FR2546308A1 - Sealed penetration (lead-in) for optical fibres - Google Patents

Abstract

The invention relates to a sealed penetration (lead-in) for a plurality of optical fibres. This penetration (lead-in) comprises an endpiece (ferrule) 12 of a given length which is pierced by a plurality of holes 14 through which the fibres pass, sealing being obtained by coating the fibres 6 with adhesive over the entire length of the holes. The invention applies especially during pressure testing of transmission cables.

Description

WATERPROOF FITTING FOR OPTICAL FIBERS
The present invention relates to a sealed crossing for a plurality of optical fibers of the same cable or of several cables.

 Generally, a sealed passage through a partition or wall is used to isolate from one another two media containing fluids which may be different and at different pressures.

 In the field of optical telecommunications or data transmission cables, intended for example for an underwater application, it is customary to test these cables or the components of these cables under pressure by installing the (s) ) fiber optic cable (s) in a box filled with a pressurized fluid, such as for example water, and closed by a cover isolating said fluid from the surrounding air. In order to carry out the various tests residing, for example, in a measure of attenuation in transmission of the optical fibers using processing and measuring equipment placed in the surrounding medium (air), it turns out to be necessary to pass through so - seals the optical fibers by the wall of the cover closing the box.

 In addition, in the case of one or more optical cables with high fiber density, also called multi-fiber cables, and with a given structure, such as for example the grooved cylindrical structure, it is desirable to make a so-called multi-fiber waterproof crossing in conditions of very reduced bulk and without modification of the geometrical position of the optical fibers at the level of the crossing.

 There are already known different types of sealed crossing, as described for example in French patent application No. 2,493,952.

According to this request, the watertight bushing is of the monofiber type, that is to say for a single optical fiber, and comprises a thin disc having a central hole for the passage of the fiber, the momentum seal, obtained by gluing two the he has dsje along the length of the hole. In addition, the sealed bushing thus formed is designed to be able to withstand high pressures, of at least 7 'bars.

 However, such a sealed crossing has drawbacks. On the one hand, the structure of this crossing is designed for.

a single fiber and can only be adapted for a high density of optical fibers by installing as many crossing structures as there are fibers to be crossed, which leads to a very large bulk and to a short of high achievement. In addition, as the cross-piece is constituted by a thin disc, a high pressure risks causing its deformation, which can create a lateral compression stress on the fiber, and consequently a breakage of this. last.

On the other hand, the fiber leaving the disc is bare, therefore unprotected, therefore risking breaking during handling.

 The present invention aims to remedy these drawbacks by proposing a sealed crossing of the multifiber type, this is done for several optical fibers, which is entirely satisfactory in terms of sealing, inexpensive, and of reduced bulk.

 To this end, the invention relates to a sealed passage for a plurality of optical fibers, characterized in that it comprises a support piece in the form of a nozzle of given length and pierced with a plurality of holes for the passage of the fibers, and in that the fibers are glued over the entire length of the holes.

 It is understood that thus the nozzle pierced with holes will allow the fibers to pass overall, that is to say all together in a single operation, through the holes in the nozzle, which will guarantee rapid implementation of crossing. In addition, as the pierced end piece is in the form of a crossing piece of a certain given length, the stresses on the glued fibers repair themselves will be drawn much better over the entire length of the fiber passage holes.

 According to another characteristic of the invention, the fibers present at the outlet of the endpiece are protected by flexible sheaths in the form of tubes, which avoids breaking the fibers during any manipulation.

Other characteristics and advantages of the invention appear better in the detailed description which follows and refers to the attached drawings given solely by way of example and in which:
- Figure 1 shows an exploded perspective view of the installation of a bushing according to the invention on a fiber optic transmission cable, the two end parts of the bushing being shown separately;
- Figure 2 shows an elevational view of the bushing according to the invention;
- Figure 3 shows a sectional view of the sealed crossing according to the invention rnontée on the optical cable of Figure I; and
FIG. 4 represents a sectional view of the assembly of the sealed bushing of FIG. 3.

 In these different figures, the same references relate to the same elements.

 In order, for example, to test an optical transmission cable under pressure, with a grooved cylindrical structure for example, and installed in a box closed by a cover and filled with a fluid, such as for example water, at a pressure given, for example of the order of 120 bars, it is customary to pass the optical fibers of the cable through the wall of the cover, out of which they open into the open air at atmospheric pressure of a bar.

 To this end, a waterproof crossing of the multifiber type is mounted at the end of the optical cable, that is to say for several optical fibers, in accordance with the invention, which will now be described with reference to Figures 1 to 3.

 In Figure 1, there is shown at 1 the end of a fiber optic transmission cable structure, for example cylindrical grooved. The cable 1, of axis XX ′, comprises a central reinforcement or core 2 intended to stiffen the cable and to absorb the longitudinal stresses, around which is disposed a dielectric cylindrical support or rod 3 hollowed out with grooves 4, for example in number of six, longitudinal or helical with single or alternating pitch, with a profile, for example in the shape of a V, and distributed uniformly on the external surface of the rod 3. Inside each groove 4 is housed an optical fiber 6, and there is shown in 7 the cable protection envelope.

 For the subsequent mounting of the waterproof bushing according to the invention on the cable I, we first prepare the end of the cable by first stripping the cable over a predetermined length, then maintaining the protective envelope 7 using an elastic sleeve 9, an outward development of the fibers 6 thus released, and a section perpendicular to its axis of the rod 3 stripped of its fibers, so that the central core 2 is longitudinally protrusion.

 According to the invention, the watertight crossing comprises means for supporting the fibers at the outlet of the cable 1, generally identified at 10 in FIGS. I and 2, and which comprise a smooth part being in the form of an end piece 12 for example cylindrical, of diameter greater than that of the rod 3 and of given length L equal to a large number of times the diameter of the optical fibers 6. This end piece 12 is for example obtained by molding of a tight material chosen so that said end piece can withstand a given pressure Thus, for example for a maximum pressure of the order of 200 bars, the molded end piece 12 is made of a non-metallic material, such as for example a cold-polymerizable resin, composed for example of methyl methacrylate , possibly charged for example with silica. For higher pressures, for example of the order of 1000 bars, the molded end piece 12 can be made of a suitable metallic material.

 As it appears in FIGS. I and 2, the end piece 12 constituting the leaktight crossing piece is pierced with a plurality of openings or holes 14 having a shape in \ ', distributed evenly in a circle centered on the longitudinal axis of the endpiece, and in number equal to that of the grooves 4 of the cable, that is to say six in the example chosen. Of course, the shape of the holes 14 can be any, round for example, without departing from the scope of the invention.

In addition, the number of holes in the endpiece 12 shown in Figure I is not at all limiting, it being understood that there are at least as many holes as grooves in the cable and that a groove can house several optical fibers.

 The support piece 10 further comprises a cylindrical intermediate piece 16, of the same diameter as the rod 3, and intended to ensure the coupling of the cable 1 with the end piece 12 during its assembly.

This intermediate piece 16 is for example made of the same material as the end piece 12 and preferably comes from molding with the latter.

 The intermediate part 16 is hollowed out with longitudinal, parallel grooves 18, of profile for example in the form of a V, and distributed uniformly over the external periphery of the part 16 at the same angular spacing as the grooves 4 of the cable and that the holes 14 of the 'tip. This part 16 also includes a central bore 20 intended to receive the central core 2 of the cable, as will be explained below.

 As it appears in FIG. 2, the intermediate piece 16 is connected to the end piece 12 by a circular groove 21 whose diameter, less than that of the piece 16, is chosen slightly greater than the diameter of the circle inscribed by the bottoms of the grooves , the depth of the grooves at the part 16 being identical to that of the rod 3 of the cable.

 After having prepared the end of the cable 1 as shown in FIG. 1, the optical fibers 6 are positioned by insertion into the grooves 18 of the coupling part 16, then the support part 10 obtained by molding is driven in translation in direclioll of the cable l, so that the line Z of the target penetrates into the bore 20 of the part 16, thus ensuring the f; - ion of the support part lú with the cable 1. In addition, during the translation of the piece 10, the optical fibers penetrate into the holes 14 of the end piece 12 to pass through it and flourish at the outlet over a given length, as it appears in FIG. 3.

 Thus, from the grooved cylindrical structure of the cable with several optical fibers, the size and the shape of this structure have been preserved in order to achieve a leaktight crossing on the one hand without discontinuity of the fibers at the crossing and on the other leaves without changing the geometric position of the fibers at the crossing.

 Then slide a flexible O-ring 23 (Figure 3), previously inserted around the rod 3, along the coupling piece 16, so as to apply and maintain the fibers at the bottom of the V-grooves, until 'it occupies its final place in the groove 21 (Figure 2).

 The following operation consists in stiffening the cable connection 1- support part 10 by means of a waterproof coating sheath 25 (FIG. 3), of given length, and for example coming from molding of an epoxy resin which is deposited around the 3-piece coupling ring assembly 16.

 As it appears in FIG. 3 representing the final stage of mounting the bushing on the optical cable, the sealing of the tip / optical fibers is obtained by gluing the fibers 6 over the entire length of the holes 14 from the tip 12 to the using an adhesive 27, such as for example an epoxy resin, injected into said holes by any suitable means, such as for example a syringe. Thus, since the fibers inside the end piece are coated with glue over the entire length of said end piece, a better distribution of the stresses on the fibers is created.

 Advantageously, the optical fibers exploded at the outlet of the sealed passage are protected by flexible sheaths 30 (FIG. 3), in the form of tubes, possibly colored, and made of a plastic material such as for example Kynar, thus ensuring mechanical protection. fibers during handling.

 Once the fibers are positioned in the sheaths 30, the latter can either be fixed by any suitable means on the outlet face of the endpiece, or engaged in the holes 14 over all or part of the length of the latter where they are glued. using for example an epoxy resin injected into the holes 14 over their entire length, as above.

 It will be noted that the sealed crossing described above in relation to an optical cable of the grooved cylindrical type can undergo various modifications which are within the reach of those skilled in the art.

Thus, in the case where the optical fibers are present alone, that is to say without cable support, it is possible to use only the pierced end piece 12 of cylindrical or other shape without the intermediate coupling piece 16. In addition, in the in the case of a crossing for optical fibers of two or more cables of any structure, it suffices to position on the support piece 10 an additional piece of suitable shape ensuring the grouping of the various fibers.

 FIG. 4 represents an example of assembly of the sealed bushing as illustrated in FIG. 3, for an enclosure or box filled with a fluid at a given pressure. The wall or partition of the enclosure is shown at 35. pass through in which is provided a housing 37 intended to receive the sealed crossing piece (cylindrical end piece 12) mounted on the optical cable 1.

The endpiece / wall seal is obtained by placing, for example, an O-ring 39 placed at the bottom of the housing 37 and against which the endpiece 12 is pressed by a cover or plug 41 pierced with a central opening 43 for the passage of the sheaths 30 for protecting the optical fibers 6. The cover 41 is made integral with the wall 35 by any suitable tightening means, such as for example screws represented schematically by their axis at 45.

It will be noted that the seal 39 could be replaced for example by a cable gland (not shown) mounted around the endpiece 12 in the housing provided 47, without departing from the scope of the invention. In this case, the cable gland seals in the end and the end piece thus creates an arched effect which does not transmit any lateral compression stress to the optical fibers.
The sealed bushing according to the invention described above was made for an optical pressure transmission cable installed in a box and intended to be subjected to tests. Of course, it is possible to envisage other applications of this sealed crossing, and in particular that for pressurizable cables requiring connections or junctions at the level of which a partition is to be crossed in a sealed manner, without departing from the scope of the invention.

 Of course, the invention is in no way limited to the embodiment described and shown and includes all the technical equivalents of the means described as well as their combinations if these are carried out according to the spirit of the invention and implemented in the context of the following claims.

Claims (13)

 1. Sealed feedthrough for a plurality of optical fibers (6), characterized in that it comprises a support piece (10) in the form of a nozzle (12) of given length and pierced with a plurality of holes (14) for the passage of the fibers, and in that the fibers (6) are glued over the entire length of the holes.  2. A bushing according to claim 1, characterized in that the end piece (12) is obtained by molding of a chosen material so that the end piece can withstand a given pressure.  3. Bushing according to claim 2, characterized in that, for a maximum pressure of the order of 200 bars, the material comprises a resin.  4. Bushing according to one of claims 1 to 3, characterized in that the fibers (6) present at the outlet of the end piece (12) are protected by flexible sheaths (30) in the form of tubes.  5. Traverse according to claim 4, characterized in that the protective sheaths (30) extend longitudinally inside the end piece (12) over at least part of the length of the holes (14) where they are engaged , and in that the sheaths are glued on their respective parts housed in the fiber passage holes.  6. Bushing according to one of claims 1 or 5, characterized in that the adhesive (27) is an epoxy resin.  7. Bushing according to one of the preceding claims, characterized in that! 'End piece (12) is cylindrical.  8. Crossing according to the Revenuica.ion 7, characterized by the fact that the holes (14) for the passage of the fibers are rr-r is regularly in a circle centered on the longitudinal axis of the end piece.  9. Bushing according to claim 8, for an optical cable comprising a cylindrical rod (3) hollowed out of grooves (4) in which the optical fibers (6) are housed, characterized in that the end piece (12) is coupled integrally to the rod by a cylindrical intermediate piece (16) hollowed out with grooves (18) at the same angular spacing as the grooves (4) of the rod and the holes (14) for passage of the fibers, so that the fibers released at the end of the rod are inserted into the grooves of the intermediate piece (16) and penetrate into the holes of the end piece (12) to pass through it.  10. Bushing according to claim 9, characterized in that the intermediate part (16) comes from molding with the end piece (12).  11. Crossing according to one of claims 9 or 10, characterized in that the rod comprises a central core (2) protruding longitudinally, and in that the intermediate piece (16) is provided with a central bore (20) receiving the soul of the rod.  12. Bushing according to one of claims 9 to 11, characterized in that the intermediate piece (16) is stiffened at the cable bead by means of a waterproof coating sheath (25) bonded to the intermediate piece and to the rush.  13. Bushing according to one of the preceding claims, characterized in that the end piece (12) is pressed by a cover (41) against a seal (39) placed at the bottom of a housing (37) formed in a wall (35) of an enclosure, and in that the cover is fixed to the wall by clamping means (45).