WO1993016361A1

WO1993016361A1 - Spectrographic device

Info

Publication number: WO1993016361A1
Application number: PCT/FR1993/000122
Authority: WO
Inventors: André BARANNE
Original assignee: Centre National De La Recherche Scientifique (Cnrs)
Priority date: 1992-02-18
Filing date: 1993-02-05
Publication date: 1993-08-19
Also published as: FR2687471B1; FR2687471A1

Abstract

A spectrographic device which may be used as a monochromator, a spectrograph, a spectrometer or a colorimeter. A monochromator or spectrograph type optical device comprising at least one collimator (1), at least a first scatterer (2), and a second scatterer (3) performing scattering in a substantially perpendicular direction relative to the scattering direction of the first scatterer, one of said scatterers consisting of an échelle network. Said device comprises reflectors so that the beams pass twice through said first and second scatterers and said collimator. The device may be used in the field of optical devices for general use in spectrography.

Description

Spectrography device.

DESCRIPTION The present invention relates to a spectrography device which can be used as a monochromator, spectrograph, spectrometer, or colorimeter.

The technical field of the invention is that of the construction of optical devices used in spectrography in general.

Spectrography devices are used to decompose light into a spectrum from which spectral bands of light radiation are selected and isolated; these more or less wide spectral bands can then be either directly observed, or be photographed or analyzed by various types of apparatus.

Among the known devices for spectrography, one can distinguish in particular assemblies called "EBERT-FASTIE" comprising a disperser and a mirror serving as a collimator, and assemblies called "CZERNY-

TURNER "comprising a disperser and two mirrors serving as a collimator.

There are also known so-called "DOUBLE" spectrography assemblies in which two monochromators or spectrographs are successively placed on the path of the analyzed light; in this type of arrangement the dispersions specific to each of the two dispersers can be added or combined with the aim of eliminating in the final spectrum obtained, unfortunate superpositions of radiation, while increasing the resolution of the device, this type of arrangement being generally called "DOUBLE ADDITIVE"; other types of assembly use two successive monochromators or spectrographs in which the proper dispersions of the two dispersers cancel each other out in order to eliminate an undesirable spectral band in the reconstituted white light, the latter type being generally called

"DOUBLE SUBTRACTIVE".

There are also known so-called "double pass" arrangements in which the parallel monochromatic beams coming from the disperser are returned on themselves by means of a plane mirror; the disperser being crossed twice by light, the spectrum obtained is twice as dispersed.

Mounts of this type are for example described in the patent FR 2,655,731 (FISONS).

It is also known devices combining characteristics of the previous basic arrangements: there is a source (a point or a slot) at the focus object of a mirror for example parabolic collimator; a disperser such as a network or a prism reflecting from its rear face, is disposed at the image focal point of the parabolic mirror and receives from it a parallel beam; the parallel beam dispersed by the disperser returns to the parabolic mirror which provides in its focal plane a spectrum, image of the source.

If we have in this spectrum a plane mirror (perpendicular to the axis) the white light is recomposed by the principle of the reverse return of the light and we obtain a white image superimposed on the source. This type of essentially double subtractive assembly has the following characteristics:

- the intermediate spectrum is in a telecentric medium (the pupil is at infinity),

- the aberrations at the center of the spectrum are zero but increase in the field as a function of the opening and of the field, which limits the use of this arrangement to the selection of a reduced spectral band in the center of the field, the spectral band being chosen by rotating the disperser,

- the source, the dispersing network, the intermediate spectrum and the image are physically superimposed.

Improvements to such systems are for example described in the document MULTILEMENT DETECTIONS SYSTEMS FOR SPECTROCHEMICAL ANALYSIS, BUCH et al., CHEMICAL ANALYSIS Volume 107, pages 187. 199. in which is provided in the intermediate focal plane, instead of the plane mirror, two mirrors arranged on the roof, the edge of which is parallel to the direction of dispersion of the disperser.

The problem posed, which is not resolved by known spectrography devices, consists in providing a device comprising a reduced number of components (so as to be compact and of low cost) which makes it possible to decompose a light beam for the purposes of analysis. certain wavelengths and / or certain spectral bands, with increased resolution. The solution to the problem posed consists in providing an optical device, of the monochromator or spectrograph type, comprising at least one collimator, at least one first disperser, at least one second disperser ensuring a dispersion whose direction (direction) is substantially perpendicular to the direction dispersing said first disperser; one of the dispersers is constituted by a ladder network and said device comprises reflection means ensuring a double passage of the beams by said first and second dispersers and by said collimator. Advantageously, said reflection means provide a spatial offset between the incident beams on said reflection means

• and the beams reflected by said reflection means, and thus ensure a corresponding offset between the image and the source of said device, which are located close to one another in the focal plane of said collimator.

Advantageously, said device comprises at least one means of selection (substantially situated in the intermediate focal plane) of at least one wavelength and / or at least one spectral band.

In a first embodiment, said reflection means comprise two substantially planar mirrors (forming a "roof") contained in ^" two" intersecting planes whose intersection (forming a straight line or edge) is perpendicular to the direction of dispersion of said scale network ; said edge may in certain cases be substantially parallel to the axis of rotation (or of pivoting) of said scale network, so that said device causes the addition of the dispersion caused, on each pass, by said scale network, thus constituting a double additive mounting relative to the dispersion of said scale network, and a subtractive double mounting relative to the dispersion of the other disperser and moreover causes the spatial shift between the image and the source.

Advantageously, said mirrors form a mobile assembly in two directions contained in a plane perpendicular to the axis of said incident beams and reflected by said mirrors, and said mirrors are of reduced size so as to constitute said means for selecting a spectral band; preferably said mirrors have a dimension (or height) substantially less than or equal to the distance separating in said intermediate focal plane, two orders successive, and said mirrors have a second dimension (or length) corresponding substantially to said spectral band to be selected.

Advantageously, said device comprises at least one window provided in a substantially planar cover located in the plane of symmetry of the assembly formed by said mirrors, and preferably said window and / or said cover is movable (preferably in said plane of symmetry ) so as to constitute said wavelength and / or spectral band selection means. In another embodiment, said reflection means comprise at least one assembly comprising three substantially planar mirrors which have a large dimension (or length) substantially parallel to the direction of dispersion of said scale network (of said first disperser). According to a particular embodiment, said reflection means and / or said wavelength selection means and / or said spatial shifting means comprise at least one optical fiber.

Advantageously, one of the dispersers is a prism, for example a zero deflection prism, placed in front of said scale network.

Advantageously, said ladder network is pivotally mounted with respect to an axis parallel to the lines with which it is provided.

Thanks to the invention, one can have an assembly ensuring a double dispersion which can be used in "double passage", which can allow, in certain cases the elimination of a pre-disperser; thanks to the device ensuring double dispersion in perpendicular directions, it is possible to select wavelengths or spectral bands with very high resolution.

The numerous advantages provided by the invention will be better understood through the following description which refers to the appended drawings, which illustrate without any limiting character, particular embodiments of device according to the invention.

FIG. 1 schematically illustrates in perspective view a first embodiment of an optical spectrography device according to the invention.

FIG. 2 illustrates in plan view a second embodiment of a device according to the invention. FIG. 3 is a view along III III of FIG. 2.

Figure 4 is a perspective view of an assembly comprising two mirrors and a cover provided in the device of Figures 2 and 3 <

Figure 5 illustrates in schematic side view a third embodiment of a device according to the invention.

FIG. 6 illustrates in schematic perspective view an alternative embodiment of a reflection device, constituting an alternative to the embodiment illustrated in FIG.

FIG. 7 illustrates in longitudinal side view a fourth embodiment of an optical device according to the invention.

FIG. 8 illustrates in simplified perspective view a third embodiment of reflection means of a device according to the invention.

FIG. 10 illustrates in plan view a fifth embodiment of a device according to the invention.

Figures 11 and 12 are respectively partial views of Figure 10 along XI and XII.

With reference to FIG. 1, it can be seen that a light beam Fi originating from a light source propagates towards a collimator 1 which may for example be constituted by a spherical mirror; said beam Fi is reflected by said collimator to constitute a parallel beam F2, which is dispersed by a first disperser 2 to form a beam F3; said first disperser 2 is advantageously a ladder network provided on its face 2a with lines 6 parallel to each other and parallel to an axis zzi around which said first disperser can optionally rotate as indicated by the arrow Ri.

Said dispersed beam F3 meets a second disperser 3 Which in the example shown in FIG. 1 can be constituted by a network provided with lines 7 parallel to each other and parallel to an axis yyi around which said network 3 can possibly rotate as indicated by the arrow R2-

A parallel beam F- is produced by the dispersion of said beam F3 by said second disperser 3. which beam F4 is reflected by a mirror ^" 8 to form a beam F5; said beam F5 is reflected successively by two substantially planar mirrors ti and t2 arranged symmetrically with respect to the axis of said beam F5 so as to constitute a reflected beam (not shown); light reflected by said mirrors ti and t2 follows a reverse optical path, that is to say substantially equivalent to said beams F5, F-4, F3, F2, Fi, so as to constitute an image in the focal plane of said collimator 1 Said mirrors t 1 and 2 are situated substantially in the intermediate focal plane of the device, that is to say in the focal plane of said mirror 8.

In said intermediate focal plane, due to the presence of said scale network 2, one can observe a spectrum represented schematically in the figure by a succession of lines L corresponding to different wavelengths of the analyzed light; said lines extend in a direction di which corresponds to the direction or direction of dispersion of said first disperser or scale network 2; due to the presence of said second disperser 3- we can observe in said intermediate focal plane a separation of order 0χ, 02, O3 ..., which orders are substantially superimposed, that is to say that the direction d2 ( or the direction) of the dispersion due to said second disperser 3 are substantially vertical.

Thanks to the position of the edge 11 of the assembly formed by said mirrors ti and t2. which edge corresponds substantially to the intersection ^" of ^{" the} substantially planar faces of said mirrors, which edge is located perpendicular to said direction of dispersion relative to said first disperser 2, a device can be used with the assembly of FIG. 1 monochromator ensuring of course a double passage of the light in said device, and ensuring an addition of the dispersion caused on each passage by said first disperser 2; in this type of device, the selection of the wavelengths which it is desired to obtain in the final image for analysis, can be carried out, as regards the selection of a wavelength or a spectral band in a given order, either by displacement in the direction di of all of said mirrors i and t2, either by rotation along the arrow Ri of said first disperser; the selection can be made, as regards the choice of order, or by moving ent of all of said mirrors ti and t2 in the direction d2 of dispersion due to said second disperser, or by the rotation along arrow R2 of said second disperser. In Figure 1, there is shown schematically the light source of said device marked S and the image obtained marked I voluntarily very far from each other; said source S and image I located in the focal plane of said collimator 1 are in fact separated by means of said reflection means constituted by said mirrors ti and t2.

With reference to FIGS. 10, 11 and 12, it can be seen that a light beam FI coming from a source S propagates towards a collimator 1, which can for example consist of a spherical mirror, said beam FI is reflected in a beam parallel F2 dispersed by a disperser 2 to form a beam F3.

Said disperser 2 is either a prism with a reflective rear face, or the network working in the first orders represented in FIG. 10, said network being provided with lines parallel to a yyl axis perpendicular to the plane of FIG. 10 and of a rotation by report to said yyl axis.

Said scattered beam F3 meets the scale network 3 provided with lines parallel to the plane of FIG. 10, with an axis of rotation zzi, in the plane of FIG. 10. The beam F3 is then dispersed in the perpendicular direction and returns successively to said network 2, on the collimator 1 to form as previously in FIG. 1, the multispectra 0i, 02ι O3 at the source S.

In the same way as before, a pair of mirrors Tl, T2 selects the spectral band chosen and by returning the light in a second passage in the device ensures the necessary spatial offset between the final image I and the source S.

Advantageously, it will be noted that the sum of the distance between said 'ispersers 2 and 3 and the distance between said disperser 2 and said collimator 1, can be equal to the radius of curvature of the mirror

(collimator 1), image I is only affected by spherical aberration and a small curvature.

Advantageously, the collimator can be parabolic. if the opening is important. The device is therefore perfectly suited to large sources with large apertures, allows the use of input grids and / or optical fibers.

Said source S and image I are strictly equal and fixed one in relation to the other; the wavelengths scrolling is advantageously obtained by means of two rotations RI and R2 of said dispersers 3 and 2 along said axes zzl and yyl respectively.

It will be noted that the device represented in FIGS. 10 to 12 is double additive with respect to the scale dispersion and double subtractive with respect to the secondary dispersion.

With reference to FIG. 2, it can be seen that in a second embodiment, said device comprises said collimator 1 constituted for example by a spherical mirror, a first disperser 2 constituted by a ladder network the front face 2a of which has parallel lines 6 between them and perpendicular to the plane of Figure 2; said device also comprises a second disperser 3 constituted by a prism located in front of said first disperser 2.

Said device also comprises two plane mirrors ti, t2 symmetrical with respect to a plane perpendicular to the plane of FIG. 2 and containing an axis yyi which can advantageously be perpendicular to the general optical axis xxi of the device.

Said device also comprises two small deflection mirrors Ni and N2 which are plane mirrors, perpendicular to the plane of FIG. 2 and superimposed along an axis (not shown) perpendicular to the plane of FIG. 2; said return mirrors Ni and N2 are advantageously positioned at ^ 5 ° relative to said optical axis xxi; a mirror Ni is provided for reflecting a light beam Fi coming from a source S to form a beam F2 which is reflected by a collimator 1 to form a parallel beam F3; said beam F3 is dispersed successively by said second disperser 3 and first disperser 2, in two crossed or perpendicular directions.

The parallel F-beam scattered and returned by said dispersers returns to said mirror 1 to be reflected and form a beam F5 which strikes a second return mirror n, so as to form a beam F6; said beam F6 is reflected successively by the front faces of said mirrors ti and t2, passes through a window provided in a cover 5 and traverses an optical path opposite to the path described above therefore corresponding substantially and in opposite directions to said beams F5, Fi |, F3, F2, Fi to form an image I in a focal plane of said collimator 1. Advantageously, said window is larger than the monochromatic image of said source S, and the height of said window is less than the interorder interval so as to isolate the desired order; in the final image plane, a monochromatic I image of said source S is collected with a magnification equal to 1

Figure 3 partially illustrates and according to III III Figure 2; in this figure 3 we see that said deflection mirrors Ni, N2 are superimposed along an axis zzi corresponding substantially to the direction of dispersion of said prism constituting said second disperser 3ι which prism is placed in front of said first disperser 2 provided with said lines 6, which first disperser 2 to a direction of dispersion substantially perpendicular to the direction of dispersion of said second disperser 3-

With reference to FIG. 4, it can be seen that said beam F6 incident on said reflection means constituted by said mirrors ti and t2 is reflected successively by said mirrors ti and t2 "so as to successively form a beam Fy passing through a window provided in a cover 5, to constitute a beam Fg ¹ which will follow the equivalent optical path (inverse) to said beams F6,

It can be seen that on the front face of said mirror t 1, the spectra formed by lines corresponding to the dispersion of said light beam by said first disperser are formed; we also see that the different orders 0ι, O2, O3 have been separated by the action of said second disperser; thanks to said cover 5 provided with said window 4, only part of said beam F6 corresponding to a spectral band BS (constituting a part of said spectrum corresponding to order 02), can pass through said cover by said window to constitute ^" the reflected beam F6 It can be seen that according to the invention, the reflecting faces of said substantially flat mirrors tl and t2 form a roof whose edge is perpendicular to the direction di of dispersion corresponding to said scale network; to select said spectral band to be analyzed, it is possible to advantageously move said mask provided with said window 4 (or an assembly comprising said mirrors Tl, T2 and said mask) in said direction di in order to select in a given order said spectral band and in a direction d2 (perpendicular to said direction di) in order to select the desired order.

The assembly illustrated in FIGS. 2, 3 and 4 being a double subtractive with respect to the dispersion given by said prism, during the movement of said cover (and possibly of said mirrors) said image I does not move; . on the other hand, this assembly being a double additive with respect to the dispersion due to said first disperser (ladder network), the dispersion obtained in image I is twice the dispersion obtained in the intermediate space, and the length scrolling wave in I is obtained by turning the scale network. FIG. 5 illustrates an alternative embodiment of a device of FIG. 2 in which said source S and the image I obtained on the one hand, said reflection and selection means constituted by said roof mirrors tl, t2 and said cover 5 on the other hand, are located on either side of an assembly comprising said first disperser 2 in front of which is located said second disperser 3 advantageously constituted by a zero deflection prism, which first and second dispersers are located on the optical axis xxi of said collimator 1; said source, said image and said reflection and selection means are located in the focal plane of said collimator 1 and are used and operate as explained in FIGS. 2, 3 and 4.

The arrangement of FIG. 5 makes it possible to separate the source S and image I assembly from the intermediate spectrum, so as to free the pupillary space occupied by said dispersers; in order to obtain a good central image of the intermediate spectrum, the concave mirror forming the collimator 1 of an "EBERT-FASTIE" type assembly is a torus whose meridians are parabolic.

With reference to FIG. 6, it can be seen that in a variant embodiment of said means of reflection and of selection of wavelength or spectral band, it is possible to use, instead of said two roof mirrors, a set of roofs and rhombes. (which are represented by the circular portions of their reflecting face) in order to offset said reflected beam Fg 'relative to said incident beam F6, in two directions of said intermediate focal plane. In the same way as for the device in FIGS. 2 to -4, a cover 5 provided with a window 4 is provided which can advantageously move in said directions or directions di and d2 corresponding to the direction of the dispersions of said first and second dispersers respectively (said roofs and rhombus-mirrors being marked 81, 82, 83, 84, 85, 86).

With reference to FIG. 7. an optical device according to the invention comprises said collimator 1, said first disperser constituted by a ladder network, said second disperser 3 constituted by a prism with zero deviation and comprises a second mirror 8 in the focal plane of which are located three mirrors Mi, M2, 3, a cover 5 provided with a window being provided in front of said mirror M2 in order to allow said spectral band or wavelength to be analyzed to be selected.

In the arrangements described in FIGS. 2 and 5, which use a basic arrangement of the "EBERT-FASTIE" type, this type of arrangement means that only radiation in the vicinity of the axis can be used; in order to analyze more distant radiations or wider spectral bands with a high resolution, one can use the assembly of the type "CZERNY-TURNER" as represented in figure 7, which makes it possible to operate in a larger image field ; advantageously, in this type of assembly, said collimator constituted by a mirror 1 is a torus with parabolic meridians, said second mirror 8 being spherical, so that the spectrum obtained in the sagittal image plane of the mirrors is devoid of aberration.

Advantageously, said mirrors Mi, M2, M3 are such that all the orders dispersed by said dispersers are drawn on said second mirror M2 which is covered with said cover provided with holes isolating the desired spectral bands; thanks to the use of a zero deviation prism, the orders obtained in the intermediate field are as curved as possible and the mechanical implantation of the components of the device is facilitated. The minimum width of a spectral band to be analyzed is limited only by the resolution of the scale network (which can be very important) and only by the dimension of the hole (source); said prism being composed of very transparent materials at all wavelengths, the spectral range in which the device is usable is very wide; this makes it possible to use a single scale network and to avoid the need for interchangeability of the dispersing means which would provide additional mechanical complications. It is interesting to note that in this case of use of the invention, a small photometric cell (or at least one optical fiber) can be provided in order to analyze the image of the source obtained by the device. Alternatively, said second mirror M2 (and possibly said other mirrors M1 and M3) can be of a height reduced to an interorder interval and of a length equal to that of the order to be observed; said cover can in this case be provided in a slightly orientable manner, in order to compensate for the slopes of orders distant from the order corresponding to a zero deviation.

In the case of using a device of this type for astronomical use such as tavelography which requires the comparison of distant spectral bands with a high resolution, said device can comprise two sets of said three mirrors Mi, M2 and M3 in order to compare parts of an order; it is thus possible, thanks to the device according to the invention, to shift symmetrically on either side of the source the white images corresponding to each of said parts of said order, which images remain stationary when the two sets of said three mirrors move perpendicular to said direction of dispersion of said scale network.

As an alternative to said mirror in the form of a parabolic torus serving as a collimator, a spherical mirror can be provided in the assembly of the type of FIG. 7 •

While the assemblies of FIGS. 1, 2, 5 constituted assemblies making it possible to add the dispersion due (to each passage) to said first disperser (ladder network) and to subtract or cancel the dispersion caused by said second disperser, the assembly of Figure 7 allows to subtract or cancel the dispersions caused by said first and second dispersers. With reference to FIG. 8 which represents a schematic perspective view of said mirrors Mi, M2, M3, of a device such as that of FIG. 7. it can be seen that an incident beam F6 is reflected successively by said mirrors M3, M2 , M1 to form a reflected beam F6 'parallel to said beam F6; said mirrors Mi, M2, M3 have a large dimension corresponding to the direction di (or direction) of dispersion due to said first disperser (scale array); a mask 5 is advantageously provided in front of said mirror 2 provided with a window 4 making it possible to select an order from 0ι .-. 0i ... 0n orders separated by said second disperser and superimposed in said direction d2, and to select in one of said orders a spectral band or a length wave, which selections can be made by moving said cover provided with said window.

In an alternative embodiment illustrated in FIG. 9, said means for reflecting, for selecting the wavelength, and for shifting said incident and reflected beams F6 and F6 '. use optical fibers. An equivalent M'i of said mirror Ml in FIG. 8 can be constituted by a plurality of optical fibers 10 provided at their end with an emission optic 10a and connected at their second end to selection and connection means 13, which means 13 receive the light transmitted by receiving optical fibers 12 provided at one end with a receiving optic 12a and connected at their second end to said means 13, said ends 12a of said optical fibers 12 constituting an equivalent M * 3 of the mirror marked M3 in Figure 8; by processing the light transmitted by each of said optical fibers 12 for the purpose of delivering light into the corresponding optical fibers 10, it is possible to constitute means ensuring both the emission of said beam F6 ′ offset from said beam Fg, thus that means for selecting said wavelength to be analyzed, and means for shifting said incident and reflected beams. It may be noted that said means 13 for transmitting light information between said receiving optical fibers and said emitting optical fibers 12 and 10 respectively, can make it possible to control the light information passing between said optical fibers and thus easily make it possible to perform the function of selection of at least one wavelength; it is in fact possible to provide a high density of transmitting and receiving optical fibers so that a wavelength can be associated with each of said pairs of optical fibers.

Claims

1. Monochromator or spectrograph type optical device comprising at least one collimator (1) and at least one first disperser (2) characterized in that it comprises a second disperser (3) ensuring a dispersion whose direction is substantially perpendicular to the direction of dispersing said first disperser, and in that one of said dispersers is constituted by a ladder network and in that said device comprises reflection means ensuring a double passage of the beams by said first and second dispersers and by said collimator.

2. Device according to claim 1 characterized in that said reflection means provide a spatial offset between the image and the source of said device, which are located close to each other in the focal plane of said collimator.

3- Device according to any one of claims 1 to 2 characterized in that it comprises at least one means for selecting at least one wavelength.

4. Device according to any one of claims 1 to 3 characterized in that said reflection means comprise two mirrors (tl, t2) substantially planes contained in two intersecting planes whose intersection is perpendicular to the direction of dispersion of said scale network.

5- Device according to claim 4 characterized in that said mirrors (tl, t2) form a movable assembly in two directions (di, d2) contained in a plane perpendicular to the axis of incident beams (F6) and reflected (F6 ' ) by said mirrors, and in that said mirrors are of reduced size so as to constitute said means for selecting a spectral band.

6. Device according to claim 4 characterized in that it comprises at least one window (4) provided in a cover (5) substantially planar located in the plane of symmetry of the assembly formed by said mirrors (ti, t2), and said window and / or said mask is movable so as to constitute said means for selecting wavelength and / or spectral bands.

7- Device according to any one of claims 1 to 3 characterized in that said reflection means comprise at least one assembly comprising three mirrors (Ml. M2 _> N) substantially planar which have a large dimension substantially parallel to the direction of dispersion of said scale network.

8. Device according to any one of claims 1 to 7 characterized in that said reflection means and / or said means for selecting a wavelength and / or said spatial shifting means comprise at least one optical fiber.

9. Device according to any one of claims 1 to 8 characterized in that said second disperser is a prism placed in front of said first disperser.

10. Device according to any one of claims 1 to 9 characterized in that at least one of the two dispersers is pivotally mounted relative to an axis perpendicular to the dispersion it causes.