NO138020B - OPTICAL GLASS FIBER AND PROCEDURE FOR COATING SUCH A FIBER WITH A PLASTIC COAT - Google Patents

Description

The present invention relates to an optical glass fiber surrounded

a<y> a protective plastic coating and method of coating

such an optical fiber with a plastic coating.

Glass fibers used as light conductors for optical transmission systems are usually fragile and brittle and therefore difficult to handle. This means that they are not suitable for processing by conventional twisting and twisting machines to produce cables containing several light conductors in the core.

The possibilities for producing fibers of a more robust type of glass are severely limited, as the glass must primarily be selected on the basis of its optical properties and not on the basis of its mechanical properties.

The most common way of reinforcing such glass fibers has so far simply been to place strain-absorbing wires next to the plastic-coated optical fibers. The idea has then been that the nearby strain-absorbing elements take part of the brunt when the fibers are subjected to mechanical strain. This has probably worked to some extent, but still the method is far from ideal. After all, the fibers will never be subjected to exactly the same mechanical stress as the nearby strain-absorbing elements, and moreover, it is difficult to find materials that have such a small degree of elastic elongation that they will really prevent a break from occurring in the thin glass fiber, which is extremely brittle and delicate. When cables containing several optical fibers are to be made, it has therefore been a major problem to get the fibers to withstand the stresses that the handling during cable production must necessarily entail.

The plastic coating that surrounds the individual fiber has primarily been used to prevent lost light energy from one fiber from being transferred to the neighboring fiber and thereby causing crosstalk. The plastic material that has been used has been of such a nature that it has not taken

up some noticeable mechanical stretch. If it has provided any mechanical protection, then it must have been against scratching and not against tensile stresses. It has also previously been associated with major problems to apply the plastic coating in question. This bellows had to be extruded around the fibre. The extrusion nozzle therefore had to be made with the highest degree of precision in order not to damage the fiber which also passes centrally through the nozzle and to simultaneously produce a plastic coating which is in good contact with the fiber all the way through.

It has long been well known that in plastic that is deformed by mechanical stress, an orientation of the molecules occurs. Similarly, it has been well known that such oriented plastic has an increased mechanical strength in the longitudinal direction of the molecules. But this knowledge has never been used in connection with optical fibres.

The purpose of the present invention is therefore to provide glass fibers which, despite the fact that the glass is selected only according to its optical qualifications, are so hard-wearing and robust that they can be twisted or twisted together into multi-conductor cables.

A further object is to produce a method for coating an optical fiber with a plastic coating which gives the fiber an increased strength, but which is not as critical in terms of nozzle dimensions and surface quality as previously known methods.

This is achieved by a fiber and a method according to the patent claims set out below. A glass fiber produced according to the present invention is therefore particularly suitable as a light conductor for optical communication.

In order to provide a clearer understanding of the invention, reference is made to the detailed description below of an embodiment and to the accompanying drawing showing a schematic representation of an apparatus for covering a glass fiber with plastic according to the present invention.

In the figure, a glass fiber 1 is drawn from a drum 2 through the extrusion head 3 to a plastic melt extruder 4 which is equipped with a tube-forming nozzle (not shown). The fiber 1 is passed through the center of the nozzle so that it is enclosed by the extruded tube 5, without being touched by it. The plastic pipe 5 with the centrally placed glass fiber is passed through a cooling zone 6 over a pulley 7 and is then drawn through a tapering, forming nozzle 8 which presses the plastic pipe together until it forms contact with the glass fiber. Finally, the coated glass fiber is passed over a pulley 9 and stored on the drum 10. The lowering of the tube's cross-section can be accompanied by heating, and for some applications the forming nozzle 8 can be omitted. Glass fibers that are be-

shot in this way, can be assembled into a multi-conductor fiber optic cable.

The drawing down of the cross-section of the plastic tube in the nozzle 8 produces a coating of strongly oriented plastic molecules on the glass fiber. This oriented plastic coating is superior in strength in the axial direction compared to coatings formed by conventional melt extrusion. The coating therefore gives the fiber lying in it a significantly increased resistance to tensile stresses.

The described production process also reduces the need for tight tolerances for the fiber's dimensions, as the glass is not in physical contact with the plastic material during extrusion.

Various thermoplastic materials can be used for this process. However, polyesters are preferred, and in particular "ARNITEG" polyethylene terephthalate.

In certain applications, handling of the fiberglass, e.g. when winding, is avoided by placing an extraction device for glass fiber in tandem with a pT.ast coating device of the type mentioned above. The glass fiber must then be cooled down to the ambient temperature before it is fed into the tubular nozzle, and the speed at which the plastic tube is drawn down in size is adjusted to the withdrawal speed for the glass fibre. In this way, it is achieved that the glass fiber is coated before its surface can be damaged.

As the plastic material is not extruded directly in contact with the glass fiber, it is not necessary to control the axial position of the glass fiber as precisely as it has previously been required to do in conventional melt extruders.

Claims (4)

1. Optical glass fiber surrounded by a protective plastic coating, characterized in that the plastic coating is axially oriented. 2. Method for coating an optical glass fiber with a plastic coating according to claim 1, which plastic coating constitutes a protective and supporting coating for the fiber, the fiber being fed forward along the geometric axis of an extruded tube of the plastic material in question so that the tube surrounds, but does not touch, the fiber, after which the tube is pulled down into contact with the fiber, characterized in that the fiber (1) and the tube (5) are pulled through a cross-section-reducing nozzle (8) so that the plastic material in the tube is axially oriented and forced into contact with the fiber (1) and that the injection-molded tube (5) is cooled (in 6) before the pull-down is carried out so that the orientation the ring becomes permanent. 3. Method according to claim 1, characterized in that the plastic material is a thermoplastic of the polyethylene terephthalate type 4. Method according to claim 2 or 3, characterized in that the fiber is drawn in tandem with the extrusion and drawing-down process.