Optical fiber cable

Details Optical fiber cable Optical fiber cable

1999-06-23

2001-05-08

Sanghavi, Hemang (Department: 2874)

Optical waveguides

Optical transmission cable

Ribbon cable

C385S112000, C385S113000

Reexamination Certificate

active

06229944

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber cable having a structure in which optical fiber ribbon stack are accommodated in a cylindrical space.
2. Related Art of the Invention
As an method for achieving a multi-core optical fiber cable, optical fiber ribbon stack each having coated optical fibers (hereinafter sometimes simply referred to as“core”) aligned and coated so as to be shaped like a tape are used. A multi-core optical fiber cable can be formed easily by stack of the optical fiber ribbon. When the optical fiber ribbons are stacked simply, however, external force is apt to act on the optical fiber cable so that micro-bending loss increases. As a countermeasure, for example, a structure in which a plurality of optical fiber ribbons are accommodated in a metallic tube is employed in an optical fiber cable disclosed in JP-A-8-278432.
It may be, therefore, thought of that the cylindrical space is enlarged and the amount of the cushioning fillers between the optical fiber ribbons and the metallic cylinder is reduced. However, if the amount of the cushioning fillers is too small, the optical fiber ribbon stack hit against the inner wall of the metallic cylinder when bending force acts on the optical cable. As a result, the optical fiber ribbons are broken or bent precipitously, so that transmission loss increases because of leakage of light.
On the other hand, the linear expansion coefficient of the optical fiber ribbon is smaller than that of the cushioning fillers around the optical fiber ribbons. Accordingly, at a low temperature, the outer tube shrinks more greatly than the optical fiber ribbons, so that the optical fiber ribbons are deformed. However, if the amount of the cushioning fillers around the optical fiber ribbons is too large, there occurs another problem that a limitless number of bends with small radii of curvature are generated, so that transmission loss increases because of leakage of light.
The background-art type structure in which only the optical fiber ribbons are directly accommodated in a cylindrical space, however, has a problem that transmission loss is apt to increase because coated optical fibers located in end portions of the optical fiber ribbons suffer side pressure from the inner wall of the metallic cylinder. Accordingly, there arises a problem on design of the optical fiber cable that a countermeasure to widen the cylindrical space, a countermeasure to limit the number of coated optical fibers (hereinafter simply referred to as “cores” in each of optical fiber ribbon stack or the member of optical fiber ribbon stack accommodated in the cylindrical space, etc. are required to prevent the side pressure from acting on the optical fiber ribbon stack.
SUMMARY OF THE INVENTION
An object of the present invention is therefore to provide an optical fiber cable in which optical fiber ribbon stack are accommodated in a cylindrical space efficiently and which has excellent loss characteristic.
In order to achieve the above object, according to an aspect of the present invention, provided is an optical fiber cable comprising optical fiber ribbon stack, cushioning fillers disposed around the stack, and a cylindrical space for accommodating the stack and the fillers, wherein the space factor of the cushioning fillers in the cylindrical space is set to be in a range of from 10% to 60%, the space factor being defined as a ratio of a sectional area occupied by the fillers in the cylindrical space to a sectional area of the cylindrical space excluding the whole of the optical fiber ribbon stack.
Preferably, in the above optical fiber cable, the optical fiber ribbon stack are constituted by a combination of one type of or different types of coated optical fibers each of which contains coated optical fibers in a range of 4 to 36 in number.
Preferably, in the above optical fiber cable, the cylindrical space is formed by a molding of a plastic material extruded on an outer circumference of the cushioning fillers.
Preferably, in the above optical fiber cable, the cylindrical space is constituted by a metallic tube formed on an outer circumference of the cushioning fillers.
It is therefore an object of the present invention not only to fulfil a sufficient cushioning function to thereby prevent breaking or precipitous bending of optical fiber ribbon stack due to collision of the optical fiber ribbon stack with the inner wall of an outer tube such as a metallic or plastic cylinder when bending force acts on the outer tube, but also to lead the optical fiber ribbon stack to make the surplus length of the optical fiber ribbon stack curved gently, that is, curved with a large radius of curvature to thereby prevent the increase of transmission loss due to leakage of light when cushioning fillers shrink at a low temperature.
According to a first aspect, provided is an optical fiber cable comprising optical fiber ribbon stack successively stacked, fiber-like cushioning fillers made of stack of strings or yarns and disposed so as to surround the coated optical fiber ribbon stack, and an outer tube forming a cylindrical inner space so as to surround the cushioning fillers, wherein a space factor S of the cushioning fillers in a remaining space after removal of the optical fiber ribbon stack from the inner space of the outer tube is set to be in a range of from 10 to 60% when the space factor S is given by the following expression:
S={B/
(&pgr;
r
2
−A
)}×100
in which A is a total sectional area of the optical fiber ribbon stack, and B is a total sectional area of cushion fillers and given by the following expression:
B=
(denier value×total number)/900000×specific gravity of the cushioning fillers.
In such a configuration, it becomes possible to provide a highly reliable optical fiber cable which is very low both in initial loss and in transmission loss at a low temperature. If the space factor is smaller than 10%, the cushioning effect of the cushioning fillers cannot be fulfilled sufficiently, and as a result, the optical fiber cable becomes weak against bending force, so that transmission loss due to the bending of the optical fiber cable, that is, initial loss increases. If the space factor is contrariwise larger than 60%, the optical fiber ribbon stack cannot be led to make the surplus length of the optical fiber ribbon stack curved gently, that is, curved with a large radius of curvature when the cushioning fillers shrink at a low temperature, and as a result, a limitless number of curves with small radii of curvature are generated to thereby increase the transmission loss due to leakage of light.
According to a second aspect, in the optical fiber cable defined in the First aspect, the space factor of the cushioning fillers is set to be in a range of from 10 to 50%.
In such a configuration, the transmission loss at a low temperature can be reduced securely.
According to a third aspect, in the optical fiber cable defined in the first aspect, the cushioning fillers are provided so as to be stranded around the optical fiber ribbon stack.
That is, stack of strings or yarns used as the cushioning fillers are stranded around the optical fiber ribbon stack, so that the cushioning effect is fulfilled more securely to reduce initial loss even in the case where the space factor is small. On the other hand, even at a low temperature, the optical fiber ribbon stack can be led to be curved gently due to the stranding of the cushioning fillers, so that low-temperature loss can be also reduced. Furthermore, because of use of a stranding method, such as an S-stranding method in which the cushioning fillers are stranded so as to be S-shaped, a Z-stranding method in which the cushioning fillers are stranded so as to be Z-shaped, an SZ-stranding method in which S-stranding and Z-stranding are alternately repeated, or the like, force acts only to unstrand the cushioning fillers when the cushioning fillers shrink, so that side pressure is little applied to the optical fiber ribbon st

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