Abstract:
A spectrometer ( 100 ) for analyzing the spectrum of an upstream light beam ( 1 ), includes an entrance slit ( 101 ) and collimating elements ( 110 ) suitable for generating, from the upstream light beam, a collimated light beam ( 10 ), characterized in that it also includes: a polarization-dependent diffraction grating ( 120 ) suitable for diffracting, at each wavelength ( 11, 12 ) of the spectrum of the upstream light beam, the collimated light beam into a first diffracted light beam ( 11, 12 ) and a second diffracted light beam ( 21, 22 ); optical recombining elements ( 130 ) including a planar optical reflecting surface ( 130 ) perpendicular to the grating and suitable for deviating at least the second diffracted light beam; and focussing elements ( 140 ) suitable for focussing, at each wavelength, the first diffracted light beam and the second diffracted light beam onto one and the same focussing area ( 141 ).

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to the field of optical metrology. 
     More particularly, it relates to a spectrometer of high diffraction efficiency for analysing the spectrum of a light beam or a light source. 
     The present invention finds a particularly advantageous application when a high efficiency of the spectrometer over a broad spectrum band is desired. 
     Description of the Related Art 
     In optical metrology, spectroscopy is a technique that consists in analysing the spectrum of an upstream light beam, whether the latter comes directly from a light source or from an object illuminated by a light source, and in deducing therefrom certain properties of this source or this object. 
     A spectrometer is an optical instrument allowing to perform such an analysis for a spectrum comprising a plurality of wavelengths. 
     It is well known that a spectrometer generally includes:
         an entrance slit letting the upstream light beam through,   collimation means generating, from the upstream light beam, a collimated light beam,   angular dispersion means intercepting the collimated light beam and angularly dispersing the collimated light beam according to a plurality of wavelengths,   detection means adapted to measure light intensities according to the plurality of wavelengths.       

     In many applications, as for example Raman spectroscopy or near-infrared spectroscopy, the quantity of light available in the spectrometer, at the detection means, for the spectrum analysis is low. Indeed, a spectrometer carries all the photons of the upstream light beam from the entrance slit to the detection means, with many losses, due in particular to the angular dispersion means. 
     Moreover, even if the angular dispersion means may be optimised so as to reduce the losses in such a spectrometer, this can be done only on a narrow spectrum band. 
     Hence, a fast or accurate measurement with such a spectrometer may prove difficult, except using high-performance but expensive detection means. 
     From documents US2010/0225856A1 and US2010/0225876A1 are known, for example, angular dispersion means comprising so-called achromatic, polarization-separation diffraction gratings, which have a very high diffraction efficiency in the diffraction orders +1 and −1 over a broad spectrum band, in particular in the domain of ultraviolet, visible and infrared wavelengths. 
     On the other hand, such achromatic polarization-separation diffraction gratings able to be used in a spectrometer are known from the article C. Oh and M. J. Escuti, “Achromatic polarization gratings as highly efficient thin-film polarizing beamsplitters for broadband light”,  Proceedings of SPIE , vol. 6682, no. 668211 (2007). 
     However, by construction, a polarization-separation diffraction grating operates as a polarization separator. Therefore, the measurement performances of a spectrometer using such a component are not uniform as a function of the polarization state of the upstream light beam, the light intensities measured by the detection means varying with the fluctuations of the polarization state of the upstream light beam. 
     BRIEF SUMMARY OF THE INVENTION 
     One of the objects of the invention is to propose a spectrometer whose angular dispersion means have a very high efficiency over a broad spectrum band, allowing to carry with a minimum of losses all the photons of the upstream light beam from the entrance slit to the detection means. 
     Another object of the invention is to propose a spectrometer having a dispersion efficiency independent of the polarization state of the light beam to be analysed. 
     Another object of the invention is to propose a spectrometer offering a greater rapidity and a better accuracy of measurement for the analysis of the upstream light beam spectrum. 
     In order to remedy the above-mentioned drawback of the state of the art, the present invention proposes a spectrometer having improved performances on a broad spectrum band. 
     For that purpose, the invention relates to a spectrometer for analysing the spectrum of an upstream light beam including:
         an entrance slit adapted to let said upstream light beam through,   collimation means adapted to generate, from said upstream light beam, a collimated light beam, and   angular dispersion means arranged so as to intercept said collimated light beam and to angularly disperse said collimated light beam according to a plurality of wavelengths,       

     wherein:
         said angular dispersion means comprise at least one polarization-separation diffraction grating that has a normal to the grating, said polarization-separation diffraction grating being adapted to diffract said collimated light beam into:
           at least one first diffracted light beam according to a first diffraction order that is either the diffraction order +1, or the diffraction order −1 of said polarization-separation diffraction grating, said first diffracted light beam being angularly diffracted according to said plurality of wavelengths and having a first polarization state that is circular, and   a second diffracted light beam according to a second diffraction order that is either the diffraction order +1, or the diffraction order −1 of said polarization-separation diffraction grating, said second diffraction order being different from said first diffraction order, said second diffracted light beam being angularly diffracted according to said plurality of wavelengths and having a second polarization state that is circular and orthogonal to said first polarization state, and including:   
           optical recombination means arranged at least on an optical path of said second diffracted light beam, downstream of said polarization-separation diffraction grating, said optical recombination means comprising at least one planar optical-reflection surface parallel to said normal to the grating that is adapted to deviate at least said second diffracted light beam, and   focussing means adapted to focus, for each wavelength of said plurality of wavelengths, said first diffracted light beam and said second diffracted light beam to a same focussing surface.       

     The spectrometer according to the invention hence combines at least one polarization-separation diffraction grating, focussing means and optical recombination means to achieve better performances over a broad spectrum band. 
     Such a polarization-separation diffraction grating, described for example in the documents US2010/0225856A1 and US2010/0225876A1, has a very high diffraction efficiency in the diffraction orders +1 and −1 over a broad spectrum band, in particular in the domain of ultraviolet, visible and infrared wavelengths. 
     The spectrometer according to the invention hence allows to exploit the very high diffraction efficiency of the polarization-separation diffraction grating over a broad spectrum band thanks to the optical recombination means that allow, for example at the focussing surface, the recombination of the light beams diffracted by the polarization-separation diffraction grating. 
     The combined use of a polarization-separation diffraction grating whose sum of diffraction efficiencies in the diffraction order +1 and in the diffraction order −1 is close to 100% for each wavelength of the plurality of wavelengths of the spectrum of the upstream light beam and of optical means for recombining the first diffracted light beam and the second diffracted light beam allows to collect on the focussing surface nearly 100% of the light intensity of the upstream light beam at each wavelength. 
     Thanks to the focussing means, the light beams, diffracted by the polarization-separation diffraction grating and recombined by the optical recombination means at one wavelength, are focussed on the focussing surface to one focussing point that is spatially separated from the focussing point to which are focussed the light beams diffracted by the polarization-separation diffraction grating and recombined by the optical recombination means at another wavelength. 
     Hence, the spectrometer according to the invention carries, with almost no loss, the upstream light beam up to the focussing surface. 
     The spectrometer according to the invention is particularly advantageous insofar as a reduced number of polarization-separation diffraction gratings is necessary to cover a relatively broad wavelength range. This allows in particular to reduce the cost of such a spectrometer. 
     In the particular case where a single polarization-separation diffraction grating is necessary, this allows not to introduce error in the measurement, wherein the angular dispersion means can be fixed in the spectrometer over the whole measurement range. 
     Moreover, other advantageous and non-limitative characteristics of the device according to the invention are the following:
         said polarization-separation diffraction grating is planar and has rectilinear and parallel lines;   said planar optical-reflection surface is parallel to said lines of the polarization-separation diffraction grating, so that said first diffracted light beam and said second diffracted light beam are focussed, for each wavelength of said plurality of wavelengths, by said focussing means to a same focussing point of said focussing surface, said focussing points being separated on said focussing surface according to said plurality of wavelengths;   said planar optical-reflection surface forms with said lines of the polarization-separation diffraction grating an angle comprised between 0° and 90°, so that said first diffracted light beam and said second diffracted light beam are focussed, for each wavelength of said plurality of wavelengths, by said focussing means to two distinct focussing points of said focussing surface, said two distinct focussing points being separated on said focussing surface;   said spectrometer includes an exit slit, fixed or mobile, arranged on said focussing surface and adapted to let through said diffracted light beams recombined by said focussing means on said focussing surface;   said spectrometer includes detection means arranged on said focussing surface and adapted to deliver, for each wavelength of said plurality of wavelengths, a signal relating to the sum of the light intensity diffracted at said wavelength in the diffraction order +1 and of the light intensity diffracted at said wavelength in the diffraction order −1;   said optical recombination means include a planar mirror arranged so as to reflect, for each wavelength of said plurality of wavelengths, said second diffracted light beam in a direction parallel to said first diffracted light beam;   said optical recombination means include a prism comprising a base, an entrance face and an exit face inclined with respect to the entrance face, said entrance face and said exit face refracting, for each wavelength of said plurality of wavelengths, said first diffracted light beam and said second diffracted light beam, said second diffracted light beam being deviated, for each wavelength of said plurality of wavelengths, by reflection on said base of the prism between said entrance face and said exit face;   said optical recombination means include:
           a quarter-wave retardation plate arranged at the exit of the polarization-separation diffraction grating, said quarter-wave retardation plate being adapted to modify said first polarization state and said second polarization state to transform their orthogonal circular polarization states into linear polarization states that are orthogonal to each other,   a first mirror and a second mirror arranged parallel to each other so as to face each other and perpendicular to said polarization-separation diffraction grating, so that said first mirror, respectively said second mirror, reflects, for each wavelength of said plurality of wavelengths, said first diffracted light beam, respectively said second diffracted light beam, and   a polarization-recombining cube having a recombination interface and placed between said first mirror and said second mirror, so that said first diffracted light beam reflected by said first mirror is incident on a first entrance face of said polarization-recombining cube and that said second diffracted light beam reflected by said second mirror is incident on a second entrance face of said polarization-recombining cube, one of said reflected diffracted light beams being reflected by said recombination interface and the other reflected diffracted light beam being transmitted by said recombination interface, said first diffracted light beam and said second diffracted light beam being parallel at the exit of said polarization-recombining cube;   
           said collimation means comprise an optical collimation system having a collimation numerical aperture and said focussing means comprise an optical focussing system having a focussing numerical aperture that is at least equal to the double of said collimation numerical aperture;   said detection means include a multi-channel detector;   said detection means include a mobile slit and a single-channel detector;   said spectrometer includes a fluorescence cell and measurement means, said fluorescence cell being arranged downstream of said exit slit so as to be illuminated by said recombined diffracted light beams and to emit a fluorescence signal, said measurement means being adapted to measure the light intensity of said fluorescence signal.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Several embodiments of the invention are described in detail with reference to the drawings, in which: 
         FIG. 1  is a schematic view of a spectrometer according to a first embodiment where the polarization-separation diffraction grating operates in transmission and where the optical recombination means comprise a planar mirror; 
         FIG. 1A  is a schematic view according to the plane A-A of  FIG. 1 , showing the planar mirror and the lines of the polarization-separation diffraction grating of  FIG. 1 ; 
         FIG. 2  is a schematic view of a spectrometer according to a first variant of the first embodiment where the polarization-separation diffraction grating operates in reflection and where the optical recombination means comprise a planar mirror; 
         FIG. 3  is a schematic view of a spectrometer according to a second variant of the first embodiment where the polarization-separation diffraction grating operates in transmission and where the optical recombination means comprise a prism; 
         FIG. 4  is a schematic view of a spectrometer according to a third variant of the first embodiment where the polarization-separation diffraction grating operates in transmission and where the optical recombination means comprise a quarter-wave plate, two mirrors and a polarization-recombining cube; 
         FIG. 5  is a schematic view of a spectrometer according to a second embodiment used as a monochromator and comprising a fluorescence cell; 
         FIG. 6  is a schematic view of a spectrometer according to a third embodiment used as a spectropolarimeter. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1  is shown a first embodiment of a spectrometer  100  according to the invention, intended to analyse the spectrum of an upstream light beam  1 . 
       FIGS. 2 to 4  relates to several variants of the first embodiment of a spectrometer  100 . 
     For the sake of simplification and for illustrating the examples of the invention, in the following of the description, two particular wavelengths of the spectrum of the upstream light beam  1  for which the light intensity is non-zero will be considered. 
     These two particular wavelengths are denoted λ 1  and λ 2 . 
     This consideration is not in any way limitative and does not presume of the precise nature of the spectrum of the upstream light beam  1 , which may for example be a continuous spectrum, a discrete spectrum, a band spectrum, a line spectrum, or a mixture of all these types of spectrum, or of the spectral extent thereof. 
     With no limitation either, it will be considered that the upstream light beam  1  is not polarized. Indeed, with no knowledge of the polarization state of the upstream light beam  1 , this case is the less restrictive. Moreover, the interest of the invention towards this ignorance a priori of the polarization state of the upstream light beam  1  will be understood from the examples. 
     Generally, the spectrometer  100  comprises, in the first embodiment and in the variants thereof, an entrance slit  101 , collimation means  110 , angular dispersion means  120  and detection means  150 . The different above-mentioned elements of the spectrometer  100  are included in a casing (not shown) that is opaque to the external light, the entrance slit  101  being located on one of the walls of this casing. 
     The entrance slit  101  is herein a planar slit of rectangular shape, centred on an optical axis A 1  that is perpendicular to the plane of the entrance slit  101 . 
     It will be considered in the different embodiments that the upstream light beam  1  is approximately a divergent light beam formed of a cone of light rays, comprising light rays at the wavelengths λ 1  and λ 2 , the cone being a cone of revolution about the optical axis A 1  and having for apex the centre of the entrance slit  101 . 
     Hence arranged, the entrance slit  101  lets the upstream light beam  1  through. 
     The spectrometer  100  also includes collimation means  110  that herein comprise an optical collimation system having a collimation numerical aperture. 
     The optical collimation system  110  has herein an optical axis that is merged with the optical axis A 1  and whose object focal point is located at the centre of the entrance slit  101 . 
     Advantageously, the optical collimation system  110  is corrected from the chromatic aberration at least over the spectral extent of the upstream light beam  1 . 
     Hence positioned along the optical axis A 1 , the collimation means  110  generate, from the upstream light beam  1 , a collimated light beam  10 . 
     The collimated light beam  10  is hence formed of light rays at the wavelengths λ 1  and λ 2 , parallel to each other and to the optical axis A 1 . 
     The spectrometer  100  further includes angular dispersion means  120  placed downstream of the collimation means  110  along the optical axis A 1 . 
     The angular dispersion means  120  intercept the collimated light beam  10  so that all the light rays at the wavelengths λ 1  and λ 2  of the collimated light beam  10  are incident on the angular dispersion means  120  according to the same angle of incidence. 
     The angular dispersion means  120  angularly disperse the collimated light beam  10  according to the wavelength. 
     It is understood thereby that the angular dispersion means  120  generate, from the collimated light beam  10  at the wavelengths λ 1  and λ 2 :
         at least one light beam diffracted at the wavelength λ 1 , and   at least one light beam diffracted at the wavelength λ 2 , that is angularly separated from the light beam diffracted at the wavelength λ 1 .       

     In the first embodiment and in the variants thereof, the spectrometer  100  finally includes detection means  150  placed on the optical path of the diffracted light beams. 
     To analyse the spectrum of the upstream light beam  1 , these detection means  150  measure the light intensity of the light beam diffracted at the wavelength λ 1  and the light intensity of the light beam diffracted at the wavelength λ 2 . 
     According to the invention, the angular dispersion means  120  comprise at least one polarization-separation diffraction grating. 
     Generally, a diffraction grating diffracts an incident light ray into one or several diffracted rays propagating in different directions. 
     With reference to the diffraction grating law, it is hence talked about diffraction orders such as the order 0 and the higher orders: orders ±1, orders ±2, etc. . . . . 
     It is well known from the physical optics laws that, when they both exist, the rays diffracted in the diffraction orders +1 and −1 propagate in two symmetrical directions with respect to the direction of propagation of the diffracted ray in the diffraction order 0. 
     It is also known that a planar diffraction grating having rectilinear and parallel lines regularly spaced apart is such that a beam of incident light rays parallel to each other is diffracted into one or several diffracted light beams of parallel light rays. 
     A polarization-separation diffraction grating is generally a holographic component formed of at least one liquid-crystal diffractive wave plate. 
     A polarization-separation diffraction grating has the particularity to diffract an incident light beam into at least one ray diffracted in the diffraction order +1 and a ray diffracted in the diffraction order −1, the two diffracted rays being circularly and orthogonally polarized. For example, if the ray diffracted in the diffraction order +1 is circularly polarized to the left, then the ray diffracted in the diffraction order −1 is circularly polarized to the right, and vice versa. 
     This particularity exists whether the incident light ray is not-polarized or polarized in any way. Indeed, the polarization state of the incident light ray governs only the distribution of the light energy in the diffraction order +1 and in the diffraction order −1. 
     Preferably, a polarization-separation diffraction grating has moreover, over the wavelength range for which it has been designed, for example over the range 400 nm-800 nm or the range 800 nm-2000 nm, diffraction efficiencies in the diffraction order +1 and in the diffraction order −1 such that their sum is very close to 100%, typically higher than or equal to 90%, and preferentially higher than or equal to 95%. 
     Indeed, not only a portion of the light incident on a polarization-separation diffraction grating is not diffracted—it is either back-reflected, or absorbed, or scattered—, but also a portion of the light incident on a polarization-separation diffraction grating may be diffracted in the diffraction order 0. 
     With no limitation, it will be considered hereinafter that the polarization-separation diffraction grating  120  of the spectrometer  100  according to the invention is herein a planar diffraction grating that has a normal N to the grating as shown in  FIGS. 1 to 4 . 
     Advantageously, the normal N is herein parallel to the optical axis A 1  so that the polarization-separation diffraction grating  120  is in the four embodiments shown in  FIGS. 1 to 4  placed perpendicular to the optical axis A 1  in such a manner that the collimated light beam  10  is in normal incidence on an entrance face  121  of the polarization-separation diffraction grating  120 . 
     By analogy with a conventional diffraction grating, such as that described hereinabove, it will also be considered herein that the polarization-separation diffraction grating  120  has rectilinear and parallel lines  122 , regularly spaced apart (see  FIG. 1A ). These lines  122  are not protrusions but correspond to lines of equal orientation of the liquid crystals constituting the diffractive wave plate of the polarization-separation diffraction grating  120 . 
     So designed, a light ray of the collimated light beam  10  incident on the polarization-separation diffraction grating  120  is diffracted into diffracted beams that are all coplanar and contained in a diffraction plane of the polarization-separation diffraction grating  120 . 
     This diffraction plane is the plane that contains the incident light ray and that is perpendicular to the lines  122  of the polarization-separation diffraction grating  120 . The diffraction plane is hence such that the normal N to the grating is parallel to this diffraction plane. 
     The optical behaviour of a polarization-separation diffraction grating as just described hereinabove remains valid for any wavelength for which the polarization-separation diffraction grating has been designed. 
     Are dependent on the wavelength: the directions of propagation of the diffracted rays and the diffraction efficiencies of the different diffraction orders. 
     Are not dependent on the wavelength: the “left” or “right” character of the polarization of the rays diffracted in the diffraction orders +1 and −1, i.e. two rays at different wavelengths diffracted in the diffraction order +1 are polarized circularly in the same direction (for example the left), the two rays diffracted in the diffraction order −1 at these two wavelengths being also polarized circularly in the same direction (herein the right) and with an orthogonal polarization. 
     It will hence be understood that the polarization-separation diffraction grating  120  of the spectrometer  100  according to the invention diffracts the collimated light beam  10  into:
         at least one first diffracted light beam  11 ,  12  according to a first diffraction order that is either the diffraction order +1 or the diffraction order −1 of the polarization-separation diffraction grating  120 , the first diffracted light beam  11 ,  12  being angularly diffracted as a function of the plurality of wavelengths and having a first polarization state that is circular, and   a second diffracted light beam  21 ,  22  according to a second diffraction order that is either the diffraction order +1 or the diffraction order −1 of said polarization-separation diffraction grating  120 , said second diffraction order being different from said first diffraction order, the second diffracted light beam  21 ,  22  being angularly diffracted as a function of the plurality of wavelengths and having a second polarization state that is circular and orthogonal to the first polarization state.       

     With no limitation, it will be considered in the four embodiments of the invention that the first diffracted light beam  11 ,  12  is diffracted in the diffraction order +1 and that the second diffracted light beam  21 ,  22  is diffracted in the diffraction order −1. 
     Hence, at the wavelength λ 1 , Ie collimated light beam  10  is diffracted into a first diffracted light beam  11  at the wavelength λ 1  in the diffraction order +1 and a second diffracted light beam  21  at the wavelength λ 1  in the diffraction order −1. 
     Likewise, at the wavelength λ 2 , the collimated light beam  10  is diffracted into a first diffracted light beam  12  at the wavelength λ 2  in the diffraction order +1 and a second diffracted light beam  22  at the wavelength λ 2  in the diffraction order −1. 
     With no limitation either (see supra), it will also be considered that the first diffracted light beam  11  at the wavelength λ 1  and the first diffracted light beam  12  at the wavelength λ 2  have a first left circular polarization state, and that the second diffracted light beam  21  at the wavelength λ 1  and the second diffracted light beam  22  at the wavelength λ 2  have a second right circular polarization state, which is hence orthogonal to the first polarization state. 
     In the optical configurations shown in  FIGS. 1 to 4 , the diffraction order 0 is parallel to the optical axis A 1 . Hence, as the collimated light beam  10  is in normal incidence on the entrance face  121  of the polarization-separation diffraction grating  120  (see supra), at the exit of the polarization-separation diffraction grating  120 , the first diffracted light beam  11  at the wavelength λ 1  in the diffraction order +1 and the second diffracted light beam  21  at the wavelength λ 1  in the diffraction order −1 are symmetrical with respect to the optical axis A 1 . 
     Likewise, at the exit of the polarization-separation diffraction grating  120 , the first diffracted light beam  12  at the wavelength λ 2  in the diffraction order +1 and the second diffracted light beam  22  at the wavelength λ 2  in the diffraction order −1 are symmetrical with respect to the optical axis A 1 . 
     Still according to the invention, the spectrometer  100  also includes focussing means  140 . 
     In the four variants, the focussing means  140  comprise an optical focussing system having a focussing numerical aperture and an optical focussing axis A 3 . 
     Preferably, the optical focussing system  140  is herein arranged and oriented in the spectrometer  100  so that its optical focussing axis A 3  is coplanar and secant with the optical axis A 1 . 
     The focussing means  140  focus the first diffracted light beam  11 ,  12  and the second diffracted light beam  21 ,  22  on a focussing surface that is for example formed of a focussing plane  141 . 
     In the different variants of the first embodiment of the spectrometer  100  shown in  FIGS. 1 to 4 , the detection means  150  are arranged on the focussing surface  141 . 
     More precisely here, the detection means  150  are planar detection means whose detection plane is the focussing plane  141  of the optical focussing system  140 . 
     The way the detection means  150  operate in the spectrometer  100  will be detailed hereinafter. 
     The spectrometer  100  moreover includes optical recombination means  130 ;  330 ;  430 . 
     The optical recombination means  130 ;  330 ;  430  are arranged at least on an optical path of the second diffracted light beam  21 ,  22 , downstream of the polarization-separation diffraction grating  120 . 
     In the first embodiment and in the variants thereof shown in  FIGS. 1 to 4 , the optical recombination means  130 ;  330 ;  430  are arranged between the polarization-separation diffraction grating  120  and the focussing means  140 . 
     The optical recombination means  130 ;  330 ;  430  comprise a planar optical-reflection surface  130 ;  331 ;  432  that is parallel to the normal N to the grating. 
     Moreover, in the first embodiment and in the variants thereof shown in  FIGS. 1 to 4 , the planar optical-reflection surface  130 ;  331 ;  432  is also parallel to the lines  122  of the polarization-separation diffraction grating  120 , as shown in  FIG. 1A . 
     Hence oriented, the optical recombination means  130 ;  330 ;  430  then deviate at least the second diffracted light beam  21 ,  22  so that the first diffracted light beam  11 ,  12  and the second diffracted light beam  21 ,  22  are focussed by the focusing means  140  on the focusing surface  141 . 
     More precisely, the first diffracted light beam  11 , 12  and the second diffracted light beam  21 ,  22  are focussed by the focusing means  140  to a same focusing point ( 31 ,  32 ) of the focusing surface  141 , and hence of the detection means  150  for a wavelength. 
     Still more precisely, the optical focussing system  140  focusses:
         the first diffracted light beam  11  at the wavelength λ 1  and the second diffracted light beam  21  at the wavelength λ 1  to a same first focussing point  31  on the focussing surface  141 , and hence on the detection means  150 , and   the first diffracted light beam  12  at the wavelength λ 2  and the second diffracted light beam  22  at the wavelength λ 2  to a same second focussing point  32  on the focussing surface  141 , and hence on the detection means  150 , this second focussing point  32  being spatially separated from the first focussing point  31 .       

     This spatial separation as a function of the wavelength of the focussing points on the focussing surface  141 , and hence on the detection means  150 , allows to measure separately the light intensity at the wavelength λ 1  and the light intensity at the wavelength λ 2 . 
     The detection means  150  hence deliver, for each of the two wavelengths λ 1  and λ 2 , a signal relating to the sum of the light intensity diffracted in the diffraction order +1 and of the light intensity diffracted at said wavelength in the diffraction order −1. 
     As indicated above, the sum of the diffraction efficiencies in the diffraction order +1 and in the diffraction order −1 of a polarization-separation diffraction grating  120  is very high. 
     Thanks to the combined use of the polarization-separation diffraction grating  120 , optical recombination means  130 ;  330 ;  430  and focussing means  140 , it is possible to exploit this particular property of the polarization-separation diffraction gratings so that the spectrometer  100  according to the invention has a high transmission between the entrance slit  101  and the detection means  150  so as to improve the rapidity and the accuracy of analysis of the spectrum of the upstream light beam  1 . 
     The different variants of the invention described hereinabove, and in particular the optical recombination means  130 ;  330 ;  430 , will be described in more detail hereinafter. 
     Moreover, in order to simplify the description, the different variants will be described hereinafter only according to a wavelength λ 1 , it being understood that the operation of the spectrometer  100 , and in particular of its optical recombination means  130 ;  330 ;  430 , is identical for each wavelength belonging to the spectrum of the upstream light beam  1  for which the spectrometer  100  has been designed. 
     In  FIG. 1  is shown a spectrometer  100  according to a first embodiment, in which the polarization-separation diffraction grating  120  operates in transmission and in which the optical recombination means  130  include a planar mirror, the planar optical-reflection surface of the optical recombination means  130  being formed by the reflective surface of the planar mirror. 
     This mirror  130  is arranged downstream of the polarization-separation diffraction grating  120 , between the latter and the optical focussing system  140 . 
     The mirror  130  is placed in the low portion of the polarization-separation diffraction grating  120 , towards which the second diffracted light beam  21  at the wavelength λ 1  propagates. 
     The mirror  130  is oriented so that it is, on the one hand, parallel to the normal N to the polarization-separation diffraction grating  120  and, on the other hand, parallel to the lines  122  of the polarization-separation diffraction grating  120  (see in particular  FIG. 1A ). 
     Advantageously, the mirror  130  comprises a lateral ridge  131  that is placed side by side with the exit face  122  of the polarization-separation diffraction grating  120 . 
     Hence positioned and oriented in the spectrometer  100 , the mirror  130  reflects the second diffracted light beam  21  at the wavelength λ 1 . 
     The size of the mirror  130  according to the optical axis A 1  is chosen great enough so that the mirror  130  reflects, at each wavelength, the second diffracted light beam  21 ,  22 . 
     By reflection, the mirror  130  deviates the second diffracted light beam  21  at the wavelength λ 1  so that the second diffracted light beam  21  is parallel to the first diffracted light beam  11  at the wavelength λ 1 . 
     The first diffracted light beam  11  and the second diffracted light beam  21  being parallel, they are focussed by the optical focussing system  140  to the first focussing point  31  of the detection means  150 . 
     In this optical configuration, the optical focussing system  140  must hence have a focussing numerical aperture that is the double of the collimation numerical aperture of the optical collimation system  110 . 
     The behaviour of the above-described spectrometer  100  is also valid for the wavelength λ 2  and the first diffracted light beam  12  at the wavelength λ 2  and the second diffracted light beam  22  at the wavelength λ 2  are focussed by the optical focussing system  140  to the second focussing point  32  of the detection means  150 . 
     The first diffracted light beam  12  at the wavelength λ 2  being not diffracted in the same direction as the first diffracted light beam  11  at the wavelength λ 1 , the second focussing point  32  is spatially separated from the first focussing point  31 . Moreover, this is true for each wavelength of the spectrum of the upstream light beam  1 . 
     In addition, the polarization-separation diffraction grating  120 , the mirror  130  and the optical focussing system  140  are herein arranged in the spectrometer  100  such that the different focussing points on the detection means  150  are aligned with each other. 
     The detection means  150  include a multi-channel detector herein formed of a linear array of CCD sensors placed such that the focussing points are aligned on the line of CCD sensors. 
     The first focussing point  31  and the second focussing point  32  are centred on different pixels of the linear array of CCD sensors so that the multi-channel detector  150  delivers:
         a signal relating to the sum of the light intensity diffracted at the wavelength λ 1  in the diffraction order +1 and the light intensity diffracted at the wavelength λ 1  in the diffraction order −1, this sum being close to 100% of the light intensity of the upstream light beam  1  at the wavelength λ 1 , and   a signal relating to the sum of the light intensity diffracted at the wavelength λ 2  in the diffraction order +1 and of the light intensity diffracted at the wavelength λ 2  in the diffraction order −1, this sum being also close to 100% of the light intensity of the upstream light beam  1  at the wavelength λ 2 .       

     This is true for all the wavelengths belonging to the spectrum of the upstream light beam  1 , the detection means  150  then measure, according to the wavelength, the light intensities of the upstream light beam  1  to deduce its spectrum therefrom. 
     It is besides known that the spectral resolution (expressed in nanometers) of the multi-channel detector is in particular function of the size of the CCD sensors and of the spacing thereof. 
     Generally, the spectral resolution of a spectrum is function of the power of dispersion of the angular dispersion means, of the possible optical aberrations of the collimation means and of the focussing means, as well as the spatial resolution of the detection means. 
     As a variant, the multi-channel detector could for example be formed of a matrix of CCD sensors. 
     As another variant, the detection means could have a mobile slit and a single-channel detector. The mobile slit has a shape and size that are those of the image of the entrance slit by the optical collimation system, the polarization-separation diffraction grating and the optical focussing system. The single-channel detector is a single detector, for example a silicon, germanium, InGaAs, InAs, InSb, PbS, PbSe or HgCdTe photodiode, an avalanche photodiode, a photo-multiplier tube. 
     The second variant of the spectrometer  100 , shown in  FIG. 2 , has an architecture similar to the first variant of the spectrometer  100  of  FIG. 1 , except that the polarization-separation diffraction grating  120  of the spectrometer  100  operates in reflection rather than in transmission. 
     Similarly to the first variant, the mirror  130  reflects the second light beam  21 ,  22  diffracted by the polarization-separation diffraction grating  120  at the different wavelengths λ 1 , λ 2  of the spectrum. 
     In these conditions, it is then provided that the distance between the optical collimation system  110  and the polarization-separation diffraction grating  120  is sufficient so that the mirror  130  can be arranged between the polarization-separation diffraction grating  120  and the optical collimation system  110 . 
     Moreover, it is also provided that the distance between the optical collimation system  110  and the polarization-separation diffraction grating  120  is great enough so that the diffracted light beams  11 ,  12 ,  21 ,  22  are not intercepted by the optical collimation system  110 . 
     In  FIG. 3  is shown a second variant of the first embodiment of the spectrometer  100  in which the polarization-separation diffraction grating  120  operates in transmission and in which the optical recombination means  330  include a prism. 
     Advantageously, this prism  330  is a mineral-glass optical prism whose faces are polished. It includes a base  331 , an entrance face  332  and an exit face  333 . 
     The entrance face  332  is herein a planar face that is parallel to the polarization-separation diffraction grating  120  (the entrance face  332  being hence perpendicular to the optical axis A 1 ) and that is centred on the optical axis A 1 . 
     The exit face  333  of the prism  330  is also a planar face. It is inclined with respect to the entrance face  332 . 
     The travel of the first diffracted light beam  11  at the wavelength λ 1  then the travel of the second diffracted light beam  21  at the wavelength λ 1  will be described hereinafter. 
     As shown in  FIG. 3 , the first diffracted light beam  11  is incident on the entrance face  332  of the prism  330  and is refracted by the latter. 
     The first diffracted light beam  11  then propagates in parallel in the prism  330 , then is incident on the exit face  333  of the prism  330  that in turn refracts it. 
     The first diffracted light beam  11  then propagates, still in parallel, towards the optical focussing system  140 , simply deviated by the prism  330  with respect to its initial direction at the exit of the polarization-separation diffraction grating  120 . 
     Still according to  FIG. 3 , the second diffracted light beam  21  is incident on the entrance face  332  of the prism  330  and is refracted by the latter. 
     The first diffracted light beam  11  then propagates in parallel in the prism  330 . 
     As the first diffracted light beam  11  and the second diffracted light beam  21  are symmetrical with respect to the optical axis A 1  at the exit of the polarization-separation diffraction grating  120 , before being incident on the entrance face  332  of the prism  330  to be refracted thereon, and as the entrance face  332  of the prism  330  is perpendicular to the optical axis A 1 , the first diffracted light beam  11  and the second diffracted light beam  21  are also symmetrical after refraction on the entrance face  332 . 
     The base  331  of the prism  330  being oriented in the plane of incidence in the same way as the mirror  130  of  FIG. 1  and being that way perpendicular to the entrance face  332 , the parallel light rays to the second diffracted light beam  21  are all incident on the base  331  of the prism  330  with an angle of incidence higher than the angle of total reflection of this prism  330 . 
     Hence, the second diffracted light beam  21  is totally reflected on the base  331  of the prism  330  by total internal reflection, between the entrance face  332  and the exit face  333 . The base  331  of the prism  330  thus constitutes, for the variant of  FIG. 3 , the planar optical-reflection surface of the optical recombination means  330 . 
     The second diffracted light beam  21  is hence thereafter incident with the same angle of incidence as the first diffracted light beam  11  on the exit face  333  of the prism  330  that in turn refracts it. 
     The second diffracted light beam  21  then propagates towards the optical focussing system  140 , in parallel to the first diffracted light beam  11 . 
     Advantageously, the entrance face  332  and the exit face  333  are coated with an anti-reflective treatment allowing, on the one hand, to reduce the losses by reflection on the entrance face  332  and on the exit face  333 , and on the other hand, to limit the formation of spurious light beams that could reduce the accuracy of the light intensity measurements performed by the detection means  150 . 
     In  FIG. 4  is shown a third variant of the first embodiment of the spectrometer  100  in which the polarization-separation diffraction grating  120  also operates in transmission and in which the optical recombination means  430  include a retardation plate  433 , a first mirror  431 , a second mirror  432 , and a polarization-recombination cube  4340 . 
     For the sake of clarity, only the light rays of the wavelength λ 1  have been shown in  FIG. 4 . 
     The retardation plate  433  is arranged at the exit of the polarization-separation diffraction grating  120 , in parallel to the latter. The retardation plate  433  is preferably located at a very close distance from the polarization-separation diffraction grating  120 , for example 1 millimeter, so that the retardation plate  433  intercepts the first diffracted light beam  11  and the second diffracted light beam  21 . 
     As a variant, the retardation plate may be placed side by side with the exit face of the polarization-separation diffraction grating so that they form only a single and same optical component. 
     The retardation plate  433  is an achromatic quarter-wave plate optimized to operate at least in the wavelength range of the polarization-separation diffraction grating  120 . 
     Advantageously, the retardation plate  433  has a great angular acceptance. 
     This retardation plate  433  modifies the respective polarization states of the first diffracted light beam  11  and of the second diffracted light beam  21  that are circular and orthogonal polarization states at the entrance of the retardation plate  433 , so that:
         the first polarization state is transformed into a linear polarization state, and   the second polarization state is transformed into a linear polarization state orthogonal to the first polarization state, the retardation plate  433  keeping the property of orthogonality.       

     The retardation plate  433  is oriented so that the first polarization state at the exit of this retardation plate  433  corresponds to the polarization state transmitted by the polarization-recombining cube  4340  (see hereinafter) and so that the second polarization state at the exit of the retardation plate  433  corresponds to the polarization state reflected by this polarization-recombining cube  4340  (see hereinafter). 
     The retardation plate  433  herein does not deviate the first diffracted light beam  11 , nor the second diffracted light beam  21 . 
     The second mirror  432  of the optical recombination means  430  is a planar mirror herein arranged perpendicular to the plane of incidence, in the same way with respect to the polarization-separation diffraction grating  120  and to the optical axis A 1  as the planar mirror  130  of the first embodiment. 
     This second mirror  432  constitutes, in this third variant, the planar optical-reflection surface of the optical recombination means  430 . 
     The first mirror  431  is a planar mirror arranged parallel to the second mirror  432  in order to face it. The first mirror  431  is hence also perpendicular to the polarization-separation diffraction grating  120 . 
     So arranged, it is understood that:
         the first mirror  431  reflects, at the wavelength λ 1 , the first light beam  11  diffracted at the wavelength λ 1 , and that   the second mirror  432  reflects, at the same wavelength λ 1 , the second light beam  21  diffracted at the wavelength λ 1 .       

     After reflection on the first mirror  431  and on the second mirror  432 , the first diffracted light beam  11  and the second diffracted light beam  21  remain symmetrical with respect to the optical axis A 1 . 
     Moreover, the reflections on the first mirror  431  and on the second mirror  432  do not modify the first polarization state nor the second polarization state that remain linear polarization states that are orthogonal to each other. 
     The polarization-recombining cube  4340  is placed between the first mirror  431  and the second mirror  432  so that:
         the first diffracted light beam  11  reflected by the first mirror  431  is incident according to a first angle of incidence on a first entrance face  4341 A of the polarization-recombining cube  4340 , the first diffracted light beam  11  being transmitted by this first entrance face  4341 A in the first prism  4341 , and that:   the second diffracted light beam  21  reflected by the second mirror  432  is incident according to a second angle of incidence equal to the first angle of incidence on a second entrance face  4342 A of the polarization-recombining cube  4340 , the second diffracted light beam  21  being transmitted by this second entrance face  4342 A in a second prism  4342 .       

     Advantageously, and for the same reasons as above, the first entrance face  4341 A and the second entrance face  4342 A of the polarization-recombining cube  4340  are coated with an anti-reflective treatment. 
     The polarization-recombining cube  4340  is hence formed of the first prism  4341  and of the second, identical, prism  4342 , which are both rectangular isosceles straight prisms. 
     The first prism  4341  comprises the entrance face  4341 A of the polarization-recombining cube  4340  and a hypotenuse face  4341 B. 
     Likewise, the second prism  4342  comprises the entrance face  4342 A of the polarization-recombining cube  4340 , a hypotenuse face  4342 B, and a last face  4340 A forming an exit face of the polarization-recombining cube  4340 . 
     The first prism  4341  and the second prism  4342  are placed side by side through their hypotenuse faces  4341 B,  4342 B by means of an optical glue to form a recombination interface  4343  of the polarization-recombining cube  4340 . 
     The first prism  4341  and the second prism  4342 , which are herein identical, are made of mineral glass, for example glass of the BK7 type, and their hypotenuse faces  4341 B,  4341 B are coated with a filter that has for function, for the wavelengths λ 1  and λ 2 , to:
         transmit the light rays that are incident on the hypotenuse faces  4341 B,  4342 B and that are linearly polarized, and to:   reflect the light rays that are incident on the hypotenuse faces  4341 B,  4342 B, and that are polarized linearly and orthogonally to the light rays transmitted by these hypotenuse faces  4341 B,  4342 B.       

     In particular, the polarization-recombining cube  4340  is such that:
         the first diffracted light beam  11 , that propagates in the first prism  4341  according to a first linear polarization state and that is incident on the hypotenuse face  4341 B, at the recombination interface  4343 , is transmitted by the recombination interface  4343 ;   the second diffracted light beam  21 , that propagates in the second prism  4342  according to a second linear polarization state, orthogonal to the first polarization state, and that is incident to the hypotenuse face  4341 B, at the level of the recombination interface  4343 , is reflected by the recombination interface  4343 .       

     Hence, as the first diffracted light beam  11  and the second diffracted light beam  21  are incident with the same angle of incidence on the entrance faces  4341 A,  4342 A of the polarization-recombining cube  4340 , after transmission of the first diffracted light beam  11  and reflection of the second diffracted light beam  21  through the recombination interface  4343 , the first diffracted light beam  11  and the second diffracted light beam  21  are herein superimposed to each other and propagate in parallel in the second prism  4342  to be incident, in normal incidence, on the exit face  4340 A of the polarization-recombining cube  4340 . 
     Transmitted by this exit face  4340 A, the first diffracted light beam  11  and the second diffracted light beam  21  are parallel, and even herein superimposed to each other, at the exit of the polarization-recombining cube  4340  and propagate in parallel towards the focussing means  140 . 
     As the first diffracted light beams and the second diffracted light beams are symmetrical before transmission or reflection by the recombination interface  4343  of the polarization-recombining cube  4340  for each wavelength, these light beams are superimposed to each other and propagate in parallel at the exit of the polarization-recombining cube  4340  at each wavelength. 
     Advantageously, and for the same reasons as above, the exit face  4340 A of the polarization-recombining cube  4340  is also coated with an anti-reflective treatment. 
     Thanks to the superimposition of the first diffracted light beam  11  and of the second diffracted light beam  21  at the exit of the polarization-recombining cube  4340 , the focussing numerical aperture of the optical focussing system may be lower than the double of the collimation numerical aperture. 
     It has been shown in  FIG. 5  a second embodiment of a spectrometer  200 . 
     This spectrometer  200  is similar to the third variant of the first embodiment shown in  FIG. 3 , in that it includes the following identical elements: the entrance slit  101 , the collimation means  110 , the angular dispersion means  120 , the optical recombination means  330  and the focussing means  140 . 
     As a variant, the spectrometer according to this second embodiment could, for example, include an entrance slit, collimation means, angular dispersion means, optical recombination means and focussing means identical to the first embodiment or to the variants thereof shown in  FIGS. 1, 2 and 4 . 
     The spectrometer  200  includes an exit slit  201  arranged on the focussing surface  141  of the focussing means  140 , i.e. in the focussing plane of the focussing lens. 
     The exit slit  201  is herein mobile and may be translated in the focussing plane  141  to select a particular wavelength of the spectrum of the upstream light beam  1 . 
     More precisely, in the configuration shown in  FIG. 5 , the exit slit  201  is arranged in the focussing plane so as to let through the diffracted light beams  11 ,  21  that are recombined by the focussing means  140  in the focussing plane. 
     As a variant, the exit slit could for example be fixed, so that, by construction, the spectrometer let through only a single and same wavelength determined by the fixed position of the exit slit. 
     The spectrometer  200  further includes a fluorescence cell  202  placed downstream of the exit slit  201 , on the optical path of the diffracted light beams  11 ,  21 . 
     The spectrometer  200  also includes an imaging lens  203  that forms the image of the exit slit  201  on a study volume  204  of the fluorescence cell  202  so that this fluorescence cell  202  is illuminated by the diffracted light beams  11 ,  21  passing through the exit slit  201 . 
     The fluorescence cell  202  herein comprises a small vat made of transparent glass containing a solution of a product of which it is desired to measure the fluorescence signal at a predefined excitation wavelength or the fluorescence spectrum. 
     When the fluorescence cell  202 , in particular the study volume  204 , is illuminated by the diffracted light beams  11 ,  21  at the wavelength λ 1 , this study volume  204  then emits a fluorescence signal in all the directions, over a wavelength band going from 300 nm to 1100 nm. 
     This fluorescence signal is then collected by measurement means  210  that measure the light intensity of the fluorescence signal emitted by the fluorescence cell  202 . 
     For that purpose, the measurement means  210  include an optical collection system  211  of the fluorescence signal allowing to collect a portion of the fluorescence signal emitted by the fluorescence cell  202 . 
     This optical collection system  211  has herein an optical collection axis A 2  perpendicular to the optical axis A 1  of the spectrometer  200 . 
     This configuration allows to collect the fluorescence signal without being bothered by the diffracted light beams  11 ,  21  incident on the fluorescence cell  202 . 
     The measurement means  210  also include a second spectrometer  212  provided with an entrance measurement slit  213 . 
     The optical collection system  211  forms the image of the study volume  204  of the fluorescence cell  202  in the plane of the entrance measurement slit  213 . 
     The second spectrometer  212  then measures the light intensity of the fluorescence signal emitted by the fluorescence cell  202  as a function of the wavelength. 
     The measurement means  210  hence allow to analyse the response of the fluorescence cell  202  for the wavelength λ 1 . 
     In  FIG. 6  is shown a third embodiment of a spectrometer  300  also allowing to determine the polarization state of the upstream light beam  1 . 
     This spectrometer  300  of  FIG. 6  is similar to the first embodiment shown in  FIG. 1 , in that it includes the following identical elements: an entrance slit  101 , an optical collimation system  110 , a polarization-separation diffraction grating  120 , a planar mirror  130  and a focussing lens  140 . 
     As a variant, the spectrometer according to this third embodiment could, for example, include an entrance slit, collimation means, angular dispersion means, optical recombination means and focussing means identical to the variants of the first embodiment shown in  FIGS. 2 to 4 . 
     In this third embodiment, and as well shown in  FIG. 6 , the mirror  130  is arranged and oriented with respect to the polarization-separation diffraction grating  120  so that the planar optical-reflection surface, consisted by the reflective surface of the mirror  130 , is not parallel to the lines  122  of the polarization-separation diffraction grating  120  but forms with them an angle comprised between 0° and 90°. 
     So arranged, the optical recombination means  130  are such that the first diffracted light beam  11  and the second diffracted light beam  21  are focussed, for the wavelength λ 1 , by the focussing lens  140  in two distinct focussing points  31 ,  31 A of the focussing surface  141 . 
     In the focussing plane, the two distinct focussing points  31 ,  31 A are then separated on said focussing surface  141 , and are aligned according to a straight line D 1 . 
     Similarly, the first diffracted light beam  12  and the second diffracted light beam  22  (beams not shown) are focussed, for the wavelength λ 2 , by the focussing lens  140  to two distinct focussing points  32 ,  32 A of the focussing surface  141 , which are aligned along a straight line D 2  of the focussing plane, parallel to the straight line D 1  for the wavelength λ 1 . 
     Moreover, the focussing points  31 ,  31 A at the wavelength  21  are spaced along the straight line D 1  by the same distance as the focussing points  32 ,  32 A at the wavelength λ 2  along the straight line D 2 . 
     It will be considered in the following that the first diffracted light beam  11  has a first intensity I 11  at the wavelength λ 1  and the second diffracted light beam  21  has a second intensity I 21  at the wavelength λ 1 . 
     According to what has been described hereinabove, the polarization-separation diffraction grating  120  is such that the ratio between the respective intensities I 11  and I 21  of the first and second diffracted light beams  11 ,  21  at the wavelength λ 1  is function of the polarization state of the upstream light beam  1  in the plane of the entrance slit  101 . 
     It is moreover also the same for the first and second diffracted light beams  12 ,  22  at the wavelength λ 2 . 
     Hence, by placing detection means (not shown) in the plane of the focussing surface  141  so as to measure separately the relative intensities of the first and second diffracted light beams  11 ,  21 ,  12 ,  22  for each of the wavelengths contained in the spectrum of the upstream light beam  1 , it is not only possible to determine the spectrum of the upstream light beam  1  for the two natural polarization states of the polarization-separation diffraction grating  120 , but especially the polarization state of the upstream light beam  1  according to the wavelength. A spectropolarimeter is hence obtained. 
     Generally, the spectrometer of the invention allows to recombine the diffracted beams by a polarization-separation diffraction grating in the orders +1 and −1. In a way, the spectrometer of the invention superimposes the diffraction orders +1 and −1, hence allowing to exploit at best the diffraction efficiency of such a grating. 
     Generally, the spectrometer according to the invention has a high efficiency, close to 100% over a broad spectrum band, and that independently of the polarization state of the upstream light beam, wherein the latter can be polarized or not. 
     A realization of a spectrometer according to the invention has shown, with respect to a standard spectrometer implementing a conventional diffraction grating, an improvement of the efficiency of the spectrometer by a multiplicative factor comprised between two and three, for different lines of a Hg—Ar lamp on the wavelength band comprised between 500 nm and 760 nm.