FR2544506A1 - Extensible cord (lead), in the form of a helix, for optical fibres and process for manufacturing such a cord (lead) - Google Patents

Abstract

The subject of the invention is an extensible fibre-optic cord (lead), which can withstand being flattened, comprising, inside an outer protective jacket 50, a flexible filler foam 40, advantageously having closed cells (especially foamed polyethylene), which is extruded over at least one optical fibre 20. This cord (lead), which advantageously comprises optical conductors 20 and electrical conductors 30, is preferably made in a single operation by tandem (double) extrusion. Application: especially waveguides (or light guides) between movable elements.

Description

 The invention relates to an extensible fiber optic cord, in particular for a waveguide (light) between mobile elements, of the type comprising an outer sheath of elastic thermoplastic material, shaped as a helix with generally contiguous turns, lined with a body. internal reinforcement capable of opposing the flattening of the cord, and an optical fiber inside the reinforcement body.

The invention also relates to a method of manufacturing such a cord.

 We know, in particular by the French patents Nos.

 1 * 916.1G9, 1.135.524 and 1.164.898 of extensible cords comprising an elastic outer sheath and a core composed of flexible insulated electrical conductors with textile stuffing, the outer sheath being conformed to the body in a helix with contiguous turns, so as to allow significant extensibility by pulling the cord with spacing of the turns, the cord resuming in original shape with contiguous turns when the traction is released, due to the elasticity of the sheath.

 French patent application N0 79 10250, published under Nb 2.455s296, describes a cord of the same kind, but with an optical fiber to replace the electrical conductors. According to this request, a tubular sheath consisting of an inner polyethylene tube and an outer coating of polyvinylchloride provides an internal channel through which an optical fiber passes freely. The sheath is then spiraled into contiguous turns on a mandrel, and steamed until it retains the helical shape with contiguous turns.

 According to the teachings of this application, the inner polyethylene tube is intended to resist crushing so that the optical fiber is not pinched in the internal channel during deformation of the cord. It is neces sary indeed that the optical fiber is not subjected to significant tensile forces, nor to bending on a radius too small. However, the helical conformation allows considerable overall extensions, with however for the interior fibers of the cord deformations corresponding to small variations in the radius of curvature and to twists and longitudinal variations of a second order importance. It is conceivable that the fiber free optics in the central channel of the helically shaped sheath can undergo only little damaging deformation.

 I1 was also known previously to have optical fibers in helical paths in cables, providing them with a certain freedom of movement, in order to avoid breakage of the fibers under the effect of traction or bending of the cables.

 However, in the extensible cord according to the aforementioned application where the consequences of the lengthening of the cord are offset by a great freedom of movement of the sheath1 this great freedom of movement imposes in return certain restrictions. It would be unwise to have several optical fibers and / or electrical conductors in the cord. The reciprocal friction of the conductors, both optical and electrical, or their jamming could lead to breakage of optical fibers.

Furthermore, it is questionable whether in a single fiber bead, the latter is not likely to be damaged by concentrations of stresses in its contacts with the tubular sheath, or by vibrations, which would bring a length of fiber into resonance. optic in the sheath.

In addition, the junctions of the optical fiber at the ends of the cord are delicate, since the fiber is not stopped at any point of the sheath, and is capable of being drawn over its entire length.

 The object of the present invention is to overcome these drawbacks and aims, on the one hand, to avoid any contact of an optical fiber both with the outer sheath and with other optical and / or electrical conductors arranged, where appropriate. , in the same sheath, and, on the other hand, to distribute optimally throughout the optical fibers the forces applied to them, during helical spiraling of the cord, then in service, and to place these fibers in a state of compression stresses essentially.

 The invention provides for this purpose an extensible bead with optical fibers, of the kind comprising an optical fiber inside an outer sheath formed at least partially in an elastic thermoplastic material shaped helically, this bead, adapted to resist 0 its enlatisscment, being characterized in that said outer sheath encloses a flexible foam stuffing extruded on at least one optical fiber.

 Thanks to its flexibility, and unlike the textile stuffing materials used in the prior art in association with electrical conductors, the foam of which, according to the invention, an optical fiber is surrounded within a cord collects the stress concentrations whose sheath of the cord can be the seat, and thus ensures a smoothing of the stresses exerted along the optical fiber.

This foam adheres to the fibers and keeps them axially, possibly by compressing them; it is of course capable of preserving an optical fiber from any contact with another optical or electrical conductor disposed if necessary in the same sheath. This function of positioning the conductors further facilitates the identification of the latter at the two ends of the cord.

 Insofar as the sheath is itself extruded around the foam, the invention also proposes to manufacture an extensible bead of the aforementioned type in a single operation, by means of a combined die supplied from two sides.

Other objects, characteristics and advantages emerge from the description which follows, given by way of example, with reference to the appended drawing in which
Figure 1 is a side view of an extensible cord according to the invention in the retracted resting configuration
Figure 2 is a sectional life of an extensible cord with four optical fibers and two electrical conductors.

 At rest, an extendable cord 10 with optical fibers according to the invention has two substantially straight ends 11 and 12 between which it is shaped as a helix 13 with contiguous turns 14. The latter are of the same dimensions and commonly admit a radius of curvature substantially greater than the diameter of the bead 10 so as not to cause excessive bending stresses to appear there.

 FIG. 2 represents an extensible cord 10 of the aforementioned type in which are represented, by way of example, four optical fibers 20 and two electrical conductors 30. The optical fibers 20 are conventionally surrounded by a protective coating 21 and reflection suitable for avoiding any lateral light leakage. Also in known manner, the electrical conductors are surrounded by an insulating layer 31o. These conductors can be twisted or parallel.

 According to the invention, these optical and electrical conductors are embedded in a ventilated flexible synthetic foam 40, covered with a protective sheath 50.

 A bead according to the invention must resist flattening. I1 must also be adapted to be stabilized in a helical shape, and generally comprises for this purpose a thermoplastic part. I1 must finally contain externally a protective material adapted to better resist the environmental conditions during the shaping and then the service, and / or satisfy tactile or aesthetic conditions.

 In the embodiment illustrated in Figure 2, the foam 40 is made of a material adapted to resist flattening while the sheath 50 is made of a protective material of the thermoplastic type. For this purpose, for the stuffing foam 4o, a synthetic foam with closed pores is chosen, preferably expanded polyethylene or polyurethane in closed pore foam, while the outer sheath is for example made of PVC, polyethylene or polyurethane. It is understood that this list of materials is not exhaustive.

 The invention particularly recommends manufacturing a cord of the aforementioned type in a single step of double extrusion by passing a bundle of optical and / or electrical conductors in suitable numbers through a common block head of extrusion with two extruders, or a combined die. supplied from two sides, the first extruder delivers the flexible stuffing material and the second extruder almost simultaneously spins the external protective sheath.

In order to obtain a closed-pore foam for stuffing, gas bubbles are injected into the chosen foam material before extrusion.

 The cord or cable thus obtained is then wound on a mandrel of appropriate dimensions before undergoing a heat treatment stabilizing it in its helical shape.

 In the case where the cord does not comprise any electrical conductor and where the spiraling mandrel is insulating, made of solid glass fibers for example, the invention particularly recommends steaming the cord after spiraling in a microwave oven, which speeds up the processing process and shortens manufacturing times.

 It is understood that the above description has been given for information only and that numerous variants can be proposed by those skilled in the art without departing from the scope of the intention.

 In particular, the invention also covers the case where the sheath is split into an external protective layer and an internal thermoplastic layer adapted to stabilize in a helical configuration, or the case where the sheath is adapted by an internal reinforcement lining to resist flattening and where the foam has more to ensure that the concepts of damping deformations and positioning of the various conductors.

Claims (8)

 1. Extensible fiber optic cord, of the kind comprising an optical fiber inside an outer sheath formed at least partially in an elastic thermoplastic material shaped helically, this cord, adapted to resist its flattening, being characterized in that that said outer sheath (50) encloses a flexible packing foam (40) extruded on at least one optical fiber (20).  2. Cord according to claim 1, characterized in that the foam (40) is a synthetic foam with closed pores resistant to flattening.  3. Cord according to claim 2, characterized in that the foam is made of a material chosen from the group comprising expanded polyethylene and polyurethane foam with closed pores.  4. Cord according to any one of claims 1 to 3, characterized in that the outer sheath (50) is a material chosen from the group comprising pVC, polyethylene and polyurethane.  5. Cord according to any one of claims 1 to 4, characterized in that the foam is further extruded on at least one electrical conductor (30).  6. A method of manufacturing an extensible fiber optic cord of the kind comprising an optical fiber disposed inside an outer sheath of elastic thermoplastic material internally lined with a body resistant to flattening, this method, according to which the sheath is extruded around the resistant body, being characterized in that, through a combined die supplied from two sides, a foam (40) is extruded, around a bundle comprising at least one optical fiber (20) resistant to flattening then an outer sheath (50) of thermoplastic material.  7. Method according to claim 6, characterized in that for the extrusion of the foam around said bundle, the die is supplied with a synthetic material suitable, after injection of gas bubbles, to form a foam with closed pores.  8. Process according to claim 6 or claim 7, according to which, after extrusion, said bead is wound in joint turns on a mandrel and the helical shape of the sheath thus obtained is stabilized by steaming, characterized in that, the mandrel and the cord having no electrical conductor, the steaming takes place in a microwave oven.