Optical cable

The present invention relates to an optical cable comprising one or more optical waveguides, which one or more optical waveguides are provided with a protective layer, a buffer tube surrounding said one or more optical waveguides, which buffer tube is surrounded by an outer sheath, while a radial interspace is present between said outer sheath and said buffer tube, which radial interspace is filled with a filler.

BACKGROUND OF THE INVENTION

The present invention relates to an optical cable comprising one or more optical waveguides, wherein the one or more optical waveguides are provided with a protective layer, a buffer tube surrounding the one or more optical waveguides, wherein the buffer tube is surrounded by an outer sheath, and while a radial interspace is present between the outer sheath and the buffer tube, and wherein the radial interspace is filled with a filler.

Such a cable is known per se from British patent application GB 2 215 480, in which six buffer tubes are arranged around a central strengthening element, with the buffer tubes in turn being surrounded by an outer sheath. Each buffer tube contains six optical fibres, wherein the fibres are provided with a coating, and while the interspace between the coated optical fibres and the inner side of the buffer tube is filled with a filler. A thermoplastic polyester, in particular Hytrel, is used as a suitable material for the aforesaid buffer tube.

The present inventors are active in the field of so-called “direct buried drop cables”, wherein these cables are placed underground for use in so-called “fibre to the home” (FTTH) networks, in which optical fibre cables are laid to homes. In a construction of this kind, optical fibres from different cables need to be connected together, with the weld housings usually being placed in the ground. The welding of optical fibres takes place aboveground, however, and some excess length of the optical fibres is needed for carrying out such welding operations. The optical fibre is frequently subject to mechanical loads, such as bending, during such operations. Said mechanical loads may cause the optical properties of the fibre to deteriorate.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to develop an optical cable, which optical cable is suitable for use in underground applications, while furthermore minimizing the risk of irreversible damage being caused to the optical fibres, in particular when making interconnections between various glass fibre cables.

Another object of the present invention is to provide an optical cable which makes it possible to have low-skilled personnel carry out the installation thereof in FTTH applications and to make connections between optical fibres that do not adversely affect the optical performance of the optical fibre.

The optical cable as referred to in the introduction is characterized in that the one or more optical waveguides are encapsulated in a buffer layer in the buffer tube.

Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein. It is to be understood that both the foregoing general description and the following detailed description present various embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawing in which an exemplary embodiment of the invention is shown. However, the invention may be embodied in many different forms and should not be construed as limited to the representative embodiment set forth herein. The exemplary embodiment is provided so that this disclosure will be both thorough and complete, and will fully convey the scope of the invention and enable one of ordinary skill in the art to make, use and practice the invention.

One or more of the above objects are accomplished by using the aforesaid buffer layer, wherein the buffer layer fully encapsulates the one or more optical waveguides contained in the buffer tube.

To give such a buffer tube sufficient tensile strength, it is desirable that one or more strengthening elements are present in the free space of the buffer tube, which strengthening elements are in particular configured as a layer, which layer abuts against the inner side of the buffer tube. Examples of the aforesaid strengthening elements are aramid yarns, with an acrylate-type filler being used as the buffer layer. A suitable filler is a UV curable resin, like acrylate type resins. The filler should encapsulate the one or more optical waveguides completely. In addition, the filler should prevent the one or more optical waveguides of touching each other within the buffer tube. The buffer layer as such has an elastic function as well because a hard layer will cause microbending problems.

In one embodiment of the present invention, the distance between the outer diameter of the buffer layer and a circle directly surrounding the one or more optical waveguides should be at least 100 micrometers. In a situation of 2-4 optical waveguides, the minimal diameter of the buffer layer is about 0.8 mm, preferably about 0.9 mm. The buffer layer should be easily strippable from the one or more optical waveguides. This means that the buffer layer may not remain on the one or more optical waveguides after removing the buffer layer from the optical cable. The amount of optical waveguides is maximally six. In an embodiment with six optical waveguides, the outer diameter of the buffer tube is about 2.1 mm.

It should be noted that the buffer layer used herein should not be confused with the protective layer surrounding the waveguides, which protective layer may consist of two coatings, for example, which coatings are applied during the drawing process of the optical fibre. To obtain very good properties, it is desirable that the material of the buffer tube be made of polypropylene, with the bending radius of the buffer tube in particular being at least 15 mm. It has been found that by selecting a special polypropylene quality, a crush resistance of the buffer tube of at least 1250 N/10 cm can be obtained. An optical fibre is mentioned as an example of an optical waveguide.

Present in the radial interspace between the outer sheath and the buffer tube is an element which is capable of swelling under the influence of moisture, which element preferably abuts against the outer sheath. A water-swellable tape is a suitable element in this regard.

The filler present in the radial interspace is preferably a material which is capable of absorbing moisture, in which connection super-absorbent polymers may be considered. It has been found, however, that in specific embodiments a yarn may be used as the filler. In addition, the function of the filler is also to position and to maintain the buffer tube in the centre of the optical cable.

The minimal outer diameter of the outer sheath is at least 6 mm, preferably 7 mm. A suitable choice of material is polypropylene, as well polyamide 12. The thickness of the outer sheath is preferably at least 1 mm, preferably 1.5 mm.

The present invention will be explained in more detail below with reference toFIG. 1, in which the present optical cable is schematically indicated at1. Two optical fibres2, provided with one or more coatings (not shown), is fully encapsulated in a buffer layer3. Said buffer layer3is in turn surrounded by a strengthening element, for example aramid yarns, configured as a layer4. The aforesaid layer4of strengthening elements is directly surrounded by a buffer tube5, which is preferably made of polypropylene. The construction of optical fibres2, buffer layer3, strengthening element layer4and buffer tube5is so configured that the buffer layer is fully surrounded by the strengthening element layer4.

The outer side of the buffer tube5is encapsulated in a filler6, which filler is fully surrounded by an outer sheath7, which is preferably made of polypropylene. Although two optical fibres2are schematically shown in the aforesaid FIGURE, it should be understood that it is also possible to use a different number of optical fibres, in which regard in particular maximally four or six optical fibres2can be mentioned. Examples of suitable optical fibres can be found in Bend-insensitive Single-Mode Optical Fiber; Provisional U.S. Patent Application Ser. No. 61/112,374, European Patent applications 07006909.1, and 06076957.7, which documents are incorporated here as a reference. The outside diameter, including any coatings, is about 250 micrometer.

The external diameter of the optical cable1is about 7 mm, whilst the external diameter of the buffer tube5is about 1.8 mm and the thickness of the outer sheath7is about 1.5 mm. The optical cable1is manufactured by means of an extrusion process. The aforesaid construction of the buffer tube5and the components2-4present therein, in particular the selection of polypropylene and the strengthening elements4, have made it possible to obtain a buffer tube5whose bending radius is at least 15 mm. The special selection of the thickness of the outer sheath7and of the polypropylene used for said outer sheath furthermore makes it possible to use the optical cable1in an underground construction, whilst bending of the buffer tube5will not lead to damage to the optical fibres2contained therein.

While the optical cable has been described with reference to specific embodiments and examples, it is envisioned that various details of the invention may be changed without departing from the scope of the invention. Furthermore, the foregoing description of the preferred embodiments of the invention and best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.