Note: Descriptions are shown in the official language in which they were submitted.
7~
-- 1 --
METHOD FOR FABRICATING ~N OPTICA~ FIBER CABL,E
Backgroun~ of the Inventlon
The invention relates to an undersea communications
cable containing optical fibers and to a method for
fabricating such a communications cable.
Coaxial undersea communications cables have been
manufactured Eor analog telecommunications systems. Those
cables have been fabricated to withstand some obvious
environmental ~actors such as low temperature, high
compressive pressure and corrosive water. Additi~nally
undersea cables have been made to withstand large tensile
and bending stresses encountered during cable laying and
reco~ery operations.
Recent advances in the field of optical fiber
communications technology have made possible some
practical optical fiber communications systems. The
characteristics of these systems, such as digital format,
wide balndwidth and long repeater spacings, lead to what
appear~' to be a relatively low cost per channel mile.
This potential low cost makes an undersea communications
cable containing optical fibers an attractive alternative
to present day analog coaxial communications cables.
Eleretofore, an undersea cable containing optical fibers
was described in U.S. Patent 4,156,104. Such cable
included stranded steel wires separated from a central
filament by a core in which the fibers are embedded.
A problem arises in the fabrication of a cable
including optical fibers for use in an undersea com-
munication system. The measured loss of the optical fibers
included in the cable is dependent upon strain in the
cable. Any large fluctuation in strain isl the cable during
manufacture, dep]Loyment, or operation of the cable system
complicates the processes of starting up, lining up and
operating the undersea communication system.
,
~.2297Z~2
Summary of the Invention
In accordance with an aspect oE the invention there is
provided a method for ~abricating an optical fiber cable
comprising steel wire strength members, characterized by
the steps of: coating a cable core including optical fibers
with an adhesive; winding at least one layer of steel wires
over the adhesive on the core; forming a conducting tube
over the layer or steel wires; and swaging the tube down
onto the layer of steel wires.
In accordance with another aspect of the invention
there is provideA an optica]. fiber cable, characterized
by a plurality of optical fibers; an elastic material
embedding the optical fibers; a sheath surrounding the
elastic material; stranded wire surrounding the sheath;
and an adhesive bonding the stranded wire to the sheath
for constraining the optical loss characteristic of each
optical fiber of said plurality of optical fibers to vary
only slightly in response to changes of strain ranging
from 0 to 1 percent in the cable.
Brief Descri~tion of the Drawings
~ better understanding of the invention may be derived
from the following detailed description when that
description is read in view of the appended drawings
wherein:
FIG, 1 is a cross-sectional view of an embodiment of a
communications cable including optical fibers;
FIG. 2 is an enlarged cross-sectional view of a core
of the cabie of FIG. l;
FIG. 3 is an enlarged cross-sectional view of the core
and parts of some strength members of the cable of FIG. l;
FIG. 4 is a diagrammatic side elevation view of a
production line for manufacturing an optical fiber cable
for communications; and
FIG. 5 is a graph showing a comparison between the
optical loss in fibers of a cable, made in accordance with
- 2a -
a prior art process, and in fibers of another cable "nade
in accordance with the disclosed process, both as a
function of the tensile strain in the cable.
Detailed Descri~tion
-
Referring now to FIG. 1, there is shown a cross-section
10 of an undersea communications cable containing optical
fibers arranged for transmission of optical signals. The
cable includes a core 12, steel strand 13, a cylindrical
conductor 14, and an insulator and protective jacket 16.
As shown in FIG. 2, the core 12 of the cable includes
a central eLongated strength member, or kingwire, 18,
optical Eibers 20 embedded in an elastomer 22, and a
polymer sheath 23 surrounding the elastomer.
f - 3 -
The central elongated stren~th member, or
kingwire, 18, shown in EIG. 2, is a circular cross-section
center wire which provides strenyth to the core 12 during
the processes of fabricating the core and the cable. A
high strength copper clad steel typically is ~sed.
typical diameter of the center wire is 0.8 millimeters.
The minimum cross-sectional size of the kingwire 18 is
determined by the tensile and bending strengths required
for cable fabrication processes. During the cable core
fabrication process, the kingwire is used as the principal
strength member. The core is fabricated in two operations.
During each operation, the kingwire is used for pulling the
growing core through various equipments as materials are
added step by step. After fabrication of the core, the
cable is fabricated in two additional operations.
After the cable is completely fabricated and while
the fiber communication system is being deployed to and
operated on the ocean floor, the center wire 18 serves as a
center conductor of a coaxial cable arrangement that is
used for low frequency signalling of surveillance,
maintenance and control information. Because of the
coa~ial center conductor function, the kingwire is selected
to have a conductivity of at least 40 percent of the
conductivity of an equal size wire of electrolytic copper.
In an alternative arrangement for use in a
terrestrial communication system not using the signalling
and operating in ambient temperatures which vary much more
widely than ocean temperatures, the central elongated
strength member may be fabricated out of high strength
glass, in particular as a bundle of high strength glass
fibers embedded in a polymer such as epoxy or polyester.
Elas~omer 22 is an optical fiber encapsulant, such
as an extrusion grade thermoplastic polyester, which is
supplied under the name *HYTREL by the E. I. du Pont de
Nemours and Co. and is applied to the kingwire 18 during
the first core fabrication operation. Detailed information
describing the family of *HYTREL polyesters is presented in
* - Trade Mark
7;~
-- 4 --
Rubber Age, 104, 3, pages 35-42 (1972); Proceedings of the
International Wire and able Symposium, pages 292-299
(1975); and Polymer Engineering and Science, Vol. 14, No.
12, pages 8~-852 (December 1974). The thermoplastic
elastomer completely encapsulates several separate optical
fibers for protecting them inside of the steel strand near
the center of the cable. In this arrangement the fibers
are located near the neutral bending axis of the cable.
When the cable is placed in service, sea bottom pressure is
applied essentially symmetrically to the cable. The steel
strand arrangement is designed to withstand sea bottom
pressure with very little deformation. Since the elastomer
completely surrounds each fiber within the core, the
elastomer forms a buffer for isolating each fiber from any
residual localized loads resulting from sea bottom
pressure. Thereby microbending of the fibers and
associated optical losses caused by such microbending are
minimized with respect to the efEects of sea bottom
pressure.
]n the first core fabrication operation, the
kingwire 1~ is unwound from a payout reel at a controllable
tension and speed. It is straightened, cleaned in
trichloroethane, and heated. Two layers of the
thermoplastic elastomer 22 are applied to the hot kingwire.
A first layer of the elastomer in a plastic state is
extruded clirectly over the hot kingwire. Some
predetermined number, say six to twelve, glass fibers are
laid helically over the first layer of the elastomer. A
second layer of the elastomer also is extruded in an
amorphous state. This second layer, however, is extruded
over the first layer of the elastomer and the glass fibers.
The second layer of elastomer merges w:ith the first layer
between the fibers thereby completely surrounding each of
the fibers wit:h the elastomer.
The iirst core fabrication operation is completed
by passing the partially completed core through a water
72Z
bath for cooling it before winding it onto a take-up reel.
In the seco~d core fabrication operation, the
outer surface of the elastomer is covered by the protective
nylon sheath 23. One type of nylon used for the sheath is
Zytel 153L NC 10 that is a nylon 6/12 which is supplied by
E. I. du Pont de Nemours and Co. This sheath has a
relatively high melting point at 213 Centigrade. The
partially completed core is unwrapped from the reel, and
the nylon for the sheath 23 is heated to its plastic state
and is extruded over the elastomer 22. This sheath 23
completes the core which again is passed through a water
bath ~or cooling before the completed core is wound onto a
tàke-up reeL.
Since the elastomer 22 completely surrounds the
fibers 20 and the nylon sheath 23 surrounds the elastomer,
the fibers track the elastomer and the nylon sheath when the
cable is stretched.
Fabrication of this complete core 12 into the
cable 10 of FIG. 1 is accomplished in two additional
operations. The first of these operations is described
with reference to FIGS. 2, 3 and 4. During the first
cabling operation which is accomplished in the manufacturing
line of FIG. 4, the core 12 is unreeled from a payout reel
40 and is pulled through a dancer 41 and a guide 42 to be
coated with a hot melt adhesive 25 of FIG. 2 such as one
named Eastman 148.
An adhesive applying station 43 of FIG. 4 heats
the adhesive 25, coats the nylon sheath and wipes off any
excess adhesive. In the station 43, the adhesive is heated
into a range of 220 - 240 Centigrade. The temperature is
hot enough for the adhesive to be pumped to flow over the
nylon and completely coat it but not hot enough to damage
the core. By means of a hinged wiping die within the
station 43, the adhesive 25 is wiped onto the nylon 23 at a
uniform thickness, as shown in PIG. 2.
After the adhesive is a,pplied to the sheath, two
layers of stranded steel are laid over the adhesive. The
-~r
* - Trad~ Mark
-- 6 --
quantity of adhesive 25 is selected to be enough for
completely coating the sheath and almost Eilling the
interstices 27 between the sheath and the wires 13 of the
first layer of stranded steel, as shown in FIG. 3. The
interstices should not be filled completely. Hardening of
the adhesive occurs over a period of several hours. The
hardened adhesive forms a tight hond between the nylon
sheath 23 and the inner layer of the steel strand 13. This
bond prevents creep and assures that the fiber core tracks
the steel strand during cable laying, cable recovery, and
in-service operations. The adhesive is selected so that
this bond does not fail during those operations.
Relferring once again to FIG. 1, the aylindrical
outer conductor of the low frequency signalling coaxial
cable arrangement is formed by the steel strand 13 and the
conductor 14, both of which are located outside of the
core. The steel strand includes two layers of stranded
steel wires of circular cross section.
An inner, or first, layer of the steel strand
inc].udes elght wires wraped directly over and in contact
with the outer surface of the core. These eight wires are
of ~;imilar cross-sectional size laid tightly in friction
cont:act with one another. They are laid by a Eirst stage
of ~ strander 45 in FIG. 4 so that they form a
cylindrically shaped pressure cage in which the stranded
wires pre~;s against one another continuously along their
surfaces without collapsing the cylinder.
The steel stranding in the cable also includes an
outer, or second, layer of sixteen steel wires which are
laid over the inner stranded wires by a second stage of the
strander 45. These sixteen wires are of alternate large
and small diameters, as shown in FIG. 1. They are laid
tightly in continuous friction contact with one another and
with the wires of the inner strand. These wires of the
second layer Eorm an additional cylindrically shaped
pressure cage which also holds the inner layer of wires in
place. The first and second layers of steel stranding are
Z~7ZZ
brought together over the adhesive coated core by a closing
die 47, shown in FIG. 4. The partially formecl cable,
including the core, adhesive and two layers oE steel strand
is cleaned in a bath 48 of trichloroethane before being
enclosed in the conducting tube.
~ nonporous conductive cylindrical tube 14 of
FIG. 1 is to be formed directly over the outer layer o
steel wires. It is formed by a welded seam tube of soft
electrolytic copper. This highly conductive tube provides
(1) a good direct current path for powering electronic
repeaters which are to be spaced along the cable, (~) a
moisture barrier for the ibers, and (3) in conjunction
with the steel wires, the cylindrical outer conductor for
the previously mentioned low frequency signalling system.
During cable fabrication in the production line of
FIG. 4, a high conductivity soft copper tape 50 is cleaned,
slit longitudinally to a uniform width, and rolled into a
tubular shape around the steel strand by a slitter and tube
forming mi]l 51. The tube is sized to fit loosely over the
ste~l strand leaving a gap between the steel and the
rolled-toclether, abutting edges oE the tape. Upon leaving
the tube forming mill 51, the edges of the tape are welded
toge!ther into the tubular conductor 14 by a continuous seam
welder 53~, Immediately the conductive tube is swaged, by
rolling and drawing through a swaging mill ~5, down onto
the outer steel strands forcing some copper into the
interstices between adjacent wires in l:he outer steel
strand, as shown in FIG. 1. This swaging of the copper
into the outer interstices of the second layer of steel
helps assure that the steel strand package retains its
cylindrical shape, especially during cable handling
operations. Swaging of the copper down onto the steel
wires produces an area of contact between each wire and the
copper to hel~? retain the cylindrical shape oE the strands
and to assure that the steel and copper track each other
during subsequent handling.
After the copper tube 14 is swaged into place, the
3'7~
growing cable is run through another cleaning bath 57 for a
final cleaning in trichloroethane. This portion of the
cable in process proceeds through a dancer 58 and is
wrapped onto a take-up reel 60.
Subsequently in a separate operation, the jacket
of insulation l6, shown in FIG. 1, is extruded over the
copper tube 14. The jacket is formed by a low density
natural polyethylene. During the process of extruding the
polyethylene, the cable including the steel stranding, and
the copper tube is heated to a temperature high enough for
producing a polyethylene to copper bond. The polyethylene
is heated to a plastic state in a temperature range of
210-230 Centigrade so that the polyethylene flows readily
during extlusion. The temperature of the copper tube is
elevated to a minimum of 80 Centigrade. A bond, formed
between the polyethylene and copper, is sufficiently strong
so that they track one another during cable laying and
recovery operations and during system service operations.
Because of this bond and the tightness between the copper
tube and the steel strand, the outer jacket of polyethylene
and the steel strand also track one another. Since the
jacket, the steel strand, and the core all track one
another, the fibers are strained as much as other
components of the cable. Because the Eibers are proof
tested to 2.0 percent strain, they can withstand the strain
of cable laying ancl recovery operations without breaking.
Optical loss in the fibers varies only slightly with
- changes of tensile strain in the cable. The change in
optical loss in the fibers varies much less with strain
than the change in loss produced by prior cable design. A
description of suitable optical fibers is presented in
Proceedings of the IEEE, pages 1280-81, September 1974;
Digest of Tech. Papers, International Conference on
Integrated ~~tics and Optical Eiber Communications, page
26, April 19~1; CLEO 1981, paper W6 6-1, June 1981; and
IEEE Journal of Quantum Electronics, Vol. QE-18, No. ~,
pages 504-510, April 1982. Optical loss in the fibers
~.~2'~ 2
varies only slightly with changes o tensile s~rain in the
cable much less than changes of loss in fibers fabricated
into a cable by prior methods.
FIG. 5 shows the change in optical loss in the
fibers with strain in the cable. The solid line 62
represents the change of optical loss characteristic for
the fibers in the c~ble arranged in accordance with the
instant invention. Change of optical loss is approximately
0.01 decibels per kilometer at a strain of 0.5 percent. A
dashed line 64 represents the change of optical loss
characteristic for fibers in a prior art cable arrangement.
The llne 32 shows the prior design change of optical loss
to be approximately 0.10 decibels per kilometer at a strain
oE 0.5 percent. Reduced change of optical loss with
respect to strain results from the new design which enables
the cable components to track one another thereby
constraining microbending which otherwise would be caused
by the strain in the cable.
