EP0028548A1 - Real-time optical correlation system - Google Patents

Real-time optical correlation system Download PDF

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Publication number
EP0028548A1
EP0028548A1 EP80401397A EP80401397A EP0028548A1 EP 0028548 A1 EP0028548 A1 EP 0028548A1 EP 80401397 A EP80401397 A EP 80401397A EP 80401397 A EP80401397 A EP 80401397A EP 0028548 A1 EP0028548 A1 EP 0028548A1
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Prior art keywords
objects
beams
correlation system
illuminating
photosensitive support
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German (de)
French (fr)
Inventor
Jean-Pierre Huignard
Jean-Pierre Herriau
Laurence Pichon
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Thales SA
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Thomson CSF SA
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    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06E—OPTICAL COMPUTING DEVICES; COMPUTING DEVICES USING OTHER RADIATIONS WITH SIMILAR PROPERTIES
    • G06E3/00—Devices not provided for in group G06E1/00, e.g. for processing analogue or hybrid data
    • G06E3/001—Analogue devices in which mathematical operations are carried out with the aid of optical or electro-optical elements
    • G06E3/005—Analogue devices in which mathematical operations are carried out with the aid of optical or electro-optical elements using electro-optical or opto-electronic means

Definitions

  • the invention relates to optical correlation systems making it possible to obtain the function of correlating one image with another.
  • Such systems allow for example to recognize a predetermined graphic in a composite pattern.
  • a known correlation method consists in recording on a photosensitive support a system of interference fringes representing the diffraction pattern provided by a lens which corresponds to two light beams on the path of which are interposed respectively two objects with non-uniform transparency, generally l object to analyze and a reference object.
  • This photosensitive support is read by a reading beam and a characteristic intensity distribution is obtained in the focal plane of a second lens in certain zones of the correlation product between the two objects.
  • the reference object carries a pattern that one seeks to find in the object to be analyzed
  • the image obtained is formed of peaks indicating the presence and the position of the reference pattern in the object to be analyzed.
  • This correlation method has been tested with interference fringe supports of photographic and thermoplastic types.
  • Such supports require a treatment, chemical or thermal, between the recording and reading phases, which implies a delay between the two operations.
  • they are generally not erasable. They therefore do not allow operation in real time.
  • the object of the invention is to use the correlation method described above in real-time applications such as automatic reading, target tracking, machine guidance, etc.
  • the correlation system according to the invention comprises a photosensitive support that is continuously recyclable, that is to say writable without development and erasable at will.
  • Particularly suitable materials are electro-optical materials such as bismuth-silicon oxide, in which spatial variations in light intensity can be converted in real time into spatial variations in refractive index. Registration is done by volume, and not on the surface, the optimal reading conditions are defined by Bragg's law which imposes a value distinct from the reading angle for each spatial frequency recorded. Knowing that the correlation peaks are linked to the recorded spatial frequencies, the invention provides an angular scanning of the read beam making it possible to explore the whole spectrum of the recorded spatial frequencies.
  • FIG. 1 represents a known optical system carrying out the recording trement of the algebraic sum of the Fourier transforms of two two-dimensional functions.
  • the two functions represent the transmittances of the two objects A and B lit by parallel beams FA and FB contiguous or not coming from the same coherent source.
  • Objects A and B are placed on either side of the optical axis z of a lens L 1 of focal distance fl, in the same plane PO perpendicular to this axis.
  • the focal plane PF of the lens L 1 an amplitude distribution proportional to the Fourier transform of the amplitude distribution in the object plane is obtained.
  • a photosensitive support 1 photographic or thermoplastic being placed in the plane PF records the superposition of fringe systems of intensity of different steps, the average step p being equal to where ⁇ 1 is the optical wavelength of the beams FA and F B and ⁇ 0 the half angle between the two beams which interfere.
  • the resulting intensity distribution along x, y axes of the PF plane is proportional to the square of the module of the Fourier transform of the amplitude distribution in the object plane PO.
  • the positions of the objects A and B in this plane are represented in figure 2.
  • the sign X expresses the correlation product.
  • k is the magnification ratio: .
  • FIG. 5 represents an embodiment of the invention. Part of the elements of the correlation device are common with those of Figures 1 and 3 and bear the same references.
  • the interference fringes resulting from the superposition of the beams F A and F B which illuminate the objects A and B, after the focusing effected by the lens L are recorded in a photosensitive plate 10 centered on the focal image plane PF of the lens L 1 and made of an electro-optical material polarized by a field electric obtained by means of a voltage source V. Its orientation is such that the electric field produces a transverse electro-optical effect. Spatial variations in light intensity existing in the PF plane are instantly reflected in the plate by spatial variations in refractive index, the interference planes being almost perpendicular to the direction of the applied electric field.
  • the thickness of the crystal must be equal to or greater than the width of the diffraction zone corresponding to the intersection of the diffraction ellipsoids of the two beams FA and FB whose dimensions depend on the numerical aperture of the lens L 1 .
  • a useful thickness can be defined, which is in any case clearly greater than the wavelength of the two beams so that the recording in the slide can be considered as three-dimensional. It is a superposition of arrays of surfaces.
  • these surfaces can be assimilated to planes perpendicular to the plane of the figure and whose pitch p and inclination f relative to the z axis depend on the angle of the interfering rays, the wavelength ⁇ 1 and the refractive index n of the plate 10.
  • the materials which can be used to form the strip 10 must be both photosensitive and electro-optical.
  • Bismuth-silicon oxide (Bi 12 Si 0 20 ) and bismuth-germanium oxide (Bi 12 Ge 020) are particularly suitable for the invention because they are very sensitive, in the range of wavelengths commonly used (visible and infrared spectrum) and it is known to obtain single crystals of sufficient dimensions (several cm) and having good optical qualities.
  • Other materials could also be suitable but generally do not have as good optical qualities: potassium niobate (KNb0 3 ), KTN, SBN.
  • the invention provides for angular scanning of the reading beam F L.
  • This is supplied by a laser 4 of low power and of wavelength ⁇ 2 chosen outside the range of wavelengths to which the material constituting the lamelO is sensitive.
  • the beam F L is deflected by a conventional acousto-optical or mechanical deflector 5 carrying out the angular scanning in a manner which will be detailed later.
  • the semi strip -transparent L is interposed on the path of the beams FA and F B and must be designed so as to allow these beams to pass. It inevitably introduces a phase shift, which is not annoying since it is identical for the two beams.
  • the orientation relative to the blade 10 of the reading beam, parallel is variable as a function of time and controlled by the deflector 5.
  • n is the refractive index of the plate 10 and d the thickness of the useful diffraction zone in the plate 10.
  • the detection of the correlation peaks is carried out by means 18 such as, for example: mosaic of detectors or vidicon tube connected to a television system.
  • the scanning beam scanning speed is advantageously equal to the television scanning speed.
  • the device was produced with a monocrystalline bismuth-silicon oxide blade of length 2 mm and thickness 1 mm polarized by a voltage V 0 of the order of 2000 V, which provides an electric field of the order of 10 kV / cm 1 , the wavelength of the illumination beams ⁇ , was 0.5 ⁇ m, which corresponds to good sensitivity of the crystal.
  • the focal length of the lens L 1 was: 30 cm and that of the lens L 2 : 10 cm.
  • the magnification k was therefore equal to 0.4.
  • the objects were 2 cm x 2 cm slides.
  • the extent of each zone II and III was thus 0.8 x 0.8 cm, observable with a vidicon tube whose diameter is typically 1.5 cm.
  • a semiconductor laser with a wavelength of 0.8 ⁇ m can also be used.
  • FIG. 5 admits numerous variants, in particular as to the means supplying the beams F A , F B , F L , to the means for detecting the correlation peaks obtained in the plane P and at the respective location of the different elements optical.
  • FIG. 6 represents an alternative embodiment concerning the means supplying the bundles F A and F B. It avoids the use of a lens L 1 with a large aperture. In fact, according to the previous embodiment, the width of the objects being typically 2 or 3 cm and the distance between their centers at least equal to this value, the necessary diameter of the lens L 1 reaches close to 10 cm.
  • the lens L 1 is replaced by two lenses LA and L B , smaller since their dimensions correspond to those of objects A and B, and whose optical axes are merged respectively with the axes of the beams FA and F B which are no longer parallel but each form with respect to the z axis an angle ⁇ ⁇ o, which remains unchanged after the lenses.
  • the beams FA and F B come from a single beam F delivered by a laser 7, argon for example, after widening by a widener 13 and separation and return by mirrors 14, 15, 16, 17.
  • Objects A and B are centered with respect to the respective axes of the two beams.
  • the correlation system is represented in the case of its application to the tracking of targets: Object A is the reference object.
  • Object B has a variable pattern. It consists of an electro-optical modulator controlled by a signal S from, for example, a television camera aiming at the object to be pursued.
  • the correlation system allows the detection of the coincidence between the target landscape and the fixed landscape.
  • the illumination due to this reference beam creates a first variation of index which is not spatially modulated, to which are added the variations due to interference systems due to the illumination beams of objects A and B. additional interference is formed but it can be ensured, by suitably choosing the inclination of the reference beam, that the reflected rays which result therefrom are clearly outside the zones examined, centered around I and J.
  • An exemplary embodiment of system in which a constant level of index modulation is created is shown in FIG. 7.
  • the reference beam FR comes from the same source 7 as the beams F A and F B.
  • a semi-reflecting plate 8 and a mirror 9 make it possible to separate the beam F R.
  • the beams F A F B on the one hand and F R on the other hand are widened by means of wideners 11 and 12.
  • the rest of the system is similar to that of FIG. 5 or of one of the variants thereof. .

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Theoretical Computer Science (AREA)
  • Mathematical Physics (AREA)
  • Nonlinear Science (AREA)
  • Optics & Photonics (AREA)
  • General Physics & Mathematics (AREA)
  • Holo Graphy (AREA)
  • Optical Recording Or Reproduction (AREA)

Abstract

L'invention concerne un système de corrélation fournissant la fonction de corrélation de deux objets (A et B) éclairés en lumière cohérente, en utilisant le principe de la double diffraction. Le système de corrélation selon l'invention se caractérise essentiellement en ce qu'il utilise comme milieu d'enregistrement une lame (10) de matériau recyclable, c'est à dire inscriptible et effacable à volonté, tel que l'oxyde de bismuth-silicium. L'enregistrement réalisé dans la lame par la superposition des faisceaux d'éclairement des objets est lu par un faisceau (FL) subissant un balayage angulaire permettant d'optimiser le rendement de diffraction pour tous les pics de corrélation dans un plan d'observation. Application à la reconnaissance de formes, à la poursuite de cibles.The invention relates to a correlation system providing the correlation function of two objects (A and B) illuminated in coherent light, using the principle of double diffraction. The correlation system according to the invention is essentially characterized in that it uses as recording medium a slide (10) of recyclable material, that is to say writable and erasable at will, such as bismuth oxide- silicon. The recording made in the slide by the superposition of the beams of illumination of the objects is read by a beam (FL) undergoing an angular scanning making it possible to optimize the diffraction efficiency for all the correlation peaks in an observation plane. Application to pattern recognition, the pursuit of targets.

Description

L'invention se rapporte aux systèmes de corrélation optique permettant d'obtenir la fonction de corrélation d'une image par une autre. De tels systèmes permettent par exemple de reconnaître un graphisme prédéterminé dans un motif composite.The invention relates to optical correlation systems making it possible to obtain the function of correlating one image with another. Such systems allow for example to recognize a predetermined graphic in a composite pattern.

Une méthode de corrélation connue consiste à enregistrer sur un support photosensible un système de franges d'interférences représentant la figure de diffraction fournie par une lentille qui correspond à deux faisceaux lumineux sur le trajet desquels sont interposés respectivement deux objets à transparence non uniforme, généralement l'objet à analyser et un objet de référence. Ce support photosensible est lu par un faisceau de lecture et on obtient, dans le plan focal d'une seconde lentille une répartition d'intensité caractéristique dans certaines zones du produit de corrélation entre les deux objets. Dans le cas où l'objet de référence porte un motif que l'on cherche à retrouver dans l'objet à analyser, l'image obtenue est formée de pics indiquant la présence et la position du motif de référence dans l'objet à analyser. Cette méthode de corrélation a été éprouvée avec des supports de franges d'interférences de types photographiques et thermoplastiques.A known correlation method consists in recording on a photosensitive support a system of interference fringes representing the diffraction pattern provided by a lens which corresponds to two light beams on the path of which are interposed respectively two objects with non-uniform transparency, generally l object to analyze and a reference object. This photosensitive support is read by a reading beam and a characteristic intensity distribution is obtained in the focal plane of a second lens in certain zones of the correlation product between the two objects. In the case where the reference object carries a pattern that one seeks to find in the object to be analyzed, the image obtained is formed of peaks indicating the presence and the position of the reference pattern in the object to be analyzed. . This correlation method has been tested with interference fringe supports of photographic and thermoplastic types.

De tels supports nécessitent un traitement, chimique ou thermique, entre les phases d'enregistrement et de lecture, ce qui implique un délai entre les deux opérations. De plus, ils ne sont généralement pas effaçables. Ils ne permettent donc pas un fonctionnement en temps réel.Such supports require a treatment, chemical or thermal, between the recording and reading phases, which implies a delay between the two operations. In addition, they are generally not erasable. They therefore do not allow operation in real time.

L'invention a pour but d'utiliser la méthode de corrélation décrite ci-dessus dans des applications en temps réel telles que la lecture automatique, la poursuite de cibles, le guidage d'engins ... A cet effet, le système de corrélation selon l'invention comprend un support photosensible continument recyclable, c'est à dire inscriptible sans développement et effacable à volonté. Des matériaux particulièrement adaptés sont des matériaux électro-optiques tels que l'oxyde de bismuth-silicium, dans lesquels des variations spatiales d'intensité lumineuses peuvent être converties en temps réel en variations spatiales d'indice de réfraction. L'enregistrement se faisant en volume, et non en surface, les conditions optimales de lecture sont définies par la loi de Bragg qui impose une valeur distincte de l'angle de lecture pour chaque fréquence spatiale enregistrée. Sachant que les pics de corrélation sont liés aux fréquences spatiales enregistrées, l'invention prévoit un balayage angulaire du faisceau de lecture permettant d'explorer tout le spectre des fréquences spatiales enregistrées.The object of the invention is to use the correlation method described above in real-time applications such as automatic reading, target tracking, machine guidance, etc. To this end, the correlation system according to the invention comprises a photosensitive support that is continuously recyclable, that is to say writable without development and erasable at will. Particularly suitable materials are electro-optical materials such as bismuth-silicon oxide, in which spatial variations in light intensity can be converted in real time into spatial variations in refractive index. Registration is done by volume, and not on the surface, the optimal reading conditions are defined by Bragg's law which imposes a value distinct from the reading angle for each spatial frequency recorded. Knowing that the correlation peaks are linked to the recorded spatial frequencies, the invention provides an angular scanning of the read beam making it possible to explore the whole spectrum of the recorded spatial frequencies.

L'objet de la présente invention est un système de corrélation optique permettant d'obtenir la fonction de corrélation d'un premier objet par un deuxième, comprenant des moyens d'éclairement des objets par respectivement deux faisceaux cohérents, des premiers moyens de focalisation projetant dans un plan focal (PF) un éclairement représentatif de la somme algébrique des transformées de Fourier des amplitudes lumineuses transmises par les deux objets respectivement, un support photosensible enregistrant cet éclairement, d'autres moyens d'éclairement du support photosensible, des seconds moyens de focalisation projetant dans un plan focal (P) un éclairement représentatif de la transformée de Fourier de l'éclairement enregistré, et des moyens de détection des pics de corrélation situés dans une zone du plan focal (P) caractérisant la fonction de corrélation, caractérisé en ce que le support photosensible est constitué d'un matériau continument recyclable dans lequel l'enregistrement constitue un réseau tridimensionnel de franges et en ce que les autres moyens d'éclairement du support photosensible comprennent des moyens de balayage angulaire assurant un rendement de diffraction optimum successivement pour les différents points de la zone observée du plan (P):

  • D'autres caractéristiques et avantages de l'invention apparaîtront dans la description ci-après, en référence aux figures annexées où :
  • - Les figures 1 et 3 représentent un système de corrélation de type connu ;
  • - Les figures 2 et 4 sont des figures explicatives du fonctionnement du système représenté sur les figures 1 et 3 ;
  • - La figure 5 représente un mode de réalisation de l'invention ;
  • - Les figures 6 et 7 représentent des variantes de réalisation de l'invention.
The object of the present invention is an optical correlation system making it possible to obtain the correlation function of a first object by a second, comprising means for illuminating the objects by respectively two coherent beams, first focusing means projecting in a focal plane (PF) an illumination representative of the algebraic sum of the Fourier transforms of the light amplitudes transmitted by the two objects respectively, a photosensitive support recording this illumination, other means of illuminating the photosensitive support, second means of focusing projecting into a focal plane (P) an illumination representative of the Fourier transform of the recorded illumination, and means for detecting correlation peaks located in an area of the focal plane (P) characterizing the correlation function, characterized in that the photosensitive support consists of a continuously recyclable material in which the recording const itue a three-dimensional network of fringes and in that the other means for illuminating the photosensitive support include angular scanning means ensuring optimum diffraction efficiency successively for the different points of the observed area of the plane (P):
  • Other characteristics and advantages of the invention will appear in the description below, with reference to the appended figures where:
  • - Figures 1 and 3 show a known type of correlation system;
  • - Figures 2 and 4 are explanatory figures of the operation of the system shown in Figures 1 and 3;
  • - Figure 5 shows an embodiment of the invention;
  • - Figures 6 and 7 show alternative embodiments of the invention.

La figure 1 représente un système optique connu réalisant l'enregistrement de la somme algébrique des transformées de Fourier de deux fonctions bidimensionnelles. Les deux fonctions représentent les transmittances des deux objets A et B éclairés par des faisceaux parallèles FA et FB contigüs ou non issus d'une même source cohérente. Les objets A et B sont placés de part et d'autre de l'axe optique z d'une lentille L1 de distance focale fl, dans un même plan PO perpendiculaire à cet axe. Dans le plan focal PF de la lentille L1, on obtient une distribution d'amplitude proportionnelle à la transformée de Fourier de la répartition d'amplitude dans le plan objet. Un support photosensible 1 : photographique ou thermoplastique étant placé dans le plan PF enregistre la superposition de systèmes de franges d'intensité de pas différents, le pas moyen p étant égal à

Figure imgb0001
où λ1 est la longueur d'onde optique des faisceaux FA et FB et α0 le demi- angle entre les deux faisceaux qui interfèrent. La répartition d'intensité résultante selon des axes x,y du plan PF est proportionnelle au carré du module de la transformée de Fourier de la répartition d'amplitude dans le plan objet PO. Les positions des objets A et B dans ce plan sont représentés sur la figure 2. x , y étant les axes parallèles à x,y dans le plan PO, on appelera centres des deux objets les points de coordonnées respectives (a, o) et (- b, o), si bien que les transmittances des deux objets peuvent s'exprimer sous la forme : A(x - a, y ) et B(x + b, y ). Leurs transformées de Fourier peuvent s'écrire respectivement : TA e2πjax, TBe-2πjbx, si bien que la répartition d'intensité dans le plan PF s'écrit :
Figure imgb0002
 FIG. 1 represents a known optical system carrying out the recording trement of the algebraic sum of the Fourier transforms of two two-dimensional functions. The two functions represent the transmittances of the two objects A and B lit by parallel beams FA and FB contiguous or not coming from the same coherent source. Objects A and B are placed on either side of the optical axis z of a lens L 1 of focal distance fl, in the same plane PO perpendicular to this axis. In the focal plane PF of the lens L 1 , an amplitude distribution proportional to the Fourier transform of the amplitude distribution in the object plane is obtained. A photosensitive support 1: photographic or thermoplastic being placed in the plane PF records the superposition of fringe systems of intensity of different steps, the average step p being equal to
Figure imgb0001
 where λ 1 is the optical wavelength of the beams FA and F B and α 0 the half angle between the two beams which interfere. The resulting intensity distribution along x, y axes of the PF plane is proportional to the square of the module of the Fourier transform of the amplitude distribution in the object plane PO. The positions of the objects A and B in this plane are represented in figure 2. x, y being the axes parallel to x, y in the plane PO, we will call centers of the two objects the points of respective coordinates (a, o) and (- b, o), so that the transmittances of the two objects can be expressed in the form: A (x - a, y) and B (x + b, y). Their Fourier transforms can be written respectively: TA e 2πjax , TBe -2πjbx , so that the intensity distribution in the plane PF is written:
Figure imgb0002

Une fois l'enregistrement sur le support photosensible réalisé, celui-ci subit le traitement chimique ou thermique approprié puis est lu par le système optique représenté sur la figure 3. La lecture s'effectue par un faisceau FL parallèle cohérent éclairant le support photosensible 1 sous

Figure imgb0003
incidence normale. Les différents réseaux enregistrées diffractent le faisceau FL selon des angles θ qui dépendent des pas p : où λ2 est la longueur d'onde du faisceau FL.Once recording on the photosensitive directed support, it undergoes chemical treatment or appropriate heat then is read by the optical system shown in Figure 3. The reading is performed by a beam F L parallel coherent illuminating the photosensitive medium 1 penny
Figure imgb0003
 normal incidence. The different networks recorded diffract the beam FL at angles θ which depend on the steps p: where λ 2 is the wavelength of the beam F L.

Une nouvelle transformation Fourier est effectuée par une deuxième lentille L2 de distance focale f2. On obtient donc dans son plan focal P par rapport à des axes X,Y parallèles aux axes x,y une distribution d'intensité I(X,Y) égale à la somme de trois termes :

Figure imgb0004
Figure imgb0005
Figure imgb0006
 A new Fourier transformation is carried out by a second lens L 2 with focal distance f 2 . We therefore obtain in its focal plane P with respect to axes X, Y parallel to the axes x, y an intensity distribution I (X, Y) equal to the sum of three terms:
Figure imgb0004
Figure imgb0005
Figure imgb0006

Le signe Ⓧ exprime le produit de corrélation. k est le rapport de grandissement :

Figure imgb0007
. On obtient les produits de corrélation des deux fonctions A et B, centrés autour des points M : (k(a+b),o) et N : (-k(a+b),o).The sign Ⓧ expresses the correlation product. k is the magnification ratio:
Figure imgb0007
 . We obtain the correlation products of the two functions A and B, centered around the points M: (k (a + b), o) and N: (-k (a + b), o).

On a représenté sur la figure 4 les limites des images dans le plan P des trois termes de l'expression ci-dessus : I, II, III dans le cas où, dans le plan PO, les deux objets sont des carrés de côté 1. En fonction de la corrélation entre les deux objets, apparaissent dans le plan P, qui est celui de la figure, des pics d'intensité lumineuse dont la position est comprise dans les cadres représentés, de côté 2K1 et est caractéristique de la présence d'un même signal dans les deux objets. A titre d'exemple, un même motif en forme de croix, représenté dans le plan PO sur la figure 2, occupe dans les deux objets les positions respectives (xA,yA) et (-xB,-yB). A la présence de ce signal correspond dans le plan image P deux pics d'intensité PII et PIII symétriques par rapport à l'axe Y de coordonnées dans les axes X,Y: ± k(xA+xB), k(yA+yB). En raison de la présence du terme TI, dont l'image I est centrée sur l'intersection O des axes X,Y, il est préférable, pour éviter toute superposition des trois termes, que la distance des centres des objets : a+b soit supérieure à leur largeur 1. Bien entendu, tout ce qui a été dit pour la direction x serait valable également pour la direction y, dans le cas général où les centres des objets A et B ne sont pas situés sur l'axe x. Dans la description de l'invention qui suit, on considérera pour simplifier que a=b et que les objets A et B sont centrés sur l'axe x.The limits of the images in the plane P of the three terms of the expression above have been represented in FIG. 4: I, II, III in the case where, in the plane PO, the two objects are squares of side 1 Depending on the correlation between the two objects, appear in the plane P, which is that of the figure, peaks of light intensity whose position is included in the frames shown, on side 2K1 and is characteristic of the presence of 'same signal in both objects. By way of example, the same cross-shaped pattern, represented in the plane PO in FIG. 2, occupies in the two objects the respective positions (x A , y A ) and (-x B , -y B ). The presence of this signal corresponds in the image plane P to two peaks of intensity P II and P III symmetrical with respect to the axis Y of coordinates in the axes X, Y: ± k (x A + x B ), k (y A + y B ). Because of the presence of the term T I , whose image I is centered on the intersection O of the axes X, Y, it is preferable, to avoid any superposition of the three terms, that the distance from the centers of the objects: a + b is greater than their width 1. Of course, all that has been said for the direction x would also be valid for the direction y, in the general case where the centers of objects A and B are not located on the x axis . In the description of the invention which follows, it will be considered for simplicity that a = b and that the objects A and B are centered on the axis x.

La figure 5 représente un mode de réalisation de l'invention. Une partie des éléments du dispositif de corrélation sont communs avec ceux des figures 1 et 3 et portent les mêmes références. Les franges d'interférences résultant de la superposition des faisceaux FA et FB qui éclairent les objets A et B, après la focalisation opérée par la lentille L sont enregistrées dans une lame photosensible 10 centrée sur le plan focal image PF de la lentille L1 et constituée d'un matériau électro-optique polarisé par un champ électrique obtenu au moyen d'une source de tension V. Son orientation est telle que le champ électrique produit un effet électro-optique transverse. Les variations spatiales d'intensité lumineuse existant dans le plan PF se traduisent instantanément dans la lame par des variations spatiales d'indice de réfraction, les plans d'interférence étant quasi perpendiculaires à la direction du champ électrique appliqué. La modulation d'indice disparait dès que sa cause, c'est à dire la présence des objets A et B sur le trajet des faisceaux disparait. On obtient donc une inscription en temps réel, effacable à volonté. Pour obtenir toute l'information avec un maximum de résolution, il est nécessaire que l'épaisseur du cristal soit égale ou supérieure à la largeur de la zone de diffraction correspondant à l'intersection des ellipsoïdes de diffraction des deux faisceaux FA et FB dont les dimensions dépendent de l'ouverture numérique de la lentille L1. On peut définir une épaisseur utile, qui est de toute façon nettement supérieure à la longueur d'onde des deux faisceaux si bien que l'enregistrement dans la lame peut être considéré comme tridimensionnel. Il s'agit d'une superposition de réseaux de surfaces. Lorsque la largeur de la lame 10 dans la direction perpendiculaire au plan de la figure n'est pas trop grande, (typiquement du même ordre de grandeur que l'épaisseur), ces surfaces peuvent être assimilées à des plans perpendiculaires au plan de la figure et dont le pas p et l'inclinaison f par rapport à l'axe z dépendent de l'angle des rayons qui interférent, de la longueur d'onde λ1 et de l'indice de réfraction n de la lame 10.FIG. 5 represents an embodiment of the invention. Part of the elements of the correlation device are common with those of Figures 1 and 3 and bear the same references. The interference fringes resulting from the superposition of the beams F A and F B which illuminate the objects A and B, after the focusing effected by the lens L are recorded in a photosensitive plate 10 centered on the focal image plane PF of the lens L 1 and made of an electro-optical material polarized by a field electric obtained by means of a voltage source V. Its orientation is such that the electric field produces a transverse electro-optical effect. Spatial variations in light intensity existing in the PF plane are instantly reflected in the plate by spatial variations in refractive index, the interference planes being almost perpendicular to the direction of the applied electric field. The index modulation disappears as soon as its cause, that is to say the presence of objects A and B in the path of the beams disappears. We therefore obtain a real-time registration, erasable at will. To obtain all the information with maximum resolution, the thickness of the crystal must be equal to or greater than the width of the diffraction zone corresponding to the intersection of the diffraction ellipsoids of the two beams FA and FB whose dimensions depend on the numerical aperture of the lens L 1 . A useful thickness can be defined, which is in any case clearly greater than the wavelength of the two beams so that the recording in the slide can be considered as three-dimensional. It is a superposition of arrays of surfaces. When the width of the blade 10 in the direction perpendicular to the plane of the figure is not too large, (typically of the same order of magnitude as the thickness), these surfaces can be assimilated to planes perpendicular to the plane of the figure and whose pitch p and inclination f relative to the z axis depend on the angle of the interfering rays, the wavelength λ 1 and the refractive index n of the plate 10.

Les matériaux utilisables pour constituer la lames 10 doivent être à la fois photosensibles et électro-optiques. L'oxyde de bismuth-silicium (Bi12Si 020) et l'oxyde de bismuth-germanium (Bi12Ge 020) conviennent particulièrement à l'invention car ils sont très sensibles, dans la gamme des longueurs d'onde couramment utilisées (spectre visible et infra-rouge) et on sait obtenir des monocristaux de dimensions suffisantes (plusieurs cm) et ayant de bonnes qualités optiques. D'autres matériaux pouraient convenir également mais n'ont pas généralement d'aussi bonnes qualités optiques : le niobate de potassium (KNb03), le KTN, le SBN.The materials which can be used to form the strip 10 must be both photosensitive and electro-optical. Bismuth-silicon oxide (Bi 12 Si 0 20 ) and bismuth-germanium oxide (Bi 12 Ge 020) are particularly suitable for the invention because they are very sensitive, in the range of wavelengths commonly used (visible and infrared spectrum) and it is known to obtain single crystals of sufficient dimensions (several cm) and having good optical qualities. Other materials could also be suitable but generally do not have as good optical qualities: potassium niobate (KNb0 3 ), KTN, SBN.

Lors de la lecture, pour obtenir un rendement optimum dans l'un des ordres de diffraction, il convient de respecter la condition de Bragg qui définit, pour chaque système d'interférence, l'angle entre le faisceau parallèle de lecture et les plans de diffraction. Cette condition ne pouvant pas être réalisée simultanément pour tous les systèmes qui se superposent, l'invention prévoit un balayage angulaire du faisceau de lecture FL. Celui-ci est fourni par un laser 4 de faible puissance et de longueur d'onde λ2 choisie en dehors de la gamme des longueurs d'onde auxquelles est sensible le matériau constituant la lamelO. Le faisceau FL est défléchi par un déflecteur 5 classique acousto-optique ou mécanique réalisant le balayage angulaire d'une façon qui sera détaillé plus loin. Le faisceau FL est représenté sur la figure dans sa position moyenne, correspondant à un réseau de plans parallèles à z (ϕ = o) de pas

Figure imgb0008
Il traverse ensuite un élargisseur de faisceau 6 et est renvoyé par une lame semi-transparente L en direction de la lame 10. L'élargissement apporté par l'élargisseur 6 permet d'éclairer toute la zone enregistrée de la lame 10. La lame semi-transparente L s'interpose sur le trajet des faisceaux FA et FB et doit être conçue de façon à laisser passer ces faisceaux. Elle introduit inévitablement un déphasage, qui n'est pas génant car identique pour les deux faisceaux. L'orientation par rapport à la lame 10 du faisceau de lecture, parallèle, est variable en fonction du temps et commandée par le déflecteur 5. Après diffraction par la deuxième lentille L2 et traversée d'un filtre 2 et d'un polariseur 3, on obtient dans le plan focal P de la lentille L2 des pics de corrélation analogues à ceux obtenus par exemple avec une plaque photographique. Toutefois, à chaque instant, pour une orientation donnée, seuls sont obtenus avec un rendement maximum les points situés sur une droite perpendiculaire au plan de la figure et auxquels on peut associer une inclinaison ϕ et un pas p de réseau de plans dans la lame 10 pour lequel l'incidence e du faisceau par rapport aux plans est l'incidence de Bragg : défini par
Figure imgb0009
. Sont également obtenus avec un rendement réduit les points voisins pour lesquels l'incidence est comprise dans une gamme
Figure imgb0010
où n est l'indice de réfraction de la lame 10 et d l'épaisseur de la zone utile de diffraction dans la lame 10. Pour examiner la totalité de la zone III (ou II) centrée autour du point N (ou M), il faut balayer toute la plage angulaire correspondante. Tous les pics de corrélation apparaissent ainsi séquentiel- lement.When reading, to obtain an optimum efficiency in one of the diffraction orders, it is necessary to respect the Bragg condition which defines, for each interference system, the angle between the paral beam the reading line and the diffraction planes. Since this condition cannot be achieved simultaneously for all of the superimposed systems, the invention provides for angular scanning of the reading beam F L. This is supplied by a laser 4 of low power and of wavelength λ 2 chosen outside the range of wavelengths to which the material constituting the lamelO is sensitive. The beam F L is deflected by a conventional acousto-optical or mechanical deflector 5 carrying out the angular scanning in a manner which will be detailed later. The beam F L is represented in the figure in its average position, corresponding to a network of planes parallel to z (ϕ = o) of step
Figure imgb0008
 It then crosses a beam expander 6 and is returned by a semi-transparent strip L in the direction of the strip 10. The widening provided by the spreader 6 makes it possible to illuminate the entire recorded area of the strip 10. The semi strip -transparent L is interposed on the path of the beams FA and F B and must be designed so as to allow these beams to pass. It inevitably introduces a phase shift, which is not annoying since it is identical for the two beams. The orientation relative to the blade 10 of the reading beam, parallel, is variable as a function of time and controlled by the deflector 5. After diffraction by the second lens L 2 and crossing of a filter 2 and a polarizer 3 , one obtains in the focal plane P of the lens L 2 correlation peaks similar to those obtained for example with a photographic plate. However, at each instant, for a given orientation, only the points located on a line perpendicular to the plane of the figure are obtained with maximum efficiency and to which an inclination ϕ and a pitch p of network of planes in the blade 10 can be associated for which the incidence e of the beam with respect to the planes is the Bragg incidence: defined by
Figure imgb0009
 . Neighboring points for which the incidence is within a range are also obtained with reduced efficiency.
Figure imgb0010
 where n is the refractive index of the plate 10 and d the thickness of the useful diffraction zone in the plate 10. To examine the whole of the zone III (or II) centered around the point N (or M), it is necessary to scan the entire corresponding angular range. All the correlation peaks thus appear sequentially.

La détection des pics de corrélation s'effectue par des moyens 18 tels que, par exemple : mosaïque de détecteurs ou tube vidicon relié à un système de télévision. Dans ce dernier cas, la vitesse de balayage du faisceau de lecture est avantageusement égale à la vitesse de balayage de télévision.The detection of the correlation peaks is carried out by means 18 such as, for example: mosaic of detectors or vidicon tube connected to a television system. In the latter case, the scanning beam scanning speed is advantageously equal to the television scanning speed.

A titre d'exemple non limitatif, le dispositif a été réalisé avec une lame monocristalline d'oxyde de bismuth-silicium de longueur 2 mm et d'épaisseur 1 mm polarisée par une tension V0 de l'ordre de 2000 V, ce qui fournit un champ électrique de l'ordre de 10 kV/cm1 , la longueur d'onde des faisceaux d'éclairement λ, était de 0,5 µm, ce qui correspond à une bonne sensibilité du cristal. Le faisceau de lecture FL provenait d'un laser Hélium-Néon de quelques mW de puissance et de longueur d'onde λ2 = 0,6 µm.By way of nonlimiting example, the device was produced with a monocrystalline bismuth-silicon oxide blade of length 2 mm and thickness 1 mm polarized by a voltage V 0 of the order of 2000 V, which provides an electric field of the order of 10 kV / cm 1 , the wavelength of the illumination beams λ, was 0.5 μm, which corresponds to good sensitivity of the crystal. The reading beam F L came from a Helium-Neon laser of a few mW of power and wavelength λ 2 = 0.6 μm.

La distance focale de la lentille L1 était : 30 cm et celle de la lentille L2 : 10 cm. Le grandissement k était donc égal à 0,4.The focal length of the lens L 1 was: 30 cm and that of the lens L 2 : 10 cm. The magnification k was therefore equal to 0.4.

Les objets étaient des diapositives de dimensions 2 cm x 2 cm. L'étendue de chaque zone Il et III était ainsi 0,8 x 0,8 cm, observable avec un tube vidicon dont le diamètre est typiquement 1,5 cm. Au lieu d'utiliser un laser HeNe, on peut aussi utiliser un laser semi-conducteur de longueur d'onde 0,8 µm.The objects were 2 cm x 2 cm slides. The extent of each zone II and III was thus 0.8 x 0.8 cm, observable with a vidicon tube whose diameter is typically 1.5 cm. Instead of using a HeNe laser, a semiconductor laser with a wavelength of 0.8 µm can also be used.

Le système représenté sur la figure 5 admet de nombreuses variantes, notamment quant aux moyens fournissant les faisceaux FA, FB, FL, aux moyens de détection des pics de corrélation obtenus dans le plan P et à l'emplacement respectif des différents éléments optiques. La figure 6 représente une variante de réalisation concernant les moyens fournissant les faisceaux FA et FB. Elle permet d'éviter l'utilisation d'une lentille L1 de grande ouverture. En effet, selon le mode de réalisation précédent, la largeur des objets étant typiquement 2 ou 3 cm et la distance entre leurs centres au moins égale à cette valeur, le diamètre nécessaire de la lentille L1 atteint près de 10 cm. Selon la variante proposée, la lentille L1 est remplacée par deux lentilles LA et LB, plus petites puisque leurs dimensions correspondent à celles des objets A et B, et dont les axes optiques sont confondus respectivement avec les axes des faisceaux FA et FB qui ne sont plus parallèles mais forment chacun par rapport à l'axe z un angle ± αo, qui reste inchangé après les lentilles. Les faisceaux FA et FB proviennent d'un faisceau unique F délivré par un laser 7, à argon par exemple, après élargissement par un élargisseur 13 et séparation et renvoi par des miroirs 14, 15, 16, 17. Les objets A et B sont centrés par rapport aux axes respectifs des deux faisceaux. Le système de corrélation est représenté dans le cas de son application à la poursuite de cibles : L'objet A est l'objet de référence. Il est par exemple constitué par une diapositive représentant un paysage fixe. L'objet B porte un motif variable. Il est constitué d'un modulateur électro-optique commandé par un signal S issu, par exemple, d'une caméra de télévision visant l'objet à poursuivre. Le système de corrélation permet la détection de la coïncidence entre le paysage visé et le paysage fixe.The system represented in FIG. 5 admits numerous variants, in particular as to the means supplying the beams F A , F B , F L , to the means for detecting the correlation peaks obtained in the plane P and at the respective location of the different elements optical. FIG. 6 represents an alternative embodiment concerning the means supplying the bundles F A and F B. It avoids the use of a lens L 1 with a large aperture. In fact, according to the previous embodiment, the width of the objects being typically 2 or 3 cm and the distance between their centers at least equal to this value, the necessary diameter of the lens L 1 reaches close to 10 cm. According to the proposed variant, the lens L 1 is replaced by two lenses LA and L B , smaller since their dimensions correspond to those of objects A and B, and whose optical axes are merged respectively with the axes of the beams FA and F B which are no longer parallel but each form with respect to the z axis an angle ± αo, which remains unchanged after the lenses. The beams FA and F B come from a single beam F delivered by a laser 7, argon for example, after widening by a widener 13 and separation and return by mirrors 14, 15, 16, 17. Objects A and B are centered with respect to the respective axes of the two beams. The correlation system is represented in the case of its application to the tracking of targets: Object A is the reference object. It is for example constituted by a slide representing a fixed landscape. Object B has a variable pattern. It consists of an electro-optical modulator controlled by a signal S from, for example, a television camera aiming at the object to be pursued. The correlation system allows the detection of the coincidence between the target landscape and the fixed landscape.

Pour obtenir une meilleure linéarité dans la réponse du cristal électro-optique, il peut être intéressant de créer un niveau constant de modulation grâce à un faisceau lumineux de référence analogue à celui présent dans un système holographique classique. L'éclairement du à ce faisceau de -référence crée une première variation d'indice non modulée spatialement, à laquelle s'ajoutent les variations dues aux systèmes d'interférences dus aux faisceaux d'éclairement des objets A et B. Des systèmes d'interférences supplémentaires se forment mais on peut faire en sorte, en choisissant convenablement l'inclinaison du faisceau de référence, que les rayons réfléchis qui en résultent se trouvent nettement en dehors des zones examinées, centrées autour de I et J. Un exemple de réalisation de système dans lequel est créé un niveau constant de modulation d'indice est représenté sur la figure 7. Le faisceau de référence FR provient de la même source 7 que les faisceaux FA et FB. Une lame semi-réfléchissante 8 et un miroir 9 permettent de séparer le faisceau FR. Les faisceaux FA FB d'une part et FR d'autre part sont élargis au moyens d'élargisseurs 11 et 12. Le reste du système est semblable à celui de la figure 5 ou d'une des variantes de celui-ci.To obtain better linearity in the response of the electro-optical crystal, it may be advantageous to create a constant level of modulation by means of a reference light beam analogous to that present in a conventional holographic system. The illumination due to this reference beam creates a first variation of index which is not spatially modulated, to which are added the variations due to interference systems due to the illumination beams of objects A and B. additional interference is formed but it can be ensured, by suitably choosing the inclination of the reference beam, that the reflected rays which result therefrom are clearly outside the zones examined, centered around I and J. An exemplary embodiment of system in which a constant level of index modulation is created is shown in FIG. 7. The reference beam FR comes from the same source 7 as the beams F A and F B. A semi-reflecting plate 8 and a mirror 9 make it possible to separate the beam F R. The beams F A F B on the one hand and F R on the other hand are widened by means of wideners 11 and 12. The rest of the system is similar to that of FIG. 5 or of one of the variants thereof. .

Claims (8)

1. Système de corrélation optique permettant d'obtenir la fonction de corrélation d'un premier objet par un deuxième, comprenant des moyens d'éclairement des objets par respectivement deux faisceaux cohérents, des premiers moyens de focalisation projetant dans un premier plan focal (PF) un éclairement représentatif de la somme algébrique des transformées de Fourier des amplitudes lumineuses transmises par les deux objets respectivement, un support photosensible, situé dans le premier plan focal (PF), enregistrant en temps réel cet éclairement, et constitué d'un matériau continûment recyclable dans lequel l'enregistrement constitue un réseau tridimensionnel de franges, d'autres moyens d'éclairement du support photosensible, des seconds moyens de focalisation projetant dans un second plan focal (P) un éclairement représentatif de la transformée de Fourier de l'éclairement enregistré, et des moyens de détection des pics de corrélation situés dans une zone du second plan focal (P) et caractérisant la fonction de corrélation, caractérisé en ce que les autres moyens d'éclairement du support photosensible comprennent des moyens de balayage angulaire assurant un rendement de diffraction optimum successivement pour les différents points de la zone observée du second plan focal (P).1. Optical correlation system making it possible to obtain the correlation function of a first object by a second, comprising means for illuminating the objects by respectively two coherent beams, first focusing means projecting into a first focal plane (PF ) an illumination representative of the algebraic sum of the Fourier transforms of the light amplitudes transmitted by the two objects respectively, a photosensitive support, located in the first focal plane (PF), recording this illumination in real time, and consisting of a material continuously recyclable in which the recording constitutes a three-dimensional network of fringes, other means of illuminating the photosensitive support, second focusing means projecting into a second focal plane (P) an illumination representative of the Fourier transform of the illumination recorded, and means for detecting correlation peaks located in an area of the second focal plane (P) and charac testing the correlation function, characterized in that the other means for illuminating the photosensitive support include angular scanning means ensuring optimum diffraction efficiency successively for the different points of the observed area of the second focal plane (P). 2. Système de corrélation optique selon la revendication 1, caractérisé en ce que les moyens d'éclairement des objets comprennent un laser et des moyens optiques fournissant deux faisceaux parallèles à l'axe des lentilles de part et d'autre de cet axe, les objets étant respectivement centrés sur les axes des deux faisceaux.2. Optical correlation system according to claim 1, characterized in that the means for illuminating the objects comprise a laser and optical means providing two beams parallel to the axis of the lenses on either side of this axis, the objects being respectively centered on the axes of the two beams. 3. Système de corrélation optique selon la revendication l, caractérisé en ce que les moyens d'éclairement des objets comprennent un laser et des moyens de séparation du faisceau laser fournissant deux faisceaux parallèles dont les axes forment entre eux un angle prédéterminé et se coupent au voisinage du centre du support photosensible, les premiers moyens de focalisation étant constitués de deux lentilles sphériques dont les axes coïncident respectivement avec les axes des deux faisceaux et dont les foyers coïncident avec le point de rencontre de ces axes, les objets étant respectivement centrés sur les axes des deux faisceaux.3. Optical correlation system according to claim l, characterized in that the means for illuminating the objects comprise a laser and means for separating the laser beam providing two parallel beams whose axes form between them a predetermined angle and intersect at the vicinity of the center of the photosensitive support, the first focusing means being made up of two spherical lenses whose axes coincide respectively with the axes of the two beams and whose focal points coincide with the meeting point of these axes, the objects being respectively centered on the axes of the two beams. 4. Système de corrélation optique selon l'une quelconque dess revendications 1 à 3, caractérisé en ce que les autres moyens d'éclairement du support photosensible comprennent un laser.4. Optical correlation system according to any one of claims 1 to 3, characterized in that the other means for illuminating the photosensitive support comprise a laser. 5. Système de corrélation optique selon l'une quelconque des revendications 1 à 4, caractérisé en ce que le support photosensible est une lame monocristalline d'oxyde de bismuth-silicium.5. Optical correlation system according to any one of claims 1 to 4, characterized in that the photosensitive support is a monocrystalline plate of bismuth-silicon oxide. 6. Système de corrélation optique selon l'une quelconque des revendications 1 à 4, caractérisé en ce que le support photosensible est une lame monocristalline d'oxyde de bismuth-germanium.6. Optical correlation system according to any one of claims 1 to 4, characterized in that the photosensitive support is a monocrystalline plate of bismuth-germanium oxide. 7. Système de corrélation optique selon l'une quelconque des revendications 1 à 6, caractérisé en ce que le support photosensible est polarisé par un champ électrique obtenu au moyen d'une source de tension.7. Optical correlation system according to any one of claims 1 to 6, characterized in that the photosensitive support is polarized by an electric field obtained by means of a voltage source. 8. Système de corrélation selon l'une quelconque des revendications 1 à 7, caractérisé en ce qu'il comprend en outre des troisièmes moyens d'éclairement du support photosensible permettant de créer dans ce support photosensible un niveau constant de modulation, superposé au réseau tridimensionnel de franges correspondant à l'enregistrement de l'éclairement représentatif de la somme algébrique des transformées de Foùrier des amplitudes lumineuses transmises par les deux objets.8. Correlation system according to any one of claims 1 to 7, characterized in that it further comprises third means for illuminating the photosensitive support making it possible to create in this photosensitive support a constant level of modulation, superimposed on the network. three-dimensional fringes corresponding to the recording of the illumination representative of the algebraic sum of the Foùrier transforms of the light amplitudes transmitted by the two objects.
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FR2468947A1 (en) 1981-05-08
JPS5675618A (en) 1981-06-22
US4383734A (en) 1983-05-17

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