FR2860599A1 - Optical coupler device for e.g. multi-core single mode fiber, has section of pure silicon fiber welded to free end of section of index gradient fiber and/or inserted between end of multi-core fiber and end of index gradient fiber - Google Patents

Abstract

The device has a section of a pure silicon fiber welded to a free end of an index gradient fiber section (41) and/or inserted between an end of multi-core fiber (40) and an end of the index gradient fiber section. Length of the silicon fiber section is between zero and one millimeter. A sparker is fixed to another end of the fiber (40) to connect each core of the fiber (40) to mono-core single mode fiber (10). An independent claim is also included for a method of manufacturing an optical coupler device.

Description

Optical coupling device for a multi-core single mode fiber, and

  corresponding manufacturing process.

  The field of the invention is that of optical telecommunications. More specifically, the invention relates to the interconnection of high density optical fibers.

  In the telecommunications field, single-mode fiber is the preferred medium for high-speed data transmission over long distances.

  The use of such single-mode fibers, however, entails significant assembly difficulties, whenever an optical interconnection between fibers, or between a single-mode fiber and an optical device, is necessary. Indeed, the emission surface of monomode fibers is small, typically of the order of 10 ycm in diameter. The scale of this emission surface makes the optical coupling very sensitive, on the one hand to the axial and transverse positioning of the elements to be interconnected, and on the other hand, to the slightest dust or end-defect of the single-mode fibers. These problems arise each time an optical assembly involving a monomode fiber is made.

  It has therefore been envisaged to widen the optical beam at the end of monomode fibers by using optical collimators. This broadening of the section of the beam makes it possible to relax the positioning constraints and the influence of the dust on the coupling.

  To do this, several technological solutions have already been proposed: - it was first considered positioning at the end of the monomode optical fiber, a glass bar having a suitable index gradient. This solution is commonly called SELFOC; it has also been proposed to modify the guiding structure (ie the core) of the monomode fiber, as described in the following articles: * K. Furaya et al., "Low loss splicing of single mode fibers by tapered butt joined method ", Trans. IECE Japan, vol. E61, pp. 957, 1978; N. Amitay et al., "Optical fiber tapered-A novel approach to self-aligning beam expansion and single mode hardware", JLT, vol. LT-5, No. 1, pp. 70-76, Jan. 1987; K. Shiraishi et al., "Beam expansing fiber using thermal diffusion of the dopant", JLT, vol.8, No. 8, pp. 1151-1161, August 1990; Finally, it has been envisaged to provide, at the end of the monomode fiber, a solder connection of a section of fiber which may be index gradient, as described in European Patent Document No. EP 0 825 464 and in US Pat. article by W. Emkey and C. Jack, "Analysis and Evaluation of Graded Index Fiber Lenses", JLT, vol. LT5, No. 9, pp. 303-306, June 1989. In addition to the constraints described above related to the optical assembly of single-mode fibers, another problem that must be solved for the use of monomode fibers is that the level of integration that can be achieved. Indeed, when making a component based on optical fibers, it is necessary to achieve a high integration, so that this component occupies a volume as low as possible.

  This second constraint has led those skilled in the art to develop very compact beam-widening solutions. It has thus been proposed to integrate several fibers (typically 2 fibers) in the same collimation element, which can have a transmission or reflection operation. The commonly used technological solution is a SELFOC type solution, in which two monomode fibers are positioned on the same side of a glass bar. This solution, illustrated in FIG. 1, constitutes the closest prior art of the present invention.

  The collimator of FIG. 1 comprises a glass bar 11 having a SELFOC index gradient at the end of which two monomode fibers 10 are positioned (by gluing, welding, etc.). The two monomode fibers 10 must be arranged. symmetrically to the axis of the index gradient bar 11. The bonding of the two monomode fibers 10 on the same side of the index gradient bar 11 makes it possible to obtain transmission and reflection operation of this collimator, as illustrated by FIG. Figures 2a and 2b.

  Figure 2a shows a transmission assembly, in which two collimators of the type of that of Figure 1 are mounted vis-a-vis. Such an arrangement therefore comprises two input optical fibers 101 and 102 and two output optical fibers 103 and 104.

  In FIG. 2b, a mirror 20 is disposed at the end of the collimator of FIG. 1, so as to carry out a reflection assembly.

  Such a solution has several advantages, such as: intrinsic losses of weak coupling, of about 0.3 dB; a fairly high beam broadening, with a spot of about 500 μm in diameter (the initial beam diameter of the monomode fiber is about 10 μm); a fairly large working distance of about 2000 ycm; a low level of signal feedback thanks to bias polishing and anti-reflective treatments (-60 dB).

  Such a solution, however, has many disadvantages.

  A first disadvantage of this solution of the prior art is that it has a very low level of integration, so that the optical interconnection system thus produced is very large volume. Conventionally, the collimator of FIG. 1 has a cylinder-type outer volume of length 9 mm and diameter 3 mm. This diameter is very much greater than that of an optical fiber of outer diameter 125 μm. The compactness of such an optical interconnection is therefore limited.

  Another disadvantage of such an optical interconnection system is that it requires a complex assembly of the fibers 10 and the index gradient rod 11 requiring a very good mechanical accuracy. This assembly must also be inserted and glued in a tube 12.

  Such a solution also has the disadvantage of requiring precise polishing of the fibers 10 and the index gradient rod 11.

  A final disadvantage of this solution of the prior art is that it requires to have a dioptre of air or glue 13 between the fibers 10 and the index gradient bar 11 which can cause parasitic optical reflections.

  Furthermore, according to another technique of the prior art, in order to cope with the increase in the demand for transmission capacity, it has also been envisaged to increase the number of optical fibers between two emission points, or of increase the modulation rate by fiber. It has thus been proposed in European patent document EP 0 611 973 to produce fibers containing several cores, called multicore fibers, which can contain 2, 4, or 9 cores in the same fiber, as illustrated in FIG. which has shown a fiber with four hearts numbered from 1 to 4.

  The invention particularly aims to overcome the disadvantages of the prior art described above.

  More precisely, an object of the invention is to provide an optical coupling technique that makes it possible to simultaneously respond to the constraints of high integration level and high transmission capacity.

  Another objective of the invention is to implement such a technique which makes it possible to facilitate the interconnection of multi-core optical fibers.

  The invention also aims to provide such a technique that is simple and inexpensive to implement.

  The invention also aims to implement such a technique that allows the realization of various optical functions at the end of a multi-core single-mode fiber.

  Yet another object of the invention is to provide such an optical coupling technique which operates in both reflection and transmission.

  The invention also aims to implement such a technique that is not subject to parasitic reflections and optical losses, so as to be adapted to intense light flux.

  These objectives, as well as others which will appear later, are achieved by means of an optical coupling device of a multi-core single-mode fiber.

  According to the invention, such a device comprises at least one index gradient fiber section reported at a first end of said multi-core fiber, so as to enlarge the size of optical beams from the cores of said multi-core fiber.

  Thus, the invention is based on an entirely new and inventive approach to optical coupling with a high integration level and a high transmission capacity. Indeed, a coupling device according to the invention is much more compact than a device of the prior art using a SELFOC type bar (registered trademark), since the widening of the section of the light beam is achieved by a graded index fiber section having an outside diameter substantially equal to that of the multi-core single mode fiber. In addition, the data transmission rate authorized by such a coupling device is much higher than according to the techniques of the prior art since it is possible to use multi-core fibers comprising up to 9 cores. According to the device previously described in connection with FIG. 1, on the other hand, two monomode fibers juxtaposed at the end of a glass bar were assembled at the most.

  It should be noted that it was not at all obvious, for a person skilled in the art, to consider assembling a single-mode multi-core fiber and a gradient-index fiber section, because of the strong difference the geometry of these two elements: while the graded index fibers are of substantially circular section, the section of a mufti-core fiber comprises a plurality of lobes, the same number as the number of hearts of the fiber. There was thus a strong prejudice for the skilled person, dissuading him from considering such an assembly: he thought indeed that such a difference in geometry would lead to optical losses of coupling, as well as the risk of parasitic reflections, to the junction between the two fibers.

  In addition, the inventors of the present patent application also overcame another obstacle which discouraged a person skilled in the art from assembling a multi-core fiber and a gradient index fiber, namely the strong difference in nature of the fibers. , both in chemical composition and in stress distribution.

  Advantageously, such a device also comprises at least one section of pure silica fiber.

  According to a first alternative embodiment, said section of pure silica fiber is inserted between said multi-core fiber and said index gradient fiber section.

  According to a second variant embodiment, said section of pure silica fiber is welded to the free end of said graded index fiber section. These two variants can be combined.

  Preferably, such a device also comprises at least one spark gap fixed at a second end of said multi-core fiber, making it possible to connect each core of said multi-core fiber to a mono-core single-mode fiber.

  Advantageously, the free end of said index gradient fiber section is cleaved at an angle.

  According to an advantageous characteristic of the invention, the free end of said index gradient fiber section has an anti-reflection treatment.

  According to another advantageous characteristic of the invention, the free end of said graded index fiber section has a shape lens.

  According to an advantageous variant of the invention, such a device comprises a mirror situated vis-à-vis the free end of said index gradient fiber section.

  According to another advantageous variant of the invention, such a device comprises a filter on the free end of said index gradient fiber section.

  Preferably, said fiber mufti-cores comprises at least two optical cores in a silica matrix.

  Advantageously, such a device has a substantially constant outside diameter equal to 125 m.

  Preferably, the length of said index gradient fiber section is optimized as a function of an optical function of collimation or focusing to achieve.

  Advantageously, said fiber mufti-cores and said monomode fibers are polarization-maintaining fibers.

  The invention also relates to the use of the optical coupling device described above in a reflection arrangement, in which a mirror is arranged opposite the free end of said index gradient fiber section. coupling is effected between two cores of said multi-core fiber arranged radially with respect to the optical axis of said device.

  In such a use, at least one optical component is inserted between said coupling device and said mirror.

  For such a use of the optical coupling device described above in a transmission arrangement, two of said coupling devices are arranged facing one another, so that coupling takes place between a first core of a multi-fiber hearts of the first device and a second core of a multi-core fiber of the second device, said first and second cores being disposed radially with respect to the optical axis of said devices.

  Preferably, at least one optical component is inserted between said coupling devices vis-à-vis.

  The invention also relates to a method of manufacturing an optical coupling device described above, comprising steps of: cleaving a fiber mufti-cores; welding a graded index fiber at the end of said cleaved multi-core fiber.

  Advantageously, such a method also comprises subsequent steps of: precision cleavage of said index gradient fiber; optimizing the length of said gradient index fiber section.

  The invention also relates to the application of the optical coupling device described above to the production of any of the optical components belonging to the group comprising: an insulator; a circulator; an optical attenuator; a switch; a polarization controller; a wavelength division multiplexing device; a channel multiplexer (interleaver); a module for inserting and extracting optical channels; a filter of lines or bands.

  Other characteristics and advantages of the invention will emerge more clearly on reading the following description of a preferred embodiment, given by way of a simple illustrative and nonlimiting example, and the appended drawings, among which: FIG. 1, already commented on in relation with the prior art, presents a block diagram of an optical collimator of the SELFOC (registered trademark) type; FIGS. 2a and 2b respectively show a transmission arrangement and a reflection arrangement of the collimator of FIG. 1; FIG. 3 shows two simplified diagrams of single-mode multicore fibers, comprising respectively 2 and 4 cores; FIGS. 4a and 4b respectively show a sectional view and a perspective view of an optical coupling device according to the invention, comprising a graded index fiber section at the end of a multi-core fiber; FIG. 5 shows an alternative embodiment of the coupling device of FIG. 4, to which two sections of silica fiber have been added on either side of the index gradient fiber section; FIGS. 6a to 6c show different treatments that can be applied to the free end of the index gradient section of the device of FIG. 4; FIGS. 7a and 7b respectively illustrate a reflection mounting and a transmission mounting of the coupling device of the invention; FIGS. 8 and 9 illustrate an example of a coupler with a 4 core fiber in reflective configuration according to the invention; FIGS. 10a and 10b describe examples of components that can be produced according to the invention, respectively in a transmission arrangement and in a reflection arrangement; Figure 11 illustrates the first step of the method of manufacturing the coupling device of the invention; FIG. 12 illustrates the difference in external geometry of the multicore and index gradient fibers assembled in the coupling device of the invention; Figure 13 shows the second step of the method of manufacturing the coupling device of the invention; Figure 14 shows the third step of the method of manufacturing the coupling device of the invention.

  The general principle of the invention is based on assembling an index gradient fiber section at the end of a multicore single mode fiber, so as to expand the beams conveyed by the single mode fiber.

  As illustrated in FIG. 3, a monomode fiber mufti-cores comprises a silica sheath 31 and a plurality of optical cores 30. In the example of FIG. 3, a two-core multi-core fiber is shown, and a multi-core fiber with 4 hearts. For the sake of simplification, the section of these fibers has been represented in circular form. Indeed, the outer diameter of these elements is close enough to a cylinder of outer diameter of 125 ycm. However, it will be noted, as illustrated in FIG. 9 for example, that the section of the multi-core fibers conventionally has a plurality of lobes, in number corresponding to that of the number of cores.

  Such fiber mufti-hearts also has at its end a spark gap (which has not been shown in Figure 3) to associate each core to a conventional monomode fiber.

  In relation to FIGS. 4a and 4b, an embodiment of a coupling device according to the invention is presented.

  The originality of such a coupling device lies in the presence of a gradient index fiber section 41, of length that is adaptable to the optical function that it is desired to carry out, at the end of a multi-core fiber. 40. This segment of graded index fiber 41 ensures the increase in the size of the modes of each core 30. The optical beam from each of the cores 30 of the fiber mufti-cores 40 is thus expanded in the fiber section. gradient index 41, which acts as a lens. As a function of the length of the section of the index gradient 41, a collimation or focusing optics is produced. At the output of the collective micro-optics, the diameter of the optical beam can reach about 80 m, whereas in the hearts of the fiber mufti-cores, the optical beam is of identical size to that of a monomode single-core fiber, about 10 m. A magnification of 8 is thus operated.

  The beam widening optics is therefore composed of a graded index fiber section 41 juxtaposed at the end of the multicore fiber 40. The length of the section of the index gradient fiber 41, and as its composition, are function of the application. This length varies from a few microns to a few millimeters. This graded index fiber is generally 125 m outside diameter.

  The spark gap located at the end of the multi-core fiber 40 makes it possible to connect a plurality of mono-core single-mode fibers 10 (in number equal to the number of cores of the multi-core fiber 40) to the multi-core fiber 40.

  Thus, as shown in perspective in FIG. 4b, the multicore fiber 40 comprises four optical cores, so that four mono-core single-mode fibers 10 are connected to it.

  In an alternative embodiment of the invention illustrated in FIG. 5, the coupling device of FIGS. 4a and 4b comprises a section of pure silica 50 of any length at the end of the index gradient section 41 and / or between the multi-core fiber 40 and index gradient fiber 41.

  A section of silica fiber 50 disposed between the mufti-hearts fiber 40 and the index gradient fiber section 41 adjusts the magnification factor and the working distance of the micro-optics. The length of such a silica section 50 is generally between zero and one millimeter. At each length of this silica section 50 is associated a specific length of the index gradient section. The micro-optics is then defined by the silica length 50 pair and index gradient 41. This option makes it possible to obtain a modularity of the optical properties.

  The addition of a section of pure silica 50 of any length at the end of the graded index section 41 allows for it to perform a final polishing, if the need arises. This last section 50 does not change the size of the transmitted mode. This option can be useful in the case of a connection of the component. Indeed, during the manufacture of the connectors, the introduction of the fiber into the ferrule is carried out without precision, and therefore exceeds a certain length. Polishing is then performed when the ferrule is mounted in the connector. This polishing is possibly biased to reduce the level of reflections.

  Other embodiments of the coupling device of the invention are illustrated in FIGS. 6a to 6c, in which an end treatment is applied to the graded index fiber section 41.

  Thus, in Fig. 6a, the index gradient fiber section 41 has been skewed at its end to limit the level of reflections.

  In FIG. 6b, this index gradient fiber section 41 has been shaped so as to have a lens at its end, to modify the magnification, the working distance of the optical beam, as well as the level of reflections (this type Lentil thus makes it possible to increase the optical qualities of the coupling device of the invention).

  Finally, in FIG. 6c, the index gradient fiber section 41 has an antireflection treatment, a mirror or a filter 44 at its end. Such anti-reflection treatment 44 minimizes optical losses if the micro-optics of the invention is used for propagation in a medium different from silica. The mirror 44 can itself be attached or offset in front of the output face of the optical beams. The optical beams then have a reflection function, and any optical function can therefore be inserted between the mirror and the mufti-hearts fiber 40, equipped with the index gradient section 41. Finally, if a filter 44 is disposed at the end of the graded index section 41, it may be an interference filter or an attenuating filter. An interference filter makes it possible to filter a broadband incident signal or at several wavelengths: the system can for example transmit the 1.6 m band contained in a broadband spectrum C + L and backscatter the band 1.5 m . An attenuating filter makes it possible to reduce the intensity of the transmitted optical signal. For example, a 3 dB filter deposited at the end of the micro-optics divides by two the optical power of the transmitted signal.

  It will also be noted that, according to the invention, the multi-core fiber 40, as well as the mono-core single-mode fibers 10 may be polarization-maintaining. In this case, the coupling device of the invention allows the interconnection to maintain polarization.

  Figures 7a and 7b illustrate two possible configurations of the coupling device of the invention.

  In the reflection configuration illustrated in FIG. 7a, the core-to-core coupling is made through the index gradient section 41 between two radially opposite cores (from channel 101 to channel 102).

  In the transmission configuration illustrated in FIG. 7b, the core-to-core coupling is carried out starting from the first index gradient section 411 of the injection fiber 401 towards the second index gradient section 412 of the receiving fiber 402. The coupling takes place between two cores opposed radially with respect to the optical axis (from channel 101 to channel 104 and from channel 102 to channel 103).

  Figures 8 and 9 illustrate more particularly an exemplary configuration in which a coupler with a four-core fiber 40 (numbered from 1 to 4 in FIG. 9) has been made in a reflective configuration. An index gradient section 41 of 950 m is welded at the end of a multi-core fiber 40. The core-to-core coupling is carried out through the index gradient section between two radially opposite cores (of the channel). 1 to track 3).

  Optical characterization of the coupler was performed using a mirror 20 placed at the focal length of the micro-optics. After alignment of the mirror 20, it is clearly observed that the optical beam coming from the channel 1 couples with the channel 3 and the beam of the channel 2 is coupled with the channel 4.

  The optical characteristics of such a coupler are as follows: Beam diameter = 80 μm Outer diameter = 125 μm - Working distance (fiber-mirror distance) = 100 μm Coupling losses 1 to 3 s 1.5 dB Loss of coupling 2 to 4 s 1.5 dB The collimation device of the invention described above has many applications in the field of optical interconnection.

  In a reflection configuration illustrated in FIG. 10b, a component 100 having inter alia a reflection function 20 is placed in front of the collimation device of the invention, comprising a multi-core fiber 40 and a gradient fiber. 41 index.

  In a transmission configuration, the component 100 is inserted between two of these collimators.

  The optical functions associated with these applications are diverse and can rely on effects in amplitude, intensity, polarization and phase on the light beams. Examples of components 100 are: insulators; circulators; switches (or switch); polarization controllers; wavelength division multiplexing components; interleavers (channel multiplexers); optical channel insertion and extraction modules; Etc ...

  The components thus produced have a strong integration (that is to say a low volume) and an easy assembly (the constraints on the alignment are released by the size of the optical beam).

  The method of manufacturing the optical device described previously in this document is now presented in relation to FIGS. 11 to 14. The manufacturing process of these micro-optics consists firstly in attaching a gradient-index fiber 41 at the end of a core fiber 40, and then in cutting a length-precise segment into this fiber. at index gradient 41.

  Specifically, the manufacturing comprises the following four main steps: Step 1 Figure 11: The cleavage of the multi-core fiber 40 must be performed accurately to obtain a perfectly straight cleavage face. The cleavage of a multi-core fiber 40 requires the realization of a rupture primer 111 112 tangentially to any one of the cores of the fiber 40 in a direction perpendicular to the axis of the fiber. The cleavage of a core-core fiber requires a perfect surface state on each of the cores to limit the optical losses and allow a good weld with an index gradient fiber 41. Unlike the cleavage of a monomode fiber having a only one core, the area having a perfect surface state after cleavage must be extended on the hearts of the multi-core fiber.

  Standard cleavers comprising a cleaving tool 110 and a movement in flexion and / or extension of the fiber are suitable for cleaving the multi-core fiber 40. However, the adjustments on the position of the cleaving tool 110, on the tension to apply on the fiber, on the bending level of the fiber should be optimized.

  2nd step FIGS. 12 and 13: The welding of a multi-core fiber 40 at the end of a graded index fiber 41 is made difficult by: different external geometries, as illustrated by FIG. 12: the index gradient fiber 41 is of circular section while the multi-core fiber 40 is composed of juxtaposed lobes; - natures of different fibers in chemical composition and stress distribution.

  As an example, the optimized welding process on the Ericsson registered FSU995 reference sealer is given. The welding of a circular fiber 41 on a multi-core fiber 40 is obtained: - by optimizing the centering of the two elements 40 and 41, by optimizing the welding program either: o a gap between the ends of the fibers of 50 ysm o a fiber penetration of 12 ycm o a first melting time of 0.3 s with a melt flow of 8 25 mA o a second melting time of 2 s with a melt flow of 10 mA o a third melting time of 2 s with a 13 mA melting current This process is transferable to equivalent equipment of another brand, with an optimization.

  As shown in FIG. 13, a gradual melting and penetration of the fibers 40 and 41 is thus performed during a sub-step referenced 130.

  An electric arc 132 is created between the electrodes vis-à-vis 133 to allow such a fusion. The two multi-core fibers 40 and index gradient 41 solidify (131) then.

  3rd step FIG. 14: The micro-lens is then produced by precision cleavage of the index gradient fiber 41 welded at the end of the multi-core fiber 40 in order to obtain a stretch of defined length 140. This cleavage is Conventional for the fiber section lamination process, it is independent of the multicore fiber and uses a cleaving tool 110.

  4th step: The calculation of the lengths 140 of the sections of index gradient fibers 41 (or pure silica fibers 50) is optimized for illumination originating from the cores of the multi-core single-mode fiber 40. Indeed the optical system presents a radial offset of the sources relative to the geometric axis of the index gradient fiber 41. This offset is taken into account to optimize the lengths of the sections.

  In view of the description above, the coupling device of the invention therefore has many advantages, among which may be mentioned: a collective enlargement of the optical beams from a multi-core fiber, facilitating its interconnection; magnification optics for integrating active or passive optical functions near the micro-optics; a low volume, with a substantially constant outer diameter close to that of the single-mode fiber, ie 125 μm; an absence of dioptre of air or glue in the optical path, by using a weld between the multi-core fiber 40 and the index gradient fiber 41. This absence of air diopter or glue limits the parasitic reflections and increases the resistance to intense light fluxes; a simple, fast and therefore inexpensive joining method through the use of fast and efficient mechanical alignment of the multi-core and index gradient fibers 41 in a conventional reflow welder. The optimization of the welding program thus allows the mufti-cores fiber 40 to be fused with the index gradient fiber 41 and optionally with the silica fiber 50; it does not require special polishing of the optical fibers but only a simple cleavage.

Claims (21)

  1. An optical coupling device for a multi-core single-mode fiber, characterized in that it comprises at least one gradient-index fiber section reported at a first end of said multi-core fiber, so as to widen the size of optical beams from the hearts of said multi-core fiber.   2. Optical coupling device according to claim 1, characterized in that it also comprises at least one section of pure silica fiber.   An optical coupling device according to claim 2, characterized in that said pure silica fiber section is inserted between said multicore fiber and said index gradient fiber section.   An optical coupling device according to any one of claims 1 to 3, characterized in that said section of pure silica fiber is soldered to the free end of said index gradient fiber section.   5. optical coupling device according to any one of claims 1 to 4, characterized in that it also comprises at least one spark gap fixed at a second end of said multi-core fiber, for connecting each core of said multi fiber -coeurs to a mono-core single-mode fiber.   6. Optical coupling device according to any one of claims 1 to 5, characterized in that the free end of said graded index fiber section is cleaved at an angle.   7. Optical coupling device according to any one of claims 1 to 6, characterized in that the free end of said index gradient fiber section has an antireflection treatment.   8. Optical coupling device according to any one of claims 1 to 7, characterized in that the free end of said graded index fiber section has a shaped lens.   9. An optical coupling device according to any one of claims 1 to 8, characterized in that it comprises a mirror located vis-à-vis the free end of said gradient index fiber section.   10. Optical coupling device according to any one of claims 1 to 9, characterized in that it comprises a filter on the free end of said gradient index fiber section.   11. Optical coupling device according to any one of claims 1 to 10, characterized in that said multi-core fiber comprises at least two optical cores in a silica matrix.   12. Optical coupling device according to any one of claims 1 to 11, characterized in that it has a substantially constant outer diameter equal to 125 m.   13. Optical coupling device according to any one of claims 1 to 12, characterized in that the length of said index gradient fiber section is optimized according to an optical function of collimation or focusing to achieve.   14. Optical coupling device according to any one of claims 1 to 13, characterized in that said multi-core fiber and said monomode fibers are polarization-maintaining fibers.   The use of the optical coupling device according to any one of claims 1 to 14 in a reflection arrangement, wherein a mirror is disposed opposite the free end of said gradient fiber section. index, so that a coupling is effected between two cores of said fiber mufti-cores arranged radially with respect to the optical axis of said device.   16. Use according to claim 15, characterized in that at least one optical component is inserted between said coupling device and said mirror.   17. Use of the optical coupling device according to any one of claims 1 to 14 in a transmission mounting, wherein is arranged in opposite two of said coupling devices, so that a coupling is carried out between a first core of a multi-core fiber of the first device and a second core of a multi-core fiber of the second device, said first and second cores being arranged radially with respect to the optical axis of said devices.   18. Use according to claim 17, characterized in that at least one optical component is inserted between said coupling devices vis-à-vis.   19. A method of manufacturing an optical coupling device according to any one of claims 1 to 14, characterized in that it comprises steps of: cleaving a multi-core fiber; welding a graded index fiber at the end of said cleaved multi-core fiber.   20. Manufacturing method according to claim 19, characterized in that it also comprises subsequent steps of: precision cleavage said index gradient fiber; optimizing the length of said gradient index fiber section.   21. Application of the optical coupling device according to any one of claims 1 to 14 to the realization of any of the optical components belonging to the group comprising: an insulator; a circulator; an optical attenuator; a switch; a polarization controller; a wavelength division multiplexing device; a channel multiplexer (interleaver); a module for inserting and extracting optical channels; - a filter of lines or bands.