Method of determining lay length of S-Z stranded buffer...

Details Method of determining lay length of S-Z stranded buffer... Method of determining lay length of S-Z stranded buffer...

2001-06-27

2004-04-27

Chowdhury, Tarifur R. (Department: 2871)

Optical waveguides

Optical transmission cable

Tightly confined

C385S111000

Reexamination Certificate

active

06728453

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to optical fiber cables that have buffer tubes arranged in S-Z strands, and in particular a method of determining the lay length of such S-Z strands during the manufacturing process.
In telecommunication cables, optical fibers or optical fiber ribbons are often used as a medium to transmit optical signals. These cables often have a central strength member, such as a steel rod or stranded steel wires, that extends longitudinally along the central axis of the cable. As shown in FIG. 1 (from U.S. Pat. No. 5,229,851, which is incorporated by reference), central strength member
2
is intended to withstand and resist any tensile or compressive force applied axially to the cable
1
. The central strength member
2
is often encircled by a covering
3
, which may serve as a cushioning material. A plurality of plastic buffer tubes
4
-
8
surround covering
3
and loosely house protect optical fibers or ribbons within them. A binder thread or threads
17
and
18
are often contrahelically applied around buffer tubes
4
-
8
to hold them in place. A water swellable tape (not shown) may be applied over the buffer tubes to block water ingress into the cable. An overall plastic jacket
20
then covers the contents of optical fiber cable
1
. If the intended installation for cable
1
requires extra mechanical strength, the cable may include additional strength members in the form of armor or strength yarns
19
placed intermediate the water swellable tape and the jacket.
As shown in
FIG. 1
, buffer tubes
4
-
8
are generally wrapped around central strength member
2
in a reverse helix or “S-Z” fashion. The locations at which the stranded tubes reverse direction (e.g. from an “S” to a “Z”) are referred to as reversal points. S-Z stranding of buffer tubes in general, and the reversal points in particular, are advantageous for accessing the cable midspan. That is, due to the S-Z stranding, one or more optical fibers within the cable may be “tapped” at the reversal points without having to sever the cable or to carry out major reconfiguration. The S-Z stranding provides sufficient excess of tube length to make the tap easy by opening the side of the cable at a point along its length without losing the desired slack in the ribbon units or optical fibers within the tube that is opened. Thus, taps in an S-Z stranded cable can be made without interrupting other tubes or ribbon units.
To ensure that the optical fibers within the buffer tubes are not subjected to bending stress, which may cause unwanted attenuation, a parameter of the S-Z stranded buffer tubes called “lay length” needs to be monitored. Bending stress is a loss mechanism in optical fibers that may occur if the cable is subjected to tensile forces, either from installation or temperature, or compression forces. Bending stress may cause signal loss in the optical fibers. The S-Z strand of buffer tubes in an optical fiber cable may take several forms. Each ‘S’ turn may be followed immediately by a reversal to a ‘Z’ stranding direction. Alternatively, there may be several helical turns between reversals. In general, then, the average lay length is defined by the distance between reversal points divided by the number of turns between reversals.
The actual lay length of each individual S-Z stranded tube will vary from the average lay length by a small amount due to additional twisting and processing conditions. That is, the lay length of any given tube, may be more or less than the average lay length, as a given tube may make more than a whole number of turns between reversals. For example, in a cable with 6 different colored buffer tubes, one being red, and all S-Z stranded around a central member, the red tube may be at the top or at the 12 o'clock position on the cable at the first reversal point. But at the next reversal point the red tube may be at the 6 o'clock position on the cable, 3 tubes removed from the 12 o'clock position. Thus, the red tube has gone one half turn more between reversals. This half-turn must be included in the lay length calculation for the most accuracy. Thus, the actual lay length of a given S-Z stranded buffer tube is comprised of several components and can be calculated to close approximation by:
Lay Length=D/N,
where:
N=N′+n/T
where D is the axial distance between the reversal points, N is the number of turns between reversals, and N′ is the number of whole turns between the reversal points; n is the number of tubes which a given tube is offset from its angular position on the previous reversal point, counted in the direction of rotation; and T is the total number of buffer tubes.
To protect against bending stress on the optical fibers, the lay length of the S-Z stranded buffer tubes is checked on finished cable to verify that the lay length is within acceptable specifications. The only way to check the lay length on finished cables is to strip back the jacket and other layers in the cable over the buffer tubes. It is not sufficient to do this on the cable ends as the start-up and finish of the stranding process may have been done at conditions that vary from the rest of the cable. Instead, lay length has been measured manually during the manufacturing process after stranding. The line operator would make the length measurement while walking alongside the progressing cable, which was fairly easy to accomplish accurately because line speeds were slow. More recently, however, line speeds have increased dramatically, making this type of manual measurement inaccurate. One alternative is to stop the line periodically to take measurements. However this is impractical and inefficient.
Many methods of determining the lay length of S-Z stranded optical fiber cables require the detection of lay reversal points of the S-Z stranded buffer tubes. One approach to the lay reversal detection problem is described in U.S. Pat. No. 5,809,194. In this patent, a process for marking an outer jacket of an oscillating lay cable (including S-Z stranding) to indicate the locations of the lay reversal points under the jacket is described. This process includes the step of providing detectable markings on an unjacketed cable core in predetermined positions relative to the lay reversal points. The process further includes the step of sensing the detectable markings with a sensor (such as a luminescence scanner) prior to extruding an outer jacket over the cable core. Next, the process includes predicting the location of the sensed markings on the cable core after a cable jacket has been extruded and providing markings on the cable jacket at predetermined positions relative to the predicted location of the sensed markings.
Another approach to the lay reversal detection problem is described in U.S. Pat. No. 5,745,628. In this patent, similar to the '194 patent, a process and apparatus for marking an outer jacket of an S-Z stranded cable to indicate the lay reversal points under the jacket is described. This process comprises passing a portion of a cable core within a field of view of an imaging means to acquire an image of that portion of the cable core. This imaging means includes a camera connected to a vision inspection/image acquisition system. The quantity of visually distinguishable conductors in the acquired image is compared to a reference value. If the reference value is exceeded, a lay reversal point is indicated. Once a lay reversal point is indicated, its position is tracked through an outer jacketing step. A marking to indicate the location of a lay reversal point is applied to the outer jacket according to the tracked position of the lay reversal point.
Yet another approach to the lay reversal detection problem is described in U.S. Pat. No. 5,729,966. In this patent, similar to the '194 and '628 patents, a method for marking sections of a fiber optic cable so that lay reversal points can be indicated on an exterior surface of the fiber optic cable is described. The method includes the steps of: 1) dete

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