes at least one inlet aperture, said at least one inlet aperture is disposed downstream or, respectively, inside the polarization rectifier optical device, said at least one inlet aperture being arranged to transmit the first light beam and the second rectified light beam or, respectively, the first light beam and the second light beam.

[0021] According to another particular and advantageous aspect, the plurality of N diffraction gratings includes at least one very high efficiency grating for TM polarization or a lamellar diffraction grating or a transmission diffraction grating.

[0022] Of course, the various features, variants, and embodiments of the invention can be combined with one another in various ways, provided they are not incompatible or mutually exclusive. Brief description of the drawings

[0023] 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:

[0024] [Fig. 1] is a schematic view of a spectrometer according to a first embodiment of the invention,

[0025] [Fig.2] is a schematic view of different masks with different openings,

[0026] [Fig.3] is a schematic view of a spectrometer according to a second mode of realization of the invention,

[0027] [Fig.4] illustrates an example of transmission curves as a function of wavenumber through a dichroic filter for an incident light beam respectively: for a beam polarized according to a first polarization state (dashed curve), for a beam polarized according to a second polarization state (solid line curve) and for an unpolarized beam (dotted line curve),

[0028] [Fig.5] illustrates another example of a dispersive optical device based on gratings of diffraction,

[0029] [Fig.6] schematically represents a device in front and top views Diffractive optics according to a first example of implementation,

[0030] [Fig.7] schematically represents a device in front and top views Diffractive optics according to a second embodiment example,

[0031] [Fig.8] schematically represents a device in front and top views Diffractive optics according to a third embodiment example,

[0032] [Fig.9] schematically represents a device in front and top views diffractive optics according to a fourth example of implementation;

[0033] [Fig. 10] schematically represents in front view and top view a diffractive optical device according to a fifth embodiment;

[0034] [Fig. 11] schematically represents in front view and top view a diffractive optical device according to a sixth embodiment;

[0035] [Fig. 12] schematically represents a lamellar network in cross-section;

[0036] [Fig. 13] illustrates an example of diffraction efficiency curves of a lamellar grating as a function of wavelength, in -1 order, respectively for TM polarization (dashed curve) and for TE polarization (mixed dashed and dotted curve);

[0037] [Fig. 14] illustrates another example of a polarization rectifier optical device arranged upstream of the spectrometer inlet;

[0038] [Fig. 15] illustrates a perspective view of a spectrometer according to an example embodiment.

[0039] 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

[0040] Generally, the optical spectrometer of this disclosure comprises an entrance mask having at least one entrance aperture, a diffractive optical device comprising an assembly of several diffraction gratings, and a two-dimensional detector. The optical spectrometer also includes an optical system for directing the light beam from the entrance mask to the diffractive optical device and another optical system for directing the diffracted light beams from the diffractive optical device to the detector. More specifically, the optical spectrometer includes a polarization-rectifying optical device and a dispersive optical device arranged upstream of the diffractive optical device. The polarization-rectifying optical device and the dispersive optical device together form a polarization-rectifying and dispersion optical device. The optical device The polarization rectifier and the dispersive optical device are arranged optically in series along the path of the light beams. In a first embodiment described in more detail in relation to [Fig. 1], the polarization rectifier is upstream of the dispersive optical device. In a second embodiment described in more detail in relation to [Fig. 3], the polarization rectifier is downstream of the dispersive optical device.

[0041] Figure 1 shows an optical spectrometer 100 according to the first embodiment of the invention. An orthonormal coordinate system XYZ is also shown in Figure 1. The optical spectrometer 100 comprises an entrance mask 70 having an entrance aperture 7, a polarization rectifier optical device 10, a collimator optical system 8, a dispersive optical device 20, a diffractive optical device 30, a focusing optical system 5, and a detector 6. The detector 6 is a two-dimensional array detector comprising at least 2N rows of photodetectors. The detector 6 comprises, for example, a CCD camera or, preferably, a CMOS camera. The advantage of a CMOS camera is that reading all the pixels is faster than reading on a CCD camera, and does not suffer from artifacts such as mixing between the signals of the different lines ("smearing").

[0042] The input mask 70 comprises an aperture 7, for example in the form of an elongated slit (see, for example, [Fig. 2]). The input aperture of the spectrometer 100 receives a light beam 1 to be spectrally analyzed. The light beam 1 propagates, for example, along the X-axis. The light beam 1 is generally unpolarized. After passing through the aperture 7, the light beam 1 is incident on the polarization rectifier optical device 10. In the example of [Fig. 1], the polarization rectifier optical device 10 comprises a polarization splitter optical device 15, an optical system comprising, for example, two lenses 16, 18, a wave plate 2, and an output mask 17.The optical polarization splitter device 15 comprises at least one polarization splitting component, such as a Savart plate, a Wollaston prism, a Rochon prism or a polarization splitter cube, and at least one element for rotating at least one polarization.

[0043] The optical polarization splitter 15, also called a polarization beam splitter, receives the light beam 1, also called the source beam, and spatially or angularly separates this source beam according to the polarization into a first light beam 11 polarized according to a first polarization state, for example linear transverse magnetic or TM, and a second light beam 12, polarized according to a second polarization state, for example linear transverse electric or TE. In the example of [Fig. 1], the optical splitter The polarization beam splitter 15 includes a polarization splitter plate that transmits the TM polarization component without angular deflection and angularly deflects the TE polarization component. At the output of the polarization splitter optical device 15, the second light beam 12 appears to originate from a virtual source offset along the Z-axis relative to the aperture 7 of the input mask 70. The wave plate 2, for example a half-wave plate, is positioned between the polarization beam splitter 15 and the output mask 17 on the optical path of the second light beam 12 only. The wave plate 2 receives the second light beam 12, polarized according to the second polarization state, and transmits it, rotating its polarization by 90 degrees, to form a second rectified light beam 120, polarized according to the first polarization state.However, this second rectified light beam 120 remains representative of the intensity of the component of the source beam according to the second polarization state. Indeed, the losses during transmission through the wave plate 2 are negligible.

[0044] The optical system consisting here of the two lenses 16, 18 forms the image of the input mask 70 on the output mask 17. As illustrated in [Fig. 2], the output mask 17 has two apertures 171, 172, each aperture being in the form of an elongated slit and arranged along the same longitudinal axis, for example parallel to the Z-axis. The spacing between the two apertures 171, 172 is adapted so that the optical system with lenses 16, 18 forms the image of the first light beam 11 on the first aperture 171 and the image of the second rectified light beam 120 on the second aperture 172. This yields the two polarization components of the source light beam 1, which are spatially separated by a center-to-center distance denoted d in the direction of the Z-axis.

[0045] According to a variant illustrated in [Fig. 14], the polarization rectifier optical device 10 can be arranged so that the polarization splitter optical device 15 is positioned upstream of the spectrometer's input mask 7. In this case, the input mask 7 comprises two apertures 71, 72 arranged to allow passage, respectively, of the first light beam 11 polarized according to the first polarization state, for example, linear transverse magnetic or TM, and the second light beam 12 polarized according to the second polarization state, for example, linear transverse electric or TE. The wave plate 2, for example, a half-wave plate, is positioned at the output of the aperture 272 on the optical path of the second light beam 12 only.Wave plate 2 receives the second light beam 12 polarized according to the second polarization state and transmits it by rotating its polarization by 90 degrees, to form a second rectified light beam 120 polarized according to the first polarization state. Optionally, a . polarizer 19 ensures that the first light beam 11 and the second rectified light beam 120 have the same polarization.

[0046] The collimator optical system 8 is based on a lens, a mirror, or an optical assembly of lenses and mirrors. In particular, the collimator optical system 8 is based on the use of spherical mirrors, toroidal mirrors, or spherical or cylindrical lenses. The collimator optical system 8 is arranged so as to collect the first light beam 11 and the second rectified light beam 120 at the output of the polarization rectifier optical device 10. The collimator optical system 8 collimates these two light beams 11 and 120 to inject them into the dispersive optical device 20, which, in this first embodiment, is arranged downstream of the polarization rectifier optical device 10.

[0047] The dispersive optical device 20 is configured to spectrally separate an incident beam having a spectral band width of approximately 4000 cm⁴, which is classically the spectral band of interest in Raman spectroscopy, into several spectral bands, for example, five spectral bands each with a spectral width of 800 cm⁻¹. The dispersive optical device 20 comprises at least one spectrally dispersive or diffractive optical element. The number of spectrally dispersive or diffractive optical elements of the dispersive optical device 20 depends on the number of diffraction gratings of the diffractive optical device 30 arranged downstream. In the example illustrated in [Fig. 1], the diffractive optical device 30 comprises three diffraction gratings 31, 32, 33 and the dispersive optical device 20 comprises two dichroic mirrors 21, 22.More generally, the diffractive optical device 30 comprises N diffraction gratings, N being an integer greater than or equal to two, and the dispersive optical device 20 comprises at least Nl spectrally dispersive or diffractive optical elements. In the example of [Fig. 1], the dispersive optical device 20 also comprises two reflective mirrors 23, 24.

[0048] We will now explain the operation of the dispersive optical device 20. The first dichroic mirror 21 receives the two light beams 11, 120 collimated by the collimator optical system 8. As a reminder, the two light beams 11, 120 represent the two polarization components of the source beam, while having the same polarization state. The first dichroic mirror 21 separates each beam spectrally into two spectral bands. More precisely, the first dichroic mirror 21 separates the first light beam 11 by transmitting a dispersed light beam 110 into one spectral band and reflecting another dispersed light beam 112 into another spectral band, referred to here as the second spectral band. Similarly, the second dichroic mirror 22 separates the dispersed light beam 110 by transmitting a light beam The first dichroic mirror 21 disperses light beam 111 into a first spectral band and reflects a dispersed light beam 113 into a third spectral band. Simultaneously, the first dichroic mirror 21 splits the second straightened light beam 120 by transmitting a dispersed light beam 220 into one spectral band and reflecting another dispersed light beam 122 into the second spectral band. Then, the second dichroic mirror 22 splits the dispersed light beam 220 by transmitting a dispersed light beam 121 into the first spectral band and reflecting a dispersed light beam 123 into the third spectral band. Thus, the second dichroic mirror 22 transmits the dispersed light beams 111 and 121 into the first spectral band to the output of the dispersive optical device 20.The first mirror 23 is arranged to receive the dispersed light beams 112 and 122 in the second spectral band and direct them towards the output of the dispersive optical device 20. The second mirror 24 is arranged to receive the dispersed light beams 113 and 123 in the third spectral band and direct them towards the output of the dispersive optical device 20. Preferably, the different spectral bands are complementary to obtain a continuous spectrum. By way of non-limiting example, the first spectral band extends from 640 nm to 701 nm, the second spectral band from 701 nm to 765 nm, and the third spectral band from 765 nm to 830 nm. Advantageously, the dichroic mirrors 21, 22 and the mirrors 23, 24 are plane mirrors. In this way, the light beams remain collimated until they exit the dispersive optical device 20.Furthermore, the dichroic mirrors 21, 22 and the mirrors 23, 24 are arranged so as to preserve the polarization state of the light beams in both reflection and transmission.

[0049] Figure 4 shows an example of transmission curves through a dichroic mirror or dichroic filter placed in the path of a light beam incident at an oblique angle of incidence, for example, 24 degrees, as a function of the Raman shift wavenumber, denoted π, and according to the polarization of the beam incident on the dichroic filter. The dashed curve represents the transmission for a beam polarized according to a first polarization state, for example TM; the solid curve represents the transmission for a beam polarized according to a second polarization state, for example TE; and the dashed curve represents the transmission for an unpolarized beam. A difference of approximately 50 cm⁻¹ is observed between the cutoff wavenumber for the TM-polarized beam and the cutoff wavenumber for the TE-polarized beam.Such a dichroic filter illuminated at oblique incidence has a spectral transition width that is small, here less than a few cm⁻¹ only if its polarization is parallel or perpendicular to the plane of incidence. For a light... The unpolarized dashed curve indicates a transition width exceeding several tens of cm⁻¹, here on the order of 50 cm⁻¹. This transition width is a disadvantage if one wants to separate the incident beam upstream of the polarization-rectifying optical device 10 into spectral bands with a spectral width of a few hundred cm⁻¹ without discontinuities between the spectral bands. On the other hand, using such a dichroic filter downstream of the polarization-rectifying optical device 10 makes it possible to obtain a dichroic mirror that is polarization-selective over a narrow and very well-defined spectral band, between two specific wavenumbers. Each dichroic mirror 21, 22 is adapted according to the spectral band considered.

[0050] The dispersive optical device 20 is also called a pre-dispersive optical device because it allows spectral dispersion of the beams upstream of the diffractive optical device 30 into N spectral bands. Thus, at the output of the dispersive optical device 20, beams 111, 121, 112, 122, 113, and 123 are obtained, which are spatially separated into three spectral bands. Furthermore, within each spectral band, the beams are spatially and / or angularly separated into two according to the two polarization components of the source beam. Advantageously, the dichroic mirrors 21, 22 and the reflective mirrors 23, 24 are configured and arranged so that the various dispersed light beams 111, 121, 112, 122, 113 and 123 are collimated, spatially separated and substantially parallel to each other at the output of the dispersive optical device 20. In addition, the dispersive optical device 20 does not modify the polarization state of the light beams.This yields dispersed light beams 111, 121, 112, 122, 113 and 123, all having the same polarization state. As indicated above, the dispersed light beams 111, 121, 112, 122, 113 and 123 are also collimated beams.

[0051] The diffractive optical device 30 is here arranged downstream of the dispersive optical device 20. The diffractive optical device 30 comprises a plurality of N diffraction gratings, forming what is also called a segmented grating. The diffraction gratings are planar gratings; however, they are not necessarily all arranged in the same plane. Various examples of segmented grating arrangements are described later, in connection with Figures 6 to 11. The properties and / or orientation of each diffraction grating are adapted so that, on the one hand, the beams are diffracted in the same angular range, and, on the other hand, the diffraction efficiency for the incident polarization is high. Each spectrum from each diffraction grating is focused onto a distinct area of ​​the detector 6.

[0052] In the example of [Fig. 1], the diffractive optical device 30 comprises three diffraction gratings 31, 32, 33. The first diffraction grating 31 is arranged to receive the light beams dispersed 111, 121 in the first band spectral. The second diffraction grating 32 is arranged to receive the scattered light beams 112 and 122 in the second spectral band. The third diffraction grating 33 is arranged to receive the scattered light beams 113 and 123 in the third spectral band. In this example, the scattered light beams 111, 112, and 113 represent the polarization component of the source beam in its first polarization state, while the scattered light beams 121, 122, and 123 represent the polarization component of the source beam in its second polarization state. For example, each diffraction grating has an efficiency that is matched, or maximized, depending on the spectral band it receives. Furthermore, the diffraction gratings are chosen to have maximum efficiency in a polarization state that corresponds to that of the light beams they receive.According to a particular and advantageous aspect, the diffraction gratings 31, 32, 33 are arranged and oriented according to the angle of incidence of the dispersed light beams 111, 121, 112, 122, 113 and 123, which are all collimated, so as to maximize the diffraction efficiency of each diffraction grating in the spectral band of the incident light beam.

[0053] Thus, the first diffraction grating 31 diffracts the dispersed light beam 111 in the first spectral band to form a first diffracted beam 131 and, respectively, diffracts the dispersed light beam 121 in the first spectral band to form a second diffracted beam 141. The output of the first diffraction grating 31 thus yields a pair of diffracted beams consisting of the first diffracted beam 131 and the second diffracted beam 141 in the first spectral band. Similarly, the second diffraction grating 32 diffracts the dispersed light beam 112 in the second spectral band to form a first diffracted beam 132 and, respectively, diffracts the dispersed light beam 122 in the second spectral band to form a second diffracted beam 142.Thus, at the output of the second diffraction grating 32, we obtain a first pair of diffracted beams consisting of the second diffracted beam 132 and the second diffracted beam 142 in the second spectral band. Similarly, the third diffraction grating 33 diffracts the dispersed light beam 113 in the third spectral band to form a first diffracted beam 133 and, respectively, diffracts the dispersed light beam 123 in the third spectral band to form a second diffracted beam 143. Thus, at the output of the third diffraction grating 33, we obtain a pair of diffracted beams consisting of the second diffracted beam 133 and the second diffracted beam 143 in the third spectral band. In other words, three pairs of diffracted light beams (131, 141), (132, 143) and (133, 143) are obtained at the output of the diffractive optical device 30.

[0054] The diffraction gratings 31, 32, 33, which together form the diffractive optical device 30, can be based on diffraction gratings operating in reflection or transmission. The diffraction gratings of the diffractive optical device 30 are preferably gratings exhibiting very high efficiency for TM polarization in the spectral band in which they are used and in the range of incidence angles in which they are used. Gratings with very high efficiency for TM polarization are, for example, used for laser pulse compression applications. A well-known type of such grating is the lamellar grating. Figure 12 shows an example of a profile view of the features of a lamellar grating.The lamellar grating here has a reflective coating, for example, gold or aluminum, or a stack of high-reflectivity dielectric materials in which the grating is embedded (multi-layer dielectric grating, or MLD). The lines here have a trapezoidal cross-section with a pitch w and a height h. In one example, the pitch w is 0.8 pm and the height h is 0.19 pm. Figure 13 shows diffraction efficiency curves, denoted E, of such a lamellar grating as a function of wavelength, denoted X, in -1 order, respectively for TM polarization (dashed line) and for TE polarization (mixed dashed and dotted line). We observe that the diffraction efficiency of this grating in the -1 order for TM polarization is very high, greater than 90% over the entire spectral band here between 800 nm and 900 nm.On the contrary, the diffraction efficiency of this grating in the -1 order for TE polarization is quite low, less than 40% over the same spectral band between 800 nm and 900 nm. Each diffraction grating 31, 32, 33 is adapted according to the spectral band considered to exhibit a very high diffraction efficiency, preferably greater than 80%, or even 90%, in the -1 order for TM polarization.

[0055] Furthermore, the relationship between the grating orientation, the number of lines, and the useful spectral band is defined such that only zero and -1 orders can exist. More precisely, the following three conditions must be met.

[0056] The condition for the existence of the order -1 is as follows:

[0057] [Math.l] 1 + sia # >

[0058] where 0 is the angle of incidence on the diffraction grating considered, G the line density and X the wavelength.

[0059] The condition for non-existence of the order +1 is as follows:

[0060] [Math.2] 1 - sia 0 <

[0061] The condition for the non-existence of the order -2 is as follows:

[0062] [Math.3] dn^<2^G

[0063] The focusing optical system 5 is arranged between the diffractive optical device 30 and the two-dimensional matrix detector 6. In the example illustrated in [Fig. 1], the focusing optical system 5 comprises a single lens. More generally, the focusing optical system 5 can be based on lenses (spherical, cylindrical, or freeform), mirrors (spherical, parabolic, cylindrical, toroidal, or freeform), or a combination of these elements. For example, the focusing optical system 5 consists of several mirrors. The focusing optical system 5 is arranged to receive the three pairs of diffracted light beams (131, 141), (132, 143) and (133, 143) and to form an image on the detector 6. More specifically, each diffracted light beam is imaged on a distinct area of ​​the detector 6. The first pair of diffracted beams (131, 141) is imaged on a first region 71 of the detector.Within this first region 71, the first diffracted beam 131 of the first spectral band is imaged on a zone 61 and the second diffracted beam 141 of the first spectral band is imaged on another zone 62. Each of the two zones 61, 62 comprises at least one line of pixels. Advantageously, the two zones 61, 62 are separated by at least one line of pixels. The detector 6 thus makes it possible to detect the two spectrally diffracted polarization components of the source beam with high spectral resolution on the first spectral band.

[0064] Similarly, the second pair of diffracted beams (132, 142) is imaged on a second region 72 of the detector, which is distinct from the first region 71 of the detector. Advantageously, the two regions 71, 72 are separated by at least one line of pixels. Within this second region 72, the first diffracted beam 132 of the second spectral band is imaged on an area 63, and the second diffracted beam 142 of the second spectral band is imaged on another area 64. Each of the two areas 63, 64 comprises at least one line of pixels. Advantageously, the two areas 63, 64 are separated by at least one line of pixels. The detector 6 thus makes it possible to detect the two spectrally diffracted polarization components of the source beam with high spectral resolution on the second spectral band.

[0065] Similarly, the third pair of diffracted beams (133, 143) is imaged on a third region 73 of the detector which is distinct from the other two regions 71, 72 of the detector. Advantageously, the two regions 72, 73 are separated by at least one line of pixels. Within this third region 73, the first diffracted beam 133 of the third spectral band is imaged in zone 65, and the second diffracted beam 143 of the third spectral band is imaged in another zone 66. Each of the two zones 65 and 66 comprises at least one line of pixels. Advantageously, the two zones 63 and 64 are separated by at least one line of pixels. Detector 6 thus enables the detection of the two spectrally diffracted polarization components of the source beam with high spectral resolution in the third spectral band.

[0066] The detector 6 simultaneously collects the two spectrally diffracted polarization components of the source beam with high spectral resolution in three spectral bands. A processing unit advantageously recombines the detected signals corresponding to the three diffracted beams 131, 132, and 133 to obtain a high-resolution spectrum of the polarization component of the source beam according to the first polarization state. Similarly, the processing unit recombines the detected signals corresponding to the three diffracted beams 141, 142, and 143 to obtain a high-resolution spectrum of the polarization component of the source beam representative of the second polarization state.It should be noted that for each spectrum, the information relating to the two input polarizations can be measured separately, in cases where the optical aberrations of the different optical components encountered are sufficiently low.

[0067] In the first embodiment described above, the polarization rectifier optical device 10 arranged upstream of the dispersive optical device 20 and the diffractive optical device 30 allows: 1- the use of diffraction gratings 31, 32, 33... exhibiting very high efficiency only for a single polarization, for example the TM polarization, and 2- the simultaneous determination of the spectra associated with two polarization components of the source beam, for example the TM polarization component and the TE polarization component.

[0068] In the case of a predisperser with dichroic elements, it also allows for a very clear spectral separation between the different spectral ranges, which is an advantage when one wants to have a spectral measurement without gaps between the sub-bands; because as shown in [Fig.4], a dichroic filter illuminated at oblique incidence has a spectral transition width that is small (less than a few cm1) only if its polarization is parallel or perpendicular to the plane of incidence; for unpolarized light, the transition width can exceed several tens of cm', which is a disadvantage if one wants to separate the incident beam into bands of a few hundred cm-1 of spectral width without gaps between the bands.

[0069] Figure 3 represents an optical spectrometer 200 according to the second embodiment of the invention. The optical spectrometer 200 here comprises a dispersive optical device 20, a polarization rectifier optical device 10, a collimator optical system 8, a diffractive optical device 30, a focusing optical system 5 and a detector 6. The detector 6 is analogous to that described in connection with Figure 1. The dispersive optical device 20 is positioned here at the inlet of the optical spectrometer 200. The dispersive optical device 20 is based on optical fiber couplers 25, 26. More precisely, the dispersive optical device 20 comprises portions of optical fiber 40, 41, 42, 43 and 44. The optical fiber couplers 25, 26 are of the Y-junction type and are configured to spectrally separate the source beam into several spectral bands.In other words, the optical fiber couplers 25 and 26 are dichroic couplers, known as wavelength-division multiplexers, or WDMs. The optical fiber portion 40 guides the light beam 1 to be spectrally analyzed, which propagates to the common branch of the first optical fiber coupler 25. The first optical fiber coupler 25 splits the light beam 1 into a light beam 114 in a first spectral band and another light beam 144 in a different spectral band. The optical fiber portion 44 guides the other light beam 144 to the common branch of the second optical fiber coupler 26. The second optical fiber coupler 26 splits the other light beam 144 into a light beam 116 in a second spectral band and a light beam 118 in a third spectral band.The optical fiber segment 41 is connected to a secondary branch of the first optical fiber coupler 25. The optical fiber segment 41 guides the light beam 114 in the first spectral band to the output of the dispersive optical device 20, which is also the input of the polarization rectifier optical device 10. Each optical fiber segment 42, respectively 43, is connected to one of the two secondary branches of the second optical fiber coupler 26. The optical fiber segment 42 guides the light beam 116 in the second spectral band to the output of the dispersive optical device 20. Similarly, the optical fiber segment 43 guides the light beam 118 in the third spectral band to the output of the dispersive optical device 20. Thus, at the output of the dispersive optical device 20, three light beams 114, 116, 118 are obtained, separated spatially and spectrally into three spectral bands.

[0070] The polarization rectifier optical device 10 is here arranged downstream of the dispersive optical device 20. The polarization rectifier optical device 10 receives the three light beams 114, 116, 118 in the three spectral bands. For each of the light beams, the polarization rectifier optical device 10 operates in the same way as that described in connection with [Fig. 1]. For example, the device The optical polarization rectifier 10 includes a polarization splitter in the optical path of each of the three light beams 114, 116, and 118. This polarization splitter is, for example, monolithic or composed of several distinct elements in the optical path of each light beam. Advantageously, a polarization splitter adapted to the first spectral band is placed in the optical path of the light beam 114 in the first spectral band. Similarly, a polarization splitter adapted to the second spectral band, and respectively the third spectral band, is placed in the optical path of the light beam 116 in the second spectral band, and of the light beam 118 in the third spectral band, respectively. The polarization splitter separates the light beam 114 into a light beam 111 in the first polarization state and a light beam 115 in the second polarization state.A wave plate 2 rectifies the polarization of the light beam 115 to form a rectified light beam 121 having the same polarization state as the light beam 111. This yields a first pair of light beams (111, 121) representing the two polarization states of the source beam in the first spectral band. Similarly, from the light beams 116 and 118, respectively, a second pair of light beams (112, 122) representing the two polarization states of the source beam in the second spectral band, and respectively a third pair of light beams (113, 123) representing the two polarization states of the source beam in the third spectral band, are obtained. Advantageously, an optical system (not shown) forms the image of the pairs of light beams (111, 121), (112, 122), and (113, 123) on an output mask 27. As schematically illustrated in [Fig. 2], the mask 27 has, for example, apertures 271, 272, 273, 274, 275, 276. The output aperture 271 is configured to receive the image of the light beam 111, the output aperture 272 is configured to receive the image of the light beam 121, the output aperture 273 is configured to receive the image of the light beam 112, the output aperture 274 is configured to receive the image of the light beam 122, the output aperture 275 is configured to receive the image of the light beam 113, and the output aperture 276 is configured to receive the image of the light beam 123. For example, the apertures 271, 272, 273, 274, 275, 276 are of circular shape. Advantageously, the optical system of the polarization rectifier optical device 10 forms the image of the ends of the portions of optical fibers 41, 42, 43 on the apertures 271, 272, 273, 274, 275, 276.

[0071] The collimator optical system 8 collects the pairs of light beams (111, 121), (112, 122) and (113, 123) at the output of the apertures 271, 272, 273, 274, 275, 276 and directs them towards the diffractive optical device 30, which works in a manner analogous to that described in connection with [Fig.1].

[0072] Figure 5 schematically represents another example of a dispersive optical device 20 based on diffraction gratings that can be used in the optical spectrometer according to either the first or second embodiment. In this example, the dispersive optical device 20 comprises a first diffraction grating 28 and a second diffraction grating 29, which are planar gratings. In the first embodiment, where the dispersive optical device 20 is arranged downstream of the polarization rectifier optical device 10 and the collimation optical system 8, the first diffraction grating 28 receives the first light beam 11 polarized according to the first polarization state and the second rectified light beam 120 representing the second polarization state. The two polarized light beams 11 and 120 are collimated by the collimation optical system 8.The two polarized light beams 11 and 120 are laterally offset from each other. The first diffraction grating 28 receives the two polarized light beams 11 and 120 at an angle of incidence ALPHA. The first diffraction grating 28 diffracts the two polarized light beams 11 and 120 in the order -1 according to their wavelength. The second diffraction grating 29 receives the two beams diffracted by the first grating 28 and diffracts them again in the order -1. This yields dispersed beams, respectively: two light beams 111, 121 dispersed in a first spectral band, two light beams 112, 122 in a second spectral band, and two light beams 113, 123 in a third spectral band. The light beams 111, 112 and 113 are formed by diffraction of the polarized light beam 11 on the two diffraction gratings 28 and 29.The light beams 121, 122, and 123 are formed by diffraction of the polarized light beam 120 on the two diffraction gratings 28 and 29. This yields, for example, six polarized beams, spectrally dispersed in three spectral bands and all spatially separated from one another. A six-aperture mask is used, for example, to spatially filter the beams 111, 112, 113, 121, 122, and 123 thus dispersed. In the second embodiment, where the dispersive optical device 20 is arranged at the entrance of the optical spectrometer, the first diffraction grating 28 receives the source beam 1 at an angle of incidence, denoted ALPHA. The first diffraction grating 28 diffracts the source beam 1 in the order -1 as a function of wavelength. The second diffraction grating 29 receives the beam diffracted by the first grating 28 and diffracts it again in the order -1.This results in beams dispersed respectively: light beam 114 in a first spectral band, light beam 116 in a second spectral band, and light beam 118 in a third spectral band. The light beams. 114, 116 and 118 are spectrally dispersed and spatially separated. A three-aperture mask is used, for example, to spatially filter the 114, 116 and 118 beams thus dispersed.

[0073] According to another aspect of this disclosure, the plurality of diffraction gratings of the diffractive optical device 30 can be arranged in different ways. We will now describe different arrangements of the plurality of diffraction gratings of the diffractive optical device 30 with reference to Figures 6 to 11.

[0074] Figure 6 schematically represents a diffractive optical device 30 in projection onto an XZ plane and onto an XY plane in a three-dimensional orthonormal coordinate system XYZ, according to a first embodiment. In this example, and without limitation, the diffractive optical device 30 comprises three diffraction gratings 31, 32, 33. Each diffraction grating is a planar grating. The normal to the plane of the diffraction grating 31, 32, and 33 is denoted 41, 42, and 43, respectively. The three diffraction gratings 31, 32, and 33 have substantially the same line density. Each grating has straight lines, parallel to each other and arranged periodically with a periodic spacing denoted w. In the example in Figure 6, the three diffraction gratings 31, 32, and 33 have the same spacing w. We note 50 the alignment axis of the network features 31, respectively 32, 33. In the example of [Fig.[6] All the diffraction gratings 31, 32, 33 have the same alignment axis 50, which is parallel to the Z-axis. The three diffraction gratings 31, 32, 33 are arranged here in a single column along an axis parallel to the alignment axis 50 and adjacent to each other. However, the three diffraction gratings 31, 32, 33 do not have the same orientation in top view. The normal 41, respectively 42, 43 to the diffraction gratings 31, respectively 32, 33 is located in the XY plane, perpendicular to the alignment axis 50, but rotates around the Z axis. For example, the normal 41 to the first diffraction grating 31 is parallel to the Y axis, while the normal 42 to the second diffraction grating 32 forms an angle, for example of about 4 degrees, with the Y axis and the normal 43 to the third diffraction grating 33 forms an angle, for example of about 7 degrees with the Y axis.Segmenting the diffractive optical device 30 into three diffraction gratings allows the beams to be dispersed into three spectral bands of limited spectral range. This arrangement makes it possible to adjust the angle of incidence of the incident light beam on each diffraction grating and thus to intercept the diffracted beams with the focusing device. The arrangement remains very compact, the gratings being arranged in a rotating column like the risers of a spiral staircase.

[0075] On [Fig.7], a diffractive optical device 30 according to a second embodiment is schematically represented in projection in an XZ plane and in projection in an XY plane in a three-dimensional orthonormal frame XYZ. In this example, without limitation, the diffractive optical device 30 comprises eight diffraction gratings 31, 32, 33, 34, 35, 36, 37, 38. Each diffraction grating is a planar grating. The normal to the plane of diffraction grating 31, 32, 33, 34, 35, 36, 37, 38 is denoted by 41, 42, 43, 44, 45, 46, 47, 48, respectively. The eight diffraction gratings 31, 32, 33, 34, 35, 36, 37, 38 have approximately the same line density. Each grating has straight lines, parallel to each other and arranged periodically with a periodic spacing denoted by w. In the example of [Fig.7], the eight diffraction gratings 31, 32, 33, 34, 35, 36, 37, 38 have the same pitch w. We denote 50 the alignment axis of the lines of the grating 31, respectively 32, 33, 34, 35, 36, 37, 38. In the example of [Fig.7], all the diffraction gratings 31, 32, 33, 34, 35, 36, 37, 38 have the same alignment axis 50 which is parallel to the Z axis.The eight diffraction gratings 31, 32, 33, 34, 35, 36, 37, 38 are here arranged in two adjacent columns of four gratings each, each column following an axis parallel to the alignment axis 50. However, the four diffraction gratings 31, 32, 33, 34 of the first column do not have the same orientation in top view and the four diffraction gratings 35, 36, 37, 38 of the second column do not have the same orientation in top view. The normals 41, 42, 43, and 44 to the diffraction gratings 31, 32, 33, and 34 respectively lie in the XY plane, perpendicular to the alignment axis 50, but rotate around the Z axis. Similarly, the normals 45, 46, 47, and 48 to the diffraction gratings 35, 36, 37, and 38 respectively lie in the XY plane, perpendicular to the alignment axis 50, but rotate around the Z axis.For example, the normal 41 to the first diffraction grating is parallel to the Y-axis, while the normal 42 to the second diffraction grating 32 forms an angle, for example, of about 2 degrees, with the Y-axis, the normal 43 to the third diffraction grating 33 forms an angle, for example, of about 4 degrees, with the Y-axis, and the normal 44 to the fourth diffraction grating 34 forms an angle, for example, of about 6 degrees with respect to the Y-axis. Similarly, the normal 45 to the fifth diffraction grating 35 is parallel to the Y-axis, while the normal 46 to the sixth diffraction grating 36 forms an angle, for example, of about 3 degrees, with the Y-axis, the normal 44 to the seventh diffraction grating 37 forms an angle, for example, of about 5 degrees, with the Y-axis, and the normal 48 to the eighth diffraction grating 38 forms an angle, for example, approximately 7 degrees relative to the Y axis.Segmenting the diffractive optical device 30 into eight diffraction gratings allows the beams to be dispersed into eight spectral bands of limited spectral range. This arrangement allows the angle of incidence of the incident light beam on each diffraction grating to be adjusted, thus intercepting the diffracted beams with the focusing device. Furthermore, the two-column arrangement reduces the length of each diffraction grating. The arrangement remains very compact. networks being arranged in two rotating columns in the manner of the risers of spiral staircases.

[0076] Figure 8 schematically represents a third embodiment of a diffractive optical device 30 in projection onto an XZ plane and onto an XY plane in a three-dimensional orthonormal coordinate system XYZ. In this example, and without limitation, the diffractive optical device 30 comprises three diffraction gratings 31, 32, 33. Each diffraction grating is a planar grating. The normal to the plane of the diffraction grating 31, 32, and 33 is denoted by 41, 42, and 43, respectively. The three diffraction gratings 31, 32, and 33 have different line densities. For example, the first diffraction grating 31 has a line density of 1699 lines / mm, the second diffraction grating 32 has a line density of 1545 lines / mm and the third diffraction grating 33 has a line density of 1405 lines / mm.Each diffraction grating 31, 32, 33 has straight lines, parallel to each other and arranged periodically with a periodic spacing denoted w1, w2, w3 respectively. In other words, in the example in [Fig. 8], the three diffraction gratings 31, 32, 33 have a different spacing from each other, taken two at a time. We denote by 50 the alignment axis of the lines of grating 31, 32, 33 respectively. In the example in [Fig. 8], all the diffraction gratings 31, 32, 33 have the same alignment axis 50, which is parallel to the Z-axis. The three diffraction gratings 31, 32, 33 are arranged here in a single column along an axis parallel to the alignment axis 50 and adjacent to each other. Furthermore, the three diffraction gratings 31, 32, 33 are in the same XZ plane and therefore have the same orientation in top view.The normals 41, 42, and 43 to the diffraction gratings 31, 32, and 33 respectively lie in the XY plane, perpendicular to the alignment axis 50, and, for example, parallel to the Y-axis. Segmenting the diffractive optical device 30 into three diffraction gratings allows the beams to be dispersed into three spectral bands of limited spectral range. This arrangement simplifies adjustments since all the gratings are illuminated at the same angle of incidence. The arrangement remains very compact, with the gratings arranged in a single column in the same plane.

[0077] Figure 9 schematically represents a diffractive optical device 30, according to a fourth embodiment, in projection onto an XZ plane and onto an XY plane in a three-dimensional orthonormal coordinate system XYZ. In this example, and without limitation, the diffractive optical device 30 comprises eight diffraction gratings 31, 32, 33, 34, 35, 36, 37, 38. Each diffraction grating is a planar grating. We note 41, 42, 43, 44, 45, 46, 47, 48 respectively as the normal to the plane of the diffraction grating 31, 32, 33, 34, 35, 36, 37, 38 respectively. The four diffraction gratings 31, 32, 33, 34, 35, 36, 37, 38 in each column are gratings having densities of different features from each other. For example, the first diffraction grating 31 has a feature density of 1756 features / mm, the second diffraction grating 32 has a feature density of 1699 features / mm, the third diffraction grating 33 has a feature density of 1642 features / mm and the fourth diffraction grating 34 has a feature density of 1585 features / mm, the fifth diffraction grating 35 has a feature density of 1528 features / mm, the sixth diffraction grating 36 has a feature density of 1471 features / mm, the seventh diffraction grating 37 has a feature density of 1414 features / mm and the eighth diffraction grating 38 has a feature density of 1358 features / mm. Each diffraction grating 31, respectively 32, 33, 34, 35, 36, 37, 38 has straight lines, parallel to each other and arranged periodically with a periodic step denoted wl, respectively w2, w3, w4, w5, w6, w7, w8. In the example of [Fig.9], on the one hand, the four diffraction gratings 31, 32, 33, 34 of the first column have a different pitch from each other, taken two by two, and, on the other hand, the four diffraction gratings 35, 36, 37, 38 of the second column have a different pitch from each other, taken two by two. The alignment axis of the grating lines 31, 32, 33, 34, 35, 36, 37, and 38 is denoted by 50. In the example in [Fig. 9], all the diffraction gratings 31, 32, 33, 34, 35, 36, 37, and 38 have the same alignment axis 50, which is parallel to the Z-axis. The eight diffraction gratings 31, 32, 33, 34, 35, 36, 37, and 38 are arranged here in two adjacent columns of four gratings each, each column following an axis parallel to the alignment axis 50. Furthermore, the eight diffraction gratings 31, 32, 33, 34, 35, 36, 37, and 38 are in the same XZ plane and therefore have the same orientation in top view.The normals 41, 42, 43, 44, 45, 46, 47, and 48 to the diffraction gratings 31, 32, 33, 34, 35, 36, 37, and 38 respectively, lie in the XY plane, perpendicular to the alignment axis 50, and, for example, parallel to the Y-axis. Segmenting the diffractive optical device 30 into eight diffraction gratings allows the beams to be dispersed into eight spectral bands of limited spectral range. This arrangement simplifies adjustments since all the gratings are illuminated at the same angle of incidence. Furthermore, the two-column arrangement reduces the length of each diffraction grating. The arrangement remains very compact, with the gratings arranged in two parallel columns in the same plane.

[0078] In the examples illustrated in Figures 8 and 9, the line density differs between each diffraction grating 31, 32, 33, 34 in the same column. It is thanks to this difference in line density that different spectral bands, illuminated at the same angle of incidence on diffraction gratings 31, 32, 33, 34 having the same orientation, can be diffracted at the same solid angle and collected by the focusing optical system 5.

[0079] In the examples illustrated in Figures 6 to 9, each diffraction grating 31, respectively 32, 33, of the diffractive optical device 30 has a diffraction plane defined by the normal 41, respectively 42, 43 to the plane of the diffraction grating and perpendicular by the alignment axis 50 of the diffraction gratings and the diffraction planes of the plurality of N diffraction gratings are all parallel to each other.

[0080] We will now describe two other embodiment examples related to Figures 10 and 11, in which the diffraction planes of the plurality of N diffraction gratings are not parallel to each other.

[0081] In [Fig. 10], a diffractive optical device 30 is schematically represented in projection onto an XZ plane and onto an XY plane in a three-dimensional orthonormal coordinate system XYZ, according to a fifth embodiment. In this example, without limitation, the diffractive optical device 30 comprises three diffraction gratings 31, 32, 33. Each diffraction grating is a planar grating. The normal to the plane of the diffraction grating 31, 32, 33, respectively, is denoted 41, 42, 43. The three diffraction gratings 31, 32, 33 are described here in connection with [Fig. 6] or [Fig. 8]. Each grating has straight lines, parallel to each other and arranged periodically with a periodic spacing denoted w. In the example of [Fig. 10], the three diffraction gratings 31, 32, 33 have the same pitch w. We denote 51, respectively 52, 53 the alignment axis of the grating lines of 31, respectively 32, 33. In the example of [Fig.

[10] The alignment axes 51, 52, 53 of the three diffraction gratings 31, 32, 33 lie in the same YZ plane but rotate around the X-axis. The three diffraction gratings 31, 32, 33 are arranged here in a single column, each grating extending along an axis parallel to the X-axis and adjacent to one another. However, the three diffraction gratings 31, 32, 33 do not have the same orientation in side view. The normal 41, respectively 42, 43, to the diffraction gratings 31, respectively 32, 33, lies in the YZ plane, perpendicular to the alignment axis 51, respectively 52, 53, and also rotates around the X axis. For example, the alignment axis 52 of the lines of the second diffraction grating 32 is parallel to the Z axis and the normal 42 to the second diffraction grating 32 is parallel to the Y axis.The alignment axis 51 of the lines of the first diffraction grating 31 forms an angle, for example, of approximately +0.2 degrees, with the Z-axis, and the normal 41 to the first diffraction grating 31 forms the same angle with respect to the Y-axis. The alignment axis 53 of the lines of the third diffraction grating 33 forms an angle, for example, of approximately -0.2 degrees, with the Z-axis, and the normal 43 to the third diffraction grating 33 forms the same angle with respect to the Y-axis. Segmenting the diffractive optical device 30 into three diffraction gratings allows the beams to be dispersed into three spectral bands of limited spectral range. In other words, the diffraction plane of the second diffraction grating 32, defined by the normal 42 to the grating and perpendicular to the alignment axis 52, lies in the XY plane. The diffraction plane of the first diffraction grating 31, defined by the normal 41 to the grating. and perpendicular to the alignment axis 51, is inclined at an angle, for example, of +0.2 degrees, with respect to the XY plane. The diffraction plane of the third diffraction grating 33, defined by the normal 43 to the grating and perpendicular to the alignment axis 53, is inclined at an angle, for example, of -0.2 degrees, with respect to the XY plane. The diffraction planes of gratings 31, 32, and 33 are oriented so as to form a non-zero angle between them in pairs. This arrangement allows each of the beams diffracted by each diffraction grating to be focused on a distinct area of ​​detector 6.

[0082] In [Fig. 11], a diffractive optical device 30 is schematically represented in projection onto an XZ plane and onto an XY plane in a three-dimensional orthonormal coordinate system XYZ, according to a sixth embodiment. In this example, without limitation, the diffractive optical device 30 comprises three diffraction gratings 31, 32, 33. Each diffraction grating is a planar grating. The normal to the plane of the diffraction grating 31, 32, 33 is denoted 41, 42, 43 respectively. The three diffraction gratings 31, 32, 33 are described here in connection with [Fig. 6] or [Fig. 8]. Each grating has straight lines, parallel to each other and arranged periodically with a periodic spacing denoted w. In the example of [Fig. 11], the three diffraction gratings 31, 32, 33 have the same pitch w. We denote 51, respectively 52, 53 the alignment axis of the lines of the grating 31, respectively 32, 33. In the example of the [Fig.

[11] The alignment axes 51, 52, 53 of the three diffraction gratings 31, 32, 33 lie in the same XZ plane but rotate around the Y axis. The three diffraction gratings 31, 32, 33 are arranged here in a single column, each grating extending in the same XZ plane and the gratings being adjacent to one another. However, the lines of the three diffraction gratings 31, 32, 33 do not have the same alignment axis in front view. The alignment axis 51, respectively 52, 53 to the diffraction grating 31, respectively 32, 33 is located in the XZ plane and has a variable inclination with respect to the Z axis. For example, the alignment axis 52 of the lines of the second diffraction grating 32 is parallel to the Z axis and the normal 42 to the second diffraction grating 32 is parallel to the Y axis.The alignment axis 51 of the lines of the first diffraction grating 31 forms an angle, for example, between +0.1 degrees and +1 degrees, with the Z-axis, and the normal 41 to the first diffraction grating 31 is parallel to the Y-axis. The alignment axis 53 of the lines of the third diffraction grating 33 forms an angle, for example, between -0.1 degrees and -1 degrees, with the Z-axis, and the normal 43 to the third diffraction grating 33 is parallel to the Y-axis. Segmenting the diffractive optical device 30 into three diffraction gratings allows the beams to be dispersed into three spectral bands of limited spectral extent. In other words, the diffraction plane of the second diffraction grating 32, defined by the normal 42 to the grating and perpendicular to the alignment axis 52, lies in the XY plane. The diffraction plane of the first diffraction grating 31, defined by the normal 41 to the grating and perpendicular to the alignment axis 51, is inclined at an angle of a few tenths of a degree, for example +0.5 degrees, with respect to the XY plane. The diffraction plane of the third diffraction grating 33, defined by the normal 43 to the grating and perpendicular to the alignment axis 53, is inclined at an angle of a few tenths of a degree, for example -0.5 degrees, with respect to the XY plane. The diffraction planes of gratings 31, 32, and 33 are oriented so as to form a non-zero angle between them in pairs. This arrangement allows each of the beams diffracted by each diffraction grating to be focused onto a distinct area of ​​the detector 6.

[0083] The configurations illustrated in figures 10 and 11, in which the lines of the gratings and / or the diffraction planes of the different gratings are not parallel, make it possible to ensure the focusing of the beam diffracted by each diffraction grating on a given area of ​​the detector 6.

[0084] Furthermore, the combination of the embodiments illustrated in Figures 8 or 9 and 11 makes it possible to obtain a fully planar diffractive optical device 30. The advantage of such a planar segmented grating is that its production can be achieved by very efficient replication processes.

[0085] It is also possible to combine the embodiment examples illustrated in figures 6 and 8 or to combine the embodiment examples illustrated in figures 7 and 9.

[0086] It is also possible to combine the embodiment examples illustrated in figures 10 and 8 or 10 and 11 or even 11 and 8.

[0087] Figure 15 illustrates an example of the implementation of an optical spectrometer 100 according to the first embodiment of this disclosure. The optical spectrometer 100 is based on a Czemy-Turner arrangement. The optical spectrometer 100 comprises an entrance mask having a circular aperture with a diameter of 40 pm, followed by a polarization-rectifying optical device 10 analogous to that described in connection with Figure 1. The optical spectrometer 100 includes a collimating optical system 8 consisting of a first mirror having a focal length of 140 mm which collimates the light from a light source. The optical spectrometer 100 includes a dispersive optical device 20 as described in connection with Figure 1. The optical spectrometer 100 includes a diffractive optical device 30 having three diffraction gratings arranged in the arrangement shown in Figure 6.The optical spectrometer 100 includes a focusing optical system 5 consisting here of a toroidal mirror, having focal lengths of 140 mm and 136.5 mm, to focus the diffracted light onto a plane called the image plane. The optical spectrometer 100 includes a matrix light detector 6, placed in the image plane, which transforms the incident light into electronic signals. The diffractive optical device 30 is segmented and consists of three sub-gratings, inclined relative to each other so that the... The light diffracted by each of the diffraction gratings 31, 32, and 33 is focused into distinct areas of the matrix detector. Furthermore, the diffraction gratings 31, 32, and 33 are rotated relative to each other so that the light diffracted by each sub-grating can reach the matrix detector. These three diffraction gratings 31, 32, and 33 have line densities G of 1200 lines / mm, 1480 lines / mm, and 1740 lines / mm, respectively. The dispersive optical device 20 here includes dichroic mirrors 21 and 22, whose cutoff wavelengths are, for TM polarization, respectively 701 nm and 765 nm, which transmit light whose wavelength is greater than the cutoff wavelength and reflect light whose wavelength is less than the cutoff wavelength.The dispersive optical device 20 also includes reflecting mirrors 23, 24 which reflect the light towards the segmented diffractive optical device 30. The mirrors 23, 24 are arranged so that the first spectral band from 640 nm to 701 nm falls on the first diffraction grating 31 of 1740 lines / mm, the second spectral band from 701 nm to 765 nm falls on the second diffraction grating 32 of 1480 lines / mm, and the third spectral band beyond 765 nm (and going at least up to 830 nm) falls on the third diffraction grating 33 of line density G of 1200 lines / mm. This optical spectrometer 100 is particularly useful for performing laser-excited Raman measurements with a wavelength of 640 nm, with excellent efficiency and excellent spectral resolution in the spectral range from 640 nm to 830 nm.

Claims

Demands

1. High spectral resolution optical spectrometer (100, 200) comprising: - an optical polarization dispersion and rectification device comprising a polarization rectifier optical device (10) and a dispersive optical device (20) arranged optically in series, the optical polarization dispersion and rectification device being arranged to receive a light beam (1) and configured to separate the light beam into N pairs of light beams dispersed in N spectral bands, where N is an integer, the N pairs of dispersed light beams having the same polarization state, each pair of dispersed light beams comprising a first dispersed light beam (111, 112, 113) and a second dispersed light beam (121, 122, 123) in the same spectral band among the N spectral bands,the first dispersed light beam and the second dispersed light beam of each pair of dispersed light beams being spatially and / or angularly separated; - a diffractive optical device (30) comprising a plurality of N diffraction gratings, each diffraction grating (31, 32, 33) of the plurality of N diffraction gratings being arranged and configured to selectively receive a pair of dispersed light beams ((111, 121), (112, 122), (113, 123)) in a spectral band and to form by diffraction a pair of diffracted beams comprising a first diffracted light beam (131, 132, 133) and a second diffracted light beam (141, 142, 143) in the spectral band, the plurality of N diffraction gratings forming a plurality of N pairs of diffracted light beams,- a two-dimensional matrix detector (6) comprising at least 2N rows of photodetectors; and - an optical focusing system (5) arranged to receive the plurality of N pairs of diffracted light beams and form an image on the detector (6), each diffracted light beam from the plurality of N pairs of diffracted light beams being imaged on a region (61, 62, 63, 64, 65, 66) distinct from the detector (6).

2. Optical spectrometer (100) according to claim 1, wherein:

3. - the optical polarization rectifier device (10) is arranged and configured to receive the light beam (1) and split the light beam into a first light beam (11) having a first polarization state and a second light beam (12) having a second polarization state, the optical polarization rectifier device (1) comprising a wave plate (2) disposed on the optical path of the second light beam (12) and adapted to modify the polarization of the second light beam (12) so as to form a second rectified light beam (120) with the same polarization state as the first light beam (11), and - the dispersive optical device (20) is arranged downstream of the polarization rectifier optical device (10) to receive the first light beam (11) and the second rectified light beam (120), the dispersive optical device (20) being configured to separate spatially and / or angularly the first light beam (11) into a plurality of N first spectrally dispersed light beams (111, 112, 113) and the second rectified light beam (120) into a plurality of N second spectrally dispersed light beams (121, 122, 123), forming said N pairs of light beams dispersed in N spectral bands. Optical spectrometer (200) according to claim 1, wherein: - the dispersive optical device (20) is arranged to receive the light beam (1), the dispersive optical device (20) being configured to spatially and / or angularly separate the light beam (1) into a plurality of N spectrally dispersed light beams (114, 116, 118) in said N spectral bands, and - the polarization rectifier optical device (10) is arranged downstream of the dispersive optical device (20) and configured to receive and polarize the plurality of N spectrally dispersed light beams (114, 116, 118) into N first light beams (111, 112, 113) having a first polarization state and N other light beams (115, 117, 119) having a second polarization state, the polarization rectifier optical device (10) comprising at least one waveplate (2) arranged on the optical path of the N other light beams (115, 117,119) and adapted to modify the polarization of the other N light beams (115, 117, 119) so as to form the second N light beams, (121, 122, 123) of the same polarization state as the first N light beams (111, 112, 113).

4. An optical spectrometer according to any one of claims 1 to 3, wherein the plurality of N diffraction gratings comprises N planar diffraction gratings, each diffraction grating having parallel lines and each diffraction grating having a direction normal (41, 42, 43, 44, 45, 46) to its plane, the plurality of N diffraction gratings being arranged so that the lines of the N diffraction gratings are parallel to the same alignment axis (50), the plurality of N diffraction gratings is arranged in one or more columns parallel to the alignment axis (50) of the grating lines and wherein the normal directions (41, 42, 43, 44, 45, 46) of the gratings in each column are oriented in a plane perpendicular to the alignment axis (50) so as to form a non-zero angle between them in pairs.

5. Optical spectrometer according to any one of claims 1 to 3, wherein the plurality of N diffraction gratings comprises N planar diffraction gratings, each diffraction grating having parallel lines, the plurality of N diffraction gratings being arranged so that the lines of the N diffraction gratings are in the same plane, defined by an alignment axis (50) and the normals (41, 42, 43) to the planes of the diffraction gratings, wherein the plurality of N diffraction gratings is arranged in one or more columns parallel to the alignment axis (50) of the lines of the gratings, and wherein each diffraction grating in each column has a line density different from that of the other gratings in the same column.

6. Optical spectrometer according to any one of claims 1 to 3, wherein the plurality of N diffraction gratings comprises N planar diffraction gratings, each diffraction grating (31, 32, 33) having lines parallel to an alignment axis (51, 52, 53), each diffraction grating (31, 32, 33) having a normal (41, 42, 43) to the plane of the diffraction grating, and wherein the normals of the plurality of N diffraction gratings are oriented so as to form a non-zero angle between them taken two by two.

7. An optical spectrometer according to any one of claims 1 to 3, wherein the plurality of N diffraction gratings comprises N planar diffraction gratings, the plurality of N diffraction gratings being arranged in the same plane, each diffraction grating having features parallel to an alignment axis (51, 52, 53) and in which the alignment axes (51, 52, 53) of the plurality of N diffraction gratings are oriented so as to form a non-zero angle between them two by two.

8. Optical spectrometer according to any one of claims 1 to 7, wherein the dispersive optical device (20) comprises at least one dichroic plate, a diffraction grating and / or a dichroic optical fiber coupler.

9. Optical spectrometer according to any one of claims 1 to 8, comprising an inlet aperture (7), wherein the inlet aperture (7) is disposed upstream of the polarization rectifier optical device (10).

10. Optical spectrometer according to any one of claims 1 to 8, comprising at least one inlet aperture (7, 71, 72), wherein said at least one inlet aperture (7, 71, 72) is disposed downstream or, respectively, inside the polarization rectifier optical device (10), said at least one inlet aperture (7, 71, 72) being arranged to transmit the first light beam (11) and the second rectified light beam (12) or, respectively, the first light beam and the second light beam (21).

11. Optical spectrometer according to any one of claims 1 to 10 wherein the plurality of N diffraction gratings comprises at least one very high efficiency TM polarization grating or a lamellar diffraction grating.

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