EP0000529B1 - Coupling device for an optical fibre - Google Patents

Coupling device for an optical fibre Download PDF

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Publication number
EP0000529B1
EP0000529B1 EP78100414A EP78100414A EP0000529B1 EP 0000529 B1 EP0000529 B1 EP 0000529B1 EP 78100414 A EP78100414 A EP 78100414A EP 78100414 A EP78100414 A EP 78100414A EP 0000529 B1 EP0000529 B1 EP 0000529B1
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EP
European Patent Office
Prior art keywords
axis
optical
fibre
light
adapter
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Expired
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EP78100414A
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German (de)
French (fr)
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EP0000529A1 (en
Inventor
Luc Jeunhomme
Jean-Paul Pocholle
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Alcatel Lucent SAS
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Compagnie Generale dElectricite SA
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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/42—Coupling light guides with opto-electronic elements
    • G02B6/4201—Packages, e.g. shape, construction, internal or external details
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/42—Coupling light guides with opto-electronic elements
    • G02B6/4201—Packages, e.g. shape, construction, internal or external details
    • G02B6/4256—Details of housings
    • G02B6/4257—Details of housings having a supporting carrier or a mounting substrate or a mounting plate
    • G02B6/4259—Details of housings having a supporting carrier or a mounting substrate or a mounting plate of the transparent type
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/42—Coupling light guides with opto-electronic elements
    • G02B6/4201—Packages, e.g. shape, construction, internal or external details
    • G02B6/4286—Optical modules with optical power monitoring
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/42—Coupling light guides with opto-electronic elements
    • G02B6/4201—Packages, e.g. shape, construction, internal or external details
    • G02B6/4287—Optical modules with tapping or launching means through the surface of the waveguide
    • G02B6/4289—Optical modules with tapping or launching means through the surface of the waveguide by inducing bending, microbending or macrobending, to the light guide

Definitions

  • the invention relates to a coupling device for optical fiber.
  • an optical fiber consists of a core with an optical index N surrounded by a sheath with an index n smaller than N and makes it possible to guide a light in the core which can, for example, be modulated for telecommunication purposes.
  • a coupling device allowing, without interrupting the optical fiber, either to introduce light into the fiber, or to derive a fraction of the light which it guides in order, for example, to have, in the vicinity of the point of diversion, information that the fiber carries further.
  • no truly effective device of this kind has been proposed. Such a device must in particular apply to multimode fibers. These fibers are those in the heart of which light can propagate in several distinct modes.
  • the change in propagation constant is obtained by inducing alternating curvatures in the fiber using two networks formed by two regular successions of hollows and projections etched in two blocks, the projections of a network being opposite each other's hollows. It is known in fact (Bell System Technical Journal 52, 1973, page 817) that, when the position of the axis of the fiber, its curvature, the index or the diameter of the core undergo fluctuations along the axis of propagation, there is an exchange of energy between different modes corresponding to different values of the propagation constant K.
  • the index adapter consists of a thin plate of a transparent material, one edge of which carries an undulation constituting a network ("grating") and is applied against the fiber so as to ensure periodic deformation of that -this.
  • the optical contact between this edge and the sheath of the fiber allows light to pass through this thin plate, in which it is directed towards a curved edge which reflects it and focuses it on a detector placed in contact with another edge.
  • This same device also makes it possible to introduce light into the fiber.
  • it is generally desired to introduce into the fiber as large a fraction as possible of the light produced by a light-emitting diode (LED).
  • LED light-emitting diode
  • the efficiency of such a light introducing device is very poor because the diode emits light from a emitting surface which is not small, and in a solid angle which is large. If we use, for example a lens which receives all the light emitted by the diode, and which concentrates it all on the surface of the section of the core of the fiber, most of this light is made with the axis of the fiber far too large an angle for this light to propagate through the fiber.
  • the object of the present invention is to provide a coupling device for optical fiber making it possible to obtain good coupling efficiency, in particular for coupling with a light emitting diode emitting light in a large solid angle, without requiring cutting the optical fiber.
  • Figure 1 shows a device according to the invention seen in section through a plane passing through its axis, the coupling means between the inside and the outside of the fiber not being shown.
  • Fig. 2 shows a view of the device of FIG. 1 in section through a plane perpendicular to its axis.
  • FIG. 3 represents a sectional view of a device for injecting light into an optical fiber, the section plane passing through the axis of this device.
  • FIG. 4 shows a view of the device of Figure 3 in section through a plane perpendicular to its axis.
  • FIG. 5 represents, in strong lines, a view of a coupling device according to the invention in section through a plane passing through its axis.
  • fine lines represent various lines and a hyperbola that can be traced in the section plane to allow a better understanding of the shape of the optical surface of the device.
  • FIG. 6 represents a view of a light extraction device according to the invention, with use of a reflecting optical surface, this device being cut by a plane passing through its axis.
  • FIG. 7 represents a view of a device for injecting and extracting light according to the invention, in section through a plane passing through its axis.
  • this device has an axis 2, shown in phantom, this axis also being that of a portion of straight optical fiber and / or the middle axis of a portion of corrugated fiber;
  • these coupling means may advantageously include means for coupling modes capable of creating alternating curvatures in a portion of the fiber and thus giving this portion an undulating shape oscillating around a mean axis. This makes it possible to couple modes with high propagation constants propagating in the core with modes with low propagation constants propagating in the sheath.
  • These mode coupling means are associated with an adapter of index 3 (FIGS. 1 and 2) consisting of a transparent medium whose index is not substantially less than. that of the sheath. This adapter makes it possible to couple the modes propagating in the sheath with light propagating in this transparent medium and constituting said set of rays.
  • optical surface is then constituted by a surface 5 of this index adapter.
  • the optical fiber is represented by a single line along the axis 2.
  • the index adapter has the shape of a solid cylinder, of revolution around the axis 2 and therefore surrounding the fiber .
  • This solid cylinder ends at one end with a cone of revolution around the same axis and forming a point towards the outside. It is the surface of this cone which constitutes said optical surface.
  • the mode coupling means which have just been indicated are not shown in FIGS. 1 and 2. They are analogous to the arrangements described in the Miller patent mentioned above. However, it has been found, according to the present invention, that a property, little exploited previously, could advantageously be used of the light which propagated in the index adapter either by exiting the sheath, or, conversely, by being able to enter it to form the previously mentioned modes.
  • This property is that these light rays all form substantially the same angle with the axis of the fiber. They therefore constitute a set of spokes of a particular type, comprising for example the spokes 4,6 8 and 10 of FIGS. 1 and 2. This set is different from a parallel beam because the rays are located in various planes passing through the axis.
  • This set is on the other hand different from a beam converging on the axis because it comprises rays passing through various points of the axis.
  • the rays of this set do not pass exactly through the axis, but only in the vicinity of it. There is therefore a difference between the real rays and those of a set of rays exactly passing through the axis. This difference is less than the radius of the fiber and does not matter for the use of light when considering what is happening at a distance from the fiber significantly greater than its diameter.
  • the previously mentioned optical surface preferably extends up to a distance from the fiber greater than ten times its diameter, so that the rays arriving through this surface are, at least in majority, deflected by this surface. practically in the same way as if their extension exactly met axis 2.
  • the angle (a) (fig. 1) that the radii of this set make with the axis 2 is not perfectly predetermined. It can for example undergo a variation of 1.5 ° in more or less. This variation is somewhat troublesome for the implementation of the invention.
  • the choice of a high index for the adapter also has the advantage that the angle (a) is increased, which avoids excessive length of the adapter.
  • the value of this angle in radians is approximately given by the formula (n) being the index of the adapter 3 and (n,) being the index of the core of the fiber.
  • the surface represented in FIG. 1 is a diopter, that is to say that it separates two media of different indices, that of the adapter (n) and that of the air and that it is crossed by the light.
  • a the angle
  • b the angle at the top
  • FIGS. 3 and following The above-mentioned mode coupling means are shown in FIGS. 3 and following, in which the optical fiber is shown with a very large diameter and with exaggerated deformations, so as to facilitate understanding of the drawing.
  • These means are notably studied in a conference by L. Jeun Subscribe and JP Pocholle "T Coupler for multimode optical fiber", (North Atlantic Treaty Organization, Advisory Group for Aerospace Research and Development, AGARD, 7 rue Ancelle 92200 NEUILLY SUR SEINE, France) .
  • the report of this conference can be obtained from ONERA 29 avenue de la Division Leclerc, 92 CHATILLON SOUS BAGNEUX, France and from the National Technical Information Service (NTIS) 5285 Port Royal Road, Springfield, VIRGINIA 22151, USA.
  • NTIS National Technical Information Service
  • the optical action of these means can be defined by two quantities: the pitch P of the regular succession of alternating curvatures, and the amplitude of the deformation. This amplitude is typically between 10 and 100 microns.
  • the length of the corrugated portion of the latter can be between 10 and 50 mm approximately, and must be followed by a long portion at least 10 mm in optical contact with the adapter.
  • this wavy portion must be in optical contact with the index adapter, and its length must in principle be sufficient for any ray coming from said optical surface arrives on this undulating portion, the latter having to extend downstream beyond the illuminated portion.
  • mode coupling means indicated above seem to be most advantageous, other means could be used, such as, for example, a regular succession of thinning and thickening of the fiber core.
  • this adapter could equally well be made of a hard, moldable transparent material with a high index.
  • the index adapter 3 consists of two parts 14 and 16 each having in section the shape of a semicircle so as to constitute the complete circle by bringing these two parts together with the aid of a pressure means such than a screw 12 (Fig. 3).
  • the combination of the two successions of projections and depressions is often called a "network".
  • the optical contact between the fiber and the adapter is improved by the use of a suitable transparent oil, of index intermediate between that.
  • the light injection device shown in Figures 3 and 4 includes an adapter having a diameter of 26 mm, and a length on the axis of 75 mm, including the conical part. This length is entirely occupied by the network.
  • a converging step lens 20 (Fresnel lens) is arranged coaxially with the adapter on the side of the cone. It has a diameter of 25.4 mm and a focal length of 10 mm. At the focal point of this lens beyond this is disposed the emitting surface of a light-emitting diode 22 of the usual type whose radiation diagram is close to Lambert's law.
  • the light extraction device shown in FIG. 6 includes an index adapter 30 similar to the previous one. Its diameter is 30 mm. It comprises a network extending from its rear face over a length of 44 mm, and extending forwards by a long zone of 16 mm in which the fiber 18 does not undergo deformation, the optical contact being preserved .
  • This adapter ends forwards by a conical convex optical surface 32 with a half angle at the top equal to This optical surface is metallized so that the light which reaches it from the fiber is reflected in the adapter by forming a beam parallel to the axis 2 which reaches the front face 34.
  • This front face constitutes a diopter convergent of a type well known to opticians, and which makes it possible to converge the beam leaving the adapter towards a receiving diode 36.
  • This converging diopter is eccentric relative to the axis 2, so as to allow the diode to be placed 36 outside the axis 2. It is thus possible not only not to cut the fiber 18, but also not to bend it outside the adapter,
  • a single index adapter comprising a single network, and provided at each of its front and rear ends a dioptric optical surface, an optical input surface 42 and an optical output surface 44.
  • These optical surfaces can be conical. They must then each be associated with a converging lens, respectively 46 and 48, if it is desired to make a coupling with elements of small dimensions, respectively a light-emitting diode 50 and a receiving diode 52.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Optical Couplings Of Light Guides (AREA)
  • Diffracting Gratings Or Hologram Optical Elements (AREA)

Description

L'invention concerne un dispositif de couplage pour fibre optique.The invention relates to a coupling device for optical fiber.

On sait qu'une fibre optique est constituée d'un coeur d'indice optique N entouré par une gaine d'indice n plus petit que N et permet de guider dans le coeur une lumière qui peut par exemple être modulée à des fins de télécommunication. Il y a alors intérêt à disposer d'un dispositif de couplage permettant, sans interrompre la fibre optique, soit d'introduire de la lumière dans la fibre, soit de dériver une fraction de la lumière qu'elle guide pour pouvoir par exemple disposer, au voisinage du point de dérivation, de l'information que la fibre transporte plus loin. Malgré ce besoin évident aucun dispositif de ce genre vraiment efficace a été proposé. Un tel dispositif doit notamment s'appliquer aux fibres multimodes. Ces fibres sont celles dans le coeur desquelles la lumière peut se propager selon plusieurs modes distincts. Ce sont les seules dont l'utilisation industrielle soit pratiquement envisagée en raison de. leur diamètre relativement grand (0,15 mm par exemple avec la gaine). Le diamètre d du coeur vérifie la relation d.f. racine de (N2-n21 supérieur à 0,7656.c, f étant la fréquence de la lumière utilisée et c.la vitesse de propagation de la lumière dans le vide.It is known that an optical fiber consists of a core with an optical index N surrounded by a sheath with an index n smaller than N and makes it possible to guide a light in the core which can, for example, be modulated for telecommunication purposes. . There is then an advantage in having a coupling device allowing, without interrupting the optical fiber, either to introduce light into the fiber, or to derive a fraction of the light which it guides in order, for example, to have, in the vicinity of the point of diversion, information that the fiber carries further. Despite this obvious need, no truly effective device of this kind has been proposed. Such a device must in particular apply to multimode fibers. These fibers are those in the heart of which light can propagate in several distinct modes. These are the only ones whose industrial use is practically envisaged due to. their relatively large diameter (0.15 mm for example with the sheath). The diameter d of the heart verifies the relation df root of (N 2- n 2 1 greater than 0.7656.c, f being the frequency of the light used and c. The speed of propagation of light in a vacuum.

Un dispositif de couplage connu est décrit dans la publication US-A-3 931 518 (Miller) et dans la publication FR-A-2 334 125 (Compagnie Générale d'Electricité). Selon ces publications, dans le but d'extraire de la lumière, on réalise préalablement un couplage entre des modes à forte constante de propagation se propageant dans le coeur et des modes à faible constante de propagation qui ne peuvent se propager que dans la gaine.A known coupling device is described in publication US-A-3,931,518 (Miller) and in publication FR-A-2,334,125 (Compagnie Générale d'Electricite). According to these publications, in order to extract light, a coupling is carried out beforehand between modes with a high propagation constant propagating in the core and modes with a low propagation constant which can only propagate in the sheath.

Le changement de constante de propaga-' tion est obtenu en induisant des courbures alternées dans la fibre à l'aide de deux réseaux constitués par deux successions régulières de creux et de saillies gravés dans deux blocs, les saillies d'un réseau étant en regard des creux de l'autre. On sait en effet (Bell System Technical Journal 52, 1973, page 817) que, lorsque la position de l'axe de la fibre, sa courbure, l'indice ou le diamètre du coeur subissent des fluctuations le long de l'axe de propagation, il y a échange d'énergie entre différents modes correspondant différentes valeurs de la constante de propagation K.The change in propagation constant is obtained by inducing alternating curvatures in the fiber using two networks formed by two regular successions of hollows and projections etched in two blocks, the projections of a network being opposite each other's hollows. It is known in fact (Bell System Technical Journal 52, 1973, page 817) that, when the position of the axis of the fiber, its curvature, the index or the diameter of the core undergo fluctuations along the axis of propagation, there is an exchange of energy between different modes corresponding to different values of the propagation constant K.

Plus précisément, si le défaut ainsi introduit est sinusoïdal avec une fréquence spatiale P en radians par unité de longueur, l'échange d'énergie se fait entre deux modes de constante de propagation K 1 et K2 tels que, K 1 - K2 = P, cet échange pouvant se faire dans les deux sens (W. J. Stewart: "Mode Conversion due to periodic distortions of the fiber axis" Optical Fibre Communications Conference, 16 au 18 Septembre 1975).More precisely, if the fault thus introduced is sinusoidal with a spatial frequency P in radians per unit of length, the energy exchange takes place between two modes of propagation constant K 1 and K2 such that, K 1 - K2 = P , this exchange can be done in both directions (WJ Stewart: "Mode Conversion due to periodic distortions of the fiber axis" Optical Fiber Communications Conference, September 16 to 18, 1975).

Les limites supérieure KM et inférieure Km de la constante de propagation K des modes susceptibles de subsister dans le coeur sont données. par les relations:

  • c. KM = 6,283. f.N.
  • c. Km = 6,283. f.n,

c étant la vitesse de la lumière dans le vide et f la fréquence de la lumière utilisée. La dérivation d'une' fraction significative de l'énergie transportée résulte d'une part du fait que l'amplitude de la déformation induite dans la fibre est suffisamment importante et d'autre part du fait que la succession des courbures alternées induit, dans la lumière se propageant dans le coeur de la fibre, une succession de transpositions de mode en nombre suffisant pour obtenir une valeur finale de constante de propagation au-dessous de la limite inférieure de propagation dans le coeur. La lumière dont la constante de propagation a été ainsi diminuée se propage désormais dans la gaine. Elle peut être aisément extraite de celle-ci grâce à un "adaptateur d'indice", constitué par un milieu transparent d'indice optique au moins sensiblement égal à celui de la gaine et en contact optique avec la surface extérieure de celle-ci. L'indice de l'adaptateur ne doit pas descendre au-dessous de 0,8 fois celui de la gaine. Cet adaptateur est constitué, dans le brevet Miller précédemment cité, par le disque de couplage (coupling disk) 18. La lumière ayant pénétré dans l'adaptateur est transmise par celui-ci à un dispositif d'utilisation, tel qu'une diode photodétectrice.The upper KM and lower Km limits of the propagation constant K of the modes likely to remain in the core are given. by relationships:
  • vs. KM = 6.283. fN
  • vs. Km = 6.283. fn,

c being the speed of light in a vacuum and f the frequency of light used. The derivation of a significant fraction of the energy transported results on the one hand from the fact that the amplitude of the deformation induced in the fiber is sufficiently large and on the other hand from the fact that the succession of alternating curvatures induces, in the light propagating in the core of the fiber, a succession of mode transpositions in sufficient number to obtain a final value of propagation constant below the lower limit of propagation in the core. The light, the propagation constant of which has thus been reduced, now propagates in the sheath. It can be easily extracted therefrom by means of an "index adapter", constituted by a transparent medium with an optical index at least substantially equal to that of the sheath and in optical contact with the external surface of the latter. The index of the adapter must not drop below 0.8 times that of the sheath. This adapter is constituted, in the Miller patent cited above, by the coupling disk (coupling disk) 18. The light having penetrated into the adapter is transmitted by the latter to a device for use, such as a photodetector diode .

Un autre dispositif d'extraction de la lumière d'une fibre est décrit dans une communication de C. et W. J. Stewart "Directional coupler for single multimode optical fibre" au "deuxième colloque européen sur les transmissions par fibres optiques", Paris, 27―30 Septembre 1976 et publiée par le "Comité du colloque international sur les transmissions par fibres optiques", 11 rue Hamelin 75783 PARIS Cédex 16 (Cables and connections, Part 2, p 267-268).Another device for extracting light from a fiber is described in a communication by C. and WJ Stewart "Directional coupler for single multimode optical fiber" at the "second European conference on transmissions by optical fibers", Paris, 27― September 30, 1976 and published by the "Committee of the international symposium on fiber optic transmissions", 11 rue Hamelin 75783 PARIS Cédex 16 (Cables and connections, Part 2, p 267-268).

Dans ce dispositif, l'adaptateur d'indice est constitué par une plaque mince d'un materiau transparent dont un bord porte une ondulation constituant un réseau ("grating") et est appliqué contre la fibre de manière à assurer une déformation périodique de celle-ci. Le contact optique entre ce bord et la gaine de la fibre permet à la lumière de passer dans cette plaque mince, dans laquelle elle se dirige vers un bord incurvé qui la réfléchit et la focalise sur un détecteur placé au contact d'un autre bord.In this device, the index adapter consists of a thin plate of a transparent material, one edge of which carries an undulation constituting a network ("grating") and is applied against the fiber so as to ensure periodic deformation of that -this. The optical contact between this edge and the sheath of the fiber allows light to pass through this thin plate, in which it is directed towards a curved edge which reflects it and focuses it on a detector placed in contact with another edge.

Ces divers dispositifs d'extraction de lumière présentent l'inconvénient de n'envoyer sur l'organe d'utilisation (détecteur) qu'une petite fraction de la lumière circulant dans la fibre. Cette fraction est celle qui sort dans un demi plan partant de l'axe de la fibre, ou plus exactement dans le petit angle dièdre formé par deux demi plans partant de l'axe de la fibre et très voisins l'un de l'autre. Dans le cas du dispositif décrit dans le brevet Miller, le plan de sortie de la lumière est un plan passant par l'axe de la fibre et perpendiculaire à la surface du disque de couplage 18. Dans le cas du dispositif décrit dans l'article de Stewart, le plan dé sortie de la lumière est celui de la plaque mince: Un autre dispositif pour extraire de la lumière de la fibre est décrit dans la publication US-A-4 021 099 (Kawasaki et al). 11- utilise un système focalisant comportant un miroir et éventuellement une lentille et présente l'inconvénient d'obliger à couper la fibre.These various light extraction devices have the disadvantage of only sending a small fraction of the light circulating in the fiber to the use member (detector). This fraction is that which leaves in a half plane starting from the axis of the fiber, or more exactly in the small dihedral angle formed by two half planes starting from the axis of the fiber and very close to each other. In the case of the device described in the Miller patent, the light exit plane is a plane passing through the axis of the fiber and perpendicular to the surface of the coupling disc 18. In the case of the device described in the article from Stewart, the light exit plane is that of the thin plate: Another device for extracting light from the fiber is described in publication US-A-4,021,099 (Kawasaki et al). 11 - uses a focusing system comprising a mirror and possibly a lens and has the drawback of forcing the fiber to be cut.

Ce même dispositif permet aussi d'introduire de la lumière dans la fibre. Or on souhaite généralement introduire dans la fibre un fraction aussi importante que possible de la lumière produite par une diode électroluminescente (LED). Il apparaît alors que le rendement d'un tel dispositif d'introduction de lumière est très mauvais car la diode émet la lumière à partir d'une surface émettrice qui n'est pas petite, et dans un angle solide qui est grand. Si on utilise, par exemple une lentille qui reçoit toute la lumière émise par la diode, et qui la concentre toute sur la surface de la section du coeur de la fibre, la plus grande partie de cette lumière fait avec l'axe de la fibre un angle beaucoup trop grand pour que cette lumière puisse se propager dans la fibre. Aucun système optique classique ne permet d'éviter cet inconvénient, en raison d'une loi connue de l'optique, appelée parfois "théorème de conservation de l'étendue géométrique d'un faisceau". Cette loi dit qu'aucun système optique ne peut, sans perte de lumière, diminuer le produit de l'angle solide de divergence d'un faisceau par l'aire de la section de ce faisceau. Cette loi est notamment exprimée dans le livre "Principle of Optics" de M. Born et E. Wolf (3ème édition, Pergamon Press, p. 120, equation 54). Compte tenu des caractéristiques des diodes électroluminescentes connues, il en résulte que le rendement d'introduction de leur lumière dans une fibre est toujours mauvais. Le rendement d'introduction de la lumière dans la fibre peut être augmenté considérablement si on utilise la lumière d'un laser, qui présente une divergence très faible. Mais d'autres inconvénients apparaissent alors tels que le prix du laser, son encombrement, etc...This same device also makes it possible to introduce light into the fiber. However, it is generally desired to introduce into the fiber as large a fraction as possible of the light produced by a light-emitting diode (LED). It then appears that the efficiency of such a light introducing device is very poor because the diode emits light from a emitting surface which is not small, and in a solid angle which is large. If we use, for example a lens which receives all the light emitted by the diode, and which concentrates it all on the surface of the section of the core of the fiber, most of this light is made with the axis of the fiber far too large an angle for this light to propagate through the fiber. No conventional optical system makes it possible to avoid this drawback, due to a known law of optics, sometimes called "theorem of conservation of the geometric extent of a beam". This law says that no optical system can, without loss of light, reduce the product of the solid angle of divergence of a beam by the area of the section of this beam. This law is expressed in particular in the book "Principle of Optics" by M. Born and E. Wolf (3rd edition, Pergamon Press, p. 120, equation 54). Given the characteristics of known light-emitting diodes, it follows that the efficiency of introducing their light into a fiber is always poor. The efficiency of introducing light into the fiber can be considerably increased if the light from a laser is used, which has a very small divergence. But other drawbacks then appear such as the price of the laser, its size, etc.

Ces dispositifs connus d'introduction de lumière présentent de plus l'inconvénient d'obliger à couper la fibre.These known devices for introducing light also have the disadvantage of forcing the fiber to be cut.

La présente invention a pour but la réalisation d'un dispositif de couplage pour fibre optique permettant d'obtenir un bon rendement de couplage, notamment pour le couplage avec une diode électroluminescente émettant de la lumière dans un grand angle solide, sans obliger à couper la fibre optique.The object of the present invention is to provide a coupling device for optical fiber making it possible to obtain good coupling efficiency, in particular for coupling with a light emitting diode emitting light in a large solid angle, without requiring cutting the optical fiber.

Elle a pour objet un dispositif de couplage pour fibre optique, applicable à une fibre optique constituée d'un coeur entouré par une gaine d'indice optique plus petit, ce dispositif comportant

  • - des moyens de couplage de modes créant des courbures alternées dans une portion de la fibre et donnant ainsi à cette portion une forme ondulée oscillant autour d'un axe moyen, de manière à coupler des modes à fortes constantes de propagation se propageant dans le coeur avec des modes à faibles constantes de propagation se propageant dans la gaine,
  • - et un adaptateur d'indice constitué d'un milieu transparent dont l'indice n'est pas sensiblement inférieur à celui de la gaine, en contact optique avec la gaine, de manière à coupler lesdits modes se propageant dans la gaine avec de la luminère se propageant dans ce milieu transparent et constituant un ensemble de rayons, les rayons de cet ensemble passant près de l'axe de la fibre avec un écart inférieur au rayon de ladite fibre, et celà sur toute la longueur d'un segment de cet axe, ce segment se trouvant à l'intérieur de l'adaptateur, les rayons faisant tous un même angle prédéterminé avec cet axe, caractérisé par le fait qu'une surface optique de cet adaptateur d'indice présente une forme en pointe de révolution autour de cet axe, pour assurer un couplage entre cet ensemble de rayons et un faisceau lumineux parallèle à cet axe ou convergent en un point aligné sur cet axe.
It relates to a coupling device for optical fiber, applicable to an optical fiber consisting of a core surrounded by a sheath of smaller optical index, this device comprising
  • - means for coupling modes creating alternating curvatures in a portion of the fiber and thus giving this portion an undulating shape oscillating around a mean axis, so as to couple modes with high propagation constants propagating in the core with modes with low propagation constants propagating in the sheath,
  • - And an index adapter consisting of a transparent medium whose index is not significantly lower than that of the sheath, in optical contact with the sheath, so as to couple said modes propagating in the sheath with light propagating in this transparent medium and constituting a set of rays, the rays of this set passing near the axis of the fiber with a difference less than the radius of said fiber, and this over the entire length of a segment of this axis, this segment being inside the adapter, the rays all making the same predetermined angle with this axis, characterized in that an optical surface of this index adapter has a shape in point of revolution around of this axis, to ensure a coupling between this set of rays and a light beam parallel to this axis or converging at a point aligned on this axis.

A l'aide des figures schématiques 1 à 7 ci- jointes, on va décrire ci-après, à titre non limitatif, comment l'invention peut être mise en oeuvre. Les éléments qui se correspondent sur plusieurs de ces figures y sont désignés par les mêmes signes de référence. Les trajets des rayons lumineux sont représentés sur ces figures par des traits tiretés, avec des flèches montrant le sens de propagation de la lumière.Using the attached schematic figures 1 to 7, a description will be given below, without implied limitation, of how the invention can be implemented. The elements which correspond in several of these figures are designated therein by the same reference signs. The paths of the light rays are represented in these figures by dashed lines, with arrows showing the direction of propagation of the light.

La figure 1 représente un dispositif selon l'invention vu en coupe par un plan passant par son axe, les moyens de couplage entre l'intérieur et l'extérieur de la fibre n'étant pas représenté.Figure 1 shows a device according to the invention seen in section through a plane passing through its axis, the coupling means between the inside and the outside of the fiber not being shown.

La fig. 2 représente une vue du dispositif de la figure 1 en coupe par un plan perpendiculaire à son axe.Fig. 2 shows a view of the device of FIG. 1 in section through a plane perpendicular to its axis.

La figure 3 représente une vue en coupe d'un dispositif d'injection de lumière dans une fibre optique, le plan de coupe passant par l'axe de ce dispositif.FIG. 3 represents a sectional view of a device for injecting light into an optical fiber, the section plane passing through the axis of this device.

La figure 4 représente une vue du dispositif de la figure 3 en coupe par un plan perpendiculaire à son axe.4 shows a view of the device of Figure 3 in section through a plane perpendicular to its axis.

La figure 5 représente, en traits forts une vue d'un dispositif de couplage selon l'invention en coupe par un plan passant par son axe. Sur cette figure des traits fins représentent diverses droites et une hyperbole que l'on peut tracer dans le plan de coupe pour permettre de mieux comprendre la forme de la surface optique du dispositif.FIG. 5 represents, in strong lines, a view of a coupling device according to the invention in section through a plane passing through its axis. In this figure, fine lines represent various lines and a hyperbola that can be traced in the section plane to allow a better understanding of the shape of the optical surface of the device.

La figure 6 représente une vue d'un dispositif d'extraction de lumière selon l'invention, avec utilisation d'une surface optique réfléchissante, ce dispositif étant coupé par un plan passant par son axe.FIG. 6 represents a view of a light extraction device according to the invention, with use of a reflecting optical surface, this device being cut by a plane passing through its axis.

La figure 7 représente une vue d'un dispositif d'injection et d'extraction de lumière selon l'invention, en coupe par un plan passant par son axe.FIG. 7 represents a view of a device for injecting and extracting light according to the invention, in section through a plane passing through its axis.

Sur ces diverses figures, ce dispositif présente un axe 2, représenté en traits mixtes, cet axe étant aussi celui d'une portion de fibre optique rectiligne et/ou l'axe moyen d'une portion de fibre ondulée;In these various figures, this device has an axis 2, shown in phantom, this axis also being that of a portion of straight optical fiber and / or the middle axis of a portion of corrugated fiber;

Il a été indiqué ci-dessus que l'invention utilise des moyens de couplage entre l'intérieur et l'extérieur de la fibre Dans le cas où l'invention s'applique à une fibre optique constituée d'un coeur entouré d'une gaine d'indice optique plus petit, ces moyens de couplage peuvent avantageusement comporter des moyens de couplage de modes propres à créer des courbures alternées dans une portion de la fibre et donner ainsi à cette portion une forme ondulée oscillant autour d'un axe moyen. Ceci permet de coupler des modes à fortes constantes de propagation se propageant dans le coeur avec des modes à faibles constantes de propagation se propageant dans le gaine. Ces moyens de couplage de modes sont associés à un adaptateur d'indice 3 (Fig. 1 et 2) constitué d'un milieu transparent dont l'indice n'est pas sensiblement inférieur à. celui de la gaine. Cet adaptateur permet de coupler les modes se propageant dans la gaine avec de la lumière se propageant dans ce milieu transparent et constituant ledit ensemble de rayons.It has been indicated above that the invention uses coupling means between the inside and the outside of the fiber. In the case where the invention applies to an optical fiber consisting of a core surrounded by a sheath with a smaller optical index, these coupling means may advantageously include means for coupling modes capable of creating alternating curvatures in a portion of the fiber and thus giving this portion an undulating shape oscillating around a mean axis. This makes it possible to couple modes with high propagation constants propagating in the core with modes with low propagation constants propagating in the sheath. These mode coupling means are associated with an adapter of index 3 (FIGS. 1 and 2) consisting of a transparent medium whose index is not substantially less than. that of the sheath. This adapter makes it possible to couple the modes propagating in the sheath with light propagating in this transparent medium and constituting said set of rays.

Ladite surface optique est alors constituée par une surface 5 de cet adaptateur d'indice. Sur les figures 1 et 2, la fibre optique est représentée par un simple trait selon l'axe 2. L'adaptateur d'indice a la forme d'un cylindre plein, de révolution autour de l'axe 2 et entourant donc la fibre. Ce cylindre plein se termine à une extrémité par -un cône de révolution autour du même axe et formant une pointe vers l'extérieur. C'est la surface de ce cône qui constitue ladite surface optique.Said optical surface is then constituted by a surface 5 of this index adapter. In FIGS. 1 and 2, the optical fiber is represented by a single line along the axis 2. The index adapter has the shape of a solid cylinder, of revolution around the axis 2 and therefore surrounding the fiber . This solid cylinder ends at one end with a cone of revolution around the same axis and forming a point towards the outside. It is the surface of this cone which constitutes said optical surface.

Les moyens de couplage de modes qui viennent d'être Indiqués ne sont pas représentés sur les figures 1 et 2. Ils sont analogues aux dispositions décrites dans le brevet Miller précédemment mentionné. Il a été cependant trouvé, selon la présente invention, que l'on pouvait avantageusement utiliser une propriété, peu exploitée antérieurement, de la lumière qui se propageait dans l'adaptateur d'indice soit en sortant de la gaine, soit, réciproquement, en étant capable d'y pénétrer pour y former les modes précédemment mentionnés. Cette propriété est que ces rayons lumineux forment sensiblement tous un même angle avec l'axe de la fibre. Ils constituent donc un ensemble de rayons d'un type particulier, comportant par exemple les rayons 4,6 8 et 10 des figures 1 et 2. Cet ensemble est différent d'un faisceau parallèle parce que les rayons sont situés dans divers plans passant par l'axe. Cet ensemble est d'autre part différent d'un faisceau convergent sur l'axe parce qu'il comporte des rayons passant par divers points de l'axe.The mode coupling means which have just been indicated are not shown in FIGS. 1 and 2. They are analogous to the arrangements described in the Miller patent mentioned above. However, it has been found, according to the present invention, that a property, little exploited previously, could advantageously be used of the light which propagated in the index adapter either by exiting the sheath, or, conversely, by being able to enter it to form the previously mentioned modes. This property is that these light rays all form substantially the same angle with the axis of the fiber. They therefore constitute a set of spokes of a particular type, comprising for example the spokes 4,6 8 and 10 of FIGS. 1 and 2. This set is different from a parallel beam because the rays are located in various planes passing through the axis. This set is on the other hand different from a beam converging on the axis because it comprises rays passing through various points of the axis.

En fait, les rayons de cet ensemble ne passent pas exactement par l'axe, mais seulement à proximité de celui-ci. Il y a donc un écart entre les rayons réels et ceux d'un ensemble de rayons passant exactement par l'axe. Cet écart est inférieur au rayon de la fibre et n'a pas d'importance pour l'utilisation de la lumière lorsqu'on considère ce qui se passe à une distance de la fibre nettement supérieure à son diamètre. C'est pourquoi, la surface optique précedemment mentionnée s'étend de préférence jusqu'à une distance de la fibre supérieure à dix fois son diamètre, de manière que les rayons arrivant par cette surface soient, au moins en majorité, défléchis par cette surface pratiquement de la même manière que si leur prolongement rencontrait exactement l'axe 2.In fact, the rays of this set do not pass exactly through the axis, but only in the vicinity of it. There is therefore a difference between the real rays and those of a set of rays exactly passing through the axis. This difference is less than the radius of the fiber and does not matter for the use of light when considering what is happening at a distance from the fiber significantly greater than its diameter. This is why, the previously mentioned optical surface preferably extends up to a distance from the fiber greater than ten times its diameter, so that the rays arriving through this surface are, at least in majority, deflected by this surface. practically in the same way as if their extension exactly met axis 2.

. Il doit d'autre part être remarqué que la propriété utile précédemment mentionnée existe même lorsque la fibre présente une forme ondulée oscillant autour d'un axe moyen rectiligne. Dans ce cas, c'est cet axe moyen qui constitue l'axe 2.. It should also be noted that the useful property mentioned above exists even when the fiber has a wavy shape oscillating around a rectilinear mean axis. In this case, it is this mean axis which constitutes axis 2.

Il doit enfin être remarqué que l'angle (a) (fig. 1) que les rayons de cet ensemble font avec l'axe 2 n'est pas parfaitement prédéterminé. Il peut par exemple subir une variation de 1,5° en plus ou en moins. Cette variation est un peu gênante pour la mise en oeuvre de l'invention. Mais elle est d'autant plus faible que l'indice du matériau constituant l'adaptateur est plus élevé. C'est pourquoi, cet indice est choisi de préférence non seulement au moins égal à celui de la gaine de la fibre, ce qui est à peu près nécessaire pour permettre un couplage entre la lumière se propageant dans l'adaptateur et celle des modes se propageant dans la gaine, mais encore au moins égal à l'indice du coeur de la fibre, ce dernier indice étant toujours nettement supérieur à celue de la gaine. Le choix d'un indice élevé pour l'adaptateur présente de plus t'avantage que l'angle (a) en est augmenté, ce qui permet d'éviter une longueur excessive de l'adaptateur. La valeur de cet angle en radians est approximativement donné par la formule

Figure imgb0001
(n) étant l'indice de l'adaptateur 3 et (n,) étant l'indice du coeur de la fibre.Finally, it should be noted that the angle (a) (fig. 1) that the radii of this set make with the axis 2 is not perfectly predetermined. It can for example undergo a variation of 1.5 ° in more or less. This variation is somewhat troublesome for the implementation of the invention. However, the lower the index of the material constituting the adapter, the higher it is. This is why this index is preferably chosen not only at least equal to that of the fiber cladding, which is roughly necessary to allow coupling between the light propagating in the adapter and that of the modes propagating in the sheath, but still at least equal to the index of the core of the fiber, the latter index always being significantly higher than that of the sheath. The choice of a high index for the adapter also has the advantage that the angle (a) is increased, which avoids excessive length of the adapter. The value of this angle in radians is approximately given by the formula
Figure imgb0001
 (n) being the index of the adapter 3 and (n,) being the index of the core of the fiber.

La surface représentée sur la figure 1 est un dioptre, c'est-à-dire qu'elle sépare deux milieux d'indices différents, celui de l'adaptateur (n) et celui de l'air et qu'elle est traversée par la lumière. Pour assurer le couplage entre l'ensemble des rayons se propageant dans l'adaptateur en formant l'angle (a) avec l'axe 2, et un faisceau extérieur parallèle à l'axe 2, il faut alors. donnera cette surface la forme d'un cône ayant un demi-angle au sommet (b), tel que

Figure imgb0002
The surface represented in FIG. 1 is a diopter, that is to say that it separates two media of different indices, that of the adapter (n) and that of the air and that it is crossed by the light. To ensure the coupling between all the rays propagating in the adapter by forming the angle (a) with the axis 2, and an external beam parallel to the axis 2, it is then necessary. will give this surface the shape a cone having a half angle at the top (b), such that
Figure imgb0002

Les moyens de couplage de modes précédemment mentionnés sont représentés sur les figures 3 et suivantes, sur lesquelles la fibre optique est représentée avec un diamètre fortement grosse et avec des déformations exagérées, de manière à faciliter la compréhension du dessin. Ces moyens sont notamment étudiés dans une conférence de L. Jeunhomme et J. P. Pocholle "T Coupler for multimode optical fiber", (North Atlantic Treaty Organization, Advisory Group for Aerospace Research and Development, AGARD, 7 rue Ancelle 92200 NEUILLY SUR SEINE, France). Le compte-rendu de cette conférence peut être obtenu, auprès de l'ONERA 29 avenue de la Division Leclerc, 92 CHATILLON SOUS BAGNEUX, France et du National Technical Information Service (NTIS) 5285 Port Royal Road, Springfield, VIRGINIA 22151, USA.The above-mentioned mode coupling means are shown in FIGS. 3 and following, in which the optical fiber is shown with a very large diameter and with exaggerated deformations, so as to facilitate understanding of the drawing. These means are notably studied in a conference by L. Jeunhomme and JP Pocholle "T Coupler for multimode optical fiber", (North Atlantic Treaty Organization, Advisory Group for Aerospace Research and Development, AGARD, 7 rue Ancelle 92200 NEUILLY SUR SEINE, France) . The report of this conference can be obtained from ONERA 29 avenue de la Division Leclerc, 92 CHATILLON SOUS BAGNEUX, France and from the National Technical Information Service (NTIS) 5285 Port Royal Road, Springfield, VIRGINIA 22151, USA.

L'action optique de ces moyens peut être définie part deux grandeurs: le pas P de la succession régulière de courbures alternées, et l'amplitude de la déformation. Cette amplitude est comprise typiquement entre 10 et 100 microns. Le pas de la succession doit être défini de manière plus précise. Si on appelle (r) le rayon du coeur de la fibre, n, l'indice de ce coeur et n2 celui de la gaine, on peut écrire que la valeur optimale de P varie comme r/2D pour une fibre à échelon d'indice, et comme r/1,41 D pour une fibre à gradient d'indice parabolique, avec

Figure imgb0003
Par exemple, pour une fibre typique pour laquelle D2 = 6 x 10-3 et r = 0,0425 mm, P doit être voisin de 2 mm. En fait, il est apparu que P pouvait être comprise entre 1,4 mm et 4 mm.The optical action of these means can be defined by two quantities: the pitch P of the regular succession of alternating curvatures, and the amplitude of the deformation. This amplitude is typically between 10 and 100 microns. The step of the succession must be defined more precisely. If we call (r) the radius of the core of the fiber, n, the index of this core and n 2 that of the cladding, we can write that the optimal value of P varies as r / 2D for a fiber with step d 'index, and as r / 1.41 D for a fiber with a parabolic index gradient, with
Figure imgb0003
 For example, for a typical fiber for which D 2 = 6 x 10- 3 and r = 0.0425 mm, P must be close to 2 mm. In fact, it appeared that P could be between 1.4 mm and 4 mm.

Lorsqu'il s'agit de réaliser un dispositif d'extraction de lumière à partir de la fibre, la longueur de la portion ondulée de celle-ci peut être comprise entre 10 et 50 mm environ, et doit être suivie d'une portion longue d'au moins 10 mm en contact optique avec l'adaptateur. Lorsqu'il s'agit d'injecter de la lumière dans la fibre, cette portion ondulée doit être en contact optique avec l'adaptateur d'indice, et sa longueur doit être en principe suffisante pour que tout rayon en provenance de ladite surface optique arrive sur cette portion ondulée, celle-ci devant se prolonger en aval au-delà de la portion éclairée.When it comes to making a device for extracting light from the fiber, the length of the corrugated portion of the latter can be between 10 and 50 mm approximately, and must be followed by a long portion at least 10 mm in optical contact with the adapter. When it comes to injecting light into the fiber, this wavy portion must be in optical contact with the index adapter, and its length must in principle be sufficient for any ray coming from said optical surface arrives on this undulating portion, the latter having to extend downstream beyond the illuminated portion.

Quoique les moyens de couplage de modes indiqués ci-dessus semblent des plus avantageux, d'autres moyens pourraient être utilisés, tels que, par exemple, une succession régulière d'amincissements et d'épaississements du coeur de la fibre.Although the mode coupling means indicated above seem to be most advantageous, other means could be used, such as, for example, a regular succession of thinning and thickening of the fiber core.

Les exemples de réalisation qui vont être maintenant décrits utilisent un adaptateur d'indice ayant la forme générale d'un cylindre de révolution plein et constitué d'un verre d'indice n = 1,4859 pour une longueur d'onde lumineuse de 6.328 angstroems. Cet adaptateur pourrait cependant tout aussi bien être constitué d'une matière plastique transparente dure, moulable et d'indice élevé. La fibre optique a un diamètre de coeur de 85 microns, un indice de coeur n, = 1,4645 et un indice de gaine n2 = 1,4565. L'angle (a) vaut a = 12,6°. Dans les cas où la surface optique est un dioptre, le demi angle au sommet du cône vaut b = 54°29'. L'adaptateur d'indice 3 est constitué de deux parties 14 et 16 ayant chacune en section la forme d'un demi cercle de manière à constituer le cercle complet par rapprochement de ces deux parties avec l'aide d'un moyen de pression tel qu'une vis 12 (Fig. 3). Les faces planes en regard de ces deux parties comportent chacune une succession régulière de saillies et de creux avec un pas . P .= 2 mm, disposées de manière que les creux d'une succession soient en regard des saillies de l'autre. On donne ainsi à la portion de la fibre 18 serrée entre ces deux parties une forme . ondulée. L'ensemble des deux successions de saillies et de creux est souvent appelé "réseau". Le contact optique entre la fibre et l'adaptateur est amélioré par l'utilisation d'une huile transparente appropriée, d'indice intermédiaire entre celui. de la gaine et celui de l'adaptateur. Cette huile est retenue par capillarité. Ce peut être par exemple la graisse silicone. Le dispositif d'injection de lumière représenté sur les figures 3 et 4 comporte un adaptateur ayant un diamètre de 26 mm, et une longueur sur l'axe de 75 mm, y compris la partie conique. Cette longueur est entièrement occupée par le réseau. Une lentille convergente à échelons 20 (lentille de Fresnel) est disposée coaxialement à l'adaptateur du côté du cône. Elle a un diamètre de 25,4 mm et une focale de 10 mm. Au foyer de cette lentille au-delà de celle-ci est disposée la surface émettrice d'une diode électroluminescente 22 de type habituel dont le diagramme de rayonnement est proche de la loi de Lambert. La plus grande partie de la lumière émise par cette diode est reçue par la lentille 20, transformée en un faisceau parallèle à l'axe 2, puis transformée par la surface optique conique de l'adaptateur en un ensemble de rayons faisant l'angle convenable avec cet axe et se propageant vers cet axe à l'intérieure de l'adaptateur. Elle rencontre donc la gaine de la fibre dans laquelle elle pénètre et se propage en y formant les modes précédemment mentionnés, qui sont rapidement transformés en modes se propageant dans le coeur grâce aux moyens de couplage de modes constitués par le réseau. La longueur de celui-ci est choisie suffisante pour que cette dernière transformation soit à peu près complète. On obtient ainsi un rendement d'injection élevé. Il n'est pas nécessaire d'interrompre la fibre 18, mais seulement de la courber à l'extérieur de l'adaptateur, pour laisser la place de la lentille 20 qui doit être aussi proche que possible de l'adaptateur.The exemplary embodiments which will now be described use an index adapter having the general shape of a full cylinder of revolution and consisting of a glass of index n = 1.4859 for a light wavelength of 6.328 angstroms . However, this adapter could equally well be made of a hard, moldable transparent material with a high index. The optical fiber has a core diameter of 85 microns, a core index n, = 1.4645 and a cladding index n 2 = 1.4565. The angle (a) is a = 12.6 °. In cases where the optical surface is a diopter, the half angle at the top of the cone is b = 54 ° 29 '. The index adapter 3 consists of two parts 14 and 16 each having in section the shape of a semicircle so as to constitute the complete circle by bringing these two parts together with the aid of a pressure means such than a screw 12 (Fig. 3). The planar facing faces of these two parts each have a regular succession of projections and hollows with a pitch. P = 2 mm, arranged so that the hollows of one succession are opposite the projections of the other. This gives the portion of the fiber 18 clamped between these two parts a shape. wavy. The combination of the two successions of projections and depressions is often called a "network". The optical contact between the fiber and the adapter is improved by the use of a suitable transparent oil, of index intermediate between that. of the sheath and that of the adapter. This oil is retained by capillarity. It can for example be silicone grease. The light injection device shown in Figures 3 and 4 includes an adapter having a diameter of 26 mm, and a length on the axis of 75 mm, including the conical part. This length is entirely occupied by the network. A converging step lens 20 (Fresnel lens) is arranged coaxially with the adapter on the side of the cone. It has a diameter of 25.4 mm and a focal length of 10 mm. At the focal point of this lens beyond this is disposed the emitting surface of a light-emitting diode 22 of the usual type whose radiation diagram is close to Lambert's law. Most of the light emitted by this diode is received by the lens 20, transformed into a beam parallel to the axis 2, then transformed by the conical optical surface of the adapter into a set of rays making the suitable angle with this axis and propagating towards this axis inside the adapter. It therefore meets the sheath of the fiber into which it penetrates and propagates by forming the previously mentioned modes, which are rapidly transformed into modes propagating in the core thanks to the mode coupling means constituted by the network. The length of the latter is chosen sufficient for this latter transformation to be almost complete. A high injection yield is thus obtained. It is not necessary to interrupt the fiber 18, but only the bend on the outside of the adapter, to make room for lens 20 which must be as close as possible to the adapter.

L'utilisation de la lentille 20 peut être évitée en donnant à la surface optique 5 une forme en pointe bombée en conservant la valeur du demi angle au sommet. Une telle disposition est représentée sur la figure 5 qui représente en traits forts des éléments ayant une existence réelle. La forme de la surface optique en pointe n'est alors plus conique et peut être définie comme suit

  • 1 - Elle est de révolution autour de l'axe 2.
  • 2 - Elle conserve le même sommet S que précédemment, qui constitue l'extrémité de la pointe.
  • 3 - Elle conserve la même valeur du demi angle au sommet, c'est-à-dire qu'au sommet elle est tangente au cône précédemment décrit.
  • 4 - Elle est engendrée par la rotation d'un arc d'hyperbole passant par le sommet S et tournant autour de l'axe 2.
  • 5 - Pour préciser la définition, on peut:
    • . appeler (e) la distance entre ce sommet S et la surface émettrice de la diode 22 et (n) l'indice optique de l'adaptateur 3.
    • . couper la surface optique par un plan passant par l'axe 2, par exemple le plan de la figure 5,
    • . tracer dans ce plan un.axe Oy passant par le centre 0 de cette surface émettrice, cet axe Oy étant parallèle à un rayon dudit ensemble de rayons dans l'adaptateur, c'est-à-dire faisant avec l'axe 2 l'angle (a) précédemment défini, et un axe Ox perpendiculaire à Oy,
    • . considérer, une moitié seulement de la section de la surface optique par ce plan, par exemple celle qui est située au-dessus de l'axe 2 sur la figure 5,
    • . On peut alors préciser que cette demi section de la surface optique par ce plan est un arc d'une hyperbole définie par l'équation:
      Figure imgb0004
The use of the lens 20 can be avoided by giving the optical surface 5 a rounded point shape while retaining the value of the half angle at the top. Such an arrangement is represented in FIG. 5 which represents in strong lines elements having a real existence. The shape of the pointed optical surface is then no longer conical and can be defined as follows
  • 1 - It is of revolution around axis 2.
  • 2 - It retains the same vertex S as above, which constitutes the end of the point.
  • 3 - It retains the same value of the half angle at the top, that is to say that at the top it is tangent to the cone previously described.
  • 4 - It is generated by the rotation of a hyperbola arc passing through the vertex S and rotating around the axis 2.
  • 5 - To clarify the definition, we can:
    • . call (e) the distance between this vertex S and the emitting surface of the diode 22 and (n) the optical index of the adapter 3.
    • . cut the optical surface by a plane passing through the axis 2, for example the plane of FIG. 5,
    • . trace in this plane an axis Oy passing through the center 0 of this emitting surface, this axis Oy being parallel to a radius of said set of rays in the adapter, that is to say forming with axis 2 the angle (a) previously defined, and an axis Ox perpendicular to Oy,
    • . consider only half of the section of the optical surface by this plane, for example that which is situated above the axis 2 in FIG. 5,
    • . We can then specify that this half section of the optical surface by this plane is an arc of a hyperbola defined by the equation:
      Figure imgb0004

On a tracé cette hyperbole sur la figure 5 en traits fins ainsi que ses axes Ox et Oy et sès asymptotes. Il doit cependant être bien compris que ces éléments tracés en traits fins ne peuvent pas être vus sur le dispositif d'injection lui-même. Ils servent seulement à aider à comprendre comment on peut définir la surface optique.We have drawn this hyperbola in Figure 5 in fine lines as well as its axes Ox and Oy and its asymptotes. It should however be clearly understood that these elements drawn in fine lines cannot be seen on the injection device itself. They only serve to help understand how one can define the optical surface.

Le dispositif d'extraction de lumière représenté sur la figure 6 comporte un adaptateur d'indice 30 analogue au précédent. Son diamètre est de 30 mm. Il comporte un réseau s'étendant à partir de sa face arrière sur une longueur de 44 mm, et se prolongeant vers l'avant par une zone longue de 16 mm dans laquelle la fibre 18 ne subit pas de déformation, le contact optique étant conservé. Cet adaptateur se termine vers l'avant par une surface optique 32 conique convexe avec un demi angle au sommet égal à

Figure imgb0005
Cette surface optique est métallisée de manière que la lumière qui lui parvient à partir de la fibre soit réfléchie dans l'adaptateur en formant un faisceau parallèle à l'axe 2 qui parvient jusqu'à la face avant 34. Cette face avant constitue un dioptre convergent d'un type bien connu des opticiens, et qui permet de faire converger le faisceau sortant de l'adaptateur vers une diode réceptrice 36. Ce dioptre convergent est excentré par rapport à l'axe 2, de manière à permettre de placer la diode 36 en dehors de l'axe 2. Il est ainsi possible non seulement de ne pas couper la fibre 18, mais encore de ne pas la courber en dehors de l'adaptateur,The light extraction device shown in FIG. 6 includes an index adapter 30 similar to the previous one. Its diameter is 30 mm. It comprises a network extending from its rear face over a length of 44 mm, and extending forwards by a long zone of 16 mm in which the fiber 18 does not undergo deformation, the optical contact being preserved . This adapter ends forwards by a conical convex optical surface 32 with a half angle at the top equal to
Figure imgb0005
 This optical surface is metallized so that the light which reaches it from the fiber is reflected in the adapter by forming a beam parallel to the axis 2 which reaches the front face 34. This front face constitutes a diopter convergent of a type well known to opticians, and which makes it possible to converge the beam leaving the adapter towards a receiving diode 36. This converging diopter is eccentric relative to the axis 2, so as to allow the diode to be placed 36 outside the axis 2. It is thus possible not only not to cut the fiber 18, but also not to bend it outside the adapter,

On vient de décrire à l'aide des figures 3 et 4 un dispositif d'injection de lumière utilisant une surface optique dioptrique, et à l'aide de la figure 6 un dispositif d'extraction de lumière utilisant une surface optique réfléchissant. Il est cependant bien évident que l'on pourrait utiliser une surface optique dioptrique dans un dispositif d'extraction de lumière, et une surface optique réfléchissante dans un dispositif d'injection de lumière.We have just described, with the aid of FIGS. 3 and 4, a light injection device using a dioptric optical surface, and with the aid of FIG. 6, a light extraction device using a reflective optical surface. It is however obvious that one could use a dioptric optical surface in a light extraction device, and a reflective optical surface in a light injection device.

Il est d'autre part possible, comme représenté sur la figure 7, d'assurer l'injection et l'extraction de lumière avec un seul adaptateur d'indice, comportant un seul réseau, et muni à chacune de ses extrémités avant et arrière d'une surface optique dioptrique, une surface optique d'entrée 42 et une surface optique de sortie 44. Ces surfaces optiques peuvent être coniques. Elles doivent alors être associées chacune à une lentille convergente, respectivement 46 et 48, si on désire réaliser un couplage avec des éléments de petites dimensions, respectivement une diode électroluminescente 50 et une diode réceptrice 52.It is also possible, as shown in Figure 7, to ensure the injection and extraction of light with a single index adapter, comprising a single network, and provided at each of its front and rear ends a dioptric optical surface, an optical input surface 42 and an optical output surface 44. These optical surfaces can be conical. They must then each be associated with a converging lens, respectively 46 and 48, if it is desired to make a coupling with elements of small dimensions, respectively a light-emitting diode 50 and a receiving diode 52.

Claims (13)

1. An optical coupling device for an optical fibre (18), applicable to an optical fibre which is constituted by a core surrounded by a cladding with a lower optical index, this device comprising
- mode coupling means (14,16,12) forming alternate bends in a portion of the fibre and this imparting to this portion an undulating form oscillating round an average axis (2), so as to couple modes with high propagation constants which propagate in the core to modes with low propagation constants which propagate in the cladding,
- and an index adapter (3) constituted by a transparent medium whose index is not substantially lower than that of the cladding, in optical contact with the cladding, so as to couple said modes which propagate in the cladding to light which propagates in this transparent medium and which constitutes a set of rays, the rays of this set passing near the axis (2) of the fibre with a distance inferior to the radius of said fibre and this along the whole length of a segment of this axis, this segment being disposed inside the adapter, all rays forming the same predetermined angle with this axis, characterized by the fact that an optical surface (5,32,42,44) of this index adapter ends in a tip' of revolution about this axis to provide coupling. between this set of rays and a light beam which is parallel to this axis or converges at a point aligned on this axis.
2. A device according to claim 1, characterized by the fact, that said optical surface (5) extends up to a distance from the fibre (18) greater than ten times the diameter of this fibre and that said index adapter (3) has an optical index at least equal to that of the fibre core.
3. A device according to claim 2, characterized by the fact that said optical surface (5) is a diopter which permits the refraction of the light.
4. A device according to claim 3, characterized by the fact that said diopter (5) is in the form of a cone of revolution about said axis (2) so as to transform mutually said set of rays and a light beam which is parallel to this axis.
5. A device according to claim 4, characterized by the fact that it further comprises a converging system (20) outside said index adapter (3) and focussing this parallel beam at a point (22) outside this adapter.
6. A device according to claim 3, characterized by the fact that said diopter (5) is in the form of a surface of revolution obtained by turning an arc of a hyperbola about said axis (2) of the fibre (18), this hyperbola being drawn in a plane which passes through this axis and having an axis of symmetry which, with said axis of the fibre, forms the same angle as the rays of said set of rays, so as to transform mutually this set of rays and a light beam which converges at a point aligned on this axis of the fibre and on said axis of symmetry of the hyperbola.
7. A device according to claim 2, characterized by the fact that said optical surface is a reflecting surface (32) for reflecting .light towards the interior of the index adapter (30) and is in the form of a cone of revolution about said axis (2) so as to transform mutually said set of rays and a light beam parallel to this axis and inside this adapter.
8. A device according to claim 7, characterized by the fact that the adapter also has in the way of said parallel interior beam a converging diopter (34) which transforms mutually this interior beam and an exterior converging beam.
9. A device according to claim 8, characterized by the fact that said converging diopter (34) provides the transformation between the interior parallel beam and an exterior converging beam at a point (36) distant from the axis (2) of the fibre (18).
10. A device according to claim 2, characterized by the fact that said means for forming alternate bends comprise two gratings constituted by two regular successions of ridges and of furrows formed respectively on two facing surfaces of said index adapter (14,16), the ridges of one grating facing the furrows of the other, and
- pressure means (12) for pressing the ridges against two opposite sides of the optical fibre,
- means assuring an optical contact between the adapter and the undulating zone of the fibres being provided.
11. A device according to claim 10, applicable to injecting light into the optical fibre (18) and characterized by the fact that said grating extends along the whole length of the zone which receives light from said optical surface (42) and extends downstream beyond this zone.
12. A device according to claim 10, applicable to extracting light from the optical fibre (18) and characterized by the fact that said index adapter (40) comprises an extra output zone about one centimetre long which provides optical contact with a rectilinear portion of the fibre consecutive to said undulating portion which bears against said grating, the axis (2) of this rectilinear portion forming the extension of the average axis (2) of this undulating portion, said optical surface (44) of revolution about this common axis being disposed so as to receive directly light leaving this rectilinear portion and from at least a part of this undulating portion close to this rectilinear portion.
13. A device according to claim 12, characterized by the fact that said index adapter (40) comprises an optical input surface (42) and an optical output surface (44) on either side of said grating so that the mode coupling means constituted by this grating can be used both for injection of light into the fibre (18) through the optical input surface and for extraction of light from the optical output surface.
EP78100414A 1977-07-25 1978-07-18 Coupling device for an optical fibre Expired EP0000529B1 (en)

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FR7722716A FR2399042A1 (en) 1977-07-25 1977-07-25 COUPLING DEVICE FOR FIBER OPTIC
FR7722716 1977-07-25

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EP0000529A1 EP0000529A1 (en) 1979-02-07
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JP (1) JPS5424045A (en)
CA (1) CA1111690A (en)
DE (1) DE2860751D1 (en)
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FR (1) FR2399042A1 (en)
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IT7825688A0 (en) 1978-07-14
IE781481L (en) 1979-01-25
IE47140B1 (en) 1983-12-28
DE2860751D1 (en) 1981-09-17
DK329178A (en) 1979-01-26
EP0000529A1 (en) 1979-02-07
US4253727A (en) 1981-03-03
CA1111690A (en) 1981-11-03
FR2399042B1 (en) 1982-01-15
FR2399042A1 (en) 1979-02-23
IT1097143B (en) 1985-08-26
DK144926C (en) 1982-11-29
JPS5424045A (en) 1979-02-23

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