Laminated Strength Members for Fiber Optic Cable

This invention relates to an improved fiber optic cable. More particularly, it relates to the use of pre-stretched strength members parallel to optical fibers for providing a force equal to or greater than the predetermined break strength of an optical fiber.

With the realization of the limited number of communication channels available in metal conductor electrical communication cable, the use of optical fibers with modulated light sources for communication is fast becoming a reality. An optical fiber provides at least one order of magnitude more channels per tranmission path than a metal conductor.

One of the major problems in the use of optical fibers is that they are normally made of glass and thus are easily broken. One type of breakage occurs when the cable, and thus optical fiber, is placed under a longitudinal load, causing longitudinal stress and strain on the fiber. Several means have been devised to overcome this problem of longitudinal stress. U.S. Pat. No. 3,855,897 shows a plurality of optical fibers arranged in a helical fashion about the core of the cable. During longitudinal stress, the pitch in each helical turn will lose amplitude to absorb the stress on the fibers rather than the fibers themselves.

U.S. Pat. No. 3,883,218 also discloses the use of a helical wound fiber bundle which also takes up stress during longitudinal load on the cable. Furthermore, this patent discloses a reinforcing member inserted in the center of the fiber bundle for providing strength to the cable as well as overcoming certain stress problems.

The use of helical fibers pose severe manufacturing problems as well as impaired light transmission.

U.S. Pat. No. 4,009,932 discloses the use of strength members parallel to the optical fiber in a optical cable core. The strength members are made of metal wires which have a much higher break strength than the fiber. The use of these metal strength members increases the cost of the resulting cable. Furthermore, the metal wire acts as an antenna which could cause electrical interference with the electronics at both ends of the cable.

While all of these above patents disclose means of protecting optical fibers from longitudinal stress, there is a need to provide a simple and relatively inexpensive means of protecting optical fibers.

It is therefore one object of this invention to provide a fiber optic cable having an improved means for protecting optical fibers from longitudinal stress.

It is another object to provide an inexpensive and relatively simple method of manufacturing a fiber optic cable having protection from longitudinal stress.

There is provided a fiber optic cable having a fiber optic core. At least one pre-stretched elastic strength member is provided parallel to the fiber optic core. The pre-stretched strength member is kept in its pre-stretched condition by adherence to at least one laminant which substantially circumscribes the fiber optic core. As a result of this stretching, the strength member provides a force in the opposite direction to longitudinal load stress that may be placed on the fiber optic core.

In accordance with another form of this invention there is provided a method for making an improved fiber optic cable. The method comprises pre-stretching at least one strength member; adhereing the strength member to a laminant; holding the pre-stretched strength member in its elongated position; wrapping the laminant about the fiber optic core.

The subject matter which is regarded as the invention is set forth in the appended claims. The invention, however, together with further objects and advantages thereof may be better understood with reference to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a partial isometric view with portions removed, of a fiber optic cable having pre-stretched strength members in accordance with one form of this invention;

FIG. 2 is a sectional end view of the fiber optic cable shown in FIG. 1;

FIG. 3 is a partial top view of the laminated pre-stretched strength members showing further aspects of this invention;

FIG. 4 is a sectional end view of the laminated fiber optic strength members shown in FIG. 3.

Referring now more particularly to FIG. 1, there is provided fiber optic cable 1 shown with its structure partially removed in a staggered manner from the inner fiber to the outer jacket. Optical fiber 2 in this embodiment is made of a glass material. A suitable glass optical fiber may be obtained from the Corning Glass Works or from other commercial sources. Optical fiber 2 may be replaced by a bundle of optical fibers without departing from the spirit of the invention.

In this embodiment, optical fiber 2 is covered by a material 3 which protects the optical fiber from abrasion and also from external forces in the perpendicular direction to the horizontal axis of fiber 2. Protection member 3 may be made from many materials such as plastics including polyvinlychloride. The protection member may be extruded over the optical fiber.

Sheath 6 may be extruded over the top of the core 7, also providing protection for the core. This sheath may be made from a synthetic resin such as polyurethane. Laminant 4 acts as a carrier for a plurality of optical strength members 5 and may be made of various materials such as polyester. The strength members may be made of various elastic materials such as Kevlar, nylon or polyester. Since these types of strength members may be elongated by a force less than the force required to break the glase fiber, the strength members 5 are stretched prior to their use. An adhesive coating 8 is placed on the top side of laminant 4 for adhering the stretched strength members to the laminant, and also for holding the strength members in their pre-stretched position.

As can be seen from FIG. 2, the strength members 5 are placed symetrically about the cable core by wrapping the laminant 4 thereabout and the restraining forces applied by the strength members are relatively even about the core to prevent bending of the fiber. The relationship between the laminant 4, the strength member 5, and the adhesive coating 8 may be more clearly seen with reference to FIG. 4.

FIG. 4 shows an end cross-sectional view of the laminant 4 having the strength members attached thereto prior to it being wrapped about the cable core. As can be seen from FIG. 4, there is provided a thin layer of adhesive 8 in which the strength members 5 are embedded. Adhesive may be any pressure-sensitive or heat-sensitive adhesive such as an epoxy resin of sufficient strength to hold a strength member such as Kevlar in a pre-stretched position of at least from 7.times.10.sup.6 pounds per square inch to 19.times.10.sup.6 pounds per square inch pressure depending on the break strength of the optical fiber and the desired protection level. A second laminant (not shown) with accompanying adhesive, like laminant 4 and adhesive 8, could be applied to the other side of the strength members without departing from the spirit of the invention.

The cable shown in FIG. 1 may be manufactured in the following manner:

An optical fiber 2 or fiber bundle is provided. Polyvinylchloride protection member 3 may be extruded over the fiber. The force required to break the fiber in its longitudinal direction is measured or calculated. A plurality of Kevlar strength members are stretched so as to apply a force in the opposite direction to the calculated break force of the optical fiber. In the preferred embodiment this stretch force will be approximately equal to the calculated break force of the optical fiber. Normally, the strength members will be stretched to a point where they will elongate less than 1% more.

The stretched strength members are symetrically applied to a tape comprising a laminate with adhesive on one side. The adhesive should be strong enough to hold the strength members in their stretched condition. The strength members may be sandwiched between a pair of such tapes to insure that they are held in their stretched condition.

The laminated strength members are placed around the core containing the optical fiber or fibers. A synthetic resin protective sheath may then be extruded over the laminated strength members.

From the foregoing description of the preferred embodiment of the invention, it would be apparent that many modifications may be made therein. It will be understood, therefore, that this embodiment is intended as an exemplification only and that the invention is not limited thereto. It is to be understood, therefore, that it is intended that the pending claims cover all such modifications that shall fall within the true spirit and scope of the invention.