FR2847668A1 - High resolution spectrometer for application to variable spectral domains, comprises point source sending beam through first optical element, a grism, a second optical element and means of detection - Google Patents

Abstract

The axial spectrometer has a point source (3,31,...39) which sends a luminous beam (2,21,...29) to a first optical element (1). The first optical element collimates the beam to a grism (4) with a dispersion axis and to a second optical element (5) which focuses the dispersed beams (9,91,92,...99) on to a means of detection (6). Each point source defines a central wavelength in the focal image plan of the second optical element Independent claims are also included for the following: 1) A Raman type spectroscope which includes the high resolution axial spectroscope 2) A photoluminescence type spectroscope which includes the high resolution axial spectroscope 3) A fluorescence type spectroscope which includes the high resolution axial spectroscope

Description

I The present invention relates to a high resolution axial spectrometer

  spatial and spectral whose spectral domain of observation is variable.

  Fixed dispersive element spectrometers have many advantages, particularly with regard to the compactness, the simplicity of the device and therefore its mechanical stability. These spectrometers are therefore naturally used in the majority of assemblies allowing the study of the spectral composition of the waves emitted by light sources (irradiated samples, Raman, Fluorescence, Photoluminescence,

  .). Among these spectrometers, the axial spectrometer has the additional qualities of an imaging spectrometer with high spatial resolution. This spatial resolution..DTD: authorizes the positioning at its entry of a large number of source points on an axis perpendicular to the axis of dispersion. The simultaneous obtaining of the spectra of each of these source points is thus made possible on the image plane of the spectrometer.

  For this, the axial spectrometer uses the qualities of grism (or Carpenter's prism). It is a dispersive element which is produced by applying a transmission network to the hypotenuse of a right angle prism. The role of the prism is then to compensate for a wavelength the deviation produced by the network. A spread of the spectrum is thus obtained on either side of the direction defined by the incident beam. In general, the non-deviated wavelength is close to the middle of the spectrum. It is said to be central. The axial spectrometer allows observation in the axis of the incident beam, the latter penetrating perpendicularly into the prism. The use of grism makes it possible to greatly improve the image qualities of the spectrometer. Indeed, the presence of the prism in front of the grating greatly reduces the aberrations at the wavelength

not deflected.

  Figure 1 is a schematic representation of an axial spectrometer as used in a device of the prior art. The axial spectrometer has a first objective 1, a collimating optic, which collimates the light 2 emitted by a light source 3 and coming from the input of the

  spectrometer to direct it towards a grisme 4 having an axis of dispersion. A second objective 5, a focusing optics makes the image of the spectrum on a detection system 6 which can be a linear or matrix detector of the charge transfer sensor (CCD) type.

  However, the major intrinsic drawback to this spectrometer lies in the fact that the spectral range of observation is frozen. A spectrometer with a fixed dispersive element in fact allows only a predetermined wavelength range X to be observed. The parameters of the spectrometer are calculated for a non-deviated wavelength, called the central wavelength, and a spectral domain which for a detector of width d along the dispersion axis is approximated by the following relation: Xi = (nd 1) x cos (A) Nxf o Xf is the maximum wavelength observed, Xi the wavelength

  minimum observed, N the number of lines / mm of the grating 7, f the focal length of the objective and At the angle at the apex of the prism 8 and n the refractive index of the material constituting the prism.

  The axial spectrometer of the prior art is therefore designed for a

  precise application, which makes it very versatile.

  The objective of the present invention is to provide an axial spectrometer which is simple in its design and in its operating mode, rapid and economical, making it possible to vary the spectral range of observation and to measure an object with high spatial and spectral resolution, in increasing the

  linear dispersion of the spectrometer for a given spectral range.

  Such a spectrometer is particularly useful for forensic analysis, analyzes of works of art, archeology and gemology. Such a spectrometer also allows online and real-time monitoring of chemical reactions in the laboratory or in facilities

industrial.

  To this end, the invention relates to an axial spectrometer comprising at least one source point sending a light beam on a first optical element having an object focal plane, each source point being placed in the object focal plane of the first optical element, said first element optical system 30 collimating said light beam (s) towards a lens having an axis of dispersion and a second optical element focusing said light beam (s) on detection means placed in the focal plane

  image of the second optical element.

  According to the invention, the source point or points can take different positions in the object focal plane of the first optical element, each position

  a source point defining a central wavelength.

  In different embodiments, the present invention also relates to the following characteristics which must be considered in isolation or according to all their technically possible combinations: - the axial spectrometer comprises means for moving a source point in the focal plane object of the first optical element, - the axial spectrometer comprises a set of source points placed in the object focal plane of the first optical element, - the set of source points forms an NxN matrix of sources Si] (i, j = 1 to N), - only the sources Sij with i = j are sources emitting a light beam, - the detection means comprise a charge transfer sensor (CCD), - the detection means comprise a single-channel detector, - the first and second optical elements comprise objectives ,

  - the source points are cores of optical fibers.

  The invention also relates to a Raman spectroscopy or fluorescence spectroscopy device for analyzing a sample. According to the invention, said device comprises an axial spectrometer as described above. The invention finally relates to a photoluminescence spectroscopy device. According to the invention, this device comprises a spectrometer

  axial as described above.

  The invention can allow the production of spectral images.

  In different possible embodiments, the invention will be described in more detail with reference to the accompanying drawings in which: - Figure 1 is a schematic representation of an axial spectrometer of the prior art; - Figure 2 is a schematic representation of a screen on which is incident, normally at the entrance face of the prism, a light beam; - Figure 3 shows a block diagram (Fig. 3a) and an example of implementation (Fig. 3b-c) of an axial spectrometer in an embodiment of the present invention. Fig. 3b) shows said spectrometer with its tubular envelope and FIG. 3c) schematically represents a section along the axis A - A of said axial spectrometer; FIG. 4 is an exemplary implementation of the invention showing the variation of the spectral range of observation (in nm) as a function of the angle of incidence (in degrees) of the light beam on the dimming of the axial spectrometer; - Figure 5 is a second example of implementation of the invention showing the simultaneous acquisition on the image plane of the axial spectrometer of spectra in different spectral observation domains; - Figure 6 is a third example of implementation of the invention showing the simultaneous acquisition on the image plane of the axial spectrometer of a

  reference spectrum and a measured spectrum.

  The axial spectrometer as shown in FIG. 3a) comprises at least one light source point 3 emitting a beam 2. These source points 3, 31, ..., 39 are, for example, optical fibers or slits. The emitted light beam (s) 2, 21, ..., 29 are sent to a first optical element 1 having an object focal plane which collimates them on a lens 4 having an axis of dispersion. At the end of the lens 4, a second optical element 5 20 focuses the said scattered light beam (s) 9, 91, ...., 99 on means

  detection 6 placed in the image focal plane of the second optical element 5.

  In one embodiment, the detection means 6 comprise a charge transfer sensor (CCD) comprising a matrix of pixels.

  Each point of the detector is identified by a pair of 25 coordinates (x, y). The intensity signals ls (x, y, XI) measured at each point for a given wavelength? J are sent to a computer. These signals are then processed digitally by software. In another embodiment, the detection means 6 comprise a single-channel detector, for example a photomultiplier. The first and second optical elements 1, 5 comprise either objectives or lenses. FIG. 3 b) shows an exemplary embodiment of an axial spectrometer according to the invention, shown with a protective tubular envelope 10. This envelope 10 is made, for example, of metal. Figure 3 c) schematically shows a cross section along the axis A - A of said spectrometer

axial.

  The source point (s) 3, 31, ..., 39 can take different positions in the object focal plane of the first optical element, each position

  from a source point 3, 31, ..., 39 defining a central wavelength.

  In one embodiment, the axial spectrometer comprises means for moving a source point 3, 31, ..., 39 in the focal plane object of the first optical element 1. By moving said source point, it is possible to make vary the angle of incidence of the light beam emitted relative to normal at the input face of the lens. This variation in the angle of incidence allows the spectral range observed in said axial spectrometer to be changed. These means for moving a source point include,

  for example, a mechanical element for translating a source.

  This characteristic opens many possibilities in particular for the measurement of the spectrum 22 studied with respect to a spectrum 21 (complex or

  simple) for reference or for calibration.

  A light flux whose spectrum 21 is taken as a reference can enter the spectrometer by a fiber 18 and be read on a line of the detector 17, the measured spectrum 22 being brought by another fiber 181 and read on another line 171. drifts produced by the common environment or by the spectrometer are translated in the same way on the reference spectrum and on the spectrum

  measured and have no effect on the measurement.

  On the contrary, the drift of the measured spectrum 22 with respect to the reference spectrum 21 may be the sign of a physical phenomenon that it is so

possible to measure.

  When the reference is a line, for example a laser line which may be that of the Raman excitation source, it is thus possible to extract with very good precision the spectral intervals between the reference line and each point of the spectrum measured. These intervals are often characteristic of the product analyzed. We thus get rid of the always possible drift of the excitation line. In another embodiment, said spectrometer comprises several source points 3, 31, ..., 39 which are placed in the object focal plane of the first optical element 1. Advantageously, the set of source points forms an N × N matrix from sources Sij (ij = 1 to N). It is then possible to choose a particular angle of incidence for a light beam by selecting a given emitting source (or set of sources) S1j, the selection of said source or sources being, for example, ensured by an optical router. The angle of incidence of each of the light beams emitted in the object focal plane of the first optical element can be independently varied. Thus not only can several spectra be observed simultaneously, but these spectra can be found in different spectral domains. In one embodiment, only the sources S1j with

  i = j are sources emitting a light beam.

  A theoretical approach has been developed to explain the variation of the wavelength X of a light beam 2 incident at an angle i on a lens 4, said angle i being identified with respect to the normal to the input face. a prism 8 of optical index n. The relation between the year. gle of incidence i and the angle of refraction r of the light beam in the prism, given by the law of SnellDescartes, is sin (i) = n sin (r). The angle of incidence on the network 7 of the beam can still be written (Ar), where A is the angle at the top of the prism. At the end of the gray 4, the wavelength Xc not deviated, called the central wavelength, then follows the following relationship: X.0 (r) = x [nxsin (A-r) -sinrA] N

  o N is the number of lines of the network 7 per unit of length (in mm).

  This relation can be rewritten to reveal the angle of incidence i of the light beam 2 on the input face of said dimming 4: C x [nx sin (A - (Arc sin (sin (i) / n))) - sin A] N This relation therefore shows that by varying the angle of incidence i by

  displacement of the emitting source in the focal plane of the first optical element 1, the central wavelength X is modified.

  The axial spectrometer also makes it possible to increase the linear dispersion and therefore the resolution of the spectrometer for a given spectral range of observation. The axial spectrometer according to the invention has

  therefore not only the qualities of a high spatial resolution imager but also a high spectral resolution. It therefore makes it possible to obtain a spectrometric image for the medical field or for the industrial field.

  The axial spectrometer as described above can advantageously be used in the field of medical imaging, Raman spectrometry and photoluminescence spectrometry.

  The axial spectrometer of the invention was the subject of several implementations presented in the following examples highlighting the quality of the results obtained:

Example 1

  Figure 4 shows a first example of implementation of such an axial spectrometer to obtain the variation of the spectral range of observation 10 initially comprised between 532 nm and 665 nm. The abscissa axis 11 represents the angle of incidence (in degrees) of a light beam emitted by a source on the height of the axial spectrometer. The angle of incidence is considered in relation to an axis normal to the entry face of the crane, L.e. that the angle of incidence O ′ corresponds to a light beam which normally penetrates the lens. The ordinate axis 12 represents the wavelength in nanometers (nm). A first curve 13 (diamond + solid line) shows the variation of the central wavelength, a second curve 14 (triangle + solid line) shows the variation of the maximum wavelength while a third curve 15 (square + solid line) represents the variation of the minimum wavelength in

  function of the angle of incidence of the light beam.

Example 2

  Figure 5 is a second example of implementation of the invention showing the simultaneous acquisition on the image plane 16 of the axial spectrometer of spectra 17, 171, ..., 175 in spectral domains of observation 25 - different. This acquisition is obtained by positioning along a diagonal of fiber cores 18, 181, ..., 185 in the object plane of the spectrometer 19. An embodiment of such a device is obtained by a set of source points comprising fiber hearts 18, 181, ..., 185 forming a matrix (6x6) of sources S1j (i, j = 1 to 6) in which only the 30 sources Si] with i = j are sources emitting a light bleam. The

  arrow 20 gives the direction of the dispersion axis.

  Thus, the invention allows the production of a compact, robust and stable spectrometer at high resolution. It does not include any mobile element and is therefore susceptible of various applications.

  Example 3 FIG. 6 is a third example of implementation of the invention showing the simultaneous acquisition on the focal plane image 16 of the axial spectrometer of a reference spectrum 21 and of a measured spectrum 22. The reference spectra 21 and measured 22 are in different spectral observation ranges, respectively [Xi, M21 and [X3, 4]. This acquisition is obtained by a suitable positioning of two fiber cores 18, 181 in the focal plane

  object of the spectrometer 19. The arrow 20 gives the direction of dispersion.

  The reference spectrum 21 is obtained by the dispersion of the beam emitted by the reference source, ie the fiber core 18. The measured spectrum 22 is obtained by the dispersion of the beam emitted by the measurement source, ie the fiber core 181. FIG. 6 b) shows the drift AI, of the reference spectrum 21, obtained at an instant t1 with respect to the reference spectrum 21 'obtained at an instant t2. Processing means make it possible to compare the reference spectrum 21 measured at time t1 with the reference spectrum 21 'measured at time t2 and thus determine the drift AI, introduced by a phenomenon independent of the substance measured, for example mechanical instability of the spectrometer, drift of the laser. The drift AI, then represents the drift introduced by the system of

complete measurement.

  FIG. 6 c) shows the drift AX2 of the measured spectrum 22 obtained at time t1 with respect to the measured spectrum 22 'obtained at time t2. Processing means make it possible to compare the spectrum 22 measured at time t1 with the spectrum 22 'measured at time t2 and thus determine the observed drift Ak2

  between times t1 and t2. The drift A% 2 is the result of a drift AX1 independent of the substance measured and possibly of an additional drift AX2 'directly linked to the measurement.

  The information obtained on the drift AI, of the reference spectrum makes it possible to distinguish said drifts and thus to obtain exact information on the real drift of the spectrum measured by eliminating the always possible measurement errors introduced by the drift of the measurement system. himself.

Claims (12)

  1. Axial spectrometer comprising at least one source point (3, 31, ....   39) sending a light beam (2, 21, ..., 29) onto a first optical element (1) having an object focal plane, each source point source points (3, 31, ..., 39) being placed in the object focal plane of the first optical element (1), said first optical element (1) collimating said light beam (s) (2, 21, ..., 29) towards a lens (4) having a dispersion axis and a second optical element (5) focusing the said scattered light beam (s) (9, 91, ..., 99) on detection means (6) placed in the image focal plane of the second optical element (5), characterized in that the or the source points source points (3, 31, ..., 39) can take different positions in the object focal plane of the first optical element (1), each position of a source point source points (3, 31,   15 ..., 39) defining a central wavelength.   2. Axial spectrometer according to claim 1 characterized in that the axial spectrometer comprises means for moving a source point (3,   31, ..., 39) in the focal plane object of the first optical element (1).   3. Axial spectrometer according to claim 1 characterized in that the axial spectrometer comprises a set of source points (3, 31, ...., 39)   placed in the object focal plane of the first optical element (1).   4. Axial spectrometer according to claim 3 characterized in that all the source points (3, 31, ..., 39) form an NxN matrix of sources Sij (i, j = 1 to N).   5. Axial spectrometer according to claim 4 characterized in that only the sources Sij with i = j are sources emitting a beam bright (2, 21, ..., 29).   6. Axial spectrometer according to any one of claims 1 to 5   characterized in that the detection means (6) comprise a sensor with charge transfer (CCD).   7. Axial spectrometer according to any one of claims 1 to 5,   characterized in that the detection means (6) comprise a single-channel detector.   8. Axial spectrometer according to any one of claims 1 to 7   characterized in that the first (1) and second optical elements (5) include goals.   9. Axial spectrometer according to any one of claims 1 to 8   characterized in that the source points (3, 31, ..., 39) are hearts of optical fiber.   10. Raman spectroscopy device for analyzing a sample, characterized in that it comprises an axial spectrometer according to any one of claims 1 to 9.   11. Photoluminescence spectroscopy device characterized in that it comprises an axial spectrometer according to any one of claims 1 to 9.   12. Fluorescence spectrometry device characterized in that it   comprises an axial spectrometer according to any one of claims 1 to 9.