WO2011098726A1 - Compact spectrograph having a wide spectral band including a plurality of diffraction gratings - Google Patents

Abstract

The present invention relates to a spectrograph having diffraction gratings including an input slit (3), suitable for transmitting an input light beam including a plurality of wavelengths; a visible matrix detector (4) including at least N rows of photodetectors, where N is no lower than two; and a plurality of N diffraction gratings, said gratings being concave reflection diffraction gratings. According to the invention, the diffraction gratings (1a, 1b) are placed and positioned such that each grating (1a, 1b) receives a part of the light beam transmitted by the input slit (3), angularly diffracts said part of the input beam according to the wavelength in a spectral domain ([λa1,... λaM], [λbi,... λbM] - - -) and forms a spectrum on the visible matrix detector, said spectrum extending according to a separate row of photodetectors respectively for each diffraction grating (1a, 1b).

Description

 Compact spectral broadband spectrograph comprising a plurality of diffraction gratings

The present invention relates to a spectral spectrometer of compact planar field and wide spectral band comprising a plurality of diffraction gratings.

 A prism or diffraction grating spectrometer makes it possible to angularly disperse the spectrum of a light beam in a direction transverse to the entrance slit of the spectrograph. The spectrum obtained at the output is located on a more or less complex surface, for example on a circle for the Rowland circle spectrographs. A planar field spectrograph is a spectrograph whose components are calculated and arranged so that the spectrum obtained at the output is located in a plane. Spectrographs of the flat-field type are particularly suitable for photodetectors of the photodiode array or CCD type.

 The document FR2334947 describes a concave holographic diffraction grating plane field spectrograph which makes it possible to obtain, in reflection, a planar spectrum over a broad spectral range. Such a spectrograph requires no optical focusing or collimation component in addition to the concave network. This document also describes the optimization of the concave holographic diffraction grating for correcting defocusing, coma and astigmatism aberrations on the plane spectrum. However, the measurable spectral range is limited in practice by the length of the detector. Moreover, a single network can hardly be optimized over a very wide spectrum.

 In order to extend the spectral range of the spectrograph or to improve the resolution on two spectral zones, the document FR2334947 proposes to juxtapose two networks on the same concave support, the two networks having lines inclined relative to each other so as to form two spectra shifted in the diffraction plane. This arrangement makes it possible to use two networks optimized each on a spectral zone and thus to extend the spectral range of the spectrograph. However, the manufacture of two networks inclined relative to each other on the same concave support poses implementation difficulties. In addition, this spectrograph requires two detectors inclined relative to each other to detect the two spectra, which increases the cost of the spectrograph.

 The object of the invention is to provide a compact planar field spectrograph having a broad spectral range of detection. Another object of the invention is to provide a spectrograph having a high resolution over a wide spectral range.

 More particularly, the invention relates to a concave diffraction grating spectrograph comprising:

an input slot, capable of transmitting an input light beam comprising wavelengths (λ ₃ ,..., λ _ΝΜ ), a matrix detector comprising at least N lines of photodetectors, with N greater than or equal to two, and

 a plurality of N diffraction gratings said gratings being concave diffraction gratings in reflection.

According to the invention, said diffraction grating is on separate supports, and each diffraction grating is placed at a distance from the entrance slit and the detector and the strokes of each grating are oriented so that each grating respectively receives a portion of the light beam transmitted by the input slot, angularly diffracts said portion of the input beam as a function of the wavelength on a spectral domain and forms a spectrum ([λ ₃ , ..., λ _3Μ ], [λ _Μ , ..., ι> Μ] .- _" . [Νΐ, ..., λ _ΝΜ ]) on the matrix detector, each spectrum ([λ _3ΐ , ..., λ _3Μ ], [ λ _Μ , ..., bM], ■■■, [N1, ..., λ _ΝΜ ]) extending along a distinct line of photodetectors respectively for each diffraction grating.

 According to a preferred embodiment, the height of the input slot is limited so that, the distance between two adjacent lines of the photodetector being equal to d, the spectral image of the slot on the detector by a network s extends over half a height h less than d.

According to a preferred embodiment, the spectrograph comprises two diffraction gratings, a first network forming a first spectrum extending over the spectral domain [λ ₃ , ..., λ _3Μ ] on a first line of photodetectors and a second network. forming a second spectrum extending over the spectral range [λ _Μ , ..., λbM] on a second line of photodetectors, the second spectrum being shifted with respect to the first spectrum by a distance d in a direction transverse to the direction spectral diffraction in the plane of the detector.

 According to a particular embodiment, the spectrograph comprises four diffraction gratings.

 In a preferred embodiment, the diffraction gratings are corrected for defocus, coma and astigmatism aberrations.

 According to a preferred embodiment, the diffraction gratings have the same radius of curvature.

 According to a particular embodiment, two adjacent lines of photodetectors of the detector are separated by a non-photosensitive zone with a width e of less than 600 microns.

 According to a particular embodiment, the diffraction efficiency of at least one diffraction grating is optimized over a spectral domain.

According to a particular embodiment, the spectral domains [λ ₃ , ..., λ _3Μ ] and [λυι, ..., λbM] are distinct.

According to another particular embodiment, the spectral domains [λ ₃ , ..., 3Μ] and [λ _Μ , ..., λbM] have a spectral overlap zone. The present invention also relates to the features which will emerge in the course of the description which follows and which will have to be considered individually or in all their technically possible combinations.

 This description, given by way of non-limiting example, will better understand how the invention can be made with reference to the accompanying drawings in which:

 FIG. 1 schematically represents a perspective view of a spectrograph with two diffraction gratings according to a first preferred embodiment of the invention;

 FIG. 2 schematically represents a transverse sectional view of the two arrays and the detector of the spectrograph of FIG. 1;

 FIG. 3 represents a front view of a matrix detector used in the spectrograph of the invention;

 FIG. 4 schematically represents a perspective view of a spectrograph with four diffraction gratings according to another embodiment of the invention;

 FIG. 5 represents an enlarged view of the detector part of the spectrograph of FIG. 4 and illustrates the formation of four spectra on four lines of the detector.

FIG. 1 schematically represents a perspective view of a spectrograph with two reflection diffraction gratings according to a preferred embodiment of the invention.

 The spectrograph (10) comprises an entrance slit (3), a first diffraction grating (1 a) in reflection on a concave support, a second diffraction grating (1 b) in reflection on another concave support and a detector (4) comprising at least two lines of photodetectors.

 The input slot (3) transmits an incident light beam towards the two networks (1 a) and (1 b).

 The grating (1a) receives a beam (2a) from the entrance slot and diffracts this beam into a beam (5a), the beam (5a) being angularly diffracted at an angle that depends on the wavelength. The diffracted beam (5a) is focused by the grating (1a) on a first line (14a) of photodetectors of the detector (4).

Similarly, the grating (1b) receives a beam (2b) coming from the entrance slot and diffracts this beam into a beam (5b), the beam (5b) being angularly diffracted at an angle that depends on the length wave. The diffracted beam (5b) is focused by the grating (1b) on a second line (14b) of photodetectors of the detector (4), distinct from the first line (14a). FIG. 1 shows the outline of some beams (2a, 2b, 5a, 5b).

 According to the preferred embodiment, shown schematically in FIG. 1, the supports of the two networks (1 a, 1 b) have the same radius of curvature and are placed at the same distance from the entrance slit (3) and the detector ( 4).

 FIG. 2 diagrammatically represents the optical operation of the spectrograph of FIG. 1 between the arrays and the matrix detector.

 The networks (1 a) and (1 b) and the photodetector are shown in FIG. 2 in transverse section, the plane of FIG. 2 comprising a median radius of the diffracted beam (5a) by the first grating (1 a) and a median radius of the diffracted beam (5b) by the second network (1b).

As indicated above, the first network (1 a) receives a portion of the input light beam, diffracts it into a beam (5a) on a first spectral domain and forms the image of this first spectrum along a line in a plane output P _a .

The second grating (1b) receives a portion of the input light beam diffracts it into a beam (5b) on a second spectral domain and forms the image of this second spectrum along a line in an output plane P _b . The first network (1 a) and the second network (1 b) are arranged so that the first spectrum diffracted by the ^1st network (1 a) and the second spectrum diffracted by the second network (1 b) s' extend in parallel lines.

 Many arrangements networks are possible while remaining within the scope of the invention. We detail a few special cases in the following description. In this document, the median plane of the spectrograph is defined as the plane passing through the center of the entrance slot (3), the center of the detector (4) and the center of gravity of the diffraction gratings.

 The detector (4) is placed in a plane Ps, including the central lines of the spectra diffracted respectively by the two diffraction gratings (1 a, 1 b), so that a first line (14a) of detectors receives the spectrum diffracted by the first network (1 a) and a second line (14b) of detectors receives the spectrum diffracted by the second network (1 b). In the case of a spectrograph with two diffraction gratings, according to the embodiment shown in FIG. 1, the arrays are disposed respectively on either side of the median plane (so-called "superimposed arrays" configuration); according to another embodiment, the networks can be arranged so that their respective centers are located in the median plane (configuration called "side-by-side networks"). In the case of a spectrograph with four diffraction gratings, some networks may be centered on the median plane and other networks centered off the median plane.

More specifically, the orientation of the lines of the networks as well as the positioning of each of the networks are optimized in order to position the central lines of the networks. respective spectra of each array on a line of photodetectors of the detector (4). If the diffraction planes (Pa), (Pb) of the networks (1 a), (1 b) respectively, are spatially separated (in the case of two superimposed networks), the plane Pa forms an angle ε ₃ with the plane Ps and the plane Pb forms an angle £ _b with plane P _s . In the case where the networks (1 a) and (1 b) are side by side, the diffraction planes (Pa) and (Pb) coincide with the plane P _s .

 Figure 3 schematically shows a front view of the matrix detector (4). The detector (4) comprises a first line (14a) of photodetectors (24a) and a second line (14b) of photodetectors (24b). In an exemplary embodiment, the distance between two adjacent lines is equal to d, and the leading space is equal to e. The space between the lines (14a) and (14b) of width e may be a non-photosensitive space. The concave reflection gratings (1a, 1b), the input slot (3) and the matrix detector (4) are placed in such a way that the spectrum of the input slot formed by the first grating (1 a) is imaged on the first line (14a) of photodetectors and the spectrum of the entrance slot formed by the second network (1 a) is imaged on the second line (14b) of photodetectors.

By optimizing the position and the orientation of each of the networks (1 a, 1 b), a first spectrum extending along a first line (14a) of photodetectors and a second spectrum extending along a second line ( 14b) of photodetectors of a single matrix detector. The first line (14a) of photodetectors can thus detect a first spectrum [λ ₃ , ..., λ _3Μ ], while the second line (14b) can detect a second spectrum [λ _Μ , A, _bM ] -

The extent of the entrance slot (3) is limited in height in order to avoid the superposition of the spectra on the adjacent lines (14a, 14b) of the detector. The maximum height of the slot depends on the separation of the detection lines (14a, 14b) and also the correction of the astigmatism of the different spectrograph networks.

The networks (1a) and (1b) have an image quality such that the first line (14a) of detectors receives only the first spectrum [λ ₃ , ..., λ _3Μ ] and the second line (14b) of detectors receives only the second spectrum [λ _Μ , _bM ]. Thus the spectra [λ ₃ , ..., λ _3Μ ] and [λ _Μ , ..., λ _b M] do not overlap in the plane of the detector (4). However, it is not excluded that part of the diffracted spectra may be superimposed in the non-photosensitive space of width e.

 The networks (1 a) and (1 b) are advantageously concave holographic networks corrected for aberrations in order to optimize the spectral resolution on each spectral domain.

The network (1 a) and the network (1 b) can be optimized so as to cover distinct and possibly complementary spectral domains. In an exemplary embodiment, the spectrograph makes it possible to cover the near UV-visible-near-infrared range.

 The spectrograph of the invention comprises a single matrix detector (4), the detector (4) comprising at least two lines of photodetectors (14a, 14b).

 The spectrograph shown in FIGS. 1 and 2 comprises two diffraction gratings (1 a) and (1 b) on two distinct supports.

 More generally, the spectrograph of the invention may comprise a number N of networks with N greater than or equal to two, the matrix detector comprising at least N lines of photodetectors.

Figure 4 schematically shows a spectrograph according to another embodiment. The spectrograph comprises four diffraction gratings (1 a, 1 b, 1 c, 1 d). The first network (1 a) receives a fraction (2a) of the input light beam, diffracts it into a beam (5a) on a first spectral domain and forms the image of this first spectrum [λ ₃ , ..., λ _3Μ ] in an output plane P _a along a line (14a) of photodetectors of a matrix detector (4). Similarly, the gratings (1b), respectively (1c), (1d) receive a fraction (2b), (2c), (2d) of the input light beam, and diffract respectively into a beam ( 5b), (5c), (5d) on spectral domains so that the grating (1b) forms the image of a spectrum [λυι, λ _b M] along a line (14b) of photodetectors, the network (1 c) forms the image of a spectrum [λ ₀ ι, ..., λ _οΜ ] along a line (14c) of photodetectors and respectively the network (1 d) forms the image of the spectrum [λ ^ , ..., λ _ϋΜ ] along a line (14d) of photodetectors. In order not to overload Figure 4, it is shown only the outline of some beams beams (2a, 2b, 2c, 2d, 5a, 5b, 5c, 5d).

As illustrated in FIG. 5, four spectra are obtained [λ ₃ , ..., λ _3Μ ], [λ _Μ , ... , λbM], [λοΐ, ... , λ _οΜ ], [λάΐ, ... , λ _άΜ ] respectively on four lines (14a, 14b, 14c, 14d) of a matrix detector (4).

 The spectrograph thus obtained offers the advantage of covering a wide spectral range while being very compact. A spectrograph with two diffraction gratings makes it possible to cover a spectral range twice as wide or the same spectral domain with a resolution twice as high as a spectrograph of the same size comprising only one diffraction grating. The greater the number of networks, the more the spectral range and / or the spectral resolution can be improved.

 The spectrograph of the invention has no moving parts, the location and orientation of the networks being optimized during manufacture. This spectrograph can use standard diffraction gratings and a single detector which makes it possible to manufacture a low cost spectrograph.

The spectrograph has the advantage of using only one input slot (3), which allows a better homogeneity of the analysis of a source over a wide band spectral and / or with a fine spectral resolution, as compared with a multi-slit spectrograph using slots of different sizes depending on the spectral range or resolution sought.

Claims

A concave diffraction grating spectrograph comprising: an input slot (3) capable of transmitting an input light beam comprising wavelengths (λ ₃ , ..., λ _ΝΜ ), a matrix detector (4) comprising at least N lines of photodetectors (14a, 14b, 14 _N ), with N greater than or equal to two, a plurality of N diffraction gratings (1a, 1b, 1c, 1d, ..., 1N), said gratings being concave diffraction gratings in reflection,  characterized in that  said diffraction grating (1a, 1b, 1c, 1d, ..., 1N) are on separate supports,  and in that : each diffraction grating (1a, 1b, 1c, 1d, ..., 1N) is placed at a distance from the entrance slot and the detector and the lines of each network are oriented so that each network ( 1a, 1b, 1c, 1d, ..., 1N) respectively receives a part (2a, 2b, 2c, 2d, ..., 2N) of the light beam transmitted by the input slot, angularly diffracts said portion of the beam input as a function of the wavelength on a spectral domain and forms a spectrum ([λ ₃ , ..., λ _3Μ ],

on the matrix detector (4), each spectrum ([λ ₃ ι, ..., λ _3Μ ], [λ _Μ , .. Μ], ---, [λ _Ν ι, ..., λΝΜ]) extending along a distinct line of photodetectors (14a, 14b, 14c, 14d, ..., 14N) respectively for each diffraction grating (1a, 1b, 1c, 1d, ..., 1N). Spectrograph according to Claim 1, characterized in that the height of the entrance slit (3) is limited so that, the distance between two adjacent lines of the photodetector being equal to d, the spectral image of the slit on the detector by a network (1a, 1b, 1c, 1d, ..., 1N) extends over a half-height h 'less than d. Spectrograph according to claim 1 or 2, characterized in that it comprises two diffraction gratings, a first grating (1a) forming a first spectrum extending over the spectral domain [λ ₃ , ..., λ _3Μ ] on a first line (14a) of photodetectors (24a) and a second network forming a second spectrum extending over the spectral domain [λ _Μ , ..., λbM] on a second line (14b) of photodetectors (24b), the second spectrum being offset from the first spectrum by a distance d in a direction transverse to the spectral diffraction direction in the plane of the detector (4). 4. Spectrograph according to one of claims 1 to 2 characterized in that it comprises four diffraction gratings (1a, 1b, 1c, 1d). 5. Spectrograph according to one of claims 1 to 4 characterized in that the diffraction gratings (1a, 1b, 1c, 1d, ..., 1N) are corrected defocusing aberrations, coma and astigmatism. 6. Spectrograph according to one of claims 1 to 5 characterized in that the diffraction gratings (1a, 1b, 1c, 1d ... 1 N) have the same radius of curvature. 7. Spectrograph according to one of claims 1 to 6 characterized in that two adjacent lines (14a, 14b) of photodetectors of the detector are separated by a non-photosensitive area of a width e less than 600 microns. 8. Spectrograph according to one of claims 1 to 7 characterized in that the diffraction efficiency of at least one diffraction grating (1a, 1b, 1c, 1d, ..., 1 N) is optimized on a domain spectral. 9. Spectrograph according to one of claims 1 to 8 characterized in that the spectral domains [λ ₃ , ..., λ _3Μ ] and [λ _Μ , ..., λbM] are distinct. 10. Spectrograph according to one of claims 1 to 9 characterized in that the spectral domains [λ ₃ , ..., λ _3Μ ] and [λ _Μ , ..., λbM] have a spectral overlap zone.