FR2717913A1 - Multi-Centre/Single fibre optic connection for multi-element fibre optic cables - Google Patents

Abstract

Four fibre optic cables (F1-F4) are free at one end, and fixed at the other end. The fixed end has the fibre optic centres (I1-I4) forming a precision square of 44 microns spacing. The outer optical sleeves (21-24) are tangential to each and have a common outer diameter of 106 microns. To effect connection, the fixed fibres are placed in a holding clamp of the same dimension as the outer sleeves, and the cables are cut to length. The cables are glued together, then the outers stripped off with Hydrofluoric acid to form the connections.

Description

 The present invention relates to a component for connecting several optical fibers to a multi-core optical fiber. It also relates to a method for producing such a component.

 Multicore optical fibers are optical waveguides comprising in a single linear matrix a plurality of parallel guide cores.

 In particular, it has recently been proposed in the French patent application filed under number 93-01674 an optical guide having, in the same matrix, a plurality of cores, each surrounded by an optical sheath.

The axes of these hearts are arranged in the matrix so as to present between them highly precise geometric relationships, the respective positions of the axes of the hearts being defined to within a few tenths of a micrometer.

FIG. 1 shows an example of an optical fiber F of this type. This fiber F comprises, in a matrix M, four elementary optical guides G1 to G4 whose axes X1 to X4 form, in cross section, the vertices of a square C1 of high precision. The contours of the matrix M are defined by four identical portions
R1 to R4 of cylinders of revolution, the axes of which respectively coincide with the axes X1 to X4 of the elementary optical guides.

 This multi-core fiber F is for example of maximum outside diameter D equal to 125 μm, the width of the square C1 formed by the axes X1 to X4 being 44 Hm.

 Multicore optical fibers of this type make it possible to envisage replacing the architectures of the sharing type, usually implemented, in particular on telecommunications networks, by architectures of a lower cost where n guides connect n subscribers. In architectures of the sharing type, in fact, a single-mode fiber is shared between n subscribers and it is necessary to provide relatively expensive amplifiers and couplers on the networks. Multi-core fiber architectures allow these components to be saved.

 To date, however, an optical component has not yet been proposed allowing the connection of a multicore fiber of the type described in FR-93 01674 to n individual fibers.

 An object of the invention is therefore to propose such a component, as well as a method for its production.

 To this end, the optical connection component proposed by the invention is characterized in that it comprises several optical fibers each comprising at least one guide core surrounded by an optical sheath, these fibers being joined at a common end so that their optical sheaths are tangent to each other and the axes of their hearts are arranged in a geometry corresponding to that of the axes of a multi-core fiber to which this common end must be connected, said fibers being free with respect to each other at their opposite ends.

 It will be noted that such a connection component preferably has a structure of high precision so as to avoid losses of power between the hearts of the multicore fiber and the hearts of the individual fibers to which this multicore fiber is connected.

To this end, according to the production method proposed by the invention: - several fibers are juxtaposed longitudinally
optics each comprising at least one core of
guidance surrounded by an optical sheath, - at least one drop of a
liquid whose surface tension brings together
optical sheaths of said fibers so as to make them
tangent to each other, - the assembled optical sheaths are frozen when the axes
of their hearts are arranged in a geometry
corresponding to that of the axes of a multicore fiber at
to which the component must be connected.

 The invention also provides a connection assembly comprising such a component and means for its connection to a multicore fiber.

The description which follows is purely illustrative and not limiting. It should be read in conjunction with the accompanying drawings on which
Figure 1 has already been commented on and is a cross-sectional view of a multicore fiber;
Figure 2 is a schematic perspective view of a multicore component according to 1 invention
Figure 3 is a cross-sectional view of a connection component according to a possible embodiment of the invention, at its end intended to be connected to the multi-core fiber of Figure 1
Figure 4 shows in perspective a rod used as a position initiator element in a possible implementation of the method according to the invention
Figures 5a and 5b illustrate two steps in this process
Figures 6 and 7 illustrate two possible variants for obtaining the reduction in the diameter of the optical sheaths of the fibers
Figures 8 to 10 illustrate the means used according to the invention to achieve the gathering of fibers
FIG. 11 illustrates a possible arrangement of the fibers after assembly
FIG. 12 represents in section view a position initiating element used in a possible embodiment of the invention
FIG. 13 illustrates a step of depositing a mechanical sheath on the component according to the invention
FIG. 14 illustrates a possible variant of means for the connection of a component according to the invention and of a multicore fiber
FIG. 15 illustrates another possible variant for the connection of a component according to the invention and of a multicore fiber
FIG. 16 illustrates a connection component according to the invention for the splitting of a four-core fiber into two two-core fibers
Figures 17 and 18 are sectional views similar to Figure 3 of fiber connection components having respectively seven and nine hearts;
FIG. 19 represents a top view of the initiator element used, in a possible implementation of the method according to the invention, to produce the component of FIG. 18
FIG. 20 illustrates a possible variant for initiating the geometry of the fibers of the connection component.

 The connection component 1 shown in FIGS. 2 and 3 consists of four fibers F1 to F4 assembled at one common end and free at their other ends.

 Each of these fibers F1 to F4 conventionally comprises a core 11 to 14 surrounded by an optical cladding 21 to 24.

As illustrated in FIG. 3, the fibers F1 to F4 are tangent at their common end, their axes being arranged in a square C2 of high precision corresponding to the square C1 of the multicore fiber
F of Figure 1, to which this component 1 must be connected.

 By high precision is meant that the axes of the hearts have positions defined to within a few tenths of a micron.

 Since the square C2 is the same width as C1, the fibers F1 to F4 each have a diameter d of 44 μm at their common end; they are inscribed in a circle Ce with a diameter De equal to 106 μm.

 We will now describe an embodiment of the method according to the invention, for the production of component 1 from four conventional silica fibers of 125 μm in diameter of optical sheath.

First, we have the four fibers
F1 to F4 to be assembled, coated with their mechanical sheath, in a pre-initiator element, so as to arrange these fibers according to a geometry corresponding by homothety to the final arrangement which one wishes to obtain. This initiating element is for example a hollow rod of the rod type 2, which is shown in FIG. 4. This rod 2 comprises four notches 3 in the shape of a cylinder arc which each have a diameter corresponding to the diameter of the mechanical sheath of a fiber .

 The fibers F1 to F4 are held in this pre-initiator 2, either by gluing, or preferably mechanically.

 Once held in the pre-initiator 2, the fibers F1 to F4 are, at their end opposite to the pre-initiator 2, cut at the same height H of said pre-initiator, as illustrated in FIG. Sa, so that their ends to be assembled coincide. In this FIG. 5a and the figures which follow, the mechanical sheaths of the fibers F1 to F4 have been shown and referenced from 31 to 34.

 The fibers F1 to F4 are then subjected to a treatment intended to reduce their end diameter.

 To this end, in accordance with a particularly advantageous variant of the invention, the fibers F1 to F4 are collectively quenched, as illustrated by the double arrows T in FIG. 5b, without stripping, in a chemical attack bath 4, for example in a hydrofluoric acid bath of 48% concentration, for a period of the order of a few minutes.

 The hydrofluoric acid then attacks the optical sheaths 21 to 24 of the fibers F1 to F4, rising by capillarity along said sheaths. As shown in Figure 6, each fiber F1 to F4 emerges from the bath 4 with an optical sheath 21 to 24 having a substantially conical end shape.

 By way of example, fibers F1 to F4 have thus been obtained, whose optical sheaths 21 to 24 have a frustoconical end with a height h of about 7 cm, flaring from a terminal diameter d1 from 20 pm to a diameter d2 corresponding to the diameter of 125 pm.

 At least one zone of these frustoconical ends therefore has a diameter equal to the diameter d of 44 μm.

 The mechanical sheath 31 to 34 of each fiber F1 to F4 is then removed either chemically, mechanically or even by combining a chemical attack and a mechanical attack. In the latter case, etching is used to separate the mechanical sheath from the optical sheath and facilitate mechanical removal.

 Of course, as a variant, it is also possible to work directly with fibers the diameter of 44 Fm of which comes from fiber drawing.

 Also, as illustrated in FIG. 7, the diameter of 44 lun can be obtained by soaking in a bath 4 of 48% hydrofluoric acid, the stripped ends of the fibers F1 to F4, the mechanical sheaths 31 to 34 having previously been removed at these ends. The optical sheaths 21 to 24 of the fibers F1 to F4 then undergo a controllable cylindrical shrinkage over time.

 Note however that the first variant described (optical sheaths of frustoconical ends) does not require a precision control of the soaking time: it suffices that the soaking time is sufficient for the terminal diameter of the optical sheaths to be less than the desired diameter (44 Hm, in the example described). This solution is therefore preferred.

 As illustrated in FIGS. 8 and 9, once the position of the fibers F1 to F4 has been initialized, and the ends of the fibers possibly prepared so that they have the desired diameter, they are brought together, in accordance the invention, by depositing one or more drops G1 of a liquid having a high surface tension. The surface tension fc represented by arrows in these figures has the effect of grouping the four fibers F1 to F4.

 The liquid is chosen for its high surface tension fc and its compatibility with the fibers on which we are working. Examples of suitable liquids are ethanol or acetone. It will be noted that water which has significant surface tensions is not recommended with the silica fibers since it is likely to attack them.

 As illustrated in FIG. 10, the drops G1 of liquid are for example deposited on the fibers F1 to F4, in a zone of these just below the pre-initiator rod 2. They slide by gravity towards the ends to gather fibers F1 to F4.

 In FIG. 10, the rod 2 is carried by a bracket 5.

Under the action of the surface tension of the drops, the four fibers F1 to F4 can take two positions of equilibrium - a square equilibrium position, which is that
desired and which is represented in FIG. 3, - either a diamond-shaped equilibrium position such that
shown in Figure 11.

 In order to necessarily obtain the desired square geometry, the combined ends of the fibers F1 to F4 are introduced into an initiator element 6 carried by the bracket 5 in line with the pre-initiator element 2, below the latter. This element 6 is advantageously, as shown in Figure 12, a glass capillary whose inner surface has a shape which is of revolution and which is flared.

This element 6 is for example obtained from a cylindrical capillary of 0.8 mm inside diameter - heated and stretched so as to give it a shape
inner narrowed and flared, - then cut at its narrowed end into an area where it
has an inside diameter precisely equal to
diameter De of 106 Sm of the circle in which the fibers
F1 to F4 must be written at their end
to assemble.

 To pinpoint the area of the stretched capillary which corresponds to the diameter of 106 Sm, a calibrated metallic wire 106 m in diameter is introduced through its opening: the end of the calibrated wire is blocked by the diameter inside 106 Ktm of said capillary.

 The stretched capillary is then cut slightly below the area where the calibrated wire is blocked, then polished, by conventional techniques known to the skilled person, until the desired diameter is reached.

 The four fibers F1 to F4 assembled by the drops of liquid G1 are introduced into the initiator element 6 thus obtained, by the flared end of the latter.

 The introduction of the fibers F1 to F4 in the initiator element 6 is accompanied by a deposit of acetone drops on the fibers F1 to F4 so as to maintain said fibers assembled by surface tension, while ensuring the lubrication of the walls internal of the initiator element 6. The risks that the fibers do not break during their handling in the initiator element 6 are thus minimized.

 When the descent of the fibers in the initiator element 6 is blocked, the operator subjects the four fibers F1 to F4 to small alternating rotational movements, so as to force them to position themselves properly.

 This operation carried out, the zones of diameter d of the optical sheaths 21 to 24 are inscribed in the circle of diameter De, which defines the opening of small diameter of said initiator element 6. The optical sheaths therefore necessarily have the desired arrangement, the axes hearts together forming square C2.

The fibers are then cut at a distance of a few minimeters (2 to 4 mm) from the small diameter opening of the initiator element 6 and the fibers are glued
F1 to F4 at their ends projecting from said initiator element 2, so as to freeze the square position thus obtained.

 These ends can, for example, as illustrated in FIG. 10, be immersed in a tank 7 of cyanolite glue. The adhesive then rises by capillary action in the direction of the initiator element 6.

 It will be noted that the cyanolite glue has the property of drying very quickly, so that it freezes, before having reached the opening of diameter De of the initiator element 6.

This operation carried out, the end of the fibers
F1 to F4 which protrudes from the initiating element 6 is polished to the diameter of 106 ssm, or, advantageously, is left as is, the polishing then taking place at the time of the connection which will be described later in more detail.

 In a possible variant of the invention, the fibers F1 to F4 are then removed from the initiating element 6. Their assembly at their common end is consolidated by depositing a drop of cyanolite glue descending along said fibers F1 to F4.

 Then, as illustrated in FIG. 13, a coating 8 of mechanical sheathing is deposited on the end of the fibers. This operation is for example carried out by programmed descent arrow P), along the zone where the fibers F1 to F4 are assembled, of a deposition tank 9 (conventionally called by the skilled person, according to English terminology, " coating tank "), and means 10 for directing towards the deposition an ultraviolet polymerization radiation.

 In the case of optical sheaths with frustoconical ends, a gradient deposition is provided, so that the final component has, at its end where the fibers are assembled, a cylindrical mechanical sheath 8 of external diameter corresponding to that of the mechanical sheaths of the fibers classic units.

 Component 1 thus obtained is therefore easily usable with conventional connection means.

 At their end where they are free from each other, the fibers F1 to F4 can each be individually connected to a conventional unitary optical fiber, by usual connection means.

 At its other end, the component 1 is connected to the multi-core fiber F, as illustrated in FIG. 14, by conventional connection techniques using end fittings 11 and 12, and an element for restoring centering 13 receiving these tips.

 The relative positions of the multicore fiber F and of the component 1 are adjusted so that the axes of the hearts of the component 1 and the axes of the hearts of the multicore fiber F coincide.

 Advantageously used to make the connection end piece 11 which terminates the connection component 1, the flared capillary 2 which serves as an initiating element.

In this hypothesis, the fibers F1 to F4 have not left the capillary 2 after bonding in their assembled position: a drop of cyanolite glue is introduced through the end of the larger diameter of the capillary in order to secure the capillary 2 and the fibers
F1 to F4.

 As return element 13, it is possible to use a zirconia-alumina ceramic rod, the end openings of which are flared, this return rod 13 receiving on either side the end pieces 11 and 12 of the connection component and the multicore fiber.

 Such a centering rendering element constitutes with the connection end pieces that it receives and the component 1, an example of a connection assembly according to the invention.

As a variant, it is also possible to use as connection means, a connector of individual fibers with polarization maintenance, of the type marketed by the
RADIALL company under the name "MP connector". Advantageously refer to the catalog in this regard.
RADIALL: "Connectors for optical fiber-Monomode-VFO-MP" -
March 1992. Such a connector conventionally allows, on the one hand, the centering of the axes of two individual single-mode fibers with polarization maintenance and, on the other hand, the adjustment of the relative angular position of these two fibers around their common axis. .

 Used with the connection component according to the invention and a corresponding multi-core fiber, it allows, on the one hand, the centering of the squares C1 and C2, and, on the other hand, the angular adjustment of these squares so that their axes coincide.

 In another variant, the multi-core fiber F and the component 1 with four fibers may be bonded, their relative position having previously been adjusted on supports with V-grooves, by means of apparatus allowing precision movements on the fibers.

 In another variant, the multi-core fiber F and the component 1 can be assembled, as illustrated in FIG. 15, by fusion of an end zone Z of the multi-core fiber F on the end of the component 1. Electric arc apparatus used conventionally for the welding of fibers is used to produce such zone fusions.

 It is also advantageously possible to use the connection techniques described in FR-2 632 735.

 The invention has just been described here in the context of a connection to a fiber F with four cores. It applies of course in general to all connections to multicore fibers: connections of p individual fibers to a fiber n cores, or of one (or more) fiber (s) p cores to a fiber n cores, with p less than or equal to n. In particular, a four-core fiber can also be split, as illustrated in FIG. 16, into two two-core fibers.

 As a further example, FIG. 17 illustrates the cross section of the end of a component 101 for connection to a fiber with seven cores.

Six of the seven fibers 102 of this connection component 101 are distributed around a central fiber. The seven fibers 102 are tangent two by two and inscribed in a circle Ce. The technique previously described for connection to a four-core fiber applies in the same way, with the only modification being the change in the internal diameter of the initiator element.

 For non-circular multi-core fiber geometries, the internal geometry of the initiator element is modified accordingly.

 Thus, in the case illustrated in FIG. 18 of a component 201 for connection to a nine-core fiber, the nine fibers 202 are inscribed in a square Ca.

 An initiator element 203 of the type shown in top view in FIG. 19 is then used, having a flared inner shape whose end of small opening has an inner contour of square section Ca. This shape is easily obtained by introducing into a cylindrical pyrex capillary a multi-core silica fiber with nine hearts, and by heating and stretching this pyrex tube. The melting temperature of the pyrex is indeed 9000C, while that of the fibers is 20000C, so that it is possible to stretch the tube without varying the internal geometry of the multicore fiber. An initiator is therefore obtained, the narrowed end section of which corresponds to that of the nine-core fiber.

It should also be noted that the initiating element is not essential. I1 can in particular be omitted when the assembled fibers have only one possible equilibrium position, which is the case for example for a connection component with two fibers. It is also possible, as illustrated in FIG. 20, to place in the space or spaces between the fibers F1 to F4 a wire or a fiber.
Fc of calibrated outside diameter on which the fibers F1 to F4 are arranged tangentially when they are gathered by the drops of liquid, this thread (or this fiber) Fc calibrated (e) imposing on the fibers the desired configuration. In the case of the four-core connection described above, there is between the fibers F1 to F4 of 44 Hm of optical sheath diameter a wire Fc of 18 µm in diameter. The fibers F1 to F4 are therefore necessarily positioned in a square when they gather on this thread Fc.

 The invention has been described in the case of silica fibers, but of course also applies to plastic optical fibers.

Claims (16)

 1. Optical connection component, characterized in that it comprises several optical fibers (F1 to F4) each comprising at least one guide core (11 to 14) surrounded by an optical sheath (21 to 24), these fibers (F1 at F4) being joined at a common end so that their optical sheaths (21 to 24) are tangent to each other and that the axes of their hearts (11 to 14) are arranged in a geometry corresponding to that of the axes of a multicore fiber (F) to which this common end is to be connected, said fibers (F1 to F4) being free relative to each other at their opposite ends.  2. Component according to claim 1, characterized in that it comprises a connection end piece in which the fibers (F1 to F4) are arranged at their common end.  3. Component according to claim 2, characterized in that the connection end piece is a flared capillary (6) which has at its end of small section an internal contour in which the optical sheaths (21 to 24) of the fibers (F1 to F4) are necessarily registered according to the arrangement which they present at their common end.  4. Component according to one of claims 1 to 3, characterized in that it comprises a mechanical sheath (8) which surrounds the fibers (F1 to F4) in the vicinity of their common end, this mechanical sheath having an outer shape substantially cylindrical.  5. Component according to one of the preceding claims, characterized in that it comprises four fibers (F1 to F4) whose axes of the hearts (11 to 14) form, in cross section at their common end, the vertices of a square.  6. Component according to claims 3 and 5 in combination, characterized in that, at its end of small section, the flared capillary (6) forming connection tip has a circular contour of diameter 2V2d, where d is the diameter of the sheaths ( 21 to 24) fiber optics (F1 to F4) at this common end.  7. Connection assembly comprising a component (1) according to one of claims 1 to 6 and means (11, 12, 13) for the connection of the end of this component where its fibers (F1 to F4) are combined at the end of a multi-core fiber (F).  8. Connection assembly according to claim 7 comprising a component (1) according to one of claims 2 or 3, a centering return element receiving the connection connector of the component, as well as a connection connector of the fiber. multicore (F).  9. Method for producing an optical connection component according to one of claims 1 to 6, characterized in that - several fibers (F1 to  F4) optics each comprising at least one core (11 to  14) for guidance surrounded by an optical sheath (21 to 24), - at least one drop is deposited on these fibers (F1 to F4)  of a liquid whose surface tension gathers  the optical sheaths (21 to 24) of said fibers (F1 to F4)  so as to make them tangent to each other, - the assembled optical sheaths (21 to 24) are frozen  when the axes of their hearts (11 to 14) are  arranged in a geometry corresponding to that of  axes of a multicore fiber (F) to which the component  (1) must be connected.  10. Method according to claim 9, characterized in that, before freezing the optical sheaths (21 to 24), the optical sheaths (21 to 24) are introduced together in an initiator element (6) intended to necessarily give them an arrangement determined.  11. Method according to claim 10, characterized in that the initiating element is a flared capillary (6) which has at its end of small section an inner contour in which the optical sheaths (21 to 24) of the fibers (F1 to F4 ) necessarily fall under a specific provision.  12. Method according to one of claims 9 to 11, characterized in that the optical sheaths (21 to 24) are frozen by gluing in the zone where they are gathered.  13. Method according to claims 11 and 12 taken in combination, characterized in that the optical sheaths (21 to 24) are glued into the capillary (6).  14. Method according to one of claims 9 to 13, characterized in that the optical sheaths (21 to 24) are attacked chemically to reduce their external diameter in the vicinity of the zone where they are gathered.  15. The method of claim 14, characterized in that the ends of the fibers (F1 to F4) are soaked in a chemical attack bath.  16. The method of claim 15, characterized in that the ends of the fibers (F1 to F4) are soaked in a chemical attack bath (4) while they are covered with a mechanical sheath, so that the sheaths (21 to 24) optics emerge from said bath with a substantially frustoconical shape.