Optical-fiber submarine cable and manufacturing method thereof

An optical-fiber submarine cable, in which a combination is provided by a thin, cylindrical pressure resisting sheath, and a reinforcing member of a cross section inserted by the thin, cylindrical pressure resisting sheath so as to divide the circular cross section of the pressure resisting sheath into a plurality of spaces and to have a required withstand pressure; and in which at least one low-loss optical fiber is inserted in each of the plurality of spaces. The combination can be fabricated by separately paying out the reinforcing member and the thin, cylindrical pressure resisting sheath, or by curving inwardly a sheet of tape being paid out.

BACKGROUND OF THE INVENTION 
This invention relates to a cable structure of a submarine cable system 
employing low-loss optical fibers as transmission media in the field of 
optical communication and a method for the manufacture of such a submarine 
cable. 
Because of its advantages of such as a low loss, a wide transmission band 
and a light weight, an optical fiber is regarded as a promising 
transmission media by which a coaxial submarine cable heretofore employed 
will be replaced. 
A submarine cable is laid under the deep sea of up to 10,000 m and exposed 
to a water pressure as high as about 1,000 atm. at maximum. When an 
optical fiber cable is received such a high pressure, the optical fiber 
slightly bends due to nonuniformity in the thickness of a material coated 
thereon, resulting in markedly degraded transmission characteristic. 
Further, the optical fiber is made of silica glass or optical glass of 
small loss and hence is brittle and may be broken when greatly bent. Since 
the optical fiber is thus made of glass, when it is immersed in sea water 
for a long period of time, its mechanical property and transmission 
characteristic are degraded. To avoid this, it has already been proposed 
to house the optical fiber for the submarine cable in a high pressure 
resisting pipe (Japanese Pat. Disc. No. 99032/76). Since the high-pressure 
resisting pipe increases its thickness with an increase in its inner 
radius, it is necessary in the manufacture of the pressure resisting pipe 
to minimize its diameter. In a case of inserting one optical fiber into 
the pipe, its diameter may be reduced, but in a case of inserting a 
plurality of optical fibers in the pipe, reduction of the pipe diameter is 
difficult if the pipe is adapted to be used both as a pressure resisting 
pipe and a power feeding conductor or the like. In the optical-fiber 
submarine cable, it is necessary from the economical point of view to 
increase the number of optical fibers utilizing their small diameter as 
well as to transmit a large quantity of information utilizing the wide 
transmission band property of the optical fibers themselves. It is also 
possible to employ such a method of increasing the number of fibers by 
protecting each optical fiber with a pipe but, in this case, a space loss 
by division occurs in the manufacture of pipes. In view of the above, it 
is desirable to house a plurality of optical fibers in one pressure 
resisting pipe, but in the prior art, since an increase in the inner 
diameter of the pipe causes an increase in the pipe thickness, it is 
difficult to manufacture a thick pipe while inserting optical fibers at 
the center thereof. 
SUMMARY OF THE INVENTION 
An object of this invention is to overcome such a defect and to provide an 
optical-fiber submarine cable which employs a pressure resisting, 
relatively thin and cylindrical sheath having inserted therein a member 
for enhancing its pressure resisting property and further to provide a 
method for the manufacture of such a submarine cable.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates an embodiment of this invention, in which four low-loss 
fibers such as single mode fibers, multimode fibers or grouped fibers and 
housed in a thin cylindrical pressure resisting layer. In FIG. 1, 
reference numeral 1 indicates optical fibers; 2 designates a thin 
cylindrical pressure resisting sheath or cover made of plastic, aluminum, 
copper or like material; and 3 identifies a reinforcing member made of 
plastic, aluminum, copper or like material for increasing the mechanical 
strength of the cylindrical pressure resisting layer 2. The fibers 1 are 
inserted in spaces defined by the abovesaid reinforcing member 3 in the 
thin cylindrical layer 2 and, in this case, a gap may also be provided 
between the pressure resisting layer or sheath 2 and the reinforcing 
member 3 for inserting an adiabatic member therein so as to prevent the 
heat produced during the fabrication of the pressure resisting layer 2 is 
transmitted to the optical fibers, or for inserting a pressure buffer 
layer therein so as to prevent that a force applied to the pressure 
resisting layer is transmitted directly to the optical fibers. It is also 
possible to replace a required one or ones of the plurality of optical 
fibers 1 by power feeding conductors or tension members. The reinforcing 
member 3 or the pressure resisting layer 2 may also be used as a power 
feeding conductor by forming it of a material having high electrical 
conductivity. 
The high-pressure resisting layer having housed therein a plurality of 
optical fibers in accordance with this invention can easily be fabricated 
by paying out the reinforcing member 3 in its finished form or while 
making it, extending the optical fibers 1 along the reinforcing member 3 
being paid out and then extruding the thin, cylindrical pressure resisting 
layer 2. In this manner, the high-pressure resisting layer can be 
associated with the optical fibers extending along the reinforcing member 
of a rectilineal configuration, by which it is possible to provide marked 
reduction in a change of optical fiber loss in the process of inserting 
the optical fibers 1 into the pressure resisting layer 2. The pressure 
resisting layer is fabricated by extruding polycarbonite or vinyl 
chloride, or forming a metallic tape by means of rollers into a 
cylindrical shape. FIG. 2 shows an example of such a manufacturing method. 
Reference numeral 1 indicates optical fibers being paid out; 2 designates 
a metallic tape as of copper being paid out to form a cylindrical pressure 
resisting layer; and 3 identifies a reinforcing member to be inserted into 
the cylindrical pressure resisting layer 2. In this example, the 
reinforcing member 3 is shown to make contact with the pressure resisting 
layer 2 at four places but may be formed to contact the layer at a desired 
number of places. 
FIG. 3 shows another embodiment of this invention, in which a sheet-like 
tape as of copper, aluminum or the like is formed into a cylindrical shape 
to produce the cylindrical pressure resisting layer 2 and the reinforcing 
member 3 in one step. In FIG. 3, reference numeral 1 indicates optical 
fibers; 3 designates a reinforcing member for increasing the mechanical 
strength of a pressure resisting layer; 2 identifies the pressure 
resisting layer; and 4 denotes a welding joint or an adhesive binder. FIG. 
4 illustrates a step in the manufacturing process for forming the pressure 
resisting layer 3 shown in FIG. 3. A sheet-like tape is formed first into 
such a shape as shown in FIG. 4 and, after insertion of a required number 
of optical fibers 1, rolled into a cylindrical shape, as indicated by 
arrows, and then fixed as by welding at the joint, as indicated by 4 in 
FIG. 3. With this method, it is very easy to fabricate a pressure 
resisting layer having built therein a reinforcing member. 
FIG. 5 shows an example in which a joint 11 is formed zigzag to increase 
the area of contact between the mating surfaces thereby providing for 
enhanced airtightness. In FIG. 3, the reinforcing member 3 is shown to 
have three legs, but it is needless to say that a reinforcing member with 
a desired number of legs can also be produced by this method. 
FIGS. 6, 7 and 8 respectively illustrate embodiments of an optical-fiber 
submarine cable employing such a pressure resisting layer for protection 
optical fibers from a high water pressure. 
In FIG. 6, an optical fiber 10 inserted in the pressure resisting layer is 
disposed at the center of a submarine cable, and either one or both of the 
reinforcing member and the pressure relating layer are formed of a 
material such as copper or aluminum to perform the functions of both the 
pressure resisting layer and a power feeding conductor. On the outside of 
the pressure resisting layer, an insulating layer 5 is covered with a 
material such, for example, as polyethylene, outside of which are disposed 
a tension member 6 and a protective jacket 7. In FIG. 7, as is the case 
with a conventional nonarmoured coaxial cable, the tension member 6 and a 
power feeding tape 8 are disposed at the center of a cable and covered 
with an insulator such as polyethylene, and optical fibers 10, each 
inserted in the pressure resisting layer, are arranged in the insulator in 
spaced relation to one another so as not to apply a pressure to their 
particular portions. In FIG. 8, an optical fiber inserted in the pressure 
resisting layer is disposed at the center of a cable and at least the 
cylindrical member of the pressure resisting layer is made of a material 
having high electrical conductivity such as, for example, copper or 
aluminum. A conductor 9 is disposed concentrically with the cylindrical 
member and an insulator such as polyethylene is packed between the 
pressure resisting layer and the conductor 9 to provide the function of a 
coaxial cable. In FIG. 8, reference numeral 6 indicates an armour line 
serving as a tension member, and 7 designates a protective jacket. The 
embodiment illustrated in FIG. 8 may be used as a submarine cable for 
transmitting information to and controlling of various submarine 
equipments, utilizing the wide transmission band property of an optical 
fiber and the simplicity of an electric circuit of the coaxial cable. 
As has been described above, according to this invention, by inserting a 
reinforcing member in a pressure resisting layer, a thin cylindrical 
member can be used and a pressure resisting layer easy to fabricate can be 
realized. Further, this pressure resisting layer can also be produced with 
a sheet of tape.With this invention, as described above, it is possible to 
produce a pressure resisting layer in which an optical fiber extends along 
a member for reinforcing the pressure resisting layer, so that the optical 
fiber can be held straight, preventing loss of the optical fiber which is 
caused by using it in a cable. By making such a reinforcing member of a 
material having high electrical conductivity, it can also be used as a 
power feeding conductor and an optical fiber submarine cable can be 
realized which withstands a high water pressure and is simple in 
construction.