Optical waveguides – Optical transmission cable – Tightly confined
Patent
1998-01-12
2000-04-18
Ngo, Hung N.
Optical waveguides
Optical transmission cable
Tightly confined
G02B 644
Patent
active
060525010
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a cable including optical fibers enclosed in an outer sheath.
BACKGROUND OF THE INVENTION
In the cable industry, it is well known that optical fibers must not suffer mechanical stress if both their transmission performance and their lifetime are not to be degraded.
To limit traction stresses on optical fibers while cables are being laid, or during temperature variations which cause expansion or retraction of the optical fibers, it is known to use reinforcing members which may be central or peripheral. In the case of a central reinforcing member it is generally necessary to twist the optical fibers or the elements containing the optical fibers. This slows down the rate of production and increases the cost of the cable obtained.
To avoid the twisting operation, it is known to make an optical fiber cable by enclosing a bundle of optical fibers in an outer sheath having reinforcing members embedded longitudinally therein. When a cable of this type is subjected to bending, the reinforcing members which are located on the outside of the bend relative to the middle of the cable, are lengthened by an amount that increases with increasing diameter of the cable. This results in resistance of the cable to bending and in the risk of the reinforcing members breaking if the force applied to the cable is greater than a force which causes the outermost reinforcing members to lengthen to breaking point.
To avoid that problem, it has been envisaged in document DE-A-38 15565 to dispose only two diametrally opposite reinforcing members, or even to dispose diametrally opposite reinforcing members in groups including three spaced-apart reinforcing members or four reinforcing members arranged in a housing filled with a material having a low coefficient of friction to enable the reinforcing members to move relative to the sheath. However, in the case of spaced-apart reinforcing members it is not possible to bend the cable about an axis parallel to the line interconnecting the reinforcing members without subjecting the reinforcing members to differential elongation which risks causing them to break. Furthermore, in the case of reinforcing members disposed in a housing, the outer sheath does not participate in the traction force and it is thus necessary to provide a greater number of reinforcing members. In both cases the sheath in which the reinforcing members are embedded must itself be relatively thick, thereby reducing the bending capacity and reduces the compactness of the cable.
OBJECTS AND SUMMARY OF THE INVENTION
The invention provides a cable including optical fibers enclosed in an outer sheath having elongate reinforcing members embedded therein, disposed in two diametrally opposite groups, each comprising a plurality of adjacent reinforcing members, wherein the reinforcing members in each group are mutually tangential.
Thus, in allowing the cable to bend about an axis parallel to a line interconnecting the two groups of reinforcing members, it is possible to obtain a total cross-section of reinforcing members in which the diameter of each reinforcing member is reduced and, consequently, the thickness of the outer sheath is reduced. The bending capacity of the cable is thus increased.
Furthermore, the distance of the reinforcing members from the middle of the cable is minimized, and the differential elongation between the reinforcing members is consequently minimized, thereby allowing greater bending.
According to an advantageous aspect of the invention, the number of reinforcing members of each group is selected so that when the cable bends around the minimum radius of curvature of the cable about an axis parallel to a line interconnecting the two groups of reinforcing members, the reinforcing member subjected to the greatest stress is elongated by an amount that is less than the breaking elongation of said reinforcing member. There is thus an optimum compromise between the number of reinforcing members that allow the thickness of the outer sheath to be minimized and the stresses to
REFERENCES:
patent: 4844575 (1989-07-01), Kinard et al.
patent: 5050957 (1991-09-01), Hamilton et al.
patent: 5838864 (1998-11-01), Patel et al.
Ngo Hung N.
SAT (Societe Anonyme de Telecommunications
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