High spectral resolution optical spectrometer

Abstract

The present invention relates to an optical spectrometer (100) with high spectral resolution. 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 polarization 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 of the plurality of N pairs of diffracted light beams being imaged on a separate area (61, 62, 63, 64, 65, 66) of the detector (6).Figure for abstract: 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 applications in particular in Raman microspectrometry which requires very high spectral resolution. State of the art

[0003] In the above field, it is known to use an optical spectrometer comprising a multichannel detector based on a pixel matrix. To obtain a high spectral resolution and a significant spectral band, 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 image phase grating (or VIPA, acronym for the expression Virtually imaged phased array, in English) advantageously allow this separation of the spectral components over several lines of the image sensor.However, these echelle grating or VIPA type spectrometers have little versatility in terms of design, and do not offer the possibility of imaging the spectrum of several spatially separated points at the spectrometer input.

[0004] Patent document FR2956204 describes a compact spectrograph with a wide spectral band comprising a plurality of concave diffraction gratings. However, this spectrograph has the disadvantage of low efficiency, because all of the available light is divided between several diffraction gratings, each grating operating in a spectral domain, so that for a given wavelength only one of these gratings directs the light towards the sensor. A spectrometer comprising a separation stage based on a dichroic element so as to illuminate each grating with the adapted spectral sub-band makes it possible to partially solve the efficiency problem, but does not apply to very high resolution spectrometers requiring the division into spectral sub-bands whose width is less than 100 nanometers (nm), because it is known that the cut-off wavelength of the filters di- chroic under oblique incidence can differ by one or several tens of nm depending on the polarization, which prevents a separation into narrow sub-bands. Finally, the efficiency of the gratings used in spectrometers also limits the efficiency of spectrometers. 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 have an efficiency greater than 90% over a spectral band, and for a given incidence range, but this efficiency is only achieved for a single linear transverse magnetic polarization, called TM, the efficiency for the other linear transverse electric polarization, called TE, being generally less than 20%, which makes the average efficiency for unpolarized light less than 55%.

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

[0006] In this context, the present invention provides an optical spectrometer with high spectral resolution.

[0007] More particularly, the invention proposes a high spectral resolution optical spectrometer comprising: an optical polarization dispersion and rectification device comprising a polarization rectifier optical device and a dispersive optical device optically arranged in series, the optical polarization dispersion and rectification device being arranged to receive a light beam and configured to polarization 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 diffractly form 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 matrix detector comprising at least 2N rows of photodetectors, each diffracted light beam of the plurality of N pairs of diffracted light beams being imaged on a separate area of ​​the detector

[0008] Thus, the optical spectrometer provides excellent spectral resolution while remaining compact. In addition, this makes it possible to obtain the spectrum of the two polarization components on each spectral band.

[0009] The spectrometer makes it possible to obtain a complete spectrum distributed in several spectral bands superimposed on a pixel matrix detector, with a high spectral resolution and a very high efficiency. This spectrometer has the additional advantage of being able to be an imager, that is to say of forming the image of several light sources placed at the level of the entrance slit of the spectrometer 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 of the source beam.

[0010] Other non-limiting and advantageous characteristics of the system according to the invention, taken individually or in all technically possible combinations, are as follows.

[0011] According to a first embodiment, the polarization rectifying optical device is arranged and configured to receive the light beam and polarization-split 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 rectifying optical device comprising a wave plate arranged 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 arranged downstream of the polarization rectifying 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 first spectrally dispersed light beams and the second rectified light beam into a plurality of N second spectrally dispersed 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 separating the light beam into a plurality of N spectrally dispersed light beams in said N spectral bands, and the polarization rectifying optical device is arranged downstream of the dispersive optical device and configured to receive and polarization separate 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 rectifying optical device comprising at least one wave plate arranged 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 N first 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 a column or in several 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 of each column has a line density different from that of the other gratings of 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 lines 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 lines 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 plate, a diffraction grating and / or a dichroic optical fiber coupler.

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

[0020] Alternatively, the optical spectrometer comprises at least one input aperture, said at least one input aperture being arranged downstream or, respectively, inside the polarization rectifying optical device, said at least one input 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 comprises at least one grating with very high efficiency 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 may be combined with each other in various combinations to the extent that they are not incompatible or mutually exclusive. Brief description of the drawings

[0023] Furthermore, various other characteristics of the invention emerge from the appended description given with reference to the drawings which illustrate non-limiting forms of embodiment of the invention and where:

[0024] [Fig.l] 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 lization of the invention,

[0027] [Fig.4] illustrates an example of transmission curves as a function of wave number 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 curve) and for an unpolarized beam (dotted curve),

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

[0029] [Fig.6] schematically represents in front view and in top view a device diffractive optics according to a first example of embodiment,

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

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

[0032] [Fig.9] schematically represents in front view and in top view a device diffractive optics according to a fourth embodiment;

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

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

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

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

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

[0038] [Fig. 15] illustrates a perspective view of a spectrometer according to an exemplary 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 the present disclosure comprises an input mask having at least one input aperture, a diffractive optical device comprising an assembly of several diffraction gratings and a two-dimensional detector. The optical spectrometer also comprises an optical system for directing the light beam from the input mask towards the diffractive optical device and another optical system for directing the diffracted light beams from the diffractive optical device onto the detector. More particularly, the optical spectrometer comprises a polarization rectifier optical device and a dispersive optical device arranged upstream of the diffractive optical device. The polarization rectifier optical device and the dispersive optical device together form a polarization dispersion and rectification optical device.The polarization rectifier optical device and the dispersive optical device are optically arranged in . series on the path of the light beams. In a first embodiment described more precisely in connection with [Fig.l], the polarization rectifier optical device is upstream of the dispersive optical device. In a second embodiment described more precisely in connection with [Fig.3], the polarization rectifier optical device is downstream of the dispersive optical device.

[0041] [Fig. 1] represents an optical spectrometer 100 according to the first embodiment of the invention. An orthonormal XYZ reference frame is also represented in [Fig. 1]. The optical spectrometer 100 comprises an input mask 70 having an input 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 matrix detector comprising at least 2N lines 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 them 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 opening 7, for example in the form of an elongated slit (see, for example, [Fig. 2]). The input opening 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 opening 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 polarization splitter optical 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 device 15, also called a polarization beam splitter, receives the light beam 1, also called a 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.l], the optical polarization splitter device 15 comprises a polarization splitter plate which transmits the component of TM polarization without angular deviation and which angularly deflects the TE polarization component. At the output of the optical polarization splitter device 15, the second light beam 12 appears to come from a virtual source offset along the Z axis relative to the opening 7 of the input mask 70. The wave plate 2, for example a half-wave plate, is arranged 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 by 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.In fact, the losses during transmission through wave plate 2 are negligible.

[0044] The optical system here consisting 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 here comprises two openings 171, 172, each opening 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 openings 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 opening 171 and the image of the second rectified light beam 120 on the second opening 172. This gives 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 rectifying optical device 10 can be arranged so that the polarization splitting optical device 15 is arranged upstream of the input mask 7 of the spectrometer. In this case, the input mask 7 comprises two openings 71, 72 arranged to allow 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, to pass respectively. The wave plate 2, for example a half-wave plate, is here arranged at the output of the opening 272 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 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 collimating optical system 8 is based on a lens, a mirror or an optical assembly of lenses and mirrors. In particular, the collimating optical system 8 is based on the use of spherical mirrors, toroidal mirrors, spherical or cylindrical lenses. The collimating 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 collimating optical system 8 collimates these two light beams 11, 120 to inject them into the dispersive optical device 20 which is, in this first embodiment, 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 of width approximately 4000 cm1, which is conventionally the spectral band of interest in Raman spectrometry, into several spectral bands, for example into five spectral bands each of 800 cm1 of spectral width. 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.l], 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.l], the dispersive optical device 20 also comprises two reflecting 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 collimating optical system 8. As a reminder, the two light beams 11, 120 are representative of 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 in one spectral band and by reflecting another dispersed light beam 112 in another spectral band, here called the second spectral band.Similarly, the second dichroic mirror 22 separates the dispersed light beam 110 by transmitting a dispersed light beam 111 in a first spectral band and reflecting a dispersed light beam 113 in a third spectral band. Simultaneously, the first dichroic mirror 21 separates the second rectified light beam 120 into . transmitting a dispersed light beam 220 in one spectral band and reflecting another dispersed light beam 122 in the second spectral band. Then, the second dichroic mirror 22 separates the dispersed light beam 220 by transmitting a dispersed light beam 121 in the first spectral band and reflecting a dispersed light beam 123 in the third spectral band. Thus, the second dichroic mirror 22 transmits the dispersed light beams 111, 121 in 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 to 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 to the output of the dispersive optical device 20.Preferably, the different spectral bands are complementary to enable a continuous spectrum to be obtained. By way of non-limiting example, the first spectral band extends from 640 nm to 701 nm, the second spectral band extends from 701 nm to 765 nm and the third spectral band extends from 765 nm to 830 nm. Particularly advantageously, the dichroic mirrors 21, 22 and the mirrors 23, 24 are plane mirrors. In this way, the light beams remain collimated until the exit of the dispersive optical device 20. In addition, the dichroic mirrors 21, 22 and the mirrors 23, 24 are arranged so as to maintain the polarization state of the light beams in reflection as in transmission.

[0049] [Fig.4] shows an example of transmission curves by a dichroic mirror or dichroic filter arranged on the path of an incident light beam at an oblique angle of incidence, for example 24 degrees, as a function of the Raman shift wave number, denoted o, and according to the polarization of the incident beam 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 dotted curve represents the transmission for an unpolarized beam. A difference of approximately 50 cm 1 is observed between the cut-off wave number for the TM polarized beam and the cut-off wave number for the TE polarized beam.Such a dichroic filter illuminated under oblique incidence has a spectral transition width which is small, here less than a few cm1 only if its polarization is parallel or perpendicular to the plane of incidence. For unpolarized light, the dotted curve indicates a transition width exceeding several tens of cm 1, here of the order of 50 cm1. This transition width is penalizing if one wants to separate the incident beam upstream of the polarization rectifier optical device 10 into spectral bands of a few hundred cm1 of spectral width. without discontinuity between the spectral bands. On the other hand, the use of such a dichroic filter downstream of the polarization rectifier optical device 10 makes it possible to obtain a polarization-selective dichroic mirror over a narrow and very well-defined spectral band, between two determined wave numbers. Each dichroic mirror 21, 22 is adapted according to the spectral band considered.

[0050] The dispersive optical device 20 is also called a predispersive 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. In addition, in each spectral band, the beams are spatially and / or angularly separated into two depending on the two polarization components of the source beam. Advantageously, the dichroic mirrors 21, 22 and the reflecting mirrors 23, 24 are configured and arranged so that the different 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 produces 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 arrangements of segmented gratings are described below, in connection with FIGS. 6 to 11. The properties and / or the 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 on a distinct zone of the detector 6.

[0052] In the example of [Fig.l], the diffractive optical device 30 comprises three diffraction gratings 31, 32, 33. The first diffraction grating 31 is arranged to receive the dispersed light beams 111, 121 in the first spectral band. The second diffraction grating 32 is arranged to receive the dispersed light beams 112 and 122 in the second spectral band. The third diffraction grating 33 is arranged to receive the dispersed light beams 113 and 123 in the third spectral band. In this example, the dispersed light beams 111, 112 and 113 are representative of the polarization component of the source beam. according to the first polarization state while the dispersed light beams 121, 122 and 123 are representative of the polarization component of the source beam according to the second polarization state. For example, each diffraction grating has an efficiency adapted, or maximized, according to the spectral band that it receives. In addition, the diffraction gratings are chosen to have a maximum efficiency in a polarization state which corresponds to that of the light beams that it receives. 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 beams.

[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. A pair of diffracted beams consisting of the first diffracted beam 131 and the second diffracted beam 141 in the first spectral band is thus obtained at the output of the first diffraction grating 31. 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, a first pair of diffracted beams is obtained, 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, a pair of diffracted beams is obtained, 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 together forming the diffractive optical device 30 may be based on diffraction gratings operating in reflection or in transmission. The diffraction gratings of the diffractive optical device 30 are preferably gratings having a very high efficiency for TM polarization in the spectral band in which they are used and in the range of incidence angles where they are used. The gratings with very high efficiency for TM polarization TM gratings are, for example, used for laser pulse compression applications. A well-known type of such gratings are lamellar gratings. [Fig. 12] shows an example of a side 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 highly reflective dielectric materials in which the grating is inscribed (multi-layer dielectric grating, or "MLD" in English, for "Multi-Layer Dielectric Grating). The features here have a trapezoidal section with a pitch w and a height h. In one example, the pitch w is equal to 0.8 pm and the height h is equal to 0.19 pm. [Fig.13] shows diffraction efficiency curves, denoted E, of such a lamellar network as a function of the wavelength, denoted X, in the order -1, respectively for TM polarization (dashed curve) and for TE polarization (dash-dotted and mixed curve).It is observed that the diffraction efficiency of this grating in order -1 for the 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 order -1 for the 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 have a very high diffraction efficiency, preferably greater than 80%, or even 90%, in order -1 for the TM polarization.

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

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

[0057] [Math.l] 1 + sis#

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

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

[0060] [Math.2] 1 - sin 0 < zAG

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

[0062] [Math.3] 1 + sin^<2^G

[0063] The optical focusing system 5 is arranged between the diffractive optical device 30 and the two-dimensional matrix detector 6. In the example illustrated in [Fig.l], the The optical focusing system 5 comprises a single lens. More generally, the optical focusing system 5 may be based on lenses (spherical, cylindrical or free-form), mirrors (spherical, parabolic, cylindrical, toroidal or free-form) or a combination of these elements. For example, the optical focusing system 5 consists of several mirrors. The optical focusing 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 precisely, each diffracted light beam is imaged on a separate 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 an area 61 and the second diffracted beam 141 of the first spectral band is imaged on another area 62. Each of two areas 61, 62 comprises at least one line of pixels. Advantageously, the two areas 61, 62 are separated by at least one line of pixels. The detector 6 thus makes it possible to detect the two polarization components of the source beam spectrally diffracted 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 a zone 63 and the second diffracted beam 142 of the second spectral band is imaged on another zone 64. Each of two zones 63, 64 comprises at least one line of pixels. Advantageously, the two zones 63, 64 are separated by at least one line of pixels. The detector 6 thus makes it possible to detect the two polarization components of the source beam spectrally diffracted 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 on a zone 65 and the second diffracted beam 143 of the third spectral band is imaged on another zone 66. Each of the two zones 65, 66 comprises at least one line of pixels. Advantageously, the two zones 63, 64 are separated by at least one line of pixels. The detector 6 thus makes it possible to detect the two polarization components of the source beam spectrally diffracted with high spectral resolution on the third band. spectral.

[0066] The detector 6 simultaneously collects the two polarization components of the source beam spectrally diffracted with high spectral resolution in three spectral bands. A processing unit advantageously makes it possible to recombine the detected signals corresponding respectively 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 makes it possible to recombine the detected signals corresponding respectively 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.Note that for each spectrum, the information relating to the two input polarizations can be measured separately, in the case 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 of the diffractive optical device 30 makes it possible: 1- to use diffraction gratings 31, 32, 33... having a very high efficiency only for a single polarization, for example the TM polarization, and 2- to simultaneously determine 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 a gap between the sub-bands; because as shown in [Fig.4], a dichroic filter illuminated under oblique incidence has a spectral transition width which is low (less than a few cm1) only if its polarization is parallel or perpendicular to the plane of incidence; for unpolarized light, the width of the transition can exceed several tens of cm1, 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] [Fig. 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 collimating optical system 8, a diffractive optical device 30, a focusing optical system 5 and a detector 6. The detector 6 is similar to that described in connection with [Fig. 1]. The dispersive optical device 20 is here arranged at the input 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 fiber optic 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 fiber optic couplers 25, 26 are dichroic couplers, known as wavelength division multiplexers, or WDM (for "Wavelength-Division Multiplexer") in English. The optical fiber portion 40 guides the light beam 1 to be spectrally analyzed which propagates to the common branch of the first fiber optic coupler 25. The first fiber optic coupler 25 separates the light beam 1 into a light beam 114 in a first spectral band and another light beam 144 in another spectral band. The optical fiber portion 44 guides the other light beam 144 to the common branch of the second fiber optic coupler 26.The second optical fiber coupler 26 separates 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 portion 41 is connected to a secondary branch of the first optical fiber coupler 25. The optical fiber portion 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 portion 42, respectively 43, is connected to one of the two secondary branches of the second optical fiber coupler 26. The optical fiber portion 42 guides the light beam 116 in the second spectral band to the output of the dispersive optical device 20. Similarly, the optical fiber portion 43 guides the light beam 118 in the third spectral band to the output of the dispersive optical device 20.Three light beams 114, 116, 118 are thus obtained at the output of the dispersive optical device 20, separated spatially and spectrally in three spectral bands.

[0070] The polarization rectifying optical device 10 is here arranged downstream of the dispersive optical device 20. The polarization rectifying optical device 10 receives the three light beams 114, 116, 118 in the three spectral bands. For each of the light beams, the polarization rectifying optical device 10 operates in the same manner as that described in connection with [Fig.l]. For example, the polarization rectifying optical device 10 comprises a polarization splitter on the optical path of each of the three light beams 114, 116, 118. This polarization splitter is, for example, monolithic or made up of several separate elements on the optical path of each of the light beams. Advantageously, a polarization splitter adapted to the first spectral band is arranged on the optical path of the light beam 114 in the first spectral band.Likewise, a polarization splitter adapted to the second spectral band, respectively third spectral band, is arranged on the optical path of the light beam 116. in the second spectral band, respectively of the light beam 118 in the third spectral band. The polarization splitter polarizes the light beam 114 into a light beam 111 according to the first polarization state and a light beam 115 according to 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. A first pair of light beams (111, 121) is thus obtained, representative of the two polarization states of the source beam in the first spectral band.Similarly, from the light beam 116, and respectively 118, a second pair of light beams (112, 122) representative of the two polarization states of the source beam in the second spectral band, and respectively a third pair of light beams (113, 123) representative of 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 illustrated schematically in [Fig. 2], the mask 27 comprises, for example, openings 271, 272, 273, 274, 275, 276.The exit opening 271 is configured to receive the image of the light beam 111, the exit opening 272 is configured to receive the image of the light beam 121, the exit opening 273 is configured to receive the image of the light beam 112, the exit opening 274 is configured to receive the image of the light beam 122, the exit opening 275 is configured to receive the image of the light beam 113 and the exit opening 276 is configured to receive the image of the light beam 123. For example, the openings 271, 272, 273, 274, 275, 276 are circular in shape. Advantageously, the optical system of the polarization rectifier optical device 10 forms the image of the ends of the optical fiber portions 41, 42, 43 on the openings 271, 272, 273, 274, 275, 276.

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

[0072] [Fig. 5] schematically represents another example of a dispersive optical device 20 based on diffraction gratings which can be used in the optical spectrometer according to 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 grating diffraction grating 28 receives the first light beam 11 polarized according to the first polarization state and the second rectified light beam 120 representative of 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 relative to each other. The first diffraction grating 28 receives the two polarized light beams 11 and 120 at the angle of incidence ALPHA. The first diffraction grating 28 diffracts the two polarized light beams 11 and 120 in the order -1 as a function of the wavelength. The second diffraction grating 29 receives the two beams diffracted by the first grating 28 and diffracts them again in the order -1.Dispersed beams are thus obtained 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 gives, for example, six polarized beams, spectrally dispersed in three spectral bands and all spatially separated from each other. For example, a six-aperture mask is used 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 input 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 the wavelength. The second diffraction grating 29 receives the beam diffracted by the first grating 28 and diffracts it again in the order -1. This produces dispersed beams respectively: the light beam 114 in a first spectral band, the light beam 116 in a second spectral band and the light beam 118 in a third spectral band. The light beams 114, 116 and 118 are spectrally dispersed and spatially separated. For example, a three-aperture mask is used to spatially filter the beams 114, 116 and 118 thus dispersed.

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

[0074] In [Fig.6], a diffractive optical device 30 according to a first exemplary embodiment is shown schematically in projection in an XZ plane and in projection in an XY plane in a three-dimensional orthonormal reference frame XYZ. In this For example, in a non-limiting manner, the diffractive optical device 30 comprises three diffraction gratings 31, 32, 33. Each diffraction grating is a plane grating. 41, respectively 42, 43 denotes the normal to the plane of the diffraction grating 31, respectively 32, 33. The three diffraction gratings 31, 32, 33 are here gratings having substantially the same line density. Each grating has rectilinear lines, parallel to each other and arranged periodically with a periodic pitch denoted w. In the example of [Fig.6], the three diffraction gratings 31, 32, 33 have the same pitch w. The alignment axis of the lines of the grating 31, respectively 32, 33, is noted 50. 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 here arranged 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 respect to the Y axis. The segmentation of the diffractive optical device 30 into three diffraction gratings makes it possible to disperse the beams into three spectral bands of limited spectral extent.This arrangement allows the angle of incidence of the incident light beam on each diffraction grating to be adjusted and thus the diffracted beams to be intercepted 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] In [Fig. 7], a diffractive optical device 30 according to a second exemplary embodiment is shown schematically in projection in an XZ plane and in projection in an XY plane in a three-dimensional orthonormal reference frame XYZ. In this example, in a non-limiting manner, 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, respectively 42, 43, 44, 45, 46, 47, 48 the normal to the plane of the diffraction grating 31, respectively 32, 33, 34, 35, 36, 37, 38. The eight diffraction gratings 31, 32, 33, 34, 35, 36, 37, 38 are here gratings having substantially the same density of lines. Each grating has rectilinear lines, parallel to each other and arranged periodically with a periodic pitch noted 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 note 50 the alignment axis of the lines of the network 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 normal 41, respectively 42, 43, 44 to the diffraction gratings 31, respectively 32, 33, 34 is located in the XY plane, perpendicular to the alignment axis 50, but rotates about the Z axis. Similarly, the normal 45, respectively 46, 47, 48 to the diffraction gratings 35, respectively 36, 37, 38 is located in the XY plane, perpendicular to the alignment axis 50, but rotates about the Z axis.For example, normal 41 to the first diffraction grating is parallel to the Y axis, while normal 42 to the second diffraction grating 32 forms an angle, for example of about 2 degrees, with the Y axis, normal 43 to the third diffraction grating 33 forms an angle, for example of about 4 degrees, with the Y axis and normal 44 to the fourth diffraction grating 34 forms an angle, for example of about 6 degrees with respect to the Y axis. Similarly, normal 45 to the fifth diffraction grating 35 is parallel to the Y axis, while normal 46 to the sixth diffraction grating 36 forms an angle, for example of about 3 degrees, with the Y axis, normal 44 to the seventh diffraction grating 37 forms an angle, for example of about 5 degrees, with the Y axis and normal 48 to the eighth diffraction grating 38 forms an angle, for example of about 5 degrees, with the Y axis and normal 48 to the eighth diffraction grating 38 forms an angle, for example of about 6 degrees with respect to the Y axis. angle, for example about 7 degrees from the Y axis.The segmentation of the diffractive optical device 30 into eight diffraction gratings makes it possible to disperse the beams into eight spectral bands of limited spectral extent. 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. In addition, the arrangement in two columns makes it possible to reduce the length of each diffraction grating. The arrangement remains very compact, the gratings being arranged in two rotating columns like the risers of spiral staircases.

[0076] In [Fig. 8], a diffractive optical device 30 according to a third embodiment example is shown schematically in projection in an XZ plane and in projection in an XY plane in a three-dimensional orthonormal reference frame XYZ. In this example, in a non-limiting manner, the diffractive optical device 30 comprises three diffraction gratings 31, 32, 33. Each diffraction grating is a plane grating. 41, respectively 42, 43 denotes the normal to the plane of the diffraction grating 31, respectively 32, 33. The three diffraction gratings 31, 32, 33 are here gratings having different line densities from one another. For example, the first grating 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 rectilinear lines, parallel to each other and arranged periodically with a periodic pitch noted wl, respectively w2, w3. In other words, in the example of [Fig.8], the three diffraction gratings 31, 32, 33 have a different pitch from each other, taken two by two. The alignment axis of the lines of the grating 31, respectively 32, 33, is noted 50. In the example of [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 here arranged 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 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, and, for example, parallel to the Y axis. The segmentation of the diffractive optical device 30 into three diffraction gratings makes it possible to disperse the beams into three spectral bands of limited spectral extent. This arrangement simplifies the adjustments since all of the gratings are illuminated under the same incidence. The arrangement remains very compact, the gratings being arranged in a single column in the same plane.

[0077] In [Fig.9], we have shown schematically in projection in an XZ plane and in projection in an XY plane in a three-dimensional orthonormal reference frame XYZ, a diffractive optical device 30 according to a fourth exemplary embodiment. In this example, in a non-limiting manner, 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, respectively 42, 43, 44, 45, 46, 47, 48 the normal to the plane of the diffraction grating 31, respectively 32, 33, 34, 35, 36, 37, 38. The four diffraction gratings 31, 32, 33, 34, respectively 35, 36, 37, 38 of each column are here gratings are here gratings having different line densities from each other.For example, the first diffraction grating 31 has a line density of 1756 lines / mm, the second diffraction grating 32 has a line density of 1699 lines / mm, the third diffraction grating 33 has a line density of 1642 lines / mm and the fourth diffraction grating 34 has a line density of 1585 lines / mm, the fifth diffraction grating 35 has a line density of 1528 lines / mm, the sixth diffraction grating 36 has a line density of 1471 lines / mm, the seventh diffraction grating 37 has a line density of 1414 lines / mm and the eighth diffraction grating 38 has a line density of 1358 lines / mm. Each diffraction grating 31, respectively 32, 33, 34, 35, 36, 37, 38 has rectilinear lines, parallel to each other and arranged periodically with a periodic pitch noted 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 lines of the grating 31, respectively 32, 33, 34, 35, 36, 37, 38, is denoted 50. In the example of [Fig.9], 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. In addition, the eight diffraction gratings 31, 32, 33, 34, 35, 36, 37, 38 are in the same XZ plane and therefore have the same orientation in view. from above.The normal 41, respectively 42, 43, 44, 44, 45, 46, 47, 48 to the diffraction gratings 31, respectively 32, 33, 34, 35, 36, 37, 38 is located in the XY plane, perpendicular to the alignment axis 50, and, for example, parallel to the Y axis. The segmentation of the diffractive optical device 30 into eight diffraction gratings makes it possible to disperse the beams into eight spectral bands of limited spectral extent. This arrangement simplifies the adjustments since all the gratings are illuminated under the same incidence. In addition, the arrangement in two columns makes it possible to reduce the length of each diffraction grating. The arrangement remains very compact, the gratings being arranged in two parallel columns in the same plane.

[0078] In the examples illustrated in Figures 8 and 9, the line density is different between each diffraction grating 31, 32, 33, 34 of the same column. It is thanks to this difference in line density that different spectral bands, illuminated with the same incidence on diffraction gratings 31, 32, 33, 34 having the same orientation, can be diffracted in the same solid angle and collected by the optical focusing 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 exemplary embodiments in connection with 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 according to a fifth exemplary embodiment is shown schematically in projection in an XZ plane and in projection in an XY plane in a three-dimensional orthonormal reference frame XYZ. In this example, in a non-limiting manner, the diffractive optical device 30 comprises three diffraction gratings 31, 32, 33. Each diffraction grating is a plane grating. 41, respectively 42, 43 denotes the normal to the plane of the diffraction grating 31, respectively 32, 33. The three diffraction gratings 31, 32, 33 are here gratings described in connection with [Fig.6] or [Fig.8]. Each grating has rectilinear lines, parallel to each other and arranged periodically with a periodic pitch denoted w. In the example of [Fig.10], the three diffraction gratings 31, 32, 33 have the same pitch w. 51, respectively 52, 53 denotes the alignment axis of the lines of the grating 31, respectively 32, 33. In the example of [Fig. 10], the alignment axes 51, 52, 53 of the three diffraction gratings 31, 32, 33 are located in the same YZ plane, but rotate around the X axis. The three diffraction gratings 31, 32, 33 are here arranged in a single column, each grating extending along an axis parallel to the X axis and adjacent to each other.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, is located 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.The segmentation of the diffractive optical device 30 into three diffraction gratings makes it possible to disperse the beams 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, is located 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 +0.2 degrees, relative 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 -0.2 degrees, relative to the XY plane. The diffraction planes of the gratings 31, 32, 33 are oriented so as to form a non-zero angle between them taken two by two.This arrangement makes it possible to focus each of the beams diffracted by each diffraction grating on a distinct zone of the detector 6.

[0082] In [Fig. 11], a diffractive optical device 30 according to a sixth embodiment is shown schematically in projection in an XZ plane and in projection in an XY plane in a three-dimensional orthonormal reference frame XYZ. In this For example, in a non-limiting manner, the diffractive optical device 30 comprises three diffraction gratings 31, 32, 33. Each diffraction grating is a plane grating. 41, respectively 42, 43 denotes the normal to the plane of the diffraction grating 31, respectively 32, 33. The three diffraction gratings 31, 32, 33 are here gratings described in connection with [Fig. 6] or [Fig. 8]. Each grating has rectilinear lines, parallel to each other and arranged periodically with a periodic pitch denoted w. In the example of [Fig. 11], the three diffraction gratings 31, 32, 33 have the same pitch w. 51, respectively 52, 53 denotes the alignment axis of the lines of the grating 31, respectively 32, 33. In the example of [Fig. 11], the alignment axes 51, 52, 53 of the three diffraction gratings 31, 32, 33 are located in the same XZ plane, but rotate around the Y axis.The three diffraction gratings 31, 32, 33 are here arranged in a single column, each grating extending in the same XZ plane and the gratings being adjacent to each other. 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 of variable inclination relative 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 degree and +1 degree, 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. The segmentation of the diffractive optical device 30 into three diffraction gratings makes it possible to disperse the beams 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, is located 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, relative to the XY plane. The diffraction planes of the gratings 31, 32, 33 are oriented so as to form a non-zero angle between them taken two by two. This arrangement makes it possible to focus each of the beams diffracted by each diffraction grating on a distinct zone 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, allow 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 network is that its production can be carried out by very efficient replication methods.

[0085] It is also possible to combine the exemplary embodiments illustrated in Figures 6 and 8 or to combine the exemplary embodiments illustrated in Figures 7 and 9.

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

[0087] [Fig. 15] illustrates an exemplary implementation of an optical spectrometer 100 according to the first embodiment of the present disclosure. The optical spectrometer 100 is based on a Czemy-Turner arrangement. The optical spectrometer 100 comprises an input mask having a circular aperture with a diameter of 40 μm, followed by a polarization rectifier optical device 10 analogous to that described in connection with [Fig.l]. The optical spectrometer 100 comprises a collimating optical system 8 consisting of a first mirror having a focal length of 140 mm which collimates light from a light source. The optical spectrometer 100 comprises a dispersive optical device 20 as described in connection with [Fig.l]. The optical spectrometer 100 comprises a diffractive optical device 30 having here three diffraction gratings arranged according to the arrangement of [Fig.6].The optical spectrometer 100 comprises an optical focusing system 5 consisting here of a toroidal mirror, having focal lengths of 140 mm and 136.5 mm, for focusing the diffracted light in a plane called the image plane. The optical spectrometer 100 comprises a light matrix detector 6, placed in the image plane and transforming the incident light into electronic signals. The diffractive optical device 30 is segmented and is composed of three sub-networks, inclined relative to each other so that the light diffracted by each of the diffraction gratings 31, 32, 33 is focused in very distinct zones of the matrix detector. In addition, the diffraction gratings 31, 32, 33 are rotated relative to each other so that the light diffracted by each of the sub-networks can arrive at the light matrix detector.These three diffraction gratings 31, 32, 33 have a line density G of respectively: 1200 lines / mm, 1480 lines / mm, and 1740 lines / mm. The dispersive optical device 20 here comprises dichroic mirrors 21 and 22, the cut-off wavelengths of which are, for the TM polarization, respectively 701 nm and 765 nm, which transmit light whose wavelength is greater than the cut-off wavelength and reflect light whose wavelength is less than the cut-off wavelength. The dispersive optical device 20 also comprises deflecting 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 100 optical spectrometer is particularly useful for performing Raman measurements excited by a 640 nm wavelength laser, with excellent efficiency, and excellent spectral resolution in the spectral range from 640 nm to 830 nm.

Claims

Claims

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) optically arranged in series, the optical polarization dispersion and rectification device being arranged to receive a light beam (1) and configured to polarization 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 separated spatially and / or angularly; - 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 scattered 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, and; - an optical focusing system (5) arranged to receive the plurality of N pairs of diffracted light beams and form an image on a two-dimensional matrix detector (6) comprising at least 2N lines of photodetectors, each diffracted light beam of the plurality of N pairs of diffracted light beams being imaged on a zone (61, 62, 63, 64, 65, 66) distinct from the detector (6).

2. Optical spectrometer (100) according to claim 1, wherein: - the polarization rectifying optical device (10) is arranged and configured to receive the light beam (1) and separate into polar-

3.

4. rization of 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 polarization rectifying optical device (1) comprising a wave plate (2) arranged 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) of the same polarization state as the first light beam (11), and - the dispersive optical device (20) is arranged downstream of the polarization rectifying 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 spatially and / or angularly separate 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 rectifying optical device (10) is arranged downstream of the dispersive optical device (20) and configured to receive and polarization-separate 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 rectifying optical device (10) comprising at least one wave plate (2) arranged on the optical path of the N other light beams (115, 117, 119) and adapted to modify the polarization of the N other light beams (115, 117, 119) so as to form the N second light beams (121, 122, 123) of the same polarization state as the N first light beams (111, 112, 113). An optical spectrometer according to one of claims 1 to 3, wherein the plurality of N diffraction gratings comprises N diffraction gratings planes, each diffraction grating having parallel lines and each diffraction grating having a normal direction (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 column or several columns parallel to the alignment axis (50) of the lines of the gratings and in which the normal directions (41, 42, 43, 44, 45, 46) of the gratings of each column are oriented in a plane perpendicular to the alignment axis (50) so as to form a non-zero angle between them two by two.

5. An optical spectrometer according to 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 of each column has a line density different from that of the other gratings of the same column.

6. An optical spectrometer according to 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 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 lines parallel to an alignment axis (51, 52, 53) and wherein 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 one of claims 1 to 7, in which the dispersive optical device (20) comprises at least one di- chroic, a diffraction grating and / or a dichroic fiber optic coupler.

9. Optical spectrometer according to one of claims 1 to 8, comprising an inlet opening (7), wherein the inlet opening (7) is arranged upstream of the polarization rectifying optical device (10).

10. Optical spectrometer according to one of claims 1 to 8, comprising at least one input aperture (7, 71, 72), wherein said at least one input aperture (7, 71, 72) is arranged downstream or, respectively, inside the polarization rectifying optical device (10), said at least one input 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 one of claims 1 to 10 wherein the plurality of N diffraction gratings comprises at least one grating with very high efficiency for TM polarization or a lamellar diffraction grating.

Images