FR2772147A1 - Multiple center fiber optic coupling cable construction - Google Patents

Abstract

The inner section of the cable has an insert (M) with projections fitting into the gaps between the inner cables, preventing relative cable rotation.

Description

The present invention relates to a device for coupling multi-core optical fibers, in particular to four cores.

Single-core optical fibers consist of an optical guide, comprising a core surrounded by an optical sheath, and a protective coating without optical function, most often composed of a primary coating in contact with the optical sheath and possibly a secondary coating in contact with the environment external to the fiber.

Multicore optical fibers, as described in European patent application EP-A-0,611,573 and which are advantageous for the transmission of signals in greater number, in particular of images, are in fact fibers with several so-called optical guides. elementary. Thus, a multi-core optical fiber for example with four cores is formed of an optical guide called multi-core coated with at least one protective coating without optical function. The multicore optical guide in turn comprises four elementary optical guides, the axes of which form, in cross section, the vertices of a square, these elementary optical guides being distant from each other and embedded in an optical matrix forming an optical sheath common to all the elementary optical guides so as to produce said multicore optical guide. The contours of the matrix include four identical portions of cylinders of revolution whose axes respectively coincide with the axes of the elementary optical guides. In practice, the multicore optical fiber generally has a mark such as a singular heart different from the others or a mark made on the periphery, which defines the orientation of the pattern of the multicore optical guide. Thus, the location on the multicore optical guide marks out one of the cylindrical portions of said guide. The protective coating generally comprises a primary coating in contact with the optical matrix and possibly a secondary coating in contact with the medium external to the fiber.

The multicore optical fibers described in the above document are indicated as having been manufactured with excellent precision, of the order of a micrometer (or micron). Such multi-core optical fibers must transmit information from a remote point to another, and the maintenance of a multi-core optical fiber network requires making long connections or even simply repairs, for example in the case of accidental cutting or tearing of such fibers.

Thus, the coupling of multicore optical fibers is necessary.

In the case of conventional single-core optical fibers, comprising a cylindrical optical guide, it is well known, as described for example in US-A-3,989,567, to couple two single-core optical fibers by means of a coupling device comprising three cylindrical rods, which allow the two stripped optical fibers to be maintained, that is to say from which the protective coating (s) has (have) been removed, in axial extension for their connection with a low loss in signal transmission. The accuracy of such systems is good for optical fibers whose optical guide is cylindrical in shape. Such a coupling device is not satisfactory for coupling multi-core optical fibers with four cores, as described above, and the cross section of the optical part of which is not cylindrical. Indeed, the device with three cylindrical rods is of order 3 symmetry, while the optical fiber with four hearts is of order symmetry 2. This is why the precision, in particular angular, of the positioning of the two multicore optical fibers , is not sufficient to allow proper positioning of the hearts to be connected one facing the other. Thus, such a device, in the case of multicore optical fibers, leads to a degradation of the optical properties of the transmitted signals incompatible with the transmission requirements on the optical networks.

We also know the European patent application EP-A-0,611,573 cited above which describes a coupling device comprising a vee system on which the multi-core fibers to be coupled rest. Such a device is not entirely satisfactory because the heart furthest from the vee, in this case the C2 heart, undergoes too great a misalignment compared to the other hearts. Indeed, taking into account the manufacturing tolerances of the vee, it may happen that the stripped multi-core optical fiber is not well wedged in the vee and in this case it performs rotational movements which modify its positioning and lead to a misalignment of the hearts to be connected with respect to the cores of the multi-core optical fiber with which it must be coupled.

The present invention therefore aims to achieve a coupling device for multi-core optical fibers to maintain them firmly, so that they do not undergo too significant positioning change, but also flexible, to avoid problems misalignment from one multi-core optical fiber to another.

The present invention proposes for this purpose a device for coupling two multi-core optical fibers each of the type comprising a multi-core optical guide with four elementary optical guides with parallel axes for transmitting the optical signals, coated with at least one protective coating, the surface exterior of said multicore optical guide comprising four portions of identical cylinders of revolution whose axes respectively coincide with the axes of the elementary optical guides, said axes being in cross section at the vertices of a four-sided polygon, said device comprising a positioning vee intended to receive one of said portions of cylinders of revolution, characterized in that said device further comprises three other vee, the set of four vee defining a central channel for housing the multicore optical guide so that each vee is intended to receive a separate cylinder portion, of so as to prevent any rotation of the multicore optical guides to be coupled around their longitudinal axes.

Thus the coupling device according to the invention allows flexible coupling of two multi-core optical fibers which minimizes or even eliminates any risk of misalignment of the cores to be connected in the case of the coupling of two different multi-core optical fibers. In addition, the device according to the invention allows a firm hold of the two multicore optical guides of the multicore optical fibers to be coupled. By making a closed channel which firmly and flexibly encloses the multi-core optical guides of two multi-core optical fibers to be coupled, the positioning accuracy and above all the positioning reproducibility for each of the multi-core optical fibers to be coupled, even different, is very good , which results in a very weak attenuation of the coupling.

Finally, the possible angular indexing of the multi-core optical fiber and therefore of the multi-core optical guide, which is generally done by a location defining the orientation of the multi-core pattern, requires that the singular hearts of the multi-core optical guides must correspond opposite in the coupling device. The coupling device according to
I'invention therefore advantageously allows to position and maintain these corners, and therefore the orientation of multicore optical fibers.

The coupling device according to the invention may further comprise a clamping means so that the multicore optical guide is immobilized in said central channel. In one embodiment of the device according to the invention, said clamping means can be a clamping ring. In another embodiment of the device according to the invention, said clamping means comprises at least one heat-shrinkable element, for example having the shape of a sheath of heat-shrinkable material.

According to one of the preferred embodiments of the device according to the invention, the ves are defined by four substantially cylindrical pins, each of said pins being in tangential contact with two of the other neighboring pins.

Such an embodiment of the cutting device according to the invention, which requires the machining of cylindrical rods, is relatively easy to produce industrially and it is inexpensive. In addition, the use of such rods allows firm maintenance of the multi-core optical guides of the two multi-core optical fibers to be coupled, while achieving precise alignment, and therefore a weak attenuation of the transmitted signals, of said fibers.

In a preferred embodiment of the device according to the invention, each of said rods comprises a bar generally made of silica glass, optionally coated with an outer sheath generally made of elastomeric material.

The invention will be better understood and other advantages and particularities will appear on reading the description which follows, given without limitation, with reference to FIGS. 1 to 7.

FIG. 1 schematically represents a multi-core optical fiber with four hearts in cross section, before stripping for coupling.

FIG. 2 shows a schematic perspective view of a first embodiment of the coupling device according to the invention before the coupling of two multicore optical fibers such as that shown in FIG. 1.

Figures 3 and 4 respectively represent schematic perspective and cross-sectional views of the first embodiment of the coupling device according to the invention, after the coupling of the two multi-core optical fibers.

FIGS. 5 and 6 schematically represent views in cross section of a second embodiment of the coupling device according to the invention, respectively before coupling and after coupling, of multicore optical fibers such as that represented in FIG. 1.

FIG. 7 schematically represents a cross-sectional view of a third embodiment of a coupling device according to the invention, after the coupling of multicore optical fibers such as that shown in FIG. 1.

Figure 1 shows a multi-core fiber 1 in cross section. comprising a multicore optical guide G surrounded by two protective coatings without optical function, the primary coating Ri and the secondary coating R2. The multicore optical guide G comprises four elementary optical guides G1, G2, G3 and G4, each comprising an optical core, respectively the hearts C1, C2, C3 and C4, the axes of which XI, X2, X3 and X4 form the vertices of a square. The elementary optical guides G1, G2, G3 and
G4 are embedded in an optical matrix M which forms an optical sheath common to all the elementary optical guides so as to produce said multi-core optical guide G. The contours of the matrix form four identical portions of cylinders of revolution whose axes coincide respectively with the axes X1, X2, X3 and X4 of the elementary optical guides G1 to G4.

In an exemplary embodiment, the side of the square formed by the centers of the elementary optical guides G1 to G4 is of dimension substantially equal to 52 μm, and the multicore optical guide is such that the dimensions x and y, representing respectively the diagonal and the short side of the square CA1 respectively CA2 cirsconcrit to the multicore optical guide G, are respectively substantially equal to 152 µm and 131 µm respectively. The precision on the dimensions is e 2 μm, corresponding to three standard deviations, the precision on the position of the cores C1 to C4 in the multicore optical guide G being + 1 μm, corresponding to three standard deviations. The outer surface of the primary coating R1 is a cylinder with a diameter substantially equal to 195 pu (+ 2 pm). The external surface of the secondary coating R2 is a cylinder with a diameter substantially equal to 245 μm (+ 2 μm). Once this multi-core optical fiber is stripped, that is to say after its protective coatings R1 and R2 have been removed from it, there remains only, for subsequent coupling, the multi-core optical guide G.

FIG. 2 is a schematic perspective view of a first embodiment of the coupling device 2 according to the invention, before the coupling of the multicore optical guides G and G 'of two multicore fibers such as that shown in FIG. 1.

Said coupling device comprises four substantially cylindrical pins 5 which are bars generally made of silica glass because this material advantageously makes it possible to obtain a substantially identical coefficient of expansion for the fibers and for said bars. The four rods 5 joined to each other by tangential contact of each of the rods with two of the other neighboring rods define a central channel C which forms four ves V1, V2, V3 and V4. FIG. 2 also shows the positioning of a substantially cylindrical fiber tool 4 in one of the ves V4 (not shown in FIG. 2) of the central channel C. A sheath 20 holds all of the four pins 5 joined together.

Said retaining sheath 20 is made of heat-shrinkable material, for example of modified polyolefin crosslinked by irradiation. To obtain coupling, we began to carry out a partial constriction of the sheath 20. This constriction is carried out at the ends, giving rise to two constricted zones 21 and 23, the zone 22 representing the part of the central sheath on which no means of outside heating was not applied. The sheath 20 therefore consists of zones 21, 22 and 23. This constriction makes it possible to pre-assemble the pins 5 in order to obtain a central channel C precalibrated on the fiber tool 4.

According to the previous embodiment of the multicore optical guide G, the rods are for example cylinders with a diameter substantially equal to 300 μm (+2 μm) and of length equal to 1.5 cm (+ 2 mm). According to this exemplary embodiment, the fiber tool 4 has an outer surface of diameter substantially equal to 129 μm (+ 2 μm).

Figures 3 and 4 respectively represent schematic perspective views and in cross section of the first embodiment of the coupling device according to the invention, after the coupling of the two multicore optical guides G and G '.

To obtain coupling, the tool fiber 4 was removed, and the respective introduction of two muttico-core optical guides G and G 'was carried out at the two ends of the rods, generally accompanied by the introduction of an index gel intended to ensure the optical contact between the two guides. The two guides G and G 'are then approached from each other as close as possible to the center of the central channel C, or even in partial contact with one another. In the case of the aforementioned digital example, the length of each guide in the channel is of the order of 7.5 mm (+ 2 mm). The pursuit and the end of the shrinking of the sheath by the heating means not shown leads to the coupling of the guides G and G ', with excellent precision. Indeed, said guides are blocked in angular rotation with the immobilization of each of the cylinders of revolution forming the outer surface of the multi-core optical guide G or G ', and the constriction on the rods allows a flexible contraction around the two guides G and G' in a similar holding envelope around G and G '. Thus, the coupling device according to the first embodiment does not add to the possible inaccuracies in positioning the hearts CI to C4 an inaccuracy in positioning a guide G relative to another guide G '. As a result the sheath is completely shrunk into a sheath 24 securely holding the pins 5 clamped around the multi-core optical guides G and G '.

FIG. 3 shows, in perspective, the positioning of the guides G and G 'in the device 2, after said coupling.

FIG. 4 shows, in cross section, such a positioning in connection with FIG. 3. We see there the central channel C forming the four ves V1 to V4, and the guide G sandwiched between the four pins 5 held in the sheath 24. Another cross section would similarly show the positioning of the guide G '.

Advantageously, the ends of each of the four pins 5 are tapered so as to form an entry cone of the multicore optical guides G and G ', facilitating their introduction into the coupling device 2 (not shown).

In the context of this first embodiment of the device according to the invention, the clamping means comprises at least one heat-shrinkable element in the form of a sheath 20 of heat-shrinkable material enveloping all four pins 5.

With the device according to the invention, a first embodiment of which has been presented in FIGS. 2 to 4, it can be seen that any multicore optical guide G or G 'maintained by all four pins 5 after the total shrinkage of the sheath 20 in a sheath 24 is held securely and is fixed angularly so as to be in front (heart to heart) of another similar guide G 'or G and to produce a coupling device 2 in which the risks of misalignment between optical fibers different multi-cores are minimized or even canceled.

With the use of the second embodiment of the coupling device according to the invention as shown in Figures 5 and 6, the average attenuation after coupling of the transmitted signals was found to be 0.17 dB with an estimated standard deviation 0.1 dB as part of an experiment using the connection of two fibers from the same production.

In another variant of this first embodiment of the device according to the invention, more than two, for example three, constricted zones are made to form the central precalibrated housing channel around the tool fiber.

FIG. 5 schematically represents a cross-sectional view of a second embodiment of the coupling device according to the invention, before coupling of the multicore optical guides G and G '(only the multicore optical guide G is visible in FIGS. 5 and 6).

The coupling device 2 ′ comprises four pins 6 which each comprise a central bar 51 made of hard material such as silica glass coated with a softer material 60 such as an elastomer. A sheath 70 holds all of the four rods 6 joined together. Said retaining sheath 70 is made of heat-shrinkable material, for example of modified polyolefin crosslinked by irradiation. The central channel C formed is similar to that of FIGS. 2 to 4. The multicore optical guide G is introduced into the central channel C, bearing against the vee V4. At the other end of the rods 6 is introduced the other multicore optical guide G 'in a similar manner. The contacting of the two guides G and G 'and then the coupling are similar to those explained above in the context of the first embodiment of the device according to the invention.

The presence of the flexible material 60 outside the pins 6 allows the compact assembly of the pins 6 in contact with each other while providing a channel C of wider section than the guide G or G ', and then allows a reduction of the section by deformation of the elastomeric material, for example silicone elastomer capable of a maximum deformation of 150%.

In the context of this second embodiment of the device according to the invention, the clamping means comprises at least one heat-shrinkable element in the form of a sheath of heat-shrinkable material 70 enveloping all four pins 5.

FIG. 6 schematically represents a cross-sectional view of the second embodiment of the coupling device 2 'according to the invention, after the coupling of the multicore optical guides G and G'. The retaining sheath 70 has shrunk into a sheath 71.

FIG. 7 schematically represents a cross-sectional view of a third embodiment of a 2 "coupling device according to the invention, after the coupling of the multicore optical guides G and G 'of two multicore optical fibers such as that shown in figure 1.

In this embodiment, the guides G and G 'are introduced into a central channel C calibrated in size and predefined in the factory by the shrinking of a heat-shrinkable sheath enclosing the four rods 5, similar to those represented in FIGS. 2 to 4, in a sheath 80. Compared with the first embodiment according to the invention, this third embodiment of the device according to the invention is such that a clearance of 2 μm at most, necessary for the introduction of the multicore optical guides G and G 'is established by construction between said guides G and G' and the surface of the pins 5. In this third embodiment of the device, there is no pre-calibration of the central channel C before the installation of the guides G and G '(only the guide G is visible in FIG. 7), that is to say that the shrinking of the envelope into an envelope 80 is directly carried out in a single step around the guides G and G' .

Thus, this third embodiment of the device according to the invention allows the guides G and G 'to be installed with play. To reduce the misalignment linked to the play of the guides G and G ', a known method and generally used in the context of said third embodiment consists in imparting a curvature to the coupling device with cylindrical rods, deformable by its small section opposite its length. This curvature is generally produced by introducing said coupling device into a mechanical reinforcing element having an appropriate longitudinal shape allowing the device to bend, which forces the multicore optical guides G and G 'to match the same vee, thus significantly reducing the defect of 'alignment.

Claims (9)

 CLAIMS 1.1 Device for coupling two multi-core optical fibers each of the type comprising a multi-core optical guide (G; G ') to four elementary optical guides (G1, G2, G3, G4) with axes (X1, X2, X3, X4), parallel for transmitting the optical signals, coated with at least one protective coating (R1; R2), the external surface of said multicore optical guide forming four portions of identical cylinders of revolution whose axes coincide respectively with the axes of the elementary optical guides, said axes being in cross section at the vertices of a four-sided polygon, said device comprising a positioning vee (V4) intended to receive one of said portions of cylinders of revolution, characterized in that said device further comprises three other ves (V1, V2, V3), all four ves defining a central channel (C) for housing the multicore optical guide so that each vee is intended to re cevoir a separate cylinder portion, so as to prevent any rotation of the multicore optical guides to be coupled around their longitudinal axes. 2.1 Device according to claim 1 such that the ves are defined by four substantially cylindrical pins (5; 6), each of said pins being in tangential contact with two of the other pins. 3.1 Coupling device according to claim 2 such that each of said rods comprises a silica glass bar (5; 51). 4. / A coupling device according to claim 3 such that each of said bars is coated with an outer sheath (60) made of an elastomeric material. 5.1 Device according to one of claims 2 to 4 such that the ends of each of said pins are tapered so as to produce an inlet cone of the multi-core optical guide. 6. / coupling device according to one of claims 1 to 5 as it further comprises a clamping means so that the multicore optical guides are immobilized in said central channel. 7. / Device according to claim 6 such that said clamping means comprises at least one heat-shrinkable element. 8.1 Coupling device according to claim 7 such that said heat-shrinkable element has the form of a sheath made of heat-shrinkable material (20; 70). 9.1 Device according to claim 6 such that said clamping means is a clamping ring.