Optical waveguides – Optical transmission cable – Ribbon cable
Reexamination Certificate
1999-05-26
2001-07-03
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Optical transmission cable
Ribbon cable
C385S110000
Reexamination Certificate
active
06256439
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a communication cable design having a central core containing a plurality of fiber optic ribbons, and to lubricants therefor.
BACKGROUND OF THE INVENTION
In the manufacture of optical communication cables, two design types are most frequently employed that are generally referred to as “central-core” and “loose-tube” designs. In the central-core design, a number of optical fibers are contained within a tube, which is located at the center of the cable. This cable typically includes strength members that are positioned between the central core and an outer plastic jacket. By comparison, loose-tube cable designs typically include a number of relatively small tubes that are positioned around a central strength member, and each tube encloses a number of optical fibers. In the loose-tube cable design, the fiber-containing tubes are longitudinally stranded around the central member, which is to say that the tubes are rotated around the central member along the length of the cable. An example of such a loose-tube cable design is disclosed in U.S. Pat. No. 5,621,841.
From a manufacturing standpoint, the central-core design is advantageous because it allows the various components of the cable to be assembled into their ultimate cable form in a single step rather than two or three steps as in the loose tube design. From an efficiency standpoint, optical fiber ribbon (i.e., a planar array of optical fibers that are bonded together as a unit) is advantageous because many ribbons can be stacked on top of each other within a small space. Accordingly, central-core cables having stacked optical fiber ribbons are highly desirable.
Longitudinal stranding is used in the optical cable industry to avoid subjecting individual fibers to undue tensile or compressive stress that may occur when the cable is bent. Such stress not only modifies the transmission characteristics of a fiber but also leads to breakage in extreme situations. In this industry, two types of longitudinal stranding are know: “continuous” and “S-Z.” Continuous stranding is a process in which one or more strands of material (e.g., optical fibers) are helically rotated in a single direction along the longitudinal axis of the cable; whereas in S-Z stranding, the direction of rotation periodically reverses. S-Z stranding is preferred because it achieves the benefits of longitudinal stranding without the need for heavy machinery to lift and rotate large reels of strand material and it removes limits on the length of component materials. (Heretofore, stranding a stack of optical fiber ribbons in a central-core cable has been undesirable because transmission loss is significantly increased. This increase is frequently referred to as “cabling” loss since it is wholly attributable to the installation of transmission media [i.e. the ribbon stack] in a cable.) Furthermore, S-Z stranding has been difficult to achieve because twisted material tends to unwind at the point of reversal owing to inherent physical forces (restorative forces) that are created when a relatively stiff body is twiste—much like a torsion spring. This difficulty is exacerbated when the stiffness is increased, such as by stacking and bonding fiber ribbons together in a single unit.
In the formation of stacked ribbons, there is a tendency for the ribbons to stick together during sheathing which can create microbending, resulting in increased losses in the cable. The addition of suitable lubricant between the ribbons has resulted in substantially eliminating such losses. However, it has been found that many lubricants, e.g. oils, can present additional problems, stemming from characteristics of the lubricant such as viscosity, surface tension, contact angle, and surface wetting. These characteristics can lead to inadequate lubricating of the ribbons with a consequent increase in losses.
SUMMARY OF THE INVENTION
The foregoing problems have been overcome by a cable having two or more optical fiber ribbons, which are stacked in an array and disposed within a tube that is positioned at the center of the cable. A lubricant is applied between adjacent ribbons of the stack in order to hold the ribbons together as a unit and to facilitate sliding movement between the ribbons. The stack of ribbons is longitudinally (i.e., twisted) along the length of the cable.
The lubricant of the present invention is, for example, a polyalphaolefin oil or a mineral oil which, in accordance with the invention, has a surface tension of below 34 dynes per centimeter and a contact angle, which will be discussed hereinafter, of less than 44 degrees. The contact angle is a measure of beading of the oil and hence, indirectly, a measure of the wetting properties of the oil. The surface tension is also a factor in the wetting of the ribbon surface, and a relatively low surface tension produces more complete wetting. In addition, a cable containing stacked ribbons is generally filled with a filling material (water blocking viscous gel) which, itself, functions as a lubricant. A surface tension of the oil below 34 dynes per centimeter with a relatively low viscosity permits the filling material to be drawn into the space between ribbons, thereby complementing the lubrication supplied by the oil, and the bonding effect between ribbons is substantially unimpaired.
The lubricating oils of the invention, therefore, function to hold the stack of ribbons together, to permit sliding of the ribbons relative to each other, to reduce transmission losses in the cable, and to insure, at least at some extent, the distribution of the water blocking gel within the stack. Also, the stack of ribbons is twisted in one direction for a first predetermined distance, and then twisted in the opposite direction for a second predetermined distance. These distances are preferably equal to each other.
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Brown Robert J.
Sheu Jim Jenqtsong
Lucent Technologies - Inc.
Rojas Omar
Sanghavi Hemang
Thomas Kayden Horstemeyer & Risley LLP
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